TELOMERE REPEAT BINDING FACTORS AND DIAGNOSTIC AND THERAPEUTIC USE THEREOF

Abstract: The present invention relates to a nucleotide sequence encoding a telomeric protein which binds a repeat region sequences, and to the protein encoded thereby. Also included within the invention are expression vectors for the production of the telomeric protein and host cells transformed with the nucleotide sequence. In addition, antibodies, probes and antagonists specific for the telomeric protein are contemplated. Methods of identifying antagonists of the telomeric protein, diagnostic methods of identifying the telomeric protein in a sample, and therapeutic uses of the telomeric protein, particularly in the treatment of aging and cancer, are also contemplated.

Description: TELOMERE REPEAT BINDING FACTORS AND DIAGNOSTIC AND THERAPEUTIC USE THEREOF TECHNICAL FIELD OF THF. INVENTION The present invention relates generally to telomenc binding proteins, in particular to telomenc repeat binding factors (TRFs), to the nucleotide and ammo acid sequences encoding the TRFs, and to diagnostic and therapeutic methods of use thereof. TRFs have particular uses m the treatment of cancer and aging. BACKGROUND OF THF INVENTION Eucaryotic chromosomes end m specialized structures, called telomeres [Muller, The Collecting Net- Woods Hole, 13 : 181 - 195 ( 1939] that are thought to fulfill at least three functions. First, telomeres protect natural double-stranded DNA ends from degradation, fusion, and recombination with chromosome-internal DNA [McChntock, Genetics, 26.234- 282 (1941)]. Second, cytogenetic observations indicate that telomeres are located at the nuclear penphery, suggesting a role for chromosome ends in the architecture of the nucleus [Agard et al , Nature, 302:676-681 (1983); Rabl, Morphol. J., 10:214-330 (1885)]. Third, telomeres must provide a solution to the end-replication problem [Watson, Nature, 239:197- 201 (1972)]: because all known polymerases require a primer and synthesize DNA from 5' to 3', the 3' ends of linear DNA pose a problem to the replication machinery. The single common structural feature of most eucaryotic telomeres is the presence of a tandem array of G-nch repeats which, according to genetic studies in Saccharomyces cerevisiae, are necessary and sufficient for telomere function [Lundblad et al , Cell, 83:633- 643 (1989); Szostak et al., Cell, 36:459-568 (1982)]. Although all telomeres of one genome are composed of the same repeats, the terminal sequences in different species vary. For instance, Oxytncha chromosomes terminate in TTTTGGGG repeats [Klobutcher et al., Proc Natl Acad. Sci. USA, 78:3015-3019 (1981)], Tetrahymena utilizes an array of (TTGGGG) n [Blackburn et al., J. Mol. Bwl , 120:33-53 (1978)], plant chromosomes carry the sequence (TTTAGGG) n (Richards et al , Cell, 53:127-136 (1988)], and trypanosomas and mammals have TTAGGG repeats at their chromosome ends [Blackburn et al , Cell, 36:447-458 (1984); Brown, Nature, 338:774-776 (1986); Cross et al, Nature, 338:771-774 (1989); Moyzis et al , Proc Natl. Acad. Sci USA, 85:6622-6626 (1988); Van der Ploeg et al, Cell 36:459-468 (1984)]. The organization of the telomenc repeats is such that the G-nch strand extends to the 3' end of the chromosome. At this position, telomerase, an RNA-dependent DNA polymerase, first demonstrated m Tetrahymena thermophila and other ci ates, can elongate telomeres, probably by using an internal RNA component as template for the addition of the appropnate G-nch sequence [Greider and Blackburn, Cell, 43:405-413 (1985)]. This activity is thought to complement the inability of polymerases to replicate chromosome ends, but other mechanisms of telomere maintenance may operate as well [Pluta et al, Nature, 337:429-433 (1989)]. Recently, it has been reported that the addition of telomerase into a cultured human cell leads to an increase of the prohferative life-span of that cell [Bodner et al, Science, 279:349-352 (1998)]. Proximal to the essential telomenc repeats, some chromosome ends harbor additional common elements called sub-telomeπc repeats or telomere-associated sequences [Chan et al , Cell, 33:563-573 (1983); Corcoran et al, Cell, 53:807-813 (1988); de Lange et al , Nucl Acids Res , 11.8149-8165 (1983); Van der Ploeg et al (1984); Dunn et al , Cell, 39:191-201 (1984)]. Unlike telomenc repeats, these sequences are not conserved and their function remains unclear [Murray et al. , Mol Cell. Bwl , 6:3166-3172 (1986)]. Chromosome ends of unicellular organisms often show structural instability. Frequent rearrangements of subtelomeπc sequences occur in trypanosomas [Borst, Annu. Rev Bwchem., 55:701-732 (1986), de Lange et al (1983)], S. cerevisiae [Carlson et al, Mol Cell Biol , 3:351-359 (1983); Horowitz et al, Mol Cell Bwl , 4:2509-2517 (1984)], and plasmodia [Corcoran et al, (1988)], and changes in the telomenc repeat region can be observed in protozoa [Bernards et al, Nature, 303:592-597 (1983); Pays, Nucl. Acids. Res , 11:8137-8147 (1983); Van der Ploeg (1984)], cihates [Larson et al , Cell, 50:477-483 (1987)], and fungi [Carson et al, Cell, 42:249-257 (1985); Lundblad et al, (1989), Lustig et al, Proc. Natl. Acad. Sci USA, 83:1398-1402 (1986)]. As much as 3.5 kilobase pairs (kb) was seen to be added to telomeres of Trypanosoma brucei in a process that appears gradual and continuous, and was calculated to result from the addition of 6 to 10 base pairs (bp) per end per cell division [Bernards et al, (1983); Pays et al, (1983); Van der Ploeg, (1984)]. A similar gradual telomere elongation, compatible with the addition of telomenc repeats by telomerase, occurs in continuously growing T. thermophila [Larson, (1987)] and a cell cycle mutant (cdcl7) of S. cerevisiae [Carson et al, (1985)]. In wild-type S. cerevisiae [Shampay et al, Proc. Natl. Acad. Sci. USA, 85:534-538 (1988)], however, and in T. thermophila grown in batch cultures [Larson et al, (1998)], the tandem array of telomenc repeats is maintained at constant length. At least four genes (CDC 17, EST1, TEL1, and TEL2 [Carson et al , (1985); Lundblad et al, (1989); Lustig et al , (1986)] govern the length and stability of yeast telomeres; their mode of action is not understood. Much less is known about the structure and behavior of chromosome ends of multicellular organisms. Mammalian telomeres have become amenable to molecular dissection with the demonstration that telomenc repeats of plants and T. thermophila species cross-hybridize to vertebrate chromosome ends [Allshire et al, Nature, 332:656-659 (1988); Richards et al, (1988)]. It has also been shown that human DNA contains tandem arrays of TTAGGG repeats, probably at the chromosome ends, providing further evidence for the evolutionary conservation of telomeres and a tool for the isolation of telomenc DNA [Moyzis et al, (1988)]. Two strategies to obtain human chromosome ends have proven successful: an indirect isolation protocol that relies on human telomeres to be functional m S. cerevisiae [Brown et al, (1989); Cross et al , (1989)] and direct cloning in E. coh. de Lange et al [Mol Cell. Biol, 10:518-527 (1990)] characterized the structure and vanabi ty of human autosomal chromosome ends. The chromosome ends they analyzed shared a sub-telomeπc repeat of at least 4 kb that is not conserved in rodent genomes. These chromosome ends were characterized by a long stretch of DNA, of up to 14 kb, that lacks restnction enzyme cutting sites and may be entirely composed of TTAGGG repeats. From this region sequences are lost dunng development, leading to shortened, heterogeneously sized telomeres m somatic tissues, pnmary tumors, and most cell lines. de Lange [EMBO J., 11:717-724 (1992)] reported that human telomeres are tightly associated with the nuclear matrix. Telomeres were demonstrated to be anchored via their TTAGGG repeats. Moreover, TTAGGG repeats at internal sites withm the chromosome do not behave as matrix-attached loci, suggesting that the telomenc position of the repeats is required for their interaction with the nuclear matrix. This evidence is consistent with the role of telomeres in a nucleoprotem complex. TRF activity was first identified in 1992 by Zhong et al. [Mol Cell. Bwl , 12:4834-4943 (1992)] as a DNA-bmding factor specific for TTAGGG repeat arrays TRF was found to be present in nuclear extracts of human, mouse and monkey cells. The optimal site for TRF binding was found to contain at least six contiguous TTAGGG repeats. However, the protein isolated by Zhong et al was not sufficiently purified from other DNA-bmdmg proteins such that its ammo acid sequence could be determined. Saltman et al [Chromosoma, 102:121-128 (1993)] charactenzed the molecular structure of telomeres of two human tumor cell lines with frequent end-to-end associations of metaphase chromosomes. Such end-to-end associations have been observed in a vanety of human tumors, aging cells and several chromosome instability syndromes. The telomeres of such end-associated chromosomes were shown by Saltman et al. to be severely reduced compared to other human cells with functional telomeres. However, other cell lines with severely shortened telomeres were not detectably compromised in their function. Thus, the investigators suggested that telomeric length was not the only determinant of the fusigenic behavior of human telomeres in tumor cells. A Xenopus laevis protein factor that specifically recognizes vertebrate telomeric repeats at DNA ends, termed Xenopus telomere end factor (XTEF) was identified by Cardenas et al. in 1993 [Genes andDevel, 7:883-894 (1993)]. The DNA-binding properties of XTEF resembled the characteristics of a class of terminus-specific telomere proteins identified in hypotrichous cihates. There has been speculation on the role of an enzyme termed telomerase in human cancer, in particular in ovarian carcinoma [de Lange, Proc. Natl. Acad. Sci. USA, 91:2882-2885 (1994)]. Telomerases use the 3' end of DNA as a primer and employ an RNA template for the synthesis of G-rich telomeric repeats. Telomerase activation appears to be an obligatory step in the immortalization of human cells. Hanish et al. [Proc. Natl. Acad. Sci. USA, 91:8861-8865 (1994)] examined the requirements for the formation of human telomeres from TTAGGG seeds, and found that telomere formation was not correlated with the ability of human telomerase to elongate telomeric sequences in vitro, and did not appear to be a result of homologous recombination. Rather, the investigators reported that the sequence dependence of telomere formation matched the in vitro binding requirements for TRF 1. Although the activity of TRFl had been identified and isolated to some extent, the purification of TRF was fraught with difficulty, both in isolating the protein away from other DNA binding proteins, and in obtaining active protein from the isolate. Therefore, there is a need to isolate and characterize vertebrate TRFl. In addition, there is a need to identify other vertebrate telomere repeat binding factors which must also serve as structural and/or functional proteins in the maintenance of normal telomere physiological processes. Further, there is a need to isolate and characterize such TRFs (including TRF2) and to distinguish their characteristics from TRFl, as well as ascertain the role such TRFs play in telomere maintenance and elongation. SUMMARY OF THE INVENTION The present invention provides vertebrate telomenc binding factors (TRFs) that bind to the TTAGGG repeat sequences of telomeres. Such TRFs compnse two key domains: a dimenzation domain, and a Myb domain. In addition, at least some TRFs, e g mammalian TRFl and TRF2 contain a third domain, a polar N-termmal domain. In specific examples, the TRF nucleotide sequence is isolated from human, munne, or avian sources. The present invention includes these nucleic acids, the TRFs they encode, the individual domains of the encoded TRFs, and the nucleic acids that encode these individual domains. In one particular embodiment of the present invention the TRF has the following charactenstics- a) it binds to telomere repeat sequences, m particular, TTAGGG repeats, b) the DNA binding activity in a punfied form requires the presence of another factor such as casern ; and c) it exhibits substantial sequence homology to Myb type DNA binding domains. The present invention includes a nucleic acid or a degenerate vanant thereof, which encodes a TRF of the present invention; preferably a recombmant DNA molecule or cloned gene. For example a recombmant DNA molecule or cloned gene, encodes a TRF such a TRFl which has a nucleotide sequence of (or complementary to) SEQ ID NO:l 1 (shown in FIGURE 2A- 2B), or SEQ ID NO.22. These nucleotide sequences encode a TRF having an ammo acid sequence of SEQ ID NO: 12 or 23 respectively which are also part of the present invention A nucleotide sequence of a TRFl having a nucleotide sequence of SEQ ID NO:24 is also part of the present invention. The present invention also provides an isolated nucleic acid encoding a vertebrate telomere repeat binding factor (TRF) which is a TRF2 having an ammo acid sequence substantially homologous to that of SEQ ID NO:27, and compnsmg the following charactenstics: a basic N-terminal domain; a dimenzation domam; and a Myb domain. In one such embodiment when the basic N-termmal domain is removed the TRF detectably binds to the telomere repeat sequence (TTAGGG) 12 . Such binding is preferably detected m an in vitro assay. In a preferred embodiment of this type, the isolated nucleic acid encodes a TRF that is a mammalian protein. More preferably the isolated nucleic acid encodes a human TRF having the ammo acid sequence of SEQ ID N0.27, or SEQ ID NO.27 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid encodes a human TRF having the ammo acid sequence of SEQ ID NO:27. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:26. In another embodiment the isolated nucleic acid encodes a munne TRF having the ammo acid sequence of SEQ ID NO:29, or SEQ ID NO:29 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid encodes a munne TRF having the ammo acid sequence of SEQ ID NO:29. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:28. Another aspect of the invention includes a nucleic acid encoding an avian TRFl having the nucleotide sequence of SEQ ID NO:24. All of the isolated nucleic acids of the present invention can further compnse a heterologous nucleotide sequence. Such heterologous nucleotide sequences can encode, for example, a fusion peptide (e g., a FLAG-tag as in Example 7 below) or a chimenc protein partner such as a fusion protein. In addition any isolated nucleic acid of the present invention e g , the corresponding recombmant DNA molecule or cloned gene can be operatively linked to an expression control sequence which may be introduced into an appropriate host. The present invention accordingly extends to unicellular hosts transformed with the cloned gene or recombmant DNA molecule compnsing a DNA sequence encoding a TRF, or a structural/functional domain of a TRF of the present invention In one such example, the present invention provides the isolated nucleic acid encoding a vertebrate TRF having an ammo acid sequence of SEQ ID NO:27 operatively linked to an expression control sequence. The present invention also provides a unicellular host transformed or transfected with the nucleic acid. In addition the present invention provides a method of expressing the TRF encoded by the nucleic acid which comprises cultunng the unicellular host m an appropriate cell culture medium under conditions that provide for expression of the protein by the cell. This method can further compnse the step of punfymg the TRF. The punfied form of the TRF obtained by such methodology is also part of the present invention. This methodology is mtended to be general and is suitable for the expression and isolation of all of the nucleic acids of the present invention. According to other preferred features of certain preferred embodiments of the present invention, a recombmant expression system is provided to produce biologically active vertebrate TRFs, including human TRFs as well as TRF structural/functional domains, TRF chimeπc proteins and the like. The present invention also includes nucleic acids that encode the dimenzation domain of a TRF and/or the basic or acidic N-termmal domain of a TRF. In one such embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:37, or SEQ ID NO:37 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid encodes a basic N-termmal domain that has the ammo acid sequence of SEQ ID NO:37. In a preferred embodiment of this type the isolated nucleic acid comprises the coding sequence of SEQ ID NO:36. In another such embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:39, or SEQ ID NO-39 with a conservative amino acid substitution. In a particular embodiment the nucleic acid encodes a basic N-terminal domain that has the ammo acid sequence of SEQ ID NO:39. In a preferred embodiment of this type the isolated nucleic acid comprises the coding sequence of SEQ ID NO:38. In another embodiment an isolated nucleic acid compnses a nucleotide sequence encoding an acidic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:33, or SEQ ID NO:33 with a conservative ammo acid substitution In a particular embodiment the nucleic acid encodes an acidic N-termmal domain that has the ammo acid sequence of SEQ ID NO:33. In a preferred embodiment of this type the isolated nucleic acid comprises the coding sequence of SEQ ID NO:32. In another such embodiment an isolated nucleic acid compnses a nucleotide sequence encoding an acidic N-terminal domain of a TRF that has the ammo acid sequence of SEQ ID NO:35, or SEQ ID NO:35 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid encodes an acidic N-termmal domain that has the ammo acid sequence of SEQ ID NO:35. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:34. In still another embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a dimenzation domain of a TRF that has the ammo acid sequence of SEQ ID NO:45, or SEQ ID NO:45 with a conservative amino acid substitution. In a particular embodiment the nucleic acid compnses a nucleotide sequence encoding a dimenzation domain that has the ammo acid sequence of SEQ ID NO:45. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:44 In a related embodiment an isolated nucleic acid comprises a nucleotide sequence encoding a dimenzation domain of a TRF that has the ammo acid sequence of SEQ ID NO:47, or SEQ ID NO:47 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid compnses a nucleotide sequence encoding a dimenzation domain that has the ammo acid sequence of SEQ ID NO:47. In a preferred embodiment of this type the isolated nucleic acid comprises the coding sequence of SEQ ID NO:46. In another embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a dimenzation domain of a TRF that has the ammo acid sequence of SEQ ID NO:43, or SEQ ID NO:43 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid compnses a nucleotide sequence encoding a dimenzation domain that has the ammo acid sequence of SEQ ID NO:43. In a preferred embodiment of this type the isolated nucleic acid comprises the coding sequence of SEQ ID NO:42. In yet another embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a dimenzation domain of a TRF that has the ammo acid sequence of SEQ ID NO:49, or SEQ ID NO:49 with a conservative amino acid substitution. In a particular embodiment the nucleic acid comprises a nucleotide sequence encoding a dimenzation domain that has the amino acid sequence of SEQ ID NO:49. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:48. In still another embodiment an isolated nucleic acid compnses a nucleotide sequence encoding a dimenzation domain of a TRF that has the amino acid sequence of SEQ ID NO:41, or SEQ ID NO:41 with a conservative ammo acid substitution. In a particular embodiment the nucleic acid compnses a nucleotide sequence encoding a dimenzation domain that has the ammo acid sequence of SEQ ID NO:41. In a preferred embodiment of this type the isolated nucleic acid compnses the coding sequence of SEQ ID NO:40. The present invention includes an isolated nucleic acid comprising a nucleotide sequence encoding a truncated vertebrate TRF that has the ammo acid sequence of SEQ ID NO:31 or SEQ ID NO:31 with a conservative amino acid substitution. In a particular embodiment the isolated nucleic acid comprises a nucleotide sequence encoding a truncated vertebrate TRF that has the amino acid sequence of SEQ ID NO: 31. In a preferred embodiment of this type the isolated nucleic acid has the nucleotide sequence of SEQ ID NO:30. The present invention also provides all of the peptides or proteins that are encoded by all of the nucleic acids of the present invention including isolated TRFs, proteolytic fragments of TRFs, truncated proteins, and peptides or proteins compnsing a particular domain of a TRF such as the dimenzation domain, the Myb domain or the basic or acidic N-termmal domain of the TRF. In addition, all of these proteins and peptides can be combined into corresponding chimenc proteins or peptides such as fusion proteins or fusion peptides. Such chimenc proteins and peptides are also part of the present invention including for example, a chimenc protein having an N-termmal domain and a dimenzation domain of a TRF2 and a Myb domain of TRFl. In one such embodiment the isolated vertebrate telomere repeat binding factor (TRF) has an ammo acid sequence substantially homologous to that of SEQ ID NO:27, and comprises the following characteristics: a basic N-terminal domain, a dimenzation domain, and a Myb domain. In a preferred embodiment when the basic N-termmal domain is removed the TRF detectably binds to the telomere repeat sequence (TTAGGG) 12 . Such binding is preferably detected in an in vitro assay. Preferably the isolated TRF is a is a mammalian protein. In one embodiment the isolated TRF is a human protein having the amino acid sequence of SEQ ID NO:27, or SEQ ID NO:27 with a conservative amino acid substitution. In a particular embodiment the isolated TRF has the amino acid sequence of SEQ ID NO.27. In another embodiment the isolated TRF is a munne protein having the ammo acid sequence of SEQ ID NO:29, or SEQ ID NO:29 with a conservative amino acid substitution. In a particular embodiment the isolated TRF has the ammo acid sequence of SEQ ID NO:29. The present invention further provides an isolated protein comprising the basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:37, or SEQ ID NO:37 with a conservative ammo acid substitution. In a particular embodiment the isolated the basic N- termmal domain has the amino acid sequence of SEQ ID NO:37. Another embodiment compnses the basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:39, or SEQ ID NO:39 with a conservative ammo acid substitution. In a particular embodiment the isolated the basic N-termmal domain has the ammo acid sequence of SEQ ID NO:39. An isolated protein compnsing a dimenzation domain of a TRF having the ammo acid sequence of SEQ ID NO:45, or SEQ ID NO:45 with a conservative ammo acid substitution is also included in the present invention. In a particular embodiment the isolated the dimenzation domain has the ammo acid sequence of SEQ ID NO:45. A related embodiment contains an isolated protein compnsing a dimenzation domain of a TRF having the ammo acid sequence of SEQ ID NO:47, or SEQ ID NO:47 with a conservative amino acid substitution. In a particular embodiment the isolated the dimenzation domain has the amino acid sequence of SEQ ID NO:47. The present invention also includes an isolated protein compnsing a dimenzation domain of a TRF having the ammo acid sequence of SEQ ID NO:41, or SEQ ID NO:41 with a conservative amino acid substitution. In a particular embodiment the isolated the dimenzation domain has the ammo acid sequence of SEQ ID NO:41. A related embodiment contains an isolated protein compnsing a dimenzation domain of a TRF having the ammo acid sequence of SEQ ID NO:43, or SEQ ID N0.43 with a conservative ammo acid substitution. In a particular embodiment the isolated the dimenzation domain has the ammo acid sequence of SEQ ID NO:43. In yet another embodiment an isolated protein compnses a dimenzation domain of a TRF that has the ammo acid sequence of SEQ ID NO:49, or SEQ ID NO:49 with a conservative ammo acid substitution. In a particular embodiment of this type, the dimenzation domain has the ammo acid sequence of SEQ ID NO:49. The present invention further provides an isolated avian TRF encoded by SEQ ID NO:24 The present invention also provides an isolated protein that is a truncated TRF having the ammo acid sequence of SEQ ID NO.31 or SEQ ID NO:31 with a conservative ammo acid substitution. In a particular embodiment the isolated the truncated TRF has the ammo acid sequence of SEQ ID NO:31. The present invention also includes antibodies to all of the TRFs and TRF domains of the present invention. One such embodiment is an antibody that recognizes a basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:37. In another such embodiment the antibody recognizes a basic N-termmal domain of a TRF that has the ammo acid sequence of SEQ ID NO:39 Such antibodies can be polyclonal, monoclonal, and/or chimenc antibodies. The present invention also includes immortal cell lines that produce the monoclonal antibodies of the present invention. In a related aspect of the present invention, a novel method for purifying telomenc binding proteins is provided, which compnses the steps of: a) isolating nuclei from tissue culture cells; b) preparmg nuclear extracts of the nuclei; c) contacting the nuclear extracts with an affinity chromatography column compnsing a bound DNA fragment, wherein the DNA fragment comprises TTAGGG repeat sequences; and d) eluting telomenc binding proteins from the column. In a particular embodiment, casein is added to the eluted telomenc binding proteins to obtain active DNA-bmdmg proteins. In another aspect of the present invention, the TRFs of the present invention or antagonists or agonists thereof may be used to counteract the shortening of telomere length which occurs dunng agmg, and to counteract the abnormal telomere physiology present m cancerous cells. Accordingly, methods of providing a TRF and/or its agonists or antagonists are contemplated Still a further aspect of the present invention extends to antibodies and ohgonucleotide probes to the TRFs of the present invention, which may be used for both diagnostic and therapeutic approaches. The DNA sequences of the TRFs of the present invention or portions thereof, may be prepared as probes to screen for complementary sequences and genomic clones m the same or alternate species. The present invention extends to probes so prepared that may be provided for screening cDNA and genomic hbranes for the TRF. For example, the probes may be prepared with a vanety of known vectors, such as the phage λ vector. The present invention also includes the preparation of plasmids including such vectors, and the use of the DNA sequences to construct vectors expressing antisense RNA or πbozymes which would attack the mRNAs of any or all of the DNA sequences in the present invention. Correspondingly, the preparation of antisense RNA and nbozymes are included herein. The present invention also includes TRF proteins having the activities noted herein, and that have the ammo acid sequences included m the present invention. The present invention provides specific factors i.e., TRFs, which bind to TTAGGG repeat sequences as descnbed earlier. Accordingly, the exact structure of each TRF will understandably vary so as to achieve this DNA binding and activity specificity. It is this specificity and the direct involvement of the TRF in the chain of events leading to telomere length regulation, that offers the promise of a broad spectrum of diagnostic and therapeutic utilities. The present invention naturally contemplates several means for preparation of a TRF, including as illustrated herein known recombmant techniques, and the invention is accordingly intended to cover such synthetic preparations withm its scope. The isolation of the cDNAs that encode a TRF ammo acid sequence disclosed herein (e g , using a nucleic acid that hybndizes with the cDNA encoding a TRF to act as a probe/bmdmg partner to detect and/or isolate a nucleic acid encoding a related TRF), facilitates the reproduction of the TRF by such recombmant techniques, and accordingly, the invention extends to expression vectors prepared from the disclosed DNA sequences for expression m host systems by recombmant DNA techniques, and to the resulting transformed hosts. The invention includes an assay system for screening of potential drugs effective to modulate TRF activity of target mammalian cells by interrupting or potentiating the activity of the TRF. In one instance, the test drug could be administered to a cellular sample containing the TRF along with telomenc sequences, to determine its effect upon the binding activity of the TRF to the DNA, or to the test drug, by companson with a control. In still another assay, the purified TRFs or a particular structural/functional domain are used as targets for testing the binding charactenstics of potential drugs. For example, the basic N-termmal domain of TRF2 and acidic N-terminal domain of TRFl can be employed to screen potential drugs for binding specificity for the two corresponding TRFs. In this way a drug can be readily identified which is likely to interfere with TRFl without interfenng with TRF2. The assay system could more importantly be adapted to identify drugs or other entities that are capable of binding to a TRF and/or other telomenc binding factors or proteins in the nucleus, thereby inhibiting or potentiating the activity of the TRF. Such assays would be useful in the development of drugs that would be specific against particular cellular activity, or that would potentiate such activity, m time or m level of activity. For example, such drugs might be used to inhibit the proliferation of cells m cancerous states, or to treat cells which are agmg, or to treat other pathologies associated with vanations m telomere length. In yet a further embodiment, the invention contemplates antagonists of the activity of a TRF, m particular, an agent or molecule that inhibits the role of TRFs in telomere function. In a specific embodiment, the antagonist can be a peptide having the sequence of a portion of a DNA binding domain of a TRF, such as that illustrated by SEQ ID NO: 13. The diagnostic utility of the present invention extends to the use of the present TRF in assays to screen for cancer and other mhented diseases associated with telomere length. The present invention likewise extends to the development of antibodies against the TRFs or to the specific structural/functional domains of the TRFs of the present invention, including naturally raised and recombmantly prepared antibodies. For example an antibody raised against the basic N-termmal domain of a TRF can be used to distinguish a TRF2 from a TRFl . In addition, the antibodies could be used to screen expression libraries to obtain the gene or genes that encode the TRFs. Such antibodies could include both polyclonal and monoclonal antibodies prepared by known genetic techniques, as well as bi-specific (chimenc) antibodies, and antibodies including other functionalities suiting them for additional diagnostic use conjunctive with their capability of modulating telomere length. Thus, the TRFs, their analogs and/or agonists, and any antagonists or antibodies that may be raised thereto, are capable of use in connection with various diagnostic techniques, including immunoassays, such as a radioimmunoassay, using for example, an antibody to the TRF that has been labelled by either radioactive addition, reduction with sodium borohydπde, or radio lodination. In an immunoassay, a control quantity of the antagonists or antibodies thereto, or the like may be prepared and labelled with an enzyme, a specific binding partner and/or a radioactive element, and may then be introduced into a cellular sample. After the labelled matenal or its binding partner(s) has had an opportunity to react with sites withm the sample, the resulting mass may be examined by known techniques, which may vary with the nature of the label attached. In the instance where a radioactive label, such as the isotopes 3 H, ,4 C, 32 P, 35 S, 36 C1, 51 Cr, 57 Co, 58 Co, 59 Fe, 90 Y, 125 1, 131 I, and 186 Re are used, known currently available counting procedures may be utilized. In the instance where the label is an enzyme, detection may be accomplished by any of the presently utilized colonmetnc, spectrophotometnc, fluorospectrophotometnc, amperometnc or gasometπc techniques known in the art. The present invention includes an assay system which may be prepared in the form of a test kit for the quantitative analysis of the extent of the presence of the TRF, or to identify drugs or other agents that may mimic or block their activity. The system or test kit may comprise a labelled component prepared by one of the radioactive and/or enzymatic techniques discussed herein, coupling a label to the TRFs, their agonists and/or antagonists, and one or more additional immunochemical reagents, at least one of which is a free or immobilized ligand, capable either of binding with the labelled component, its binding partner, one of the components to be determined or their binding partner(s). In a further embodiment, the present invention relates to certain therapeutic methods which would be based upon the activity of the TRF(s), its (or their) subunits, or active fragments thereof, or upon agents or other drugs determined to possess the same activity. A first therapeutic method is associated with the prevention of the manifestations of conditions causally related to or following from the binding activity of the TRF or its subunits, and comprises administering an agent capable of modulating the production and/or activity of the TRF or subunits thereof, either individually or in mixture with each other in an amount effective to prevent the development of those conditions in the host. For example, drugs or other binding partners to the TRF or proteins may be administered to inhibit or potentiate TRF activity, as in the potentiation of TRF activity in aging, or the inhibition or modulation of TRF activity in cancer therapy. More specifically, the therapeutic method generally referred to herein could include the method for the treatment of various pathologies or other cellular dysfunctions and derangements by the administration of pharmaceutical compositions that may comprise effective inhibitors or enhancers of activation of the TRF or its subunits, or other equally effective drugs developed for instance by a drug screening assay prepared and used in accordance with a further aspect of the present invention. For example, drugs or other binding partners to the TRF or proteins, as represented by SEQ ID NO: 12, may be administered to inhibit or potentiate telomere lengthening activity, as in the potentiation of TRF in cancer therapy. In particular, the isolated TRFs, proteolytic fragments of TRFs, truncated proteins, and peptides or proteins which comprise a particular structural/functional domain of a TRF, their antibodies, agonists, antagonists, or active fragments thereof, could be prepared in pharmaceutical formulations for administration in instances where appropriate, such as to treat cancer or counteract the aging process. Accordingly, it is a pnncipal object of the present invention to provide TRFs in punfied form that exhibits certain charactenstics and activities associated with telomere lengthening activity. It is a further object of the present invention to provide antibodies to the TRFs, and methods for their preparation, including by recombmant means. It is a further object of the present invention to provide a method for detecting the presence of the TRF and its subunits in mammals m which invasive, spontaneous, or ldiopathic pathological states are suspected to be present. It is a further object of the present invention to provide a method and associated assay system for screening substances such as drugs, agents and the like, potentially effective m either mimicking the activity or combating the adverse effects of the TRFs and/or its subunits in mammals. It is a still further object of the present invention to provide a method for the treatment of mammals to control the amount or activity of the TRF or subunits thereof, so as to alter the adverse consequences of such presence or activity, or where beneficial, to enhance such activity. It is a still further object of the present invention to provide a method for the treatment of mammals to control the amount or activity of the TRF or its subunits, so as to treat or avert the adverse consequences of invasive, spontaneous or ldiopathic pathological states. It is a still further object of the present invention to provide pharmaceutical compositions for use in therapeutic methods which compnse or are based upon the TRF, its subunits, their binding partner(s), or upon agents or drugs that control the production, or that mimic or antagonize the activities of the TRF. Other objects and advantages will become apparent to those skilled the art from a review of the ensuing description which proceeds with reference to the following illustrative drawings. BRTF.F DESCRIPTION OF THF. PR AWTNfiS FIGURE 1 A- IB depicts peptide sequences derived from the purified TRF preparation (SEQ ID NOS:1-10). FIGURE 2A-2B depicts the nucleotide sequence of human TRF mRNA (SEQ ID NO:l 1). The Xho I site used for construction of expression constructs (in vitro and in vivo) is indicated. The sequence contains a Met ATG codon with the surrounding sequence AACΔT-GG, which is as expected for an initiation codon. The encoded protein is 439 amino acids in length, with a predicted molecular weight of about 51 kD. The fact that this is somewhat smaller than the 67 kD protein observed on SDS-PAGE indicated that TRF may be modified or run anomalously on SDS-PAGE. FIGURE 3 depicts the deduced amino acid sequence of human TRF (SEQ ID NO: 12). In italics is a region with a high percentage acidic residues ("acidic domain"), a hallmark of transcription factors. Underlined are the regions determined by peptide sequencing. Bolded is the region of homology to Myb type DNA binding repeats. FIGURE 4 depicts the alignment of human TRF sequence to Myb type DNA binding domains (SEQ ID NOS : 13- 19). Hu = human, Mu = mouse and Dro = Drosophila. FIGURE 5 depicts a gel-shift experiment showing telomeric repeat binding activity of protein encoded by the human HTRF4.1 cDNA. The human cDNA was fused onto a His6 tag and a initiator codon and expressed by in vitro transcription (T7 RNA polymerase) and translation in a rabbit reticulocyte lysate. No gel shift complex is formed in the absence of human TRF cDNA. The probe is a (TTAGGG) 12 containing restriction fragment. Competitions were done with plasmids containing long arrays of telomeric DNAs with the indicated sequence. Gel-shift methods are as described by [Zhong et al, Mol. Cell Biol, 13:4834-4843 (1992)]. The data indicate that the human TRF binds TTAGGG repeats with the same sequence specificity as HeLa TRF activity. FIGURE 6 depicts a gel-shift experiment showing telomeric repeat binding activity of protein encoded by the mouse MTRF12 cDNA. The cDNA was transcribed using T3 RNA polymerase and translated in a rabbit reticulocyte lysate. The probe is a (TTAGGG) 12 - containing restriction fragment. Competitions were done with plasmids containing long arrays of telomeric DNAs with the indicated sequence. Gel-shift methods are as described by (Zhong et al, 1992). FIGURE 7 depicts various staining patterns on HeLa cells. Figure 7A shows a FLAG epitope tagged mouse TRF expressed in human HeLa cells. Shown is two interphase nuclei in which the anti-FLAG antibodies detect a speckled TRF pattern (green) against the background of D API stained DNA (blue). Figure 7B shows in situ hybridization of telomeric TTAGGG DNA (red) in the nuclei shown in A. Figure 7C shows superimposed images from Figures 7A and 7B, demonstrating that all signals co-localize. Figure 7D shows HA epitope tagged mouse TRF expressed in human HeLa cells. Shown is a metaphase spread in which the anti-HA antibody detects TRF at each chromosome end (green). The DNA is stained with DAPI (blue). FIGURE 8: Characterization of the expression of TRF2. Figure 8 A depicts a Northern blot (Clontech) containing poly-adenylated RNAs from the indicated human tissues probed with hTRF2, hTRFl, and a β-actin probe. Figure 8b shows an SDS-PAGE of 35 S-methionine labelled in vitro translation products obtained with cDNAs encoding mouse and human TRFl and TRF2 (as indicated above the lanes). Figure 9A shows the alignment of the primary sequences of mouse and human TRFl and TRF2. Identical amino acids are highlighted. The likely positions of the three helices constituting the Myb domain, as inferred from the NMR structure of c-Myb,[Ogata et al, Cell 79:639-648 (1994)] are indicated below the sequence. Figure 9B is a schematic representation of the domain structure of hTRFl and hTRF2 and their sequence similarity. The data on TRFl is from Broccoli et al.[Hum. Mol. Genetics 6:69-76 (1997)] A small alternatively spliced exon (ALT SPL) is indicated in the hTRFl sequence. The asterisk in c denotes an alanine residue that was found to be absent in one of the HeLa cDNAs sequenced. Figure 10: TRF2 binds duplex telomeric DNA with the same sequence specificity as TRFl. Figure 10A shows the results of a gel-shift assay [Zhong et al. Mol. Cell. Biol. 13:4834-4843 (1992)] with whole cell extracts [Bianchi et al, EMBO J. 16:1785-1794 (1997)] from HeLa cells transfected with FLAG-tagged full length mTRF2 or the vector backbone as indicated below the lanes. Incubations contained either of the following additions: antibody 5 to hTRFl [Luderus et al, J. Cell. Biol. 135:867-883 (1996)]. FLAG-antibody M2, or 200 ng of a plasmid (pTH5 19 ) containing [TTAGGG] 27 as indicated above the lanes. Figure 10B shows the results of a gel-shift assay with extracts of cells transfected with FLAG-tagged mTRF2ΔN-49 or the vector backbone. Additions as m a. Asterisks in a and b denote mTRF2-DNA complexes supershifted with the FLAG antibody. Figure IOC shows the results of a gel-shift assay with mTRF2ΔN-49 as in b but in the presence of 50 and 100 ng circular plasmids containing 1-2 kb stretches of the indicated telomeric sequences [Hanish et al, Proc. Natl Acad Sci USA 91:8861-8865 (1994)] (lanes 3-14) or single-stranded [TTAGGG], and [CCCTAA] 6 oligonucleotides (lanes 15-18). The reaction m lane 2 contained 50 ng pTH5. The probe used in Figures 10 A- IOC is a double-stranded restriction fragment containing the sequence [TTAGGG] 12 [Zhong et al, 1992, supra]. Figure 11: Telomenc localization of TRF2 m transfected HeLa cells. Figures 11A-11C show the co-localization of FLAG-epitope tagged mTRF2 protein with telomeric DNA in mterphase. Figure 11A shows the detection of mTRF2 with the FLAG-antibody M2 (green) in transiently transfected HeLa cell. Figure 1 IB shows the detection of telomenc DNA the same nucleus with a [CCCUAA] 27 RNA probe (red), and Figure 11C depicts the supenmposition of Figures 11A-1 IB. Figure 1 ID shows the detection of mTRF2 at the ends of metaphase chromosomes with the M2 anti-FLAG antibody (green). DNA was stained with 4, 6-dιamιdιno-2-phenyhndole (blue) in Figures 11A-1 ID. Figure 12: TRF2 interacts with TRF2 but not with TRFl. Figure 12A shows the results from two hybrid assays [Fields et al, Nature 340:245-246 (1989)] for interactions between LexA and GAD fusion proteins containing the mdicated portions of hTRFl and hTRF2 constructed as described previously [Bianchi et al , 1997, supra], β-galactosidase activity levels were measured as descnbed previously [Bianchi et al. , 1997, supra] for strains containing the indicated plasmids. Values represent an average of three to nine independent transformants. Figure 12B depicts in vitro translated 35 S-methιonιne labelled TRFl and TRF2 proteins with or without an N-termmal FLAG tag (indicated above the lanes) were lmmunoprecipitated using the M2 anti-FLAG antibodies and analyzed by autoradiography. Figure 13. Inducible expression of TRF2 proteins in HTC75 cells. Figure 13A depicts a schematic of full length human TRF2 [Broccoli et al , Nature Gen., 17:231-235 (1997)] and the deletion mutants TRF2 ΛB and TRF2 ΔBΔM . The approximate position of the peptide used to raise the polyclonal αTRF2 antibody #508 is indicated. The two TRF2 deletion mutants carry an N-termmal FLAG epitope. Figure 13B shows a Western analysis for the mducible expression of the three forms of TRF2 shown m Figure 13 A. Whole cell extracts were prepared from clonal HTC75-denved cell lines expressing the full length TRF2 (clone PI 2), TRF2 ΔB (clone SI 3), TRF2 ΔBΔM (clone T4), and control cell line B27, which contains the empty vector. Extracts were prepared from cells grown in parallel in the presence (umnduced) or absence (induced) of doxycychn for the indicated time. For each extract 20 μg of protein was fractioned, blotted and incubated with the pnmary antibodies indicated m the Figure. Figure 14. In vivo effects of the TRF2 mutants on telomere binding of endogenous wild-type TRFl and TRF2 in transiently transfected HeLa cells. Figure 14A-B show the localization of endogenous wild-type TRF2 using antibody #508 (green/yellow) m an mterphase nucleus (Fig. 14A) and on mitotic chromosomes (Fig. 14B) of HeLa cells. DNA was stained with DAPI (shown in red). Figure 14C-D show HeLa cells transiently transfected with wild-type TRF were dual-labelled for TRF2 using antibody #508 (green in Fig. 14C) and endogenous TRFl using mouse serum #2 (end TRFl, red in Fig. 14D). Three transfected cells overexpressing TRF2 are indicated by arrowheads; the other three cells were probably not transfected and showed levels of endogenous TRF2 similar to untransfected control cells. Figure 14 E-H show HeLa cells transiently transfected with TRF2 ΔB that were dual-labelled for FLAG-tagged mutant protein using antibody M2 (green in Fig. 14E and Fig. 14G) and either endogenous TRF2 (endTRF2, red in Fig. 14F) or endogenous TRFl (endTRFl, red in Figure 14I-L show HeLa cells transiently transfected with TRF2 ΔBΔM that were dual-labelled for FLAG-tagged mutant protein using antibody M2 (green in Fig 141 and 14K) and either endogenous TRF2 (red m Fig. 14J) or endogenous TRFl (red m Fig. 14L). DAPI staining of nuclear DNA m C-L is shown m blue. Figure 15. Growth arrest and induction of a senescent phenotype in response to TRF2 mutants. Figure 15A and 15B contain graphs showing the effect of induction of full length TRF2 (clone P12), TRF2 ΔB (clone S13), and TRF2 ΔBΔM (clone T4) on the growth of HTC75 cells. B27 is a clonal HTC75 cell line containing the vector. Figure 15C-H show the morphological changes of the indicated HTC75 clones expressing the indicated TRF2 alleles grown for 9 days in the presence or absence (umnduced and induced, respectively) of doxycychn. Cells were stained for β-galactosidase activity at pH 6.0 and photographed using DIC optics. Figure 16 depicts the induction of anaphase bndges and metaphase fusions by TRF2 ΔBΔM . Figure 16A-C shows three anaphase cells displaying TRF2 ΔBΔM -mduced anaphase bridges and a lagging chromosome (cell on left). DNA was stained with DAPI. Figure 16D shows the mduction of anaphase bridged and lagging chromosomes (together referred to as "events" on the y-axis) in T4 cells expressing the TRF2 ΔBΔM and lack of induced fusions in SI 3 cells expressing TRF2 ΔB . For each time point one hundred anaphase cells were scored for anaphase bridges and lagging chromosomes. Figure 16E-G shows telomere fusions in metaphase chromosomes from T4 cells induced to express TRF2 ΔBΔM (Fig. 16E) Metaphase chromosomes showing end-to-end fusions stained with DAPI. Several fusion events (F) and a ring chromosome (R) are indicated. Figure 16F shows metaphase chromosomes G-banded with Trypsm showing multiple end-to-end fusions. Figure 16G shows the detection of telomeric TTAGGG repeats at the sites of telomere fusion (arrowheads). TTAGGG repeats were detected using a fluorescently labelled PNA [CCCTAA] 3 probe (green). DNA was stained with DAPI. Figure 17. Detection of telomere fusions in naked DNA. Figure 17A shows the telomere structure m clonal HTC75 lines expressing wildtype TRF2 (P clones) and in a vector control cell line (clone B27) grown m the presence and absence of doxycychn (- and + induction respectively) for eight population doublings. Figure 17B shows the telomere structure in clonal lines expressing the indicated deletion of alleles of TRF2 grown with and without doxycychn for 9 days (- and + induction respectively). Figure 17C depicts the time course of changes m telomere structure in T4 cells induced to express TRF2 ΔBΔM and in S24 cells induced to express TRF2 ΔB . Figure 17D shows the Bal31 exonuclease digestion of DNA from T4 cells induced to express TRF2 ΔBΔM for 9 days. Figure 17E shows the quantitation of Bal31 exonuclease experiment similar to that shown in Figure 17D performed with DNA from T19 cells induced to express TRF2 ΔBΔM for 9 days. Figure 17F shows the heat stability of the telomeric fusions. DNA derived from the same cells used in Figure 17D was treated for 10 minutes at the indicated temperatures and immediately loaded on an agarose gel. Figure 17A-F shows all genomic DNA samples were digested with Hinfl and Rsal and analyzed by blotting using a TTAGGG repeat scientific probe (see Experimental Procedures in Example 8). The position of λHindlll DNA marker fragments (23, 9.4, 6.6, 4.4, 2.3, and 2.0 kb) is indicated next to each blot. Figure 18. Expression of TRF2 ΔBΔM causes loss of G-strand overhang signals in the presence of telomerase activity. Figure 18A shows the G-strand overhang assays performed on DNA derived from the indicated cell lines (grown in the presence or absence of doxycychn for 9 days as indicated) expressing the indicated TRF2 polypeptides. Figure 18B depicts the time course of the loss of G-tails in the T4 clone expressing r τ "C ΔBΔM Figure 18C shows the quantitation of the loss of G-strand overhangs upon induction of TRF2 ΔBΔM in two independents experiments performed with the T4 clone. The data were derived from two experiments similar to those shown in Figure 18B and the average value was plotted. Figure 18D shows the similar telomerase activity in four HTC75 clonal lines expressing the indicated TRF2 polypeptides grown for 9 days in the presence or absence of doxycychn (- and + induction respectively). For each extract, identical amounts of protein (0.5 μg) were tested using the TRAP assay. Figure 19 illustrates the role of human telomeric proteins in telomere protection and telomere length regulation. TRFl is depicted as a negative regulator of telomere maintenance, proposed to act by inhibiting telomerase at individual chromosome ends [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264, filed February 13, 1997, herein incorporated by reference in its entirety] TRF2 is involved in the protection of chromosome ends by inhibiting end-to-end fusions. Thus, in the process of adding TTAGGG repeats, telomerase synthesizes binding sites for two proteins onto chromosome ends, one of which ensures telomere mtegnty and the other regulates the length of the telomeres. DETAILED DESCRIPTION The present invention provides vertebrate telomeric binding factors (TRFs) that bind specifically to duplex TTAGGG repeats in vitro. Human TRFs, for example, are localized to all human telomeres in metaphase. The binding of the TRFs to telomeres plays an important role m telomere elongation and maintenance. The TRFs of the present invention contain a distinctive polar N-termmal domain, a dimenzation domain, and a Myb domain. TRFs function as homodimers in solution and require two Myb motifs to bind to telomenc DNA in vitro and in vivo. There are two classes of TRFs, TRFls and TRF2s. TRFl and TRF2 do not readily form heterodimers with each other, if at all. Further, although the structural motif of the both classes of TRFs contain the same three structural/functional domains listed above, both the dimenzation domain and the polar N-termmal domains of TRFs differ appreciably. Indeed, whereas mammalian TRFl has an acidic N-termmal domain, mammalian TRF2 has a basic N-terminal domain. Similarly, the lack of heterodimers indicates that the dimenzation domains of the two classes of TRFs do not bind to each other. The structural features of the proteins and more particularly the individual domains allows the two classes to be readily distinguished in either functional assays or in drug assays in which the object would be to attenuate the function of one of the classes of TRFs while minimally effecting the function of the other class. As descnbed herein, TRF2 is a human telomenc protein that is required to maintain the correct structure at telomere termini, and protects against end-to-end fusions. In addition, TRF2 plays a role m the successful progression through the cell division cycle. As such, TRF2 is involved in the main functions ascnbed to telomeres in somatic human cells and is therefore a likely player in the loss of telomere function and growth arrest that accompanies telomere shortening m normal and transformed human cells. The mechanism by which telomeres prevent end-to-end fusion has heretofore remained elusive. Human telomeres are bound by two TTAGGG repeat binding factors: TRFl, a negative regulator of telomere maintenance, and TRF2, a homolog of TRFl. TRF2 is shown herein to be required for cellular proliferation and for the protection of chromosome ends in human cells. Overexpression of two deletion denvatives of TRF2 lacking its basic N-terminus induced an irreversible growth arrest with characteristics of cellular senescence A strong dommant negative allele causing the loss of endogenous TRF2 from telomeres, induced end-to-end chromosome fusions detectable m metaphase and anaphase cells. Telomeric DNA persisted at the fusions, demonstrating that TTAGGG repeat arrays per se are not sufficient to maintain telomere mtegnty. Telomenc fusions were detectable in native genomic DNA as joined terminal restriction fragments and molecular analysis suggested that they represented hgation of chromosome ends that have lost their smgle-stranded G-tails. TRF2 protects chromosome ends from fusion, through the maintenance of the correct structure at telomere termini. The chromosome end fusions and growth arrest observed m senescent pnmary human cells and certain malignant cells are caused by the loss of TRF2 from the critically shortened telomeres in these cells. A general view of the logic underlying the function of human telomeres is now emerging (Figure 19). Human telomerase has long been understood to maintain the terminal sequences of human chromosome ends and thus counter DNA attrition with cell divisions. The need for telomere length maintenance is particularly obvious m immortalized cells and in the germhne. The results described herein reveal a second function for telomerase. In addition to balancing the terminal sequence loss that accompanies DNA replication, in the process of synthesizing arrays of TTAGGG repeats, telomerase also ensures the continued presence of TRF2 binding sites at chromosome ends. Since TRF2 is required to prevent telomere fusions, telomerase thus maintains the protective activity of telomeres by constantly replenishing TRF2 binding sites that are lost from telomere termini with DNA replication. This second function of telomerase critically depends on the sequence of the telomenc repeats its synthesizes and this model predicts that the exact sequence of the telomerase products is a key aspect of the mechanism of telomere function. In this regard, mutations of the telomerase template RNA in Tetrahymena has given nse to dramatic cellular phenotypes [Yu et al , Nature, 344: 126-132 (1990); Kirk et al, Science, 275:1478-1481 (1997)], including occasional anaphase bndges that may well represent telomere fusions of the type reported herein. Tetrahymena telomeric binding proteins that could have been displaced by the altered telomenc repeats have not yet been identified. Addition of TTAGGG repeats to chromosome ends also ensures the binding of a second telomenc protein, TRFl, that acts as a negative regulator of telomerase, and modulates the length of the TTAGGG repeats arrays at chromosome ends [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997, herein incorporated by reference m its entirety]. Thus, the telomerase-mediated maintenance of telomeric TTAGGG repeats secures a functional and regulated telomenc complex required for the mtegnty of chromosome ends. Selective chromosome end fusions and growth arrest in, malignant cells, for example can be effected by inhibiting TRF2 by agents identified by methods and compositions disclosed herein. Since both TRFl and TRF2 have the ability to bind to double-stranded TTAGGG repeats, they can be used as tools for the targeting of protein domains to TTAGGG repeats in vitro and in vivo. For example, the Green Fluorescent Protein (GFP) can be targeted to telomeres by fusion to TRFl . Such specific targeting of protein domains to TTAGGG repeat regions in the genome can act as important research tools for telomere length monitoring etc. Furthermore, TRFl and TRF2 can also be used m the construction of mammalian artificial chromosomes. Artificial chromosomes have great utility in gene therapy and basic research. Artificial chromosomes depend on the functionality of their telomere ends. Since telomeres depend on the function of both TRFl and TRF2, coating the telomere termini of artificial chromosomes with TRFl and/or TRF2 strongly enhances the frequency of artificial chromosome formation. In particular the activity of TRF2 as a telomere protection protein can be crucial m the generation of functional telomeres. Overexpression of TRFl in human HTC75 cells has resulted m cell lines with cntically shortened telomeres (e.g the D4 cell line of U.S. Patent Application No. 08/800,264 filed February 13, 1997). Because their telomeres are unusually short, such cell lines are expected to show a highly increased sensitivity to the inhibition of telomerase and could be used as a screening tool for the in vivo effects of telomerase inhibitors identified by in vitro screening strategies. Therefore, the present invention provides nucleic acids, antibodies, proteins and fragments thereof and methodology using the same which can be utilized in basic research and/or the clinical setting, e.g., to monitor, probe, diagnose and potentially treat conditions involving telomere maintenance. The direct correlation shown between telomere maintenance and cellular senescence, for example [Bodner et al , Science, 279:349-352 (1998)] indicates that the compositions of matter and processes provided by the present invention can also play a direct role in preventing and/or treating (1) atrophy of the skin through loss of extracellular matrix homeostasis in dermal fϊbroblasts [Takeda et al, Arch. Dermatol, 130:87 (1994)]; (2) age- related macular degeneration [Boulton et al, J. Neuroscl, 15:4992 (1995)]; and (3) atherosclerosis [Kamazaki et al, J. Med. Sci., 42:97 (1993)]. In addition, Bodner et al. [supra] have pointed out that cells having an extended life-span can also have important ex vivo applications in the production of bioengineered products and even in gene therapy. Nucleic Acids, Peptides and Proteins In accordance with the present invention there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, Second Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York (herein "Sambrook et al, 1989); DNA Cloning: A Practical Approach, Volumes I and II (D.N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed. 1984); Nucleic Acid Hybridization [B.D. Hames & S. J. Higgins eds (1985)]; Transcription and Translation [B.D. Hames & S. J. Higgins, eds. (1984)]; Animal Cell Culture [R. I. Freshney, ed. (1986)]; Immobilized Cells and Enzymes [IRL Press, (1986)]; B. Perbal, A Practical Guide to Molecular Cloning (1984); F. M. Ausubel et al. (eds.), Current Protocols in Molecular Biology, John Wiley & Sons, Inc. (1994). Therefore, if appearing herein, the following terms shall have the definitions set out below. The terms "telomere repeat binding factor", "telomeric repeat binding factor", "telomeric binding factor", "TTAGGG repeat binding factor", and "TRF," and any variants not specifically listed, may be used herein interchangeably, and as used throughout the present application and claims refer to proteinaceous material including single or multiple proteins, including the functional dimeric form of the protein and extends to those proteins having the amino acid sequences described herein, and the profile of activities set forth herein. Accordingly, proteins displaying substantially equivalent or altered activity are likewise contemplated. These modifications may be deliberate, for example, such as modifications obtained through site-directed mutagenesis, or may be accidental, such as those obtained through mutations in hosts that are producers of the complex or its named subunits. Also, the terms "telomere repeat binding factor," and "TRF" are intended to include within their scope proteins specifically recited herein as well as all substantially homologous analogs and allelic variations. There are two classes of TRFs, TRFls and TRF2s both of which are contained in the group of proteins labelled as TRFs. A "structural/functional domain" is used herein to indicate a specific domain of a TRF which can identified by a structural and/or a functional characteristic such as a polar N-terminal domain, a dimerization domain, and a Myb domain. The polar N-terminal domain of TRFl is acidic, whereas that of TRF2 is basic. A "basic N-terminal domain" as used herein comprises an amino acid sequence at the N- terminal segment of a TRF that contains 30 to 90 amino acids, preferably 40 to 60 amino acids which has a ratio of basic amino acids to acidic amino acids of greater than 1.5:1 and preferably greater than 2:1. Examples of such basic N-terminal amino acids include SEQ ID NOs:37 and 39. An "acidic N-terminal domain" as used herein comprises an amino acid sequence at the N- terminal segment of a TRF that contains 30 to 90 amino acids, preferably 40 to 75 amino acids which has a ratio of acidic amino acids to basic amino acids of greater than 2:1 and preferably greater than 3:1. Examples of such basic N-terminal amino acids include SEQ ID NOs:32 and 35. The amino acid residues described herein are preferred to be in the "L" isomeric form. However, residues in the "D" isomeric form can be substituted for any L-amino acid residue, as long as the desired functional property of the protein is retained by the polypeptide. A "replicon" is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo; i.e., capable of replication under its own control. A "vector" is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A "DNA molecule" refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) in its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences may be descnbed herein according to the normal convention of giving only the sequence in the 5' to 3' direction along the nontranscnbed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). An "ongin of replication" refers to those DNA sequences that participate in DNA synthesis. A DNA "coding sequence" is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropnate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5' (ammo) terminus and a translation stop codon at the 3' (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcnption termination sequence will usually be located 3' to the coding sequence. Transcnptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers, polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. A "promoter sequence" is a DNA regulatory region capable of binding RNA polymerase m a cell and initiating transcnption of a downstream (3' direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3' terminus by the transcnption initiation site and extends upstream (5' direction) to include the minimum number of bases or elements necessary to initiate transcnption at levels detectable above background. Withm the promoter sequence will be found a transcnption initiation site (conveniently defined by mapping with nuclease SI), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters will often, but not always, contain "TATA" boxes and "CAT" boxes. Prokaryotic promoters contain Shme-Dalgarno sequences m addition to the -10 and -35 consensus sequences. An "expression control sequence" is a DNA sequence that controls and regulates the transcnption and translation of another DNA sequence. A coding sequence is "under the control" of transcnptional and translational control sequences in a cell when RNA polymerase transcnbes the coding sequence into mRNA, which is then translated into the protein encoded by the coding sequence. A "signal sequence" can be included before the coding sequence. This sequence encodes a signal peptide, N-termmal to the polypeptide, that communicates to the host cell to direct the polypeptide to the cell surface or secrete the polypeptide into the media, and this signal peptide is clipped off by the host cell before the protein leaves the cell. Signal sequences can be found associated with a vanety of protems native to prokaryotes and eukaryotes. The term "oligonucleotide," as used herein in referring to the probe of the present invention, is defined as a molecule comprised of about 15 or more nucleotides, preferably more than about 24. Its exact size will depend upon many factors which, m turn, depend upon the ultimate function and use of the oligonucleotide. The term "pnmer" as used herein refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product, which is complementary to a nucleic acid strand, is induced, i.e., m the presence of nucleotides and an inducing agent such as a DNA polymerase and at a suitable temperature and pH. The pnmer may be either smgle-stranded or double-stranded and must be sufficiently long to pnme the synthesis of the desired extension product in the presence of the inducing agent. The exact length of the pnmer will depend upon many factors, including temperature, source of pnmer and use of the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide pnmer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. The pnmers herein are selected to be "substantially" complementary to different strands of a particular target DNA sequence. This means that the primers must be sufficiently complementary to hybndize with their respective strands. Therefore, the pnmer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5' end of the pnmer, with the remainder of the primer sequence being complementary to the strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the pnmer, provided that the primer sequence has sufficient complementanty with the sequence of the strand to hybndize therewith and thereby form the template for the synthesis of the extension product. Mutations can be made in nucleotide sequences of the present invention such that a particular codon is changed to a codon which codes for a different amino acid. Such a mutation is generally made by making the fewest nucleotide changes possible. A substitution mutation of this sort can be made to change an amino acid in the resulting protein in a non- conservative manner (i.e., by changing the codon from an amino acid belonging to a grouping of amino acids having a particular size or characteristic to an amino acid belonging to another grouping) or in a conservative manner (i.e., by changing the codon from an amino acid belonging to a grouping of amino acids having a particular size or characteristic to an amino acid belonging to the same grouping). Such conservative amino acid changes define the term "a conservative amino acid substitution" as used herein, which is used to denote one or more conservative changes. A conservative change generally leads to less change in the structure and function of the resulting protein. A non-conservative change is more likely to alter the structure, activity or function of the resulting protein. The present invention should be considered to include sequences containing conservative changes which do not significantly alter the activity or binding characteristics of the resulting protein. The following is one example of various groupings of amino acids: Amino acids with nonpolar R groups Alanine Valine Leucine Isoleucine Proline Phenylalanine Tryptophan Methionine Amino acids with uncharged polar R groups Glycine Serine Threonine Cysteine Tyrosme Asparagme Glutamme Ammo acids with charged polar R groups (negatively charged at Ph 6.0) Aspartic acid Glutamic acid Basic ammo acids (positively charged at pH 6.0) Lysine Arginme Histidme (at pH 6.0) Another grouping may be those ammo acids with phenyl groups: Phenylalanme Tryptophan Tyrosme Particularly preferred substitutions are: - Lys for Arg and vice versa such that a positive charge may be maintained; - Glu for Asp and vice versa such that a negative charge may be maintained; - Ser for Thr such that a free -OH can be maintained; and - Gin for Asn such that a free NH, can be maintained. Amino acid substitutions may also be introduced to substitute an ammo acid with a particularly preferable property. For example, a Cys may be introduced a potential site for disulfide bridges with another Cys. A His may be introduced as a particularly "catalytic" site (i.e., His can act as an acid or base and is the most common amino acid in biochemical catalysis). Pro may be introduced because of its particularly planar structure, which induces β-turns m the protein's structure. A "heterologous nucleotide sequence" as used herein is a nucleotide sequence that is added to a nucleotide sequence of the present invention by recombmant methods to form a nucleic acid which is not naturally formed m nature. Such nucleic acids can encode chimenc and/or fusion proteins. Thus the heterologous nucleotide sequence can encode peptides and/or proteins which contain regulatory and/or structural properties. In another such embodiment the heterologous nucleotide can encode a protein or peptide that functions as a means of detecting the protein or peptide encoded by the nucleotide sequence of the present invention after the recombmant nucleic acid is expressed. In still another such embodiment the heterologous nucleotide can function as a means of detecting a nucleotide sequence of the present invention. A heterologous nucleotide sequence can compnse non-codmg sequences including restnction sites, regulatory sites, promoters and the like. A "heterologous region" of the DNA construct is an identifiable segment of DNA withm a larger DNA molecule that is not found m association with the larger molecule in nature. Thus, when the heterologous region encodes a mammalian gene, the gene will usually be flanked by DNA that does not flank the mammalian genomic DNA m the genome of the source organism. Another example of a heterologous coding sequence is a construct where the coding sequence itself is not found in nature (e.g., a cDNA where the genomic coding sequence contains nitrons, or synthetic sequences having codons different than the native gene). Allelic variations or naturally-occurnng mutational events do not give rise to a heterologous region of DNA as defined herein. A cell has been "transformed" by exogenous or heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into chromosomal DNA making up the genome of the cell. In prokaryotes, yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a plasmid. With respect to eukaryotic cells, a stably transformed cell is one m which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing the transforming DNA. A "clone" is a population of cells derived from a single cell or common ancestor by mitosis. A "cell line" is a clone of a pnmary cell that is capable of stable growth in vitro for many generations. As used herein, the terms "restnction endonucleases" and "restnction enzymes" refer to bactenal enzymes, each of which cut double-stranded DNA at or near a specific nucleotide sequence. Two DNA sequences are "substantially homologous" when at least about 60% (preferably at least about 80%, and most preferably at least about 90 or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybndization expenment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybndization conditions is withm the skill of the art. See, e.g., Mamatis et al, supra; DNA Cloning, Vols. I & II, supra; Nucleic Acid Hybndization, supra. Likewise, two polypeptide sequences are "substantially homologous" when at least about 60% (preferably at least about 80%, and most preferably at least about 90 or 95%) of the ammo acids are either identical or contain conservative changes, as defined above, over the defined length of the polypeptide sequences. A DNA sequence is "operatively linked" to an expression control sequence when the expression control sequence controls and regulates the transcnption and translation of that DNA sequence. The term "operatively linked" includes having an appropnate start signal (e.g., ATG) in front of the DNA sequence to be expressed and maintaining the correct reading frame to permit expression of the DNA sequence under the control of the expression control sequence and production of the desired product encoded by the DNA sequence. If a gene that one desires to insert into a recombmant DNA molecule does not contain an appropnate start signal, such a start signal can be inserted in front of the gene. A nucleic acid molecule is "hybndizable" to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropnate conditions of temperature and solution ionic strength (see Sambrook et al, supra). The conditions of temperature and ionic strength determine the "stringency" of the hybndization. For preliminary screening for homologous nucleic acids, low stringency hybndization conditions are used corresponding to 50°C as descnbed by Church and Gilbert [Proc. Natl. Acad Sci. USA, 81:1991-1995 (1984)] Washes are performed in 2X SSC/0.1% SDS at 50°C. Moderate stringency hybridization conditions correspond to a higher temperature e g , 60 °C High stnngency hybridization conditions are performed at 65 °C. Washes m this case are performed in 0.3X SSC/0.1% SDS at 65 °C. Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stnngency of the hybridization, mismatches between bases are possible. The appropnate stnngency for hybndizmg nucleic acids depends on the length of the nucleic acids and the degree of complementation, vanables well known in the art. The greater the degree of similanty or homology between two nucleotide sequences, the greater the value of T m for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher T m ) of nucleic acid hybndizations decreases m the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybnds of greater than 100 nucleotides in length, equations for calculating T m have been denved (see Sambrook et al, supra, 9.50-0.51). For hybndization with shorter nucleic acids, i.e., ohgonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook et al, supra, 11.7-11.8). Preferably a minimum length for a hybndizable nucleic acid is at least about 12 nucleotides; preferably at least about 18 nucleotides; and more preferably the length is at least about 27 nucleotides; and most preferably 36 nucleotides or more. In a specific embodiment, the term "standard hybndization conditions" refers to a T m of 55 °C, and utilizes conditions as set forth above. In a preferred embodiment, the T m is 60 °C; in a more preferred embodiment, the T m is 65 °C. The term "standard hybridization conditions" refers to salt and temperature conditions substantially equivalent to 5 x SSC and 65 °C for both hybndization and wash. The term "approximately" is used interchangeably with the term "about" and means that the value may vary by 10%, preferably no more than 5%, and most preferably no more than 2%. Another feature of this invention is the expression of the DNA sequences disclosed herein. As is well known in the art, DNA sequences may be expressed by operatively linking them to an expression control sequence in an appropnate expression vector and employing that expression vector to transform an appropnate unicellular host. Such operative linking of a DNA sequence of this invention to an expression control sequence, of course, includes, if not already part of the DNA sequence, the provision of an initiation codon, ATG, m the correct readmg frame upstream of the DNA sequence. A wide vanety of host expression vector combinations may be employed in expressing the DNA sequences of this invention. Useful expression vectors, for example, may consist of segments of chromosomal, non-chromosomal and Synthetic DNA sequences. Suitable vectors include derivatives of SV40 and known bactenal plasmids, e.g., E. coli plasmids col El, pCRl, pBR322, pMB9 and their denvatives, plasmids such as RP4; phage DNAS, e.g., the numerous derivatives of phage λ, e.g., NM989, and other phage DNA, e.g., M13 and Filamentous smgle stranded phage DNA; yeast plasmids such as the 2μ plasmid or derivatives thereof; vectors useful in eukaryotic cells, such as vectors useful in insect or mammalian cells; vectors derived from combinations of plasmids and phage DNAs, such as plasmids that have been modified to employ phage DNA or other expression control sequences; and the like. Any of a wide vanety of expression control sequences — sequences that control the expression of a DNA sequence operatively linked to it ~ may be used m these vectors to express the DNA sequences of this invention. Such useful expression control sequences include, for example, the early or late promoters of SV40, CMV, vaccinia, polyoma or adenovirus, the lac system, the trp system, the TAC system, the TRC system, the LTR system, the major operator and promoter regions of phage λ, the control regions of fd coat protein, the promoter for 3-phosphoglycerate kmase or other glycolytic enzymes, the promoters of acid phosphatase (e.g., Pho5), the promoters of the yeast α-matmg factors, and other sequences known to control the expression of genes of prokaryotic or eukaryotic cells or their viruses, and various combinations thereof A wide vanety of unicellular host cells are also useful in expressing the DNA sequences of this invention. These hosts may include well known eukaryotic and prokaryotic hosts, such as strains of E coli, Pseudomonas, Bacillus, Streptomyces, fungi such as yeasts, and animal cells, such as CHO, Rl.l, B-W and L-M cells, African Green Monkey kidney cells (e.g., COS 1, COS 7, BSC1, BSC40, and BMT10), insect cells (e.g., Sf9), and human cells and plant cells in tissue culture. It will be understood that not all vectors, expression control sequences and hosts will function equally well to express the DNA sequences of this invention. Neither will all hosts function equally well with the same expression system However, one skilled in the art will be able to select the proper vectors, expression control sequences, and hosts without undue expenmentation to accomplish the desired expression without departing from the scope of this invention. For example, in selecting a vector, the host must be considered because the vector must function in it. The vector's copy number, the ability to control that copy number, and the expression of any other proteins encoded by the vector, such as antibiotic markers, will also be considered. In selecting an expression control sequence, a vanety of factors will normally be considered These include, for example, the relative strength of the system, its controllability, and its compatibility with the particular DNA sequence or gene to be expressed, particularly as regards potential secondary structures. Suitable unicellular hosts will be selected by consideration of, e.g., their compatibility with the chosen vector, their secretion charactenstics, their ability to fold proteins correctly, and their fermentation requirements, as well as the toxicity to the host of the product encoded by the DNA sequences to be expressed, and the ease of punfication of the expression products. Considenng these and other factors a person skilled in the art will be able to construct a vanety of vector/expression control sequence/host combinations that will express the DNA sequences of this invention on fermentation or m large scale animal culture. In a specific embodiment, an TRF fusion protein or peptide can be expressed. A TRF fusion protein compnses at least a functionally active portion of a non-TRF protein joined via a peptide bond to a TRF or a structural/functional domain of a TRF. Similarly a TRF fusion peptide can be expressed. The non-TRF sequences can be amino- or carboxyl-termmal to the TRF sequences. For stable expression of a TRF fusion protein, the portion of the non-TRF fusion protein or peptide can be joined via a peptide bond to the ammo terminus of the TRF protein. A recombmant DNA molecule encoding such a fusion protein comprises a sequence encoding at a functionally active portion of a non-TRF protein or peptide joined in- frame to the TRF coding sequence, and preferably encodes a cleavage site for a specific protease, e.g., thrombm or Factor Xa, preferably at the TRF-non-TRF juncture. In a specific embodiment, the fusion protein is expressed in Escherichia coli. An example of a fusion peptide is a TRF having a FLAG-tag as descnbed in Example 7 below. An example of a fusion protein is a TRF or a structural/functional domain of a TRF joined with a green fluorescent protein or modified green fluorescent protein as descnbed in U.S. Patent 5,625,048, Issued 4/29/97 herein incorporated by reference in its entirety. Such fusion proteins and peptides may also be classified as chimenc proteins or peptides which further include TRFs having switched structural/functional domains such as a TRF having an acidic N-termmal domain and a Myb domain of a TRFl, and a dimenzation domain of a TRF2. All of such chimenc TRFs including the fusion proteins and peptides are contemplated in the present invention. It is further intended that TRF analogs may be prepared from nucleotide sequences of the protein complex/subumt denved withm the scope of the present invention. Analogs, such as fragments, may be produced, for example, by pepsin digestion of TRF material. Other analogs, such as mutems, can be produced by standard site-directed mutagenesis of TRF coding sequences. Analogs exhibiting "TRF activity" such as small molecules, whether functioning as promoters or inhibitors, may be identified by known in vivo and/or in vitro assays. As mentioned above, a DNA sequence encoding a TRF can be prepared synthetically rather than cloned. The DNA sequence can be designed with the appropnate codons for the TRF amino acid sequence. In general, one will select preferred codons for the intended host if the sequence will be used for expression. The complete sequence is assembled from overlapping ohgonucleotides prepared by standard methods and assembled into a complete coding sequence. See, e.g., Edge, Nature, 292:756 (1981); Nambair et al , Science, 223:1299 ( 1984); Jay et al , J Bwl Chem , 259:6311 (1984). Synthetic DNA sequences allow convenient construction of genes which will express TRF analogs or "mutems". Alternatively, DNA encoding mutems can be made by site-directed mutagenesis of native TRF genes or cDNAs, and mutems can be made directly using conventional polypeptide synthesis. A general method for site-specific incorporation of unnatural ammo acids into proteins is descnbed in Chnstopher J. Noren, Spencer J. Anthony-Cahill, Michael C. Griffith, Peter G. Schultz, Science, 244:182-188 (April 1989) This method may be used to create analogs with unnatural ammo acids Antibodies An "antibody" is any lmmunoglobuhn, including antibodies and fragments thereof, that binds a specific epitope. The term encompasses polyclonal, monoclonal, and chimenc antibodies, the last mentioned descnbed in further detail in U.S Patent Nos 4,816,397 and 4,816,567 One antibody of the present invention is exemplified by an antibody to human TRF2 in Example 8. An "antibody combinmg site" is that structural portion of an antibody molecule compnsed of heavy and light chain vanable and hypervanable regions that specifically binds antigen The phrase "antibody molecule" in its vanous grammatical forms as used herein contemplates both an intact lmmunoglobuhn molecule and an immunologically active portion of an lmmunoglobuhn molecule Exemplary antibody molecules are intact immunoglobulin molecules, substantially intact immunoglobulin molecules and those portions of an immunoglobulin molecule that contains the paratope, including those portions known in the art as Fab, Fab', F(ab') 2 and F(v), which portions are preferred for use in the therapeutic methods described herein. Fab and F(ab') 2 portions of antibody molecules are prepared by the proteolytic reaction of papain and pepsin, respectively, on substantially intact antibody molecules by methods that are well-known. See for example, U.S. Patent No. 4,342,566 to Theofilopolous et al. Fab' antibody molecule portions are also well-known and are produced from F(ab') 2 portions followed by reduction of the disulfide bonds linking the two heavy chain portions as with mercaptoethanol, and followed by alkylation of the resulting protein mercaptan with a reagent such as iodoacetamide. An antibody containing intact antibody molecules is preferred herein. The phrase "monoclonal antibody" in its various grammatical forms refers to an antibody having only one species of antibody combining site capable of immunoreacting with a particular antigen. A monoclonal antibody thus typically displays a single binding affinity for any antigen with which it immunoreacts. A monoclonal antibody may therefore contain an antibody molecule having a plurality of antibody combining sites, each immunospecific for a different antigen; e.g., a bispecific (chimeric) monoclonal antibody. The general methodology for making monoclonal antibodies by hybridomas is well known. Immortal, antibody-producing cell lines can also be created by techniques other than fusion, such as direct transformation of B lymphocytes with oncogenic DNA, or transfection with Epstein-Barr virus. See, e.g., M. Schreier et al, "Hybridoma Techniques" (1980); Hammerling et al, "Monoclonal Antibodies And T-cell Hybridomas" (1981); Kennett et al, "Monoclonal Antibodies" (1980); see also U.S. Patent Nos. 4,341,761; 4,399,121; 4,427,783; 4,444,887; 4,451,570; 4,466,917; 4,472,500; 4,491,632; 4,493,890. Panels of monoclonal antibodies produced against TRF peptides can be screened for various properties; i.e., isotype, epitope, affinity, etc. Of particular interest are monoclonal antibodies that neutralize the binding activity of the TRF or its subunits. Such monoclonals can be readily identified in, for example, gel-shift assays. High affinity antibodies are also useful when immunoaffϊnity purification of native or recombmant TRF is possible. Preferably, the anti-TRF antibody used m the diagnostic methods of this invention is an affinity punfied polyclonal antibody. More preferably, the antibody is a monoclonal antibody (mAb). In addition, it is preferable for the anti-TRF antibody molecules used herein be in the form of Fab, Fab', F(ab') 2 or F(v) portions of whole antibody molecules. Methods for producing polyclonal anti-polypeptide antibodies are well-known in the art. See U.S. Patent No. 4,493,795 to Nestor et al. A monoclonal antibody, typically containing Fab and/or F(ab') 2 portions of useful antibody molecules, can be prepared using the hybndoma technology descnbed in Antibodies - A Laboratory Manual, Harlow and Lane, eds., Cold Spnng Harbor Laboratory, New York (1988), which is incorporated herein by reference. Bnefly, to form the hybndoma from which the monoclonal antibody composition is produced, a myeloma or other self-perpetuating cell line is fused with lymphocytes obtained from the spleen of a mammal hypenmmumzed with a TRF-bindmg portion thereof, or TRF, or a DNA-bmdmg portion thereof. Splenocytes are typically fused with myeloma cells using polyethylene glycol (PEG) 6000. Fused hybrids are selected by their sensitivity to HAT. Hybridomas producing a monoclonal antibody useful m practicing this invention are identified by their ability to immunoreact with the present TRF and their ability to inhibit specified activity at telomeres in target cells. A monoclonal antibody useful in practicing the present invention can be produced by initiating a monoclonal hybndoma culture comprising a nutrient medium containing a hybndoma that secretes antibody molecules of the appropnate antigen specificity. The culture is maintained under conditions and for a time period sufficient for the hybndoma to secrete the antibody molecules into the medium. The antibody-containmg medium is then collected. The antibody molecules can then be further isolated by well-known techniques. Media useful for the preparation of these compositions are both well-known in the art and commercially available and include synthetic culture media, mbred mice and the like. An exemplary synthetic medium is Dulbecco's minimal essential medium (DMEM) [Dulbecco et al, Virol 8:396 (1959)] supplemented with 4.5 gm/1 glucose, 20 mm glutamme, and 20% fetal calf serum. An exemplary bred mouse strain is the Balb/c. Methods for producing monoclonal anti-TRF antibodies are also well-known in the art. See Niman et al. [Proc. Natl. Acad. Sci USA, 80:4949-4953 (1983)]. Typically, the present TRF or a peptide analog is used either alone or conjugated to an lmmunogenic carrier, as the immunogen in the before-described procedure for producing anti-TRF monoclonal antibodies. The hybndomas are screened for the ability to produce an antibody that immunoreacts with the TRF peptide analog and the present TRF. Diagnostics and Therapeutics The phrase "pharmaceutically acceptable" refers to molecular entities and compositions that are physiologically tolerable and do not typically produce an allergic or similar untoward reaction, such as gastric upset, dizziness and the like, when administered to a human. The phrase "therapeutically effective amount" is used herein to mean an amount sufficient to prevent, and preferably reduce by at least about 30 percent, more preferably by at least 50 percent, most preferably by at least 90 percent, a clinically significant change the S phase activity of a target cellular mass, or other feature of pathology such as for example, elevated blood pressure, fever or white cell count as may attend its presence and activity. The possibilities both diagnostic and therapeutic that are raised by the existence of the TRF, derive from the fact that the factors appear to participate in direct and causal protem-DNA interaction between the repeat sequences that are bound by their binding factors, and those factors that thereafter directly interface with the DNA repeat sequence and effect telomere length and accordingly the health and/or prohferative capacity of the cell. As suggested earlier and elaborated further on herein, the present invention contemplates pharmaceutical intervention in the cascade of reactions m which the TRF is implicated, to modulate the activity initiated by the binding factor bound to the repeat sequence. In instances where it is desired to modulate the length or function of a telomere resulting from a particular stimulus or factor, an appropriate modulator of the TRF could be introduced to block the interaction of the TRF with those repeat sequences causally connected with telomere maintenance. As discussed earlier, the TRF or their binding partners or other ligands or agents exhibiting either mimicry or antagonism to the TRF or control over their production, may be prepared in pharmaceutical compositions, with a suitable carrier and at a strength effective for administration by various means to a patient expenencing an adverse medical condition associated with abnormal telomere length, stimulation for the treatment thereof. A variety of administrative techniques may be utilized, among them parenteral techniques such as subcutaneous, intravenous and mtrapentoneal injections, cathetenzations and the like. Average quantities of the TRFs or their subunits may vary and in particular should be based upon the recommendations and prescription of a qualified physician or veterinarian. Also, antibodies including both polyclonal and monoclonal antibodies, and drugs that modulate the production or activity of the TRF and/or their subunits may possess certain diagnostic applications and may for example, be utilized for the purpose of detecting and/or measuring conditions such as viral infection or the like. For example, the TRF or its structural/functional domains may be used to produce both polyclonal and monoclonal antibodies to themselves in a variety of cellular media, by known techniques such as the hybridoma technique utilizing, for example, fused mouse spleen lymphocytes and myeloma cells. Likewise, small molecules that mimic or antagonize the activity(ies) of the TRF of the invention may be discovered or synthesized, and may be used in diagnostic and/or therapeutic protocols. As suggested earlier, the diagnostic method of the present invention comprises examining a cellular sample or medium by means of an assay including an effective amount of an antagonist to a TRF protein, such as an anti-TRF antibody, preferably an affinity-purified polyclonal antibody, and more preferably a mAb. In addition, it is preferable for the anti- TRF antibody molecules used herein be in the form of Fab, Fab', F(ab') 2 or F(v) portions or whole antibody molecules. As previously discussed, patients capable of benefiting from this method include those suffering from cancer, a pre-cancerous lesion, a viral infection or other like pathological derangement. Methods for isolating and inducing anti-TRF antibodies and for determining and optimizing the ability of anti-TRF antibodies to assist in the examination of the target cells are all well-known in the art. The present invention further contemplates therapeutic compositions useful in practicing the therapeutic methods of this invention. A subject therapeutic composition includes, in admixture, a pharmaceutically acceptable excipient (carrier) and one or more of a TRF, polypeptide analog thereof or fragment thereof, as described herein as an active ingredient. In a preferred embodiment, the composition comprises an antigen capable of modulating the specific binding of the present TRF within a target cell. The preparation of therapeutic compositions which contain polypeptides, analogs or active fragments as active ingredients is well understood in the art. Typically, such compositions are prepared as injectables, either as liquid solutions or suspensions, however, solid forms suitable for solution in, or suspension in, liquid prior to injection can also be prepared. The preparation can also be emulsified. The active therapeutic ingredient is often mixed with excipients which are pharmaceutically acceptable and compatible with the active ingredient. Suitable excipients are, for example, water, saline, dextrose, glycerol, ethanol, or the like and combinations thereof. In addition, if desired, the composition can contain minor amounts of auxiliary substances such as wetting or emulsifying agents, pH buffering agents which enhance the effectiveness of the active ingredient. A polypeptide, analog or active fragment can be formulated into the therapeutic composition as neutralized pharmaceutically acceptable salt forms. Pharmaceutically acceptable salts include the acid addition salts (formed with the free amino groups of the polypeptide or antibody molecule) and which are formed with inorganic acids such as, for example, hydrochloric or phosphoric acids, or such organic acids as acetic, oxalic, tartaric, mandelic, and the like. Salts formed from the free carboxyl groups can also be derived from inorganic bases such as, for example, sodium, potassium, ammonium, calcium, or ferric hydroxides, and such organic bases as isopropylamine, trimethylamine, 2-ethylamino ethanol, histidine, procaine, and the like. The therapeutic polypeptide-, analog- or active fragment-containing compositions are conventionally administered intravenously, as by injection of a unit dose, for example. The term "unit dose" when used in reference to a therapeutic composition of the present invention refers to physically discrete units suitable as unitary dosage for humans, each unit containing a predetermined quantity of active material calculated to produce the desired therapeutic effect in association with the required diluent; i.e., carrier, or vehicle. The compositions are administered in a manner compatible with the dosage formulation, and in a therapeutically effective amount. The quantity to be administered depends on the subject to be treated, capacity of the subject's immune system to utilize the active ingredient, and degree of inhibition or neutralization of TRF binding capacity desired. Precise amounts of active ingredient required to be administered depend on the judgment of the practitioner and are peculiar to each individual. However, suitable dosages may range from about 0.1 to 20, preferably about 0.5 to about 10, and more preferably one to several, milligrams of active ingredient per kilogram body weight of individual per day and depend on the route of administration. Suitable regimes for initial administration and booster shots are also variable, but are typified by an initial administration followed by repeated doses at one or more hour intervals by a subsequent injection or other administration. Alternatively, continuous intravenous infusion sufficient to maintain concentrations of ten nanomolar to ten micromolar m the blood are contemplated. The present invention also relates to a vanety of diagnostic applications, including methods for detecting the presence of stimuli such as the earlier referenced polypeptide ligands, by reference to their ability to elicit the activities which are mediated by the present TRF. As mentioned earlier, the TRFs can be used to produce antibodies to itself by a vanety of known techniques, and such antibodies could then be isolated and utilized as in tests for the presence of particular activity of TRF at telomeres in suspect target cells. Antisense, Gene Targeting and Ribozymes The functional activity of TRF can be evaluated transgenically. In this respect, a transgemc mouse model can be used. The TRF gene can be used in complementation studies employing transgemc mice. Transgemc vectors, including viral vectors, or cosmid clones (or phage clones) corresponding to the wild type locus of candidate gene, can be constructed using the isolated TRF gene. Cosmids may be introduced into transgemc mice using published procedures [Jaemsch, Science, 240-1468-1474 (1988)]. In a genetic sense, the transgene acts as a suppressor mutation. Alternatively, a transgemc animal model can be prepared m which expression of the TRF gene is disrupted. Gene expression is disrupted, according to the invention, when no functional protein is expressed. One standard method to evaluate the phenotypic effect of a gene product is to employ knock-out technology to delete the gene (see U.S. Patent 5,464,764 Issued 11/7/95 herein incorporated by reference in its entirety.) The present invention also extends to the preparation of antisense nucleotides and ribozymes that may be used to interfere with the expression of the TRF at the translational level. This approach utilizes antisense nucleic acid and nbozymes to block translation of a specific mRNA, either by masking that mRNA with an antisense nucleic acid or cleaving it with a πbozyme. Antisense nucleic acids are DNA or RNA molecules that are complementary to at least a portion of a specific mRNA molecule. [See Wemtraub, (1990); Marcus-Sekura, (1988)]. In the cell, they hybndize to that mRNA, forming a double stranded molecule. The cell does not translate an mRNA m this double-stranded form. Therefore, antisense nucleic acids interfere with the expression of mRNA into protein. Ohgomers of about fifteen nucleotides and molecules that hybndize to the AUG initiation codon will be particularly efficient, since they are easy to synthesize and are likely to pose fewer problems than larger molecules when introducing them into TRF-producing cells. Antisense methods have been used to inhibit the expression of many genes in vitro [Marcus-Sekura, (1988); Hambor et al , (1988)]. Ribozymes are RNA molecules possessing the ability to specifically cleave other single stranded RNA molecules in a manner somewhat analogous to DNA restnction endonucleases. Ribozymes were discovered from the observation that certain mRNAs have the ability to excise their own mtrons. By modifying the nucleotide sequence of these RNAs, researchers have been able to engineer molecules that recognize specific nucleotide sequences m an RNA molecule and cleave it [Cech, (1988)]. Because they are sequence-specific, only mRNAs with particular sequences are inactivated. Investigators have identified two types of nbozymes, Tetrahymena-type and "hammerhead"-type. [Hasselhoff and Gerlach, (1988)] Tetrahymena-type ribozymes recognize four-base sequences, while "hammerhead"-type recognize eleven- to eighteen-base sequences. The longer the recognition sequence, the more likely it is to occur exclusively m the target mRNA species. Therefore, hammerhead-type nbozymes are preferable to Tetrahymena-type ribozymes for inactivating a specific mRNA species, and eighteen base recognition sequences are preferable to shorter recognition sequences The DNA sequences described herein may thus be used to prepare antisense molecules against, and nbozymes that cleave mRNAs for TRF and their ligands. Labels The TRFs, structural functional domains of TRFs, and their antibodies, nucleic acids encoding TRFs, structural/functional domains of TRFs and probes to the nucleic acids may all be labelled The labels most commonly employed for these studies are radioactive elements, enzymes, chemicals which fluoresce when exposed to ultraviolet light, and others A number of fluorescent matenals are known and can be utilized as labels. These include, for example, fluorescem, rhodamine, auramme, Texas Red, AMCA blue and Lucifer Yellow A particular detecting matenal is anti-rabbit antibody prepared m goats and conjugated with fluorescem through an isothiocyanate. The TRF or its binding partner(s) can also be labelled with a radioactive element or with an enzyme. The radioactive label can be detected by any of the currently available counting procedures. The preferred isotope may be selected from 3 H, 14 C, 32 P, 35 S, 36 C1, 51 Cr, 57 Co, 58 Co, 59 Fe, 90 Y, ,25 1, 131 I, and 186 Re. Enzyme labels are likewise useful, and can be detected by any of the presently utilized colorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques. The enzyme is conjugated to the selected particle by reaction with bridging molecules such as carbodiimides, diisocyanates, glutaraldehyde and the like. Many enzymes which can be used in these procedures are known and can be utilized. The preferred are peroxidase, β-glucuronidase, β-D-glucosidase, β-D-galactosidase, urease, glucose oxidase plus peroxidase and alkaline phosphatase. U.S. Patent Nos. 3,654,090; 3,850,752; and 4,016,043 are referred to by way of example for their disclosure of alternate labeling material and methods. Assays for Agonists and Antagonists of TRFs Identification and isolation of a gene encoding a TRF of the invention provides for expression of TRF in quantities greater than can be isolated from natural sources, or in indicator cells that are specially engineered to indicate the activity of TRF expressed after transfection or transformation of the cells. Accordingly, in addition to rational design of agonists and antagonists based on the structure of TRF, the present invention contemplates an alternative method for identifying specific ligands of TRF using various screening assays known in the art. Any screening technique known in the art can be used to screen for TRF agonists or antagonists. The present invention contemplates screens for small molecules that bind to TRF and agonize or antagonize TRF in vitro and/or in vivo. For example, natural products libraries can be screened using assays of the invention for molecules that agonize or antagonize the activity of TRF (see Example 7 for assays which test the ability of a TRF to bind a telomeric repeat sequence). Knowledge of the primary sequence of the TRF, and the similarity of that sequence with other DNA binding proteins, can provide an initial clue as the inhibitors or antagonists of the TRF. Identification and screening of antagonists is further facilitated by determining structural features of the protein, e.g., using X-ray crystallography, neutron diffraction, nuclear magnetic resonance spectrometry, and other techniques for structure determination. These techniques provide for the rational design or identification of agonists and antagonists. Another approach uses recombinant bacteriophage to produce large libraries. Using the "phage method" [Scott and Smith, Science, 249:386-390 (1990); Cwirla, et al, Proc. Natl. Acad. Sci., 87:6378-6382 (1990); Devlin et al, Science, 249:404-406 (1990)], very large libraries can be constructed (10 6 -10 8 chemical entities). A second approach uses primarily chemical methods, of which the Geysen method [Geysen et al, Molecular Immunology, 23:709-715 (1986); Geysen et al. J. Immunologic Method, 102:259-274 (1987)] and the method of Fodor et al. [Science, 251:767-773 (1991)] are examples. Furka et al. [14th International Congress of Biochemistry, Volume 5, Abstract FR:013 (1988); Furka, Int. J. Peptide Protein Res., 37:487-493 (1991)], Houghton [U.S. Patent No. 4,631,211, issued December 1986] and Rutter et al. [U.S. Patent No. 5,010,175, issued April 23, 1991] describe methods to produce a mixture of peptides that can be tested as agonists or antagonists. In another aspect, synthetic libraries [Needels et al, Proc. Natl. Acad. Sci. USA, 90:10700-4 (1993); Ohlmeyer et al, Proc. Natl. Acad. Sci. USA, 90:10922-10926 (1993); Lam et al, International Patent Publication No. WO 92/00252; Kocis et al, International Patent Publication No. WO 9428028, each of which is incorporated herein by reference in its entirety], and the like can be used to screen for TRF ligands according to the present invention. Screening can be performed with recombinant cells that express one or more TRFs of the present invention, or alternatively, using purified protein, and/or specific structural functional domains of TRFs e.g., produced recombinantiy, as described above. For example, a labelled TRF2 dimerization domain can be used to screen libraries, as described in the foregoing references for small molecules that will inhibit the dimerization of the TRF2. Similarly, antagonists or agonists to the acidic or basic domains of TRFl and TRF2 respectively, can be identified in analogous screens. Indeed, essentially all of the nucleic acids, peptides and proteins, and antibodies can be employed in such drug assays. In one such case, the activity of a specific TRF or fragment thereof (as disclosed herein) can be monitored or determined in the presence and absence of a potential drug. A candidate drug can then be selected on the basis of a measurable change in the activity determined, for example, which is found in the presence of the potential drug relative to in its absence. Such assays may be performed in vitro, in situ, and or in vivo as desired. Gene Therapy and Transgenic Vectors In one embodiment, a gene encoding a TRF or structural/functional domain thereof is introduced in vivo in a viral vector. Such vectors include an attenuated or defective DNA virus, such as but not limited to herpes simplex virus (HSV), papilloma virus, Epstein Barr virus (EBV), adenovirus, adeno-associated virus (AAV), and the like. Defective viruses, which entirely or almost entirely lack viral genes, are preferred. Defective virus is not infective after introduction into a cell. Use of defective viral vectors allows for administration to cells in a specific, localized area, without concern that the vector can infect other cells. Thus, any tissue can be specifically targeted. Examples of particular vectors include, but are not limited to, a defective herpes virus 1 (HSV1) vector [Kaplirt et al, Molec. Cell. Neurosci., 2:320-330 (1991)], an attenuated adenovirus vector, such as the vector described by Stratford-Perricaudet et al. [J. Clin. Invest., 90:626-630 (1992)], and a defective adeno-associated virus vector [Samulski et al, J. Virol, 61:3096-3101 (1987); Samulski et al, J. Virol, 63:3822-3828 (1989)]. Preferably, for in vitro administration, an appropriate immunosuppressive treatment is employed in conjunction with the viral vector, e.g. , adenovirus vector, to avoid immuno- deactivation of the viral vector and transfected cells. For example, immunosuppressive cytokines, such as interleukin-12 (IL-12), interferon-γ (IFN-γ), or anti-CD4 antibody, can be administered to block humoral or cellular immune responses to the viral vectors [see, e.g., Wilson, Nature Medicine (1995)]. In addition, it is advantageous to employ a viral vector that is engineered to express a minimal number of antigens. In another embodiment the gene can be introduced in a retroviral vector, e.g., as described in Anderson et al, U.S. Patent No. 5,399,346; Mann et al, Cell, 33:153 (1983); Temin et al, U.S. Patent No. 4,650,764; Temin et al, U.S. Patent No. 4,980,289; Markowitz et al, J. Virol, 62: 1120 (1988); Temin et al, U.S. Patent No. 5,124,263; International Patent Publication No. WO 95/07358, published March 16, 1995, by Dougherty et al; and Kuo et al, Blood, 82:845 (1993). Targeted gene delivery is described in International Patent Publication WO 95/28494, published October 1995. Alternatively, the vector can be introduced in vivo by lipofection. For the past decade, there has been increasing use of liposomes for encapsulation and transfection of nucleic acids in vitro. Synthetic cationic lipids designed to limit the difficulties and dangers encountered with posome mediated transfection can be used to prepare liposomes for in vivo transfection of a gene encoding a marker [Feigner et. al., Proc. Natl Acad. Sci U S.A., 84:7413-7417 (1987); see Mackey et al, Proc. Natl. Acad. Sci U.S.A., 85:8027-8031 (1988)]. The use of canonic lipids may promote encapsulation of negatively charged nucleic acids, and also promote fusion with negatively charged cell membranes [Feigner and Rmgold, Science, 337:387-388 (1989)]. The use of hpofection to introduce exogenous genes into the specific organs in vivo has certain practical advantages. Molecular targeting of liposomes to specific cells represents one area of benefit. It is clear that directing transfection to particular cell types would be particularly advantageous in a tissue with cellular heterogeneity, such as pancreas, liver, kidney, and the brain. Lipids may be chemically coupled to other molecules for the purpose of targeting [see Mackey et. al., supra]. Targeted peptides, e.g., hormones or neurotransmitters, and proteins such as antibodies, or non-peptide molecules could be coupled to liposomes chemically. It is also possible to introduce the vector in vivo as a naked DNA plasmid. Naked DNA vectors for gene therapy can be introduced into the desired host cells by methods known in the art, e.g., transfection, electroporation, micromjection, transduction, cell fusion, DEAE dextran, calcium phosphate precipitation, use of a gene gun, or use of a DNA vector transporter [see, e g, Wu et al, J. Bwl Chem., 267:963-967 (1992); Wu and Wu, J. Biol Chem., 263:14621-14624 (1988); Hartmut et al , Canadian Patent Application No. 2,012,311, filed March 15, 1990]. In a preferred embodiment of the present invention, a gene therapy vector as descnbed above employs a transcnption control sequence operably associated with the sequence for the TRF inserted in the vector. That is, a specific expression vector of the present invention can be used in gene therapy. Such an expression vector is particularly useful to regulate expression of a therapeutic TRF gene. In one embodiment, the present invention contemplates constitutive expression of the TRF gene, even if at low levels. Various therapeutic heterologous genes can be inserted in a gene therapy vector of the invention such as but not limited to adenosme deammase (ADA) to treat severe combined immunodeficiency (SCID); marker genes or lymphokme genes into tumor infiltrating (TIL) T cells [Kasis et al , Proc. Natl. Acad. Sci U.S A., 87:473 (1990); Culver et al, ibid. 88:3155 (1991)]; genes for clotting factors such as Factor VIII and Factor LX for treating hemophilia [Dwarki et al, Proc. Natl. Acad. Sci. USA, 92:1023-1027 (1995); Thompson, Thromb. andHaemostatis, 66:119-122 (1991)]; and vanous other well known therapeutic genes such as, but not limited to, β-globin, dystrophin, insulin, erythropoietin, growth hormone, glucocerebrosidase, β-glucuronidase, α-antitrypsin, phenylalanine hydroxylase, tyrosine hydroxylase, ornithine transcarbamylase, apolipoproteins, and the like. In general, see U.S. Patent No. 5,399,346 to Anderson et al. Kits In a further embodiment of this invention, commercial test kits suitable for use by a medical specialist may be prepared to determine the presence or absence of predetermined telomere- binding activity or predetermined telomere lengthening activity capability in suspected target cells. In accordance with the testing techniques discussed above, one class of such kits will contain at least the labelled TRF or its binding partner, for instance an antibody specific thereto, and directions, of course, depending upon the method selected, e.g., "competitive", "sandwich", "DASP" and the like. The kits may also contain peripheral reagents such as buffers, stabilizers, etc. Accordingly, a test kit may be prepared for the demonstration of the presence or capability of cells for binding activity to telomeres, comprising: (a) a predetermined amount of at least one labelled immunochemically reactive component obtained by the direct or indirect attachment of the present TRF or a specific binding partner thereto, to a detectable label; (b) other reagents; and (c) directions for use of said kit. More specifically, the diagnostic test kit may comprise: (a) a known amount of the TRF as described above (or a binding partner) generally bound to a solid phase to form an immunosorbent, or in the alternative, bound to a suitable tag, or plural such end products, etc. (or their binding partners) one of each; (b) if necessary, other reagents; and (c) directions for use of said test kit. In a further variation, the test kit may be prepared and used for the purposes stated above, which operates according to a predetermined protocol (e.g. "competitive", "sandwich", "double antibody", etc.), and comprises: (a) a labelled component which has been obtained by coupling the TRF to a detectable label; (b) one or more additional immunochemical reagents of which at least one reagent is a ligand or an immobilized ligand, which ligand is selected from the group consisting of: (i) a ligand capable of binding with the labelled component (a); (ii) a ligand capable of binding with a binding partner of the labelled component (a); (iii) a ligand capable of binding with at least one of the component(s) to be determined; and (iv) a ligand capable of binding with at least one of the binding partners of at least one of the component(s) to be determined; and (c) directions for the performance of a protocol for the detection and/or determination of one or more components of an immunochemical reaction between the TRF and a specific binding partner thereto. In accordance with the above, an assay system for screening potential drugs effective to modulate the activity of the TRF may be prepared. The TRF may be introduced into a test system, and the prospective drug may also be introduced into the resulting cell culture, and the culture thereafter examined to observe any changes in the telomere lengths of chromosomes in the cells, due either to the addition of the prospective drug alone, or due to the effect of added quantities of the known TRF. PRELIMINARY CONSIDERATIONS Vertebrate telomeres contain 2- 100 kb of tandem GGTTAG repeats. This telomeric nucleotide sequence is specified by the 5' CUAACC 3' template within the RNA component of vertebrate telomerases. Telomerase-mediated addition of GGTTAG repeats to the 3' chromosome ends can balance the loss of terminal sequences that occurs during replication of linear DNAs. The telomeric repeat array is essential for the stability of mammalian chromosomes. Without this protective cap, chromosome termini might activate DNA damage checkpoints, or be attacked by DNA repair functions leading to chromosome end fusion and degradation. This protective function of vertebrate telomeres is thought to result from the interaction of the telomeric GGTTAG repeats with telomere specific proteins. In support of this notion, telomeres in yeast and hypotrichous ciliates are known to contain protein components, telomeres have a unique chromatin structure [Tommerup et al, Mol. Cell Biol, 14:5777-5785 (1994)], and the sequence requirements for formation of new telomeres in human cells are highly specific [Hanish et al, Proc. Natl. Acad. Sci. USA, 91:8861-8865 (1994)], as would be expected if the GGTTAG repeats interact with a sequence specific DNA binding protein [de Lange, Seminars in Cell Biology 7, in press (1995)]. A search for telomeric proteins in vertebrate cells was therefore initiated. Since homologs of the telomenc proteins from unicellular organisms have not been found in vertebrates, biochemical approaches were taken to identify GGTTAG sequence specific DNA binding proteins. Several groups identified abundant proteins that interact with smgle-stranded GGTTAG repeats but these factors turned out to be hnRNP components that probably have a function in RNA metabolism rather than at telomeres [Ishikawa et al , Mol. Cell Biol, 13:4301-4310 (1993); McKay et al , Nucl Acids Res., 20:1387-1391 (1992); McKay et al , Nucl. Acids Res., 20:6461-6464 (1992)]. The present invention uses double-stranded telomenc sequences to probe HeLa nuclear extracts for the presence of sequence-specific DNA binding proteins and has identified one candidate activity, called here Telomenc Repeat Binding Factor or TRF [Zhong et al , Mol Cell Biol, 13:4834-4843 (1992), incorporated herein by reference m its entirety]. TRF was identified as a gel-shift activity that forms a specific complex with probes containing 3 or more tandem TTAGGG repeats. The sequence specificity of TRF is apparent from competition expenments in which the TRF gel-shift complex is easily competed out with TTAGGG repeat DNA but not with closely related telomeric sequences from other organisms, such as TTGGGG, and TTAGGC repeats. A TRF-hke activity was found in all mammalian nuclear extracts that were examined, including extracts from a vanety of human cell lines, human peripheral blood leukocytes, and extracts from monkey, mouse, hamster, and chicken cells [Zhong et al, (1992); unpublished observations by Chong and de Lange]. The sequence specificity of TRF and its ubiquitous expression were as expected for a vertebrate telomenc protein. A series of molecular genetic expenments also suggested that TRF interacts with telomenc DNA in human cells [Hanish et al, Proc Natl. Acad. Sci. USA, 91:8861-8865 (1994)]. In these expenments de novo formation of human telomeres was induced by transfection of telomenc repeat sequences into human cells. Upon transfection of 0.8 kb or more of TTAGGG repeat DNA into human HeLa cells, approximately 70% of the transfected cell lines will carry a new telomere. However, when stretches of TTGGGG repeats, TTAGGC repeats or other closely related sequences are transfected, telomere formation is not observed m any of the cell lines examined. These stnngent sequence requirements for telomere formation human cells closely follow the sequence preference of TRF [Hanish et al, (1994)]. No other factor that is currently known can explain this dependence on precise TTAGGG repeat m the process of de novo telomere formation. Therefore, it seems likely that the incoming TTAGGG repeats require the interaction with TRF to form a new telomere. This is as expected if TRF is an integral component of mammalian telomeres. TRFl is the first telomenc protein isolated from human, any other vertebrate cell, or any other multicellular organism. Uses for the TRF include those related to the involvement of telomeres in human cancer and agmg. Human telomeres shorten dunng normal cell divisions and telomere shortening may eventually limit cell proliferation and lead to ag g. In cancer cells, telomere shortening may lead to genome instability. Many human cancer cells contain the enzyme telomerase that can restore telomere length Inhibition of TRF in human tumors is expected to lead to loss of telomere function. This loss of telomere function could limit the growth of tumor cells. Inhibition of TRF could be achieved by anti-sense approaches. In addition, TRF inhibition could be used m combination with anti-telomerase therapy. Anti-telomerase drugs are presently being developed. However, such drugs may not have an acute cytotoxic phenotype because it takes some time to lose enough telomeric DNA after inhibition of telomerase. Dual inhibition of both TRF and telomerase may synergize the effects of either drug. The presence of TRF on telomeres may be a good indicator of the function of human telomeres. Since telomeres change m length during tumoπgenesis, TRF staining of chromosome ends in human tumors may be able to reveal aspects of the stage of the tumor. Moreover, loss of TRF function or changes in TRF function are predicted to destabilize the genome and may contnbute to tumongenic transformation. TRF may therefore be an oncogene. As such, TRF has diagnostic and therapeutic uses m cancer diagnosis and treatment. By analogy to the function of the telomeric protein RAPl m yeast, TRF is expected to control telomere length. Therefore, TRF could be a target for therapies that aim to change telomere length. In addition, it is possible that mutations in TRF would be responsible for certain genome instability syndromes. In the cases of families with mutations in TRF, TRF could be useful for diagnostic purposes and also for gene therapy. The present invention may be better understood by reference to the following non-limiting Examples, which are provided as exemplary of the invention. The following examples are presented in order to more fully illustrate the preferred embodiments of the invention. They should in no way be construed, however, as limiting the broad scope of the invention. EXAMPLE 1 Purification of TRF Protein TRF was isolated from nuclear extract from HeLa cells (see Zhong et al, 1992, for preparation of nuclear extract). The following general strategy was used: 940 liters of HeLa cells 4 9.2 x 10" nuclei 4 4 grams of protein in 1000 ml nuclear extract 4 DEAE, elute at 0.3 M KC1 4 Phophocellulose, elute at 0.6 M 4 CM Sepharose, elute at 0.3 M 4 Streptavidin with E. coli DNA, elute at 0.2 M 4 Streptavidin with [TTAGGG] 27 , elute at 0.5 M 4 batch concentration on DEAE CM, PI 1, [TTAGGG] 27 4 precipitate with TCA/DOC The use of a [TTAGGG] 27 repeat column greatly facilitated the purification of TRF. The column contains restriction fragments derived from a plasmid that was constructed by the present inventors, p[TTAGGG] 27x6 . This plasmid contains an tandem array of six restriction fragments that are identical and each carry 27 TTAGGG repeats. The plasmid is digested with Asp 718 restriction enzyme and the ends of the fragments are labelled with Bio-dUTP and coupled onto Sepharose-Streptavidin. This is the only column material which allowed the separation of TRF from other DNA binding proteins. A significant finding was that TRF does not bind DNA when it is highly punfied. Thus, dunng isolation of the protein all DNA binding activity disappears. It was discovered that the DNA binding activity could be rescued by adding back bovine b-casem and a select set of other proteins. The addition of casein to TRF preparations dunng the punfication thus appeared to be a necessary element of the isolation. Detailed descnption of the punfication TRF was isolated from a total of 9.2 x 10" HeLa Cells, equivalent to 940 liters of culture (in Jokhk's Media, supplemented with bovine calf serum, 2 mM L-glutamme, 0.1 mMi non- essential ammo acids, and penicillin/streptomycin). A nuclear extract containing 4 grams of protein and about 650 pmol of TRF (1 pmol TRF is defined as the amount of protein required to complex 1 pmol of labelled probe), as determined by quantitation using a Phospholmager, was obtained from the cells and the soluble fraction was then purified over a senes of ion exchange and affinity columns, as noted above. For each column, the sample was loaded m buffer containing 100 mM KC1 and the flow-through was reloaded. The columns were developed m buffer with increasing potassium chlonde salt concentration, with a final stripping of the column with 1 M salt. All fractions were dialyzed to 100 mM KC1, 20 mM HEPES, 3 mM MgCl 2 , 20% glycerol, and 0.1% NP-40 (nomdet-P40). Activity was determined by gel shift assay using a probe of 145 bp fragment containing (TTAGGG) 12 having the following sequence (SEQ ID NO:20)- 5' - GTA CCC GGG GATCGTGAC TCTAGAGGG GCC CTAACC CTAACC CTA ACC CTAACC CTAACC CTAACC CTA ACC CTAACC CTAACC CTA ACC CTA ACC CTA ACC CTAACC CGG GTC GAATTC GAT CTC TAG AGTCGA CCT GCA GGCATG C - 3' The nuclear extract was divided in half and the first 4 columns were run in duplicate. A 530 ml sample was applied onto a 200 ml DEAE column (diethenyl benzene, amon exchanger with 0.45-0.90 μm particle size, EM Separations) and TRF eluted at 0.2-0.3 M KC1. Fractions were collected in 100 ml ahquots. The active fractions were pooled (226 ml, 201 mg protein, 131 pmol TRF) and applied onto a 200 ml PI 1, phosphocellulose column (cation exchanger with fiber length 50-250 μm, Whatman). To activate the column the PI 1 resm was incubated m NaOH for 1 minute, HC1 for 1 mmute, then neutralized in HEPES, pH 7.9 TRF activity was found in 0.6 M fractions. These fractions were then combined (125 ml, 14 mg protein) and then run over a 4 ml CM Sepharose column (cross-linked agarose, 6%, cation exchanger, 45-165 μm). TRF eluted at 0.4-0.5 M KC1. The 4.5 ml of active fractions collected were run on a 4.5 ml non-specific DNA column. This column is composed of Biotm-labelled, Hint I digested E coli chromosomal DNA bound to streptavidin beads. TRF activity eluted at 0.2-0.3 mM KC1. At this point, the active fractions were combined from the duplicate purification schemes run m parallel and the rest of the purification was completed using 1 of each subsequent column. The affinity column is a biotm-labelled, Asp 718 digested DNA bound to streptavidin beads, with (TTAGGG) 27 plasmid DNA. A 4.5 ml sample was loaded onto a 0.5 ml affinity column and the activity eluted into 0.5 M KC1 fractions. This fraction was supplied with casein (50 μg) and batch-wise bound to DEAE, CM, and PI 1 columns The final fraction was bound to 100 μl of (TTAGGG) 27 column matenal and eluted at 0.5 M KC1. This active sample was precipitated with 20% tπchloroacetic acid and 0.015% deoxycholate and then run on an SDS-PAGE gel and transferred to nitrocellulose The 67 kD band was cut out, trypsimzed, and sequenced. EXAMPLE 2 Peptide Sequence Analysis and Isolation of TRF cDNAs Approximately 3 micrograms TRF protein of a MW of 67 kD was isolated. The partial amino acid sequences of a number of TRF peptides was determined (Figure 1). One of these sequences (T29) was used for a search of the databases and identified two anonymous human cDNA fragments present in the databases at that time. The Genbank accession number of these sequence are. Z 19923 and Z45971 Based on the nucleotide sequence of the anonymous cDNA fragment Z 19923, a 33 nucleotide synthetic DNA probe was synthesized that overlaps the T29 peptide sequence. This probe was end-labelled and used to screen a commercial HeLa cDNA library (from Stratagene). The sequence of this probe is: 5' GTCAAAAACTGACATATGTATATCGTTCTCAAC 3' (SEQ ID NO:22) From the screen of the HeLa cDNA library a candidate TRF cDNA was isolated (clone 11.2) The insert m this clone was subsequently used to rescreen the HeLa cDNA library. Sequence analysis of the longest human cDNAs resulted m the identification of an open reading frame that contained all peptide sequences previously identified. The human TRF cDNA 4.1 was used to screen a Stratagene mouse cDNA library. One of the resulting clones, #12, was partially sequenced and showed a high degree of sequence similarity to the human cDNA. The MTRF12 cDNA contains an open reading frame that begins with an initiator codon that conforms to the Kozak rules, indicating that this is the N-terminus of the mouse TRF reading frame. That this clone indeed encodes full length mouse TRF proteins is further substantiated by the fact that TRF gel-shift complex encoded by this cDNA co-migrates with genuine mouse TRF from cultured J558 cells (see below). The mouse and human sequences are very similar, allowing alignment of the two reading frames. The cDNA sequence of the human cDNA sequence and deduced open reading frame are given in Figures 2 and 3, respectively. EXAMPLE 3 TRF Contains a Myb Type DNA Binding Domain The TRF sequence was used to execute database searches and a similarity to Myb type DNA binding domains was noted (Figure 4). EXAMPLE 4 Proof That the cDNAs Encode TRF Activity One of the human cDNAs (HTRF4.1) was used to construct a fusion protein in which part of the TRF open reading frame from the N-terminal Xho I site to the natural termination codon at the Hind III site was inserted into the pET28b HisόTag expression vector from Novagen (Madison, WI). This new, chimeric open reading frame contained an initiator ATG codon with "Kozak rules" environment at its 5' end as well as an T7 RNA polymerase promoter sequence upstream. This construct was used for in vitro coupled transcription translation (using a kit from Promega) resulting in synthetic protein (labelled with 35 S methionine) with an apparent molecular weight (MW) of 60 kD. The in vitro synthesized protein was used in a gel-shift assay with a double stranded (TTAGGG) 12 repeat probe and shown to form a complex that migrates close to the HeLa TRF gel-shift complex. The in vitro synthesized protein was shown to have the same sequence specificity as TRF; i.e., it bound to TTAGGG repeats but not to TTGGGG or TTAGGC repeats (Figure 5). The mouse cDNA (MTRF12) contained its own initiator codon and was transcribed and translated without further modification using the T3 promoter in its pBluescript vector. Similar to the human cDNA, a -60 kD protein was synthesized that bound TTAGGG repeats but not TTGGGG or TTAGGC repeats (Figure 6). The gel-shift complex obtained with MTRF-12 encoded protein co-migrates with the gel-shift complex formed with J558 mouse TRF, indicating that MTRF12 encodes full length TRF protein. EXAMPLE 5 Proof That TRF is a Telomeric Protein Staining of metaphase chromosome ends: Using the mouse TRF cDNA (MTRF12), a gene was constructed encoding the TRF protein tagged at the N-terminus with the HA antibody tag. This gene was endowed with the cytomegalovirus promoter and a Bovine Growth Hormone poly A addition site and transfected into a HeLa cell line which had previously been shown to have long telomeres [de Lange, EMBO J., 11:717-724 (1992); de Lange et al, Mol. Cell Biol, 10:518-527 (1990)]. The construct also contained a neomycin marker gene, allowing selection of stably transfected HeLa cells in the presence of 300 μg/ml G418. Clonal cell lines that expressed the HA-tagged TRF were isolated and used to make metaphase chromosome spreads (using colcemid block and cytospin technique for spreading). Staining for the HA-tagged TRF was achieved using a monoclonal anti-HA antibody and a FITC labelled secondary goat anti-mouse antibody. Signals are noted at the ends of all metaphase chromosomes (Figure 7). No other signals are seen. Control experiments with HeLa cells without the HA-TRF construct do not show telomeric staining. EXAMPLE 6 Co-localization of TRF and Telomeric DNA in Interphase Nuclei In a second line of evidence that TRF is exclusively located at telomeres it was shown that TTAGGG repeat DNA and epitope TRF co-localize in interphase cells. For these experiments MTRF12 was used to construct a FLAG tagged derivative fusion protein expressed from the CMV promoter. This construct was transiently transfected (by electroporation) into HeLa cells with long telomeres (see preceding paragraph) and the cells were fixed 24 hours post-transfection with 2% formaldehyde. The telomeric DNA in these nuclei was detected through the annealing of a 150 nucleotide RNA containing CCCUAA repeats (the template is one of the TTAGGG repeat clones described in de Lange et al, 1990) that was synthesized in vitro in the presence of digoxygenin labelled rUTP and detected with sheep anti-digoxygenin primary antibody and a TRITC labelled donkey anti-sheep secondary antibody. The FLAG-tagged TRF was detected with a monoclonal mouse anti-FLAG antibody followed by a FITC labelled goat anti-mouse antibody. Both signals (TRITC and FITC) showed a speckled fully overlapping pattern, indicating complete co-localization of TRF with telomeric DNA (Figure 7). The following is a list of documents related to the above disclosure and particularly to the experimental procedures and discussions. 1. Tommerup, H., Dousmanis, A & de Lange, T., Mol. Cell Biol, 14:5777-5785 (1994). 2. Hanish, J.P., Yanowitz, J. & de Lange, T., Proc. Natl. Acad. Sci. USA, 91:8861-8865 (1994). 3. de Lange, T., Seminars in Cell Biology, 7, in press (1996). 4. Ishikawa, F., Matunis, M. J., Dreyfuss, G & Cech, T. R., Mol. Cell Biol, 13:4301-4310 (1993). 5. McKay, S.J. & Cooke, H., Nucl. Acids Res., 20:1387-1391 (1992). 6. Mckay, S.J. & Cooke, H., Nucl. Acids Res., 20:6461-6464 (1992). 7. Zhong, Z., Shiue, L., Kaplan, S. & de Lange, T., Mol. Cell Biol, 13:4834-4843 (1992). 8. de Lange, T., EMBO J., 11:717-724 (1992). 9. de Lange, T., Shiue, L., Myers, R. M., Cox, D.R., Naylor, S.L., Killery, A.M. & Varmus, H.E., Mol. Cell Biol, 10:518-527 (1990). 10. de Lange, T., "Telomere Dynamics and Genome Instability in Human Cancer," in Telomeres, Cold Spring Harbor Monograph, E. H. Blackburn and C. W. Greider, Eds (1995). 11. Broccoli, D., Young, J. W., de Lange, T., "Telomerase activity in normal and malignant hematopoietic cells," Proc. Natl. Acad. Sci. USA, in press (1995). HUMAN TELOMERES CONTAIN TWO DISTINCT MYB-RELATED PROTEINS, TRFl AND TRF2 Introduction Human telomeres are composed of long arrays of TTAGGG repeats that form a nucleoprotem complex required for the protection and replication of chromosome ends. One component of human telomeres is the TTAGGG repeat binding factor 1 (TRFl) disclosed above, a ubiquitously expressed, Myb-related protein present at telomeres throughout the cell cycle [Zhong et al , Mol. Cell Bwl, 13:4834-4843 (1992); Chong et al , Science, 270: 1663- 1667 (1995); Ludέrus et al , J. Cell Bwl , 135:867-883 (1996); Broccoli et al, Hum Mol Genetics, 6:69-76 (1997); Smith et al , Trends in Genetics, 13:21-26 (1997); and Bianchi et al , supra, (1997)]. Recent evidence has implicated TRFl the control of telomere length [van Steensel, et al , Nature, 385:740-743 (1997)]. TRFl is proposed to be an inhibitor of telomerase, actmg in cis to limit the elongation of individual chromosome ends. The cloning of TRF2, a related homologue of TRFl that carries a very similar Myb-type DNA binding motif is reported herein. Like TRFl, TRF2 is ubiquitously expressed, bound specifically to duplex TTAGGG repeats in vitro, and localized to all human telomeres in metaphase. TRF2 is shown to have a similar architecture to TRFl in that it carries a C-termmal Myb motif and a large TRFl -related dimenzation domain near its N- terminus. However, the dimenzation domains of TRFl and TRF2 do not interact, suggesting that these proteins predominantly exist as homodimers. While having identical telomere binding activity and similar domain organization, TRF2 differs from TRFl in that its N- terminus is basic rather than acidic, and TRF2 is much more conserved than TRFl . The results indicate that the TTAGGG repeat arrays at the ends of human and mouse chromosomes bind to two related proteins. Since TRFl and TRF2 showed significant differences, suggesting that these factors have distinct functions at telomeres. Methods Cloning and sequence analysis. The sequence of the 3 ' UTR of a TRFl-related EST (GenBank T58911) was used a nested PCR strategy to clone a human TRF2 cDNA fragment from a HeLa cDNA library (Stratagene). Additional hTRF2 cDNAs were isolated by hybndization screening from a Namalwa [Scheidereit et al , Nature, 336:551-557 (1988)] and a breast cancer cell line [Kratzschmar et al , J. Biol Chem , 271:4593-4596 (1996)] library The latter library yielded a cDNA that carried the complete open reading frame (designated hTRF2-16.1). This cDNA contains 1,282 bp of 3' UTR but appears to lack a canonical poly(A) addition sequence. The cDNA denved from HeLa cells and the hTRF2-16.1 cDNAs differed at amino acid position 433 where the HeLa cDNA encoded two alanine residues whereas the 16.1 cDNA encoded only a single alanme (Fig. 10A). A mouse cDNA (designated mTRF2-26) carrying the full open reading frame of mTRF2 was isolated from a mouse bram cDNA library (Stratagene) by hybridization to the human TRF2 cDNA. Sequences were determined on both strands using duplex templates and have been deposited m the GenBank database. A comparison to the databases indicated that the TRF2 proteins are not homologous to previously identified proteins other than TRFl and other Myb-related factors. Alignment of the TRFl and 2 sequences was achieved using Clustalw 1.5 with a gap opening penalty of 20 and a gap extension penalty of 0.1 and the results were displayed using SeqVu l.01. Expression studies. mTRF2 containing an N-termmal FLAG epitope tag was constructed by cloning a Notl - Apal fragment from mTRF2-26 representing the entire open reading frame into a modified pRc/CMV expression vector (Invitrogen) carrying the FLAG epitope 5' of the cloning site. The resulting construct contained 10 amino acids derived from the pBluescnpt poly linker and an additional 22 ammo acids from the mTRF2 5' UTR as well as the mTRF2 start codon. FLAG-tagged mTRF2ΔN49 was constructed by PCR amplification of the region between amino acids 49 and the stop codon and cloning the purified PCR product into pRc/CMV. HeLa cells were transfected with the constructs by electroporation and processed for immunofluorescence of metaphase spreads or for simultaneous detection of telomenc DNA by FISH and mTRF2 by immunofluorescence as described previously [Chong et al , Science, 270:1663-1667 (1995); van Steensel et al , Nature, 385:740-743 (1997)]. FLAG-tagged protein was detected with monoclonal antibody M2 (Kodak) followed by FITC-labelled donkey anti-mouse. For FISH, digoxigenm-labelled [CCCUAA] 27 RNA was detected with a sheep anti-digoxigenm antibody (Boehnnger) and a TRITC-conjugated donkey anti-sheep IgG Whole cell extracts were prepared from transfected cells in a buffer containing 400 mM KC1 and 0.2% NP-40 as described [van Steensel et al, Nature, 385:740-743 (1997)] and used under standard conditions [Chong et al, Science, 270:1663-1667 (1995); van Steensel et al, Nature, 385:740-743 (1997)] m electrophoretic mobility shift assays for duplex [TTAGGG] 12 binding activity [Zhong et al , Mol. Cell Bwl , 13:4834-4843 (1992)]. Yeast 2 -hybrid analysis Constructs for 2-hybnd analysis [Fields et al , Nature, 340:245-246 (1989)] containing the region between ammo acid 45 and ammo acid 246, representing the dimenzation domain of hTRF2, were built by PCR-amplification and cloning into vectors pBTMl 16 [Bartel et al , Cellular interactions in development: a practical approach 9ed Harley, D.A.) 153-179 IRL Perss, Oxford (1993)] and pACT2 (Clontech) to create GAD and LexA fusions respectively. The other constructs used as well as the procedures followed for yeast transformation and analysis of b-galactosidase activity were described previously [Bianchi et al, supra, (1997)]. Co-immunoprecipitation experiments. The human and mouse TRFl constructs used were previously descnbed [Chong et al, Science, 270:1663-1667 (1995); Broccoli et al, Hum. Mol Genetics, 6:69-76 (1997); Bianchi et al, supra (1997)]. The FLAG-tagged mTRF2 and untagged mTRF2 were translated from cDNAs described above. 35 S-methιomne labelled proteins [Chong et al, Science, 270:1663-1667 (1995); Broccoli et al, Hum. Mol. Genetics, 6:69-76 (1997); Bianchi et al, supra, (1997)] were immunoprecipitated using the M2 anti- FLAG antibody in buffer D (20 mM Hepes-KOH pH 7.9, 100 mM KC1, 20% glycerol, 0.2 mM EDTA, 0.2 mM EGTA, 0.1 % NP40, 0.1 % Tπton-X-100, 0.5 mM DTT, 0.5 mM PMSF) followed by four washes m buffer D with detergents and two washes in buffer D without detergent. Pellets were suspended m SDS/PAGE loading buffer and resolved on a 9% SDS-polyacrylamide gel. Results An anonymous cDNA fragment encoding a TRFl related-Myb motif was reported in the database (GenBank T58911) [Bilaud et α/ , Λføc/ Acids Res , 24:1294-303 (1996)]. This sequence information was used in a combination of PCR and hybridization strategies to isolate the full length human and mouse cDNAs representing this protein (called TRF2 for TTAGGG repeat binding factor 2) (see Methods above). The human TRF2 (hTRF2) cDNA hybndized to a 3.1 kb mRNA with the same ubiquitous expression pattern as hTRFl (Fig. 8A). Conceptual translation of the mouse and human TRF2 cDNAs revealed two open reading frames starting with Kozak consensus initiation sites that predicted two closely related proteins of approximately 500 amino acids. In agreement with the slightly longer primary sequence of TRF2, in vitro translated human and mouse TRF2 proteins displayed a larger apparent MW than human and mouse TRFl (Fig. 8B). Alignment of the predicted sequences of TRFl and TRF2 showed that the strongest conservation between these proteins is in their C-terminal Myb-domams (Figs. 9A and 9B). In addition, a moderate level of conservation (27% identity) was observed m the region from positions 45 to 245 m hTRF2, coinciding with the previously identified dimenzation domain of TRFl [Bianchi et al, supra, (1997)] (Fig. 9B). Thus, TRF2 had a similar overall architecture as TRFl with a C-termmal Myb-type DNA binding motif and a dimenzation domain in the N-termmal half of the protein (Fig. 9B). However, a striking difference was that while both mouse and human TRFl have an acidic amino terminus, TRF2 proteins contained predominantly basic residues m the N-terminus and the predicted pi of the two proteins is rather different (estimated to be 6.06 for hTRFl and 9.11 for hTRF2). A second unexpected distinction was that TRFl and 2 appeared to evolve at different rates. While TRFl diverged rapidly leading to as little as 65% sequence identity between the human and mouse proteins [Broccoli et al , Hum Mol Genetics, 6-69-76 (1997)], TRF2 showed a much higher level of conservation (82% identity) more m keeping with the rate of divergence of other mammalian proteins (for which 85% identity is the norm [Makalowski et al , Genome Res , 6-846-857 (1996)]) (Fig. 9B). As TRF2 earned a Myb-type DNA binding domain with considerable sequence similarity to the TRFl Myb motif, it was of interest to compare the DNA binding properties of these proteins. To address this issue, gel-shift assays were performed with extracts from HeLa cells transiently expressing mouse TRF2 (mTRF2) endowed with an N-termmal FLAG epitope (Fig. 10 A) Using conditions previously optimized for the detection of TRFl activity [Zhong et al , 1992, supra], a discrete DNA-protem complex that could represent mTRF2 bound to the duplex [TTAGGG] 12 probe was not observed. This effect is not due to co-migration of the hTRFl and mTRF2 complexes because supershifting of the hTRFl complex using a TRFl -specific anti-peptide antibody (antibody 5 [Luderus et al , J Cell Bwl , 135:867-883 (1996)]) did not reveal a residual DNA binding activity that could represent TRF2 (Fig. 10A, lane 3). We also failed to detect a discrete TRF2 complex with a TRF2 protein lacking the FLAG-tag or when using a variety of other gel-shift systems. However, when the mTRF2-contaιnmg extracts were assayed m the presence of the anti-FLAG antibody, two discrete mTRF2-specιfic complexes were observed (Fig. 10 A, lane 5). These supershifted complexes did not appear with extract from cells transfected with the vector backbone (Fig. 2a, lane 11). Possibly the binding of the FLAG antibody stabilized the mTRF2-DNA complex, altered its mobility this gel-system, or removed an inhibitory activity such as interacting with the juxtaposed basic N-terminus and thereby preventing its potentially inhibiting effect, allows detection of the DNA binding activity of mTRF2. Since the most striking difference between TRF2 and TRFl is found m the charge of the N-termmus, it was determined if deletion of this part of TRF2 resulted in a detectable gel-shift complex. Transfection of deletion mutant mTRF2ΔN-49 into HeLa cells resulted extracts with a readily detectable new TTAGGG repeat binding activity (Fig. 10B). This new activity was present in excess over the endogenous hTRFl activity and could be super-shifted with the FLAG antibody (Fig. 10B, lane 4). The mTRF2ΔN-49 mutant protein was also found to localize to telomeric loci in transfected cells (see below). The results suggested that TRF2 has the ability to bind to TTAGGG repeats in vitro but that its basic N-terminus somehow interfered with detection of a discrete TRF2-DNA complexes in this assay. These findings are in agreement with the previous demonstration by Bilaud et al. that a C-terminal fragment of hTRF2 can bind TTAGGG repeat DNA in a Southwestern assay [Bilaud et al, Nucl. Acids Res., 24:1294-303 (1996)]. In order to address the sequence specificity of TRF2, we assayed the [TTAGGG] 12 binding activity of mTRF2ΔN-49 in the presence of six different duplex telomeric DNAs (Fig. IOC). The competition experiments showed that the sequence preference of mTRF2ΔN-49 is indistinguishable from TRFl, both proteins binding much better to TTAGGG repeats than to TTAGGC, TTAGGGGG, and TAGGG repeats. Similar to TRFl [Zhong et al, Mol. Cell. Biol, 13:4834-4843 (1992); Hanish et al, Proc. Natl. Acad. Sci. USA, 91:8861-8865 (1994)], mTRF2 DN-49 bound weakly to the TTGGGG repeats from Tetrahymena telomeres (Fig. IOC, lanes 5 and 6). As mTRF2 DN-49 binding is competed for by circular plasmids carrying TTAGGG repeat arrays (Fig. IOC), there is no in vitro requirement for a DNA end near the TRF2 binding site. Competition experiments also showed that, like TRFl [Zhong et al, Mol. Cell. Biol, 13:4834-4843 (1992)], mTRF2 DN-49 failed to bind to single-stranded [TTAGGG], and [CCCTAA] 6 oligonucleotides (Fig. IOC, lanes 15-18). These results are consistent with the high degree of sequence similarity of the Myb motifs in TRFl and TRF2, and suggest that TRF2, like TRFl, could bind specifically to telomeric DNA in vivo. To determine whether TRF2 is a component of the mammalian telomeric complex, full length FLAG-tagged mouse TRF2 was expressed in transiently-transfected HeLa cells. Indirect immunofluorescence with a monoclonal antibody to the FLAG epitope (M2) revealed a punctate pattern in interphase nuclei (Fig. 11A), as expected if the epitope-tagged mTRF2 protein specifically localized to telomeres. Labeling of the telomeres in the same nuclei by fluorescence in situ hybridization with a TTAGGG repeat specific RNA probe (Fig. 1 IB) revealed most telomeric loci coincided with an mTRF2 signal (Fig. 1 IC), indicative of a telomeric localization of the epitope-tagged protein. However, a minority of the mTRF2 signals was not obviously associated with telomeric DNA and some of the telomeric loci did not contain detectable mTRF2 (Fig. 11). Transfected mTRF2 was also demonstrable at the ends of mitotic chromosomes. All HeLa metaphase chromosomes were found to contain an mTRF2 signal at their termini (Fig. 1 ID) and no other location for the FLAG-tagged mTRF2 protein was noted. Similarly, FLAG-tagged mTRF2ΔN-49 was found to localize to HeLa chromosome ends and the localization of epitope-tagged and GFP-tagged mTRF2 to telomeres in transfected NIH3T3 cells was observed. Thus, according to these indirect methods, TRF2 appears to be a integral component of the telomenc complex associated with telomenc DNA m interphase and mitosis. The telomenc localization of TRF2 was corroborated by detection of the endogenous TRF2 protein using a TRF2 specific antibody TRFl is a homodimer in solution and requires two Myb motifs to bind to telomenc DNA in vitro and in vivo [Bianchi et al , supra, (1997)]. Using the yeast 2-hybπd assay [Fields et al , Nature, 340:245-246 (1989)], the homodimenzation domain of TRFl was previously mapped to the region from position 66 to 263 [Bianchi et al , supra, (1997)] (see Fig. 12 A) Since TRF2 displayed considerable sequence similanty to the dimenzation domain of TRFl (Fig. 9A), the dimenzation domain of TRF2 was examined to see if it has the ability to interact with itself m the yeast 2-hybnd assay Interaction of LexA and GAD fusion proteins beanng the TRF2 dimenzation domain resulted in readily detectable b-galactosidase activity m the same range as what is observed for TRFl -TRFl interaction (Fig. 12A), suggesting that TRF2 has the ability to form homodimers with a similar architecture as TRFl . The similar migration rate of TRFl and mTRF2ΔN-49 complexes in gel-shift assays (Fig. 10B) is consistent with both proteins binding to DNA as a dimer of approximately the same mass Furthermore, addition of FLAG antibody to mTRF2ΔN-49 gelshifts resulted in two supershifted complexes (Fig. 10B), m agreement with the presence of two FLAG epitopes in each DNA bound TRF2 complex Next the yeast 2-hybnd assay was used to determine whether the dimenzation domains of TRFl and TRF2 can interact. In a large number of trials, no convincing interactions between these proteins m two settings were detected (Fig. 12A). No activation occurred when the dimenzation domain of TRF2 was probed for interactions with full length TRFl In addition, no significant interactions were observed between fusion proteins containing the isolated TRFl and TRF2 dimenzation domains. The weak b-galactosidase activity detected with these proteins is probably due to the mherent slight transactivation activity of the TRFl dimenzation domain (Fig. 12A) which has been noted previously [Bianchi et al, supra, (1997)] These data indicated that while TRF2 has the ability to interact with itself, the TRF1-TRF2 interaction was either absent or much weaker. Absence of heterodimer formation was further corroborated by co-immunoprecipitation of in vitro translation products (Fig. 12B). In these expenments, FLAG-hTRFl (human TRF) could be lmrnunoprecipitated together with GFP-hTRFl and FLAG-mTRF2 (mouse TRF) was found to associate with an untagged version of mTRF2, consistent with homodimerization by both proteins. However, FLAG-mTRFl (mouse TRF) failed to form a stable association with mTRF2 (Fig. 12B). The lack of heterodimerization is also consistent with the gel-shift assays on extracts from cells over-expressing mTRF2ΔN-49 (Fig. 10B). In these experiments, expression of an excess of mTRF2ΔN-49 did not reduce the presence of hTRFl homodimer in the extract, indicating that mTRF2ΔN-49 did not form heterodimers with hTRFl . The unaltered level of the TRFl gel-shift complex was most clearly demonstrated in reactions in which the mTRF2ΔN-49 complex was supershifted with the FLAG antibody (Fig. 10B, lane 4). In similar experiments executed with FLAG-tagged hTRF 1 , heterodimerization between the endogenous TRFl and the transfected protein is consistently detected. These results demonstrate that human telomeres contain two distinct distantly-related telomeric DNA binding proteins, TRFl and TRF2. TRFl and TRF2 have several features in common. They both carry a C-terminal Myb-type DNA binding domain and have the ability to form homodimers through interactions in an N-terminal dimerization domain. Both proteins show specificity for the duplex TTAGGG repeats typical of vertebrate telomeres and they have the same expression pattern. However, TRFl and TRF2 are clearly distinguishable based on the difference in the amino acid composition of their N-terminus and their rates of evolution, suggesting that these two factors may have distinct functions, in the regulation of telomere regulation. Unlike the situation in human cells, in the yeast S. cerevisiae, the duplex part of the telomere is bound to a single DNA binding protein, Raplp. While Raplp is structurally and functionally similar to TRFl [Smith et al, Trends in Genetics, 13:21-26 (1997); van Steensel et al, Nature, 385:740-743 (1997); Kyrlon et al, Mol. Cell. Biol, 12:5159-5173 (1992); Konig et al, Cell, 85:125-136 (1996); Marcand et al, Science, 275:986-990 (1997)], these two proteins show little or no amino acid sequence similarity. Recent data suggest that Raplp bound to telomeres exists in two alternative states, one associated with the Sir proteins and another complexed with Rif proteins (ref. [Marcand et al, Science, 275:986-990 (1997)]). The two forms of Raplp are proposed to have different functions at telomeres. The presence of two distinct telomeric proteins on human telomeres could similarly reflect two different tasks for duplex telomeric binding factors. EXAMPLE 8 TRF2 PROTECTS HUMAN TELOMERES FROM END TO END FUSIONS Introduction Based on genetic and cytological observations Muller and McClintock reasoned that telomeres protect chromosomes from end-to-end fusion [Muller, The Collecting Net - Woods Hole, 13:181-195 (1938); McClintock, Genetics, 26:234-282 (1941); McClintock, Proc. Natl Acad. Sci. USA, 28:458-463 (1942)]. Telomeres are now understood to be terminal complexes of repetitive sequences and associated proteins that distinguish natural chromosome ends from damaged DNA. Despite their extensive characterization in yeasts, ciliates, and mammals, the molecular mechanism by which telomeres prevent end-to-end fusions has heretofore been unclear. This issue is studied herein in human cells by direct visualization of chromosome behavior after interference with the function of a telomeric protein. Human chromosome ends carry 2-30 kb of double-stranded TTAGGG repeats, which are necessary and sufficient for telomere function in somatic cells [Fair et al, Proc. Natl. Acad. Sci. USA, 88:7006-7010 (1991); Hanish et al, Proc. Natl. Acad. Sci. USA, 91:8861-8865 (1994)]. In the germline and in immortalized cells, this sequence can be maintained by telomerase, a reverse transcriptase that adds TTAGGG repeats onto the 3 ' ends of chromosomes (see [Morin, Seminars in CellDev. Biol, 7:5-15 (1996)] for review). The termini of human telomeres carry long (-150 nt) protrusions of single-stranded TTAGGG repeats [Makarov et al, Cell, 88:657-666 (1997); McElligott and Wellinger, EMBOJ, 16:3705-3714 (1997); Wright et al, Genes Dev., 11:2801 (1997)], which are an effective substrate for telomerase in vitro. According to one analysis [Makarov et al, Cell, 88:657- 666 (1997)], G-strand overhangs appear to be present at most chromosome ends and are maintained in cells lacking telomerase, suggesting that a 5 '-3' exonuclease acting on the C-rich telomeric strand may be responsible for their formation. However, other experiments suggest that long G-strand tails are only present on half of the chromosome ends, consistent with their being generated by incomplete lagging strand synthesis during DNA replication [Wright et al, Genes Dev., 11:2801 (1997)]. Telomeres in somatic human cells shorten by 50-200 bp per cell division [Cooke and Smith, Cold Spring Harbor Symp. Quant. Biol, L 213-219 (1986); Harley et al, Nature, 345:458- 460 (1990); Hastie et al, Nature, 346:866-868 (1990); reviewed in Harley et al, In Telomeres, Cold Spring Harbor Press, Cold Spring Harbor, NY (1995)]. Programmed telomere shortening in normal human cells may function as a tumor suppresser mechanism that limits the growth potential of transformed cells [reviewed in de Lange, In Telomeres, Cold Spring Harbor Press, Cold Spring Harbor, NY (1995)]. In agreement, telomere length is strongly correlated with the proliferative capacity of normal human cells and the catalytic subunit of telomerase (hTRT/hEst2p) is upregulated in human tumors and immortalized cells [AΑsopp et al, Proc. Natl. Acad. Sci. USA, 89:10114-10118 (1992); Meyerson et α/., Cell, 90:785-795 (1997); Nakamura et al, Science, 277:955-959 (1997)]. Loss of telomere function in human cells results in the formation of dicentric chromosomes and other abnormalities created through end-to-end fusions [Counter et al, EMBO J., 11:1921-1929 (1992)]. Both in senescent cells and in tumor cells, dicentric chromosomes, rings, and sister-chromatid fusions are correlated with critically shortened telomeres [reviewed in de Lange, In Telomeres, Cold Spring Harbor Press, Cold Spring Harbor (1995)]. These observations, taken together with evidence for a protective role of telomeres from yeast, ciliates, flies, and maize, have led to the supposition that chromosome ends lacking telomeric DNA fail to recruit a terminal protein complex required for their protection. However, there has been no direct evidence for telomeric proteins that protect chromosome ends from end-to-end fusion and it has remained obscure how such factors might act. Two human telomeric DNA binding proteins are identified herein. TRFl was isolated as a double-stranded TTAGGG repeat binding protein from HeLa cells. This factor is a homodimeric protein with a C-terminal helix-turn helix motif similar to the Myb and homeodomain DNA binding folds [Bianchi et al, EMBO J., 16:1785-1794 (1997)] [reviewed in Smith and de Lange, Trends in Genetics, 13:21-26 (1997); and Konig and Rhodes, Trends Biochem. Sci., 22:43-47 (1997)]. TRF2 carries a similar C-terminal Myb motif but is different from TRFl in that its N-terminus is very basic rather than acidic [Bilaud et al, Nature Genetics, 17:236-239 (1997); Broccoli et al, Nature Gen., 17:231-235 (1997)] (Figure 13 A). Both proteins bind specifically to double-stranded TTAGGG repeats in vitro and are located at telomeres in vivo. The two TRFs are ubiquitously expressed and current evidence is consistent with most human telomeres containing both factors bound simultaneously throughout the cell cycle [Chong et al, Science, 270:1663-1667 (1995); Broccoli et al, Nature Gen., 17:231-235 (1997); Smith and de Lange, Trends in Genetics, 13:21-26 (1997); van Steensel and de Lange, Nature, 385:740-743; U.S. Patent Application No. 08/800,264 filed February 13, 1997, hereby incorporated herein by reference it its entirety]. TRFl has been shown to be a negative regulator of telomere length maintenance [van Steensel and de Lange, Nature, 385:740-473; U.S. Patent Application No. 08/800,264 filed February 13, 1997]. Here a key role for TRF2 is demonstrated for a second function of telomeres, the protection of chromosome ends from end-to-end fusion. Experimental Procedures Expression vectors. The cDNA encoding full-length human TRF2 was placed under the tetracyclin-controlled promoter by cloning the EcoRI fragment of plasmid phTRF216-l [Broccoli et al, 1997; Example 7] into vector pUHD10-3, resulting in plasmid pTethTRF2. To facilitate the creation of constructs encoding truncated proteins with an N-terminal FLAG-tag, expression vector pTetNFLAG was constructed by inserting a linker encoding a FLAG tag and an EcoRI site into the SacII and BamHI sites of pUHD 10-3. Next, TRF2 ΔB (containing amino acids 45-500 of SEQ ID NO:27) and TRF2 ΔBΔM (containing amino acids 45-454 of SEQ ED NO:27) were each cloned into the EcoRI and BamHI sites of pTetNFLAG (in-frame with the FLAG-tag) by PCR cloning, using Pfu-polymerase, plasmid phTRF216-l as template, with 5'TTGAATTCGAGGCACGGCTGGAAGAG3' (SEQ ID NO:51) as forward primer for both constructs, 5 ' CGGGATCCTGTTTCAGTTCATGCCAA3 ' (SEQ ID NO:51) as backward primer for TRF2 45"500 and 5'CGGGATCCTCATTCTACAGTCCACTTCTGCT3' (SEQ ID NO:52) as backward primer for TRF2 45 - 454 . Induction of TRF2 polypeptides in HTC75 cells. The empty vector pUHD10-3 and the pUHD10-3-derived constructs for expression of the TRF2 alleles were each co-transfected with neomycin resistance plasmid pNY-HI into cell line HTC75 using the calcium phosphate co-precipitation. HTC75 is a hygromycin resistant HT1080-derived clonal cell line that stably expresses the tetracyclin-controlled transactivator (tTA) [Gossen and Bujard, Proc. Natl. Acad. Sci. USA, 89:5547-5551 (1992); van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997]. Transfected cells were grown in the presence of doxycychn (100 ng/ml) and G418 (600 (μg/ml). For each construct approximately 25 G418-resistant cell lines were isolated by ring cloning and tested for expression of TRF2 polypeptides after 24 hours of induction. Expression of TRF2 ΔBΔM and TRF2 ΔB was tested by immunofluorescence microscopy and western blotting using anti-FLAG antibody M2 (Eastman Kodak); expression of wild-type TRF2 was tested by gelshift assays using a TTAGGG repeat probe and by western blotting using affinity-purified serum #508 (see below). All clones were grown in DMEM supplemented with 10% bovine calf serum or bovine fetal serum and 150 μg G418 per ml. All clones were grown in parallel with or without doxycychn (100 ng/ml) . Polyclonal antibody against TRF2. A 28-mer peptide (pep28) encompassing amino acid residues 16-42 of human TRF2 (SEQ ID NO:27) with an additional N-terminal cysteine was synthesized (BioSynthesis, Lewisville, TX) and conjugated to maleimide-activated Keyhole Limpet Haemocyanin (KLH, Pierce, Rockford, IL). Serum from a rabbit immunized with the pep28-KLH conjugate was affinity-purified against pep28 cross-linked to SulfoLink coupling gel (Pierce) using standard procedures [Harlow and Lane, Antibodies, a laboratory manual, Cold Spring Harbor Press (1988)]. The resulting purified antibody #508 reacts specifically with TRF2 in western blotting and immunofluorescence labeling assays. The antibody does not cross-react with TRFl . Whole-cell extracts. Cells grown in 10 cm dishes were washed with 5 ml cold phosphate buffered saline (PBS), harvested by scraping in 1 ml PBS per dish and centrifuged 2 minutes in an Eppendorf microfuge at setting 4,000 g. Subsequent steps were all carried out on ice or at 4°C. The cell pellets (~4 million cells) were resuspended in 200 μl buffer C (20 mM Hepes-KOH pH 7.9, 420 mM KC1, 25% glycerol, 0.1 mM EDTA, 5 mM MgCl 2 , 1 mM dithiothreitol, 0.5 mM phenylmethylsulfonyl fluoride, 0.2% Nonidet P-40, 1 μg leupeptin per ml, 1 μg pepstatin per ml, 1 μg aprotinin per ml), incubated for 30 minutes and centrifuged for 10 minutes in an Eppendorf microfuge at 14,000 g. The supernatant was dialyzed 2-5 hrs against 100 ml of buffer D (20 mM Hepes-KOH pH 7.9, 100 mM KC1, 20% glycerol, 0.2 mM EDTA, 0.2 mM EGTA, 0.5 mM dithiothreitol, 0.5 mM phenylmethylsulfonyl fluoride), frozen in liquid nitrogen and stored at -80 °C. Protein content of the extracts was measured using the Bradford assay (BioRad, Hercules, CA) using bovine serum albumin as a standard. Western blotting. Twenty μg of whole-cell extract proteins were separated on 10% SDS-polyacrylamide gels and transferred to nitrocellulose by electroblotting. Ponceau S staining confirmed equal loading of the samples. Blots were pre-incubated 30 minutes in 10% non-fat milk powder and 0.5% Tween-20 in PBS. All subsequent incubations and washing steps were carried out in 0.1% non-fat dry milk powder and 0.1% (w/v) Tween-20 in PBS. Blots were incubated for 12-16 hours at 4°C with either anti-FLAG antibody M2 or anti-TRF2 antibody #508, followed by three 10 minute washing steps. Next, blots were incubated 45 minutes with horseradish peroxidase conjugated sheep-anti-mouse (Jackson Immuno Research Labs) or donkey-anti-rabbit antibody (Amersham) and washed three times for 10 minutes. Bound antibody was detected using the ECL kit (Amersham). Immunofluorescence labeling and microscopy. The HeLaI.2.11 cell line, a subclone of HeLal [Saltman et al, Chromosoma, 102: 121-128 (1993)] bearing telomeres of >25 kb, was transfected by electroporation with pTethTRF2, pTetFLAGhTRF2 ΔB , or pTetFLAGhTRF2 ΔBΔM together with the tTA-expression vector pUHD15-l [Gossen and Bujard, Proc. Natl. Acad. Sci. USA, 89:5547-5551 (1992)]. Cells were grown for 24 hours on Alcian Blue coated coverslips in the absence of doxycychn. Fixation and immunostaining were carried out as described [Chong et al, Science, 270:1663-1667 (1995); van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997]. TRF2 was detected with polyclonal antibody #508 (see above) raised and affinity purified against an N-terminal peptide of TRF2. The FLAG epitope tag was detected with the M2 anti-FLAG monoclonal antibody (Eastman Kodak). TRFl was detected with a mouse polyclonal serum (#2) directed against the full length protein or with antibody 371C2 . Rabbit antibodies were detected with FITC- or Cy3-conjugated donkey-anti-rabbit antibodies (Jackson ImmunoResearch Labs). Mouse antibodies were detected with FITC-conjugated donkey-anti-mouse antibody (Jackson ImmunoResearch Labs). Control experiments indicated that secondary antibodies did not show any cross-reaction. To exclude that binding of anti-TRFl (#371C2, [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997]) and anti-TRF2 (#508) antibodies to endogenous TRF proteins was prevented by anti-FLAG antibody M2 through steric hindrance, the cells were pre-inducated overnight with #371C2 or #508 before adding M2. Micrographs were recorded on a Zeiss Axioplan microscope with a Kodak DCS200 digital camera. Images were noise-filtered, corrected for background and merged using Adobe Photoshop. Cell growth curves and β-galactosidase assay. Cells were plated in duplicate at various densities (-0.1-4.0 x 10 6 cells/15 cm dish) the day before the experiment. On day 0, all plates were washed three times with medium containing G418 (150 μg/ml), with or without doxycychn (100 ng/ml). On indicated days cells were harvested and counted, and cell pellets were frozen at -80°C for isolation of genomic DNA. Whole-cell extracts were prepared from dishes grown in parallel. In most experiments, cells were split at day 4 (1:32-1:4) for day 6 and day 9 time points. Cells induced for 9 days were stained for β-galactosidase using the method described in Dimri et al. [Proc. Natl. Acad. Sci. USA, 92:9363-9367 (1995)] , but with phosphate buffer instead of citrate/phosphate buffer. Cells were washed in PBS, pH 7.2, fixed for 5 minutes in 2% formaldehyde/0.2% glutaraldehyde solution in PBS , washed again in PBS (pH 7.2) and stained with X-gal (1 mg/ml) in 150 mM NaCl, 2 mM MgCl 2 , 5 mM K 3 Fe(CN) 6 , 5 mM K 4 Fe(CN) 6 , and 40 mM NaPi pH 6.0, pH 4 0 or pH 7 0, for 6 to 12 hours at 37°C Chromosome analysis in metaphase and anaphase cells Four to six days after induction (as indicated in the text) cells were incubated with 0.1 μg demecolcme per ml for 90 minutes, harvested by trypsimzation, incubated for 7 minutes at 37 °C m 0 075 M KC1, and fixed m freshly prepared methanol: glacial acidic acid (3:1 vol/vol). Cells were stored at 4°C and when needed dropped onto wet slides and air dried For DAPI staining of DNA, slides with metaphase spreads were incubated 10 minutes in 0.5 μg 4', 6-dιammo-2-phenylmdole (DAPI)(Sιgma) per ml PBS, washed for 2 minutes in PBS, and mounted in 90% glycerol 10% PBS containing 1 mg p-phenylene diamme (Sιgma)/ml For trypsin banding, metaphase spreads prepared as above were incubated m banding solution (2X trypsm-EDTA (Gibco), IX Hanks Balanced Salt Solutιon(Gιbco) in water) for 45 to 75 seconds at 37 C C and stained with filtered staining solution (16% Giemsa Blood Staining Solution (J.T.Baker), 4% Giemsa Solution (Fisher) in Tπs-Maleic acid buffer pH 5.6) for 60-75 seconds at room temperature Anaphase cells were visualized by DAPI staining of cells grown on cover slips for the indicated number of days in the presence or absence of doxycychn FISH. In situ hybridization was executed according to Lansdorp et al [Hum Mol Gen , 5:685-691 (1996)]. Hybridization was performed with 0 5 μg/ml FITC-conjugated(C 3 TA 2 ) 3 peptide nucleic acid (PNA) probe (Biotech GmbH), and after washing, the cells were embedded in 90% glycerol/10% PBS containing 1 mg p-phenylene diamme (Sigma Chemical Company, Inc.) per ml, supplemented with 0.2 μg 4',6-dιammo-2-phenylmdole (DAPI) per ml Genomic blotting and Bal31 digestion Isolation of genomic DNA, genomic blotting and telomere-length estimation were carried out as described [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S Patent Application No 08/800,264 filed February 13, 1997]. For the Bal31 nuclease experiment, about 65 μg undigested genomic DNA was incubated at 30°C with 13 units Bal31 nuclease (mixed, New England Biolabs, Beverly, MA) in 390 μl buffer containing 600 mM NaCl, 12 mM CaCl 2 , 12 mM MgCl 2 , 20 mM Tπs-HCl, 1 mM EDTA, pH 8 0 At indicated time points, 30 μl samples were taken and inactivated by addition of 2μl 0.5 M EGTA and incubation for 10 minutes at 65°C. Bal31 treated DNA samples were extracted with phenol/chloroform, precipitated with ethanol and digested with Hmfl and Rsal. To ensure equal loading on agarose gels, all DNA samples were quantified after restnction enzyme digestion by fluorometry using Hoechst 33258 dye. G-strand overhang assay The non-denatunng hybridization assay to detect G-strand overhangs was carried out essentially as described [Makarov et al , Cell, 88:657-666 (1997)]. [TTAGGG] 4 and [CCCTAA] 4 oligonucleotide probes were end-labelled using γ- 32 P-ATP (3000 Ci/mmol, Amersham) and T4 polynucleotide kmase. Depending on the experiment, 2.5-5.0 μg Hmfl/Rsal digested genomic DNA was ethanol-precipitated, resuspended m 21 μl hybndization buffer (50 mM Tns-HCl pH 8.0, 50 mM NaCl, 1 mM EDTA), added to 4 μl labelled probe (8 nM) and incubated 10-16 hrs at 50 C C in a Perkin-Elmer PCR-thermocycler. Hybridized samples were size-fractionated on 0.8% agarose gels in lxTAE. The gels were dried on Whatman DE-81 fϊlterpaper and exposed to autoradiography film or a Phosphorlmager screen. As a control, 4 μg Hmfl/Rsal digested DNA was treated for 30 minutes at 30°C with 0, 10 or 40 Units Mung Bean nuclease (New England Biolabs) m MB buffer (50 mM sodium acetate, 30 mM NaCl, 1 mM ZnS04, pH 5.0), inactivated by addition of 0 01% SDS, and ethanol-precipitated before carrying out the overhang assay. Inspection of the ethidium bromide stained gel confirmed that Mung Bean nuclease did not have any detectable endonuclease activity. Treatment with 40 U of Mung Bean nuclease completely abolished the overhang signal. Annealing with a [TTAGGG] 4 probe did not reveal a signal at the position of the telomeres. For quantitation of the G-strand overhangs, hybridization intensity was measured using ImageQuant software by integration of the signal of the entire lane between -1.5 and 30 kb. DNA samples from cells grown with or without doxycychn were always analyzed m parallel and run on the same gel. TRAP assay Reactions were performed with whole cell extracts as described elsewhere [Broccoli et al , Proc Natl Acad Sci USA, 92:9082-9086 (1995)]. Protein concentrations m the extracts were determined by Bradford assay (BioRad) and 0.5 μg protein was used per extract. RNase digestions were done in parallel to the untreated reaction by addition of 0.2 μg DNase free RNAse A to the telomerase extension reaction. Results Overexpresswn and Inhibition ofTRF2 To further examine the role of TRF2 at human telomeres an mducible expression system based on the cell line HTC75, a Tetracychn-inducible derivative of the human fibrosarcoma cell line HT1080 was used. This expression system was previously employed for the functional analysis of TRFl [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997]. Using this approach, a full length TRF2 protein was expressed. Two truncated alleles in a doxycyclin-controlled fashion were also expressed. One allele (TRF2 ΔB ) lacked the N-termmal basic domain and the second allele (TRF2 ΔBΔM ) also lacked the C-termmal Myb domain (Figure 13 A). The two truncated proteins were endowed with an N-terminal FLAG tag allowing their detection with a FLAG-specific monoclonal antibody. For detection of full length TRF2, a polyclonal antibody directed against amino acids 16-42 of SEQ ID NO:27 was raised and affinity purified (antibody #508, see Figure 13B). Clonal HTC75 cells transfected with each of the three TRF2 constructs were derived and shown to express appropnately-sized TRF2 polypeptides in an mducible manner with expression reaching plateau levels 1-2 days post-induction (Figure 13B). Expression of the endogenous TRF2 protein was not affected by doxycychn (Figure 13B) Overexpression of full length TRF2 was also demonstrated by a gel-shift assay for the detection of TTAGGG repeat binding activity. Consistent with previous experiments using epitope-tagged protein [Broccoli et al , Nature Gen , 17:231-235 (1997)], endogenous TRF2 protein localized to telomeres throughout the cell cycle as evident from the punctate pattern in interphase and the terminal localization of TRF2 signals in metaphase (Figure 14A and 14B) Furthermore, TRF2 co-localized with TRFl in interphase nuclei (Figure 14C and 14D). Transient overexpression of TRF2 did not significantly affect the localization of TRFl at telomeres, although a minor effect was noted in a few transfected cells with very high levels of TRF2 (Figure 14C and 14D). Similarly to full length TRF2, TRF2 ΔB accumulated at telomeres (Figure 14E), consistent with previous evidence that the basic domain is not required for the localization of this protein to chromosome ends [Broccoli et al , Nature Gen , 17:231-235 (1997)] Cells expressing high levels of TRF2 ΔB showed diminished levels of the endogenous full length TRF2 on telomeres, evidencing a weak dominant interfering activity for this allele (Figure 14F). The effect of TRF2 ΔB on the binding of endogenous TRFl to telomeres was much less conspicuous than the effect on TRF2 and loss of TRFl signal was only obvious m transiently transfected cells expressing extremely high levels of TRF2 ΔB (Figure 14G and 14H). The fact that overexpression of TRF2 ΛB caused displacement of TRF2 but not TRFl from telomeres indicates that the accumulation of the TRFs at telomeres involves more than their simple binding to TTAGGG repeats. TRFl binds to telomeric DNA as a homodimer, requiring two Myb domains for stable association with its target site in vitro and in vivo [Bianchi et al, EMBO J., 16: 1785-1794 (1997)]. This architecture has allowed the design of a dominant negative allele of TRFl containing the dimerization domain and the Nuclear Localization Sequence (NLS), but lacking the Myb DNA binding domain [van Steensel and de Lange, Nature, 385:740-743 (1997); U.S. Patent Application No. 08/800,264 filed February 13, 1997]. Since TRF2 carried a similar dimerization domain [Broccoli et al, Nature Gen., 17:231-235 (1997)], it was asked whether expression of an allele of TRF2 that lacked the Myb motif also acted in a dominant negative fashion. Expression of this version of TRF2 (TRF2 ΔBΔM , Figure 13 A) resulted in a diffuse nuclear staining without evidence for accumulation of this protein at telomeres as expected from the absence of its DNA binding domain (Figure 141 and 14K). The expression of TRF2 ΔBΔM clearly interfered with the accumulation of the endogenous TRF2 protein at telomeres (Figure 14J). While TRF2 could be readily demonstrated at telomeres in untransfected control cells, no or little TRF2 protein was observed at telomeric sites in cells expressing the TRF2 ΔBΔM , attesting to the dominant negative activity of this protein. Consistent with the earlier finding that the dimerization domains of TRFl and TRF2 do not show strong interactions in vitro [Broccoli et al, Nature Gen., 17:231-235 (1997); Example 7], TRF2 ΔBΔM did not affect the accumulation of the endogenous TRFl protein on telomeres (Figure 14K and 14L). TRF2 ΔBΔM and TRF2 ΔB Induce a Growth Arrest in HTC75 Cells. While overexpression of full length TRF2 had no significant effect on the short term growth of HTC75 cells, induction of TRF2 ΔBΔM and TRF2 ΔB led to nearly complete inhibition of growth after approximately 4 days of culturing in the absence of doxycychn (Figure 15 A). This growth arrest was accompanied by induction of a β-galactosidase activity detectable at pH 6 (Figure 15C-H), an indication that the cells were undergoing changes akin to senescence [Dimri et al, Proc. Natl. Acad. Sci. USA, 92:9363-9367 (1995)] although the staining of the arrested HTC75 cells was less intense than senescent primary human fibroblasts. In addition, the cells became enlarged, had a vacuolated cytoplasm, and often showed multiple small nuclei (Figure 15C-H), all morphological phenomena associated with senescence of human cells [Hayfhck and Moorhead, Exp. Cell Res., 25:585-621 (1961); Sherwood et al, Proc. Natl. Acad. Sci. USA, 85:9086-9090 (1988)]. Consistent with senescence, the arrest appeared irreversible since addition of doxycychn to the media (to repress synthesis of the TRF2 mutant proteins) on day 12 did not alter the morphology or the proliferative arrest of the cells over a period of 9 days. A substantial proportion of the cells in each culture failed to show convincing morphological alteration and did not stain with β-galactosidase at pH 6.0 (Figure 15C-H). Most of these cells expressed very low levels of the TRF2 deletion derivatives. Collectively, the data suggested that TRF2 ΔB and χRF2 ΔBΔM induced a growth arrest with phenotypic characteristics of senescence. JRJP2 ΔBΔM Induces Chromosome End Fusions. Microscopic analysis of DAPI-stamed cells expressing TRF2 ΔBΔM revealed the frequent occurrence of anaphase bridges and lagging chromosomes (Figure 16A). This phenotype was not observed after induction of control cells not expressing TRF2 proteins or m cells induced for full length TRF2 or TRF2 ΔB (Figure 16B). (However cells expressing TRF2 ΛB often contained small DAPI-positive fragments that were detectable in anaphase). The incidence of anaphase bridges and lagging chromosomes was quantitated m a total of 100 anaphase cells expressing TRF2 ΔBΔM , umnduced control cells, and in a cell line expressing TRF2 ΔB . At day 4 after induction of TRF2 ΔBΔM , 40% of the cells had one or more aberrant chromosomes (a bridge or a lagging chromosome) and the culture showed on average 0.7 fusions per anaphase cell (Figure 16B). By contrast, the level of anaphase bridges and lagging chromosomes was low (<0.1 per cell) in the umnduced control cells and m a cell line expressing TRF2 ΔB (Figure 16B). Chromosome end fusions induced by TRF2 ΔBΔM were also detected in metaphase spreads. Colcemid treated cells showed dicentncs fused at one or both chromatids, multiple fused chromosomes, and ring chromosomes (Figure 16E-G and Table 1). After induction for 6 days, 88% of the metaphases showed at least one fusion (Table 1). Several cells showed trains of 3 or 4 chromosomes (Figure 16E-G) and one cell showed as many as 30 individual fusion events. On average there were 2.4 fusion events per cell in cultures of the T4 clone when induced to express TRF2 ΔBΔM for 4 or 6 days. Umnduced T4 cells showed only 0.4 events per cell (Table 1). Similarly, a second cell line (T19) expressing TRF2 ΔBΔM showed an increase of the fusion frequency from 0.6 to 3.5 per cell upon induction of this dominant negative allele of TRF2. Cells with end-to-end fusions were rare in a control HTC75 cell line transfected with the vector (B27) or in cells expressing TRF2 ΔB (SI 3) (Table 1). In each case less then 0.3 fusions were observed per cell and fusions were only seen in 10-20% of the cells. Some of these fusion events may actually represent fortuitous juxtaposition of chromosome ends dunng spreading. Thus, expression of the dominant negative allele of TRF2 increases the frequency of telomere fusions by at least 10 fold. The relatively high frequency of telomere fusions in the cell lines T4 and T19 in the presence of doxycychn may be due to leaky expression of the TRP2 ΛBΔM protein in a fraction of the cells. Table 1. Induction of Chromosome End Fusions by Mutant TRF2 Proteins Cell line Inducible Gene Induction Growth Period Number of Fraction with Fusions (days) Cells Examined Fusions per Cell B27 - - 4 50 22% 0.2 T4 TRF2(ΔBΔM) - 4 100 38% 0.4 T4 TRF2(ΔBΔM) + 4 100 77% 2.4 T4 TRF2(ΔBΔM) + 6 50 88% 2.4 T19 TRF2(ΔBΔM) - 4 50 52% 0.6 T19 TRF2(ΔBΔM) + 4 50 78% 3.5 S13 TRF2(ΔB) - 4 50 10% 0.1 S13 TRF2(ΔB) + 4 50 20% 0.2 It should be stressed that the detection of chromosome end fusions anaphase and metaphase cells likely represent an underestimate of the actual number of events. For instance, metaphase cells we do not score for sister-chromatid fusions or fusions that have been followed by chromosome breakage and fusions are only detectable m anaphase cells when a bndge or lagging chromosome results. Thus, our quantitation of chromosome ends fusions probably reflects a minimal estimate of the actual fusion frequency m the cells. Taken together the cytogenetic analysis indicated that the removal of TRF2 from telomeres leads to loss of telomeric protection detectable as end-to-end fusion anaphase and metaphase chromosomes It was unlikely that this phenotype was caused by the presence of excess TRF2 protein in the nucleoplasm because overexpression of full length TRF2 similarly resulted in the presence of TRF2 throughout the nucleus, yet induction of anaphase bridges was not noted m such cells. Fused Chromosome Ends Contain Telomenc DNA Fusion of chromosome ends has been documented in cells containing DNA damage and in cells that have depleted their reservoir of telomeric DNA. In those cases, telomenc DNA is usually not detectable at the site of fusion [Blasco et al , Cell , 91:25-34 (1997)]. Therefore it was determined whether the fusions m response to TRF2 ΔBΔM were similarly correlated with loss of telomenc DNA from individual chromosome ends. Using a fluoresceine-labelled peptide nucleic acid (PNA) [CCCTAA] 3 probe specific for telomenc DNA, in situ hybridizations were carried out on metaphase spreads from cells displaying the chromosome ends fusions. The results in Figure 16E-G showed that telomenc DNA was preserved at the site of chromosome end fusion. In the majority of cases, the signal at the fused ends was substantially stronger than that found at free telomeres, consistent with the telomenc stretches of both fused chromosome ends remaining intact. TRF2 ΔBΔM Induces Molecular Joining of Telomenc DNA Sequences In order to establish whether the joining of telomeres in TRF2 ΔBΛM -expressιng cells depended on a protemaceous bridge, evidence for telomere fusion m naked genomic DNA was sought. Detection of telomenc restnction fragments m genomic DNA from vector control cells and cells expressing full length TRF2 or TRF2 ΔB showed no change in telomere structure over the course of the induction period (Figure 17A and B). By contrast, cells induced for TRF2 ΔBΔM revealed a dramatic alteration in the pattern of Hmfl/Rsal fragments detectable with TTAGGG repeat probes (Figure 17B). A new class of longer restnction fragments first became apparent at 4 days post-induction (Figure 17C) and this set of new fragments increased in intensity, but not in length over the course of the 9 day experiment. The new class of TTAGGG repeat fragments was observed m four independent clonal TRF2 ΔBΔM cell lines and in each case they migrated at a MW exactly twice (ratio of 2.0± 0.2 (n=4)) that of the length of the original population of telomenc fragments. Quantitation of genomic blots indicated that up to 22% (average value 13.8±6.1% (n=4)) of the TTAGGG repeat signal was found in the larger class of hybndizmg matenal at day 9 post-induction. The fact that the TRF2 ΔBΔM -mduced new TTAGGG repeat fragments were twice the size of the original telomeres suggested that these molecules might represent the chromosome end fusions that were first detected by cytogenetic analysis of metaphase and anaphase cells. Such structures would be expected to be resistant to exonuclease Bal31 treatment of mtact genomic DNA, whereas this exonuclease should readily attack the new class of larger TTAGGG repeat fragments if they represented elongated telomeres. Bal31 digestion of genomic DNA from T4 cells expressing χRf2 ΔBΔM indeed showed the resistance of the longer TTAGGG repeat fragments to this exonuclease (Figure 17D). Quantitation of a second data set obtained with TRF2 ΔBΔM -expressιng T19 cells (Figure 17E) showed that while the ongmal telomenc loci were gradually shortened by Bal31, the TRF2 ΔBΔM -mduced longer fragments were not affected by the enzyme. This result indicated that the new class of TTAGGG repeat fragments did not represent elongated telomeres Therefore these longer species are denved from the fused chromosome ends. Since the detection of fused ends in naked DNA indicates that the telomeres are held together by nucleic acid interactions, these end-joming events are referred to as telomeric fusions. The telomere fusions might be mediated by (Hoogsteen) base-pairmg between the G-strand overhangs at human telomeres. Such a configuration was previously shown to temporarily link the termini of yeast chromosomes which carry long G-tails m late S-phase. Since this type of association was shown to be labile at 72-78°C, it was determined as to whether the fused human telomeres could be similarly resolved by treatment at that temperature. As shown in Figure 17F, the fused telomeres derived from TRF2 ΔBΔM -expressιng cells are resistant to a temperature of 85°C and only melt out at higher temperatures. Such temperatures also denature bulk DNA. This observation argues against the presence of G-G basepainng in the 3 ' overhang as the main mechanism by which telomere fusions occur However, it is conceivable that the human G-tails form more stable G-G base paired structures than yeast telomere overhangs. However, the observation that the fused telomeric fragments are resistant to Bal31 nuclease constitutes further evidence against G-tail interactions in the fused telomeres. Since Bal31 readily cleaves smgle-stranded DNA, including very short regions of unpaired sequences such as those occurring due to pyπmidme dimers [Linn and Roberts, Nucleases, Cold Spnng Harbor Laboratory, Cold Spring Harbor (1982)], this enzyme would be expected to digest smgle-stranded regions within G-G basepaired telomeric tails and resolve the joins. Therefore the telomere fusions appear to be the result of end-to-end hgations of one or both telomenc strands. Telomeric Fusions Correlate with the Loss of G-strand Overhangs Ligation of telomere termini would be unexpected if, as proposed by Makarov et al. [Cell, 88:657-666 (1997)], all or most human telomeres contain long regions of smgle-stranded TTAGGG repeat DNA. It was therefore prudent to appraise the status of the telomere termini in cells displaying telomere fusions. Makarov et al.[Ce//, 88:657-666 (1997)] have developed a method for the quantitative detection of smgle-stranded TTAGGG repeats at the ends of human chromosomes. In this technique, Hmfl/Rsal digested non-denatured genomic DNA is annealed to labelled [CCCTAA] 4 oligonucleotide and the indirectly labelled telomenc fragments are detected by autoradiography of size fractionated DNA. This technique evaluates the relative amount of unpaired smgle-stranded TTAGGG repeats in genomic DNA but does not discnminate between loss of signal due to shortening of the G-tails, complete disappearance of G-tails, or reduced detection of G-tails due to G-G base pairing m the overhangs. Using the [CCCTAA] 4 probe on DNA denved from the control cell line B27, G-strand overhangs at the ends of wildtype telomeres were readily detected (Figure 18). To validate the method, it was verified that the probe did not anneal to DNA that was pretreated with Mung Bean nuclease and that annealing of a [TTAGGG] 4 probe did not result in a telomeric pattern. When the amount of unpaired TTAGGG repeat DNA m cells grown in the presence and absence of doxycychn was compared, no alteration the signal was noted in cells induced to express full length TRF2 or the TRF2 ΔB allele (Figure 18 A). Similarly, overexpression of TRFl or a dominant negative allele of TRFl [van Steensel and de Lange, Nature, 385:740-743 (1997); U S. Patent Application No. 08/800,264 filed February 13, 1997] did not affect the presence of unpaired TTAGGG repeats at telomere termini. By contrast, cells expressing r TR , 2 ΔBΔM displayed a consistent reduction in the amount of detectable G-tail sequences and no signal was present at the position of the larger terminal fragments representing the fused telomeres (Figure 18A and 18B) Quantitation of the data on four clonal cell lines showed that induction of TRF2 ΔBΔM for 6-9 days resulted m a 40-60% decline in the total smgle-stranded TTAGGG repeat signals at chromosome ends (Figure 18C) These data indicated that TRF2 ΔBΔM expression resulted in the loss of detectable smgle-stranded TTAGGG repeats at chromosome ends TRF2 does not Affect Telomerase Expression The loss of G-tail sequences in TRF2 ΔBΔM -expressmg cells could be explained if TRF2 is a positive regulator of telomerase expression Therefore the telomerase levels in extracts of cells induced for the three types of TRF2 protein used m this study were examined along with the matching umnduced controls using the PCR-based TRAP assay [Kim et al , Science, 266:2011 -2015 ( 1994)] The result revealed similar levels of robust telomerase activity in each cell type regardless of the presence of doxycychn in the media (Figure 18D), indicating that the telomerase activity is not affected by TRF2 in this setting and that the loss of G-tail DNA occurs through some other mechanism Telomere Protection by TRF2 and the Role ofG-Tails A striking consequence of loss of TRF2 function is the formation of end-to-end fusions detectable in metaphase and anaphase chromosomes In contrast with the first documented end-to-end fusions which involved broken chromosome ends m Drosophila and maize [Muller, The Collecting Net - Woods Hole, 13: 181-195 (1938), McClintock, Genetics, 26:234-282 (1941), McClintock, Proc Natl Acad Sci USA, 28:458-463 (1942)], the fusions induced by loss of TRF2 carry telomenc DNA The presence of telomenc sequences at the fusions was demonstrated by in situ hybridization and the fused telomenc fragments were detectable m protem-free genomic DNA Yet while the telomenc TTAGGG repeats persisted, the telomeres failed to protect the chromosome ends from fusion, indicating that the duplex stretch of TTAGGG repeats itself is insufficient for telomere protection m human cells Therefore the protective function of telomeres is conferred by a nucleoprotem complex containing TRF2 The data also reveal a crucial role for TRF2 in the maintenance of unpaired G-strand overhangs at telomere termini Loss of TRF2 from telomeres caused by expression of the dominant negative TRF2 ΔBΔM allele resulted in a -50% reduction m the single-stranded TTAGGG repeat signal Inhibition of TRF2 appears to result m an actual loss of G-tail DNA sequences from human chromosome ends Such G-tail loss could be the consequence of a failure to protect the overhangs from degradation or could result from a deficiency in creating new G-tails after DNA replication TRF2 is the first telomere associated protein implicated in the maintenance of the correct DNA configuration of the telomenc 3 ' overhang It was previously shown that telomerase is not involved in the maintenance of G-tails in yeast and mammals [Dionne and Wellmger, Proc. Natl. Acad. Sci USA, 93:13902-13907 (1996)] and none of the other telomeric proteins identified m eukaryotes are known to affect this aspect of telomere synthesis. Our data indicate that changes in telomerase expression are unlikely to be involved in this process. Collectively, the data are consistent with a model m which TRF2 protects telomeres from fusion through the maintenance of their smgle-strand TTAGGG repeat overhangs. This view is consistent with the finding that G-strand overhangs are a universal feature of eukaryotic telomeres [reviewed in Wellmger and Sen, Eur J Cancer, 33:735-749 (1997)] and identification of G-strand binding proteins in several systems. Thus, one of the mam objectives of the transactions at telomeres may be to create and maintain a protrusion of smgle-stranded telomeric repeats that can bind specific proteins. This terminal complex could constitute the unique aspect of telomeres that allows cells to distinguish natural chromosome ends from broken DNA. The 3 ' extension of TTAGGG repeats at human chromosome ends are likely to serve as a binding site for smgle-strand specific telomenc proteins but the actual factors involved m this function are still elusive. A candidate activity that could cap the TTAGGG repeats has been identified in Xenopus extracts [Cardenas et al , Genes Dev , 7:883-894 (1993)], G-strand overhangs are bound by terminus specific proteins in cihates [Gottschhng and Zakian, Cell, 47: 195-205 (1986); Price, Mol Cell Biol, 10:3241-3431 (1990)], and budding yeast telomeres are protected from degradation by Cdcl3p [Garvik, Mol Cell Biol , 15:6128-6138 (1995)], a protein with G-tail binding activity in vitro [Lin and Zakian, Proc Natl. Acad. Sci USA, 93: 13760-13765 (1996); Nugent et al , Science, 274:249-252 (1996)]. However, human homologs of these factors have not been identified yet It should also be noted that G-nch telomenc repeats have the ability to form G-G (Hoogsteen) basepaired folded structures with several alternative conformations [reviewed by Henderson, In Telomeres, Cold Spring Harbor Press, Cold Spnng Harbor (1995)] that could potentially contribute to the protection of chromosome ends. The telomenc fusions are probably the consequence of processing of unmasked telomere termini by enzymes normally acting on broken DNA. A possible scenario is that loss of TRF2 from the chromosome ends leads to disappearance of the G-tail overhangs and activation of a DNA damage response by the denuded telomeres. A cell cycle arrest might ensue under these conditions and those cells that process the offending ends into fused telomeres may preferentially continue in the cell cycle leading to the observed metaphase abnormalities and anaphase bridges. The occurrence of fused telomeres m turn creates problems in mitosis due to the mechanical difficulties in segregating dicentric chromosomes which require either a break in the spmdle or a break m a chromatid. Thus, the loss of telomenc protection may well lead to activation of checkpoints at several stages of the cell cycle. Chromosome End Fusions in Cells with Critically Shortened Telomeres. Telomere associations have been observed by cytogenetic inspection of chromosome behavior in a number of different settings, including senescent primary cells, cells transformed with viral agents, and in a large variety of tumor specimen [reviewed in de Lange, In Telomeres, Cold Spnng Harbor Press, Cold Spring Harbor (1995)]. Although this was not always established m these studies, data accumulated over the past decade suggest that in most cases where telomere associations were observed, the telomeres may have been fairly short. Indeed in studies that measured telomere length directly, there is a correlation between shortened telomeres and their association in metaphase [Counter et al . EMBO J, 11:1921-1929 (1992), Saltman et al , Chromosoma, 102:121-128 (1993)]. Our observations on the behavior of chromosome ends after loss of the duplex telomenc DNA binding protein, TRF2, are consistent with a molecular mechanism underlying these telomere associations: When the telomeres reach a critical minimal length their ability to recruit sufficient TRF2 is diminished and end-to-end fusions result The length setting in which this aspect of telomere function becomes compromised has not been fully established. Telomeres and Cellular Senescence Expression of two mutant TRF2 polypeptides induced a growth arrest in the human fibrosarcoma cells used in this study. This arrest had several features consistent with the induction of senescence, including a specific cellular morphology, expression of a β-galactosidase activity correlated with senescence, and the irreversible nature of the arrest. This senescent-like phenotype m these transformed cells is likely to be directly related to the rephcative senescence described m primary human cells These results indicate that transformed human cells are rather sensitive to the status of their telomeres and that interference with telomere function can inhibit proliferation of malignant cells Two mechanisms for the induction of the observed growth arrest can be entertained at this stage. Since the growth arrest m HTC75 cells is accompanied by chromosomal abnormalities, one possibility is that the arrest is a response to DNA damage arising from the altered TRF2 activity at telomeres. The second possibility is that there exists a specific pathway allowing cells to evaluate the status of their telomeres For instance, the presence of the basic N-terminus of TRF2 on telomeres may be required to suppress cellular senescence Overexpression of a mutant protein lacking this domain would then be expected to induce arrest by displacing the endogenous TRF2 Similarly, the strong dominant negative activity of TRF2 ΔBΔM would result in telomeres lacking the basic N-termmal domain of TRF2 and cause an arrest signal Such a mechanism would allow cells to monitor the length of their telomeres and initiate a growth arrest and senescence program in response to critical shortening of the telomeres Requirements for Telomere Formation in Human Cells Transfection of TTAGGG repeats into human cells leads to efficient de novo formation of fully functional telomeres [Fair et al , Proc Natl Acad Sci USA, 88:7006-7010 (1991)] An excellent correlation between the binding specificity of TRF 1 and the cis-acting requirements for de novo telomere formation in human cells has been determined [Hanish et al , Proc Natl Acad Sci USA, 91:8861-8865 (1994)], indicating that the acquisition of TRFl might be an essential step m telomere healing However, the subsequent cloning of TRF2 revealed that this protein has the same sequence preference as TRFl [Broccoli et al , Nature Gen , 17:231-235 (1997)], raising the possibility that TRF2, rather than TRFl is involved in the conversion of the transfected DNA into a functional telomere The finding that TRF2 is important for the protection of chromosome ends now indicates an alternative scenario for de novo telomere formation in human cells Acquisition of TRF2 may be one of the early steps as the transfected telomere seed enters the nucleus According to the current data, TRF2 has the ability to protect the TTAGGG repeats from hgation to other DNA As a result, the TRF2-bound end of the transfected linear plasmid may be prevented from undergoing the recombination reaction that normally leads to chromosome-internal integration of transfected DNA Recombination of the other (non-telomenc) end of the transfected DNA with a resident chromosomal locus will then lead to the observed chromosome fragmentation Telomerase subsequently elongates the telomere seed to form a fully functional telomere This invention may be embodied in other forms or carried out in other ways without departing from the spmt or essential characteristics thereof The present disclosure is therefore to be considered as m all respects illustrative and not restrictive, the scope of the invention being indicated by the appended Claims, and all changes which come withm the meaning and range of equivalency are intended to be embraced therein Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties.

Claims: WHAT TS CLAIMED IS: 1. An isolated nucleic acid encoding a vertebrate telomere repeat binding factor (TRF) having an amino acid sequence substantially homologous to that of SEQ ID NO:27, and compnsing the following characteristics: a) a basic N-termmal domain; b) a dimenzation domain; and c) a Myb domain; wherein when the basic N-terminal domain is removed, the TRF detectably binds to the telomere repeat sequence (TTAGGG) 12 . 2. The isolated nucleic acid of Claim 1 wherein the TRF is a mammalian protein. 3. The isolated nucleic acid of Claim 2 wherein the TRF is a human protein having the amino acid sequence of SEQ ID NO:27, or SEQ ID NO.27 with a conservative ammo acid substitution. 4. The isolated nucleic acid of Claim 3 wherein the nucleic acid comprises the coding sequence of SEQ ID NO:26. 5. The isolated nucleic acid of Claim 1 further comprising a heterologous nucleotide sequence. 6. The isolated nucleic acid of Claim 2 wherein the TRF is a munne protein having the ammo acid sequence of SEQ ID NO:29, or SEQ ID NO:29 with a conservative ammo acid substitution. 7. The isolated nucleic acid of Claim 6 wherein the nucleic acid comprises the coding sequence of SEQ ID NO:28. 8. The isolated nucleic acid of Claim 7 further comprising a heterologous nucleotide sequence. 9. The isolated nucleic acid of Claim 1 operatively linked to an expression control sequence. 10 A unicellular host transformed or transfected with the nucleic acid of Claim 9. 11. A method of expressing the TRF encoded by the nucleic acid of Claim 10 compnsing cultunng the unicellular host in an appropriate cell culture medium under conditions that provide for expression of the protein by the cell. 12. The method of Claim 11 further comprising the step of purifying the TRF. 13. The purified form of the TRF obtained by the method of Claim 12. 14. An isolated nucleic acid compnsing a nucleotide sequence encoding a basic N- terminal domain of a vertebrate telomere repeat binding factor (TRF), that has the ammo acid sequence of SEQ ID NO:37, or SEQ ID NO:37 with a conservative ammo acid substitution. 15 The isolated nucleic acid of Claim 14 wherein the nucleic acid comprises the coding sequence of SEQ ID NO:36. 16. The isolated nucleic acid of Claim 14 further comprising a heterologous nucleotide sequence. 17. An isolated nucleic acid compnsing a nucleotide sequence encoding a dimenzation domain of a vertebrate telomere repeat binding factor (TRF) that has the amino acid sequence of SEQ ID NO:45, or SEQ ID NO:45 with a conservative ammo acid substitution 18. The isolated nucleic acid of Claim 17 further comprising a heterologous nucleotide sequence. 19. The isolated nucleic acid of Claim 17 wherein the nucleic acid comprises the coding sequence of SEQ ID NO:44. 20. An isolated nucleic acid comprising a nucleotide sequence encoding a dimenzation domain of a vertebrate telomere repeat binding factor (TRF) that has the amino acid sequence of SEQ ID NO:47, or SEQ ID NO:47 with a conservative ammo acid substitution. 21. The isolated nucleic acid of Claim 20 further comprising a heterologous nucleotide sequence. 22. The isolated nucleic acid of Claim 20 wherein the nucleic acid comprises the coding sequence of SEQ ID NO:46. 23. An isolated nucleic acid compnsing a nucleotide sequence encoding a truncated vertebrate telomere repeat binding factor (TRF) that has the ammo acid sequence of SEQ ID NO:31 or SEQ ID NO:31 with a conservative ammo acid substitution. 24. The isolated nucleic acid of Claim 23 having the nucleotide sequence of SEQ ID NO:30. 25. The isolated nucleic acid of Claim 23 further comprising a heterologous nucleotide sequence. 26. An isolated vertebrate telomere repeat binding factor (TRF) having an ammo acid sequence substantially homologous to that of SEQ ID NO.27, and comprising the following characteristics: a) a basic N-termmal domain; b) a dimenzation domain; and c) a Myb domain; wherein when the basic N-termmal domain is removed, the TRF detectably binds to the telomere repeat sequence (TTAGGG) 12 . 27. The isolated TRF of Claim 26 that is a mammalian protein 28. The isolated TRF of Claim 26 that is a fusion protein 29. The isolated TRF of Claim 27 that is a human protein having the ammo acid sequence of SEQ ID NO:27, or SEQ ID NO:27 with a conservative amino acid substitution. 30. A proteolytic fragment of the TRF of Claim 29. 31. The isolated TRF of Claim 27 that is a munne protein having the amino acid sequence of SEQ ID NO:29, or SEQ ID NO:29 with a conservative ammo acid substitution. 32. An isolated protein comprising the basic N-termmal domain of a vertebrate telomere repeat binding factor (TRF) that has the ammo acid sequence of SEQ ID NO:37, or SEQ ID NO:37 with a conservative amino acid substitution. 33. The isolated protein of Claim 32 that is a fusion protein. 34. An isolated protein comprising a dimenzation domain of a vertebrate telomere repeat binding factor (TRF) that has the amino acid sequence of SEQ ID NO:45, or SEQ ID NO:45 with a conservative ammo acid substitution. 35. The isolated protein of Claim 34 that is a fusion protein. 36. An isolated protein that is a truncated vertebrate telomere repeat binding factor (TRF) that has the ammo acid sequence of SEQ ID NO:31 or SEQ ID NO:31 with a conservative ammo acid substitution. 37. The isolated protein of Claim 36 that is a fusion protein. 38. An antibody that recognizes a basic N-termmal domain of a vertebrate telomere repeat binding factor (TRF) that has the ammo acid sequence of SEQ ID NO:37, or SEQ ID NO:37 with a conservative amino acid substitution. 39. The antibody of Claim 38 which is a polyclonal antibody 40. The antibody of Claim 39 which is a monoclonal antibody. 41 An immortal cell line that produced a monoclonal antibody according to Claim 40.

Inventors: BROCCOLI DOMINIQUE, DE LANGE TITIA, SMOGORZENSKA AGATA, BROCCOLI, DOMINIQUE, DE LANGE, TITIA, SMOGORZENSKA, AGATA
Applicants: UNIV ROCKEFELLER, THE ROCKEFELLER UNIVERSITY
Application year: 1998
Main classification: C12N15
IPC codes: C07K14, C12N15