ARF-p19, A NOVEL REGULATOR OF THE MAMMALIAN CELL CYCLE

Abstract: The INK4A (MTS1, CDKN2) gene encodes a specific inhibitor (InK4a-p16) of the cyclin D-dependent kinases CDK4 and CDK6. InK4a-p16 can block these kinase from phosphorylating the retinoblastoma protein (pRb), preventing exit from the G1 phase of the cell cycle. Deletions and mutations involving the gene encoding InK4a-p16, INK4A, occur frequently in cancer cells, implying that INK4a-p16, like pRb, suppresses tumor formulation. However, a completely unrelated protein (ARF-p19) arises in major part from an alternative reading frame of the mouse INK4A gene. Expression of an ARF-p19 cDNA (SEQ ID NO:1) in rodent fibroblasts induces both G1 and G2 phase arrest. Economical reutilization of protein coding sequences in this manner is without precedent in mammalian genomes, and the unitary inheritance of INK4a-p16 and ARF-p19 may reflect a dual requirement for both proteins in cell cycle control.

Description: ARF-pl9, A Novel Regulator of the Mammalian Cell Cycle Field ofthe Invention This invention relates to cancer detection and treatment and, more particularly, to a novel protein called "pl9 ARF protein," "p^^" or simply "ARF-pl9" that is involved in regulation of the eukaryotic cell cycle. Protein ARF-pl9 is encoded by a nucleic acid derived from the gene, INK4A, which also encodes an inhibitor of D-type cyclin-dependent kinases called u pl6 InK a protein," "pl6 InK4a " or simply "InK4a-pl6." Transcripts encoding InK4a-pl6 originate from a first promoter, Elα; the present invention is based on the observation that some INK4A transcripts initiate from a second promoter, Elβ, and contain an Alternative Reading Frame, ARF, which overlaps the InK4a-pl6 reading frame to some degree. ARF transcripts direct the production of a protein that has ARF-pl9 amino acid sequences instead of the previously-known InK4a-pl6 sequences. Like InK4a-pl6, ARF-pl9 regulates the eukaryotic cell cycle. When overexpressed, ARF-pl9 inhibits cells from proceeding past both the Gl and G2 phases of the cell cycle. However, the mechanism(s) by which ARF-pl9 acts are unlike those of InK4a-pl6, which acts by directly and specifically interacting with CDK (cyclin D-dependent kinase) proteins and thus preventing CDK-cyclin D interactions. In addition to (1) ARF-pl9 proteins, this invention further relates to (2) nucleic acids that encode ARF-pl9 isolated from mice, humans and other mammals; (3) antibodies that specifically bind ARF-pl9 protein or polypeptides derived therefrom; (4) methods for detecting one or more nucleic acids encoding ARF-pl9, or alterations in such nucleic acids; (5) methods for producing ARF-pl9 proteins using nucleic acids that encode ARF-pl9; (6) purified ARF-pl9 proteins, or fusion proteins derived from the joining of an ARF-pl9 polypeptide sequence with a second polypeptide sequence; (7) methods of treating cancer using purified ARF-pl9 proteins or fusion proteins derived therefrom; (8) methods of inducing cell cycle arrest using ARF-pl9 proteins or nucleic acids encoding ARF-pl9 proteins; (9) methods for detecting ARF-pl9 proteins using antibodies that specifically bind ARF-pl9 proteins; (10) methods of selectively killing cells having uncontrolled growth using antibodies that specifically bind ARF-pl9 proteins, or conjugates derived from such antibodies; (11) methods of stimulating cell growth using antibodies that specifically bind ARF-pl9 proteins, or fragments derived from such antibodies; and (12) transgenic non¬ human animals that have a genetically engineered alteration in one or more nucleic acids encoding ARF- pl9 proteins but which express normal levels of wild-type InK4a-pl6 protein, or which overexpress human ARF-pl9 or mutant forms of ARF-pl9. Statement as to -Rights to Inventions Made Under Federally-Sponsored Research and Development This invention was made with U.S. Government support under Cancer Center CORE grant 5P30CA21765-18, awarded by the National Cancer Institute. The U.S. Government has certain rights in this invention. Support for this invention was also provided by the Howard Hughes Medical Institute and the American Lebanese Syrian Associated Charities of St. Jude Children's Research Hospital. Background ofthe Invention Neoplasia, the pathological process by which tumors develop, necessarily involves unregulated, or at best misregulated, cellular growth and division. The molecular pathways that regulate cellular growth must inevitably intersect with those that regulate the cell cycle. The cell cycle consists of a cell division phase and the events that occur during the period between successive cell divisions, known as inteφhase. Inteφhase is composed of successive Gl, S, and G2 phases, and normally comprises 90% or more of the total cell cycle time. Most cell components are made continuously throughout inteφhase; it is therefore difficult to define distinct stages in the progression of the growing cell through inteφhase. One exception is DNA synthesis, since the DNA in the cell nucleus is replicated only during a limited portion of inteφhase. This period is denoted as the S phase (S = synthesis) of the cell cycle. The other distinct stage of the cell cycle is the cell division phase, which includes both nuclear division (mitosis) and the cytoplasmic division (cytokinesis) that follows. The entire cell division phase is denoted as the M phase (M = mitotic). This leaves the period between the M phase and the start of DNA synthesis, which is called the Gl phase (G = gap), and the period between the completion of DNA synthesis and the next M phase, which is called the G2 phase (Alberts, B. et al. , Molecular Biology of the Cell. Garland Publishing, Inc. , New York & London (1983), pages 611-612.). Progression through different transitions in the eukaryotic cell cycle is positively regulated by a family of master enzymes, the cyclin-dependent kinases (reviewed by Sherr, C , Cell 73: 1059-1065 (1993)). These holoenzymes are composed of two proteins, a regulatory subunit (the cyclin), and an associated catalytic subunit (the actual cyclin-dependent kinase or CDK), the levels of which vary with different phases of the cell cycle (Peters, G. , Nature 577:204-205 (1994)). Both cyclins and CDKs represent molecular families that encompass a variety of genetically related but functionally distinct proteins. Generally, different types of cyclins are designated by letters (i.e., cyclin A, cyclin B, cyclin D, cyclin E, etc.); CDKs are distinguished by numbers (CDK1. CDK2, CDK3, CDK4. CDKS, etc.; CDK1 is a.k.a. CDC2). CDK-cyclin D complexes regulate the decision of cells to replicate their chromosomal DNA (Sherr, Cell 73: 1059-1065 (1993)). As cells enter the cycle from quiescence, the accumulation of CDK- cyclin D holoenzymes occurs in response to mitogenic stimulation, with their kinase activities being first detected in mid-Gl phase and increasing as cells approach the Gl/S boundary (Matsushime et al. , Mol. Cell. Biol. 14:2066-2016 (1994); Meyerson and Harlow, Mol. Cell. Biol. 74:2077-2086 (1994)). The cyclin D regulatory subunits are highly labile, and premature withdrawal of growth factors in Gl phase results in a rapid decay of CDK-cyclin D activity that correlates with the failure to enter S phase. In contrast, removal of growth factors late in Gl phase, although resulting in a similar collapse of CDK- cyclin D activity, has no effect on further progression through the cell cycle (Matsushime et al. , Cell 65:701-713 (1991)). Microinjection of antibodies to cyclin Dl into fibroblasts during Gl prevents entry into the S phase, but injections performed at or after the Gl - S transition are without effect (Baldin et al. , Genes & Devel. 7:812-821 (1993); Quelle et al.. Genes & Devel. 7: 1559-1571 (1993)). Therefore, CDK- cyclin D complexes execute their critical functions at a late Gl checkpoint, after which cells become independent of mitogens for completion of the cycle. In mammals, cells enter the cell cycle and progress through Gl phase in response to extracellular growth signals which trigger the transcriptional induction of D-type cyclins. The accumulation of D cyclins leads to their association with two distinct catalytic partners, CDK4 and CDK6, to form kinase holoenzymes. Several observations argue for a significant role of the cyclin D-dependent kinases in phosphorylating the retinoblastoma protein, pRb, leading to the release of pRB-associated transcription factors that are necessary to facilitate progression through the Gl - S transition. First, CDK-cyclin D complexes have a distinct substrate preference for pRb but do not phosphorylate the canonical CDK substrate, histone HI (Matsushime et al. , Cell 77:323-334 (1992); Matsushime et al. , Mol. Cell. Biol. 74:2066-2076 (1994); Meyerson and Harlow, Mol. Cell. Biol. 74:2077-2086 (1994)). Their substrate specificity may be mediated in part by the ability of D-type cyclins to bind to pRb directly, an interaction which is facilitated by a Leu-X-Cys-X-Glu pentapeptide that the D cyclins share with DNA oncoproteins that also bind pRb (Dowdy et al.. Cell 75:499-51 1 (1993); Ewen et al. , Cell 73:481-497 (1993); Kato et al.. Genes & Devel. 7:331-342 (1993)). Second, cells in which pRb function has been disrupted by mutation, deletion, or after transformation by DNA tumor viruses are no longer inhibited from entering 5 phase by microinjection of antibodies to D cyclin, indicating that they have lost their dependency on the cyclin D-regulated Gl checkpoint (Lukas et al. , 3. Cell. Biol. 725:625-638 (1994); Tarn et al , Oncogene 9:2663-2674 (1994)). However, introduction of pRb into such cells restores their requirement for cyclin D function (Lukas et al., J. Cell. Biol. 725:625-638 (1994)). Third, pRb-negative cells synthesize elevated levels of a 16 kDa polypeptide inhibitor of CDK4, "plό 11 * 43 " (a.k.a. "InK4a-pl6" or simply "pl6"), which is a member of a recently discovered class of cell cycle regulatory proteins (Nasmyth and Hunt, Nature 566:634-635 (1993); Peters, G., Nature 371:204-205 (1994)) and which is found in complexes with CDK4 at the expense of D-type cyclins during Gl phase (Bates et al. , Oncogene 9: 1633-1640 (1994); Serrano et al. , Nature 366:104-101 (1993); Xiong et al.. Genes & Devel. 7: 1572-1583 (1993)). The fact that such cells cycle in the face of apparent CDK4 inhibition again implies that D-type cyclins are dispensable in the Rb-negative setting. The InK4 gene family ("InK4" signifies /πhibitors of CDK4) is known to include at least three other low molecular weight polypeptides, InK4b-pl5, induced in human epithelial cells treated by transforming growth factor-β (TGF-β) (Hannon, G.J. , and Beach, D. , Nature 577:257-261 ( 1994)), InK4d-pl9 (Hirai, H. , et al , Mol. Cell. Biol. 75:2672-2681 (1995)) and InK4c-pl8 (Guan et al.. Genes 6 Develop. 8:2939-2952 (1994); Hirai. H., et l, Mol. Cell. Biol. 75:2672-2681 (1995)). InK4d-pl9 and InK4c-pl8 are described in detail in Serial No. 08/384, 106, filed February 6, 1995, which is hereby incoφorated by reference. Members of the InK4 family are typically composed of repeated ankyrin motifs, each of about 32 amino acids in length. All known members of the InK4 family act to specifically inhibit enzymatic activities of D-type cyclin-dependent kinases such as CDK4 and CDK6. Unlike other universal CDK inhibitors, such as p 21 c,P1 Wafl (El-Deiry et al . Cell 75:817-825 (1993); Gu et al.. Nature 566:707-710 (1993); Haφer et al . Cell 75:805-816 (1993); Xiong et al.. Nature 566:701-704 (1993)) and p27 ,pl (Polyak et al.. Genes & Devel. 8:9-22 (1994); Polyak et al. , Cell 78:59-66 (1994): Toyoshima and Hunter, Cell 78:61-14 (1994)), the InK4 proteins selectively inhibit the activities of CDK4 and CDK6, but do not inhibit the activities of other CDKs (Guan et al . Genes & Devel. 8:2939-2952 (1994); Hannon and Beach, Nature 577:257-261 (1994); Serrano et al.. Nature 566:704-707 (1993)). Like many CDK inhibitors (CKIs) (Nasmyth and Hunt, Nature 566:634-635 (1993)). InK4 family members negatively regulate progression through the mammalian cell cycle, in part in response to anti- proliferative extracellular signals. The InK4 proteins, by inhibiting the activities of a specific class of the D-type cyclin-dependent kinases (i.e., CDK4 and/or CDK6), arrest cell cycle progression in Gl phase and thus prevent cells from replicating their chromosomal DNA. Thus, in contradistinction to the positive regulation of D-type cyclin synthesis by growth factors, extracellular inhibitors of Gl progression can negatively regulate the activity of D-type cyclin-dependent kinases by inducing InK4 proteins. Related Art Mullis etal.. U.S. Patent 4,965,188 (Oct. 23, 1990), describe methods for amplifying nucleic acid sequences using the polymerase chain reaction (PCR). Beach, published PCT patent application WO 92/20796 (Nov. 26, 1992), describes genes encoding D cyclins and uses thereof. Berns, U.S. Patent 5,174,986 (Dec. 29, 1992), describes methods for determining the oncogenic potential of chemical compounds using a transgenic mouse predisposed to develop T-cell lymphomas. Crissman et al. , U.S. Patent 5, 185,260 (Feb. 9, 1993), describe methods for distinguishing and selectively killing transformed (neoplastic) cells using synthetic Gl kinase inhibitors. Stone, S., et al, Cancer Research 55:2988-2994 (1995), describe two cDNAs derived from the human INK4A gene, including an "α form" encoding InK4a-pl6 and a "β form" that includes an open reading frame (designated "ORF 2") that overlaps the reading frame encoding the ARF-pl9 protein described herein. Stone et al. state that it "is unknown if ORF 2 encodes a protein" (legend to Figure 1 , page 2990) and indicate that "ORF 2 has not been selectively maintained and probably does not encode a protein" (page 2989, column 2, lines 20-21). Mao, L., et al, Cancer Research 55:2995-2997 (1995), describe two transcripts and corresponding cDNAs derived from the human INK4A gene, designated "pl6" and "pl6β." The pl6 transcript is stated to encode the InK4a-pl6 protein, while the pl6β transcript is stated to contain a "theoretical open reading frame" (page 1996, column 1 , line 47) that is not further defined, and suggest this sequence "probably represents an untranslated open reading frame" (page 2997, column 2, lines 9-10). Mao et al. state that the in vitro transcription and translation (TNT) product of the pl6β cDNA is recognized by an antibody to InK4a-pl6 polypeptide sequences (page 2997. column 1 , lines 6). suggesting that the pl6β transcript encodes an amino-terminal truncated InK4a-pl6 polypeptide rather than a protein having, as ARF-pl9 does, an amino acid sequence unrelated to that of InK4a-pl6. However, Mao et al. also state that, using InK4a-pl6 antiserum, they are unable to identify an amino-terminal truncated pl6β protein in cell lines (page 2997, column 2, lines 1-2). Thus, Mao et al. are silent regarding the ARF-pl9 protein described herein. Summary ofthe Invention The present invention relates to the discovery in mammalian cells of a novel cell cycle regulatory protein, having a predicted molecular mass of 19 kDa, here designated "ARF-pl9 protein" or more simply "ARF-pl9" . In particular, the invention relates to ARF-pl9 proteins isolated from cells derived from a mouse or a human. Although derived from the gene encoding the previously-known lnK4a-pl6 protein, ARF-pl9 arises by differential transcription and translation of InK4a-pl6 sequences. That is, ARF-pl9 is encoded by an alternative reading frame (ARF) and has an amino acid sequence (SEQ ID NO:2; SEQ ID NO:4) that is wholly unrelated to that of InK4a-pl6. Suφrisingly. however. ARF-pl9 protein functions to regulate the cell cycle in a similar but less specific manner than, and by a mechanism distinct from that of, InK4a-pl6 protein. Thus, one aspect of the invention is directed to methods of using the ARF-pl9 proteins of the invention to inhibit the growth of cancer cells and/or to prevent cancer cells from replicating their chromosomal DNA. Both InK4-pl6 and InK4-pl5 appear to act as tumor suppressors (Noburi, T. et al. , Nature 568:753-756 (1994); Kamb, A. et al . Science 264:436-440 (1994)). The genes encoding pl6 and pl5 map in a tandem array to the short arm of human chromosome 9 within a region that is frequently deleted in cancer cells, and the resulting loss of their anti-proliferative functions can contribute to tumorigenesis (Noburi et al., Nature 568:753-756 (1994); Okuda, T., et al. Blood 85:2321-2330 (1995)). The novel ARF-pl9 protein described herein (1) plays a role in preventing the Gl - S and G2 - M phase transitions in normal mammalian cells, and (2) if having reduced or altered activity due to one or more mutations affecting the alternative reading frame encoding ARF-pl9, could contribute to oncogenesis in some cancers, even if such mutations have no effect on the reading frame encoding InK4a-pl6. In another aspect, the invention provides nucleic acid sequences encoding ARF-pl9 polypeptides from mice, humans and other mammals. The nucleic acid sequences of the invention may be expressed in the form of isolated nucleic acids, such as cDNA clones, genomic DNA clones, mRNA transcribed from either cDNA or genomic DNA clones, synthetic oligonucleotides, and/or synthetic amplification products resulting from PCR, and may be single-stranded or double-stranded. In a related aspect, the invention provides methods for detecting nucleic acids encoding wild-type or mutant ARF-pl9 proteins using the nucleic acid sequences of the invention described above. The detection of point mutations, deletions of, or other mutations in, the reading frame encoding ARF-pl9 is predictive of a predisposition to, or diagnostic of, certain types of cancer. In another related aspect, the DNA molecules of the invention described above may be cloned into expression vectors and placed in an appropriate host in order to produce ARF-pl9 proteins or fusion proteins containing ARF-pl9 polypeptide sequences. When placed in an animal that has cancer, this aspect of the invention relates to gene therapy for certain types of cancers. In another aspect, the invention provides antibody compositions that bind specifically to ARF-pl9 proteins and/or polypeptides derived therefrom. The antibody compositions of the invention may be polyclonal, monoclonal, or monospecific. Although all of the antibody compositions of the invention bind specifically to ARF-pl9, some compositions bind to a specific epitope of ARF-pl9 and thereby inhibit a specific function of ARF-pl9. In a related aspect, the invention provides methods for detecting ARF-pl9 proteins using the antibody compositions described above. The detection of reduced amounts of, or altered forms of, ARF- pl9 proteins is predictive of a predisposition to, or diagnostic of, certain types of cancer. In another aspect, the invention provides transgenic non-human animals which have one or more mutations in the endogenous reading frame encoding ARF-pl9, wherein said mutation results in the production of a mutant ARF-pl9 protein or results in a loss of ARF-pl9 expression but does not significantly affect the InK4a-pl6 gene product or expression thereof. Additionally or alternatively, the transgenic non-human animals of the invention express a human wild-type or mutant ARF-pl9. Because of the transgene(s) introduced into the genome of the non-human animals of the invention. the animals have a reduced and/or altered ARF-pl9 activity compared to wild-type animals, and consequentially develops certain types of cancers, particularly melanomas, in a reproducible and thus predictable manner. In a related aspect, compositions are evaluated for their potential to enhance or inhibit certain types of cancers, particularly melanomas, using the transgenic non-human animals of the invention. Brief Description ofthe Figures Figure 1 shows the sequence of a murine cDNA molecule (SEQ ID NO: l) homologous to human INK4A β transcripts and the sequence of the polypeptide, ARF-pl9 (SEQ ID NO: 2), encoded thereby; the partial carboxy-terminal amino acid sequence of InK4a-pl6 (SEQ ID NO:5), encoded by exons 2 and 3 in INK4A a transcripts, is also indicated. Sequences from exon lβ (nucleotides 1-232) are spliced to exon 2 to create an open reading frame capable of encoding a novel 169 amino acid protein (ARF-pl9). The initiator codon for ARF-pl9 occurs at nucleotides 43-45, and a putative UAG stop codon is found at nt 550-552. Nucleotides 436-438 (double underlined CCA) are replaced by a TGA termination codon in corresponding position in the human INK4A gene (Serrano et al.. Nature 566:704-707 (1993)); accordingly, the human ARF-pl9 protein is truncated relative to the murine ARF-pl9 protein. Unrelated sequences from exon lα (not shown) are spliced to the same exon 2 acceptor site to open another reading frame that encodes InK4a-pl6. Exon 2-coded InK4a-pl6 amino acid sequences are shown below those of ARF-pl9. The carboxyl terminus of mouse InK4a-pl6 is 20 residues longer than the human polypeptide (Quelle et al. , Oncogene 77:635-645 (1995)), with the last four amino acids of the latter encoded by a third exon (Kamb, A. et al , Science 264:436-440 (1994a)). The location of residues corresponding to the primers used to specifically amplify β transcripts by RT-PCR (Figure 2) are underlined. Figure 2 (panels A-D) shows the results of RT-PCR assays of INK4A α and β mRNA transcripts in mouse tissues and the mouse MEL cell line. Equivalent quantities of RNA from the indicated tissues were amplified in parallel by RT-PCR using 5' primers specific for a or β transcripts and a common 3' primer (Figure 1). Products from β (panels A, B) and o (panels C, D) RT templates were hybridized with specific exon lβ (panel A), exon lα (panel C). or exon 2 probes (panels B, D). Auioradiographic exposure times were 2 hrs. Figure 3 shows the results of immunoassays using antibodies to the carboxyl-termmal portion of ARF-pl9 (pl9 antiserum) or antibodies to InK4a-pl6 (pl6 antiserum) and the detection of InK4a-pl6 (pl6), ARF-pl9 (pl9) or ARF-pl9 tagged with hemagglutinin (HA-pl9). cDNAs encoding InK4a-pl6, ARF-pl9, and HA-tagged ARF-pl9 (as indicated below the panel) were transcribed and translated in vitro. Proteins labeled with [ 35 S]-Methionine, normalized for equal input of radioactivity, were precipitated with nonimmune rabbit serum (NRS) or with antisera to InK4a-pl6 or ARF-pl9 (as indicated at the top) and separated on denaturing gels. The positions of marker proteins of known molecular mass are indicated at the right. Figure 4 (panels A-C) shows the results of immunoassays using antibodies specific for ARF-pl9, InK4a-pl6, or hemagglutinin (HA). Cell lysates (indicated at the bottom of panel C) were divided into three equal aliquots, separated on denaturing gels, and immunoblotted with antibodies specific for ARF- pl9 (panel A), HA (panel B) or InK4a-pl6 (panel C). The cell lines indicated on the bottom of the Figure (Sf9 = insect cells in which baculoviral expression occurs: N3T3-d = NIH-3T3 cells genetically engineered to overexpress cyclin Dl ; B3T3 = derivative of Balb-3T3 cells; MEL = mouse erythroleukemia cells) were either uninfected ("none") or infected with appropriate expression constructs expressing ARF-pl9 ("pl9"), ARF-pl9 tagged with hemagglutinin ("HA-pl9"). or InK4a-pl6 ("p!6"). Cells were infected for 48 hrs before lysis with control vectors lacking inserts (lanes 1 , 3, and 7) or containing the indicated cDNAs (top, panel A). The positions of marker proteins are shown at the left and positions of ARF-pl9 or InK4a-pl6 at the right and by arrowheads in panels A and B. Blots were developed using enhanced fluorography (exposure time, 3 sees), allowing only approximate comparisons of signal intensities between the different panels. Figure 5 (panels A-C) shows the localization of ARF-pl9 and pl9-hemagglutinin fusion proteins to cellular nuclei. Cytospin preparations of NIH-3T3 cells infected for 48 hrs with a vector encoding HA- tagged ARF-pl9 were fixed and stained with antiserum to ARF-pl9 (panel A), anti-ARF-pl9 plus cognate peptide (panel B). or anti-HA serum (panel C). Matched exposures are shown at 600X magnification. The addition of polypeptides having ARF-pl9 sequences blocks the signal produced by the antibodies specific for ARF-pl9 (panel B). Figure 6 summarizes mutant amino acid residues in ARF-pl9 predicted from mutations in the gene that encodes both pl6-InK4a and ARF-pl9 compiled from data from primary tumors, xenografts, and established cell lines. The majority of INK4A mutations so far described target the 5' portion of INK4A exon 2 (Hirama and Koeffler, Blood 86:841-854 (1995)), which encodes portions of both InK4a-pl6 and ARF-pl9. Comparison of the mouse (upper) and human (lower) ARF-pl9 amino acid sequences defines conserved residues (bold type). Residues in the human gene that have sustained mutations in cancer cells are doubly underlined and the mutant amino acids are indicated below them. Mutations that are silent with regard to the InK4a-pl6 coding frame but which are predicted to affect the primary structure of ARF-pl9 are indicated by asterisks (e.g., P71T*). Superscripts note multiple substitutions of the same residue (e.g. G68L 3 was independently observed in 3 cases), and closed squares define microdeletions plus frame shifts. No nonsense mutations were found. All mutations were detected in sporadic cancers except for Rl 14L (G101W in pl6), which has been genetically implicated in familial melanoma in 3 of 9 kindreds (Hussussian et al. , Nature Genet 8: 15-21 (1994); Kamb et al. , Nature Genet 8:22-26 (1994b)). Known sequence polymoφhisms have been excluded. The remaining data were taken from Caldas et al. , Nature Genet 8:27-32 (1994), Hayashi et al. , Biochem. Biophys. Res Commun. 202: 1426-1430 (1994), Kamb, A. et al . Science 264:4 6-440 (1994), Mori et al . Cancer Res. 54:3396-3397 (1994), Ohta et al . Cancer Res 54:5269-5272 (1994), and Zhang et al . Cancer Res 54:5050-5053 (1994). Numbering of InK4a-pl6 amino acid sequences in the text is based on the corrected N-terminus (Hannon and Beach, Nature 577:257-261 (1994)) which includes 8 residues beyond those originally identified (Serrano et al.. Nature 566:704-707 (1993)). Figure 7 shows the sequence of a human cDNA molecule (3; see also Mao et al.. Cancer Research 55:2995-2997 91995)) corresponding to human INK4A β transcripts and the sequence of the polypeptide, ARF-pl9 (SEQ ID NO:4, denoted "arf" in the Figure), which is (as described herein) encoded thereby. The partial carboxyl-terminal amino acid sequence of human InK4a-pl6 ("pl6") is also indicated. Detailed Description ofthe Disclosure Terms and Symbols. For puφoses of this disclosure, the following abbreviations and definitions are used herein unless otherwise indicated. The following list indicates the correspondence between the one-letter amino acid code (used in Figures 1 , 6 and 7 and Example 6) and the three-letter amino acid code (used elsewhere herein, in accordance with 37 C.F.R. § 1.822, revised as of July 1 , 1994): A = Ala C = Cys D = Asp E = Glu F = Phe G = Gly H = His I = He K = Lys = Leu M = Met N = Asn P = Pro Q = Gin R = Arg S = Ser T = Thr V = Val = Trp Y = Tyr ABBREVIATIONS CDK = cyclin-dependent kinase (protein); cdk = gene cDNA = complementary deoxyribonucleic acid DNA = deoxyribonucleic acid DMEM = Dulbecco's modified Eagle's medium EDTA = ethylenediamine tetraacetic acid ES = embryonic stem FISH = fluorescent in situ hybridization InK = inhibitor of CDK (protein); INK = gene kb = kilobase(s) kDa = kilodalton(s) MEL = mouse erythroleukemia (cell line) nt =- nucleotide(s) PBS = phosphate-buffered saline PCR = polymerase chain reaction pRB = retinoblastoma protein RT = reverse transcriptase SDS = sodium dodecyl sulfate Sf9 = Spodoptera frugiperda (cell line) Tg = transgenic TK = thymidine kinase Throughout the disclosure, abbreviations for nucleotide residues present in nucleic acid sequences are as described in 37 C.F.R. § 1.822, revised as of July 1, 1994. Glossary Amino acid sequence: The sequence of a polypeptide given in the order of from amino terminal (N-terminal), to carboxyl terminal (C-terminal). Synonymous with "polypeptide sequence, " "peptide sequence," "protein sequence," or "primary protein sequence." Animal: (1) Excludes human beings, individually and collectively, in all stages of development, including embryonic and fetal stages, unless otherwise indicated; and (2) includes all other vertebrate animals, including an individual animal in any stage of development, including embryonic and fetal stages. "Non-human animal" has the same meaning as "animal. " Animal model: A non-human animal that faithfully mimics a human disease and in which potential therapeutic compositions or potentially harmful compositions may be evaluated for their effect on the disease. Antibody: A protein molecule synthesized by a B-cell upon exposure to antigen capable of combining specifically with that antigen. Synonymous with immunoglobulin (Ig). Antibody, polyclonal: A composition that comprises an assortment of different antibodies that all recognize a particular antigen. Antibody, monoclonal: A unique, isolated antibody molecule produced by a hybridoma. Antibody, monospecific: A polyclonal antibody produced in immunological response to a single or few epitopes found in (1) a short, isolated, synthetic antigen or (2) a short, isolated, carrier-bound hapten. Antigen: A molecule or composition of matter which (1) induces an immune response in an animal, and (2) interacts specifically with antigen-recognizing components of an immune animal's immune system. Asyntactic: Not having the same arrangement (syntax); "out of register." In particular, note that fusion proteins cannot result from the asyntactic linkage of two (or more) open reading frames. Carrier: A molecule required in combination with a hapten in order for an immune response to the hapten to occur. That is, a molecule which puts a hapten in a molecular context in which the hapten has enhanced immunogenicity. Detectable label: A chemical moiety that is coupled to a biomolecule to enable detection of the biomolecule and which may be selected from the group consisting of a radiolabel, an enzyme such as horseradish peroxidase or alkaline phosphatase. streptavidin, biotin, an epitope recognized by an antibody, and equivalents thereof. Detectably labeled: A state of a biomolecule in which the biomolecule has covalently attached to it a detectable label. Disease: (1) Excludes pregnancy per se but not autoimmune and other diseases associated with pregnancy; (2) includes any abnormal condition of an organism or part, especially as a consequence of infection, inherent weakness, environmental stress, that impairs normal physiological functioning; and (3) includes cancers and rumors. DNA sequence: The sequence of contiguous nucleotide bases of a strand of DNA as read from 5' to 3'. Synonymous with "DNA molecule. " Enzyme: Protein that is a catalyst for a specific chemical reaction, often one involving one or more biomolecules as substrates and/or products. Unlike non-biologically derived catalysts, enzymes may recognize a substrate with stereospecificity, i.e., some enzymes are capable of recognizing, and thus catalyzing the chemical reaction of, only one of a pair of L- and D-enantiomers. Epitope: A part of an antigen that interacts specifically with antigen-recognizing components of an animal's immune system. In a polypeptidic antigen, epitopes may correspond to short sequences of contiguous amino acids; the remainder of the antigen is called the carrier. Synonymous with antigenic determinant. Expression vector: An artificial DNA sequence, or a naturally-occurring DNA sequence that has been artificially modified, into which foreign or abnormal genes can be inserted and that contains transcription and translation signals that direct the expression of the inserted genes in host cells appropriate for the expression vector, and the DNA of which is replicated, either extra- or intra- chromosomally, in such appropriate host cells. Expression construct: A construct consisting essentially of an expression vector and one or more foreign or abnormal genes inserted therein in such a manner that the expression vector's transcription and translation signals are operably linked to the inserted gene(s). Foreign or abnormal: Not endogenous to a healthy, wild-type organism. "Foreign or abnormal genes" designates nucleic acid sequences that are not endogenous to an organism's genome, or originally endogenous nucleic acid sequences that have been rearranged, mutated, or otherwise genetically engineered so as to possess properties (i.e., genomic location, regulation of expression, copy number, etc.) not possessed by the endogenous nucleic acid sequences from which they were derived. Gene: A DNA sequence that consists of a structural gene, e.g. , a reading frame that encodes a polypeptide sequence, according to the standard genetic code; and expression elements, e.g. , promoters, terminators, enhancers, etc. , required for transcription of the structural gene. Genetically engineered: Subject to human manipulation intended to introduce genetic change. Hapten: A small molecule which (1) cannot, by itself, induce an immune response in an animal, (2) can, in combination to a carrier to which it is bound, induce an immune response in an animal, and (3) interacts specifically with the antigen-recognizing components of an immune animal's immune system. Host animal: An animal that harbors foreign and/or abnormal genes introduced as a result of (1) invasion of cells of the animal by a naturally occurring or genetically engineered intracellular parasite; or (2) introduction into cells of foreign or abnormal genes by human manipulation. Immune animal: An animal which has been presented with an immunizing amount of antigen and has generated a humoral and/or cell-mediated immune response thereto. Mammal: (1) Excludes human beings, individually and collectively, in all stages of development, including embryonic and fetal stages, unless otherwise indicated; and (2) includes all other animals that are members of the vertebrae class Mammalia, including an individual animal in any stage of development, including embryonic and fetal stages, wherein members of the class are distinguished by self-regulating body temperature, hair, and, in the females, milk-producing mammae. Microorganism: A single-celled organism (e.g. , a bacterium) or an intracellular parasite (e.g., a rickettsia or a virus); includes both "live" and "attenuated" microorganisms. Operably linked: Arranged so as to have a functional relationship; in expression constructs, inserted foreign or abnormal genes that are properly positioned with regard to the signals that control transcription and translation so that efficient expression of the inserted genes occurs are said to be operably linked to such signals (and vice-versa). Polypeptide: A p ^ lymer of amino acid residues. Protein: A biomolecule comprising one or more polypeptides arranged into a functional, three- dimensional form. Restriction endonuclease: An endonuclease that cleaves DNA at each occurrence therein of a specific recognition sequence. Synonymous with "restriction enzyme. " Syntactic: Having the same arrangement (syntax); "in register. " In particular, note that fusion proteins can result from the syntactic linkage of two (or more) open reading frames. Transgene: A gene that does not occur naturally in an animal, i.e., a foreign or abnormal gene, introduced into an animal by nonnatural means, i.e., by human manipulation. Transgenic animal: An animal into which has been introduced, by nonnatural means, i.e., by human manipulation, one or more transgenes. Detailed Description ofthe Preferred Embodiments In its broadest form, the invention comprises a novel mammalian protein known as "ARF-pl9," that regulates the cell cycle; nucleic acid molecules having sequences encoding polypeptide sequences of ARF-pl9; antibodies specific for ARF-pl9; transgenic non-human animals with alterations in the gene encoding ARF-pl9; methods of making ARF-pl9 nucleic acids and polypeptides; methods of making ARF- pl9-specific antibodies; methods of making transgenic non-human animals with alterations in the gene encoding ARF-pl9; and methods of using the nucleic acids, proteins, antibodies and transgenic animals of the invention to detect ARF-pl9 nucleic acids or proteins in a sample, to diagnose cancers or predispositions thereto, to evaluate compositions for their therapeutic or oncogenic potential, and to prepare therapeutic compositions for the treatment of tumors and cancers. Nucleic Acids and Related Embodiments. In one embodiment, the invention comprises nucleic acids having sequences encoding mouse ARF-pl9, human ARF-pl9 or ARF-pl9 polypeptides from other mammals. For example, the invention provides cDNA molecules encoding mouse ARF-pl9 (SEQ ID NO: 1). The ARF-pl9 cDNAs of the invention are in turn used to isolate additional nucleic acids that encode ARF-pl9 polypeptide sequences, such as mouse and human genomic DNA clones. Moreover, because the homology between the nucleotide sequences of mouse and human ARF-pl9 genes is quite high, the mouse and human nucleic acids may be used to design probes or degenerate primers for PCR in order to isolate cDNA and genomic clones of ARF-pl9 genes from other mammals. One skilled in the art can readily adapt the nucleic acid sequences of the invention to any system which is capable of producing nucleic acids to produce the nucleic acids of the invention. The nucleic acids of the invention, which may optionally comprise a detectable label, may be prepared as cDNA clones, genomic clones, RNA transcribed from either cDNA or genomic clones, synthetic oligonucleotides, and/or synthetic amplification products resulting, e.g., from PCR. The nucleic acids of the invention may be prepared in either single- or double-stranded form. Methods of preparing cDNA clones are known in the art (see, for example, Chapter 8 in Sambrook et al. , Molecular Cloning: A Laboratory Manual. Vol. 2, 2d. Ed.. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989), pages 8.1-8.86). Methods of analyzing genomic DNA sequences and preparing genomic clones are known in the art (see, for example. Chapter 9 in Sambrook et al , Molecular Cloning- A Laboratory Manual, Vol 2, 2d Ed , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989), pages 9 1-9 62, and Chapter 2 in Current Protocols in Molecular Biology, Vol 1 , Ausubel et al . eds , John Wiley & Sons, Inc , Boston, MA (1994), pages 2 1 1-2 14 8) Genomic DNA sequences, 1 e , chromosomally-deπved nucleic acids, are isolated (see Example 9) from mice and other non-human animals and used for the production of transgenic non-human animals RNA containing ARF-pl9 sequences may be prepared from cells expressing ARF-pl9 according to methods known in the art (see, e g , Chapter 4 in Current Protocols in Molecular Biolog , Vol 1 , Ausubel et al , eds , John Wiley & Sons, Inc , Boston, MA (1994), pages 4 1 1-4 10 11). or may be generated by in vitro transcription using the DNA molecules of the invention (see, e g , Chapter 10 in Sambrook et al , Molecular Cloning A Laboratory Manual, Vol 2, 2d Ed , Cold Spring Harbor Laboratory Press, Cold Spπng Harbor, NY (1989), pages 10 1-10 70) Synthetic oligonucleotides having ARF-pl9-specιfic nucleotide sequences can be prepared using the nucleic acid sequences of the invention by known methods (see, e g . Chapter 1 1 m Sambrook et al , Molecular Cloning A Laboratory Manual, Vol 2, 2d Ed , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989), pages 11 1-11 61) When used as pπmers in the polymerase chain reaction (PCR), the synthetic oligonucleotides preferably contain from about 15 to about 30 contiguous nucleotides exactly corresponding to unique portions of the ARF-pl9 sequences of the invention, but may optionally contain additional nucleotides 5' therefrom (Innis, M A and Gelfand, D H , Chapter 1 in PCR Protocols A Guide to Methods and Applications , Innis et al , eds , Academic Press, Inc , New York (1990), pages 3-12, Saiki, R K , Chapter 2 in PCR Protocols A Guide to Methods and Applications , Innis et al , eds , Academic Press, Inc , New York (1990), pages 13-20) Synthetic amplification products are prepared using the synthetic oligonucleotides of the invention in amplification systems such as PCR (see, e g , U S Patent 4,965,188 to Mul s et al (Oct 23, 1990), Scharf, S J , Chapter 11 in PCR Protocols A Guide to Methods and Applications, Innis et al , eds , Academic Press. Inc , New York (1990), pages 84-98, Chapter 15 in Current Protocols in Molecular Biology, Vol 2, Ausubel et al eds , John Wiley & Sons, Inc , Boston, MA (1994), pages 15 0 1-15 8 8, and Chapter 14 in Sambrook et al , Molecular Cloning A Laboratory Manual, Vol 2, 2d Ed , Cold Spring Harbor Laboratory Press, Cold Spπng Harbor, NY (1989), pages 14 1-14 35) Those of skill in the art will appreciate that chemical derivatives of nucleotide structures can be substituted for natural nucleotides in the nucleic acids of the invention Methods of nucleic acid expression: In one aspect of this embodiment of the invention, the nucleic acids of the invention are used to prepare ARF-pl9 proteins, or fusion proteins derived from ARF- pl9, via recombinant DNA technology By inserting any of the nucleic acids of the invention that encode ARF-pl9 polypeptide sequences into an appropriate expression vector, and introducing the resultant expression vector construct into appropriate host cells, those skilled in the art can produce large quantities of ARF-p 19 polypeptides There are numerous host/expression vector systems available for the generation of proteins from the isolated nucleic acids of the invention These include, but are not limited to, bacteπa/plasmid systems, bacteπa/phage systems, eukaryotic cell/plasmid systems, eukaryotic cell/virus systems, and the like (see, for example, U S Patent 4,440,859 to Rutter et al (Apr 3, 1984), Chapter 16 in Current Protocols in Molecular Biology, Vol 2, Ausubel et al , eds , John Wiley & Sons, Inc , Boston, MA (1994), pages 16 0 5-16 20 16, and Sambrook et al , Molecular Cloning A Laboratorv Manual Vol 3. 2d Ed , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989)) One skilled in the art can readily adapt the nucleic acids of the invention to any host/expression vector system which is capable of propagating and expressing heterologous nucleic acids to produce the proteins or polypeptides of the invention. Preferred host/expression systems include bacteria/plasmid systems and insect cell/baculoviral expression vector systems. Diagnostic methods and kits: In another aspect of this embodiment, ARF-pl9 nucleic acid sequences are used to prepare oligonucleotide probes, or PCR primers, to serve as materials for diagnostic tests for ARF-pl9 expression, mutation, or deletion in samples of cells isolated from mammals. Deletions of the genes encoding pl5 and pl6 occur frequently in cancer cells, and the resulting loss of their anti¬ proliferative functions can contribute to tumorigenesis (Noburi et al, Nature 568:753-756 (1994)). Similarly, point mutations, deletions or other mutations in the genes encoding ARF-pl9 are diagnostic of cancer or indicative of a predisposition to develop certain types of cancers. Mutations in the human gene for ARF-pl9 are detected by any of a variety of methods depending in part on the nature of the mutation of interest. Deletions and insertions of about 100 base pairs (bp) or more are detected by electrophoretic separation of genomic DNA and hybridization analysis using nucleic acid probes derived from unique portions of the nucleotide sequence of the human ARF-pl9 coding sequence (SEQ ID NO:3; see also Figure 7), or by PCR of genomic DNA using synthetic oligonucleotides derived from the unique portions of the nucleotide sequence of the human ARF-pl9 coding sequence as primers. The term "the unique portions of the nucleotide sequence of human ARF-pl9" is intended to encompass nucleotide sequences that occur in molecules encoding ARF-pl9 but which are not found in pl6-InK4a mRNAs. In one aspect, the invention comprises methods of detecting the presence of a nucleic acid polymoφhism associated with a predisposition to develop cancer by analyzing DNA or RNA from a mammal using nucleic acid molecules containing part or all of the unique portions of the nucleotide sequences from an ARF-pl9 gene from a mammal, such as a mouse or a human, or the reverse complement thereof. Such methods are used in conjunction with any procedure which will detect the nucleic acids of the invention. Examples of such procedures include hybridization analysis using the nucleic acids of the invention, i.e., isolation of nucleic acids from the cells of a mammal, followed by restriction digestion, separation by a means such as gel electrophoresis, transfer to nitrocellulose or a comparable material, and detection of ARF-pl9 nucleic acid sequences thereon by exposure to detectably labeled nucleic acid probes which contain nucleotide sequences encoding ARF-pl9 polypeptide sequences. In one embodiment of the present invention, the preferred method of detecting the presence of a DNA polymoφhism associated with a predisposition to develop cancer involves RFLP (restriction fragment length polymoφhism) techniques based on amplification of ARF-pl9 sequences via PCR, followed by restriction digestion and agarose gel electrophoresis. In this method, a biological sample containing nucleated cells, preferably leukocytes, is obtained from a human. Suitable biological samples having nucleated cells that may be used in this invention include, but are not limited to, blood and tissue. The method of obtaining the biological sample will vary depending upon the nature of the sample. By the term "nucleated cells" is meant any cell containing a nucleus. Examples of such cells include, but are not limited to, white blood cells, epithelial cells, or mesenchymal cells. The cells are then isolated from the sample and the DNA from the nucleated cells is purified using conventional methods known in the art such as phenol-chloroform extraction, lytic enzymes, chemical solutions and centrifugation, or size exclusion chromatography (see, for example, Blin and Stafford, Nucl. Acid Res. 5:2303-2308 (1976); Maniatis, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York (1982)). Following isolation, the DNA sequences of interest are amplified using conventional PCR methods (see, for example, Innis et al , PCR Protocols, Academic Press, NY (1990); Mullis et al , Cold Spring Harbor Symp. Quant. Biol. 57:263-273 (1986); Mullis and Faloona, Methods Enzymol. 755:335- 350 (1987); and Mullis et al , U.S. Patent 4,965,188 (Oct. 23, 1990)). In one aspect of this embodiment, sequences comprising the unique portions of nucleotide sequences for ARF-pl9 are utilized as primers for specific amplification of ARF-pl9 nucleic acids (see Example 2). In such an embodiment the amplified product is subjected to restriction digestion prior to visualization. Different alleles of ARF-INK4a will yield amplified fragments of differing size after digestion with an appropriate restriction endonuclease. The amplified DNA is then precipitated, and digested with a restriction enzyme, such as BamHl, Bglll , Pstl, or EcoRI. Digested DNA fragments are separated according to their molecular weights to form a pattern, typically using agarose gel electrophoresis. Following electrophoresis, the gel is stained with an appropriate agent, such as ethidium bromide, using standard protocols, and photographed under ultraviolet transillumination. Polymoφhisms result in the appearance of additional bands (i.e. , bands not found in the wild-type ARF-lnK4a allele) on the gel. In an alternative aspect of this embodiment, the DNA isolated from the cells' nuclei is digested with a given restriction endonuclease, utilizing PCR amplification. The restriction endonucleases that may be used in this invention include, but are not limited to, BamWl, Bglll, Pstl, or EcoRI. After a digest is obtained, and the DNA is separated by standard technique, for example by agarose gel electrophoresis, the separated bands are probed with one or more DNA fragments containing a unique portion of the nucleotide sequences encoding human ARF-pl9 polypeptide sequences. In one aspect of this embodiment, the preferred probe of the invention is based on the cDNA or genomic sequence from the gene for human ARF-pl9. The use of RFLP technology is only one preferred embodiment of detecting polymoφhisms in the nucleic acids of the invention. Since, ultimately, the use of RFLP depends on polymoφhism in DNA restriction sites along the nucleic acid molecule, other methods of detecting the polymoφhism can also be used. Any method of analysis which allows one skilled in the art to determine the linkage between the polymoφhism detected by the probes and primers of the present invention can be utilized. Techniques such as direct location of a polymoφhism affecting ARF-pl9 at its chromosomal location by in situ hybridization (e.g. , FISH) using radiolabeled, fluorescence-labeled, or enzyme-labeled probes may be employed. Other suitable techniques include, but are not limited to, amplification methods such as the ribonuclease mis-match cleavage assay and direct oligonucleotide hybridization. Any size fragment of the human InK4A gene (SΕQ ID NO: 3) can be utilized as a probe as long as it is capable of hybridizing to a restriction fragment which displays a polymoφhism within an intron or an exon required for ARF-pl9 expression. The hybridization probes can be labeled by standard labeling techniques such as with a radiolabel, enzyme label, fluorescent label, biotiπ-avidin label, chemiluminescence, and the like. After hybridization, the probes are visualized using known methods. Comparison of the RFLP or RFLP's for the subject under investigation will quickly reveal the presence or absence of polymoφhisms in the gene encoding human ARF-pl9 linked to a predisposition to cancer. Polymoφhisms that may be detected by the methods of the invention include RFLPs, point mutations, insertions, deletions, inversions, alternately spliced mRNAs. and the like. The materials for use in this aspect of the invention are ideally suited for the preparation of a kit. Specifically, the invention provides a compartmentalized kit to receive in close confinement, one or more containers which comprises: (a) a first container comprising one or more of the probes or amplification primers of the present invention; and (b) one or more other containers comprising one or more of the following: a sample reservoir, wash reagents, reagents capable of detecting presence of bound probe from the first container, or reagents capable of amplifying sequences hybridizing to the amplification primers. In detail, a compartmentalized kit includes any kit in which reagents are contained in separate containers. Such containers include small glass containers, plastic containers or strips of plastic or paper. Such containers allows one to efficiently transfer reagents from one compartment to another compartment such that the samples and reagents are not cross-contaminated and the agents or solutions of each container can be added in a quantitative fashion from one compartment to another. Such containers will include a container which will accept the test sample, a container which contains the probe or primers used in the assay, containers which contain wash reagents (Tris-buffers, etc.), and containers which contain the reagents used to detect the bound probe or amplified product. Types of detection reagents include labeled secondary probes, or in the alternative, if the primary probe is labeled, the enzymatic, or antibody binding reagents which are capable of reacting with the labeled probe. One skilled in the art will readily recognize that the disclosed probes and amplification primers of the present invention can readily be incoφorated into one of the established kit formats which are well known in the art. In one example, a first container may contain a hybridization probe. The second container may contain the restriction enzyme to be used in the digest. Other containers may contain reagents useful in the localization of labeled probes, such as enzyme substrates such as x-gal tagged avidin if a biotinylated probe is utilized. Still other containers may contain buffers, etc. Gene therapy: In another embodiment of this invention, ARF-pl9 nucleic acid sequences are used for gene therapy, i.e., to inhibit, enhance or restore expression of ARF-pl9 in cells with reduced, altered or no ARF-pl9 activity, using the nucleic acid sequences of the invention. 1. In order to enhance or restore ARF-pl9 activity to cells in need of growth regulation, ARF-pl9 expression constructs are prepared. An expression construct consists of nucleic acid sequences encoding a protein having ARF-pl9 polypeptide sequences operably linked to nucleic acid sequences required for genetic expression in a cell (such as promoters) in an expression vector. The expression constructs are introduced into cells, wherein they direct expression of proteins having ARF-pl9 polypeptide sequences. The expressed proteins may be fusion proteins that additionally include polypeptide sequences designed to improve the in vivo activity, targeting and/or stability of the gene products expressed by the expression construct. The expressed proteins function to restore or enhance ARF-pl9 function in their host cells and thus negatively regulate the progression of the cell through the cell cycle. The disclosure demonstrates that, even in cells genetically engineered to overexpress cyclin D and thus possessing 5-10 fold greater levels of CDKs than corresponding wild-type cells, the constitutive expression of ARF-pl9 in a cell results in Gl or G2 phase arrest (see Example 5). Thus, even in cells with runaway cyclin D expression, the introduction of ARF-pl9 function in excess inhibits the progression of the cells through the cell cycle and thus prevents their further growth. 2. In order to inhibit ARF-pl9 activity in cells in need of growth stimulation, synthetic antisense oligonucleotides are prepared from the coding sequences for ARF-pl9 found in cDNA clones. An antisense oligonucleotide consists of nucleic acid sequences corresponding to the reverse complements of ARF-pl9 coding sequences or other sequences required to be present in ARF-pl9 mRNA molecules for in vivo expression. The antisense oligonucleotides are introduced into cells, wherein they specifically bind to ARF-pl9 mRNA molecules (and thus inhibit translation of ARF-pl9 gene products), or to double- stranded DNA molecules to form triplexes (see U.S. Patent 5,190,931 to Inouye (Mar. 2, 1993); Riordan and Martin, Nature 550:442-443 (1991)). Because antisense oligonucleotides bind with high specificity to their targets, selectivity is high and toxic side effects resulting from misdirection of the compounds are minimal, particularly given the present state of the art with regard to the design of, preparation and chemical modification of, and means of delivery to cells for, oligonucleotides (see, e.g. , Wagner, R.W. , Nature 572:333-335 (1994); Tseng and Brown, Cancer Gene Therapy 7:65-71 (1994); Morishita, R. , et al, J. Clin. Invest. 95: 1458-1464 (1994); Stein and Cheng, Science 267: 1004-1012 (1993); Lisziewicz, J. , et al , Proc. Natl. Acad. Sci. (USA) 90:3860-3864 (1993); Watson, P.H. , et al . Cancer Res. 57:3996-4000 (1991); Han, L., et al., Proc. Natl. Acad. Sci. (USA) 88:4313-4317 (1991); Florini and Ewton. J. Biol. Chem. 265: 13435-13437 (1990); and Uhlmann and Peyman, Chem. Reviews 90:543-583 (1990)). Means for the delivery of oligonucleotides to cells include, but are not limited to, liposomes (see, e.g. , Renneisen, K., et al , J. Biol. Chem. 265:16337-16342 (1990)) and introduction of expression constructs that direct the transcription of antisense oligoribonucleotides in vivo (see, e.g. , Shohat, O., et al , Oncogene 7 :277-283 (1987)). Polypeptides and Related Embodiments. In one embodiment, the invention comprises proteins having amino acid sequences of mouse ARF-pl9 protein, human ARF-pl9 protein and ARF-pl9 polypeptides from other mammals. For example, the invention provides the complete amino acid sequences of mouse ARF-pl9 (SEQ ID NO:2) and of human ARF-pl9 (SEQ ID NO:4). When introduced into mammalian cells ARF-pl9 proteins induce cell cycle arrest or, at lower concentrations, slow cell growth to a desired rate. One skilled in the art can readily adapt the amino acid sequences of the invention to a variety of known applications. For example, fusion proteins that comprise amino acid sequences from ARF-pl9 and a second polypeptide can be produced by recombinant DNA technology to generate novel proteins having properties of both parent proteins (see Example 4). Similarly, the proteins of the invention can be conjugated to other proteins in order to target the conjugated protein to CDK-cyclin complexes in a cell. Synthetic oligopeptides (a.k.a. "peptides") generally contain from about 5 to about 100 contiguous amino acids exactly corresponding to the polypeptide sequence of ARF-pl9 of the invention, but may optionally contain additional amino acids at the carboxyl terminus, the amino terminus, or both. Moreover, those of skill in the art will appreciate that substitution of endogenous amino acids for chemical derivatives and/or isomers of amino acids will yield peptides with properties that are enhanced relative to the native ARF-pl9 proteins. Properties that may be so altered include, for example, in vivo stability. Antibodies and Related Embodiments. In another embodiment of the invention, ARF-pl9 proteins, or oligopeptide sequences derived therefrom, are used to create antibody compositions that specifically recognize (bind) ARF-pl9 epitopes. Antibodies to ARF-pl9 serve as probes for diagnostic tests for ARF-pl9 expression or as diagnostic materials. Antibodies to ARF-pl9 can also be conjugated to toxins to generate specific immunotoxins for use in mammalian therapy. Methods of generating antibodies using purified proteins or synthetic oligopeptides are known in the art (see Antibodies : A Laboratory Manual, Harlow, E., and Lane, D. , Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1988)). The antibody compositions of the invention may be polyclonal, monospecific or monoclonal. Diagnostic methods and kits: In one aspect of this embodiment, the concentration of ARF-pl9 protein in a sample of cells from a mammal is determined by contacting the sample with a detectably labeled antibody composition specific to ARF-pl9, qualitatively or quantitatively determining the amount of label bound or not bound in the sample, and calculating therefrom the concentration of ARF-pl9 in the sample. The sample of cells is obtained from a mammal and are washed in an appropriate buffer such as Hank's balanced salt solution. The cells are lysed and incubated with a detectably labeled ARF-pl9- specific antibody composition for an appropriate amount of time. The cells are washed with the buffer a second time to remove unbound antibody. The amount of bound or unbound labeled antibody is then detected by conventional means. Alternatively, unlabeled ARF-pl9-specific antibody compositions, bound or unbound in a sample, are detected using a secondary antibody or protein which is specific for an immunoglobulin, e.g. , protein A, protein G, anti-IgM or anti-IgG antibodies. In this alternative embodiment, the secondary (anti- immunoglobulin) antibodies, which may be monoclonal or polyclonal, are detectably labeled and are detected in the course of carrying out the method. Alternatively, ARF-pl9 levels in a sample of mammalian cells are determined by detecting the level of soluble ARF-pl9 in a sample of lysed cells. In this aspect, a sample of lysed cells obtained from a mammal is contacted with an ARF-pl9-specific antibody composition which is immobilized onto a solid matrix, and allowed to incubate so as to form an ARF-pl9/ARF-pl9-specific antibody complex. Following a wash step with suitable buffers to remove the unbound antibody, a detectably labeled molecule which binds to the ARF-pl9-specific antibody composition is added. The amount of bound label then is detected to determine the concentration of ARF-pl9 present in the sample. Suitable types of immunoassays for detecting ARF-pl9 include sandwich immunoassay and competition assays, performed using conventional methods. Naturally, other ligands specific for ARF-pl9 may be used in lieu of ARF- pl9-specific antibody compositions. Of course, the specific amounts of ARF-pl9-specific antibody compositions and detectably labeled second antibodies, the temperature and time of incubation, as well as other assay conditions may be varied, depending on various factors including the concentration of ARF-pl9 in the sample, the nature of the sample, and the like. Those skilled in the art will be able to determine operative and optimal assay conditions for each determination by employing routine experimentation. Other such steps as washing, stirring, shaking, filtering and the like may be added to the assays as is customary or necessary for the particular situation. A variety of means may be used to detectably label antibody compositions for use in the methods of the invention. For example, one means by which an ARF-pl9-specific antibody composition, or secondary antibodies, can be detectably labeled is by conjugation to an enzyme. The conjugated enzyme, when later exposed to its substrate, will react with the substrate in such a manner as to produce a chemical moiety which can be detected, for example, by spectrophotometric, fluorometric or by visual means. Enzymes which can be used to detectably label antibody compositions include, but are not limited to, malate dehydrogenase, staphylococcal nuclease, delta- v-steroid isomerase, yeast alcohol dehydrogenase, alpha-glycerophosphate dehydrogenase, triose phosphate isomerase, horseradish peroxidase, alkaline phosphatase, asparaginase, glucose oxidase, beta-galactosidase, ribonuclease. urease. catalase, glucose-VI- phosphate dehydrogenase, glucoamylase and acetylcholine esterase. Antibody compositions may also be labeled with a radioactive isotope which can be determined by such means as the use of a gamma counter or a scintillation counter or by autoradiography. It is also possible to label antibody compositions with a fluorescent compound. When fluorescently labeled antibody is exposed to light of the proper wave length, its presence can then be detected due to the fluorescence of the dye. Among the most commonly used fluorescent labeling compounds are fluorescein isothiocyanate, rhodamine, phycoerythrin, phycocyanin, allophycocyanin, o-phthaldehyde and fluorescamine. Antibodies can also be detectably labeled using fluorescence emitting metals such as 152 Eu, or others of the lanthanide series. These metals can be attached to ARF-pl9-specific antibodies using such metal chelating groups as diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA). Antibodies also can be detectably labeled by coupling to a chemiluminescent compound. The presence of chemiluminescent-tagged antibodies is then determined by detecting the presence of luminescence that arises during the course of a chemical reaction. Examples of particularly useful chemiluminescent labeling compounds are luminol, isoluminol, theromatic acridium ester, imidazole, acridinium salt and oxalate ester. Likewise, a bioluminescent compound may be used to label antibody compositions for use in the methods of the present invention. Bioluminescence is a type of chemiluminescence found in biological systems in which a catalytic protein increases the efficiency of the chemiluminescent reaction. The presence of a bioluminescent protein is determined by detecting the presence of luminescence. Important bioluminescent compounds for puφoses of labeling are luciferin, luciferase and aequorin. Detection of bound or unbound antibodies may be accomplished by a scintillation counter, for example, if the detectable label is a radioactive gamma emitter, or by a fluorometer, for example, if the label is a fluorescent material. In the case of an enzyme label, the detection can be accomplished by colorimetric methods which employ a substrate for the enzyme. Detection may also be accomplished by visual comparison of the extent of enzymatic reaction of a substrate in comparison with similarly prepared standards. In another embodiment of the present invention, kits are provided which contain the necessary reagents to carry out the previously described immunoassays with ARF-pl9-specific antibodies, in order to diagnose certain types of cancers, or to detect a predisposition for certain types of cancers, in a mammal. Specifically, the invention provides a compartmentalized kit to receive, in close confinement, one or more containers which comprises: (a) a first container containing an ARF-pl9-specific antibody; and (b) one or more other containers containing one or more of the following: wash reagents, and reagents capable of detecting presence of bound or unbound ARF-pl9-specific antibodies. In detail, a compartmentalized kit includes any kit in which reagents are contained in separate containers. Such containers include small glass containers, plastic containers or strips of plastic or paper. Such containers allow one to efficiently transfer reagents from one compartment to another compartment such that the samples and reagents are not cross-contaminated, and the agents or solutions of each container can be added in a quantitative fashion from one compartment to another. Such containers will include a container which will accept the test sample, a container which contains the antibodies used in the assay, containers which contain wash reagents (such as phosphate buffered saline, Tris-buffers, etc.), and containers which contain the reagents used to detect the bound antibody. Types of detection reagents include detectably labeled secondary antibodies, or in the alternative, if the primary antibody is detectably labeled, the appropriate enzymatic or antibody binding reagents which are capable of reacting with the labeled antibody. One skilled in the art will readily recognize that the disclosed antibodies of the present invention can readily be incoφorated into any one of the variety of established kit formats which are well known in the art. Therapeutics and Related Embodiments. Another embodiment of the invention includes screening for and producing new compounds that inhibit the activity of ARF-pl9, to be applied to mammalian cells in need of reduced regulation of their cell cycles, cellular growth, and/or DNA replication. For example, in order to promote cellular growth in, e.g., healing processes, "negative-dominant" (Herskowitz, I. , Nature 529:219-222 (1987)) ARF-pl9 variants are prepared which competitively inhibit endogenous ARF- pl9 proteins and thereby reduce ARF-pl9 activity within a cell. As another example of a means by which cellular growth may be promoted, antibodies that bind regions of ARF-pl9 involved in its biological action are introduced into a cell and prevent endogenous ARF-pl9 proteins from functioning, thereby reducing ARF-pl9 activity within a cell. In a related embodiment, proteins, fusion proteins, conjugates, or synthetic oligopeptides having ARF-pl9 function can be introduced into eukaryotic cells to arrest their progression from Gl to S phases, or from G2 to M phases, during inteφhase and thus inhibit growth of undesired cells, e.g. , cancer cells (see Example 5). ARF-pl9, or derivatives thereof, can be employed in combination with conventional excipients, i.e. , pharmaceutically acceptable organic or inorganic carrier substances suitable for parenteral application which do not deleteriously react with the active compound. Suitable pharmaceutically acceptable carriers include, but are not limited to, water, salt solutions, alcohol, vegetable oils, polyethylene glycols, gelatin, lactose, amylose, magnesium stearate, talc, silicic acid, viscous paraffin, perfume oil, fatty acid monoglycerides and diglycerides, petroethral fatty acid esters, hydroxymethylcellulose, polyvinylpyrrolidone, etc. The pharmaceutical preparations can be sterilized and if desired, mixed with auxiliary agents, e.g. , lubricants, preservatives, stabilizers, wetting agents, emulsifiers, salts for influencing osmotic pressure, buffers, colorings, flavoring and/or aromatic substances and the like which do not deleteriously react with the active compounds. For parenteral application, particularly suitable vehicles consist of solutions preferably oily or aqueous solutions, as well as suspensions, emulsions, or implants. Aqueous suspensions may contain substances which increase the viscosity of the suspension and include, for example, sodium carboxymethyl cellulose, sorbitol, and/or dextran. Optionally, the suspension may also contain stabilizers. The term "therapeutically effective amount," for the puφoses of the invention, refers to the amount of ARF-pl9 or derivatives thereof which is effective to achieve its intended puφose. While individual needs vary, determination of optimal ranges for effective amounts of ARF-pl9 or its derivatives is within the skill of the art. Generally, the dosage required to provide an effective amount of the composition, and which can be adjusted by one of ordinary skill in the art will vary, depending on the age, health, physical condition, weight, extent of disease of the recipient, frequency of treatment and the nature and scope of the desired effect. Transgenic Animals and Related Embodiments. In another embodiment of the invention, ARF- pl9 nucleic acid sequences can be used to create transgenic non-human animals to serve as animal models for ARF-pl9 overexpression (transgenic expression) or mutations such as multiple stop codons ("knockouts" or "null alleles") or other mutations which alter one or more ARF-pl9 activities without affecting InK4a-pl6 activity For example, transgenic mice having little or no ARF-pl9 activity due to mutations in one or both alleles of the gene encoding ARF-pl9 (INK4A) are prone to develop certain types of tumors The non-human animals of the invention comprise any animal having a deficiency of ARF-pl9 activity as a result of the transgenic alteration of the gene(s) encoding ARF-p 19 Such non-human animals include vertebrates such as rodents, non-human primates, sheep, dog, cow, amphibians, reptiles, etc Preferred non-human animals are selected from non-human mammalian species of animals, most preferably, animals from the rodent family including rats and mice, most preferably mice The transgenic animals of the invention are animals into which has been introduced by nonnatural means (I e , by human manipulation), one or more genes that do not occur naturally in the animal, e g , foreign genes, genetically engineered endogenous genes, etc The nonnaturally introduced genes, known as transgenes, may be from the same or a different species as the animal but not naturally found in the animal in the configuration and/or at the chromosomal locus conferred by the transgene Transgenes may comprise foreign DNA sequences, 1 e , sequences not normally found in the genome of the host animal Alternatively or additionally, transgenes may comprise endogenous DNA sequences that are abnormal in that they have been rearranged or mutated in vitro in order to alter the normal in vivo pattern of expression of the gene, or to alter or eliminate the biological activity of an endogenous gene product encoded by the gene (Watson, J D , et al , in Recombinant DNA, 2d Ed , W H Freeman & Co , New York (1992), pages 255-272, Gordon, J W , Intl Rev Cytol 115 171-229 (1989), Jaenisch, R , Science 240 1468-1474 (1989), Rossant, J , Neuron 2 323-334 (1990)) Methods of preparing transgenic animals: In one aspect of this embodiment of the invention, the nucleic acids of the invention are used to prepare transgenic constructs to be introduced into non-human animals in order to generate the transgenic animals of the invention Specificall) , ARF-p 19 sequences deπved from the genome of the non-human animal of choice are used to create such transgenic constructs The transgenic non-human animals of the invention are produced by introducing ARF-p 19 transgenic constructs into the germline of the non-human animal Embryonic target cells at various developmental stages are used to introduce the transgenes of the invention Different methods are used depending on the stage of development of the embryonic target cell(s) 1. Microinjection of zygotes is the preferred method for incoφorating transgenes into animal genomes in the course of practicing the invention A zygote, a fertilized ovum that has not undergone pronuclei fusion or subsequent cell division, is the preferred target cell for microinjection of transgenic DNA sequences The murine male pronucleus reaches a size of approximately 20 micrometers in diameter, a feature which allows for the reproducible injection of 1-2 picohters of a solution containing transgenic DNA sequences The use of a zygote for introduction of transgenes has the advantage that, in most cases, the injected transgenic DNA sequences will be incoφorated into the host animal's genome before the first cell division (Brinster, et al , Proc Natl Acad Set (USA) 82 4438-4442 ( 1985)) As a consequence, all cells of the resultant transgenic animals (founder animals) stably carry an incoφorated transgene at a particular genetic locus, referred to as a transgenic allele The transgenic allele demonstrates Mende an inheritance half of the offspring resulting from the cross of a transgenic animal with a non-transgenic animal will inherit the transgenic allele, in accordance with Mendel s rules of random assortment 2. Viral integration can also be used to introduce the transgenes of the invention into an animal. The developing embryos are cultured in vitro to the developmental stage known as a blastocyte. At this time, the blastomeres may be infected with appropriate retroviruses (Jaenich, R., Proc. Natl. Sci. (USA) 75: 1260-1264). Infection of the blastomeres is enhanced by enzymatic removal of the zona pellucida (Hogan, et al. , in Manipulating the Mouse Embryo, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1986)). Transgenes are introduced via viral vectors which are typically replication- defective but which remain competent for integration of viral-associated DNA sequences, including transgenic DNA sequences linked to such viral sequences, into the host animal's genome (Jahner, et al. , Proc. Natl. Acad. Sci. (USA) 82:6927-6931 (1985); Van der Putten, et al , Proc. Natl. Acad. Sci. (USA) 82:6148-6152 (1985)). Transfection is easily and efficiently obtained by culture of blastomeres on a mono¬ layer of cells producing the transgene-containing viral vector (Van der Putten, et al. , Proc. Natl. Acad. Sci. (USA) 82:6148-6152 (1985); Stewart, et al , EMBO Journal 6:383-388 (1987)). Alternatively, infection may be performed at a later stage, such as a blastocoele (Jahner, D., et al. , Nature 298:623-628 (1982)). In any event, most transgenic founder animals produced by viral integration will be mosaics for the transgenic allele; that is, the transgene is incoφorated into only a subset of all the cells that form the transgenic founder animal. Moreover, multiple viral integration events may occur in a single founder animal, generating multiple transgenic alleles which will segregate in future generations of offspring. Introduction of transgenes into germline cells by this method is possible but probably occurs at a low frequency (Jahner, D. , et al. , Nature 298:623-628 (1982)). However, once a transgene has been introduced into germline cells by this method, offspring may be produced in which the transgenic allele is present in all of the animal's cells, i.e. , in both somatic and germline cells. 3. Embryonic stem (ES) cells can also serve as target cells for introduction of the transgenes of the invention into animals. ES cells are obtained from pre-implantation embryos that are cultured in vitro (Evans, M.J., et al , Nature 292:154-156 (1981); Bradley, M. O., et al , Nature 509:255-258 (1984); Gossler, et al. , Proc. Natl. Acad. Sci. (USA) 85:9065-9069 (1986); Robertson et al. , Nature 522:445-448 (1986); Robertson, E.J., in Teratocarcinomas and Embryonic Stem Cells: A Practical Approach, Robertson, E.J. , ed. , IRL Press, Oxford (1987), pages 71-112). ES cells, which are commercially available (from, e.g. , Genome Systems, Inc. , St. Louis, MO), can be transformed with one or more transgenes by established methods (Lovell-Badge, R.H., in Teratocarcinomas and Embryonic Stem Cells: A Practical Approach. Robertson, E.J., ed., IRL Press, Oxford (1987), pages 153-182). Transformed ES cells can be combined with an animal blastocyst, whereafter the ES cells colonize the embryo and contribute to the germline of the resulting animal, which is a chimera (composed of cells derived from two or more animals) (Jaenisch, R., Science 240: 1468-1474 (1988); Bradley, A. , in Teratocarcinomas and Embryonic Stem Cells: A Practical Approach, Robertson, E.J., ed., IRL Press, Oxford (1987), pages 113- 151). Again, once a transgene has been introduced into germline cells by this method, offspring may be produced in which the transgenic allele is present in all of the animal's cells, i.e. , in both somatic and germline cells. However it occurs, the initial introduction of a transgene is a Lamarckian (non-Mendelian) event. However, the transgenes of the invention may be stably integrated into germ line cells and transmitted to offspring of the transgenic animal as Mendelian loci. Other transgenic techniques result in mosaic transgenic animals, in which some cells carry the transgenes and other cells do not. In mosaic transgenic animals in which germ line cells do not carry the transgenes, transmission of the transgenes to offspring does not occur Nevertheless, mosaic transgenic animals are capable of demonstrating phenotypes associated with the transgenes Transgenes may be introduced into animals in order to provide animal models for human diseases Transgenes that result in such animal models include, e g , transgenes that encode mutant gene products associated with an inborn error of metabolism in a human genetic disease and transgenes that encode a human factor required to confer susceptibility to a human pathogen (1 e , a bacterium, virus, or other pathogenic microorganism) (Leder et al , U S Patent 5, 175,383 (Dec 29, 1992), Kindt et al , U S Patent 5,183,949 (Feb 2, 1993), Small et al , Cell 46 13-18 (1986), Hooper et al , Nature 326 292-295 (1987), Stacey et al , Nature 332 131-136 (1988); Windle et al . Nature 343 665-669 (1990), Katz et al , Cell 74 1089-1100 (1993)) Transgenic animals that are predisposed to a disease may be used to identify compositions that induce the disease and to evaluate the pathogenic potential of compositions known or suspected to induce the disease (Berns, A J M , U S Patent 5, 174,986 (Dec 29, 1992)) Offspring that have inherited the transgenes of the invention are distinguished from littermates that have not inherited transgenes by analysis of genetic material from the offspring for the presence of biomolecules that comprise unique sequences corresponding to sequences of, or encoded by, ihe transgenes of the invention For example, biological fluids that contam polypeptides uniquely encoded by the transgenes of the invention may be immunoassayed for the presence of the polypeptides A more simple and reliable means of identifying transgenic offspring comprises obtaining a tissue sample from an extremity of an animal, e g , a tail, and analyzing the sample for the presence of nucleic acid sequences corresponding to the DNA sequence of a unique portion or portions of the transgenes of the invention The presence of such nucleic acid sequences may be determined by, e g , hybridization ("Southern") analysis with DNA sequences corresponding to unique portions of the transgene, analysis of the products of PCR reactions using DNA sequences in a sample as substrates and oligonucleotides derived from the transgene 's DNA sequence, etc Null alleles: A preferred embodiment is a transgenic animal that is homozygous for a null (a k a "knock-out") allele of ARF-INK4A but which has a wild-type INK4A-pl6 allele For example, selective interruption of INK4A exon lβ eliminates ARF-INK4A expression but does not affect sequences encoding InK4a-pl6 Additionally or alternatively, one or more point mutations that create stop codons in the ARF- pl9 reading frame, but which result in silent mutations in the InK4a-pl6 reading frame, aie introduced by site-directed mutagenesis into cloned INK4A genomic nucleic acid sequences which are then reintroduced into the genome of an animal to generate a transgenic ARF-p 19-deficιent animal The transgenic ARF-p 19 null or ARF-p 19-deficιent animals of the invention are predisposed to develop certain types of cancers, including but not limited to melanomas, in a reproducible and thus reliable manner In order to generate null alleles in embryonic stem cells, the positive-negative selection strategy of Mansour et al (Nature 336 348-352 (1988)) is applied A positive selectable marker, for example the hygromycin phosphotransferase cassette (van Deursen and Wieπnga, Nucl Acids Res 29 3815-3820 (1992)), is inserted into a 5 portion of an INK4 gene This position for the positive selectable marker is chosen to obtain a genuine null mutant allele, I e , to avoid translation of a truncated polypeptide In the resulting targeting vector the hygromycin gene is flanked 5 and 3' by several kb of homologous murine genomic sequences In addition, a negative selectable marker, for example the Heφes Simplex Virus (HSV) thymidine kinase (TK) gene, is placed in a 3' position flanking the region of homologous sequences in order to enable selection against nonhomologous integrants Both the positive and negative selectable markers are inserted in the antisense orientation with respect to the transcriptional orientation of the Ink4 gene, and are expressed due to the TK promoter and Py F441 Polyoma enhancer. Linearized targeting construct is introduced into ES cells by eiectroporation or other suitable means and selection with hygromycin and FIAU (l-[2-deoxy, 2-fluoro-β-D-arabinofuranosyl]) is carried out for 7 to 10 days. Resistant colonies are expanded in 24- well plates; half of the cells in each well are cryo-preserved and the other half expanded for genotype analysis. Positive clones are stored in liquid nitrogen and thawed at least 3 days prior to blastocyst injection. Blastocysts are isolated, for example, at day 3.5 postcoitum by flushing the uterine horns of naturally mated C57BL/6 pregnant females with DMEM + 10% FBS. Approximately 10 to 15 ES cells from each homologous recombinant clone with a normal karyotype are microinjected into recipient blastocysts, and about 10 to 20 embryos are transferred into the uterine horns of (C57BL/6 x CBA/Ca) Fl pseudopregnant fosters (Bradley, A. , in Teratocarcinomas and Embryonic Stem Cells: A Practical Approach, Robertson, E.J. , ed., IRL Press, Oxford (1987), pages 1 13-151). Chimeric males are mated with C57BL/6 or FVB/J females and germline transmission of the mutant allele is verified by Southern blot analysis of tail DNA from Fl offspring with either agouti or gray coat color. F2 offspring from interbred heterozygotes are genotyped by Southern blotting to identify homozygous null mutants. Methods of evaluating the therapeutic or oncogenic potential of compositions: Using the transgenic animals of the invention, it is possible to evaluate a variety of compositions for their therapeutic or oncogenic potential. 1. Generally, methods for determining the therapeutic potential of a composition to treat cancer comprise the step of administering a known dose of the composition to a transgenic animal having a phenotype of reduced or altered ARF-p 19 activity, monitoring resulting biological or biochemical parameters correlated with cancer, and comparing the symptoms of treated animals to those of untreated animals. A first method of assessing the therapeutic potential of a composition using the transgenic animals of the invention comprises the steps of: (1) Administering a known dose of the composition to a first transgenic animal having a phenotype of reduced or altered ARF-p 19 activity; (2) Detecting the time of onset of cancer in the first transgenic animal; and (3) Comparing the time of onset of cancer in the first transgenic animal to the time of onset of cancer in a second transgenic animal having a phenotype of reduced or altered ARF- pl9 activity, which has not been exposed to the composition, wherein a statistically significant decrease in the time of onset of cancer in the first transgenic animal relative to the time of onset of the symptoms in the second transgenic animal indicates the therapeutic potential of the composition for treating cancer. A second method of assessing the therapeutic potential of a composition using the transgenic animals of the invention comprises the steps of: (1) Administering a known dose of the composition to a first transgenic animal having a phenotype of reduced or altered ARF-p 19 activity, at an initial time, t,,; (2) Determining the extent of cancer in the first transgenic animal at a later time, t,; and (3) Comparing, at t,, the extent of cancer in the first transgenic animal to the extent of neurological symptoms in a second transgenic animal having a phenotype of reduced or altered ARF-p 19 activity, which has not been exposed to the composition at to, wherein a statistically significant decrease in the extent of cancer at t, in the first transgenic animal relative to the extent of the symptoms at t, in the second transgenic animal indicates the therapeutic potential of the composition for treating cancer. A third method of assessing the therapeutic potential of a composition using the transgenic animals of the invention comprises the steps of: (1 ) Administering a known dose of die composition to a first transgenic animal having a phenotype of reduced or altered ARF-p 19 activity; (2) Measuring the lifespan of the first transgenic animal; and (3) Comparing the lifespan of the first transgenic animal to the lifespan of a second transgenic animal having a phenotype of reduced or altered ARF-p 19 activity, which has not been exposed to the composition, wherein a statistically significant increase in the lifespan of the first transgenic animal relative to the lifespan of the second transgenic animal indicates the therapeutic potential of the composition for treating cancer. 2. Generally, methods for determining the potential of a composition to cause or exacerbate cancer comprise the step of administering a known dose of the composition to a transgenic animals having a phenotype of reduced or altered ARF-pl9 activity, monitoring resulting biological or biochemical parameters correlated with cancer, and comparing the symptoms of treated animals to those of untreated animals. A first method of assessing the oncogenic potential of a composition using the transgenic animals of the invention comprises the steps of: (1) Administering a known dose of the composition to a first transgenic animal having a phenotype of reduced or altered ARF-p 19 activity; (2) Detecting the time of onset of cancer in the first transgenic animal; and (3) Comparing the time of onset of cancer in the first transgenic animal to the time of onset of cancer in a second transgenic animal having a phenotype of reduced or altered ARF- pl9 activity, which has not been exposed to the composition, wherein a statistically significant increase in the time of onset of cancer in the first transgenic animal relative to the time of onset of the symptoms in the second transgenic animal indicates the potential of the composition for causing or exacerbating cancer. A second method of assessing the oncogenic potential of a composition using the transgenic animals of the invention comprises the steps of: ( 1 ) Administering a known dose of the composition to a first transgenic animal having a phenotype of reduced or altered ARF-pl9 activity, at an initial time, to; (2) Determining the extent of cancer in the first transgenic animal at a later time, t, ; and (3) Comparing, at t,, the extent of cancer in the first transgenic animal to the extent of neurological symptoms in a second transgenic animal having a phenotype of reduced or altered ARF-pl9 activity, which has not been exposed to the composition at t < ,, wherein a statistically significant increase in the extent of cancer at t, in the first transgenic animal relative to the extent of the symptoms at t, in the second transgenic animal indicates the potential of the composition for causing or exacerbating cancer. A third method of assessing the oncogenic potential of a composition using the transgenic animals of the invention comprises the steps of: ( 1 ) Administering a known dose of the composition to a first transgenic animal having a phenotype of reduced or altered ARF-p 19 activity; (2) Measuring the lifespan of the first transgenic animal; and (3) Comparing the lifespan of the first transgenic animal to the lifespan of a second transgenic animal having a phenotype of reduced or altered ARF-p 19 activity, which has not been exposed to the composition, wherein a statistically significant decrease in the lifespan of the first transgenic animal relative to the lifespan of the second transgenic animal indicates the potential of the composition for causing or exacerbating cancer. In both of the above sets of methods, the composition may comprise a chemical compound administered by circulatory injection or oral ingestion. The composition being evaluated may alternatively comprise a polypeptide administered by circulatory injection of an isolated or recombinant bacterium or virus that is live or attenuated, wherein the polypeptide is present on the surface of the bacterium or virus prior to injection, or a polypeptide administered by circulatory injection of an isolated or recombinant bacterium or virus capable of reproduction within a mouse, and the polypeptide is produced within a mouse by genetic expression of a DNA sequence encoding the polypeptide. Alternatively, the composition being evaluated may comprise one or more nucleic acids, including a gene from the human genome or a processed RNA transcript thereof. Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. The entire text of all publications cited above and below are hereby incoφorated by reference. Examples Example 1: cDNA Sequences Encoding ARF-pl9 Tandemly linked INK4A (MTSl, CDKN2) and INK4B (MTS2) genes on the short arm of human chromosome 9 encode distinct 16 kDa and 15 kDa inhibitors (InK4a-pl6 and InK4b-pl5, respectively) of the Gl cyclin D-dependent kinases CDK4 and CDK6 (Serrano et al, Nature 566:704-707 (1993); Hannon and Beach, Nature 371:251-261 (1994)). Homozygous co-deletion of INK4A and INK4B, hemizygous deletions of INK4A together with point mutations within the remaining allele, and de novo methylation of an CpG island extending into exon 1 of INK4A (Merlo et al, Nature Med. 7:686-692 (1995)) are commonly observed in human cancers, suggesting that InK4a-pl6, and perhaps InK4b-pl5, function as tumor suppressors (Kamb, A. et al , Science 264:436^40 (1994); Noburi, T. et al . Nature 568:753-756 (1994); Sheaff and Roberts, Curr. Biol. 5:28-31 (1994); Hunter and Pines, Cell 79:573-582 (1995)). Two other members of the 1NK4 gene family, InK4c-pl8 and INK4d-pl9, map to different human chromosomes (Guan et al . Genes & Develop. 8:2939-2952 ((1994); Chan et al . Mol. Cell. Biol. 75:2682-2688 (1995); Hirai et al , Mol. Cell. Biol. 75:2672-2681 (1995); Okuda et al, Genomics (in press) (1995)). The human INK4A gene yields transcripts that initiate at two promoters, the first (Elα) located in close proximity to other InK4a-pl6 coding exons and the second (Elβ) mapping centromerically in close proximity to the lNK4b gene (Stone, S.. et al, Cancer Res. 55:2988-2994 (1995); Mao, L., et al, Cancer Res. 55:2995-2997 (1995)). A long microsatellite (CA N ) repeat downstream of exon lβ is highly polymoφhic. The nucleotide sequence of mRNAs derived from exon lβ include a 5 'AUG codon which, if used to initiate protein synthesis in fully spliced transcripts, can yield another polypeptide (here designated ARF-p 19) derived from a theoretical Alternative Reading Frame that includes most of the exon 2 coding sequences represented in INK4a-pl6 mRNA. Two classes of transcripts (α and β, containing 5' sequences derived from exons lα and lβ, respectively), although virtually identical in length, have been successfully identified by reverse transcription and polymerase chain reactions (RT-PCR) using mRNA templates from a variety of human tissues, but others inferred that the β transcript is unlikely to encode a protein (Stone, S. , et al, Cancer Res. 55:2988-2994 (1995); Mao, L. , et al. Cancer Res. 55:2995-2997 (1995)). However, as described herein, mouse exon lβ sequences are spliced to exon 2 of the 1NK4A gene to generate transcripts encoding a polypeptide that is completely different in amino acid sequence from InK4a-pl6. A mouse erythroleukemia (MEL) cell DNA library (5' Stretch λgtlO, Clontech, Palo Alto, CA) was screened with a full-length human InK4a-pl6 probe, and twelve hybridizing cDNAs subcloned into pBluescript (Stratagene, La Jolla, CA) were sequenced. One cDNA represented mouse InK4a-pl6 (Quelle et al , Oncogene 77:635-645 (1995)) while the remaining clones, designated ARF-pl9, contained alternative sequences derived from exon lβ (Figure 1 ; SEQ ID NO: 1). As confirmed by Southern blotting analysis, the unrelated sequences from the lα and lβ exons hybridized to distinct genomic DNA fragments. The mouse β mRNA, which directs the expression of ARF-pl9 in a variety of tissues (see Examples 2 to 4), contains an AUG codon at nucleotides 43-45 flanked by Kozak consensus sequences, and translation from this initiator would yield a 169 amino acid polypeptide of 19.349 daltons (ARF-pl9; SEQ ID NO:2). Splicing of exon lβ to exon 2 of the INK4A gene occurs at the same acceptor site as that used by exon lβ but changes the exon 2 reading frame to generate an entirely novel protein containing 105 exon 2-derived residues. The mouse and human lβ exons are conserved in length, except for one additional arginine codon in the human β mRNA located just 5' of the splice donor site. The reading frame (SEQ ID NO:3; Figure 7) for the human ARF-pl9 protein (SEQ ID NO:4) is only 132 codons in length due to a predicted TGA terminator in place of CCA at nucleotides 436-438 (Figure 1). Mouse and human ARF-p 19 polypeptides are 44% identical through their exon lβ segments and 46% identical overall. By comparison, INK4A exon lα segments are 72% identical, with mouse and human InK4a-pl6 proteins sharing 65 % overall identity. The ARF-pl9 proteins are highly basic, as indicated by their high arginine content (human, 21 % Arg; mouse 22% Arg) and are unrelated to known proteins in searchable databases. However, several known RNA-binding proteins, while not having amino acid sequences that are per se homologous to that of ARF-pl9, nonetheless resemble ARF-pl9 by having stretches of arginine-rich sequences. Moreover, in at least some instances, these arginine-rich regions have been implicated in the binding of these proteins to specific RNA sequences (Craven, M.G., et al, J. Bacteriol. 776: 1394-1404 (1994): Calnan, B.J. , et al, Science 252: 1167-1171 (1991), and erratum, 255:665 (1992)); Lazinski, D., Cell 59:207-218 (1989); Tao, J., et al, Proc. Natl. Acad. Sci. (U.S.A.) 89:2723-2726 (1992)). There are precedents for dual utilization of coding sequences in small virus genomes which cannot exceed a complexity that prevents their packaging into virions (Lamb and Horvath, Trends in Genetics 7:261-266 (1991); Cullen, Annu. Rev. Microbiol. 45:219-250 (1991)). In contrast, eukaryotic genes are composed largely of introns and are more widely spaced, presumably relieving them from evolutionarily imposed size constraints. In Saccharomyces cerevisiae, the SNF6 locus and genes encoding certain glycolytic enzymes exhibit overlapping reading frames on opposite strands (Estruch and Carlson, Mol. Cell. Biol. 70:2544-2553 (1990); Boles and Zimmermann, Mol. Ben. Genet. 245:363-368 (1994)), whereas the stress response gene, DDR48. includes two overlapping but asyntactic reading frames, each with a capacity to encode a protein of -45 kDa (Treger and McEntee, Mol. Cell. Biol. 70:3174-3184 (1990)). In these cases, however, only one of the two asyntactic reading frames appears to be expressed. Recently, Labarriere et al. (J. Biol. Chem. 270: 19205-19208 (1995)) reported that transcripts originating from a novel promoter in the human growth hormone (GH) gene have the potential to specify a 107 amino acid protein, the C-terminal half of which arises from a second reading frame in GH exons 1 and 2. Antibodies to the C-terminus of this predicted polypeptide histochemically stained a subpopulation of pituitary cells, arguing for limited focal translation of this mRNA. In general, however, overlapping genes noted as such in the available databases have not been assigned dual protein products. That two INK4A- coded polypeptides can each induce cell cycle arrest, albeit at different points in the cycle and via apparently distinct mechanisms (see Example 5), suggests that their unitary inheritance has functional significance. It is noteworthy that alternative splicing of transcripts from another cell cycle-controlling gene, that encoding integrin β,, produces a second gene product, β ιc , that also functions to regulate cell cycle arrest (Merdith et al. Science 269: 1570-1572 (1995)). However, unlike ARF-pl9 and InK4a-pl6, the integrin β, and β ιc reading frames overlap syntactically rather than asyntactically. Specifically, β- c contains a carboxyl-terminal 48-amino acid sequence that replaces the carboxyl-terminal 21 amino acids found in β- ; otherwise, however, the sequences of the two proteins are identical. If alternative splicing of RNA transcripts is a recurring motif of cell cycle gene expression then, conceivably, a limited number of common RNA splicing factors could affect the expression of many proteins involved in the regulation of progression through the cell cycle. Example 2: ARF-pl9 is Expressed in Mouse Cells and Tissues In a survey of mouse tissues and cells lines α and β mRNAs of similar length ( ~ 1 kb) were detected by Northern blotting with specific exonlα and exon lβ probes and, in agreement, by RT-PCR using specific 5' primers (Figure 2). For RT-PCR Analysis, polyadenylated mRNA was prepared from tissues excised from normal female mice (C3H/HEJ, Jackson Laboratories, Bar Harbor, ME), and cDNA was synthesized from 50 ng of polyA mRNA templates according to manufacturer's instructions (StrataScript RT-PCR Kit, Stratagene) (Quelle et al. , Oncogene 77:635-645 (1995)). PCR amplification of either mouse ARF-pl9 β transcripts, or of InK4a-pl6 α transcripts, was performed using a common antisense primer having the sequence 5'-GCAAAGCTTGAGGCCGGATTTAGCTCTGCTC SEQ ID NO : 5 and either an ARF-p 19 specific sense primer having the sequence 5' -AGGGATCCTTGGTCACTGTGAGGATTC SEQ ID NO : 6 or an InK4a-pl6 specific sense primer having the sequence 5' - CGGGATCCGCTGCAGACAGACTGGCCAG SEQ ID NO : 7 with 35 cycles of denaturation (95°C, 1 min), annealing (65°C, 45 sec), and extension (72°C, 2 min). Products ( - 0.5 kb, 10 μl per lane) were electrophoresed on 1.5 % agarose gels and blotted onto nylon membranes (Hybond N, Amersham, Arlington Heights, IL). 32 P-labeled probes which specifically recognized exon lβ (ARF-pl9, bases 1-228 of SEQ ID NO: l), exon lα (InK4a-pl6, bases 58-182), or an antisense oligonucleotide derived from exon 2 and having the sequence 5' -CGTCTAGAGCGTGTCCAGGAAGCCTTCC SEQ ID NO : 8 were hybridized at 50°C in neutral pH buffer containing 0.9M NaCl and washed in 0.015M NaCl, 0.1 % SDS at the same temperature. Specific amplification of β transcripts and hybridization of the products with exon 1 β (Figure 2(A)) and exon 2 (Figure 2(B)) probes revealed their ubiquitous expression in various organs. The signal was particularly high using mRNA templates from MEL cells from which both α and β cDNAs have been cloned. By contrast, α mRNAs encoding INK4a-pl6 were expressed at relatively high levels in only few tissues (Quelle et al. , Oncogene 77:635-645 (1995)), and the more restricted patterns of hybridization observed with the exon lα and exon 2 probes confirmed the specificity of the PCR primers (Figures 2(C) and 2(D)). Importantly, amplified α transcripts generated no signal at all after hybridization with the exon lβ probe and vice versa, and no products were obtained in the absence of mRNA templates (Figure 2). ARF-p 19-encoding β transcripts were also detected in other cell lines, including CTLL-2 and RL-12 T cells, and NFS112 B cells, but were absent from NIH-3T3 fibroblasts and BAC1.2F5 macrophages, both of which have sustained deletions of the INK4A locus (data not shown). Example 3: Antibodies to ARF-pl9 and Detection of ARF-pl9 Proteins An antiserum directed to the unique carboxy-terminal amino acid sequences was generated using a synthetic, conjugated carboxy terminal oligopeptide derived from the ARF-p 19 protein using techniques previously described for InK4a-pl6-derived oligopeptides (Quelle et al , Oncogene 77 :635-645 (1995)). Specifically, a synthetic peptide having the sequence (SEQ ID NO:9): NH 2 - Val - Phe -Val -Tyr -Arg -Trp -Gl -Arg -Arg - Pro -Asp -Arg -Arg -Ala corresponding to residues 156-169 of murine ARF-pl9 protein (SEQ ID NO:2) was used. The antibody to ARF-p 19 carboxy-terminal sequences encoded by the β mRNA precipitated a protein with an apparent molecular mass of about 22 kDa (i.e., ARF-pl9) after transcription and translation of the β cDNA in vitro (Figure 3, lane 8). There is no evidence that ARF-p 19 undergoes post- translational modification(s) in vivo that detectably alter its mobility on denaturing gels (see Example 4), so the apparent disparity between the masses of the predicted and translated proteins likely reflect the unusual amino acid composition of ARF-p 19 (Figure 1). An antiserum to the C-terminus of mouse InK4a- pl6 detected the cognate protein (Figure 3, lane 6) but did not cross-react with ARF-p 19 (Figure 3, lane 9). Sequences encoding a hemagglutinin (HA) epitope tag were added to the 5' end of he ARF-pl9 cDNA by polymerase chain reaction (PCR) using a forward primer containing a 5' BamHl site (underlined) having the sequence (SEQ ID NO: 10): 5 ' - C G G G A T C C G A A T T C A G C C A T G G G T T A C C C A T A C G A C G - TCCCAGACTACGCTACCGGTCGCAGGTTCTTGGTCAC and a reverse primer extending over the single BssHll site (underlined) in exon lβ having the sequence of the reverse complement of residues 68-87 of SEQ ID NO: 1 , i.e. , 5 ' - GCCCGCGCGCTGAATCCTCA SEQ ID NO : 11 The PCR product was digested with BamHl and Z ϊHII, subcloned into the original cDNA in pBluescript, and resequenced. The resultant ARF-pl9 fusion protein, in which the amino terminus of ARF-pl9 is tagged with a hemagglutinin (HA) epitope, has a mobility that is slightly retarded compared to that of wild-type ARF-p 19 (Figure 3, lane 2). The HA-tagged ARF-p 19 protein could be detected with anti-HA serum (see below). The endogenous ARF-p 19 protein could also be detected in lysates of mouse MEL cells (Figure 4(A), lane 9, arrow), which synthesize high levels of both a and β mRNAs (Figure 2). Protein InK4a-pl6, expressed in MEL cells (Figure 4(C), lane 9), was not detected with antiserum to ARF-pl9, nor vice versa (compare Figures 4(A) and 4(C)), confirming the specificity of the antisera and indicating as well that an N-terminally truncated InK4a-pl6 protein, which would have carboxylterminal ARF-pl9 sequences, does not normally arise by initiation from internal AUG codons within the β mRNA. In cytospins prepared from cells harvested 48 hours after infection with viruses encoding HA-tagged ARF-p 19, immunofluorescence using detectably labeled antibodies to the ARF-p 19 C-terminus (Figure 5(A)) or to the amino-terminal HA epitope (Figure 5(C)) demonstrate that both untagged and HA- tagged ARF-p 19 localize to the cell nucleus. This reflects the strictly nuclear localization of wild-type ARF-p 19, which is normally expressed in derivatives of Balb/c fibroblasts (data not shown). Example 4: Production of Recombinant ARF-pl9 Native and Fusion Proteins Baculoviral Expression in Insect Sf9 Cells For expression in insect Sf9 cells (Kato et al . Genes & Devel. 7:331-342 (1993)). an EcoRI fragment encoding tagged ARF-pl9 was inserted into the pVL1393 baculovirus vector (Pharmingen. San Diego, CA). Insect Sf9 cells were infected and harvested as previously described (Kato et al.. Genes ά Devel. 7:331-342 (1993); see also Richardson, CD. , ed.. Baculovirus Expression Protocols, Methods in Molecular Biology Vol. 39, Walker, J.M. , series ed., Humana Press, Totowa, New Jersey (1995), Chapters 1-5 and 1 1). When the cDNA encoding HA-tagged ARF-pl9 was expressed under baculoviral control in insect Sf9 cells, a protein of slightly slower mobility than that of the endogenous protein in MΕL cells was detected with antibody to ARF-pl9 (Figure 4(A), lane 2) or to the hemagglutinin (HA) tag (Figure 4(B), lane 2). Virus Production and Infection of Mammalian Cells. Both untagged and HA-tagged ARF-pl9 cDNAs were subcloned into the EcoRI site of the SRα-MSV-tk-neo retroviral vector (Muller et al. , Mol. Cell. Biol. 77: 1785-1792 (1994)) for production of virus. Human kidney 293T cells were transfected with 15 μg ecotropic helper virus DNA plus 15 μg SRα vector DNA using a modified calcium phosphate precipitation technique (Chen and Okayama, Mol. Cell. Biol. 7:2745-2752 (1987)). Cell supernatants containing infectious retroviral pseudotypes were harvested 24-60 hours post-transfection, pooled on ice, and filter (0.45 μ) sterilized. Virus infections of exponentially growing mouse fibroblasts in 100 mm diameter culture dishes were performed at 37°C in a 4% C0 2 atmosphere using 2 ml virus-containing supernatants containing 8 μg/ml polybrene (Sigma, St. Louis. MO). After 3 hours, 10 ml fresh medium was added. Cells were harvested 48 hours after infection and their DNA content was analyzed by flow cytometry (Matsushime et al , Cell 65:701-703 (1991)). Mammalian cell lines were maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum, 2 mM glutamine, and 100 units/ml penicillin and streptomycin (Gibco, Grand Island, NY). NIH-3T3 cells (Rb + , INK4A ' , p53 status uncertain) were transfected with vectors encoding D-type cyclins alone or with CDK4, and polyclonal populations derived from pooled, drug resistant transformants were used (Quelle et al . Genes & Devel. 7: 1559-1571 (1993)). Derivatives of Balb-3T3 cells (Rb status uncertain, INK4a-pl6 + , p53 " ) were provided by G. Zambetti (St. Jude Children's Research Hospital), and the 293T retrovirus packaging line (Pear et al , Proc. Natl. Acad. Sci. USA 90:8392-8396 (1993)) was obtained from Charles Sawyers (UCLA) with permission from David Baltimore (MIT). For analysis of ARF-pl9 or InK4a-pl6 expression, pelleted mammalian cells were disrupted in ice-cold cell lysis buffer (1 x IO 7 cells/ml) for 1 hour on ice. Nuclei and debris were removed by centrifugation in a microfuge at 12,000 φm for 10 min at 4°C. Supernatants were boiled in gel loading buffer, and proteins (2 x 10 5 cell equivalents per lane) were separated on denaturing gels as above and transferred onto nitrocellulose (Quelle et al. , Genes & Devel. 7: 1559-1571 (1993)). Proteins were detected by enhanced chemiluminescence (ECL, Amersham) according to manufacturer's specifications with ARF-pl9 or InK4a-pl6 antisera or with 12CA5 monoclonal antibody to the HA tag (ICN, Costa Mesa, CA). Assays for CDC2 and CDK2-associated histone HI kinase activity were performed as previously described (Matsushime et al. , Cell 77:323-334 (1992)). In some experiments, specifically immunoprecipitated CDKs or cyclins were separated on denaturing gels and immunoblotted with anti-ARF- p!9 as above. Infection of 7MT4-negative NIH-3T3 cells engineered to overexpress cyclin Dl with a retrovirus containing the β cDNA led to ectopic ARF-p 19 synthesis (Figure 4A. lane 4). When the HA-tagged ARF- pl9 protein was introduced, the polypeptide again migrated with a slightly slower mobility than the wild- type protein (Figure 4A, lane 5 versus 4) and was also revealed with anti-HA serum (Figure 4B, lane 5). Unlike NIH-3T3 cells which lack the INK4A gene and do not express ARF-p 19 (Figure 4A, lane 3) or InK4a-pl6 (Figure 4C, lanes 3-5). derivatives of Balb-3T3 cells synthesize both proteins (Figures 4A and 4C, lanes 7 and 8). Therefore, the β transcript encodes an authentic ARF-pl9 protein which is coexpressed with InK4a-pl6 in MEL and Balb-3T3-derived cell lines. Example 5: Induction of Cell Cycle Arrest by ARF-pl9 In the experiments shown in Figure 4, the DNA content of cells expressing ARF-p 19 was concomitantly measured in order to assess the effect(s) of ARF-pl9 on the cell cycle. Suφrisingly, infection of NIH-3T3 cells for 48 hours with retroviruses encoding ARF-pl9 induced cell cycle arrest in both the Gl (2N DNA content) and G2/M (4N DNA content) phases of the cell cycle with a proportional loss of cells in S phase (DNA content between 2N and 4N) (Table 1). Cells infected with the empty vector were distributed throughout the cycle in a manner indistinguishable from uninfected cells. In similar experiments performed with cells engineered to overexpress cyclin Dl , a greater proportion of ARF-p 19 expressing cells arrested in G2/M versus Gl (Table 1). Proliferating cyclin Dl overexpressors have a 20- 30% contracted Gl phase interval and a compensatory shortening of their doubling time, so that their overall cell cycle distribution is unchanged (Quelle et al. , Genes ά Devel. 7: 1559-1571 (1993)). However, the relative increase in the G2/M versus the Gl phase fraction suggests that cyclin Dl overexpression partially overcomes the pl9-induced Gl block. Table 1: Cell Cycle Arrest by ARF-pl9 Cell Line Vector Cell Cycle Distribution (No. Expts) cDNA Insert (48 hrs post-infection) % G, G1 % S % G2/M NIH-3T3 (7) None 42 4 ± 3 3 42 0 ±2 3 15 6 ± 3 3 NIH-3T3 (8) pl 9 ARF 66 6 ± 5 2 12 5 ± 2 6 20 9 ± 5 1 3T3-D1 (8) None 41 1 ± 6 6 39 1 ± 4 4 19 8 ± 2 7 3T3-D1 (9) pl 9 ARF 53 0 ± 4 3 1 1 6 ± 2 8 35 4 ± 4 2 BALB-3T3 (3) None 36 5 ± 8 9 49 0 ± 8 8 14 5 ± 1 2 BALB-3T3 (3) p ] 9 ARF 57 1 ± 4 9 260 ± 2 8 16 9 =fc 1 4 In these experiments, the DNA content of NIH-3T3 cells infected for 48 hours with a control vector or with retroviruses encoding ARF-pl9 was determined by FACS analysis. Proliferating polyclonal derivatives overexpressing cyclin Dl (3T3-D1) are not redistributed through the cell cycle, because the shortening of their Gl phase by 20-30% is compensated by a reduced generation time (Quelle et al. , Genes & Devel. 7: 1559-1571 (1993)). ARF-p 19-induced G2 - M arrest in all cyclin D overexpressors was more marked than in parental cells. Arrested cells were fully viable, lacked metaphases. and contained intact nuclei (Figure 5). features all indicative of arrest in inteφhase. Infection of NIH-3T3 or 3T3-D1 overexpressors with retroviruses expressing InK4a-pl6 or InK4d- pl9 leads only to Gl phase arrest (Hirai et al, Mol. Cell. Biol. 75:2672-2681 (1995); Quelle et al , Oncogene 77:635-645 (1995)). The fact that a significant proportion of cells ectopically expressing ARF- pl9 accumulate in G2/M suggests that the action of the ARF-pl9 protein is not limited to effects relating to InK4a-pl6 activity. The phenotype of the ARF-p 19 expressors was unusual in that many of the infected NIH-3T3 cells appeared rounded and highly refractile, superficially similar to those in mitosis. However, cells enforced to express ARF-pl9 exhibited only a minor ( <0.2%) metaphase fraction compared to cells infected with the vector control ( — 3%). Nor did the cells undergo apoptosis, as determined by the following sensitive flow cytometric assay and by their lack of DNA fragmentation. Apoptosis Assay. Trypsinized cell suspensions were washed and suspended in 0.5 ml PBS and pipetted dropwise into 5 ml of 1 % buffered paraformaldehyde on ice with gentle mechanical mixing. After 15 min incubation on ice, cells were pelleted, washed with 10 ml cold PBS, and the pellets were resuspended in 1 ml 70& ethanol pre-cooled at -20°C. Samples stored at -20°C overnight were resuspended, divided into two equal aliquots, collected by centrifugation, and washed twice with ice cold PBS Duplicate samples were resuspended in 50 μl reaction mixtures containing lx terminal deoxynucleotidyl transferase (TdT) buffer, CoCl 2 , and digoxigenin-1 1-UTP with or without 0.5 μl TdT (all supplied as a TdT kit by Boehringer Mannheim Coφ., Indianapolis, IL). After 30 min incubation at 37°C, 1 ml of ice cold PBS was added, and recentrifuged cells were suspended in 100 μl of a 1 :40 dilution in PBS of anti-digoxigenin-FITC monoclonal antibody and incubated for 30 minutes in the dark at room temperature. Cells were sequentially washed in 1 ml of ice cold PBS containing 2mM sodium azide and 0.35% bovine serum albumin (BSA), and then in 1 ml 0.1 % Triton X-100 in PBS, and resuspended in 1 ml PBS-azide-BSA containing 50 μg/ml propidum iodide. RNAse (50 μg/ml) was added, and after 30 minute incubation at room temperature, samples were filtered for flow cytometry and analyzed for DNA content (red PI fluorescence) and TdT-labeled DNA fragments (green FITC) on a Becton Dickinson FACScan (Matsushime et al , 1991). DNA fragmentation was quantitated by determining the difference in FITC fluorescence between duplicate samples incubated with and without TdT. Human Jurkat T cells treated with 100 μM etoposide for 6 hrs were routinely included as positive controls for cells undergoing apoptosis. Immunofluorescence . Cells were harvested 48 hours after infection and spun onto glass slides (5 x IO 4 cells/slide) using a Scimetrics Cytospin3 at 500 φ for 5 min. Following fixation for 10 min at room temp in 3% paraformaldehyde, slides were washed 3 times with phosphate-buffered saline (PBS), permeabilized in 0.2% Triton X-100 for 10 min at room temperature, and washed 3 more times with PBS. After 30 min incubation in blocking solution (PBS containing 1 % dry milk), cells were incubated at room temperature in a humidified chamber for 1 hour with primary antibody (5 μg/ml 12CAS MAb or 1 :600 ARF-p 19 polyclonal antiserum diluted in blocking solution). To confirm the specificity of ARF-p 19 antiserum, primary antibody was incubated with ARF-pl9 peptide for 1 hour at temperature prior to incubation with cells. After six washes with blocking solution, secondary antibody incubations were performed in blocking solution for 30 min, using 1 :50 dilutions of either FITC-conjugated sheep anti- mouse or FITC-conjugated donkey anti-rabbit IgG (Amersham, Arlington Heights, IL). After six washes with PBS, cells were stained with Hoescht dye 33258 (1 μg/ml), wet mounted with vectashield medium (Vector, Burlingame, CA), and photographed at 600X magnification through a microscope equipped with epifluorescence optic (Olympus, Lake Success, NY). When cytospins were prepared from cells harvested 48 hours after infection with viruses encoding HA-tagged ARF-p 19, immunofluorescence performed with antibodies to the ARF-p 19 C-terminus (Figure 5(A)) or to the N-terminal HA epitope (Figure 5(C)) revealed the protein in the cell nucleus. Greater than 80% of the infected cells stained brightly compared to uninfected cells (not shown) or to those stained with peptide blocked serum (Figure 5(B)). No nuclear dissolution or mitotic figures were observed, confirming that the cells arrested in inteφhase. Example 6: Effects of ARF-pl9 on CDK Activity When lysates of ARF-pl9-infected NIH-3T3 cells were precipitated with specific antisera to CDC2 or CDK2 and histone HI kinase activity was measured in immune complexes, the activities of both CDKs were greatly reduced relative to proliferating populations infected with the control vector (data not shown). The observed — 10- fold decrease in CDC2 kinase is consistent with the loss of the mitotic fraction, whereas the ~ 5 fold drop in CDK2 activity likely reflects a redistribution of cells from S phase into Gl and G2. Indeed, given the dissimilarity in structure between ARF-pl9 and known CDK inhibitors, it seems unlikely that the protein interacts directly with CDKs or cyclins. In agreement, when CDC2, CDK2, CDK4, CDK6, cyclins Dl , D2, E. and A immunoprecipitates from NML cell lysates were blotted with anti-ARF-pl9, no coprecipitation of ARF-pl9 was observed. Reconstruction experiments with baculovirus expression vectors in insect Sf9 cells also failed to provide convincing evidence for associations between ARF-pl9 and these cell cycle regulators. The mechanism(s) by which ARF-pl9 induces cell cycle arrest remains unclear, although the propensity of some cells to arrest in G2 argues that these ARF-p 19-induced effects are pRb- independent. Example 7: ARF-pl9 Mutations in Human Cancers Current evidence indicates that InK4a-pl6 functions upstream of cyclin D-dependent kinases and pRb in a biochemical pathway that regulates exit of Gl phase cells into S phase. InK4a-pl6 cannot induce Gl arrest in cells that lack pRb function (Guan et al . Genes Devel. 8:2932-2952 (1994); Tarn et al , Oncogene 9:2663-2674 (1994); Lukas et al . Nature 575:503-506 (1995); Medema et al , Proc. Natl. Acad. Sci. USA 92:6289-6293 (1995); Koh et al , Nature 575:506-510 (1995)), and in lung cancer, INK4A deletions are restricted to tumors that retain pRb activity and vice versa, implying that a loss of either gene makes compromise of the other irrelevant (Otterson et al , Oncogene 9:3375-3378 (1994)). Increased levels of InK4a-pl6 are generally observed in pRb-negative tumor cells (Serrano et al, Nature 366:104- 707 (1993); Bates et al, Oncogene 9:1633-1640 (1994); Lukas et al , J. Cell. Biol. 725:625-638 (1994); Tarn et al. , Oncogene 9:2663-2674 (1994)), suggesting that InK4a-pl6 expression may be somehow governed by pRb itself or, alternatively, that pRb provides a predisposing selective pressure that favors elimination of pRb-mediated controls (Sherr and Roberts, Genes Devel. 9: 1149-1163 (1995)). Of the numerous InK4a-pl6 mutations found in human cancers, only a few have been experimentally evaluated for effects on the cell cycle. However, two which clearly abrogate InK4a-pl6 inhibitory functions (R87P and H98P in Koh et al , Nature 575:506-510 ( 1995)) are silent with regard to ARF-pl9, and another (P114L in Lukas et al . Nature 575:503-506 (1995)) falls outside the region of overlap between InK4a-pl6 and ARF-p 19, indicating that the latter is not a target of inactivating mutations in these cases. Nevertheless, most mutations involving INK4A in cancer cells fall within the 5' half of exon 2, raising the possibility that some may dually affect InK4a-pl6 and ARF-pl9 or, conceivably, ARF-pl9 alone. About 60% of mutations in InK4a-pl6 cluster within the region overlapping ARF-pl9 (Hirama and Koeffler. Blood 86:841-854 (1995)), and more than 80% of these affect ARF-pl9 primary structure. Figure 6 shows the predicted ARF-pl9 mutations within this segment, compiled from data obtained with primary tumors, xenografts, and established cell lines (Kamb, A. et al. , Science 264:436-440 (1994a), Nature Genet 8:22-26 (1994b); Caldas et al . Nature Genet. 8:27-32 (1994); Hussussian et al . Nature Genet. 8: 15-21 (1994); Ohta et al . Cancer Res 54:5269-5272 (1994); Zhang et al . Cancer Res 54:5050- 5053 (1994); Mori et al , Cancer Res. 54:3396-3397 (1994); Hayashi et al , Biochem. Biophys. Res. Commun. 202: 1426-1430 (1994)). Of 50 missense and frame shift mutations, 39 involve codons conserved between human and mouse ARF-p 19 (residues in bold type) and four (marked by asterisks) are silent in InK4a-pl6. The most frequently mutated ARF-pl9 residues in sporadic cancers are Gly-68, Pro-93, Arg- 97, each of which is conserved in humans and mice, and the most common disease-related alteration in melanoma kindreds (Hussussian et al, Nature Genet. 8:15-21 (1994); Kamb et al.. Nature Genet. 8:22-26 ( 1994b)) converts conserved Arg-114 of ARF-pl9 to Leu. A further complication is that frame shift mutations have the potential to produce chimeric proteins. For example, those involving ARF-pl9 Gln-69 (Hayashi et al . Biochem. Biophys. Res Commun. 202: 1426-1430 (1994)) and Gly-72 (Ohta et al . Cancer Res. 54:5269-5212 (1994)) should result in INK4A a transcripts encoding InK4a-pl6/ARF-pl9 fusions in which the majority of exon-2 sequences encode ARF-p 19 residues. Conversely, an ARF-p 19 frame shift involving Gly-101 (Hayashi et al , Biochem. Biophys. Res Commun. 202: 1426-1430 (1994)) would yield a β transcript encoding the C-terminal half of InK4a-pl6. To the extent that mutations in ARF-pl9 contribute to aberrant growth control and tumorigenesis, detection and analysis of ARF-p 19-specific nucleic acids (Example 2) in a mammal serves to diagnose, or assist in the diagnosis of, existing mmors in the mammal, or to predict the mammal's predisposition for developing certain forms of cancer.

Claims: What Is Claimed Is: 1. An isolated nucleic acid encoding a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases. 2. The isolated nucleic acid of claim 1 , wherein said nucleic acid is a cDNA molecule which has the nucleotide sequence of SEQ ID NO: l and encodes murine ARF-p 19 polypeptide, or which has the nucleotide sequence of SEQ ID NO: 3 and encodes the human ARF-p 19 polypeptide. 3. The isolated nucleic acid of claim 2, wherein said nucleic acid is a cDNA molecule which has the nucleotide sequence of SEQ ID NO: 3 and encodes a human ARF-p 19 having one or more mutations that alters a codon that encodes Gly-68, Gln-69, Pro-71 , Gly-73, Gly-74, Pro-79, Arg-81 , Gly- 82. Leu-85, Arg-87, Ser-91, Pro-93, Ala-95, Arg-96, Gly-101 , Gly-105, His-106, Gly-108, Gly-113, Arg- 1 14, Ala-117, Gly-1 18, Ala- 120 or Arg- 121 of SEQ ID NO:4. 4. An isolated polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases. 5. The isolated polypeptide of claim 4, wherein said polypeptide is murine ARF-pl9 having the amino acid sequence of SEQ ID NO:2 or human ARF-pl9 having the amino acid sequence of SEQ ID NO:4. 6. The isolated polypeptide of claim 5, wherein said human ARF-pl9 has a mutant amino acid residue at one or more of the following positions of SEQ ID NO:4: Gly-68, Gln-69, Pro-71 , Gly-73, Gly-74, Pro-79, Arg-81 , Gly-82, Leu-85, Arg-87, Ser-91, Pro-93, Ala-96, Arg-97, Gly-101 , Gly-105, His-106, Gly-108, Gly-113, Arg-114, Ala-117, Gly-118, Ala-120 or Arg-121. 7. An isolated fusion protein, wherein said fusion protein has from 10 to 169 contiguous amino acids derived from SEQ ID NO:2 or from 10 to 132 contiguous amino acids derived from SEQ ID NO:4. 8. An isolated antibody that specifically binds the isolated polypeptide of claim 4. 9. An isolated antibody that specifically binds the isolated polypeptide of claim 6. 10. A method of detecting a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases, wherein said method comprises the step of contacting said polypeptide with the isolated antibody of claim 8. 11. A method of detecting a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases, wherein said method comprises the step of contacting said polypeptide with the isolated antibody of claim 9. 12. An isolated oligonucleotide, wherein said isolated oligonucleotide consists essentially of from about 15 to about 30 contiguous nucleotides present in residues 1 to 250 of SEQ ID NO: l , from about 15 to about 30 contiguous nucleotides present in residues 1 to 160 of SEQ ID NO: 3, or from about 15 to about 30 contiguous nucleotides present in the isolated nucleic acid of claim 3. 13. A method of detecting a nucleic acid encoding a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases, wherein said method comprises the step of contacting said nucleic acid with the isolated oligonucleotide of claim 12. 14. A vector construct containing the isolated nucleic acid molecule of claim 1. 15. The vector construct of claim 14, wherein said vector construct is an expression vector construct. 16. A cultured host cell harboring the expression vector construct of claim 15. 17. A method of producing a polypeptide comprising the steps of culturing the cultured host cell of claim 16. 18. A vector construct containing an isolated nucleic acid molecule having the sequence of SEQ ID NO: 1 , SEQ ID NO:3 or that of the isolated nucleic acid of claim 3. 19. The vector construct of claim 18, wherein said vector construct is an expression vector construct. 20. A cultured host cell harboring the expression vector construct of claim 19. 21. A method of producing a polypeptide comprising the steps of culturing and harvesting the cultured host cell of claim 20. 22. A method of inducing cell cycle arrest in a eukaryotic cell comprising the step of introducing into said cell the isolated polypeptide of claim 4. 23. A method of inducing cell cycle arrest in a eukaryotic cell comprising the step of introducing into said cell the expression vector construct of claim 15. 24. A method of inducing cell cycle arrest in a eukaryotic cell comprising the step of introducing into said cell the expression vector construct of claim 19. 25. A method of selectively killing cells undergoing uncontrolled growth comprising the step of introducing into said cells the antibody of claim 8 conjugated to a toxin. 26. A non-human transgenic animal which has been altered to express a nucleic acid encoding the isolated polypeptide of claim 5. 27. A non-human transgenic animal which has been altered to express a nucleic acid encoding the isolated polypeptide of claim 6. 28. A non-human transgenic animal which has been altered so that said mammal does not express an endogenous gene encoding a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases. 29. A non-human transgenic animal which has been altered so that said mammal expresses a nucleic acid encoding the isolated polypeptide of claim 6 but does not express an endogenous gene encoding a polypeptide, wherein said polypeptide (a) has an amino acid sequence that is composed of at least about 20% arginine residues; (b) induces cell cycle arrest when overexpressed in eukaryotic cells; and (c) is encoded by a first reading frame that asyntactically overlaps a second reading frame, wherein said second reading frame encodes a polypeptide that inhibits one or more D-type cyclin-dependent kinases. 30. A cultured cell line derived from the non-human transgenic animal of claim 28.

Inventors: QUELLE DAWN E, SHERR CHARLES J, QUELLE, DAWN, E., SHERR, CHARLES, J.
Applicants: ST JUDE CHILDRENS RES HOSPITAL, ST. JUDE CHILDREN'S RESEARCH HOSPITAL
Application year: 1996
Main classification: C12Q1
IPC codes: A01K67, A61K31, A61K38, A61K39, A61K48, A61P35, A61P43, C07K7, C07K14, C07K16, C07K19, C12N1, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33