NOVEL CELL CYCLE CHECKPOINT GENES AND PROTEINS ENCODED THEREBY

Abstract: The present invention encompasses novel mammalian cell cycle checkpoint genes/DNA repairgenes, cDNA or genomic DNA, isolated nucleic acids corresponding thereto, expression vectors comprising said nucleic acids, host cells transformed with said expression vectors, pharmaceutical compositions and the formulation of such compositions comprising said nucleic acids or proteins expressed therefrom, methods for treating a cell using such nucleic acids, proteins or pharmaceutical compositions, and the use of such nucleic acids or proteins in formulating a pharmaceutical preparation.

Description: NOVEL CELL CYCLE CHECKPOINT GENES AND PROTEINS ENCODED THEREBY Field of the Invention The present invention relates generally to the field of medicine, and relates specifically to methods and compositions for modulating cell growth and death, including cell formation of tissues, using novel proteins, variants of these proteins and nucleic acids encoding them. Background of the Invention The integrity of the genome is of prime importance to a dividing cell. In response to DNA damage, eukaryotic cells rely upon a complex system of controls to delay cell-cycle progression. The normal eukaryotic cell-cycle is divided into 4 phases (sequentially Gl, S, G2, M) which correlate with distinct cell morphology and biochemical activity. Cells withdrawn from the cell-cycle are said to be in GO, or non- cycling state. When cells within the cell-cycle are actively replicating, duplication of DNA occurs in the S phase, and active division of the cell occurs in M phase. See generally Benjamin Lewin, GENES VI (Oxford University Press, Oxford, GB, Chapter 36, 1997). DNA is organized in the eukaryotic cell into successively higher levels of order that result in the formation of chromosomes. Non-sex chromosomes are normally present in pairs, and during cell division, the DNA of each chromosome replicates resulting in paired chromatids. (See generally Benjamin Lewin, GENES VI (Oxford University Press, Oxford, GB, Chapter 5, 1997). The eukaryotic cell cycle is tightly regulated by mtrinsic mechanisms that ensure ordered progression through its various phases and surveillance mechanisms that prevent cycling in the presence of aberrant or incompletely assembled structures. These negative regulatory surveillance mechanisms have been termed checkpoints (Hartwell and Weinert, 1989, "Checkpoints: controls that ensure the order of cell cycle events" Science, 246: 629-634). The mitotic checkpoint prevents cells from undergoing mitosis until all chromosomes have been attached to the mitotic spindle whereas the DNA structure checkpoint, which can be subdivided into the replication and DNA damage checkpoint, result in arrests at various points in the cell cycle in the presence of DNA damage or incompletely replicated DNA (Elledge, 1996, "Cell cycle checkpoints: preventing an identity crisis." Science. 274: 1664-1672). These arrests are believed to allow time for replication to be completed or DNA repair to take place. Cell cycling in the presence of DNA damage, incompletely replicated DNA or improper mitotic spindle assembly can lead to genomic instability, an early step in tumorigenesis. Defective checkpoint mechanisms, resulting from inactivation of the p53, ATM, and Bubl checkpoint gene products have been implicated in several human cancers. Checkpoint delays provide time for repair of damaged DNA prior to its replication in S-phase and prior to segregation of chromatids in M-phase (Hartwell and Weinert, 1989, supra.). In many cases the DNA-damage response pathways cause arrest by inhibiting the activity of the cyclin-dependent kinases (Elledge, 1997, supra.). In human cells the DNA-damage induced G2 delay is largely dependent on inhibitory phosphorylation of Cdc2 (Blasina et al., 1997, "The role of inhibitory phosphorylation of cdc2 following DNA replication block and radiation induced damage in Human cells." Mol. Biol. Cell 8: 1013-1023; Jin et al., 1997, "Role of inhibiting cdc2 phosphorylation in radiation-induced G2 arrest in human cells." J. Cell Biol., 134: 963-970), and is therefore likely to result from a change in the activity of the opposing kinases and phosphatases that act on Cdc2. However, evidence that the activity of these enzymes is substantially altered in response to DNA damage is lacking (Poon et al., 1997, "The role of cdc2 feedback loop control in the DNA damage checkpoint in mammalian cells." Cancer Res., 57: 5168-5178). Three distinct Cdc25 proteins are expressed in human cells. Cdc25A is specifically required for the Gl-S transition (Hoffmann et al., 1994, "Activation of the phosphatase activity of human CDC25A by a cdk2-cyclin E dependent phosphorylation at the G-l/S transition." EMBO J.. 13: 4302-4310; Jinno et al., 1994, "Cdc25A is a novel phosphatase functioning early in the cell cycle" EMBO J., 13: 1549- 1556), whereas Cdc25B and Cdc25C are required for the G2-M transition (Gabrielli et al., 1996, "Cytoplasmic accumulation of cdc25B phosphatase in mitosis triggers centrosomal microtubule mucleation in HeLa cells" J. Cell Sci., 109(5): 1081-1093; Galaktionov et al., 1991, "Specific activation of cdc25 tyrosine phosphatases by B-type cyclins: evidence for multiple roles of mitotic cyclins" Cell, 67: 1181-1194; Millar et al., 1991, "p55CDC25 is a nuclear protein required for the initiation of mitosis in human cells" Proc. Natl. Acad. Sci. USA, 88: 10500-10504; Nishijima et al., 1997, J. Cell Bioi, 138: 1105-1116). The exact contribution of Cdc25B and Cdc25C to M-phase progression is not known. Much of our current knowledge about checkpoint control has been obtained from studies using budding (Saccharo yces cerevisiae) and fission (Schizosaccharomyces pombe) yeast. A number of reviews of our current understanding of cell cycle checkpoint in yeast and higher eukaryotes have recently been published (Hartwell & Kastan, 1994, "Cell cycle control and Cancer" Science, 266: 1821-1828; Murray, 1994, "Cell cycle checkpoints" Current Opinions in Cell Biologv, 6: 872-876; Elledge, 1996, supra; Kaufmann & Paules, 1996, "DNA damage and cell cycle checkpoints" FASEB J., 10: 238-247). In the fission yeast six gene products, rad , rad3 + , rad , radl7 , rad26 + , and husl + have been identified as components of both the DNA-damage dependent and DNA-replication dependent checkpoint pathways. In addition cdsl + has been identified as being required for the DNA-replication dependent checkpoint and rad27 + /chkl + has been identified as required for the DNA-damage dependent checkpoint in yeast. Several of these genes have structural homologues in the budding yeast. Further conservation across eukaryotes has recently been suggested with the cloning of several human homologues of S. pombe checkpoint genes, including two related to S. pombe rad3 + : ATM (ataxia telangiectasia mutated) (Savitsky et al, 1995, "A single ataxia telangiectasia gene with a product similar to PI-3 kinase" Science, 268: 1749- 1753) and ATR (ataxia telangiectasia and rad3 + related) (Bentley et al, 1996, "The Schizosaccharomyces pombe rad3 checkpoint genes" EMBO J., 15: 6641-6651; Cimprich et al, "cDNA cloning and gene mapping of a candidate human cell cycle checkpoint protein" 1996, Proc. Natl. Acad. Sci. USA, 93: 2850-2855); and human homologues of S. pombe rad9+, Hrad9 (Lieberman et al, 1996, "A human homolog of the Schizosaccharomyces pombe rad9+ checkpoint control gene" Proc. Natl. Acad. Sci. USA, 93: 13890-13895), Hradl (Parker et al. , 1998, "Identification of a human homologue of the Schizosaccharomyces pombe radl7+ checkpoint gene" J. Biol. Chem. 273: 18340-18346; Freire et al, 1998, "Human and mouse homologs of Schizosaccharomyces pombe radl(+) and Saccharomyces cerevisia RAD17: linkage to checkpoint control and mammalian meiosis" Genes Dev. 12:2560-2573; Udell et al, 1998, "Hradl and Mradl encode mammalian homologues of the fission yeast radl(+) cell cycle checkpoint control gene" Nucleic Acids Res. 26:2971-3976), Hradl7 (Parker et al, 1998, supra), Hhusl (Kostrab et al., 1998, "Huslp, a conserved fission yeast checkpoint protein, interacts with Radlp and is phosphorylated in response to DNA damage" EMBO J. 17:2055-2066), Hchkl (Sanchez et al, 1997, "Conservation of the Chkl checkpoint pathway in mammals: linkage of DNA damage to Cdk regulation through Cdc25" Science 277: 1497-1501) and Hcdsl (Matusoka et al, 1998, "Linkage of ATM to cell cycle regulation by the Chk2 protein kinase" Science 282(5395): 1893-1897; Blasina et al, 1999, "A human homologue of the checkpoint kinase Cdsl directly inhibits Cdc25 phosphatase" Curr. Biology 9(1): 1-10). Genetic and biochemical analysis of the checkpoint proteins in yeast and mammalian cells suggests that the checkpoint response is transmitted through a conventional signal transduction pathway. Hradl, Hrad9, Hradl 7, and Hhusl transmit the signal emanating from damaged or incompletely replicated DNA to the central kinases ATM and ATR, which in turn activate the downstream kinases, Chkl and Cdsl . The DNA structure checkpoint responses ultimately lead to phosphorylation of the mitosis inducing phosphatase Cdc25 by Chkl or Cdsl. This phosphorylation event creates a binding site for 14-3-3 proteins that target Cdc25 for export from the nucleus to the cytoplasm, thus preventing it from removing an inhibitory phosphate from the cyclin dependent kinase, Cdc2. Removal of this inhibitory phosphate is required for passage from G2 to mitosis in every cell cycle. The DNA structure checkpoint responses prevent this from occurring and result in a G2/M arrest. Whereas the Chkl protein has been shown to be required for the G2/M DNA damage checkpoint in S. pombe, the replication checkpoint requires the activity of both Cds 1 and Chkl . When replication is blocked by treatment with the ribonucleotide reductase inhibitor hydroxyurea (HU), wild type cells arrest prior to mitosis. A cdsl chkl double mutant fails to arrest in the presence of HU while both single mutants arrest normally (Russell, 1998, "Checkpoints on the road to mitosis" Trends in Biochemical Sciences 23(10):399-402). S. pombe Chkl and Cdsl are both capable of phosphorylating Cdc25 and targeting it for binding by 14-3-3 proteins. Activation of the S. pombe Cdsl protein kinase by HU also results in enhanced binding to and phosphorylation of Wee 1 , and accumulation of Mikl . These two protein kinases are required for the inhibitory phosphorylation of Cdc2 that prevents cells from entering mitosis suggesting an alternative to Cdc25C phosphorylation for checkpoint mediated cell cycle arrest. Recently, Cdsl has also been shown to be required for a DNA damage checkpoint in S-phase (Rhind and Russell, 1998, "The Schizosaccharomyces pombe S-phase checkpoint differentiates between different types of DNA damage" Genetics 149(4): 1729-1737; Lindsay et al, 1998, "S-phase- specific activation of Cdsl kinase defines a subpathway of the checkpoint response in Schizosaccharomyces pombe" Genes Dev. 12(3):382-395). A human homologue of S. pombe Cds 1 that is activated by DNA damage and HU in an ATM-dependent manner and is capable of phosphorylating Cdc25C in vitro was recently identified (Matsuoka et al, 1998, supra; Blasina et al, 1999, supra). The human cDNA encodes a 543 amino acid protein which like its S. pombe homologue, contains a forkhead associated (FHA) domain N-terminal to the kinase domain. FHA domains are found in several other proteins including the S. cerevisiae Cdsl orthologue Rad53. Rad53 contains two FHA domains, one of which is required for interaction with the DNA damage checkpoint protein Rad9 in the presence of DNA damage (Sun et al, 1998, "Rad53 FHA domain associated with phosphorylated Rad9 in the DNA damage checkpoint" Science 281(5374):272-274). In order to develop new and more effective treatments and therapeutics for the amelioration of the effects of aging or disease such as cancer, it is important to identify and characterize mammalian, and in particular human, checkpoint proteins and to identify mediators of their activity. The present invention teaches the identification and characterization of human and murine nucleic acids encoding human Mus81 (Hmus81) and murine Mus81 (Mmus81) protein with significant homology to the S. pombe Mus81 protein that interacts with the S. pombe Cds 1 FHA domain. The S. cerevisiae orthologue is reported to be involved in meiosis and DNA repair. As described below, a Hmus81 gene acts as a checkpoint/repair gene and is involved with DNA repair. The checkpoint/repair delays provide time for repair of damaged DNA prior to its replication in S-phase and prior to segregation of chromatids in M-phase, and Hmus81 appears to act in both aspects, similarly to other known checkpoint/repair genes. In many cases, the DNA-damage response pathways will cause arrest, and the cell will fail to divide. However, a functional DNA repair mechanism will allow the damage to be corrected, and thus allow eventual cell division to occur. In humans, excision repair is an important defense mechanism against two major carcinogens, sunlight and cigarette smoke. It has been found that individuals defective in excision repair exhibit a high incidence of cancer, (see Sancar, A, 1996, "DNA Excision Repair" Ann. Rev. Biochem. 65:43-81). Other mechanisms also act in a simiar manner to repair DNA, such as mismatch repair which stabilizes the cellular genome by correcting DNA replication errors and by blocking recombination events between divergent DNA sequences. Inactivation of genes encoding these activities results in a large increase in spontaneous mutability and predisposition to tumor development, (see Modrich & Lahue, 1996, "Mismatch Repair in Replication Fidelity, Genetic Recombination and Cancer Biology" Ann. Rev. Biochem. 65: 101- 33). The importance of maintaining fidelity in the DNA is amply illustrated by the many mechanisms for repair, and if unrepairable, arrest of cell division, (see Wood, RD, 1996, "DNA Repair in Eukaryotes" Ann. Rev. Biochem. 65: 135-67). Many chemotherapeutic agents are designed to disrupt or otherwise cause damage to the DNA of the targeted malignant cells. Antineoplastic agents such as alkylating agents, antimetabolites, and other chemical analogs and substances typically act by inhibiting nucleotide biosynthesis or protein synthesis, cross-linking DNA, or intercalating with DNA to inhibit replication or gene expression. Bleomycin and etoposide for example, specifically damage DNA and prevent repair. The inhibition of Hmus81 gene or protein activity amplifies the potency of antineoplastic agents, and enhances the efficacy of their use as chemotherapeutic agents. This enhancement is beneficial in not only more thoroughly affecting the targeted cells, but by allowing for reduced dosages to be used in proportion to the increased efficacy, thus reducing unwanted side effects. Inhibition of Hmus81 or Mmus81 gene activity via anti-sense nucleic acid pharmaceuticals can be effected using the nucleic acids of the invention as the template for constructing the anti -sense nucleic acids. It is preferred to target the amino terminal end of the nucleic acid for anti-sense binding, and thus inhibition, as this reduces translation of the mRNA. Inhibition of Hmus81 protein activity can be effected by the use of altered or fragments of Hmus81 or Mmusδl protein to competitively inhibit the biochemical cascade that results in the repair of damaged DNA, or to cause cell arrest. Disease can also result from defective DNA repair mechanisms, and include hereditary nonpolyposis colorectal cancer (defect in mismatch repair), Nijmegen breakage syndrome (defect in double strand break repair), Xeroderma pigmentosum, Cockayne syndrome, and Trocothiodystrophy (defect in nuclear excision repair), (see for example Lengauer et al., 1998, "Genetic instabilities in human cancers" Nature 396(6712):643-649; Kanaar et al., 1998, "Molecular mechanisms of DNA double stranded repair" Trends Cell Biol. 8(12):483-489). It is further envisioned that the transient inhibition of Hmus81 gene or protein activity can be sufficient to effect improved treatment of cell behavior due to aging or disease. For example, the transient inhibition of DNA checkpoint/DNA damage arrest of cell division may allow the combined use of lower doses of chemotherapeutic agents to effect greater damage to targeted cells in the treatment of diseases such as cancer. Summary of the Invention Novel genes and proteins encoded thereby are useful for modifying cell growth, division and death. One aspect of the invention is a novel mammalian, e.g., human or murine checkpoint/repair protein, the nucleic acids which encode for it and its protein variants, nucleic acid constructs, and methods for the production and use of mammalian Mus81 encoding gene and protein. As used herein, "checkpoint gene" means a gene which encodes for a protein which acts in the checkpoint/repair regulation of cell division. Such protein can effect both replication and DNA damage checkpoint activity, ie. having checkpoint/repair activity. Specific characterization of the mammalian Mus81 protein encoding nucleic acids and their role in cell cycle regulation provides for novel and useful compounds for modulating the mammalian cell cycle in a target cell. As used herein, the terms "human Mus81 gene", "Hmus81 encoding gene" and "Hmus81 gene" encompas human Mus81 encoding genes, including the allelic variants of the gene which will occur in a human population, but still encode for the same protein, splice variants of the gene, as well as the transcripts from such genomic genes, cDNA encoding for the transcript, and other nucleic acids which will encode a Hmus81 protein. As used herein, the terms "human Mus81 protein", "Hmus81" and "Hmus81 protein" refer generally to the protein expressed from a Hmus81 encoding nucleic acid, and includes splice variants and glycosylation variants of the protein which are generated by the translation and processing of the protein encoded by a Hmus81 encoding gene, and in particular to a human Mus81 protein having an amino acid sequence corresponding to that depicted as SEQ ID NO.: 2, 4, 8, and 10. In a preferred embodiment, the isolated nucleic acids of the invention correspond to a cDNA that encodes for a human Mus81 protein. Any particular isolated nucleic acid of the invention, preferably encodes for only one form of a human Mus81 protein. As described in detail below, the human Mus81 encoding nucleic acids of the invention encompasses isolated nucleic acids comprising a nucleotide sequence corresponding to the nucleotide sequences disclosed herein and specifically identified as Human Musδ^ ("Hmus81(l)"; SEQ O NO.: 1), Human Mus81 2 ("Hmus81(2)"; -9- SEQ ID NO.: 3), Human Mus81 3 ("Hmus81(3)"; SEQ ID NO: 7), and Human Mus81 4 ("Hmus81(4)"; SEQ LD NO: 9). All of the foregoing nucleic acids encode for a human Mus81 protein, and its equivalents. Thus, the present invention encompasses a nucleic acid having a nucleotide sequence which encodes for a Hmus81 protein and specifically encompasses a nucleotide sequence corresponding to the coding domain segment of the sequences that are depicted as SEQ ID NO.: 1, 3, 7, 9 and 25. The present invention also encompasses a nucleic acid which encodes for two versions of Hmus81 protein having a nucleotide sequence corresponding to that depicted as SEQ LD NO.:25. This nucleic acid encodes for a Hmusδl protein having an amino acid residue sequence depicted as SEQ LD NO.: 4, wherein the 201 nucleotides from position 1274 to 1474 of the sequence of SEQ LD NO.: 25 containing a stop codon, have been deleted, thus allowing translation of the longer coding domain segment sequence of DNA. The nucleic acid having a corresponding nucleotide sequence as that depicted as SEQ ID NO.: 25 also encodes for the shorter Hmus81 protein having the amino acid sequence depicted as SEQ LD NO.: 2, from a shorter coding domain segment, leaving the intron in place. Thus, in a preferred embodiment, the present invention encompasses nucleic acids which encode for human Mus81 proteins, and in particular, nucleic acids having a coding domain segment sequence corresponding to that represented by nucleotides 23-1675 of the nucleotide sequence depicted as SEQ LD NO.: 1; to that represented by nucleotides 185-1549 of the nucleotide sequence depicted as SEQ ID NO.:3; to that represented by nucleotides 26-1297 of the nucleotide sequence depicted as SEQ ID NO.:7; to that represented by nucleotides 26-1681 of the nucleotide sequence depicted as SEQ ID NO.:9; or as identified in SEQ LD NO.: 25. The terms "murine Mus81 gene" and "Mmus81 gene" are used herein to refer to the novel murine Mus81 encoding genes. The terms "murine Mus81 protein", "Mmusδl" and "Mmus81 protein" refer generally to the protein product of the Mmusδl genes and in particular, to murine Mus81 proteins having an amino acid residue sequence corresponding to that depicted * as SEQ ID NO.: 12, 14, 16, and 18. The terms "murine Mus81 gene", "Mmus81 gene" and "Mmus81 encoding gene" encompass the Mmus81 genes, and in particular isolated nucleic acids comprising a nucleotide sequence corresponding to the nucleotide sequences disclosed herein and identified as Mouse (murine) Mus81 t ("Mmus81(l)"; SEQ LD NO.: 11), Mouse Mus81 2 ("Mmus81(2)"; SEQ ID NO.: 13), Mouse Mus81 3 ("Mmus81(3)"; SEQ ID NO: 15), and Mouse Mus81 4 ("Mmus81(4)"; SEQ LD NO: 17), and the protein coding domain segments encoded for therein. In a preferred embodiment, the isolated nucleic acids of the invention correspond to a cDNA that encodes for a murine Mus81 protein. Any particular isolated nucleic acid of the invention, preferably encodes for only one form of a murine Mus81 protein. In another preferred embodiment, the present invention encompasses nucleic acids which encode for murine Mus81 proteins, and in particular, nucleic acids which have a coding domain segment sequence corresponding to that represented by nucleotides 42-1694 of the nucleotide sequence depicted as SEQ LD NO.: 11; to that represented by nucleotides 15-1323 of the nucleotide sequence depicted as SEQ ID NO.: 13; to that represented by nucleotides 52-1644 of the nucleotide sequence depicted as SEQ LD NO.: 15; or to that represented by nucleotides 52-1614 of the nucleotide sequence depicted as SEQ ID NO.: 17. The present invention also encompasses nucleic acid constructs, vectors, plasmids, cosmids, retrovirus or viral constructs and the like which contain a nucleotide sequence encoding for a human Mus81 or murine Mus81 protein. In particular, the present invention provides for nucleic acid vector constructs which contain the nucleotide sequence of the Hmus81 coding domain segments of the nucleic acid depicted as SEQ ID NO.: 1, 3, 7, 9 or 25 and which are expressible as a protein. The present invention also provides for nucleic acid vector constracts which contain the Mmus81 coding domain segments of the nucleic acids depicted as SEQ ID NO.: 11, 13, 15, or 17. The term "transgene capable of expression" as used herein means a suitable nucleotide sequence which leads to expression of Hmus81 or Mmus81 proteins, having the same function and/or the same or similar biological activity as such protein. The transgene can include, for example, genomic nucleic acid isolated from mammalian cells (e.g. human or mouse) or synthetic nucleic acid, including DNA integrated into the genome or in an extrachromosomal state. Preferably, the transgene comprises the nucleotide sequence encoding the proteins according to the invention as described herein, or a biologically active portion of said protein. A biologically active protein should be taken to mean, and not limited to, a fusion product, fragment, digestion fragment, segment, domain or the like of a Mus81 protein having some if not all of the protein activity as a whole Musδl protein. A biologically active protein thus contains a biologically functional portion of a mammalian Mus81 protein conveying a biochemical function thereof. The present invention encompasses nucleic acid vectors that are suitable for the transformation of host cells, whether eukaryotic or prokaryotic, suitable for incorporation into viral vectors, or suitable for in vivo or in vitro protein expression. Particularly preferred host cells for prokaryotic expression of protein include, and are not limited to bacterial cells such as E. coll. Suitable host cells for eukaryotic expression of protein include, and are not limited to mammalian cells of human or murine origin and the like, or yeast cells. In a preferred embodiment, expression of protein, as described below, is accomplished by viral vector transformation of immortalized human cells. The present invention further embodies a nucleotide sequence which encodes for a human Mus81 or murine Mus81 protein, in tandem with, or otherwise in conjunction with additional nucleic acids for the generation of fusion protein products. Human Mus81 fusion proteins will contain at least one segment of the protein encoded for by the nucleic acid depicted as the coding domain segment depicted in the nucleotide sequence described as SΕQ ID NO.: 1, 3, 7, and 9. Similarly, murine Mus81 fusion protein will contain at least one segment of protein encoded for by the coding domain segments of the nucleic acid depicted as SΕQ LD NO.: 11, 13, 15, and 17. The present invention also encompasses isolated nucleic acids or nucleic acid vector constracts containing nucleic acid segments, adapted for use as naked DNA transformant vectors for incorporation and expression in target cells. Also provided are inhibitors of human Mus81 or murine Mus81 encoding nucleic acid transcripts, such as anti-sense DNA, triple-helix nucleic acid, double-helix RNA or the like. Biologically active anti-sense DNA molecule formulations are those which are the complement to the nucleotide sequence of the human Mus81 or murine Mus81 encoding genes or fragments thereof, whether complementary to contiguous or discontinuous portions of the targeted nucleotide sequence, and are inhibitors of the human Mus81 or murine Mus81 protein expression in cells. Such inhibitors and inhibition are useful for many purposes including and not limited to, in vitro analysis of the cell-cycle checkpoint pathway, detection and/or evaluation of inhibiting or potentiating compounds, and for in vivo therapy. The present invention also provides for compositions incorporating modified nucleotides or substitute backbone components which encode for the nucleotide sequence of a human Mus81 or murine Mus81 encoding gene, or fragments thereof. The present invention encompasses the use of anti-sense nucleic acids which comprise a nucleic acid that is the complement of at least a portion of a nucleic acid encoding for a human Mus81 or murine Mus81 protein. Also envisioned are biologically active analogs of this antisense molecule selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. An antisense molecule of the invention can also be a phosphorothioate analog. Also encompassed are pharmaceutical preparations for inhibiting Hmus81 protein expression or function in a cell which comprises an antisense nucleic acid analog which is capable of entering said cell and binding specifically to a nucleic acid molecule encoding for Hmus81 protein. The antisense nucleic acid is present in a pharmaceutically acceptable carrier and has a nucleotide sequence complementary to at least a portion of the nucleic acid of SEQ ID NO.: 1, 3, 7, 9 or 25. It is also envisioned that this pharmaceutical preparation can comprise a nucleic acid having a sequence complementary to at least the nucleotides encoding for amino acid residues 1-50 of the amino acid residue sequence of SEQ ID NO.: 2, 4, 8, or 10. In a preferred embodiment, the pharmaceutical preparation comprises a nucleic acid having a nucleotide sequence complementary to at least nucleotides 1-20 of a coding domain segment in the nucleotide sequence depicted as SEQ ID NO.: 1, 3, 7, 9 or 25. In a most preferred embodiment, the antisense nucleic acid comprises a nucleic acid having a sequence complementary to at least nucleotides 1-10 of a coding domain segment in the nucleotide sequence depicted as SEQ LD NO.: 1, 3, 7, 9 or 25. The present invention also encompasses nucleotide sequences which would encode for the Hmus81 protein having an amino acid sequence as that depicted by that of SEQ ID NO.: 2, 4, 8 or 10 based upon synonymous codon substitution given the knowledge of the triplet codons and which amino acids they encode, based upon the coding domain segment of the nucleotide sequence depicted in SEQ ID NO. 1, 3, 7, 9 or 25. The equivalent synonymous nucleic acid code for generating any nucleotide sequence which will encode for a protein having a particular amino acid sequence is known and predictable to one of skill in the art. In a preferred embodiment codon usage is optimized to increase protein expression as desired for the target host cell, such as where a nucleic acid is modified so that it comprises a protein coding domain segment of the nucleotide sequence depicted in SEQ ID NO.: 1, 3, 7, 9, 11, 13, 15, 17 or 25, wherein the least preferred codons are substituted with those that are most preferred in the target host cell. In the case of human target host cells, the least preferred codons are ggg, art, etc, tec, and gtc. The invention also provides for methods of generating human Mus81 or murine Mus81 protein, fusion proteins, or fragments thereof by using recombinant DNA technology and the appropriate nucleic acid encoding for human Mus81 or murine Mus81 protein. The invention provides for incorporating an appropriate nucleotide sequence into a suitable expression vector, the incorporation of suitable control elements such as a ribosome binding site, promoter, and/or enhancer element, either inducible or constitutively expressed. The invention provides for the use of expression vectors with or without at least one additional selectable marker or expressible protein. The invention provides for methods wherein a suitably constructed expression vector is transformed or otherwise introduced into a suitable host cell, and protein is expressed by such a host cell. The present invention also provides transformed host cells, which are capable of producing human Musδl or murine Mus81 protein, fusion protein, or fragments thereof. The expression vector including said nucleic acid according to the invention may advantageously be used in vivo, such as in, for example, gene therapy. The invention encompasses mammalian, e.g. human or murine Mus81 protein, fusion products, and biologically active portions thereof produced by recombinant DNA technology and expressed in vivo or in vitro. A biologically active portion of a protein is protein segment or fragment having the enzymatic activity of, or at least a some enzymatic activity of the whole mammalian Mus81 protein, when compared under similar conditions. For example, it will be readily apparent to persons skilled in the art that nucleotide substitutions or deletions may be introduced using routine techniques, which do not affect the protein sequence encoded by said nucleic acid, or which encode a biologically active, functional protein according to the invention. Manipulation of the protein to generate fragments as a result of enzyme digestion, or the modification of nucleic acids encoding for the protein can similarly result in biologically active portions of the mammalian Mus81 protein. Complete protein, fusion products and biologically active portions thereof of the mammalian Mus81 protein are useful for therapeutic formulations, diagnostic testing, and as immunogens, as for example to generate antibodies thereto. The invention thus encompasses Hmus81 and Mmus81 protein produced by transformed host cells in small-scale or large-scale production. The invention encompasses complete Hmus81 and Mmus81 protein, in either glycosylated or unglycosylated forms, produced by either eukaryotic or prokaryotic cells. The present invention provides for Hmus81 and Mmus81 protein expressed from mammalian, insect, plant, bacterial, fungal, or any other suitable host cell using the appropriate transformation vector as known in the art. The present invention encompasses Hmusδl and Mmus81 protein that is produced as a fusion protein product, conjugated to a solid support, or Hmus81 and Mmusδl protein which is labeled with any chemical, radioactive, fluorescent, chemiluminescent or otherwise detectable marker. The present invention also provides Hmus81 and Mmus81 proteins isolated from natural sources and enriched in purity over that found in nature. Also provided are pharmaceutical formulations of Hmus81 and Mmus81 protein as well as formulations of the Hmusδl and Mmus81 protein in pharmaceutically acceptable carriers or excipients. The present invention also encompasses the use of human Musδl or murine Mus81 protein, fusion protein, or biololgically active fragments thereof to generate specific antibodies which bind specifically to the human Mus81 or murine Mus81 protein, or both, as either polyclonal or monoclonal antibodies generated by the immunization of a mammal with human Mus81 protein having the amino acid residue sequence, or an immunogenic fragment of the amino acid residue sequence shown as SEQ ID NO.: 2, 4, 8, or 10, or the murine Mus81 protein having the amino acid residue sequence shown as SEQ LD NO.: 12, 14, 16 or 18. An immunogenic fragment is one which will elicit an immune response, when injected into a immunologically competent host under immunogenic conditions, and generate antibodies specific for the immunogemc fragment. The present invention also encompasses equivalent proteins where substitutions of amino acids for amino acid residues as shown in the amino acid sequence encoding for human Mus81 protein (SEQ LD NO.: 2, 4, 8, 10) or murine Mus81 protein (SEQ ID NO.: 12, 14, 16, 18) are made. Such amino acid substitutions include conservative substitutions of similar amino acid residues that are reasonably predictable as being equivalent, or semi-conservative substitutions which have a reasonably predictable effect on solubility, glycosylation, or protein expression. For example, non-polar (hydrophobic side-chain) amino acids alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, methionine; uncharged polar amino acids glycine, serine, threonine, cysteine, tyrosine, asparagine, glutamine; charged polar amino acids aspartic acid, glutamic acid; basic amino acids lysine, arginine, and histidine are understood by those in the art to have functionally predictable effects when substituted. Amino acid substitutions also include replacement of amino acid residues with modified amino acid residues or chemically altered substitutes. The present invention also encompasses nucleic acids which encode for such equivalent proteins and the embodiments thereof which encode for the human Mus81 proteins or murine Mus81 proteins. Specific modification can be made of codons used in the nucleic acids corresponding to the human Mus81 or murine Mus81 encoding genes of the invention such that the modified nucleic acids utilize codons preferred by the target host cell, while still encoding for the human Mus81 or murine Mus81 protein. This can be accomplished by conservative synonymous codon substitutions that reduce the number of less preferred codons and/or an increase in the number of preferred codons used by the target host cell The present invention also encompasses modified nucleic acids which incorporate, for example, internucleotide linkage modification, base modifications, sugar modification, nonradioactive labels, nucleic acid cross-linking, and altered backbones including PNAs (polypeptide nucleic acids). The knowledge that Hmus81 acts as a checkpoint/repair protein and is most likely involved in DNA repair, allows for the use of the compounds of the invention in therapeutic treatment of diseases which involve abnormal DNA damage checkpoint/repair function, or that would advantageously inhibit DNA repair in a targeted cell. The present invention further provides for the use of the compounds of the present invention as therapeutics for the treatment of cancer. In one embodiment, inhibitors or agents which inhibit the function of the normal proteins and/or genes of the invention would be useful to sensitize cells for treatment with chemotherapeutics, radiation, DNA damaging agents, or replication inhibitors. The present invention also encompasses methods for screening test compounds for efficacy in effecting the Mus81 mediated checkpoint/repair function of eukaryotic cells. These methods comprise contacting a test compound to eukaryotic cells, and detecting any change in mammalian Mus81 expression or function. Also encompassed are methods of screening wherein a compound is administered, and detection of change in Hmus81 or Mmus81 gene expression or function is accomplished by assaying for Hmus81 or Mmus81 mRNA production or by assaying for Hmus81 or Mmusδl protein expression. Methods for detection of changes in expression level of a particular gene are known in the art. In particular, the present invention allows for the screening of candidate substances for efficacy in modifying the mammalian Mus81 mediated DNA damage checkpoint/repair or DNA repair function by screening for any change in nuclease, phosphorylation or kinase activity of mammalian Mus81 protein. The compounds or substances identified by the assays of the invention, or compounds corresponding to such compounds or substances, can be used for the manufacture of pharmaceutical therapeutics. Methods of identifying a chemical compound that modulates the Mus81 dependent cell cycle pathway are provided for as well. Such methods comprise administering the chemical compound to be tested to a host cell, and detecting the amount of mammalian Mus81 protein in said cell, and comparing the amount detected with that of a normal untreated cell. Further provided for is a method of identifying a chemical compound that modulates the Mus81 dependent cell cycle pathway, which method comprises administering the chemical compound to be tested to a biochemical mixture of Hmus81 protein and a suitable substrate, and detecting the level of Hmus81 protein activity in said mixture, and comparing the detected activity with that of a normal untreated biochemical mixture of Hmusδl protein. As shown in the examples below, isolated Hmus81 protein and suitable substrates can be measured in isolated chemical reactions. In one embodiment, the present invention also provides for pharmaceutical compositions which comprise the Hmus81 protein, Hmus81 nucleic acid, or Hmus81 anti-sense nucleic acids. The therapeutic Hmus81 protein can be normally glycosylated, modified, or unglycosylated depending upon the desired characteristics for the protein. Similarly, Hmus81 protein includes the complete long or short protein, fusion product, or functional or immunogenic fragment thereof. Hmus81 nucleic acids include those encoding for the entire long or short protein, portions of the protein, fusion protein products, and fragments thereof. Also included are modified forms of nucleic acids including those incorporating substitute base analogs, modified bases, PNAs and those incorporating preferred codon usage. Anti-sense nucleic acids include complementary nucleic acids which can bind specifically to the targeted nucleic acids, having full, part or discontinuous segments of complementary nucleic acid which can be DNA, RNA or analog compounds thereof. In another embodiment, the present invention provides for compounds or substances identified as suitable for use as a therapeutic in pharmaceutical formulations by the assays of the invention. These pharmaceutical compositions can further include chemotherapeutic agents for the use in treating cancer, or be administered in a regimen coordinated with the administration of other anti-cancer therapies. The present invention, in one embodiment, encompasses methods for combined chemotherapy using the Hmus81 derived pharmaceuticals independently, and in combination with other chemotherapeutic agents, and in a second embodiment as admixtures with other anti-cancer therapeutics for single dose administration. Similarly, murine Mus81 protein, or nucleic acids encoding for the protein can be used to modulate the cell cycle of murine or non-murine mammalian cells. Nucleic acids encoding for the murine Mus81 protein, can be used to produce murine Mus81 protein by recombinant means for use in pharmaceuticals, detection methods and kits, and assay systems in the same manner as human Mus81 protein. The invention provides for a transgenic cell, transformed cell, tissue or organism comprising a transgene capable of expressing human Mus81 protein, which protein comprises the amino acid sequence illustrated in FIG. 1A (SEQ ID NO.:2), FIG. 1B (SEQ ID N0.:4), FIG. 1C (SEQ ID NO.: 8), FIG. ID (SEQ LD NO.: 10), or a murine Mus81 protein, which protein comprises the amino acid sequence illustrated in FIG. 2A (SEQ LD NO.: 12), FIG. 2B (SEQ LD NO.: 14), FIG. 2C (SEQ LD NO.: 16), FIG. 2D (SEQ LD NO.: 18), or the amino acid sequence of a biologically active functional equivalent or bioprecursor or biologically active fragment therefor. And for the isolated protein produced by such transformed host cells. Brief Description of the Drawings The invention may be better understood by reference to one or more of the following drawings in combination with the detailed description of specific embodiments, and claims presented herein. FIG. 1 depicts nucleotide sequences of human Mus81 cDNA molecules and amino acid sequences of their translation products. FIG. 1A depicts the nucleotide sequence of a PCR product from cerebellum cDNA library encoding a 551 amino acid protein Hmus81(l) (SEQ LD NO.: 1 and 2). FIG. IB depicts the nucleotide sequence of IMAGE 128349 cDNA encoding a 455 amino acid protein Hmus81(2) (SEQ ID NO. : 3 and 4). FIG. 1 C depicts Sequence of a PCR product from cerebellum cDNA library encoding a 424 amino acid protein Hmus81(3) (SEQ LD NO.: 7 and 8). FIG. ID depicts a nucleotide sequence of a PCR product from cerebellum cDNA library encoding a 552 amino acid protein Hmus81(4) (SEQ ID NO.: 9 and 10). FIG.2 depicts nucleic acid nucleotide sequences of mouse Mus81 cDNA molecules and amino acid sequences of their translation products. FIG. 2A depicts a nucleic acid encoding for Mmus81(1) and the amino acid sequence for the translated protein of 551 amino acids in length (SEQ LD NO.: 11 and 12). FIG. 2B depicts a nucleic acid encoding for Mmus81(2) and the amino acid sequence for the translated protein of 424 amino acids in length (SEQ ID NO.: 13 and 14). FIG. 2C depicts a nucleic acid encoding for Mmus81(3) and the amino acid sequence for the translated protein of 531 amino acids in length (SEQ LD NO. : 15 and 16). FIG. 2D depicts a nucleic acid encoding for Mmus81(4) and the amino acid sequence for the translated protein of 521 amino acids in length (SEQ LD NO.: 17 and 18). FIG. 3 graphically presents an alignment of mouse (Mm) (Mmusδl; SEQ ID NO.: 12), human (Hs) (Hmus81; SEQ ID NO.: 10), S. pombe (Sp) (Spmus81; SEQ ID NO.:6), and S. cerevisiae (Sc) Mus81 (Scmusδl; SEQ LD NO.:5) amino acid sequences. Amino acids conserved in all proteins are highlighted in black and in two or more proteins in grey. Sequences underlined in red correspond to the conserved catalytic domain of the XPF family of endonucleases. FIG. 4. Genomic structure and splicing variations of human Musδl. Solid line represents genomic sequence and boxes indicate positions of exons. Sizes of exons and introns (in bp) are indicated above and below the genomic fragment, respectively. Alternative splicing that occurs around exons 13 and 14 corresponds to Mus81 l5 Mus81 4 , and Mus81 3 , is shown by thin lines. Mus81 2 utilizes all the identified exons. FIG. 5. Chromosomal localization of human Mus81 by FISH analysis. (A) Chromosome metaphase spread labelled with a fluorescent Mus81 cDNA probe (left panel) and corresponding DAPI staining (right panel). (B) Idiogram of chromosome 11 with location of Hybridisation signal from 10 representative metaphase spreads. FIG. 6. Northern blot analysis of human Mus81. Human tissues (HI and H2) and cancer cell lines (C). FIG.7. Cellular localization of a Mus81 -GFP (GFP: Green Fluorescent Protein e.g. from Aequorea victoria) fusion protein. A549 cells infected with a retrovirus expressing a Hmus81-GFP fusion at 3 days after induction. FIG.8. Co-irnmunoprecipitation of human Musδl and Cdsl. Western blots of lysates (L) and immunoprecipitates (IP) from cells expressing tagged forms of Mus81 and Cdsl separately and together. Bands corresponding to Mus81 and Cdsl are indicated with arrows. Bands corresponding to a protein that cross-reacts with the HA antibody in the upper panel indicated by an asterisk. Immunoglobulin heavy chains in the lower panel are indicated by an arrowhead. Detailed Description of the Invention The present invention, in one aspect, provides for isolated nucleic acids which encode for novel mammalian cell cycle check-point/repair proteins, such as human Mus81 proteins and murine Mus81 proteins and the like. The nucleic acids of the invention are useful for generating human Mus81 or murine Mus81 proteins using recombinant DNA techniques, for transforming target host cells as naked nucleic acid vectors, or when constructed in combination with nucleic acid regulatory elements such as promoters, enhancers, or supressors as expression vector constracts. Advantageously, the nucleic acid molecules according to the invention can be used as a medicament, or in the preparation of a medicament for modulating cell cycle checkpoint/repair functions of a target cell, for the treatment of cancer and other proliferative diseases. The present invention also provides for isolated and/or recombinantly produced human and murine Mus81 proteins, and protein analogs. Recombinantly produced human Mus81 or murine Mus81 proteins of the invention can be used advantageously in vitro or in vivo for modulating the cell cycle and/or checkpoint/repair pathway of a targeted host cell. Isolated human Mus81 or murine Mus81 protein of the present invention may be utilized to generate antibodies which bind specifically to the human Mus81 protein and/or murine Mus81 protein, where such antibodies can be either polyclonal or monoclonal. Advantageously, the protein molecules according to the invention can be used as a medicament, or in the preparation of a medicament for modulating cell cycle checkpoint/repair functions of a target cell, for the treatment of cancer and other proliferative diseases. Isolated recombinantly produced human and/or murine Mus81 proteins can also be used in combination with other proteins as in vitro biochemical systems for modeling enzymatic steps of an in vivo cell cycle checkpoint/repair pathway for testing and/or evaluating chemical or protein compounds for the ability to modulate the cell cycle checkpoint/repair mechanism associated with human Mus81 or murine Mus81 protein. A biochemical mixture of human Mus81 or murine Mus81 protein will comprise the isolated enzyme, appropriate ions and/or cofactors, and suitable substrate. A preferred biochemical mixture will comprise a suitable substrate which will detectably change or signal a change in state, when the enzymatic activity of the Mus81 protein has been applied to the substrate, for example by emission of energy, or flourescent light, or an alteration in the wavelength of emitted light energy, or by a change in binding by a antibody molecule specific for a particular form of Mus81 protein. The isolated nucleic acids of the invention, and the nucleotide sequence encoded by them, provide for isolated DNA, RNA, modified nucleotide analog, or labeled nucleic acid constructs which can mimic, complementarily bind to, and/or otherwise label nucleic acids comprising the same or highly related nucleotide sequences in nucleic acids in vitro or in vivo. It is envisioned that the nucleic acids of the invention can incorporate modified nucleotides and nucleic acid base analogs, which are known in the art (see for example Verma et al, 1998, "Modified Oligonucleotides" Ann. Rev. Biochem. 67: 99-134). The isolated nucleic acids of the present invention can be a biologically active antisense molecule, which is one capable of hybridizing to a target nucleic acid upon the complementary binding of nucleic acids and thereby modulate the expression of the targeted nucleic acid. Advantageously, the antisense molecule according to the invention can be used as a medicament, or in the preparation of a medicament for modulating cell cycle checkpoint/repair functions of a target cell, for the treatment of cancer and other proliferative diseases. Suitable biologically active antisense nucleic acids comprise modified nucleotide bases or the like for improving the stabilization of such nucleic acids or resistance to nucleases, such as (2'-O-(2-methoxy)ethyl (2'-MOE) modification of oligonucleotides (McKay et al., 1999, "Characterization of a potent and specific class of antisense oligonucleotide inhibitors of human PKC-alpha expression" J. Biol. Chem. 274:1715-1722). Preferred antisense nucleic acid molecules are at least 10 residues in length, preferably 20 residues in length, and are directed to a portion of the gene transcript that will result in the inhibition of translation of a functional protein from the gene transcript. The present invention also advantageously provides for nucleotide sequences of at least approximately 15 nucleotides which are complementary to a contiguous portion of a nucleic acid according to the invention. These complementary sequences can be used as probes or primers to initiate replication, to detect the presence of nucleic acids having the nucleotide sequence of the invention, or to specifically amplify segments of the desired nucleic acid from a sample. Such complementary nucleotide sequences can be produced according to techniques well known in the art, such as by recombinant or synthetic means. The prepared primers, properly coordinated to specifically amplify a portion of a target nucleic acid in a sample may be used in diagnostic kits, or the like, for detecting the presence of a nucleic acid according to the invention. These tests generally comprise contacting the probe with the sample under hybridizing conditions and detecting for the presence of any duplex or triplex formation between the probe and any nucleic acid in the sample. Advantageously, the nucleotide sequences embodying the invention can be produced using such recombinant or synthetic means, such as for example using PCR cloning mechanisms which generally involve making a pair of primers, which may be from approximately 15 to 50 nucleotides to a region of the gene which is desired to be cloned, bringing the primers into contact with mRNA, cDNA, or genomic DNA from a human cell, performing a polymerase chain reaction under conditions which bring about amplification of the desired region (and where necessary first performing a reverse transcription step), isolating the amplified region or fragment and recovering the amplified DNA. Advantageously, human allelic variants of the nucleic acid according to the invention can be obtained by for example, probing genomic DNA libraries from a range of individuals for example from different populations, and other genotyping techniques. Furthermore, nucleic acids and probes according to the invention may be used to sequence genomic DNA from patients, using techniques well known in the art, for example, the Sanger dideoxy chain termination method, which can advantageously ascertain any predisposition of a patient to certain proliferative disorders. Specific modification of codons used in the nucleic acids corresponding to SEQ ID NO.: 1, 3, 7, and 9 can be such that the modified nucleic acids utilize codons preferred by the target host cell, while still encoding for the Hmusδl protein. Similarly, the present invention encompasses specific modification of codons used in the nucleic acids corresponding to SEQ LD NO.: 11, 13, 15, and 17 such that the modified nucleic acids utilize codons preferred by the target host cell, while still encoding for the Mmus81 protein. The present invention also encompasses modified nucleic acids which incorporate, for example, internucleotide linkage modification, base modifications, sugar modification, nonradioactive labels, nucleic acid cross- linking, and altered backbones including PNAs (polypeptide nucleic acids), as well as codon substitutions to reduce the number of less preferred codons and/or an increase in the number of preferred codons used by the target host cell (see Zhang et al., 1991, "Graphic analysis of codon usage strategy in 1490 human proteins" Gene 105(1):61- 72; Zhang et al., 1993, "Low-usage codons in Escherichia coli, yeast, fruit fly and primates" J. Protein Chemistry 12(3):329-335). According to one aspect of the present invention, there is provided a nucleic acid encoding Hmus81 protein having the amino acid residue sequence as illustrated as SEQ ID NO.: 2, 4, 8, or 10, or encoding a functionally equivalent fragment, or bioprecursor of said protein. According to another aspect of the present invention, there is provided a nucleic acid encoding Mmus81 protein having the amino acid residue sequence as illustrated as SEQ ID NO.: 12, 14, 16, or 18, or encoding a functionally equivalent fragment, or bioprecursor of said protein. Preferably, the nucleic acid is a DNA molecule such as a genomic DNA molecule, and even more preferably a cDNA molecule. However, it may also be RNA. As is well known to those skilled in the art, due to the degeneracy of the triplet codon genetic code, the present nucleotide sequences can include substitutions therein yet which still encode the same amino acid residue sequence. The nucleotide sequences defined herein are capable of hybridizing under low stringency conditions to nucleotide sequences derived from a nucleic acid of the invention, to identify homologs therefrom or alternatively to identify nucleotide sequences from other species. The present nucleic acids can be incorporated into an expression vector and subsequently used to transform, transfect or infect a suitable host cell. In such an expression vector the nucleic acid according to the invention is operably linked to a control sequence, such as a suitable promoter or the like, ensuring expression of the proteins according to the invention in a suitable host cell. The expression vector can be a plasmid, cosmid, virus or other suitable vector. The expression vector and the host cell transfected, transformed or infected with the vector also form part of the present invention. Preferably, the host cell is a eukaryotic cell or a bacterial cell and even more preferably a mammalian cell or insect cell. Mammalian host cells are particularly advantageous because they provide the necessary post-translational modifications to the expressed proteins according to the invention, such as glycosylation or the like, which modifications confer optimal biological activity of said proteins, which when isolated can advantageously be used in diagnostic kits or the like. The recombinant vectors of the invention generally comprise a Hmusδl gene or Mmus81 operatively positioned downstream from a promoter. The promoter is capable of directing expression of the human Mus81 or murine Mus81 encoding nucleic acid in a mammalian, e.g. human cell. Such promoters are thus "operative" in mammalian cells, e.g. human cells. Expression vectors and plasmids embodying the present invention comprise one or more constitutive promoters, such as viral promoters or promoters from mammalian genes that are generally active in promoting transcription. Examples of constitutive viral promoters include the HSV, TK, RSV, SV40 and CMV promoters, of which the CMV promoter is a currently preferred example. Examples of constitutive mammalian promoters include various housekeeping gene promoters, as exemplified by the β-actin promoter. Inducible promoters and/or regulatory elements are also contemplated for use with the expression vectors of the invention. Examples of suitable inducible promoters include promoters from genes such as cytochrome P450 genes, heat shock protein genes, metallothionein genes, hormone-inducible genes, such as the estrogen gene promoter, and such like. Promoters that are activated in response to exposure to ionizing radiation, such asfos un and erg-1, are also contemplated. The tetVP16 promoter that is responsive to tetracycline is a currently preferred example. Tissue-specific promoters and/or regulatory elements will be useful in certain embodiments. Examples of such promoters that can be used with the expression vectors of the invention include promoters from the liver fatty acid binding (FAB) protein gene, specific for colon epithelial cells; the insulin gene, specific for pancreatic cells; the transphyretin, . alpha.1-antitrypsin, plasminogen activator inhibitor type 1 (PAI-1), apolipoprotein Al and LDL receptor genes, specific for liver cells; the myelin basic protein (MBP) gene, specific for oligodendrocytes; the glial fibrillary acidic protein (GFAP) gene, specific for glial cells; OPSIN, specific for targeting to the eye; and the neural-specific enolase (NSE) promoter that is specific for nerve cells. The construction and use of expression vectors and plasmids is well known to those of skill in the art. Virtually any mammalian cell expression vector can thus be used in connection with the genes disclosed herein. Preferred vectors and plasmids are constructed with at least one multiple cloning site. In certain embodiments, the expression vector will comprise a multiple cloning site that is operatively positioned between a promoter and a human Mus81 or murine Mus81 encoding gene sequence. Such vectors can be used, in addition to uses in other embodiments, to create N-terminal or C-terminal fusion proteins by cloning a second protein-encoding DNA segment into the multiple cloning site so that it is contiguous and in-frame with the mammalian Mus81 encoding nucleotide sequence. In other embodiments, expression vectors comprise a multiple cloning site that is operatively positioned downstream from the expressible human Mus81 or murine Mus81 encoding sequence. These vectors are useful, in addition to their uses, in creating C-terminal fusion proteins by cloning a second protein-encoding DNA segment into the multiple cloning site so that it is contiguous and in-frame with the human Musδl or murine Mus81 encoding sequence. Vectors and plasmids in which additional protein- or RNA-encoding nucleic acid segment(s) is(are) also present are, of course, also encompassed by the invention, irrespective of the nature of the nucleic acid segment itself. A second reporter gene can be included within an expression vector of the present invention. The second reporter gene can be comprised within a second transcriptional unit. Suitable second reporter genes include those that confer resistance to agents such as neomycin, hygromycin, puromycin, zeocin, mycophenolic acid, histidinol and methotrexate. Expression vectors can also contain other nucleotide sequences, such as LRES elements, polyadenylation signals, splice donor/splice acceptor signals, and the like. Particular examples of suitable expression vectors are those adapted for expression using a recombinant adenoviral, recombinant adeno-associated viral (AAV) or recombinant retroviral system. Vaccinia virus, herpes simplex virus, cytomegalovirus, and defective hepatitis B viruses, amongst others, can also be used. In one specific embodiment, the present invention encompasses isolated nucleic acids which encode for novel mammalian checkpoint/repair proteins. In another specific embodiment, the invention encompasses novel mammalian checkpoint/repair proteins derived from nucleic acids isolated from a human source called Hmus81 (human Mus81), and from a murine source called Mmusδl (murine Mus81). Further provided by the present invention are isolated proteins having an amino acid residue sequence corresponding to that illustrated as SEQ ID NO.: 2, 4, 8 or 10, or the amino acid sequence of a functionally equivalent fusion protein product, fragment or bioprecursor of said protein. Also provided by the present invention are isolated proteins having an amino acid sequence corresponding to that illustrated as SEQ ID NO.: 12, 14 16 or 18, or the amino acid residue sequence of a functionally equivalent, fusion protein product, biologically active fragment or bioprecursor of said protein. Also envisioned is the use of such protein for the generation of antibodies, monoclonal or polyclonal capable of specifically binding to the amino acid sequences of these proteins or fragments thereof. As is well known to those of skill in the art, the proteins according to the invention can comprise conservative or semi-conservative substitutions, deletions or insertions wherein the protein comprises different amino acids than those disclosed in FIG. 1 and FIG. 2. A protein of the invention can be in a substantially purified form, in which case it will generally comprise the polypeptide in a preparation in which more than 90%, e.g. 95%, 98%, or 99% of the polypeptide in the preparation is a polypeptide of the invention. Protems of the invention can be modified, for example by the addition of histidine residues to assist their purification or by the addition of a signal sequence to promote their secretion from a cell. Proteins having at least 90% sequence identity, for example at least 95%, 98% or 99% sequence identity to the polypeptide protein depicted in SEQ ID NO.: 2 , 4, 8 or 10 may be proteins which are amino acid sequence variants, alleles, derivatives, or mutants of the protein depicted in SEQ ID NO.: 2, 4, 8 or 10, and are also provided by the present invention. Similarly, proteins having at least 90% sequence identity, for example at least 95%, 98% or 99% sequence identity to the polypeptide protein depicted in SEQ ID NO.: 12 , 14, 16 or 18 can be proteins which are amino acid sequence variants, alleles, derivatives, or mutants of the protein depicted in SEQ ID NO.: 12, 14, 16 or 18, and are also provided by the present invention. The percentage identity of protein amino acid residue sequences can be calculated by using commercially available algorithms which compare a reference sequence (i.e. SEQ ID NO.: 2, 4, 8, 10, 12, 14, 16, 18) with a query sequence. The following programs (provided by the National Center for Biotechnology Information, NCBI) may be used to determine homologies: BLAST, gapped BLAST, BLASTN and psi-BLAST, which may be used with default parameters. Use of either of the terms "homology" or "homologous" herein does not imply any necessary evolutionary relationship between compared sequences, in keeping with standard use of such terms as "homologous recombination" which merely requires that two nucleotide sequence are sufficiently similar to recombine under the appropriate conditions. Another method for determining the best overall match between a nucleotide sequence or portion thereof, and a query sequence is the use of the FASTDB computer program based on the algorithm of Brutlag et al., (1990, "Improved sensitivity of biological sequence database searches" Compt Appl. Biosci., 6:237-245). The program provides a global sequence alignment. The result of such a global sequence alignment is expressed as percent identity. Suitable parameters used in a FASTDB search of a nucleotide sequence to calculate the degree of identity are known. Where a query sequence is determined to have an identity to that of SEQ LD NO.: 2, 4, 8 or 10 of at least 90%, said sequence being that of a protein retaining the same activity as Hmus81 , such a sequence is encompassed by the present invention. Similarly, where a query sequence is determined to have an identity to that of SEQ ID NO.: 12, 14, 16 or 18 of at least 90%, said sequence being that of a protein retaining the same activity as Mmus81, such a sequence is encompassed by the present invention. Preferred fragments include those comprising an epitope of the proteins according to the invention. The epitopes can be determined using, for example, peptide scanning techniques as described in the art (see e.g. Geysen et. al, 1986, "A priori determination of a peptide which mimics a discontinuous antigenic determinant" Mol. Immunol., 23; 709-715). The polyclonal and monoclonal antibodies according to the invention can be produced according to techniques which are known to those skilled in the art (e.g. Immunochemical Protocols. 2nd. edition, Pound, J.D. ed., 1998, Methods in Molecular Biology Vol. 80, Humana Press, Totowa, N.J.). For example, polyclonal antibodies can be generated by inoculating a host animal, such as a mouse, rabbit, goat, pig, cow, horse, hamster, rat or the like, with a protein or epitope according to the invention and recovering the immune serum. The present invention also includes fragments of whole antibodies which maintain their binding activity, such as for example, Fv, F(ab') and F(ab') 2 fragments as well as single chain antibodies. The nucleic acid and/or the proteins according to the invention can be included in a pharmaceutical composition together with a pharmaceutically acceptable carrier, diluent or excipient therefor. The pharmaceutical composition containing said nucleic acids according to the invention can, for example, be used in gene therapy. Such nucleic acids, according to the invention, can be administered naked, or packaged in protein capsules, lipid capsules, liposomes, membrane based capsules, virus protein, whole viras, cell vectors, bacterial cell hosts, altered mammalian cell hosts, or such suitable means for administration. There is further provided by the present invention a method for detecting for the presence or absence of a nucleic acid according to the invention, in a biological sample, which method comprises, (a) bringing said sample into contact with a probe comprising a nucleic acid or probe according to the invention under hybridizing conditions, and (b) detecting for the presence of hybridization, for example, by the presence of any duplex or triplex formation between said probe and any nucleic acid present in said sample. Proteins according to the invention can also be detected by (a) contacting said sample with an antibody to an epitope of a protein according to the invention under conditions which allow for the formation of an antibody-antigen complex, (b) monitoring for the presence of any antigen-antibody complex. Kits for detecting nucleic acids and proteins are also provided by the present invention. A kit for detecting for the presence of a nucleic acid according to the invention in a biological sample can comprise (a) means for contacting the sample with a probe comprising a nucleic acid or a probe according to the invention and means for detecting for the presence of any duplex or triplex formation between said probe and any nucleic acid present in the sample. Likewise, a kit for detecting for the presence of a protein according to the invention in a biological sample can comprise (a) means for contacting said sample with an antibody to an epitope of a protein according to the invention under conditions which allow for the formation of an antibody - protein complex, and (b) means for monitoring said sample for the presence of any protein - antibody complex. A further aspect of the present invention provides a method of determining whether a compound is an inhibitor or an activator of expression or activity of the proteins of the mammalian cell cycle checkpoint/repair pathway. The method comprises contacting a cell expressing the proteins in said pathway with said compound and comparing the level of expression of any of the proteins of the checkpoint/repair pathway of said cell against a cell which has not been contacted with said compound. Any compounds identified can then advantageously be used as a medicament or in the preparation of a medicament for treating cancer or proliferative disorders. Alternatively, the compounds can be included in a pharmaceutical composition together with a pharmaceutically acceptable carrier, diluent or excipient therefor. Any compound identified as, or any compound corresponding to a compound identified as an inhibitor of the cell checkpoint/repair pathway can be included in a pharmaceutical composition according to the invention together with a cytotoxic agent, such as a DNA damaging chemotherapeutic agent, and a pharmaceutically acceptable carrier diluent or excipient therefor. Thus, the cell cycle checkpoint/repair inhibitor can enhance the chemotherapeutic effect of cytotoxic agents used in, for example, anti-cancer therapy. There is also provided by the present invention a method for screening candidate substances for anti-cancer therapy, which method comprises (a) providing a protein according to the present invention exhibiting kinase activity together with a substrate for said protein under conditions such that the kinase will act upon the substrate, (b) bringing the protein and substrate into contact with a candidate substance, (c) measuring the degree of any increase or decrease in the kinase activity of the protein, (d) selecting a candidate substance which provides a decrease or increase in activity. Such a candidate substance can also be used as a medicament, or in the preparation of a medicament for the treatment of cancer or other such proliferative cell disorders. The present invention thus provides inter alia, for therapeutic compositions comprising (i) Hmus81 protein, fusion protein product, or biologically active fragments thereof, (ii) nucleic acids encoding for Hmus81 protein, fusion protein or fragments thereof, (iii) expression vector constructs having an expressible nucleic acid encoding for Hmus81 protein, fusion protein, or fragments thereof, (iv) anti-sense nucleic acids which correspond to the complement of nucleic acids encoding for Hmus81 protein, (v) modified Hmus81 proteins, (vi) antibodies that specifically bind to a portion of an Hmusδl protein, (vii) transformed host cells capable of expressing Hmus81 protein, fusion protein, or fragments thereof, and (viii) therapeutic agents identified by screening for the ability to bind to and/or affect the activity of Hmus81 protein. The present invention also provides for therapeutic compositions comprising (i) Mmusδl protein, fusion protein product, or biologically active fragments thereof, (ii) nucleic acids encoding for Mmus81 protein, fusion protein or fragments thereof, (iii) expression vector constracts having an expressible nucleic acid encoding for Mmus81 protein, fusion protein, or fragments thereof, (iv) anti-sense nucleic acids which correspond to the complement of nucleic acids encoding for Mmusδl protein, (v) modified Mmusδl proteins, (vi) antibodies that specifically bind to a portion of an Mmus81 protein, (vii) transformed host cells capable of expressing Mmusδl protein, fusion protein, or fragments thereof, and (viii) therapeutic agents identified by screening for the ability to bind to and/or affect the activity of Mmus81 protein. Therapeutic compositions of the present invention can combine mixtures of two or more species of Mus81 protein, nucleic acid encoding such protein, antibodies to such protein, or inhibitors of the nucleic acid transcripts of such proteins. A therapeutic composition of the present invention can be utilized to make a pharmaceutical preparation for the treatment of an individual in need of modulation of the DNA checkpoint/repair mediated by the activity of Hmus81. Another aspect of the present invention is the use of a therapeutic composition of the present invention in the formulation of a pharmaceutical preparation for the treatment of an individual in need of anti-neoplastic treatment. It is further envisioned that a therapeutic composition of the present invention is useful in the formulation of a pharmaceutical preparation in combination with at least one other anti-neoplastic agent for the treatment of an individual in need of anti-neoplastic treatment. Therapeutic compositions, or pharmaceutical formulations containing such therapeutic compositions, can be used to treat an individual in need of a treatment which involves the Hmus81 mediated activity of targeted cells. Illustrative are treatment for neoplastic conditions, comprising contacting a cell of the individual in need of such treatment with at least one therapeutic composition of the invention. Such therapeutic methods can include the administration of one or more therapeutic composition sequentially, simultaneously, or in combination with other therapeutics for treating a neoplastic condition. As would be understood by one of skill in the art, many variations and equivalents to the compositions of the present invention are easily obtained and generated through the application of routine methods known in the art using the teaching of the present invention. Many of the methods and materials for carrying out the basic molecular biology manipulations as described in the examples below are known in the art, and can be found in such references as Sambrook et al., Molecular Cloning, 2nd edition, Cold Spring Harbor Laboratory Press (1989); Berger et al., Guide to Molecular Cloning Techniques, Methods in Enzymology, Vol. 152, Academic Press, Inc., (1987); Davis et al., Basic Methods in Molecular Biology, Elsevier Science Publishing Co., Inc. (1986); Ausubel et al., Short Protocols in Molecular Biologv, 2nd ed., John Wiley & Sons, (1992); Goeddel Gene Expression Technology, Methods in Enzymology, Vol. 185, Academic Press, Inc., (1991); Guthrie et al., Guide to Yeast Genetics and Molecular Biology, Methods in Enzymology, Vol. 194, Academic Press, Inc., (1991); McPherson et al., PCR Volume 1. Oxford University Press, (1991); McPherson et al., PCR Volume 2, Oxford University Press, (1995); Richardson, CD. ed., Baculoviras Expression Protocols, Methods in Molecular Biology, Vol. 39, Humana Press, Inc. (1995). The invention in its several aspects can be more readily understood with reference to the following examples. Example 1 Human Mus81 (Hmus 1) Cloning Oligonucleotide primers HmusδlFW (GACATGGCGGCCCCGGTCCG) (SEQ LD NO.: 21) and Hmus81REV (GACTCAGGTCAAGGGGCCGTAG) (SEQ ID NO.: 22) corresponding to the 5' (ATGGCGGCCCCGGTCCG) (SEQ LD NO.: 19) and 3' (CTACGGCCCCTTGACCTGA) (SEQ ID NO.: 20) ends of the putative human Mus81 ORF were used to amplify DNA products from a Marathon-Ready human cerebellum cDNA library (Clontech, Palo Alto CA) by polymerase chain reaction (PCR). PCR was done with Pfu polymerase and the following reaction conditions: 95°C for 30", 68°C for 30", 72°C for 1 - 30" (35x). The resulting DNA products were cloned into the pCR2.1-TOPO plasmid as recommended by the manufacturer (Invitrogen, Carlsbad CA) and the DNA sequenced. Oligonucleotide primers corresponding to the 5' and 3' ends of Hmus81, from a putative ORF constracted using the identified yeast sequences were used to amplify sequences from a human cerebellum cDNA library. A 1653 nucleotide sequence was obtained which encodes a 551 amino acid protein (SEQ LD NO.: 2) with significant similarity to the yeast Mus81 sequences (SEQ ID NO.: 5). A longer 1857 nucleotide sequence encodes for a shorter variant of Hmus81 that is a 455 amino acid protein (SEQ LD NO.:4). This results from the presence of a stop codon within a DNA insert from position 1274 to 1474 of the nucleotide sequence (SEQ LD NO.: 25). Example 2 Mouse Mus81 Cloning Oligonucleotide primers RJH030 (GAGACTCTGAAGGAGCCAG) (SEQ ID NO.: 23) and RJH031 (GCTAAAAGGCTAGCCAGCC) (SEQ ID NO.: 24) corresponding to sequences flanking the 5' and 3' ends of the putative mouse Mus81 ORF were used to amplify DNA products from a Marathon-Ready mouse brain cDNA library (Clontech) by PCR. The following conditions were used: 95°C for 60", 60°C for 60", 72 °C for 2 '30" (35x). The resulting PCR products were cloned into the pCR2.1-TOPO plasmid (Invitrogen) and the DNA sequenced. The human cDNA sequences were used to search for homologous mouse sequences in the public databases. Several ESTs with significant homology to the 5' and 3' ends of the human sequence were identified. This resulted in the amplification of several sequences (probably representing splicing variants) encoding proteins from 424 to 551 amino acids (FIG. 2). The translation products of the human and mouse cDNAs have significant similarity to the yeast Mus81 amino acid sequences. The longest human (Hmus81 4 ) and mouse (MmusSl^ translation products are 17-20% identical and 30-40%) similar to the yeast proteins. No other mammalian proteins had high similarity with the yeast proteins indicating that this had identified the closest homologues. The mouse sequence is 81% identical and 87% similar to the human protein. Alignment of the mammalian and yeast proteins demonstrates that there is similarity throughout, with more highly conserved regions in the central and C-terminal regions of the proteins (FIG. 3). The conserved central region is found in the XPF family of endonucleases and corresponds to the catalytic site (Aravind et al., "Conserved domains in DNA repair protein and evolution of repair systems" Nucleic Acids Res 27(5): 1223-1242, 1999). Example 3 Northern Blot Hybridisation Human multiple tissue and cancer cell line blots (Clontech) were hybridized with a 1.7 kb probe corresponding to human Mus81 cDNA using the QuickHyb method as described by the manufacturer (Clontech). The blots were washed at high stringency (0.1XSSC, 0.1% SDS, 50°C, 2 x 20 min) and signals were detected by autoradiography . Northern blot analysis using the Hmus81 cDNA as probe demonstrated that specific transcripts of approximately 2.5-3.0 kb were present in most human tissues with lower levels in lung, liver and kidney (FIG. 6). Example 4 Identification of a Cdsl FHA domain-binding protein A yeast two-hybrid screen was employed using the S. pombe Cdsl FHA domain as bait and a S. pombe cDNA library as prey. Transformants that grew in the selection conditions for interaction between the bait and prey proteins were isolated and tested in secondary screens for specificity of interaction. One of the transformants that was isolated from this screen contained a cDNA sequence that encoded a 572 amino acid hypothetical protein (PID g2213548). The amino acid sequences encoded by the S. pombe ORF SPCC4G3.05c (Spmus81) (SEQ ID NO: 6) and S. cerevisiae ORF YDR386W (ScMus81) (SEQ ID NO: 5) were compared and alignment of the translation products of the yeast and human sequences for amino acid sequence comparison was performed with the program CLUSTALW. The translation product of this S. pombe ORF (mus81 + ) was found to have significant homology to the S. cerevisiae hypothetical protein encoded by ORF YDR386w (25% identity, 42% similarity). This protein has been annotated as Mus81 in the Saccharomyces Genome Database and is reported to be in a complex with the DNA repair protein Rad54. A null mutant is reported to be viable, but defective in meiosis and sensitive to the DNA damaging agents MMS and UV light. The genomic copy of S. pombe mus81 + was tagged at the 3' end with three tandem copies of the haemoinfluenza HA epitope through site-directed recombination. Antibodies directed against the HA epitope detected polypeptides from an asynchronous culture which migrated through SDS-PAGE with a mobility of approximately 65-70 kDa. The calculated predicted molecular weight being about 65 kDa. The presence of multiple polypeptides demonstrates that the protein can be post-translationally modified, possibly by phosphorylation. The proportion of slower migrating polypeptides in asynchronous cultures was increased by treatment of the cells with hydroxyurea, a ribonucleotide reductase inhibitor that causes a cell cycle arrest in S-phase. This shows that the post-translational modification is cell cycle regulated, and may be checkpoint dependent. The increased modification of Mus81 was not observed in Cds 1 and Rad3 checkpoint mutant strains, but did occur in a rad54 mutant strain. The physical interaction between Cdsl and Mus81 was confirmed in vivo by co-immunoprecipitation of the two proteins. Inactivation of Mus81 makes fission yeast more sensitive to UV irradiation. This is also observed in yeast strains that are defective for the two repair pathways that account for all detectable repair of UV induced damage (nucleotide excision repair and UV excision repair). This suggests that Musδl is required for tolerating UV damage. In order to determine whether the product encoded by this gene is involved in checkpoint/repair responses, a S. pombe strain was generated in which the entire ORF for mus81 was deleted by site-directed recombination. This mutant strain had increased sensitivity to UV irradiation, but appeared to have an intact checkpoint/repair response in the presence of DNA damage. Example 5 Interaction between human Mus81 and Cdsl The Hmus81 x ORF was cloned into the mammalian transient expression vector pYCIHA (Fu et al., "TNLK, a novel member of the germinal center kinase family that activates the c-Jun N-terminal kinase pathway and regulates the cytoskeleton" J. Biol. Chem. 274(43):30729-30737, 1999) immediately downstream of and in frame with the HA epitope tag. Similarly, the human Cdsl ORF was cloned into the pYClFLAG (Fu et al., supra 1999) expression vector downstream of and in frame with the FLAG epitope. Plasmid DNA was used to transfect HEK293 cells using Superfect reagent as described by the manufacturer (Qiagen). After 24 hours, the cells were collected in lysis buffer (1% NP40, 50 mM TrisHCl pH 7.5, 150 mM NaCl, lmM DTT) supplemented with Pefabloc ® SC and Complete ™ protease inhibitors as recommended by the manufacturer (Boehringer Mannheim). The lysates were cleared of debris by centrifugation at lOOOOg for 15 min. (4"C). Cleared supernatants from cells transiently expressing epitope-tagged proteins were incubated several hours at 4°C with agarose bead-linked antibodies directed against the HA (Santa Cruz Biotechnologies) or FLAG (OctA-probe™, Santa Cruz Biotechnologies) epitope. The agarose beads were then washed 3 times with lysis buffer, resuspended in SDS denaturing buffer and incubated at 95°C for 5 min. Supernatants and immunoprecipitates were resolved by SDS-PAGE and transferred to PVDF membranes. The membranes were blocked with TTBS (150 mM NaCl, 100 mM Tris-HCl pH7.5, 0.1% Tween 20) containing 5% skimmed milk. For detection of HA-tagged Mus81 protein, the blots were incubated for two hours at room temperature with horseradish peroxidase conjugated anti-HA antibodies (Santa Cruz Biotechnologies) diluted to 1: 1000 in TTBS containing 0.1 % milk. For detection of FLAG-tagged Cdsl, blots were incubated with anti-FLAG ® M2 antibody (Sigma) diluted to 1:3000 in TTBS. The blot was washed with TTBS and then incubated for 1 hour at room temperature with horseradish peroxidase conjugated anti-mouse Ig antibody diluted to 1 : 3000 in TTBS. Finally, the blots were washed with TTBS and signals detected using the ECL-Plus chemoluminescence detection system as described by the manufacturer (Amersham Pharmacia Biotech). In order to determine whether human Mus81 and Cds 1 are capable of interacting, the proteins were tagged with the HA and FLAG epitopes, respectively, and expressed transiently in mammalian cells alone or in combination. Cell lysates were prepared from the transfected cells and immunoprecipitations were carried out with antibodies against the epitope tags. The resulting immunoprecipitates were subjected to western blot analysis with the reciprocal antibody. The HA antibodies recognized a 65 kDa protein, the expected size for the tagged version of human Mus81 , only in lysates from cells transfected with the Mus81 construct. Similarly, the FLAG antibodies recognized a 65 kDa protein corresponding to tagged Cdsl only in lysates from cells expressing Cdsl-FLAG (FIG. 8). Mus81 was also detected in precipitates obtained with the FLAG antibody from lysates of cells transfected with both Cdsl and Mus81. However, Mus81 was not present in precipitates from cells expressing only Mus81 or Cdsl. Conversely, Cdsl was only detected in precipitates obtained with the HA antibody from cells expressing both tagged proteins. These results indicate that the human Mus81 and Cdsl proteins are capable of interacting in mammalian cells. This suggests that the Hmus81 protein is involved in UV DNA damage repair. The present invention identifies the human and mouse homologues of the yeast Mus81 protein, which are involved in UV damage tolerance and interacts with the FHA domain of fission yeast Cds 1. Human Mus81 is present as various splicing isoforms and is expressed in most human tissues and cancer cell lines. Analysis of a Mus81-GFP fusion protein suggests that it is predominantly nuclear while co-immunoprecipitation of tagged forms of human Mus81 and Cdsl indicate that they form a complex in mammalian cells. Example 6 Genomic structure and chromosomal localization of human Mus81 The human cDNAs were used to identify contiguous genomic sequences containing Mus81 in the public databases. Comparison of the genomic sequence confirmed that the various cDNA forms corresponded to different splice variants. Examination of the results identified 18 exons encoding Mus81 sequences within a 5.8 kb genomic region (FIG. 4). The splicing differences in the cDNAs identified occurred in the region encompassing exons 13 and 14. The nucleic acid encoding for human Mus81 2 (SEQ LD NO.:3) was composed of all of the exons identified. The nucleic acid encoding for human Musδl ! (SEQ ID NO.: 1) did not contain exon 13 and the nucleic acid encoding for human Mus81 3 (SEQ LD NO.: 7) was lacking exons 13 and 14. Splicing of the nucleic acid encoding for human Mus81 4 (SEQ ID NO.: 9) was identical to that found in the nucleic acid encoding for human Musβ^ (SEQ ID NO.: 1) except that it contained three additional nucleotides (CAG) at the 5'end of exon 14 due to utilization of an alternative splice acceptor site. Splicing of all introns utilized the consensus donor and acceptor sites. Fluorescence in situ Hybridisation (FISH) analysis was carried out using standard procedures. Briefly, human lymphocytes isolated from blood were synchronized by culturing in the presence of 0.18 mg/ml BrdU. The BrdU was washed off to release the block and the cells were cultured for 6 hours prior to harvesting and fixation. FISH detection was carried out with a Mus81 cDNA probe labelled with biotinylated dATP. Chromosomal localization was determined by comparison of FISH signals to DAPI banding pattern. FISH analysis using human Mus81 cDNA as a probe resulted in staining of a single pair of chromosomes at 1 lql3 in 70 out of 100 mitotic spreads (FIG. 5). This localization was confirmed by the previous assignment of a public EST (WI- 18484), which is identical to part of the Mus81 sequence, to chromosome 11 on the WICGR radiation hybrid map. Example 7 Expression and intracellular localization of human Mus81 The human Mus81 4 cDNA was cloned downstream and in frame with the green fluorescent protein (GFP) encoding open reading frame gene (ORF) in a retroviras expression vector. The retroviras expression vector is chosen to allow for the regulated expression of proteins of interest, and in a preferred embodiment allows fusion of the protein of interest to the GFP or modified GFP for visualization of expression. It is also possible to express both the Mus81 protein and GFP protein as separate proteins from the same expression vector. Commercially available vectors suitable for expression of Mus81 protein include and are not limited to, for example, pRevTRE (Clontech) which are derived from the pLNCX (Clontech) retroviral expression vector (Gossen, M. & Bujard, H., 1992, "Tight control of gene expression in mammalian cells by tetracycline-responsive promoters" PNAS(USA) 89:5547-5551), or GFP fusion protein expressing retroviral expression vectors pLEGFP-Nl andpLEGFP-Cl (Clontech). The Human Mus81-GFP expressing retroviras vector was used to infect A549 lung carcinoma cells containing an integrated copy of the tTA trans activator for regulated expression of the fusion protein. The cells were grown to allow expression of the fusion protein, and visualized by fluorescence microscopy three days after infection. Human Mus81 was expressed as a fusion with the GFP protein in A549 cells. Fluorescence was detected primarily in the nuclei of these cells (FIG. 7). The nuclear localization of Hmus81 is in agreement with its role in DNA repair associated functions. The invention, having been fully described in many of its aspects and claimed herein can be made and executed without undue experimentation by one of skill in the art according to the teaching herein. While the compositions and methods of this invention have been described by way of example above, it will be apparent to those of skill in the art that many variations and modifications can be applied to the compositions and methods described herein without departing from the concept, spirit and scope of the invention.

Claims: We Claim: 1 An isolated nucleic acid encoding for a mammalian Mus81 protein. 2. A nucleic acid of claim 1 encoding for a human Mus81 protein or a biologically active portion thereof. 3. A nucleic acid of claim 2, wherein said nucleic acid encodes for a human Mus81 protein having the amino acid sequence depicted by SEQ ID NO.:2. 4. A nucleic acid of claim 2, wherein said nucleic acid encodes for a human Musδl protein having the amino acid sequence depicted by SEQ ID NO.:4. 5. A nucleic acid of claim 2, wherein said nucleic acid encodes for a human Mus81 protein having the amino acid sequence depicted by SEQ ID NO.:8. 6. A nucleic acid of claim 2, wherein said nucleic acid encodes for a human Mus81 protein having the amino acid sequence depicted by SEQ ID NO.: 10. 7. A nucleic acid of claim 2 having a nucleotide sequence corresponding to that represented by nucleotides 23-1675 of the nucleotide sequence depicted in SEQ ID NO.: l. 8. A nucleic acid of claim 2 having a nucleotide sequence corresponding to that represented by nucleotides 185-1549 of the nucleotide sequence depicted in SEQ ID NO.:3. 9. A nucleic acid of claim 2 having a nucleotide sequence corresponding to that represented by nucleotides 26-1297 of the nucleotide sequence depicted in SEQ ID NO.:7. 10. A nucleic acid of claim 2 having a nucleotide sequence corresponding to that represented by nucleotides 26-1681 of the nucleotide sequence depicted in SEQ ID NO.:9. 11. An expression vector comprising a nucleic acid of claim 2. 12. A host cell transformed with a vector of claim 11. 13. An expression vector comprising a nucleic acid of claim 3. 14. A host cell transformed with a vector of claim 13. 15. An expression vector comprising a nucleic acid of claim 4. 16. A host cell transformed with a vector of claim 15. 17. An expression vector comprising a nucleic acid of claim 5. 18. A host cell transformed with a vector of claim 17. 19. An expression vector comprising a nucleic acid of Claim 6. 20. A host cell transformed with a vector of claim 19. 21. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 3. 22. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 4. 23. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 5. 24. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 6. 25. A biologically active analog of the antisense molecule of claim 21 , said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 26. A biologically active analog of the antisense molecule of claim 22, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 27. A biologically active analog of the antisense molecule of claim 23, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 28. A biologically active analog of the antisense molecule of claim 24, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 29. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 21. 30. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 22. 31. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 23. 32. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 24. 33. A pharmaceutical preparation for inhibiting human Mus81 protein expression or function in a cell which comprises an antisense nucleic acid analog capable of entering said cell and binding specifically to a nucleic acid molecule encoding for said human Mus81 protein, said antisense nucleic acid being present in a pharmaceutically acceptable carrier and having a nucleotide sequence complementary to at least a portion of a nucleic acid of claim 2. 34. An isolated mammalian Mus81 protein. 35. An isolated protein of claim 34 which is either a human Mus81 protein or murine Mus81 protein. 36. An isolated human Mus81 protein of claim 35 selected from the group of proteins having the amino acid residue sequence corresponding to that depicted by SEQ ID NO.: 2, SEQ ID NO : 4, SEQ ID NO.: 8, or SEQ ID NO.: 10. 37. A fusion protein comprising a protein of claim 35 or biologically active portion thereof. 38. A protein of claim 37 identifiable as Hmus81-GFP. 39. An antibody which specifically binds to a portion of a protein of Claim 36. 40. An antibody of claim 39 that is a polycolonal antibody. 41. An antibody of claim 39 that is a monoclonal antibody. 42. A method for identifying a compound as an inhibitor or activator of expression of the human Mus81 cell cycle checkpoint/repair pathway protein which method comprises contacting a cell expressing the protein in said pathway with said compound and comparing the level of expression of the human Mus81 human cell cycle checkpoint/repair pathway protein of said cell with that of a cell which has not been contacted with said compound. 43. A compound corresponding to one identified by the method of claim 42. 44. A compound identifiable by the method of claim 42, useful for treating cancer or other proliferative diseases. 45. The use of a compound of claim 43 in the preparation of a medicament for the treatment of cancer or proliferative diseases. 46. A pharmaceutical composition comprising a compound according to claim 43, together with a pharmaceutically acceptable carrier, diluent or excipient therefor. 47. A pharmaceutical composition comprising a compound of claim 43 together with a DNA damaging chemotherapeutic agent and a pharmaceutically acceptable carrier, diluent or excipient therefor. 48. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of a candidate substance identifiable by the method of claim 42. 49. The method of treating cancer or proliferative disease in a patient, comprising administering a compound of claim 43 , to a patient in need of treatment. 50. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 21. 51. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 22. 52. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 23. 53. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 24. 54. The method of treating cancer or proliferative disease in a patient, comprising administering a pharmaceutical preparation of claim 47, to a patient in need of treatment. 55. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an inhibitor compound that inhibits the activity of the human Mus81 protein. 56. A method of identifying a chemical compound that modulates Mus81 dependent cell cycle pathway, which method comprises administering a chemical compound to be tested to a host cell, detecting the level of human Mus81 protein or subsequent cell cycle protein in said cell, and comparing the detected level to that of a normal untreated cell. 57. A chemical compound corresponding to one identified by the method of claim 56. 58. A method of identifying a chemical compound that modulates Mus81 dependent cell cycle pathway, which method comprises administering a chemical compound to be tested to a host cell, detecting the level of human Mus81 protein encoding nucleic acid or subsequent cell cycle protein encoding nucleic acid in said cell, and comparing the detected level to that of a normal untreated cell. 59. A chemical compound corresponding to one identified by the method of claim 58. 60. A method of identifying a chemical compound that modulates Mus81 dependent cell cycle pathway, which method comprises administering a chemical compound to be tested to a biochemical mixture of human Mus81 protein and a suitable substrate, detecting the level of human Mus81 protein activity in said mixture, and comparing the detected level to that of a normal untreated biochemical mixture of human Mus81 protein. 61. A chemical compound corresponding to one identified by the method of claim 60. 62. A nucleic acid of claim 1 encoding for a murine Mus81 protein or biologically active fragment thereof. 63. A nucleic acid of claim 62, wherein said nucleic acid encodes for a murine Mus81 protein having the amino acid sequence depicted by SEQ LD NO.: 12. 64. A nucleic acid of claim 62, wherein said nucleic acid encodes for a murine Mus81 protein having the amino acid sequence depicted by SEQ LD NO.: 14. 65. A nucleic acid of claim 62, wherein said nucleic acid encodes for a murine Mus81 protein having the amino acid sequence depicted by SEQ ID NO.: 16. 66. A nucleic acid of claim 62, wherein said nucleic acid encodes for a murine Mus81 protein having the amino acid sequence depicted by SEQ LD NO.: 18. 67. A nucleic acid of claim 62 having a nucleotide sequence corresponding to that represented by nucleotides 42-1694 of the nucleotide sequence depicted in SEQ ID NO.: 11. 68. A nucleic acid of claim 62 having a nucleotide sequence corresponding to that represented by nucleotides 15-1323 of the nucleotide sequence depicted in SEQ ID NO.: 13. 69. A nucleic acid of claim 62 having a nucleotide sequence corresponding to that represented by nucleotides 52-1644 of the nucleotide sequence depicted in SEQ ID NO.: 15. 70. A nucleic acid of claim 62 comprising nucleic acid having a nucleotide sequence corresponding to that represented by nucleotides 52-1614 of the nucleotide sequence depicted in SEQ ID NO.: 17. 71. An expression vector comprising a nucleic acid of claim 62. 72. A host cell transformed with a vector of claim 71. 73. An expression vector comprising a nucleic acid of claim 63. 74. A host cell transformed with a vector of claim 73. 75. An expression vector comprising a nucleic acid of claim 64. 76. A host cell transformed with a vector of claim 75. 77. An expression vector comprising a nucleic acid of claim 65. 78. A host cell transformed with a vector of claim 77. 79. An expression vector comprising a nucleic acid of Claim 66. 80. A host cell transformed with a vector of claim 79. 81. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 63. 82. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 64. 83. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 65. 84. A biologically active antisense molecule comprising a nucleic acid that is the complement of at least a portion of the nucleic acid of claim 66. 85. A biologically active analog of the antisense molecule of claim 81 , said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 86. A biologically active analog of the antisense molecule of claim 82, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 87. A biologically active analog of the antisense molecule of claim 83, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 88. A biologically active analog of the antisense molecule of claim 84, said analog being selected from the group consisting of peptide nucleic acids, methylphosphonates and 2-O-methyl ribonucleic acids. 89. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 81. 90. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 82. 91. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 83. 92. A biologically active phosphorothioate analog of the antisense oligonucleotide of claim 84. 93. A pharmaceutical preparation for inhibiting Mus81 protein expression or function in a mammalian cell which comprises an antisense nucleic acid analog capable of entering said cell and binding specifically to a nucleic acid molecule encoding for said Mus81 protein, said antisense nucleic acid being present in a pharmaceutically acceptable carrier and having a nucleotide sequence complementary to at least a portion of the nucleic acid of claim 62. 94. An isolated murine Mus81 protein of claim 35 selected from the group of proteins having the amino acid residue sequences depicted by SEQ LD NO.: 12, SEQ ID NO.: 14, SEQ ID NO.: 16, or SEQ ID NO.: 18. 95. An antibody which specifically binds to a portion of a protein of Claim 94. 96. The antibody of claim 95 that is a polycolonal antibody. 97. The antibody of claim 95 that is a monoclonal antibody. 98. A method for identifying a compound as an inhibitor or activator of expression of mammalian Mus81 cell cycle checkpoint/repair pathway protein which method comprises, contacting a cell expressing the protein in said pathway with said compound, detecting the level of expression of the mammalian Mus81 cell cycle checkpoint/repair pathway protein in said cell, and comparing the detected level with that of a cell which has not been contacted with said compound. 99. A compound corresponding to a compound identified by the method of claim 98. 100. A compound identifiable by the method of claim 98, useful for treating cancer or other proliferative diseases. 101. The us e of a compound of claim 99 in the preparation of a medicament for the treatment of cancer or proliferative diseases. 102. A pharmaceutical composition comprising a compound according to claim 99, together with a pharmaceutically acceptable carrier, diluent or excipient therefor. 103. A pharmaceutical composition comprising a compound of claim 99 together with a DNA damaging chemotherapeutic agent and a pharmaceutically acceptable carrier, diluent or excipient therefor. 104. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of a candidate substance identifiable by the method of claim 98. 105. The method of treating cancer or proliferative disease in a patient, comprising administering a compound of claim 99, to a patient in need of treatment. 106. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 81. 107. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 82. 108. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 83. 109. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an antisense nucleic acid of claim 84. 110. The method of treating cancer or proliferative disease in a patient, comprising administering a pharmaceutical composition of claim 100, to a patient in need of treatment. 111. A method of increasing susceptibility of cancer cells to chemotherapy or radiotherapy, which method comprises administering to a patient a therapeutically effective amount of an inhibitor compound wherein said inhibitor inhibits the activity of the Mmus81 protein. 112. A method of identifying a chemical compound that modulates the Musδl dependent cell cycle pathway, which method comprises administering a chemical compound to be tested to a host cell, detecting the level of murine Mus81 protein or subsequent cell cycle protein in said cell, and comparing said detected level with that of a normal untreated cell. 113. A chemical compound corresponding to one identifiable by the method of claim 112. 114. A method of identifying a chemical compound that modulates the Mus81 dependent cell cycle pathway, which method comprises administering a chemical compound to be tested to a host cell, detecting the level of murine Mus81 protein encoding nucleic acid or subsequent cell cycle protein encoding nucleic acid in said cell, and comparing said detected level with that of a normal untreated cell. 115. A chemical compound corresponding to one identifiable by the method of claim 114. 116. A method of identifying a chemical compound that modulates the Mus81 dependent cell cycle checkpoint pathway, which method comprises administering the chemical compound to be tested to a biochemical mixture of murine Musδl protein and a suitable substrate, and detecting if the level of murine Mus81 protein activity in said mixture is altered from that of a normal untreated biochemical mixture of murine Mus81 protein. 117. A chemical compound identifiable by the method of claim 112.

Inventors: BODDY MICHAEL N, DENIS CECILE-MARIE D D, LOPEZ-GIRONA ANTONIA, RUSSELL PAUL R, SHANAHAN PAUL A, VIALARD JORGE E, BODDY, MICHAEL, N., DENIS, CECILE-MARIE, D., D., LOPEZ-GIRONA, ANTONIA, RUSSELL, PAUL, R., SHANAHAN, PAUL, A., VIALARD, JORGE, E.
Applicants: BODDY MICHAEL N, DENIS CECILE MARIE D D, JANSSEN PHARMACEUTICA NV, LOPEZ GIRONA ANTONIA, RUSSELL PAUL R, SCRIPPS RESEARCH INST, SHANAHAN PAUL A, VIALARD JORGE E, BODDY, MICHAEL, N., DENIS, CECILE-MARIE, D., D., JANSSEN PHARMACEUTICA N.V., LOPEZ-GIRONA, ANTONIA, RUSSELL, PAUL, R., SHANAHAN, PAUL, A., THE SCRIPPS RESEARCH INSTITUTE, VIALARD, JORGE, E.
Application year: 2000
Main classification: C12Q1
IPC codes: A61K31, A61K35, A61K38, A61K45, A61K48, A61P35, A61P43, C07K14, C07K16, C07K19, C12N1, C12N5, C12N9, C12N15, C12P21, C12Q1, G01N33