Double Strand Break Repair (1997)

This was the most unkindest cut of all…Then I,and you,and all of us fell down,Whilst bloody treason flourished over us(1).DNA double strand breaks (DSBs) 1The abbreviations used are: DSB, double strand break; DEB, DNA end binding; DNA-PK, DNA-dependent protein kinase; DNA-PKcs, catalytic subunit of DNA-dependent protein kinase; RAG1 andRAG2, recombination activating genes 1 and 2; TdT, terminal deoxynucleotidyltransferase; XRCC, x-ray cross-complementing; bp, base pair(s).1The abbreviations used are: DSB, double strand break; DEB, DNA end binding; DNA-PK, DNA-dependent protein kinase; DNA-PKcs, catalytic subunit of DNA-dependent protein kinase; RAG1 andRAG2, recombination activating genes 1 and 2; TdT, terminal deoxynucleotidyltransferase; XRCC, x-ray cross-complementing; bp, base pair(s). may be the most disruptive form of DNA damage. If left unrepaired, they lead to broken chromosomes and cell death. If repaired improperly, they can lead to chromosome translocations and cancer. Humans are at risk for DSBs from exogenous agents. The paradigm agent, ionizing radiation, is present in the environment mainly from the decay of radon gas, which accumulates in homes to different levels depending on the uranium content of the underlying soil. Ionizing radiation is also utilized in medicine for diagnostic x-rays and for treating cancer patients. Anticancer drugs will generate DSBs as well: bleomycin produces oxidative free radicals, which induce strand breaks; etoposide and adriamycin inhibit topoisomerase II to create protein-bridged DSBs. Humans are also at risk for DSBs from endogenous agents. Oxidative metabolism generates free radicals and subsequent strand breaks. V(D)J recombination generates DSBs during rearrangement of genes encoding B cell immunoglobulins and T cell receptors (2Smider V. Chu G. Semin. Immunol. 1997; 9: 189-197Crossref PubMed Scopus (53) Google Scholar). In response to the threat of DSBs, cells have evolved at least two independent pathways for repairing DSBs, by homologous recombination or by nonhomologous DNA end joining. This review will focus on recent progress in understanding the dominant mechanism in mammalian cells, nonhomologous end joining. Immunological diversity is generated by V(D)J recombination (Fig.1), a site-specific cleavage of the chromosome followed by an end-joining reaction to bring the free DNA ends together (2Smider V. Chu G. Semin. Immunol. 1997; 9: 189-197Crossref PubMed Scopus (53) Google Scholar). The possibility that DSB repair and V(D)J recombination might share the same biochemical pathway was first recognized in the severe combined immunodeficiency (scid) mouse. The scid mouse lacks mature B and T cells due to a defect in V(D)J recombination (3Lieber M.R. Hesse J.E. Lewis S. Bosma G.C. Rosenberg N. Mizuuchi K. Bosma M.J. Gellert M. Cell. 1988; 55: 7-16Abstract Full Text PDF PubMed Scopus (365) Google Scholar) and is also hypersensitive to ionizing radiation due to a defect in DSB repair (4Biedermann K. Sun J. Giaccia A. Tosto L. Brown J.M. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 1394-1397Crossref PubMed Scopus (448) Google Scholar, 5Fulop G. Phillips R. Nature. 1990; 347: 479-482Crossref PubMed Scopus (433) Google Scholar, 6Hendrickson E. Qin X.Q. Bump E. Schatz D. Oettinger M. Weaver D. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 4061-4065Crossref PubMed Scopus (283) Google Scholar). Screening of other x-ray-sensitive cell lines (TableI) led to the identification of three genetic complementation groups with defects in both DSB repair and V(D)J recombination (7Pergola F. Zdzienicka M.Z. Lieber M.R. Mol. Cell. Biol. 1993; 13: 3464-3471Crossref PubMed Scopus (173) Google Scholar, 8Taccioli G. Rathbun G. Oltz E. Stamato T. Jeggo P. Alt F. Science. 1993; 260: 207-210Crossref PubMed Scopus (417) Google Scholar). Since efforts were under way to complement these rodent cell lines with human genes, the genes for the complementation groups were designated XRCC, for x-ray cross-complementing. XRCC7 mutant cells (scidmouse cells and V3 hamster cells) were severely defective in coding joint formation but only mildly defective in signal joint formation. By contrast, XRCC4 and XRCC5 mutant cells were severely defective in both coding and signal joint formation.Table IGenetics of double strand break repairMutated geneV(D)J defectMutant cellsMutationXRCC4Coding/signalXR-1Gene deletion (39Li Z. Otevrel T. Gao Y. Cheng H.-L. Seed B. Stamato T.D. Taccioli G.E. Alt F.W. Cell. 1995; 83: 1079-1089Abstract Full Text PDF PubMed Scopus (395) Google Scholar)XRCC5(Ku86)Coding/signalXR-V9BInternal a.a. deletion (30Errami A. Smider V. Rathmell W.K. He D. Hendrickson E.A. Zdzienicka M. Chu G. Mol. Cell. Biol. 1996; 16: 1519-1526Crossref PubMed Scopus (156) Google Scholar)XR-V15BInternal a.a. deletion (30Errami A. Smider V. Rathmell W.K. He D. Hendrickson E.A. Zdzienicka M. Chu G. Mol. Cell. Biol. 1996; 16: 1519-1526Crossref PubMed Scopus (156) Google Scholar)xrs4Truncated 287-a.a. protein (31Singleton B. Priestly A. Steingrimsdottir H. Gell D. Blunt T. Jackson S. Lehmann A. Jeggo P. Mol. Cell. Biol. 1997; 17: 1264-1273Crossref PubMed Scopus (163) Google Scholar)xrs5?xrs6Truncated 24-a.a. protein (31Singleton B. Priestly A. Steingrimsdottir H. Gell D. Blunt T. Jackson S. Lehmann A. Jeggo P. Mol. Cell. Biol. 1997; 17: 1264-1273Crossref PubMed Scopus (163) Google Scholar)sxi1?sxi2?sxi3?XRCC6(Ku70)Coding/signalES cellsKnockout (73Gu Y. Jin S. Gao Y. Weaver D. Alt F. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 8076-8081Crossref PubMed Scopus (346) Google Scholar)XRCC7(DNA-PK cs)CodingscidTerminal 83-a.a. truncation (37Blunt T. Gell D. Fox M. Taccioli G. Jackson S. Lehman A. Jeggo P. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10285-10290Crossref PubMed Scopus (303) Google Scholar, 38Danska J. Holland D. Mariathasan S. Williams K. Guidos C. Mol. Cell. Biol. 1996; 10: 6472-6481Google Scholar)V3?Genetic complementation groups, corresponding to the x-ray cross-complementing genes, XRCC4–XRCC7, have been assigned for cell lines hypersensitive to ionizing radiation and defective in V(D)J recombination. The XRCC6 mutant cell line was generated by targeted knockout of mouse embryonic stem cells. a.a., amino acid. Open table in a new tab Genetic complementation groups, corresponding to the x-ray cross-complementing genes, XRCC4–XRCC7, have been assigned for cell lines hypersensitive to ionizing radiation and defective in V(D)J recombination. The XRCC6 mutant cell line was generated by targeted knockout of mouse embryonic stem cells. a.a., amino acid. The path toward isolating the XRCC5 andXRCC7 genes began with identification of the Ku protein as an autoantigen in scleroderma-polymyositis overlap syndrome (9Mimori T. Akizuki M. Yamagata H. Inada S. Yoshida S. Homma M. J. Clin. Invest. 1981; 68: 611-620Crossref PubMed Scopus (342) Google Scholar, 10Reeves W. Rheum. Dis. Clin. North Am. 1992; 18: 391-415PubMed Google Scholar). In the hope of gaining insight into the pathogenesis of autoimmune disease, Ku was extensively characterized. Ku is a heterodimer of 70 and 86 kDa (Ku70 and Ku86) that binds with strong affinity to DNA ends, stem-loop or bubble structures, or transitions between double-stranded DNA and two single strands (11Mimori T. Hardin J.A. J. Biol. Chem. 1986; 261: 10375-10379Abstract Full Text PDF PubMed Google Scholar,12Falzon M. Fewell J.W. Kuff E.L. J. Biol. Chem. 1993; 268: 10546-10552Abstract Full Text PDF PubMed Google Scholar). Once Ku binds to DNA, it can translocate along the DNA, so that three or more Ku molecules can bind to a single linear DNA fragment (13de Vries E. van Driel W. Bergsma W.G. Arnberg A.C. van der Vliet P.C. J. Mol. Biol. 1989; 208: 65-78Crossref PubMed Scopus (216) Google Scholar, 14Paillard S. Strauss F. Nucleic Acids Res. 1991; 19: 5619-5624Crossref PubMed Scopus (188) Google Scholar). If the linear DNA is then ligated into a circle, Ku can no longer dissociate, consistent with a model in which Ku molecules bind to DNA like beads on a string. Ku is a DNA-dependent ATPase that is activated by both double- and single-stranded DNA (15Cao Q. Pitt S. Leszyk J. Baril E. Biochemistry. 1994; 33: 8548-8557Crossref PubMed Scopus (80) Google Scholar). Both Ku70 and Ku86 contain motifs for potential ATP binding sites. Ku has also been reported to have an ATP-dependent 3′ to 5′ helicase activity (16Tuteja N. Tuteja R. Ochem A. Taneja P. Huang N. Simoncsits A. Susic S. Rahman K. Marusic L. Chen J. Zhang J. Wang S. Pongor S. Falaschi A. EMBO J. 1994; 13: 4991-5001Crossref PubMed Scopus (216) Google Scholar). Ku is the regulatory subunit for DNA-dependent protein kinase (DNA-PK), which has the unusual property of remaining quiescent until activated by DNA ends (17Gottlieb T. Jackson S. Cell. 1993; 72: 131-142Abstract Full Text PDF PubMed Scopus (1011) Google Scholar). DNA-PK contains an enormous catalytic subunit of 465 kDa (DNA-PKcs) that is activated when Ku binds to DNA. DNA-PK will phosphorylate serine or threonine residues that immediately precede glutamine in a large number of protein substrates in vitro (18Anderson C. Trends Biochem. Sci. 1993; 18: 433-437Abstract Full Text PDF PubMed Scopus (234) Google Scholar). Interestingly, DNA-PK will phosphorylate Ku in vitro, activating its ATPase activity (15Cao Q. Pitt S. Leszyk J. Baril E. Biochemistry. 1994; 33: 8548-8557Crossref PubMed Scopus (80) Google Scholar). However, attempts to identify in vivo substrates have been inconclusive. For example, DNA-PK will phosphorylate p53 in vitro but is not required for the accumulation or activation of p53 (19Fried L.M. Koumenis C. Peterson S.R. Green S.L. van Zijl P. Allalunis-Turner J. Chen D.C. Fishel R. Giaccia A.J. Brown J.M. Kirchgessner C.U. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13825-13830Crossref PubMed Scopus (90) Google Scholar, 20Rathmell W.K. Kaufmann W.K. Hurt J.C. Byrd L.L. Chu G. Cancer Res. 1997; 57: 68-74PubMed Google Scholar). A DNA end binding (DEB) factor was detected by an electrophoretic mobility shift assay (21Rathmell W.K. Chu G. Mol. Cell. Biol. 1994; 14: 4741-4748Crossref PubMed Scopus (147) Google Scholar). This assay was used to screen a large number of x-ray-sensitive cell lines, and DEB factor was absent in three different cell lines, all belonging to the complementation group forXRCC5 (21Rathmell W.K. Chu G. Mol. Cell. Biol. 1994; 14: 4741-4748Crossref PubMed Scopus (147) Google Scholar). DEB factor proved to be both biochemically and antigenically similar to Ku (22Rathmell W.K. Chu G. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7623-7627Crossref PubMed Scopus (193) Google Scholar, 23Getts R. Stamato T. J. Biol. Chem. 1994; 269: 15981-15984Abstract Full Text PDF PubMed Google Scholar). Furthermore, the XRCC5and Ku86 genes were independently mapped to the same human chromosome locus, 2q33-35 (24Chen D.J. Marrone B.L. Nguyen T. Stackhouse M. Zhao Y. Siciliano M.J. Genomics. 1994; 21: 423-427Crossref PubMed Scopus (23) Google Scholar, 25Hafezparast M. Kaur G. Zdzienicka M. Athwal R. Lehmann A. Jeggo P. Somatic Cell Mol. Genet. 1993; 19: 413-421Crossref PubMed Scopus (38) Google Scholar, 26Cai Q.-Q. Plet A. Imbert J. Lafage-Pochitaloff M. Cerdan C. Blanchard J.-M. Cytogenet. Cell Genet. 1994; 65: 221-227Crossref PubMed Scopus (86) Google Scholar). To demonstrate that Ku86 and XRCC5 are identical, an expression vector for Ku86 was transfected into the mutantXRCC5 hamster cells. Transfection of human Ku86rescued the mutant hamster cells for DEB activity, DNA-PK enzymatic activity, x-ray resistance, and V(D)J recombination (27Taccioli G. Gottlieb T. Blunt T. Priestly A. Demengeot J. Mizuta R. Lehmann A. Alt F. Jackson S. Jeggo P. Science. 1994; 265: 1442-1445Crossref PubMed Scopus (589) Google Scholar, 28Smider V. Rathmell W.K. Lieber M. Chu G. Science. 1994; 266: 288-291Crossref PubMed Scopus (319) Google Scholar). Transfection of Ku86 also restored resistance to the topoisomerase II inhibitor etoposide (29He D.M. Lee S.E. Hendrickson E.A. Mutat. Res. 1996; 363: 43-56Crossref PubMed Scopus (28) Google Scholar). Thus Ku is involved in the repair of DNA DSBs produced by ionizing radiation, V(D)J recombination, or etoposide. XRCC5 cells were found to contain mutations in theKu86 gene (30Errami A. Smider V. Rathmell W.K. He D. Hendrickson E.A. Zdzienicka M. Chu G. Mol. Cell. Biol. 1996; 16: 1519-1526Crossref PubMed Scopus (156) Google Scholar, 31Singleton B. Priestly A. Steingrimsdottir H. Gell D. Blunt T. Jackson S. Lehmann A. Jeggo P. Mol. Cell. Biol. 1997; 17: 1264-1273Crossref PubMed Scopus (163) Google Scholar), leading to functionally significant alterations in Ku86 (Table I). As expected, the generation of Ku86 knockout mice produced severe immunodeficiency due to an absence of both T and B cells (32Nussenzweig A. Chen C. da Costa Soares V. Sanchez M. Nussenzweig M. Li G. Nature. 1996; 382: 551-555Crossref PubMed Scopus (569) Google Scholar,33Zhu C. Bogue M. Lim D.S. Hasty P. Roth D. Cell. 1996; 86: 379-389Abstract Full Text Full Text PDF PubMed Scopus (392) Google Scholar). Both coding and signal joint formation were impaired, and the coding joint defect was accompanied by accumulation of hairpin coding ends in Ku86 knockout thymocytes. Unexpectedly, theKu86 knockout mice also displayed marked growth retardation (32Nussenzweig A. Chen C. da Costa Soares V. Sanchez M. Nussenzweig M. Li G. Nature. 1996; 382: 551-555Crossref PubMed Scopus (569) Google Scholar), suggesting that Ku might have an additional unforseen role. The discovery that Ku86 was defective in XRCC5mutant cells raised the possibility that DNA-PKcs might be defective in XRCC7 cells (34Blunt T. Finnie N. Taccioli G. Smith G. Demengeot J. Gottlieb T. Mizuta R. Varghese A. Alt F. Jeggo P. Jackson S. Cell. 1995; 80: 813-823Abstract Full Text PDF PubMed Scopus (773) Google Scholar, 35Kirchgessner C. Patil C. Evans J. Cuomo C. Fried L. Carter T. Oettinger M. Brown J.M. Science. 1995; 267: 1178-1185Crossref PubMed Scopus (586) Google Scholar, 36Peterson S. Kurimasa A. Oshimura M. Dynan W. Bradbury E. Chen D. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 3171-3174Crossref PubMed Scopus (267) Google Scholar). Indeed, DNA-PK enzymatic activity and DNA-PKcs protein levels were severely reduced in both scid and V3 cells. When genomic yACs containing theDNA-PK cs gene were transfected intoscid and V3 cells, DNA-PK enzymatic activity, ionizing radiation resistance, and coding joint formation were restored. Furthermore, the DNA-PK cs gene was mutated inscid cells, producing a premature termination codon in the putative kinase domain that truncates the C-terminal 83 amino acids (37Blunt T. Gell D. Fox M. Taccioli G. Jackson S. Lehman A. Jeggo P. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10285-10290Crossref PubMed Scopus (303) Google Scholar, 38Danska J. Holland D. Mariathasan S. Williams K. Guidos C. Mol. Cell. Biol. 1996; 10: 6472-6481Google Scholar). As in Ku86 knockout thymocytes, scid thymocytes accumulate hairpin coding ends (33Zhu C. Bogue M. Lim D.S. Hasty P. Roth D. Cell. 1996; 86: 379-389Abstract Full Text Full Text PDF PubMed Scopus (392) Google Scholar). Therefore, Ku and DNA-PK are required for the proper processing of hairpin ends. Ku or DNA-PK do not have an endonuclease activity that opens the hairpin ends directly. Instead, DNA-PK enzymatic activity may make the hairpin accessible to a still unidentified hairpin endonuclease. The only known XRCC4 cell line, XR-1, is rescued by transfection of a cDNA encoding a 37-kDa protein, restoring both ionizing radiation resistance and V(D)J recombination to wild-type levels (39Li Z. Otevrel T. Gao Y. Cheng H.-L. Seed B. Stamato T.D. Taccioli G.E. Alt F.W. Cell. 1995; 83: 1079-1089Abstract Full Text PDF PubMed Scopus (395) Google Scholar). XRCC4 is a novel gene, lacking homology to other known genes. It is deleted in XR-1 cells and therefore not essential for growth. The sensitivity of XR-1 cells increases dramatically in the G1 phase of the cell cycle (40Giaccia A. Weinstein R. Hu J. Stamato T.D. Somatic Cell Mol. Genet. 1985; 11: 485-491Crossref PubMed Scopus (136) Google Scholar), and any proposed role for XRCC4 must account for this phenomenon. End joining of naked DNA has been studied by introducing linear DNA into intact cells. When linearized plasmid DNA was injected intoXenopus oocytes, a low level of plasmid recircularization occurred (41Grzesiuk E. Carroll D. Nucleic Acids Res. 1987; 15: 971-985Crossref PubMed Scopus (24) Google Scholar). The end-joining reaction resulted in junctions containing deletions back to regions of microhomology of 1–10 bases within 20 bp of the ends. When linearized SV40 DNA was transfected into mammalian cells, end joining either occurred by joining of the ends or by the deletions back to regions of microhomology of bases Mol. Cell. Biol. 1985; PubMed Scopus Google Scholar, Mol. Cell. Biol. 1986; PubMed Scopus Google Scholar). joining occurred for ends with ends of and 3′ The joining microhomology were proposed to be by base of the microhomology regions the DNA ends for subsequent in the end-joining Interestingly, end joining also occurred with the of from a number of free in the the DSB by and repair and of A residues during repair of a 5′ single-stranded Mol. Cell. Biol. 1989; 9: PubMed Scopus Google Scholar). To end joining of DNA, have been used to DSBs at sites. were into hamster cells, which are at the J. W. Mol. Cell. Biol. 1994; 14: PubMed Scopus Google Scholar). of cells containing mutations were then at the DNA of deletions to bp, and of and deletions at the cleavage sites. of the deletions involved the of microhomology of bases at the recombination this for mutations in the of end-joining not be the end-joining with transfected naked DNA and DNA and deletions by have been used to a single or DSBs into the mammalian F. P. Weaver D. Jeggo P. M. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: PubMed Scopus Google Scholar). hamster cells were to contain a single an of the DSB an intact gene the end joining occurred with deletion of the ends back to a of In this XRCC5 mutant cells were severely defective in the DSB by at least of with wild-type hamster cells. V(D)J recombination junctions additional insight into end joining of broken chromosomes (2Smider V. Chu G. Semin. 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Genetic that the of these two is different in and mammalian cells. In for to ionizing radiation have produced mutant of genes involved in homologous recombination G.C. W. DNA and for D. Scholar), but not genes involved in nonhomologous end joining. Furthermore, for the genes homologous to mammalian Ku have sensitivity to ionizing radiation S. Jackson S. Nucleic Acids Res. 1996; PubMed Scopus Google Scholar, G. D. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar, Jin S. Weaver Mol. Cell. Biol. 1996; 16: PubMed Scopus Google Scholar, W. A. F. E. 1996; PubMed Google Scholar). Ku is involved in DSB Ku70 to linearized S. Jackson S. Nucleic Acids Res. 1996; PubMed Scopus Google Scholar, Jin S. Weaver Mol. Cell. Biol. 1996; 16: PubMed Scopus Google Scholar). also to when to which DSBs at the same in both repair by homologous recombination G. D. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar). 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The of Ku and DNA-PK can be used to a model for these might DNA end joining When the chromosome is Ku the DNA ends from until end joining is In of when Ku86 mutant cells were for V(D)J recombination, the signal joining that were large deletions G. Rathbun G. Oltz E. Stamato T. Jeggo P. Alt F. Science. 1993; 260: 207-210Crossref PubMed Scopus (417) Google Scholar). when Ku mutant cells were transfected with linearized plasmid DNA, they of the DNA ends F. M. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). Ku protein free DNA ends from in vitro R. Stamato T. J. Biol. Chem. 1994; 269: 15981-15984Abstract Full Text PDF PubMed Google Scholar). binding to the DNA ends, Ku of the two DNA ends might be by two independent DNA binding on DNA-PK or by the of two DNA-PK DNA-PK both DNA-PKcs and Ku vitro D. S. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). that this in so that the kinase on DNA end the DNA-PK on the other DNA in the kinase activity so that processing of the DNA ends only of the two ends is by DNA-PK in vitro, DNA-PKcs from Ku D. S. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar), and Ku helicase activity (15Cao Q. Pitt S. Leszyk J. Baril E. Biochemistry. 1994; 33: 8548-8557Crossref PubMed Scopus (80) Google Scholar, N. Tuteja R. Ochem A. Taneja P. Huang N. Simoncsits A. Susic S. Rahman K. Marusic L. Chen J. Zhang J. Wang S. Pongor S. Falaschi A. EMBO J. 1994; 13: 4991-5001Crossref PubMed Scopus (216) Google Scholar). that this helicase activity DNA ends in vivo so that regions of microhomology can by base The DNA then be either by an or a endonuclease. A has the enzymatic activity Lieber M.R. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar), its role in DSB repair and V(D)J recombination has not been are then in by a DNA and the by to the end-joining The that DNA-PK in has for V(D)J recombination. that the joining of signal ends and not intact kinase When a of signal and coding ends is by DNA-PK on the signal end and phosphorylate Ku to the coding activating the Ku The hairpin end then be to cleavage by a hairpin endonuclease. The reaction is DNA-PK on the hairpin end as a kinase in Ku to the signal end processing of the signal ends. Therefore, the the joining of signal ends is and it is by the To this are to the of Ku helicase of the ATP binding in Ku86 has no (31Singleton B. Priestly A. Steingrimsdottir H. Gell D. Blunt T. Jackson S. Lehmann A. Jeggo P. Mol. Cell. Biol. 1997; 17: 1264-1273Crossref PubMed Scopus (163) Google Scholar), but a similar has not been for the ATP binding in must also the helicase activity of Ku will DNA ends and Ku is an in vivo for A knockout of the DNA-PKcs gene is to the enormous catalytic subunit has additional biochemical To the must be is the role of the XRCC4 protein in the end-joining protein has hairpin endonuclease are required for of the ends by microhomology base the DNA The pathway for nonhomologous DNA end joining has evolved for other its role in joining broken It has been that RAG1 and were into the mammalian by an 1995; Full Text PDF Scopus Google Scholar). V(D)J recombination then have evolved by the cleavage activity of with the end-joining activity for broken a of end joining is also in which Ku for both and DNA repair E. D. 1996; 10: PubMed Scopus Google Scholar). cells may have additional for mice marked growth retardation (32Nussenzweig A. Chen C. da Costa Soares V. Sanchez M. Nussenzweig M. Li G. Nature. 1996; 382: 551-555Crossref PubMed Scopus (569) Google Scholar), suggesting a role for Ku other in DNA repair or V(D)J recombination. in by C. A. B. S. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: PubMed Scopus Google Scholar, A. Gottlieb T. Jackson S. 1995; 9: PubMed Scopus Google Scholar) or II M. Fewell J.W. Kuff E.L. J. Biol. Chem. 1993; 268: 10546-10552Abstract Full Text PDF PubMed Google Scholar, W. H. D. G. L. R. Nature. 1996; PubMed Scopus Google Scholar). from in and in G. D. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar). In the of cells repair double strand breaks has a end-joining to not the repair of broken chromosomes but other pathways in DNA and for and of the