Alternative titles; symbols
HGNC Approved Gene Symbol: XRCC5
Cytogenetic location: 2q35 Genomic coordinates (GRCh38): 2:216,109,348-216,206,293 (from NCBI)
The human XRCC5 DNA repair gene complements the radiosensitive mutant xrs-6, derived from Chinese hamster ovary cells which are defective in DNA double-strand break repair and in ability to undergo V(D)J recombination. The XRCC5 gene encodes the 80-kD subunit of the Ku autoantigen, a heterodimer which contributes to genomic integrity through its ability to bind DNA double-strand breaks and facilitate repair by the nonhomologous end joining (NHEJ) pathway.
A DNA double-strand break is a major lesion that destroys the integrity of the DNA molecule. Such damage is introduced by ionizing radiation. A number of mutants defective in the repair of DNA double-strand breaks have been identified in rodent cells and classified into distinct complementation groups. The repair gene defective in one group of mutants was designated XRCC5. Using the method of microcell-mediated chromosome transfer, Jeggo et al. (1992) achieved complementation of the repair defect in hamster xrs mutants by transfer of human chromosome 2. The sensitivity of these cells to ionizing radiation and their impaired ability to rejoin radiation-induced DNA double-strand breaks were corrected by chromosome 2, although the correction of radiation sensitivity was only partial. Complementation was observed in 1 hybrid which contained only the long arm of chromosome 2.
Taccioli et al. (1994) showed through genetic and biochemical approaches that the XRCC5 is the 80-kilodalton subunit of the Ku protein. Ku binds to free double-stranded DNA ends and is the DNA-binding component of the DNA-dependent protein kinase. Thus, the Ku protein is involved in DNA repair and in V(D)J recombination, and the Ku-DNA-dependent protein kinase complex may have a role in those same processes. See 152690 for discussion of the Ku p70 subunit.
Tuteja et al. (1994) purified from HeLa cells an enzyme they called DNA helicase II, an ATP-dependent DNA unwinding enzyme. They showed that it is a heterodimer of 72 and 87 kD polypeptides. Sequencing showed that it is identical to the Ku autoantigen. The exclusively nuclear location of this particular DNA helicase II/Ku antigen, its highly specific affinity for double-stranded DNA, its abundance, and its exclusive DNA-duplex unwinding activity pointed to additional roles for this molecule in DNA metabolism.
Li et al. (2002) constructed a human somatic cell line containing a targeted disruption of the Ku86 locus. Human colon cancer cells heterozygous for Ku86 were haploinsufficient with an increase in polyploid cells, a reduction in cell proliferation, elevated p53 levels, and a slight hypersensitivity to ionizing radiation. Functional inactivation of the second Ku86 allele resulted in cells with a drastically reduced doubling time. These cells were capable of undergoing only a limited number of cell divisions, after which they underwent apoptosis. These experiments demonstrated that the Ku86 locus is essential in human somatic tissue culture cells.
Using human and hamster cells, Mari et al. (2006) showed that Ku heterodimers on DNA ends were in dynamic equilibrium with Ku70/Ku80 in solution, suggesting that formation of the NHEJ complex is reversible. Accumulation of XRCC4 (194363) on DNA double-strand breaks depended on the presence of Ku70/Ku80, but not PRKDC (600899). Mari et al. (2006) found that XRCC4 interacted directly with Ku70, and they hypothesized that XRCC4 serves as a flexible tether between Ku70/Ku80 and LIG4 (601837).
Guirouilh-Barbat et al. (2007) studied NHEJ in Xrcc4- and Ku80-null XR-1 CHO cells and showed differences in the effects of these mutations. While significant end joining existed in Xrcc4-null cells due to the use of microhomologies distal from the double-strand break, the efficiency of NHEJ was reduced. In contrast, knockout of Ku80 barely affected the efficiency of end joining. In both mutant cell lines, however, the accuracy of end joining was reduced. Guirouilh-Barbat et al. (2007) concluded that the KU80/XRCC4 pathway is conservative and can accommodate non-fully complementary ends at the cost of limited mutagenesis.
Roberts et al. (2010) demonstrated, in vitro and in cells, that accurate and efficient repair by NHEJ of double-strand breaks with nucleotide damage requires 5-prime-deoxyribose-5-phosphate/apurinic/apyrimidinic (dRP/AP) lyase activity. Classically defined NHEJ is moreover uniquely effective at coupling this end-cleaning step to joining in cells, helping to distinguish this pathway from otherwise robust alternative NHEJ pathways. The NHEJ factor Ku was identified as an effective 5-prime-dRP/AP lyase. In a similar manner to other lyases, Ku nicks DNA 3-prime of an abasic site by a mechanism involving a Schiff-base covalent intermediate with the abasic site. Roberts et al. (2010) showed, by using cell extracts, that Ku is essential for the efficient removal of AP sites near double-strand breaks and, consistent with this result, that joining of such breaks is specifically decreased in cells complemented with a lyase-attenuated Ku mutant. While Ku had previously been presumed only to recognize ends and recruit other factors that process ends, the data of Roberts et al. (2010) supported an unexpected direct role for Ku in end-processing steps as well.
Crystal Structure
Walker et al. (2001) determined the crystal structure of the human Ku heterodimer both alone and bound to a 55-nucleotide DNA element at 2.7- and 2.5-angstrom resolution, respectively. Ku70 and Ku80 share a common topology and form a dyad-symmetrical molecule with a preformed ring that encircles duplex DNA. The binding site can cradle 2 full turns of DNA while encircling only the central 3-4 base pairs. Ku makes no contacts with DNA bases and few with the sugar-phosphate backbone, but it fits sterically to major and minor groove contours so as to position the DNA helix in a defined path through the protein ring. Walker et al. (2001) concluded that these features are well designed to structurally support broken DNA ends and to bring the DNA helix into phase across the junction during end processing and ligation.
Chen et al. (1992) used analysis of somatic cell hybrids and microcell-mediated chromosome transfer analyzed by fluorescence in situ hybridization (FISH) to assign the XRCC5 gene to 2p. They concluded that the location is probably between 2p21 and 2p12. For the regional mapping of the XRCC5 gene, Chen et al. (1994) constructed a panel of x-ray hybrids and a panel of microcell-mediated chromosome 2 hybrids in the mutant background that contained only fragments of human chromosome 2. By using FISH, chromosome banding, and physical mapping of these x-ray hybrids, they localized the XRCC5 gene to 2q35. This result was further confirmed by segregation analysis indicating that the radiation-resistant phenotype of a repair-proficient hybrid cosegregated with the human 2q35 chromosome fragment. Koike et al. (1996) mapped the homologous genes to mouse chromosome 1 and rat chromosome 9.
Blunt et al. (1995) assembled a YAC contig for the region 2q33-q34 encompassing XRCC5.
Difilippantonio et al. (2000) demonstrated that mouse cells deficient for Ku80 display a marked increase in chromosomal aberrations, including breakage, translocations, and aneuploidy. Despite the observed chromosome instabilities, Ku80 -/- mice have only a slightly earlier onset of cancer. Loss of p53 (191170) synergizes with Ku80 to promote tumorigenesis such that all Ku80 -/-/p53 -/- mice succumb to disseminated pro-B-cell lymphoma before 3 months of age. Tumors result from a specific set of chromosomal translocations and gene amplifications involving IgH and c-Myc, reminiscent of Burkitt lymphoma (113970). Difilippantonio et al. (2000) concluded that Ku80 is a caretaker gene that maintains the integrity of the genome by a mechanism involving suppression of chromosomal rearrangements.
Couedel et al. (2004) created Rad54 (604289)/Xrcc5 double-mutant mice and determined that homologous recombination and nonhomologous end joining components collaborate to repair DNA damage. Tissue and cells from double-mutant mice showed spontaneous DNA damage. The authors concluded that even a mild repair deficiency can have profound effects in the context of other mutations.
Blunt, T., Taccioli, G. E., Priestley, A., Hafezparast, M., McMillan, T., Liu, J., Cole, C. C., White, J., Alt, F. W., Jackson, S. P., Schurr, E., Lehmann, A. R., Jeggo, P. A. A YAC contig encompassing the XRCC5 (Ku80) DNA repair gene and complementation defective cells by YAC protoplast fusion. Genomics 30: 320-328, 1995. [PubMed: 8586433] [Full Text: https://doi.org/10.1006/geno.1995.9871]
Chen, D. J., Marrone, B. L., Nguyen, T., Stackhouse, M., Zhao, Y., Siciliano, M. J. Regional assignment of a human DNA repair gene (XRCC5) to 2q35 by x-ray hybrid mapping. Genomics 21: 423-427, 1994. [PubMed: 8088837] [Full Text: https://doi.org/10.1006/geno.1994.1287]
Chen, D. J., Park, M. S., Campbell, E., Oshimura, M., Liu, P., Zhao, Y., White, B. F., Siciliano, M. J. Assignment of a human DNA double-strand break repair gene (XRCC5) to chromosome 2. Genomics 13: 1088-1094, 1992. [PubMed: 1505945] [Full Text: https://doi.org/10.1016/0888-7543(92)90023-l]
Couedel, C., Mills, K. D., Barchi, M., Shen, L., Olshen, A., Johnson, R. D., Nussenzweig, A., Essers, J., Kanaar, R., Li, G. C., Alt, F. W., Jasin, M. Collaboration of homologous recombination and nonhomologous end-joining factors for the survival and integrity of mice and cells. Genes Dev. 18: 1293-1304, 2004. [PubMed: 15175261] [Full Text: https://doi.org/10.1101/gad.1209204]
Difilippantonio, M. J., Zhu, J., Chen, H. T., Meffre, E., Nussenzweig, M. C., Max, E. E., Ried, T., Nussenzweig, A. DNA repair protein Ku80 suppresses chromosomal aberrations and malignant transformation. Nature 404: 510-514, 2000. [PubMed: 10761921] [Full Text: https://doi.org/10.1038/35006670]
Guirouilh-Barbat, J., Rass, E., Plo, I., Bertrand, P., Lopez, B. S. Defects in XRCC4 and KU80 differentially affect the joining of distal nonhomologous ends. Proc. Nat. Acad. Sci. 104: 20902-20907, 2007. [PubMed: 18093953] [Full Text: https://doi.org/10.1073/pnas.0708541104]
Jeggo, P. A., Hafezparast, M., Thompson, A. F., Broughton, B. C., Kaur, G. P., Zdzienicka, M. Z., Athwal, R. S. Localization of a DNA repair gene (XRCC5) involved in double-strand-break rejoining to human chromosome 2. Proc. Nat. Acad. Sci. 89: 6423-6427, 1992. [PubMed: 1631138] [Full Text: https://doi.org/10.1073/pnas.89.14.6423]
Koike, M., Matsuda, Y., Mimori, T., Harada, Y.-N., Shiomi, N., Shiomi, T. Chromosomal localization of the mouse and rat DNA double-strand break repair genes Ku p70 and Ku p80/XRCC5 and their mRNA expression in various mouse tissues. Genomics 38: 38-44, 1996. [PubMed: 8954777] [Full Text: https://doi.org/10.1006/geno.1996.0589]
Li, G., Nelsen, C., Hendrickson, E. A. Ku86 is essential in human somatic cells. Proc. Nat. Acad. Sci. 99: 832-837, 2002. [PubMed: 11792868] [Full Text: https://doi.org/10.1073/pnas.022649699]
Mari, P.-O., Florea, B. I., Persengiev, S. P., Verkaik, N. S., Bruggenwirth, H. T., Modesti, M., Giglia-Mari, G., Bezstarosti, K., Demmers, J. A. A., Luider, T. M., Houtsmuller, A. B., van Gent, D. C. Dynamic assembly of end-joining complexes requires interaction between Ku70/80 and XRCC4. Proc. Nat. Acad. Sci. 103: 18597-18602, 2006. [PubMed: 17124166] [Full Text: https://doi.org/10.1073/pnas.0609061103]
Roberts, S. A., Strande, N., Burkhalter, M. D., Strom, C., Havener, J. M., Hasty, P., Ramsden, D. A. Ku is a 5-prime-dRP/AP lyase that excises nucleotide damage near broken ends. Nature 464: 1214-1217, 2010. [PubMed: 20383123] [Full Text: https://doi.org/10.1038/nature08926]
Taccioli, G. E., Gottlieb, T. M., Blunt, T., Priestley, A., Demengeot, J., Mizuta, R., Lehmann, A. R., Alt, F. W., Jackson, S. P., Jeggo, P. A. Ku80: product of the XRCC5 gene and its role in DNA repair and V(D)J recombination. Science 265: 1442-1445, 1994. [PubMed: 8073286] [Full Text: https://doi.org/10.1126/science.8073286]
Tuteja, N., Tuteja, R., Ochem, A., Taneja, P., Huang, N. W., Simoncsits, A., Susic, S., Rahman, K., Marusic, L., Chen, J., Zhang, J., Wang, S., Pongor, S., Falaschi, A. Human DNA helicase II: a novel DNA unwinding enzyme identified as the Ku autoantigen. EMBO J. 13: 4991-5001, 1994. [PubMed: 7957065] [Full Text: https://doi.org/10.1002/j.1460-2075.1994.tb06826.x]
Walker, J. R., Corpina, R. A., Goldberg, J. Structure of the Ku heterodimer bound to DNA and its implications for double-strand break repair. Nature 412: 607-614, 2001. [PubMed: 11493912] [Full Text: https://doi.org/10.1038/35088000]