Alternative titles; symbols
HGNC Approved Gene Symbol: XRCC6
Cytogenetic location: 22q13.2 Genomic coordinates (GRCh38): 22:41,621,295-41,664,041 (from NCBI)
The XRCC6 gene encodes subunit p70 of the p70/p80 autoantigen. The p70/p80 autoantigen consists of 2 proteins of molecular mass of approximately 70,000 and 80,000 daltons that dimerize to form a 10 S DNA-binding complex. See 194364 for discussion of the gene encoding the p80 subunit. Exchange of immunologic reagents showed that the p70/p80 autoantigen is identical to the Ku antigen, the Ki antigen, and the 86- to 70-kD protein complex. The p70/p80 complex binds to the ends of double-stranded DNA in a cell cycle-dependent manner, being associated with chromosomes of interphase cells, followed by complete dissociation from the condensing chromosomes in early prophase. Both p70 and p80 contain phosphoserine residues. A role for the antigen in DNA repair or transposition has been proposed.
Some patients with systemic lupus erythematosus (152700) produce very large amounts of autoantibodies to p70 and p80. Reeves and Sthoeger (1989) used autoantibodies from the serum of such a person to isolate cDNA clones encoding p70, the protein that is thought to mediate binding of the Ku complex to DNA. Analysis of the predicted amino acid sequence of p70 suggested structural similarities with other DNA-binding proteins. The information may be useful in examining the function of the Ku complex, as well as the causes of autoimmunity to this antigen. The presence of a 61-residue acidic region rich in serine raised the possibility that its charge might be modulated by phosphorylation. The predicted amino acid sequence also contains 2 regions with periodic repeats of either leucine alone or leucine alternating with serine every seventh position. The latter repeat showed structural similarities with the 'leucine zipper' regions of the MYC oncogene product. The p70 antigen and the p80 antigen did not appear to have extensive sequence homology, according to analysis of RNA blots and immunologic criteria.
The autoantibody that was thought to identify the TSH receptor and which was used for isolating cDNA clones that were mapped to 22q11-q13 by Southern analysis of somatic cell hybrid DNAs and by linkage of RFLPs to CYP2D (124030) (McBride et al., 1987; Mitchell et al., 1989) was shown not to represent TSHR (603372) but rather a separate, 70-kD thyroid autoantigen (Chan et al., 1989). The autoantibody has been found in patients with autoimmune thyroid disease (Graves disease) as well as in those with lupus.
Takiguchi et al. (1996) showed that the mouse Ku70 gene contains 13 exons (exon 1 is untranslated) spanning about 24 kb of genomic DNA and encodes a predicted 608-amino acid polypeptide (compared to 609 residues in humans).
Takiguchi et al. (1996) mapped the mouse Ku70 gene to chromosome 15E by fluorescence in situ hybridization. Koike et al. (1996) mapped the Ku70 gene to mouse chromosome 1 and rat chromosome 9. The human Ku70 gene resides on chromosome 22q11-q13 (Chan et al., 1989).
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. Hartley et al. (1995) noted that the Ku autoantigen is a component of DNA-dependent protein kinase (see PRKDC; 600899).
Double-strand DNA breaks (DSBs) pose a major threat to living cells, and several mechanisms for repairing these lesions have evolved. Eukaryotes can process DSBs by homologous recombination (HR) or nonhomologous end joining (NHEJ). NHEJ connects DNA ends irrespective of their sequence, and it predominates in mitotic cells, particularly during G1 (Takata et al., 1998). HR requires interaction of the broken DNA molecule with an intact homologous copy, and allows restoration of the original DNA sequence. HR is active during G2 of the mitotic cycle and predominates during meiosis, when the cell creates DSBs, which must be repaired by HR to ensure proper chromosome segregation. How the cell controls the choice between the 2 repair pathways was investigated by Goedecke et al. (1999). They demonstrated a physical interaction between the mammalian Ku70, which is essential for NHEJ (Baumann and West, 1998), and MRE11 (600814), which functions both in NHEJ and meiotic HR. Moreover, they showed that irradiated cells deficient for Ku70 are incapable of targeting Mre11 to subnuclear foci that may represent DNA-repair complexes. Nevertheless, Ku70 and Mre11 were differentially expressed during meiosis. In the mouse testis, Mre11 and Ku70 colocalized in nuclei of somatic cells and in the XY bivalent. In early meiotic prophase, however, when meiotic recombination is most probably initiated, Mre11 was abundant, whereas Ku70 was not detectable. Goedecke et al. (1999) proposed that Ku70 acts as a switch between the 2 DSB repair pathways. When present, Ku70 destines DSBs for NHEJ by binding to DNA ends and attracting other factors for NHEJ, including Mre11; when absent, it allows participation of DNA ends and Mre11 in the meiotic HR pathway.
The Drosophila Dmblm locus is a homolog of the human Bloom syndrome gene (604610). Kusano et al. (2001) demonstrated that mutant Dmblm phenotypes were partially rescued by an extra copy of the DNA repair gene Ku70, indicating that the 2 genes functionally interact in vivo.
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. 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).
Pace et al. (2010) found a genetic interaction between the Fanconi anemia gene FANCC (227645) and the NHEJ factor Ku70. Disruption of both FANCC and Ku70 suppressed sensitivity to crosslinking agents, diminished chromosome breaks, and reversed defective homologous recombination. Ku70 binds directly to free DNA ends, committing them to NHEJ repair. Pace et al. (2010) showed that purified FANCD2 (227646), a downstream effector of the Fanconi anemia pathway, might antagonize Ku70 activity by modifying such DNA substrates. Pace et al. (2010) concluded that these results reveal a function for the Fanconi anemia pathway in processing DNA ends, thereby diverting double-strand break repair from abortive NHEJ and toward homologous recombination.
Congenital muscular dystrophy type 1A (MDC1A; 607855) is caused by mutations in the gene encoding laminin-alpha-2 (LAMA2; 156225). Bax (600040)-mediated muscle cell death is a significant contributor to the severe neuromuscular pathology seen in the Lama2-null mouse model of MDC1A. Vishnudas and Miller (2009) analyzed molecular mechanisms of Bax regulation in normal and LAMA2-deficient muscles and cells, including myogenic cells from MDC1A patients. In mouse myogenic cells, Bax coimmunoprecipitated with the multifunctional protein Ku70. In addition, cell-permeable pentapeptides designed from Ku70, termed Bax-inhibiting peptides (BIPs), inhibited staurosporine-induced Bax translocation and cell death in mouse myogenic cells. Acetylation of Ku70, which can inhibit binding to Bax and can be an indicator of increased susceptibility to cell death, was more abundant in Lama2-null mouse muscles than in normal mouse muscles. Myotubes formed in culture from human LAMA2-deficient patient myoblasts produced high levels of activated caspase-3 (CASP3; 600636) when grown on poly-L-lysine, but not when grown on a LAMA2-containing substrate or when treated with BIPs. Cytoplasmic Ku70 in human LAMA2-deficient myotubes was both reduced in amount and more highly acetylated than in normal myotubes. Vishnudas and Miller (2009) concluded that increased susceptibility to cell death appears to be an intrinsic property of human LAMA2-deficient myotubes and that Ku70 is a regulator of Bax-mediated pathogenesis.
By screening human cells transfected with foreign DNA, Zhang et al. (2011) observed expression of IFNL1 (IL29; 607403), a type III interferon (IFN), rather than type I IFN (e.g., IFNB1; 147640). Pull-down analysis with cytosolic proteins identified Ku70 and Ku80 as the DNA-binding proteins. Knockdown and reporter analyses revealed that Ku70 functioned as a DNA sensor that induced IFNL1 activation. Analysis of the IFNL1 promoter indicated that positive-regulatory domain I and IFN-stimulated response element sites were predominantly involved in DNA-mediated IFNL1 activation. Pull-down assays showed that IFNL1 induction was associated with activation of IRF1 (147575) and IRF7 (605047). Zhang et al. (2011) concluded that Ku70 mediates the induction of type III IFN by DNA, as may occur in viral infections or DNA vaccination.
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.
Li et al. (1998) presented evidence that inactivation of the Ku70 gene by targeted disruption in mice and derived cell lines leads to a propensity for malignant transformation both in vitro and in vivo. In vitro, Ku70 -/- mouse fibroblasts displayed an increased rate of sister chromatid exchange and a high frequency of spontaneous neoplastic transformation. In vivo, Ku70 -/- mice, known to be defective in B- but not T-lymphocyte maturation, developed thymic and disseminated T-cell lymphomas at a mean age of 6 months with CD4+/CD8+ tumor cells. In addition, many of the knockout mice showed segmental aganglionosis affecting the small intestine and the colon. These findings demonstrated that Ku70 deficiency facilitates neoplastic growth and suggested a role of the Ku70 locus in tumor suppression.
An article by Sawada et al. (2003) discussing the function of Ku70 was retracted because it contained 'a significant number of digital image manipulations in the published figures.' The retraction superceded the previously published erratum for this article.
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Kusano, K., Johnson-Schlitz, D. M., Engels, W. R. Sterility of Drosophila with mutations in the Bloom syndrome gene--complementation by Ku70. Science 291: 2600-2602, 2001. [PubMed: 11283371] [Full Text: https://doi.org/10.1126/science.291.5513.2600]
Li, G. C., Ouyang, H., Li, X., Nagasawa, H., Little, J. B., Chen, D. J., Ling, C. C., Fuks, Z., Cordon-Cardo, C. Ku70: a candidate tumor suppressor gene for murine T cell lymphoma. Molec. Cell 2: 1-8, 1998. [PubMed: 9702186] [Full Text: https://doi.org/10.1016/s1097-2765(00)80108-2]
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]
McBride, O. W., Chan, J. Y. C., Notkins, A. L., Kohn, L. D., Lerman, M. The TSH receptor gene is located on human chromosome 22 and homologous sequences are present on chromosomes 1q, 8, 10, and Xq. (Abstract) Am. J. Hum. Genet. 41: A177, 1987.
Mitchell, A. L., Bale, A. E., Chan, J., Kohn, L., Gonzalez, F., McBride, O. W. Localization of TSHR gene and cytochrome p450 IID subfamily on chromosome 22 by linkage analysis. (Abstract) Cytogenet. Cell Genet. 51: 1045, 1989.
Pace, P., Mosedale, G., Hodskinson, M. R., Rosado, I. V. Sivasubramaniam, M., Patel, K. J. Ku70 corrupts DNA repair in the absence of the Fanconi anemia pathway. Science 329: 219-223, 2010. [PubMed: 20538911] [Full Text: https://doi.org/10.1126/science.1192277]
Reeves, W. H., Sthoeger, Z. M. Molecular cloning of cDNA encoding the p70 (Ku) lupus autoantigen. J. Biol. Chem. 264: 5047-5052, 1989. [PubMed: 2466842]
Sawada, M., Sun, W, Hayes, P., Leskov, K., Boothman, D. A., Matsuyama, S. K70 suppresses the apoptotic translocation of Bax to mitochondria. Nature Cell Biol. 5: 320-329, 2003. Note: Erratum: Nature Cell Biol. 6: 373 only, 2004; Retraction: Nature Cell Biol. 9: 480 only, 2007. [PubMed: 12652308] [Full Text: https://doi.org/10.1038/ncb950]
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Takiguchi, Y., Kurimasa, A., Chen, F., Pardington, P. E., Kuriyama, T., Okinaka, R. T., Moyzis, R., Chen, D. J. Genomic structure and chromosomal assignment of the mouse Ku70 gene. Genomics 35: 129-135, 1996. [PubMed: 8661113] [Full Text: https://doi.org/10.1006/geno.1996.0331]
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Vishnudas, V. K., Miller, J. B. Ku70 regulates Bax-mediated pathogenesis in laminin-alpha-2-deficient human muscle cells and mouse models of congenital muscular dystrophy. Hum. Molec. Genet. 18: 4467-4477, 2009. [PubMed: 19692349] [Full Text: https://doi.org/10.1093/hmg/ddp399]
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]
Zhang, X., Brann, T. W., Zhou, M., Yang, J., Oguariri, R. M., Lidie, K. B., Imamichi, H., Huang, D.-W., Lempicki, R. A., Baseler, M. W., Veenstra, T. D., Young, H. A., Lane, H. C., Imamichi, T. Cutting edge: Ku70 is a novel cytosolic DNA sensor that induces type III rather than type I IFN. J. Immun. 186: 4541-4545, 2011. [PubMed: 21398614] [Full Text: https://doi.org/10.4049/jimmunol.1003389]