Alternative titles; symbols
HGNC Approved Gene Symbol: XRCC2
Cytogenetic location: 7q36.1 Genomic coordinates (GRCh38): 7:152,644,776-152,676,141 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 7q36.1 | ?Fanconi anemia, complementation group U | 617247 | Autosomal recessive | 3 |
| ?Premature ovarian failure 17 | 619146 | Autosomal recessive | 3 | |
| Spermatogenic failure 50 | 619145 | Autosomal recessive | 3 |
The XRCC2 gene is a member of the RAD51 gene family (see, e.g., 179617), which encode proteins involved in homologous recombination repair of DNA damage (summary by Tambini et al., 1997). The XRCC2 gene acts late in the Fanconi anemia (FA)-BRCA (see, e.g., BRCA1; 113705) pathway of DNA repair (summary by Park et al., 2016).
Thacker et al. (1995) fused the V79 hamster cell line irs1, which is a repair-deficient mutant that shows hypersensitivity to a number of different DNA-damaging agents (Jones et al., 1987), to normal human cells, resulting in complementation of the defect. The resultant hybrids were analyzed by Alu-PCR, chromosome painting, and DNA markers to map the complementing gene, designated XRCC2, to a specific chromosome region. The hybrid cells showed correction of sensitivity to both x-rays and mitomycin C and contained human chromosome 7, often as their only human component. Hybrids showing unstable retention of human chromosomes were subcloned to show that loss of chromosome 7 and loss of resistance to mitomycin C occurred concordantly. Two separate hybrids were found to have a smaller piece of chromosome 7, and specific DNA probes and microsatellite markers defined this as a contiguous region at 7q35-q36. Hybrid irradiation-fusion methods were used to reduce further the size of the complementing genomic region and to localize the gene to an approximately 3- to 5-Mb region at 7q36.1.
Jones et al. (1995) mapped the XRCC2 gene to 7q36 by studying complementation of the defect in the irs1 hamster cell line described by Jones et al. (1987). Jones et al. (1995) formed somatic cell hybrids by fusing irs1 cells with human lymphocytes and selecting for complementation in medium containing concentrations of mitomycin C that are toxic to irs1 cells but not their human fusion partners. Retention of chromosome 7 or of the region 7q36 resulted in cells that were resistant to mitomycin C.
Tambini et al. (1997) took the radiation reduction of human/hamster hybrids further to locate the XRCC2 gene to a small genomic region defined by a single microsatellite marker D7S483. Yeast artificial chromosomes (YACs) carrying that marker were then fused to the irs1 hamster cell line and a YAC that carried the complementing gene was identified. This YAC was used for direct cDNA selection experiments to identify the XRCC2 gene. The gene was found to share homology with the yeast RAD51 gene and its human homolog (179617), which are involved in the recombinational repair of DNA damage. Strong support for the candidacy of this gene as XRCC2 was obtained from its refined map position and by the full complementation of irs1 sensitivity with a 40-kb cosmid carrying the gene. Tambini et al. (1997) noted that, although the XRCC2 candidate gene on chromosome 7 showed homology to yeast RAD51, it must be distinct from the human RAD51 homolog, which is located on chromosome 15.
Thacker et al. (1995) and Jones et al. (1995) mapped the XRCC2 gene to chromosome 7q36.1.
Johnson et al. (1999) demonstrated that XRCC2 is essential for the efficient repair of DNA double-strand breaks by homologous recombination between sister chromatids. Hamster cells deficient in XRCC2 showed a more than 100-fold decrease in homologous recombination induced by double-strand breaks compared with the parental cell line. This defect was corrected to almost wildtype levels by transient transfection with a plasmid expressing XRCC2. The repair defect in XRCC2 mutant cells appeared to be restricted to recombinational repair because nonhomologous end joining was normal. Johnson et al. (1999) concluded that XRCC2 is involved in the repair of DNA double-strand breaks by homologous recombination.
Using a yeast 2-hybrid assay, Braybrooke et al. (2000) identified a direct interaction between XRCC2 and RAD51L3 (602954), and they confirmed the interaction by pull-down assays between recombinant XRCC2 and endogenous RAD51L3 in HeLa cell extracts. Size-exclusion chromatography followed by Western blot analysis suggested that the 2 proteins exist as a heterodimer of about 70 kD.
Masson et al. (2001) found that antibody directed against RAD51L3 immunoprecipitated a complex from HeLa cell lysates that included XRCC2, RAD51B (RAD51L1; 602948), and RAD51C (602774), along with RAD51L3. Interactions between these proteins were confirmed in pull-down assays using recombinant proteins expressed in sf9 insect cells. Gel filtration of the complexes indicated an apparent molecular mass of about 180 kD, suggesting a 1:1:1:1 stoichiometry of the 4 subunits. Binding assays, confirmed by electron microscopy, indicated that the purified complex bound single-stranded or nicked DNA. This binding was dependent on Mg(2+) but independent of ATP. The DNA-stimulated ATPase activity of the complex was extremely low. Masson et al. (2001) also identified a second, heterodimeric protein complex between RAD51C and XRCC3 (600675). Using coprecipitation and multiple pull-down assays, Liu et al. (2002) confirmed interaction between the same RAD51 paralogs in the same 2 distinct protein complexes.
In a yeast 2-hybrid screen of a human brain cDNA library using XRCC2 as bait, Kurumizaka et al. (2002) also found that RAD51L3 interacts directly with XRCC2. Using a D-loop formation assay, they found that RAD51L3 and XRCC2, coexpressed and purified from bacterial cultures, catalyze homologous pairing between a single-stranded oligonucleotide and a superhelical double-stranded DNA. Significant single- and double-stranded DNA were bound by the complex in the absence of ATP, but homologous pairing was dependent on ATP and Mg(2+). By electron microscopy, they found that RAD51L3 and XRCC2 form a multimeric ring structure in the absence of DNA, and they form filamentous structures in the presence of single-stranded DNA.
Adelman et al. (2013) reported that Helq (606769) helicase-deficient mice exhibit subfertility, germ cell attrition, interstrand crosslink (ICL) sensitivity, and tumor predisposition, with Helq heterozygous mice exhibiting a similar, albeit less severe, phenotype than the null, indicative of haploinsufficiency. Adelman et al. (2013) established that HELQ interacts directly with the RAD51 paralog complex BCDX2 (RAD51B, RAD51C, RAD51D, and XRCC2) and functions in parallel to the Fanconi anemia pathway to promote efficient homologous recombination at damaged replication forks. Adelman et al. (2013) concluded that their results revealed a critical role for HELQ in replication-coupled DNA repair, germ cell maintenance, and tumor suppression in mammals.
Fanconi Anemia, Complementation Group U
In a boy, born of consanguineous Saudi Arabian parents, with an atypical form of Fanconi anemia, complementation group U (FANCU; 617247), Shamseldin et al. (2012) identified a homozygous truncating mutation in the XRCC2 gene (R215X; 600375.0001). The mutation was found by whole-exome sequencing followed by autozygome filtering. Chromosome testing in patient fibroblasts showed a marked increase in the frequency of dsDNA breaks in response to damage, indicating a defect in homologous recombination repair. Complementation studies were not performed. Shamseldin et al. (2012) noted the phenotypic similarities to Xrcc2-null mice (Deans et al., 2000). Park et al. (2016) found that expression of wildtype XRCC2 in cells derived from the patient reported by Shamseldin et al. (2012) corrected all 3 abnormal cellular phenotypes that were apparent in patient cells: cellular sensitivity to DNA interstrand crosslinking agents, chromosome instability, and accumulation of cells at the G2/M stage of the cell cycle. Patient cells showed normal levels of monoubiquitinated FANCD2 (613984), a central step in the FA pathway, and decreased assembly of RAD51 (179617) foci, suggesting that XRCC2 acts downstream of this event. Patient cells showed defective assembly of the components of the BCDX2 complex, particularly RAD51C (602774). Patient cells also showed increased sensitivity to ionizing radiation, consistent with a defect in proteins that act downstream in the FA pathway.
Spermatogenic Failure 50 and Premature Ovarian Failure 17
In a consanguineous Chinese family in which 2 infertile brothers had azoospermia due to meiotic arrest (SPGF50; 619145), Yang et al. (2018) identified homozygosity for a missense mutation in the XRCC2 gene (L14P; 600375.0002) that segregated with disease. The authors noted that there was no evidence of cancer in the family, and that examination of the affected brothers by a neurologist, hematologist, and orthopedist did not detect any additional signs of disease.
In a Chinese woman with premature ovarian failure (POF17; 619146) and her brother, who was infertile due to azoospermia, Zhang et al. (2019) identified homozygosity for the L14P mutation in XRCC2. Their first-cousin parent were heterozygous for the mutation.
Role In Malignancy
Kuschel et al. (2002) performed genetic association studies in a population-based breast cancer case-control study analyzing polymorphisms in 7 genes involved in DNA repair. The association of a rare variant in XRCC2 (R188H) was marginally significant. In a comparable English cohort, Rafii et al. (2002) found that carriage of R188H was associated with breast cancer overall, and this association was enhanced when younger-onset cases with a positive family history were compared with older controls with no family history. Using site-directed mutagenesis of XRCC2, Rafii et al. (2002) further showed that nonconservative substitution or deletion of amino acid 188 of XRCC2 could significantly affect cellular sensitivity to DNA damage. The authors hypothesized that subtle variation in DNA repair capacity may influence cancer susceptibility in the population.
Loss of mismatch repair (MMR) leads to a complex mutator phenotype that appears to drive the development of a subset of colon cancers (see Peltomaki, 2001). Mohindra et al. (2002) showed that MMR-defective tumor cell lines were defective in homologous recombination repair (HRR) induced by DNA double-strand breaks (DSBs). A frameshift mutation (342delT) in XRCC2 found in the MMR-deficient uterine tumor cell line, SKUT-1, conferred thymidine sensitivity when introduced into an MMR-proficient line. Like other cells with defective XRCC2, SKUT-1 cells were sensitive to mitomycin C, and MMR-proficient cells expressing the mutant XRCC2 allele also became more sensitive to this agent. The authors suggested that the thymidine sensitivity of MMR-deficient tumor cell lines may be a consequence of defects in the homologous recombination repair pathway. Mohindra et al. (2004) introduced 342delT into HRR-proficient cells containing a recombination reporter substrate. In 1 set of transfectants, expression of 342delT conferred sensitivity to thymidine and mitomycin C and suppressed HRR induced at the recombination reporter by thymidine, but not by DSBs. In a second set of transfectants, expression of 342delT was accompanied by a decreased level of full-length XRCC2, and these cells were defective in induction of HRR by either thymidine or DSBs. Mohindra et al. (2004) concluded that 342delT suppresses recombination induced by thymidine in a dominant-negative manner, while recombination induced by DSBs appears to depend upon the level of XRCC2, as well as expression of the mutant XRCC2 allele. The authors suggested that HRR pathways responding to stalled replication forks or DSBs are genetically distinguishable and that XRCC2 has a critical role in HRR at replication forks, possibly in the loading of RAD51 onto gapped DNA.
Park et al. (2012) performed an exome-sequencing study of families with multiple breast cancer-affected individuals and identified 2 families with mutations, 1 with a protein-truncating mutation and 1 with a probable deleterious missense mutation in the XRCC2 gene. Park et al. (2012) then performed a population-based case-control mutation screening study that identified 6 probably pathogenic coding variants in 1,308 cases with early-onset breast cancer and no variants in 1,120 controls (the severity grading was p less than 0.02). They then performed additional mutation screening in 689 multiple-case families and identified 10 breast cancer-affected families with protein-truncating or probably deleterious rare missense variants in XRCC2.
Deans et al. (2000) found that most homozygous Xrcc2-null mice die midgestation. The few mice that survived to later stages showed developmental abnormalities and died at birth. Neonatal lethality, apparently due to respiratory failure, was associated with a high frequency of apoptotic death of postmitotic neurons in the developing brain, leading to abnormal cortical structure. Embryonic cells showed genetic instability, revealed by a high level of chromosomal aberrations, and were sensitive to gamma-rays. The findings demonstrated that homologous recombination has an important role in endogenous damage repair in the developing embryo.
Yang et al. (2018) generated mice homozygous for an L14P mutation (600375.0002) in the Xrcc2 gene. The homozygous males were infertile and had smaller testes than wildtype mice. Histologic examination of the infertile males' testes showed that nearly all of the seminiferous tubules were depleted, with no mature sperm present in any of the tubules after 7 days postpartum (dpp), consistent with complete meiotic arrest. The tubules contained multiple layers of spermatocytes arrested at the zygotene and pachytene states of meiotic prophase I. In addition, half of female homozygotes were infertile, and the other half had significantly reduced litter sizes. Infertile females had bilateral small, atrophic, and fibrotic ovaries without identifiable follicles, whereas homozygotes with reduced fertility had unilateral atrophic ovaries. Analysis of L14P homozygous ovaries from 7 dpp to 180 dpp showed gradual reduction in the percentage of ovaries with follicles, from 100% to 20%, consistent with premature ovarian failure.
In a 2.5-year-old boy, born of consanguineous Saudi Arabian parents, with an atypical form of Fanconi anemia, complementation group U (FANCU; 617247), Shamseldin et al. (2012) identified a homozygous c.643C-T transition (c.643C-T, NM_005431) in the XRCC2 gene, resulting in an arg215-to-ter (R215X) substitution predicted to remove the C terminus and abolish XRCC2 activity. The mutation was found by whole-exome sequencing followed by autozygome filtering. Chromosome testing in patient fibroblasts showed a marked increase in the frequency of dsDNA breaks in response to damage, indicating a defect in homologous recombination repair. Complementation studies were not performed. Shamseldin et al. (2012) noted the phenotypic similarities to Xrcc2-null mice (Deans et al., 2000).
Park et al. (2016) performed detailed studies on cells derived from the patient reported by Shamseldin et al. (2012). The mutant protein was not found by Western blot analysis, indicating that it is unstable, but likely not subject to nonsense-mediated mRNA decay since the mutation occurs in the last exon. Expression of wildtype XRCC2 corrected all 3 abnormal cellular phenotypes that were apparent in patient cells: cellular sensitivity to DNA interstrand crosslinking agents, chromosome instability, and accumulation of cells at the G2/M stage of the cell cycle. Patient cells showed normal levels of monoubiquitinated FANCD2 (613984), a central step in the FA pathway, and decreased assembly of RAD51 (179617) foci, suggesting that XRCC2 acts downstream of this event. Patient cells showed defective assembly of the components of the BCDX2 complex, particularly RAD51C (602774). Patient cells also showed increased sensitivity to ionizing radiation, consistent with a defect in proteins that act downstream in the FA pathway.
Spermatogenic Failure 50
In 2 infertile brothers from a consanguineous Chinese family with azoospermia due to meiotic arrest (SPGF50; 619145), Yang et al. (2018) identified homozygosity for a c.41T-C transition (c.41T-C, NM_005431) in the XRCC2 gene, resulting in a leu14-to-pro (L14P) substitution at a highly conserved residue near the splicing site of exon 2. The mutation segregated fully with disease in the family. Immunohistochemical analysis of seminiferous tubules from 1 of the affected brothers showed that XRCC2 protein was present, and RNA and protein assay on peripheral blood lymphocytes from the brothers confirmed the presence of the full XRCC2 transcript and the full XRCC2 protein. Quantification of chromosomal breaks induced by 2 chromosomal crosslinking agents, MMC or DPP, showed a similar frequency of chromosomal breaks in patient cells compared to control cells.
Premature Ovarian Failure 17
In a Chinese woman with premature ovarian failure (POF17; 619146) and her infertile brother, who had azoospermia due to meiotic arrest, Zhang et al. (2019) identified homozygosity for the L14P mutation in XRCC2. Their first-cousin parent were heterozygous for the mutation.
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