Correction of chromosomal instability and sensitivity to diverse mutagens by a cloned cDNA of the XRCC3 DNA repair gene

ATM dysfunction in Chinese hamster XRCC8 mutants

XRCC8 is a member of the X-ray cross-complementing (XRCC) family, whose responsible gene has not been identified. Previous studies suggested ATM and other genes were potential candidates for XRCC8, but this was not confirmed. In this study, we characterized three V79-derived XRCC8 mutant cells: V-C4, V-E5, and V-G8. Western blot analysis showed reduced expression of the ATM protein in three XRCC8 mutants, and radiation-induced phosphorylated ATM foci were not detected by fluorescence immunocytochemistry. Both ATM knockout cells and XRCC8 mutants exhibited hypersensitivity to camptothecin. Through a cell fusion-based complementation test, we found that XRCC8 mutants were complemented by ATM-proficient cells, but not by ATM knockout cells, in terms of camptothecin sensitivity. Comprehensive sequencing of the ATM genome in XRCC8 mutants revealed unique mutations in each mutant. These results suggest that XRCC8 mutants carry ATM mutations, and their ATM is not properly functional, despite protein expression being detected. This is similar to missense mutations in some Ataxia Telangiectasia patients.

Mechanism of BCDX2-mediated RAD51 nucleation on short ssDNA stretches and fork DNA

Homologous recombination (HR) factors are crucial for DSB repair and processing stalled replication forks. RAD51 paralogs, including RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3, have emerged as essential tumour suppressors, forming two subcomplexes, BCDX2 and CX3. Mutations in these genes are associated with cancer susceptibility and Fanconi anaemia, yet their biochemical activities remain unclear. This study reveals a linear arrangement of BCDX2 subunits compared to the RAD51 ring. BCDX2 shows a strong affinity towards single-stranded DNA (ssDNA) via unique binding mechanism compared to RAD51, and a contribution of DX2 subunits in binding branched DNA substrates. We demonstrate that BCDX2 facilitates RAD51 loading on ssDNA by suppressing the cooperative requirement of RAD51 binding to DNA and stabilizing the filament. Notably, BCDX2 also promotes RAD51 loading on short ssDNA and reversed replication fork substrates. Moreover, while mutants defective in ssDNA binding retain the ability to bind branched DNA substrates, they still facilitate RAD51 loading onto reversed replication forks. Our study provides mechanistic insights into how the BCDX2 complex stimulates the formation of BRCA2-independent RAD51 filaments on short stretches of ssDNA present at ssDNA gaps or stalled replication forks, highlighting its role in genome maintenance and DNA repair.

Comparative analyses reveal potential genetic mechanisms for high-altitude adaptation of Schizopygopsis fishes based on chromosome-level genomes

The schizothoracine fishes, widely distributed in the Qinghai-Tibetan Plateau and its adjacent areas, are considered as ideal models for investigation of high-altitude adaptation. Schizophygopsis are one group of the highly specialized schizothoracine fishes, and the genetic basis for their high-altitude adaptation is poorly understood. In this study, we performed comparative genomics analyses to investigate the potential genetic mechanisms for high-altitude adaptation of Schizopygopsis malacanthus and Schizopygopsis pylzovi based on the chromosome-level genomes. Functional enrichment analysis revealed that many expanded gene families in Schizopygopsis were associated with immune response while many contracted gene families were functionally associated with olfaction. Among the 123 positively selected genes (PSGs), angpt2a was detected in HIF-1 signaling pathway and possibly related to the hypoxia adaptation of Schizopygopsis. Furthermore, two PSGs cox15 and ndufb10 were distributed in thermogenesis, and there was a Schizopygopsis-specific missense mutation in cox15 (Gln115Glu), which possibly contributed to the cold temperature adaptation of the Schizopygopsis. Kyoto Encyclopedia of Genes and Genomes enrichment of the PSGs revealed three significant pathways including metabolic pathways, cell cycle, and homologous recombination and Gene Ontology enrichment analysis of the PSGs revealed several categories associated with DNA repair, cellular response to DNA damage stimulus, and metabolic process. Chromosome-scale characterization of olfactory receptor (OR) repertoires indicated that Schizopygopsis had the least number of OR genes, and the OR gene contraction was possibly caused by the limited food variety and the environmental factors such as lower air pressure, lower humidity, and lower temperature. Our study will help expand our understanding of the potential adaptive mechanism of Schizopygopsis to cope with the high-altitude conditions.

X-ray cross-complementing family: the bridge linking DNA damage repair and cancer

Genomic instability is a common hallmark of human tumours. As a carrier of genetic information, DNA is constantly threatened by various damaging factors that, if not repaired in time, can affect the transmission of genetic information and lead to cellular carcinogenesis. In response to these threats, cells have evolved a range of DNA damage response mechanisms, including DNA damage repair, to maintain genomic stability. The X-ray repair cross-complementary gene family (XRCC) comprises an important class of DNA damage repair genes that encode proteins that play important roles in DNA single-strand breakage and DNA base damage repair. The dysfunction of the XRCC gene family is associated with the development of various tumours. In the context of tumours, mutations in XRCC and its aberrant expression, result in abnormal DNA damage repair, thus contributing to the malignant progression of tumour cells. In this review, we summarise the significant roles played by XRCC in diverse tumour types. In addition, we discuss the correlation between the XRCC family members and tumour therapeutic sensitivity.

Association between XRCC3 p.Thr241Met polymorphism and risk of glioma: A systematic review and meta-analysis

BACKGROUND

The XRCC3 p.Thr241Met (rs861539) polymorphism has been extensively studied for its association with glioma risk, but results remain conflicting. Therefore, we performed a systematic review and meta-analysis to resolve this inconsistency.

METHODS

Studies published up to June 10, 2022, were searched in PubMed, Web of Science, Scopus, VIP, Wanfang, and China National Knowledge Infrastructure databases and screened for eligibility. Then, the combined odds ratio (OR) of the included studies was estimated based on five genetic models, i.e., homozygous (Met/Met vs. Thr/Thr), heterozygous (Thr/Met vs. Thr/Thr), dominant (Thr/Met + Met/Met vs. Thr/Thr), recessive (Met/Met vs. Thr/Thr + Thr/Met) and allele (Met vs. Thr). The study protocol was preregistered at PROSPERO (registration number: CRD42021235704).

RESULTS

Overall, our meta-analysis of 14 eligible studies involving 12,905 subjects showed that the p.Thr241Met polymorphism was significantly associated with increased glioma risk in both homozygous and recessive models (homozygous, OR = 1.381, 95% CI = 1.081-1.764, P = 0.010; recessive, OR = 1.305, 95% CI = 1.140-1.493, P<0.001). Subgroup analyses by ethnicity also revealed a statistically significant association under the two aforementioned genetic models, but only in the Asian population and not in Caucasians (P>0.05).

CONCLUSION

We demonstrated that the XRCC3 p.Thr241Met polymorphism is associated with an increased risk of glioma only in the homozygous and recessive models.

Noncanonical NF-κB factor p100/p52 regulates homologous recombination and modulates sensitivity to DNA-damaging therapy

Homologous recombination (HR) serves multiple roles in DNA repair that are essential for maintaining genomic stability, including double-strand DNA break (DSB) repair. The central HR protein, RAD51, is frequently overexpressed in human malignancies, thereby elevating HR proficiency and promoting resistance to DNA-damaging therapies. Here, we find that the non-canonical NF-κB factors p100/52, but not RelB, control the expression of RAD51 in various human cancer subtypes. While p100/p52 depletion inhibits HR function in human tumor cells, it does not significantly influence the proficiency of non-homologous end joining, the other key mechanism of DSB repair. Clonogenic survival assays were performed using a pair DLD-1 cell lines that differ only in their expression of the key HR protein BRCA2. Targeted silencing of p100/p52 sensitizes the HR-competent cells to camptothecin, while sensitization is absent in HR-deficient control cells. These results suggest that p100/p52-dependent signaling specifically controls HR activity in cancer cells. Since non-canonical NF-κB signaling is known to be activated after various forms of genomic crisis, compensatory HR upregulation may represent a natural consequence of DNA damage. We propose that p100/p52-dependent signaling represents a promising oncologic target in combination with DNA-damaging treatments.

XRCC3 loss leads to midgestational embryonic lethality in mice

RAD51 paralogs are key components of the homologous recombination (HR) machinery. Mouse mutants have been reported for four of the canonical RAD51 paralogs, and each of these mutants exhibits embryonic lethality, although at different gestational stages. However, the phenotype of mice deficient in the fifth RAD51 paralog, XRCC3, has not been reported. Here we report that Xrcc3 knockout mice exhibit midgestational lethality, with mild phenotypes beginning at about E8.25 but severe developmental abnormalities evident by E9.0-9.5. The most obvious phenotypes are small size and a failure of the embryo to turn to a fetal position. A knockin mutation at a key ATPase residue in the Walker A box results in embryonic lethality at a similar stage. Death of knockout mice can be delayed a few days for some embryos by homozygous or heterozygous Trp53 mutation, in keeping with an important role for XRCC3 in promoting genome integrity. Given that XRCC3 is a unique member of one of two RAD51 paralog complexes with RAD51C, these results demonstrate that both RAD51 paralog complexes are required for mouse development.

Association between XRCC3 Thr241Met polymorphism and risk of gynecological malignancies: A meta-analysis

Studies have investigated the relationship between the X-ray cross- complementing group 3 (XRCC3) Thr241Met polymorphism and the risk of gynecological malignancies (GM) with the contradictory conclusions. Here, a meta-analysis was performed to provide clear picture of the association between Thr241Met and GM risk. The Pubmed and Chinese National Knowledge Infrastructure (CNKI) databases were searched for published eligible studies. The pooled odds ratios (OR) with their corresponding 95% confidence interval (CI) was used to assessed the strength of association. Totally, 15 publications with 5,740 cases and 9,931 controls were included. In the overall analysis, the results of meta-analysis showed no significant association between the Thr241Met and the risk of GM. However, in the Asians subgroup, significant increased risks were found in the comparisons of TT/CT+TT vs. CC(TT vs. CC: OR=3.25, 95% CI=1.47-7.18; CT+TT vs. CC: OR=1.51, 95%CI=1.10-2.09) in Asians; additionally, stratified analysis by cancer type in Asians, significantly increased risks was found in cervical carcinoma (CT vs. CC: OR=1.50, 95%CI=1.04-2.14; TT vs. CC: OR=3.14, 95%CI=1.38-7.14; CT+TT vs. CC: OR=1.64, 95% CI=1.17-2.31). It suggests that the risk of GM might be significantly increased by the XRCC3 Thr241Met polymorphism according to ethnicity and cancer types. Further studies with larger sample size in different ethnic populations and different sites of GM are needed to verify the findings.

ZmRAD51C is Essential for Double-Strand Break Repair and Homologous Recombination in Maize Meiosis

Radiation sensitive 51 (RAD51) recombinases play crucial roles in meiotic double-strand break (DSB) repair mediated by homologous recombination (HR) to ensure the correct segregation of homologous chromosomes. In this study, we identified the meiotic functions of ZmRAD51C, the maize homolog of Arabidopsis and rice RAD51C. The Zmrad51c mutants exhibited regular vegetative growth but complete sterility for both male and female inflorescence. However, the mutants showed hypersensitivity to DNA damage by mitomycin C. Cytological analysis indicated that homologous chromosome pairing and synapsis were rigorously inhibited, and meiotic chromosomes were often entangled from diplotene to metaphase I, leading to chromosome fragmentation at anaphase I. Immunofluorescence analysis showed that although the signals of the axial element absence of first division (AFD1) and asynaptic1 (ASY1) were normal, the assembly of the central element zipper1 (ZYP1) was severely disrupted. The DSB formation was normal in Zmrad51c meiocytes, symbolized by the regular occurrence of γH2AX signals. However, RAD51 and disrupted meiotic cDNA 1 (DMC1) signals were never detected at the early stage of prophase I in the mutant. Taken together, our results indicate that ZmRAD51C functions crucially for both meiotic DSB repair and homologous recombination in maize.

Differential Requirements for the RAD51 Paralogs in Genome Repair and Maintenance in Human Cells

Deficiency in several of the classical human RAD51 paralogs [RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3] is associated with cancer predisposition and Fanconi anemia. To investigate their functions, isogenic disruption mutants for each were generated in non-transformed MCF10A mammary epithelial cells and in transformed U2OS and HEK293 cells. In U2OS and HEK293 cells, viable ablated clones were readily isolated for each RAD51 paralog; in contrast, with the exception of RAD51B, RAD51 paralogs are cell-essential in MCF10A cells. Underlining their importance for genomic stability, mutant cell lines display variable growth defects, impaired sister chromatid recombination, reduced levels of stable RAD51 nuclear foci, and hyper-sensitivity to mitomycin C and olaparib, with the weakest phenotypes observed in RAD51B-deficient cells. Altogether these observations underscore the contributions of RAD51 paralogs in diverse DNA repair processes, and demonstrate essential differences in different cell types. Finally, this study will provide useful reagents to analyze patient-derived mutations and to investigate mechanisms of chemotherapeutic resistance deployed by cancers.

Epigenome-wide association study of leukocyte telomere length

Telomere length is associated with age-related diseases and is highly heritable. It is unclear, however, to what extent epigenetic modifications are associated with leukocyte telomere length (LTL). In this study, we conducted a large-scale epigenome-wide association study (EWAS) of LTL using seven large cohorts (n=5,713) - the Framingham Heart Study, the Jackson Heart Study, the Women's Health Initiative, the Bogalusa Heart Study, the Lothian Birth Cohorts of 1921 and 1936, and the Longitudinal Study of Aging Danish Twins. Our stratified analysis suggests that EWAS findings for women of African ancestry may be distinct from those of three other groups: males of African ancestry, and males and females of European ancestry. Using a meta-analysis framework, we identified DNA methylation (DNAm) levels at 823 CpG sites to be significantly associated (P<1E-7) with LTL after adjusting for age, sex, ethnicity, and imputed white blood cell counts. Functional enrichment analyses revealed that these CpG sites are near genes that play a role in circadian rhythm, blood coagulation, and wound healing. Weighted correlation network analysis identified four co-methylation modules associated with LTL, age, and blood cell counts. Overall, this study reveals highly significant relationships between two hallmarks of aging: telomere biology and epigenetic changes.

Optimization of Drug Candidates That Inhibit the D-Loop Activity of RAD51

RAD51 is the central protein in homologous recombination (HR) repair, where it first binds ssDNA and then catalyzes strand invasion via a D-loop intermediate. Additionally, RAD51 plays a role in faithful DNA replication by protecting stalled replication forks; this requires RAD51 to bind DNA but may not require the strand invasion activity of RAD51. We previously described a small-molecule inhibitor of RAD51 named RI(dl)-2 (RAD51 inhibitor of D-loop formation #2, hereafter called 2 h), which inhibits D-loop activity while sparing ssDNA binding. However, 2 h is limited in its ability to inhibit HR in vivo, preventing only about 50 % of total HR events in cells. We sought to improve upon this by performing a structure-activity relationship (SAR) campaign for more potent analogues of 2 h. Most compounds were prepared from 1-(2-aminophenyl)pyrroles by forming the quinoxaline moiety either by condensation with aldehydes, then dehydrogenation of the resulting 4,5-dihydro intermediates, or by condensation with N,N'-carbonyldiimidazole, chlorination, and installation of the 4-substituent through Suzuki-Miyaura coupling. Many analogues exhibited enhanced activity against human RAD51, but in several of these compounds the increased inhibition was due to the introduction of dsDNA intercalation activity. We developed a sensitive assay to measure dsDNA intercalation, and identified two analogues of 2 h that promote complete HR inhibition in cells while exerting minimal intercalation activity.

A new perspective on oxidation of DNA repair proteins and cancer

Reactive oxygen and nitrogen species (RONS) are formed as byproducts of many endogenous cellular processes, in response to infections, and upon exposure to various environmental factors. An increase in RONS can saturate the antioxidation system and leads to oxidative stress. Consequently, macromolecules are targeted for oxidative modifications, including DNA and protein. The oxidation of DNA, which leads to base modification and formation of abasic sites along with single and double strand breaks, has been extensively investigated. Protein oxidation is often neglected and is only recently being recognized as an important regulatory mechanism of various DNA repair proteins. This is a review of the current state of research on the regulation of DNA repair by protein oxidation with emphasis on the correlation between inflammation and cancer.

Dynamin impacts homology-directed repair and breast cancer response to chemotherapy

After the initial responsiveness of triple-negative breast cancers (TNBCs) to chemotherapy, they often recur as chemotherapy-resistant tumors, and this has been associated with upregulated homology-directed repair (HDR). Thus, inhibitors of HDR could be a useful adjunct to chemotherapy treatment of these cancers. We performed a high-throughput chemical screen for inhibitors of HDR from which we obtained a number of hits that disrupted microtubule dynamics. We postulated that high levels of the target molecules of our screen in tumors would correlate with poor chemotherapy response. We found that inhibition or knockdown of dynamin 2 (DNM2), known for its role in endocytic cell trafficking and microtubule dynamics, impaired HDR and improved response to chemotherapy of cells and of tumors in mice. In a retrospective analysis, levels of DNM2 at the time of treatment strongly predicted chemotherapy outcome for estrogen receptor-negative and especially for TNBC patients. We propose that DNM2-associated DNA repair enzyme trafficking is important for HDR efficiency and is a powerful predictor of sensitivity to breast cancer chemotherapy and an important target for therapy.

SETD2 alterations impair DNA damage recognition and lead to resistance to chemotherapy in leukemia

Mutations in SETD2, encoding the histone 3 lysine 36 trimethyltransferase, are enriched in relapsed acute lymphoblastic leukemia and MLL-rearranged acute leukemia. We investigated the impact of SETD2 mutations on chemotherapy sensitivity in isogenic leukemia cell lines and in murine leukemia generated from a conditional knockout of Setd2. SETD2 mutations led to resistance to DNA-damaging agents, cytarabine, 6-thioguanine, doxorubicin, and etoposide, but not to a non-DNA damaging agent, l-asparaginase. H3K36me3 localizes components of the DNA damage response (DDR) pathway and SETD2 mutation impaired DDR, blunting apoptosis induced by cytotoxic chemotherapy. Consistent with local recruitment of DDR, genomic regions with higher H3K36me3 had a lower mutation rate, which was increased with SETD2 mutation. Heterozygous conditional inactivation of Setd2 in a murine model decreased the latency of MLL-AF9-induced leukemia and caused resistance to cytarabine treatment in vivo, whereas homozygous loss delayed leukemia formation. Treatment with JIB-04, an inhibitor of the H3K9/36me3 demethylase KDM4A, restored H3K36me3 levels and sensitivity to cytarabine. These findings establish SETD2 alteration as a mechanism of resistance to DNA-damaging chemotherapy, consistent with a local loss of DDR, and identify a potential therapeutic strategy to target SETD2-mutant leukemias.

Association between the DNA Repair Gene XRCC3 rs861539 Polymorphism and Risk of Osteosarcoma: a Systematic Review and Meta-Analysis

Objective: Although there are a few studies investigating the relation between X-Ray Repair Cross Complementing 3 (XRCC3) gene rs861539 polymorphism and osteosarcoma (OSA), the results are inconsistent. Therefore, we performed this systematic review and meta-analysis to clarify the associations between XRCC3 rs861539 polymorphism and OSA risk. Methods: We have retrieved published literature from PubMed, Google scholar, and ISI Web of Knowledge up to 25 January 2017. Odds ratios were pooled using either fixed-effects or random effects models. Overall and subgroup analyses were performed. Statistical analysis was performed running comprehensive meta-analysis (CMA) 2.0 software. Results: A total of four studies with 515 cases and 1,109 controls were identified in order to investigate the association between XRCC3 rs861539 polymorphism and OSA risk. The results showed that XRCC3 rs861539 polymorphism was associated with OSA in allelic (T vs. C: OR= 1.563, 95% CI: 1.244-1.963, p= <0.001), homozygote (TT vs. CC: OR= 2.574, 95% CI: 1.573-4.212, p= <0.001), dominant (TT+TC vs. CC: OR= 1.255, 95% CI: 1.011-1.558, p= 0.039), and recessive (TT vs. TC+CC: OR= 2.224, 95% CI: 1.393-3.552, p= 0.001), but not with heterozygote (TC vs. CC: OR= 1.361, 95% CI: 0.982-1.885, p= 0.064). The XRCC3 rs861539 polymorphism conferred susceptibility to OSA in Asians, but not in Caucasians. Additionally, we observed no evidence of publication bias. Conclusion: To the best of our knowledge, this is the first meta-analysis investigating the association between XRCC3 rs861539 polymorphism and OSA risk. Our results revealed a significant association between the XRCC3 rs861539 polymorphism and risk of OSA, especially in Asian populations. Future more comprehensive and well-designed case control studies with larger sample size are needed to warrant these findings.

Recent Developments Using Small Molecules to Target RAD51: How to Best Modulate RAD51 for Anticancer Therapy?

Homologous recombination (HR) is an evolutionarily conserved DNA repair process. Overexpression of the key HR protein RAD51 is a common feature of malignant cells. RAD51 plays two distinct genome-stabilizing roles, including HR-mediated repair of double-strand breaks (DSBs) and the promotion of replication fork stability during replication stress. Because upregulation of RAD51 in cancer cells can promote tumor resistance to DNA-damaging oncologic therapies, we and others have worked to develop cancer therapeutics that target various aspects of RAD51 protein function. Herein, we provide an overview of recent developments in this field, together with our perspectives on the challenges associated with these evolving anticancer strategies.

The Shu complex is a conserved regulator of homologous recombination

Homologous recombination (HR) is an error-free DNA repair mechanism that maintains genome integrity by repairing double-strand breaks (DSBs). Defects in HR lead to genomic instability and are associated with cancer predisposition. A key step in HR is the formation of Rad51 nucleoprotein filaments which are responsible for the homology search and strand invasion steps that define HR. Recently, the budding yeast Shu complex has emerged as an important regulator of Rad51 along with the other Rad51 mediators including Rad52 and the Rad51 paralogs, Rad55-Rad57. The Shu complex is a heterotetramer consisting of two novel Rad51 paralogs, Psy3 and Csm2, along with Shu1 and a SWIM domain-containing protein, Shu2. Studies done primarily in yeast have provided evidence that the Shu complex regulates HR at several types of DNA DSBs (i.e. replication-associated and meiotic DSBs) and that its role in HR is highly conserved across eukaryotic lineages. This review highlights the main findings of these studies and discusses the proposed specific roles of the Shu complex in many aspects of recombination-mediated DNA repair.

Development of Small Molecules that Specifically Inhibit the D-loop Activity of RAD51

RAD51 is the central protein in homologous recombination (HR) DNA repair and represents a therapeutic target in oncology. Herein we report a novel class of RAD51 inhibitors that were identified by high throughput screening. In contrast to many previously reported RAD51 inhibitors, our lead compound 1 is capable of blocking RAD51-mediated D-loop formation (IC50 21.3 ± 7.8 μM) at concentrations that do not influence RAD51 binding to ssDNA. In human cells, 1 inhibits HR (IC50 13.1 ± 1.6 μM) without blocking RAD51's ability to assemble into subnuclear foci at sites of DNA damage. We determined that the active constituent of 1 is actually an oxidized derivative (termed RI(dl)-1 or 8) of the original screening compound. Our SAR campaign also yielded RI(dl)-2 (hereafter termed 9h), which effectively blocks RAD51's D-loop activity in biochemical systems (IC50 11.1 ± 1.3 μM) and inhibits HR activity in human cells (IC50 3.0 ± 1.8 μM).

BRCA2 is needed for both repair and cell cycle arrest in mammalian cells exposed to S23906, an anticancer monofunctional DNA binder

Repair of DNA-targeted anticancer agents is an active area of investigation of both fundamental and clinical interest. However, most studies have focused on a small number of compounds limiting our understanding of both DNA repair and the DNA damage response. S23906 is an acronycine derivative that shows strong activity toward solid tumors in experimental models. S23906 forms bulky monofunctional DNA adducts in the minor groove which leads to destabilization of the double-stranded helix. We now report that S23906 induces formation of DNA double strand breaks that are processed through homologous recombination (HR) but not Non-Homologous End-Joining (NHEJ) repair. Interestingly, S23906 exposure was accompanied by a higher sensitivity of BRCA2-deficient cells compared to other HR deficient cell lines and by an S-phase accumulation in wild-type (wt), but not in BRCA2-deficient cells. Recently, we have shown that S23906-induced S phase arrest was mediated by the checkpoint kinase Chk1. However, its activated phosphorylated form is equally induced by S23906 in wt and BRCA2-deficient cells, likely indicating a role for BRCA2 downstream of Chk1. Accordingly, override of the S phase arrest by either 7-hydroxystaurosporine (UCN-01) or AZD7762 potentiates the cytotoxic activity of S23906 in wt, but not in BRCA2-deficient cells. Together, our findings suggest that the pronounced sensitivity of BRCA2-deficient cells to S23906 is due to both a defective S-phase arrest and the absence of HR repair. Tumors with deficiencies for proteins involved in HR, and BRCA2 in particular, may thus show increased sensitivity to S23906, thereby providing a rationale for patient selection in clinical trials.

Quantitative assessment of the influence of X-ray repair cross-complementing group 3 rs861539 polymorphism and cutaneous melanoma susceptibility

Previous studies evaluating the association between X-ray repair cross-complementing group 3 (XRCC3) rs861539 polymorphism and cutaneous melanoma susceptibility reported conflicting findings. To draw a more precise association between XRCC3 rs861539 polymorphism and cutaneous melanoma susceptibility, we searched PubMed, EMBASE and Web of Science for case-control studies. A total of eight case-control studies including 3463 cases of melanoma and 4216 controls were included in the meta-analysis. Overall, no significant associations were found between XRCC3 rs861539 polymorphism and cutaneous melanoma susceptibility under all four genetic models (TT vs CC: OR 0.99, 95 % CI 0.86-1.14; TC vs CC: OR 0.92, 95 % CI 0.83-1.01; dominant model: OR 0.94, 95 % CI 0.85-1.03; recessive model: OR 1.05, 95 % CI 0.92-1.19). In the subgroup by source of control, no significant associations were found in hospital-based and population-based subgroup. This meta-analysis suggested that the XRCC3 rs861539 polymorphism was not a risk factor for cutaneous melanoma susceptibility.

Homologous recombination and human health: the roles of BRCA1, BRCA2, and associated proteins

Homologous recombination (HR) is a major pathway for the repair of DNA double-strand breaks in mammalian cells, the defining step of which is homologous strand exchange directed by the RAD51 protein. The physiological importance of HR is underscored by the observation of genomic instability in HR-deficient cells and, importantly, the association of cancer predisposition and developmental defects with mutations in HR genes. The tumor suppressors BRCA1 and BRCA2, key players at different stages of HR, are frequently mutated in familial breast and ovarian cancers. Other HR proteins, including PALB2 and RAD51 paralogs, have also been identified as tumor suppressors. This review summarizes recent findings on BRCA1, BRCA2, and associated proteins involved in human disease with an emphasis on their molecular roles and interactions.

Association between the XRCC3 Thr241Met polymorphism and risk of colorectal cancer: a meta analysis of 5,193 cases and 6,645 controls

BACKGROUND

Many studies have reported associations of the X-ray repair cross-complementing group 3 (XRCC3) Thr241Met polymorphism with colorectal cancer (CRC) risk, but the results remained controversial. Hence, we performed the present meta-analysis with different inheritance models.

MATERIALS AND METHODS

We searched the PubMed and Google scholar databases for studies relating to associations between XRCC3 Thr241Met polymorphism and risk of CRC. 16 studies with 5,193 cases and 6,645 controls were finally included into the meta-analysis.

RESULTS

We found that the XRCC3 Thr241Met polymorphism was associated with increased CRC risk only under a dominant genetic model (CC+CT vs. TT: OR 0.575, 95%CI 0.498-1.665, p<0.001, Pheterogeneity=0.00, I2=83%). There was a significant association between XRCC3 Thr241Met polymorphism and CRC risk in Caucasian in the overall 8 studies under only in the heterozygote genetic model (CT vs. TT: OR=0.929, 95%CI=0.806-1.070, P=0.308, Pheterogeneity=0.002, I2=57%). Four studies evaluated the XRCC3 Thr241Met polymorphism and CRC risk in Asians. Two genetic models of the XRCC3 polymorphism were significantly correlated with increasing risk in Asians (dominant model: CC+CT vs. TT: OR= 0.609, 95%CI=411-0.902, P=0.013, Pheterogeneity=0.54, I2=0.00%; Allele model: C vs. T: OR=0.708, 95 %=CI 0.605-0.829, p=0.000, Pheterogeneity=0.000, I2=92%). The sensitivity analysis suggested stability of this meta-analysis and no publication bias was detected.

CONCLUSIONS

In conclusion, this meta-analysis indicates that XRCC3 Thr241Met shows an increased CRC risk, particularly in Asians rather than Caucasians.

The major DNA repair pathway after both proton and carbon-ion radiation is NHEJ, but the HR pathway is more relevant in carbon ions

The purpose of this study was to identify the roles of non-homologous end-joining (NHEJ) or homologous recombination (HR) pathways in repairing DNA double-strand breaks (DSBs) induced by exposure to high-energy protons and carbon ions (C ions) versus gamma rays in Chinese hamster cells. Two Chinese hamster cell lines, ovary AA8 and lung fibroblast V79, as well as various mutant sublines lacking DNA-PKcs (V3), X-ray repair cross-complementing protein-4 [XRCC4 (XR1), XRCC3 (irs1SF) and XRCC2 (irs1)] were exposed to gamma rays ((137)Cs), protons (200 MeV; 2.2 keV/μm) and C ions (290 MeV; 50 keV/μm). V3 and XR1 cells lack the NHEJ pathway, whereas irs1 and irs1SF cells lack the HR pathway. After each exposure, survival was measured using a clonogenic survival assay, in situ DSB induction was evaluated by immunocytochemical analysis of histone H2AX phosphorylation at serine 139 (γ-H2AX foci) and chromosome aberrations were examined using solid staining. The findings from this study showed that clonogenic survival clearly depended on the NHEJ and HR pathway statuses, and that the DNA-PKcs(-/-) cells (V3) were the most sensitive to all radiation types. While protons and γ rays yielded almost the same biological effects, C-ion exposure greatly enhanced the sensitivity of wild-type and HR-deficient cells. However, no significant enhancement of sensitivity in cell killing was seen after C-ion irradiation of NHEJ deficient cells. Decreases in the number of γ-H2AX foci after irradiation occurred more slowly in the NHEJ deficient cells. In particular, V3 cells had the highest number of residual γ-H2AX foci at 24 h after C-ion irradiation. Chromosomal aberrations were significantly higher in both the NHEJ- and HR-deficient cell lines than in wild-type cell lines in response to all radiation types. Protons and gamma rays induced the same aberration levels in each cell line, whereas C ions introduced higher but not significantly different aberration levels. Our results suggest that the NHEJ pathway plays an important role in repairing DSBs induced by both clinical proton and C-ion beams. Furthermore, in C ions the HR pathway appears to be involved in the repair of DSBs to a greater extent compared to gamma rays and protons.

Genetic variation in a DNA double strand break repair gene in saudi population: a comparative study with worldwide ethnic groups

DNA repair capacity is crucial in maintaining cellular functions and homeostasis. However, it can be altered based on DNA sequence variations in DNA repair genes and this may lead to the development of many diseases including malignancies. Identification of genetic polymorphisms responsible for reduced DNA repair capacity is necessary for better prevention. Homologous recombination (HR), a major double strand break repair pathway, plays a critical role in maintaining the genome stability. The present study was performed to determine the frequency of the HR gene XRCC3 Exon 7 (C18067T, rs861539) polymorphisms in Saudi Arabian population in comparison with epidemiological studies by "MEDLINE" search to equate with global populations. The variant allelic (T) frequency of XRCC3 (C>T) was found to be 39%. Our results suggest that frequency of XRCC3 (C>T) DNA repair gene exhibits distinctive patterns compared with the Saudi Arabian population and this might be attributed to ethnic variation. The present findings may help in high-risk screening of humans exposed to environmental carcinogens and cancer predisposition in different ethnic groups.

RAD51B Activity and Cell Cycle Regulation in Response to DNA Damage in Breast Cancer Cell Lines

Common genetic variants mapping to two distinct regions of RAD51B, a paralog of RAD51, have been associated with breast cancer risk in genome-wide association studies (GWAS). RAD51B is a plausible candidate gene because of its established role in the homologous recombination (HR) process. How germline genetic variation in RAD51B confers susceptibility to breast cancer is not well understood. Here, we investigate the molecular function of RAD51B in breast cancer cell lines by knocking down RAD51B expression by small interfering RNA and treating cells with DNA-damaging agents, namely cisplatin, hydroxyurea, or methyl-methanesulfonate. Our results show that RAD51B-depleted breast cancer cells have increased sensitivity to DNA damage, reduced efficiency of HR, and altered cell cycle checkpoint responses. The influence of RAD51B on the cell cycle checkpoint is independent of its role in HR and further studies are required to determine whether these functions can explain the RAD51B breast cancer susceptibility alleles.

Genetic risk score and acute skin toxicity after breast radiation therapy

Genetic predisposition has been shown to affect the severity of skin complications in breast cancer patients after radiotherapy. Limited data exist regarding the use of a genetic risk score (GRS) for predicting risk of tissue radiosensitivity. We evaluated the impact of different single-nucleotide polymorphisms (SNPs) in genes related to DNA repair mechanisms and oxidative stress response combined in a GRS on acute adverse effects induced by breast radiation therapy (RT). Skin toxicity was scored according to the Radiation Therapy Oncology Group (RTOG) criteria in 59 breast cancer patients who received RT. After genotyping, a multilocus GRS was constructed by summing the number of risk alleles. The hazard ratio (HR) for GSTM1 was 2.4 (95% confidence intervals [CI]=1.1-5.3, p=0.04). The other polymorphisms were associated to an increased adverse radiosensitivity, although they did not reach statistical significance. GRS predicted roughly 40% risk for acute skin toxicity per risk allele (HR 1.37, 95% CI=1.1-1.76, p<0.01). Patients in the top tertile had a fivefold higher risk of skin reaction (HR 5.1, 95% CI=1.2-22.8, p=0.03). Our findings demonstrate that the joint effect of SNPs from oxidative stress and DNA damage repair genes may be a promising approach to identify patients with a high risk of skin reaction after breast RT.

Analyzing association of the XRCC3 gene polymorphism with ovarian cancer risk

This meta-analysis aims to examine whether the XRCC3 polymorphisms are associated with ovarian cancer risk. Eligible case-control studies were identified through search in PubMed. Pooled odds ratios (ORs) were appropriately derived from fixed effects models. We therefore performed a meta-analysis of 5,302 ovarian cancer cases and 8,075 controls from 4 published articles and 8 case-control studies for 3 SNPs of XRCC3. No statistically significant associations between XRCC3 rs861539 polymorphisms and ovarian cancer risk were observed in any genetic models. For XRCC3 rs1799794 polymorphisms, we observed a statistically significant correlation with ovarian cancer risk using the homozygote comparison (T2T2 versus T1T1: OR = 0.70, 95% CI = 0.54–0.90, P = 0.005), heterozygote comparison (T1T2 versus T1T1: OR = 1.10, 95% CI = 1.00–1.21, P = 0.04), and the recessive genetic model (T2T2 versus T1T1+T1T2: OR = 0.67, 95% CI = 0.52–0.87, P = 0.002). For XRCC3 rs1799796 polymorphisms, we also observed a statistically significant correlation with ovarian cancer risk using the heterozygote comparison (T1T2 versus T1T1: OR = 0.91, 95% CI = 0.83–0.99, P = 0.04). In conclusion, this meta-analysis shows that the XRCC3 were associated with ovarian cancer risk overall for Caucasians. Asian and African populations should be further studied.

The RAD51-stimulatory compound RS-1 can exploit the RAD51 overexpression that exists in cancer cells and tumors

RAD51 is the central protein that catalyzes DNA repair via homologous recombination, a process that ensures genomic stability. RAD51 protein is commonly expressed at high levels in cancer cells relative to their noncancerous precursors. High levels of RAD51 expression can lead to the formation of genotoxic RAD51 protein complexes on undamaged chromatin. We developed a therapeutic approach that exploits this potentially toxic feature of malignancy, using compounds that stimulate the DNA-binding activity of RAD51 to promote cancer cell death. A panel of immortalized cell lines was challenged with the RAD51-stimulatory compound RS-1. Resistance to RS-1 tended to occur in cells with higher levels of RAD54L and RAD54B, which are Swi2/Snf2-related translocases known to dissociate RAD51 filaments from dsDNA. In PC3 prostate cancer cells, RS-1-induced lethality was accompanied by the formation of microscopically visible RAD51 nuclear protein foci occurring in the absence of any DNA-damaging treatment. Treatment with RS-1 promoted significant antitumor responses in a mouse model, providing proof-of-principle for this novel therapeutic strategy.

Comparative analysis of the gonadal transcriptomes of the all-female species Poecilia formosa and its maternal ancestor Poecilia mexicana

Background

The Amazon molly, Poecilia formosa (Teleostei: Poeciliinae) is an unisexual, all-female species. It evolved through the hybridisation of two closely related sexual species and exhibits clonal reproduction by sperm dependent parthenogenesis (or gynogenesis) where the sperm of a parental species is only used to activate embryogenesis of the apomictic, diploid eggs but does not contribute genetic material to the offspring.

Here we provide and describe the first de novo assembled transcriptome of the Amazon molly in comparison with its maternal ancestor, the Atlantic molly Poecilia mexicana. The transcriptome data were produced through sequencing of single end libraries (100 bp) with the Illumina sequencing technique.

Results

83,504,382 reads for the Amazon molly and 81,625,840 for the Atlantic molly were assembled into 127,283 and 78,961 contigs for the Amazon molly and the Atlantic molly, respectively. 63% resp. 57% of the contigs could be annotated with gene ontology terms after sequence similarity comparisons. Furthermore, we were able to identify genes normally involved in reproduction and especially in meiosis also in the transcriptome dataset of the apomictic reproducing Amazon molly.

Conclusions

We assembled and annotated the transcriptome of a non-model organism, the Amazon molly, without a reference genome (de novo). The obtained dataset is a fundamental resource for future research in functional and expression analysis. Also, the presence of 30 meiosis-specific genes within a species where no meiosis is known to take place is remarkable and raises new questions for future research.

Nuclear proteomics with XRCC3 knockdown to reveal the development of doxorubicin-resistant uterine cancer

The nucleus is a key organelle in mammary cells, which is responsible for several cellular functions including cell proliferation, gene expression, and cell survival. In addition, the nucleus is the primary targets of doxorubicin treatment. In the current study, low-abundance nuclear proteins were enriched for proteomic analysis by using a state-of-the-art two-dimensional differential gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) strategy to compare and identify the nuclear protein profiling changes responsible for the development of doxorubicin resistance in human uterine cancer cells. The results of the nuclear proteomic analysis indicated that more than 2100 protein features were resolved from an equal pooled amount of three purified nuclear proteins and 117 differentially expressed spots were identified. Of these 117 identified proteins, 48 belonged to nuclear proteins and a positive correlation was observed between the expression levels of 32 of these nuclear proteins and an increase in drug resistance. According to our review of relevant research, nuclear proteins such as DNA repair protein XRCC3 (XRCC3) have not been reported to play roles in the formation of doxorubicin resistance. Previous studies have used RNA interference and cell viability analysis to evidence the essential roles of XRCC3 on its potency in the formation of doxorubicin resistance. To sum up, our nuclear proteomic approaches enabled us to identify numerous proteins, including XRCC3, involved in various drug-resistance-forming mechanisms. Our results provide potential diagnostic markers and therapeutic candidates for treating doxorubicin-resistant uterine cancer.

Association of XRCC3 Thr241Met polymorphisms and gliomas risk: evidence from a meta-analysis

The relationship between the X-ray repair cross-complementing group 3 (XRCC3) Thr241Met polymorphism and gliomas remains inclusive or controversial. For better understanding of the effect of XRCC3 Thr241Met polymorphism on glioma risk, a meta-analysis was performed. All eligible studies were identified through a search of PubMed, Elsevier Science Direct, Excerpta Medica Database (Embase) and Chinese Biomedical Literature Database (CBM) before May 2013. The association between the XRCC3 Thr241Met polymorphism and gliomas risk was conducted by odds ratios (ORs) and 95% confidence intervals (95% CIs). A total of nine case-control studies including 3,533 cases and 4,696 controls were eventually collected. Overall, we found that XRCC3 Thr241Met polymorphism was significantly associated with the risk of gliomas (T vs. C: OR=1.10, 95%CI=1.01-1.20, P=0.034; TT vs. CC: OR=1.30, 95%CI=1.03-1.65, P=0.027; TT vs.

TC/CC

OR=1.29, 95%CI=1.01-1.64, P=0.039). In the subgroup analysis based on ethnicity, the significant association was found in Asian under four models (T vs. C: OR=1.17, 95%CI=1.07-1.28, P=0.00; TT vs. CC: OR=1.79, 95%CI=1.36- 2.36, P=0.00; TT vs.

TC/CC

OR=1.75, 95%CI=1.32-2.32, P=0.00; TT/TC vs. CC: OR=1.11,95% CI=1.02-1.20). This meta-analysis suggested that the XRCC3 Thr241Met polymorphism is a risk factor for gliomas, especially for Asians. Considering the limited sample size and ethnicities included in the meta-analysis, further large scale and well-designed studies are needed to confirm our results.

Therapy-related myelodysplasia and acute myeloid leukemia

Therapy-related leukemia (myelodysplasia and acute myeloid leukemia-t-MDS/AML) is a well-known complication of conventional chemoradiotherapy used to treat a variety of primary malignancies including Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL), acute lymphoblastic leukemia (ALL), sarcoma, and ovarian and testicular cancers. The median time to development of t-MDS/AML is 3-5 years, with the risk decreasing markedly after the first decade. t-MDS/AML is the major cause of non-relapse mortality after autologous hematopoietic cell transplantation (HCT) for HL or NHL. The magnitude of risk of t-MDS/AML is higher, and the latency is shorter after HCT, compared to conventional therapy. Two types of t-MDS/AML are recognized depending on the causative therapeutic exposure: an alkylating agent/radiation-related type and a topoisomerase II inhibitor-related type. Inter-individual variability in the risk for development of t-MDS/AML suggests a role for genetic variation in susceptibility to genotoxic exposures. Treatment of t-MDS/AML with conventional therapy is associated with a uniformly poor prognosis, with a median survival of 6 months. Because of the poor response to conventional chemotherapy, allogeneic HCT is recommended. Current research is focused on developing risk prediction and risk reduction strategies.

Polymorphisms in human DNA repair genes and head and neck squamous cell carcinoma

Genetic polymorphisms in some DNA repair proteins are associated with a number of malignant transformations like head and neck squamous cell carcinoma (HNSCC). Xeroderma pigmentosum group D (XPD) and X-ray repair cross-complementing proteins 1 (XRCC1) and 3 (XRCC3) genes are involved in DNA repair and were found to be associated with HNSCC in numerous studies. To establish our overall understanding of possible relationships between DNA repair gene polymorphisms and development of HNSCC, we surveyed the literature on epidemiological studies that assessed potential associations with HNSCC risk in terms of gene-environment interactions, genotype-induced functional defects in enzyme activity and/or protein expression, and the influence of ethnic origin on these associations.We conclude that large, well-designed studies of common polymorphisms in DNA repair genes are needed. Such studies may benefit from analysis of multiple genes or polymorphisms and from the consideration of relevant exposures that may influence the likelihood of HNSCC when DNA repair capacity is reduced.

Roles of XRCC2, RAD51B and RAD51D in RAD51-independent SSA recombination

The repair of DNA double-strand breaks by recombination is key to the maintenance of genome integrity in all living organisms. Recombination can however generate mutations and chromosomal rearrangements, making the regulation and the choice of specific pathways of great importance. In addition to end-joining through non-homologous recombination pathways, DNA breaks are repaired by two homology-dependent pathways that can be distinguished by their dependence or not on strand invasion catalysed by the RAD51 recombinase. Working with the plant Arabidopsis thaliana, we present here an unexpected role in recombination for the Arabidopsis RAD51 paralogues XRCC2, RAD51B and RAD51D in the RAD51-independent single-strand annealing pathway. The roles of these proteins are seen in spontaneous and in DSB-induced recombination at a tandem direct repeat recombination tester locus, both of which are unaffected by the absence of RAD51. Individual roles of these proteins are suggested by the strikingly different severities of the phenotypes of the individual mutants, with the xrcc2 mutant being the most affected, and this is confirmed by epistasis analyses using multiple knockouts. Notwithstanding their clearly established importance for RAD51-dependent homologous recombination, XRCC2, RAD51B and RAD51D thus also participate in Single-Strand Annealing recombination.

Inhibition of homologous recombination with vorinostat synergistically enhances ganciclovir cytotoxicity

The nucleoside analog ganciclovir (GCV) elicits cytotoxicity in tumor cells via a novel mechanism in which drug incorporation into DNA produces minimal disruption of replication, but numerous DNA double strand breaks occur during the second S-phase after drug exposure. We propose that homologous recombination (HR), a major repair pathway for DNA double strand breaks, can prevent GCV-induced DNA damage, and that inhibition of HR will enhance cytotoxicity with GCV. Survival after GCV treatment in cells expressing a herpes simplex virus thymidine kinase was strongly dependent on HR (>14-fold decrease in IC50 in HR-deficient vs. HR-proficient CHO cells). In a homologous recombination reporter assay, the histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA; vorinostat), decreased HR repair events up to 85%. SAHA plus GCV produced synergistic cytotoxicity in U251tk human glioblastoma cells. Elucidation of the synergistic mechanism demonstrated that SAHA produced a concentration-dependent decrease in the HR proteins Rad51 and CtIP. GCV alone produced numerous Rad51 foci, demonstrating activation of HR. However, the addition of SAHA blocked GCV-induced Rad51 foci formation completely and increased γH2AX, a marker of DNA double strand breaks. SAHA plus GCV also produced synergistic cytotoxicity in HR-proficient CHO cells, but the combination was antagonistic or additive in HR-deficient CHO cells. Collectively, these data demonstrate that HR promotes survival with GCV and compromise of HR by SAHA results in synergistic cytotoxicity, revealing a new mechanism for enhancing anticancer activity with GCV.

Comprehensive assessment of the association between DNA repair gene XRCC3 Thr241Met polymorphism and leukemia risk

The XRCC3 gene has been suggested to play an important role in the pathogenesis of leukemia risk. But the findings of publications are contradictory. To derive a more precise estimation of the association, we performed a meta-analysis. The PubMed, Embase, and China National Knowledge Infrastructure (CNKI) databases were searched for case-control studies published up to August 2013. The pooled odds ratio (OR) and its corresponding 95% confidence interval (CI) were calculated by using a fixed- or random-effect model. A total of 15 case-control studies met the inclusion criteria and were selected. The pooled OR showed that there was no statistically significant association between XRCC3 Thr241Met polymorphism and leukemia risk in overall including studies, while a risky association was observed for acute myeloid leukemia (AML) (dominant model TT/TC vs. CC: OR = 1.240, 95% CI = 1.018-1.511, P = 0.032). The XRCC3 Thr241Met polymorphism might be associated with risk of leukemia in AML. More studies with larger sample sizes are needed to validate this result.

Oxidative stress in mammalian cells impinges on the cysteines redox state of human XRCC3 protein and on its cellular localization

In vertebrates, XRCC3 is one of the five Rad51 paralogs that plays a central role in homologous recombination (HR), a key pathway for maintaining genomic stability. While investigating the potential role of human XRCC3 (hXRCC3) in the inhibition of DNA replication induced by UVA radiation, we discovered that hXRCC3 cysteine residues are oxidized following photosensitization by UVA. Our in silico prediction of the hXRCC3 structure suggests that 6 out of 8 cysteines are potentially accessible to the solvent and therefore potentially exposed to ROS attack. By non-reducing SDS-PAGE we show that many different oxidants induce hXRCC3 oxidation that is monitored in Chinese hamster ovarian (CHO) cells by increased electrophoretic mobility of the protein and in human cells by a slight decrease of its immunodetection. In both cell types, hXRCC3 oxidation was reversed in few minutes by cellular reducing systems. Depletion of intracellular glutathione prevents hXRCC3 oxidation only after UVA exposure though depending on the type of photosensitizer. In addition, we show that hXRCC3 expressed in CHO cells localizes both in the cytoplasm and in the nucleus. Mutating all hXRCC3 cysteines to serines (XR3/S protein) does not affect the subcellular localization of the protein even after exposure to camptothecin (CPT), which typically induces DNA damages that require HR to be repaired. However, cells expressing mutated XR3/S protein are sensitive to CPT, thus highlighting a defect of the mutant protein in HR. In marked contrast to CPT treatment, oxidative stress induces relocalization at the chromatin fraction of both wild-type and mutated protein, even though survival is not affected. Collectively, our results demonstrate that the DNA repair protein hXRCC3 is a target of ROS induced by environmental factors and raise the possibility that the redox environment might participate in regulating the HR pathway.

Association between XRCC1 and XRCC3 polymorphisms with lung cancer risk: a meta-analysis from case-control studies

Many studies have reported the association of X-ray repair cross-complementing group 1 (XRCC1) Arg399Gln, Arg194Trp, Arg280His, −77T>C, and X-ray repair cross-complementing group 3 (XRCC3) T241M polymorphisms with lung cancer risk, but the results remained controversial. Hence, we performed a meta-analysis to investigate the association between lung cancer risk and XRCC1 Arg399Gln (14,156 cases and 16,667 controls from 41 studies), Arg194Trp (7,426 cases and 9,603 controls from 23 studies), Arg280His (6,211 cases and 6,763 controls from 16 studies), −77T>C (2,487 cases and 2,576 controls from 5 studies), and XRCC3 T241M (8,560 cases and 11,557 controls from 19 studies) in different inheritance models. We found that −77T>C polymorphism was associated with increased lung cancer risk (dominant model: odds ration [OR] = 1.45, 95% confidence interval [CI] = 1.27–1.66, recessive model: OR = 1.73, 95% CI = 1.14–2.62, additive model: OR = 1.91, 95% CI = 1.24–1.94) when all the eligible studies were pooled into the meta-analysis. In the stratified and sensitive analyses, significantly decreased lung cancer risk was observed in overall analysis (dominant model: OR = 0.83, 95% CI = 0.78–0.89; recessive model: OR = 0.90, 95% CI = 0.81–1.00; additive model: OR = 0.82, 95% CI = 0.74–0.92), Caucasians (dominant model: OR = 0.82, 95% CI = 0.76–0.87; recessive model: OR = 0.89, 95% CI = 0.80–0.99; additive model: OR = 0.81, 95% CI = 0.73–0.91), and hospital-based controls (dominant model: OR = 0.81, 95% CI = 0.76–0.88; recessive model: OR = 0.89, 95% CI = 0.79–1.00; additive model: OR = 0.80, 95% CI = 0.71–0.90) for XRCC3 T241M. In conclusion, this meta-analysis indicates that XRCC1 −77T>C shows an increased lung cancer risk and XRCC3 T241M polymorphism is associated with decreased lung cancer risk, especially in Caucasians.

Association of XRCC3 and XRCC4 gene polymorphisms, family history of cancer and tobacco smoking with non-small-cell lung cancer in a Chinese population: a case-control study

Single-nucleotide polymorphisms (SNPs) of DNA repair genes have been reported to modify cancer risk. This study aimed to determine SNPs of the DNA repair genes X-ray repair cross-complementing group 3 (XRCC3) and X-ray cross-complementing group 4 (XRCC4) and their association with non-small-cell lung cancer (NSCLC) susceptibility in a Chinese population. A total of 507 NSCLC patients and 662 healthy controls were recruited for genotyping. Epidemiological and clinical data were also collected for association studies. The data showed that the rs1799794 G allele in the XRCC3 gene and minor allele carriers of XRCC4, including rs1056503 and rs9293337, were inversely associated with NSCLC risk (GG vs homozygote AA), whereas the rs861537 AG or AA genotype and XRCC4 rs6869366 had a significantly increased NSCLC risk. Furthermore, tobacco smoking over 26 pack-years, a family history of lung cancer, exposure to environmental tobacco smoke (ETS) and negative mental status were risk factors for developing NSCLC. This study suggests that SNPs of XRCC3 and XRCC4 and other environmental factors are risk factors for developing NSCLC in this Chinese Han population.

The HsRAD51B-HsRAD51C stabilizes the HsRAD51 nucleoprotein filament

There are six human RAD51 related proteins (HsRAD51 paralogs), HsRAD51B, HsRAD51C, HsRAD51D, HsXRCC2, HsXRCC3 and HsDMC1, that appear to enhance HsRAD51 mediated homologous recombinational (HR) repair of DNA double strand breaks (DSBs). Here we model the structures of HsRAD51, HsRAD51B and HsRAD51C and show similar domain orientations within a hypothetical nucleoprotein filament (NPF). We then demonstrate that HsRAD51B-HsRAD51C heterodimer forms stable complex on ssDNA and partially stabilizes the HsRAD51 NPF against the anti-recombinogenic activity of BLM. Moreover, HsRAD51B-HsRAD51C stimulates HsRAD51 mediated D-loop formation in the presence of RPA. However, HsRAD51B-HsRAD51C does not facilitate HsRAD51 nucleation on a RPA coated ssDNA. These results suggest that the HsRAD51B-HsRAD51C complex plays a role in stabilizing the HsRAD51 NPF during the presynaptic phase of HR, which appears downstream of BRCA2-mediated HsRAD51 NPF formation.

Polymorphisms in DNA repair genes and susceptibility to glioma in a chinese population

The excision repair cross-complementing rodent repair deficiency complementation group 1 (ERCC1), and X-ray repair cross-complementing group 1 (XRCC1) genes appear to protect mammalian cells from the harmful effects of ionizing radiation. We conducted a large case-control study to investigate the association of polymorphisms in ERCC1 C118T, ERCC1 C8092A, XRCC1 A194T, XRCC1 A194T, and XRCC3 C241T, with glioma risk in a Chinese population. Five single nucleotide polymorphisms (SNPs) were genotyped, using the MassARRAY IPLEX platform, in 443 glioma cases and 443 controls. Association analyses based on an χ² test and binary logistic regression were performed to determine the odds ratio (OR) and a 95% confidence interval (95% CI) for each SNP. For XRCC1 Arg194Trp, the variant genotype T/T was strongly associated with a lower risk of glioma cancer when compared with the wild type C/C (OR = 2.45, 95% CI = 1.43–4.45). Individuals carrying the XRCC1 399A allele had an increased risk of glioma (OR = 1.33, 95% CI = 1.02–1.64). The XRCC3 241T/T genotype was associated with a strong increased glioma risk (OR = 3.78, 95% CI = 1.86–9.06). Further analysis of the interactions of two susceptibility-associated SNPs, XRCC1 Arg194Trp and XRCC3 Thr241Met, showed that the combination of the XRCC1 194T and XRCC3 241T alleles brought a large increase in glioma risk (OR = 2.75, 95% CI = 1.54–4.04). XRCC1 Arg194Trp, XRCC1 Arg399Gln, and XRCC3 C241T, appear to be associated with susceptibility to glioma in a Chinese population.

BRCA2 is epistatic to the RAD51 paralogs in response to DNA damage

Homologous recombination plays an important role in the high-fidelity repair of DNA double-strand breaks. A central player in this process, RAD51, polymerizes onto single-stranded DNA and searches for homology in a duplex donor DNA molecule, usually the sister chromatid. Homologous recombination is a highly regulated event in mammalian cells: some proteins have direct enzymatic functions, others mediate or overcome rate-limiting steps in the process, and still others signal cell cycle arrest to allow repair to occur. While the human BRCA2 protein has a clear role in delivering and loading RAD51 onto single-stranded DNA generated after resection of the DNA break, the mechanistic functions of the RAD51 paralogs remain unclear. In this study, we sought to determine the genetic interactions between BRCA2 and the RAD51 paralogs during DNA DSB repair. We utilized siRNA-mediated knockdown of these proteins in human cells to assess their impact on the DNA damage response. The results indicate that loss of BRCA2 alone imparts a more severe phenotype than the loss of any individual RAD51 paralog and that BRCA2 is epistatic to each of the four paralogs tested.

XRCC3 Thr241Met gene polymorphisms and lung cancer risk: a meta-analysis

Many studies have examined the association between the XRCC3 Thr241Met gene polymorphism and lung cancer risk in various populations, but their results have been inconsistent. To assess this relationship more precisely, a meta-analysis was performed. The PubMed, Embase, Web of Science, and CNKI database was searched for case–control studies published up to July 2012. Data were extracted and pooled odds ratios (OR) with 95% confidence intervals (CI) were calculated.

Ultimately, 17 studies, comprising 4123 lung cancer cases and 5597 controls were included. Overall, for T allele carriers (TC + TT) versus the wild-type homozygotes (CC), the pooled OR was 0.95 (95% CI = 0.87-1.04 P = 0.228 for heterogeneity), for TT versus CC the pooled OR was 0.99 (95% CI = 0.86-1.15 P = 0.315 for heterogeneity). In the stratified analysis by ethnicity, histological types of lung cancer and smoking status, no any significantly risks were found for (C/T + T/T) vs C/C or T/T vs C/C. No publication bias was found by using the funnel plot and Egger's test.

Overall, there is no evidence showing a significant correlation between XRCC3 Thr241Met polymorphism and lung cancer risk stratified analysis by ethnicity, histology and smoking status.

Association between XRCC1 and XRCC3 polymorphisms and colorectal cancer risk: a meta-analysis of 23 case-control studies

Several potential functional polymorphisms in the DNA repair gene X-ray repair cross-complementing group 1 (XRCC1) Arg399Gln (rs25487), Arg194Trp (rs1799782), Arg280His (rs25489) and X-ray repair cross-complementing group 3 (XRCC3) T241M (rs861539) have been implicated in colorectal cancer (CRC) risk, but the results are conflicting. Here, we performed a meta-analysis of 23 published case control datasets and assessed genetic heterogeneity between those datasets. All the case-control studies published from January 2000 to June 2012 on the association between those polymorphisms and CRC risk were identified by searching the electronic literature Medline. Statistical analysis was performed with the software programs Review Manager (version 4.2). For overall CRC, no significant association was observed, the pooled odds ratios for XRCC1 Arg399Gln, Arg194Trp, Arg280His, and XRCC3 T241M were 1.02 (95 % CI: 0.93, 1.12), 1.03 (95 % CI: 0.94, 1.14), 0.98 (95 % CI: 0.85, 1.13) and 1.03 (95 % CI: 0.85, 1.26), respectively. Furthermore, no significant association was observed in subgroup analyses based on ethnicity. The results suggested that these four SNPs evaluated are not associated with risk of CRC.