The importance of XRCC2 in RAD51-related DNA damage repair

Research progress of the Fanconi anemia pathway and premature ovarian insufficiency†

The Fanconi anemia pathway is a key pathway involved in the repair of deoxyribonucleic acidinterstrand crosslinking damage, which chiefly includes the following four modules: lesion recognition, Fanconi anemia core complex recruitment, FANCD2-FANCI complex monoubiquitination, and downstream events (nucleolytic incision, translesion synthesis, and homologous recombination). Mutations or deletions of multiple Fanconi anemia genes in this pathway can damage the interstrand crosslinking repair pathway and disrupt primordial germ cell development and oocyte meiosis, thereby leading to abnormal follicular development. Premature ovarian insufficiency is a gynecological clinical syndrome characterized by amenorrhea and decreased fertility due to decreased oocyte pool, accelerated follicle atresia, and loss of ovarian function in women <40 years old. Furthermore, in recent years, several studies have detected mutations in the Fanconi anemia gene in patients with premature ovarian insufficiency. In addition, some patients with Fanconi anemia exhibit symptoms of premature ovarian insufficiency and infertility. The Fanconi anemia pathway and premature ovarian insufficiency are closely associated.

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.

Genetic variants underlying spermatogenic arrests in men with non-obstructive azoospermia

Spermatogenic arrest is a severe form of non-obstructive azoospermia (NOA), which occurs in 10-15% of infertile men. Interruption in spermatogenic progression at premeiotic, meiotic, or postmeiotic stage can lead to arrest in men with NOA. Recent studies have intensively focused on defining genetic variants underlying these spermatogenic arrests by making genome/exome sequencing. A number of variants were discovered in the genes involving in mitosis, meiosis, germline differentiation and other basic cellular events. Herein, defined variants in NOA cases with spermatogenic arrests and created knockout mouse models for the related genes are comprehensively reviewed. Also, importance of gene panel-based screening for NOA cases was discussed. Screening common variants in these infertile men with spermatogenic arrests may contribute to elucidating the molecular background and designing novel treatment strategies.

Postnatal nutrition environment reprograms renal DNA methylation patterns in offspring of maternal protein-restricted stroke-prone spontaneously hypertensive rats

Maternal malnutrition hampers the offspring health by manipulating the epigenome. Recent studies indicate that the changes in DNA methylation could be reversed by afterbirth nutrition supplementation. In this study, we used DNA methylation arrays to comprehensively investigate the DNA methylation status of the renal promoter regions and the effects of postnatal protein intake on DNA methylation. We fed stroke-prone spontaneously hypertensive (SHRSP) rat dams a normal diet or a low-protein diet during pregnancy, and their 4-week-old male offspring were fed a normal diet or a high-/low-protein diet for 2 weeks. We found that the methylation status of 2,395 differentially methylated DNA regions was reprogrammed, and 34 genes were reset by different levels of postnatal protein intake in the offspring. Among these genes, Adora2b, Trpc5, Ar, Xrcc2, and Atp1b1 are involved in renal disease and blood pressure regulation. Our findings indicate that postnatal nutritional interventions can potentially reprogram epigenetic changes, providing novel therapeutic and preventive epigenetic targets for salt-sensitive hypertension.

Advances in the study of HSP70 inhibitors to enhance the sensitivity of tumor cells to radiotherapy

The 70 kDa heat shock protein (HSP70) is one of the most conserved proteins and a ubiquitous molecular chaperone that plays a role in the folding, remodeling, and degradation of various proteins to maintain proteostasis. It has been shown that HSP70 is abundantly expressed in cancer and enhances tumor resistance to radiotherapy by inhibiting multiple apoptotic pathways, such as interfering with the cellular senescence program, promoting angiogenesis, and supporting metastasis. Thus, HSP70 provides an effective target for enhancing the effects of radiation therapy in the clinical management of cancer patients. Inhibition of HSP70 enhances the radiation-induced tumor-killing effect and thus improves the efficacy of radiotherapy. This article reviews the sensitivity of Hsp70 and its related inhibitors to radiotherapy of tumor cells.

Establishing and characterizing a novel doxorubicin-resistant acute myeloid leukaemia cell line

Drug resistance is a major setback in cancer treatment, thus models to study its mechanisms are needed. Our work aimed to establish and characterize a resistant cell line from a sensitive acute myeloid leukaemia (AML) cell line - HL60 - by treating the sensitive cells with increasing concentrations of doxorubicin. We confirmed (cell viability assays) that the established subline, HL60-CDR, was resistant to doxorubicin for at least 30 days without drug treatment. The HL60-CDR cells were also resistant to three other drugs (cisplatin, etoposide and daunorubicin), exhibiting a multidrug resistant (MDR) profile. We verified (Western Blotting) that the MDR cells do not express drug efflux pumps, nor present altered expression of apoptotic proteins, when compared with the parental cell line. HL60-CDR cells presented alterations in the cell cycle profile, and in the expression levels of proteins involved in DNA repair mechanisms and drug metabolism, when compared with their drug sensitive counterpart. Proteomic analysis revealed that HL60-CDR cells presented an upregulation of proteins involved in oncogenic pathways, such as TSC2, PDPK1, Annexin A2, among others. Overall, we established an AML MDR subline - HL60-CDR - which presents several resistance mechanisms, providing an in vitro model to test new compounds to circumvent MDR in AML.

PARP inhibitor olaparib enhances the efficacy of radiotherapy on XRCC2-deficient colorectal cancer cells

The use of PARP inhibitors in combination with radiotherapy is a promising strategy to locally enhance DNA damage in tumors. Loss of XRCC2 compromises DNA damage repairs, and induced DNA damage burdens may increase the reliance on PARP-dependent DNA repairs of cancer cells to render cell susceptibility to PARP inhibitor therapy. Here we tested the hypothesis that XRCC2 loss sensitizes colorectal cancer (CRC) to PARP inhibitor in combination with radiotherapy (RT). We show that high levels of XRCC2 or PARP1 in LARC patients were significantly associated with poor overall survival (OS). Co-expression analyses found that low levels of PARP1 and XRCC2 were associated with better OS. Our in vitro experiments indicated that olaparib+IR led to reduced clonogenic survival, more DNA damage, and longer durations of cell cycle arrest and senescence in XRCC2-deficient cells relative to wild-type cells. Furthermore, our mouse xenograft experiments indicated that RT + olaparib had greater anti-tumor effects and led to long-term remission in mice with XRCC2-deficient tumors. These findings suggest that XRCC2-deficient CRC acquires high sensitivity to PARP inhibition after IR treatment and supports the clinical development for the use of olaparib as a radiosensitizer for treatment of XRCC2-deficient CRC.

Homologous recombination DNA repair gene RAD51, XRCC2 & XRCC3 polymorphisms and breast cancer risk in South Indian women

BACKGROUND

Homologous recombination repair (HRR) accurately repairs the DNA double-strand breaks (DSBs) and is crucial for genome stability. Genetic polymorphisms in crucial HRR pathway genes might affect genome stability and promote tumorigenesis. Up to our knowledge, the present study is the first to investigate the impact of HRR gene polymorphisms on BC development in South Indian women. The present population-based case-control study investigated the association of polymorphisms in three key HRR genes (XRCC2-Arg188His, XRCC3-Thr241Met and RAD51-G135C) with BC risk.

MATERIALS AND METHODS

Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method was used for genotyping the HRR variants in 491 BC cases and 493 healthy women.

RESULTS

We observed that the XRCC3 Met allele was significantly associated with BC risk [OR:1.27 (95% CI: 1.02-1.60); p = 0.035]. In addition, the homozygous mutant (C/C) genotype of RAD51 G135C variant conferred 2.19 fold elevated risk of BC [OR: 2.19 (95% CI: 1.06-4.54); p = 0.034]. Stratified analysis of HRR variants and BC clinicopathological features revealed that the XRCC3-Thr241Met and RAD51-G135C variants are associated with BC progression. Combined SNP analysis revealed that the individuals with RAD51-C/C, XRCC2-Arg/Arg, and XRCC3-Thr/Thr genotype combination have three-fold increased BC risk.

CONCLUSION

The present study imparts additional evidence that genetic variants in crucial HRR pathway genes might play a pivotal role in modulating BC risk in South Indian women.

FDI-6 and olaparib synergistically inhibit the growth of pancreatic cancer by repressing BUB1, BRCA1 and CDC25A signaling pathways

Inducing homologous recombination (HR) deficiency is a promising strategy to broaden the indication of PARP1/2 inhibitors in pancreatic cancer treatment. In addition to inhibition kinases, repression of the transcriptional function of FOXM1 has been reported to inhibit HR-mediated DNA repair. We found that FOXM1 inhibitor FDI-6 and PARP1/2 inhibitor Olaparib synergistically inhibited the malignant growth of pancreatic cancer cells in vitro and in vivo. The results of bioinformatic analysis and mechanistic study showed that FOXM1 directly interacted with PARP1. Olaparib induced the feedback overexpression of PARP1/2, FOXM1, CDC25A, CCND1, CDK1, CCNA2, CCNB1, CDC25B, BRCA1/2 and Rad51 to promote the acceleration of cell mitosis and recovery of DNA repair, which caused the generation of adaptive resistance. FDI-6 reversed Olaparib-induced adaptive resistance and inhibited cell cycle progression and DNA damage repair by repressing the expression of FOXM1, PARP1/2, BUB1, CDC25A, BRCA1 and other genes-involved in cell cycle control and DNA damage repair. We believe that targeting FOXM1 and PARP1/2 is a promising combination therapy for pancreatic cancer without HR deficiency.

DNA damage response proteins synergistically affect the cancer prognosis and resistance

Amplification of oxidative stress can be utilized as a strategy to attenuate cancer progression by instigating apoptosis. However, the duration of positive response to such therapies is limited, as cancer cells eventually develop resistance. The underlying molecular mechanisms of cancer cells to escape apoptosis under oxidative stress is unknown. Employing big data, and its integration with transcriptome, proteome and network analysis in six cancer types revealed system-level interactions between DNA damage response (DDR) proteins, including; DNA damage repair, cell cycle checkpoints and anti-apoptotic proteins. Cancer system biology is used to elucidate mechanisms for cancer progression, but networks defining mechanisms causing resistance is less explored. Using system biology, we identified DDR hubs between G1-S and M phases that were associated with bad prognosis. The increased expression of DDR network was involved in resistance under high oxidative stress. We validated our findings by combining H₂O₂ induced oxidative stress and DDR inhibitors in human lung cancer cells to conclude the necessity of targeting a 'disease-causing network'. Collectively, our work provides insights toward designing strategies for network pharmacology to combat resistance in cancer research.

FDI-6 inhibits the expression and function of FOXM1 to sensitize BRCA-proficient triple-negative breast cancer cells to Olaparib by regulating cell cycle progression and DNA damage repair

Inducing homologous-recombination (HR) deficiency is an effective strategy to broaden the indications of PARP inhibitors in the treatment of triple-negative breast cancer (TNBC). Herein, we find that repression of the oncogenic transcription factor FOXM1 using FOXM1 shRNA or FOXM1 inhibitor FDI-6 can sensitize BRCA-proficient TNBC to PARP inhibitor Olaparib in vitro and in vivo. Mechanistic studies show that Olaparib causes adaptive resistance by arresting the cell cycle at S and G2/M phases for HR repair, increasing the expression of CDK6, CCND1, CDK1, CCNA1, CCNB1, and CDC25B to promote cell cycle progression, and inducing the overexpression of FOXM1, PARP1/2, BRCA1/2, and Rad51 to activate precise repair of damaged DNA. FDI-6 inhibits the expression of FOXM1, PARP1/2, and genes involved in cell cycle control and DNA damage repair to sensitize TNBC cells to Olaparib by blocking cell cycle progression and DNA damage repair. Simultaneously targeting FOXM1 and PARP1/2 is an innovative therapy for more patients with TNBC.

miR-27b-3p a Negative Regulator of DSB-DNA Repair

Understanding the regulation of DNA repair mechanisms is of utmost importance to identify altered cellular processes that lead to diseases such as cancer through genomic instability. In this sense, miRNAs have shown a crucial role. Specifically, miR-27b-3 biogenesis has been shown to be induced in response to DNA damage, suggesting that this microRNA has a role in DNA repair. In this work, we show that the overexpression of miR-27b-3p reduces the ability of cells to repair DNA lesions, mainly double-stranded breaks (DSB), and causes the deregulation of genes involved in homologous recombination repair (HRR), base excision repair (BER), and the cell cycle. DNA damage was induced in BALB/c-3T3 cells, which overexpress miR-27b-3p, using xenobiotic agents with specific mechanisms of action that challenge different repair mechanisms to determine their reparative capacity. In addition, we evaluated the expression of 84 DNA damage signaling and repair genes and performed pathway enrichment analysis to identify altered cellular processes. Taken together, our results indicate that miR-27b-3p acts as a negative regulator of DNA repair when overexpressed.

Meeting report from the 2018 12th Biennial Ovarian Cancer Research Symposium detection and prevention of ovarian cancer

The objective of this review is to summarize recent research advances in the detection and prevention of ovarian cancer and discuss the experts' opinions of future directions. The 12th Biennial Ovarian Cancer Research Symposium was held in Seattle, Washington, in September 2018. At this meeting, experts in ovarian cancer research gathered to present and discuss recent breakthroughs and their visions of future ovarian cancer research. Session 1 of the symposium focused on the detection and prevention of ovarian cancer. It included two invited oral presentations from Ranjit Manchanda, MD, PhD (Barts Cancer Institute) and Rosana Risques, PhD (University of Washington). Another eight oral presentations were selected from abstract submissions. Fifteen abstracts were presented in poster format. These presentations covered topics including cellular origin of high-grade serous cancer, risk factors for ovarian cancer, new methods for early detection of ovarian cancer, mechanisms underlying ovarian cancer development, and new therapeutic approaches for preventing ovarian cancer from forming or progressing. In conclusion, a clear understanding of the cellular origin and molecular mechanisms underlying the initiation of high-grade serous cancer is essential for developing effective means for early detection and prevention of this most devastating type of ovarian cancer. Recognizing the complexity of ovarian cancer and appreciating that ovarian cancer is not a single disease will help us to generate proper models, design rational experiments, and collect and analyze patient data in a meaningful way. A concerted effort in the field will help to bridge the basic science and clinical applications and lead to more precise and effective detection and treatment.

Mechanisms of different response to ionizing irradiation in isogenic head and neck cancer cell lines

Background

Treatment options for recurrent head and neck tumours in the previously irradiated area are limited, including re-irradiation due to radioresistance of the recurrent tumour and previous dose received by surrounding normal tissues. As an in vitro model to study radioresistance mechanisms, isogenic cells with different radiosensitivity can be used. However, they are not readily available. Therefore, our objective was to establish and characterize radioresistant isogenic human pharyngeal squamous carcinoma cells and to evaluate early radiation response in isogenic parental, radioresistant and radiosensitive cells.

Methods

Radioresistant cells were derived from parental FaDu cells by repeated exposure to ionizing radiation. Radiosensitivity of the established isogenic radioresistant FaDu-RR cells was evaluated by clonogenic assay and compared to isogenic parental FaDu and radiosensitive 2A3 cells. Additional phenotypic characterization of these isogenic cells with different radiosensitivity included evaluation of chemosensitivity, cell proliferation, cell cycle, radiation-induced apoptosis, resolution of DNA double-strand breaks, and DNA damage and repair signalling gene expression before and after irradiation.

Results

In the newly established radioresistant cells in response to 5 Gy irradiation, we observed no alteration in cell cycle regulation, but delayed induction and enhanced resolution of DNA double-strand breaks, lower induction of apoptosis, and pronounced over-expression of DNA damage signalling genes in comparison to parental cells. On the other hand, radiosensitive 2A3 cells were arrested in G₂/M-phase in response to 5 Gy irradiation, had a prominent accumulation of and slower resolution of DNA double-strand breaks, and no change in DNA damage signalling genes expression.

Conclusions

We concluded that the emergence of the radioresistance in the established radioresistant isogenic cells can be at least partially attributed to the enhanced DNA double-strand break repair, altered expression of DNA damage signalling and repair genes. On the other hand, in radiosensitive isogenic cells the reduced ability to repair a high number of induced DNA double-strand breaks and no transcriptional response in DNA damage signalling genes indicate on a lack of adaptive response to irradiation. Altogether, our results confirmed that these isogenic cells with different radiosensitivity are an appropriate model to study the mechanisms of radioresistance.

Exploring the druggable space around the Fanconi anemia pathway using machine learning and mechanistic models

BACKGROUND

In spite of the abundance of genomic data, predictive models that describe phenotypes as a function of gene expression or mutations are difficult to obtain because they are affected by the curse of dimensionality, given the disbalance between samples and candidate genes. And this is especially dramatic in scenarios in which the availability of samples is difficult, such as the case of rare diseases.

RESULTS

The application of multi-output regression machine learning methodologies to predict the potential effect of external proteins over the signaling circuits that trigger Fanconi anemia related cell functionalities, inferred with a mechanistic model, allowed us to detect over 20 potential therapeutic targets.

CONCLUSIONS

The use of artificial intelligence methods for the prediction of potentially causal relationships between proteins of interest and cell activities related with disease-related phenotypes opens promising avenues for the systematic search of new targets in rare diseases.

Genetic Variant Screening of DNA Repair Genes in Myelodysplastic Syndrome Identifies a Novel Mutation in the XRCC2 Gene

BACKGROUND

We aimed to detect single nucleotide polymorphisms (SNPs) and mutations in DNA repair genes and their possible association with myelodysplastic syndrome (MDS).

METHODS

Targeted enrichment resequencing of 84 DNA repair genes was initially performed on a screening cohort of MDS patients. Real-time polymerase chain reaction was used for genotyping selected SNPs in the validation cohort of patients.

RESULTS

A heterozygous frameshift mutation in the XRCC2 gene was identified. It leads to the formation of a truncated non-functional protein and decreased XRCC2 expression level. Decreased expression levels of all DNA repair genes functionally connected with mutated XRCC2 were also present. Moreover, a synonymous substitution in the PRKDC gene and 2 missense mutations in the SMUG1 and XRCC1 genes were also found. In the screening cohort, 6 candidate SNPs were associated with the tendency to develop MDS: rs4135113 (TDG, p = 0.03), rs12917 (MGMT, p = 0.003), rs2230641 (CCNH, p = 0.01), rs2228529 and rs2228526 (ERCC6, p = 0.04 and p = 0.03), and rs1799977 (MLH1, p = 0.04). In the validation cohort, only a polymorphism in MLH1 was significantly associated with development of MDS in patients with poor cytogenetics (p = 0.0004).

CONCLUSION

Our study demonstrates that genetic variants are present in DNA repair genes of MDS patients and may be associated with susceptibility to MDS.

Response of DNA damage genes in acrolein-treated lung adenocarcinoma cells

Acrolein is a α-β-unsaturated aldehyde and is toxic to human upon its exposure from the environment. Sources of exposure to acrolein can be from heating cooking oil, automobile exhaust, tobacco smoke, and plastic waste. Acrolein exposure to lung is a major concern because of its volatile nature and due to its presence in the urban atmospheric air. Acrolein being highly reactive forms DNA and protein adducts, thereby making the cells vulnerable to long-term damage. Such long-term effect can lead to high susceptibility towards malignant transformation as has been reported in cigarette smokers. The response of DNA damaging genes by acrolein can perhaps give an insight to the cause of damage in the DNA by acrolein. The aim of this study was to examine the response of the DNA damage responsive genes by acrolein in A549 lung adenocarcinoma cells. Acrolein treatment at IC50 concentration showed a robust response of the DNA repair genes but eventually failed to rescue the cells from undergoing apoptosis. The cells pretreated with acrolein and followed by growing the same cells in fresh medium in the absence of acrolein did not help the cells to proliferate. These results conclude that exposure to acrolein marks long-lasting damage to DNA, irrespective of the DNA repair response.

Differential expression of homologous recombination DNA repair genes in the early and advanced stages of myelodysplastic syndrome

BACKGROUND

The high incidence of mutations and cytogenetic abnormalities in patients with myelodysplastic syndrome (MDS) suggests that defects in DNA repair mechanisms. We monitored DNA repair pathways in MDS and their alterations during disease progression.

METHODS

Expression profiling of DNA repair genes was performed on CD34+ cells, and paired samples were used for monitoring of RAD51 and XRCC2 gene expression during disease progression. Immunohistochemical staining for RAD51 was done on histology samples.

RESULTS

RAD51 and XRCC2 showed differential expression between low-risk and high-risk MDS (P<.0001), whereas RPA3 was generally decreased among the entire cohort (FC=-2.65, P<.0001). We demonstrated that RAD51 and XRCC2 expression gradually decreased during the progression of MDS. Down-regulation of XRCC2 and RAD51 expression was connected with abnormalities on chromosome 7 (P=.0858, P=.0457). Immunohistochemical staining revealed the presence of RAD51 only in the cytoplasm in low-risk MDS, while in both the cytoplasm and nucleus in high-risk MDS. The multivariate analysis identified RAD51 expression level (HR 0.49; P=.01) as significant prognostic factor for overall survival of patients with MDS.

CONCLUSIONS

Our study demonstrates that the expression of DNA repair factors, primarily RAD51 and XRCC2, is deregulated in patients with MDS and presents a specific pattern with respect to prognostic categories.

The relationship of single-strand breaks in DNA to breast cancer risk and to tissue concentrations of oestrogens

CONTEXT

Clinical study of breast cancer patients in Chicago, IL, USA.

OBJECTIVE

Ascertain the utility of measurements of single-strand breaks (SSB) in DNA for assessment of breast cancer risk.

METHODS

Fine-needle aspirates of the breast, SSB by nick translation, percent breast density (PBD), Gail model risk, cumulative methylation index (CMI), enzymes of DNA repair and tissue antioxidants.

RESULTS

DNA repair enzymes and 4-hydroxyestradiol were negatively associated with SSB; CMI and PBD were positively associated.

CONCLUSIONS

Quantitative measurement of SSBs by this procedure indicates the relative number of SSBs and is related to promoter methylation, antioxidant availability and percent breast density.

Associations between XRCC2 rs3218536 and ERCC2 rs13181 polymorphisms and ovarian cancer

Recent studies explored XRCC2 rs3218536 and ERCC2 rs13181 polymorphisms and ovarian cancer (OC) risk. However, the association between these two single nucleotide polymorphisms and OC risk remains conflicting. Thus, we conducted a comprehensive systematic review and meta-analysis to investigate the association. We searched the databases of PubMed, and Embase. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) were calculated by using fixed-effect or random-effect models. 15 case-control studies published in 11 papers including 4,757 cases and 8,431 controls were included in this meta-analysis. No associations were obtained between XRCC2 rs3218536 and ERCC2 rs13181 polymorphisms and OC risk. Stratification analyses of Hardy–Weinberg equilibrium status indicated that rs3218536 polymorphism was associated with the decreased risk of OC when in analysis of combined HWE positive studies. In conclusion, this meta-analysis indicates that XRCC2 rs3218536 and ERCC2 rs13181 polymorphisms may not be associated with the risk of OC.

MiRNA-binding site functional polymorphisms in DNA repair genes RAD51, RAD52, and XRCC2 and breast cancer risk in Chinese population

RAD51, RAD52, and XRCC2 are all involved in DNA homologous recombinational repair, and there are interactions among those genes. Polymorphisms in 3'-UTR of DNA repair genes may change DNA repair capacity by regulating gene expression. However, potential regulatory variants affecting their expression remain largely unexplored. Five miRNA-binding site SNPs (rs7180135 and rs45549040 in RAD51, rs1051669 and rs7963551 in RAD52 and rs3218550 in XRCC2) selected by bioinformatics method were genotyped in 498 breast cancer (BC) patients and 498 matched controls in Chinese population. Association between SNPs and BC risk was analyzed by adjusted odds ratios (ORs) and 95 % confidence intervals (CIs) in unconditional logistic regression model. Quantitative real-time (qRT) PCR and Western Blot assays were used to calculate the relative expression of RAD52 in recombinant plasmid-pGenesil-1-let-7b group and let-7b-inhibitor group. Gene-reproductive factors interactions were evaluated by multifactor dimensionality reduction (MDR) method. We found that individuals with AC (OR 0.684, 95%CI 0.492-0.951) and CC (OR 0.317, 95%CI 0.200-0.503) genotypes of rs7963551 had a significantly lower risk of breast cancer and qRT-PCR and Western Blot revealed that let-7b might downregulate the expression of RAD52 in MCF-7 and SKBR-3 cells. A significant interaction between the number of pregnancy (≥2) and rs7963551 (Ars7963551) was found to increase breast cancer risk by 2.63-fold (OR 2.63; 95%CI 2.03-3.42). In summary, the miRNA-binding SNPs in DNA repair genes RAD51, RAD52, and XRCC2 and their interaction with reproductive factors might play important roles in the development of BC, and let-7b might downregulate RAD52 expression in MCF-7 and SKBR-3 cells.

XRCC2 as a predictive biomarker for radioresistance in locally advanced rectal cancer patients undergoing preoperative radiotherapy

XRCC2 has been shown to increase the radioresistance of some cancers. Here, XRCC2 expression was investigated as a predictor of preoperative radiotherapy (PRT) treatment response in locally advanced rectal cancer (LARC). XRCC2 was found to be overexpressed in rectal cancer tissues resected from patients who underwent surgery without PRT. In addition, overall survival for LARC patients was improved in XRCC2-negative patients compared with XRCC2-positive patients after treatment with PRT (P < 0.001). XRCC2 expression was also associated with an increase in LARC radioresistance. Conversely, XRCC2-deficient cancer cells were more sensitive to irradiation in vitro, and a higher proportion of these cells underwent cell death induced by G2/M phase arrest and apoptosis. When XRCC2 was knocked down, the repair of DNA double-strand breaks caused by irradiation was impaired. Therefore, XRCC2 may increases LARC radioresistance by repairing DNA double-strand breaks and preventing cancer cell apoptosis. Moreover, the present data suggest that XRCC2 is a useful predictive biomarker of PRT treatment response in LARC patients. Thus, inhibition of XRCC2 expression or activity represents a potential therapeutic strategy for improving PRT response in LARC patients.

Functional Analysis of Missense Variants in the Putative Breast Cancer Susceptibility Gene XRCC2

XRCC2 genetic variants have been associated with breast cancer susceptibility. However, association studies have been complicated because XRCC2 variants are extremely rare and consist mainly of amino acid substitutions whose grouping is sensitive to misclassification by the predictive algorithms. We therefore functionally characterized variants in XRCC2 by testing their ability to restore XRCC2-DNA repair deficient phenotypes using a cDNA-based complementation approach. While the protein-truncating variants p.Leu117fs, p.Arg215*, and p.Cys217* were unable to restore XRCC2 deficiency, 19 out of 23 missense variants showed no or just a minor (<25%) reduction in XRCC2 function. The remaining four (p.Cys120Tyr, p.Arg91Trp, p.Leu133Pro, and p.Ile95Leu) had a moderate effect. Overall, measured functional effects correlated poorly with those predicted by in silico analysis. After regrouping variants from published case-control studies based on the functional effect found in this study and reanalysis of the prevalence data, there was no longer evidence for an association with breast cancer. This suggests that if breast cancer susceptibility alleles of XRCC2 exist, they are likely restricted to protein-truncating variants and a minority of missense changes. Our study emphasizes the use of functional analyses of missense variants to support variant classification in association studies.

Association between Genetic Variants in DNA Double-Strand Break Repair Pathways and Risk of Radiation Therapy-Induced Pneumonitis and Esophagitis in Non-Small Cell Lung Cancer

Radiation therapy (RT)-induced pneumonitis and esophagitis are commonly developed side effects in non-small cell lung cancer (NSCLC) patients treated with definitive RT. Identifying patients who are at increased risk for these toxicities would help to maximize treatment efficacy while minimizing toxicities. Here, we systematically investigated single nucleotide polymorphisms (SNPs) within double-strand break (DSB) repair pathway as potential predictive markers for radiation-induced esophagitis and pneumonitis. We genotyped 440 SNPs from 45 genes in DSB repair pathways in 250 stage I–III NSCLC patients who received definitive radiation or chemoradiation therapy, followed by internal validation in 170 additional patients. We found that 11 SNPs for esophagitis and 8 SNPs for pneumonitis showed consistent effects between discovery and validation populations (same direction of OR and reached significance in meta-analysis). Among them, rs7165790 in the BLM gene was significantly associated with decreased risk of esophagitis in both discovery (OR = 0.59, 95% CI: 0.37–0.97, p = 0.037) and validation subgroups (OR = 0.45, 95% CI: 0.22–0.94, p = 0.032). A strong cumulative effect was observed for the top SNPs, and gene-based tests revealed 12 genes significantly associated with esophagitis or pneumonitis. Our results support the notion that genetic variations within DSB repair pathway could influence the risk of developing toxicities following definitive RT in NSCLC.

Analysis of XRCC2 and XRCC3 gene polymorphisms in pancreatic cancer

The double-strand break DNA repair pathway, including XRCC2 and XRCC3 genes, is implicated in maintaining genomic stability and therefore could affect the pancreatic cancer risk. The aim of the present study was to evaluate the clinical significance of the XRCC2 and XRCC3 gene polymorphisms in patients with pancreatic cancer. The present study included 203 patients: 101 with pancreatic cancer and 102 healthy controls. The Arg188His XRCC2 and the Thr241Met XRCC3 gene polymorphisms have been studied in DNA isolated from blood samples. The associations of the analysed genotypes and clinical data at diagnosis have been evaluated. The frequencies of the genotypes of the Arg188His XRCC2 and Thr241Met XRCC3 polymorphisms did not differ significantly between patients and controls. The study did not identify a correlation between the XRCC2 and XRCC3 genes polymorphisms and tumor size or localisation. Analysed polymorphisms were also not associated with the gender and age of the patient, or the presence of regional or distant metastases. In conclusion, the present study did not suggest an association between the Arg188His XRCC2 and the Thr241Met XRCC3 polymorphisms and the clinical data of patients with pancreatic cancer.

Correlation between selected XRCC2, XRCC3 and RAD51 gene polymorphisms and primary breast cancer in women in Pakistan

Genetic polymorphisms in homologous recombination repair genes cause an abnormal development of cancerous cells. In the present study we evaluated the possibility of breast cancer association with single nucleotide polymorphisms of RAD51, XRCC2 and XRCC3 genes. Polymorphisms selected in this study were RAD51 135G/C, XRCC2 Arg188His; and XRCC3 Thr241Met. Each polymorphism was genotyped using Polymerase chain reaction-restriction fragment length polymorphism in study cohort of 306 females (156 breast cancer patients and 150 controls). We observed that heterozygous variant genotype (GC) of RAD51 135 G/C polymorphism was associated with a significantly (OR=2.70; 95%CI (0.63-1.79); p<0.03) increased risk of breast cancer. In case of the XRCC3 gene we observed that frequency of heterozygous (OR=2.88; 95%CI (1.02-8.14); p<0.02) and homozygous (OR=1.46; 95%CI (0.89-2.40); p<0.04) genotype of Thr241Met polymorphism were significantly higher in breast cancer patients. For the Arg188His polymorphism of XRCC2, ~2fold increase in breast cancer risk (OR=1.6, 95%CI = 0.73-3.50) was associated with GA genotype with a p value for trend of 0.03. Our results suggest that the 135G/C polymorphism of the RAD51, Thr241Met polymorphism of XRCC3 and Arg188His polymorphism of XRCC2 can be independent markers of breast cancer risk in Pakistan.

Association of reduced XRCC2 expression with lymph node metastasis in breast cancer tissues

The main purpose of this study was to evaluate the association between reduction in XRCC2 gene and involvement of lymph node metastasis in breast cancer. In first part of the study, meta-analysis of 14 published XRCC2 studies was performed to define the role of XRCC2 gene as diagnostic marker and in second part of the study XRCC2 gene expression was observed using real time PCR in study cohort of 100 females (50 breast cancer patients and 50 controls). A statistically significant down regulation of XRCC2 (p < 0.04) and up-regulation of ki-67 (p < 0.05) was observed in breast cancer tissues compared to non-cancerous healthy tissues. In order to explore gene-gene and gene-clinicopathological parameters relationship Spearmen correlation was performed. We observed a significantly negative correlation between XRCC2 and Ki-67 expression (r = -0.376**, p < 0.01). In case of gene-clinicopathological parameters relationship, we observed a significant correlation between XRCC2 expression and lymph node status (r = -0.521***, p < 0.002) and metastatic status (r = -0.303*, p < 0.04) of breast cancer patients. Our data suggests that deregulation of XRCC2 in breast cancer has the potential to predict lymph node metastasis and may serve as a therapeutic target for breast cancer patients at risk of metastasis.

Increased risk of differentiated thyroid carcinoma with combined effects of homologous recombination repair gene polymorphisms in an Iranian population

Homologous recombination (HR) repair has a crucial role to play in the prevention of chromosomal instability, and it is clear that defects in some HR repair genes are associated with many cancers. To evaluate the potential effect of some HR repair gene polymorphisms with differentiated thyroid carcinoma (DTC), we assessed Rad51 (135G>C), Rad52 (2259C>T), XRCC2 (R188H) and XRCC3 (T241M) polymorphisms in Iranian DTC patients and cancer-free controls. In addition, haplotype analysis and gene combination assessment were carried out. Genotyping of Rad51 (135G>C), Rad52 (2259C>T) and XRCC3 (T241M) polymorphisms was determined by PCR-RFLP and PCR-HRM analysis was carried out to evaluate XRCC2 (R188H) . Separately, Rad51, Rad52 and XRCC2 polymorphisms were not shown to be more significant in patients when compared to controls in crude, sex-adjusted and age-adjusted form. However, results indicated a significant difference in XRCC3 genotypes for patients when compared to controls (p value: 0.035). The GCTG haplotype demonstrated a significant difference (p value: 0.047). When compared to the wild type, the combined variant form of Rad52/XRCC2/XRCC3 revealed an elevated risk of DTC (p value: 0.007). It is recommended that Rad52 2259C>T, XRCC2 R188H and XRCC3 T241M polymorphisms should be simultaneously considered as contributing to a polygenic risk of differentiated thyroid carcinoma.

XRCC2 gene polymorphisms and its protein are associated with colorectal cancer susceptibility in Chinese Han population

XRCC2 is an essential part of the homologous recombination repair pathway. However, relatively little is known about the effect of XRCC2 gene C41657T and G4234C polymorphisms on the individual susceptibility to colorectal cancer (CRC). The purpose of this study was to investigate the association between XRCC2 gene C41657T and G4234C polymorphisms and CRC and to explore the relationship among the polymorphisms and clinicopathologic parameters and protein expression levels of XRCC2. A hospital-based case-control study was conducted with 246 CRC cases and 262 healthy controls. The genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism. XRCC2 protein was analyzed by immunohistochemistry for the paraffin sections of 120 CRC cases. The study data showed that the C41657T genotypes were associated with the risk of CRC. The CT/TT genotypes and T allele were overrepresented among the CRC cases. Compared with CC, CT/TT enhanced the risk of CRC (odds ratio = 1.646, 95 % confidence interval = 1.127-2.404, P = 0.010). XRCC2 protein expression of CRC patients with CT/TT genotypes was significantly higher than that of the patients with CC genotype (χ (2) = 4.887, P = 0.027). XRCC2 gene G4234C polymorphisms have no relevance to the risk of CRC. Our findings suggest that XRCC2 C41657T polymorphism may adjust the XRCC2 expression and might influence susceptibility of CRC.

MicroRNA regulation of DNA repair gene expression in 4-aminobiphenyl-treated HepG2 cells

We examined the role of miRNAs in DNA damage response in HepG2 cells following exposure to 4-aminobiphenyl (4-ABP). The arylamine 4-ABP is a human carcinogen. Using the Comet assay, we showed that 4-ABP (18.75-300μM) induces DNA damage in HepG2 cells after 24h. DNA damage signaling pathway-based PCR arrays were used to investigate expression changes in genes involved in DNA damage response. Results showed down-regulation of 16 DNA repair-related genes in 4-ABP-treated cells. Among them, the expression of selected six genes (UNG, LIG1, EXO1, XRCC2, PCNA, and FANCG) from different DNA repair pathways was decreased with quantitative real-time PCR (qRT-PCR). In parallel, using the miRNA array, we reported that the expression of 27 miRNAs in 4-ABP-treated cells was at least 3-fold higher than that in the control group. Of these differential 27 miRNAs, the most significant expression of miRNA-513a-5p and miRNA-630 was further validated by qRT-PCR, and was predicted to be implicated in the deregulation of FANCG and RAD18 genes, respectively, via bioinformatic analysis. Both FANCG and RAD18 proteins were found to be down-regulated in 4-ABP-treated cells. In addition, overexpression and knockdown of miRNA-513a-5p and miRNA-630 reduced and increased the expression of FANCG and RAD18 proteins, respectively. Based on the above results, we indicated that miRNA-513a-5p and miRNA-630 could play a role in the suppression of DNA repair genes, and eventually lead to DNA damage.

shRNA-mediated XRCC2 gene knockdown efficiently sensitizes colon tumor cells to X-ray irradiation in vitro and in vivo

Colon cancer is one of the most common tumors of the digestive tract. Resistance to ionizing radiation (IR) decreased therapeutic efficiency in these patients’ radiotherapy. XRCC2 is the key protein of DNA homologous recombination repair, and its high expression is associated with enhanced resistance to DNA damage induced by IR. Here, we investigated the effect of XRCC2 silencing on colon tumor cells’ growth and sensitivity to X-radiation in vitro and in vivo. Colon tumor cells (T84 cell line) were cultivated in vitro and tumors originated from the cell line were propagated as xenografts in nude mice. The suppression of XRCC2 expression was achieved by using vector-based short hairpin RNA (shRNA) in T84 cells. We found that the knockdown of XRCC2 expression effectively decreased T84 cellular proliferation and colony formation, and led to cell apoptosis and cell cycle arrested in G2/M phase induced by X-radiation in vitro. In addition, tumor xenograft studies suggested that XRCC2 silencing inhibited tumorigenicity after radiation treatment in vivo. Our data suggest that the suppression of XRCC2 expression rendered colon tumor cells more sensitive to radiation therapy in vitro and in vivo, implying XRCC2 as a promising therapeutic target for the treatment of radioresistant human colon cancer.

Resection activity of the Sgs1 helicase alters the affinity of DNA ends for homologous recombination proteins in Saccharomyces cerevisiae

The RecQ helicase family is critical during DNA damage repair, and mutations in these proteins are associated with Bloom, Werner, or Rothmund-Thompson syndromes in humans, leading to cancer predisposition and/or premature aging. In the budding yeast Saccharomyces cerevisiae, mutations in the RecQ homolog, SGS1, phenocopy many of the defects observed in the human syndromes. One challenge to studying RecQ helicases is that their disruption leads to a pleiotropic phenotype. Using yeast, we show that the separation-of-function allele of SGS1, sgs1-D664Δ, has impaired activity at DNA ends, resulting in a resection processivity defect. Compromising Sgs1 resection function in the absence of the Sae2 nuclease causes slow growth, which is alleviated by making the DNA ends accessible to Exo1 nuclease. Furthermore, fluorescent microscopy studies reveal that, when Sgs1 resection activity is compromised in sae2Δ cells, Mre11 repair foci persist. We suggest a model where the role of Sgs1 in end resection along with Sae2 is important for removing Mre11 from DNA ends during repair.

Effects of XRCC2 and RAD51B mutations on somatic and meiotic recombination in Arabidopsis thaliana

Homologous recombination is key to the maintenance of genome integrity and the creation of genetic diversity. At the mechanistic level, recombination involves the invasion of a homologous DNA template by broken DNA ends, repair of the break and exchange of genetic information between the two DNA molecules. Invasion of the template in eukaryotic cells is catalysed by the RAD51 and DMC1 recombinases, assisted by a number of accessory proteins, including the RAD51 paralogues. Eukaryotic genomes encode a variable number of RAD51 paralogues, ranging from two in yeast to five in animals and plants. The RAD51 paralogues form at least two distinct protein complexes, believed to play roles in the assembly and stabilization of the RAD51-DNA nucleofilament. Somatic recombination assays and immunocytology confirm that the three 'non-meiotic' paralogues of Arabidopsis, RAD51B, RAD51D and XRCC2, are involved in somatic homologous recombination, and that they are not required for the formation of radioinduced RAD51 foci. Given the presence of all five proteins in meiotic cells, the apparent absence of a meiotic role for RAD51B, RAD51D and XRCC2 is surprising, and perhaps simply the result of a more subtle meiotic phenotype in the mutants. Analysis of meiotic recombination confirms this, showing that the absence of XRCC2, and to a lesser extent RAD51B, but not RAD51D, increases rates of meiotic crossing over. The roles of RAD51B and XRCC2 in recombination are thus not limited to mitotic cells.

Hereditary breast cancer: the era of new susceptibility genes

Breast cancer is the most common malignancy among females. 5%-10% of breast cancer cases are hereditary and are caused by pathogenic mutations in the considered reference BRCA1 and BRCA2 genes. As sequencing technologies evolve, more susceptible genes have been discovered and BRCA1 and BRCA2 predisposition seems to be only a part of the story. These new findings include rare germline mutations in other high penetrant genes, the most important of which include TP53 mutations in Li-Fraumeni syndrome, STK11 mutations in Peutz-Jeghers syndrome, and PTEN mutations in Cowden syndrome. Furthermore, more frequent, but less penetrant, mutations have been identified in families with breast cancer clustering, in moderate or low penetrant genes, such as CHEK2, ATM, PALB2, and BRIP1. This paper will summarize all current data on new findings in breast cancer susceptibility genes.

BLM and RAD51 genes polymorphism and susceptibility to breast cancer

DNA repair by homologous recombination is one of the main processes of DNA double strand breaks repair. In the present work we performed a case-control study (304 cases and 319 controls) to check an association between the genotypes of the c.-61 G>T and the g.38922 C>G polymorphisms of the RAD51 gene and the g.96267 A>C and the g.85394 A>G polymorphisms of the BLM gene and breast cancer occurrence. Genotypes were determined in DNA from peripheral blood by PCR-RLFP and by PCR-CTPP. We observed an association between breast cancer occurrence and the T/G genotype (OR 4.41) of the c.-61 G>T-RAD51 polymorphism, the A/A genotype (OR 1.69) of the g.85394 A>G-BLM polymorphism and the A/A genotype (OR 2.49) of the g.96267 A>C-BLM polymorphism. Moreover, we demonstrated a correlation between intra- and intergenes genotypes combinations and breast cancer occurrence. We found a correlation between progesterone receptor expression and the T/G genotype (OR 0.57) of the c.-61 G>T- RAD51 polymorphism. We also found a correlation between the T/G genotype (OR 1.86) and the T/T genotype (OR 0.56) of the c.-61 G>T- RAD51 polymorphism and the lymph node metastasis. We showed an association between the A/A genotype (OR 2.45) and the A/C genotype (OR 0.41) of the g.96267 A>C-BLM polymorphism and G3 grade of tumor. Our results suggest that the variability of the RAD51 and BLM genes may play a role in breast cancer occurrence. This role may be underlined by a common interaction between these genes.

Repair of DNA strand breaks in a minichromosome in vivo: kinetics, modeling, and effects of inhibitors

To obtain an overall picture of the repair of DNA single and double strand breaks in a defined region of chromatin in vivo, we studied their repair in a ∼170 kb circular minichromosome whose length and topology are analogous to those of the closed loops in genomic chromatin. The rate of repair of single strand breaks in cells irradiated with γ photons was quantitated by determining the sensitivity of the minichromosome DNA to nuclease S1, and that of double strand breaks by assaying the reformation of supercoiled DNA using pulsed field electrophoresis. Reformation of supercoiled DNA, which requires that all single strand breaks have been repaired, was not slowed detectably by the inhibitors of poly(ADP-ribose) polymerase-1 NU1025 or 1,5-IQD. Repair of double strand breaks was slowed by 20–30% when homologous recombination was supressed by KU55933, caffeine, or siRNA-mediated depletion of Rad51 but was completely arrested by the inhibitors of nonhomologous end-joining wortmannin or NU7441, responses interpreted as reflecting competition between these repair pathways similar to that seen in genomic DNA. The reformation of supercoiled DNA was unaffected when topoisomerases I or II, whose participation in repair of strand breaks has been controversial, were inhibited by the catalytic inhibitors ICRF-193 or F11782. Modeling of the kinetics of repair provided rate constants and showed that repair of single strand breaks in minichromosome DNA proceeded independently of repair of double strand breaks. The simplicity of quantitating strand breaks in this minichromosome provides a usefull system for testing the efficiency of new inhibitors of their repair, and since the sequence and structural features of its DNA and its transcription pattern have been studied extensively it offers a good model for examining other aspects of DNA breakage and repair.

Recognition, signaling, and repair of DNA double-strand breaks produced by ionizing radiation in mammalian cells: the molecular choreography

The faithful maintenance of chromosome continuity in human cells during DNA replication and repair is critical for preventing the conversion of normal diploid cells to an oncogenic state. The evolution of higher eukaryotic cells endowed them with a large genetic investment in the molecular machinery that ensures chromosome stability. In mammalian and other vertebrate cells, the elimination of double-strand breaks with minimal nucleotide sequence change involves the spatiotemporal orchestration of a seemingly endless number of proteins ranging in their action from the nucleotide level to nucleosome organization and chromosome architecture. DNA DSBs trigger a myriad of post-translational modifications that alter catalytic activities and the specificity of protein interactions: phosphorylation, acetylation, methylation, ubiquitylation, and SUMOylation, followed by the reversal of these changes as repair is completed. "Superfluous" protein recruitment to damage sites, functional redundancy, and alternative pathways ensure that DSB repair is extremely efficient, both quantitatively and qualitatively. This review strives to integrate the information about the molecular mechanisms of DSB repair that has emerged over the last two decades with a focus on DSBs produced by the prototype agent ionizing radiation (IR). The exponential growth of molecular studies, heavily driven by RNA knockdown technology, now reveals an outline of how many key protein players in genome stability and cancer biology perform their interwoven tasks, e.g. ATM, ATR, DNA-PK, Chk1, Chk2, PARP1/2/3, 53BP1, BRCA1, BRCA2, BLM, RAD51, and the MRE11-RAD50-NBS1 complex. Thus, the nature of the intricate coordination of repair processes with cell cycle progression is becoming apparent. This review also links molecular abnormalities to cellular pathology as much a possible and provides a framework of temporal relationships.

Rare mutations in XRCC2 increase the risk of breast cancer

An exome-sequencing study of families with multiple breast-cancer-affected individuals identified two families with XRCC2 mutations, one with a protein-truncating mutation and one with a probably deleterious missense mutation. We performed a population-based case-control mutation-screening study that identified six 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 < 0.02). We also performed additional mutation screening in 689 multiple-case families. We identified ten breast-cancer-affected families with protein-truncating or probably deleterious rare missense variants in XRCC2. Our identification of XRCC2 as a breast cancer susceptibility gene thus increases the proportion of breast cancers that are associated with homologous recombination-DNA-repair dysfunction and Fanconi anemia and could therefore benefit from specific targeted treatments such as PARP (poly ADP ribose polymerase) inhibitors. This study demonstrates the power of massively parallel sequencing for discovering susceptibility genes for common, complex diseases.

Assessment of genetic mutations in the XRCC2 coding region by high resolution melting curve analysis and the risk of differentiated thyroid carcinoma in Iran

Homologous recombination (HR) is the major pathway for repairing double strand breaks (DSBs) in eukaryotes and XRCC2 is an essential component of the HR repair machinery. To evaluate the potential role of mutations in gene repair by HR in individuals susceptible to differentiated thyroid carcinoma (DTC) we used high resolution melting (HRM) analysis, a recently introduced method for detecting mutations, to examine the entire XRCC2 coding region in an Iranian population. HRM analysis was used to screen for mutations in three XRCC2 coding regions in 50 patients and 50 controls. There was no variation in the HRM curves obtained from the analysis of exons 1 and 2 in the case and control groups. In exon 3, an Arg¹⁸⁸His polymorphism (rs3218536) was detected as a new melting curve group (OR: 1.46; 95%CI: 0.432–4.969; p = 0.38) compared with the normal melting curve. We also found a new Ser¹⁵⁰Arg polymorphism in exon 3 of the control group. These findings suggest that genetic variations in the XRCC2 coding region have no potential effects on susceptibility to DTC. However, further studies with larger populations are required to confirm this conclusion.

DNA repair proteins are directly involved in regulation of gene expression during plant immune response

Systemic acquired resistance (SAR), an inducible plant-defense response to local infection, requires the signaling molecule salicylic acid (SA) and the transcriptional coactivator NPR1, with concerted activation of pathogenesis-related (PR) genes. Arabidopsis sni1 is an npr1 suppressor and derepression of defense genes in sni1 causes reduced growth and fertility and increased homologous recombination. Characterizing suppressors of sni1, we identify the DNA damage repair proteins SSN2 and RAD51D as genetic and physical interactors with SNI1. During plant defense, SSN2 and possibly RAD51D replace the transcription repressor SNI1 at pathogenesis-related gene promoters. In the presence of SNI1, NPR1 is also required for SSN2 binding. Thus, coordinated action of SNI1, SSN2-RAD51D, and NPR1 ensures the tight control of plant immune gene expression. Given that the SSN2-RAD51D complex is conserved in eukaryotes, their dual function in homologous recombination and transcription regulation of plant-defense genes suggests a general link between these two stress responses.

Polymorphisms of homologous recombination genes and clinical outcomes of non-small cell lung cancer patients treated with definitive radiotherapy

The repair of DNA double-strand breaks (DSBs) is the major mechanism to maintain genomic stability in response to irradiation. We hypothesized that genetic polymorphisms in DSB repair genes may affect clinical outcomes among non-small cell lung cancer (NSCLC) patients treated with definitive radio(chemo)therapy. We genotyped six potentially functional single nucleotide polymorphisms (SNPs) (i.e., RAD51 −135G>C/rs1801320 and −172G>T/rs1801321, XRCC2 4234G>C/rs3218384 and R188H/rs3218536 G>A, XRCC3 T241M/rs861539 and NBN E185Q/rs1805794) and estimated their associations with overall survival (OS) and radiation pneumonitis (RP) in 228 NSCLC patients. We found a predictive role of RAD51 −135G>C SNP in RP development (adjusted hazard ratio [HR] = 0.52, 95% confidence interval [CI], 0.31–0.86, P = 0.010 for CG/CC vs. GG). We also found that RAD51 −135G>C and XRCC2 R188H SNPs were independent prognostic factors for overall survival (adjusted HR = 1.70, 95% CI, 1.14–2.62, P = 0.009 for CG/CC vs. GG; and adjusted HR = 1.70; 95% CI, 1.02–2.85, P = 0.043 for AG vs. GG, respectively) and that the SNP-survival association was most pronounced in the presence of RP. Our study suggests that HR genetic polymorphisms, particularly RAD51 −135G>C, may influence overall survival and radiation pneumonitis in NSCLC patients treated with definitive radio(chemo)therapy. Large studies are needed to confirm our findings.

Kinetic analysis of DNA double-strand break repair pathways in Arabidopsis

Double-strand breaks in genomic DNA (DSB) are potentially lethal lesions which separate parts of chromosome arms from their centromeres. Repair of DSB by recombination can generate mutations and further chromosomal rearrangements, making the regulation of recombination and the choice of recombination pathways of the highest importance. Although knowledge of recombination mechanisms has considerably advanced, the complex interrelationships and regulation of pathways are far from being fully understood. We analyse the different pathways of DSB repair acting in G2/M phase nuclei of irradiated plants, through quantitation of the kinetics of appearance and loss of γ-H2AX foci in Arabidopsis mutants. These analyses show the roles for the four major recombination pathways in post-S-phase DSB repair and that non-homologous recombination pathways constitute the major response. The data suggest a hierarchical organisation of DSB repair in these cells: C-NHEJ acts prior to B-NHEJ which can also inhibit MMEJ. Surprisingly the quadruple ku80 xrcc1 xrcc2 xpf mutant can repair DSB, although with severely altered kinetics. This repair leads to massive genetic instability with more than 50% of mitoses showing anaphase bridges following irradiation. This study thus clarifies the relationships between the different pathways of DSB repair in the living plant and points to the existence of novel DSB repair processes.

Effect of gold nanorod surface chemistry on cellular response

Gold nanorods (GNRs) stabilized with cetyltrimethylammonium bromide (CTAB) and GNR functionalized via a ligand exchange method with either thiolated polyethylene glycol (PEG(5000)) or mercaptohexadecanoic acid (MHDA) were investigated for their stability in biological media and subsequent toxicological effects to HaCaT cells. GNR-PEG and GNR-MHDA exhibited minimal effects on cell proliferation, whereas GNR-CTAB reduced cell proliferation significantly due to the inherent toxicity of the cationic surfactant to cells. Cell uptake studies indicated relatively low uptake for GNR-PEG and high uptake for GNR-MHDA. Reverse transcriptase polymerase chain reaction (RT-PCR) revealed that GNR-PEG induced less significant and unique changes in the transcription levels of 84 genes related to stress and toxicity compared to GNR-MHDA. The results demonstrate that, although cell proliferation was not affected by both particles, there is a significant difference in gene expression in GNR-MHDA exposed cells, suggesting long-term implications for chronic exposure.

Homologous recombination proteins are associated with centrosomes and are required for mitotic stability

In response to DNA damage, cells need robust repair mechanisms to complete the cell cycle successfully. Severe forms of DNA damage are repaired by homologous recombination (HR), in which the XRCC2 protein plays a vital role. Cells deficient in XRCC2 also show disruption of the centrosome, a key component of the mitotic apparatus. We find that this centrosome disruption is dynamic and when it occurs during mitosis it is linked directly to the onset of mitotic catastrophe in a significant fraction of the XRCC2-deficient cells. However, we also show for the first time that XRCC2 and other HR proteins, including the key recombinase RAD51, co-localize with the centrosome. Co-localization is maintained throughout the cell cycle, except when cells are finishing mitosis when RAD51 accumulates in the midbody between the separating cells. Taken together, these data suggest a tight functional linkage between the centrosome and HR proteins, potentially to coordinate the deployment of a DNA damage response at vulnerable phases of the cell cycle.

Structural and functional characterization of the N-terminal domain of human Rad51D

Rad51D, one of five Rad51 paralogs, is required for homologous recombination and disruption of Holliday junctions with bloom syndrome protein (BLM) in vertebrates. The N-terminal domain of Rad51D is highly conserved in eukaryotic Rad51D orthologs and is essential for protein-protein interaction with XRCC2, but nothing is known about its individual structure or function. In this study, we determined the solution structure of the human Rad51D N-terminal domain (residues 1-83), which consists of four short helices flanked by long N- and C-terminal tails. Interestingly, the position of the N-terminal tail (residues 1-13) is fixed within the domain structure via several hydrophobic interactions between Leu4 and Thr27, Leu4 and Val28, and Val6 and Ile17. We show that the domain preferentially binds to ssDNA versus dsDNA and does not bind to a mobile Holliday junction by electrophoretic mobility shift assay. NMR titration and dynamics studies showed that human Rad51D-N interacts with ssDNA by positively charged and hydrophobic residues on its surface. The results suggest that the N-terminal domain of Rad51D is required for the ssDNA-specific binding function of human Rad51D and that the conserved N-terminal domains of other Rad51 paralogs may have distinguishable functions from each other in homologous recombination.