XRCC2 is a nuclear RAD51-like protein required for damage-dependent RAD51 focus formation without the need for ATP binding

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.

Relationship between polymorphisms in homologous recombination repair genes RAD51 G172T、XRCC2 & XRCC3 and risk of breast cancer: A meta-analysis

Background

Genetic variability in DNA double-strand break repair genes such as RAD51 gene and its paralogs XRCC2、XRCC3 may contribute to the occurrence and progression of breast cancer. To obtain a complete evaluation of the above association, we performed a meta-analysis of published studies.

Methods

Electronic databases, including PubMed, EMBASE, Web of Science, and Cochrane Library, were comprehensively searched from inception to September 2022. The Newcastle-Ottawa Scale (NOS) checklist was used to assess all included non-randomized studies. Odds ratios (OR) with 95% confidence intervals (CI) were calculated by STATA 16.0 to assess the strength of the association between single nucleotide polymorphisms (SNPs) in these genes and breast cancer risk. Subsequently, the heterogeneity between studies, sensitivity, and publication bias were performed. We downloaded data from The Cancer Genome Atlas (TCGA) and used univariate and multivariate Cox proportional hazard regression (CPH) models to validate the prognostic value of these related genes in the R software.

Results

The combined results showed that there was a significant correlation between the G172T polymorphism and the susceptibility to breast cancer in the homozygote model (OR= 1.841, 95% CI=1.06-3.21, P=0.03). Furthermore, ethnic analysis showed that SNP was associated with the risk of breast cancer in Arab populations in homozygous models (OR=3.52, 95% CI=1.13-11.0, P= 0.003). For the XRCC2 R188H polymorphism, no significant association was observed. Regarding polymorphism in XRCC3 T241M, a significantly increased cancer risk was only observed in the allelic genetic model (OR=1.05, 95% CI= 1.00-1.11, P=0.04).

Conclusions

In conclusion, this meta-analysis suggests that Rad51 G172T polymorphism is likely associated with an increased risk of breast cancer, significantly in the Arab population. The relationship between the XRCC2 R188H polymorphism and breast cancer was not obvious. And T241M in XRCC3 may be associated with breast cancer risk, especially in the Asian population.

Characterization of an archaeal recombinase paralog that exhibits novel anti-recombinase activity

The process of homologous recombination is heavily dependent on the RecA family of recombinases for repair of DNA double-strand breaks. These recombinases are responsible for identifying homologies and forming heteroduplex DNA between substrate ssDNA and dsDNA templates, activities that are modified by various accessory factors. In this work we describe the biochemical functions of the SsoRal2 recombinase paralog from the crenarchaeon Sulfolobus solfataricus. We found that the SsoRal2 protein is a DNA-independent ATPase that, unlike the other S. solfataricus paralogs, does not bind either ss- or dsDNA. Instead, SsoRal2 alters the ssDNA binding activity of the SsoRadA recombinase in conjunction with another paralog, SsoRal1. In the presence of SsoRal1, SsoRal2 has a modest effect on strand invasion but effectively abrogates strand exchange activity. Taken together, these results indicate that SsoRal2 assists in nucleoprotein filament modulation and control of strand exchange in S. solfataricus.

RAD51D splice variants and cancer-associated mutations reveal XRCC2 interaction to be critical for homologous recombination

The proficiency of cancer cells to repair DNA double-strand breaks (DSBs) by homologous recombination (HR) is a key determinant in predicting response to targeted therapies such as PARP inhibitors. The RAD51 paralogs work as multimeric complexes and act downstream of BRCA1 to facilitate HR. Numerous epidemiological studies have linked RAD51 paralog mutations with hereditary cancer predisposition. Despite their substantial links to cancer, RAD51 paralog HR function has remained elusive. Here we identify isoform 1 as the functional isoform of RAD51D, whereas isoform 4 which has a large N-terminal deletion (including the Walker A motif), and isoform 6 which includes an alternate exon in the N-terminus, are non-functional. To determine the importance of this N-terminal region, we investigated the impact of cancer-associated mutations and SNPs in this variable RAD51D N-terminal region using yeast-2-hybrid and yeast-3-hybrid assays to screen for altered protein-protein interactions. We identified two cancer-associated mutations close to or within the Walker A motif (G96C and G107 V, respectively) that independently disrupt RAD51D interaction with XRCC2. We validated our yeast interaction data in human U2OS cells by co-immunoprecipitation and determined the impact of these mutations on HR-proficiency using a sister chromatid recombination reporter assay in a RAD51D knock-out cell line. Our investigation reveals that the interaction of RAD51D with XRCC2 is required for DSB repair. By characterizing the impact of cancer-associated mutations on RAD51D interactions, we aim to develop predictive models for therapeutic sensitivity and resistance in patients who harbor similar mutations in RAD51D.

XRCC2 (X-ray repair cross complementing 2)

XRCC2 is one of five somatic RAD51 paralogs, all of which have Walker A and B ATPase motifs. Each of the paralogs, including XRCC2, has a function in DNA double-strand break repair by homologous recombination (HR). However, their individual roles are not as well understood as that of RAD51 itself. The XRCC2 protein forms a complex (BCDX2) with three other RAD51 paralogs, RAD51B, RAD51C and RAD51D. It is believed that the BCDX2 complex mediates HR downstream of BRCA2 but upstream of RAD51, as XRCC2 is involved in the assembly of RAD51 into DNA damage foci. XRCC2 can bind DNA and, along with RAD51D, can promote homologous pairing in vitro. Consistent with its role in HR, XRCC2-deficient cells have increased levels of spontaneous chromosome instability, and exhibit hypersensitivity to DNA interstrand crosslinking agents such as mitomycin C and cisplatin as well as ionizing radiation, alkylating agents and aldehydes. XRCC2 also functions in promoting DNA replication and chromosome segregation. Biallelic mutation of XRCC2 (FANCU) causes the FA-U subtype of FA, while heterozygosity for deleterious mutations in XRCC2 may be associated with an increased breast cancer risk. XRCC2 appears to function 'downstream' in the FA pathway, since it is not required for FANCD2 monoubiquitination, which is the central step in the FA pathway. Clinically, the only known FA-U patient in the world exhibits severe congenital abnormalities, but had not developed, by seven years of age, the bone marrow failure and cancer that are often seen in patients from other FA complementation groups.

The homologous recombination protein RAD51D protects the genome from large deletions

Homologous recombination (HR) is a DNA double-strand break (DSB) repair pathway that protects the genome from chromosomal instability. RAD51 mediator proteins (i.e. paralogs) are critical for efficient HR in mammalian cells. However, how HR-deficient cells process DSBs is not clear. Here, we utilized a loss-of-function HR-reporter substrate to simultaneously monitor HR-mediated gene conversion and non-conservative mutation events. The assay is designed around a heteroallelic duplication of the Aprt gene at its endogenous locus in isogenic Chinese hamster ovary cell lines. We found that RAD51D-deficient cells had a reduced capacity for HR-mediated gene conversion both spontaneously and in response to I-SceI-induced DSBs. Further, RAD51D-deficiency shifted DSB repair toward highly deleterious single-strand annealing (SSA) and end-joining processes that led to the loss of large chromosomal segments surrounding site-specific DSBs at an exceptionally high frequency. Deletions in the proximity of the break were due to a non-homologous end-joining pathway, while larger deletions were processed via a SSA pathway. Overall, our data revealed that, in addition to leading to chromosomal abnormalities, RAD51D-deficiency resulted in a high frequency of deletions advancing our understanding of how a RAD51 paralog is involved in maintaining genomic stability and how its deficiency may predispose cells to tumorigenesis.

BLM helicase regulates DNA repair by counteracting RAD51 loading at DNA double-strand break sites

The BLM gene product, BLM, is a RECQ helicase that is involved in DNA replication and repair of DNA double-strand breaks by the homologous recombination (HR) pathway. During HR, BLM has both pro- and anti-recombinogenic activities, either of which may contribute to maintenance of genomic integrity. We find that in cells expressing a mutant version of BRCA1, an essential HR factor, ablation of BLM rescues genomic integrity and cell survival in the presence of DNA double-strand breaks. Improved genomic integrity in these cells is linked to a substantial increase in the stability of RAD51 at DNA double-strand break sites and in the overall efficiency of HR. Ablation of BLM also rescues RAD51 foci and HR in cells lacking BRCA2 or XRCC2. These results indicate that the anti-recombinase activity of BLM is of general importance for normal retention of RAD51 at DNA break sites and regulation of HR.

Distinct cellular phenotype linked to defective DNA interstrand crosslink repair and homologous recombination

Repair of DNA interstrand crosslinks (ICLs) predominantly involves the Fanconi anemia (FA) pathway and homologous recombination (HR). The HR repair system eliminates DNA double strand breaks (DSBs) that emerge during ICLs removal. The current study presents a novel cell line, CL-V8B, representing a new complementation group of Chinese hamster cell mutants hypersensitive to DNA crosslinking factors. CL-V8B exhibits increased sensitivity to various DNA-damaging agents, including compounds leading to DSBs formation (bleomycin and 6-thioguanine), and is extremely sensitive to poly (ADP-ribose) polymerase inhibitor (>400-fold), which is typical for HR-defective cells. In addition, this cell line exhibits a reduced number of spontaneous and induced sister chromatid exchanges, which suggests likely impairment of HR in CL-V8B cells. However, in contrast to other known HR mutants, CL-V8B cells do not show defects in Rad51 foci induction, but only slight alterations in the focus formation kinetics. CL-V8B is additionally characterized by a considerable chromosomal instability, as indicated by a high number of spontaneous and MMC-induced chromosomal aberrations, and a twice as large proportion of cells with abnormal centrosomes than that in the wild type cell line. The molecular defect present in CL-V8B does not affect the efficiency and stabilization of replication forks. However, stalling of the forks in response to replication stress is observed relatively rarely, which suggests an impairment of a signaling mechanism. Exposure of CL-V8B to crosslinking agents results in S-phase arrest (as in the wild type cells), but also in larger proportion of G₂/M-phase cells and apoptotic cells. CL-V8B exhibits similarities to HR- and/or FA-defective Chinese hamster mutants sensitive to DNA crosslinking agents. However, the unique phenotype of this new mutant implies that it carries a defect of a yet unidentified gene involved in the repair of ICLs.

Impact of RAD51C-mediated Homologous Recombination on Genomic Integrity in Barrett's Adenocarcinoma Cells

BACKGROUND

In normal cells, RAD51-mediated homologous recombination (HR) is a precise DNA repair mechanism which plays a key role in the maintenance of genomic integrity and stability. However, elevated (dysregulated) RAD51 is implicated in genomic instability and is a potential target for treatment of certain cancers, including Barrett’s adenocarcinoma (BAC). In this study, we investigated genomic impact and translational significance of moderate vs. strong suppression of RAD51 in BAC cells.

METHODS

BAC cells (FLO-1 and OE33) were transduced with non-targeting control (CS) or RAD51-specific shRNAs, mediating a moderate (40–50%) suppression or strong (80-near 100%) suppression of the gene. DNA breaks, spontaneous or following exposure to DNA damaging agent, were examined by comet assay and 53BP1 staining. Gene expression was monitored by microarrays (Affymetrix). Homologous recombination (HR) and single strand annealing (SSA) activities were measured using plasmid based assays.

RESULTS

We show that although moderate suppression consistenly inhibits/reduces HR activity, the strong suppression is associated with increase in HR activity (by ~15 – ≥ 50% in various experiments), suggesting activation of RAD51-independent pathway. Contrary to moderate suppression, a strong suppression of RAD51 is associated with a significant induced DNA breaks as well as altered expression of genes involved in detection/processing of DNA breaks and apoptosis. Stronger RAD51 suppression was also associated with mutagenic single strand annealing mediated HR. Suppression of RAD51C inhibited RAD51-independent (SSA-mediated) HR in BAC cells.

CONCLUSION

Elevated (dysregulated) RAD51 in BAC is implicated in both the repair of DNA breaks as well as ongoing genomic rearrangements. Moderate suppression of this gene reduces HR activity, whereas strong or near complete suppression of this gene activates RAD51C-dependent HR involving a mechanism known as single strand annealing (SSA). SSA-mediated HR, which is a mutagenic HR pathway, further disrupts genomic integrity by increasing DNA breaks in BAC cells.

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.

ZNF281 contributes to the DNA damage response by controlling the expression of XRCC2 and XRCC4

ZNF281 is a zinc-finger factor involved in the control of cellular stemness and epithelial-mesenchymal transition (EMT). Here, we report that ZNF281 expression increased after genotoxic stress caused by DNA-damaging drugs. Comet assays demonstrated that DNA repair was delayed in cells silenced for the expression of ZNF281 and treated with etoposide. Furthermore, the expression of 10 DNA damage response genes was downregulated in cells treated with etoposide and silenced for ZNF281. In line with this finding, XRCC2 and XRCC4, two genes that take part in homologous recombination and non-homologous end joining, respectively, were transcriptionally activated by ZNF281 through a DNA-binding-dependent mechanism, as demonstrated by luciferase assays and Chromatin crosslinking ImmunoPrecipitation experiments. c-Myc, which also binds to the promoters of XRCC2 and XRCC4, was unable to promote their transcription or to modify ZNF281 activity. Of interest, bioinformatic analysis of 1971 breast cancer patients disclosed a significant correlation between the expression of ZNF281 and that of XRCC2. In summary, our data highlight, for the first time, the involvement of ZNF281 in the cellular response to genotoxic stress through the control exercised on the expression of genes that act in different repair mechanisms.

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.

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.

Roles of Rad51 paralogs for promoting homologous recombination in Leishmania infantum

To achieve drug resistance Leishmania parasite alters gene copy number by using its repeated sequences widely distributed through the genome. Even though homologous recombination (HR) is ascribed to maintain genome stability, this eukaryote exploits this potent mechanism driven by the Rad51 recombinase to form beneficial extrachromosomal circular amplicons. Here, we provide insights on the formation of these circular amplicons by analyzing the functions of the Rad51 paralogs. We purified three Leishmania infantum Rad51 paralogs homologs (LiRad51-3, LiRad51-4 and LiRad51-6) all of which directly interact with LiRad51. LiRad51-3, LiRad51-4 and LiRad51-6 show differences in DNA binding and annealing capacities. Moreover, it is also noteworthy that LiRad51-3 and LiRad51-4 are able to stimulate Rad51-mediated D-loop formation. In addition, we succeed to inactivate the LiRad51-4 gene and report a decrease of circular amplicons in this mutant. The LiRad51-3 gene was found to be essential for cell viability. Thus, we propose that the LiRad51 paralogs play crucial functions in extrachromosomal circular DNA amplification to circumvent drug actions and preserve survival.

A mRad51-GFP antimorphic allele affects homologous recombination and DNA damage sensitivity

Accurate DNA double-strand break repair through homologous recombination is essential for preserving genome integrity. Disruption of the gene encoding RAD51, the protein that catalyzes DNA strand exchange during homologous recombination, results in lethality of mammalian cells. Proteins required for homologous recombination, also play an important role during DNA replication. To explore the role of RAD51 in DNA replication and DSB repair, we used a knock-in strategy to express a carboxy-terminal fusion of green fluorescent protein to mouse RAD51 (mRAD51-GFP) in mouse embryonic stem cells. Compared to wild-type cells, heterozygous mRad51(+/wt-GFP) embryonic stem cells showed increased sensitivity to DNA damage induced by ionizing radiation and mitomycin C. Moreover, gene targeting was found to be severely impaired in mRad51(+/wt-GFP) embryonic stem cells. Furthermore, we found that mRAD51-GFP foci were not stably associated with chromatin. From these experiments we conclude that this mRad51-GFP allele is an antimorphic allele. When this allele is present in a heterozygous condition over wild-type mRad51, embryonic stem cells are proficient in DNA replication but display defects in homologous recombination and DNA damage repair.

DNA repair pathways in trypanosomatids: from DNA repair to drug resistance

All living organisms are continuously faced with endogenous or exogenous stress conditions affecting genome stability. DNA repair pathways act as a defense mechanism, which is essential to maintain DNA integrity. There is much to learn about the regulation and functions of these mechanisms, not only in human cells but also equally in divergent organisms. In trypanosomatids, DNA repair pathways protect the genome against mutations but also act as an adaptive mechanism to promote drug resistance. In this review, we scrutinize the molecular mechanisms and DNA repair pathways which are conserved in trypanosomatids. The recent advances made by the genome consortiums reveal the complete genomic sequences of several pathogens. Therefore, using bioinformatics and genomic sequences, we analyze the conservation of DNA repair proteins and their key protein motifs in trypanosomatids. We thus present a comprehensive view of DNA repair processes in trypanosomatids at the crossroads of DNA repair and drug resistance.

Replication fork dynamics and the DNA damage response

Prevention and repair of DNA damage is essential for maintenance of genomic stability and cell survival. DNA replication during S-phase can be a source of DNA damage if endogenous or exogenous stresses impair the progression of replication forks. It has become increasingly clear that DNA-damage-response pathways do not only respond to the presence of damaged DNA, but also modulate DNA replication dynamics to prevent DNA damage formation during S-phase. Such observations may help explain the developmental defects or cancer predisposition caused by mutations in DNA-damage-response genes. The present review focuses on molecular mechanisms by which DNA-damage-response pathways control and promote replication dynamics in vertebrate cells. In particular, DNA damage pathways contribute to proper replication by regulating replication initiation, stabilizing transiently stalled forks, promoting replication restart and facilitating fork movement on difficult-to-replicate templates. If replication fork progression fails to be rescued, this may lead to DNA damage and genomic instability via nuclease processing of aberrant fork structures or incomplete sister chromatid separation during mitosis.

The RAD51 paralogs ensure cellular protection against mitotic defects and aneuploidy

The interplay between homologous DNA recombination and mitotic progression is poorly understood. The five RAD51 paralogs (RAD51B, -C, -D, XRCC2, XRCC3) are key enzymes for DNA double-strand break repair. In our search for specific functions of the various RAD51 paralogs, we found that inhibition of XRCC3 elicits checkpoint defects, while inhibition of RAD51B and RAD51C induces G2/M cell cycle arrest in Hela cells. Using live-cell microscopy we show that XRCC3-knockdown cells displayed persistent spindle assembly checkpoint and a higher frequency of chromosome misalignments, anaphase bridges, and aneuploidy. We observed centrosome defects in the absence of XRCC3. While RAD51B and RAD51C act early in HR, XRCC3 functions jointly with GEN1 later in the pathway at the stage of Holliday junction resolution. Our data demonstrate that Holliday junction resolution has critical functions for preventing aberrant mitosis and aneuploidy in mitotic cells.

A role for XRCC2 gene polymorphisms in breast cancer risk and survival

BACKGROUND

The XRCC2 gene is a key mediator in the homologous recombination repair of DNA double strand breaks. It is hypothesised that inherited variants in the XRCC2 gene might also affect susceptibility to, and survival from, breast cancer.

METHODS

The study genotyped 12 XRCC2 tagging single nucleotide polymorphisms (SNPs) in 1131 breast cancer cases and 1148 controls from the Sheffield Breast Cancer Study (SBCS), and examined their associations with breast cancer risk and survival by estimating ORs and HRs, and their corresponding 95% CIs. Positive findings were further investigated in 860 cases and 869 controls from the Utah Breast Cancer Study (UBCS) and jointly analysed together with available published data for breast cancer risk. The survival findings were further confirmed in studies (8074 cases) from the Breast Cancer Association Consortium (BCAC).

RESULTS

The most significant association with breast cancer risk in the SBCS dataset was the XRCC2 rs3218408 SNP (recessive model p=2.3×10(-4), minor allele frequency (MAF)=0.23). This SNP yielded an OR(rec) of 1.64 (95% CI 1.25 to 2.16) in a two-site analysis of SBCS and UBCS, and a meta-OR(rec) of 1.33 (95% CI 1.12 to 1.57) when all published data were included. This SNP may mark a rare risk haplotype carried by two in 1000 of the control population. Furthermore, the XRCC2 coding R188H SNP (rs3218536, MAF=0.08) was significantly associated with poor survival, with an increased per-allele HR of 1.58 (95% CI 1.01 to 2.49) in a multivariate analysis. This effect was still evident in a pooled meta-analysis of 8781 breast cancer patients from the BCAC (HR 1.19, 95% CI 1.05 to 1.36; p=0.01).

CONCLUSIONS

These findings suggest that XRCC2 SNPs may influence breast cancer risk and survival.

Interactions among Trypanosoma brucei RAD51 paralogues in DNA repair and antigenic variation

Homologous recombination in Trypanosoma brucei is used for moving variant surface glycoprotein (VSG) genes into expression sites during immune evasion by antigenic variation. A major route for such VSG switching is gene conversion reactions in which RAD51, a universally conserved recombinase, catalyses homology-directed strand exchange. In any eukaryote, RAD51-directed strand exchange in vivo is mediated by further factors, including RAD51-related proteins termed Rad51 paralogues. These appear to be ubiquitously conserved, although their detailed roles in recombination remain unclear. In T. brucei, four putative RAD51 paralogue genes have been identified by sequence homology. Here we show that all four RAD51 paralogues act in DNA repair, recombination and RAD51 subnuclear dynamics, though not equivalently, while mutation of only one RAD51 paralogue gene significantly impedes VSG switching. We also show that the T. brucei RAD51 paralogues interact, and that the complexes they form may explain the distinct phenotypes of the mutants as well as observed expression interdependency. Finally, we document the Rad51 paralogues that are encoded by a wide range of protists, demonstrating that the Rad51 paralogue repertoire in T. brucei is unusually large among microbial eukaryotes and that one member of the protein family corresponds with a key, conserved eukaryotic Rad51 paralogue.

RAD51C: a novel cancer susceptibility gene is linked to Fanconi anemia and breast cancer

Germline mutations in many of the genes that are involved in homologous recombination (HR)-mediated DNA double-strand break repair (DSBR) are associated with various human genetic disorders and cancer. RAD51 and RAD51 paralogs are important for HR and in the maintenance of genome stability. Despite the identification of five RAD51 paralogs over a decade ago, the molecular mechanism(s) by which RAD51 paralogs regulate HR and genome maintenance remains obscure. In addition to the known roles of RAD51C in early and late stages of HR, it also contributes to activation of the checkpoint kinase CHK2. One recent study identifies biallelic mutation in RAD51C leading to Fanconi anemia-like disorder. Whereas a second study reports monoallelic mutation in RAD51C associated with increased risk of breast and ovarian cancer. These reports show RAD51C is a cancer susceptibility gene. In this review, we focus on describing the functions of RAD51C in HR, DNA damage signaling and as a tumor suppressor with an emphasis on the new roles of RAD51C unveiled by these reports.

Xrcc2 modulates spontaneous and radiation-induced tumorigenesis in Apcmin/+ mice

XRCC2 has an important role in repair of DNA damage by homologous recombination. Adult Apc(min/+) (min, multiple intestinal neoplasia) mice, wild-type or heterozygous for Xrcc2 deficiency, were sham-irradiated or 2-Gy X-irradiated. Spontaneous mammary and intestinal tumor incidences are lower in Apc(min/+) Xrcc2(+/-) mice than in Apc(min/+) Xrcc2(+/+) mice (mammary tumors: 14% and 38%, respectively, χ(2) P = 0.03; intestinal adenomas in mice reaching full life span: 108.6 and 130.1, respectively, t-test P = 0.005). Following irradiation, the increase in mammary tumors was greatest in female mice heterozygous for Xrcc2 (7.25 ± 0.50-fold in Apc(min/+) Xrcc2(+/-) mice compared with 2.57 ± 0.35-fold in Apc(min/+) Xrcc2(+/+) mice; t-test P < 0.001). The increase in intestinal tumor multiplicity following irradiation was significantly greater in Apc(min/+) Xrcc2(+/-) mice (Apc(min/+) Xrcc2(+/-), 4.14 ± 0.05-fold, versus Apc(min/+) Xrcc2(+/+), 3.30 ± 0.05-fold; t-test P < 0.001). Loss of heterozygosity of all chromosome 18 markers was greater in intestinal tumors from Apc(min/+) Xrcc2(+/-) mice than in tumors from Apc(min/+) Xrcc2(+/+) mice. These findings indicate that Xrcc2 haploinsufficiency reduces spontaneous tumor incidence on an Apc(min/+) background but increases the tumorigenic response to radiation.

Choreography of recombination proteins during the DNA damage response

Genome integrity is frequently challenged by DNA lesions from both endogenous and exogenous sources. A single DNA double-strand break (DSB) is lethal if unrepaired and may lead to loss of heterozygosity, mutations, deletions, genomic rearrangements and chromosome loss if repaired improperly. Such genetic alterations are the main causes of cancer and other genetic diseases. Consequently, DNA double-strand break repair (DSBR) is an important process in all living organisms. DSBR is also the driving mechanism in most strategies of gene targeting, which has applications in both genetic and clinical research. Here we review the cell biological response to DSBs in mitotically growing cells with an emphasis on homologous recombination pathways in yeast Saccharomyces cerevisiae and in mammalian cells.

XRCC2 and XRCC3 regulate the balance between short- and long-tract gene conversions between sister chromatids

Sister chromatid recombination (SCR) is a potentially error-free pathway for the repair of DNA lesions associated with replication and is thought to be important for suppressing genomic instability. The mechanisms regulating the initiation and termination of SCR in mammalian cells are poorly understood. Previous work has implicated all the Rad51 paralogs in the initiation of gene conversion and the Rad51C/XRCC3 complex in its termination. Here, we show that hamster cells deficient in the Rad51 paralog XRCC2, a component of the Rad51B/Rad51C/Rad51D/XRCC2 complex, reveal a bias in favor of long-tract gene conversion (LTGC) during SCR. This defect is corrected by expression of wild-type XRCC2 and also by XRCC2 mutants defective in ATP binding and hydrolysis. In contrast, XRCC3-mediated homologous recombination and suppression of LTGC are dependent on ATP binding and hydrolysis. These results reveal an unexpectedly general role for Rad51 paralogs in the control of the termination of gene conversion between sister chromatids.

Transcription-associated recombination in eukaryotes: link between transcription, replication and recombination

Homologous recombination (HR) is an important DNA repair pathway and is essential for cellular survival. It plays a major role in repairing replication-associated lesions and is functionally connected to replication. Transcription is another cellular process, which has emerged to have a connection with HR. Transcription enhances HR, which is a ubiquitous phenomenon referred to as transcription-associated recombination (TAR). Recent evidence suggests that TAR plays a role in inducing genetic instability, for example in the THO mutants (Tho2, Hpr1, Mft1 and Thp2) in yeast or during the development of the immune system leading to genetic diversity in mammals. On the other hand, evidence also suggests that TAR may play a role in preventing genetic instability in many different ways, one of which is by rescuing replication during transcription. Hence, TAR is a double-edged sword and plays a role in both preventing and inducing genetic instability. In spite of the interesting nature of TAR, the mechanism behind TAR has remained elusive. Recent advances in the area, however, suggest a link between TAR and replication and show specific genetic requirements for TAR that differ from regular HR. In this review, we aim to present the available evidence for TAR in both lower and higher eukaryotes and discuss its possible mechanisms, with emphasis on its connection with replication.

Transcription-associated recombination is independent of XRCC2 and mechanistically separate from homology-directed DNA double-strand break repair

It has previously been shown that transcription greatly enhances recombination in mammalian cells. However, the proteins involved in catalysing this process and the recombination pathways involved in transcription-associated recombination (TAR) are still unknown. It is well established that both the BRCA2 protein and the RAD51 paralog protein XRCC2 are required for homologous recombination. Here, we show that the BRCA2 protein is also required for TAR, while the XRCC2 protein is not involved. Expression of the XRCC2 gene in XRCC2 mutated irs1 cells restores the defect in homologous recombination repair of an I-SceI-induced DNA double-strand break, while TAR is unaffected. Interestingly, the XRCC2-deficient irs1 cells are also proficient in recombination induced at slowed replication forks, suggesting that TAR is mechanistically linked with this recombination pathway. In conclusion, we show that TAR depends on BRCA2 but is independent of XRCC2, and that this recombination pathway is separate from that used to repair a two-ended DNA double-strand break.

Mismatch repair-dependent processing of methylation damage gives rise to persistent single-stranded gaps in newly replicated DNA

O(6)-Methylguanine ((Me)G) is a highly cytotoxic DNA modification generated by S(N)1-type methylating agents. Despite numerous studies implicating DNA replication, mismatch repair (MMR), and homologous recombination (HR) in (Me)G toxicity, its mode of action has remained elusive. We studied the molecular transactions in the DNA of yeast and mammalian cells treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Although replication fork progression was unaffected in the first cell cycle after treatment, electron microscopic analysis revealed an accumulation of (Me)G- and MMR-dependent single-stranded DNA (ssDNA) gaps in newly replicated DNA. Progression into the second cell cycle required HR, while the following G(2) arrest required the continued presence of (Me)G. Yeast cells overcame this block, while mammalian cells generally failed to recover, and those that did contained multiple sister chromatid exchanges. Notably, the arrest could be abolished by removal of (Me)G after the first S phase. These new data provide compelling support for the hypothesis that MMR attempts to correct (Me)G/C or (Me)G/T mispairs arising during replication. Due to the persistence of (Me)G in the exposed template strand, repair synthesis cannot take place, which leaves single-stranded gaps behind the replication fork. During the subsequent S phase, these gaps cause replication fork collapse and elicit recombination and cell cycle arrest.

Genetic polymorphisms in the DNA repair genes XRCC1, XRCC2 and XRCC3 and risk of breast cancer in Cyprus

Population-based studies have reported significant associations between specific genetic polymorphisms and breast cancer susceptibility. A number of studies have demonstrated that common variants of genes involved in the DNA repair pathway act as low penetrance breast cancer susceptibility alleles. We aimed to investigate the association of single nucleotide polymorphisms (SNPs) in the DNA repair genes XRCC1, XRCC2 and XRCC3 and breast cancer in MASTOS, a population-based case-control study of 1,109 Cypriot women with breast cancer diagnosed between 40 and 70 years and 1,177 age-matched healthy controls. Five coding SNPs were genotyped including rs1799782, rs25489 and rs25487 in XRCC1, rs3218536 in XRCC2 and rs861539 in XRCC3. Homozygous XRCC1 280His carriers had an increased risk of breast cancer (odds ratio 4.68; 95% CI 1.01-21.7; P = 0.03). The XRCC2 188His allele was associated with a marginal protective effect for breast cancer (odds ratio 0.79; 95% CI 0.62-1.00; P = 0.05). No significant associations were observed between the other three SNPs and breast cancer. This study suggests that genetic variation in SNPs in XRCC1 and XRCC2 genes may influence breast cancer susceptibility.

An investigation on the polymorphisms of two DNA repair genes and susceptibility to ESCC and GCA of high-incidence region in northern China

AIM

To investigate the possible association of three SNPs, XRCC2 C41657T, XRCC2 G4234C and XRCC3 A17893G with susceptibility to esophageal squamous cell carcinoma (ESCC) and gastric cardia adenocarcinoma (GCA) in a population of northern China.

METHODS

XRCC2 C41657T, XRCC2 G4234C and XRCC3 A17893G SNP were genotyped by polymerase-chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analysis in 583 cancer patients (329 ESCC and 254 GCA) and 614 healthy controls.

RESULTS

The genotype distribution of the XRCC2 C41657T in ESCC and GCA patients were significantly different from that in healthy controls (P values = 0.04 and 0.04 respectively). And a significant difference was found in the allele distribution of GCA patients from that in controls (P = 0.01). The XRCC2 C41657T polymorphism was associated with a modest enhancement in ESCC risk and GCA risk: OR for C/T genotype was 1.38 (1.01-1.89) in GCA risk and for T/T genotype was 2.24 (1.10-4.57) in ESCC risk. When stratified for age, smoking status and family history of UGIC, the C/T genotype showed a modest significant trend on the risk of GCA patients in the groups of age < or =50 years and non-smokers, the adjusted OR were 2.84 (1.21-6.66) and 1.62 (1.06-2.49). The T/T genotype significantly increased the susceptibility of GCA patients in negative family history of UGIC (3.04, 1.02-8.32) and to ESCC patients in the group of age >50 years (3.03, 1.31-6.98), Negative family of UGIC (3.03, 1.12-7.07) and smokers (2.64, 1.02-6.83). The genotype and allele distribution of XRCC2 G4234C and XRCC3 A17893G in ESCC and GCA patients were not significantly different from that in healthy controls (all P values were above 0.05).

CONCLUSION

In this study, we found that the C41657T polymorphism of XRCC2 genes might modify the risk of ESCC and GCA development.

Double-stranded DNA breaks and gene functions in recombination and meiosis

Meiotic prophase I is a long and complex phase. Homologous recombination is an important process that occurs between homologous chromosomes during meiotic prophase I. Formation of chiasmata, which hold homologous chromosomes together until the metaphase I to anaphase I transition, is critical for proper chromosome segregation. Recent studies have suggested that the SPO11 proteins have conserved functions in a number of organisms in generating sites of double-stranded DNA breaks (DSBs) that are thought to be the starting points of homologous recombination. Processing of these sites of DSBs requires the function of RecA homologs, such as RAD51, DMC1, and others, as suggested by mutant studies; thus the failure to repair these meiotic DSBs results in abnormal chromosomal alternations, leading to disrupted meiosis. Recent discoveries on the functions of these RecA homologs have improved the understanding of the mechanisms underlying meiotic homologous recombination.

Xrcc2 deficiency sensitizes cells to apoptosis by MNNG and the alkylating anticancer drugs temozolomide, fotemustine and mafosfamide

DNA double-strand breaks (DSBs) are potent killing lesions, and inefficient repair of DSBs does not only lead to cell death but also to genomic instability and tumorigenesis. DSBs are repaired by non-homologous end-joining and homologous recombination (HR). A key player in HR is Xrcc2, a Rad51-like protein. Cells deficient in Xrcc2 are hypersensitive to X-rays and mitomycin C and display increased chromosomal aberration frequencies. In order to elucidate the role of Xrcc2 in resistance to anticancer drugs, we compared Xrcc2 knockout (Xrcc2-/-) mouse embryonic fibroblasts with the corresponding isogenic wild-type and Xrcc2 complemented knockout cells. We show that Xrcc2-/- cells are hypersensitive to the killing effect of the simple methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). They undergo apoptosis after MNNG treatment while necrosis is only marginally enhanced. Complementation of Xrcc2 deficient cells by Xrcc2 cDNA transfection conferred resistance to the cytotoxic and apoptosis-inducing effect of MNNG. The hypersensitivity of Xrcc2-/- cells to MNNG prompted us to investigate their killing and apoptotic response to various methylating, chloroethylating and crosslinking drugs used in anticancer therapy. Xrcc2 deficient cells were found to be hypersensitive to temozolomide, fotemustine and mafosfamide. They were also hypersensitive to cisplatin but not to taxol. The data reveal that Xrcc2 plays a role in the protection against a wide range of anticancer drugs and, therefore, suggest Xrcc2 to be a determinant of anticancer drug resistance. They also indicate that HR is involved in the processing of DNA damage induced by simple alkylating agents.

Disparate requirements for the Walker A and B ATPase motifs of human RAD51D in homologous recombination

In vertebrates, homologous recombinational repair (HRR) requires RAD51 and five RAD51 paralogs (XRCC2, XRCC3, RAD51B, RAD51C and RAD51D) that all contain conserved Walker A and B ATPase motifs. In human RAD51D we examined the requirement for these motifs in interactions with XRCC2 and RAD51C, and for survival of cells in response to DNA interstrand crosslinks (ICLs). Ectopic expression of wild-type human RAD51D or mutants having a non-functional A or B motif was used to test for complementation of a rad51d knockout hamster CHO cell line. Although A-motif mutants complement very efficiently, B-motif mutants do not. Consistent with these results, experiments using the yeast two- and three-hybrid systems show that the interactions between RAD51D and its XRCC2 and RAD51C partners also require a functional RAD51D B motif, but not motif A. Similarly, hamster Xrcc2 is unable to bind to the non-complementing human RAD51D B-motif mutants in co-immunoprecipitation assays. We conclude that a functional Walker B motif, but not A motif, is necessary for RAD51D's interactions with other paralogs and for efficient HRR. We present a model in which ATPase sites are formed in a bipartite manner between RAD51D and other RAD51 paralogs.

Sws1 is a conserved regulator of homologous recombination in eukaryotic cells

Rad52-dependent homologous recombination (HR) is regulated by the antirecombinase activities of Srs2 and Rqh1/Sgs1 DNA helicases in fission yeast and budding yeast. Functional analysis of Srs2 in Schizosaccharomyces pombe led us to the discovery of Sws1, a novel HR protein with a SWIM-type Zn finger. Inactivation of Sws1 suppresses the genotoxic sensitivity of srs2Delta and rqh1Delta mutants and rescues the inviability of srs2Delta rqh1Delta cells. Sws1 functions at an early step of recombination in a pro-recombinogenic complex with Rlp1 and Rdl1, two RecA-like proteins that are most closely related to the human Rad51 paralogs XRCC2 and RAD51D, respectively. This finding indicates that the XRCC2-RAD51D complex is conserved in lower eukaryotes. A SWS1 homolog exists in human cells. It associates with RAD51D and ablating its expression reduces the number of RAD51 foci. These studies unveil a conserved pathway for the initiation and control of HR in eukaryotic cells.

The ATPase motif in RAD51D is required for resistance to DNA interstrand crosslinking agents and interaction with RAD51C

Homologous recombination (HR) is a mechanism for repairing DNA interstrand crosslinks and double-strand breaks. In mammals, HR requires the activities of the RAD51 family (RAD51, RAD51B, RAD51C, RAD51D, XRCC2, XRCC3 and DMC1), each of which contains conserved ATP binding sequences (Walker Motifs A and B). RAD51D is a DNA-stimulated ATPase that interacts directly with RAD51C and XRCC2. To test the hypothesis that ATP binding and hydrolysis by RAD51D are required for the repair of interstrand crosslinks, site-directed mutations in Walker Motif A were generated, and complementation studies were performed in Rad51d-deficient mouse embryonic fibroblasts. The K113R and K113A mutants demonstrated a respective 96 and 83% decrease in repair capacity relative to wild-type. Further examination of these mutants, by yeast two-hybrid analyses, revealed an 8-fold reduction in the ability to associate with RAD51C whereas interaction with XRCC2 was retained at a level similar to the S111T control. These cell-based studies are the first evidence that ATP binding and hydrolysis by RAD51D are required for efficient HR repair of DNA interstrand crosslinks.

Genetic interactions between RAD51 and its paralogues for centrosome fragmentation and ploidy control, independently of the sensitivity to genotoxic stresses

We evaluate here whether RAD51 and its paralogues XRCC2 and XRCC3 act via a common pathway for sensitivity to genotoxic stress, centrosome fragmentation and chromosome stability. We expressed the RAD51 dominant-negative SMRAD51 in irs1 and irs1SF cells, defective for XRCC2 and XRCC3, respectively, and in their corresponding wild-type cells (V79 and AA8, respectively). V79-SMRAD51 cells are sensitive to mitomycin C (MMC), but SMRAD51 did not further sensitize irs1 cells to MMC, showing that SMRAD51 and XRCC2 act on the same pathway for resistance to MMC. However, in contrast to irs1 and irs1SF cells, SMRAD51-V79 and SMRAD51-AA8 cells are not sensitive to gamma-rays or UV-C. Despite these differences in sensitivity, SMRAD51-expressing cells and xrcc2- or xrcc3-defective cells show similar increased levels of centrosome fragmentation. This spontaneous centrosome fragmentation is resistant to caffeine, suggesting that ATM and ATR are not involved. Consistent with centrosome fragmentation, increased aneuploidy was measured in irs1 and SMRAD51-expressing cells. Expression of SMRAD51 in irs1 or irs1SF cells did not increase further the frequency of multipolar cells. Thus, RAD51, XRCC2 and XRCC3 act in the same pathway for centrosome fragmentation, independently of the sensitivity to exogenous genotoxic stresses and of the ATM/ATR pathway.

ASCIZ regulates lesion-specific Rad51 focus formation and apoptosis after methylating DNA damage

Nuclear Rad51 focus formation is required for homology-directed repair of DNA double-strand breaks (DSBs), but its regulation in response to non-DSB lesions is poorly understood. Here we report a novel human SQ/TQ cluster domain-containing protein termed ASCIZ that forms Rad51-containing foci in response to base-modifying DNA methylating agents but not in response to DSB-inducing agents. ASCIZ foci seem to form prior to Rad51 recruitment, and an ASCIZ core domain can concentrate Rad51 in focus-like structures independently of DNA damage. ASCIZ depletion dramatically increases apoptosis after methylating DNA damage and impairs Rad51 focus formation in response to methylating agents but not after ionizing radiation. ASCIZ focus formation and increased apoptosis in ASCIZ-depleted cells depend on the mismatch repair protein MLH1. Interestingly, ASCIZ foci form efficiently during G1 phase, when sister chromatids are unavailable as recombination templates. We propose that ASCIZ acts as a lesion-specific focus scaffold in a Rad51-dependent pathway that resolves cytotoxic repair intermediates, most likely single-stranded DNA gaps, resulting from MLH1-dependent processing of base lesions.

Identification and characterization of the rlp1+, the novel Rad51 paralog in the fission yeast Schizosaccharomyces pombe

A new DNA repair gene from fission yeast Schizosaccharomyces pombe rlp1+ (RecA-like protein) has been identified. Rlp1 shows homology to RecA-like proteins, and is the third S. pombe Rad51 paralog besides Rhp55 and Rhp57. The new gene encodes a 363 aa protein with predicted Mr of 41,700 and has NTP-binding motif. The rlp1Delta mutant is sensitive to methyl methanesulfonate (MMS), ionizing radiation (IR), and camptothecin (CPT), although to a lesser extent than the deletion mutants of rhp55+ and rhp51+ genes. In contrast to other recombinational repair mutants, the rlp1Delta mutant does not exhibit sensitivity to UV light and mitomycin C (MMC). Mitotic recombination is moderately reduced in rlp1 mutant. Epistatic analysis of MMS and IR-sensitivity of rlp1Delta mutant indicates that rlp1+ acts in the recombinational pathway of double-strand break (DSB) repair together with rhp51+, rhp55+, and rad22+ genes. Yeast two-hybrid analysis suggests that Rlp1 may interact with Rhp57 protein. We propose that Rlp1 have an accessory role in repair of a subset of DNA damage induced by MMS and IR, and is required for the full extent of DNA recombination and cell survival under condition of a replication fork collapse.

The RAD51 gene family, genetic instability and cancer

Inefficient repair or mis-repair of DNA damage can cause genetic instability, and defects in some DNA repair genes are associated with rare human cancer-prone disorders. In the last few years, homologous recombination has been found to be a key pathway in human cells for the repair of severe DNA damage such as double-strand breaks. The RAD51 family of genes, including RAD51 and the five RAD51-like genes (XRCC2, XRCC3, RAD51L1, RAD51L2, RAD51L3) are known to have crucial non-redundant roles in this pathway. Current knowledge of the functions of the RAD51 gene family is reviewed, as well as the evidence for extensive genetic instability arising from loss of their activity. Reports of potential associations between variants of RAD51 family genes and specific forms of cancer are summarized, but it is seen that many of these studies have relatively low statistical power. As yet these data provide only tantalizing suggestions of modified cancer risks arising from polymorphisms, mutations, or changes in expression of the RAD51 gene family, and there is still a lot to learn before firm conclusions can be made.

Ionizing radiation-induced foci formation of mammalian Rad51 and Rad54 depends on the Rad51 paralogs, but not on Rad52

Homologous recombination is of major importance for the prevention of genomic instability during chromosome duplication and repair of DNA damage, especially double-strand breaks. Biochemical experiments have revealed that during the process of homologous recombination the RAD52 group proteins, including Rad51, Rad52 and Rad54, are involved in an essential step: formation of a joint molecule between the broken DNA and the intact repair template. Accessory proteins for this reaction include the Rad51 paralogs and BRCA2. The significance of homologous recombination for the cell is underscored by the evolutionary conservation of the Rad51, Rad52 and Rad54 proteins from yeast to humans. Upon treatment of cells with ionizing radiation, the RAD52 group proteins accumulate at the sites of DNA damage into so-called foci. For the yeast Saccharomyces cerevisiae, foci formation of Rad51 and Rad54 is abrogated in the absence of Rad52, while Rad51 foci formation does occur in the absence of the Rad51 paralog Rad55. By contrast, we show here that in mammalian cells, Rad52 is not required for foci formation of Rad51 and Rad54. Furthermore, radiation-induced foci formation of Rad51 and Rad54 is impaired in all Rad51 paralog and BRCA2 mutant cell lines tested, while Rad52 foci formation is not influenced by a mutation in any of these recombination proteins. Despite their evolutionary conservation and biochemical similarities, S. cerevisiae and mammalian Rad52 appear to differentially contribute to the DNA-damage response.

Arabidopsis Rad51B is important for double-strand DNA breaks repair in somatic cells

Rad51 paralogs belong to the Rad52 epistasis group of proteins and are involved in homologous recombination (HR), especially the assembly and stabilization of Rad51, which is a homolog of RecA in eukaryotes. We previously cloned and characterized two RAD51 paralogous genes in Arabidopsis, named AtRAD51C and AtXRCC3, which are considered the counterparts of human RAD51C and XRCC3, respectively. Here we describe the identification of RAD51B homologue in Arabidopsis, AtRAD51B. We found a higher expression of AtRAD51B in flower buds and roots. Expression of AtRAD51B was induced by genotoxic stresses such as ionizing irradiation and treatment with a cross-linking reagent, cisplatin. Yeast two-hybrid analysis showed that AtRad51B interacted with AtRad51C. We also found and characterized T-DNA insertion mutant lines. The mutant lines were devoid of AtRAD51B expression, viable and fertile. The mutants were moderately sensitive to gamma-ray and hypersensitive to cisplatin. Our results suggest that AtRAD51B gene product is involved in the repair of double-strand DNA breaks (DSBs) via HR.

The BRCA2-interacting protein BCCIP functions in RAD51 and BRCA2 focus formation and homologous recombinational repair

Homologous recombinational repair (HRR) of DNA damage is critical for maintaining genome stability and tumor suppression. RAD51 and BRCA2 colocalization in nuclear foci is a hallmark of HRR. BRCA2 has important roles in RAD51 focus formation and HRR of DNA double-strand breaks (DSBs). We previously reported that BCCIPalpha interacts with BRCA2. We show that a second isoform, BCCIPbeta, also interacts with BRCA2 and that this interaction occurs in a region shared by BCCIPalpha and BCCIPbeta. We further show that chromatin-bound BRCA2 colocalizes with BCCIP nuclear foci and that most radiation-induced RAD51 foci colocalize with BCCIP. Reducing BCCIPalpha by 90% or BCCIPbeta by 50% by RNA interference markedly reduces RAD51 and BRCA2 foci and reduces HRR of DSBs by 20- to 100-fold. Similarly, reducing BRCA2 by 50% reduces RAD51 and BCCIP foci. These data indicate that BCCIP is critical for BRCA2- and RAD51-dependent responses to DNA damage and HRR.

How Fanconi anemia proteins promote the four Rs: replication, recombination, repair, and recovery

The genetically complex disease Fanconi anemia (FA) comprises cancer predisposition, developmental defects, and bone marrow failure due to elevated apoptosis. The FA cellular phenotype includes universal sensitivity to DNA crosslinking damage, symptoms of oxidative stress, and reduced mutability at the X-linked HPRT gene. In this review article, we present a new heuristic molecular model that accommodates these varied features of FA cells. In our view, the FANCA, -C, and -G proteins, which are both cytoplasmic and nuclear, have an integrated dual role in which they sense and convey information about cytoplasmic oxidative stress to the nucleus, where they participate in the further assembly and functionality of the nuclear core complex (NCCFA= FANCA/B/C/E/F/G/L). In turn, NCCFA facilitates DNA replication at sites of base damage and strand breaks by performing the critical monoubiquitination of FANCD2, an event that somehow helps stabilize blocked and broken replication forks. This stabilization facilitates two kinds of processes: translesion synthesis at sites of blocking lesions (e.g., oxidative base damage), which produces point mutations by error-prone polymerases, and homologous recombination-mediated restart of broken forks, which arise spontaneously and when crosslinks are unhooked by the ERCC1-XPF endonuclease. In the absence of the critical FANCD2 monoubiquitination step, broken replication forks further lose chromatid continuity by collapsing into a configuration that is more difficult to restart through recombination and prone to aberrant repair through nonhomologous end joining. Thus, the FA regulatory pathway promotes chromosome integrity by monitoring oxidative stress and coping efficiently with the accompanying oxidative DNA damage during DNA replication.

Differing requirements for the Arabidopsis Rad51 paralogs in meiosis and DNA repair

In addition to the recombinase Rad51, vertebrates have five paralogs of Rad51, all members of the Rad51-dependent recombination pathway. These paralogs form two complexes (Rad51C/Xrcc3 and Rad51B/C/D/Xrcc2), which play roles in somatic recombination, DNA repair and chromosome stability. However, little is known of their possible involvement in meiosis, due to the inviability of the corresponding knockout mice. We have recently reported that the Arabidopsis homolog of one of these Rad51 paralogs (AtXrcc3) is involved in DNA repair and meiotic recombination and present here Arabidopsis lines carrying mutations in three other Rad51 paralogs (AtRad51B, AtRad51C and AtXrcc2). Disruption of any one of these paralogs confers hypersensitivity to the DNA cross-linking agent Mitomycin C, but not to gamma-irradiation. Moreover, the atrad51c-1 mutant is the only one of these to show meiotic defects similar to those of the atxrcc3 mutant, and thus only the Rad51C/Xrcc3 complex is required to achieve meiosis. These results support conservation of functions of the Rad51 paralogs between vertebrates and plants and differing requirements for the Rad51 paralogs in meiosis and DNA repair.