Germline mutation in the RAD51B gene confers predisposition to breast cancer

Mammalian RAD51 paralogs protect nascent DNA at stalled forks and mediate replication restart

Mammalian RAD51 paralogs are implicated in the repair of collapsed replication forks by homologous recombination. However, their physiological roles in replication fork maintenance prior to fork collapse remain obscure. Here, we report on the role of RAD51 paralogs in short-term replicative stress devoid of DSBs. We show that RAD51 paralogs localize to nascent DNA and common fragile sites upon replication fork stalling. Strikingly, RAD51 paralogs deficient cells exhibit elevated levels of 53BP1 nuclear bodies and increased DSB formation, the latter being attributed to extensive degradation of nascent DNA at stalled forks. RAD51C and XRCC3 promote the restart of stalled replication in an ATP hydrolysis dependent manner by disengaging RAD51 and other RAD51 paralogs from the halted forks. Notably, we find that Fanconi anemia (FA)-like disorder and breast and ovarian cancer patient derived mutations of RAD51C fails to protect replication fork, exhibit under-replicated genomic regions and elevated micro-nucleation. Taken together, RAD51 paralogs prevent degradation of stalled forks and promote the restart of halted replication to avoid replication fork collapse, thereby maintaining genomic integrity and suppressing tumorigenesis.

Rad51 Paralogs Remodel Pre-synaptic Rad51 Filaments to Stimulate Homologous Recombination

Repair of DNA double strand breaks by homologous recombination (HR) is initiated by Rad51 filament nucleation on single-stranded DNA (ssDNA), which catalyzes strand exchange with homologous duplex DNA. BRCA2 and the Rad51 paralogs are tumor suppressors and critical mediators of Rad51. To gain insight into Rad51 paralog function, we investigated a heterodimeric Rad51 paralog complex, RFS-1/RIP-1, and uncovered the molecular basis by which Rad51 paralogs promote HR. Unlike BRCA2, which nucleates RAD-51-ssDNA filaments, RFS-1/RIP-1 binds and remodels pre-synaptic filaments to a stabilized, "open," and flexible conformation, in which the ssDNA is more accessible to nuclease digestion and RAD-51 dissociation rate is reduced. Walker box mutations in RFS-1, which abolish filament remodeling, fail to stimulate RAD-51 strand exchange activity, demonstrating that remodeling is essential for RFS-1/RIP-1 function. We propose that Rad51 paralogs stimulate HR by remodeling the Rad51 filament, priming it for strand exchange with the template duplex.

Mutation Analysis of the RAD51C and RAD51D Genes in High-Risk Ovarian Cancer Patients and Families from the Czech Republic

Recent studies have conferred that the RAD51C and RAD51D genes, which code for the essential proteins involved in homologous recombination, are ovarian cancer (OC) susceptibility genes that may explain genetic risks in high-risk patients. We performed a mutation analysis in 171 high-risk BRCA1 and BRCA2 negative OC patients, to evaluate the frequency of hereditary RAD51C and RAD51D variants in Czech population. The analysis involved direct sequencing, high resolution melting and multiple ligation-dependent probe analysis. We identified two (1.2%) and three (1.8%) inactivating germline mutations in both respective genes, two of which (c.379_380insG, p.P127Rfs*28 in RAD51C and c.879delG, p.C294Vfs*16 in RAD51D) were novel. Interestingly, an indicative family cancer history was not present in four carriers. Moreover, the ages at the OC diagnoses in identified mutation carriers were substantially lower than those reported in previous studies (four carriers were younger than 45 years). Further, we also described rare missense variants, two in RAD51C and one in RAD51D whose clinical significance needs to be verified. Truncating mutations and rare missense variants ascertained in OC patients were not detected in 1226 control samples. Although the cumulative frequency of RAD51C and RAD51D truncating mutations in our patients was lower than that of the BRCA1 and BRCA2 genes, it may explain OC susceptibility in approximately 3% of high-risk OC patients. Therefore, an RAD51C and RAD51D analysis should be implemented into the comprehensive multi-gene testing for high-risk OC patients, including early-onset OC patients without a family cancer history.

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.

RAD51, XRCC3, and XRCC2 mutation screening in Finnish breast cancer families

Majority of the known breast cancer susceptibility genes have a role in DNA repair and the most important high-risk genes BRCA1 and BRCA2 are specifically involved in the homologous recombination repair (HRR) of DNA double-strand breaks. A central player in HRR is RAD51 that binds DNA at the damage site. The RAD51 paralogs RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3 facilitate the binding of RAD51 to DNA. While germline mutations in RAD51C and RAD51D are associated with high ovarian cancer risk and RAD51B polymorphisms with breast cancer, the contribution of RAD51, XRCC3, and XRCC2 is more unclear. To investigate the role of RAD51, XRCC3, and XRCC2 in breast cancer predisposition and to identify putative recurrent founder mutations in the Finnish population where such mutations have been observed in most of the currently known susceptibility genes, we screened 182 familial Finnish breast or ovarian cancer patients for germline variation in the RAD51and XRCC3 genes and 342 patients for variation in XRCC2, with a subset of the patients selected on the basis of decreased RAD51 protein expression on tumors. We also performed haplotype analyses for 1516 breast cancer cases and 1234 controls to assess the common variation in these genes. No pathogenic mutations were detected in any of the genes and the distribution of haplotypes was similar between cases and controls. Our results suggest that RAD51, XRCC3, and XRCC2 do not substantially contribute to breast cancer predisposition in the Finnish population.

A DNA repair variant in POLQ (c.-1060A > G) is associated to hereditary breast cancer patients: a case-control study

BACKGROUND

One of the hallmarks of cancer is the occurrence of high levels of chromosomal rearrangements as a result of inaccurate repair of double-strand breaks (DSB). Germline mutations in BRCA and RAD51 genes, involved in DSB repair, are strongly associated with hereditary breast cancer. Pol θ, a translesional DNA polymerase specialized in the replication of damaged DNA, has been also shown to contribute to DNA synthesis associated to DSB repair. It is noteworthy that POLQ is highly expressed in breast tumors and this expression is able to predict patient outcome. The objective of this study was to analyze genetic variants related to POLQ as new population biomarkers of risk in hereditary (HBC) and sporadic (SBC) breast cancer.

METHODS

We analyzed through case-control study nine SNPs of POLQ in hereditary (HBC) and sporadic (SBC) breast cancer patients using Taqman Real Time PCR assays. Polymorphisms were systematically identified through the NCBI database and are located within exons or promoter regions. We recruited 204 breast cancer patients (101 SBC and 103 HBC) and 212 unaffected controls residing in Southern Brazil.

RESULTS

The rs581553 SNP located in the promoter region was strongly associated with HBC (c.-1060A > G; HBC GG = 15, Control TT = 8; OR = 5.67, CI95% = 2.26-14.20; p < 0.0001). Interestingly, 11 of 15 homozygotes for this polymorphism fulfilled criteria for Hereditary Breast and Ovarian Cancer (HBOC) syndrome. Furthermore, 12 of them developed bilateral breast cancer and one had a familial history of bilateral breast cancer. This polymorphism was also associated with bilateral breast cancer in 67 patients (OR = 9.86, CI95% = 3.81-25.54). There was no statistically significant difference of age at breast cancer diagnosis between SNP carriers and non-carriers.

CONCLUSIONS

Considering that Pol θ is involved in DBS repair, our results suggest that this polymorphism may contribute to the etiology of HBC, particularly in patients with bilateral breast cancer.