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Abstract: Initial BRCA1 and BRCA2 analyses conducted in breast and ovarian cancer families were focused on identification of mutations in coding sequences and splicing sites of the genes. Large genomic rearrangements as well as mutations in promoter or untranslated regions have been missed by standard detection strategies. Nevertheless, in Western countries, a detailed study of families with strong linkage to BRCA1 identified large genomic deletions and rearrangements in this gene as early as 1997. To date, no such gene alteration has been described in Central and Eastern European populations. In our study of BRCA1/2 genes in the Czech population, we have detected a complex genomic rearrangement in BRCA1 using RNA-based analysis for mutation screening. This rearrangement involves exons 21 and 22 and results in a protein product lacking BRCT domain important for its function. [Copyright &y& Elsevier]

Abstract: Alternative pre-mRNA splicing is a fundamental post-transcriptional regulatory mechanism. Cancer-specific misregulation of the splicing process may lead to formation of irregular alternative splicing variants (ASVs) with a potentially negative impact on cellular homeostasis. Alternative splicing of BRCA1 pre-mRNA can give rise to BRCA1 protein isoforms that possess dramatically altered biological activities compared with full-length wild-type BRCA1. During the screening of high-risk breast cancer (BC) families we ascertained numerous BRCA1 ASVs, however, their clinical significance for BC development is largely unknown. In this study, we examined the influence of the BRCA1Δ17–19 ASV, which lacks a portion of the BRCT domain, on DNA repair capacity using human MCF-7 BC cell clones with stably modified BRCA1 expression. Our results show that overexpression of BRCA1Δ17–19 impairs homologous recombination repair (sensitizes cells to mitomycin C), delays repair of ionizing radiation-induced DNA damage and dynamics of the ionizing radiation-induced foci (IRIF) formation, and undermines also the non-homologous end joining repair (NHEJ) activity. Mechanistically, BRCA1Δ17–19 cannot interact with the partner proteins Abraxas and CtIP, thus preventing interactions known to be critical for processing of DNA lesions. We propose that the observed inability of BRCA1Δ17–19 to functionally replace wtBRCA1 in repair of DNA double-strand breaks (DDSB) reflects impaired capacity to form the BRCA1-A and -C repair complexes. Our findings indicate that expression of BRCA1Δ17–19 may negatively influence genome stability by reducing the DDSB repair velocity, thereby contributing to enhanced probability of cancer development in the affected families. [Copyright &y& Elsevier]

Abstract: The BRCA1 gene codes for a protein involved in the DNA double strand break (DDSB) repair. Alongside the dominant full-length splicing form of BRCA1, numerous endogenously expressed alternative splicing variants of unknown significance have been described in various tissues. Some of them retain the original BRCA1 reading frame but lack several critical BRCA1 structural domains, suggesting an altered function of the resulting protein in the BRCA1-regulated processes. To characterize the effect of the BRCA1Δ14-15 splicing variant (with an in-frame deletion affecting the regulatory serine-containing domain) on the DDSB repair, we constructed the MCF-7 clones stably expressing the analyzed variant with/without a shRNA-mediated downregulation of the endogenous full-length wild-type BRCA1 expression. Our results show that the expression of the BRCA1Δ14-15 variant delays the γ-radiation-induced DDSB repair, alters the kinetics of irradiation-induced foci formation/decomposition and reduces the non-homologous end-joining capacity in MCF-7 cells. Therefore, the BRCA1Δ14-15 is not able to functionally replace the full-length wt BRCA1 in the DDSB repair. Our findings indicate that the endogenously expressed BRCA1 alternative splicing variants may negatively influence genome stability and support the growing evidence of the pathological potential of the sequence variants generated by an altered or misregulated alternative splicing in the process of mammary malignant transformation. [Copyright &y& Elsevier]