Decreased DNA double‑strand break repair and enhanced chromosomal radiosensitivity in irradiated non‑tumorigenic human breast epithelial cells with a partial BRCA1 or BRCA2 knockdown

It has previously been demonstrated that peripheral blood lymphocytes of healthy women carrying BRCA1 or BRCA2 mutations exhibit increased chromosomal radiosensitivity, which is characterized by an enhanced formation of micronuclei. These results suggest that the deficient repair of DNA double‑strand breaks may also occur in breast epithelial cells of women exhibiting a reduced expression of wild‑type BRCA1/BRCA2 proteins due to the presence of germline mutations in BRCA1/2 genes. The aim of this study was to further investigate in vitro the effects of the reduced expression of BRCA1 and BRCA2 in MCF10A human non‑tumorigenic breast epithelial cells, tentatively mimicking the phenotype of heterozygous cells of carriers of BRCA1/2 mutations. By lentivirus‑mediated RNA interference, the stable reduction of BRCA1 and BRCA2 expression at the mRNA and protein level was achieved, thus generating the BRCA1i and BRCA2i cell lines. In these cells, homologous recombination was impaired, as significantly lower yields of RAD51 foci were obtained following exposure to 2 Gy ionizing radiation compared to the control MCF10A cells (BRCA1i cells, 58% reduction; BRCA2i cells, 64% reduction). Moreover, in the BRCA1i and BRCA2i cells, a dose‑dependent increase in micronuclei was observed compared to the cells which were not subjected to gene knockdown. Cell viability was also affected by partial BRCA1/2 knockdown. On the whole, the findings of this study indicated that in cells with a reduced BRCA1 or BRCA2 expression, the impairment of homologous recombination resulted in a >50% decrease in RAD51 foci following irradiation and increased chromosomal abnormalities (micronuclei). These findings suggest that the healthy breast tissue of BRCA1/2 mutation carriers may be prone to neoplastic transformation upon exposure to diagnostic or therapeutic radiation, and that the RAD51 foci assay may be useful for the assessment of the functionality of HR repair and radiosensitivity in these women.