Reduction in BRCA1 gene dose modulates DNA replication stress level

BRCA1 is a well-known breast cancer risk gene, involved in DNA damage repair via homologous recombination (HR) and replication fork protection. Therapy resistance has been linked to loss and amplification of the BRCA1 gene causing inferior survival of breast cancer patients. Most studies were focused on the analysis of complete loss or mutations in functional domains of BRCA1. How mutations in non-functional domains contribute to resistance mechanisms, remains elusive and has been the focus of this study. Therefore, MCF7 clones with mutations in BRCA1 exon 9 and 14 were generated, using CRISPR/Cas9. Clones with successful introduced BRCA1 mutations were evaluated regarding their capacity to perform HR, how they handle DNA replication stress (RS) and the consequences on the sensitivity to MMC, PARP1 inhibition and ionizing radiation. Unexpectedly, BRCA1 mutations resulted in both, increased sensitivity and resistance to exogenous DNA damage, despite a reduction of HR capacity in all clones. Resistance was associated with improved DNA double strand repair and reduction in RS. Lower RS was accompanied by increased activation and interaction of proteins essential for the S phase-specific DNA damage response consisting of HR proteins, FANCD2, and CHK1.