Abstract
Abstract BRCA1, BRCA2 and RAD51 are key players of the homologous recombination (HR) DNA repair pathway, but are also involved in stalled DNA replication fork protection and repair. BRCA1-deficiency is encountered in 25% of Triple Negative Breast Cancer (TNBC). We, thus, tested whether BRCA1-deficient TNBC cell models showed increased sensitivity to replication poisons and in particular to the frequently used RNR inhibitor and a chain terminator gemcitabine. Compared with isogenic BRCA1-proficient cells, gemcitabine treatment of BRCA1-deficient models resulted in large scale replication catastrophe and massive cell death. Cells undergoing replication catastrophe manifested a strong accumulation of single strand DNA (ssDNA), DNA double strand breaks (DSB) and an absence of RPA and/or RAD51 signals. We show here that ssDNA accumulation in BRCA1-deficient cells resulted from uncontrolled MRE11 resection, suggesting that replication fork reversal was a leading response to replication poisoning. Furthermore, we demonstrate that, in addition to replication catastrophe, gemcitabine treated BRCA1-deficient cells are prone to undergo mitotic catastrophe, producing strongly BrdU and γH2AX stained micronuclei (MN) and mitotic bridges. Noticeably these BrdU-positive MN and DNA bridges triggered cGAS activation. Our data, thus, strongly suggest that gemcitabine treatment could be beneficial in BRCA1-deficient TNBC both in terms of cancer cell death, but possibly as well in terms of antitumor immune response.