Replication poison treated BRCA1-deficient breast cancers are prone to MRE11 over-resection resulting in single strand DNA accumulation and mitotic catastrophe

BRCA1, BRCA2 and RAD51, key players of homologous recombination (HR) repair, are also involved in stalled DNA replication fork protection and repair. BRCA1-deficiency is encountered in 25% of Triple Negative Breast Cancer (TNBC). Here we investigated the sensitivity of BRCA1-deficient TNBC cell models to gemcitabine a frequently used replication poison that does not alter DNA structure. We show that BRCA1-deficient models, in contrast to their isogenic BRCA1-proficient counterparts, are superiorly sensitive to gemcitabine, accumulate massive levels of single strand DNA (ssDNA), in absence of RPA and RAD51 signals and elevated double strand break (DSB) numbers leading to cell death. Remarkably, ssDNA accumulation in gemcitabine-treated BRCA1-deficient cells was strongly diminished by the MRE11 inhibitor mirin, while it did not affect ssDNA levels resulting from PARP inhibitor olaparib treatment. The central role of MRE11 DNA resection strongly suggested that replication fork reversal may be important in response to replication poisoning by gemcitabine in BRCA1-deficient models. Furthermore, we demonstrate that gemcitabine-treated BRCA1-deficient cells showing massive ssDNA accumulation slipped into mitosis and produced mitotic bridges and micronuclei (MN) showing strong BrdU and {gamma}H2AX staining. Noticeably these BrdU-positive MN and DNA bridges triggered cGAS sensing. 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.