The nuclease EXO1 promotes genomic instability by degrading nascent DNA in BRCA-proficient cells

Genomic instability promotes carcinogenesis. DNA repair genes are generally considered tumor suppressors, as their inactivation is observed in tumors and is associated with carcinogenesis. Mutations in BRCA1 and BRCA2 genes are observed in breast, ovarian, and other cancers. BRCA pathway inactivation results in defective homologous recombination DNA repair, as well as in degradation of nascent DNA during replication stress. This degradation is catalyzed by nucleases including MRE11 and EXO1, and occurs at two types of DNA structures which are formed upon replication stress, namely single stranded DNA (ssDNA) gaps and reversed replication forks, eventually causing double strand DNA break (DSB) formation. However, most tumors are BRCA pathway-proficient. Here, we show that EXO1 is overexpressed in a significant proportion of tumors. EXO1 overexpression causes the degradation of nascent DNA at both ssDNA gaps and reversed forks, through its exonuclease catalytic activity. Importantly, EXO1-mediated nascent strand degradation occurs efficiently in BRCA-proficient cells, through its cooperation with MRE11. This results in increased DSB formation and hypersensitivity to genotoxic agents. We thus identify increased EXO1 activity as a mechanism of genomic instability similar to BRCA pathway inactivation, but occurring more frequently in tumors compared to BRCA inactivation.