Structure and repair of replication-coupled DNA breaks.

Using CRISPR/Cas9 nicking enzymes, we examine the interaction between the replication machinery and single strand breaks, one of the most common forms of endogenous DNA damage. We show that replication fork collapse at leading strand nicks generates resected single-ended double-strand breaks (seDSBs) that are repaired by homologous recombination (HR). If these seDSBs are not promptly repaired, arrival of adjacent forks creates double ended DSBs (deDSBs), which could drive genomic scarring in HR-deficient cancers. deDSBs can also be generated directly when the replication fork bypasses lagging strand nicks. Unlike deDSBs produced independently of replication, end-resection at nick-induced se/deDSBs is BRCA1-independent. Nevertheless, BRCA1 antagonizes 53BP1 suppression of RAD51 filament formation. These results highlight unique mechanisms that maintain replication fork stability.