BRCA2-dependent maturation of nascent strands during DNA replication.

A major source of poly(ADP-ribose) polymerase (PARP) activity in proliferating cells is unligated Okazaki fragments. Consequently, the anti-cancer PARP inhibitor olaparib impedes the maturation of nascent DNA strand fragments during DNA replication. Here, we show that wild-type human cells overcome this impediment by triggering a process that facilitates nascent strand maturation in the presence of olaparib. We show that this process operates on very large nascent strand fragments and repairs thousands of olaparib-induced DNA single-strand breaks/gaps per genome. Critically, this process is dependent on the tumor suppressors BRCA1 and BRCA2 and is associated with the BRCA2-dependent accumulation of RAD51 recombinase in chromatin. Our data identify nascent strand gaps that are induced by olaparib independently of replication fork reversal and/or PRIMPOL-mediated repriming and that are repaired by a BRCA2-dependent process that we propose is daughter-strand gap protection and/or repair occurring hundreds of kilobases behind DNA replication forks.