<p class="first" id="d7499538e241">Mutations in BRCA1 or BRCA2 (BRCA) is synthetic
lethal with poly(ADP-ribose) polymerase
inhibitors (PARPi). Lethality is thought to derive from DNA double-stranded breaks
(DSBs) necessitating BRCA function in homologous recombination (HR) and/or fork protection
(FP). Here, we report instead that toxicity derives from replication gaps. BRCA1-
or FANCJ-deficient cells, with common repair defects but distinct PARPi responses,
reveal gaps as a distinguishing factor. We further uncouple HR, FP, and fork speed
from PARPi response. Instead, gaps characterize BRCA-deficient cells, are diminished
upon resistance, restored upon resensitization, and, when exposed, augment PARPi toxicity.
Unchallenged BRCA1-deficient cells have elevated poly(ADP-ribose) and chromatin-associated
PARP1, but aberrantly low XRCC1 consistent with defects in backup Okazaki fragment
processing (OFP). 53BP1 loss resuscitates OFP by restoring XRCC1-LIG3 that suppresses
the sensitivity of BRCA1-deficient cells to drugs targeting OFP or generating gaps.
We highlight gaps as a determinant of PARPi toxicity changing the paradigm for synthetic
lethal interactions.
</p>