Cong, KePeng, MinKousholt, Arne NedergaardLee, Wei Ting C.Lee, SilvianaNayak, SumeetKrais, JohnVanderVere-Carozza, Pamela S.Pawelczak, Katherine S.Calvo, JenniferPanzarino, Nicholas J.Turchi, John J.Johnson, NeilJonkers, JosRothenberg, EliCantor, Sharon B.2023-06-192023-06-192021Cong K, Peng M, Kousholt AN, et al. Replication gaps are a key determinant of PARP inhibitor synthetic lethality with BRCA deficiency [published correction appears in Mol Cell. 2021 Aug 5;81(15):3227]. Mol Cell. 2021;81(15):3128-3144.e7. doi:10.1016/j.molcel.2021.06.011https://hdl.handle.net/1805/33851Mutations 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.en-USPublisher PolicyBRCA1BRCA2Fanconi anemia (FA)Okazaki fragment processingPARP inhibitorFork protectionHomologous recombinationParylationReplication gapsssDNASynthetic lethalArticle