The ssDNA Theory of BRCAness and Genotoxic Agents

Cancers that are deficient in BRCA1 or BRCA2 are thought to be hypersensitive to genotoxic agents because they cannot prevent or repair DNA double strand breaks, but observations in patients suggest this dogma may no longer agree with experiment. Here, we propose that single stranded DNA underlies the hypersensitivity of BRCA deficient cancers, and that defects in double strand break repair and prevention do not. Specifically, in BRCA deficient cells, ssDNA gaps developed because replication was not effectively restrained in response to stress. In addition, we observed gaps could be suppressed by either restored fork restraint or by gap filling, both of which conferred therapy resistance in tissue culture and BRCA patient tumors. In contrast, restored double strand break repair and prevention did not confer therapy resistance when gaps were present. Critically, double strand breaks were not detected after therapy when apoptosis was inhibited, supporting a framework in which double strand breaks are not directly induced by genotoxic agents, but instead are created by cell death nucleases and are not fundamental to genotoxic agents. Together, these data indicate that ssDNA replication gaps underlie the BRCA cancer phenotype, BRCAness, and we propose are fundamental to the mechanism-of-action of genotoxic chemotherapy

Opposing Roles of FANCJ and HLTF Protect Forks and Restrain Replication during Stress

The DNA helicase FANCJ is mutated in hereditary breast and ovarian cancer and Fanconi anemia (FA). Nevertheless, how loss of FANCJ translates to disease pathogenesis remains unclear. We addressed this question by analyzing proteins associated with replication forks in cells with or without FANCJ. We demonstrate that FANCJ-knockout (FANCJ-KO) cells have alterations in the replisome that are consistent with enhanced replication stress, including an aberrant accumulation of the fork remodeling factor helicase-like transcription factor (HLTF). Correspondingly, HLTF contributes to fork degradation in FANCJ-KO cells. Unexpectedly, the unrestrained DNA synthesis that characterizes HLTF-deficient cells is FANCJ dependent and correlates with S1 nuclease sensitivity and fork degradation. These results suggest that FANCJ and HLTF promote replication fork integrity, in part by counteracting each other to keep fork remodeling and elongation in check. Indicating one protein compensates for loss of the other, loss of both HLTF and FANCJ causes a more severe replication stress response

Replication Fork Stability Confers Chemoresistance in BRCA-deficient Cells

Brca1- and Brca2-deficient cells have reduced capacity to repair DNA double-strand breaks (DSBs) by homologous recombination (HR) and consequently are hypersensitive to DNA damaging agents, including cisplatin and poly(ADP-ribose) polymerase (PARP) inhibitors. Here we show that loss of the MLL3/4 complex protein, PTIP, protects Brca1/2-deficient cells from DNA damage and rescues the lethality of Brca2-deficient embryonic stem cells. However, PTIP deficiency does not restore HR activity at DSBs. Instead, its absence inhibits the recruitment of the MRE11 nuclease to stalled replication forks, which in turn protects nascent DNA strands from extensive degradation. More generally, acquisition of PARPi and cisplatin resistance is associated with replication fork (RF) protection in Brca2-deficient tumor cells that do not develop Brca2 reversion mutations. Disruption of multiple proteins, including PARP1 and CHD4, leads to the same end point of RF protection, highlighting the complexities by which tumor cells evade chemotherapeutic interventions and acquire drug resistance

Opposing Roles of FANCJ and HLTF Protect Forks and Restrain Replication during Stress

Summary: The DNA helicase FANCJ is mutated in hereditary breast and ovarian cancer and Fanconi anemia (FA). Nevertheless, how loss of FANCJ translates to disease pathogenesis remains unclear. We addressed this question by analyzing proteins associated with replication forks in cells with or without FANCJ. We demonstrate that FANCJ-knockout (FANCJ-KO) cells have alterations in the replisome that are consistent with enhanced replication stress, including an aberrant accumulation of the fork remodeling factor helicase-like transcription factor (HLTF). Correspondingly, HLTF contributes to fork degradation in FANCJ-KO cells. Unexpectedly, the unrestrained DNA synthesis that characterizes HLTF-deficient cells is FANCJ dependent and correlates with S1 nuclease sensitivity and fork degradation. These results suggest that FANCJ and HLTF promote replication fork integrity, in part by counteracting each other to keep fork remodeling and elongation in check. Indicating one protein compensates for loss of the other, loss of both HLTF and FANCJ causes a more severe replication stress response. : Peng et al. find that loss of FANCJ enhances the replisome association of helicase-like transcription factor (HLTF). HLTF depletion suppresses fork degradation in FANCJ-deficient cells, and FANCJ depletion suppresses aberrant fork elongation in HLTF-deficient cells. However, the combined loss of HLTF and FANCJ causes severe replication stress. Keywords: FANCJ/BACH1/BRIP1, Fanconi anemia, hereditary breast cancer, DNA replication, replication stress response, helicase, replisome, fork protection, fork degradation, iPON

Replication gaps are a key determinant of PARP inhibitor synthetic lethality with BRCA deficiency

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