XRCC1 promotes replication restart, nascent fork degradation and mutagenic DNA repair in BRCA2-deficient cells

Abstract

Homologous recombination/end joining (HR/HEJ)-deficient cancers with BRCA mutations utilize alternative DNA double-strand break repair pathways, particularly alternative non-homologous end joining or microhomology-mediated end joining (alt-EJ/MMEJ) during S and G2 cell cycle phases. Depletion of alt-EJ factors, including XRCC1, PARP1 and POLQ, is synthetically lethal with BRCA2 deficiency; yet, XRCC1 roles in HR-deficient cancers and replication stress are enigmatic. Here, we show that after replication stress, XRCC1 forms an active repair complex with POLQ and MRE11 that supports alt-EJ activity in vitro. BRCA2 limits XRCC1 recruitment and repair complex formation to suppress alt-EJ at stalled forks. Without BRCA2 fork protection, XRCC1 enables cells to complete DNA replication at the expense of increased genome instability by promoting MRE11-dependent fork resection and restart. High XRCC1 and MRE11 gene expression negatively impacts Kaplan–Meier survival curves and hazard ratios for HR-deficient breast cancer patients in The Cancer Genome Atlas. The additive effects of depleting both BRCA2 and XRCC1 indicate distinct pathways for replication restart. Our collective data show that XRCC1-mediated processing contributes to replication fork degradation, replication restart and chromosome aberrations in BRCA2-deficient cells, uncovering new roles of XRCC1 and microhomology-mediated repair mechanisms in HR-deficient cancers, with implications for chemotherapeutic strategies targeting POLQ and PARP activities.

An atypical BRCT–BRCT interaction with the XRCC1 scaffold protein compacts human DNA Ligase IIIα within a flexible DNA repair complex

“…Future structural and biochemical studies are needed to address how the binding of other XRCC1 partner proteins impact the flexibility of the XRCC1 scaffold and the repair of base damage as well as both single- and double-strand breaks. For example, in BRCA2-defective cells, there are increased interactions between XRCC1 and both MRE11 and DNA polymerase theta that may contribute to the observed genome rearrangements in BRCA-deficient cancers ( 91 ), so these structural results may inform ongoing investigations of existing and novel chemotherapeutic strategies.…”

Targeting SARS-CoV-2 Nsp3 macrodomain structure with insights from human poly(ADP-ribose) glycohydrolase (PARG) structures with inhibitors

“…PARG has been the focus of concentrated inhibitor development for cancer, both as a complement to and substitute for clinical PARP inhibitors (PARPi) ( Chen and Yu, 2019 ; Houl et al, 2019 ; Slade, 2020 ), which act in part by trapping PARP1 on damaged DNA ( Zandarashvili et al, 2020 ) and by acting synergistically to kill cancer cells with defective homology-directed repair ( Syed and Tainer, 2018 ) or alternative end joining ( Eckelmann et al, 2020 ). Following DNA damage, PARG reverses the signaling response initiated by PARP1 at ssDNA breaks by hydrolyzing the ‘cloud’ of poly (ADP-ribose) (PAR) into mono-nucleotide ADP-ribose (ADPr) ( Pourfarjam et al, 2020 ; Slade et al, 2011 ).…”

Heritable pattern of oxidized DNA base repair coincides with pre-targeting of repair complexes to open chromatin

“…We surveyed the Human Gene Mutation Database (HGMD) to assess the frequency of variants at G4 DNA motifs in 5′-UTRs known to cause or predispose toward inherited disease. A total of 13 loci were identified ( Supplementary Table S3 ); however, two of them, located respectively within the DNA repair genes RAD51 and XRCC1 with pivotal roles at the replication-repair interface ( 46 , 47 ), may confer an increased burden of morbidity upon the human population by increasing genome instability and hence susceptibility to cancer.…”