We thank S. cancers, and render tumours hypersensitive to PARP inhibitors. To understand resistance mechanisms, we conducted whole-genome CRISPR-Cas9 synthetic-viability/resistance screens in BRCA1-deficient breast cancer cells treated with PARP inhibitors. We identified two previously uncharacterized proteins, C20orf196 and FAM35A, whose inactivation confers strong PARP-inhibitor resistance. Mechanistically, we show C20orf196 and FAM35A form a complex, Shieldin (SHLD1/2), with FAM35A interacting with single-stranded DNA via its C-terminal OB fold region. We Bacitracin establish that Shieldin acts as the downstream effector of 53BP1/RIF1/MAD2L2 to promote DNA double-strand break (DSB) end-joining through restricting DSB resection and counteract homologous recombination by antagonising BRCA2/RAD51 loading in BRCA1-deficient cells. Notably, Shieldin inactivation further sensitises BRCA1-deficient cells to cisplatin, suggesting how defining the SHLD1/2 status of BRCA1-deficient tumours might aid patient stratification and yield new treatment opportunities. Highlighting this potential, we document reduced SHLD1/2 expression in human breast cancers displaying intrinsic or acquired PARP-inhibitor resistance. Intro DNA double-strand breaks (DSBs) are highly cytotoxic cellular lesions that must be efficiently and accurately repaired to keep up genome stability and prevent premature ageing, neurodegeneration, immunodeficiency, malignancy and other diseases1C3. In response to DSB detection, the apical kinases ATM, ATR and PRKDC (DNA-PKcs) become activated and phosphorylate several substrates to initiate the cellular DNA damage response (DDR)4. The ensuing cascade of molecular DDR events, which are advertised by numerous post-translational modifications including protein phosphorylation, ubiquitylation, sumoylation and poly (ADP-ribosyl)ation, effects on a myriad of cellular components, amongst other things leading to assembly of DDR factors at DNA-damage sites, arrest or slowing of cell-cycle progression, and activation of DNA restoration mechanisms4, 5. The two main types of DSB-repair pathway are non-homologous end-joining (NHEJ) which is definitely active throughout the cell cycle, and homologous recombination (HR), which normally requires a sister chromatid like a template and hence only operates in S and G2 phases of the cell cycle. DSB-repair pathway choice is definitely partly determined by functional antagonism between the HR-promoting element BRCA1 and NHEJ-promoting proteins such as TP53BP1 (53BP1), RIF1 and MAD2L2 (REV7)6C13. Inherited or acquired mutations in the or genes that result in protein loss or a mutant BRCA1/2 protein cause breast, ovarian, prostate and additional cancers, and render tumours hypersensitive to PARP-inhibitor medicines such as olaparib14C17. Unfortunately, intrinsic or acquired PARP-inhibitor resistance regularly prospects to lack-of-response or to patient relapse and tumour regrowth15, 18. In the medical center, the most common PARP-inhibitor resistance mechanisms reported to day are repair of BRCA1/2 manifestation or function. Notably, 53BP1 manifestation is lost in various triple-negative breast IKZF2 antibody cancers7, which may account for particular clinically relevant examples of PARP-inhibitor resistance. Nevertheless, the mechanisms driving PARP-inhibitor resistance in a large proportion of BRCA1/2-deficient tumours remain unexplained18, 19. To systematically survey for genetic mechanisms of PARP-inhibitor resistance, we carried out whole-genome CRISPR-Cas9 synthetic-viability/resistance screens in human being BRCA1-deficient breast malignancy cells treated with PARP inhibitors. In addition to identifying known resistance factors such as 53BP1, RIF1 and MAD2L2 loss6C13, we recognized two previously uncharacterized proteins, C20orf196 and FAM35A, whose inactivation confers PARP-inhibitor resistance to BRCA1-deficient cells. Our ensuing work lead us to define the Shieldin (SHLD1C20orf196/SHLD2FAM35A) complex that promotes NHEJ by providing as the downstream effector of 53BP1, RIF1 and MAD2L2, restricts DSB resection, and counteracts HR in BRCA1-deficient cells by antagonising alternative of replication protein A (RPA) with BRCA2 and RAD51 Bacitracin on resected single-stranded DNA (ssDNA). Finally, we statement that SHLD1C20orf196/SHLD2FAM35A loss confers hypersensitivity to the DNA-crosslinking agent cisplatin, and that reduced SHLD1C20orf196 or SHLD2FAM35A manifestation is associated with development of PARP-inhibitor resistance inside a patient-derived BRCA1-deficient breast malignancy xenograft model and in BRCA1-mutant cancers showing intrinsic PARP-inhibitor resistance. Results FAM35A or C20orf196 loss suppresses Bacitracin PARP-inhibitor level of sensitivity of and whose products form a complex21, we recognized several fresh suppressor candidates (Supplementary Table 1, Supplementary Fig 1d-e). These included DYNLL1, a known 53BP1 connection partner22, and TEN1, a component of.