Shieldin complex promotes DNA end joining and counters homologous recombination in BRCA1-null cells

ABSTRACT: BRCA1 deficiencies cause breast, ovarian and other 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. Thus, we identified two previously uncharacterized proteins, C20orf196 and FAM35A, whose inactivation confers strong PARP-inhibitor resistance. Mechanistically, we show that C20orf196 and FAM35A form a complex, termed “Shieldin” (SHLD1/2), with FAM35A interacting with single-stranded DNA via its C-terminal OB-fold region. We establish that Shieldin promotes DNA double-strand break (DSB) end-joining, acting as the downstream effector of 53BP1/RIF1/MAD2L2 to restrict DSB resection and counteract homologous recombination in BRCA1-deficient cells by antagonising BRCA2/RAD51 loading. 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.

INSTRUMENT(S): Q Exactive

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Cell Culture

SUBMITTER: Petra Beli  

LAB HEAD: Petra Beli

PROVIDER: PXD009830 | Pride | 2018-05-31

REPOSITORIES: Pride