Balmus Laboratory

Shieldin


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


Dev H, Chiang TW, Lescale C, de Krijger I, Martin AG, Pilger D, Coates J, Sczaniecka-Clift M, Wei W, Ostermaier M, Herzog M, Lam J, Shea A, Demir M, Wu Q, Yang F, Fu B, Lai Z, Balmus G, Belotserkovskaya R, Serra V, O’Connor MJ, Bruna A, Beli P, Pellegrini L, Caldas C, Deriano L, Jacobs JJL, Galanty Y, Jackson SP
Nature Cell Biology (2018) 

Abstract

BRCA1 deficiencies cause breast, ovarian, prostate and other cancers, and render tumours hypersensitive to poly(ADP-ribose) polymerase (PARP) inhibitors. To understand the 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 that C20orf196 and FAM35A form a complex, ‘Shieldin’ (SHLD1/2), with FAM35A interacting with single-stranded DNA through its C-terminal oligonucleotide/oligosaccharide-binding fold region. We establish that Shieldin acts as the downstream effector of 53BP1/RIF1/MAD2L2 to promote DNA double-strand break (DSB) end-joining by restricting DSB resection and to counteract homologous recombination by antagonizing BRCA2/RAD51 loading in BRCA1-deficient cells. Notably, Shieldin inactivation further sensitizes 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.


featured in Nature Cell Biology: News and Views: Assembling a protective shield by Roger A Greenberg

A previously unidentified protein complex termed Shieldin acts with the nucleosome-binding protein 53BP1 to limit end resection at DNA double-strand breaks, impacting myriad biological outcomes, from immunology to cancer therapy, and highlighting the importance of chromatin responses to DNA damage in vertebrates.