The shieldin complex mediates 53BP1-dependent DNA repair.

Abstract

53BP1 is a chromatin-binding protein that regulates DNA double-strand break (DSB) repair by suppressing the nucleolytic resection of DNA termini1,2. This function of 53BP1 requires interactions with PTIP3 and RIF14–9, the latter also recruiting REV7/MAD2L2 to break sites10,11. How 53BP1-pathway proteins shield DNA ends is unknown but two models best explain their action: in one model, the 53BP1 complex strengthens the nucleosomal barrier to end-resection nucleases12,13, whereas in a second model, 53BP1 recruits effector proteins with end-protection activity. Here we describe the identification of such a 53BP1 effector complex, Shieldin, which includes C20orf196 (SHLD1), FAM35A (SHLD2), CTC-534A2.2 (SHLD3) and REV7. Shieldin localizes to DSB sites in a 53BP1- and RIF1-dependent manner and its SHLD2 subunit binds to ssDNA via OB-fold domains analogous to those of RPA1 and POT1. Loss of Shieldin impairs non-homologous end-joining (NHEJ), leads to defective immunoglobulin class switching and causes hyper-resection. Mutations in Shieldin subunit genes also cause resistance to poly(ADP-ribose) polymerase (PARP) inhibition in BRCA1-deficient cells and tumours due to restoration of homologous recombination (HR). Finally, we show that ssDNA binding by SHLD2 is critical for Shieldin function, consistent with a model where Shieldin protects DNA ends to mediate 53BP1-dependent DNA repair.