53BP1/shieldin-dependent DSB processing in BRCA1-deficient cells requires CST/Polα/primase fill-in synthesis

PARPi efficacy in BRCA1-deficient cells depends on 53BP1 and shieldin, which have been proposed to limit single-stranded DNA at DSBs by blocking resection and/or through CST/Polα/primase-mediated fill-in. We show that—like 53BP1/shieldin and CST/Polα—primase promotes radial chromosome formation in PARPi-treated BRCA1-deficient cells and demonstrate shieldin/CST/Polα/primase-dependent incorporation of BrdU at DSBs. In the absence of 53BP1 or shieldin, radial formation in BRCA1-deficient cells was restored by tethering of CST near DSBs, arguing that in this context shieldin acts primarily by recruiting CST. Furthermore, a SHLD1 mutant defective in CST binding (SHLD1Δ) was non-functional in BRCA1-deficient cells and its function was restored upon reconnecting SHLD1Δ to CST. Interestingly, at dysfunctional telomeres and DNA breaks in class switch recombination where CST has been implicated, SHLD1Δ was fully functional, perhaps because these DNA ends carry CST recognition sites that afford SHLD1-independent binding of CST. The data establish that in BRCA1-deficient cells, CST/Polα/primase is the major effector of shieldin-dependent DSB processing.