53BP1-independent Shieldin-BRCA1 antagonism at replication-coupled double-strand breaks

BRCA1-deficient tumors are hypersensitive to PARP inhibitors (PARPi) due to impaired homologous recombination (HR). The 53BP1-Shieldin complex inhibits end resection at replication-independent DNA double-strand breaks (DSBs), promoting PARPi sensitivity in BRCA1-deficient cells. However, its role in replication-coupled DSBs is less clear. Here, we show that loss of Shld2 or Shld3, but not 53bp1, confers PARPi resistance in mouse embryonic stem cells lacking a functional BRCA1 BRCT domain. Unlike 53bp1 loss, deletion of Shld2 or Shld3 partially restores HR at replication-coupled DSBs, reduces BRCA1-linked insertion/deletion signatures, and promotes RAD51 loading. This 53BP1-independent function requires the CST complex and counteracts residual BRCA1 coiled-coil domain activity in RAD51 loading. Shld2 loss also confers PARPi resistance in Bard1-null cells retaining residual BRCA1, but not in cells expressing RING-less BRCA1 with degraded BARD1. These findings identify a 53BP1-independent function for Shieldin in sustaining HR deficiency in BRCA1-deficient cells, providing new mechanistic insights into PARPi resistance. The 53BP1-Shieldin pathway is known to influence PARP inhibitor resistance in BRCA1 deficient tumors by altering DNA repair. Here, the authors show that Shieldin proteins independently of 53BP1 maintain specific DNA repair defects, and their loss restores repair and drives PARP inhibitor resistance.