The CST complex mediates a post-resection non-homologous end-joining repair pathway and promotes local deletions

Repair of a DNA double-strand break (DSB) by non-homologous end-joining (NHEJ) generally leaves an intact or minimally modified DNA sequence. Resection initiation exposes single-stranded DNA and directs repair towards homology-dependent pathways and away from NHEJ. Therefore, NHEJ is not thought to be an available repair pathway once the DSB is resected. Here, we report that the Cdc13/Stn1/Ten1 (CST) complex, well characterized for its telomere-associated functions, acts after resection initiation to mediate a backup NHEJ repair. We found a CST-specific mutation signature after DSB repair, characterized by deletions of 5-85 bp, mostly dependent on NHEJ. In contrast, NHEJ-mediated small deletions of 1-4 bp and insertions are not affected in CST mutants. The interaction between CST and Polα-primase is critical for these intermediate size deletions, suggesting a role for fill-in synthesis. Consistently, in stn1Δ and Polα-primase mutant deficient for interaction with CST, resection is increased, leading to larger deletions of several kilobases mediated by microhomologies. Collectively, these results depict a more complex picture of repair pathway choice where CST allows a post-resection NHEJ repair, promoting local deletions but guarding against much larger and potentially more deleterious deletions and rearrangements.