Mus81 and converging forks limit the mutagenicity of replication fork breakage

TLDR

Replication‑fork breakage is a major source of spontaneous DNA double‑strand breaks, and although break‑induced replication involving Pol32/POLD3 has been proposed to repair such breaks, the mechanisms that prevent genome destabilization remain unclear. The study proposes that a lack of a timely converging fork or Mus81 activity can drive the genome instability observed in cancer. Broken‑fork repair initiates with error‑prone Pol32‑dependent synthesis, but Mus81 endonuclease and a converging fork limit mutagenic synthesis to a few kilobases from the break and suppress template switches between homologous and divergent Alu elements, highlighting Mus81’s role in maintaining genome stability.

Abstract

Most spontaneous DNA double-strand breaks (DSBs) result from replication-fork breakage. Break-induced replication (BIR), a genome rearrangement-prone repair mechanism that requires the Pol32/POLD3 subunit of eukaryotic DNA Polδ, was proposed to repair broken forks, but how genome destabilization is avoided was unknown. We show that broken fork repair initially uses error-prone Pol32-dependent synthesis, but that mutagenic synthesis is limited to within a few kilobases from the break by Mus81 endonuclease and a converging fork. Mus81 suppresses template switches between both homologous sequences and diverged human Alu repetitive elements, highlighting its importance for stability of highly repetitive genomes. We propose that lack of a timely converging fork or Mus81 may propel genome instability observed in cancer.