SMARCAD1-mediated active replication fork stability maintains genome integrity.

The stalled fork protection pathway mediated by breast cancer 1/2 (BRCA1/2) proteins is critical for replication fork stability. However, it is unclear whether additional mechanisms are required to maintain replication fork stability. We describe a hitherto unknown mechanism, by which the SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily-A containing DEAD/H box-1 (SMARCAD1) stabilizes active replication forks, that is essential to maintaining resistance towards replication poisons. We find that SMARCAD1 prevents accumulation of 53BP1-associated nucleosomes to preclude toxic enrichment of 53BP1 at the forks. In the absence of SMARCAD1, 53BP1 mediates untimely dissociation of PCNA via the PCNA-unloader ATAD5, causing frequent fork stalling, inefficient fork restart, and accumulation of single-stranded DNA. Although loss of 53BP1 in SMARCAD1 mutants rescues these defects and restores genome stability, this rescued stabilization also requires BRCA1-mediated fork protection. Notably, fork protection-challenged BRCA1-deficient naive- or chemoresistant tumors require SMARCAD1-mediated active fork stabilization to maintain unperturbed fork progression and cellular proliferation.