Stalled Fork Rescue via Dormant Replication Origins in Unchallenged S Phase Promotes Proper Chromosome Segregation and Tumor Suppression

Eukaryotic cells license far more origins than are actually used for DNA replication, thereby generating a large number of dormant origins. Accumulating evidence suggests that such origins play a role in chromosome stability and tumor suppression, though the underlying mechanism is largely unknown. Here, we show that a loss of dormant origins results in an increased number of stalled replication forks, even in unchallenged S phase in primary mouse fibroblasts derived from embryos homozygous for the Mcm4Chaos3 allele. We found that this allele reduces the stability of the MCM2-7 complex, but confers normal helicase activity in vitro. Despite the activation of multiple fork recovery pathways, replication intermediates in these cells persist into M phase, increasing the number of abnormal anaphase cells with lagging chromosomes and/or acentric fragments. These findings suggest that dormant origins constitute a major pathway for stalled fork recovery, contributing to faithful chromosome segregation and tumor suppression. [Display omitted] ► An unstable MCM2-7 complex results in a loss of dormant origins in Mcm4Chaos3 cells ► A loss of dormant origins impairs stalled fork recovery in unchallenged S phase ► A loss of dormant origins increases replication intermediates in prophase ► Replication intermediates in M phase are a likely cause of chromosome instability