Making sense of chaos: replication and segregation defects in the yeast mcm4Chaos3 mutant

Abstract

Genome instability is a defining feature of cancer and is strongly influenced by alterations in the fidelity and timing of DNA replication. The conserved Mini-Chromosome Maintenance (MCM 2-7) helicase is a central component of the eukaryotic DNA replication machinery, playing critical roles in origin licensing and activation. Genetic screens for genome instability mutations in mice identified mcm4Chaos3, a hypomorphic allele of MCM4 associated with the formation of micronuclei and tumorigenesis, suggesting that the mcm4Chaos3 allele causes genome instability. To define the mechanistic basis of this instability, we examined the effects of the corresponding mutation in Saccharomyces cerevisiae. We found that mcm4Chaos3 selectively compromises origin firing at a subset of early S-phase origins, with pronounced effects at many centromere-proximal origins. Chromosome V, containing the sensitive centromere-adjacent ARS510, exhibited elevated loss rates that were rescued by replacement with the robust early-firing origin ARS305, restoring both origin activity and chromosome stability. Together these findings demonstrate that mcm4Chaos3 compromises genome stability by impairing a subset of early, centromere-proximal origins that are intrinsically sensitive to origin activation capacity. Our results establish that robust origin function near centromeres is critical for accurate chromosome segregation and reveal how subtle perturbations in helicase function can lead to chromosome instability.