Unscheduled DNA replication in G1 causes genome instability and damage signatures indicative of replication collisions

Abstract

DNA replicates once per cell cycle. Interfering with the regulation of DNA replication initiation generates genome instability through over-replication and has been linked to early stages of cancer development. Here, we engineered genetic systems in budding yeast to induce unscheduled replication in the G1-phase of the cell cycle. Unscheduled G1 replication initiated at canonical S-phase origins. We quantified the composition of replisomes in G1- and S-phase and identified firing factors, polymerase α, and histone supply as factors that limit replication outside S-phase. G1 replication per se did not trigger cellular checkpoints. Subsequent replication during S-phase, however, resulted in over-replication and led to chromosome breaks and chromosome-wide, strand-biased occurrence of RPA-bound single-stranded DNA indicating head-to-tail replication fork collisions as key mechanism generating genome instability upon G1 replication. Low-level, sporadic induction of G1 replication induced an identical response, indicating findings from synthetic systems are applicable to naturally occurring scenarios of unscheduled replication initiation.