Mitotic systemic genomic instability in yeast

Conventional models of genome evolution generally include the assumption that mutations accumulate gradually and independently over time. We characterized the occurrence of sudden spikes in the accumulation of genome-wide loss-of-heterozygosity (LOH) in Saccharomyces cerevisiae, suggesting the existence of a mitotic systemic genomic instability process (mitSGI). We characterized the emergence of a rough colony morphology phenotype resulting from an LOH event spanning a specific locus (ACE2/ace2-A7). Surprisingly, half of the clones analyzed also carried unselected secondary LOH tracts elsewhere in their genomes. The number of secondary LOH tracts detected was 20-fold higher than expected assuming independence between mutational events. Secondary LOH tracts were not detected in control clones without a primary selected LOH event. We then measured the rates of single and double LOH at different chromosome pairs and found that coincident LOH accumulated at rates 30-100 fold higher than expected if the two underlying single LOH events occurred independently. These results were consistent between two different strain backgrounds, and in mutant strains incapable of entering meiosis. Our results indicate that a subset of mitotic cells within a population experience systemic genomic instability episodes, resulting in multiple chromosomal rearrangements over one or few generations. They are reminiscent of early reports from the classic yeast genetics literature, as well as recent studies in humans, both in the cancer and genomic disorder contexts, all of which challenge the idea of gradual accumulation of structural genomic variation. Our experimental approach provides a model to further dissect the fundamental mechanisms responsible for mitSGI.\n\nSIGNIFICANCE STATEMENTPoint mutations and alterations in chromosome structure are generally thought to accumulate gradually and independently over many generations. Here, we combined complementary genetic approaches in budding yeast to track the appearance of chromosomal changes resulting in loss-of-heterozygosity (LOH). Contrary to expectations, our results provided evidence for the occurrence of non-independent accumulation of multiple LOH events over one or a few cell generations. These results are analogous to recent reports of bursts of chromosomal instability in humans. Our experimental approach provides a framework to further dissect the fundamental mechanisms underlying systemic chromosomal instability processes, including in the human cancer and genomic disorder contexts.