The sister chromatid cohesion pathway suppresses multiple chromosome gain and chromosome amplification.

Gain or loss of chromosomes resulting in aneuploidy can be important factors in cancer and adaptive evolution. Although chromosome gain is a frequent event in eukaryotes, there is limited information on its genetic control. Here we measured the rates of chromosome gain in wild-type yeast and sister chromatid cohesion (SCC) compromised strains. SCC tethers the newly replicated chromatids until anaphase via the cohesin complex. Chromosome gain was measured by selecting and characterizing copper-resistant colonies that emerged due to increased copies of the metallothionein gene CUP1. Although all defective SCC diploid strains exhibited increased rates of chromosome gain, there were 15-fold differences between them. Of all mutants examined, a hypomorphic mutation at the cohesin complex caused the highest rate of chromosome gain while disruption of WPL1, an important regulator of SCC and chromosome condensation, resulted in the smallest increase in chromosome gain. In addition to defects in SCC, yeast cell type contributed significantly to chromosome gain, with the greatest rates observed for homozygous mating-type diploids, followed by heterozygous mating type, and smallest in haploids. In fact, wpl1-deficient haploids did not show any difference in chromosome gain rates compared to wild-type haploids. Genomic analysis of copper-resistant colonies revealed that the "driver" chromosome for which selection was applied could be amplified to over five copies per diploid cell. In addition, an increase in the expected driver chromosome was often accompanied by a gain of a small number of other chromosomes. We suggest that while chromosome gain due to SCC malfunction can have negative effects through gene imbalance, it could also facilitate opportunities for adaptive changes. In multicellular organisms, both factors could lead to somatic diseases including cancer.

Gene copy-number variation in haploid and diploid strains of the yeast Saccharomyces cerevisiae.

The increasing ability to sequence and compare multiple individual genomes within a species has highlighted the fact that copy-number variation (CNV) is a substantial and underappreciated source of genetic diversity. Chromosome-scale mutations occur at rates orders of magnitude higher than base substitutions, yet our understanding of the mechanisms leading to CNVs has been lagging. We examined CNV in a region of chromosome 5 (chr5) in haploid and diploid strains of Saccharomyces cerevisiae. We optimized a CNV detection assay based on a reporter cassette containing the SFA1 and CUP1 genes that confer gene dosage-dependent tolerance to formaldehyde and copper, respectively. This optimized reporter allowed the selection of low-order gene amplification events, going from one copy to two copies in haploids and from two to three copies in diploids. In haploid strains, most events involved tandem segmental duplications mediated by nonallelic homologous recombination between flanking direct repeats, primarily Ty1 elements. In diploids, most events involved the formation of a recurrent nonreciprocal translocation between a chr5 Ty1 element and another Ty1 repeat on chr13. In addition to amplification events, a subset of clones displaying elevated resistance to formaldehyde had point mutations within the SFA1 coding sequence. These mutations were all dominant and are proposed to result in hyperactive forms of the formaldehyde dehydrogenase enzyme.