Heterozygous mutations cause genetic instability in a yeast model of cancer evolution

Abstract

Genetic instability, a heritable increase in the mutation rate, accelerates evolutionary adaptation1 and is widespread in cancer2,3. In mammals, instability can arise from damaging both copies of genes involved in DNA metabolism and cell cycle regulation4 or from inactivating one copy of a gene whose product is present in limiting amounts (haploinsufficiency5), but determining the relative importance of these two mechanisms is difficult. In E. coli6, applying repeated, strong selection enriches for genetic instability. We used this approach to evolve genetic instability in diploid cells of the budding yeast, Saccharomyces cerevisiae and isolated clones with increased rates of point mutation, mitotic recombination, and chromosome loss. We identified candidate, heterozygous, instability-causing mutations and engineering these mutations, as heterozygotes, into the ancestral diploid strain caused genetic instability. Mutations that inactivate one copy of haploinsufficient genes are more common than those that dominantly alter the function of the mutated gene copy. The mutated genes are enriched for genes functioning in transport, protein quality control, and DNA metabolism, and reveal new targets for genetic instability7-11, including essential genes. Although only a minority (10 out of 57 genes with orthologs or close homologs) of the targets we identified have homologous human genes implicated in cancer2, the remainder are candidates to contribute to human genetic instability. To test this hypothesis, we inactivated six examples in a near haploid human cell line: five of these mutations increased instability. We conclude that single genetic events cause genetic instability in diploid yeast cells, and propose that similar, heterozygous mutations in mammalian homologs initiate genetic instability in cancer.