A cancer-associated DNA polymerase δ variant modeled in yeast causes a catastrophic increase in genomic instability

Accurate DNA synthesis by the replicative DNA polymerases alpha, delta, and epsilon is critical for genome stability in eukaryotes. In humans, over 20 SNPs were reported that result in amino-acid changes in Pol delta or Pol epsilon. In addition, Pol delta variants were found in colon-cancer cell lines and in sporadic colorectal carcinomas. Using the yeast-model system, we examined the functional consequences of two cancer-associated Pol delta mutations and four polymorphisms affecting well-conserved regions of Pol delta or Pol epsilon. We show that the R696W substitution in Pol delta (analog of the R689W change in the human cancer-cell line DLD-1) is lethal in haploid and homozygous diploid yeast. The cell death results from a catastrophic increase in spontaneous mutagenesis attributed to low-fidelity DNA synthesis by Pol delta-R696W. Heterozygotes survive, and the mutation rate depends on the relative expression level of wild-type versus mutant alleles. Based on these observations, we propose that the mutation rate in heterozygous human cells could be regulated by transient changes in gene expression leading to a temporary excess of Pol delta-R689W. The similarities between the mutational spectra of the yeast strains producing Pol delta-R696W and DLD-1 cells suggest that the altered Pol delta could be responsible for a significant proportion of spontaneous mutations in this cancer cell line. These results suggest that the highly error-prone Pol delta-R689W could contribute to cancer initiation and/or progression in humans.