A cancer-associated DNA polymerase delta 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 Poldelta or Polepsilon. In addition, Poldelta 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 Poldelta mutations and four polymorphisms affecting well-conserved regions of Poldelta or Polepsilon. We show that the R696W substitution in Poldelta (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 Poldelta-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 Poldelta-R689W. The similarities between the mutational spectra of the yeast strains producing Poldelta-R696W and DLD-1 cells suggest that the altered Poldelta could be responsible for a significant proportion of spontaneous mutations in this cancer cell line. These results suggest that the highly error-prone Poldelta-R689W could contribute to cancer initiation and/or progression in humans.