Human Pol ɛ-dependent replication errors and the influence of mismatch repair on their correction

• An active site mutant of human Pol ɛ is a strong mutator for base pair substitutions.
• Proofreading corrects almost all replication errors made by this mutant.
• The proofreading-deficient mutant drives mutagenesis in MMR-deficient human cells.
• MMR is fully able to correct proofreading-deficient Pol ɛ replication errors.
• Pol ɛ proofreading-deficient cancer mutants likely have an underlying MMR defect.

Mutations in human DNA polymerase (Pol) ɛ, one of three eukaryotic Pols required for DNA replication, have recently been found associated with an ultramutator phenotype in tumors from somatic colorectal and endometrial cancers and in a familial colorectal cancer. Possibly, Pol ɛ mutations reduce the accuracy of DNA synthesis, thereby increasing the mutational burden and contributing to tumor development. To test this possibility in vivo, we characterized an active site mutant allele of human Pol ɛ that exhibits a strong mutator phenotype in vitro when the proofreading exonuclease activity of the enzyme is inactive. This mutant has a strong bias toward mispairs opposite template pyrimidine bases, particularly T
• dTTP mispairs. Expression of mutant Pol ɛ in human cells lacking functional mismatch repair caused an increase in mutation rate primarily due to T
• dTTP mispairs. Functional mismatch repair eliminated the increased mutagenesis. The results indicate that the mutant Pol ɛ causes replication errors in vivo, and is at least partially dominant over the endogenous, wild type Pol ɛ. Since tumors from familial and somatic colorectal patients arise with Pol ɛ mutations in a single allele, are microsatellite stable and have a large increase in base pair substitutions, our data are consistent with a Pol ɛ mutation requiring additional factors to promote tumor development.