HYPERMUTABILITY OF HOMONUCLEOTIDE RUNS IN MISMATCH REPAIR AND DNA-POLYMERASE PROOFREADING YEAST MUTANTS

Homonucleotide runs in coding sequences are hot spots for frameshift m utations and potential sources of genetic changes leading to cancer in humans having a mismatch repair defect. We examined frameshift mutati ons in homonucleotide runs of deoxyadenosines ranging from 4 to 14 bas es at the same position in the LYS2 gene of the yeast Saccharomyces ce revisiae, In the msh2 mismatch repair mutant, runs of 9 to 14 deoxyade nosines are 1,700-fold to 51,000-fold, respectively, more mutable for single nucleotide deletions than are runs of 4 deoxyadenosines. These frameshift mutations can account for up to 99% of all forward mutation s inactivating the 4-kh LYS2 gene. Based on results with single and do uble mutations of the POL2 and MSH2 genes, both DNA polymerase epsilon proofreading and mismatch repair are efficient for short runs while o nly the mismatch repair system prevents frameshift mutations in runs o f greater than or equal to 8 nucleotides. Therefore, coding sequences containing long homonucleotide runs are likely to be at risk for mutat ional inactivation in cells lacking mismatch repair capability.