title: Multiple mutations are common at mouse Aprt in genotoxin-exposed mismatch repair deficient cells.
abstract: Mismatch repair deficiency is known to contribute to elevated rates of mutations, particularly at mono- and dinucleotide repeat sequences. However, such repeats are often missing from the coding regions of endogenous genes. To determine the types of mutations that can occur within an endogenous gene lacking highly susceptible repeat sequences, we examined mutagenic events at the 2.3 kb mouse Aprt gene in kidney cell lines derived from mice deficient for the PMS2 and MLH1 mismatch repair proteins. The Aprt mutation rate was increased 33-fold and 3.6-20-fold for Mlh1 and Pms2 null cell lines, respectively, when compared with a wild-type kidney cell line. For the Pms2 null cells this increase resulted from both intragenic events, which were predominantly base-pairs substitutions, and loss of heterozygosity events. Almost all mutations in the Mlh1 null cells were due to base-pair substitutions. A:T-->G:C transitions (54% of small events) were predominant in the Pms2 null cells whereas G:C-->A:T transitions (36%) were the most common base-pair change in the Mlh1 null cells. Interestingly, 4-9% of the spontaneous mutant alleles in the mismatch repair deficient cells exhibited two well-separated base-pair substitution events. The percentage of mutant alleles with two and occasionally three base-pair substitutions increased when the Pms2 and Mlh1 null cells were treated with ultraviolet radiation (15-21%) and when the Mlh1 null cells were treated with hydrogen peroxide (35%). In most cases the distance separating the multiple base-pair substitutions on a given allele was in excess of 100 base-pairs, suggesting that the two mutational events were not linked directly to a single DNA lesion. The significance of these results is discussed with regards to the roles for the PMS2 and MLH1 proteins in preventing spontaneous and genotoxin-related mutations.
authors.affiliation: Center for Research on Occupational and Environmental Toxicology, Oregon Health Sciences University, Portland, Oregon, OR 97201, USA.
authors.forename: Chi Y
authors.initials: CY
authors.lastname: Shin
authors.forename: Isabel
authors.initials: I
authors.lastname: Mellon
authors.forename: Mitchell S
authors.initials: MS
authors.lastname: Turker
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000228:Adenine Phosphoribosyltransferase / Q000235:genetics*; D000251:Adenosine Triphosphatases / Q000502:physiology*; D000818:Animals; D020137:Base Pair Mismatch; D002352:Carrier Proteins; D002460:Cell Line; D004260:DNA Repair; D045643:DNA Repair Enzymes; D004268:DNA-Binding Proteins / Q000502:physiology*; D006861:Hydrogen Peroxide / Q000494:pharmacology; D019656:Loss of Heterozygosity; D051379:Mice; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D009154:Mutation; D009363:Neoplasm Proteins / Q000502:physiology*; D009687:Nuclear Proteins; D014466:Ultraviolet Rays
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; C501486:MLH1 protein, human; C501485:Mlh1 protein, mouse; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D006861:Hydrogen Peroxide; D000228:Adenine Phosphoribosyltransferase; D000251:Adenosine Triphosphatases; C499533:Pms2 protein, mouse; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D045643:DNA Repair Enzymes