|Abstract or Summary
- DNA damage, if not repaired, can become a mutation. Mutation
accumulation is associated with initiation and progression of tumorigenesis. DNA
mismatch repair (MMR) is required for maintaining genetic stability by repairing
replication errors. Biochemical studies have shown that MMR also recognizes
mismatch-causing DNA lesions, suggesting the role of MMR in preventing
mutations after the exposure to DNA damaging agents. I hypothesized that MMR
plays a role in preventing genotoxin induced mutations and that the abilities of
two MMR proteins, PMS2 and MLH1, to prevent mutations are not equal. The
mutagenic effects of ultraviolet and oxidative stress were studied with two genetic
tools: forward and reversion mutational assays using the 2.3 Kb mouse Aprt gene in
kidney cells of PMS2 or MLH1 null mice. I report that loss of MLH1 resulted in a
more significant spontaneous mutator phenotype with an increase in all types of
base pair substitutions. In constrast, loss of PMS2 resulted in increased
spontaneous levels of base pair substitutions and large mutational events (loss of heterozygosity). Interestingly, half of all base pair substitutions in the Pms2 null
cells were A:T G:C transitions. Upon the exposure to DNA damaging agents,
both Pms2 and Mihi deficient cells significantly increased the percentage of mutant
alleles with multiple mutations. The reversion assay was used to demonstrate that
the increase in A:T G:C transitions was due to an elevated rate of formation of
these mutations. Using the specificity of reversion assay to detect either C -* T or
CC -* U mutations, I also showed that the tandem CC TT double substitution
is the signature mutation caused by hydrogen peroxide and metal mixture (Cu/Fe).
I also identified a mouse kidney cell line with G:C -> C:G mutator phenotype.
Cytotoxicity and microsatellite instability studies suggested that either an atypical
MMR pathway or a non-MMR pathway is involved in this rare mutator phenotype.
Taken together, these data suggest that MMR prevents genotoxin - induced
mutations by post- replicationally repairing mismatched bases opposite DNA
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