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Base transitions dominate the mutational spectrum of a transgenic reporter gene in MSH2 deficient mice
S E Andrew1, A H Reitmair, J Fox
1Biomedical Research Centre and Centre for Molecular Medicine and Therapeutics, University of British Columbia, Vancouver, Canada.
Abstract:
Tumors derived from individuals with hereditary nonpolyposis colorectal cancer syndrome frequently demonstrate mutations in both alleles of hMSH2, a key gene in DNA mismatch repair (MMR). Sporadic tumors also frequently exhibit MMR deficiency. In keeping with the role of MMR in the maintenance of genome integrity, mice deficient in MSH2 via gene targeting demonstrate a high incidence of thymic lymphomas and small intestinal adenocarcinomas. To investigate the effects of MSH2 deficiency in normal tissues, mice containing a retrievable transgenic lacI reporter gene for mutation detection were crossed with MSH2-/- mice. Mice homozygous for MSH2 deficiency revealed 4.8, 11.0 and 15.2-fold elevations in spontaneous mutation frequency in DNA obtained from brain, small intestine, and thymus, respectively, as compared to heterozygous or wild-type mice. Mutations most frequently recovered from MSH2-/- mice were single base substitutions (77%), particularly base transitions (64%). Frameshifts occurred less frequently (19%) and fell within very short (3-5 bp) mononucleotide runs. Thus the number of key growth control genes potentially impacted by MMR deficiency extends beyond those containing repetitive sequences. These results highlight the capacity for MSH2 deficiency to serve as a potent driving force during the multi-step evolution of tumors.
Insights
DNA mismatch repair (MMR) deficiency, specifically in the MSH2 gene, significantly elevates mutation rates in normal tissues. This MSH2 gene deficiency drives tumor development by increasing spontaneous mutations in critical genes.
Area of Science:
- Genetics
- Molecular Biology
- Cancer Research
Background:
- Hereditary nonpolyposis colorectal cancer syndrome and sporadic tumors often show mutations in the MSH2 gene, a key player in DNA mismatch repair (MMR).
- MMR is crucial for maintaining genome integrity.
- MSH2-deficient mice exhibit high incidences of lymphomas and adenocarcinomas.
Purpose of the Study:
- To investigate the impact of MSH2 deficiency on mutation frequency in normal tissues.
- To characterize the types of mutations that arise in the absence of functional MSH2.
Main Methods:
- Mice lacking functional MSH2 (MSH2-/-) were crossed with mice carrying a lacI reporter gene.
- Mutation frequency was assessed in DNA from brain, small intestine, and thymus of MSH2-deficient mice compared to controls.
- Mutation types (base substitutions, frameshifts) were analyzed.
Main Results:
- Mice homozygous for MSH2 deficiency showed 4.8 to 15.2-fold increases in spontaneous mutation frequency across different tissues.
- Single base substitutions, primarily transitions, constituted the majority of mutations (77%).
- Frameshift mutations (19%) were observed in short mononucleotide runs.
Conclusions:
- MSH2 deficiency profoundly increases mutation rates in normal tissues, impacting a broader range of genes than previously thought.
- The loss of MSH2 function acts as a significant driver in the multi-step process of tumor evolution.