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Efficient repair of A/C mismatches in mouse cells deficient in long-patch mismatch repair
S Oda1, O Humbert, S Fiumicino
1Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Herts EN6 3LD, UK.
Abstract:
A previously unrecognized mismatch repair activity is described. Extracts of immortalized MSH2-deficient mouse fibroblasts did not correct most single base mispairs. The same extracts carried out efficient repair of A/C mismatches. A/G mispairs were less efficiently corrected and there was no significant repair of A/A. MLH1-defective mouse extracts also repaired an A/C mispair. A/C correction by Msh2(-/-) mouse cell extracts was not affected by antibodies against the PMS2 protein, which inhibited long-patch mismatch repair. A/C repair activity is thus independent of MutSalpha, MutSbeta and MutLalpha. A/C mismatches were corrected 5-fold more efficiently by extracts of Msh2 knockout mouse cells than by comparable extracts prepared from hMSH2- or hMLH1-deficient human cells. MSH2-independent A/C correction by mouse cell extracts did not require a nick in the circular duplex DNA substrate. Repair involved replacement of the A and was associated with the resynthesis of a limited stretch of
Insights
A novel DNA repair pathway corrects A/C mismatches independently of known mismatch repair proteins. This MSH2-independent activity in mouse cells efficiently repairs specific DNA errors.
Area of Science:
- Molecular Biology
- Genetics
- DNA Repair Mechanisms
Background:
- Mismatch repair (MMR) is crucial for genomic stability, correcting errors arising during DNA replication.
- Key MMR proteins include MutS and MutL homologs, forming complexes like MutSalpha, MutSbeta, and MutLalpha.
- Deficiencies in MMR genes (e.g., MSH2, MLH1) are linked to hereditary nonpolyposis colorectal cancer (HNPCC) and microsatellite instability.
Purpose of the Study:
- To characterize a previously unrecognized DNA repair activity observed in MSH2-deficient mouse cell extracts.
- To determine the substrate specificity and protein dependence of this novel repair mechanism.
- To compare the efficiency of this activity with known MMR pathways in both mouse and human cells.
Main Methods:
- Utilized immortalized MSH2-deficient and MLH1-defective mouse fibroblast extracts.
- Assessed repair of various single base mispairs (A/C, A/G, A/A) using in vitro DNA substrates.
- Investigated the role of PMS2 protein using specific antibodies and examined the requirement for DNA nicks.
- Quantified repair efficiency by comparing extracts from knockout mouse cells and deficient human cells.
Main Results:
- MSH2-deficient mouse extracts efficiently repaired A/C mismatches but not A/A or most other single base mispairs.
- MLH1-defective extracts also showed A/C repair activity.
- A/C repair was independent of MutSalpha, MutSbeta, and MutLalpha complexes, as it was unaffected by PMS2 antibodies.
- Mouse cell extracts exhibited 5-fold higher A/C repair efficiency than deficient human cell extracts.
- Repair involved A replacement and limited DNA resynthesis (<=25 bases) without requiring a DNA nick.
Conclusions:
- A novel, MSH2- and MLH1-independent DNA mismatch repair pathway exists in mouse cells.
- This pathway specifically targets and repairs A/C mismatches through a nick-independent mechanism.
- The findings reveal a new layer of DNA repair complexity and potential therapeutic targets for MMR-deficient cancers.