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The repair of DNA methylation damage in Saccharomyces cerevisiae
W Xiao1, B L Chow, L Rathgeber
1Department of Microbiology, University of Saskatchewan, Saskatoon, SK, S7N 5E5, Canada.
Abstract:
The major genotoxicity of methyl methanesulfonate (MMS) is due to the production of a lethal 3-methyladenine (3MeA) lesion. An alkylation-specific base-excision repair pathway in yeast is initiated by a Mag1 3MeA DNA glycosylase that removes the damaged base, followed by an Apn1 apurinic/ apyrimidinic endonuclease that cleaves the DNA strand at the abasic site for subsequent repair. MMS is also regarded as a radiomimetic agent, since a number of DNA radiation-repair mutants are also sensitive to MMS. To understand how these radiation-repair genes are involved in DNA methylation repair, we performed an epistatic analysis by combining yeast mag1 and apn1 mutations with mutations involved in each of the RAD3, RAD6 and RAD52 groups. We found that cells carrying rad6, rad18, rad50 and rad52 single mutations are far more sensitive to killing by MMS than the mag1 mutant, that double mutants were much more sensitive than either of the corresponding single mutants, and that the effects of the double mutants were either additive or synergistic, suggesting that post-replication and recombination-repair pathways recognize either the same lesions as MAG1 and APN1, or else some differ- ent lesions produced by MMS treatment. Lesions handled by recombination and post replication repair are not simply 3MeA, since over-expression of the MAG1 gene does not offset the loss of these pathways. Based on the above analyses, we discuss possible mechanisms for the repair of methylation damage by various pathways.
Insights
Methyl methanesulfonate (MMS) genotoxicity involves 3-methyladenine (3MeA) lesions. Yeast DNA repair pathways, including post-replication and recombination repair, are crucial for repairing MMS-induced DNA damage beyond just 3MeA.
Area of Science:
- Molecular Biology
- Genetics
- DNA Repair
Background:
- Methyl methanesulfonate (MMS) is a genotoxic agent primarily causing 3-methyladenine (3MeA) DNA lesions.
- Yeast possesses an alkylation-specific base-excision repair pathway initiated by Mag1 3MeA DNA glycosylase and Apn1 endonuclease.
- MMS is also considered radiomimetic, indicating a role for radiation-repair genes in its DNA damage response.
Purpose of the Study:
- To investigate the involvement of radiation-repair genes in DNA methylation repair.
- To perform an epistatic analysis of MMS sensitivity in yeast by combining mag1 and apn1 mutations with mutations in RAD3, RAD6, and RAD52 epistasis groups.
Main Methods:
- Epistatic analysis was conducted by creating double mutants combining yeast mag1 and apn1 mutations with mutations from RAD3, RAD6, and RAD52 groups.
- Sensitivity to MMS killing was assessed for single and double mutants.
- The effect of MAG1 gene overexpression was evaluated in the context of other repair pathway mutations.
Main Results:
- Single mutations in rad6, rad18, rad50, and rad52 conferred greater MMS sensitivity than the mag1 mutation.
- Double mutants exhibited significantly higher MMS sensitivity compared to their respective single mutants, with additive or synergistic effects.
- Overexpression of MAG1 did not compensate for the loss of recombination and post-replication repair pathways, indicating that lesions repaired by these pathways are not solely 3MeA.
Conclusions:
- Post-replication and recombination-repair pathways recognize DNA lesions induced by MMS, which may include 3MeA or other distinct lesions.
- These pathways play a significant role in mitigating the genotoxicity of MMS, acting in concert with or parallel to the base-excision repair pathway.
- The findings suggest complex mechanisms for repairing methylation damage involving multiple DNA repair systems in yeast.