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Mammalian DNA repair methyltransferases shield O4MeT from nucleotide excision repair
1Department of Molecular and Cellular Toxicology, Harvard School of Public Health, Boston, MA 02115, USA.
Abstract:
O6-Methylguanine (O6MeG) and O4-methylthymine (O4MeT) are potentially mutagenic DNA lesions that cause G:C-->A:T and A:T-->G:C transition mutations by mispairing during DNA replication, and the repair of O6MeG and O4MeT by DNA repair methyltransferases (MTases) is therefore expected to prevent methylation-induced transitions. The efficiency of O6MeG and O4MeT repair by different MTases can vary by several hundred-fold and the aim of this study was to establish the biological consequences of such differences in the efficiency of repair. The ability of three microbial and two mammalian MTases to prevent methylation-induced G:C-->A:T and A:T-->G:C transitions is taken as a measure of their ability to repair O6MeG and O4MeT in vivo respectively. All five MTases give complete protection against G:C-->A:T transitions. However, while the microbial MTases give complete protection against A:T-->G:C transitions, the mammalian MTases actually sensitize cells to A:T-->G:C transitions. We hypothesize that the mammalian MTases bind O4MeT lesions in vivo but that, because they are extremely slow at subsequent methyl transfer, binding shields O4MeT from repair by the nucleotide excision repair pathway. Results are presented to support this hypothesis.
Insights
DNA repair methyltransferases (MTases) differ in their efficiency. While all tested MTases repaired O6-methylguanine (O6MeG) lesions, mammalian MTases unexpectedly increased A:T-->G:C mutations from O4-methylthymine (O4MeT) lesions.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- O6-methylguanine (O6MeG) and O4-methylthymine (O4MeT) are mutagenic DNA lesions.
- DNA repair methyltransferases (MTases) are crucial for repairing these lesions and preventing mutations.
Purpose of the Study:
- To investigate the biological consequences of varying O6MeG and O4MeT repair efficiencies by different MTases.
- To assess the in vivo efficacy of microbial and mammalian MTases in preventing methylation-induced mutations.
Main Methods:
- Assessed the ability of three microbial and two mammalian MTases to prevent G:C-->A:T and A:T-->G:C transitions in vivo.
- Used mutation prevention as a proxy for O6MeG and O4MeT repair efficiency.
Main Results:
- All five MTases effectively prevented G:C-->A:T transitions, indicating efficient O6MeG repair.
- Microbial MTases prevented A:T-->G:C transitions, but mammalian MTases sensitized cells to these mutations.
- Mammalian MTases appear to shield O4MeT lesions from nucleotide excision repair due to slow methyl transfer.
Conclusions:
- Differential repair efficiencies of MTases have significant biological consequences.
- Mammalian MTases exhibit a novel, detrimental role in O4MeT mutagenesis, distinct from microbial counterparts.