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O6-methylguanine residues elicit DNA repair synthesis by human cell extracts
P Karran1, P Macpherson, S Ceccotti
1Clare Hall Laboratories, Imperial Cancer Research Fund, South Mimms, United Kingdom.
Abstract:
We have investigated the processing of O6-methylguanine (O6-MeGua) in plasmid DNA by extracts of human cells defective in O6-MeGua-DNA methyltransferase. Cell extracts of HeLaMR cells performed viral T antigen-independent DNA synthesis on plasmids that had been treated with low concentrations of methylating agents. The in vitro DNA synthesis was non-semiconservative and depended on the presence of O6-MeGua in the substrate. The involvement of DNA polymerase delta or epsilon and proliferating cell nuclear antigen but not single-strand binding protein was indicated by partial fractionation, inhibitor, and antibody studies. Processing of O6-MeGua is not via the UV nucleotide excision repair pathway since additional component(s) are apparently required to perform repair synthesis on the methylated substrate. This is the first direct demonstration of DNA repair synthesis provoked by O6-MeGua in DNA. Since O6-MeGua is not excised from DNA by Mex- cells, it represents a novel type of processing of the methylated base that may be involved in its cytotoxicity.
Insights
Human cells defective in O6-methylguanine-DNA methyltransferase initiate DNA repair synthesis on O6-methylguanine (O6-MeGua) lesions. This novel processing pathway may contribute to O6-MeGua cytotoxicity.
Area of Science:
- Molecular Biology
- DNA Repair Mechanisms
- Cellular Toxicology
Background:
- O6-methylguanine (O6-MeGua) is a DNA adduct formed by methylating agents.
- Defects in O6-MeGua-DNA methyltransferase (MGMT) can lead to increased cellular sensitivity to methylating agents.
- The precise processing and repair pathways for O6-MeGua in human cells are not fully elucidated.
Purpose of the Study:
- To investigate the processing of O6-MeGua in plasmid DNA using extracts from human cells deficient in O6-MeGua-DNA methyltransferase.
- To characterize the DNA synthesis associated with O6-MeGua lesions in vitro.
- To determine the molecular machinery involved in O6-MeGua processing.
Main Methods:
- In vitro DNA synthesis assays using plasmid DNA treated with methylating agents.
- Utilizing cell extracts from HeLaMR cells (deficient in O6-MeGua-DNA methyltransferase).
- Employing partial fractionation, inhibitor studies, and antibody inhibition to identify involved proteins.
Main Results:
- Cell extracts from O6-MeGua-DNA methyltransferase-deficient cells exhibited viral T antigen-independent DNA synthesis on O6-MeGua-containing plasmids.
- The observed DNA synthesis was non-semiconservative and specifically dependent on the presence of O6-MeGua.
- Evidence suggested the involvement of DNA polymerase delta or epsilon and proliferating cell nuclear antigen, but not single-strand binding protein.
- This processing did not follow the canonical UV nucleotide excision repair pathway.
Conclusions:
- This study provides the first direct demonstration of DNA repair synthesis triggered by O6-MeGua in DNA.
- O6-MeGua is processed via a novel pathway in cells lacking O6-MeGua-DNA methyltransferase, distinct from excision.
- This alternative processing mechanism may contribute to the cytotoxic effects of O6-MeGua.