Transcription-coupled repair removes cytotoxic DNA methylation damage and promotes mutational strand asymmetry in gliomas

MMS induces N7-methylguanine (7meG) and N3-methyladenine (3meA). Interestingly, the repair asymmetry was only found for 3meA, but not for 7meG, which is less cytotoxic and mutagenic than 3meA. Further knockout of TC-NER genes, CSB or UVSSA, significantly sensitized AAG-deficient cells to MMS and abolished the repair asymmetry, indicating that the TS-biased repair of 3meA is dependent on TC-NER.

The methylating agent temozolomide (TMZ) is used for glioma treatment. By analyzing glioma mutation data, we found that TMZ-treated gliomas accumulated fewer G>A mutations on the TS relative to the NTS, a phenomenon that can be attributed to strand-biased repair of O6-methylguanine (O6meG) by TC-NER. Finally, we show that knockout of the TC-NER factor CSB significantly sensitized glioma cells to TMZ.

These findings indicate that TC-NER repairs cytotoxic methylation damage (e.g., 3meA and O6meG) on the TS, promotes mutational strand asymmetry in TMZ-treated gliomas, and maintains resistance to alkylating agents.