O6-methylguanine DNA lesion (O6-MeG)

O6-methylguanine (O6-MeG) is a highly cytotoxic and mutagenic DNA lesion formed by the alkylation of the O6 position of guanine. It is the primary therapeutic lesion generated by alkylating chemotherapeutic agents such as temozolomide and dacarbazine. While it represents a small fraction of total DNA alkylation, its persistence is critical for the efficacy of these drugs in treating various cancers, particularly glioblastoma. The lesion's toxicity stems from its tendency to mispair with thymine during DNA replication; the subsequent recognition of this mismatch by the cellular mismatch repair (MMR) machinery leads to repetitive, unsuccessful repair cycles that eventually cause lethal DNA double-strand breaks and trigger apoptosis. The clinical impact of O6-MeG is heavily modulated by the repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), which can remove the methyl group and restore the guanine base, thereby conferring resistance to alkylating therapies (PMID: 15343335, PMID: 11412110).

Other names

O6-methylguanineO6-MeG adductO6-alkylguanine lesionO(6)-methylguanine

Alkylating agents transfer methyl groups to the O6 position of guanine, creating O6-methylguanine lesions. These lesions mispair with thymine during replication, triggering the mismatch repair (MMR) system. Because the MMR system removes the mismatched thymine but leaves the O6-MeG intact, a cycle of futile repair ensues, leading to double-strand breaks and apoptosis (PMID: 15343335, PMID: 21953770).