O6-alkylguanine DNA alkyltransferase (also known as AGT, MGMT or AGAT) is a protein that in humans is encoded by the O6-methylguanine DNA methyltransferase (MGMT) gene. O6-methylguanine DNA methyltransferase is crucial for genome stability. It repairs the naturally occurring mutagenic DNA lesion O6-methylguanine back to guanine and prevents mismatch and errors during DNA replication and transcription. Accordingly, loss of MGMT increases the carcinogenic risk in mice after exposure to alkylating agents. The two bacterial isozymes are Ada and Ogt. Function and mechanism Although alkylating mutagens preferentially modify the guanine base at the N7 position, O6-alkyl-guanine is a major carcinogenic lesion in DNA. This DNA adduct is removed by the repair protein O6-alkylguanine DNA alkyltransferase through an SN2 mechanism. This protein is not a true enzyme since it removes the alkyl group from the lesion in a stoichiometric reaction and the active enzyme is not regenerated after it is alkylated (referred to as a suicide enzyme). The methyl-acceptor residue in the protein is a cysteine. Demethylation of 6-O-methylguanosine to Guanosine Clinical significance In patients with glioblastoma, a severe type of brain tumor, the cancer medicine temozolomide is more effective in those with a methylation of the gene's promoter. Overall, MGMT methylation is associated with prolonged patient survival in clinical prediction models. For testing of the MGMT promoter methylation status in the clinical setting, DNA-based methods such as methylation-specific polymerase chain reaction (MS-PCR) or pyrosequencing are preferred over immunohistochemical or RNA- based assays. MGMT has also been shown to be a useful tool increasing gene therapy efficiency. By using a two component vector consisting of a transgene of interest and MGMT, in vivo drug selection can be utilized to select for successfully transduced cells. Mutagens in the environment, in tobacco smoke, food, as well as endogenous metabolic products generate reactive electrophilic species that alkylate or specifically methylate DNA, generating 6-O-methylguanine (m6G). In 1985 Yarosh summarized the early work that established m6G as the alkylated base in DNA that was the most mutagenic and carcinogenic. In 1994 Rasouli-Nia et al. showed that about one mutation was induced for every eight unrepaired m6Gs in DNA. Mutations can cause progression to cancer by a process of natural selection. Expression in cancer Epigenetic repression Only a minority of sporadic cancers with a DNA repair deficiency have a mutation in a DNA repair gene. However, a majority of sporadic cancers with a DNA repair deficiency do have one