Effect of 5-azacytidine on expression of the human DNA repair enzyme O6-methylguanine-DNA methyltransferase.

The role of methylation of CpG dinucleotides in the regulation of O6-methylguanine-DNA methyltransferase (MGMT) gene expression has been investigated. A previous observation, that cell lines deficient in MGMT (Mer-) are methylated in a SmaI site in the MGMT gene promoter whereas MGMT-expressing cells (Mer+) are unmethylated in the same site, has been extended to a total of 30 cell lines, tumors and normal tissues. To examine further the association between methylation in the MGMT promoter and the Mer- phenotype we have treated Mer+ and Mer- cell lines with 5-azacytidine to inhibit DNA methylation. Reduced methylation in the SmaI site coincided with induction of MGMT expression for one of three Mer- cell lines. MGMT increased several-fold further upon continued culture of the induced cells in the absence of 5-azacytidine, coincident with an abrupt increase in methylation in the body of the MGMT gene even though the SmaI site remained demethylated. These results, and those of other previous studies, suggest that methylation of sequences within the MGMT gene promoter and methylation within the body of the gene have opposite effects.