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

The role of methylatlon of CpG dinucleotides in the regulationof O⁶-methylguanine-DNA methyltransferase (MGMT) gene expressionhas been investigated. A previous observation, that cell linesdeficient in MGMT (Men^– are methylated in a SmaI sitein the MGMT gene promoter whereas MGMT expressing cells (Mer⁺)are unmethylated in the same site, has been extended to a totalof 30 cell lines, tumors and normal tissues. To examine furtherthe association between methylation in the MGMT promoter andthe Mer^– phenotype we have treated Mer⁺ and Mer^–cell lines with 5-azacytidine to inhibit DNA methylation. Reducedmethylation in the SmaI site coincided with induction of MGMTexpression for one of three Mer^– cell lines. MGMT increasedseveral-fold further upon continued culture of the induced cellsin the absence of 5-azacytidlne, coincident with an abrupt increasein methylation in the body of the MGMT gene even though theSmaI site remained demethylated. These results, and those ofother previous studies, suggest that methylation of sequenceswithin the MGMT gene promoter and methylation within the bodyof the gene have opposite effects.