GRADED METHYLATION IN THE PROMOTER AND BODY OF THE O-6-METHYLGUANINE DNA METHYLTRANSFERASE (MGMT) GENE CORRELATES WITH MGMT EXPRESSION IN HUMAN GLIOMA-CELLS

Expression of the O-6-methylguanine DNA methyltransferase (MGMT) gene in human glioma cell lines is strongly associated with resistance to t he chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea. To exa mine the possibility that methylation of the body and promoter regions of the MGMT gene is associated with MGMT expression in a graded, rath er than a completely on/off fashion, the present study analyzed the me thylation status of the MGMT gene in human glioma cell lines exhibitin g a wide range of MGMT expression. Methylation in the body of the gene was uniform within each cell line and correlated directly with MGMT e xpression. The level of MGMT promoter methylation was also graded acro ss the cell lines, at 21 of 25 CpGs tested, but correlated inversely w ith MGMT expression. Two sites in the MGMT promoter were also much mor e accessible to restriction enzyme digestion, and thus in a more open chromatin conformation, in nuclei from high MGMT expressors relative t o nuclei from cells with little or no MGMT expression. We conclude tha t the level of methylation, in both the body and promoter of the MGMT gene, is associated with MGMT expression in a graded fashion and may b e important in setting the transcriptional state of the MGMT promoter through changes in chromatin structure.