ALTERED METHYLATION OF THE HUMAN MDR1 PROMOTER IS ASSOCIATED WITH ACQUIRED MULTIDRUG-RESISTANCE

One of the most important forms of drug resistance in acute myeloid le ukemia is the multidrug resistance (MDR) phenotype, which is character ized by the expression of the MDR1 gene product, P-glycoprotein. Altho ugh a number of factors affect MDR1 gene expression, the genetic event s that ''switch on'' the human MDR1 gene in tumor cells that were prev iously P-glycoprotein negative have remained elusive. Here, we report evidence that the methylation status of the human MDR1 promoter may se rve as a basis for this ''switch.'' Based on Southern analysis using m ethylation-sensitive and methylation-insensitive restriction enzymes, a tight correlation was found between MDR phenotype and demethylation of the 5' region of the MDR1 gene in a human T cell leukemia cell line . Similar results were obtained from the analysis of P-glycoprotein-po sitive and P-glycoprotein-negative samples of chronic lymphocytic leuk emia. Treatment of the cell lines with the demethylating agent 5'-azad eoxycytidine altered the methylation pattern of the MDR1 promoter in P -glycoprotein-negative cells to resemble that of P-glycoprotein-positi ve cells and activated the promoter such that MDR1 mRNA was now detect able. Treatment also resulted in an increased resistance to epirubicin and decreased daunomycin accumulation, both of which were reversible by verapamil, a characteristic of the classical MDR phenotype in cells expressing P-glycoprotein. These results suggest that the MDR phenoty pe may be acquired as a result of changes in methylation of the MDR1 p romoter.