Promoter-Dependent Mechanism Leading to Selective Hypomethylation within the 5′ Region of Gene MAGE-A1 in Tumor Cells

Abstract

Several male germ line-specific genes, including MAGE-A1, rely on DNA methylation for their repression in normal somatic tissues. These genes become activated in many types of tumors in the course of the genome-wide demethylation process which often accompanies tumorigenesis. We show that in tumor cells expressing MAGE-A1, the 5′ region is significantly less methylated than the other parts of the gene. The process leading to this site-specific hypomethylation does not appear to be permanent in these tumor cells, since in vitro-methylated MAGE-A1 sequences do not undergo demethylation after being stably transfected. However, in these cells there is a process that inhibits de novo methylation within the 5′ region of MAGE-A1, since unmethylated MAGE-A1 transgenes undergo remethylation at all CpGs except those located within the 5′ region. This local inhibition of methylation appears to depend on promoter activity. We conclude that the site-specific hypomethylation of MAGE-A1 in tumor cells relies on a transient process of demethylation followed by a persistent local inhibition of remethylation due to the presence of transcription factors.

We are grateful to Adrian Bird, Francis Brasseur, and Etienne De Plaen for advice and comments on the manuscript.

This work was supported by the Belgian Program on Inter-University Poles of Attraction initiated by the Belgian State Prime Minister's Office, Science Policy Programming. A.L. is supported by the Fonds pour la Recherche Scientifique dans l'Industrie et l'Agriculture (Brussels, Belgium).