Genome hypomethylation and activation of MAGE genes in tumor cells

Genome hypomethylation is a common feature of human cancers. We have shown previously that this epigenetic alteration in tumors is accompanied by the ectopic activation of a group of germline-specific genes, including gene MAGE-A1, whose repression in normal somatic tissues relies on DNA methylation. MAGE-A1 has a CpG-rich promoter, which unlike classical CpG-rich promoters, is methylated in most healthy tissues in the adult. Its expression in both male germ cells and tumor cells is associated with extensive demethylation of a restricted region of less than 3 Kb centered around the transcription initiation site. The other parts of the locus are only partially unmethylated in tumors. The process leading to this site-specific hypomethylation does not appear to be due to a permanent demethylating activity targeted to the 5' region of MAGE-A1, as in vitro methylated MAGE-A1 sequences do not undergo demethylation after being stably transfected in cells that normally express the gene. These cells however, contain transcriptional activators that protect the 5' region of the gene against remethylation , since unmethylated MAGE-A1 transgenes undergo remethylation at all CpGs except those located within the 5' region. These observations suggest that the site-specific hypomethylation in the 5' region of MAGE-A1 results from a sequence of two distinct processes: a transient DNA demethylation process, followed by a persistent site-specific protection of this region against remethylation. We are now searching to identify the DNA methyltransferases that are normally involved in maintaining the MAGE-A1 gene methylated. Knock-down experiments using anti-sense oligonucleotides suggest that DNMT1 plays a dominant role in this process