Hypomethylation in human cancers is causally related to transcriptional activation of MAGE-type genes

MAGE-type genes comprise several unrelated families of X-linked genes that code for tumor-specific antigens recognized by cytolytic T lymphocytes. These genes are normally expressed exclusively in testicular germ cells, but become aberrantly activated in many tumors. Interestingly, MAGE-type genes were found to be expressed in tumor cell lines that show a decrease in the overall DNA methylation level, suggesting that their activation may be a consequence of the genome-wide demethylation that frequently occurs in cancer cells. MAGE-type genes are indeed methylated in all normal adult somatic tissues from either male or female origin, but are demethylated in expressing tumor cells. They are also unmethylated in spermatogenic cells. Detailed analysis of the methylation status of gene MAGEA1 in expressing cells indicated that demethylation is restricted to a region of less than 3 Kb centered around the transcription initiation site. Interestingly, promoters of MAGE-type genes have an intermediate density of CpGs and may constitute a unique class of promoters which fall between the constitutively unmethylated CpG island-promoter and the conditionally methylated CpG poor-promoter. Treatment with the demethylating agent 5'-aza-deoxycytidine suffices to induce stable expression of MAGE-type genes, and this induction was shown to correlate with promoter demethylation. Unmethylated MAGEA1 promoter constructs exert transcriptional activity after transfection in cells that do not express the gene. Conversely, in vitro methylated promoter constructs are completely and stably repressed in cells that normally express gene MAGEA1. Our data indicate that MAGE-type genes have promoters that are driven by ubiquitous transcription factors. DNA methylation appears therefore to be an essential repression mechanism ensuring the germ-cell specific expression of these genes. Hence, MAGE-type genes constitue targets for gene activation consequently to genome-wide hypomethylation in cancer cells.