Dependence of histone modifications and gene expression on DNA hypermethylation in cancer

We examined the relationship between aberrant DNA hypermethylation and key histone code components at a hypermethylated, silenced tumor suppressor gene promoter in human cancer. In lower eukaryotes, methylated H3-lysine 9 (methyl-H3-K9) determines DNA methylation and correlates with repressed gene transcription. Here we show that a zone of deacetylated histone H3 plus methyl-H3-K9 surrounds a hypermethylated, silenced hMLH1 promoter, which, when unmethylated and active, is embedded in methyl-H3-K4 and acetylated H3. Inhibiting DNA methyltransferases, but not histone deacetylases, leads first to promoter demethylation, second to gene reexpression, and finally to complete histone code reversal. Our findings suggest a new paradigm-DNA methylation may directly, or indirectly by inhibiting transcription, maintain key repressive elements of the histone code at a hypermethylated gene promoter in cancer.