Figure 1. DNA methylation and histone modifications at promoters of normal and cancer cells.
(A) At promoters of genes that are actively transcribed, the tails of histone proteins are marked with acetylation and the CpG dinucleotides are unmethylated. (B) Silenced genes can be marked by aberrant methylation of CpG dinucleotides alone, or (C) by histone H3 lysine 27 trimethylation and methylation of CpG dinucleotides or (D) By histone H3 lysine 27 trimethylation in the absence of CpG methylation. Initial reports in cancer cell lines suggest this pattern (D) is found in far fewer genes compared with aberrant DNA methylation. It is quite possible that other histone modifications change coordinately or inversely with DNA methylation, but have not yet been studied in gliomas.