Figure 1. DNA methylation and histone modification patterns are altered in cancers.

A, Approximately 70% of genes possess promoter-associated CpG islands that mostly remain unmethylated in normal cells unlike the remainder of the genome which tends to be heavily methylated. Maintenance of an unmethylated promoter CpG island positively contributes to a high transcriptional potential and is associated with active histone modifications including histone H3 and H4 acetylation and methylation at H3K4. B, Cancer cells, on the other hand, exhibit dense hypermethylation of up to 10% of CpG islands as well as hypomethylation of bulk chromatin including intergenic regions and repetitive elements. A densely methylated CpG island is capable of driving chromatin compaction and repressing gene expression in association with repressive modifications including H3K9me3, H3K27me3, and/or H4K20me3.