Non-CG Methylation in the Human Genome

DNA methylation is a chemical modification that occurs predominantly on CG dinucleotides in mammalian genomes. However, recent studies have revealed that non-CG methylation (mCH) is abundant and nonrandomly distributed in the genomes of pluripotent cells and brain cells, and is present at lower levels in many other human cells and tissues. Surprisingly, mCH in pluripotent cells is distinct from that in brain cells in terms of sequence specificity and association with transcription, indicating the existence of different mCH pathways. In addition, several recent studies have begun to reveal the biological significance of mCH in diverse cellular processes. In reprogrammed somatic cells, mCH marks megabase-scale regions that have failed to revert to the pluripotent epigenetic state. In myocytes, promoter mCH accumulation is associated with the transcriptional response to environmental factors. In brain cells, mCH accumulates during the establishment of neural circuits and is associated with Rett syndrome. In this review, we summarize the current understanding of mCH and its possible functional consequences in different biological contexts.