DNA methyltransferase

If DNA methylation is a system of sticky-note labels, then DNA methyltransferases are the hands that place them. These enzymes attach methyl groups onto specific DNA sites, writing the methylation patterns that help decide which genes are read.

Chemically, a DNA methyltransferase transfers a methyl group from a donor molecule onto a cytosine base, usually at a CpG site. In mammals these enzymes fall into two broad roles. Some establish new methylation patterns on previously unmarked DNA, a job called de novo methylation; others act after DNA replication to copy an existing pattern onto the newly made strand, called maintenance methylation.

Maintenance methylation is what allows a methylation pattern, and therefore a silenced state, to be inherited faithfully through cell division. Without it, marks would be diluted away after a few divisions and epigenetic memory would fade.

Because these enzymes control such a powerful layer of regulation, they are studied intensively in development and disease. Drugs that inhibit DNA methyltransferases are used to reactivate wrongly silenced genes in certain cancers, an example of epigenetic therapy.