Demethylation is relevant to epigenetics.  Demethylation of DNA is catalyzed by demethylases.  These enzymes oxidize N-methyl groups, which occur in histones, in lysine derivatives, and in some forms of DNA.  
:RN-CH  +  O   →   RN-H   +  CHO
One family of such oxidative enzymes is the cytochrome P450. Alpha-ketoglutarate-dependent hydroxylases are also active for demethylation of DNA, operating by a similar stoichiometry.  These reactions, which proceed via hydroxylation, exploit the slightly weakened C-H bonds of methylamines and methyl ethers.
Demethylation of some sterols are steps in the biosynthesis of testosterone and cholesterol.  Methyl groups are lost as formate.