THE DRUGGABLE EPIGENOME
Current Approaches
All current approaches to manipulating DNA methylation levels in some way target the endoge-
nous enzymatic machinery that is responsible for the addition or removal of mCs from DNA.
Constitutive and conditional gene knockout mouse models as well as viral-mediated RNA knock-
down or overexpression techniques have elucidated much about the importance of active DNA
methylation during neurodevelopment and in the functioning adult CNS (Figure 3). For example,
researchers now understand that whereas DNMT3a/b are often responsible for de novo methyl-
ation and DNMT1 for its maintenance, these assignments are not exclusive, and the knockout of
both DNMT1 and DNMT3a in adult forebrain neurons is necessary to elicit dysfunction in long-
term plasticity and deficits in learning and memory (67). Recently, a Tet1 knockout mouse model
and RNA knockdown experiments demonstrated that mC oxidation by Tet1 is critical for memory
and regulation of activity-related genes in the dorsal hippocampus, such as Fos and Arc (40, 41).