Possibility 3: trigger increased methylation to activate a gene. It has become clear that,
although cytosine methylation has historically been viewed as a gene suppression mechanism,
methylation also has the capacity to increase transcription as well (10, 107). This may be most
common for methylation events that occur within the gene body, i.e., within the coding region
of the gene. Thus, as a third type of methylome-based therapeutic, one might also target the
appropriate methylation sites within a gene to elicit gene-activating methylation. To illustrate this
possibility, we can consider Pitt-Hopkins Syndrome (PTHS). PTHS is an autosomal dominant
disease characterized by heterozygous inactivating mutation or deletion of the TCF4 transcription
factor, which leads to pronounced intellectual disability, autistic behaviors, and loss of language
acquisition (108, 109). Conceptually, one might trigger activating methylation of the remaining
normal allele of TCF4 in PTHS patients to upregulate transcription of the remaining normal
allele (i.e., move it above its normal transcriptional set point) to restore near-normal levels of
active TCF4 in cells.