DNA methylation: activation of a suppressed imprinted gene through cytosine demethylation.
For example, with a developmentally imprinted gene, one copy of the gene (the maternal or
paternal allele) is silenced by methylation. So, in the case that the expressed copy is mutated,
the imprinting of the remaining functional allele silences what would otherwise be a perfectly
functional copy of the gene. De-imprinting and activating transcription of the imprinted allele
could provide a strong beneficial effect. A good example of an application for this type of ap-
proach is Angelman syndrome (AS). AS is caused by an inactivating mutation of the maternal
copy of the UBE3A ubiquitin ligase, and the paternal allele is silenced in many areas of the CNS.
Thus, functionally, the AS brain manifests a homozygous UBE3A deficiency in critical learn-
ing and memory regions (105). Derepression of the paternal allele of UBE3A in the CNS, via
demethylation of that allele, could restore a normal transcriptional level of an active UBE3A
transcript.