DNA methylation is the process by which methyl groups are placed on the fifth position of pyramidine rings of cytosine-guanine (CpG) dinucleotides,[7] which are common in gene promoter regions.[8] The reaction is catalyzed by a group of enzymes, known as DNA methyltransferases (DNMTs).[7] CpG methylation at gene promoter regions impedes the binding of transcription factors, and attracts methyl-binding proteins that ultimately compact chromatin and suppress gene expression.[9] Another common epigenetic mechanism involves the alteration of histones, proteins around which DNA is wrapped. Covalent modifications of histones are involved in mediating changes in the structure of chromatin to facilitate or impede access to DNA by transcription factors, and therefore facilitate or impede expression.[10,11] Predictably, epigenetic modification, including CpG methylation and histone modification, is fundamental to cell differentiation and specialization throughout eukaryotic embryogenesis and development.[12] A third mechanism involves imprinting by non-coding RNAs (ncRNA) and/or protein gene products. In this circumstance, these macromolecules act to alter chromatin structure, transcription, RNA splicing, editing, and translation—ultimately shaping gene expression.[13] The best-known example of epigenetic alteration of gene expression by RNA is the inactivation of
