Epigenetic biomarkers have the potential to address these critical issues. Epigenetics is the study of potentially heritable marks that provide structural and regulatory functions to the genome, but are distinct from changes in the base pair sequence of the genome [19]. Epigenetic marks include DNA methylation at CpG residues, histone tail modifications, small non-coding RNAs, and open versus closed chromatin packing. The basic concepts of epigenetics have been reviewed previously [12,18,20] and will not be discussed in detail here. Functionally, epigenetic changes affect the expression of genes, as measured by RNA and protein production, which in turn may affect cellular structure and function, which in turn may in turn lead to changes in higher level phenomena such as behavior. With respect to drug taking and drug seeking behavior, preclinical literature has demonstrated that changes in the expression of genes such as BDNF [21] and OPRM1 [22,23] alter the reinforcement properties of drugs such as alcohol, cocaine and heroin. Thus, in addition to having utility as biomarkers, epigenetic changes and resultant changes in gene expression can also contribute to our mechanistic understanding of addictions.
