This review identified fifteen candidate gene methylation studies ranging across all of the above areas. Loci studied included alpha-synuclein [77], DNA methyltransferase 3b [78], homocysteine-induced endoplasmic reticulum protein [79], NMDA receptor subtype 2b [80], monoamine oxidase A [36], the serotonin transporter [81,82,83], the dopamine transporter [83,84,85,86], the H19 and IG differentially methylated regions [87], vasopressin and atrial natriuretic peptide [88], proopiomelanocortin [89], orexin A [90], nerve growth factor [91], MeCP2 [83], leptin [92], and the mu opioid receptor [93]. Studies generally reported small but significant changes in methylation at the above loci. A few loci, however, have had consistently negative results [81,82], failed to replicate previous associations [85,86], or found associations in subgroups only [36].
