The goal of this pilot study was to explore the putative molecular mechanisms underlying the phenotypic discordance of monozygotic (MZ) twins. Thus, patterns of epigenetic DNA modification were investigated in the 5'-regulatory region of the dopamine D2 receptor gene (DRD2) in two pairs of monozygotic twins, one concordant and one discordant for schizophrenia. The bisulfite DNA modification-based approach was used to fine-map methylated cytosines in DRD2 in genomic DNA extracted from lymphocytes. Numerous DNA methylation differences were identified in the analyzed region both within and between the pairs of MZ twins. "Epigenetic distances" between MZ twins were calculated and used for the comparison of twin DRD2 methylation profiles. It was detected that the affected twin from the pair discordant for schizophrenia was epigenetically "closer" to the affected concordant twins than to his unaffected MZ co-twin. Although the epigenetic analysis was conducted for only several hundred base pairs of DRD2, the fact that numerous studies identified nonuniform methylation patterns across the clones of bisulfite-modified DNA from the same individual, as well as nonuniform patterns across different individuals, argues for the universality of intra- and interindividual epigenetic variation. Epigenetic studies should provide insight into the molecular causes of differential susceptibility to a disease in genetically identical organisms that may generalize to singletons.

Elucidation of genetic factors in schizophrenia and bipolar disorder remains a challenging task to psychiatric researchers. As a rule, data from genetic linkage and association studies are quite controversial. In this article, we further explore the possibility that in addition to DNA sequence variation, a putative epigenetic dysregulation of brain genes plays an important role in the etiopathogenesis of major psychosis. We provide an epigenetic interpretation of unclear genetic findings specifically pertaining to chromosome 22 in schizophrenia and bipolar disorder. It is suggested that epigenetic strategies, when applied in conjunction with traditional genetic ones, may significantly expedite the uncovering of the molecular causes of major psychosis.