Comparative analysis of epigenetic status in monozygotic twins

Epigenetic differences in genetically identical organisms such as monozygotic twins (MZ) and inbred animals, may help to understand the molecular mechanisms of phenotypic discordance which represents one of the key features of complex traits. We applied the bisulphite modification technique for mapping of methylated cytosines in the regulatory region of the human dopamine D2 receptor gene (DRD2) in lymphocyte DNA from six pairs of monozygotic twins. Due to the metastable nature of DNA methylation, the patterns of DNA methylation across individuals were complex, and comparison of epigenetic differences/similarities across individuals is not a trivial task. We applied two criteria: 1) density of methylated cytosines, and 2) position of methylated cytosines. In the first analysis, we detected that two out of six sets of MZ twins exhibited a higher intra-pair similarity than any other randomly selected pair of unrelated individuals. For the second analysis, we encoded methylated cytosines as a fifth base and used phylogeny generation programs such as MegAlign and PHYLIP. The idea behind this approach is that the lower the difference between the positions of methylated cytosines, the closer the "epigenetic" distance between individuals. In our study, three pairs out of six exhibited maximal similarity, i.e. two individuals from an MZ pair were more similar to each other than to any other unrelated individual. The above studies may serve as an example for the statistical comparative analysis of epigenetic status in genetically identical organisms.