Monozygotic twins share the same DNA and often the same environment, and are considered identical, but sometimes they show striking phenotypic differences. In light of recent epigenetic studies, investigators believe it is time to revisit the problem.23-25 DNA methylation, which tags cytosine with a methyl group, and chromatin modification, by acetylation, methylation, phosphorylation, etc., are heritable epigenetic mechanisms that together store information and modulate gene expression throughout the genome. Whereas genomic information is uniform among different cells of a complex organism, epigenomic information varies from tissue to tissue, providing a specific identity to each cell type (see Chapter 6).
Recently, Fraga and colleagues, on examining epigenetic differences arising during the lifetime of twins, find that monozygotic twins are epigenetically indistinguishable during the early years of life, but with aging, their epigenetic patterns tend to diverge and exhibit remarkable differences in overall content and genomic distribution of methylated cytosine and modified histones.26 They found that about a third of monozygotic twins harbored epigenetic differences in DNA methylation and chromatin (histone) modifications. Smoking habits, physical activity, or diet are some environmental factors that might influence epigenetic modifications over the long term. It is also possible that the accumulation of small defects in transmitting through successive cell divisions, or in maintaining it in differentiated cells, might contribute to differences associated with aging. Fraga's comparison of identical twins suggests that environmental and intrinsic factors can affect the phenotype by altering the pattern of epigenetic modification and thereby modulate genetic information. Further studies that address the specific mechanisms responsible for these epigenetic modifications will be of great interest.
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