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Epigenetics Reveals Unexpected, and Some Identical, Results

2009-06-19 Author:Tina Hesman Saey Source:The Twins Foundation

Now a team led by Arturas Petronis of the University of Toronto has explored all of the CpG islands dotting the genome to see which sport methylation flags. The team compared the methylation patterns of twins from monozygotic pairs - twins created when a single embryo splits. Although the twins had identical DNA, their methylation of CpG islands varied. But the methylation patterns in monozygotic twins were more similar than those in fraternal twins, who develop from separate eggs. And the group found that the amount of variation between monozygotic twins correlates with the time the embryo split: Counterintuitively, twins from an early-splitting embryo have more similar methylation patterns than twins from a later split.

Epigenetic patterns established in the early embryo are carried throughout life, with some differences introduced by the environment and others by random chance and error in replicating the patterns as a person develops. DNA is reproduced with high fidelity - mistakes happen in about one in a million bases - but the process of reproducing epigenetic patterns in piding cells is more error-prone, with one in a thousand epigenetic marks going awry.

Petronis thinks the similarity between monozygotic twins results not from shared DNA sequences but from having come from the same embryo. "We don't see any reason to think that the DNA sequence makes up the epigenetic profile," Petronis says.

But swimming away from CpG islands may offer a different perspective. Andrew Feinberg, director of the Epigenetics Center at Johns Hopkins University in Baltimore, and colleagues embarked on a genome-wide tour to chart DNA methylation in different human tissues. The researchers had expected that each tissue would have a characteristic methylation pattern, indicating which genes are turned off and which are turned on to build a liver, spleen, brain or other tissue. Often researchers examine methylation only at CpG islands, but Feinberg says that most islands are surprisingly free of methylation in most tissues.

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