Epigenetics Reveals Unexpected, and Some Identical, Results

Tattoos on the skin can say a lot about a person. On a deeper level, chemical tattoos on a person's DNA are just as distinctive and inpidual - and say far more about a person's life history.
A pair of reports published online January 18 in Nature Genetics show just how important one type of DNA tattoo, called methylation, can be. Researchers at Johns Hopkins University report the unexpected finding that DNA methylation - a chemical alteration that turns off genes - occurs most often near, but not within, the DNA regions scientists have typically studied. The other report, from researchers at the University of Toronto and collaborators, suggests that identical twins owe their similarities not only to having the same genetic makeup, but also to certain methylation patterns established in the fertilized egg.
 
Methylation is one of many epigenetic signals - chemical changes to DNA and its associated proteins - that modify gene activity without altering the genetic information itself. Methylation and other epigenetic signals help guide stem cells as they develop into other types of cells. Mistakes in methylation near certain critical genes can lead to cancer.
The Johns Hopkins group has now shown that DNA methylation is more common at what they call "CpG island shores" instead of at the CpG islands that most researchers have focused on. CpG islands are short stretches of DNA rich in the paired bases cytosine and guanine, letters "C" and "G" in the genetic alphabet. Methyl groups attach to cytosine bases in DNA.
CpG islands are located near the start sites of genes and help control a gene's activity. It's been thought that planting a methyl group on an island declares the nearby gene off-limits, blocking activity.
Researchers have thought of methylation as a type of long-term memory, preserving environmental effects on genes long after those cues have disappeared, says Rolf Ohlsson, a geneticist at the Karolinska Institute in Stockholm.