Like anyone else, identical and fraternal twins change a lot as they get older. Just look at Mary-Kate and Ashley Olsen (fraternal twins, believe it or not). Perhaps not-so-surprisingly, researchers are reporting that DNA methylation patterns of identical and fraternal twins change a lot in their first 18 months. The kicker is that even though people thought twins’ patterns would get more different from each other over time, some pairs’ DNA methylations actually became more similar.
The Australian-Finnish team says that most studies on this topic are cross-sectional, or just look at subjects at one moment in time. But cross-sectional twin studies on DNA methylation sometimes find that older twins differ more from each other (“epigenetic drift”) compared with younger twin pairs, while other reports say the opposite.
In an attempt to settle this question, the researchers conducted a longitudinal study, taking cheek-cell samples from 10 pairs of identical twins and five pairs of fraternal twins at birth and again at 18 months. Here’s some of what they found:
- About a third of the analyzed CpGs changed from birth to 18 months. Some changed by more than 20%.
- Most changes involved increases in DNA methylation.
- Almost 75% of the genes associated with the DNA methylation changes are involved in development and morphogenesis.
“What was most surprising is that we found that, as well as some pairs showing the expected epigenetic drift between birth and 18 months of age, some twin pairs became more epigenetically similar,” says Jeffrey Craig, who is the corresponding author and is at the University of Melbourne and Murdoch Childrens Research Institute. “Although ours is a small study, it suggests that epigenetic drift may not be a universal phenomenon and that the relationship between twins is a little more complex, especially in early life. My guess is that some pairs may change from having quite different intrauterine environments to having very similar environments after they are born.”
Read more at Genome Biology, May 2013.