It depends on which tissues you’re talking about. And epigenetic modification — or at least methylation — in newborns is dictated by genetic, environmental, and tissue-specific factors, according to research published this month by Lavinia Gordon and colleagues at the Murdoch Children’s Research Institute and six other universities, research centers, and hospitals.
First the tissue-based factor. Using statistical clustering, the mostly Australian multi-institutional group found that the methylation profiles of identical twins segregated into the same categories — the same general patterns of DNA modification — more often or less often, depending on the DNA’s tissue of origin. in placental tissue, 71 percent of twin pairs’ methylation patterns “clustered” together statistically. In cord blood mononuclear cells (CBMC), identical twins shared similar methylation profiles 58 percent of the time. And in human umbilical vasclar endothilial cells (HUVEC), these twins’ methylation profiles clustered only 29 percent of the time. Those three tissue types were the only ones they tested for this part of the experiment.
Now for genetic factors. Compared to fraternal (non-identical) twins, the epigenetic modifications of identical twins showed more overall within-pair similarity — less discordance — as evaluated by the investigators using Euclidean distance and Pearson’s correlation coefficient.
That trend wasn’t always perfect, though. Here’s where environmental factors come in. Some newborns had methylation profiles that were less similar to twins’ profiles than they were to the profiles of another, unrelated baby in the study. In fact, when the researchers limited their focus to identical twin pairs, they found that those who developed in separate chorionic and amniotic sacs showed a lot more epigenetic similarity than the twin pairs who shared the same sacs. At least, this was true in analyses of CBMC and HUVEC tissue — the researchers didn’t have enough test subjects for a reliable analysis of their placental tissue.
The research group tested 27,578 CpGs associated with 14,475 transcription start sites in each tissue type using an Illumina Infinium Human Methylation27 BeadChip array. The CBMC tissue came from 18 identical and nine fraternal twins, the HUVEC tissue from 14 and 10, respectively, and the placental tissue from eight and seven, also respectively.
In other interesting findings, although previous studies have shown that methylation discordance increases with the age of identical twins — including one paper by some of this same group — that didn’t appear to be the case in this study. Using Euclidean distance on data from newborn twins all the way to 73-year-old twins, 94 identical and 17 fraternal . . .
We found no evidence for epigenetic drift throughout the life course in either MZ or DZ pairs.
It’s an interesting result, and the authors suggest that the question of how environment changes epigenetic modifications over time should be further investigated to figure out just what happens and when. They’re dealing with a small sample size throughout the study, after all, particularly in the tissue-specific methylation profiling.
[The newborn twin picture at the top is by Flickr user Beautiful Freaks, and it's used here under a Creative Commons license.]
Gordon L, Joo JE, Powell JE, Ollikainen M, Novakovic B, Li X, Andronikos R, Cruickshank MN, Conneely KN, Smith AK, Alisch RS, Morley R, Visscher PM, Craig JM, & Saffery R (2012). Neonatal DNA methylation profile in human twins is specified by a complex interplay between intrauterine environmental and genetic factors, subject to tissue-specific influence. Genome research PMID: 22800725