By birth, genetic doubles use their DNA differently

By Tina Hesman Saey

Web edition: July 17, 2012
Print edition: August 25, 2012; Vol.182 #4 (p. 14)

Identical twins aren’t perfect carbon copies of each other even at birth.

Twins emerge from the womb carrying different chemical marks on their DNA that influence the activity of individual genes, a new study shows. Known as epigenetic markers, these alterations don’t change the underlying genetic information. But by regulating the activity of certain genes, they can profoundly influence how the DNA blueprint is used to create and operate a living organism.

Past research has shown that identical twins bear some differences in epigenetic markers. But those differences were thought to arise after birth, as twins have different life experiences and encounter different environments.

The new study — the first to measure epigenetic profiles in newborns — suggests that subtle differences in conditions within the womb can leave marks on fetal DNA that may have long-term consequences for adult health.

These differing chemical tags may help explain why identical twins look slightly different, have their own personalities and may have different susceptibility to diseases. Jeffrey Craig, a molecular and cell biologist at Murdoch Childrens Research Institute in Parkville, Australia, and his colleagues report the findings online July 15 in Genome Research.

Identical twins are on average more epigenetically similar than fraternal twins, the researchers found. The similarity was probably not due to sharing a womb, but could be attributed partially to genetics and partially to chance, they suggest.

Arturas Petronis, an epigenetics researcher at the University of Toronto, has another idea about why identical twins are more epigenetically similar than fraternal twins. It’s not because they have the same genes, but because they inherit the same epigenetic signature. Just as eggs and sperm carry different combinations of parental genes, the cells are also marked with different DNA tags. So identical twins, which result when a single embryo splits, start out with the same epigenetic tags. Fraternal twins, who are the product of separate fertilization events, begin with slightly different epigenetic signatures, Petronis says.

Craig and his colleagues found that some of the greatest epigenetic differences between twins were near genes involved in development and metabolism. Birth weight was associated with differing epigenetic tags on genes involved in metabolism, growth and cardiovascular disease. Low birth weight has previously been shown to be a risk factor for obesity and heart disease in adults.

Factors in the womb that also influence how well fetuses grow, such as the size of the umbilical cord, may create epigenetic discrepancies between twins, Craig says. In the study, identical twins who shared a placenta were more epigenetically different than twins who each had their own, probably because one twin got slightly more nutrition from the single source, he says.

Petronis says it is too early to tell whether birth weight sets epigenetic marks or if the DNA tags influence birth weight.

Either way, says Karen Lillycrop, an epigeneticist at the University of Southampton in England, “the evidence suggests that in terms of metabolism these epigenetic changes can have very long-term effects.”