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Identical twins start to differ in the womb, study shows

23 July 2012
Appeared in BioNews 666

Despite sharing the same womb, identical twins are born with different alterations to their DNA that can affect the activity of individual genes.

These modifications, known as epigenetic markers, are thought to be caused by environmental factors. The process adds chemical tags to the DNA, which alters a gene's activity, but not its sequence.

Although previous studies have shown that identical twins do have different sets of epigenetic markers, it was thought that the changes occurred after birth, when the twins experience different environments. However, in this, the first analysis of the epigenetic profiles of newborn human babies, the team from Murdoch Children's Research Institute, Australia show that differences are already apparent straight after birth.

'Twins, like the rest of us, sit in their own amniotic sac and have their own individual experiences', lead researcher Dr Jeffrey Craig told International Business Times.

The study, published in Genome Research, suggests that even small differences in womb environment, such as availability of nutrients or the influence of the placenta and umbilical cord, could be responsible.

Dr Craig added: 'Sometimes one placenta could be in the best place in the womb, while the other twin might be shunted off to the side somewhere'.

The results also showed that twins who shared a placenta were even more likely to be epigenetically different, potentially because the twins would have had to share the same source of nutrition, and so one would potentially get more than the other.

The study, which analysed the umbilical cords, cord blood and placentas from 22 identical and 12 non-identical pairs of twins, showed that the epigenetic profiles of identical twins were more similar than those of non-identical twins.

Additionally, differences in birth weight between the twins corresponded to differences in epigenetic markers on genes known to be associated with metabolism, growth and heart disease.

Dr Karen Lillycrop, an epigeneticist at the University of Southampton, told Science News that current evidence suggests that 'in terms of metabolism these epigenetic changes can have very long-term effects'.

Co-author Dr Richard Saffery hopes the work will add to our understanding of how epigenetic changes influence future health: 'This has potential to identify and track disease risk early in life, or even to modify risk through specific environmental or dietary interventions'.

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