You are what you eat

Embryonic development comes with a program that is basically written in epigenetics - genes that are no longer needed are turned off for good once they have done their job, such as those needed for growing an arm, and others keep working, Craig says.

"The genes are turned off mainly by DNA methylation and histone modification. We and others now strongly suspect that environmental conditions can affect these genetic changes and push the programming off course and that this happens via epigenetics - sometimes called re-programming or epigenetic de-regulation."

Nutrition is an important aspect of environment, and folate is an example of a dietary component that has been shown to influence DNA methylation and gene expression. For example, studies of famine events reveal that if a foetus receives less food it re-programs its metabolism to deal more efficiently with less food - to expect less so make the most out of everything you get. There is one hypothesis that when the baby is born and gets an excess of food, conditions such as obesity can result. However, the timing and the overall contribution of environmental influences to epigenetic de-regulation remain largely unknown.

There is also some evidence from the famine studies of a transgenerational effect on health - meaning that an individual's health problems are seen in their grandchildren despite improvements in diet and other aspects of environment. The most critical times for the programming seem to be in utero in males and females and just before puberty in men. If the program is disturbed at these crucial points, not only can it re-program your life but the epigenetic changes and any associated health predisposition can pass on to the next generation.

To address their suspicions, Craig and his team are studying a group of women pregnant with twins. Participating mothers are asked to complete a number of questionnaires throughout the pregnancy and submit samples for testing of factors such as nutritional status. It is known that folate and other vitamins as well as omega 3 and 6 affect gene activity, although the precise involvement of epigenetics in these effects is not known.

Further samples are taken at birth, including placenta, cord blood and whole cords, which are very important for analysis of complex diseases. "We are interested in a few things, the simplest being what are the epigenetic differences between identical twins (monozygotic) at birth compared to non-identical twins (dizygotic)," Craig says. "We think there will be some differences, especially in twins with significantly different birth weights. It is hoped that genes exhibiting the largest epigenetic differences within these pairs will provide some clue to the underlying pathways linking low birth weight and risk of chronic disease in later life."

Once all mothers and twins are recruited and tested, differences in several target genes will be examined. A whole-genome microarray approach will be used to assess changes in gene expression (expression array studies are currently underway on same birth-weight twins to get baselines) and DNA methylation state (using a technique called methylation-dependent immunoprecipitation to attain essentially a methylation signature).

"We really do not know quite what to expect until we do it, but we hypothesise that the discordant-weight twins will show some differences, and the epigenetics differences we find may reflect the twins' health in later life," Craig says. "The current plan is to track them to 18 months of age. No other studies have monitored epigenetics changes over time and with different environments."

Ideally, the group would like to follow the twin pairs further into life but this will depend on the first set of results and of course on funding. The group is looking at expression and methylation data for two twin pairs by microarray and RT-PCR analysis to back up any of the interesting microarray data. Already, subtle expression differences are starting to emerge, although the significance of these differences will become apparent only when more twin pairs are born and analysed.