Identical twins are not as similar as they might appear. In fact, according to a recent report in Proceedings of the National Academy of Sciences, the genes expressed by twin siblings can differ markedly. And, as monozygotic twins share a common genotype, these differences are due to epigenetic variation — specifically, differing patterns of histone and DNA modifications.

The study conducted by a multinational team of scientists quantified the global DNA methylation content and histone acetylation levels in samples from 40 pairs of monozygotic twins ranging in age from 3 to 74 years.

Remarkably, 35% of the twin pairs showed significant differences in these epigenetic marks, and the older the twins and the more disparate their lifestyles or medical histories, the greater the differences.

Importantly, a 50-year-old twin pair with the greatest difference in DNA methylation and histone acetylation levels varied markedly in their gene expression profiles, whereas the gene expression profiles of a 3-year-old twin pair were almost identical. This provides some of the strongest evidence so far for the impact of the environment on gene expression.

As lead author Mario Fraga points out, “Most people had the hypothesis that changes in DNA methylation are effected by the environment ... This is the first time that somebody has demonstrated that this is the case.” (news@nature.com, 4 July 2005).

Arturas Petronis, research scientist at the University of Toronto, Canada, comments that the study has proved its worth by “...quantifying how genetically identical individuals could differ in gene expression on a global level due to epigenetics.” (The Scientist, 7 July 2005).

It is hoped that future studies will reveal the mechanisms responsible for these epigenetic differences.