Forget nature versus nurture, new research on twins hints at a crack in the conventional view that environment and genes combine to influence our traits.

“Environment makes twins different – that’s part of our traditional paradigm that is pretty well known,” says Art Petronis, a geneticist at the University of Toronto, who led the new study. “Unfortunately, most of the attempts to identify what specific environmental factors make twins different failed.”

Instead, inherited chemical adornments to DNA letters might play a substantial role in shaping individual differences between all people.

Petronis’ team found that “identical” twins, who by definition share DNA, exhibit considerably different patterns of one kind of chemical modification across their genomes. These modifications, known as methylations, can affect gene activation, as well as DNA replication and recombination.


Functional variations

Dubbed the epigenome, scientists have found increasing evidence that such changes underlie susceptibility to cancer, mental illness and, in mice, coat colour.

Researchers, including Petronis, have noticed individual genes and swathes of DNA that seem to differ between identical twins, but this is the first effort to look across the entire human genome.

Using DNA microarrays designed to measure methylation, Petronis’ team compared the patterns in tissues belonging to 57 pairs of identical twins. This method interrogated just 1 or 2% of the total genome, but at sites peppered across the 22 non-sex chromosomes.

“We detected that epigenetic differences are a universal phenomenon. We see them in various locations across the entire genome,” he says.

What’s more, twins spawned from fertilised eggs that separated after just a few cell divisions differed more than twins who split days later. This suggests that some epigenetic divergence occurs very early in embryo development, Petronis says.

Fraternal or non-identical twins share even fewer methylations than identical twins, Petronis’ team found. He theorises that these epigenetic variations are vestiges of methylation patterns initially present in sperm or egg cells.

Developmental differences

His team did not examine the result of these epigenetic changes, but other scientists have little doubt they contribute to traits such as disease susceptibility. “Some fraction of them I’m sure will lead to functional differences,” says Andrew Chess, a biologist at Harvard Medical School.

Randy Jirtle, a geneticist at Duke University Medical Center in Durham, North Carolina, thinks meeting twins in person offers ample proof of the epigenome at work. “Behaviourally they are a little different, but not so much in the way they look,” he says.

The importance of epigenetic changes doesn’t stop with twins. Jirtle speculates that developmental differences between organisms lies principally in epigenetic modifications, not DNA sequences: “If we had the same epigenome as a mouse, we’d have a snout and a long tail.”

Journal reference: Nature Genetics (DOI: 10.1038/ng.286)