Not so similar. Epigenetic patterns can separate twins over time.

Identical Twins Not So Identical

Twins who share the same DNA sequences may still end up looking and acting differently. Biologists haven't been sure what causes these variations. Now researchers report that some of them may be due to environmental influences that switch genes on and off.

The key lies in chemicals that attach to the genes and determine which ones get turned on, or expressed. Like genes themselves, these chemicals, called epigenetic marks, are passed from parent to child, and changes in their patterns can trigger genetically linked diseases such as some cancers. Since twins with identical genes do not always develop the same diseases, Manel Esteller, a geneticist at the Spanish National Cancer Centre in Madrid, wondered if environment might have affected their epigenetic patterns.

Esteller studied 49 sets of identical twins in Europe and recorded background information such as their smoking histories and diets. To check for epigenetic differences, Esteller and his team sequenced the twins' DNA and injected chemicals that identified known epigenetic markers. The researchers found that twins have indistinguishable epigenetic patterns at birth but develop differences later. "I think the CSI [Crime Scene Investigation] will be happy with this finding," says Esteller, explaining that epigenetic analyses could distinguish twins in criminal cases.

Environment seemed to play a critical role: Twins reared apart showed more epigenetic differences than those reared together. A serious illness in one twin seemed to correlate with greater differences between epigenetic profiles. Epigenetics may explain how DNA and the environment interact to give organisms their unique characteristics, says Esteller, who reports his findings online this week in the Proceedings of the National Academy of Sciences.

This study gives a remarkable demonstration of how environment affects our DNA "packaging," says Stephen Baylin, a cancer biologist at John Hopkins School of Medicine in Baltimore, Maryland. It may also herald a new approach to genetic medicine, adds Arturas Petronis, a geneticist at the University of Toronto in Canada. "Despite billions of dollars pumped into [researching] complex diseases, the results are quite modest," Petronis says. "If we shift the emphasis from DNA sequence variation to epigenetics misregulation as a key mechanism of disease origin, we can perhaps better explain the non-Mendelian features" of diseases.

Related sites
Esteller's Cancer Epigenetics Laboratory
Human Epigenome Consortium