Genetic differences between identical twins can start very early in embryonic development, according to a new study that researchers say has implications for studying the effects of nature versus creation.
Identical – or monozygotic – twins come from a single fertilized egg that splits in two. They are important research objects because they are believed to have minimal genetic differences. This means that when physical or behavioral differences arise, environmental factors are assumed to be the likely cause.
But the new research, published on Thursday in the journal Nature Genetics, suggests that the role of genetic factors in the formation of these differences has been underestimated.
“The classic model has been to use identical twins to help you separate the influence of genetics versus environment in disease analysis,” said Kari Stefansson, co-author of the article and head of Iceland’s genetic deCODE, a US subsidiary pharmaceutical company Amgen.
“So, if you take identical twins raised separately and one of them develops autism, the classic interpretation is that this is caused by the environment.
“But this is an extraordinarily dangerous conclusion,” he said, adding that there was a possibility that the disease was caused by an early genetic mutation in one twin, but not the other.
A mutation means a change in a DNA sequence – a small change that is not inherently good or bad, but can influence physical characteristics or susceptibility to certain diseases.
Jan Dumanski, a geneticist at the University of Uppsala in Sweden, who was not involved in the new article, praised it as “a clear and important contribution” to medical research. “The implication is that we have to be very careful when using twins as a model” to separate the influences of nature and creation, he said.
Stefansson and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track genetic mutations.
They measured the mutations that occurred during embryonic growth and found that identical twins differed by an average of 5.2 initial developmental mutations. In 15% of the twins, the number of divergent mutations was higher.
When a mutation occurred in the first weeks of embryonic development, it would be expected to spread both in the cells of an individual and those of his descendants.
In one of the pairs of twins studied, for example, a mutation was present in every cell in a sibling’s body – meaning that it probably happened very early in development – but not in the other twin.
Stefansson said that of the initial mass that would form the individuals, “one of the twins is made up of the descendants of the cell where the mutation occurred and nothing else”, while the other is not.
“These mutations are interesting because they allow you to start exploring the way twinning happens.”
Given the genetic differences found, the identical term itself can be misleading to describe the brothers. “I’m more inclined to call them monozygotic twins today than identical ones,” said Stefansson.
Previous studies, including a 2008 article in the American Journal of Human Genetics, have identified some genetic differences between identical twins.
The new study goes beyond previous work, including DNA from parents, children and spouses of identical twins. This allowed researchers to identify when genetic mutations occurred in two different types of cells: those present in just one individual and those inherited by that person’s children.
Nancy Segal, a psychologist who studies twins at California State University Fullerton and was not involved in the article, called the research “heroic and really significant.”
“This will force scientists to refine our thinking about the influences of genetics and the environment,” she said. “The twins are very similar, but it is not a perfect resemblance.”
With Agence France-Presse and Associated Press