Paris (AFP)
Genetic differences between monozygotic twins, commonly referred to as "identical twins," can appear very early in embryonic development, according to a study released Thursday, which could question how twins help researchers unravel what falls under innate and acquired.
A birth of monozygotes, sometimes also called identical twins, occurs when the egg (or zygote) resulting from fertilization separates into two, thus forming two embryos.
They occupy an important place in research, because it is commonly accepted that they have a similar genetic makeup and that their physical or behavioral differences result from the environment which surrounds them.
The study by Icelandic researchers, published in the journal Nature Genetics, looked at mutations that occur at an early stage of embryonic development.
She concludes that monozygotes have an average of 5.2 different mutations compared to their twin's genome.
In 15% of them, the number of these different early mutations is even "substantial", stress the authors.
According to them, these findings suggest that the role of genetic factors in the differences between monozygotic twins has been underestimated.
“The classic model is to use identical twins to help distinguish the influence of genetics from that of the environment in analyzing disease. So if you take identical twins that were raised separately and the one of them develops autistic disorders, the classic interpretation is that they are caused by the environment, "Kari Stefansson, boss of deCODE genetics, an Icelandic subsidiary of the American pharmaceutical group Amgen, told AFP.
"But this is an extremely dangerous conclusion," because there is the possibility that the disease is due to an early genetic mutation, which occurred in one of the twins but not in the other, he added.
His research team sequenced the complete genome of 387 pairs of monozygotic twins as well as that of their parents, spouses and children, to detect genetic mutations.
When a mutation occurs in the first weeks of the development of the embryo, it tends to be found in all the cells of an individual (somatic cells such as those of muscles, organs, etc. but also reproductive cells or gametes), and to be transmitted to its offspring.
In one of the pairs of twins studied, the researchers detected a mutation in all of the cells of one of them - which means that it is a mutation that occurred at an early stage of the development - but not at all in his twin.
In the cluster of cells resulting from the first divisions of the egg, "one of the twins is the product of the divisions of the cell where the mutation occurred, and only that one", while the other twin formed from the other cells, says Kari Stefansson.
"These mutations are interesting because they allow us to explore the way in which twin pregnancies occur," he says.
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