For a long time scientists thought that the birth of identical, or monozygotic, twins occurred at random because it rarely runs in families. Earlier this year a group of international scientists finally found an explanation for the existence of identical twins and, as with most scientific discoveries, this discovery will impact other research such as that related to congenital defects.
About every four out of 1000 births all around the world involve identical twins. They are the result of a single fertilized egg cell, called a zygote, splitting into two or more embryos early in its development. Past research into why the single egg cell splits has focused on the analyses of possible genetic predispositions and possible environmental exposures. Neither of which was able to shine a light on the origins of identical twins.
While identical twins remained a mystery, scientists learned a lot about nonidentical (or dizygotic) twins, which do run strongly in families and are therefore likely the result of a genetic influence. New research has now shown that when it comes to monozygotic twins scientists should move their focus away from the DNA of identical twins and towards small chemical marks associated with DNA, namely the levels of methylation.
The building blocks of your DNA are surrounded by control elements, of which methylation is one. These elements determine how genes are tuned and how strongly they are expressed, this is the so-called epigenome. The epigenome controls, for example, how dark or light your hair is without changing your DNA.
Twin registries from the Netherlands, Great Britain, Finland, and Australia participated in this recent study into the origins of identical twins. The researchers measured the level of methylation at over 400,000 sites in the DNA of more than 6,000 twins and found 834 locations in the DNA where the methylation level was different in identical twins compared to the rest of the human population.
The next step to unravelling the mystery of the origin of identical twins will be to discover why their levels of methylation are different. The study has also revealed an unexpected consequence. Because identical twins keep a lifelong molecular signature (the changed methylation levels), we can retrospectively diagnose if a person was conceived as an identical twin or not, and thus whether or not they lost a twin early on in the pregnancy, also known as vanishing twin syndrome.