Identical siblings are used to sharing a lot with their twin, including their DNA. But new research suggests that they also share a distinct marker of their twin status, not encoded within their DNA, but rather on it. This marker is found within the epigenome, consisting of chemical tags along the DNA that regulate gene activity without changing the genetic sequence. Researchers discovered that identical twins universally exhibit a common set of these markers, which remains consistent from birth through adulthood.
The shared epigenetic markers can help identify individuals who were conceived as identical twins but lost their sibling in the womb or were separated at birth. This research lays the groundwork for understanding the process of monozygotic twinning, which remains a mystery despite its long-standing fascination. Identical twins form when a fertilized egg splits into two embryos, a process with unknown triggers. Jenny van Dongen, an epigeneticist at Vrije Universiteit Amsterdam, says the biological process that generates twins “is an enigma.”
During early development, both twins and non-twins undergo numerous epigenetic changes that activate or deactivate genes as the embryo forms. Some of these changes might explain minor distinctions between identical twins. So to better understand what makes a zygote split to form identical twins, “it makes sense to look at epigenetics,” van Dongen says.
By analyzing over 450,000 genomic sites in nearly 6,000 monozygotic and dizygotic twins, they compared identical twins with fraternal ones, eliminating influences from shared womb experiences. They identified 834 spots where identical twins exhibited remarkable similarity in epigenetic marks, particularly in centromere and telomere regions and near genes governing early developmental processes, like cell adhesion regulation. The health implications of these differences remain uncertain. These shared epigenetic patterns were consistent across twins of various ages and geographical locations- and even in different cell types. The researchers developed a test with an 80 percent accuracy rate to identify identical twins, including those affected by vanishing twin syndrome or separated at birth.
“This is a very, very important finding that opens up a lot of avenues of inquiry,” says Segal, the developmental psychologist. For example, identical twins are predisposed for a variety of conditions, from left-handedness to certain congenital disorders such as spina bifida, where the spine fails to develop properly. Perhaps, for some portion of people, these conditions stem from being an unknown identical twin, she says.
Genetic research, and quite frankly all research, regarding twins always seems so interesting! It's quite cool how twins have piqued interest across cultures and traditions throughout history- and it’s even cooler to see how science develops deeper into understanding twins and the biological mechanisms behind it.
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