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Identical twins come from an egg that divides and gives rise to two embryos, but during development, a twin sometimes “disappears”, leaving only one baby to be born. Now, a new study suggests your DNA can reveal whether you started out as an identical twin in the womb, even if your twin passed away long before you were born.
In the new study, published Tuesday (September 28) in the journal Nature Communication, the researchers zoomed in on what is called epigenetic modifications found in twin DNA. The term “epigenetics” refers to factors that can turn genes on or off without changing their underlying. DNA sequence. For example, small molecules called methyl groups can attach like sticky notes to specific genes and prevent the cell from reading those genes, effectively turning them off.
The DNA of identical twins is adorned with a characteristic pattern of sticky methyl groups, according to the new study. This model spans 834 genes and can be used to differentiate identical twins from fraternal twins and non-twins, the authors found. And, in fact, based on these results, the team developed a computer algorithm that can reliably identify an identical twin based solely on the location of methyl groups in their DNA.
In theory, such a tool would also be able to spot someone who had had an endangered twin, although the new study did not test this idea.
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Essentially, this methyl group pattern is sort of a “molecular scar” left by the early embryonic development of identical twins, said Robert Waterland, professor of pediatrics and genetics at Baylor College of Medicine who was not involved in the news. study. “The authors discovered an epigenetic signature of monozygous twinning,” meaning a twinning that arises from a single fertilized egg, or zygote, he said.
The genes coated with these methyl groups play various roles in cell development, growth and adhesion, which means that they help cells stick to each other. That said, based on the current study, it’s unclear how these methylated genes, in particular, might influence the growth, development or health of identical twins, Waterland said.
By investigating these early developmental scars, the authors wanted to better understand why an identical pairing occurs in the first place. Scientists know that the zygote divides at some stage of development, but why division sometimes occurs remains a mystery. “[The study] was motivated by the fact that we knew very little about why monozygotic twins occur, ”said first author Jenny van Dongen, assistant professor in the department of biological psychology at Vrije University (VU) in Amsterdam .
An estimated 12% of human pregnancies start as multiple pregnancies, but less than 2% are carried to term, meaning the rest end up in a so-called endangered twin, according to a 1990 report in the International Journal of Fertility and Infertility. Overall, in cases where both twins are coming to term, fraternal twins are usually more common than identical twins.
Evidence suggests that genetic influences the likelihood of a mother bearing fraternal twins, which occurs when two eggs are fertilized at the same time. For example, studies show that sibling twinning can run in families and that the genes involved in hyperovulation appear to be at play, van Dongen said. By comparison, the prevalence of identical twins is fairly constant around the world, occurring in about 3 to 4 in 1,000 births, suggesting that genetics are not the cause. The question is, what is it?
“It really is a mystery in developmental biology,” said lead author Dorret Boomsma, professor in the Department of Biological Psychology at VU Amsterdam.
The team wondered if the solution to this mystery could be encoded in the methyl groups decorating a person’s DNA, since the molecules help control embryonic development in its very early stages. And thanks to special proteins called methyltransferases, the methyl groups added to our developing DNA are copied as our cells continue to divide, meaning they can stay into adulthood.
For the new study, the team extracted epigenetic data from six large cohorts of twins, totaling more than 6,000 people. The cohorts included both identical twins and fraternal twins as well as some non-twin family members of these individuals. By including fraternal twins, the team was able to verify whether the epigenetic patterns seen in identical twins were actually unique to them and not common to all kinds of twins.
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Most of the data on DNA methylation comes from some blood samples taken from adults, but one data set consisted of swab samples from children. And in all of the samples, the team found the same distinct patterns of methylation in the DNA of identical twins.
“The fact that they are seeing the same things in these cells is reassuring,” because it shows that the model is not specific to a cell type, said Waterland. This implies that telltale methylation took place very early in development, before specialized tissues, like the heart Where lungs, began to form. When methyl groups stick to DNA at this point, methyltransferases pass the molecules on to all subsequent daughter cells, regardless of what cell type they ultimately become.
Since some of the datasets included DNA samples collected at multiple points in time, the team was able to verify how stable these methylation patterns were over several years. “They discovered that these methylation states are very stable in an individual,” which further reinforces the idea that these methyl groups could possibly remain from fertilization into adulthood, said Waterland.
“It seems like something happens very early in development and it stays in the methylation pattern of different cell types in our body,” van Dongen said. “It remains archived in our cells.” That said, as of yet, it’s unclear what effect these methyl groups have on gene expression (turning a gene on or off), or whether the methylation pattern represents a cause, an effect. or a by-product of an identical match, it is noted.
“To really understand the exact stages that take place during early embryonic development that lead to the formation of monozygotic twins, we really need functional studies,” van Dongen said, referring to research on how these changes affect actual cells. The team plans to conduct such studies using animal models and human cells in laboratory dishes; they can also use models of the human embryo known as blastoids.
In the future, the team may also study a larger sample of epigenetic genome changes, to see if the methylation pattern extends beyond the roughly 800 genes already identified, Waterland said. The new study has covered hundreds of thousands of potential methyl group sticking points, but there are many more to explore, he said.
Originally posted on Live Science.
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