One egg cell can produce identical twins. However, two embryos are formed from the same egg cell. During development, one twin "vanishes" and leaves only one baby.
A new study suggests that DNA can reveal whether your twin was identical in conception, even though your twin died long before you were born.
The researchers focused on epigenetic modifications in twin DNA in their new study published in Nature Communications Tuesday, Sept. 28th.
Epigenetic refers to factors that switch genes on or off without altering their underlying DNA sequence. Small molecules known as methyl groups, for example, can stick like sticky notes to certain genes and stop the cell from reading them, effectively turning them off.
The DNA of identical twins is adorned with a distinctive pattern of sticky methyl group, according to the study. The authors discovered that this pattern can be used to distinguish identical twins from fraternal and non-twins. Based on these results, the researchers developed a computer algorithm to reliably identify identical twins based only on their location in the DNA.
Although the new study did not test this theory, it is possible to detect if someone has a vanishing twin.
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This methyl group pattern, which is basically a type of "molecular scar", is what identical twins have left behind from their early embryonic development. Robert Waterland, a Baylor College of Medicine professor of genetics and pediatrics, was not involved with the new study.
He said that the epigenetic signature for monozygotic twining was discovered by the authors. This means that twinning stems from one fertilized egg or zygote.
These methyl groups are responsible for various functions in cell growth, development and adhesion. They help cells stick together. Waterland stated that it is not clear how these methylated gene might impact the health, growth, and development of identical twins based on current research.
The authors sought to understand the reasons for identical twinning by looking at the scars of early development. Scientists know that the zygote splits at certain points in development. However, it is still a mystery why this happens sometimes.
"[The study] was motivated by the fact we knew very little about how monozygotic twins are born," stated Jenny van Dongen (assistant professor, Department of Biological Psychology, Vrije University Amsterdam).
According to an International Journal of Fertility & Sterility report, approximately 12 percent of pregnancies are multiple pregnancies. However, less than 2 percent of pregnancies end up being carried to term.
Fraternal twins are more common in cases where both of the twins survive to term.
There is evidence that genetics can influence a mother's chances of having fraternal twins. This happens when two eggs are fertilized simultaneously. Van Dongen stated that studies have shown that fraternal twins can be a common trait in families, and that the genes responsible for hyperovulation may also play a role.
The prevalence of identical twins in the world is much more consistent than that of the United States. It occurs in approximately 3 to 4 of every 1,000 births. This suggests that the phenomenon is not genetic. What is the answer?
Dorret Boomsma (a senior author and professor at the Department of Biological Psychology, VU Amsterdam) said that "it's really a mystery within developmental biology."
The team wondered whether the solution might lie in the methyl group decorations in a person's DNA. These molecules are crucial in controlling embryonic development from its earliest stages. Special proteins called methyltransferases allow the methyl groups in our DNA to be copied down by our cells as they divide. This allows them to stay with us into adulthood.
The team analyzed epigenetic data from six large cohorts that included more than 6,000 twins for the new study. These cohorts were made up of identical and fraternal twins, as well as non-twin relatives. The fraternal twins were included to allow the team to determine if any epigenetic patterns observed in identical twins are unique and not common across all types of twins.
Similar: Having a Baby: The Stages of Pregnancy by Trimester
The majority of DNA methylation data was derived from blood samples taken from adults. However, one set included cheek swab data from children. The team discovered the same patterns of methylation in identical twin DNA across all samples.
Waterland stated that "the fact that they see similar things in these cells is reassuring" because it shows that the pattern doesn't just apply to one cell type.
This means that the methylation was detected very early in the development process, long before the formation of specialized tissues like the heart and lungs. The methyltransferases transmit the molecules to any daughter cells that methyl groups stick with DNA when this happens.
The team was able to double-check the stability of methylation patterns over many years because some data sets contained DNA samples taken at different times. Waterland stated that they found these methylation states to be very stable in an individual, which strengthens the possibility that these methyl groups could stick around after fertilization.
Van Dongen stated, "It appears that something happens very early in development, and this remains written in methylation patterns of different cell types within our body." It is still preserved in our cells.
She noted that it is not yet clear what effect these methyl groups have upon gene expression (the turning on or off of a gene) or whether the methylation pattern is a cause, effect, or byproduct of identical twinning.
Van Dongen stated that functional studies are needed to understand how cells react to these changes in order to fully comprehend the embryonic development process that leads to monozygotic twins.
These studies will be conducted using human cells and animal models in laboratory dishes. They may also use models of human embryos known as blastoids.
Waterland stated that the team may also be able to survey larger amounts of epigenetic modifications to genomes in the future to determine if the methylation pattern is wider than the 800-odd genes currently identified.
He said that the new study included hundreds of thousands potential sticking points for the methyl group, but that there are many more to be explored.
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