Did You Have A 'Vanishing Twin'? Study Sheds Light On Identical Twin Mystery

Some people may not know that they had an identical twin while still in the womb, but a team of researchers has now come up with a test that will determine if someone actually had a "vanishing twin." The researchers' work sheds light on the still-mysterious origins of identical twins.

Identical twinning appears randomly at a rate of four per a thousand births, Vrije Universiteit Amsterdam (VU) noted in a news release. And while it is known that identical twins, also called monozygotic twins (MZ), occur when a fertilized egg, called a zygote, splits into two embryos during development, the exact mechanisms behind it have remained a mystery.

"Overall, as many as 12% of human pregnancies may start as multiple pregnancies, but under 2% carry to term, resulting in a vanishing twin (the contribution of MZ and dizygotic multiples to these numbers is unknown)," the researchers of a new study, published Tuesday in the journal Nature Communications, wrote.

Looking at the epigenome

The researchers focused on the epigenome or the elements around the building blocks of the DNA that determine how the genes are expressed and tuned. These epigenetic markers are not a part of the DNA itself but are "dotted" in many spots on the DNA.

"A useful analogy is how the holding the shift key on a keyboard, can make the letter 'a' become capitalized 'A' allowing another level of regulation on how each letter or number on the keyboard can be displayed," VU explained. "Likewise, DNA methylation (like pressing the shift key) controls which genes are 'on' and which genes are 'off' in each cell of the body."

The researchers looked at 6,000 adult twins from the Netherlands, Great Britain, Finland and Australia. They used fraternal twins (dizygotic twins or DZ) as the control element instead of comparing the identical twins to non-twins to rule out any changes that may have arisen from the "unique prenatal condition."

They looked at the level of methylation at over 400,000 spots in the DNA and found that identical twins actually have a "unique" epigenetic profile. The DNA methylation level was different at 834 locations in identical twins.

"MZ twin correlations of the methylation level of the 834 sites were on average almost three times larger compared to DZ twin correlations," the researchers wrote.

Do you have a vanishing twin?

"These locations in the DNA are involved in functions in early embryonic development," study lead Dr. Jenny van Dongen, of VU, said in the news release. "In addition to insights into the fabrics of monozygotic twins, our results may lead to a better understanding of congenital abnormalities that occur more often in monozygotic twins in the future. A particularly surprising finding in this study is that we can determine from the epigenetic profile of a person whether he/she is an identical twin or has lost a monozygotic twin sibling early in pregnancy, also known as vanishing twin syndrome."

In fact, the marks were so unique that the researchers were able to devise a test that could determine whether someone is an identical twin with up to 80% accuracy, Science News reported.

The findings provide an "important breakthrough" in understanding monozygotic twins. It "remains to be determined" whether the epigenetic signature is exactly a "cause, effect, or by-product" of the monozygotic twinning process, the researchers said.

"This is a very big discovery. The origin and birth of identical twins have always been a complete mystery," study co-author, Dorret Boomsma of the Netherlands Twins Register, said in the VU news release. "This is the first time that we have found a biological marker of this phenomenon in humans. The explanation appears not to lie in the genome, but in its epigenome."

"The next step will be to find out why this happens," study co-author Bruno Reversade, of the Agency for Science, Technology and Research in Singapore, added.