12% of pregnancies begin with more than one embryo, but in most cases, the embryos mysteriously disappear without a trace. Researchers have now discovered a genetic signature common to identical twins that could lead to a test that shows if you had a twin during intrauterine development.
Without knowing it, you could have been so close to having identical twins that did not develop. Sometimes a twin appears on an ultrasound scan and then disappears, but in other cases, it can be absorbed without a trace. Up to 12% of pregnancies begin with multiple embryos (identical or twins), but only 2% of these result in multiple birth pregnancies. Identical twins resulting from early embryo splitting account for 3 to 4 cases per 1000 births. The reasons for their loss are mostly unknown; it is likely due to a lack of space or nutrients.
Unique epigenetic signature for identical twins
Jenny van Dongen and her colleagues at the University of Amsterdam in the Netherlands have just discovered a unique epigenetic change in identical twins that could lead to a test to find out if you had a twin that disappeared in utero at some point during your development. The researchers scanned the genomes of more than 6,000 identical twins, regular twins, and parents of twins to find the unique “signature” of the twins.
They found no less than 834 DNA sequences with an epigenetic profile that is different in identical twins compared to identical twins. These changes are linked to DNA methylation which is known to directly affect certain cellular mechanisms, including embryogenesis. Studies show that methylation occurs very early (a few days after fertilization) and therefore is transmitted during mitosis to all other daughter cells, regardless of their nature.
Some of the epigenetic changes observed in identical twins make total sense because they directly affect genes involved in cell adhesion. They may contribute to embryo displacement and thus lead to multiple pregnancies. However, other methylations, such as those at the ends of chromosomes, do not play a clear role. “Our study was motivated by the fact that we know very little about why monozygotic twins occur,” says Jenny van Dongen. It is known that in fraternal twins, certain genetic factors increase the probability of developing multiple embryos at the same time. However, the case of identical twins remains a mystery, as neither genetic nor geographical factors seem to influence their distribution.
DNA test to find out if you had lost identical twins.
Because the study involved samples collected at various ages, the team was able to verify that this genetic signature persists over time, which could lead to a test in adulthood to find out if you had a missing twin during pregnancy. However, the reliability of the DNA test is only 70-80%, which would need to be improved further for commercial use. It could, however, give a more accurate estimate of the number of twins that died before birth.
This discovery could also shed light on certain diseases linked to epigenetic changes. For example, Beckwith-Wiedemann syndrome a disease characterized by excessive growth, tumor predisposition, and birth defects is more common in identical twins than in the general population. This may even confirm that certain diseases are only present in identical twins and explain why some children carry these diseases without being born as identical twins (the identical twin died in utero). The study was published in the journal Nature communication.