Scientists Gain New Insight Into Identical Twins
We're all familiar with identical twins. Despite significant progress into most areas of science, exactly what causes identical (monozygotic or MZ) twins is unknown. MZ twins appear randomly in families at a rate of about four per 1000 births. Scientists have investigated genetic predispositions and environmental exposures as a cause for MZ twins, but the studies have yet to determine exactly what leads to MZ twinning.
By comparison, the other type of twins known as nonidentical, or dizygotic (DZ) twins do appear to run strongly within families suggesting a genetic component. Researchers know processes leading to the types of twins are significantly different. DZ twins arise from double ovulation while identical twins come from a single embryo that splits in two.
It's unknown why the single embryo splits in two at this time. Researchers have made a discovery showing that epigenetic information in chromosomes does differ between identical twins and other people. Differences aren't in the DNA code itself but are found in chemical marks associated with it. In the study, conducted by researchers at Boomsma of the Vrije Universiteit Amsterdam (VU), registries of twins from the Netherlands, Great Britain, Finland, and Australia were used.
Researchers measured the level of methylation in over 400,000 sites of DNA in 6000 sets of twins. They discovered 834 locations in DNA where methylation levels differed in identical twins compared to non-twins. Researchers say this is a big discovery and the study is the first time a biological marker for twins has been discovered in humans.
Project scientists say the explanation for identical twins is apparently not in the genome but in the epigenome. The epigenome is described as control elements determining how genes are tuned and how strongly they're expressed located around the building blocks of DNA. Researchers believe the study could lead to an improved understanding of congenital abnormalities that occur more often in MZ twins.