An international group of researchers has discovered a new phenomenon that occurs in identical twins: independent of their identical genes, they share an additional level of molecular similarity that influences their biological characteristics. The researchers propose a mechanism to explain the extra level of similarity and show that it is associated with risk of cancer in adulthood. The results appear in the journal Genome Biology.
“The characteristics of an individual depend not only on genes inherited from the parents but also on epigenetics, which refers to molecular mechanisms that determine which genes will be turned on or off in different cell types. If we view one’s DNA as the computer hardware, epigenetics is the software that determines what the computer can do,” said senior author Dr. Robert A. Waterland, associate professor of pediatrics — nutrition at the USDA/ARS Children’s Nutrition Research Center and Texas Children’s Hospital and of molecular and human genetics at Baylor College of Medicine.
Epigenetics works by adding or removing chemical tags to genes to mark which ones should be used in different cell types. One of the better studied tags, known to play an important role in development and cancer, is the methyl chemical group. Here, in a large group of identical and fraternal twin pairs, Waterland and his colleagues studied a group of genes called metastable epialleles. Previous work indicated that methyl tags are randomly added to metastable epialleles during early embryonic development and maintained throughout life.
One reason it matters:
“Our findings should prompt a re-evaluation of previous genetic studies on twins,” Waterland said. “For decades, researchers have studied genetically identical twins to estimate what proportion of disease risk is determined by one’s genes. To the extent that epigenetic supersimilarity affects risk of disease, as our results indicate, genetic risk estimates based on twin studies have been inflated.” Paper. (public access) – Timothy E. Van Baak, Cristian Coarfa, Pierre-Antoine Dugué, Giovanni Fiorito, Eleonora Laritsky, Maria S. Baker, Noah J. Kessler, Jianrong Dong, Jack D. Duryea, Matt J. Silver, Ayden Saffari, Andrew M. Prentice, Sophie E. Moore, Akram Ghantous, Michael N. Routledge, Yun Yun Gong, Zdenko Herceg, Paolo Vineis, Gianluca Severi, John L. Hopper, Melissa C. Southey, Graham G. Giles, Roger L. Milne, Robert A. Waterland. Epigenetic supersimilarity of monozygotic twin pairs. Genome Biology, 2018; 19 (1) DOI: 10.1186/s13059-017-1374-0 More.
Twin studies have often been used to demonstrate genetic determinism (it’s in your genes!). But epigenetics is actually shifting and generational, clouding the picture in the present. Stay tuned.
Stephen Fleischfresser at Cosmos Magazine puts it like this:
The team zeroed in on a class of epigenetic markers that are stable and exist in all cell types, called “metastable epialleles” (MEs). The epigenetic variation at MEs is determined randomly and is influenced by many environmental factors, ranging from the nutritional breakdown of the mother’s diet to the season. Consequently, it was expected that levels of epigenetic similarities and differences for MEs would be similar for both identical and fraternal, or non-identical, twins.
But identical twins had the same markers.
The striking finding had a simple explanation: “If, in this group of genes, the epigenetic markers are established before the embryo splits into two, then the markers will be the same in both twins,” says Waterland. More.
See also: There’s a gene for that… or is there?
Epigenetic change: Lamarck, wake up, you’re wanted in the conference room!