How similar are identical twins? This question has occupied scientists for over a century.1 At first, you may not notice any differences. Their hair colour, facial features and height are often indistinguishable. This is explained by their origin: identical twins are derived from one fertilised egg, which ultimately forms two cells carrying the same sequence of genetic information.2 This is stored in the form of deoxyribonucleic acid (DNA) and acts as a code to produce cellular components essential for cell survival. However, despite their mutual origin, differences have been reported between identical twins, such as in their disease susceptibility.3 A rapidly expanding field called epigenetics is providing insight into the molecular basis of some of these differences.4,5 Scientists have identified chemical modifications on DNA and on the proteins that DNA is wrapped around, which do not affect the genetic sequence but alter its interpretation.
Imagine the genetic code as a music score.6 Two violinists are given the same score and are asked to play the notes they see in sequence. A bystander notices that the resulting melodies sound very different. One violinist plays the notes softly, whereas the other accentuates each note heavily, creating a sense of suspense. In the absence of any guidance on how loudly the music should be played, two contrasting interpretations result. Like musical annotations, epigenetic modifications are thought to act as signals, controlling how the DNA sequence is interpreted by the cell’s machinery. If the levels of these signals vary between identical twins, their DNA sequences may be interpreted differently, modulating the levels of specific cellular components. Epigenetic differences could therefore help explain the distinctions observed between these twins.5 But why would identical twins be epigenetically different? External stimuli are thought to affect the levels of epigenetic modifications and therefore contrasting lifestyles, such as diet, exercise or smoking habits, could all play a role in shaping an epigenetic profile that is unique to one twin.
Epigenetics has wider implications on society. A very recent study indicates that these modifications can be inherited, with reports that trauma-induced epigenetic changes in Holocaust survivors are detectable in their children.7 This raises important questions about the impact our experiences and lifestyle choices have on future generations. Great care is taken during pregnancy to ensure that the developing embryo is not exposed to potentially harmful substances such as alcohol. However, perhaps we are responsible for the health of our children well before they are conceived? The last generation solved the structure of DNA,8 leading to major advances in our understanding of human biology, inheritance and disease. Now it is our chance to make a contribution to this field by delving deeper into the intricacies of our DNA modifications and their potential impact on future generations.
- Galton, F. History or Twins. Inquiries into Human Faculty and its Development, Macmillan, 1883.
- Singh, V. Textbook of Clinical Embryology; Elsevier, 2012.
- Poulsen, P.; Esteller, M.; Vaag, A.; Fraga, M. F. Paediatric Research 2007, 61, 38R.
- Flintoft, L. Rev. Genet. 2005, 6, 667.
- Fraga, M. F.; Ballestar, E.; Paz, M. F.; Ropero, S.; Setien, F.; et al. PNAS 2005, 102, 10604.
- Jablonka, E.; Lamb, M. J. Evolution in four dimensions: genetic, epigenetic, behavioral, and symbolic variation in the history of life; MIT Press, 2005.
- Yehuda, R.; Daskalakis, N. P.; Bierer, L. M.; Bader, H. N.; Klengel, T.; et al. Psychiatry (in press).
- Watson, J. D.; Crick, F. H. C. Cold Spring Harbor Symp. Quant. Biol. 1953, 18, 123.