Identical twins start to differ in the womb, study shows

Dr Rebecca Hill Progress Educational Trust BioNews, London Despite sharing the same womb, identical twins are born with different alterations to their DNA that can affect the activity of individual genes. These modifications, known as epigenetic markers, are thought to be caused by environmental factors. The process adds chemical tags to the DNA, which alters a gene's activity, but not its sequence. Although previous studies have shown that identical twins do have different sets of epigenetic markers, it was thought that the changes occurred after birth, when the twins experience different environments. However, in this, the first analysis of the epigenetic profiles of newborn human babies, the team from Murdoch Children's Research Institute, Australia show that differences are already apparent straight after birth. 'Twins, like the rest of us, sit in their own amniotic sac and have their own individual experiences', lead researcher Dr Jeffrey Craig told International Business Times. The study, published in Genome Research, suggests that even small differences in womb environment, such as availability of nutrients or the influence of the placenta and umbilical cord, could be responsible. Dr Craig added: 'Sometimes one placenta could be in the best place in the womb, while the other twin might be shunted off to the side somewhere'. The results also showed that twins who shared a placenta were even more likely to be epigenetically different, potentially because the twins would have had to share the same source of nutrition, and so one would potentially get more than the other. The study, which analysed the umbilical cords, cord blood and placentas from 22 identical and 12 non-identical pairs of twins, showed that the epigenetic profiles of identical twins were more similar than those of non-identical twins. Additionally, differences in birth weight between the twins corresponded to differences in epigenetic markers on genes known to be associated with metabolism, growth and heart disease. Dr Karen Lillycrop, an epigeneticist at the University of Southampton, told Science News that current evidence suggests that 'in terms of metabolism these epigenetic changes can have very long-term effects'. Co-author Dr Richard Saffery hopes the work will add to our understanding of how epigenetic changes influence future health: 'This has potential to identify and track disease risk early in life, or even to modify risk through specific environmental or dietary interventions'.

Every sperm is special

Every sperm is special, genome sequencing shows

Antony Blackburn-Starza Progress Educational Trust 2 August 2012 Results of the first study to sequence the genomes of individual sperm cells obtained from one person have revealed significant genetic differences between them, confirming the belief that each sperm is unique. It is hoped the technique could be applied in fertility treatments to identify genetic mutations that may occur in the recombination process. 'For the first time we were able to generate an individual recombination map and mutation rate for each of several sperm from one person', said Professor Barry Behr from Stanford University in California, USA, who was involved in the study. 'Now we can look at a particular individual, make some calls about what they would likely contribute genetically to an embryo, and perhaps even diagnose or detect potential problems'. The team overcame significant technical obstacles to sequence each individual sperm cell, and by using a novel technique the researchers managed to separate 91 sperm cells and amplify their DNA for sequencing. The results showed that recombination - a naturally occurring process whereby chromosomes in the sperm are shuffled around during meiosis - was observed in unexpected places, suggesting this shuffling process is unique in each cell. The findings could explain why siblings have significant genetic differences between them, confirming the recombination process as being partly responsible for genetic diversity. Whereas previous studies have only looked at recombination across populations, this was the first study to observe the process in individuals. The team now hopes its findings can be applied to fertility treatments, with some commentators saying the technique could also be used to study cancer. Study leader Stephen Quake, professor of bioengineering at Stanford University, said: 'People have difficulty conceiving children due to reproductive disorders, and this will provide a very effective way to analyse when there are problems with their sperm'. Jianbin Wang, a member of Professor Quake's team, explained the technique could also be used to select eggs for IVF. 'We can use those cells to analyse an egg's genome, and screen for genetic diseases', he said. This would be an improvement on existing, more difficult techniques, explained the New Scientist. 'Our technique may make the process easier', said Wang. The research was published in the journal Cell.