What can identical twins tell us about the genomics of mental health?
Researchers running the largest ever international twin study have identified genetic markers that might explain why some of us are more susceptible to developing psychiatric conditions
Why might two people respond to the same challenging life experience in completely different ways? Researchers from around the world have come together to find out whether nature or nurture plays more of a role in our susceptibility to mental health problems and neurodevelopmental conditions – and they think that identical twins might hold some answers.
We already know that genetics and the environment both play a key role in mental health, and that different people respond differently to the same environmental pressures. Now, the identification of several genetic markers that are being linked to certain life experience responses could help explain why this might be. In short, the presence of these genetic markers could help identify how susceptible a person is to psychiatric and/or neurodevelopmental conditions.
Why twins?
Identical twins – known as monozygotic twins – result from the fertilisation of a single egg by a single sperm (with the fertilised egg then splitting into two). They share the same genomes and are nearly identical genetically.
The scientific studies that focus specifically on the genomes of identical twins (known as twin studies) are usually looking to understand the roles played by both genetics and the environment when it comes to various traits, behaviours and health conditions. Essentially, because identical twins’ genes are so similar, any phenotypic differences within one pair will most likely be caused by environmental factors. Being able to identify these causes is part of what makes twin studies so useful.
Mental health: Is it nature or nurture?
Researchers from 26 universities around the world, including King’s College London in the UK, the University of Mercia in Spain and the University of New South Wales in Australia, have conducted the largest ever international twin study: a genome-wide association study looking at the phenotypic differences between 10,896 pairs of identical twins of various ages. Specifically, they have investigated the development and severity of psychiatric and neurodevelopmental conditions, including ADHD, autism spectrum disorder, anxiety, depression and psychotic experiences. The findings from the study have been published in Nature Human Behaviour.
During the study, researchers linked a number of genetic markers to a person’s sensitivity to environmental pressures. These markers could explain why two people’s mental health will be affected in vastly different ways by the same life experience. Senior study author Dr Elham Assary, a postdoctoral researcher at King’s College London, says that their findings “suggest that specific genetic variants influence how environmental exposures impact psychiatric and neurodevelopmental symptoms.”
Among the genetic markers identified by researchers were genes encoding growth factors (which were associated with autistic traits), genes linked to stress reactivity (which were associated with depression), genes that help regulate hormones involved in the response to stress (which were associated with psychotic experiences) and a gene that encodes a protein that regulates appetite (which was associated with anxiety).
“These findings confirm that genes influence psychiatric and neurodevelopmental traits partly through affecting how people respond to the world around them,” says Professor Thalia Eley, professor of developmental behavioural genetics at King’s College London and joint senior study author. “Some people are more sensitive to their circumstances, and this can be positive in good circumstances but can make life more challenging than for others in stressful circumstances.”
Study limitations
The researchers acknowledge that there are limitations to their study – a significant one being that the twins were all of European descent, which means that the findings may not be generalisable in a non-European population. This underlines the pressing need for funding to encourage genomic data collection in multi-ancestry twin cohorts.
Where the study’s findings are likely to be most useful is in the development of our understanding of correlations and associations, as opposed to cause and effect. In other words, while the study does not determine whether a person’s life experience responses are actually caused by these genetic markers, it should help us understand how the connection between genetics and environmental pressure response shapes people’s mental health outcomes.
