Twin studies can help geneticists determine whether some trait or condition is caused by the environment, or by genetics. Since identical twins carry basically the same genome, if a trait is due to genetics, both twins will exhibit that trait, such as eye color; but if a trait is affected by the environment, twins will not always have it in common, like depression. The degree to which the environment plays a role can also be reflected in what proportion of identical twins share a trait.
Scientists wanted to know more about how genes can affect an individual's sensitivity to their environment, and how much that sensitivity can influence mental health traits including symptoms of autism, anxiety, ADHD, depression, neuroticism, and psychosis. The work showed that genes can make people more sensitive to their environments, and that some of those sensitivity-affecting genes are related to the risk of developing some of these disorders. The findings have been reported in Nature Human Behavior.
"Differences in individuals' sensitivity to life experiences can explain why the same negative or positive experiences may have varying effects on people's mental health, depending on their genetic makeup," explained first study author Dr. Elham Assary, a postdoctoral researcher at King's College London's Institute of Psychiatry, Psychology & Neuroscience (IoPPN). "Our findings suggest that specific genetic variants influence how environmental exposures impact psychiatric and neurodevelopmental symptoms."
This work analyzed genetic and health data from 10,896 pairs of identical twins (21,792 people), from eleven research studies to look for variations in genetic sequences that were associated with sensitivity to the environment. The interplay between these genetic variants and environmental conditions that people are exposed to may help explain why some people may be more or less susceptible to developing certain mental health conditions.
If a pair of identical twins is more sensitive to something in the environment compared to other twins, the sensitive twin pair may become more unlike their twin through their individual, unique, life experiences, compared to the less sensitive twin pair. The researchers used this approach to find genes that could explain some variations in the rates of anxiety, autism, depression, neuroticism, and psychotic-like experiences, through differences in environmental sensitivity.
This work suggested that genes associated with growth factors, which are related to the central nervous system, neurodevelopment, and immunity were linked to variation in autistic traits. Genes connected to stress sensitivity were linked to variations in depression risk. Genes that help control stress hormones known as catecholamines were associated with variations in psychotic-like experiences.
"These findings confirm that genes influence psychiatric and neurodevelopmental traits partly through affecting how people respond to the world around them," said co-senior study author Thalia Eley, a Professor at King's IoPPN. "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."
Sources: Kings College London, Nature Human Behavior