Identical twins are genetically alike - so why do life experiences affect their mental health differently? New genetic evidence has answers.
An international group of scientists, including researchers from UNSW's Centre for Healthy Brain Ageing (CHeBA), have identified genetic factors that may make some individuals more or less sensitive to the environments they experience.
Published in Nature Human Behaviour, the twin study examined how individuals' varying sensitivity to environmental factors - such as stress, relationships and trauma - can influence levels of ADHD symptoms, autistic traits, anxiety and depression symptoms, psychotic experiences and neuroticism.
The researchers combined data from up to 21,792 identical twins (10,896 pairs) from 11 studies to discover genetic variants linked with environmental sensitivity - that is, the way people react to life experiences and the environment around them. This is the largest genome wide association study (GWAS) of identical twins to date. GWAS is a research approach used to identify genetic variants linked to specific traits.
"This study presents a compelling advance in disentangling the genetic basis of environmental sensitivity and its role in shaping psychological traits," says CHeBA co-author Professor Perminder Sachdev.
In their paper, the researchers identified several genetic factors that were linked to differences in environmental sensitivity within identical twin pairs. They say the interaction between these genetic factors and environmental exposures could explain differences in susceptibility to psychiatric and neurodevelopmental conditions.
Dr Elham Assary, Postdoctoral Researcher at King's IoPPN and first author of the study, says 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 make-up.
Our findings suggest that specific genetic variants influence how environmental exposures impact psychiatric and neurodevelopmental symptoms.
Professor Perminder Sachdev
Nature vs nurture too simplistic
Both genes and life experiences shape a person's characteristics, including the expression of conditions such as depression, anxiety, ADHD and autism. The interaction between genetics and environmental exposures are thought to contribute to diversity in a wide range of traits within all species. But identifying the genes that are involved in this pathway has proved challenging, especially for complex psychological traits.
Prof. Sachdev says the study underscores the importance of moving beyond simplistic nature-versus-nurture models toward a more integrated, dynamic framework.
"In doing so, we begin to understand how genes shape our response to the environment which in turn interacts with other genes to lead to the pathway to mental illness or health," he says.
Identical, or monozygotic, twins are almost 100 per cent genetically identical, meaning that any differences in their characteristics are likely to be due to the environments they each experience. If a monozygotic twin pair carries genes that make them more sensitive to the effects of the range of unique environments they each experience - for example, relationships or traumatic events - they will be more dissimilar to their co-twin, compared to another pair that is less sensitive to these experiences. Using this information, it is possible to scan the genome to identify the genes that impact variations in environmental sensitivity.
Among the genetically identical twins, we discovered genes that explained variations in autistic traits, anxiety, depression, psychotic-like experiences and neuroticism, reflecting heightened environmental sensitivity.
Professor Perminder Sachdev
Genes linked to traits
The researchers found that genes linked with growth factors - biological molecules which play important roles in neurodevelopment, immune function and the central nervous system - were associated with variation in autistic traits. Genes related to reactivity to stress were linked to variation in depression symptoms. Genes involved in regulating catecholamines - a group of hormones involved in response to stress - were linked to variation in psychotic-like experiences.
Professor Thalia Eley, Professor of Developmental Behavioural Genetics at King's IoPPN and joint senior author of the study, says these findings confirm that genes influence psychiatric and neurodevelopmental traits partly through affecting how people respond to the world around them.
Some people are more sensitive to their circumstances, and this can be positive in good circumstances but can make life more challenging for others in stressful circumstances.
Professor Perminder Sachdev
"The findings also suggest that environmental manipulation may be important to prevent the development of mental disorders, and this needs to be tailored to the needs of the individual. Attention to the environment must begin with child rearing practices and continue through the period of development of the individual, taking individual sensitivity into consideration," Prof. Sachdev says.
The study received funding from Wellcome and used twin datasets from around the world: the Danish Twin Registry, Finnish Twin Cohort, Murcia Twin Registry, Netherlands Twin Registry, Older Australian Twins Study, Swedish Twin Registry, Twins Early Development Study, TwinsUK and QIMR Berghofer twin studies.
The study was a collaboration between researchers at the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King's College London, University College London, Queen Mary University of London and 23 universities around the world .
