Environmental factors are more important than previously thought in understanding the causes of autism, and equally as important as genes, according to the largest study to date to look at how autism runs in families.
The study also shows that children with a brother or sister with autism are 10 times more likely to develop autism; 3 times if they have a half-brother or sister; and 2 if they have a cousin with autism, providing much needed information for parents and clinicians for assessing individual risk.
The study, which looked at over 2 million people, was led by researchers at King's College London, Karolinska Institutet in Sweden and Mount Sinai in the US, and is published in JAMA today.
Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder defined by impairments in social interaction and communication and the presence of restrictive and repetitive behaviours. The exact causes are unknown but evidence has shown it is likely to include a range of genetic and environmental risk factors.
Using Swedish national health registers, the researchers analysed anonymous data from all 2 million children born in Sweden in between 1982 and 2006, 14,516 of which had a diagnosis of ASD. The researchers analysed pairs of family members: identical and non-identical twins, siblings, maternal and paternal half-siblings and cousins.
The study involved two separate measures of autism risk – heritability, which is the proportion of risk in the population that can be attributed to genetic factors; and Relative Recurrent Risk which measures individual risk for people who have a relative with autism.
Most previous studies have suggested that heritability of autism may be as high as 80-90%, but one study has hinted at a lower estimate. The new study is the largest and most comprehensive to date and estimates heritability of autism to be 50%, with the other 50% explained by non-heritable or environmental factors.
Environmental factors are split into 'shared environments' which are shared between family members (such as family socio-economic status), and 'non-shared environments' which are unique to the individual (such as birth complications or maternal infections or medication during the pre and perinatal period). In this study, factors which are unique to the individual, or 'non-shared environments' were the major source of environmental risk.
Professor Avi Reichenberg, author of the study from Mount Sinai Seaver Center for Autism Research, who led the study whilst at King's College London, says: "Heritability is a population measure, so whilst it does not tell us much about risk at an individual level, it does tell us where to look for causes. We were surprised by our findings as we did not expect the importance of environmental factors in autism to be so strong. Recent research efforts have tended to focus on genes, but it's now clear that we need much more research to focus on identifying what these environmental factors are. In the same way that there are multiple genetic factors to consider, there will likely be many different environmental factors contributing to the development of autism."
In the other part of the study, the researchers looked at individual risk. In the general population, autism affects approximately 1 in 100 children. The researchers found that children with a brother or sister with autism were 10.3 times more likely to develop autism; 3.3-2.9 times if they had a half-brother or sister with autism; and 2.0 times if they had a cousin with autism. There were no differences in relative risk between genders. This is the first study to provide such a comprehensive and far reaching analysis of individual risk extended as far as cousins.
Dr Sven Sandin, author of the study from King's College London and Karolinska, says: "Our study was prompted by a very basic question which parents often ask: 'if I have a child with autism, what is the risk my next child will too?' Our study shows that at an individual level, the risk of autism increases according to how close you are genetically to other relatives with autism. We can now provide accurate information about autism risk which can comfort and guide parents and clinicians in their decisions."
The study was funded by the Swedish Research Council, the US National Institutes of Health and the Beatrice and Samuel A. Seaver Foundation, New York.
For an embargoed copy of the paper or interviews with the author(s), please contact Seil Collins, Press Officer, Institute of Psychiatry, King's College London email@example.com / (+44) 0207 848 5377 / (+44) 07718 697 176
Paper reference: Sandin, S. et al. 'The familial risk of autism' is published in JAMA
About King's College London:
King's College London is one of the top 20 universities in the world (2013/14 QS World University Rankings) and the fourth oldest in England. It is The Sunday Times 'Best University for Graduate Employment 2012/13'. A research-led university based in the heart of London, King's has more than 25,000 students (of whom more than 10,000 are graduate students) from nearly 140 countries, and more than 6,500 employees. King's is in the second phase of a £1 billion redevelopment programme which is transforming its estate.
King's has an outstanding reputation for providing world-class teaching and cutting-edge research. In the 2008 Research Assessment Exercise for British universities, 23 departments were ranked in the top quartile of British universities; over half of our academic staff work in departments that are in the top 10 per cent in the UK in their field and can thus be classed as world leading. The College is in the top seven UK universities for research earnings and has an overall annual income of nearly £554 million.
King's has a particularly distinguished reputation in the humanities, law, the sciences (including a wide range of health areas such as psychiatry, medicine, nursing and dentistry) and social sciences including international affairs. It has played a major role in many of the advances that have shaped modern life, such as the discovery of the structure of DNA and research that led to the development of radio, television, mobile phones and radar.
King's College London and Guy's and St Thomas', King's College Hospital and South London and Maudsley NHS Foundation Trusts are part of King's Health Partners. King's Health Partners Academic Health Sciences Centre (AHSC) is a pioneering global collaboration between one of the world's leading research-led universities and three of London's most successful NHS Foundation Trusts, including leading teaching hospitals and comprehensive mental health services. For more information, visit: http://www.kingshealthpartners.org.
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About Karolinska Institutet:
Karolinska Institutet is one of the world's leading medical universities. It accounts for over 40 per cent of the medical academic research conducted in Sweden and offers the country's broadest range of education in medicine and health sciences. Since 1901 the Nobel Assembly at Karolinska Institutet has selected the Nobel laureates in Physiology or Medicine. More on: ki.se/english.
About the Mount Sinai Health System:
The Mount Sinai Health System is an integrated health system committed to providing distinguished care, conducting transformative research, and advancing biomedical education. Structured around seven member hospital campuses and a single medical school, the Health System has an extensive ambulatory network and a range of inpatient and outpatient services-from community-based facilities to tertiary and quaternary care.
The System includes approximately 6,600 primary and specialty care physicians, 12-minority-owned free-standing ambulatory surgery centers, over 45 ambulatory practices throughout the five boroughs of New York City, Westchester, and Long Island, as well as 31 affiliated community health centers. Physicians are affiliated with the Icahn School of Medicine at Mount Sinai, which is ranked among the top 20 medical schools both in National Institutes of Health funding and by U.S. News & World Report. For more information, visit http://www.mountsinai.org/ or find us on Facebook, Twitter or YouTube.