The largest and most rigorous autism twin study thus far conducted has confirmed what many, researchers, doctors and parents alike, had suspected for some time - that shared environmental influences play a large part in the development of the spectrum of developmental disorders.
In fact the study tips the balance for the first time towards environmental factors being a more important determining factor for a child developing an autism spectrum disorder (ASD) than their inherited genetic make-up.
The study involved a total of 192 pairs of twins drawn from the general population with the study being supported by the National Institutes of Health (NIH) and involving researchers from the Autism Genetic Resource Exchange, California Department of Public Health, Kaiser Permanente, and the University of California, Davis. The research was partially funded by advocacy group Autism Speaks. The findings of the study, known as the California Autism Twins Study (CATS), were reported in the July 2011 issue of the journal Archives of General Psychiatry.
The team of investigators drew upon California state records, initially identifying 1,156 twin pairs, with at least one member affected by an ASD, born between 1987 and 2004. The children were all at least 4 years old and diagnosed using the latest standard in diagnosing autism, which requires structured clinical assessments based on interviews with the parents as well as direct observation of the child. Ultimately, the group of 1,156 pairs of twins was wittled down to 192 as specificinclusion/exclusion criteria were applied. The final study itself involved 54 identical and 138 fraternal twins who were subjected to genetic analysis. Since autism disproportionately affects males, males outnumbered females by four to five times, with 80 of the pairs including both sexes.
The new study is the first to analyse a large sample of twins drawn from the general population; previous twin studies have been based on more limited samples, such as patients in treatment. It is also the first to use the more rigorous diagnostic requirements outlined above.
The outcome of the study may surprise those still firmly convinced that the cause of autism lies purely in a child's genes. The results show that shared environmental factors – experiences and exposures common to both twin individuals – accounted for 55 percent of strict autism and 58 percent of the broader group of ASDs. Genetic heritability accounted for 37 percent of autism and 38 percent of ASD.
"High fraternal twin concordance [both members of a twin pair having the disorder] relative to identical twin concordance underscores the importance of both the environment and moderate genetic heritability in predisposing for autism," explained Joachim Hallmayer, M.D., of Stanford University, Palo Alto, Calif. a grantee of the NIH's National Institute of Mental Health. "Both types of twin pairs are more often [in this study] concordant than what would be expected from the frequency of autism in the general population. However, the high concordance among individuals who share only half their genes relative to those who share all of their genes implies a bigger role for shared environmental factors."
Studies are currently underway to determine if autism may be traceable, in part, to environmental exposures early during pregnancy. It is expected that the developing foetal brain would be most susceptible to environmental influences - whether they be chemicals the mother is exposed to, her diet, or factors such as levels of various hormones circulating in her bloodstream.
NIMH director Thomas R. Insel, M.D. said "These new findings are in line with other recent observations supporting both environmental and genetic contributions to ASD, with the environmental factors likely prenatal and the genetic factors highly complex and sometimes not inherited."
This statement alludes to the role of 'epigenetics' - the ability of environmental factors to influence the expression of individual genes and the genetic makeup of an individual as a whole. Recent evidence also suggests that spontaneous genetic mutations occur at a surprisingly high rate in autism.
This study underlines the complexity of autism and ASDs and importantly will shift more focus to environmental factors in future research.
Source: Hallmayer J Cleveland S Torres A Phillips J Cohen B Torigoe T Miller J Fedele A Collins J Smith K Lotspeich L Croen LA Ozonoff S Lajonchere C Grether JK Risch N (2011) Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism Archives of General Psychiatry [Epub Ahead of print] PMID:21727249