Early environment shared by twins contributes to autism risk
For years, scientists have been following twins to help determine the contributions of genetic and non-genetic factors in the development of autism. But until this year, only three small autism twin studies, with just 66 twin pairs total, had been completed. Together they suggested that autism’s development stemmed almost entirely from inherited genes.
When one identical twin developed an autism spectrum disorder (ASD), these studies suggested, the chances were 9 out of 10 that the other twin would do so as well. Identical twins (who arise from the same fertilized egg) share 100 percent of their genes. By contrast, the studies found little or no autism overlap, or “concordance,” between fraternal twins. Fraternal twins arise from different eggs fertilized at the same time and, as a consequence, share about 50 percent of their genes.
Then came the game changer: In July we learned the results of the largest study to directly assess twins with autism (192 twin pairs). It revealed a significantly lower autism concordance between identical twins—just 70 percent. Even more surprising, the researchers discovered a much higher than expected overlap between fraternal twins—around 35 percent. That’s considerably more than the overlap seen among different-age siblings, which numerous studies have shown to be lower than 15 percent.
The conclusion: In the presence of an underlying genetic predisposition, the environment shared by twins—but not different age siblings—appears to significantly affect the risk that a baby will develop autism. In particular, this suggests that autism’s non-genetic, or “environmental,” risk factors involve the environment of the womb—from conception through birth. Further research is needed to pinpoint the nature of these influences and how they affect early brain development.
The large sample size of the study, led by Joachim Hallmayer, M.D., of Stanford University School of Medicine, was made possible by Autism Speaks Autism Genetic Resource Exchange (AGRE) and its volunteer families. The study was also co-funded by Autism Speaks and the National Institute of Mental Health.
Hallmayer J, Cleveland S, Torres A, et al. Genetic heritability and shared environmental factors among twin pairs with autism. Arch Gen Psychiatry. 2011;68(11):1095-102 [Jul 4 Epub ahead of print].