Environmental Factors May Significantly Impact Autism Risk
It is estimated that about one in 110 American children have an autism spectrum disorder. As we don’t know yet what it the cause of the rise in autism diagnoses, a vast amount of research has been focused on genetic factors. However, the rapid increase in cases cannot be fully explained by genetic aberrations. A new study focuses on environmental factors, saying their contribution could be substantial.
Study Does Not Discount Genetics, but More Research Should Focus on Environmental Factors
Joachim Hallmayer MD from the Stanford University School of Medicine used data from the Caifornia Department of Developmental Services to identify twins where at least one of the children had an autism spectrum disorder (ASD). Diagnosis of autism was verified using the Autism Diagnostic Interview – Revised, a structured parent interview, the Autism Diagnostic Observation Schedule and other tests that determined verbal and nonverbal cognitive abilities.
The final analysis included 54 pairs of identical twins and 238 pairs of fraternal twins, of which 80 were boy/girl. Overall, autism rates among both identical and fraternal twins were higher than in the general population. The likelihood of both children having autism or ASD was higher among identical twins than among fraternal twins, but genetic factors couldn’t account for all of the cases. The researchers’ analysis determined that about 55% could be explained by shared environmental factors versus genetic heritability.
"The fact that both groups have elevated rates suggests that something is making the two groups of twins similar to each other," says Neil Risch, director of the Institute for Human Genetics at University of California San Francisco and senior author on the paper. "Whether it occurs in utero, during childbirth or soon thereafter, we can't differentiate. But it suggests that something environmental is causing the twins to be alike."
The study only modeled risk and did not specify which environmental factors contributed to autism. Other research has implicated factors such as increased maternal and paternal age, low birth weight, multiple pregnancies and medications or infections to which the mother is exposed to during pregnancy.
The study also does not discount genetic factors as a cause of autism. "It's not either-or in terms of genetics or environment," Risch says. "We're not saying autism isn't genetic, because the huge majority of twins don't have autism. Obviously something is priming the risk, and it looks like that may be a genetic predisposition. So a genetic base and environmental factors together may explain autism better."
Halllmayer J, et al "Genetic heritability and shared environmental factors among twin pairs with autism" Arch Gen Psychiatry 2011; DOI: 10.1001/archgenpsychiatry.2011.76.