Scientists Rethink Autism's Roots
A surprising twist from the autism research front: Non-genetic factors may play a larger role in the development of the disorder than previously believed.
Comparisons of the rates of shared autism among identical and fraternal twins had led scientists to estimate the proportion of autism risk attributable to genes to be as high as 90 percent. But a recent analysis by Stanford scientists, in collaboration with colleagues from UC-San Francisco, UC-Davis, the advocacy organization Autism Speaks, the California Department of Public Health and Kaiser Permanente Northern California, downgrades the genetic contribution to about 40 percent.
Because twins share both DNA (100 percent for identical; about 50 percent for fraternal) and an environment, they can help tease apart the relative contributions of genetic and non-genetic factors in a complex disorder like autism. In the largest study of its kind, the researchers examined 192 twin pairs (54 identical; 138 fraternal) where at least one twin had been diagnosed with an autism spectrum disorder.
The findings for identical twins were similar to previous studies: In about 60 percent of the pairs, if one twin had autism, the other one did too. But the real surprise was the degree to which autism was shared by fraternal twins—pointing to something other than genes at work.
"Our work suggests that the role of environmental factors has been underestimated," said Joachim Hallmayer, an associate professor of psychiatry and behavioral sciences at Stanford and first author of the paper, which appeared in the Archives of General Psychiatry.
This amounts to a major reversal for Hallmayer, who for 20 years has worked to identify the genes responsible for the disorder. In 2010 alone, he and his Autism Genome Project colleagues identified hundreds of suspect genes that may be involved in some cases of autism.
"Our research shows us that we need to be studying both genetic and environmental factors as well as how they interact with each other," Hallmayer said. What exactly those environmental factors may be, "that's the multimillion-dollar question."
Antonio Hardan, a colleague who works with autistic patients at Lucile Packard Children's Hospital but was not directly involved in this study, suggested several potential non-genetic contributors to elevated autism risk: older parental age, shorter between-pregnancy interval, lack of prenatal care, low birth weight, multiple birth and obstetric complications. "These factors have been linked to other disorders such as schizophrenia and bipolar disorder. If we're going to think about future directions in research, we should investigate these further."
Other Stanford researchers who contributed to the project include: Linda Lotspeich, a clinical professor of psychiatry and behavioral sciences; Jennifer Phillips, a clinical associate professor of psychiatry and behavioral sciences; and research assistants Sue Cleveland and Andrea Torres.
