CHICAGO - Environmental factors may play a greater role in autism than previously thought, tipping the scale away from a strict focus on genetics, two studies released on Monday suggest.
In one, a team at Stanford University compared cases of autism in identical and fraternal twins and found that fraternal twins -- who share only half of the same genes -- have unusually high rates of autism, suggesting that factors other than genetics may be triggering the disease.
In another, researchers at health insurer Kaiser Permanente found mothers of children with autism were twice as likely to have been prescribed a common antidepressant during the year before their pregnancy than mothers of healthy children.
And the risk was even greater -- a threefold increase -- when the drug was taken in the first trimester of pregnancy.
The findings, released in the Archives of General Psychiatry, suggest that something in the birth environment -- drugs, chemicals or infections -- may be triggering autism in children who are already genetically predisposed to develop the disease.
"It has been well-established that genetic factors contribute to risk for autism," Clara Lajonchere, a study co-author and vice president of clinical programs for Autism Speaks, said in a statement.
"We now have strong evidence that, on top of genetic heritability, a shared prenatal environment may have a greater than previously realised role in the development of autism."
Autism is a spectrum of disorders ranging from a profound inability to communicate and mental retardation to relatively mild symptoms such as with Asperger's syndrome.
It affects one in every 150 children born today in the United States, or about 1 per cent of the population.
Shared environment, antidepressants
The Stanford study involved 54 pairs of identical twins, who share 100 per cent of the same genes, and 138 pairs of fraternal twins, who share half of the same genes.
In each pair, at least one of the twins had been diagnosed with autism.
The researchers found the chances of both children having autism spectrum disorder was higher among identical twins than among fraternal twins. But fraternal twins were much more likely to develop autism than studies of children in families where a sibling has autism.
According to the study, environmental factors common to twins explain about 55 per cent of the cases of autism, and while genetic factors still play a role, it is much lower than seen in other studies of twins and autism.
"Environmental factors play a bigger role than previously thought," said Dr. Joachim Hallmayer of Stanford University School of Medicine in California, who led the study.
Recent studies have suggested genetics played the biggest role in autism, but his findings suggest something different, he said in a telephone interview.
"We have to study both the genetics and the environment," Hallmayer said. "If we look only at one side, I don't think that will lead us to the right answer."
In a separate study in the same journal, a team led by Lisa Croen, director of the Autism Research Program at the Kaiser Permanente Division of Research in Oakland, California, looked to see whether antidepressants known as selective serotonin reuptake inhibitors, or SSRIs, contributed to autism risk.
The team studied nearly 300 children with autism and 1,500 randomly selected children and then checked their mothers' medical records.
They found mothers of the children with autism were twice as likely to have taken an antidepressant in the year before delivery than children in the control group.
And the effect was strongest -- three times higher -- when the drugs were taken in the first trimester of pregnancy.
"Our results suggest a possible, albeit small, risk to the unborn child associated with in utero exposure to SSRIs," Croen said in a statement.
But she said this risk must be balanced with the risk to the mother of having untreated depression.
The team cautioned that the SSRI study was preliminary and said much more work was needed to understand the link between antidepressants and autism.
"There are real risks to not being treated for a serious illness like depression. You have to weigh the options," said Dr. Thomas Insel, director of the National Institute of Mental Health.
"A threefold increase in risk is not insignificant. It is worth taking that into account with other factors," Insel said in a telephone interview.
Insel, whose agency funded the twins study, said it is not yet clear what environmental factors may be triggering autism.
"It could be a range of things from infection to chemical exposures. We simply don't know."
What is becoming clear, he said, is that the exposure is likely occurring before childbirth.
"From all the studies that so far have concluded, that is where the evidence seems to be going," he said.