New autism study implicates environmental factors

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A new study of twins suggests that environmental factors, including conditions in the womb, may be at least as important as genes in autism.

The researchers did not specify what environmental influences might be at work. But other experts said the new study, published online on Monday, marked a significant shift in thinking about the causes of autism, which is now thought to affect at least 1% of the population in the developed world.

“This is a very important study because it confirms that genetic factors are involved in the cause of the disorder,” said Dr. Peter Szatmari, a leading autism researcher who leads the study. child psychiatry and behavioral neuroscience at McMaster University in Ontario. “But that shifts the focus to the possibility that environmental factors could also be very important.”

Just a few decades ago, psychiatrists believed that autism was caused by a lack of maternal warmth. And while that notion has been discarded in favor of genetic explanations, there is growing acceptance that genes don’t tell the whole story, in part because autism rates seem to have risen much faster than our genes cannot evolve.

“I think we now understand that genetic and environmental factors need to be taken seriously,” said Dr. Joachim Hallmayer, associate professor of psychiatry and behavioral sciences at Stanford and lead author of the new study, which is to be published. in the November issue of the Archives of General Psychiatry.

Other experts cited factors such as parental age, multiple pregnancies, low birth weight, and exposure to drugs or maternal infection during pregnancy.

In the new study, the largest of its kind among twins, researchers looked at 192 pairs of identical and fraternal twins whose cases were pulled from California databases. At least one twin from each pair had the classic form of autism, characterized by extreme social withdrawal, communication problems and repetitive behaviors. In many cases, the other twin also had classic autism or a milder “autism spectrum” disorder like Asperger’s syndrome.

Identical twins share 100% of their genes; fraternal twins share 50% of their genes. Thus, comparing autism rates in the two types of twins may allow researchers to gauge the importance of genes in relation to the shared environment.

The study found that autism or autism spectrum disorder occurred in both children in 77% of identical male twins and 50% of identical female twins. As expected, rates in fraternal twins were lower: 31% of males and 36% of females.

But surprisingly, mathematical modeling suggested that only 38% of cases could be attributed to genetic factors, compared to 90% suggested by previous studies.

And even more surprisingly, shared environmental factors seemed to be at work in 58% of cases.

“We, like everyone else, were very surprised because we didn’t expect it to be this high,” said study lead author Neil Risch, a geneticist and epidemiologist at the University of California. in San Francisco.

The rate of autism in two siblings who aren’t twins is much lower, suggesting conditions the twins shared in the womb, rather than what they were exposed to after birth, contributed. to the development of autism.

A second paper, also published early on the journal’s website, found an elevated risk of autism in children whose mothers took a popular type of antidepressant in the year before giving birth. But the authors reassured women taking these drugs – so-called SSRIs like Prozac, Zoloft, Celexa and Lexapro – that the risk was still quite low: 2.1% in children whose mothers used them. in the year preceding childbirth, and 2.3% in the first trimester of pregnancy.

Dr Joseph Coyle, editor of the journal of psychiatry, called the two studies “game changers”.

Clara Lajonchere, author of the twin study and vice president of clinical programs for research and advocacy organization Autism Speaks, said “there will be a lot more emphasis on looking at prenatal and perinatal factors as regarding susceptibility to autism”.

She added: “We not only need to study environmental factors, but the relationship between genes and the environment.”

“For pregnant women or those planning to start a family,” she said, “prenatal care is essential, and if a pregnant woman is on medication, she should work closely with a doctor.”

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