Study of autism downplays DNA

Environmental factors may be more important than genes in determining whether a child develops autism, according to a controversial new analysis of the disorder in twins.

That finding runs counter to decades of prior research, which has generally found that genetic inheritance is the biggest determinant of a child’s risk of autism. The authors of the new study, published online Monday by the journal Archives of General Psychiatry, came to their conclusion after studying 192 pairs of identical and fraternal twins in which at least one twin met clinical criteria for the neurodevelopment disorder.

But the authors’ conclusion that environmental influences – perhaps chemical exposures, infections, diet or stress levels – could be so influential was roundly criticized by other autism experts.

“It’s a massive claim,” said Angelica Ronald, a behavior geneticist at Birkbeck University of London. “It flies in the face of the previous data. I don’t see why the results have come out the way they have.”

The study authors acknowledged that their calculations were subject to a very wide margin of error and thus could be incorrect. Still, they said that the analysis highlights the need for more research into environmental factors that may contribute to autism.

“Genetics don’t explain it,” said coauthor Neil Risch, a genetic epidemiologist at UC San Francisco. “They’re part of the story, but only part of the story.”

Starting in the 1970s, a series of studies looking at twins cast autism as a genetic disorder. They found that if one twin had autism, the odds that the other had it too – a figure known as the concordance rate – depended primarily on how much DNA they shared.

If the twins were identical, the odds were 80 percent or higher. But if the twins were fraternal – and shared only half their DNA, on average – the odds were 10 percent or lower.

Those studies led scientists to zero in on genetics as the main cause of disorder.

In the latest study, researchers used records from the California Department of Developmental Services to identify children labeled with autism. They conducted their own exams on as many of those children as they could, ultimately building a database of 192 twin pairs in which at least one twin has the disorder.

For boys with any form of autism – the biggest group in the sample – the researchers found concordance rates of 77 percent for identical twin pairs and 31 percent for fraternal twin pairs. Those figures were in line with other recent studies.

Then they plugged those figures into a computer model that used statistical methods to account for the contributing roles of genetics, environmental factors that were shared by both twins and other environmental influences that weren’t shared. They calculated that 38 percent of the risk for autism came from genes and 58 percent came from the environment that twins shared.