Outside influences may play a greater role in autism than previously thought, researchers reported Monday in a study certain to re-open the debate over what is causing the condition in so many children.
The study of nearly 200 sets of twins suggests that differences in environment – meaning everything outside the genes – account for 58 percent of autism cases in general and 55 percent of the most severe types.
The study may not be the last word on the issue but it conflicts with other research that shows genetics are the most important factor.
“The environment plays a larger role in autism than we had previously thought. The interplay between genes and the environment is important,” Thomas Lehner, chief of the Genomics Research Branch at the National Institute for Mental Health, said in a telephone interview.
“Autism had been thought to be the most heritable of all neurodevelopmental disorders, with a few small twin studies suggesting a 90 percent link,” added Neil Risch of the University of California, San Francisco, who helped lead the study.
“It turns out the genetic component still plays an important role, but in our study, it was overshadowed by the environmental factors shared by twins.”
U.S. research institutes are focusing on the causes of autism, a term that encompasses conditions ranging from severe disability and mental illness to milder instances such as Asperger’s syndrome. Studies by the U.S. Centers for Disease Control and Prevention show it affects one in 110 children in the United States, or about 730,000 people aged up to 21.
Members of Congress are interested in the debate. The Combating Autism Act of 2006 funds research into the issue and several bipartisan bills reauthorizing the act are in the works, backed by Sens. Richard Durbin, D-Ill., Robert Menendez, D-N.J., Mike Enzi, R-Wyo., and Scott Brown, R-Mass., and Reps. Chris Smith, R-N.J. and Mike Doyle, D-Pa.
Writing in the Archives of General Psychiatry, Hallmayer’s team said influences during pregnancy might be to blame, but their study does not show what the cause is.
They looked at California records on autism and compared them to state birth certificates for children born between 1987 and 2004, then interviewed parents of twins with autism. They found a boy with a fraternal twin with autism spectrum disorder had a 31 percent chance of also having the disorder, while a boy with an identical twin with autism had a 77 percent chance of sharing it.
For girls, who are less likely to have autism, the risk was 50 percent for identical twins of an autism patient and 36 percent of fraternal pairs. Identical twins share all their DNA while fraternal twins are only genetically as close as any sibling.
Either way, a twin had much more risk of autism than someone in the general population. This is what makes the researchers think they may share a risk factor in the womb.
Some parents fear vaccines may be a factor but several big studies have shown this is not the case. Lehner said it will be important to look elsewhere.
“All the research that has been done on vaccines as a potential contributor has come up negative,” said Lehner, who was not involved in the study. “To assume that the vaccines are back in the picture is logically flawed. It has nothing to do with vaccines,” he said.