For years, parents and researchers have wondered whether genes or environment are the true cause of autism.
Geneticists have searched for 15 years for a genetic smoking gun. Many parents, meanwhile, have long believed that the apparent rise in autism means their kids were harmed by something in the environment.
A new study out today in the Archives of General Psychiatry comes down firmly on the side of both arguments, with an extra dollop of environment. There’s no question that genes play a role in autism, the researchers say. But environment matters even more.
Researchers, led by Dr. Joachim Hallmayer, a geneticist at Stanford University, came to this conclusion by studying twins with autism born in California between 1987 and 2004. The condition was overrepresented in fraternal twin pairs (who are genetically like siblings, but have the same environment), suggesting that some factor or factors in the womb, delivery or infants’ early surroundings triggered autism.
Specifically, by working backwards from the number of twins found to have autism, researchers determined that there were more fraternal twins with autism than would be expected by chance or genes alone, meaning that the environment shared by the twins played a significant role in the development of their condition. Fraternal twins share about half their genes, while identical twins share all of them.
The study didn’t look at causes, but other research is exploring whether medications taken during pregnancy, early infections, the parents’ age, a mother’s immune problems, or other factors might lead to autism. Both Hallmayer and Clara Lajonchere, vice president for clinical programs at the advocacy group Autism Speaks, said they don’t think vaccines in infancy or pregnancy play a role, though neither could entirely rule out that possibility.
So, why does it matter whether autism is driven by genes or environment?
Women who are planning to get pregnant certainly want to know if there’s something they can do prevent autism. Knowing how much early environment matters will give researchers a better idea of where to look for possible triggers.
Then, have we wasted 15 years hunting for genes? Hallmayer and Lajonchere said definitely not. Though scientists haven’t found a clear set of genes involved in the condition, they have learned a lot about autism and potentially how to treat it by looking at the genes.
For example, Fragile X, a genetic disorder closely associated with autism, is caused by damage to a single gene called FMR1, which acts as a kind of dimmer switch or brake pedal. By understanding what happens when the gene is damaged, researchers have begun to develop medications to treat the ensuing brain problems. Early indications are that these medications might help others with autism, too, even though they don’t have obvious problems with the FMR1 gene.
In the new study, researchers examined the autism status of 200 pairs of twins. This is the largest study of twin pairs to date, Lajonchere said, and each twin was diagnosed according to the current standards. The study only considered children whose autism was not caused by known genetic factors like those responsible for Fragile X and Rett syndromes.
The database set up for the study will now be used for additional research, Hallmayer and Lajonchere said. They hope to conduct brain scans on study participants to see if they can find anatomical differences between family members with autism and those without. They also want to understand whether the autism symptoms in twins and affected siblings are the same, or whether some develop more serious symptoms and why that might be.
The ultimate goal of the research, of course, is to figure out what in the environment is contributing to the development of autism.
“The scientific community needs to do what it can to accelerate the pace of research in this particular area,” Lajonchere said. “We don’t know which factors [influence autism] and we don’t know the interplay between those environmental factors and genetics.”
Karen Weintraub is a freelance health writer who is working on a book on autism to come out next year.