A recent study shows the causes of autism to be more environmentally influenced than previously thought.
“This is a very significant study because it confirms that genetic factors are involved in the cause of the disorder,” said Dr. Peter Szatmari, a leading autism researcher who is the head of child psychiatry and behavioral neuroscience at McMaster University in Ontario. “But it shifts the focus to the possibility that environmental factors could also be really important.”
Little is known about the causes of autism and as recently as a few decades ago, psychiatrists thought autism was caused by a lack of maternal warmth. While it is currently thought that there are genetic explanations, there has been growing acceptance that genes do not paint the whole picture, partially because incidences of autism appear to be increasing faster than our genes can evolve.
“I think we now understand that both genetic and environmental factors have to be taken seriously,” said Dr. Joachim Hallmayer, an associate professor of psychiatry and behavioral sciences at Stanford and the lead author of the new study, which is to be published in the November issue of Archives of General Psychiatry.
Other experts have cited factors like parental age, multiple pregnancies, low birth weight and exposure to medications or maternal infection during pregnancy.
In Dr Hallmayer’s study, the largest of its kind among twins, looked at 192 pairs of identical and fraternal twins whose cases were drawn from California databases. At least one twin per pair had the classic form of autism. In many cases, the other twin also had classic autism or a milder “autism spectrum” disorder like Asperger’s syndrome.
Identical twins share 100 percent of their genes while fraternal twins share only half of their genes. So comparing autism rates in both types of twins can enable researchers to measure the importance of genes versus shared environment.
The study found autism spectrum disorders occurred in both children in 77 percent of the male identical twins and in 50 percent of the female identical twins. As expected, the rates among fraternal twins were lower: 31 percent of males and 36 percent of females.
The researchers concluded that only 38 percent of the cases could be attributed to genetic factors, compared with the 90 percent suggested by previous studies.
And more surprising still, shared environmental factors appeared to be at work in 58 percent of the cases.
There has been some critique of these findings. “Overall, I think the authors of this study were speculating a lot and their findings go against previous data,” Max Wiznitzer, MD, associate professor of pediatrics and neurology at Case Western Reserve University in Cleveland, Ohio, and pediatric neurologist at Rainbow Babies and Children’s Hospital in Cleveland, told Medscape Medical News.
“There are wide margins of how to apply the information and the authors appear to be leaning in 1 direction when their data could have easily allowed them to go in another,” he added.
“In other words, their conclusions are exceeding what their data can support. It doesn’t support their enthusiastic interpretation of the prominence of environmental factors,” Dr. Wiznitzer asserted.
He pointed to a recent twin study by researchers from Johns Hopkins Medical Institutions.
“Their raw data looked very similar except that they had a higher concordance rate in the twins than this study shows. And they didn’t reach the conclusion that there is a significant environmental factor, which is very interesting,” said Dr. Wiznitzer.
“I don’t think they’re saying anything new. They’re just putting a spin on it. There are a lot of environmental factors out there. But I think all of us would agree that there is still a very strong genetic basis to autism that is different than heritability.”
“I think the big question is this: how do environmental factors interact with the child’s innate genetic infrastructure? Is it just that those with a specific genetic vulnerability are susceptible to the environment? That’s the discussion now,” concluded Dr. Wiznitzer.