A new study shows that half the risk of getting autism is genetic and comes from inheriting common genetic variations from parents.
Th research shows that families that have only one child with autism, there is a 40% chance that the disorder is inherited but there is a 60% chance in families that have two or more children with autism.
The study analyzed variations of genes known as nucleotide polymorphisms in thousands of families that had donated genetic material. Families that had only one child with autism donated material to the Simons Simplex Collection, and families that had two or more children with autism donated genetic material to the Autism Genome Project.
“I can understand the frustration,” said Bernard Devlin, a psychiatry professor at the University of Pittsburgh and the main author of the multi-institution study. “It would be nice to have a simple explanation, and in fact, it’s a cause for concern in the research community, because one way to view it is that autism and other psychiatric disorders are so complex they will be intractable for any kind of drug therapy.
“At the same time,” he said, “there is another point of view — that while there are a large number of genes involved, they do seem to be converging on a few brain systems,” so that drugs that target those parts of the brain may one day be effective.
The purpose of the latest study was not to explore possible treatments, but instead was designed to measure how common genetic traits might contribute to autism.
To understand how autism might be partly inherited and why not all children in a family get it, Mr. Devlin used height as an analogy.
Although there are hundreds of genes that contribute to tallness, scientists know that about 80 percent of a person’s height is inherited. But almost everyone knows of a family with tall parents where at least one child is shorter than average. In those cases, he explained, the child received a different combination of genetic variants from his parents than his siblings did.
Similarly, the child in a family who gets autism may have received as many as 500 genetic variants associated with the disorder from his parents, while his sister who is unaffected may have gotten far fewer.
Even though height within families is mostly inherited, people throughout the world have been getting taller from generation to generation, suggesting that some environmental factors — better diets or lack of pollution — are making an impact. Mr. Devlin used the height analogy to talk about possible environmental contributions to autism.
“Environmental factors could be an important source of risk above and beyond the genetic factors” in autism. It is believed that father’s age–older fathers have been associated with a greater risk for autism in children. There may be other factors such as air pollution or the hormones found in plastics, but these are only possibilities, Devlin said.