Everyone carries the genes that predispose people to autism, according to a new study published in Nature Genetics, but the results do not mean that everybody is “a little autistic.” The results of the study suggest these genes influence behavior in the wider population and that there is no clear cutoff point to the spectrum of autism. The research suggests it may be better to view social skills on a sliding scale across the population rather than viewing a single person as having or not having a disorder.
“The primary implication is that the line at which we say people are affected or unaffected is arbitrary,” said Dr. Elise Robinson, a lead author on the paper and Harvard University instructor in an interview with The Guardian. “There is no clear objective point either in terms of genetic risk or in terms of behavioral traits, where you can say quite simply or categorically that you’re affected or unaffected. It’s like trying to pick a point where you say someone is tall or not.”
Data and Statistics
Autism spectrum disorder (ASD) is a group of developmental disabilities that can significantly challenge social, behavioral and communication. Estimates of the incidence of autism varies but has increased steadily, according to the Centers for Disease Control and Prevention (CDC), skyrocketing from 1 in 150 children in 2000 to 1 in 68 in 2010.
ASD occurs in all racial, socioeconomic and ethnic groups. The disorder is about five times more common among boys in the United States, affecting about 1 in 42 boys as compared to 1 in 189 girls.
The CDC points to earlier studies discussing the possible genetic roots of ASD. Studies of identical twins show that if one child has ASD, the other will show symptoms about 36 to 95 percent of the time. In cases of non-identical twins, if one has ASD, the other is affected up to 31 percent of the time. Parents of a child with ASD have a 2 to 18 percent chance of having a second child showing signs of the disorder.
The CDC also points to previous studies that show ASD tends to occur more often in people with certain genetic or chromosomal conditions. About one in ten children with autism also have Down syndrome, fragile X syndrome, tuberous sclerosis, or another genetic or chromosomal disorder.
Symptoms of ASD include difficulty with social interaction, impaired communication and repetitive behavior. Learning abilities range widely among those with ASD, with 46 percent of children identified with the disorder demonstrating above average intelligence.
Details of the Study
The researchers used several large ASD consortium and population-based resources to study approximately 38,000 subjects. They compared data from individuals affected by ASD and those without symptoms of the disorder. The team studied polygenic risk factors, which are the cumulative effects of thousands of genetic differences across the genome, to explore the possibility of overlapping genetic factors. They also considered de novo risk factors, which are characteristics not seen in parents, to look at rare genetic variants that produce a large effect.
These data sets clarified the very wide range of ASD-associated traits. The study results imply that the number of children currently diagnosed with clinical depression represent only the “severe tail” of the spectrum.
In other words, there is no real cut-off point on the autism spectrum. Instead, it is a range of complex genetic factors that can affect behavior.
The results of this large international study also suggest genome-wide genetic links between autism spectrum disorders and normal variations in social behavior and adaptive functioning. A small percentage of people have more gene variants, however, and the increased number of gene variants increases the likelihood the individuals will demonstrate social and behavioral trains associated with clinical ASD. The results support previous strong, but indirect evidence that polygenic risk and specific de novo mutations known to contribute to ASD.
In a press release issued by the University of Bristol, researcher Mark Daly, co-director of the Broad Institute's Medical and Population Genetics (MPG) program said that the information helped the scientists “make an incontrovertible case that the genetic risk contributing to autism is genetic risk that exists in all of us, and influences our behavior and social communication.”
While everybody carries some degree of genetic risk for ASD, the effects of this risk are unclear for those without neuropsychiatric symptoms. Having these ASD gene variants does not automatically mean the individual finds social interaction difficult. Furthermore, genetic makeup is not the only reason a person might develop autism-related traits.
The study provides a broader framework on which researchers can examine how ASD develops. In Nature Genetics, the researchers say, “A continuum model should inform the design and interpretation of studies of neuropsychiatric disease biology.”
“We can use behavioural and cognitive data in the general population to untangle the mechanisms through which different types of genetic risk are operating,” said Robinson. “We now have a better path forward in terms of expecting what types of disorders and traits are going to be associated with certain types of genetic risk.”