Autism spectrum disorders include everything from the relatively mild Asperger syndrome (characterized by more mild social and language impairments) to full-fledged autism (characterized by severe social and communicative handicaps, limited interests, and repetitive behaviors).
The Genetics of Autism
Austism is relatively common, and in the United States it is estimated that one out of every 54 boys is affected (the frequency in girls is considerably lower, averaging one affected girl out of every 252).
Although genetics certainly plays a role in these conditions, exactly how is not fully understood, and this newest study, led by Richard Anney (Trinity College of Dublin) and Bernie Devlin (University of Pittsburgh School of Medicine), is the next a long line of scientific inquiries to this point.
Their study, known as the Autism Genome Project (AGP), was conducted in two stages. The first stage of consisted of a genome-wide association study using genetic data from 1400 families affected by autism; the second stage checked the associations discovered in the first stage using the genetic data from an additional 1301 autism-affected families and included another new genome-wide association study which combined the study subjects from both stages.
A Lack of Common SNPs Associated with Autism
When all the analyses were said and done, no SNPs (common genetic variations) were significantly associated with austism. Furthermore, when some of the SNPs that had been identified in the first study as possibly associated with autism were tested in the second-stage families, the associations failed to hold up. This lack of common SNPs associated with autism is both disappointing and enlightening.
Knowledge of what is not true, paradoxically, is knowledge of what is true. For instance, if I tell you that my pet Tyger is not a dog, you are one step closer to knowing Tyger’s a cat. Most of science progresses through “not trues” — the failed hypotheses that bring us closer to real understanding. A perfect example of this mode of scientific progress is this recent genetic study. Their lack of findings was quite a finding.
The fact that no SNPs were associated with autism in such a large study suggests that the effect of any one common SNP is quite small. Even when all the SNPs were used together to predict whether a study subject would have autism, the model explained less than 1 percent of differences in risk for the condition.
Heritability
The fact that these common SNPs have little power to predict autism does not mean that genetics is unimportant.
Twin studies suggest that autism is at least moderately heritable, suggesting that genetics does play a role. This current study’s lack of findings supports the idea that common genetic variations may play a smaller role than rare mutations or copy number variation (the number of copies of a given gene) in autism risk. Thus, the lack of autism associations today has, hopefully, brought us closer to tomorrow’s discoveries.