They found that autistic people did not have more rare copy-number variants than those without the disorder, but their variants were more often found within genes rather than in the vast amount of DNA located between genes. Specifically, 20% more genes contained a rare copy-number variant in autistic participants in the study. And among genes previously linked to autism spectrum disorder or intellectual disability, 70% more of them contained a rare copy number variant.

Deletions in one region of the X-chromosome, called the DDX53-PTCHD1 locus, were associated with a three-fold higher risk of autism spectrum disorder.

Diagnostic tests based on the work will not be clearcut. Co-author Stephen Scherer of the Hospital for Sick Children in Toronto estimated that genetic clues to the disorder were present in only about ten percent of the families with an autistic member in the Canadian cohort of the study. And, the researchers noted, each patient carried their own unique assortment of copy number variations. Of nearly a thousand variants studied, the most prevalent was still only present in less than 1% of the participants with autism.

But a new test would nevertheless be welcome. At present, diagnosing autism can take months or longer – an agonizing wait for anxious parents that can delay the start of behavioural therapies. Early therapy has been shown in some cases to lessen the effects of the disorder.

Meanwhile, the hunt continues. The consortium has enrolled another 1,500 families, and hopes to use next-generation sequencing to sequence full genomes and exomes (the part of the genome that codes for RNA or protein).

Image: Ingram Publishing