In the largest study of its kind, scientists have identified genetic variants associated with attention-deficit/hyperactivity disorder (ADHD) for the very first time. The researchers have not really found "ADHD genes" but rather a cluster of genetic variations that, taken together, may put a person at risk of developing ADHD, as well as other conditions.
Researchers have been aware for some time that there are genetic components to ADHD since there's a higher chance that the siblings of a person with ADHD will have the condition as well. Until recently, genome-wide analyses failed to come up with any statistically significant associations between gene variants and ADHD. However, thanks to advances in genetic sequencing, researchers were able to gain access to a huge sample size, enabling them to finally tease out 12 different regions of DNA that seem to play a role in ADHD risk.
Stephen Faraone, Anders Børglum, Benjamin Neale, and colleagues analyzed the genomes of over 55,000 individuals, of which 20,000 had ADHD. Their analysis uncovered 304 gene variants that may be involved in ADHD, the authors reported in the journal Nature Genetics.
Each gene variant may make a person slightly more prone to developing ADHD, a complex condition affecting around 1 in 20 children. The tiny risk is negligible on its own, but when you stack up all the potentially problematic gene variants, you end up with a "polygenic risk score" that can be significant. Collectively, common genetic factors accounted for approximately 22% of the risk of ADHD.
These gene variants may put people at risk of a wide range of diseases and conditions. The authors found that 44 different diseases and traits share common genetic signals with ADHD, including depression, anorexia nervosa, and insomnia.
The candidate genes identified by the researchers play various roles, including involvement in synapse formation, speech development, learning and the regulation of dopamine. The researchers also found that diagnosed ADHD appears to share much of the same genetic background as the traits of ADHD, like inattention and fidgetiness, that can be measured in the general population.
"The correlation between these rather different definitions of ADHD suggests that clinically diagnosed ADHD may be the severe end of a continuous distribution of symptoms in the general population," the researchers explained.
The genome-wide association study does not carry any practical implications for people with ADHD -- not yet at least. What it does, however, is open the door to new research that will investigate these candidate gene variants more closely. Studies done in the future, which will certainly include even more people, will be able to identify other genome-wide associations. Ultimately, such work will have a profound impact on the development of new and better therapies that treat ADHD in both children and adults.