For the first time, researchers have conducted a large genetic analysis of attention deficit hyperactivity disorder (ADHD), which is a condition that reportedly affects around 6 million children in the United States.

Benjamin M. Neale from the Harvard Medical School in Boston, MA, Anders D. Børglum from Aarhus University in Denmark, and Stephen V. Faraone from the State University of New York led the international team working on this research.

As the authors explain in their paper, ADHD is “a highly heritable childhood behavioral disorder” that affects 5 percent of children in the U.S. but also 2.5 percent of adults.

As it is a “highly heritable” disorder, there are numerous genetic variants that raise the risk of ADHD. Although researchers believe that 74 percent of ADHD risk is genetic, they have not yet firmly linked any genes with the disorder.

In this context, Neale’s team set out to examine the genome of over 50,000 people across the globe, including more than 20,000 people with an ADHD diagnosis. In total, they analyzed about 10 million genetic loci.

Study is first to find 12 genetic locations

Study co-author Bru Cormand, who is also the head of the Research Group on Neurogenetics at the University of Barcelona in Spain, states that the team found common genetic variants, called single nucleotide polymorphisms (SNPs), that account for “21 percent of the total ADHD genetics.”

“In addition,” Cormand continues, “most genetic alterations that were identified are found in regions of the genome that [have remained throughout] evolution, which highlights [their] functional relevance.”

Specifically, the researchers identified 12 genomic segments that could make a person susceptible to ADHD. Many of the genetic changes that ADHD involves affect the expression of certain genes in the brain, say the researchers.

For instance, one of the DNA fragments corresponds to FOXP2 — a gene that plays a key role in human language development. FOXP2 encodes a protein that helps create neural synapses and thus facilitates learning.

The study found a second gene called DUSP6. This gene contributes to the regulation of dopamine, a neurotransmitter that makes learning possible.

Finally, the researchers also identified the SEMA6D gene as one that seems to raise ADHD risk. SEMA6D expression occurs during the development of the embryo, and some researchers believe that it helps develop neural branches.

Overall, the study found that ADHD shares a genetic background with several other psychiatric and non-psychiatric conditions.

“[The] results reveal a genetic overlap between ADHD and major depression, anorexia, level of education, obesity, reproductive success, smoking, or insomnia, among others.”

Bru Cormand

“[T]his study reinforces […] the idea that ADHD is a disorder with a solid biological basis, where genetics mean[s] a lot,” adds the author.

This study is the first to start identifying the specific genes that relate to ADHD risk.

“These results show the importance of promoting large scale-studies — which [are] only possible through big international consortiums — to explore the genetic basis of complex brain diseases,” Cormand concludes.

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