Attention-deficit hyperactivity disorderA vast study seeks to understand the genetic underpinnings of ADHD

TRUE DISORDER or mythical modern ailment? Attention-deficit hyperactivity disorder (ADHD) is one of the most controversial topics in psychiatry. Not for the first time, the internet is brimming with conflicting information. Some deplore a lack of recognition and insufficient diagnoses and treatment. Others denounce overdiagnosis and overtreatment, and argue that the condition is bandied about willy-nilly as an excuse for poor parenting.

Identifying the inherited underpinnings of ADHD has proved challenging. Studies of twins with and without its eponymous symptoms suggest that genetic factors bear between 70% and 80% of the blame for causing it. There is not, though—as there is in the case of (say) colour blindness—a clear gene or genes on which that blame can be pinned. Instead, the presumption is that a range of small and hard-to-detect tweaks in the sequence of DNA letters (known as nucleotides) combine to bring about susceptibility to the disorder. Environmental factors such as social deprivation and low birthweight play a greater or lesser role (or sometimes none at all) depending on exactly which genetic tweaks are present.

Now, a dozen of those tweaks have been identified by what is the first large-scale investigation of the matter. An international consortium of more than 200 geneticists and ADHD experts have published, in Nature Genetics, what is known as a genome-wide association study (GWAS). The tweaks being sought, called single-nucleotide polymorphisms, or SNPs, are the simplest possible differences between two genomes—namely variations between individuals of single genetic letters in particular spots in the DNA sequence. The researchers looked at SNP patterns in the genomes of 55,000 Europeans, more than 20,000 of whom had been diagnosed with ADHD, seeking SNP variants consistently associated with the condition.

Understanding the roles of the genes these dozen SNPs affect will lead to better understanding of ADHD’s causes. Some, for instance, are variations in genes, or the control systems of genes, involved in determining how brains develop in the womb and in early childhood. Others relate to how brain cells communicate with each other. Such understanding may also lead to ideas for new drugs to treat the condition.

Comparing this study with similar ones on what may be related conditions can also be informative. The SNP pattern found, for instance, overlaps with those discovered in GWASs of insomnia. As Stephen Faraone of Upstate Medical University in Syracuse, New York, one of the ADHD study’s leaders, observes, “we’ve known for years that children with ADHD have sleep problems.”

The dozen identified variants do not, on their own, confer the disorder. The findings will not, therefore, lead directly to genetic tests for ADHD. What they do do, though, is dispel the idea that ADHD is merely bad behaviour, or even a mythical condition. And that, of itself, may help to change attitudes towards children who have it, and towards their parents.