Thanks to a new study published in Nature Genetics, we’re now a substantial step closer to understanding the genetics of ADHD.
We’ve known that ADHD is highly heritable – one review estimates the heritability as 70-80 percent. Less clear, though, is precisely which genes are involved in determining ADHD risk.
Enter the new study in Nature Genetics. The study was the first genome-wide association study to identify common genetic variants that likely contribute to ADHD risk. A genome-wide association study (GWAS) is, as the name suggests, a type of study that involves scanning the entire genome for genetic variants that are associated with a certain trait or condition – in this case, ADHD.
These studies require very large sample sizes. The current study involved 20,183 people diagnosed with ADHD and 35,191 people without the condition, giving it a scale that I think can appropriately be described as “massive.”
The study found 12 places in the genome that were independently associated with risk for ADHD. It’s important to note that the study focused on common genetic variants – meaning it doesn’t say anything about rare genetic variants that might contribute to ADHD risk. It’s also not a be-all-end-all summary of which common genetic variants determine ADHD risk because other variants could contribute to ADHD risk in ways that are more subtle or complex, or just weren’t captured in the study’s sample of people with ADHD.
That said, the study gives us further reason to believe that common genetic variants are a significant factor in determining people’s risk for ADHD. Just how “common” are these genetic variants? Among the study’s participants, their prevalence ranged from 11 percent to 96 percent.
In other words, having one of these genetic variants is nothing to write home about. But if you have a lot, it significantly ups your chances of having ADHD. Overall, the researchers found that the genetic variants explored in the study explained a little over a fifth of the variance in people’s ADHD risk.
As the researchers highlight, this finding is consistent with the idea that ADHD is, to some extent, an “extreme” expression of traits that are present in the normal population. Almost everyone has some of the risk variants identified in the study. But some people have more of them than others, and people with ADHD tend to have even more of them.
Interestingly, the researchers found that these common variants accounted for some of the genetic overlap between ADHD and other traits. These other traits included smoking, obesity, early death, insomnia, anorexia and depression. In other words, common genetic variants are behind some of the shared risk between ADHD on one hand and other mental health conditions and physical health problems on the other.
A big gap remaining in our knowledge is that while we know these genes seem to be associated with ADHD diagnoses, we don’t know really know why that’s the case. The authors of the paper suggest that an important next step, now that we’ve identified these genetic variants in a genome-wide study, is to hone in on what they do with more narrowly targeted studies. In that sense, the newly published study is a significant step forward partly because it lays the ground for potentially even more important future work on the genetics of ADHD.