Often, a person only interacts with genetics when it impacts their health or that of a loved one, so it’s natural that an important finding related to the genetics of attention deficit/hyperactivity disorder (ADHD) brought a new group of interested parties to the table. After all, ADHD is estimated to affect 5% of all children worldwide and 2% of adults.
Headlines describing advances in genetics often use phrases like “identified the gene that causes…” or “therapy to correct the disease-causing gene …” The underlying genetics of ADHD appear to be much more complicated than that. The results of this study reinforce the magnitude of that complexity
In some cases, genetic tests can offer answers that elude patients for years. These patients often have what are known as “Mendelian disorders,” where a change in a single gene is sufficient to cause the ailment. However, most common diseases (think heart disease, neurodegenerative or psychiatric disorders) are known as “complex diseases,” meaning there can be dozens if not hundreds of genetic contributors, plus environmental factors.
That doesn’t mean we don’t know anything about the genetics of complex disease. We just can’t assign responsibility to a single gene or imply that everyone with the disease has the same set of genetic risk factors.
ADHD fits firmly in the camp of “complex disease.” A recent study in Nature Genetics details the discovery of the first “significant risk loci” for ADHD. What that means is that scientists have identified a list of places in the genome that, when altered, are linked to a risk of developing ADHD.
This was a massive study, involving data from more than 50,000 people from all over the world. It identified 304 suspicious variants at 12 different locations in the genome; that’s 304 different changes in the genetic code that show up more in patients with ADHD than those without.
Our Brain, Our World
One of the most important takeaways from the study is that the implicated regions include or are nearby genes we already knew were related to healthy brain function. In addition, many of the locations have also been linked to intellectual disability, schizophrenia, depression and other other conditions. In genetics, you often look to link genes to “phenotypes,” which are physical manifestations — think symptoms of a disease or characteristics in the body. In this study, researchers found genetic overlap between ADHD and 43 other phenotypes, from major depressive disorder to insomnia to smoking habits. Many of these have been previously associated with patients with ADHD, suggesting a shared genetic basis.
While these genetic locations represent the first common risk factors for ADHD, they frustratingly capture only a fraction of the expected genetic variants. Further study is needed to uncover rarer genetic changes, as well as environmental risks.
This illustrates the primary difficulty of identifying the genetic contributors to disorders like ADHD. The concept of a single ADHD gene is a common public misconception. There are likely dozens, if not hundreds of genetic and environmental risk factors, and each affected individual has a different combination of risks. Additional changes at other genes lead to the presence or absence of other phenotypes.
These results can broaden the ways we think about treatment for these conditions, as a wide variety of mental disorders appear to derive their genetic underpinnings from similar sets of genes. Understanding the relationship between these disorders could nuance our understanding of their treatment as well. Over time, therapies — both psychological and medical — may be shaped in part by an understanding of the underlying genetic factors..
Pharmacogenomic testing can help prioritize potential treatments on the front-end of the process. Mental health issues — from ADHD to depression — are known to have a wide array of applicable drugs with results that vary dramatically from patient to patient, even if those patients have largely matching symptoms. Genetic tests can sometimes reveal which drugs a patient will respond to, what side effects a treatment might lead to for them and which medicines might have little effect at all.
We’re constantly adding to our understanding of the incredibly complex set of variants that result in disorders like ADHD. These same genetic contributors seem broadly linked to many different parts of our intellectual development and psychological experience. Through these studies, we glimpse the genetics of both our individuality and our common heritage, and gain some insight to help with diagnosis, prognosis and treatment.
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