Oct 04 2010

ADHD and Genetics

A new study published in the Lancet purports to show a potential genetic link to ADHD (Attention Deficit Hyperactivity Disorder). While the genetic link found in this study has been somewhat overhyped in the media, the results are interesting.

The main problem with media reporting is that they tend to oversimplify the concept of a genetic disorder. The worst offenders speak of “the gene” for whatever is being discussed, like ADHD. There are purely genetic disorders that are the result of a mutation in a single gene, but more often diseases and disorders that have a genetic component are the complex result of multiple genes and their interaction with the environment. Therefore there is no single gene for ADHD, autism, migraines, obesity or other complex condition.

Saying that there is a “genetic link” to a disorder is more reasonable, but always must be put into context. What this study shows is an increased risk of copy number variants (CNVs) in people with a confirmed diagnosis of ADHD. A CNV is either a deletion or duplication of genetic material. The researchers found that 78 out of 1047 control had such CNVs (7%), while 57 out of 366 subjects with ADHD did (15%). This was a statistically significant increase. Further, CNV were more likely to occur on genes previous associated with both autism and schizophrenia (and therefore likely to be involved in brain development).

The authors conclude:

“Our findings provide genetic evidence of an increased rate of large CNVs in individuals with ADHD and suggest that ADHD is not purely a social construct.”

That conclusion needs to be parsed very carefully – they are saying that this study is evidence that ADHD is not purely social. They are not saying that it is purely or even mostly genetic. In fact, only 15% of subjects with ADHD demonstrated increased CNVs. This study is a proof of concept more than anything, demonstrating that genetic makeup can contribute, at least in some cases, to the clinical syndrome of ADHD.

This makes sense in light of our understanding of similar neurological disorders. Many such disorders are defined clinically by a collection signs and symptoms, which in turn relate to some particular function or functions of the brain. However, there are likely to be many underlying causes (from a neuro-anatomical point of view) of such altered function. In other words, there may be many underlying alterations of brain function that lead to the final common pathway that results in the recognized signs and symptoms. Many neurological syndromes are defined by that final common pathway, not the underlying disorder that led to it.

And so complex syndromes like ADHD, autism, and schizophrenia are likely many underlying disorders all with a similar clinical manifestation that we recognize and name. It is helpful to understand what the nature of the clinical manifestation is, specifically with respect to whether it is mostly a construct of environment or an effect of brain development and hard-wiring.

This study supports the plausibility that ADHD can be a problem of brain development, genetically determined. The fact that only 15% of subjects showed the genetic changes the researchers were looking for does not mean that the other 85% have no genetic link – they could have other genetic changes that this study was not designed to detect.

Previous studies have also shown genetic alterations in some subjects with ADHD (for example). Researchers are also beginning to identify genetic subtypes that may respond better to certain medications.

We are still a long way from understanding the relevant genetics, but it is reasonable to say that genetic research into ADHD has been fruitful and is likely to continue to bear fruit. But the picture that is emerging (as with many other disorders) is that of a complex relationship among many genes and interaction with the environment. This makes it complex to talk about what “causes” ADHD (which itself is likely not a single discrete entity). This further means that accurately reporting about ADHD research requires some nuance and care – something which seems to be on the wane in science journalism.

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