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Mind

Genes for ADHD, eh?

NeuroskepticBy NeuroskepticOctober 1, 2010 6:10 PM

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The first direct evidence of a genetic link to attention-deficit hyperactivity disorder has been found, a study says.

Wow! That's the headline. What's the real story?

The research was published in The Lancet, and it's brought to you by Wilson et al from Cardiff University: Rare chromosomal deletions and duplications in attention-deficit hyperactivity disorder.

The authors looked at copy-number variations (CNVs) in 410 children with ADHD, compared to 1156 healthy controls. A CNV is simply a catch-all term for when a large chunk of DNA is either missing ("deletions") or repeated ("duplications"), compared to normal human DNA. CNVs are extremely common - we all have a handful - and recently there's been loads of interest in them as possible causes for psychiatric disorders.

What happened? Out of everyone with high quality data available, 15.6% of the ADHD kids had at least one large, rare CNV, compared to 7.5% of the controls. CNVs were especially common in children with ADHD who also suffered mental retardation (defined as having an IQ less than 70) - 36% of this group carried at least one CNV. However, the rate was still elevated in those with normal IQs (11%).

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A CNV could occur anywhere in the genome, and obviously what it does depends on where it is - which genes are deleted, or duplicated. Some CNVs don't cause any problems, presumably because they don't disrupt any important stuff.

The ADHD variants were very likely to affect genes which had been previously linked to either autism, or schizophrenia. In fact, no less than 6 of the ADHD kids carried the same 16p13.11 duplication, which has been found in schizophrenic patients too.

So...what does this mean? Well, the news has been full of talking heads only too willing to tell us. Pop-psychologist Oliver James was on top form - by his standards - making a comment which was reasonably sensible, and only involved one error:

Only 57 out of the 366 children with ADHD had the genetic variant supposed to be a cause of the illness. That would suggest that other factors are the main cause in the vast majority of cases. Genes hardly explain at all why some kids have ADHD and not others.

Well, there was no single genetic variant, there were lots. Plus, unusual CNVs were also carried by 7% of controls, so the "extra" mutations presumably only account for 7-8%. James also accused The Lancet of "massive spin" in describing the findings. While you can see his point, given that James's own output nowadays consists mostly of a Guardian column in which he routinely over/misinterprets papers, this is a bit rich.

The authors say that

the findings allow us to refute the hypothesis that ADHD is purely a social construct, which has important clinical and social implications for affected children and their families.

But they've actually proven that "ADHD" is a social construct. Yes, they've found that certain genetic variants are correlated with certain symptoms. Now we know that, say, 16p13.11-duplication-syndrome is a disease, and that its symptoms include (but aren't limited to) attention deficit and hyperactivity. But that doesn't tell us anything about all the other kids who are currently diagnosed with "ADHD", the ones who don't have that mutation.

"ADHD" is evidently an umbrella term for many different diseases, of which 16p13.11-duplication-syndrome is one. One day, when we know the causes of all cases of attention deficit and hyperactivity symptoms, the term "ADHD" will become extinct. There'll just be "X-duplication-syndrome", "Y-deletion-syndrome" and (because it's not all about genes) "Z-exposure-syndrome".

When I say that "ADHD" is a social construct, I don't mean that people with ADHD aren't ill. "Cancer" is also a social construct, a catch-all term for hundreds of diseases. The diseases are all too real, but the concept "cancer" is not necessarily a helpful one. It leads people to talk about Finding The Cure for Cancer, for example, which will never happen. A lot of cancers are already curable. One day, they might all be curable. But they'll be different cures.

So the fact that some cases of "ADHD" are caused by large rare genetic mutations, doesn't prove that the other cases are genetic. They might or might not be - for one thing, this study only looked at large mutations, affecting at least 500,000 bases. Given that even a deletion or insertion of just one base in the wrong place could completely screw up a gene, these could be just the tip of the iceberg.

But the other problem with claiming that this study shows "a genetic basis for ADHD" is that the variants overlapped with the ones that have recently been linked to autism, and schizophrenia. In other words, these genes don't so much cause ADHD, as protect against all kinds of problems, if you have the right variants.

If you don't, you might get ADHD, but you might get something else, or nothing, depending on... we don't know. Other genes and the environment, presumably. But "7% of cases of ADHD associated with mutations that also cause other stuff" wouldn't be a very good headline...

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N. M. Williams et al (2010). Rare chromosomal deletions and duplications in attention deficit hyperactivity disorder: a genome-wide analysis The Lancet

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