Attention-deficit hyperactivity disorder (ADHD) is a behavioural disorder estimated to affect between 3% and 9% of British school children (NHS Choices website). Common symptoms include a short attention span, restlessness and constant fidgeting. The causes of ADHD are poorly understood, but there are believed to be a number of risk factors including exposure to toxins during pregnancy, excessive exposure to television, food intolerance and brain function and anatomy.
ADHD tends to run in families, and many people believe that genetic factors are most likely to be the cause of the condition. However, until now there has been no firm evidence of any specific genetic factors associated with the condition.
Treatment for ADHD often involves a combination of medication and behaviour therapy and counselling. Medication is usually by psychostimulant drugs such as methylphenidate, whose mechanism of action is not entirely clear.
Copy number variation
Copy number variants (CNV) are large portions of DNA which may be duplicated or missing within the chromosomes of an individual. There are two copies of most genes in the DNA of most people; however during the Human Genome Project, it was discovered that there is variation between individuals resulting in some portions of DNA being duplicated in some cases and missing in others. This could mean an individual has three copies of a particular gene, or just one. Portions of DNA which are susceptible to this kind of variation are called CNVs.
CNVs have now been linked with a variety of genetic conditions, and there is a database of these associations available to medical researchers. CNVs have previously been linked with schizophrenia and autism.
The link with ADHD
Research published in The Lancet suggests that CNVs may be linked with ADHD, thus strengthening views that the condition has genetic as well as environmental risk factors. Large and rare CNVs were found in 15% of subjects with ADHD, compared with 7% in the control group. CNVs previously associated with schizophrenia and autism were also elevated in the subjects with ADHD. These findings were replicated in a second study group in Iceland.
Implications of this research
This study provides good evidence of a potential genetic risk factor for ADHD. Many genetic conditions are influenced by a wide range of genetic factors rather than a single gene, and it is likely that ADHD is one of these cases.
There is currently a great deal of stigma surrounding ADHD with many people believing poor parenting and other avoidable environmental factors are mostly to blame. As well as opening up new avenues for further research, this study's findings will add credence to claims of a genetic predisposition. However, since the CNVs identified are not universally found among ADHD sufferers this cannot be considered to be a genetic cause for the condition. Rather, the discovery should direct researchers towards understanding the condition better.
Sources
N M Wiiliams, I Zaharieva, A Martin et al " Rare chromosomal deletions and duplications in attention-deficit hyperactivity disorder: a genome wide analysis " Lancet 30 September 2010
DECIPHER "Database or Chromosomal Imbalances and Phenotype in Humans Using Ensembl Resources" Wellcome Trust Sanger Institute
P Farley "Copy Number Variations in Schizophrenia: Rare But Powerful" Schizophrenia Research Forum
D Pinto, A T Pagnamenta, L Klei et al "Functional impact of global rare copy number variation in autism spectrum disorders" Nature 2010 466; 368-372
"What is Copy Number Variation" Gene Quantification
Veronica Mitchell - Veronica studied veterinary medicine at Cambridge University before becoming a secondary school science teacher. She enjoys writing about ...
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