What genes are involved in ADHD?

You may be aware that many functions in our body, including production of hormones and other body and brain chemicals, are controlled by specific genes—the molecules of DNA that tell our cells how to develop and behave. You may not, however, have a clear idea of how this really works, and the fact is that scientists did not either until fairly recently. Mapping the human genome has helped determine some of the genes controlling specific functions, but many genes affect body systems in ways that scientists have yet to figure out.

In some cases, multiple genes may be involved in complex interactions to cause an organ or a system to function properly (or improperly, as in the case of ADHD and many other disorders).

Genetic studies of ADHD have focused largely on genes
involved in controlling the neurotransmitter dopamine
(discussed in Question 51). This is logical because medications
that increase dopamine are effective treatments
for ADHD. Furthermore, brain-imaging studies have
identified abnormalities in the dopamine-rich frontal
and striatal regions in individuals with ADHD. In animal
models used to investigate ADHD, “knock-out”
mice—mice missing a gene important for increasing
dopamine—are hyperactive and do not respond to stimulant
treatment. Their dopamine can not be increased,
and they remain hyperactive.

Currently the genes most likely to cause ADHD are
thought to involve dopamine regulation. The dopamine
transporter (DAT) gene is the prime candidate. This
gene regulates the amount of dopamine in the synapse

by determining how much dopamine is reabsorbed
into the presynaptic neurons. In controls, the dopamine
transporter keeps the level of dopamine in the synapse
relatively high. In ADHD, the DAT “overfunctions”
and lowers the level of synaptic dopamine. Stimulants
inhibit DAT. As a result, more dopamine remains in
the synapse. Other possible causal
genes control postsynaptic dopamine receptors. They
affect the sensitivity of the receptors to dopamine. It
may take more dopamine to activate the postsynaptic
receptors in children with ADHD.

So what does this knowledge mean for treating children
with ADHD? First, it may help scientists design
better medications for treating ADHD. They can target
the cause of the neurotransmitter problem. Second,
scientists can work toward treatments, called gene
therapy, that correct the genetic abnormalities by replacing
the abnormal gene. Gene treatment is currently
being tried for a number of serious progressive neurological
disorders.
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