ADHD gene found
If a specific gene is destroyed, it causes an ADHD-like condition, reveals research on mice. That insight could lead to a better medical treatment, say scientists.
Children and young people are finding it harder and harder to be quiet and attentive. At least judging from the statistics, which show that a growing number of children and young people are diagnosed with ADHD. Researchers have yet to determine exactly what causes the attention deficit disorder.
A new Danish study carried out on mice and published in the scientific journal Neuron shows that an ADHD-like condition could be caused by a defect in a specific gene that affects the brain.
“If the gene is defective, it’s 100 percent certain that it will lead to ADHD-like symptoms in mice. But humans also have this gene and it has been suspected that it could be involved in the development of ADHD in people. Our study confirms this,” says the senior researcher behind the study, Professor Anders Nykjær of the Department of Biomedicine at Aarhus University.
The gene prevents the nerve cells in the brain’s reward system from creating the right contacts and communicating properly.
The effect most likely applies to humans
Once we know all the exact chemical processes behind the development of ADHD, it will be much easier to develop a medical treatment that can improve the disturbance in the brain. If ADHD in some cases is caused by the absence of the protein that enables the dopamine cells to communicate, then synthesising this protein for a medical drug could be an option.
Professor Anders Nykjær
It seems highly likely that the effect found in mice will apply to humans, says Professor Albert Gjedde of the Department of Neuroscience and Pharmacology at the University of Copenhagen after reading the study.
“It’s long been assumed that ADHD in people is connected to a disturbance in the dopaminergic system [also know as the reward system]. So when researchers show that this gene contributes to the maintenance of the dopaminergic system in mice, and that gene is found in people, too, it would seem that there may be something to it,” says Gjedde, who carries out research in the brain’s reward system.
“Dopamine is related to learning and rewarding; it motivates us for a certain behaviour and rewards us when we succeed. It is meant to promote behaviour that promotes our welfare and survival. Without dopamine it would be much harder to learn from our surroundings. We wouldn’t know in the same way what meaningful behaviour is.”
“At the same time, the absence of dopamine also causes attention to be more scattered and sporadic. And that’s what the researchers have found in the mice that have the defective gene,” he says.
Reward system motivates our behaviour
Dopamine is a neurotransmitter that is created in the brain’s basic areas and in the gastrointestinal tract.
Neurons in the brain’s dopaminergic system uses the neurotransmitter to send signals back and forth.
In the brain, dopamine influences physical and psychological functioning such as learning, motor control, mood, thought and memory.
The gene in question is called sorcs2, and if this gene is defective, the brain cells do not create an important protein that aids the dopamine cells in reaching each other, and that prevents the cells from communicating with the transmitter substance dopamine, the study shows.
“We could see that this gene is necessary for the cells’ transmitters to stop growing when they reach each other. Only few nerve cells made the right contact. This means that a defect in the sorcs2-gene has a very great effect on the dopaminergic system, and thus on our behaviour,” says Nykjær.
In the study, the Danish team of researchers from Aarhus University supressed the gene at the time when the mice were at an embryonic stage in order to study how this affected the animals’ behaviour. The result showed that the mice were inattentive, hyperactive, and were less structured in their way of moving around than the control group.
According to Gjedde, these are typical signs of a disturbance in the dopaminergic system -- which is also believed to be present in people with ADHD.
Not all ADHD cases are caused by the defect
Even though a defect in the sorcs2-gene is very likely to lead to ADHD, far from all cases of ADHD can be explained by this genetic defect, says Nykjær.
“ADHD is a complex condition and research suggests that it can be caused by genetic predisposition as well as the surroundings and what goes on in early childhood. The question is how many of the people who are diagnosed with ADHD have a defect in this specific gene,” he says.
The team of researchers are hoping that knowledge about the role of the sorcs2-gene in a functional dopaminergic system will contribute to the development of better medical treatments.
“If we know all the exact biological mechanisms behind the development of ADHD, it is likely that it will be much easier to develop a more targeted medical treatment in the long run. If in some cases ADHD is caused by the absence of the protein that enables the dopamine cells to communicate, then treating patients by supplying them with a functional gene could be an option,” says Nykjær.
“Certain ADHD patients have a higher risk of developing schizophrenia and other psychiatric illnesses, and we suspect that these people are to be found among the ADHD patients that have a defect in the sorcs2-gene. In which case supplying a functional gene would be a particularly appealing option.”
Gjedde is also optimistic about the options that research has to offer.
“With gene therapy it may be possible to rectify any defects or deficiencies in the gene and thereby prevent the symptoms of ADHD. But time will tell when there’s been more research in this area,” says Gjedde.
Translated by: Iben Gøtzsche Thiele
- "SorCS2 Regulates Dopaminergic Wiring and Is Processed into an Apoptotic Two-Chain Receptor in Peripheral Glia" DOI: http://dx.doi.org/10.1016/j.neuron.2014.04.022