Mutant flies confirm genetic link to restless legs syndrome

In a lab in Atlanta, a group of flies is sleeping fitfully. Their naps are fragmented, and their legs are twitching. Their behaviour is uncannily similar to people who have a condition called restless leg syndrome (RLS). When such people are awake, they experience uncomfortable sensations in their limbs that compel them to move to get some relief. Their sleep, which is fragmented and disturbed, is characterised by the same involuntary movements.

There’s a good reason for these similarities. Amanda Freeman from the Emory University School of Medicine has engineered the flies so that they have a faulty copy of BTBD9, a gene that has been linked to RLS in humans. The fact that they show the same constellation of symptoms strongly suggests that this gene is genuinely involved in the condition.

In 2007, two teams of scientists linked BTBD9 to the repeated limb movements that occur during RLS. A single change in the gene’s sequence increased the risk of such movements by more than 50 per cent, and was probably involved in around half of such cases. One of the teams wrote that the discovery “provides evidence that periodic limb movements in sleep is a genuine syndrome” with a detectable genetic basis. That’s important, especially since critics have suggested that many RLS cases are the product of “disease-mongering” by the pharmaceutical industry in order to sell more drugs.

But showing a correlation between a gene and a symptom is just the first step. You also need to work out what the gene is doing and that was unclear. The gene was switched on throughout the brain, but no one really knew what it did. Freeman has gone some way to solving that mystery, and cementing BTBD9’s connection with RLS, by studying fruit flies.

Flies also have a version of BTBD9, which is also switched on throughout the nervous system. When Freeman mutated the gene so it could no longer be used, it affected how the flies slept. (Like use, flies stay still for distinct periods throughout the day, when they become unresponsive to the outside world; if they’re deprived of such bouts, they need more rest later.) Those with inactive copies of BTBD9 slept for the same amount of time as normal flies, but in fragmented bouts.

They also walked more during their sleep, moving their limbs in a way that mirrored the “restlessness” of people with RLS. In the video above, the flies in the blue lanes are normal, while those in the red lanes are the mutants. Note how much more active they are.

“This suggests that the original human studies were pointing in the right direction,” says Subhabrata Sanyal, who led the new research. However, he cautions that “it is too early to say whether this gene does exactly the same things in flies and humans.” The symptoms look superficially similar, but they’re not an exact match. People with RLS also rhythmically flex their feet, something that Sanyal says is “virtually impossible” to see in flies.

On top of that, we still understand very little about RLS as a human condition. Its diagnosis involves a questionnaire rather than a clinical test, and it’s still unclear if it is one syndrome with a consistent set of symptoms, or many. “It is conceivable that not all RLS patients have the same disorder,” says Sanyal.

This is another area where basic science could help. In humans, RLS has been linked to a lack of dopamine (a signalling chemical in the brain), and a deficiency of iron. Freeman found evidence to support both ideas. Her mutant flies had around half as much dopamine in their brains as normal ones, and they slept more soundly once she gave them a dopamine-boosting drug. In human cells, she also found that BTBD9 controls the levels of ferritin, a protein that stores and releases iron.

It’s a start, and Sanyal emphasises that it’s “a tiny step”. He also wants to study the role of the gene in rodents, and he suspects that it is involved in a process called ubiquitination, where small chemicals are attached to proteins to control where they are sent and when they are destroyed. “Studying [BTBD9] in much greater detail is necessary to understand exactly what it does in neurons,” he says.

Research like this might offer more legitimacy to a condition that has been shrouded by debates over pharma industry tactics  (This study received no direct pharmaceutical funding, although one of its several funders—the Restless Legs Foundation— receives support from GlaxoSmithKline.)

“As a basic research scientist, this has always been a valid concern,” says Sanyal. “However, there are patients who have severe RLS where they are unable to get even a single good night’s worth of sleep. These patients cannot sit still, drive a car, travel in a plane or do simple things like attend a meeting or watch a movie. It is hard to ignore that they have had a lifetime of suffering.”

Reference: Freeman, Pranski, Miller, Radmard, Bernhard, Jinnah, Betarbet, Rye & Sanyal. 2012. Report Sleep Fragmentation and Motor Restlessness in a Drosophila Model of Restless Legs Syndrome. Current Biology

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