Solving RLS: Largest Genetic Study to Date May Help

For decades, scientists have been trying to unravel the mysteries of restless legs syndrome (RLS), a poorly understood and underdiagnosed neurological disorder causing itching, crawling, and aching sensations in the limbs that can only be relieved with movement.

A sweeping new genetic study, coauthored by an international team of 70 — including the world's leading RLS experts — marks a significant advance in that pursuit. Published on June 5 in Nature Genetics, it is the largest genetic study of the disease to date.

"It's a huge step forward for patients as well as the scientific community," said lead author Juliane Winkelmann, MD, a neurologist and geneticist with the Technical University of Munich, Munich, Germany, who's been studying and treating patients with RLS for 30 years. "We believe it will allow us to better predict the likelihood of developing RLS and investigate new ways to prevent and modify it."

The common condition, affecting about one in 10 adults, was first described centuries ago — by English physician Thomas Willis in the late 1600s. And while we know a lot more about it today — it's familial in about half of all patients and has been linked to iron deficiency, among other conditions — its exact cause remains unknown.

With gold-standard drugs long prescribed to quell symptoms shown in recent years to actually worsen the disorder over time, doctors and patients are hungry for alternatives to treat or prevent the sleep-sabotaging condition.

"The main treatments that everybody continues to use are actually making people worse," said Andrew Berkowski, MD, a Michigan-based neurologist and RLS specialist not involved in the study. These drugs — dopamine agonists such as levodopa and pramipexole — can also potentially cause drug dependence, Berkowski said.

How This Could Lead to New Treatments

In the new study, the group analyzed three genome-wide association studies, collectively including genetic information from 116,647 patients with RLS and more than 1.5 million people without it.

They identified 161 gene regions believed to contribute to RLS, about a dozen of which are already targets for existing drugs for other conditions. Previously, scientists knew of only 22 associated genes.

"It's useful in that it identifies new genes we haven't looked at yet and reinforces the science behind some of the older genes," said Berkowski. "It's given us some ideas for different things we should look into more closely."

Among the top candidates are genes that influence glutamate — a key chemical messenger that helps move signals between nerve cells in the brain.

Several anticonvulsant and anti-seizure drugs, including perampanel, lamotrigine, and gabapentin, target glutamate receptors. And at least one small study has shown perampanel prescribed off-label can improve RLS symptoms.

"Compared to starting at the beginning and developing an entirely new chemical entity, we could run clinical trials using these alternatives in RLS patients," said the study's first author Steven Bell, PhD, an epidemiologist with the University of Cambridge, Cambridge, England.

The study also confirmed the MIES1 gene, which is related to dopamine expression and iron homeostasis, as a key genetic contributor to RLS risk. Low levels of iron in the blood have long been thought to trigger RLS.

The Role of Gene-Environment Interactions

Through additional data analysis, the team confirmed that many of the genes associated with RLS play a role in development of the central nervous system.

"This strongly supports the hypothesis that restless legs syndrome is a neurodevelopmental disorder that develops during the embryo stage but doesn't clinically manifest until later in life," said Winkelmann.

About half of people with RLS report some family history of it.

But not all with a genetic predisposition will develop symptoms.

For instance, the study found that while the same gene regions seem to be associated with risk in both men and women, in practice, RLS is twice as common among women. This suggests that something about women's lives — menstruation, childbirth, metabolism — may switch a preexisting risk into a reality.

"We know that genetic factors play an important role in making people susceptible to the disease," said Winkelmann, "but in the end, it is the interaction between genetic and environmental factors that may lead to its manifestation."

The study also found associations between RLS and depression and suggests that RLS may increase the risk for type 2 diabetes.

Improving RLS Care

A potentially useful tool coming out of the study was a "polygenic risk score," which the researchers developed based on the genes identified. When they tested how accurately the score could predict whether someone would develop RLS within the next 5 years, the model got it right about 90% of the time.

Winkelmann imagines a day when someone could use such a polygenic risk score to flag the high risk for RLS early enough to take action to try to prevent it. More research is necessary to determine precisely what that action would be.

As for treatments, Berkowski thinks it's unlikely that doctors will suddenly begin using existing, glutamate-targeting drugs off-label to treat RLS, as many are prohibitively expensive and wouldn't be covered by insurance. But he's optimistic that the study can spawn new research that could ultimately help fill the treatment gap.

Shalini Paruthi, MD, an adjunct professor at Saint Louis University, St. Louis, Missouri, and chair of the Restless Legs Syndrome Foundation's board of directors, sees another benefit.

"The associations found in this study between RLS and other medical disorders may help patients and their physicians take RLS more seriously," Paruthi said, "as treating RLS can lead to multiple other downstream improvements in their health."