In a transcriptome-wide study, researchers pinpoint PPP3R1 and other novel genetic risk factors of interest in RLS.

Interview by Bushraa Khatib

Restless leg syndrome (RLS) causes exhaustion and sleepiness, which affect work productivity, personal relationships, and daily activities, says Guy A. Rouleau, MD, PhD, director of Montreal Neurological Institute and Hospital. Because up to 15% of European populations experience the condition,¹ RLS results in important social and economic burdens, thus inspiring Rouleau to investigate the genes associated with RLS. “We aimed to investigate this disease at a fundamental level that could open new research directions for therapeutic interventions,” Rouleau says. 

Previous studies have identified some common risk regions for RLS, but causal genes have yet to be fully elucidated.² Rouleau and his research team found genes observed to be associated with RLS that were not reported in previous genetic studies. “For instance, PPP3R1, among others, is a novel genetic risk factor of interest as it is the first RLS-associated gene directly implicated in dopaminergic pathways,” he says.

Rouleau discussed the study with Sleep Review over email. The transcript has been lightly edited for clarity and style.

[Editor’s Note: Read the full study, Transcriptome-wide association study for restless legs syndrome identifies new susceptibility genes, in Communications Biology.]

What does your study add to existing research on genetic causes of RLS?  

GAR: Unlike previous methods that are focused on the effect of each genetic variant on gene expression individually, this study examined the association between overall expression of genes in different tissues and RLS. This approach led to the identification of candidate genes and pathways for RLS that were not detected by previous genome-wide association studies (GWAS). 

What surprised you?

GAR: In previous studies, GWAS signals close to MEIS1 gene had been identified as one of the strongest associations in RLS.^3,4 It was surprising to see that an additional gene, PPP3R1, was prioritized close to the region where MEIS1 GWAS signals are located.

How do genome-wide and transcriptome-wide association studies complement each other?

GAR: Transcriptome-wide association studies (TWAS) are complementary to GWAS. GWAS identify genetic variants associated with a trait, while TWAS integrate the results of GWAS and tissue-specific gene-expression datasets together to detect differentially expressed genes. In our study, we used summary-level RLS-GWAS data as an input to identify genes whose differential expression is associated with RLS.

Does your research lead you to believe that one gene or set of genes leads to the manifestation of RLS more than others?

GAR: Genetic variants reported to be associated with RLS are usually common in the general population,³ and each has a small effect on the phenotype. None of the identified genetic factors so far were shown to have a large impact on the phenotype itself.

How would studying populations other than those of European ancestry used in this study add to what is known about genetic causes of RLS?

GAR: Considering differential prevalence of RLS across different populations, such as very low prevalence in Asian countries compared to Europe or North America,¹ studies in non-European populations would help identify new genetic factors, not only the ones associated with an increased risk of RLS, but also the ones that may decrease the risk or protect from the disease. Therefore, it would expand our knowledge on the genetic and epidemiological background of RLS. 

What are the clinical implications of your research?

GAR: Although direct clinical implications are not the goal of such an association study, this study will provide impetus for further functional research that would lead to the identification of disease mechanisms, and therefore, drug targets and the development of new therapeutic interventions.

What further research should be done?

GAR: Most of the genetic variants predisposing to RLS remain to be identified. The genetic variants at the risk loci identified so far account for only a small proportion of the genetically determined susceptibility to RLS. Follow up studies should search for other types of variations, such as large genomic deletions or repetitive sequence elements at these loci.

The Study: 

Akçimen F, Sarayloo F, Liao C, et al. Transcriptome-wide association study for restless legs syndrome identifies new susceptibility genes. Commun Biol. 2020 Jul 10;3(1):373.

References

​​1. Picchietti DL, Van Den Eeden SK, Inoue Y, Berger K. Achievements, challenges, and future perspectives of epidemiologic research in restless legs syndrome (RLS). Sleep Med. 2017 Mar;31:3-9. 

2. Jiménez-Jiménez FJ, Alonso-Navarro H, García-Martín E, Agúndez JA. Genetics of restless legs syndrome: an update. Sleep Med Rev. 2018 Jun;39:108-21.

3. Schormair B, Zhao C, Bell S, et al. Identification of novel risk loci for restless legs syndrome in genome-wide association studies in individuals of European ancestry: a meta-analysis. Lancet Neurol. 2017 Nov;16(11):898-907. 

4. Didriksen M, Nawaz MS, Dowsett J, et al. Large genome-wide association study identifies three novel risk variants for restless legs syndrome. Commun Biol. 2020 Nov 25;3(1):1-9.

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Sleep researchers interested in participating in a Q&A should email editor[at]sleepreviewmag.com with a link to their relevant study.