MEIS1 p.R272H in familial restless legs syndrome

BTBD9 attenuates manganese-induced oxidative stress and neurotoxicity by regulating insulin growth factor signaling pathway

Manganese (Mn) is an essential mineral, but excess exposure can cause dopaminergic neurotoxicity. Restless legs syndrome (RLS) is a common neurological disorder, but the etiology and pathology remain largely unknown. The purpose of this study was to identify the role of Mn in the regulation of an RLS genetic risk factor BTBD9, characterize the function of BTBD9 in Mn-induced oxidative stress and dopaminergic neuronal dysfunction. We found that human subjects with high blood Mn levels were associated with decreased BTBD9 mRNA levels, when compared with subjects with low blood Mn levels. In A549 cells, Mn exposure decreased BTBD9 protein levels. In Caenorhabditis elegans, loss of hpo-9 (BTBD9 homolog) resulted in more susceptibility to Mn-induced oxidative stress and mitochondrial dysfunction, as well as decreased dopamine levels and alternations of dopaminergic neuronal morphology and behavior. Overexpression of hpo-9 in mutant animals restored these defects and the protection was eliminated by mutation of the forkhead box O (FOXO). In addition, expression of hpo-9 upregulated FOXO protein levels and decreased protein kinase B levels. These results suggest that elevated Mn exposure might be an environmental risk factor for RLS. Furthermore, BTBD9 functions to alleviate Mn-induced oxidative stress and neurotoxicity via regulation of insulin/insulin-like growth factor signaling pathway.

Investigation of dopaminergic signalling in Meis homeobox 1 (Meis1) deficient mice as an animal model of restless legs syndrome

Restless legs syndrome (RLS) is a common neurological disorder in which sensorimotor symptoms lead to sleep disturbances with substantial impact on life quality. RLS is caused by a combination of genetic and environmental factors, and Meis homeobox 1 (MEIS1) was identified as the main genetic risk factor. The efficacy of dopaminergic agonists, including dopamine D₂ receptor (DRD2) agonists, for treating RLS led to the hypothesis of dopaminergic impairment. However, it remains unclear whether it is directly involved in the disease aetiology and what the role of MEIS1 is considering its developmental and postnatal expression in the striatum, a critical structure in motor control. We addressed the role of MEIS1 in striatal dopaminergic signalling in Meis1^+/- mice, a valid animal model of RLS, and in Meis1^{Drd2 -/-} mice carrying a somatic null mutation of Meis1 in Drd2⁺ neurones. Motor behaviours, pharmacological exploration of DRD2 signalling, and quantitative analyses of DRD2⁺ and DRD1⁺ expressing neurones were investigated. Although Meis1^+/- mice displayed an RLS-like phenotype, including motor hyperactivity at the beginning of the rest phase, no reduction of dopaminoceptive neurones was observed in the striatum. Moreover, the null mutation of Meis1 in DRD2⁺ cells did not lead to RLS-like symptoms and dysfunction of the DRD2 pathway. These data indicate that MEIS1 does not modulate DRD2-dependent signalling in a cell-autonomous manner. Thus, the efficiency of D₂ -like agonists may reflect the involvement of other dopaminergic receptors or normalisation of motor circuit abnormalities downstream from defects caused by MEIS1 dysfunction.

MEIS1 and Restless Legs Syndrome: A Comprehensive Review

Restless legs syndrome (RLS) is a common sleep-related disorder for which the underlying biological pathways and genetic determinants are not well understood. The genetic factors so far identified explain less than 10% of the disease heritability. The first successful genome-wide association study (GWAS) of RLS was reported in 2007. This study identified multiple RLS associated risk variants including some within the non-coding regions of MEIS1. The MEIS1 GWAS signals are some of the strongest genetic associations reported for any common disease. MEIS1 belongs to the homeobox containing transcriptional regulatory network (HOX). Work in C. elegans showed a link between the MEIS1 ortholog and iron homeostasis, which is in line with the fact that central nervous system (CNS) iron insufficiency is thought to be a cause of RLS. Zebrafish and mice have been used to study the MEIS1 gene identifying an RLS-associated-SNP dependent enhancer activity from the highly conserved non-coding regions (HCNR) of MEIS1. Furthermore, this gene shows a lower expression of mRNA and protein in blood and thalamus of individuals with the MEIS1 RLS risk haplotype. Simulating this reduced MEIS1 expression in mouse models resulted in circadian hyperactivity, a phenotype compatible with RLS. While MEIS1 shows a strong association with RLS, the protein's function that is directly linked to an RLS biological pathway remains to be discovered. The links to iron and the enhancer activity of the HCNRs of MEIS1 suggest promising links to RLS pathways, however more in-depth studies on this gene's function are required. One important aspect of MEIS1's role in RLS is the fact that it encodes a homeobox containing transcription factor, which is essential during development. Future studies with more focus on the transcriptional regulatory role of MEIS1 may open novel venues for RLS research.

[Frequent neurological diseases associated with the restless legs syndrome]

BACKGROUND

Restless legs syndrome (RLS) is a common neurological disease. Studies have shown that RLS is associated with a variety of medical and neurological disorders.

OBJECTIVES

Using the example of three associated neurological diseases, the significance for everyday therapy decisions is assessed.

MATERIAL AND METHODS

A systematic search was carried out in PubMed for all studies with the keyword "RLS" in combination with polyneuropathies (PNP), Parkinson's disease (PD) and multiple sclerosis (MS) and classified according to the methodology in high, medium or low study quality.

RESULTS

Of 16 studies on RLS and MS, 10 were rated as "high". The high association frequency of RLS in MS between 13.3% and 65.1% (the variability possibly originates from different methods) prevents further statements about the prevalence. Within 30 studies on Parkinson's disease 17 were classified as having a high quality. In patients with Parkinson disease RLS occurs most frequently during therapy and is related to the duration of dopaminergic treatment. In patients with polyneuropathy, only 5 out of 24 studies were classified as being of high quality and an increased RLS prevalence was detected for acquired polyneuropathies with heterogeneous data for hereditary forms.

CONCLUSION

There is an increased prevalence of association with RLS for the diseases discussed. This prevalence is possibly determined by the pathophysiology of these disorders. These diseases are possibly characterized by genetic predispositions as well, which can hopefully be classified more accurately in the future.

Role of MEIS1 in restless legs syndrome: From GWAS to functional studies in mice

MEIS1 is a transcription factor playing an important role in the development of several organs, including central and peripheral nervous systems. A genetic locus spanning the MEIS1 coding region has been associated with the risk of RLS in genome-wide association studies, with increasing evidence that MEIS1 is the causal RLS gene. The RLS-linked genetic signal has been mapped to an intronic regulatory element within MEIS1. This element plays a role in the ganglionic eminences of the developing forebrain, with the RLS risk allele related to a reduced activation of the enhancer. This suggests that the ganglionic eminences play an important role in the development of genetic susceptibility to RLS. In addition, rare variants within MEIS1 have been shown to contribute to the disease risk. These variants were identified first in RLS families and later found in further RLS cases by targeted sequencing. Some of these variants alone are sufficient to suppress MEIS1 function in neural development, providing further evidence of the importance of neurodevelopmental processes in the pathological mechanism of MEIS1 in RLS. Heterozygous Meis1 inactivation in mice causes hyperactivity at the onset of the inactive period, consistent with human RLS. In addition, these mice revealed an effect of MEIS1 on the dopaminergic system at both the spinal and supraspinal level. More studies are needed in human genetics to determine the exact role of MEIS1 variants in the risk of RLS, as well as in functional genetics and animal studies to further elucidate the pathological mechanism of MEIS1 in RLS.

Caenorhabditis elegans and its applicability to studies on restless legs syndrome

Restless legs syndrome (RLS) is a common neurological disorder in the United States. This disorder is characterized by an irresistible urge to move the legs, although the symptoms vary in a wide range. The pathobiology of RLS has been linked to iron (Fe) deficiency and dopaminergic (DAergic) dysfunction. Several genetic factors have been reported to increase the risk of RLS. Caenorhabditis elegans (C. elegans) is a well-established animal model with a fully sequenced genome, which is highly conserved with mammals. Given the detailed knowledge of its genomic architecture, ease of genetic manipulation and conserved biosynthetic and metabolic pathways, as well as its small size, ease of maintenance, speedy generation time and large brood size, C. elegans provides numerous advantages in studying RLS-associated gene-environment interactions. Here we will review current knowledge about RLS symptoms, pathology and treatments, and discuss the application of C. elegans in RLS study, including the worm homologous genes and methods that could be performed to advance the pathophysiology RLS.

Animal models of RLS phenotypes

Restless legs syndrome (RLS) is a complex disorder that involves sensory and motor systems. The major pathophysiology of RLS is low iron concentration in the substantia nigra containing the cell bodies of dopamine neurons that project to the striatum, an area that is crucial for modulating movement. People who have RLS often present with normal iron values outside the brain; recent studies implicate several genes are involved in the syndrome. Like most complex diseases, animal models usually do not faithfully capture the full phenotypic spectrum of "disease," which is a uniquely human construct. Nonetheless, animal models have proven useful in helping to unravel the complex pathophysiology of diseases such as RLS and suggesting novel treatment paradigms. For example, hypothesis-independent genome-wide association studies (GWAS) have identified several genes as increasing the risk for RLS, including BTBD9. Independently, the murine homolog Btbd9 was identified as a candidate gene for iron regulation in the midbrain in mice. The relevance of the phenotype of another of the GWAS identified genes, MEIS1, has also been explored. The role of Btbd9 in iron regulation and RLS-like behaviors has been further evaluated in mice carrying a null mutation of the gene and in fruit flies when the BTBD9 protein is degraded. The BTBD9 and MEIS1 stories originate from human GWAS research, supported by work in a genetic reference population of mice (forward genetics) and further verified in mice, fish flies, and worms. Finally, the role of genetics is further supported by an inbred mouse strain that displays many of the phenotypic characteristics of RLS. The role of animal models of RLS phenotypes is also extended to include periodic limb movements.

Restless legs syndrome associated with major diseases: A systematic review and new concept

Recent publications on both the genetics and environmental factors of restless legs syndrome (RLS) defined as a clinical disorder suggest that overlapping genetic risk factors may play a role in primary (idiopathic) and secondary (symptomatic) RLS. Following a systematic literature search of RLS associated with comorbidities, we identified an increased prevalence of RLS only in iron deficiency and kidney disease. In cardiovascular disease, arterial hypertension, diabetes, migraine, and Parkinson disease, the methodology of studies was poor, but an association might be possible. There is insufficient evidence for conditions such as anemia (without iron deficiency), chronic obstructive pulmonary disease, multiple sclerosis, headache, stroke, narcolepsy, and ataxias. Based on possible gene-microenvironmental interaction, the classifications primary and secondary RLS may suggest an inappropriate causal relation. We recognize that in some conditions, treatment of the underlying disease should be achieved as far as possible to reduce or eliminate RLS symptoms. RLS might be seen as a continuous spectrum with a major genetic contribution at one end and a major environmental or comorbid disease contribution at the other.

The Molecular Genetics of Restless Legs Syndrome

Restless legs syndrome (RLS) is a common sensorimotor trait defined by symptoms that interfere with sleep onset and maintenance in a clinically meaningful way. Nonvolitional myoclonus while awake and asleep is a sign of the disorder and an informative endophenotype. The genetic contributions to RLS/periodic leg movements are substantial, are among the most robust defined to date for a common disease, and account for much of the variance in disease expressivity. The disorder is polygenic, as revealed by recent genome-wide association studies. Experimental studies are revealing mechanistic details of how these common variants might influence RLS expressivity.

Targeted resequencing and systematic in vivo functional testing identifies rare variants in MEIS1 as significant contributors to restless legs syndrome

Restless legs syndrome (RLS) is a common neurologic condition characterized by nocturnal dysesthesias and an urge to move, affecting the legs. RLS is a complex trait, for which genome-wide association studies (GWASs) have identified common susceptibility alleles of modest (OR 1.2-1.7) risk at six genomic loci. Among these, variants in MEIS1 have emerged as the largest risk factors for RLS, suggesting that perturbations in this transcription factor might be causally related to RLS susceptibility. To establish this causality, direction of effect, and total genetic burden of MEIS1, we interrogated 188 case subjects and 182 control subjects for rare alleles not captured by previous GWASs, followed by genotyping of ∼3,000 case subjects and 3,000 control subjects, and concluded with systematic functionalization of all discovered variants using a previously established in vivo model of neurogenesis. We observed a significant excess of rare MEIS1 variants in individuals with RLS. Subsequent assessment of all nonsynonymous variants by in vivo complementation revealed an excess of loss-of-function alleles in individuals with RLS. Strikingly, these alleles compromised the function of the canonical MEIS1 splice isoform but were irrelevant to an isoform known to utilize an alternative 3' sequence. Our data link MEIS1 loss of function to the etiopathology of RLS, highlight how combined sequencing and systematic functional annotation of rare variation at GWAS loci can detect risk burden, and offer a plausible explanation for the specificity of phenotypic expressivity of loss-of-function alleles at a locus broadly necessary for neurogenesis and neurodevelopment.

Association studies of variants in MEIS1, BTBD9, and MAP2K5/SKOR1 with restless legs syndrome in a US population

BACKGROUND

A genome-wide association study (GWAS) identified significant association between variants in MEIS1, BTBD9, and MAP2K5/SKOR1 and restless legs syndrome (RLS). However, many independent replication studies are needed to unequivocally establish a valid genotype-phenotype association across various populations. To further validate the GWAS findings, we investigated three variants, rs2300478 in MEIS1, rs9357271 in BTBD9, and rs1026732 in MAP2K5/SKOR1 in 38 RLS families and 189 RLS patients/560 controls from the US for their association with RLS.

METHOD

Both family-based and population-based case-control association studies were carried out.

RESULTS

The family-based study showed that SNP rs1026732 in MAP2K5/SKOR1 was significantly associated with RLS (P=0.01). Case-control association studies showed significant association between all three variants and RLS (P=0.0001/OR=1.65, P=0.0021/OR=1.59, and P=0.0011/OR=1.55 for rs2300478, rs9357271, and rs1026732, respectively).

CONCLUSION

Variants in MEIS1, BTBD9, and MAP2K5/SKOR1 confer a significant risk of RLS in a US population.

Update on sleep and its disorders

Inadequate sleep and sleep disorders have important adverse consequences on multiple systems. This review covers three areas: (a) Genetic determinants of sleep disorders. Common gene variants with small effects have been identified for both restless legs syndrome and narcolepsy with cataplexy. Rare variants with large effects have been found in familial phase advance syndrome and in subjects with short sleep durations. (b) Obstructive sleep apnea (OSA). OSA is an oxidative stress disorder. Prospective cohort studies show an increased risk of cardiovascular events in patients with untreated severe OSA. (c) The impact of sleep disorders on obesity and diabetes. Inadequate sleep results in changes in insulin resistance and in hormone levels leading to increases in appetite. Hence, inadequate sleep is associated with development of obesity. OSA is also an independent risk factor for insulin resistance; treatment of OSA can improve insulin sensitivity.

Whole genome association studies in complex diseases: where do we stand?

Hundreds of genome-wide association studies have been performed in recent years in order to try to identify common variants that associate with complex disease. These have met with varying success. Some of the strongest effects of common variants have been found in lateonset diseases and in drug response. The major histocompatibility complex has also shown very strong association with a variety of disorders. Although there have been some notable success stories in neuropsychiatric genetics, on the whole, common variation has explained little of the high heritability of these traits. In contrast, early studies of rare copy number variants have led rapidly to a number of genes and loci that strongly associate with neuropsychiatric disorders. It is likely that the use of whole-genome sequencing to extend the study of rare variation in neuropsychiatry will greatly advance our understanding of neuropsychiatric genetics.

Update on the management of restless legs syndrome: existing and emerging treatment options

Restless legs syndrome (RLS) is a sensorimotor disorder, characterized by a circadian variation of symptoms involving an urge to move the limbs (usually the legs) as well as paresthesias. There is a primary (familial) and a secondary (acquired) form, which affects a wide variety of individuals, such as pregnant women, patients with end-stage renal disease, iron deficiency, rheumatic disease, and persons taking medications. The symptoms reflect a circadian fluctuation of dopamine in the substantia nigra. RLS patients have lower dopamine and iron levels in the substantia nigra and respond to both dopaminergic therapy and iron administration. Iron, as a cofactor of dopamine production and a regulator of the expression of dopamine type 2-receptor, has an important role in the RLS etiology. In the management of the disease, the first step is to investigate possible secondary causes and their treatment. Dopaminergic agents are considered as the first-line therapy for moderate to severe RLS. If dopaminergic drugs are contraindicated or not efficacious, or if symptoms are resistant and unremitting, gabapentin or other antiepileptic agents, benzodiazepines, or opioids can be used for RLS therapy. Undiagnosed, wrongly diagnosed, and untreated RLS is associated with a significant impairment of the quality of life.