SUSCEPTIBILITY TO DYT1 DYSTONIA IN EUROPEAN PATIENTS IS MODIFIED BY THE D216H POLYMORPHISM

Genetic testing for non-parkinsonian movement disorders: Navigating the diagnostic maze

Genetic testing has become a valuable diagnostic tool for movement disorders due to substantial advancements in understanding their genetic basis. However, the heterogeneity of movement disorders poses a significant challenge, with many genes implicated in different subtypes. This paper aims to provide a neurologist's perspective on approaching patients with hereditary hyperkinetic disorders with a focus on select forms of dystonia, paroxysmal dyskinesia, chorea, and ataxia. Age at onset, initial symptoms, and their severity, as well as the presence of any concurrent neurological and non-neurological features, contribute to the individual clinical profiles of hereditary non-parkinsonian movement disorders, aiding in the selection of appropriate genetic testing strategies. There are also more specific diagnostic clues that may facilitate the decision-making process and may be highly specific for certain conditions, such as diurnal fluctuations and l-dopa response in dopa-responsive dystonia, and triggering factors, duration and frequency of attacks in paroxysmal dyskinesia. While the genetic and mutational spectrum across non-parkinsonian movement disorders is broad, certain groups of diseases tend to be associated with specific types of pathogenic variants, such as repeat expansions in many of the ataxias. Some of these pathogenic variants cannot be detected by standard methods, such as panel or exome sequencing, but require the investigation of intronic regions for repeat expansions, such as Friedreich's or FGF14-linked ataxia. With our advancing knowledge of the genetic underpinnings of movement disorders, the incorporation of precise and personalized diagnostic strategies can enhance patient care, prognosis, and the application and development of targeted therapeutic interventions.

Clinical relevance and translational impact of reduced penetrance in genetic movement disorders

Reduced penetrance is an important but underreported aspect in monogenic diseases. It refers to the phenomenon that carriers of pathogenic variants do not manifest with an overt disease. Clinical expressivity, on the other hand, describes the degree to which certain disease characteristics are present. In this article, we discuss the implications of reduced penetrance on genetic testing and counseling, outline how penetrance can be estimated in rare diseases using large cohorts and review the ethical, legal and social implications of studying non-manifesting carriers of pathogenic mutations. We highlight the interplay between reduced penetrance and the prodromal phase of a neurodegenerative disorder through the example of monogenic Parkinson's disease and discuss the therapeutic implications.

Striatal and cerebellar vesicular acetylcholine transporter expression is disrupted in human DYT1 dystonia

Early-onset torsion dystonia (TOR1A/DYT1) is a devastating hereditary motor disorder whose pathophysiology remains unclear. Studies in transgenic mice suggested abnormal cholinergic transmission in the putamen, but this has not yet been demonstrated in humans. The role of the cerebellum in the pathophysiology of the disease has also been highlighted but the involvement of the intrinsic cerebellar cholinergic system is unknown. In this study, cholinergic neurons were imaged using PET with 18F-fluoroethoxybenzovesamicol, a radioligand of the vesicular acetylcholine transporter (VAChT). Here, we found an age-related decrease in VAChT expression in the posterior putamen and caudate nucleus of DYT1 patients versus matched controls, with low expression in young but not in older patients. In the cerebellar vermis, VAChT expression was also significantly decreased in patients versus controls, but independently of age. Functional connectivity within the motor network studied in MRI and the interregional correlation of VAChT expression studied in PET were also altered in patients. These results show that the cholinergic system is disrupted in the brain of DYT1 patients and is modulated over time through plasticity or compensatory mechanisms.

Integrated Genome and Transcriptome Sequencing to Solve a Neuromuscular Puzzle: Miyoshi Muscular Dystrophy and Early Onset Primary Dystonia in Siblings of the Same Family

BACKGROUND

Neuromuscular disorders (NMD), many of which are hereditary, affect muscular function. Due to advances in high-throughput sequencing technologies, the diagnosis of hereditary NMDs has dramatically improved in recent years.

METHODS AND RESULTS

In this study, we report an family with two siblings exhibiting two different NMD, Miyoshi muscular dystrophy (MMD) and early onset primary dystonia (EOPD). Whole exome sequencing (WES) identified a novel monoallelic frameshift deletion mutation (dysferlin: c.4404delC/p.I1469Sfs∗17) in the Dysferlin gene in the index patient who suffered from MMD. This deletion was inherited from his unaffected father and was carried by his younger sister with EOPD. However, immunostaining staining revealed an absence of dysferlin expression in the proband's muscle tissue and thus suggested the presence of the second underlying mutant allele in dysferlin. Using integrated RNA sequencing (RNA-seq) and whole genome sequencing (WGS) of muscle tissue, a novel deep intronic mutation in dysferlin (dysferlin: c.5341-415A > G) was discovered in the index patient. This mutation caused aberrant mRNA splicing and inclusion of an additional pseudoexon (PE) which we termed PE48.1. This PE was inherited from his unaffected mother. PE48.1 inclusion altered the Dysferlin sequence, causing premature termination of translation.

CONCLUSION

Using integrated genome and transcriptome sequencing, we discovered hereditary MMD and EOPD affecting two siblings of same family. Our results added further weight to the combined use of RNA-seq and WGS as an important method for detection of deep intronic gene mutations, and suggest that integrated sequencing assays are an effective strategy for the diagnosis of hereditary NMDs.

The evolution of dystonia-like movements in TOR1A rats after transient nerve injury is accompanied by dopaminergic dysregulation and abnormal oscillatory activity of a central motor network

TOR1A is the most common inherited form of dystonia with still unclear pathophysiology and reduced penetrance of 30-40%. ∆ETorA rats mimic the TOR1A disease by expression of the human TOR1A mutation without presenting a dystonic phenotype. We aimed to induce dystonia-like symptoms in male ∆ETorA rats by peripheral nerve injury and to identify central mechanism of dystonia development. Dystonia-like movements (DLM) were assessed using the tail suspension test and implementing a pipeline of deep learning applications. Neuron numbers of striatal parvalbumin⁺, nNOS⁺, calretinin⁺, ChAT⁺ interneurons and Nissl⁺ cells were estimated by unbiased stereology. Striatal dopaminergic metabolism was analyzed via in vivo microdialysis, qPCR and western blot. Local field potentials (LFP) were recorded from the central motor network. Deep brain stimulation (DBS) of the entopeduncular nucleus (EP) was performed. Nerve-injured ∆ETorA rats developed long-lasting DLM over 12 weeks. No changes in striatal structure were observed. Dystonic-like ∆ETorA rats presented a higher striatal dopaminergic turnover and stimulus-induced elevation of dopamine efflux compared to the control groups. Higher LFP theta power in the EP of dystonic-like ∆ETorA compared to wt rats was recorded. Chronic EP-DBS over 3 weeks led to improvement of DLM. Our data emphasizes the role of environmental factors in TOR1A symptomatogenesis. LFP analyses indicate that the pathologically enhanced theta power is a physiomarker of DLM. This TOR1A model replicates key features of the human TOR1A pathology on multiple biological levels and is therefore suited for further analysis of dystonia pathomechanism.

Epidemiology of DYT1 dystonia: Estimating prevalence via genetic ascertainment

Objective

To estimate the prevalence of TOR1A sequence variants associated with DYT1 dystonia.

Methods

We determined the frequency of the common trinucleotide deletion that causes DYT1 in the Genome Aggregation Database and the Penn Medicine Biobank, totaling exomes from over 135,000 individuals. We also evaluated the prevalence of other possible pathogenic variants in this gene and asked whether the D216H polymorphism is linked to a higher diagnostic rate for dystonia independent of the DYT1-causing mutation.

Results

The estimated range of prevalence of the most common pathogenic variant that causes DYT1 is ∼17.6–26.1 carriers per 100,000 individuals. Based on the different data sets used, we predict that there are between 54,366 and 80,891 mutation carriers in the United States, which, due to the reduced penetrance of this variant, would translate into 16,475–24,513 DYT1 patients.

Conclusions

Our data provide a prevalence estimate of the most common DYT1 mutation in the general population. This information is specifically important for those with interest in the development of precision therapeutics for dystonia.

Risk Factor Genes in Patients with Dystonia: A Comprehensive Review

Background

Dystonia is a movement disorder with high heterogeneity regarding phenotypic appearance and etiology that occurs in both sporadic and familial forms. The etiology of the disease remains unknown. However, there is increasing evidence suggesting that a small number of gene alterations may lead to dystonia. Although pathogenic variants to the familial type of dystonia have been extensively reviewed and discussed, relatively little is known about the contribution of single-nucleotide polymorphisms (SNPs) to dystonia. This review focuses on the potential role of SNPs and other variants in dystonia susceptibility.

Methods

We searched the PubMed database for peer-reviewed articles published in English, from its inception through January 2018, that concerned human studies of dystonia and genetic variants. The following search terms were included: “dystonia” in combination with the following terms: 1) “polymorphisms” and 2) “SNPs” as free words.

Results

A total of 43 published studies regarding TOR1A, BDNF, DRD5, APOE, ARSG, NALC, OR4X2, COL4A1, TH, DDC, DBH, MAO, COMT, DAT, GCH1, PRKRA, MR-1, SGCE, ATP1A3, TAF1, THAP1, GNAL, DRD2, HLA-DRB, CBS, MTHFR, and MS genes, were included in the current review.

Discussion

To date, a few variants, which are possibly involved in several molecular pathways, have been related to dystonia. Large cohort studies are needed to determine robust associations between variants and dystonia with adjustment for other potential cofounders, in order to elucidate the pathogenic mechanisms of dystonia and the net effect of the genes.

Diagnosis and treatment of pediatric onset isolated dystonia

Isolated dystonia refers to a genetic heterogeneous group of progressive conditions with onset of symptoms during childhood or adolescence, progressive course with frequent generalization and marked functional impairment. There are well-known monogenic forms of isolated dystonia with pediatric onset such as DYT1 and DYT6 transmitted with autosomal dominant inheritance and low penetrance. Genetic findings of the past years have widened the etiological spectrum and the phenotype. The recently discovered genes (GNAL, ANO-3, KTM2B) or variant of already known diseases, such as Ataxia-Teleangectasia, are emerging as another causes of pediatric onset dystonia, sometimes with a more complex phenotype, but their incidence is unknown and still a considerable number of cases remains genetically undetermined. Due to the severe disability of pediatric onset dystonia treatment remains unsatisfactory and still mainly based upon oral pharmacological agents. However, deep brain stimulation is now extensively applied with good to excellent results especially when patients are treated early during the course of the disease.

New THAP1 mutation and role of putative modifier in TOR1A

OBJECTIVES

The prevalence of DYT1 (mutation in TOR1A) and DYT6 (mutation in THAP1) may vary in different populations, which can have important implications in clinical investigation. Our goal was to characterize patients with inherited and isolated dystonia and determine the frequency of mutations responsible for DYT1 and DYT6 in Brazilian patients.

METHODS

Two movement disorder specialists examined 78 patients with idiopathic isolated dystonia using a standardized questionnaire, before sequencing TOR1A and THAP1 genes.

RESULTS

Clinically, our cohort was similar to those described in the international literature. Molecular studies of 68 subjects revealed only one potentially deleterious variant in THAP1 (1/68 patients, 1.47%). This was a novel 10-bp deletion at the end of exon 1, g.5308_5317del (ng_011837.1), which is predicted to create an alternative splicing and the insertion of a premature stop codon. Although we did not observe any potentially deleterious mutations in TOR1A, we found the missense variant rs1801968 (TOR1A p.D216H), previously reported as either a modifier of dystonia phenotype or a predisposing factor for dystonia. However, we did not identify any phenotypic impact related to the missense variant rs1801968 (P = 0.3387).

CONCLUSIONS

Although clinically similar to most cohorts with dystonia worldwide, the classical mutation (c.907_909delGAG) in TOR1A (causing DYT1) is absent in our patients. However, we found a potentially deleterious THAP1 mutation not previously reported. In addition, we found no association of rs1801968 with dystonia.

The Role of TOR1A Polymorphisms in Dystonia: A Systematic Review and Meta-Analysis

Importance

A number of genetic loci were found to be associated with dystonia. Quite a few studies have been contacted to examine possible contribution of TOR1A variants to the risk of dystonia, but their results remain conflicting. The aim of the present study was to systematically evaluate the effect of TOR1A gene SNPs on dystonia and its phenotypic subtypes regarding the body distribution.

Methods

We performed a systematic review of Pubmed database to identify all available studies that reported genotype frequencies of TOR1A SNPs in dystonia. In total 16 studies were included in the quantitative analysis. Odds ratios (ORs) were calculated in each study to estimate the influence of TOR1A SNPs genotypes on the risk of dystonia. The fixed-effects model and the random effects model, in case of high heterogeneity, for recessive and dominant mode of inheritance as well as the free generalized odds ratio (OR_G) model were used to calculate both the pooled point estimate in each study and the overall estimates.

Results

Rs1182 was found to be associated with focal dystonia in recessive mode of inheritance [Odds Ratio, OR (95% confidence interval, C.I.): 1.83 (1.14–2.93), Pz = 0.01]. In addition, rs1801968 was associated with writer’s cramp in both recessive and dominant modes [OR (95%C.I.): 5.99 (2.08–17.21), Pz = 0.00009] and [2.48 (1.36–4.51), Pz = 0.003) respectively and in model free-approach [OR_G (95%C.I.): 2.58 (1.45–4.58)].

Conclusions

Our meta-analysis revealed a significant implication of rs1182 and rs1801968 TOR1A variants in the development of focal dystonia and writer’s cramp respectively. TOR1A gene variants seem to be implicated in dystonia phenotype.

Membrane defects and genetic redundancy: Are we at a turning point for DYT1 dystonia?

Heterozygosity for a 3-base pair deletion (ΔGAG) in TOR1A/torsinA is one of the most common causes of hereditary dystonia. In this review, we highlight current understanding of how this mutation causes disease from research spanning structural biochemistry, cell science, neurobiology, and several model organisms. We now know that homozygosity for ΔGAG has the same effects as Tor1a^KO , implicating a partial loss of function mechanism in the ΔGAG/+ disease state. In addition, torsinA loss specifically affects neurons in mice, even though the gene is broadly expressed, apparently because of differential expression of homologous torsinB. Furthermore, certain neuronal subtypes are more severely affected by torsinA loss. Interestingly, these include striatal cholinergic interneurons that display abnormal responses to dopamine in several Tor1a animal models. There is also progress on understanding torsinA molecular cell biology. The structural basis of how ΔGAG inhibits torsinA ATPase activity is defined, although mutant torsinA^ΔGAG protein also displays some characteristics suggesting it contributes to dystonia by a gain-of-function mechanism. Furthermore, a consistent relationship is emerging between torsin dysfunction and membrane biology, including an evolutionarily conserved regulation of lipid metabolism. Considered together, these findings provide major advances toward understanding the molecular, cellular, and neurobiological pathologies of DYT1/TOR1A dystonia that can hopefully be exploited for new approaches to treat this disease. © 2016 International Parkinson and Movement Disorder Society.

Structures of TorsinA and its disease-mutant complexed with an activator reveal the molecular basis for primary dystonia

The most common cause of early onset primary dystonia, a neuromuscular disease, is a glutamate deletion (ΔE) at position 302/303 of TorsinA, a AAA+ ATPase that resides in the endoplasmic reticulum. While the function of TorsinA remains elusive, the ΔE mutation is known to diminish binding of two TorsinA ATPase activators: lamina-associated protein 1 (LAP1) and its paralog, luminal domain like LAP1 (LULL1). Using a nanobody as a crystallization chaperone, we obtained a 1.4 Å crystal structure of human TorsinA in complex with LULL1. This nanobody likewise stabilized the weakened TorsinAΔE-LULL1 interaction, which enabled us to solve its structure at 1.4 Å also. A comparison of these structures shows, in atomic detail, the subtle differences in activator interactions that separate the healthy from the diseased state. This information may provide a structural platform for drug development, as a small molecule that rescues TorsinAΔE could serve as a cure for primary dystonia.

Dystonia--new advances in classification, genetics, pathophysiology and treatment

Dystonia is a heterogeneous movement disorder and has been defined as 'a syndrome of sustained muscle contractions, frequently causing twisted and repetitive movements, or abnormal postures'. The classification of dystonia has developed along with increasing knowledge, and different schemes have been suggested, including age at onset, body distribution, and etiology as the main differentiating factors. A revised definition and a new classification of dystonia have now been proposed by a group of leading dystonia experts and will be referred here. The discovery of the first two gene mutations causing primary generalized dystonia (DYT1-TOR1A and DYT6-THAP1) has facilitated studies on pathogenesis and pathophysiology of primary dystonias, by comparing neurophysiology between manifesting and non-manifesting carriers, and by studying the molecular biology of the mutant gene products. During recent years, several other gene mutations causing primary dystonia, dystonia-plus, and paroxysmal dystonia disorders have been discovered. Only during the last year, by the use of whole-exome sequencing techniques, mutations in three different genes in families with predominantly cervical dystonia were found, which may lead to improved insight into the pathogenesis also of the more frequent focal dystonias. Botulinum neurotoxin (BoNT) and deep brain stimulation (DBS) have revolutionized the symptomatic treatment for dystonia during the last two decades and continue to be refined to improve efficacy and expand their indications. Unfortunately, no progress has been made in the oral medication of dystonia. Current and future new insights into pathogenetic and pathophysiological mechanisms of dystonia will hopefully lead to improvement also in this area soon.

TOR1A polymorphisms in a Russian cohort with primary focal/segmental dystonia

PURPOSE/AIM OF THE STUDY

To analyze contribution of rs3842225 and rs1182 single nucleotide polymorphisms (SNP) in TOR1A gene, the causative gene for the DYT1 form of hereditary early-onset generalized dystonia, to the development of focal and segmental dystonia in Russian patients.

MATERIALS AND METHODS

We analyzed associations between rs3842225 and rs1182 polymorphisms in TOR1A and focal/segmental dystonia in 254 patients from Russian population, including 218 Slavic patients and 36 patients of mixed ethnic background.

RESULTS

Stratification of patients based on age at the disease onset (≤ 30 years and > 30 years) showed statistically significant prevalence of the del-allele at the rs3842225 locus in Slavic patients with earlier age of onset of dystonia (36.96% vs. 21.39% in patients with late age of onset, p = 0.002) and an overrepresentation of the T-allele at the rs1182 locus (36.96% vs. 21.69%, p = 0.003). In Slavs, we also observed an overrepresentation of the homozygous genotypes, T/T (general sample of dystonia, 9.17% and focal dystonia, 10.28%) or G/G (general sample of dystonia, 60.55% and focal dystonia, 58.86%), compared to controls (T/T, 4.27% and G/G, 55.49%). In non-Slavic patients, we revealed neither significant associations, nor statistical tendencies regarding any of the clinical features.

CONCLUSIONS

Our data in an Eastern Slavic (Russian) population correspond well to results of other studies from different countries and confirm that certain TOR1A genotypes may be regarded as factors predisposing to focal and segmental dystonia.

Biochemical and cellular analysis of human variants of the DYT1 dystonia protein, TorsinA/TOR1A

Early-onset dystonia is associated with the deletion of one of a pair of glutamic acid residues (c.904_906delGAG/c.907_909delGAG; p.Glu302del/Glu303del; ΔE 302/303) near the carboxyl-terminus of torsinA, a member of the AAA(+) protein family that localizes to the endoplasmic reticulum lumen and nuclear envelope. This deletion commonly underlies early-onset DYT1 dystonia. While the role of the disease-causing mutation, torsinAΔE, has been established through genetic association studies, it is much less clear whether other rare human variants of torsinA are pathogenic. Two missense variations have been described in single patients: R288Q (c.863G>A; p.Arg288Gln; R288Q) identified in a patient with onset of severe generalized dystonia and myoclonus since infancy and F205I (c.613T>A, p.Phe205Ile; F205I) in a psychiatric patient with late-onset focal dystonia. In this study, we have undertaken a series of analyses comparing the biochemical and cellular effects of these rare variants to torsinAΔE and wild-type (wt) torsinA to reveal whether there are common dysfunctional features. The results revealed that the variants, R288Q and F205I, are more similar in their properties to torsinAΔE protein than to torsinAwt. These findings provide functional evidence for the potential pathogenic nature of these rare sequence variants in the TOR1A gene, thus implicating these pathologies in the development of dystonia.

The BiP molecular chaperone plays multiple roles during the biogenesis of torsinA, an AAA+ ATPase associated with the neurological disease early-onset torsion dystonia

Early-onset torsion dystonia (EOTD) is a neurological disorder characterized by involuntary and sustained muscle contractions that can lead to paralysis and abnormal posture. EOTD is associated with the deletion of a glutamate (ΔE) in torsinA, an endoplasmic reticulum (ER) resident AAA(+) ATPase. To date, the effect of ΔE on torsinA and the reason that this mutation results in EOTD are unclear. Moreover, there are no specific therapeutic options to treat EOTD. To define the underlying biochemical defects associated with torsinAΔE and to uncover factors that might be targeted to offset defects associated with torsinAΔE, we developed a yeast torsinA expression system and tested the roles of ER chaperones in mediating the folding and stability of torsinA and torsinAΔE. We discovered that the ER lumenal Hsp70, BiP, an associated Hsp40, Scj1, and a nucleotide exchange factor, Lhs1, stabilize torsinA and torsinAΔE. BiP also maintained torsinA and torsinAΔE solubility. Mutations predicted to compromise specific torsinA functional motifs showed a synthetic interaction with the ΔE mutation and destabilized torsinAΔE, suggesting that the ΔE mutation predisposes torsinA to defects in the presence of secondary insults. In this case, BiP was required for torsinAΔE degradation, consistent with data that specific chaperones exhibit either pro-degradative or pro-folding activities. Finally, using two independent approaches, we established that BiP stabilizes torsinA and torsinAΔE in mammalian cells. Together, these data define BiP as the first identified torsinA chaperone, and treatments that modulate BiP might improve symptoms associated with EOTD.

Genetics of dystonia: what's known? What's new? What's next?

Although all forms of dystonia share the core clinical features of involuntary dystonic dyskinesia, there is not only marked phenotypic but also etiologic heterogeneity. Isolated dystonia can be caused by mutations in TOR1A (DYT1), TUBB4 (DYT4), THAP1 (DYT6), CIZ1 (DYT23), ANO3 (DYT24), and GNAL (DYT25). Combined dystonias (with parkinsonism or myoclonus) are further subdivided into persistent (TAF1 [DYT3], GCHI [DYT5], SGCE [DYT11], ATP1A3 [DYT12]), PRKRA (DYT16), and paroxysmal (MR-1 [DYT8], PRRT2 [DYT10], SLC2A1 [DYT18]. With the advent of next-generation sequencing, an unprecedented number of new dystonia genes have recently been described, including 4 in the past 12 months. Despite the need for independent confirmation, these recent findings raise 2 important questions regarding (1) the role of genetics in dystonia overall and (2) the role of different molecular mechanisms in dystonia pathogenesis. The genetic contribution to dystonia represents a continuum ranging from genetic susceptibility factors of small effect to causative genes with markedly reduced penetrance to those with full penetrance. Equally diverse and complex are the pathways and neuronal function(s) putatively involved in dystonia pathogenesis including dopamine signaling, intracellular transport, cytoskeletal dynamics, transcriptional regulation, cell-cycle control, ion channel function, energy metabolism, signal transduction, and detoxification mechanisms. In the next decade of dystonia research, we expect to see the discovery of additional dystonia genes and susceptibility factors. In this context, it will be of great interest to explore whether the diverse cellular functions of the known dystonia proteins may be linked to shared pathways and thus complete the complex puzzle of dystonia pathogenesis. © 2013 Movement Disorder Society.

Hereditäre Dystonien

Dystonien sind eine klinisch und ätiologisch heterogene Gruppe von Bewegungsstörungen. Charakteristisch sind unwillkürliche Muskelkontraktionen, die zu drehenden, schraubenden und repetitiven Bewegungen führen und sehr schmerzhaft sein können. Die Dystonie kann dabei das einzige Symptom sein („isolierte Dystonie“) oder von anderen Symptomen begleitet werden („kombinierte Dystonie“), sie kann aber auch eine Manifestation jedweder das Zentralnervensystem betreffenden Erkrankung sein, die das motorische System in Mitleidenschaft zieht (z. B. neurodegenerative, ischämische, traumatische Prozesse). In den letzten 20 Jahren hat die Entwicklung neuer molekulargenetischer Technologien zur Entdeckung neuer Gene geführt, die vielen Dystoniesubtypen zugrunde liegen, und eine verbesserte Klassifizierung sowie einen tieferen Einblick in die Pathophysiologie ermöglicht. Es wird eine aktuelle Übersicht über die genetisch determinierten Dystonien mit Fokus auf den sog. isolierten bzw. kombinierten Formen vorgelegt. Die Zusammenstellung phänotypischer Charakteristika zu spezifischen genetischen Veränderungen soll dem Kliniker ermöglichen, anhand konkreter klinischer Manifestationen eine entsprechende molekulargenetische Abklärung in die Wege zu leiten.

The genetics of dystonia: new twists in an old tale

Dystonia is a common movement disorder seen by neurologists in clinic. Genetic forms of the disease are important to recognize clinically and also provide valuable information about possible pathogenic mechanisms within the wider disorder. In the past few years, with the advent of new sequencing technologies, there has been a step change in the pace of discovery in the field of dystonia genetics. In just over a year, four new genes have been shown to cause primary dystonia (CIZ1, ANO3, TUBB4A and GNAL), PRRT2 has been identified as the cause of paroxysmal kinesigenic dystonia and other genes, such as SLC30A10 and ATP1A3, have been linked to more complicated forms of dystonia or new phenotypes. In this review, we provide an overview of the current state of knowledge regarding genetic forms of dystonia—related to both new and well-known genes alike—and incorporating genetic, clinical and molecular information. We discuss the mechanistic insights provided by the study of the genetic causes of dystonia and provide a helpful clinical algorithm to aid clinicians in correctly predicting the genetic basis of various forms of dystonia.

Genetic issues in the diagnosis of dystonias

Dystonias are heterogeneous hyperkinetic movement disorders characterized by involuntary muscle contractions which result in twisting and repetitive movements and abnormal postures. Several causative genes have been identified, but their genetic bases still remain elusive. Primary Torsion Dystonias (PTDs), in which dystonia is the only clinical sign, can be inherited in a monogenic fashion, and many genes and loci have been identified for autosomal dominant (DYT1/TOR1A; DYT6/THAP1; DYT4/TUBB4a; DYT7; DYT13; DYT21; DYT23/CIZ1; DYT24/ANO3; DYT25/GNAL) and recessive (DYT2; DYT17) forms. However most sporadic cases, especially those with late-onset, are likely multifactorial, with genetic and environmental factors interplaying to reach a threshold of disease. At present, genetic counseling of dystonia patients remains a difficult task. Recently non-motor clinical findings in dystonias, new highlights in the pathophysiology of the disease, and the availability of high-throughput genome-wide techniques are proving useful tools to better understand the complexity of PTD genetics. We briefly review the genetic basis of the most common forms of hereditary PTDs, and discuss relevant issues related to molecular diagnosis and genetic counseling.

Is TOR1A a risk factor in adult-onset primary torsion dystonia?

BACKGROUND

Studies of genetic association between TOR1A and adult-onset primary torsion dystonia have contradictory results.

METHODS

The authors genotyped TOR1A single nucleotide polymorphisms rs1801968, rs2296793, rs1182 and rs3842225 in a cohort of clinically well characterized cervical dystonia patients (n=367) and constructed haplotypes. The authors systematically reviewed the published case-control TOR1A association studies in adult-onset primary torsion dystonia.

RESULTS

In this Dutch cervical dystonia cohort, no significant association was found with TOR1A variants. In the meta-analysis (eight studies, 1332 adult-onset primary dystonia patients) no variant reached overall significance. However, in a selection of familial cases the functional variant p.Asp216His (rs1801968) was associated with increased dystonia risk (odds ratio 1.43; 95%CI 1.01-2.02).

CONCLUSIONS

Meta-analysis does not show association with common variants in TOR1A in adult-onset primary dystonia, except for the functional variant rs1801968 in familial focal dystonia cases.

Invertebrate models of dystonia

The neurological movement disorder dystonia is an umbrella term for a heterogeneous group of related conditions where at least 20 monogenic forms have been identified. Despite the substantial advances resulting from the identification of these loci, the function of many DYT gene products remains unclear. Comparative genomics using simple animal models to examine the evolutionarily conserved functional relationships with monogenic dystonias represents a rapid route toward a comprehensive understanding of these movement disorders. Current studies using the invertebrate animal models Caenorhabditis elegans and Drosophila melanogaster are uncovering cellular functions and mechanisms associated with mutant forms of the well-conserved gene products corresponding to DYT1, DYT5a, DYT5b, and DYT12 dystonias. Here we review recent findings from the invertebrate literature pertaining to molecular mechanisms of these gene products, torsinA, GTP cyclohydrolase I, tyrosine hydroxylase, and the alpha subunit of Na+/K ATPase, respectively. In each study, the application of powerful genetic tools developed over decades of intensive work with both of these invertebrate systems has led to mechanistic insights into these human disorders. These models are particularly amenable to large-scale genetic screens for modifiers or additional alleles, which are bolstering our understanding of the molecular functions associated with these gene products. Moreover, the use of invertebrate models for the evaluation of DYT genetic loci and their genetic interaction networks has predictive value and can provide a path forward for therapeutic intervention.

Phenotypic overlap in familial and sporadic primary adult-onset extracranial dystonia

We recently demonstrated that familial and sporadic blepharospasms share several phenotypic features (including age of dystonia onset, sex, and tendency to spread) believed to reflect the etiology of a blepharospasm. To investigate whether familial and sporadic forms of primary adult-onset dystonia other than the blepharospasm also share phenotypic features, we studied the families of 98 probands with primary adult-onset dystonia other than blepharospasms using a validated two-step procedure (questionnaire and clinical examination) that yields 95 % sensitivity and 100 % specificity when used to identify dystonia among relatives. The 98 probands provided a population of 402 living first-degree relatives aged 20 years or more, 336 of whom (83 %, 111 parents, 152 siblings, and 73 children) were screened for dystonia. The screening procedure identified 26 affected relatives (five parents, 16 siblings, and five children; 11 men/15 women; age at dystonia onset, 51 ± 11.7 years) from 24/98 families (25 %). No causes of secondary dystonia were found in the relatives who suffered from various forms of dystonia. When familial and sporadic patients were compared, no significant differences emerged in age, education, family size, sex distribution, age at dystonia onset, or tendency to spread. The phenotypic overlap we observed between the study groups suggests that familial and sporadic patients with primary adult-onset dystonia other than blepharospasm probably share a common etiological background.

Assessment of D216H DYT1 polymorphism in a Chinese primary dystonia patient cohort

BACKGROUND

The D216H single-nucleotide polymorphism (SNP) (rs1801968) in DYT1 exon 4 has been suggested to be a genetic modifier in primary dystonia.

METHODS

To further explore this question, we assessed rs1801968 variations in a cohort of 210 Chinese patients with primary dystonia devoid of DYT1 mutations.

RESULTS

We found that focal dystonia, specifically cervical dystonia, was the most common form of dystonia, with 8.1% of all the patients having a positive family history of dystonia. No association of the D216H SNP with primary dystonia was identified. In a subsequent subgroup analysis, the 216H allele was found to occur more frequently in patients with writer's cramp, but no correlation was found between the allele and other forms of dystonia or age of onset.

CONCLUSIONS

Our findings do not confirm that the allele contributes to the risk of D216H SNP primary dystonia.

Primary dystonia and dystonia-plus syndromes: clinical characteristics, diagnosis, and pathogenesis

The dystonias are a heterogeneous group of hyperkinetic movement disorders characterised by involuntary sustained muscle contractions that lead to abnormal postures and repetitive movements. Dystonia syndromes represent common movement disorders and yet are often misdiagnosed or unrecognised. In recent years, there have been substantial advances in the understanding of the spectrum of clinical features that encompass dystonia syndromes, from severe generalised childhood dystonia that is often genetic in origin, to adult-onset focal dystonias and rarer forms of secondary dystonias, to dystonia as a feature of other types of CNS dysfunction. There has also been a rationalisation of the classification of dystonia and a greater understanding of the causes of dystonic movements from the study of genetics, neurophysiology, and functional imaging in the most prevalent form of dystonia syndrome, primary dystonia.

Milestones in dystonia

The last 25 years have seen remarkable advances in our understanding of the genetic etiologies of dystonia, new approaches into dissecting underlying pathophysiology, and independent progress in identifying effective treatments. In this review we highlight some of these advances, especially the genetic findings that have taken us from phenomenological to molecular-based diagnoses. Twenty DYT loci have been designated and 10 genes identified, all based on linkage analyses in families. Hand in hand with these genetic findings, neurophysiological and imaging techniques have been employed that have helped illuminate the similarities and differences among the various etiological dystonia subtypes. This knowledge is just beginning to yield new approaches to treatment including those based on DYT1 animal models. Despite the lag in identifying genetically based therapies, effective treatments, including impressive benefits from deep brain stimulation and botulinum toxin chemodenervation, have marked the last 25 years. The challenge ahead includes continued advancement into understanding dystonia's many underlying causes and associated pathology and using this knowledge to advance treatment including preventing genetic disease expression.

Clinical neuroimaging and electrophysiological assessment of three DYT6 dystonia families

The purpose of the study was to delineate clinical and electrophysiological characteristics as well as laryngoscopical and transcranial ultrasound (TCS) findings in THAP1 mutation carriers (MutC). According to recent genetic studies, DYT6 (THAP1) gene mutations are an important cause of primary early-onset dystonia. In contrast to DYT1 mutations, THAP1 mutations are associated with primary early-onset segmental or generalised dystonia frequently involving the craniocervical region and the larynx. Blood samples from twelve individuals of three German families with DYT6 positive index cases were obtained to test for THAP1 mutations. Eight THAP1 MutC were identified. Of these, six (three symptomatic and three asymptomatic) THAP1 MutC could be clinically evaluated. Laryngoscopy was performed to evaluate laryngeal dysfunction in patients. Brainstem echogenicity was investigated in all MutC using TCS. Two of the patients had undergone bilateral pallidal DBS. In all three symptomatic MutC, early-onset laryngeal dystonia was a prominent feature. Laryngeal assessment demonstrated adductor-type dystonia in all of them. On clinical examination, the three asymptomatic MutC also showed subtle signs of focal or segmental dystonia. TCS revealed increased substantia nigra (SN) hyperechogenicity in all MutC. Intraoperative microelectrode recordings under general anesthesia in two of the patients showed no difference between THAP1 and previously operated DYT1 MutC. The presence of spasmodic dysphonia in patients with young-onset segmental or generalised dystonia is a hallmark of DYT6 dystonia. SN hyperechogenicity on TCS may represent an endophenotype in these patients. Pallidal DBS in two patients was unsatisfactory.

Genetic evidence for an association of the TOR1A locus with segmental/focal dystonia

Polymorphisms in the TOR1A/TOR1B region have been implicated as being associated with primary focal and segmental dystonia. In a cohort of subjects with either focal or segmental dystonia affecting the face, larynx, neck, or arm, we report a strong association of a single nucleotide polymorphism (SNP), the deletion allele at the Mtdel SNP (rs3842225), and protection from focal dystonia. In contrast, we did not find an association of either allele at the D216H SNP (rs1801968) with focal or segmental dystonia in the same cohort.

Genetic and clinical features of primary torsion dystonia

Primary torsion dystonia (PTD) is defined as a syndrome in which dystonia is the only clinical sign (except for tremor), and there is no evidence of neuronal degeneration or an acquired cause by history or routine laboratory assessment. Seven different loci have been recognized for PTD but only two of the genes have been identified. In this review we will describe the phenotypes associated with these loci and discuss the responsible gene. This article is part of a Special Issue entitled "Advances in dystonia".

Genetics of primary torsion dystonia

Advances in the genetics of dystonia have further elucidated the pathophysiology of this clinically and etiologically heterogeneous group of movement disorders. Currently, 20 monogenic forms of dystonia, designated by the acronym DYT, are grouped as 1) pure dystonias, 2) dystonia-plus syndromes, and 3) paroxysmal dystonias/dyskinesias. We summarize recently discovered genes and loci, including the 1) detection of two primary dystonia genes (DYT6, DYT16), 2) identification of the DYT17 locus, 3) association of a dystonia/dyskinesia phenotype with a gene previously linked to GLUT1 (glucose transporter of the blood-brain barrier) deficiency syndrome (DYT18), 4) designation of paroxysmal kinesigenic and nonkinesigenic dyskinesia as DYT19 and DYT20, and 5) redefinition of DYT14 as DYT5. Further, we review current knowledge regarding genetic modifiers and susceptibility factors. Because recognizing and diagnosing monogenic dystonias have important implications for patients and their families with regard to counseling, prognosis, and treatment, we highlight clinical "red flags" of individual subtypes and review guidelines for genetic testing.

Advances in the genetics of primary torsion dystonia

Knowledge about the genetics of primary torsion dystonia (PTD) has been progressing at a very slow pace compared with other movement disorders. For many years, only one causative gene was known, DYT1/TOR1A, yet the recent identification of a second PTD causative gene (DYT6/THAP1), the detection of subclinical alterations caused by mutations in PTD genes in some healthy non-penetrant individuals, and functional studies on TOR1A and THAP1 protein products have significantly improved mutation detection, genotype-phenotype correlates, and our understanding of the cellular mechanisms underlying the development of dystonia.

Expression profiling in peripheral blood reveals signature for penetrance in DYT1 dystonia

DYT1 dystonia is an autosomal-dominantly inherited movement disorder, which is usually caused by a GAG deletion in the TOR1A gene. Due to the reduced penetrance of approximately 30-40%, the determination of the mutation in a subject is of limited use with regard to actual manifestation of symptoms. In the present study, we used Affymetrix oligonucleotide microarrays to analyze global gene expression in blood samples of 15 manifesting and 15 non-manifesting mutation carriers in order to identify a susceptibility profile beyond the GAG deletion which is associated with the manifestation of symptoms in DYT1 dystonia. We identified a genetic signature which distinguished between asymptomatic mutation carriers and symptomatic DYT1 patients with 86.7% sensitivity and 100% specificity. This genetic signature could correctly predict the disease state in an independent test set with a sensitivity of 87.5% and a specificity of 85.7%. Conclusively, this genetic signature might provide a possibility to distinguish DYT1 patients from asymptomatic mutation carriers.

TorsinA and dystonia: from nuclear envelope to synapse

A GAG deletion in the DYT1 gene is responsible for the autosomal dominant movement disorder, early onset primary torsion dystonia, which is characterised by involuntary sustained muscle contractions and abnormal posturing of the limbs. The mutation leads to deletion of a single glutamate residue in the C-terminus of the protein torsinA, a member of the AAA+ ATPase family of proteins with multiple functions. Since no evidence of neurodegeneration has been found in DYT1 patients, the dystonic phenotype is likely to be the result of neuronal functional defect(s), the nature of which is only partially understood. Biochemical, structural and cell biological studies have been performed in order to characterise torsinA. These studies, together with the generation of several animal models, have contributed to identify cellular compartments and pathways, including the cytoskeleton and the nuclear envelope, the secretory pathway and the synaptic vesicle machinery where torsinA function may be crucial. However, the role of torsinA and the correlation between the dysfunction caused by the mutation and the dystonic phenotype remain unclear. This review provides an overview of the findings of the last ten years of research on torsinA, a critical evaluation of the different models proposed and insights towards future avenues of research.