Strong genetic evidence for association of TOR1A/TOR1B with idiopathic dystonia

Peripheral nerve injury elicits microstructural and neurochemical changes in the striatum and substantia nigra of a DYT-TOR1A mouse model with dystonia-like movements

The relationship between genotype and phenotype in DYT-TOR1A dystonia as well as the associated motor circuit alterations are still insufficiently understood. DYT-TOR1A dystonia has a remarkably reduced penetrance of 20-30%, which has led to the second-hit hypothesis emphasizing an important role of extragenetic factors in the symptomatogenesis of TOR1A mutation carriers. To analyze whether recovery from a peripheral nerve injury can trigger a dystonic phenotype in asymptomatic hΔGAG3 mice, which overexpress human mutated torsinA, a sciatic nerve crush was applied. An observer-based scoring system as well as an unbiased deep-learning based characterization of the phenotype showed that recovery from a sciatic nerve crush leads to significantly more dystonia-like movements in hΔGAG3 animals compared to wildtype control animals, which persisted over the entire monitored period of 12 weeks. In the basal ganglia, the analysis of medium spiny neurons revealed a significantly reduced number of dendrites, dendrite length and number of spines in the naïve and nerve-crushed hΔGAG3 mice compared to both wildtype control groups indicative of an endophenotypical trait. The volume of striatal calretinin+ interneurons showed alterations in hΔGAG3 mice compared to the wt groups. Nerve-injury related changes were found for striatal ChAT+, parvalbumin+ and nNOS+ interneurons in both genotypes. The dopaminergic neurons of the substantia nigra remained unchanged in number across all groups, however, the cell volume was significantly increased in nerve-crushed hΔGAG3 mice compared to naïve hΔGAG3 mice and wildtype littermates. Moreover, in vivo microdialysis showed an increase of dopamine and its metabolites in the striatum comparing nerve-crushed hΔGAG3 mice to all other groups. The induction of a dystonia-like phenotype in genetically predisposed DYT-TOR1A mice highlights the importance of extragenetic factors in the symptomatogenesis of DYT-TOR1A dystonia. Our experimental approach allowed us to dissect microstructural and neurochemical abnormalities in the basal ganglia, which either reflected a genetic predisposition or endophenotype in DYT-TOR1A mice or a correlate of the induced dystonic phenotype. In particular, neurochemical and morphological changes of the nigrostriatal dopaminergic system were correlated with symptomatogenesis.

Second hit hypothesis in dystonia: Dysfunctional cross talk between neuroplasticity and environment?

One of the great mysteries in dystonia pathophysiology is the role of environmental factors in disease onset and development. Progress has been made in defining the genetic components of dystonic syndromes, still the mechanisms behind the discrepant relationship between dystonic genotype and phenotype remain largely unclear. Within this review, the preclinical and clinical evidence for environmental stressors as disease modifiers in dystonia pathogenesis are summarized and critically evaluated. The potential role of extragenetic factors is discussed in monogenic as well as adult-onset isolated dystonia. The available clinical evidence for a "second hit" is analyzed in light of the reduced penetrance of monogenic dystonic syndromes and put into context with evidence from animal and cellular models. The contradictory studies on adult-onset dystonia are discussed in detail and backed up by evidence from animal models. Taken together, there is clear evidence of a gene-environment interaction in dystonia, which should be considered in the continued quest to unravel dystonia pathophysiology.

Association of TOR1A and GCH1 Polymorphisms with Isolated Dystonia in India

Isolated dystonia is a common movement disorder often caused by genetic mutations, although it is predominantly sporadic in nature. Common variants of dystonia-related genes were reported to be risk factors for idiopathic isolated dystonia. In this study, we aimed to analyse the roles of previously reported GTP cyclohydrolase (GCH1) and Torsin family 1 member A (TOR1A) polymorphisms in an Indian isolated dystonia case-control group. A total of 292 sporadic isolated dystonia patients and 316 control individuals were genotyped for single-nucleotide polymorphisms (SNPs) of GCH1 (rs3759664:G > A, rs12147422:A > G and rs10483639:C > G) and TOR1A (rs13300897:G > A, rs1801968:G > C, rs1182:G > T and rs3842225:G > Δ) using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and confirmed by direct Sanger sequencing. The statistical significance of allelic, genotypic and haplotypic associations of all of the SNPs were evaluated using the two-tailed Fisher exact test. The minor allele (A) of rs3759664 is significantly associated with isolated limb dystonia as a risk factor (p = 0.005). The minor allele (C) of rs1801968 is strongly associated with isolated dystonia (p < 0.0001) and most of its subtypes. The major allele of rs3842225 (G) may act as a significant risk factor for Writer's cramp (p = 0.03). Four different haplogroups comprising of either rs1182 or rs3842225 or in combination with rs1801968 and rs13300897 were found to be significantly associated with isolated dystonia. No other allelic, genotypic or haplotypic association was found to be significant with isolated dystonia cohort or its endophenotype stratified groups. Our study suggests that TOR1A common variants have a significant role in isolated dystonia pathogenesis in the Indian population, whereas SNPs in the GCH1 gene may have a limited role.

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.

Inherited dystonias: clinical features and molecular pathways

Recent decades have witnessed dramatic increases in understanding of the genetics of dystonia - a movement disorder characterized by involuntary twisting and abnormal posture. Hampered by a lack of overt neuropathology, researchers are investigating isolated monogenic causes to pinpoint common molecular mechanisms in this heterogeneous disease. Evidence from imaging, cellular, and murine work implicates deficiencies in dopamine neurotransmission, transcriptional dysregulation, and selective vulnerability of distinct neuronal populations to disease mutations. Studies of genetic forms of dystonia are also illuminating the developmental dependence of disease symptoms that is typical of many forms of the disease. As understanding of monogenic forms of dystonia grows, a clearer picture will develop of the abnormal motor circuitry behind this relatively common phenomenology. This chapter focuses on the current data covering the etiology and epidemiology, clinical presentation, and pathogenesis of four monogenic forms of isolated dystonia: DYT-TOR1A, DYT-THAP1, DYT-GCH1, and DYT-GNAL.

Dissecting the genetic components of a quantitative trait locus for blood pressure and renal pathology on rat chromosome 3

BACKGROUND

We have previously confirmed the importance of rat chromosome 3 (RNO3) genetic loci on blood pressure elevation, pulse pressure (PP) variability and renal pathology during salt challenge in the stroke-prone spontaneously hypertensive (SHRSP) rat. The aims of this study were to generate a panel of RNO3 congenic sub-strains to genetically dissect the implicated loci and identify positional candidate genes by microarray expression profiling and analysis of next-generation sequencing data.

METHOD AND RESULTS

A panel of congenic sub-strains were generated containing Wistar-Kyoto (WKY)-introgressed segments of varying size on the SHRSP genetic background, focused within the first 50 Mbp of RNO3. Haemodynamic profiling during salt challenge demonstrated significantly reduced systolic blood pressure, diastolic blood pressure and PP variability in SP.WKYGla3a, SP.WKYGla3c, SP.WKYGla3d and SP.WKYGla3e sub-strains. Only SBP and DBP were significantly reduced during salt challenge in SP.WKYGla3b and SP.WKYGla3f sub-strains, whereas SP.WKYGla3g rats did not differ in haemodynamic response to SHRSP. Those sub-strains demonstrating significantly reduced PP variability during salt challenge also demonstrated significantly reduced renal pathology and proteinuria. Microarray expression profiling prioritized two candidate genes for blood pressure regulation (Dnm1, Tor1b), localized within the common congenic interval shared by SP.WKYGla3d and SP.WKYGla3f strains, and one candidate gene for salt-induced PP variability and renal pathology (Rabgap1), located within the region unique to the SP.WKYGla3d strain. Comparison of next-generation sequencing data identified variants within additional positional genes that are likely to affect protein function.

CONCLUSION

This study has identified distinct intervals on RNO3-containing genes that may be important for blood pressure regulation and renal pathology during salt challenge.

Deletion variant rs35153737 in TOR1A is associated with isolated dystonia in a Southwestern Chinese Population

BACKGROUND

TOR1A plays a very important role in early-onset isolated dystonia. Studying the association between the common variants of this gene and dystonia can help us understand the connection between TOR1A mutations and this disease.

METHODS

The TOR1A exon 5 was sequenced in 223 isolated dystonia patients and 210 age-adjusted controls. Patients and controls all came from Southwest China.

RESULTS

The following two common variants were found in the 3'-UTR of TOR1A: NM_000113.2:c.*414delG (rs35153737) and NM_000113.2:c.*824delG (rs3842225). The rs35153737 variant showed a statistically significant association with dystonia using the allele model (P=0.035) and the dominant genetic model (P=0.018); however, no association between rs3842225 and dystonia was found.

CONCLUSION

Our study suggests that there is an association between rs35153737 and dystonia in a southwestern Chinese population, and it may be caused by high linkage disequilibrium between this deletion and potential pathogenic variants in TOR1A.

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.

Association analysis of TOR1A polymorphisms rs2296793 and rs3842225 in a Chinese population with cervical dystonia

BACKGROUND

TOR1A (torsinA, DYT1) is the leading cause of early-onset generalized dystonia, however, the associations between common TOR1A single nucleotide polymorphisms (SNPs) and primary adult-onset focal dystonia are controversial.

METHODS

In a cohort of 201 focal cervical dystonia (CD) patients, we genotyped rs2296793 and rs3842225 SNPs in TOR1A using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis. We also included 289 unrelated, age- and sex-matched healthy controls (HCs) from the same region.

RESULT

No significant differences were found in either the genotype distributions or minor allele frequencies (MAFs) of rs2296793 and rs3842225 between CD patients and HCs. There were no significant differences between early-onset and late-onset CD patients, between patients with and without a positive family history of dystonia, or between patients with and without tremor or sensory tricks.

CONCLUSION

Our study suggests that the common rs2296793 and rs3842225 SNPs of TOR1A do not play a major role in CD in a Chinese population.

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.

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.

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.

Association of rs1182 polymorphism of the DYT1 gene with primary dystonia in Chinese population

BACKGROUND

The deletion mutation of glutamate codon (GAG) in the TOR1A gene is a major cause of primary generalized dystonia. Recent genetic studies suggest that the rs1182 polymorphism in the same gene may represent a risk factor for primary dystonia. However, this finding has been inconsistent. Furthermore, no data on such an association in a Chinese population have been published.

METHODS

A total of 291 patients with primary dystonia from the Department of Neurology, West China Hospital of Sichuan University were included. From the same region, 294 healthy individuals were recruited as a control group. The SNP was identified by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

In the present study, focal dystonia was the most common presented form. No difference was found in the genotype frequency, minor allele frequencies, and "G" allele frequency between all dystonia patients and controls. No difference was found either, between early- and late-onset dystonia patients, patients with and without a positive family history, patients with pain and without pain, and patients with and without sensory trick. Moreover, no significant differences in the genotype and allele frequencies were found among different dystonia subtypes.

CONCLUSION

No association of the rs1182 of TOR1A with Chinese primary dystonia was found. More studies on such an association involving a larger number of participants, especially from Asian populations, are needed to confirm the present findings.

Gene expression changes in a transgenic mouse model overexpressing human wildtype and mutant torsinA

Primary torsion dystonia is an autosomal-dominantly inherited, neurodevelopmental movement disorder caused by a GAG deletion (ΔGAG) in the DYT1 gene, encoding torsinA. This mutation is responsible for approximately 70% of cases of early-onset primary torsion dystonia. The function of wildtype torsinA is still unknown, and it is unsolved how the deletion in the DYT1 gene contributes to the development of the disease. To better understand the molecular processes involved in torsinA pathology, we used genome-wide oligonucleotide microarrays to characterize gene expression patterns in the striatum of mouse models overexpressing the human wildtype and mutant torsinA. By this approach we were able to detect gene expression changes that seem to be specific for torsinA pathology. We found an impact of torsinA, independent from genotype, on vesicle trafficking, exocytosis, and neurotransmitter release in our mouse model. In addition, we were able to identify several new pathways and processes involved in the development of the nervous system that are affected by wildtype and mutant torsinA. Furthermore, we have striking evidence from our gene expression data that glutamate receptor mediated synaptic plasticity in the striatum is the affected underlying cellular process for impaired motor learning in human ΔGAG torsinA transgenic mice.

Genetic background modulates the phenotype of a mouse model of DYT1 dystonia

DYT1 dystonia is a debilitating neurological disease characterized by involuntary twisting movements. The disease is caused by an in-frame deletion (GAG, "ΔE") mutation in the TOR1A gene that encodes the torsinA protein. Intriguingly, only 30% of mutation carriers exhibit motor symptoms despite the fact that functional brain imaging studies show abnormal brain metabolism in all carriers. Because genetic modifiers may be a determinant of this reduced penetrance, we examined the genetic contribution of three different inbred strains of mice on the DYT1 mutation in animals that are homozygous (Tor1a(ΔE/ΔE)) or heterozygous (Tor1a(ΔE/+); disease state) for the disease-causing ΔE mutation. We find that the DBA/2J, C57BL/6J, and CD1-ICR contribution of genes significantly alter lifespan in Tor1a(ΔE/ΔE) mice, which die during the first few days of life on the 129S6/SvEvTac (129) background. The C57BL/6J (B6) strain significantly decreases life expectancy of Tor1a(ΔE/ΔE) animals but, like 129S6/SvEvTac Tor1a(ΔE/+) mice, congenic C57BL/6J Tor1a(ΔE/+) mice do not exhibit any motor abnormalities. In contrast, the DBA/2J (D2) strain significantly increases life expectancy. This effect was not present in congenic DBA/2J Tor1a(ΔE/ΔE) mice, indicating that the extended lifespan of F2 129/D2 mice was due to a combination of homozygous and heterozygous allelic effects. Our observations suggest that genetic modifiers may alter the penetrance of the ΔE mutation, and that mapping these modifiers may provide fresh insight into the torsinA molecular pathway.

Common polymorphisms in dystonia-linked genes and susceptibility to the sporadic primary dystonias

Genes involved in familial dystonia syndromes (DYT genes) are ideal candidates for investigating whether common genetic variants influence the susceptibility to sporadic primary dystonia. To date, there have been few candidate gene studies for primary dystonia and only two DYT genes, TOR1A and THAP1, have been assessed. We therefore employed a haplotype-tagging strategy to comprehensively assess if common polymorphisms in eight DYT genes (TOR1A, TAF1, GCH1, THAP1, MR-1 (PNKD), SGCE, ATP1A3 and PRKRA) confer risk for sporadic primary dystonia. The 230 primary dystonia cases were matched for age and gender to 228 controls, recruited from movement disorder clinics in Brisbane, Australia and the Australian electoral roll. All subjects were genotyped for 56 tagging SNPs and genotype associations were investigated. Modest genotypic associations (P<0.05) were observed for three GCH1 SNPs (rs12147422, rs3759664 and rs10483639) when comparing all cases against controls. Associations were also seen when the cases were stratified based on presentation. Overall, our findings do not support the hypothesis that common TOR1A variants affect susceptibility for sporadic primary dystonia, and that it is unlikely that common variants around the DYT genes confer substantial risk for sporadic primary dystonia. Further work is warranted to follow up the GCH1 SNPs and the subgroup analyses.

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.

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.

Chemical enhancement of torsinA function in cell and animal models of torsion dystonia

Movement disorders represent a significant societal burden for which therapeutic options are limited and focused on treating disease symptomality. Early-onset torsion dystonia (EOTD) is one such disorder characterized by sustained and involuntary muscle contractions that frequently cause repetitive movements or abnormal postures. Transmitted in an autosomal dominant manner with reduced penetrance, EOTD is caused in most cases by the deletion of a glutamic acid (DeltaE) in the DYT1 (also known as TOR1A) gene product, torsinA. Although some patients respond well to anticholingerics, therapy is primarily limited to either neurosurgery or chemodenervation. As mutant torsinA (DeltaE) expression results in decreased torsinA function, therapeutic strategies directed toward enhancement of wild-type (WT) torsinA activity in patients who are heterozygous for mutant DYT1 may restore normal cellular functionality. Here, we report results from the first-ever screen for candidate small molecule therapeutics for EOTD, using multiple activity-based readouts for torsinA function in Caenorhabditis elegans, subsequent validation in human DYT1 patient fibroblasts, and behavioral rescue in a mouse model of DYT1 dystonia. We exploited the nematode to rapidly discern chemical effectors of torsinA and identified two classes of antibiotics, quinolones and aminopenicillins, which enhance WT torsinA activity in two separate in vivo assays. Representative molecules were assayed in EOTD patient fibroblasts for improvements in torsinA-dependent secretory function, which was improved significantly by ampicillin. Furthermore, a behavioral defect associated with an EOTD mouse knock-in model was also rescued following administration of ampicillin. These combined data indicate that specific small molecules that enhance torsinA activity represent a promising new approach toward therapeutic development for EOTD, and potentially for other diseases involving the processing of mutant proteins.

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.

Functional evidence implicating a novel TOR1A mutation in idiopathic, late-onset focal dystonia

BACKGROUND

TOR1A encodes a chaperone-like AAA-ATPase whose Delta GAG (Delta E) mutation is responsible for an early onset, generalised dystonia syndrome. Because of the established role of the TOR1A gene in heritable generalised dystonia (DYT1), a potential genetic contribution of TOR1A to the more prevalent and diverse presentations of late onset, focal dystonia has been suggested.

RESULTS

A novel TOR1A missense mutation (c.613T-->A, p.F205I) in a patient with late onset, focal dystonia is reported. The mutation occurs in a highly evolutionarily conserved region encoding the AAA-ATPase domain. Expression assays revealed that expression of F205I or Delta E, but not wildtype TOR1A, produced frequent intracellular inclusions.

CONCLUSIONS

A novel, rare TOR1A variant has been identified in an individual with late onset, focal dystonia and evidence provided that the mutation impairs TOR1A function. Together these findings raise the possibility that this novel TOR1A variant may contribute to the expression of dystonia. In light of these findings, a more comprehensive genetic effort is warranted to identify the role of this and other rare TOR1A variants in the expression of late onset, focal dystonia.

Genetics and treatment of dystonia

The torsion dystonias encompass a broad collection of etiologic subtypes, often divided into primary and secondary classes. Tremendous advances have been made in uncovering the genetic basis of dystonia, including discovery of a gene causing early onset primary torsion dystonia-a GAG deletion in exon 5 of the DYT1 gene that encodes torsinA. Although the exact function of torsinA remains elusive, evidence suggests aberrant localization and interaction of mutated protein; this may result in an abnormal response to stress or interference with cytoskeletal events and the development of neuronal brain pathways. Breakthroughs include the discovery of a genetic modifier that protects against clinical expression in DYT1 dystonia and the identification of the gene causing DYT6, THAP1. The authors review genetic etiologies and discuss phenotypes as well as counseling of patients regarding prognosis and progression of the disease. They also address pharmacologic and surgical treatment options for various forms of dystonia.

The TOR1A polymorphism rs1182 and the risk of spread in primary blepharospasm

We studied the influence of the rs1182 polymorphism of the TOR1A gene on the risk of dystonia spread in two representative cohorts of patients presenting with primary blepharospasm (BSP), one from Italy and the other from the United States of America. The relationship between rs1182 polymorphism and spread was estimated by Kaplan-Meier survival curves and Cox proportional hazard regression models adjusted by age and sex, age of BSP onset. In both series, patients carrying the T allele (G/T or T/T) in the rs1182 polymorphism were more likely to have dystonia spread as compared with the homozygous carriers of the common G allele. The comparable findings obtained in two independent cohorts support a genetic contribution to BSP spread.

High-throughput mutational analysis of TOR1A in primary dystonia

Background

Although the c.904_906delGAG mutation in Exon 5 of TOR1A typically manifests as early-onset generalized dystonia, DYT1 dystonia is genetically and clinically heterogeneous. Recently, another Exon 5 mutation (c.863G>A) has been associated with early-onset generalized dystonia and some ΔGAG mutation carriers present with late-onset focal dystonia. The aim of this study was to identify TOR1A Exon 5 mutations in a large cohort of subjects with mainly non-generalized primary dystonia.

Methods

High resolution melting (HRM) was used to examine the entire TOR1A Exon 5 coding sequence in 1014 subjects with primary dystonia (422 spasmodic dysphonia, 285 cervical dystonia, 67 blepharospasm, 41 writer's cramp, 16 oromandibular dystonia, 38 other primary focal dystonia, 112 segmental dystonia, 16 multifocal dystonia, and 17 generalized dystonia) and 250 controls (150 neurologically normal and 100 with other movement disorders). Diagnostic sensitivity and specificity were evaluated in an additional 8 subjects with known ΔGAG DYT1 dystonia and 88 subjects with ΔGAG-negative dystonia.

Results

HRM of TOR1A Exon 5 showed high (100%) diagnostic sensitivity and specificity. HRM was rapid and economical. HRM reliably differentiated the TOR1A ΔGAG and c.863G>A mutations. Melting curves were normal in 250/250 controls and 1012/1014 subjects with primary dystonia. The two subjects with shifted melting curves were found to harbor the classic ΔGAG deletion: 1) a non-Jewish Caucasian female with childhood-onset multifocal dystonia and 2) an Ashkenazi Jewish female with adolescent-onset spasmodic dysphonia.

Conclusion

First, HRM is an inexpensive, diagnostically sensitive and specific, high-throughput method for mutation discovery. Second, Exon 5 mutations in TOR1A are rarely associated with non-generalized primary dystonia.

TorsinA binds the KASH domain of nesprins and participates in linkage between nuclear envelope and cytoskeleton

A specific mutation (DeltaE) in torsinA underlies most cases of the dominantly inherited movement disorder, early-onset torsion dystonia (DYT1). TorsinA, a member of the AAA+ ATPase superfamily, is located within the lumen of the nuclear envelope (NE) and endoplasmic reticulum (ER). We investigated an association between torsinA and nesprin-3, which spans the outer nuclear membrane (ONM) of the NE and links it to vimentin via plectin in fibroblasts. Mouse nesprin-3alpha co-immunoprecipitated with torsinA and this involved the C-terminal region of torsinA and the KASH domain of nesprin-3alpha. This association with human nesprin-3 appeared to be stronger for torsinADeltaE than for torsinA. TorsinA also associated with the KASH domains of nesprin-1 and -2 (SYNE1 and 2), which link to actin. In the absence of torsinA, in knockout mouse embryonic fibroblasts (MEFs), nesprin-3alpha was localized predominantly in the ER. Enrichment of yellow fluorescent protein (YFP)-nesprin-3 in the ER was also seen in the fibroblasts of DYT1 patients, with formation of YFP-positive globular structures enriched in torsinA, vimentin and actin. TorsinA-null MEFs had normal NE structure, but nuclear polarization and cell migration were delayed in a wound-healing assay, as compared with wild-type MEFs. These studies support a role for torsinA in dynamic interactions between the KASH domains of nesprins and their protein partners in the lumen of the NE, with torsinA influencing the localization of nesprins and associated cytoskeletal elements and affecting their role in nuclear and cell movement.

Experimental therapeutics for dystonia

Dystonia is a neurological syndrome characterized by excessive involuntary muscle contractions leading to twisting movements and unnatural postures. It has many different clinical manifestations, and many different causes. More than 3 million people worldwide suffer from dystonia, yet there are few broadly effective treatments. In the past decade, progress in research has advanced our understanding of the pathogenesis of dystonia to a point where drug discovery efforts are now feasible. Several strategies can be used to develop novel therapeutics for dystonia. Existing therapies have only modest efficacy, but may be refined and improved to increase benefits while reducing side effects. Identifying rational targets for drug intervention based on the pathogenesis of dystonia is another strategy. The surge in both basic and clinical research discoveries has provided insights at all levels, including etiological, physiological and nosological, to enable such a targeted approach. The empirical approach to drug discovery, whereby compounds are identified using a nonmechanistic strategy, is complementary to the rational approach. With the recent development of multiple animal models of dystonia, it is now possible to develop assays and perform drug screens on vast numbers of compounds. This multifaceted approach to drug discovery in dystonia will likely provide lead compounds that can then be translated for clinical use.

TorsinA and DYT1 early-onset dystonia

Early-onset torsion dystonia is a severe generalized form of primary dystonia, with most cases caused by a specific mutation (ΔGAG) in the DYT1 gene encoding torsinA. This mutation is autosomal dominant and is thought to result in reduced torsinA activity. TorsinA is an AAA protein located in the lumen of the endoplasmic reticulum and nuclear envelope of most cells (with high levels in some brain neurons). It is thought to serve as a chaperone protein and/or a link between these membranes and the cytoskeleton. Other sequence variations in DYT1 can affect penetrance of the ΔGAG mutation and may be associated with more common, late-onset focal forms of dystonia. Animal models of DYT1 dystonia are emerging that will allow preclinical evaluation of drugs that can be used to prevent or treat this non-neurodegenerative neurologic disease.

Intragenic Cis and Trans modification of genetic susceptibility in DYT1 torsion dystonia

A GAG deletion in the DYT1 gene is a major cause of early-onset dystonia, but clinical disease expression occurs in only 30% of mutation carriers. To gain insight into genetic factors that may influence penetrance, we evaluated three DYT1 single-nucleotide polymorphisms, including D216H, a coding-sequence variation that moderates the effects of the DYT1 GAG deletion in cellular models. We tested DYT1 GAG-deletion carriers with (n=119) and without (n=113) clinical signs of dystonia and control individuals (n=197) and found the frequency of the 216H allele to be increased in GAG-deletion carriers without dystonia and to be decreased in carriers with dystonia, compared with the control individuals. Analysis of haplotypes demonstrated a highly protective effect of the H allele in trans with the GAG deletion; there was also suggestive evidence that the D216 allele in cis is required for the disease to be penetrant. Our findings establish, for the first time, a clinically relevant gene modifier of DYT1.

Overexpression of human wildtype torsinA and human DeltaGAG torsinA in a transgenic mouse model causes phenotypic abnormalities

Primary torsion dystonia is an autosomal-dominant inherited movement disorder. Most cases are caused by an in-frame deletion (GAG) of the DYT1 gene encoding torsinA. Reduced penetrance and phenotypic variability suggest that alteration of torsinA amino acid sequence is necessary but not sufficient for development of clinical symptoms and that additional factors must contribute to the factual manifestation of the disease. We generated 4 independent transgenic mouse lines, two overexpressing human mutant torsinA and two overexpressing human wildtype torsinA using a strong murine prion protein promoter. Our data provide for the first time in vivo evidence that not only mutant torsinA is detrimental to neuronal cells but that also wildtype torsinA can lead to neuronal dysfunction when overexpressed at high levels. This hypothesis is supported by (i) neuropathological findings, (ii) neurochemistry, (iii) behavioral abnormalities and (iv) DTI-MRI analysis.