The TOR1A (DYT1) gene family and its role in early onset torsion dystonia

TOR2A Variants in Blepharospasm

Background

Genetic factors have been implicated in the pathogenesis of blepharospasm (BSP), a dystonia characterized by excessive blinking and involuntary eyelid closure. Previous research identified a co-segregating deleterious TOR2A variant (GRCh38/hg38, NC_000009.12: g.127733410G>A, NM_001085347.3:c.568C>T, p. Arg190Cys) in three subjects with BSP and three carriers within a multi-generation pedigree. Other TOR2A variants have been reported in patients with dystonia.

Methods

Sanger sequencing was used to screen a cohort of 307 subjects with isolated BSP or BSP-plus dystonia affecting additional anatomical segments (BSP+). We also utilized computational tools to uniformly assess the deleteriousness and potential pathogenicity of previously reported TOR2A variants.

Results

There were no highly deleterious TOR2A variants in the coding or contiguous splice site regions of TOR2A within our cohort of 307 subjects.

Discussion

Highly deleterious variants in TOR2A are rare in patients with BSP/BSP+ phenotypes.

Highlights

Over 300 patients with BSP were screened for variants in TOR2A, a TOR1A (DYT1) homologue. No highly deleterious variants were identified in our cohort. The role of TOR2A in BSP and other forms of dystonia remains indeterminant.

Upregulated expression of a TOR2A gene product-salusin-β in the paraventricular nucleus enhances sympathetic activity and cardiac sympathetic afferent reflex in rats with chronic heart failure induced by coronary artery ligation

AIM

Enhanced cardiac sympathetic afferent reflex (CSAR) promotes sympathetic hyperactivation in chronic heart failure (CHF). Salusin-β is a torsin family 2 member A (TOR2A) gene product and a cardiovascular active peptide closely associated with cardiovascular diseases. We aimed to determine the roles of salusin-β in the paraventricular nucleus (PVN) in modulating enhanced CSAR and sympathetic hyperactivation in rats with CHF induced by coronary artery ligation and elucidate the underlying molecular mechanisms.

METHODS

CSAR was evaluated based on the responses of mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) to the epicardial administration of capsaicin in rats under anesthesia.

RESULTS

Salusin-β protein expression was upregulated in the PVN of the CHF compared with sham-operated rats. Salusin-β microinjection into the PVN dose-dependently increased MAP and RSNA and enhanced CSAR, while anti-salusin-β IgG exerted opposite effects. The effect of salusin-β was inhibited by reactive oxygen species (ROS) scavenger or NAD(P)H oxidase inhibitor but promoted by superoxide dismutase inhibitor. The effect of anti-salusin-β IgG was interdicted by nitric oxide (NO) synthase inhibitor. Furthermore, chronic salusin-β gene knockdown in PVN attenuated CSAR, reduced sympathetic output, improved myocardial remodeling and cardiac function, decreased NAD(P)H oxidase activity and ROS levels, and increased NO levels in the CHF rats.

CONCLUSION

Increased salusin-β activity in the PVN contributes to sympathetic hyperactivation and CSAR in CHF by inhibiting NO release and stimulating NAD(P)H oxidase-ROS production. Reducing endogenous central salusin-β expression might be a novel strategy for preventing and treating CHF in the future.

Pathophysiology of Dyt1-Tor1a dystonia in mice is mediated by spinal neural circuit dysfunction

Dystonia, a neurological disorder defined by abnormal postures and disorganized movements, is considered to be a neural circuit disorder with dysfunction arising within and between multiple brain regions. Given that spinal neural circuits constitute the final pathway for motor control, we sought to determine their contribution to this movement disorder. Focusing on the most common inherited form of dystonia in humans, DYT1-TOR1A, we generated a conditional knockout of the torsin family 1 member A (Tor1a) gene in the mouse spinal cord and dorsal root ganglia (DRG). We found that these mice recapitulated the phenotype of the human condition, developing early-onset generalized torsional dystonia. Motor signs emerged early in the mouse hindlimbs before spreading caudo-rostrally to affect the pelvis, trunk, and forelimbs throughout postnatal maturation. Physiologically, these mice bore the hallmark features of dystonia, including spontaneous contractions at rest and excessive and disorganized contractions, including cocontractions of antagonist muscle groups, during voluntary movements. Spontaneous activity, disorganized motor output, and impaired monosynaptic reflexes, all signs of human dystonia, were recorded from isolated mouse spinal cords from these conditional knockout mice. All components of the monosynaptic reflex arc were affected, including motor neurons. Given that confining the Tor1a conditional knockout to DRG did not lead to early-onset dystonia, we conclude that the pathophysiological substrate of this mouse model of dystonia lies in spinal neural circuits. Together, these data provide new insights into our current understanding of dystonia pathophysiology.

Serum salusin-β levels in patients with systemic lupus erythematosus

OBJECTIVE

Endothelial dysfunction (ED) has an important role in the pathogenesis of systemic lupus erythematosus (SLE). Studies on other inflammatory diseases show that salusin-β with various mechanisms may play a role in the promotion of ED and inflammation. The aim of this study was to measure serum salusin-β levels in SLE patients and evaluate it as a potential biomarker in assessing SLE activity and predicting organ involvement.

METHODS

In a cross-sectional study, 60 patients diagnosed with SLE and 30 age- and sex-matched healthy controls were enrolled. Disease activity of SLE patients was assessed by the systemic lupus erythematosus disease activity index 2000 (SLEDAI-2 K). Serum levels of salusin-ß were measured using a human salusin-ß enzyme-linked immunosorbent assay kit.

RESULTS

Serum salusin-β levels in SLE and control groups were 474.2 ± 117.1 pg/ml and 157.7 ± 88.7 pg/ml, respectively. The difference was significant (P = 0.001). There was no significant correlation between serum salusin-β levels with age (r =  - 0.06, P = 0.632) and SLEDAI (r =  - 0.185, P = 0.158). In patients with nephritis and thrombosis, serum salusin-β was significantly higher. In addition, in patients with serositis, serum salusin-β was significantly lower. Multiple linear regression analysis showed that serum salusin-β levels retained a significant association with nephritis and thrombosis after model adjustment for serositis, nephritis, and thrombosis.

CONCLUSIONS

Our findings showed that salusin-β might have a possible role in the pathogenesis of SLE. Salusin-β may be a potential biomarker for nephritis and thrombosis in SLE. Key Points • Serum salusin-β levels were significantly higher in SLE patients than the control group. • There was no significant correlation between serum salusin-β levels with age and SLEDAI. • Serum salusin-β levels retained a significant association with nephritis and thrombosis.

TorsinA is essential for the timing and localization of neuronal nuclear pore complex biogenesis

Nuclear pore complexes (NPCs) regulate information transfer between the nucleus and cytoplasm. NPC defects are linked to several neurological diseases, but the processes governing NPC biogenesis and spatial organization are poorly understood. Here, we identify a temporal window of strongly upregulated NPC biogenesis during neuronal maturation. We demonstrate that the AAA+ protein torsinA, whose loss of function causes the neurodevelopmental movement disorder DYT-TOR1A (DYT1) dystonia, coordinates NPC spatial organization during this period without impacting total NPC density. Using a new mouse line in which endogenous Nup107 is Halo-Tagged, we find that torsinA is essential for correct localization of NPC formation. In the absence of torsinA, the inner nuclear membrane buds excessively at sites of mislocalized, nascent NPCs, and NPC assembly completion is delayed. Our work implies that NPC spatial organization and number are independently regulated and suggests that torsinA is critical for the normal localization and assembly kinetics of NPCs.

TOR1AIP1-Associated Nuclear Envelopathies

Human TOR1AIP1 encodes LAP1, a nuclear envelope protein expressed in most human tissues, which has been linked to various biological processes and human diseases. The clinical spectrum of diseases related to mutations in TOR1AIP1 is broad, including muscular dystrophy, congenital myasthenic syndrome, cardiomyopathy, and multisystemic disease with or without progeroid features. Although rare, these recessively inherited disorders often lead to early death or considerable functional impairment. Developing a better understanding of the roles of LAP1 and mutant TOR1AIP1-associated phenotypes is paramount to allow therapeutic development. To facilitate further studies, this review provides an overview of the known interactions of LAP1 and summarizes the evidence for the function of this protein in human health. We then review the mutations in the TOR1AIP1 gene and the clinical and pathological characteristics of subjects with these mutations. Lastly, we discuss challenges to be addressed in the future.

Salusins: advance in cardiovascular disease research

Salusins are discovered in 2003 and divided into salusin-α and salusin-β, which are bioactive peptides with hemodynamic and mitotic activity and mainly distributed in plasma, urine, endocrine glands and kidneys. A large number of studies have shown that salusins can regulate lipid metabolism, inflammatory response and vascular proliferation. Despite the profound and diverse physiological properties of salusins, the exact mechanism of their cardiovascular effects remains to be determined. The potential mechanisms of action of salusins in cardiovascular-related diseases such as atherosclerosis, hypertension, heart failure, myocardial infarction and myocarditis, and their use as biomarkers of cardiovascular disease are discussed. This review aims to provide a new strategy for the diagnosis and prevention of clinical cardiovascular diseases.

Synaptic Dysfunction in Dystonia: Update From Experimental Models

Dystonia, the third most common movement disorder, refers to a heterogeneous group of neurological diseases characterized by involuntary, sustained or intermittent muscle contractions resulting in repetitive twisting movements and abnormal postures. In the last few years, several studies on animal models helped expand our knowledge of the molecular mechanisms underlying dystonia. These findings have reinforced the notion that the synaptic alterations found mainly in the basal ganglia and cerebellum, including the abnormal neurotransmitters signalling, receptor trafficking and synaptic plasticity, are a common hallmark of different forms of dystonia. In this review, we focus on the major contribution provided by rodent models of DYT-TOR1A, DYT-THAP1, DYT-GNAL, DYT/ PARK-GCH1, DYT/PARK-TH and DYT-SGCE dystonia, which reveal that an abnormal motor network and synaptic dysfunction represent key elements in the pathophysiology of dystonia.

Effect Modification between Genes and Environment and Parkinson's Disease Risk

Parkinson's disease (PD) is a complex neurodegenerative condition in which genetic and environmental factors interact to contribute to its etiology. Remarkable progress has been made in deciphering disease etiology through genetic approaches, but there is limited data about how environmental and genetic factors interact to modify penetrance, risk, and disease severity. Here, we provide insights into environmental modifiers of PD, discussing precedents from other neurological and non-neurological conditions. Based on these examples, we outline genetic and environmental factors contributing to PD and review potential environmental modifiers of penetrance and clinical variability in monogenic and idiopathic PD. We also highlight the potential challenges and propose how future studies might tackle these important questions. ANN NEUROL 2022;92:715-724.

Alpha-Synuclein is Involved in DYT1 Dystonia Striatal Synaptic Dysfunction

Background

The neuronal protein alpha‐synuclein (α‐Syn) is crucially involved in Parkinson's disease pathophysiology. Intriguingly, torsinA (TA), the protein causative of DYT1 dystonia, has been found to accumulate in Lewy bodies and to interact with α‐Syn. Both proteins act as molecular chaperones and control synaptic machinery. Despite such evidence, the role of α‐Syn in dystonia has never been investigated.

Objective

We explored whether α‐Syn and N‐ethylmaleimide sensitive fusion attachment protein receptor proteins (SNAREs), that are known to be modulated by α‐Syn, may be involved in DYT1 dystonia synaptic dysfunction.

Methods

We used electrophysiological and biochemical techniques to study synaptic alterations in the dorsal striatum of the Tor1a⁺/^Δgag mouse model of DYT1 dystonia.

Results

In the Tor1a^+/Δgag DYT1 mutant mice, we found a significant reduction of α‐Syn levels in whole striata, mainly involving glutamatergic corticostriatal terminals. Strikingly, the striatal levels of the vesicular SNARE VAMP‐2, a direct α‐Syn interactor, and of the transmembrane SNARE synaptosome‐associated protein 23 (SNAP‐23), that promotes glutamate synaptic vesicles release, were markedly decreased in mutant mice. Moreover, we detected an impairment of miniature glutamatergic postsynaptic currents (mEPSCs) recorded from striatal spiny neurons, in parallel with a decreased asynchronous release obtained by measuring quantal EPSCs (qEPSCs), which highlight a robust alteration in release probability. Finally, we also observed a significant reduction of TA striatal expression in α‐Syn null mice.

Conclusions

Our data demonstrate an unprecedented relationship between TA and α‐Syn, and reveal that α‐Syn and SNAREs alterations characterize the synaptic dysfunction underlying DYT1 dystonia. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Knockdown of Salusin-β Improves Cardiovascular Function in Myocardial Infarction-Induced Chronic Heart Failure Rats

Salusin-β is a biologically active peptide with 20 amino acids that exerts several cardiovascular activity-regulating effects, such as regulating vascular endothelial function and the proliferation of vascular smooth muscle cells. However, the regulatory effects of salusin-β in myocardial infarction-induced chronic heart failure (CHF) are still unknown. The current study is aimed at investigating the effects of silencing salusin-β on endothelial function, cardiac function, vascular and myocardial remodeling, and its underlying signaling pathways in CHF rats induced by coronary artery ligation. CHF and sham-operated (Sham) rats were subjected to tail vein injection of adenoviral vectors encoding salusin-β shRNA or a control-shRNA. The coronary artery (CA), pulmonary artery (PA), and mesenteric artery (MA) were isolated from rats, and isometric tension measurements of arteries were performed. Compared with Sham rats, the plasma salusin-β, leptin and visfatin levels and the salusin-β protein expression levels of CA, PA, and MA were increased, while the acetylcholine- (ACh-) induced endothelium-dependent vascular relaxation of CA, PA, and MA was attenuated significantly in CHF rats and was improved significantly by salusin-β gene knockdown. Salusin-β knockdown also improved cardiac function and vascular and myocardial remodeling, increased endothelial nitric oxide synthase (eNOS) activity and nitric oxide (NO) levels, and decreased NAD(P)H oxidase activity, NOX-2 and NOX-4 expression, and reactive oxygen species (ROS) levels in arteries in CHF rats. The effects of salusin-β knockdown in CHF rats were attenuated significantly by pretreatment with the NOS inhibitor L-NAME. These results indicate that silencing salusin-β contributes to the improvement of endothelial function, cardiac function, and cardiovascular remodeling in CHF by inhibiting NAD(P)H oxidase-ROS generation and activating eNOS-NO production.

Effect of Gabapentin in a Neuropathic Pain Model in Mice Overexpressing Human Wild-Type or Human Mutated Torsin A

BACKGROUND

DYT1 dystonia is the most common form of early-onset inherited dystonia, which is caused by mutation of torsin A (TA) belonging to the "ATPases associated with a variety of cellular activities" (AAA + ATPase). Dystonia is often accompanied by pain, and neuropathic pain can be associated to peripherally induced movement disorder and dystonia. However, no evidence exists on the effect of gabapentin in mice subjected to neuropathic pain model overexpressing human normal or mutated TA.

METHODS

Mice subjected to L5 spinal nerve ligation (SNL) develop mechanical allodynia and upregulation of the α2δ-1 L-type calcium channel subunit, forming a validated experimental model of neuropathic pain. Under these experimental conditions, TA is expressed in dorsal horn neurons and astrocytes and colocalizes with α2δ-1. Similar to this subunit, TA is overexpressed in dorsal horn 7 days after SNL. This model has been used to investigate (1) basal mechanical sensitivity; (2) neuropathic pain phases; and (3) the effect of gabapentin, an α2δ-1 ligand used against neuropathic pain, in non-transgenic (NT) C57BL/6 mice and in mice overexpressing human wild-type (hWT) or mutant (hMT) TA.

RESULTS

In comparison to non-transgenic mice, the threshold for mechanical sensitivity in hWT or hMT does not differ (Kruskal-Wallis test = 1.478; p = 0.4777, although, in the latter animals, neuropathic pain recovery phase is delayed. Interestingly, gabapentin (100 mg/Kg) reduces allodynia at its peak (occurring between post-operative day 7 and day 10) but not in the phase of recovery.

CONCLUSIONS

These data lend support to the investigation on the role of TA in the molecular machinery engaged during neuropathic pain.

Resolving the TorsinA Oligomerization Conundrum: The Glycan Hypothesis

TorsinA is a AAA+ ATPase involved in the severe neurological disease Early Onset Torsion Dystonia. Despite the impressive progress in the field in the recent years, the structural organization and function of this intriguing molecule is still not clear. One outstanding difference between torsinA and other AAA+ ATPases is that torsinA is a glycoprotein. TorsinA N-linked glycans impact torsinA biogenesis and subcellular localization. Here, we propose that torsinA glycans also modulate torsinA oligomerization properties. We used structural modeling to test this idea, and show that N-linked glycans appear to restrict torsinA's ability to form closed homohexameric ring assemblies, and instead promote an open hexameric conformation that allows torsinA interaction with key cofactors required for ATP hydrolysis. This mechanism would make torsinA a prime example of Nature's sophisticated molecular glycoengineering.

Mutant Allele-Specific CRISPR Disruption in DYT1 Dystonia Fibroblasts Restores Cell Function

Most individuals affected with DYT1 dystonia have a heterozygous 3-bp deletion in the TOR1A gene (c.907_909delGAG). The mutation appears to act through a dominant-negative mechanism compromising normal torsinA function, and it is proposed that reducing mutant torsinA may normalize torsinA activity. In this study, we used an engineered Cas9 variant from Streptococcus pyogenes (SpCas9-VRQR) to target the mutation in the TOR1A gene in order to disrupt mutant torsinA in DYT1 patient fibroblasts. Selective targeting of the DYT1 allele was highly efficient with most common non-homologous end joining (NHEJ) edits, leading to a predicted premature stop codon with loss of the torsinA C terminus (delta 302–332 aa). Structural analysis predicted a functionally inactive status of this truncated torsinA due to the loss of residues associated with ATPase activity and binding to LULL1. Immunoblotting showed a reduction of the torsinA protein level in Cas9-edited DYT1 fibroblasts, and a functional assay using HSV infection indicated a phenotypic recovery toward that observed in control fibroblasts. These findings suggest that the selective disruption of the mutant TOR1A allele using CRISPR-Cas9 inactivates mutant torsinA, allowing the remaining wild-type torsinA to exert normal function.

TorsinB overexpression prevents abnormal twisting in DYT1 dystonia mouse models

Genetic redundancy can be exploited to identify therapeutic targets for inherited disorders. We explored this possibility in DYT1 dystonia, a neurodevelopmental movement disorder caused by a loss-of-function (LOF) mutation in the TOR1A gene encoding torsinA. Prior work demonstrates that torsinA and its paralog torsinB have conserved functions at the nuclear envelope. This work established that low neuronal levels of torsinB dictate the neuronal selective phenotype of nuclear membrane budding. Here, we examined whether torsinB expression levels impact the onset or severity of abnormal movements or neuropathological features in DYT1 mouse models. We demonstrate that torsinB levels bidirectionally regulate these phenotypes. Reducing torsinB levels causes a dose-dependent worsening whereas torsinB overexpression rescues torsinA LOF-mediated abnormal movements and neurodegeneration. These findings identify torsinB as a potent modifier of torsinA LOF phenotypes and suggest that augmentation of torsinB expression may retard or prevent symptom development in DYT1 dystonia.

The Role of Torsin AAA+ Proteins in Preserving Nuclear Envelope Integrity and Safeguarding Against Disease

Torsin ATPases are members of the AAA+ (ATPases associated with various cellular activities) superfamily of proteins, which participate in essential cellular processes. While AAA+ proteins are ubiquitously expressed and demonstrate distinct subcellular localizations, Torsins are the only AAA+ to reside within the nuclear envelope (NE) and endoplasmic reticulum (ER) network. Moreover, due to the absence of integral catalytic features, Torsins require the NE- and ER-specific regulatory cofactors, lamina-associated polypeptide 1 (LAP1) and luminal domain like LAP1 (LULL1), to efficiently trigger their atypical mode of ATP hydrolysis. Despite their implication in an ever-growing list of diverse processes, the specific contributions of Torsin/cofactor assemblies in maintaining normal cellular physiology remain largely enigmatic. Resolving gaps in the functional and mechanistic principles of Torsins and their cofactors are of considerable medical importance, as aberrant Torsin behavior is the principal cause of the movement disorder DYT1 early-onset dystonia. In this review, we examine recent findings regarding the phenotypic consequences of compromised Torsin and cofactor activities. In particular, we focus on the molecular features underlying NE defects and the contributions of Torsins to nuclear pore complex biogenesis, as well as the growing implications of Torsins in cellular lipid metabolism. Additionally, we discuss how understanding Torsins may facilitate the study of essential but poorly understood processes at the NE and ER, and aid in the development of therapeutic strategies for dystonia.

TOR1A variants cause a severe arthrogryposis with developmental delay, strabismus and tremor

See Ginevrino and Valente (doi:10.1093/brain/awx260) for a scientific commentary on this article. Autosomal dominant torsion dystonia-1 is a disease with incomplete penetrance most often caused by an in-frame GAG deletion (p.Glu303del) in the endoplasmic reticulum luminal protein torsinA encoded by TOR1A. We report an association of the homozygous dominant disease-causing TOR1A p.Glu303del mutation, and a novel homozygous missense variant (p.Gly318Ser) with a severe arthrogryposis phenotype with developmental delay, strabismus and tremor in three unrelated Iranian families. All parents who were carriers of the TOR1A variant showed no evidence of neurological symptoms or signs, indicating decreased penetrance similar to families with autosomal dominant torsion dystonia-1. The results from cell assays demonstrate that the p.Gly318Ser substitution causes a redistribution of torsinA from the endoplasmic reticulum to the nuclear envelope, similar to the hallmark of the p.Glu303del mutation. Our study highlights that TOR1A mutations should be considered in patients with severe arthrogryposis and further expands the phenotypic spectrum associated with TOR1A mutations.

Structure of the Golgi apparatus is not influenced by a GAG deletion mutation in the dystonia-associated gene Tor1a

Autosomal-dominant, early-onset DYT1 dystonia is associated with an in-frame deletion of a glutamic acid codon (ΔE) in the TOR1A gene. The gene product, torsinA, is an evolutionarily conserved AAA+ ATPase. The fact that constitutive secretion from patient fibroblasts is suppressed indicates that the ΔE-torsinA protein influences the cellular secretory machinery. However, which component is affected remains unclear. Prompted by recent reports that abnormal protein trafficking through the Golgi apparatus, the major protein-sorting center of the secretory pathway, is sometimes associated with a morphological change in the Golgi, we evaluated the influence of ΔE-torsinA on this organelle. Specifically, we examined its structure by confocal microscopy, in cultures of striatal, cerebral cortical and hippocampal neurons obtained from wild-type, heterozygous and homozygous ΔE-torsinA knock-in mice. In live neurons, the Golgi was assessed following uptake of a fluorescent ceramide analog, and in fixed neurons it was analyzed by immuno-fluorescence staining for the Golgi-marker GM130. Neither staining method indicated genotype-specific differences in the size, staining intensity, shape or localization of the Golgi. Moreover, no genotype-specific difference was observed as the neurons matured in vitro. These results were supported by a lack of genotype-specific differences in GM130 expression levels, as assessed by Western blotting. The Golgi was also disrupted by treatment with brefeldin A, but no genotype-specific differences were found in the immuno-fluorescence staining intensity of GM130. Overall, our results demonstrate that the ΔE-torsinA protein does not drastically influence Golgi morphology in neurons, irrespective of genotype, brain region (among those tested), or maturation stage in culture. While it remains possible that functional changes in the Golgi exist, our findings imply that any such changes are not severe enough to influence its morphology to a degree detectable by light microscopy.

Biallelic TOR1A mutations cause severe arthrogryposis: A case requiring reverse phenotyping

Heterozygous mutations in TOR1A gene are known to be responsible for DYT1 dystonia with incomplete penetrance. Autosomal recessive TOR1A disease is a very recently described syndrome characterized by severe arthrogryposis, developmental delay, strabismus and tremor. A 2 month-old boy with severe arthrogryposis and developmental delay was referred to our department for genetic counseling. Dystonic movements were observed on physical examination. Whole exome sequencing revealed a homozygous nonsense variant in exon 5 of TOR1A (c.862C > T, p.Arg288*). Our results expand the phenotypic and mutational spectrum of biallelic TOR1A disease, while emphasizing the importance of reverse phenotyping in the diagnosis of rare genetic disorders.

Whole-exome sequencing for variant discovery in blepharospasm

BACKGROUND

Blepharospasm (BSP) is a type of focal dystonia characterized by involuntary orbicularis oculi spasms that are usually bilateral, synchronous, and symmetrical. Despite strong evidence for genetic contributions to BSP, progress in the field has been constrained by small cohorts, incomplete penetrance, and late age of onset. Although several genetic etiologies for dystonia have been identified through whole-exome sequencing (WES), none of these are characteristically associated with BSP as a singular or predominant manifestation.

METHODS

We performed WES on 31 subjects from 21 independent pedigrees with BSP. The strongest candidate sequence variants derived from in silico analyses were confirmed with bidirectional Sanger sequencing and subjected to cosegregation analysis.

RESULTS

Cosegregating deleterious variants (GRCH37/hg19) in CACNA1A (NM_001127222.1: c.7261_7262delinsGT, p.Pro2421Val), REEP4 (NM_025232.3: c.109C>T, p.Arg37Trp), TOR2A (NM_130459.3: c.568C>T, p.Arg190Cys), and ATP2A3 (NM_005173.3: c.1966C>T, p.Arg656Cys) were identified in four independent multigenerational pedigrees. Deleterious variants in HS1BP3 (NM_022460.3: c.94C>A, p.Gly32Cys) and GNA14 (NM_004297.3: c.989_990del, p.Thr330ArgfsTer67) were identified in a father and son with segmental cranio-cervical dystonia first manifest as BSP. Deleterious variants in DNAH17, TRPV4, CAPN11, VPS13C, UNC13B, SPTBN4, MYOD1, and MRPL15 were found in two or more independent pedigrees. To our knowledge, none of these genes have previously been associated with isolated BSP, although other CACNA1A mutations have been associated with both positive and negative motor disorders including ataxia, episodic ataxia, hemiplegic migraine, and dystonia.

CONCLUSIONS

Our WES datasets provide a platform for future studies of BSP genetics which will demand careful consideration of incomplete penetrance, pleiotropy, population stratification, and oligogenic inheritance patterns.

Human TorsinA can function in the yeast cytosol as a molecular chaperone

TorsinA (TorA) is an AAA+ (ATPases associated with diverse cellular activities) ATPase linked to dystonia type 1 (DYT1), a neurological disorder that leads to uncontrollable muscular movements. Although DYT1 is linked to a 3 bp deletion in the C-terminus of TorA, the biological function of TorA remains to be established. Here, we use the yeast Saccharomyces cerevisiae as a tractable in vivo model to explore TorA function. We demonstrate that TorA can protect yeast cells against different forms of environmental stress and show that in the absence of the molecular disaggregase Hsp104, TorA can refold heat-denatured luciferase in vivo in an ATP-dependent manner. However, this activity requires TorA to be translocated to the cytoplasm from the endoplasmic reticulum in order to access and process cytoplasmic protein aggregates. Furthermore, mutational or chemical inactivation of the ATPase activity of TorA blocks this activity. We also find that TorA can inhibit the propagation of certain conformational variants of [PSI⁺], the aggregated prion form of the endogenous Sup35 protein. Finally, we show that while cellular localisation remains unchanged in the dystonia-linked TorA mutant ΔE302-303, the ability of this mutant form of TorA to protect against cellular stress and to facilitate protein refolding is impaired, consistent with it being a loss-of-function mutation.

Strength of cholinergic tone dictates the polarity of dopamine D2 receptor modulation of striatal cholinergic interneuron excitability in DYT1 dystonia

Balance between cholinergic and dopaminergic signaling is central to striatal control of movement and cognition. In dystonia, a common disorder of movement, anticholinergic therapy is often beneficial. This observation suggests there is a pathological increase in cholinergic tone, yet direct confirmation is lacking. In DYT1, an early-onset genetic form of dystonia caused by a mutation in the protein torsinA (TorA), the suspected heightened cholinergic tone is commonly attributed to faulty dopamine D2 receptor (D2R) signaling where D2R agonists cause excitation of striatal cholinergic interneurons (ChIs), rather than the normal inhibition of firing observed in wild-type animals, an effect known as "paradoxical excitation". Here, we provide for the first time direct measurement of elevated striatal extracellular acetylcholine (ACh) in a knock-in mouse model of human DYT1 dystonia (TorA^∆E/+ mice), confirming a striatal hypercholinergic state. We hypothesized that this elevated extracellular ACh might cause chronic over-activation of muscarinic acetylcholine receptors (mAChRs) and disrupt normal D2R function due to their shared coupling to G_i/o-proteins. We tested this concept in vitro first using a broad-spectrum mAChR antagonist, and then using a M2/M4 mAChR selective antagonist to specifically target mAChRs expressed by ChIs. Remarkably, we found that mAChR inhibition reverses the D2R-mediated paradoxical excitation of ChIs recorded in slices from TorA^∆E/+ mice to a typical inhibitory response. Furthermore, we recapitulated the paradoxical D2R excitation of ChIs in striatal slices from wild-type mice within minutes by simply increasing cholinergic tone through pharmacological inhibition of acetylcholinesterase (AChE) or by prolonged agonist activation of mAChRs. Collectively, these results show that enhanced mAChR tone itself is sufficient to rapidly reverse the polarity of D2R regulation of ChI excitability, correcting the previous notion that the D2R mediated paradoxical ChI excitation causes the hypercholinergic state in dystonia. Further, using a combination of genetic and pharmacological approaches, we found evidence that this switch in D2R polarity results from a change in coupling from the preferred G_i/o pathway to non-canonical β-arrestin signaling. These results highlight the need to fully understand how the mutation in TorA leads to pathologically heightened extracellular ACh. Furthermore the discovery of this novel ACh-dopamine interaction and the participation of β-arrestin in regulation of cholinergic interneurons is likely important for other basal ganglia disorders characterized by perturbation of ACh-dopamine balance, including Parkinson and Huntington diseases, l-DOPA-induced dyskinesia and schizophrenia.

A role for cerebellum in the hereditary dystonia DYT1

DYT1 is a debilitating movement disorder caused by loss-of-function mutations in torsinA. How these mutations cause dystonia remains unknown. Mouse models which have embryonically targeted torsinA have failed to recapitulate the dystonia seen in patients, possibly due to differential developmental compensation between rodents and humans. To address this issue, torsinA was acutely knocked down in select brain regions of adult mice using shRNAs. TorsinA knockdown in the cerebellum, but not in the basal ganglia, was sufficient to induce dystonia. In agreement with a potential developmental compensation for loss of torsinA in rodents, torsinA knockdown in the immature cerebellum failed to produce dystonia. Abnormal motor symptoms in knockdown animals were associated with irregular cerebellar output caused by changes in the intrinsic activity of both Purkinje cells and neurons of the deep cerebellar nuclei. These data identify the cerebellum as the main site of dysfunction in DYT1, and offer new therapeutic targets.

DOI:http://dx.doi.org/10.7554/eLife.22775.001

Dystonia is the third most common type of movement disorder after Parkinson’s disease and tremor. Patients with dystonia experience prolonged involuntary contractions of their muscles, often causing uncontrollable postures or repetitive movements. Almost thirty years ago, genetic studies revealed that a mutation in the gene that encodes a protein called torsinA causes the most common type of dystonia, called DYT1.

Exactly how mutations that affect the torsinA protein give rise to DYT1 remains unclear, and there are still no effective treatments for the disorder. Part of the problem is that we do not fully understand how torsinA works, or which of its many proposed functions is relevant to dystonia. Moreover, attempts to study DYT1 using genetically modified mice have proved largely unsuccessful. This is because mice that simply express the same genetic mutations that cause dystonia in humans do not show the overt symptoms of dystonia.

Fremont, Tewari et al. have now generated a mouse ‘model’ that does show symptoms of dystonia, and used these model mice to investigate the role of torsinA in the disorder. Acutely reducing the amount of torsinA protein in a region of the brain called the cerebellum induced the symptoms of dystonia in the mice. Conversely, reducing the amount of torsinA in a different brain area known as the basal ganglia had no such effect, even though both the cerebellum and the basal ganglia contribute to movement. Furthermore, neither manipulation had any effect in juvenile mice, which suggests that, in contrast to humans, young mice can compensate for the loss of torsinA.

Fremont, Tewari et al. also found that the loss of torsinA causes the cerebellum to generate incorrect output signals, which in turn trigger the abnormal movements seen in dystonia. In the future, further studies of the model mice could identify the exact changes that occur in neurons following the loss of torsinA from the cerebellum. Understanding these changes could potentially pave the way for developing effective treatments for DYT1 and other dystonias.

DOI:http://dx.doi.org/10.7554/eLife.22775.002

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.

Systematic TOR1A non-c.907_909delGAG variant analysis in isolated dystonia and controls

BACKGROUND

An increasing number of rare, functionally relevant non-c.907_909delGAG (non-ΔGAG) variants in TOR1A have been recognized, associated with phenotypic expressions different from classic DYT1 childhood-onset generalized dystonia. Only recently, DYT1 genotype-phenotype correlations have been proposed, awaiting further elucidation in independent cohorts.

METHODS

We screened the entire coding sequence and the 5'-UTR region of TOR1A for rare non-ΔGAG sequence variants in a large series of 940 individuals with various forms of isolated dystonia as well as in 376 ancestry-matched controls. The frequency of rare, predicted deleterious non-ΔGAG TOR1A variants was assessed in the European sample of the Exome Aggregation Consortium (ExAC) dataset.

RESULTS

In the case cohort, we identified a rare 5'-UTR variant (c.-39G > T), a rare splice-region variant (c.445-8T > C), as well as one novel (p.Ile231Asn) and two rare (p.Ala163Val, p.Thr321Met) missense variants, each in a single patient with adult-onset focal/segmental isolated dystonia. Of these variants, only p.Thr321Met qualified as possibly disease-related according to variant interpretation criteria. One novel, predicted deleterious missense substitution (p.Asn208Ser) was detected in the control cohort. Among European ExAC individuals, the carrier rate of rare, predicted deleterious non-ΔGAG variants was 0.4%.

CONCLUSIONS

Our study does not allow the establishment of genotype-specific clinical correlations for DYT1. Further large-scale genetic screening accompanied by comprehensive segregation and functional studies is required to conclusively define the contribution of TOR1A whole-gene variation to the pathogenesis of isolated dystonia.

LINCing defective nuclear-cytoskeletal coupling and DYT1 dystonia

Mechanical forces generated by nuclear-cytoskeletal coupling through the LINC (linker of nucleoskeleton and cytoskeleton) complex, an evolutionarily conserved molecular bridge in the nuclear envelope (NE), are critical for the execution of wholesale nuclear positioning events in migrating and dividing cells, chromosome dynamics during meiosis, and mechanotransduction. LINC complexes consist of outer (KASH (Klarsicht, ANC-1, and Syne homology)) and inner (SUN (Sad1, UNC-84)) nuclear membrane proteins. KASH proteins interact with the cytoskeleton in the cytoplasm and SUN proteins in the perinuclear space of the NE. In the nucleoplasm, SUN proteins interact with A-type nuclear lamins and chromatin-binding proteins. Recent structural insights into the KASH-SUN interaction have generated several questions regarding how LINC complex assembly and function might be regulated within the perinuclear space. Here we discuss potential LINC regulatory mechanisms and focus on the potential role of AAA+ (ATPases associated with various cellular activities) protein, torsinA, as a LINC complex regulator within the NE. We also examine how defects in LINC complex regulation by torsinA may contribute to the pathogenesis of the human neurological movement disorder, DYT1 dystonia.

Torsin ATPases: structural insights and functional perspectives

Torsin ATPases are the only members of the AAA+ ATPase family that localize to the endoplasmic reticulum and contiguous perinuclear space. Accordingly, they are well positioned to perform essential work in these compartments, but their precise functions remain elusive. Recent studies have deciphered an unusual ATPase activation mechanism relying on Torsin-associated transmembrane cofactors, LAP1 or LULL1. These findings profoundly change our molecular view of the Torsin machinery and rationalize several human mutations in TorsinA or LAP1 leading to congenital disorders, symptoms of which have recently been recapitulated in mouse models. Here, we review these recent advances in the Torsin field and discuss the most pressing questions in relation to nuclear envelope dynamics.

Torsins: not your typical AAA+ ATPases

Torsin ATPases (Torsins) belong to the widespread AAA+ (ATPases associated with a variety of cellular activities) family of ATPases, which share structural similarity but have diverse cellular functions. Torsins are outliers in this family because they lack many characteristics of typical AAA+ proteins, and they are the only members of the AAA+ family located in the endoplasmic reticulum and contiguous perinuclear space. While it is clear that Torsins have essential roles in many, if not all metazoans, their precise cellular functions remain elusive. Studying Torsins has significant medical relevance since mutations in Torsins or Torsin-associated proteins result in a variety of congenital human disorders, the most frequent of which is early-onset torsion (DYT1) dystonia, a severe movement disorder. A better understanding of the Torsin system is needed to define the molecular etiology of these diseases, potentially enabling corrective therapy. Here, we provide a comprehensive overview of the Torsin system in metazoans, discuss functional clues obtained from various model systems and organisms and provide a phylogenetic and structural analysis of Torsins and their regulatory cofactors in relation to disease-causative mutations. Moreover, we review recent data that have led to a dramatically improved understanding of these machines at a molecular level, providing a foundation for investigating the molecular defects underlying the associated movement disorders. Lastly, we discuss our ideas on how recent progress may be utilized to inform future studies aimed at determining the cellular role(s) of these atypical molecular machines and their implications for dystonia treatment options.

Mutant human torsinA, responsible for early-onset dystonia, dominantly suppresses GTPCH expression, dopamine levels and locomotion in Drosophila melanogaster

Dystonia represents the third most common movement disorder in humans with over 20 genetic loci identified. TOR1A (DYT1), the gene responsible for the most common primary hereditary dystonia, encodes torsinA, an AAA ATPase family protein. Most cases of DYT1 dystonia are caused by a 3 bp (ΔGAG) deletion that results in the loss of a glutamic acid residue (ΔE302/303) in the carboxyl terminal region of torsinA. This torsinAΔE mutant protein has been speculated to act in a dominant-negative manner to decrease activity of wild type torsinA. Drosophila melanogaster has a single torsin-related gene, dtorsin. Null mutants of dtorsin exhibited locomotion defects in third instar larvae. Levels of dopamine and GTP cyclohydrolase (GTPCH) proteins were severely reduced in dtorsin-null brains. Further, the locomotion defect was rescued by the expression of human torsinA or feeding with dopamine.

Here, we demonstrate that human torsinAΔE dominantly inhibited locomotion in larvae and adults when expressed in neurons using a pan-neuronal promoter Elav. Dopamine and tetrahydrobiopterin (BH₄) levels were significantly reduced in larval brains and the expression level of GTPCH protein was severely impaired in adult and larval brains. When human torsinA and torsinAΔE were co-expressed in neurons in dtorsin-null larvae and adults, the locomotion rates and the expression levels of GTPCH protein were severely reduced. These results support the hypothesis that torsinAΔE inhibits wild type torsinA activity. Similarly, neuronal expression of a Drosophila DtorsinΔE equivalent mutation dominantly inhibited larval locomotion and GTPCH protein expression. These results indicate that both torsinAΔE and DtorsinΔE act in a dominant-negative manner. We also demonstrate that Dtorsin regulates GTPCH expression at the post-transcriptional level. This Drosophila model of DYT1 dystonia provides an important tool for studying the differences in the molecular function between the wild type and the mutant torsin proteins.

Engineering animal models of dystonia

Dystonia is a neurological disorder characterized by abnormal involuntary movements that are prolonged and often cause twisting and turning. Several genetically modified worms, fruit flies, and rodents have been generated as models of genetic dystonias, in particular DYT1, DYT11, and DYT12 dystonias. Although these models do not show overt dystonic symptoms, the rodent models exhibit motor deficits in specialized behavioral tasks, such as the rotarod and beam-walking tests. For example, in a rodent model of DYT12 dystonia, which is generally stress triggered, motor deficits are observed only after the animal is stressed. Moreover, in a rodent model of DYT1 dystonia, the motor and electrophysiological deficits can be rescued by trihexyphenidyl, a common anticholinergic medication used to treat dystonic symptoms in human patients. Biochemically, the DYT1 and DYT11 animal models also share some similarities to patients, such as a reduction in striatal D2 dopamine receptor and binding activities. In addition, conditional knockout mouse models for DYT1 and DYT11 dystonia demonstrate that loss of the causal dystonia-related proteins in the striatum leads to motor deficits. Interestingly, loss of the DYT1 dystonia causal protein in Purkinje cells shows an improvement in motor performance, suggesting that gene therapy targeting of the cerebellum or intervention in its downstream pathways may be useful. Finally, recent studies using DYT1 dystonia worm and mouse models led to a potential novel therapeutic agent, which is currently undergoing clinical trials. These results indicate that genetic animal models are powerful tools to elucidate the pathophysiology and to further develop new therapeutics for dystonia.

The metazoan protein disaggregase and amyloid depolymerase system: Hsp110, Hsp70, Hsp40, and small heat shock proteins

A baffling aspect of metazoan proteostasis is the lack of an Hsp104 ortholog that rapidly disaggregates and reactivates misfolded polypeptides trapped in stress induced disordered aggregates, preamyloid oligomers, or amyloid fibrils. By contrast, in bacteria, protozoa, chromista, fungi, and plants, Hsp104 orthologs are highly conserved and confer huge selective advantages in stress tolerance. Moreover, in fungi, the amyloid remodeling activity of Hsp104 has enabled deployment of prions for various beneficial modalities. Thus, a longstanding conundrum has remained unanswered: how do metazoan cells renature aggregated proteins or resolve amyloid fibrils without Hsp104? Here, we highlight recent advances that unveil the metazoan protein-disaggregase machinery, comprising Hsp110, Hsp70, and Hsp40, which synergize to dissolve disordered aggregates, but are unable to rapidly solubilize stable amyloid fibrils. However, Hsp110, Hsp70, and Hsp40 exploit the slow monomer exchange dynamics of amyloid, and can slowly depolymerize amyloid fibrils from their ends in a manner that is stimulated by small heat shock proteins. Upregulation of this system could have key therapeutic applications in various protein-misfolding disorders. Intriguingly, yeast Hsp104 can interface with metazoan Hsp110, Hsp70, and Hsp40 to rapidly eliminate disease associated amyloid. Thus, metazoan proteostasis is receptive to augmentation with exogenous disaggregases, which opens a number of therapeutic opportunities.

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.

The genetics of dystonias

Dystonia has been defined as a syndrome of involuntary, sustained muscle contractions affecting one or more sites of the body, frequently causing twisting and repetitive movements or abnormal postures. Dystonia is also a clinical sign that can be the presenting or prominent manifestation of many neurodegenerative and neurometabolic disorders. Etiological categories include primary dystonia, secondary dystonia, heredodegenerative diseases with dystonia, and dystonia plus. Primary dystonia includes syndromes in which dystonia is the sole phenotypic manifestation with the exception that tremor can be present as well. Most primary dystonia begins in adults, and approximately 10% of probands report one or more affected family members. Many cases of childhood- and adolescent-onset dystonia are due to mutations in TOR1A and THAP1. Mutations in THAP1 and CIZ1 have been associated with sporadic and familial adult-onset dystonia. Although significant recent progress had been made in defining the genetic basis for most of the dystonia-plus and heredodegenerative diseases with dystonia, a major gap remains in understanding the genetic etiologies for most cases of adult-onset primary dystonia. Common themes in the cellular biology of dystonia include G1/S cell cycle control, monoaminergic neurotransmission, mitochondrial dysfunction, and the neuronal stress response.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

The zebrafish homologue of the human DYT1 dystonia gene is widely expressed in CNS neurons but non-essential for early motor system development

DYT1 dystonia is caused by mutation of the TOR1A gene, resulting in the loss of a single glutamic acid residue near the carboxyl terminal of TorsinA. The neuronal functions perturbed by TorsinA[ΔE] are a major unresolved issue in understanding the pathophysiology of dystonia, presenting a critical roadblock to developing effective treatments. We identified and characterized the zebrafish homologue of TOR1A, as a first step towards elucidating the functions of TorsinA in neurons, in vivo, using the genetically-manipulable zebrafish model. The zebrafish genome was found to contain a single alternatively-spliced tor1 gene, derived from a common ancestral locus shared with the dual TOR1A and TOR1B paralogues found in tertrapods. tor1 was expressed ubiquitously during early embryonic development and in multiple adult tissues, including the CNS. The 2.1 kb tor1 mRNA encodes Torsin1, which is 59% identical and 78% homologous to human TorsinA. Torsin1 was expressed as major 45 kDa and minor 47 kDa glycoproteins, within the cytoplasm of neurons and neuropil throughout the CNS. Similar to previous findings relating to human TorsinA, mutations of the ATP hydrolysis domain of Torsin1 resulted in relocalization of the protein in cultured cells from the endoplasmic reticulum to the nuclear envelope. Zebrafish embryos lacking tor1 during early development did not show impaired viability, overt morphological abnormalities, alterations in motor behavior, or developmental defects in the dopaminergic system. Torsin1 is thus non-essential for early development of the motor system, suggesting that important CNS functions may occur later in development, consistent with the critical time window in late childhood when dystonia symptoms usually emerge in DYT1 patients. The similarities between Torsin1 and human TorsinA in domain organization, expression pattern, and cellular localization suggest that the zebrafish will provide a useful model to understand the neuronal functions of Torsins in vivo.

Neurogenesis and neuronal migration in the forebrain of the TorsinA knockout mouse embryo

Early-onset generalized torsion dystonia, also known as DYT1 dystonia, is a childhood onset heritable neurological movement disorder involving painful, involuntary muscle contractions, sustained abnormal postures, and repetitive movements. It is caused by a GAG deletion in the Tor1A gene located on chromosome 9. TorsinA, the product of the Tor1A gene, is expressed throughout the brain beginning early in embryonic development. It plays a role in the regulation of nuclear envelope-cytoskeletal interactions, and presumably nuclear translocation. Since nuclear translocation, powered by cytoskeletal traction, is critical for cell proliferation and migration, we examined whether neurogenesis and neuronal migration are affected in Tor1A-/- mouse brain. Our data show that interkinetic nuclear migration and the pattern of migration of newly generated neurons are impaired in the dorsal forebrain of the Tor1A-/- embryo. However, neurogenesis is not altered significantly. The rate of migration of cells from explants of the medial ganglionic eminence is also impaired in the Tor1A-/- embryo. Thus, loss of torsinA results in subtle but significant alterations in cell proliferation and migration in the embryonic forebrain. These subtle developmental changes are consistent with a lack of significant changes in neuronal numbers, neuronal positioning or size of brain regions in DYT1 dystonia patients.

Miniature release events of glutamate from hippocampal neurons are influenced by the dystonia-associated protein torsinA

TorsinA is an evolutionarily conserved AAA+ ATPase, and human patients with an in-frame deletion of a single glutamate (ΔE) codon from the encoding gene suffer from autosomal-dominant, early-onset generalized DYT1 dystonia. Although only 30-40% of carriers of the mutation show overt motor symptoms, most experience enhanced excitability of the central nervous system. The cellular mechanism responsible for this change in excitability is not well understood. Here we show the effects of the ΔE-torsinA mutation on miniature neurotransmitter release from neurons. Neurotransmitter release was characterized in cultured hippocampal neurons obtained from wild-type, heterozygous, and homozygous ΔE-torsinA knock-in mice using two approaches. In the first approach, patch-clamp electrophysiology was used to record glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in the presence of the Na⁺ channel blocker tetrodotoxin (TTX) and absence of GABA(A) receptor antagonists. The intervals between mEPSC events were significantly shorter in neurons obtained from the mutant mice than in those obtained from wild-type mice. In the second approach, the miniature exocytosis of synaptic vesicles was detected by imaging the unstimulated release of FM dye from the nerve terminals in the presence of TTX. Cumulative FM dye release was higher in neurons obtained from the mutant mice than in those obtained from wild-type mice. The number of glutamatergic nerve terminals was also assessed, and we found that this number was unchanged in heterozygous relative to wild-type neurons, but slightly increased in homozygous neurons. Notably, in both heterozygous and homozygous neurons, the unitary synaptic charge during each mEPSC event was unchanged. Overall, our results suggest more frequent miniature glutamate release in neurons with ΔE-torsinA mutations. This change may be one of the underlying mechanisms by which the excitability of the central nervous system is enhanced in the context of DYT1 dystonia. Moreover, qualitative differences between heterozygous and homozygous neurons with respect to certain synaptic properties indicate that the abnormalities observed in homozygotes may reflect more than a simple gene dosage effect.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

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.

Dtorsin, the Drosophila ortholog of the early-onset dystonia TOR1A (DYT1), plays a novel role in dopamine metabolism

Dystonia represents the third most common movement disorder in humans. At least 15 genetic loci (DYT1-15) have been identified and some of these genes have been cloned. TOR1A (formally DYT1), the gene responsible for the most common primary hereditary dystonia, encodes torsinA, an AAA ATPase family protein. However, the function of torsinA has yet to be fully understood. Here, we have generated and characterized a complete loss-of-function mutant for dtorsin, the only Drosophila ortholog of TOR1A. Null mutation of the X-linked dtorsin was semi-lethal with most male flies dying by the pre-pupal stage and the few surviving adults being sterile and slow moving, with reduced cuticle pigmentation and thin, short bristles. Third instar male larvae exhibited locomotion defects that were rescued by feeding dopamine. Moreover, biochemical analysis revealed that the brains of third instar larvae and adults heterozygous for the loss-of-function dtorsin mutation had significantly reduced dopamine levels. The dtorsin mutant showed a very strong genetic interaction with Pu (Punch: GTP cyclohydrolase), the ortholog of the human gene underlying DYT14 dystonia. Biochemical analyses revealed a severe reduction of GTP cyclohydrolase protein and activity, suggesting that dtorsin plays a novel role in dopamine metabolism as a positive-regulator of GTP cyclohydrolase protein. This dtorsin mutant line will be valuable for understanding this relationship and potentially other novel torsin functions that could play a role in human dystonia.

Identifying the genetic components underlying the pathophysiology of movement disorders

Movement disorders are a heterogeneous group of neurological conditions, few of which have been classically described as bona fide hereditary illnesses (Huntington’s chorea, for instance). Most are considered to be either sporadic or to feature varying degrees of familial aggregation (parkinsonism and dystonia). In the late twentieth century, Mendelian monogenic mutations were found for movement disorders with a clear and consistent family history. Although important, these findings apply only to very rare forms of movement disorders. Already in the twenty-first century, and taking advantage of the modern developments in genetics and molecular biology, growing attention is being paid to the complex genetics of movement disorders. The search for risk genetic variants (polymorphisms) in large cohorts and the identification of different risk variants across different populations and ethnic groups are under way, with the most relevant findings to date corresponding to recent genome wide association studies in Parkinson’s disease. These new approaches focusing on risk variants may enable the design of screening tests for early or even preclinical disease, and the identification of likely therapeutic targets.

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".