Mutant torsinA Research Articles

P 8 Eliciting a dystonic phenotype in genetically predisposed rodents for DYT-TOR1A dystonia via an overuse paradigm

Introduction: Abnormal postures or movements caused by sustained or intermittent muscle contractions typically characterize the hyperkinetic movement disorder dystonia. DYT-TOR1A dystonia is the most common form of monogenic dystonia and follows an autosomal dominant pattern of inheritance. Although a trinucleotide deletion in the torsinA encoding gene, TOR1A, forms the genetic basis of this disorder, its reduced penetrance of 30-40% suggests the presence of disease modifying factors. Indeed, the induction of task specific forms of dystonia such as musician’s dystonia and writer’s cramp upon repetitive peripheral limb overuse lends strong support to a “second-hit” hypothesis, which states that exposure to environmental triggers could result in overt manifestation of dystonia in mutation carriers. Based on the hypothesis that an environmental trigger in the form of repetitive limb overuse induces dystonia-like movements in asymptomatic, but genetically predisposed rats expressing mutant human TOR1A, we have been working towards developing a clinically relevant model of dystonia. Materials and methods: Rats overexpressing mutant torsinA protein (ΔETorA) and their wildtype (wt) littermates were trained to perform a single-limb overuse task. The animals learned to press a lever by receiving a sugar water reward for each successful press in an operant testing chamber. Two overuse protocols were tested: 1000 presses/day, 6 days/week and 2000 presses/day, 3 days/week. Results: 1000 presses/day, 6 days/week did not affect motor performance of ΔETorA rats compared to the wt control group. In contrast, 2000 presses/day, 3 days/week over 5 weeks resulted in pronounced differences between wt and ΔETorA animals. Wt rats showed a consistently high success rate of ∼80% for pressing the lever. The velocity of forelimb movement and the accuracy in hitting the lever remained on a stable level. ΔETorA rats, however, were increasingly and significantly less successful at pressing the lever (∼75%) as compared to their wt littermates over the observational period. In addition, the average velocity of the movement towards the lever was shown to be reduced in the ΔETorA group compared to wt animals. Moreover, ΔETorA animals needed more trials to successfully hit the lever for the first time in a series of presses. Discussion/Conclusion: Our results indicate that the 1000 presses, 6 days/week protocol is insufficient to induce behavioural changes. The 2000 presses, 3 days/week protocol revealed that ΔETorA rats show a lower success rate in pressing the lever, a lower average velocity of movement of the right forelimb and difficulties in hitting the target successfully compared to their wt littermates. This points towards motor abnormalities induced by the overuse paradigm in ΔETorA rats compared to the wt control group.

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.

A Novel Transgenic Mouse Model to Investigate the Cell-Autonomous Effects of torsinA(ΔE) Expression in Striatal Output Neurons

Dystonia is a disabling neurological syndrome characterized by abnormal movements and postures that result from intermittent or sustained involuntary muscle contractions; mutations of DYT1/TOR1A are the most common cause of childhood-onset, generalized, inherited dystonia. Patient and mouse model data strongly support dysregulation of the nigrostriatal dopamine neurotransmission circuit in the presence of the DYT1-causing mutation. To determine striatal medium spiny neuron (MSN) cell-autonomous and non-cell autonomous effects relevant to dopamine transmission, we created a transgenic mouse in which expression of mutant torsinA in forebrain is restricted to MSNs. We assayed electrically evoked and cocaine-enhanced dopamine release and locomotor activity, dopamine uptake, gene expression of dopamine-associated neuropeptides and receptors, and response to the muscarinic cholinergic antagonist, trihexyphenidyl. We found that over-expression of mutant torsinA in MSNs produces complex cell-autonomous and non-cell autonomous alterations in nigrostriatal dopaminergic and intrastriatal cholinergic function, similar to that found in pan-cellular DYT1 mouse models. These data introduce targets for future studies to identify which are causative and which are compensatory in DYT1 dystonia, and thereby aid in defining appropriate therapies.

Efficient RNA interference-based knockdown of mutant torsinA reveals reversibility of PERK-eIF2α pathway dysregulation in DYT1 transgenic rats in vivo

DYT1 dystonia is a neurological disease caused by a dominant mutation that results in the loss of a glutamic acid in the endoplasmic reticulum-resident protein torsinA. Currently, treatments are symptomatic and only provide partial relief. Multiple reports support the hypothesis that selectively reducing expression of mutant torsinA without affecting levels of the wild type protein should be beneficial. Published cell-based studies support this hypothesis. It is unclear, however, if phenotypes are reversible by targeting the molecular defect once established in vivo. Here, we generated adeno-associated virus encoding artificial microRNA targeting human mutant torsinA and delivered them to the striatum of symptomatic transgenic rats that express the full human TOR1A mutant gene. We achieved efficient suppression of human mutant torsinA expression in DYT1 transgenic rats, partly reversing its accumulation in the nuclear envelope. This intervention rescued PERK-eIF2α pathway dysregulation in striatal projection neurons but not behavioral abnormalities. Moreover, we found abnormal expression of components of dopaminergic neurotransmission in DYT1 rat striatum, which were not normalized by suppressing mutant torsinA expression. Our findings demonstrate the reversibility of translational dysregulation in DYT1 neurons and confirm the presence of abnormal dopaminergic neurotransmission in DYT1 dystonia.

Mutant torsinA in the heterozygous DYT1 state compromises HSV propagation in infected neurons and fibroblasts

Most cases of early onset torsion dystonia (DYT1) are caused by a 3-base pair deletion in one allele of the TOR1A gene causing loss of a glutamate in torsinA, a luminal protein in the nuclear envelope. This dominantly inherited neurologic disease has reduced penetrance and no other medical manifestations. It has been challenging to understand the neuronal abnormalities as cells and mouse models which are heterozygous (Het) for the mutant allele are quite similar to wild-type (WT) controls. Here we found that patient fibroblasts and mouse neurons Het for this mutation showed significant differences from WT cells in several parameters revealed by infection with herpes simplex virus type 1 (HSV) which replicates in the nucleus and egresses out through the nuclear envelope. Using a red fluorescent protein capsid to monitor HSV infection, patient fibroblasts showed decreased viral plaque formation as compared to controls. Mouse Het neurons had a decrease in cytoplasmic, but not nuclear HSV fluorescence, and reduced numbers of capsids entering axons as compared to infected WT neurons. These findings point to altered dynamics of the nuclear envelope in cells with the patient genotype, which can provide assays to screen for therapeutic agents that can normalize these cells.

Dynamic nuclear envelope phenotype in rats overexpressing mutated human torsinA protein.

ABSTRACTA three-base-pair deletion in the human TOR1A gene is causative for the most common form of primary dystonia: the early-onset dystonia type 1 (DYT1 dystonia). The pathophysiological consequences of this mutation are still unknown. To study the pathology of the mutant torsinA (TOR1A) protein, we have generated a transgenic rat line that overexpresses the human mutant protein under the control of the human TOR1A promoter. This new animal model was phenotyped with several approaches, including behavioral tests and neuropathological analyses. Motor phenotype, cellular and ultrastructural key features of torsinA pathology were found in this new transgenic rat line, supporting that it can be used as a model system for investigating the disease’s development. Analyses of mutant TOR1A protein expression in various brain regions also showed a dynamic expression pattern and a reversible nuclear envelope pathology. These findings suggest the differential vulnerabilities of distinct neuronal subpopulations. Furthermore, the reversibility of the nuclear envelope pathology might be a therapeutic target to treat the disease.

Phosphodiesterase-10A Inverse Changes in Striatopallidal and Striatoentopeduncular Pathways of a Transgenic Mouse Model of DYT1 Dystonia.

We report that changes of phosphodiesterase-10A (PDE10A) can map widespread functional imbalance of basal ganglia circuits in a mouse model of DYT1 dystonia overexpressing mutant torsinA. PDE10A is a key enzyme in the catabolism of second messenger cAMP and cGMP, whose synthesis is stimulated by D1 receptors and inhibited by D2 receptors preferentially expressed in striatoentopeducuncular/substantia nigra or striatopallidal pathways, respectively. PDE10A was studied in control mice (NT) and in mice carrying human wild-type torsinA (hWT) or mutant torsinA (hMT). Quantitative analysis of PDE10A expression was assessed in different brain areas by rabbit anti-PDE10A antibody immunohistochemistry and Western blotting. PDE10A-dependent cAMP hydrolyzing activity and PDE10A mRNA were also assessed. Striatopallidal neurons were identified by rabbit anti-enkephalin antibody.In NT mice, PDE10A is equally expressed in medium spiny striatal neurons and in their projections to entopeduncular nucleus/substantia nigra and to external globus pallidus. In hMT mice, PDE10A content selectively increases in enkephalin-positive striatal neuronal bodies; moreover, PDE10A expression and activity in hMT mice, compared with NT mice, significantly increase in globus pallidus but decrease in entopeduncular nucleus/substantia nigra. Similar changes of PDE10A occur in hWT mice, but such changes are not always significant. However, PDE10A mRNA expression appears comparable among NT, hWT, and hMT mice.In DYT1 transgenic mice, the inverse changes of PDE10A in striatoentopeduncular and striatopallidal projections might result over time in an imbalance between direct and indirect pathways for properly focusing movement. The decrease of PDE10A in the striatoentopeduncular/nigral projections might lead to increased intensity and duration of D1-stimulated cAMP/cGMP signaling; conversely, the increase of PDE10A in the striatopallidal projections might lead to increased intensity and duration of D2-inhibited cAMP/cGMP signaling.SIGNIFICANCE STATEMENT In DYT1 transgenic mouse model of dystonia, PDE10A, a key enzyme in cAMP and cGMP catabolism, is downregulated in striatal projections to entopeduncular nucleus/substantia nigra, preferentially expressing D1 receptors that stimulate cAMP/cGMP synthesis. Conversely, in DYT1 mice, PDE10A is upregulated in striatal projections to globus pallidus, preferentially expressing D2 receptors that inhibit cAMP/cGMP synthesis. The inverse changes to PDE10A in striatoentopeduncular/substantia nigra and striatopallidal pathways might tightly interact downstream to dopamine receptors, likely resulting over time to increased intensity and duration respectively of D1-stimulated and D2-inhibited cAMP/cGMP signals. Therefore, PDE10A changes in the DYT1 model of dystonia can upset the functional balance of basal ganglia circuits, affecting direct and indirect pathways simultaneously.

Disruption of Protein Processing in the Endoplasmic Reticulum of DYT1 Knock-in Mice Implicates Novel Pathways in Dystonia Pathogenesis.

Dystonia type 1 (DYT1) is one of the different forms of inherited dystonia, a neurological disorder characterized by involuntary, disabling movements. DYT1 is caused by mutations in the gene that encodes the endoplasmic reticulum (ER)-resident protein torsinA. How mutant torsinA causes neuronal dysfunction remains unknown. Here, we show the behavioral and molecular consequences of stressing the ER in DYT1 mice by increasing the amount of misfolded proteins. This resulted in the generation of a reduced number of animals, evidence of abnormal ER protein processing and dysregulation of translational control pathways. The work described here proposes a shared mechanism for different forms of dystonia, links for the first time known biological pathways to dystonia pathogenesis, and uncovers potential pharmacological targets for its treatment.

Cerebellar synaptogenesis is compromised in mouse models of DYT1 dystonia

Early-onset torsion dystonia (DYT1) is an autosomal-dominant movement disorder characterized by sustained muscle contractions and abnormal posturing. It is caused by a three base-pair deletion (ΔGAG) in the gene encoding the AAA+ protein torsinA, which gives rise to a loss of function mutation responsible of neuronal functional abnormalities. Symptoms typically appear during childhood, suggesting the presence of an early critical period of sensorimotor circuit susceptibility to torsinA dysfunction. Here, we identified in two different DYT1 mouse strains, heterozygous torsinA knockout mice (Tor1a+/−) and human ΔGAG mutant torsinA transgenic mice (hMT), the anatomical abnormalities in the cerebellum, during a critical age for synaptogenesis (postnatal day 14, P14). By means of immunofluorescence, confocal analysis and western blot quantification, we observed a reduced inhibitory input on Purkinje cells (PCs) as well as an unbalanced excitatory innervation; a significant reduction of the parallel fiber (PF) synaptic terminals and an increase of the climbing fiber (CF) inputs. Finally, in support of the in vivo results, we also provide evidence of an impaired PF synaptogenesis in a co-culture system. Of note, these alterations were rescued and in part over-compensated in the adult age in both mouse strains, suggesting that torsinA dysfunction can induce an altered maturation of cerebellar synaptic contacts. Altogether these results indicate that a loss of function of torsinA during cerebellar synaptogenesis induces important developmental alterations, that might contribute to the age-dependent susceptibility to develop dystonia in mutation carriers.

TorsinA hypofunction causes abnormal twisting movements and sensorimotor circuit neurodegeneration.

Lack of a preclinical model of primary dystonia that exhibits dystonic-like twisting movements has stymied identification of the cellular and molecular underpinnings of the disease. The classical familial form of primary dystonia is caused by the DYT1 (ΔE) mutation in TOR1A, which encodes torsinA, AAA⁺ ATPase resident in the lumen of the endoplasmic reticular/nuclear envelope. Here, we found that conditional deletion of Tor1a in the CNS (nestin-Cre Tor1a(flox/-)) or isolated CNS expression of DYT1 mutant torsinA (nestin-Cre Tor1a(flox/ΔE)) causes striking abnormal twisting movements. These animals developed perinuclear accumulation of ubiquitin and the E3 ubiquitin ligase HRD1 in discrete sensorimotor regions, followed by neurodegeneration that was substantially milder in nestin-Cre Tor1a(flox/ΔE) compared with nestin-Cre Tor1a(flox/-) animals. Similar to the neurodevelopmental onset of DYT1 dystonia in humans, the behavioral and histopathological abnormalities emerged and became fixed during CNS maturation in the murine models. Our results establish a genetic model of primary dystonia that is overtly symptomatic, and link torsinA hypofunction to neurodegeneration and abnormal twisting movements. These findings provide a cellular and molecular framework for how impaired torsinA function selectively disrupts neural circuits and raise the possibility that discrete foci of neurodegeneration may contribute to the pathogenesis of DYT1 dystonia.

Arresting a Torsin ATPase Reshapes the Endoplasmic Reticulum

Torsins are membrane-tethered AAA+ ATPases residing in the nuclear envelope (NE) and endoplasmic reticulum (ER). Here, we show that the induction of a conditional, dominant-negative TorsinB variant provokes a profound reorganization of the endomembrane system into foci containing double membrane structures that are derived from the ER. These double-membrane sinusoidal structures are formed by compressing the ER lumen to a constant width of 15 nm, and are highly enriched in the ATPase activator LULL1. Further, we define an important role for a highly conserved aromatic motif at the C terminus of Torsins. Mutations in this motif perturb LULL1 binding, reduce ATPase activity, and profoundly limit the induction of sinusoidal structures.

Ablation of rat substantia nigra is a good acute model of Parkinson's disease for cell transplantation therapy

WCN 2013 No: 1235 Topic: 2 — Movement Disorders Ablation of rat substantia nigra is a good acute model of Parkinson's disease for cell transplantation therapy S.-I. Hirano, Y. Tokumoto, Y. Nishikawa. Physiology, Osaka Dental University, Osaka, Japan; Tissue Engineering & Cell Therapy, Institute of Biomedical Research & Innovation, Kobe, Japan; Biochemistry & Integrative Medical Biology, School of Medicine, Keio University, Tokyo, Japan A valuable rat model of Parkinson's disease (PD) to study cell therapy was established. Forty-five female rats were examined. Using a stereotactic technique, we created an acute model of PD by highfrequency electric ablation of bilateral substantia nigra pars compacta. The ablation probe used was a 100 mm-long rigid bipolar needle electrode (1.0 mm in diameter). The rats developed persistent symptoms of PD, especially akinesia and postural instability. We tried all sorts of surgical protocols. The size and shape of the lesion that were ablated in a typical setting were evaluated. Then we determined that the optimal surgical parameter to place a lesionwas 1 Wof power for 15 s. The longterm viability of transplanted neural stem/progenitor cells was also examined and confirmed to be viable for more than 60 days following surgery. As the animal model was designed to suit the study of cell transplantation therapy, it did not utilize neurotoxic chemicals to make lesions and could avoid the harmful effects of them. The benefits of our ablationmodel of PDwere the rapidity and uniformity of appearance of the characteristics of Parkinsonism. Therefore we can identify the onset of PD strictly. This feature is a unique merit that is not shared by the othermethods. The technique described heremay provide a good acute model of PD for the research of intracerebral cell transplantation. This model will contribute to the progress in the field of cell transplantation therapy. doi:10.1016/j.jns.2013.07.414 Abstract — WCN 2013 No: 1201 Topic: 2 — Movement Disorders Distribution of wild and mutant torsinA in living cells WCN 2013 No: 1201 Topic: 2 — Movement Disorders Distribution of wild and mutant torsinA in living cells I. Toyoshima, E. Abe, S. Kamada. Neurology, Akita National Hospital of NHO, Yurihonjo, Japan; Neurology, Akita University School of Medicine,

Distribution of wild and mutant torsinA in living cells

Distribution of wild and mutant torsinA in living cells

Cholinergic dysregulation produced by selective inactivation of the dystonia-associated protein torsinA

DYT1 dystonia, a common and severe primary dystonia, is caused by a 3-bp deletion in TOR1A which encodes torsinA, a protein found in the endoplasmic reticulum. Several cellular functions are altered by the mutant protein, but at a systems level the link between these and the symptoms of the disease is unclear. The most effective known therapy for DYT1 dystonia is the use of anticholinergic drugs. Previous studies have revealed that in mice, transgenic expression of human mutant torsinA under a non-selective promoter leads to abnormal function of striatal cholinergic neurons. To investigate what pathological role torsinA plays in cholinergic neurons, we created a mouse model in which the Dyt1 gene, the mouse homolog of TOR1A, is selectively deleted in cholinergic neurons (ChKO animals). These animals do not have overt dystonia, but do have subtle motor abnormalities. There is no change in the number or size of striatal cholinergic cells or striatal acetylcholine content, uptake, synthesis, or release in ChKO mice. There are, however, striking functional abnormalities of striatal cholinergic cells, with paradoxical excitation in response to D2 receptor activation and loss of muscarinic M2/M4 receptor inhibitory function. These effects are specific for cholinergic interneurons, as recordings from nigral dopaminergic neurons revealed normal responses. Amphetamine stimulated dopamine release was also unaltered. These results demonstrate a cell-autonomous effect of Dyt1 deletion on striatal cholinergic function. Therapies directed at modifying the function of cholinergic neurons may prove useful in the treatment of the human disorder.

Allelic Imbalance in TOR1A mRNA Expression in Manifesting and Non-Manifesting Carriers of the GAG-Deletion

Early onset dystonia (EOD) is associated with a 3bp-(ΔGAG) in-frame deletion in the TOR1A gene, which encodes for torsinA. Carriers of the mutant (ΔGAG) allele can either develop or escape a dystonic phenotype (~30% penetrance). The expression ratio of the two alleles could be important for the manifestation or prevention of the disease since wild-type (WT) torsinA is thought to have protective function. Absence of an antibody discriminating WT from ΔE torsinA has precluded the determination ΔE and WT torsinA levels in manifesting and nonmanifesting carriers. We performed quantitative analysis of TOR1A allele expression in manifesting (MC) and nonmanifesting (NMC) carriers using quantitative allele-specific PCR (qASPCR) to determine the levels of mutant versus WT torsinA mRNA. The technique described showed high degree of specificity in detecting the two alleles. The present study represents the first comprehensive analysis of biallelic expression of the TOR1A gene in lymphoblast and brain samples from patients and NMC relatives. We demonstrate that mRNA is transcribed from both the WT and ΔGAG allele in peripheral and neural tissues with a trend for increased expression of the ΔGAG allele compared to the WT in carriers regardless of their phenotype and thus cannot account for the reduced penetrance.

Developmental Profile of the Aberrant Dopamine D2 Receptor Response in Striatal Cholinergic Interneurons in DYT1 Dystonia

BackgroundDYT1 dystonia, a severe form of genetically determined human dystonia, exhibits reduced penetrance among carriers and begins usually during adolescence. The reasons for such age dependence and variability remain unclear.Methods and ResultsWe characterized the alterations in D2 dopamine receptor (D2R) signalling in striatal cholinergic interneurons at different ages in mice overexpressing human mutant torsinA (hMT). An abnormal excitatory response to the D2R agonist quinpirole was recorded at postnatal day 14, consisting of a membrane depolarization coupled to an increase in spiking frequency, and persisted unchanged at 3 and 9 months in hMT mice, compared to mice expressing wild-type human torsinA and non-transgenic mice. This response was blocked by the D2R antagonist sulpiride and depended upon G-proteins, as it was prevented by intrapipette GDP-β-S. Patch-clamp recordings from dissociated interneurons revealed a significant increase in the Cav2.2-mediated current fraction at all ages examined. Consistently, chelation of intracellular calcium abolished the paradoxical response to quinpirole. Finally, no gross morphological changes were observed during development.ConclusionsThese results suggest that an imbalanced striatal dopaminergic/cholinergic signaling occurs early in DYT1 dystonia and persists along development, representing a susceptibility factor for symptom generation.

TorsinA participates in endoplasmic reticulum-associated degradation

TorsinA is an AAA+ ATPase located within the lumen of the endoplasmic reticulum and nuclear envelope, with a mutant form causing early onset torsion dystonia (DYT1). Here we report a new function for torsinA in endoplasmic reticulum-associated degradation (ERAD). Retro-translocation and proteosomal degradation of a mutant cystic fibrosis transmembrane conductance regulator (CFTRΔF508) was inhibited by downregulation of torsinA or overexpression of mutant torsinA, and facilitated by increased torsinA. Retro-translocation of cholera toxin was also decreased by downregulation of torsinA. TorsinA associates with proteins implicated in ERAD, including Derlin-1, VIMP, and p97. Further, torsinA reduces endoplasmic reticulum stress in nematodes overexpressing CFTRΔF508, and fibroblasts from DYT1 dystonia patients are more sensitive than controls to endoplasmic reticulum stress and less able to degrade mutant CFTR. Therefore, compromised ERAD function in the cells of DYT1 patients may increase sensitivity to endoplasmic reticulum stress with consequent alterations in neuronal function contributing to the disease state.

Cerebellothalamocortical pathway abnormalities in torsinA DYT1 knock-in mice.

The factors that determine symptom penetrance in inherited disease are poorly understood. Increasingly, magnetic resonance diffusion tensor imaging (DTI) and PET are used to separate alterations in brain structure and function that are linked to disease symptomatology from those linked to gene carrier status. One example is DYT1 dystonia, a dominantly inherited movement disorder characterized by sustained muscle contractions, postures, and/or involuntary movements. This form of dystonia is caused by a 3-bp deletion (i.e., ΔE) in the TOR1A gene that encodes torsinA. Carriers of the DYT1 dystonia mutation, even if clinically nonpenetrant, exhibit abnormalities in cerebellothalamocortical (CbTC) motor pathways. However, observations in human gene carriers may be confounded by variability in genetic background and age. To address this problem, we implemented a unique multimodal imaging strategy in a congenic line of DYT1 mutant mice that contain the ΔE mutation in the endogenous mouse torsinA allele (i.e., DYT1 knock-in). Heterozygous knock-in mice and littermate controls underwent microPET followed by ex vivo high-field DTI and tractographic analysis. Mutant mice, which do not display abnormal movements, exhibited significant CbTC tract changes as well as abnormalities in brainstem regions linking cerebellar and basal ganglia motor circuits highly similar to those identified in human nonmanifesting gene carriers. Moreover, metabolic activity in the sensorimotor cortex of these animals was closely correlated with individual measures of CbTC pathway integrity. These findings further link a selective brain circuit abnormality to gene carrier status and demonstrate that DYT1 mutant torsinA has similar effects in mice and humans.

The DYT1 carrier state increases energy demand in the olivocerebellar network

DYT1 dystonia is caused by a GAG deletion in TOR1A, the gene which encodes torsinA. Gene expression studies in rodents and functional imaging studies in humans suggest that DYT1 dystonia may be a network disorder of neurodevelopmental origin. To generate high resolution metabolic maps of DYT1 dystonia and pinpoint dysregulated network elements, we performed 2-deoxyglucose autoradiography and cytochrome oxidase (CO) histochemistry in transgenic mice expressing human mutant (hMT1) torsinA and wild-type littermates. In comparison with controls, hMT1 mice showed increased glucose utilization (GU) in the inferior olive (IO) medial nucleus (IOM), IO dorsal accessory nucleus and substantia nigra compacta, and decreased GU in the medial globus pallidus (MGP) and lateral globus pallidus. The hMT1 mice showed increased CO activity in the IOM and Purkinje cell layer of cerebellar cortex, and decreased CO activity in the caudal caudate-putamen, substantia nigra reticulata and MGP. These findings suggest that (1) the DYT1 carrier state increases energy demand in the olivocerebellar network and (2) the IO may be a pivotal node for abnormal basal ganglia-cerebellar interactions in dystonia.

The role of torsinA in dystonia

DYT1 dystonia is an autosomal-dominant movement disorder, characterised by early onset of involuntary sustained muscle contractions. It is caused by a 3-bp deletion in the DYT1 gene, which results in the deletion of a single glutamate residue in the C-terminus of the protein torsinA. TorsinA is a member of the AAA-ATPase family of; chaperones with multiple functions in the cell. There is no evidence of neurodegeneration in DYT1 dystonia, which suggests that mutant torsinA leads to functional neuronal abnormalities leading to dystonic movements. In the recent years, different functional roles have been attributed to torsinA, including being a component of the cytoskeleton and the nuclear envelope, and involvement in the secretory pathway and synaptic vesicle machinery. The aim of this review is to summarise these findings and the different models proposed, which have contributed to our current understanding of the function of torsinA.