Reduced striatal D2 receptor binding in myoclonus-dystonia

A Study of Dopaminergic Pathway in Neurologic Wilson Disease with Movement Disorder

Movement disorder (MD) is an important manifestation of neurologic Wilson disease (NWD), but there is a paucity of information on dopaminergic pathways. We evaluate dopamine and its receptors in patients with NWD and correlate the changes with MD and MRI changes. Twenty patients with NWD having MD were included. The severity of dystonia was assessed using BFM (Burke-Fahn-Marsden) score. The neurological severity of NWD was categorized as grades I to III based on the sum score of 5 neurological signs and activity of daily living. Dopamine concentration in plasma and CSF was measured using liquid chromatography-mass spectrometry, and D1 and D2 receptor expression at mRNA by reverse transcriptase polymerase chain reaction in patients and 20 matched controls. The median age of the patients was 15 years and 7 (35%) were females. Eighteen (90%) patients had dystonia and 2 (10%) had chorea. The CSF dopamine concentration (0.08 ± 0.02 vs 0.09 ± 0.017 pg/ml; p = 0.42) in the patients and controls was comparable, but D2 receptor expression was reduced in the patients (0.41 ± 0.13 vs 1.39 ± 1.04; p = 0.01). Plasma dopamine level correlated with BFM score (r = 0.592, p < 0.01) and D2 receptor expression with the severity of chorea (r = 0.447, p < 0.05). The neurological severity of WD correlated with plasma dopamine concentration (p = 0.006). Dopamine and its receptors were not related to MRI changes. The central nervous system dopaminergic pathway is not enhanced in NWD, which may be due to structural damage to the corpus striatum and/or substantia nigra.

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.

Synaptic effects of ethanol on striatal circuitry: therapeutic implications for dystonia

Alcohol consumption affects motor behavior and motor control. Both acute and chronic alcohol abuse have been extensively investigated; however, the therapeutic efficacy of alcohol on some movement disorders, such as myoclonus-dystonia or essential tremor, still does not have a plausible mechanistic explanation. Yet, there are surprisingly few systematic trials with known GABAergic drugs mimicking the effect of alcohol on neurotransmission. In this brief survey, we aim to summarize the effects of EtOH on striatal function, providing an overview of its cellular and synaptic actions in a 'circuit-centered' view. In addition, we will review both experimental and clinical evidence, in the attempt to provide a plausible mechanistic explanation for alcohol-responsive movement disorders, with particular emphasis on dystonia. Different hypotheses emerge, which may provide a rationale for the utilization of drugs that mimic alcohol effects, predicting potential drug repositioning.

Basal ganglia: From the bench to the bed

The basal ganglia (BG) encompass a set of archaic structures of the vertebrate brain that have evolved relatively little during the phylogenetic process. From an anatomic point of view, they are widely distributed throughout brain from the telencephalon to the mesencephalon. The fact that they have been preserved through evolution suggests that they may play a critical role in behavioral monitoring. Indeed, a line of evidence suggests that they are involved in the building of behavioral routines and habits that drive most of our activities in everyday life. In this article, we first examine the organization and physiology of the basal ganglia to explain their function in the control of behavior. Then, we show how disruption of the putamen, and to a lesser extent of the cerebellum, might lead to various dystonic syndromes that frequently arise during childhood.

Dopaminergic and serotonergic alterations in plasma in three groups of dystonia patients

INTRODUCTION

In dystonia, dopaminergic alterations are considered to be responsible for the motor symptoms. Recent attention for the highly prevalent non-motor symptoms suggest also a role for serotonin in the pathophysiology. In this study we investigated the dopaminergic, serotonergic and noradrenergic metabolism in blood samples of dystonia patients and its relation with (non-)motor manifestations.

METHODS

Concentrations of metabolites of dopaminergic, serotonergic and noradrenergic pathways were measured in platelet-rich plasma in 41 myoclonus-dystonia (M-D), 25 dopa-responsive dystonia (DRD), 50 cervical dystonia (CD) patients and 55 healthy individuals. (Non-)motor symptoms were assessed using validated instruments, and correlated with concentrations of metabolites.

RESULTS

A significantly higher concentration of 3-methoxytyramine (0.03 vs. 0.02 nmol/L, p < 0.01), a metabolite of dopamine, and a reduced concentration of tryptophan (50 vs. 53 μmol/L, p = 0.03), the precursor of serotonin was found in dystonia patients compared to controls. The dopamine/levodopa ratio was higher in CD patients compared to other dystonia groups (p < 0.01). Surprisingly, relatively high concentrations of levodopa were found in the untreated DRD patients. Low concentrations of levodopa were associated with severity of dystonia (r_s = -0.3, p < 0.01), depression (r_s = -0.3, p < 0.01) and fatigue (r_s = -0.2, p = 0.04).

CONCLUSION

This study shows alterations in the dopaminergic and serotonergic metabolism of patients with dystonia, with dystonia subtype specific changes. Low concentrations of levodopa, but not of serotonergic metabolites, were associated with both motor and non-motor symptoms. Further insight into the dopaminergic and serotonergic systems in dystonia with a special attention to the kinetics of enzymes involved in these pathways, might lead to better treatment options.

Response to "isolated head tremor: A DAT SPECT and somatosensory temporal discrimination study."

In response to Ferrazano and colleagues' observation of normal DAT binding in patients with isolated head tremor but with abnormal STDT, we report normal 123-IBZM SPECT in a cohort of patients with adult-onset idiopathic focal dystonia with cervical dystonia and their unaffected first-degree relatives both with normal and abnormal TDTs. We discuss molecular imaging findings in dystonia.

Functional and Molecular Properties of DYT-SGCE Myoclonus-Dystonia Patient-Derived Striatal Medium Spiny Neurons

Myoclonus-dystonia (DYT-SGCE, formerly DYT11) is characterized by alcohol-sensitive, myoclonic-like appearance of fast dystonic movements. It is caused by mutations in the SGCE gene encoding ε-sarcoglycan leading to a dysfunction of this transmembrane protein, alterations in the cerebello-thalamic pathway and impaired striatal plasticity. To elucidate underlying pathogenic mechanisms, we investigated induced pluripotent stem cell (iPSC)-derived striatal medium spiny neurons (MSNs) from two myoclonus-dystonia patients carrying a heterozygous mutation in the SGCE gene (c.298T>G and c.304C>T with protein changes W100G and R102X) in comparison to two matched healthy control lines. Calcium imaging showed significantly elevated basal intracellular Ca2+ content and lower frequency of spontaneous Ca2+ signals in SGCE MSNs. Blocking of voltage-gated Ca2+ channels by verapamil was less efficient in suppressing KCl-induced Ca2+ peaks of SGCE MSNs. Ca2+ amplitudes upon glycine and acetylcholine applications were increased in SGCE MSNs, but not after GABA or glutamate applications. Expression of voltage-gated Ca2+ channels and most ionotropic receptor subunits was not altered. SGCE MSNs showed significantly reduced GABAergic synaptic density. Whole-cell patch-clamp recordings displayed elevated amplitudes of miniature postsynaptic currents and action potentials in SGCE MSNs. Our data contribute to a better understanding of the pathophysiology and the development of novel therapeutic strategies for myoclonus-dystonia.

Investigating the role of striatal dopamine receptor 2 in motor coordination and balance: Insights into the pathogenesis of DYT1 dystonia

DYT1 or DYT-TOR1A dystonia is early-onset, generalized dystonia. Most DYT1 dystonia patients have a heterozygous trinucleotide GAG deletion in DYT1 or TOR1A gene, with a loss of a glutamic acid residue of the protein torsinA. DYT1 dystonia patients show reduced striatal dopamine D2 receptor (D2R) binding activity. We previously reported reduced striatal D2R proteins and impaired corticostriatal plasticity in Dyt1 ΔGAG heterozygous knock-in (Dyt1 KI) mice. It remains unclear how the D2R reduction contributes to the pathogenesis of DYT1 dystonia. Recent knockout studies indicate that D2R on cholinergic interneurons (Chls) has a significant role in corticostriatal plasticity, while D2R on medium spiny neurons (MSNs) plays a minor role. To determine how reduced D2Rs on ChIs and MSNs affect motor performance, we generated ChI- or MSN-specific D2R conditional knockout mice (Drd2 ChKO or Drd2 sKO). The striatal ChIs in the Drd2 ChKO mice showed an increased firing frequency and impaired quinpirole-induced inhibition, suggesting a reduced D2R function on the ChIs. Drd2 ChKO mice had an age-dependent deficient performance on the beam-walking test similar to the Dyt1 KI mice. The Drd2 sKO mice, conversely, had a deficit on the rotarod but not the beam-walking test. Our findings suggest that D2Rs on Chls and MSNs have critical roles in motor control and balance. The similarity of the beam-walking deficit between the Drd2 ChKO and Dyt1 KI mice supports our earlier notion that D2R reduction on striatal ChIs contributes to the pathophysiology and the motor symptoms of DYT1 dystonia.

Isolated head tremor: A DAT-SPECT and somatosensory temporal discrimination study

INTRODUCTION

To better understand the pathophysiology of isolated head tremor, we investigated somatosensory temporal discrimination threshold (STDT) and dopaminergic neurotransmission with ¹²³I-FP-CIT (DAT-SPECT) in patients with isolated head tremor, as well as potential correlations with their clinical features.

METHODS

We enrolled 14 patients with isolated head tremor who underwent clinical examination, STDT testing, and DAT-SPECT.

RESULTS

We found normal radiotracer uptake in both striata as assessed with DAT-SPECT examination, but higher STDT values in patients as compared to healthy subjects. No significant correlations emerged between STDT values, specific binding ratios of radiotracer uptake, and demographic or clinical features.

CONCLUSION

Our study found normal radiotracer uptake with DAT-SPECT examination, suggesting dopaminergic neurotransmission integrity in patients with isolated head tremor. Patients with isolated head tremor exhibited an abnormally elevated STDT. Both results support the hypothesis that isolated head tremor should be considered a form of dystonia.

Medical management of myoclonus-dystonia and implications for underlying pathophysiology

Myoclonus-dystonia is an early onset genetic disorder characterised by subcortical myoclonus and less prominent dystonia. Its primary causative gene is the epsilon-sarcoglycan gene but the syndrome of "myoclonic dystonia" has been shown to be a heterogeneous group of genetic disorders. The underlying pathophysiology of myoclonus-dystonia is incompletely understood, although it may relate to dysfunction of striatal monoamine neurotransmission or disruption of cerebellothalamic networks (possibly via a GABAergic deficit of Purkinje cells). A broad range of oral medical therapies have been used in the treatment of myoclonus-dystonia with a varying response, and limited data relating to efficacy and tolerability, yet this condition responds dramatically to alcohol. Few well conducted randomized controlled trials have been undertaken leading to an empirical ad hoc approach for many patients. We review the current evidence for pharmacological therapies in myoclonus-dystonia, discuss implications for underlying pathogenesis of the condition and propose a treatment algorithm for these patients.

Twenty years on: Myoclonus-dystonia and ε-sarcoglycan - neurodevelopment, channel, and signaling dysfunction

Myoclonus-dystonia is a clinical syndrome characterized by a typical childhood onset of myoclonic jerks and dystonia involving the neck, trunk, and upper limbs. Psychiatric symptomatology, namely, alcohol dependence and phobic and obsessive-compulsive disorder, is also part of the clinical picture. Zonisamide has demonstrated effectiveness at reducing both myoclonus and dystonia, and deep brain stimulation seems to be an effective and long-lasting therapeutic option for medication-refractory cases. In a subset of patients, myoclonus-dystonia is associated with pathogenic variants in the epsilon-sarcoglycan gene, located on chromosome 7q21, and up to now, more than 100 different pathogenic variants of the epsilon-sarcoglycan gene have been described. In a few families with a clinical phenotype resembling myoclonus-dystonia associated with distinct clinical features, variants have been identified in genes involved in novel pathways such as calcium channel regulation and neurodevelopment. Because of phenotypic similarities with epsilon-sarcoglycan gene-related myoclonus-dystonia, these conditions can be collectively classified as "myoclonus-dystonia syndromes." In the present article, we present myoclonus-dystonia caused by epsilon-sarcoglycan gene mutations, with a focus on genetics and underlying disease mechanisms. Second, we review those conditions falling within the spectrum of myoclonus-dystonia syndromes, highlighting their genetic background and involved pathways. Finally, we critically discuss the normal and pathological function of the epsilon-sarcoglycan gene and its product, suggesting a role in the stabilization of the dopaminergic membrane via regulation of calcium homeostasis and in the neurodevelopmental process involving the cerebello-thalamo-pallido-cortical network. © 2019 International Parkinson and Movement Disorder Society.

Dystonia and dopamine: From phenomenology to pathophysiology

A line of evidence suggests that the pathophysiology of dystonia involves the striatum, whose activity is modulated among other neurotransmitters, by the dopaminergic system. However, the link between dystonia and dopamine appears complex and remains unclear. Here, we propose a physiological approach to investigate the clinical and experimental data supporting a role of the dopaminergic system in the pathophysiology of dystonic syndromes. Because dystonia is a disorder of motor routines, we first focus on the role of dopamine and striatum in procedural learning. Second, we consider the phenomenology of dystonia from every angle in order to search for features giving food for thought regarding the pathophysiology of the disorder. Then, for each dystonic phenotype, we review, when available, the experimental and imaging data supporting a connection with the dopaminergic system. Finally, we propose a putative model in which the different phenotypes could be explained by changes in the balance between the direct and indirect striato-pallidal pathways, a process critically controlled by the level of dopamine within the striatum. Search strategy and selection criteria References for this article were identified through searches in PubMed with the search terms « dystonia », « dopamine", « striatum », « basal ganglia », « imaging data », « animal model », « procedural learning », « pathophysiology », and « plasticity » from 1998 until 2018. Articles were also identified through searches of the authors' own files. Only selected papers published in English were reviewed. The final reference list was generated on the basis of originality and relevance to the broad scope of this review.

Loss of the dystonia gene Thap1 leads to transcriptional deficits that converge on common pathogenic pathways in dystonic syndromes

Dystonia is a movement disorder characterized by involuntary and repetitive co-contractions of agonist and antagonist muscles. Dystonia 6 (DYT6) is an autosomal dominant dystonia caused by loss-of-function mutations in the zinc finger transcription factor THAP1. We have generated Thap1 knock-out mice with a view to understanding its transcriptional role. While germ-line deletion of Thap1 is embryonic lethal, mice lacking one Thap1 allele-which in principle should recapitulate the haploinsufficiency of the human syndrome-do not show a discernable phenotype. This is because mice show autoregulation of Thap1 mRNA levels with upregulation at the non-affected locus. We then deleted Thap1 in glial and neuronal precursors using a nestin-conditional approach. Although these mice do not exhibit dystonia, they show pronounced locomotor deficits reflecting derangements in the cerebellar and basal ganglia circuitry. These behavioral features are associated with alterations in the expression of genes involved in nervous system development, synaptic transmission, cytoskeleton, gliosis and dopamine signaling that link DYT6 to other primary and secondary dystonic syndromes.

Myoclonus-dystonia: classification, phenomenology, pathogenesis, and treatment

PURPOSE OF REVIEW

The present study will highlight recent advances in the field of myoclonus-dystonia with a focus on clinical aspects, pathogenesis, and treatment. We will also discuss genetics, classification issues, and diagnostic criteria.

RECENT FINDINGS

Myoclonus-dystonia is a clinical syndrome corresponding to the phenotype linked to SGCE, the main causative gene. Childhood-onset myoclonus that predominates over dystonia with prominent upper body involvement, an absence of truncal dystonia, associated anxiety or compulsivity, and a positive family history are helpful diagnostic clues. Recent studies demonstrated that zonisamide is an interesting therapeutic option in myoclonus-dystonia, and that bilateral pallidal stimulation has major and lasting therapeutic effects. Accumulating evidence suggests that an alteration in cerebello-thalamic pathway function may play a prominent role and that this is possibly related to a GABAergic deficit reflecting Purkinje cell dysfunction. Impaired striatal plasticity and disturbed serotonin homeostasis may also be implicated. Newly available cellular and rodent models may further assist in investigating the pathogenesis of this disorder.

SUMMARY

Comprehensive analysis of the phenotype and precise classification are important in patients with myoclonus and dystonia to identify homogeneous groups of patients. This is critical to guide tailored therapeutic strategies and promote effective research.

Abnormal striatal plasticity in a DYT11/SGCE myoclonus dystonia mouse model is reversed by adenosine A2A receptor inhibition

Striatal dysfunction is implicated in many movement disorders. However, the precise nature of defects often remains uncharacterized, which hinders therapy development. Here we examined striatal function in a mouse model of the incurable movement disorder, myoclonus dystonia, caused by SGCE mutations. Using RNAseq we found surprisingly normal gene expression, including normal levels of neuronal subclass markers to strongly suggest that striatal microcircuitry is spared by the disease insult. We then functionally characterized Sgce mutant medium spiny projection neurons (MSNs) and cholinergic interneurons (ChIs). This revealed normal intrinsic electrophysiological properties and normal responses to basic excitatory and inhibitory neurotransmission. Nevertheless, high-frequency stimulation in Sgce mutants failed to induce normal long-term depression (LTD) at corticostriatal glutamatergic synapses. We also found that pharmacological manipulation of MSNs by inhibiting adenosine 2A receptors (A_2AR) restores LTD, again pointing to structurally intact striatal circuitry. The fact that Sgce loss specifically inhibits LTD implicates this neurophysiological defect in myoclonus dystonia, and emphasizes that neurophysiological changes can occur in the absence of broad striatal dysfunction. Further, the positive effect of A_2AR antagonists indicates that this drug class be tested in DYT11/SGCE dystonia.

The relationship between the dopaminergic system and depressive symptoms in cervical dystonia

PURPOSE

Cervical dystonia (CD) is associated with tremor/jerks (50%) and psychiatric complaints (17-70%). The dopaminergic system has been implicated in the pathophysiology of CD in animal and imaging studies. Dopamine may be related to the motor as well as non-motor symptoms of CD. CD is associated with reduced striatal dopamine D_2/3 (D2/3) receptor and increased dopamine transporter (DAT) binding. There are differences in the dopamine system between CD patients with and without jerks/tremor and psychiatric symptoms.

METHODS

Patients with CD and healthy controls underwent neurological and psychiatric examinations. Striatal DAT and D2/3 receptor binding were assessed using [¹²³I]FP-CIT and [¹²³I]IBZM SPECT, respectively. The ratio of specific striatal to non-specific binding (binding potential; BP_ND) was the outcome measure.

RESULTS

Twenty-seven patients with CD and 15 matched controls were included. Nineteen percent of patients fulfilled the criteria for a depression. Striatal DAT BP_ND was significantly lower in depressed versus non-depressed CD patients. Higher DAT BP_ND correlated significantly with higher scores on the Unified Myoclonus Rating Scale (UMRS). The striatal D2/3 receptor BP_ND in CD patients showed a trend towards lower binding compared to controls. The D2/3 BP_ND was significantly lower in depressed versus non-depressed CD patients. A significant correlation between DAT and D2/3R BP_ND was found in both in patients and controls.

CONCLUSIONS

Alterations of striatal DAT and D2/3 receptor binding in CD patients are related mainly to depression. DAT BP_ND correlates significantly with scores on the UMRS, suggesting a role for dopamine in the pathophysiology of tremor/jerks in CD.

Non-motor symptoms in genetically defined dystonia: Homogenous groups require systematic assessment

INTRODUCTION

Dystonia is a movement disorder involving sustained or intermittent muscle contractions resulting in abnormal movements and postures. Identification of disease causing genes has allowed examination of genetically homogenous groups. Unlike the motor symptoms, non-motor characteristics are less clearly defined, despite their impact on a patient's quality of life. This review aims to examine the evidence for non-motor symptoms, addressing cohort size and methods of assessment in each study.

METHODS

A systematic and standardised search strategy was used to identify the published literature relating to psychiatric symptoms, cognition, sleep disorders, sensory abnormalities and pain in each of the genetically determined dystonias. Studies were divided according to cohort size, method of assessment and whether comparison was made to an appropriate control group.

RESULTS

Ninety-five articles were identified including reported clinical histories (n = 42), case reports and smaller case series (n = 12), larger case series (n = 23) and case-control cohorts (n = 18). Psychiatric symptoms were the most frequently investigated with anxiety, depression and Obsessive-Compulsive disorder being most common. Cognitive impairment involved either global deficits or isolated difficulties in specific domains. Disturbances to sleep were most common in the dopa-responsive dystonias. Sensory testing in DYT1 cases identified an intermediate subclinical phenotype.

CONCLUSION

Non-motor symptoms form an integral component of the dystonia phenotype. However, future studies should involve a complete assessment of all symptom subtypes in order to understand the frequency and gene-specificity of these symptoms. This will enable early symptom identification, appropriate clinical management, and provide additional outcome measures in future clinical trials.

The role of dopamine and dopaminergic pathways in dystonia: insights from neuroimaging

Background

Dystonia constitutes a heterogeneous group of movement abnormalities, characterized by sustained or intermittent muscle contractions causing abnormal postures. Overwhelming data suggest involvement of basal ganglia and dopaminergic pathways in dystonia. In this review, we critically evaluate recent neuroimaging studies that investigate dopamine receptors, endogenous dopamine release, morphology of striatum, and structural or functional connectivity in cortico-basal ganglia-thalamo-cortical and related cerebellar circuits in dystonia.

Method

A PubMed search was conducted in August 2014.

Results

Positron emission tomography (PET) imaging offers strong evidence for altered D2/D3 receptor binding and dopaminergic release in many forms of idiopathic dystonia. Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) data reveal likely involvement of related cerebello-thalamo-cortical and sensory-motor networks in addition to basal ganglia.

Discussion

PET imaging of dopamine receptors or transmitter release remains an effective means to investigate dopaminergic pathways, yet may miss factors affecting dopamine homeostasis and related subcellular signaling cascades that could alter the function of these pathways. fMRI and DTI methods may reveal functional or anatomical changes associated with dysfunction of dopamine-mediated pathways. Each of these methods can be used to monitor target engagement for potential new treatments. PET imaging of striatal phosphodiesterase and development of new selective PET radiotracers for dopamine D3-specific receptors and Mechanistic target of rampamycin (mTOR) are crucial to further investigate dopaminergic pathways. A multimodal approach may have the greatest potential, using PET to identify the sites of molecular pathology and magnetic resonance methods to determine their downstream effects.

Prefronto-striatal physiology is associated with schizotypy and is modulated by a functional variant of DRD2

“Schizotypy” is a latent organization of personality related to the genetic risk for schizophrenia. Some evidence suggests that schizophrenia and schizotypy share some biological features, including a link to dopaminergic D2 receptor signaling. A polymorphism in the D2 gene (DRD2 rs1076560, guanine > thymine (G > T)) has been associated with the D2 short/long isoform expression ratio, as well as striatal dopamine signaling and prefrontal cortical activity during different cognitive operations, which are measures that are altered in patients with schizophrenia. Our aim is to determine the association of schizotypy scores with the DRD2 rs1076560 genotype in healthy individuals and their interaction with prefrontal activity during attention and D2 striatal signaling. A total of 83 healthy subjects were genotyped for DRD2 rs1076560 and completed the Schizotypal Personality Questionnaire (SPQ). Twenty-six participants underwent SPECT with [¹²³I]IBZM D2 receptor radiotracer, while 68 performed an attentional control task during fMRI. We found that rs1076560 GT subjects had greater SPQ scores than GG individuals. Moreover, the interaction between schizotypy and the GT genotype predicted prefrontal activity and related attentional behavior, as well as striatal binding of IBZM. No interaction was found in GG individuals. These results suggest that rs1076560 GT healthy individuals are prone to higher levels of schizotypy, and that the interaction between rs1076560 and schizotypy scores modulates phenotypes related to the pathophysiology of schizophrenia, such as prefrontal activity and striatal dopamine signaling. These results provide systems-level qualitative evidence for mapping the construct of schizotypy in healthy individuals onto the schizophrenia continuum.

The anatomical basis of dystonia: current view using neuroimaging

This review will consider the knowledge that neuroimaging studies have provided to the understanding of the anatomy of dystonia. Major advances have occurred in the use of neuroimaging for dystonia in the past 2 decades. At present, the most developed imaging approaches include whole-brain or region-specific studies of structural or diffusion changes, functional imaging using fMRI or positron emission tomography (PET), and metabolic imaging using fluorodeoxyglucose PET. These techniques have provided evidence that regions other than the basal ganglia are involved in dystonia. In particular, there is increasing evidence that primary dystonia can be viewed as a circuit disorder, involving the basal ganglia-thalamo-cortical and cerebello-thalamo-cortical pathways. This suggests that a better understanding of the dysfunction in each region in the network and their interactions are important topics to address. Current views of interpretation of imaging data as cause or consequence of dystonia, and the postmortem correlates of imaging data are presented. The application of imaging as a tool to monitor therapy and its use as an outcome measure will be discussed. © 2013 Movement Disorder Society.

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.

Prominent psychiatric comorbidity in the dominantly inherited movement disorder myoclonus-dystonia

BACKGROUND

Neurological and psychiatric disorders show clinical overlap suggesting a shared pathophysiological background. We evaluated myoclonus-dystonia, a monogenic movement disorder as a disease model for inherited psychopathology.

METHOD

We investigated 12 SGCE mutation carriers using standardized neurological and psychiatric examinations to assign DSM-IV diagnoses. Furthermore, we analyzed all studies in the Medline database which included psychiatric information on SGCE mutation-positive patients.

RESULTS

Of our twelve SGCE mutation carriers, 10 were older than 16 years. Two of them (20%) reported psychiatric diagnoses before our examination, which resulted in at least one psychiatric diagnosis in seven (70%) patients, most frequently anxiety (60%), depression (30%) or both. Substance abuse was observed in 20%, whereas obsessive-compulsive disorders were absent. One mutation carrier showed Axis 2 features. In the literature analysis, the ten studies using standardized tools covering DSM-IV criteria reported prevalences similar to those in our sample. This was three times the frequency of psychiatric disorders detected in 13 studies using clinical history or patient report only.

CONCLUSION

About two thirds of SGCE mutation carriers develop psychiatric comorbidity and >80% are previously undiagnosed.

Alteration of striatal dopaminergic neurotransmission in a mouse model of DYT11 myoclonus-dystonia

Background

DYT11 myoclonus-dystonia (M-D) syndrome is a neurological movement disorder characterized by myoclonic jerks and dystonic postures or movement that can be alleviated by alcohol. It is caused by mutations in SGCE encoding ε-sarcoglycan (ε-SG); the mouse homolog of this gene is Sgce. Paternally-inherited Sgce heterozygous knockout (Sgce KO) mice exhibit myoclonus, motor impairment and anxiety- and depression-like behaviors, modeling several clinical symptoms observed in DYT11 M-D patients. The behavioral deficits are accompanied by abnormally high levels of dopamine and its metabolites in the striatum of Sgce KO mice. Neuroimaging studies of DYT11 M-D patients show reduced dopamine D2 receptor (D2R) availability, although the possibility of increased endogenous dopamine, and consequently, competitive D2R occupancy cannot be ruled out.

Methodology/Principal Findings

The protein levels of striatal D2R, dopamine transporter (DAT), and dopamine D1 receptor (D1R) in Sgce KO mice were analyzed by Western blot. The striatal dopamine release after amphetamine injection in Sgce KO mice were analyzed by microdialysis in vivo. The striatal D2R was significantly decreased in Sgce KO mice without altering DAT and D1R. Sgce KO mice also exhibited a significant increase of dopamine release after amphetamine injection in comparison to wild-type (WT) littermates.

Conclusion/Significance

The results suggest ε-SG may have a role in the regulation of D2R expression. The loss of ε-SG results in decreased striatal D2R, and subsequently leads to increased discharge of dopamine which could contribute to the behavioral impairment observed in DYT11 dystonia patients and in Sgce KO mice. The results suggest that reduction of striatal D2R and enhanced striatal dopamine release may contribute to the pathophysiology of DYT11 M-D patients.

Deep Brain Stimulation of the Pallidum is Effective and Might Stabilize Striatal D(2) Receptor Binding in Myoclonus-Dystonia

Purpose: To assess clinical efficacy of deep brain stimulation (DBS) of the pallidum in Myoclonus–Dystonia (M–D) patients, and to compare pre- and post-operative striatal dopamine D2 receptor availability. Methods: Clinical parameters were scored using validated rating scales for myoclonus and dystonia. Dopamine D2 receptor binding of three patients was studied before surgery and approximately 2 years post-operatively using 123-I-iodobenzamide Single Photon Emission Computed Tomography. Two patients who did not undergo surgery served as controls. Results: Clinically, the three M–D patients improved 83, 17, and 100%, respectively on the myoclonus rating scale and 78, 23, and 65% on the dystonia rating scale after DBS. Dopamine D2 receptor binding did not change after surgery. In the two control subjects, binding has lowered further. Conclusion: These findings confirm that DBS of the pallidum has beneficial effects on motor symptoms in M–D and suggest this procedure might stabilize dopamine D2 receptor binding.

The dopaminergic system in patients with functional dyspepsia analysed by single photon emission computed tomography (SPECT) and an alpha-methyl-para-tyrosine (AMPT) challenge test

Purpose

Functional dyspepsia (FD) is a chronic condition characterized by upper abdominal symptoms without an identifiable cause. While the serotonergic system is thought to play a key role in the regulation of gut physiology, the role of the dopaminergic system, which is important in the regulation of visceral pain and stress, is under-studied. Therefore, this study investigated the dopaminergic system and its relationship with drinking capacity and symptoms in FD patients.

Methods

In FD patients and healthy volunteers (HV) the dopaminergic system was investigated by in-vivo assessment of central dopamine D2 receptors (D2Rs) with [¹²³I]IBZM SPECT and by an acute, but reversible, dopamine depletion alpha-methyl-para-tyrosine (AMPT) challenge test. A nutrient drink test was performed to investigate the association between maximal ingested volume, evoked symptoms, and D2Rs.

Results

The HV subjects comprised 12 women and 8 men (mean age 31 ± 3 years), and the FD patients comprised 5 women and 3 men (mean age 39 ± 5 years). The FD patients had a lower left plus right average striatal binding potential (BP_NP) for the caudate nucleus (p = 0.02), but not for putamen (p = 0.15), which in the FD patients was correlated with maximal ingested volume (r = 0.756, p = 0.03). The D2R BP_NP in the putamen was correlated with nausea (r = 0.857, p = 0.01). The acute dopamine depletion test, however, failed to reveal differences in prolactin release between the FD patients and the HV subjects.

Conclusion

These preliminary data suggest that chronic rather than acute alterations in the dopaminergic system may be involved in the pathogenesis of FD. Further studies are required to reproduce our novel findings and to evaluate to what extent the dopaminergic changes may be secondary to abnormalities in serotonergic pathways.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-011-2015-6) contains supplementary material, which is available to authorized users.

Abnormal nuclear envelopes in the striatum and motor deficits in DYT11 myoclonus-dystonia mouse models

DYT11 myoclonus-dystonia (M-D) is a movement disorder characterized by myoclonic jerks with dystonic symptoms and caused by mutations in paternally expressed SGCE, which codes for ε-sarcoglycan. Paternally inherited Sgce heterozygous knock-out (KO) mice exhibit motor deficits and spontaneous myoclonus. Abnormal nuclear envelopes have been reported in cellular and mouse models of early-onset DYT1 generalized torsion dystonia; however, the relationship between the abnormal nuclear envelopes and motor symptoms are not clear. Furthermore, it is not known whether abnormal nuclear envelope exists in non-DYT1 dystonia. In the present study, abnormal nuclear envelopes in the striatal medium spiny neurons (MSNs) were found in Sgce KO mice. To analyze whether the loss of ε-sarcoglycan in the striatum alone causes abnormal nuclear envelopes, motor deficits or myoclonus, we produced paternally inherited striatum-specific Sgce conditional KO (Sgce sKO) mice and analyzed their phenotypes. Sgce sKO mice exhibited motor deficits in both beam-walking and accelerated rotarod tests, while they did not exhibit abnormal nuclear envelopes, alteration in locomotion, or myoclonus. The results suggest that the loss of ε-sarcoglycan in the striatum contributes to motor deficits, while it alone does not produce abnormal nuclear envelopes or myoclonus. Development of therapies targeting the striatum to compensate for the loss of ε-sarcoglycan function may rescue the motor deficits in DYT11 M-D patients.

The cerebellum in dystonia - help or hindrance?

Dystonia has historically been considered a disorder of the basal ganglia. This review aims to critically examine the evidence for a role of the cerebellum in the pathophysiology of dystonia. We compare and attempt to link the information available from both clinical and experimental studies; work detailing cerebellar connectivity in primates; data that suggests a role for the cerebellum in the genesis of dystonia in murine models; clinical observation in humans with structural lesions and heredodegenerative disorders of the cerebellum; and imaging studies of patients with dystonia. The typical electrophysiological findings in dystonia are the converse to those found in cerebellar lesions. However, certain subtypes of dystonia mirror cerebellar patterns of increased cortical inhibition. Furthermore, altered cerebellar function can be demonstrated in adult onset focal dystonia with impaired cerebellar inhibition of motor cortex and abnormal eyeblink classical conditioning. We propose that abnormal, likely compensatory activity of the cerebellum is an important factor within pathophysiological models of dystonia. Work in this exciting area has only just begun but it is likely that the cerebellum will have a key place within future models of dystonia.

Dose-dependent and sustained effects of varenicline on dopamine D2/3 receptor availability in rats

Imaging studies in drug-dependent subjects show reduced striatal dopamine D(2/3) receptor (DRD2/3) availability, and it is hypothesized that increasing DRD2/3 availability is a promising strategy to treat drug dependence. We recently showed that rats treated for two weeks with 2mg/kg/day varenicline (a partial agonist at α4β2 nicotinic acetylcholine receptors) showed higher striatal DRD2/3 availability compared to control rats. The present study examined the effects of lower varenicline doses as well as the duration of the effect after treatment discontinuation. DRD2/3 availability in striatal areas was studied in 80 rats following two-week treatment with 0.5, 1 or 2mg/kg/day varenicline or vehicle and survival of the effects of varenicline on DRD2/3 availability up to 2 weeks after treatment discontinuation using (123)I-IBZM storage phosphor imaging. For all varenicline doses, varenicline treated rats showed a comparable significantly higher DRD2/3 availability in the ventral striatum of approximately 11% compared to control rats, while only the rats treated with 1 and 2mg/kg/day dose showed significantly higher DRD2/3 availability in the dorsal striatum by 12.5% and 13.2% compared to control rats, respectively. Two weeks after discontinuation of the active treatment with 2mg/kg/day varenicline, DRD2/3 binding in ventral, but not dorsal, striatum was still significantly higher (11.7%) compared to vehicle. Varenicline induces dose-dependent and sustained increases in striatal DRD2/3 in rats, particularly in the ventral striatum. These observations suggest that increased DRD2/3 availability may contribute to varenicline's efficacy for smoking cessation and show promise for varenicline as a treatment of other types of drug dependence.

Severity of dystonia is correlated with putaminal gray matter changes in myoclonus-dystonia

BACKGROUND

Myoclonus-dystonia (M-D) is an autosomal dominantly inherited movement disorder characterized by myoclonic jerks and dystonic postures or movements. Morphometric studies have been performed in other, mainly heterogenous, types of dystonia producing conflicting results. However, all these studies agree on abnormalities in sensorimotor structures, mainly in the basal ganglia. We aimed to study gray matter (GM) volumes in sensorimotor brain structures with magnetic resonance imaging (MRI) in a genetically homogeneous form of dystonia, M-D.

METHODS

Twenty-five clinically affected DYT11 mutation carriers (MC) and 25 matched control subjects were studied using T1-weighted 3D anatomical images of the entire brain, obtained with a 3.0 Tesla MRI. MC were clinically scored using the Burke Fahn Marsden dsytonia rating scale (BFMDRS) and the unified myoclonus rating scale (UMRS). GM volumes in sensorimotor cortices and basal ganglia of patients and controls were compared, and multiple regression analyses were used to correlate the GM volumes of patients with the clinical rating scales BFMDRS and UMRS.

RESULTS

No significant differences were found between groups, but dystonia severity in MC was strongly correlated with increased GM volume in bilateral putamina.

CONCLUSIONS

This study provides further evidence for the involvement of putamina as important motor structures in the pathophysiology of (myoclonus-) dystonia. Changes in these structures are associated with the severity of dystonia.

Myoclonus-dystonia syndrome

Myoclonus dystonia syndrome (MDS) refers to a group of heterogeneous nondegenerative clinical conditions characterized by the association of myoclonus and dystonia as the only or prominent symptom. The "core" of MDS is represented by inherited myoclonus-dystonia (M-D), a disorder with autosomal-dominant inheritance and reduced penetrance, beginning in early childhood with a relatively benign course, with myoclonus as the most predominant and disabling symptom. Alcohol responsiveness and psychiatric symptoms are characteristic features. Mutations in the epsilon-sarcoglycan gene (SGCE, DYT11) represent the major genetic cause, but M-D is genetically heterogeneous. In a variable proportion of M-D patients no mutation is found, and at least one other locus (DYT15) has been linked to the disease. Patients with primary dystonia, with or without the DYT1 mutation, may show irregular and arrhythmic jerky movements associated with dystonia. Usually dystonia is the prominent symptom and the myoclonic jerk involves the same body region; this condition, currently defined as "myoclonic dystonia," is included in the spectrum of MDS. Dopa-responsive dystonia due to mutation in the GTP-CH gene and vitamin E deficiency can present with a phenotype of dystonia and myoclonus in combination; both conditions should be considered in the diagnostic approach to patients since they are potentially treatable.

SGCE isoform characterization and expression in human brain: implications for myoclonus-dystonia pathogenesis?

Myoclonus-dystonia (M-D) is a neurological movement disorder with involuntary jerky and dystonic movements as major symptoms. About 50% of M-D patients have a mutation in ɛ-sarcoglycan (SGCE), a maternally imprinted gene that is widely expressed. As little is known about SGCE function, one can only speculate about the pathomechanisms of the exclusively neurological phenotype in M-D. We characterized different SGCE isoforms in the human brain using ultra-deep sequencing. We show that a major brain-specific isoform is differentially expressed in the human brain with a notably high expression in the cerebellum, namely in the Purkinje cells and neurons of the dentate nucleus. Its expression was low in the globus pallidus and moderate to low in caudate nucleus, putamen and substantia nigra. Our data are compatible with a model in which dysfunction of the cerebellum is involved in the pathogenesis of M-D.

Decreased striatal dopamine receptor binding in primary focal dystonia: a D2 or D3 defect?

Dystonia is an involuntary movement disorder characterized by repetitive patterned or sustained muscle contractions causing twisting or abnormal postures. Several lines of evidence suggest that abnormalities of dopaminergic pathways contribute to the pathophysiology of dystonia. In particular, dysfunction of D2-like receptors that mediate function of the indirect pathway in the basal ganglia may play a key role. We have demonstrated with positron emission tomography that patients with primary focal cranial or hand dystonia have reduced putamenal specific binding of [(18)F]spiperone, a nonselective D2-like radioligand with nearly equal affinity for serotonergic 5-HT(2A) sites. We then repeated the study with [(18)F]N-methyl-benperidol (NMB), a more selective D2-like receptor radioligand with minimal affinity for 5-HT(2A). Surprisingly, there was no decrease in NMB binding in the putamen of subjects with dystonia. Our findings excluded reductions of putamenal uptake greater than 20% with 95% confidence intervals. The analysis of the in vitro selectivity of NMB and spiperone demonstrated that NMB was highly selective for D2 receptors relative to D3 receptors (200-fold difference in affinity), whereas spiperone has similar affinity for all three of the D2-like receptor subtypes. These findings when coupled with other literature suggest that a defect in D3, rather than D2, receptor expression may be associated with primary focal dystonia.

[Monogenetic dystonia: revisiting the dopaminergic hypothesis]

Dystonias are clinically and genetically heterogeneous neurological disorders that affect movement, and are the focus of much investigative work. The recent identification of mutations in the gene THAP1 in DYT6 dystonia reopens the very interesting question of the in fine involvement of dopamine in the different types of dystonia. In this review, we will go through the recent literature in order to evaluate the many contributions to this theory as well as to highlight the difficulties in identifying a global regulatory pathway for the different forms of this disease that we are just starting to decipher.

Genetically determined measures of striatal D2 signaling predict prefrontal activity during working memory performance

BACKGROUND

Variation of the gene coding for D2 receptors (DRD2) has been associated with risk for schizophrenia and with working memory deficits. A functional intronic SNP (rs1076560) predicts relative expression of the two D2 receptors isoforms, D2S (mainly pre-synaptic) and D2L (mainly post-synaptic). However, the effect of functional genetic variation of DRD2 on striatal dopamine D2 signaling and on its correlation with prefrontal activity during working memory in humans is not known.

METHODS

Thirty-seven healthy subjects were genotyped for rs1076560 (G>T) and underwent SPECT with [123I]IBZM (which binds primarily to post-synaptic D2 receptors) and with [123I]FP-CIT (which binds to pre-synaptic dopamine transporters, whose activity and density is also regulated by pre-synaptic D2 receptors), as well as BOLD fMRI during N-Back working memory.

RESULTS

Subjects carrying the T allele (previously associated with reduced D2S expression) had striatal reductions of [123I]IBZM and of [123I]FP-CIT binding. DRD2 genotype also differentially predicted the correlation between striatal dopamine D2 signaling (as identified with factor analysis of the two radiotracers) and activity of the prefrontal cortex during working memory as measured with BOLD fMRI, which was positive in GG subjects and negative in GT.

CONCLUSIONS

Our results demonstrate that this functional SNP within DRD2 predicts striatal binding of the two radiotracers to dopamine transporters and D2 receptors as well as the correlation between striatal D2 signaling with prefrontal cortex activity during performance of a working memory task. These data are consistent with the possibility that the balance of excitatory/inhibitory modulation of striatal neurons may also affect striatal outputs in relationship with prefrontal activity during working memory performance within the cortico-striatal-thalamic-cortical pathway.