Dystonia is one of the most common movement disorders, a core component of the isolated and combined dystonias as well as contributing to the motor phenotype of several neurodegenerative movement disorders, such as Parkinson’s disease and Huntington’s disease. In spite of this, the pathophysiological mechanisms underlying dystonia remain poorly understood with the current thinking centered on a circuit-based disorder, with altered synaptic plasticity impacting higher neuronal circuits and networks. In this review, we discuss three publications with distinct approaches in attempting to elucidate these mechanisms. These articles also highlight how the combination of gene discovery, cellular assays, and systems-based analysis can each contribute to building an understanding of complex disorders

Peall, Kathryn J

Robertson, Neil

PubMed

K. J. Peall

N. P. Robertson

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Dystonia: opportunities to gain insights into underlying pathophysiological mechanisms

Online Research @ Cardiff

JOURNAL CLUBDystonia: opportunities to gain insights into underlyingpathophysiological mechanismsK. J. Peall1 • N. P. Robertson1Published online: 10 February 2017 The Author(s) 2017. This article is published with open access at Springerlink.comIntroductionDystonia is one of the most common movement disorders,a core component of the isolated and combined dystoniasas well as contributing to the motor phenotype of severalneurodegenerative movement disorders, such as Parkin-son’s disease and Huntington’s disease. In spite of this, thepathophysiological mechanisms underlying dystoniaremain poorly understood with the current thinking cen-tered on a circuit-based disorder, with altered synapticplasticity impacting higher neuronal circuits and networks.In this review, we discuss three publications with distinctapproaches in attempting to elucidate these mechanisms.These articles also highlight how the combination of genediscovery, cellular assays, and systems-based analysis caneach contribute to building an understanding of complexdisorders.Mutations in the histone methyltransferase geneKMT2B cause complex early onset dystoniaThis paper describes a stepwise approach to a novel genediscovery and confirmation of pathogenesis in a cohort ofpatients with an undiagnosed childhood-onset dystonia.Ten individuals were identified with overlappingheterozygous interstitial microdeletions involving a regionon chromosome 19 containing two genes, KMT2B andZBTB32. The next generation sequencing techniquesidentified KMT2B heterozygous variants in six furthercases, a single case by Sanger sequencing, and a further tencases from national and international collaborations.Meyer and colleagues provide a detailed description ofthe clinical characteristics and investigational findings ofthose identified with KMT2B mutations. The majority hadan early childhood onset (1–9 years) of dystonia symptomswith no evidence of gender bias. The dystonic symptomsbegan predominantly in the craniocervical region or limbs,progressing to a more generalized form 2–11 years afterpresentation. Additional clinical features included charac-teristic facial appearances (elongated face and bulbousnasal tip), developmental delay, intellectual disability, andsystemic symptoms involving skin, renal, and respiratorysymptoms. Cerebral imaging identified symmetricalhypointensity of the globus pallidi, although no abnor-malities were observed following neurotransmitter analysisof CSF. Deep brain stimulation (DBS) in ten cases pro-vided symptomatic improvement, whilst oral medicaltherapy was ineffective. No overt genotype–phenotyperelationship was immediately observed, although thosewith the rarer missense mutations tended to developsymptoms at a later age.A key component of a novel gene discovery is demon-strating that the mutations are likely to impact the normalfunction of the encoded protein. KMT2B encodes a histonelysine methyltransferase that is involved in the methylationof histone 3 at lysine 4 (H3K4), an important epigeneticmodification associated with active gene transcription. Insilico homology modeling techniques predicted that anumber of the identified variants would impact the struc-ture–function properties of the KMT2B protein. Analysisof healthy donor adult and foetal tissue found the protein tobe ubiquitously expressed throughout the brain, with thehighest levels observed in the cerebellum, while& N. P. Robertsonrobertsonnp@cardiff.ac.uk1 Institute of Psychological Medicine and Clinical,Neurosciences, Cardiff University, Cardiff CF14 4XW, UK123J Neurol (2017) 264:616–618DOI 10.1007/s00415-017-8411-5quantitative RT-PCR of patient-derived fibroblastsdemonstrated overall lower KMT2B levels than controls.Fibroblast and CSF protein expression analysis were alsoused to determine whether KMT2B mutations could impacton other known dystonia-related pathways. These identi-fied reduced THAP1 and TorsinA transcripts (genesinvolved in DYT6 and DYT1 dystonia, respectively), areduction in CSF dopamine-2-receptor (D2R) andincreased Tyrosine Hydroxylase levels.Comment: KMT2B has been given the DYT28 identifier,making it the most recent in a growing number of disease-causing genes for dystonia. The ongoing development andreducing costs of the next generation sequencing tech-niques suggest that this number is likely to continue to rise,providing increasing opportunity to understand the mech-anisms that underpin this complex set of disorders.Although mutations in each of these individual genesremain rare, their identification and subsequent interroga-tion of function on a cellular and system-based level pro-vide an opportunity for the identification of overlappingpathways, or points of commonality that may be amenableto therapeutic intervention.Meyer E et al (2017) Nat Genet 49(2):223–237.Functional genomic analyses of mendelianand sporadic disease identify impaired elF2asignaling as a generalizable mechanismfor dystoniaThis paper outlines the stepwise approach taken to identi-fying, and building upon, the evidence for elF2a signalingpathway involvement in Mendelian and seemingly spo-radic forms of dystonia.An initial step involved a successful validation of acellular-based assay for DYT1 dystonia (GAG deletion inTorsinA) based on a known Torsin1a biology. Expressionof wild type and mutant Torsin1a in human embryonickidney (HEK) cells demonstrated a distinct and readilyidentifiable pattern of protein expression. Having estab-lished a reliable cellular assay, small interfering RNAs(siRNAs) were utilized to undertake a hypothesis-freegenome-wide screen. siRNAs are double-stranded RNAmolecules that interfere with gene expression by degradingpost-transcriptional mRNA and prevent translation to theprotein product. Using a quality control (QC) index of aminimum three standard deviation improvement to Tor-sin1a localization, whole genome screen and subsequentpathway analysis identified 93 high stringency and high-quality genetic hits. Further validation identified 11 over-represented signaling pathways with the elF2a signalingpathway the most enriched.The elF2a pathway is a ubiquitous cellular pathwayknown as the integrated stress response (ISR). ElF2aphosphorylation is thought to contribute to synaptic plas-ticity, being required for long-term depression (LTD) atcortico-striatal synapses. The rate-limiting elF2a complexsubunit is phosphorylated by one of four upstream stress-sensitive kinases, knock down of which resulted in increasedabnormal Torsin1a localization, indicating that it isdecreased activity of the signaling pathway that contributesto pathogenesis. Pharmacological testing provided furtherconfirmation still with elF2a phosphorylation inhibitors(e.g., Salubrinal) normalizing Tosrin1a localization in adose-dependent manner. Compounds targeting other sig-naling pathways identified in the whole genome screen(notch and glucocorticoid signaling) found no effect, sug-gesting that these effects were specific to the elF2a pathway.ElF2a phosphorylation also appears to be involved inthe function of other genes implicated in the pathogenesisof dystonia. Whole exome sequencing of 20 unrelatedindividuals with sporadic isolated dystonia identified 12exons with rare coding variants, one of which being thefirst exon of ATF4, one of several genes upregulated byelF2a phosphorylation. Sanger sequencing of 239 sporadiccervical dystonia cases found ATF4 functional variants in8.8% (3/239) cases compared to 0.5% in controls. Withinthe DYT1 sample, patient-derived fibroblasts also demon-strated reduced ATF4 upregulation with basal increases innegative feedback proteins (CReP and GADD34) attenu-ating the acute phase elF2a signaling. Interestingly,PRKRA (mutations of which are seen in DYT16 dystonia)encodes an upstream elF2a kinase activator, with the mostcommon PRKRA mutations thought to reduce elF2aphosphorylation.Comment: Overall, this extensive series of experimentsprovides some preliminary evidence of a common mech-anistic pathway, disrupted in distinct ways to give rise tomultiple forms of dystonia. This provides a platform todetermine not only whether disrupted elF2a signaling iscentral to other forms of dystonia, but also whether there isa selective brain and/or regional vulnerability to theseprocesses. Moreover, this work provides an exciting inroadfor therapeutic development, potentially necessitatingtreatments that are able to target multiple points within theelF2a pathway.Rittiner JE et al (2016) Neuron 92:1238–1251.Cervical dystonia: a neural integrator disorderIn this publication, Shaikh et al. seek to re-explore the roleof the midbrain in the control of head movements, andmore specifically cervical dystonia and dystonic tremor.J Neurol (2017) 264:616–618 617123The idea that the midbrain may play a role in the three-dimensional orientation of the head was first suggested inthe 1950s, but has gained further traction over the pastdecade following work addressing the direction andvelocity of head movements in individuals with cervicaldystonia. This has led to a body of work suggesting a rolefor a ‘head neural integrator’ located within the midbrain.The authors first set out their theory from over a decadeago, providing several analogies to ocular motor integratorsinvolved in the control of eye movements, and disruptionto which leads to gaze-evoked nystagmus. They also detailthe importance of neuronal integration in the medulla andmidbrain, more specifically the interstitial nucleus of cajal(INC), in regulating horizontal, vertical, and torsional eyemovements. A clear description of the individual compo-nents of disrupted eye movements is outlined, with cor-rective saccades/quick phases being seen when the eyeslook in one direction, drifting back involuntarily, and thenquickly corrected back to the original direction of gaze. Incontrast, the slow-phase velocity changes are dependent onthe position of the eye in the orbit, with changes decreasingas the eye position gets closer to the central/straight aheadposition, and increasing as they move further from thispoint.Klier and colleagues were the first to suggest that ananalogous system might be involved in dystonia followingpharmacological inactivation of the INC in non-humanprimates. However, this hypothesis struggled to gain trac-tion at the beginning of the century owing to the lack ofinvolvement of the basal ganglia and/or cerebellum, tworegions traditionally considered pivotal in the generationand propagation of dystonia. However, this publicationseeks to address some of these conceptual difficulties,extrapolating this original hypothesis to include feedbackmechanisms involving the visual system, neck proprio-ception, and cerebellum to the head neural integrator.Previous work from the same group has shown thatvibration of the paraspinal neck muscles increases cen-tripetal head drift during eccentric loading, while othershave shown that removal of visual feedback causes thehead to drift from the eccentric position towards a centralnull. The combination of retained visual feedback withvibration-induced distorted proprioception caused the headto drift with rapid corrective movements, resembling dys-tonic tremor or ‘head nystagmus’.Comment: In spite of the theme of the article, theauthors clearly state that these research findings and pro-posed mechanisms do not necessarily imply that thepathophysiology underlying cervical dystonia is intrinsic tothese midbrain regions. Neurophysiological and imagingstudies of this region also support an integrative, ratherthan generator, role for the midbrain with abnormalities inneural excitatory outflow and asymmetry of white-matterprojections between the pallidum and the midbrain.Although potentially generating some controversy inchallenging the more traditional models of dystonia, thiswork invites a re-thinking or adjusted view of how dystoniais generated and that similar end phenotypes may be causedby heterogenous underlying biological disruption con-verging on a common anatomical pathway. Arguably thishypothesis has an additional clinical importance, indicatingthe need to explore alternative or additional targets forstimulation with DBS and future targeted therapies.Shaikh AG et al (2016) Brain 139:2590–2599.Open Access This article is distributed under the terms of theCreative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,distribution, and reproduction in any medium, provided you giveappropriate credit to the original author(s) and the source, provide alink to the Creative Commons license, and indicate if changes weremade.618 J Neurol (2017) 264:616–618123

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Dystonia: opportunities to gain insights into underlying pathophysiological mechanisms

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