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Jeng CJ, Fu SJ, You CY, Peng YJ, Hsiao CT, Chen TY, Tang CY. Defective Gating and Proteostasis of Human ClC-1 Chloride Channel: Molecular Pathophysiology of Myotonia Congenita. Front Neurol 2020; 11:76. [PMID: 32117034 PMCID: PMC7026490 DOI: 10.3389/fneur.2020.00076] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2019] [Accepted: 01/22/2020] [Indexed: 01/17/2023] Open
Abstract
The voltage-dependent ClC-1 chloride channel, whose open probability increases with membrane potential depolarization, belongs to the superfamily of CLC channels/transporters. ClC-1 is almost exclusively expressed in skeletal muscles and is essential for stabilizing the excitability of muscle membranes. Elucidation of the molecular structures of human ClC-1 and several CLC homologs provides important insight to the gating and ion permeation mechanisms of this chloride channel. Mutations in the human CLCN1 gene, which encodes the ClC-1 channel, are associated with a hereditary skeletal muscle disease, myotonia congenita. Most disease-causing CLCN1 mutations lead to loss-of-function phenotypes in the ClC-1 channel and thus increase membrane excitability in skeletal muscles, consequently manifesting as delayed relaxations following voluntary muscle contractions in myotonic subjects. The inheritance pattern of myotonia congenita can be autosomal dominant (Thomsen type) or recessive (Becker type). To date over 200 myotonia-associated ClC-1 mutations have been identified, which are scattered throughout the entire protein sequence. The dominant inheritance pattern of some myotonia mutations may be explained by a dominant-negative effect on ClC-1 channel gating. For many other myotonia mutations, however, no clear relationship can be established between the inheritance pattern and the location of the mutation in the ClC-1 protein. Emerging evidence indicates that the effects of some mutations may entail impaired ClC-1 protein homeostasis (proteostasis). Proteostasis of membrane proteins comprises of biogenesis at the endoplasmic reticulum (ER), trafficking to the surface membrane, and protein turn-over at the plasma membrane. Maintenance of proteostasis requires the coordination of a wide variety of different molecular chaperones and protein quality control factors. A number of regulatory molecules have recently been shown to contribute to post-translational modifications of ClC-1 and play critical roles in the ER quality control, membrane trafficking, and peripheral quality control of this chloride channel. Further illumination of the mechanisms of ClC-1 proteostasis network will enhance our understanding of the molecular pathophysiology of myotonia congenita, and may also bring to light novel therapeutic targets for skeletal muscle dysfunction caused by myotonia and other pathological conditions.
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Affiliation(s)
- Chung-Jiuan Jeng
- Institute of Anatomy and Cell Biology, School of Medicine, National Yang-Ming University, Taipei, Taiwan.,Brain Research Center, National Yang-Ming University, Taipei, Taiwan
| | - Ssu-Ju Fu
- Institute of Anatomy and Cell Biology, School of Medicine, National Yang-Ming University, Taipei, Taiwan.,Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Chia-Ying You
- Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Yi-Jheng Peng
- Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Cheng-Tsung Hsiao
- Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan.,Department of Neurology, Taipei Veterans General Hospital, Taipei, Taiwan
| | - Tsung-Yu Chen
- Center for Neuroscience, University of California, Davis, Davis, CA, United States
| | - Chih-Yung Tang
- Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan.,College of Medicine, Graduate Institute of Brain and Mind Sciences, National Taiwan University, Taipei, Taiwan
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Wang K, Preisler SS, Zhang L, Cui Y, Missel JW, Grønberg C, Gotfryd K, Lindahl E, Andersson M, Calloe K, Egea PF, Klaerke DA, Pusch M, Pedersen PA, Zhou ZH, Gourdon P. Structure of the human ClC-1 chloride channel. PLoS Biol 2019; 17:e3000218. [PMID: 31022181 PMCID: PMC6483157 DOI: 10.1371/journal.pbio.3000218] [Citation(s) in RCA: 48] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2018] [Accepted: 03/22/2019] [Indexed: 11/18/2022] Open
Abstract
ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue ("fast gate") known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule ("slow gating"). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1-related diseases.
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Affiliation(s)
- Kaituo Wang
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
- Department of Microbiology, Immunology & Molecular Genetics, University of California at Los Angeles, Los Angeles, California
- California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California
| | | | - Liying Zhang
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Yanxiang Cui
- Department of Microbiology, Immunology & Molecular Genetics, University of California at Los Angeles, Los Angeles, California
- California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California
| | - Julie Winkel Missel
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Christina Grønberg
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Kamil Gotfryd
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Erik Lindahl
- Department of Biochemistry & Biophysics, Stockholm University, Stockholm, Sweden
| | | | - Kirstine Calloe
- Department of Veterinary and Animal Sciences, University of Copenhagen, Frederiksberg, Denmark
| | - Pascal F. Egea
- Department of Biological Chemistry, University of California at Los Angeles, Los Angeles, California
| | - Dan Arne Klaerke
- Department of Veterinary and Animal Sciences, University of Copenhagen, Frederiksberg, Denmark
| | - Michael Pusch
- Institute of Biophysics, Consiglio Nazionale delle Ricerche, Genova, Italy
| | | | - Z. Hong Zhou
- Department of Microbiology, Immunology & Molecular Genetics, University of California at Los Angeles, Los Angeles, California
- California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California
| | - Pontus Gourdon
- Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
- Department of Experimental Medical Science, Lund University, Lund, Sweden
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Jentsch TJ, Pusch M. CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease. Physiol Rev 2018; 98:1493-1590. [DOI: 10.1152/physrev.00047.2017] [Citation(s) in RCA: 214] [Impact Index Per Article: 35.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.
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Affiliation(s)
- Thomas J. Jentsch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
| | - Michael Pusch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
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Nondystrophic Myotonic Disorders. Neuromuscul Disord 2018. [DOI: 10.1007/978-981-10-5361-0_27] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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Imbrici P, Altamura C, Camerino GM, Mangiatordi GF, Conte E, Maggi L, Brugnoni R, Musaraj K, Caloiero R, Alberga D, Marsano RM, Ricci G, Siciliano G, Nicolotti O, Mora M, Bernasconi P, Desaphy JF, Mantegazza R, Camerino DC. Multidisciplinary study of a new ClC-1 mutation causing myotonia congenita: a paradigm to understand and treat ion channelopathies. FASEB J 2016; 30:3285-3295. [PMID: 27324117 PMCID: PMC5024700 DOI: 10.1096/fj.201500079r] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2015] [Accepted: 06/07/2016] [Indexed: 11/11/2022]
Abstract
Myotonia congenita is an inherited disease that is characterized by impaired muscle relaxation after contraction caused by loss-of-function mutations in the skeletal muscle ClC-1 channel. We report a novel ClC-1 mutation, T335N, that is associated with a mild phenotype in 1 patient, located in the extracellular I-J loop. The purpose of this study was to provide a solid correlation between T335N dysfunction and clinical symptoms in the affected patient as well as to offer hints for drug development. Our multidisciplinary approach includes patch-clamp electrophysiology on T335N and ClC-1 wild-type channels expressed in tsA201 cells, Western blot and quantitative PCR analyses on muscle biopsies from patient and unaffected individuals, and molecular dynamics simulations using a homology model of the ClC-1 dimer. T335N channels display reduced chloride currents as a result of gating alterations rather than altered surface expression. Molecular dynamics simulations suggest that the I-J loop might be involved in conformational changes that occur at the dimer interface, thus affecting gating. Finally, the gene expression profile of T335N carrier showed a diverse expression of K+ channel genes, compared with control individuals, as potentially contributing to the phenotype. This experimental paradigm satisfactorily explained myotonia in the patient. Furthermore, it could be relevant to the study and therapy of any channelopathy.-Imbrici, P., Altamura, C., Camerino, G. M., Mangiatordi, G. F., Conte, E., Maggi, L., Brugnoni, R., Musaraj, K., Caloiero, R., Alberga, D., Marsano, R. M., Ricci, G., Siciliano, G., Nicolotti, O., Mora, M., Bernasconi, P., Desaphy, J.-F., Mantegazza, R., Camerino, D. C. Multidisciplinary study of a new ClC-1 mutation causing myotonia congenita: a paradigm to understand and treat ion channelopathies.
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Affiliation(s)
- Paola Imbrici
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy; paola.imbrici@uniba
| | - Concetta Altamura
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
| | | | | | - Elena Conte
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
| | - Lorenzo Maggi
- Division of Neurology, Neuroimmunology and Neuromuscular Diseases Unit, Istituti di Ricovero e Cura a Carattere Scientifico Fondazione Istituto Neurologico "Carlo Besta," Milan, Italy
| | - Raffaella Brugnoni
- Division of Neurology, Neuroimmunology and Neuromuscular Diseases Unit, Istituti di Ricovero e Cura a Carattere Scientifico Fondazione Istituto Neurologico "Carlo Besta," Milan, Italy
| | - Kejla Musaraj
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
| | - Roberta Caloiero
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
| | - Domenico Alberga
- Department of Physics "M. Merlin," Istituto Nazionale di Fisica Nucleare and Centro di Tecnologie Innovative per la Rilevazione e l'Elaborazione del Segnale, University of Bari "Aldo Moro," Bari, Italy
| | | | - Giulia Ricci
- Department of Clinical and Experimental Medicine, Section of Neurology, University of Pisa, Pisa, Italy
| | - Gabriele Siciliano
- Department of Clinical and Experimental Medicine, Section of Neurology, University of Pisa, Pisa, Italy
| | - Orazio Nicolotti
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
| | - Marina Mora
- Division of Neurology, Neuroimmunology and Neuromuscular Diseases Unit, Istituti di Ricovero e Cura a Carattere Scientifico Fondazione Istituto Neurologico "Carlo Besta," Milan, Italy
| | - Pia Bernasconi
- Division of Neurology, Neuroimmunology and Neuromuscular Diseases Unit, Istituti di Ricovero e Cura a Carattere Scientifico Fondazione Istituto Neurologico "Carlo Besta," Milan, Italy
| | - Jean-Francois Desaphy
- Department of Biomedical Sciences and Human Oncology, University of Bari "Aldo Moro," Bari, Italy
| | - Renato Mantegazza
- Division of Neurology, Neuroimmunology and Neuromuscular Diseases Unit, Istituti di Ricovero e Cura a Carattere Scientifico Fondazione Istituto Neurologico "Carlo Besta," Milan, Italy
| | - Diana Conte Camerino
- Department of Pharmacy, Drug Sciences, University of Bari "Aldo Moro," Bari, Italy
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Impaired surface membrane insertion of homo- and heterodimeric human muscle chloride channels carrying amino-terminal myotonia-causing mutations. Sci Rep 2015; 5:15382. [PMID: 26502825 PMCID: PMC4621517 DOI: 10.1038/srep15382] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2015] [Accepted: 09/23/2015] [Indexed: 12/03/2022] Open
Abstract
Mutations in the muscle chloride channel gene (CLCN1) cause myotonia congenita, an inherited condition characterized by muscle stiffness upon sudden forceful movement. We here studied the functional consequences of four disease-causing mutations that predict amino acid substitutions Q43R, S70L, Y137D and Q160H. Wild-type (WT) and mutant hClC-1 channels were heterologously expressed as YFP or CFP fusion protein in HEK293T cells and analyzed by whole-cell patch clamp and fluorescence recordings on individual cells. Q43R, Y137D and Q160H, but not S70L reduced macroscopic current amplitudes, but left channel gating and unitary current amplitudes unaffected. We developed a novel assay combining electrophysiological and fluorescence measurements at the single-cell level in order to measure the probability of ion channel surface membrane insertion. With the exception of S70L, all tested mutations significantly reduced the relative number of homodimeric hClC-1 channels in the surface membrane. The strongest effect was seen for Q43R that reduced the surface insertion probability by more than 99% in Q43R homodimeric channels and by 92 ± 3% in heterodimeric WT/Q43R channels compared to homodimeric WT channels. The new method offers a sensitive approach to investigate mutations that were reported to cause channelopathies, but display only minor changes in ion channel function.
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URSU SIMONAFELICIA, ALEKOV ALEXI, MAO NINGHUI, JURKAT-ROTT KARIN. ClC1 chloride channel in myotonic dystrophy type 2 and ClC1 splicing in vitro. ACTA MYOLOGICA : MYOPATHIES AND CARDIOMYOPATHIES : OFFICIAL JOURNAL OF THE MEDITERRANEAN SOCIETY OF MYOLOGY 2012; 31:144-53. [PMID: 23097607 PMCID: PMC3476861] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Myotonic dystrophy type 2 (DM2) is caused by CCTG-repeat expansions. Occurrence of splicing and mutations in the muscle chloride channel gene CLCN1 have been reported to contribute to the phenotype. To examine the effect of CLCN1 in DM2 in Germany, we determined the frequency of a representative ClC1 mutation, R894X, and its effect on DM2 clinical features. Then, we examined CLCN1 mRNA splice variants in patient muscle functionally expressed the most abundant variant, and determined its subcellular localization. Finally, we established a cellular system for studying mouse clcn1 pre-mRNA splicing and tested effects of expression of (CCUG)₁₈, (CUG)₂₄ and (AAG)₂₄ RNAs. The R894X mutation was present in 7.7% of DM2 families. DM2 R894X-carriers had more myotonia and myalgia than non-carriers. The most abundant CLCN1 splice variant in DM2 (80% of all transcripts) excluded exons 6-7 and lead to a truncated ClC1(236X) protein. Heterologous ClC1(236X) expression did not yield functional channels. Co-expression with ClC1 did not show a dominant negative effect, but a slightly suppressive effect. In C₂C₁₂ cells, the clc1 splice variants generated by (CCUG)₁₈-RNA resembled those in DM2 muscle and differed from those generated by (CUG)₂₄ and (AAG)₂₄. We conclude that ClC1 mutations exert gene dose effects and enhance myotonia and pain in DM2 in Germany. Additionally, the ClC1(236X) splice variant may contribute to myotonia in DM2. Since splice variants depend on the types of repeats expressed in the cellular C₂C₁₂ model, similar cell models of other tissues may be useful for studying repeatdependent pathogenetic mechanisms more easily than in transgenic animals.
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Affiliation(s)
| | | | | | - KARIN JURKAT-ROTT
- Address for correspondence: Karin Jurkat-Rott, MD -Division of Neurophysiology, Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm, Germany. E-mail:
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Richman DP, Yu Y, Lee TT, Tseng PY, Yu WP, Maselli RA, Tang CY, Chen TY. Dominantly inherited myotonia congenita resulting from a mutation that increases open probability of the muscle chloride channel CLC-1. Neuromolecular Med 2012; 14:328-37. [PMID: 22790975 DOI: 10.1007/s12017-012-8190-1] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2012] [Accepted: 06/22/2012] [Indexed: 10/28/2022]
Abstract
Myotonia congenita-inducing mutations in the muscle chloride channel CLC-1 normally result in reduced open probability (P (o)) of this channel. One well-accepted mechanism of the dominant inheritance of this disease involves a dominant-negative effect of the mutation on the function of the common-gate of this homodimeric, double-barreled molecule. We report here a family with myotonia congenita characterized by muscle stiffness and clinical and electrophysiologic myotonic phenomena transmitted in an autosomal dominant pattern. DNA sequencing of DMPK and ZNF9 genes for myotonic muscular dystrophy types I and II was normal, whereas sequencing of CLC-1 encoding gene, CLCN1, identified a single heterozygous missense mutation, G233S. Patch-clamp analyses of this mutant CLC-1 channel in Xenopus oocytes revealed an increased P (o) of the channel's fast-gate, from ~0.4 in the wild type to >0.9 in the mutant at -90 mV. In contrast, the mutant exhibits a minimal effect on the P (o) of the common-gate. These results are consistent with the structural prediction that the mutation site is adjacent to the fast-gate of the channel. Overall, the mutant could lead to a significantly reduced dynamic response of CLC-1 to membrane depolarization, from a fivefold increase in chloride conductance in the wild type to a twofold increase in the mutant-this might result in slower membrane repolarization during an action potential. Since expression levels of the mutant and wild-type subunits in artificial model cell systems were unable to explain the disease symptoms, the mechanism leading to dominant inheritance in this family remains to be determined.
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Affiliation(s)
- David P Richman
- Department of Neurology and Center for Neuroscience, University of California, Davis, CA 95616, USA.
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Burgunder JM, Huifang S, Beguin P, Baur R, Eng CS, Seet RC, Lim EC, Ong BK, Hunziker W, Sigel E. Novel chloride channel mutations leading to mild myotonia among Chinese. Neuromuscul Disord 2008; 18:633-40. [DOI: 10.1016/j.nmd.2008.05.007] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2007] [Revised: 05/10/2008] [Accepted: 05/15/2008] [Indexed: 10/21/2022]
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Ryan AM, Matthews E, Hanna MG. Skeletal-muscle channelopathies: periodic paralysis and nondystrophic myotonias. Curr Opin Neurol 2007; 20:558-63. [PMID: 17885445 DOI: 10.1097/wco.0b013e3282efc16c] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
PURPOSE OF REVIEW To provide a current review of clinical phenotypes, genetics, molecular pathophysiology, and electro-diagnostic testing strategies of periodic paralysis and nondystrophic myotonias. RECENT FINDINGS The number of pathogenic mutations causing periodic paralysis and nondystrophic myotonias continues to increase. Important insight into the molecular pathogenesis of muscle sodium channelopathies has been revealed by the finding of 'leaky' closed sodium channels. Previously, alterations in sodium-channel activation or inactivation have been identified as important disease mechanisms. The recent discovery that substitutions of key arginine residues in the voltage-sensing segment of the channel may lead to a 'pore leak' when the channel is closed suggests a new mechanism. Since similar mutations exist in corresponding positions of other channels, this mechanism may apply to other channel diseases. The recognition of different electrophysiological patterns that are specific to muscle ion-channel genotypes will be useful in diagnosis and in guiding genetic testing. Recent studies demonstrate that magnetic resonance imaging may be used to detect intramuscular accumulation of sodium during episodes of weakness. SUMMARY Recent advances have refined our ability to make a precise molecular diagnosis in muscle channelopathies. The description of a pore leak with voltage-sensor mutations may represent a new disease mechanism.
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Affiliation(s)
- Aisling M Ryan
- MRC Centre for Neuromuscular Disease, Department of Molecular Neuroscience, Institute of Neurology, University College London, London, UK
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Abstract
Myotonia congenita is a hereditary chloride channel disorder characterized by delayed relaxation of skeletal muscle (myotonia). It is caused by mutations in the skeletal muscle chloride channel gene CLCN1 on chromosome 7. The phenotypic spectrum of myotonia congenita ranges from mild myotonia disclosed only by clinical examination to severe and disabling myotonia with transient weakness and myopathy. The most severe phenotypes are seen in patients with two mutated alleles. Heterozygotes are often asymptomatic but for some mutations heterozygosity is sufficient to cause pronounced myotonia, although without weakness and myopathy. Thus, the phenotype depends on the mutation type to some extent, but this does not explain the fact that severity varies greatly between heterozygous family members and may even vary with time in the individual patient. In this review, existing knowledge about phenotypic variability is summarized, and the possible contributing factors are discussed.
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Affiliation(s)
- Eskild Colding-Jørgensen
- Department of Clinical Neurophysiology 19, Glostrup Hospital, University of Copenhagen DK-2600 Glostrup, Denmark.
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