Dominant Myotonia Congenita Research Articles

Introduction

Introduction

Electrical myotonia in heterozygous carriers of recessive myotonia congenita.

We investigated electrophysiologically the unaffected parents of patients with recessive myotonia congenita. We studied 18 families, in nine of which the diagnosis was confirmed by molecular genetics. Brief myotonic discharges were present in at least one parent in 67% of the families. Fathers were more likely than mothers to show these discharges. The difficulty in distinguishing very mildly affected parents with dominant myotonia congenita from the heterozygous carriers of recessive myotonia congenita is stressed.

Transient weakness and compound muscle action potential decrement in myotonia congenita.

Twenty-five Turkish patients with recessive myotonia congenita (RMC), 16 of whom had genetic confirmation, were studied. Nineteen had transient weakness. In the upper extremities, onset age of transient weakness was usually in the early teens. All untreated RMC patients had a compound muscle action potential decrement of > or =25%, usually above 50%, with repetitive nerve stimulation at 10/s for 5 s. Patients with other nondystrophic diseases with myotonia, except 1 patient with dominant myotonia congenita, had no transient weakness and a CMAP decrement below 25%.

ClC-1 chloride channel mutations in myotonia congenita: variable penetrance of mutations shifting the voltage dependence.

Mutations in the ClC-1 muscle chloride channel cause either recessive or dominant myotonia congenita. Using a systematic screening procedure, we have now identified four novel missense mutations in dominant (V286A, F307S) and recessive myotonia (V236L, G285E), and have analysed the effect of these and other recently described mutations (A313T, I556N) on channel properties in the Xenopus oocyte expression system. Mutations V286A, F307S and A313T displayed a 'classical' dominant phenotype: their voltage dependence was shifted towards positive potentials and displayed a dominant-negative effect by significantly imparting a voltage shift on mutant-wild-type heteromeric channels as found in heterozygous patients. In contrast, the recessive mutation V236L also shifted the voltage dependence to positive values, but co-expression with wild-type ClC-1 gave almost wild-type currents. I556N, a mutation found in patients with benign dominant myotonia, drastically shifts the voltage dependence, but only a slight shift is seen when co-expressed with wild-type ClC-1. Thus, the voltage dependence of mutant heteromeric channels is not always intermediate between those of the constituent homomeric channel subunits, a conclusion further supported by mixing different ClC-1 mutants. These complex interactions correlate clinically with various inheritance patterns, ranging from autosomal dominant with various degrees of penetrance to autosomal recessive.

Identification of five new mutations and three novel polymorphisms in the muscle chloride channel gene (CLCN1) in 20 Italian patients with dominant and recessive myotonia congenita

Autosomal dominant myotonia congenita or Thomsen's disease and autosomal recessive myotonia congenita or Becker'sare rare nondystrophic disorders due to allelic mutations of the muscle chloride channel gene, CLCN1. We have analysed all 24 exons of the CLCN1 gene, in a panel of 20 unrelated patients (9 with dominant and 11 with recessive myotonia congenita). We have found five novel mutations including two missense (V563I, F708L), one nonsense (C481X), one splicing (IVS19+2T→A), and one frameshift (2264delC), and also detected the recurrent R894X mutation. These account for 10 of the 22 recessive alleles examined, while no mutations were found in the dominant form. We report three novel polymorphisms (-134 T/G, 898 C/A and 2154T/C). Our results support high molecular heterogeneity of these myotonias in Italian population and provide new insight for the diagnosis and genetic counselling of these diseases. Hum Mutat 11:331, 1998. © 1998 Wiley-Liss, Inc.

Identification of five new mutations and three novel polymorphisms in the muscle chloride channel gene (CLCN1) in 20 Italian patients with dominant and recessive myotonia congenita

Autosomal dominant myotonia congenita or Thomsen's disease and autosomal recessive myotonia congenita or Becker'sare rare nondystrophic disorders due to allelic mutations of the muscle chloride channel gene, CLCN1. We have analysed all 24 exons of the CLCN1 gene, in a panel of 20 unrelated patients (9 with dominant and 11 with recessive myotonia congenita). We have found five novel mutations including two missense (V563I, F708L), one nonsense (C481X), one splicing (IVS19+2T→A), and one frameshift (2264delC), and also detected the recurrent R894X mutation. These account for 10 of the 22 recessive alleles examined, while no mutations were found in the dominant form. We report three novel polymorphisms (-134 T/G, 898 C/A and 2154T/C). Our results support high molecular heterogeneity of these myotonias in Italian population and provide new insight for the diagnosis and genetic counselling of these diseases. Hum Mutat 11:331, 1998. © 1998 Wiley-Liss, Inc.

A mutation in autosomal dominant myotonia congenita affects pore properties of the muscle chloride channel.

Autosomal dominant myotonia congenita is an inherited disorder of skeletal muscle caused by mutations in a voltage-gated Cl- channel gene (CLCN1, 7q35). Here, we report that a mutation predicting the substitution of Gly 230 by glutamic acid (G230E) between segments D3 and D4 dramatically alters the pore properties of a recombinant human muscle Cl- channel (hCIC-1) expressed in a mammalian cell line (tsA201). The G230E mutation causes substantial changes in anion and cation selectivity as well as a fundamental change in rectification of the current-voltage relationship. Whereas wild-type channels are characterized by pronounced inward rectification and a Cl > thiocyanate > Br > NO(3) > I > CH(3)SO(3) selectivity, G230E exhibits outward rectification at positive potentials and a thiocyanate > NO(3) > I > Br > Cl > CH(3)SO(3) selectivity. Furthermore, the cation-to-anion permeability ratio of the mutant is much greater than that of the wild-type channel. Voltage-dependent blocks by intracellular and extracellular iodide help to distinguish two distinct ion binding sites within the hClC-1 conduction pathway. Both binding sites are preserved in the mutant but have decreased affinities for iodide. These findings suggest that Gly 230 is critical for normal ion conductance in hClC-1 and that this residue resides within the channel pore.

Mutations in the human skeletal muscle chloride channel gene (CLCN1) associated with dominant and recessive myotonia congenita.

Myotonia, defined as delayed relaxation of muscle after contraction, is seen in a group of genetic disorders that includes autosomal dominant myotonia congenita (Thomsen's disease) and autosomal recessive myotonia congenita (Becker's disease). Both disorders are characterized electrophysiologically by increased excitability of muscle fibers, reflected in clinical myotonia. These diseases are similar except that transient weakness is seen in patients with Becker's, but not Thomsen's disease. Becker's and Thomsen's diseases are caused by mutations in the skeletal muscle voltage-gated chloride channel gene (CLCN1). Genetic screening of a panel of 18 consecutive myotonia congenita (MC) probands for mutation in CLCN1 revealed that a novel Gln-68-Stop nonsense mutation predicts premature truncation of the chloride channel protein. Four previously reported mutations, Arg-894-stop, Arg-338-Gln, Gly-230-Glu, and del 1437-1450, were also noted in our sample set. The Arg-338-Gln and Gly-230-Glu mutations were found in patients with different phenotypes from those of previous reports. Further study of the Arg-338-Gln and Gln-230-Glu alleles may shed light on variable modes of transmission (dominant versus recessive) in different families. Physiologic study of these mutations may lead to better understanding of the pathophysiology of myotonia in these patients and of voltage-gated chloride channel structure/function relationships in skeletal muscles.

Mutations in dominant human myotonia congenita drastically alter the voltage dependence of the CIC-1 chloride channel

Autosomal dominant myotonia congenita (Thomsen's disease) is caused by mutations in the muscle chloride channel CIC-1. Several point mutations found in affected families (1290M, R317Q, P480L, and G1552R) dramatically shift gating to positive voltages in mutant/WT heterooligomeric channels, and, when measurable, even more so in mutant homooligomers. These channels can no longer contribute to the repolarization of action potentials, fully explaining why they cause dominant myotonia. Most replacements of the isoleucine at position 290 shift gating toward positive voltages. Mutant/WT heterooligomers can be partially activated by repetitive depolarizations, suggesting a role in shortening myotonic runs. Remarkably, a human mutation affecting an adjacent residue (E291 K) is fully recessive. Large shifts in the voltage dependence of gating may be common to many mutations in dominant myotonia congenita.

Absence of the Skeletal Muscle Sarcolemma Chloride Channel ClC-1 in Myotonic Mice

The voltage-dependent chloride channel ClC-1 stabilizes resting membrane potential in skeletal muscle. Mutations in the ClC-1 gene are responsible for both human autosomal recessive generalized myotonia and autosomal dominant myotonia congenita. To understand the tissue distribution and subcellular localization of ClC-1 and to evaluate its role in an animal model of myotonia, antibodies were raised against the carboxyl terminus of this protein. Expression of the 130-kDa ClC-1 protein is unique to skeletal muscle, consistent with its mRNA tissue distribution. Immunolocalization shows prominent ClC-1 antigen in the sarcolemma of both type I and II muscle fibers. Sarcolemma localization is confirmed by Western analysis of skeletal muscle subcellular fractions. The ADR myotonic mouse (phenotype ADR, genotype adr/adr), in which defective ClC-1 mRNA has been identified, is shown here to be absent in ClC-1 protein expression, whereas other skeletal muscle sarcolemma protein expression appears normal. Immunohistochemistry of skeletal muscle from ADR and other mouse models of human muscle disease demonstrate that the absence of ClC-1 chloride channel is a defect specific to ADR mice.

Myotonia levior is a chloride channel disorder.

The group of dominant non-dystrophic myotonias, comprising disorders characterized by clinically similar forms of myogenic muscle stiffness, is genetically inhomogeneous. Dominant myotonia congenita (Thomsen's disease) is linked to CLCN1, the gene encoding the major muscle chloride channel, localized on chromosome 7q35. In contrast, dominant myotonias sensitive to potassium are caused by point mutations in SCN4A on chromosome 17q, the gene for the alpha subunit of the adult skeletal muscle sodium channel. No linkage or molecular genetic data are as yet available on 'myotonia levior' characterized by milder symptoms and later onset of myotonia than in Thomsen's disease, and absence of muscle hypertrophy. We report a CLCN1 Gln-552-Arg substitution for a family with dominant inheritance previously diagnosed to have myotonia levior. Thus, this disorder appears as a variant of Thomsen's disease due to mutations leading to low clinical expressivity. In addition, we report a novel Ile-290-Met CLCN1 mutation for a typical Thomsen pedigree. In another family previously diagnosed as having Thomsen's disease, we unexpectedly found a CLCN1 14 bp deletion known to cause recessive myotonia, and a rare Trp-118-Gly polymorphism.

Multimeric structure of ClC-1 chloride channel revealed by mutations in dominant myotonia congenita (Thomsen).

Voltage-gated ClC chloride channels play important roles in cell volume regulation, control of muscle excitability, and probably transepithelial transport. ClC channels can be functionally expressed without other subunits, but it is unknown whether they function as monomers. We now exploit the properties of human mutations in the muscle chloride channel, ClC-1, to explore its multimeric structure. This is based on analysis of the dominant negative effects of ClC-1 mutations causing myotonia congenita (MC, Thomsen's disease), including a newly identified mutation (P480L) in Thomsen's own family. In a co-expression assay, Thomsen's mutation dramatically inhibits normal ClC-1 function. A mutation found in Canadian MC families (G230E) has a less pronounced dominant negative effect, which can be explained by functional WT/G230E heterooligomeric channels with altered kinetics and selectivity. Analysis of both mutants shows independently that ClC-1 functions as a homooligomer with most likely four subunits.

Linkage analysis of candidate loci in autosomal dominant myotonia congenita.

Electrophysiologic studies in patients with autosomal dominant myotonia congenita (ADMC) have implicated defects of both muscle membrane sodium and chloride channels. An adult skeletal muscle sodium channel (ASkM1) gene maps to chromosome 17q23-25, and defects in this gene are almost certainly responsible for at least three variants of hyperkalemic periodic paralysis (HPP)--myotonic HPP, nonmyotonic HPP, and paramyotonia congenita. A gene for a muscle chloride channel has not yet been mapped in humans, but has been identified in the mouse. The gene for the cystic fibrosis transmembrane regulator (CFTR), which has chloride channel properties, is located on chromosome 7q31. This region is syntenic with the area of mouse chromosome 6 that contains the muscle chloride channel gene, a defect in which is responsible for the ADR phenotype, a murine model of myotonia. We performed linkage analysis using chromosome 17q polymorphisms at D17S74, SCN4A, and GH1, two chromosome 7q31 restriction fragment length polymorphisms, and a dinucleotide repeat polymorphism within the CFTR gene (CFTR-DNR), in three pedigrees with ADMC. The lod scores obtained show that the locus for ADMC is not at ASkM1 and is excluded from a region of at least 24 cM on either side of the CFTR gene.

Inactivation of muscle chloride channel by transposon insertion in myotonic mice.

MYOTONIA (stiffness and impaired relaxation of skeletal muscle) is a symptom of several diseases caused by repetitive firing of action potentials in muscle membranes. Purely myotonic human diseases are dominant myotonia congenita (Thomsen) and recessive generalized myotonia (Becker), whereas myotonic dystrophy is a systemic disease. Muscle hyperexcitability was attributed to defects in sodium channels and/or to a decrease in chloride conductance (in Becker's myotonia and in genetic animal models). Experimental blockage of Cl- conductance (normally 70-85% of resting conductance in muscle) in fact elicits myotonia. ADR mice are a realistic animal model for recessive autosomal myotonia. In addition to Cl- conductance, many other parameters are changed in muscles of homozygous animals. We have now cloned the major mammalian skeletal muscle chloride channel (ClC-1). Here we report that in ADR mice a transposon of the ETn family has inserted into the corresponding gene, destroying its coding potential for several membrane-spanning domains. Together with the lack of recombination between the Clc-1 gene and the adr locus, this strongly suggests a lack of functional chloride channels as the primary cause of mouse myotonia.

Altered sodium channel behaviour causes myotonia in dominantly inherited myotonia congenita

The cause of increased excitability in autosomal dominant myotonia congenita (MyC) was studied in resealed > 3-cm long segments of muscle fibres from eight patients. Three hours after biopsy only about 50% of the fibre segments had regained a normal resting potential. This differs from our experiences with normal muscle or other disorders of myotonia (e.g. recessive generalized myotonia) where nearly all cut fibres reseal and repolarize during this time. When the depolarized MyC fibre segments were placed in a solution containing 1 μM tetrodotoxin (TTX) they repolarized to −80 to −90 mV. In fibre segments with normal resting potential, in the absence of TTX, spontaneous myotonic runs were recorded intracellularly, occasionally with double spikes. For only one of the eight patients, the Cl − conductance was reduced (50% of the total membrane conductance vs the usual 75%), for the rest of the patients the steady-state current-voltage relationship was normal. Sodium currents through single membrane channels were recorded with a patch clamp. For every patient re-openings of the Na + channels were observed throughout 10-ms depolarizing pulses. These are very uncommon in normal muscle. At potentials positive to the resting potential, the duration of the re-openings increased, but the current amplitude was the same. It is concluded that in myotonia congenita re-openings of Na + channels are the major cause of hyperexcitability and that Cl − conductance is normal. If it is reduced in rare cases, it may potentiate the myotonia.

Dominant myotonia congenita: pedigree with skipping of one generation.

The skipping of one generation in a family pedigree with dominant myotonia congenita is reported. It is suggested that non-penetrance in this condition occurs and should be considered in genetic counselling as a rare, but realistic possibility.