Neutral lipid storage disease with myopathy and myotonia associated to pathogenic variants on PNPLA2 and CLCN1 genes: case report.

BACKGROUND: Neutral lipid storage disease with myopathy (NLSD-M) is an autosomal recessive disease that manifests itself around the 3rd to 4th decade with chronic myopathy predominantly proximal in the shoulder girdle. Clinical myotonia is uncommon. We will report a rare case of association of pathogenic variants on PNPLA2 and CLCN1 genes with a mixed phenotype of NLSD-M and a subclinical form of Thomsen's congenital myotonia. CASE PRESENTATION: We describe a patient with chronic proximal myopathy, subtle clinical myotonia and electrical myotonia on electromyography (EMG). Serum laboratory analysis disclosure hyperCKemia (CK 1280 mg/dL). A blood smear analysis showed Jordan's anomaly, a hallmark of NLSD-M. A genetic panel was collected using next-generation sequencing (NGS) technique, which identified two pathogenic variants on genes supporting two different diagnosis: NLSD-M and Thomsen congenital myotonia, whose association has not been previously described. CONCLUSIONS: Although uncommon, it is important to remember the possibility of association of pathogenic variants to explain a specific neuromuscular disease phenotype. The use of a range of complementary methods, including myopathy genetic panels, may be essential to diagnostic definition in such cases.

An unusual case of sodium channel myotonia with transient weakness upon initiating movements which is characteristic in Becker disease.

We reported a 32-year-old man who was a sporadic case of myotonic syndrome with muscle stiffness or transient weakness of limbs upon initiating movements after rest. On examination, he showed painless myotonia with warm-up phenomenon, Hercules-like hypertrophic musculature and myotonic discharges in EMG. The clinical findings resembled to those of Becker disease rather than Thomsen disease. But electrodiagnosis suggested sodium channel myotonia instead of chloride channelopathy. Genetic testing detected a novel missense mutation (p.V1166A) in the SCN4A gene but not in the CLCN1 gene. Transient weakness upon initiating movements is usually observed in Becker disease but rare in Thomsen disease, which is not reported in sodium channel myotonia so far. He was probably the first case of sodium channel myotonia with transient weakness upon initiating movements, which was confirmed by 10 Hz repetitive nerve stimulation test as depolarization block.

Neuromuscular excitability changes produced by sustained voluntary contraction and response to mexiletine in myotonia congenita.

OBJECTIVE: To investigate the cause of transient weakness in myotonia congenita (MC) and the mechanism of action of mexiletine in reducing weakness. METHODS: The changes in neuromuscular excitability produced by 1min of maximal voluntary contractions (MVC) were measured on the amplitude of compound muscle action potentials (CMAP) in two patients with either recessive or dominant MC, compared to control values obtained in 20 healthy subjects. Measurements were performed again in MC patients after mexiletine therapy. RESULTS: Transient reduction in maximal CMAP amplitude lasting several minutes after MVC was evident in MC patients, whereas no change was observed in controls. Mexiletine efficiently reduced this transient CMAP depression in both patients. DISCUSSION: Transient CMAP depression following sustained MVC may represent the electrophysiological correlate of the weakness clinically experienced by the patients. In MC, the low chloride conductance could induce self-sustaining action potentials after MVC, determining progressive membrane depolarization and a loss of excitability of muscle fibers, thus resulting in transient paresis. Mexiletine may prevent conduction block due to excessive membrane depolarization, thus reducing the transient CMAP depression following sustained MVC.

Electrophysiological characteristics of R47W and A298T mutations in CLC-1 of myotonia congenita patients and evaluation of clinical features

Myotonia congenita (MC) is a genetic disease that displays impaired relaxation of skeletal muscle and muscle hypertrophy. This disease is mainly caused by mutations of CLCN1 that encodes human skeletal muscle chloride channel (CLC-1). CLC-1 is a voltage gated chloride channel that activates upon depolarizing potentials and play a major role in stabilization of resting membrane potentials in skeletal muscle. In this study, we report 4 unrelated Korean patients diagnosed with myotonia congenita and their clinical features. Sequence analysis of all coding regions of the patients was performed and mutation, R47W and A298T, was commonly identified. The patients commonly displayed transient muscle weakness and only one patient was diagnosed with autosomal dominant type of myotonia congenita. To investigate the pathological role of the mutation, electrophysiological analysis was also performed in HEK 293 cells transiently expressing homo- or heterodimeric mutant channels. The mutant channels displayed reduced chloride current density and altered channel gating. However, the effect of A298T on channel gating was reduced with the presence of R47W in the same allele. This analysis suggests that impaired CLC-1 channel function can cause myotonia congenita and that R47W has a protective effect on A298T in relation to channel gating. Our results provide clinical features of Korean myotonia congenita patients who have the heterozygous mutation and reveal underlying pathophyological consequences of the mutants by taking electrophysiological approach.

Miotonía congénita, una miopatía no distrófica / Myotonia congenita, a non-dystrophic myopathy

La miotonía congénita es la forma más común de miotonía no distrófica. Esta miopatía está causada por mutaciones en el gen CLCN1, codificante del principal canal de iones cloruro del músculo esquelético (ClC-1); la alteración de la función de este canal, regulado por voltaje, da lugar al fenómeno de miotonía. La enfermedad se puede heredar con un tipo de herencia dominante (enfermedad de Thomsen) o recesiva (enfermedad de Becker o miotonía congénita generalizada). El fenotipo clínico de ambas formas de la enfermedad es similar aunque la forma recesiva se caracteriza por una mayor gravedad de los síntomas. El diagnóstico clínico de miotonía congénita debe sospecharse cuando encontramos en un paciente episodios de rigidez muscular (miotonía), remisión o alivio de la rigidez con el ejercicio (fenómeno warm-up), miotonía clínica, un patrón electromiográfico característico y/o historia familiar. El diagnóstico molecular de miotonía congénita consiste en el análisis por secuenciación del gen CLCN1 (AU)

Myotonia congenita is the most common form of non-dystrophic myotonia. This myopathy is caused by mutations in the CLCN1 gene, encoding the main skeletal muscle chloride ion channel (ClC-1). Altering the function of this voltage-gated channel, leads to the phenomenon of myotonia. The disease can be inherited with a dominant (Thomsen disease) or recessive type (Becker disease or congenital generalised myotonia). The clinical phenotype of both forms of the disease is similar, although the recessive form is characterised by more severe symptoms. The clinical diagnosis of congenital myotonia should be suspected in a patient who presents with episodes of muscle stiffness (myotonia), remission or relief from stiffness with exercise (warm-up phenomenon), and a characteristic electromyography pattern, and/or family history. Sequencing the CLCN1 gene is the present approach for molecular diagnosis of myotonia congenita (AU)

Phenotypic Difference of CLCN1 Gene Variant (A313T) in a Korean Family with Myotonia Congenita

Myotonia congenita (MC) is a hereditary disease of the chloride channels of skeletal muscle caused by mutation of CLCN1. It characteristically manifests as delayed relaxation of the skeletal muscle or myotonia. It has a wide phenotypic variability, ranging from asymptomatic to severe disability. However, it is uncommon for a phenotypic difference to appear within a family. We report the first Korean family with the p.A313T mutation exhibiting marked phenotypic variability.

A novel CLCN1 mutation: P480T in a Japanese family with Thomsen's myotonia congenita.

At least 50 disease-causing mutations in the skeletal muscle voltage-gated chloride channel gene (CLCN1), almost all of which originate from Caucasian families, have been identified. We investigated a Japanese family with Thomsen's myotonia congenita that included 16 affected individuals (8 men and 8 women) through five generations. Polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) screening of 11 members showed an aberrant conformer in exon 13 of CLCN1 complementary DNA (cDNA) in 8 affected and 1 unaffected members. By sequence analysis, we identified a C-to-A transition at nucleotide position 1438, resulting in a substitution of proline for threonine at amino acid position 480 (P480T), the same position of the original mutation (P480L) in Thomsen's disease. The P480T mutation was novel and absent in 100 normal controls. Seven of the 8 affected individuals were heterozygous; another, from affected parents, was homozygous. Clinically, myotonia in the homozygous patient was more severe than that in heterozygous patients, probably due to the gene dosage effect. On a long-train nerve-stimulation test at a rate of 3 Hz, M-wave responses in the homozygous patient showed marked decrement followed by recovery. In contrast, the heterozygous patients showed just a slight decrement or no changes, and none of 2 patients with myotonic muscular dystrophy or 2 normal controls revealed any decrement. Thus, the long-train nerve-stimulation test at a low stimulus frequency may be a useful tool to assess the disease-severity/genotype relationship in myotonia congenita.

[Muscle weakness or rigidity due to hereditary ion channel diseases]. / Spierzwakte of -verstijving door hereditaire kanalopathieën.

Three men, aged 36, 16 and 66 years, had suffered for several years from muscular weakness; after a low serum potassium level had been established, supplementary examination revealed hypokalaemic periodic paralysis. A woman aged 25 had suffered since youth from muscular stiffness on sudden movements; she suffered from hereditary myotonia of the recessive type (Becker's disease). Both rare skeletal muscle ion channel diseases are characterized by the fact that the variable clinical expression complicates making the diagnosis. Since the causal mutations are known, genetic analysis is an essential step in confirming the diagnosis. Additional EMG procedures may be of diagnostic value, even in cases that cannot be clarified genetically.

[Congenital myotonia. Report of 7 patients]. / Miotonia congênita. Relato de sete pacientes.

Myotonia is the phenomenon of decrease of muscular relaxation rate, after either a contraction or a mechanical or electrical stimulus. Congenital myotonias are hereditary affections and do not present muscular dystrophy. The current trend is to group them as ionic channels diseases, together with the periodic paralysis. The authors accompanied the cases of seven patients, six males and one female, with ages ranging from 16 to 48 years (average 27 years) and onset of symptoms between 1 and 10 years (average 5 years). These patients presented a myotonic phenomenon unleashed by intensive contraction and global muscular hypertrophy. Three patients were diagnosed as cases of Becker type generalized myotonia because they presented a recessive autosomic heredity and/or transient episodes of muscular weakness. Two patients fitted the description of Thomsen congenital myotonia, with a pattern of dominating autosomic heredity and/or absence of weakness episodes or worsening factors for their condition. Two patients presented fluctuating myotonia, which because worse in cold weather or at potassium intake. The clinical diagnosis was confirmed through complementary tests (electroneuromyography, muscle biopsy and DNA study). Each of the patients made use of different drugs, in the search of optimal lessening of their myotonia. There were five reports of amelioration with the use of diphenilhydantoine; one report with the use of carbamazepine; three reports with the use of acetazolamide; one report with the use of a calcium channel blocker; one report with the use of a beta-adrenergic; one report with the use of thiazide; and none with the use of quinidine/procainamide.

Two Cases of Becker's Type Congenital Myotonia

Congenital myotonia is a benign familial disorder, main problem is muscle stiffness, delayed relaxation of skeletal muscles after voluntary contraction or following mechanical or electrical stimulation. Although weakness is always present with progression of myotonic dystrophy, many patients with myotonia congenita never develop weakness. In the autosomal dominantly inherited form of congenital myotonia (Thomsen's disease), symptoms revolve around myotonia but weakness is not present. However, in the autosomal recessive (Becker's) type congenital myotonia, mild weakness and marked muscle hypertrophy is common. We report two cases of sporadic developing Becker's type congenital myotonia with electrophysiologic and muscle biopsy findings and review of literatures.

An aspartic acid residue important for voltage-dependent gating of human muscle chloride channels.

A point mutation (D136G) predicting the substitution of glycine for aspartate in position 136 of the human muscle Cl- channel (hClC-1) causes recessive generalized myotonia. Heterologous expression of a recombinant D136G produces functional Cl- channels with profound alterations in voltage-dependent gating, without concomitant changes in pore properties. The mutant exhibits slowly activating current upon hyperpolarization, in contrast to wild-type channels, which display time-dependent current decay (deactivation) at negative membrane potentials. Steady-state activation of D136G depends upon the transmembrane Cl- gradient, reaching zero at voltages positive to the Cl- reversal potential in physiological Cl- distribution. This explains the reduced sarcolemmal Cl- conductance that causes myotonia. The functional disturbances exhibited by D136G may stem from a defect in the ClC-1 voltage sensor.

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.

The skeletal muscle chloride channel in dominant and recessive human myotonia.

Autosomal recessive generalized myotonia (Becker's disease) (GM) and autosomal dominant myotonia congenita (Thomsen's disease) (MC) are characterized by skeletal muscle stiffness that is a result of muscle membrane hyperexcitability. For both diseases, alterations in muscle chloride or sodium currents or both have been observed. A complementary DNA for a human skeletal muscle chloride channel (CLC-1) was cloned, physically localized on chromosome 7, and linked to the T cell receptor beta (TCRB) locus. Tight linkage of these two loci to GM and MC was found in German families. An unusual restriction site in the CLC-1 locus in two GM families identified a mutation associated with that disease, a phenylalanine-to-cysteine substitution in putative transmembrane domain D8. This suggests that different mutations in CLC-1 may cause dominant or recessive myotonia.

Dependence of membrane potential on extracellular ionic concentrations in myotonic goats and rats.

Intracellular potassium concentration([K]i) and the resting sodium-to-potassium permeability ratio (alpha = PNa/PK) were determined by recording resting potentials at 25 degrees C in six solutions with variable potassium concentrations, constant [K+] + [Na+], and constant [K+] X [Cl-] product and fitting the results by nonlinear least squares to the Goldman-Hodgkin-Katz equation. Fits to models including chloride permeability terms suggest that chloride is in Donnan equilibrium in the muscles studied. External intercostal muscle was biopsied from anesthetized normal goats and goats with hereditary myotonia. In normal goat muscle, alpha was 0.012 +/- 0.001 (mean +/- SE, n = 11), and [K]i was 133 +/- 6 mM (n = 9); myotonic goat muscle did not differ from this. Extensor digitorum longus (EDL) muscle was removed from anesthetized male Wistar rats and from like rats pretreated with a single large dose of 20,25-diazacholesterol (DAC). In normal rat EDL, alpha was 0.019 +/- 0.002 (n = 12), and [K]i was 142 +/- 4 mM (n = 12). In the 16-day DAC-treated rats, alpha was significantly reduced (P < 0.002) to 0.010 +/- 0.002 (n = 8), and [K]i was slightly reduced by DAC. It is concluded that DAC reduced PNa in rat fibers. Tetrodotoxin at 5 microM did not affect either parameter in the rats.