Myotonia fluctuans. A third type of muscle sodium channel disease

The changing role of pediatric electrodiagnosis

Electrodiagnosis is one of several useful diagnostic tests in infants and children who have anterior horn cell disease, neuropathy, neuromuscular junction disorders, or myopathy. It is also used for intraoperative monitoring in children. For hypotonic infants and for older children with a nonspecific presentation of weakness, EDX may provide direction for more specific diagnostic testing, such as DNA testing with or without muscle biopsy. Genetic testing has an increasingly important role in the diagnosis of children with neuromuscular disorders. Future improvements in motor unit quantitation, which do not require active patient cooperation and require less time than current methodologies, may make EDX more specific and useful for diagnosing neuromuscular disease in children.

Phenotypic variation of a Thr704Met mutation in skeletal sodium channel gene in a family with paralysis periodica paramyotonica

OBJECTIVES

Patients with paralysis periodica paramyotonica exhibit a clinical syndrome with characteristics of both hyperkalaemic periodic paralysis and paramyotonia congenita. In several types of periodic paralysis associated with hyperkalaemia, mutations in the skeletal muscle sodium channel (SCN4A) gene have been previously reported. Phenotypic variations of mutations in SCN4A, however, have not been described yet. The present study aimed to evaluate genetic variations in a family with clinical and electrophysiological characteristics of paralysis periodica paramyotonia.

METHODS

Seven members of a family affected with symptoms of paralysis periodica paramyotonia were studied by electrophysiological and genetic analyses. There were increased serum potassium concentrations in four members during paralytic attacks induced by hyperkalaemic periodic paralysis provocation tests. Short exercise tests before and after cold immersion were carried out in four patients to distinguish electrophysiological characteristics of hyperkalaemic periodic paralysis and paramyotonia. Sequencing analyses of SCN4A were performed on one patient and a normal control to identify polymorphisms. Restriction fragment length polymorphism (RFLP) analysis was then performed at the identified polymorphic sites.

RESULTS

Electrophysiological studies showed both exercise sensitivity and temperature sensitivity. Compound motor action potential (CMAP) amplitudes were decreased (7.3%-28.6%) after short exercise tests. The CMAP amplitudes were even more severely decreased (21.7%-56.5%) in short exercise tests after cold exposure. Three polymorphic sites, Gln371Glu, Thr704Met, and Aspl376Asn were identified in SCN4A. RFLP analyses showed that all affected patients carried the Thr704Met mutation, whereas unaffected family members and a normal control did not.

CONCLUSION

Phenotypic variation of the Thr704Met mutation, which was previously reported in patients with hyperkalaemic periodic paralysis, is described in a family affected with paralysis periodica paramyotonia.

Identification of the Arg1086His mutation in the alpha subunit of the voltage-dependent calcium channel (CACNA1S) in a North American family with malignant hyperthermia

Individuals from a large North American population were screened for the presence of the mutation in the alpha1 subunit of the voltage-dependent calcium channel (CACNA1S) that has recently been associated with malignant hyperthermia (MH). This Arg1086His mutation was screened for in 154 MH normal (MHN) individuals and 112 MH susceptible (MHS) individuals, who were diagnosed by the North American protocol of the in vitro contracture test. PCR and restriction enzyme analysis was used to test for the mutation. The Arg1086His mutation in the CACNA1S was not found in any of the MHN individuals. In contrast, two related individuals (grandfather and grandson, father and son of the MH proband) among the MHS group exhibited this mutation. However, a third MHS individual in the same family (granddaughter, cousin of the grandson) did not exhibit this mutation. These results indicate that this mutation may be associated with MH in this family. Genetic alterations in the CACNA1S associated with MH are present in approximately 1% of this North American MHS population.

[Spontaneous electromyographic activity. Practical importance]

Spontaneous activities are a major semeiologic sign in electromyography. The present article deals with the different aspects recorded in practice in normal and pathological cases. There are two types of spontaneous activities, those related to motor unit hyperactivity (fasciculations and myokymia) and those related to the hyperactivity of one or more muscle fibers: fibrillations, positive sharp waves, myotonic discharges and complex repetitive discharges. In the first case the lesion is located in the axone and in the second in the membrane of the muscle fibers; All theories related to the cells' abnormalities share a common feature: spontaneous activities result from abnormal firing of the membrane action potential of muscular fibers. This functional abnormality may results from different types of lesions within the cells' membrane and determines the aspects of spontaneous activities. Impaired function of muscular cells' membranes can be produced by denervation or lesion of the membrane structure itself. The latter can be multiple and linked with the membrane proteins (such as laminine or dystrophin as in AIDS diseases) or with ion channel disturbances. Multiple membrane cell alterations may produce the same kind of spontaneous activity; for instance, myotonic discharges have the same morphology in Thomsen and Steinert's disease despite their different mechanisms and fibrillations seen in denervations and myopathies. The practical consequences are discussed and a new classification of these spontaneous activities is proposed.

Channelopathies: ion channel defects linked to heritable clinical disorders

Electrical signals are critical for the function of neurones, muscle cells, and cardiac myocytes. Proteins that regulate electrical signalling in these cells, including voltage gated ion channels, are logical sites where abnormality might lead to disease. Genetic and biophysical approaches are being used to show that several disorders result from mutations in voltage gated ion channels. Understanding gained from early studies on the pathogenesis of a group of muscle diseases that are similar in their episodic nature (periodic paralysis) showed that these disorders result from mutations in a gene encoding a voltage gated Na(+) channel. Their characterisation as channelopathies has served as a paradigm for other episodic disorders. For example, migraine headache and some forms of epilepsy have been shown to result from mutations in voltage gated Ca(2+) channel genes, while long QT syndrome is known to result from mutations in either K(+) or Na(+) channel genes. This article reviews progress made in the complementary fields of molecular genetics and cellular electrophysiology which has led to a better understanding of voltage gated ion channelopathies in humans and mice.

Genetics and pathogenesis of malignant hyperthermia

Malignant hyperthermia (MH) is a potentially life-threatening event in response to anesthetic triggering agents, with symptoms of sustained uncontrolled skeletal muscle calcium homeostasis resulting in organ and systemic failure. Susceptibility to MH, an autosomal dominant trait, may be associated with congenital myopathies, but in the majority of the cases, no clinical signs of disease are visible outside of anesthesia. For diagnosis, a functional test on skeletal muscle biopsy, the in vitro contracture test (IVCT), is performed. Over 50% of the families show linkage of the IVCT phenotype to the gene encoding the skeletal muscle ryanodine receptor and over 20 mutations therein have been described. At least five other loci have been defined implicating greater genetic heterogeneity than previously assumed, but so far only one further gene encoding the main subunit of the voltage-gated dihydropyridine receptor has a confirmed role in MH. As a result of extensive research on the mechanisms of excitation-contraction coupling and recent functional characterization of several disease-causing mutations in heterologous expression systems, much is known today about the molecular etiology of MH.

Different effects of mexiletine on two mutant sodium channels causing paramyotonia congenita and hyperkalemic periodic paralysis

Effects of the antiarrhythmic and antimyotonic drug mexiletine were studied on two sodium channel mutants causing paramyotonia congenita (R1448H) and an overlap paramyotonic and hyperkalemic paralytic syndrome (M1360V). Channels were expressed in human embryonic kidney cells and studied electrophysiologically, using the whole-cell patch-clamp technique. Compared to the wild-type, channel, both mutants showed alterations of inactivation, i.e. slower inactivation, left shift of steady-state inactivation and faster recovery from inactivation. Mexiletine caused a significantly larger use-dependent block of the R1448H mutant when compared to M1360V and wild-type channels. This can be explained by a prolonged recovery from mexiletine block as observed for R1448H channels, since the affinity of mexiletine for the inactivated state was similar for all three clones. The use-dependent block of sodium channels by mexiletine reduces repetitive series of action potentials and therefore improves muscle stiffness in myotonic patients. The enhanced use-dependent block as seen with R1448H may explain the extraordinary therapeutic efficacy of mexiletine in most patients with paramyotonia congenita.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

The dominant chloride channel mutant G200R causing fluctuating myotonia: clinical findings, electrophysiology, and channel pathology

Clinical, electrophysiological, and molecular findings are reported for a family with dominant myotonia congenita in which all affected members have experienced long-term fluctuations of the symptom of myotonia. In some patients myotonia is combined with myalgia. The myotonia-causing mutation in this family is in the gene encoding the muscular chloride channel, hCIC-1, predicting the amino acid exchange G200R. We have constructed recombinant DNA vectors for expression of the mutant protein in tsA201 cells and investigation of the properties of the mutant channel. The most prominent alteration was a +100-mV shift of the midpoint of the activation curve. Therefore, within the physiological range the open probability of the mutant channel is markedly smaller than in wild-type. This shift is likely to be responsible for the myotonia in the patients. The fluctuating symptoms of this chloride channelopathy are discussed with respect to short-term fluctuations of myotonia in the sodium channelopathy of potassium-aggravated myotonia.

Human sodium channel gating defects caused by missense mutations in S6 segments associated with myotonia: S804F and V1293I

1. Missense mutations in the alpha-subunit of the human skeletal muscle sodium channel (hSkM1) have been detected in some heritable forms of myotonia. By recording Na+ currents from cells transfected with cDNA encoding either wild-type or mutant hSkM1, we characterized the functional consequences of two myotonia-associated mutations that lie at the cytoplasmic end of the sixth transmembrane segment in domain II (S804F) or domain III (V1293I). 2. Both mutations caused modest, but unequivocal, alterations in the voltage-dependent gating behaviour of hSkM1. For S804F, the abnormalities were limited to fast inactivation: the persistent Na+ current at the end of a 50 ms depolarization was increased 3-fold, the rate of inactivation from the open state was slowed 2-fold, and the voltage dependence of fast inactivation (h) was shifted by +3 mV. V1293I also disrupted fast inactivation, as evidenced by a 3-fold faster rate of recovery at hyperpolarized potentials (-70 mV). Activation was altered as well for V1293I: the voltage dependence was shifted by -6 mV (hyperpolarized). 3. Slow inactivation was not altered by S804F or V1293I. 4. We conclude that S804F and V1293I are not benign polymorphisms. Either mutation causes detectable alterations in channel gating and, in model simulations, the magnitude of the defects is sufficient to produce runs of myotonic discharges.

In vitro contracture test for diagnosis of malignant hyperthermia following the protocol of the European MH Group: results of testing patients surviving fulminant MH and unrelated low-risk subjects. The European Malignant Hyperthermia Group

BACKGROUND

Determination of sensitivity and specificity of the in vitro contracture test (IVCT) for malignant hyperthermia (MH) susceptibility using the European MH Group (EMHG) protocol has been performed in some laboratories but only on a small sample from the combined EMHG. Thus, the purpose of the present study was to determine combined EMHG sensitivity and specificity of the test.

METHODS

Results of IVCT of patients with previous fulminant MH and normal, low-risk subjects (controls) were collected from 22 centres of the EMHG. IVCT was performed according to the EMHG protocol. Patients were included in the study if the clinical crisis had a score of at least 50 points with the Clinical Grading Scale. Low-risk subjects were included provided they did not belong to a family with known MH susceptibility, they had not developed any signs of MH at previous anaesthetics, and they did not suffer from any neuromuscular disease. For inclusion of both MH patients and low-risk subjects, at least 1 muscle bundle in the IVCT should have twitches of 10 mN (1 g) or more. For evaluation of individual tests, only muscle bundles with twitch heights of 10 mN (1 g) or more were used.

RESULTS

A total of 1502 probands had undergone IVCT because of a previous anaesthesia with symptoms and signs suggestive of MH. Of these, 119 had clinical scores of 50 and above. From these 119 MH-suspected patients and from 202 low-risk subjects, IVCT data were collected. Subsequently, 14 MH-suspected patients were excluded from further analysis for the following reasons: In 3 patients, the suspected MH episode could be fully explained by diseases other than MH; in 11 MHS patients, IVCT was incomplete (n = 1), data were lost (n = 3), or none of the muscle bundles fulfilled twitch criteria (n = 7). Of the remaining 105 MH-suspected patients, 89 were MHS, 10 MHEh, 5 MHEc, and one MHN. Thus, we observed a diagnostic sensitivity of the IVCT of 99.0% if the MHE group is considered susceptible (95% confidence interval 94.8-100.0%). Of the 202 low-risk subjects, 3 were MHS, 5 MHEh, 5 MHEc, and 189 MHN. This gives a specificity of the IVCT of 93.6% (95% confidence interval 89.2-96.5%).

CONCLUSION

The IVCT for diagnosis of MH susceptibility in Europe has a high sensitivity and a satisfactory specificity.

Hypokalemic periodic paralysis: an autosomal dominant muscle disorder caused by mutations in a voltage-gated calcium channel

Hypokalemic periodic paralysis (hypoPP) is an autosomal dominant disorder characterized by acute attacks of muscle weakness concomitant to a drop in blood potassium levels. Recent molecular work has shown that hypoPP is due to mutations in a skeletal muscle voltage-gated calcium channel: the dihydropyridine receptor (DHP receptor). Mutations affect segments S4 of domains II and IV, changing an arginine in position 528 and 1239 into an histidine, or an histidine or a glycine respectively. Surprisingly, expressing in vitro mutants channels in a non-muscular environment resulted in functional calcium channels with minor modifications in electrophysiological properties. Expressing mutant channels in a muscular environment or transgenic mice might help to bridge the gap between the knowledge of the molecular defect and the understanding of the pathophysiology of the disease.

From mutation to myotonia in sodium channel disorders

Hyperkalemic periodic paralysis, paramyotonia congenita, and the potassium-aggravated myotonias are all caused by point mutations in the alpha-subunit of a sodium channel expressed selectively in skeletal muscle. This review updates the growing list of genotype-phenotype correlations for these mutations and summarizes the alterations in channel function they produce. A toxin-based in vitro model demonstrates that subtle defects in sodium channel inactivation are sufficient to cause myotonia and computer modeling suggests that specific types of inactivation defect may predispose to paralysis or myotonia.

Neuropathology of degenerative cell death in Caenorhabditis elegans

In Caenorhabditis elegans necrosis-like neuronal death is induced by gain-of-function (gf) mutations in two genes, mec-4 and deg-1, that encode proteins similar to subunits of the vertebrate amiloride-sensitive epithelial Na+ channel. We have determined the progress of cellular pathology in dying neurons via light and electron microscopy. The first detectable abnormality is an infolding of the plasma membrane and the production of small electron-dense whorls. Later, cytoplasmic vacuoles and larger membranous whorls form, and the cell swells. More slowly, chromatin aggregates and the nucleus invaginates. Mitochondria and Golgi are not dramatically affected until the final stages of cell death when organelles, and sometimes the cells themselves, lyse. Certain cells, including some muscle cells in deg-1 animals, express the abnormal gene products and display a few membrane abnormalities but do not die. These cells either express the mutant genes at lower levels, lack other proteins needed to form inappropriately functioning channels, or are better able to compensate for the toxic effects of the channels. Overall, the ultrastructural changes in these deaths suggest that enhanced membrane cycling precedes vacuolation and cell swelling. The pathology of mec-4(gf) and deg-1(gf) cells shares features with that of genetic disorders with alterations in channel subunits, such as hypokalemic periodic paralysis in humans and the weaver mouse, and with degenerative conditions, e.g., acute excitotoxic death. The initial pathology in all of these conditions may reflect attempts by affected cells to compensate for abnormal membrane proteins or functions.

[Congenital myotonia. Report of 7 patients]

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.

Paramyotonia congenita: genotype to phenotype correlations in two families and report of a new mutation in the sodium channel gene

Sodium channel disorders include hyperkalemic periodic paralysis (hyperPP), paramyotonia congenita (PC) and potassium-aggravated myotonia (PAM). PC is a myotonic syndrome characterized by cold-induced muscle stiffness and weakness. In this paper, we report two families. The first is affected by PC with cold-induced stiffness and no weakness, in addition to hyperPP. This family displays the Arg1448Cys mutation in the sodium channel gene originally described in pure PC families. The fact that families with the same mutation present distinct phenotypes indicates that other factors, genetic or environmental, may modulate the expression of the disease in sodium channel disorders. The second family was unusual because patients presented cold-induced weakness without stiffness. A mutation was found in the sodium channel gene that changed an isoleucine into a threonine at position 693. These two families demonstrate that sodium channel mutations may cause either cold-induced stiffness or weakness.

Channelopathies: the nondystrophic myotonias and periodic paralyses

The term channelopathy does not indicate a new group of neuromuscular conditions, but a re-orientation of well- and long-known muscular conditions, the congenital myotonias, and the periodic paralyses. Although, in the past, they have overlapped clinically here and there, both groups were classified differently, as myotonias and as metabolic myopathies, respectively. The discovery of mutations in several ion channels has rewritten nosography of these disorders and procured a new term, the channelopathy-clinical, electrophysiological, and molecular genetic details of which are discussed in this chapter.

Inactivation defects caused by myotonia-associated mutations in the sodium channel III-IV linker

Missense mutations in the skeletal muscle Na+ channel alpha subunit occur in several heritable forms of myotonia and periodic paralysis. Distinct phenotypes arise from mutations at two sites within the III-IV cytoplasmic loop: myotonia without weakness due to substitutions at glycine 1306, and myotonia plus weakness caused by a mutation at threonine 1313. Heterologous expression in HEK cells showed that substitutions at either site disrupted inactivation, as reflected by slower inactivation rates, shifts in steady-state inactivation, and larger persistent Na+ currents. For T1313M, however, the changes were an order of magnitude larger than any of three substitutions at G1306, and recovery from inactivation was hastened as well. Model simulations demonstrate that these functional difference have distinct phenotypic consequences. In particular, a large persistent Na+ current predisposes to paralysis due to depolarization-induced block of action potential generation.

Proximal myotonic myopathy: mini-review of a recently delineated clinical disorder

This mini-review describes proximal myotonic myopathy, a recently delineated, dominantly inherited disorder that is similar to but distinct from myotonic dystrophy. Proximal myotonic myopathy is not linked to the gene locus for myotonic dystrophy or to the loci of the genes of the muscle sodium and chloride channels associated with other myotonic disorders. Patients often present with myotonia and peculiar muscle pain in early adulthood and develop weakness of the thigh muscles later in life. Cataracts that are indistinguishable from those in myotonic dystrophy also occur commonly. The gene defect responsible for proximal myotonic myopathy awaits discovery. Because of the clinical similarities between proximal myotonic myopathy and myotonic dystrophy, clarification of the genetic differences will not only shed light on the pathomechanism of proximal myotonic myopathy, but may also increase our understanding of myotonic dystrophy.

Investigation of muscle disease

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Different effects on gating of three myotonia-causing mutations in the inactivation gate of the human muscle sodium channel

1. Three mutations at the same site in the inactivation gate of the alpha-subunit of the human muscle Na+ channel, G1306E, G1306V and G1306A, cause three phenotypes of K(+)-aggravated myotonia: G1306E as the most severe and G1306A as the most benign form. 2. Recombinant wildtype (WT) and mutant (G1306E, G1306V and G1306A) human Na+ channels were expressed in human embryonic kidney cells (HEK293). G1306E and G1306V channels showed a distinct increase in the time constants of inactivation (tau h1 and tau h2) and in the ratios of steady-state to peak currents (Iss/Ipeak) (e.g. at 0 mV, G1306E vs. WT; tau h1, 1.29 +/- 0.10 vs. 0.52 +/- 0.01 ms; Iss/Ipeak, 2.90 +/- 0.40 vs. 0.93 +/- 0.19%). G1306A channels showed only an increase in tau h1 (0.74 +/- 0.07 ms). For G1306E and G1306V channels, the steady-state inactivation curves, as well as the voltage dependence of the rate of recovery from inactivation, were shifted by +15 mV. For G1306A the h infinity curve was shifted by only +5 mV. 3. G1306E and G1306V channels showed prolonged current rise times and later first openings suggesting slowing of activation. For G1306E channels only, the steady-state activation curve was shifted by -7 mV. For all mutants the deactivation time constants were increased. 4. We conclude that (i) the combination of alterations in inactivation and activation produces the slowing of the current decay, (ii) the slowed inactivation is most responsible for myotonia, and (iii) the shift of the steady-state activation curve, seen only with G1306E channels, may explain the severity of this phenotype. 5. The results suggest that two of the mutations in the Na+ channel inactivation gate also alter channel activation and deactivation.

Adenovirus-mediated expression of a voltage-gated potassium channel in vitro (rat cardiac myocytes) and in vivo (rat liver). A novel strategy for modifying excitability

Excitability is governed primarily by the complement of ion channels in the cell membrane that shape the contour of the action potential. To modify excitability by gene transfer, we created a recombinant adenovirus designed to overexpress a Drosophila Shaker potassium channel (AdShK). In vitro, a variety of mammalian cell types infected with AdShK demonstrated robust expression of the exogenous channel. Spontaneous action potentials recorded from cardiac myocytes in primary culture were abbreviated compared with noninfected myocytes. Intravascular infusion of AdShK in neonatal rats induced Shaker potassium channel mRNA expression in the liver, and large potassium currents could be recorded from explanted hepatocytes. Thus, recombinant adenovirus technology has been used for in vitro and in vivo gene transfer of ion channel genes designed to modify cellular action potentials. With appropriate targeting, such a strategy may be useful in gene therapy of arrhythmias, seizure disorders, and myotonic muscle diseases.