Sodium Channel Myotonia Research Articles

A new case of autosomal dominant myotonia associated with the V1589M missense mutation in the muscle sodium channel gene and its phenotypic classification

We report a phenotype associated with the Val1589Met substitution in SCN4A gene in a French family which would be better classified as paramyotonia congenita. The proband was a 48-year-old woman, who described muscle stiffness and occasional flaccid weakness, both symptoms being induced by exercise, cold and heat. Severe muscle stiffness affected facial, oropharyngeal and limb muscles leading to transient paralysis of these muscles. One sister, two nephews and the son of the proband had similar symptoms. Molecular analysis of the muscle sodium channel gene (SCN4A) by nucleotide sequencing revealed a G-to-A transition of cDNA nucleotide at position 4765 predicting a substitution of methionine for valine at position 1589. This shows that the Val1589Met mutation in the SCN4 gene may cause different phenotypes, either potassium-aggravated myotonia or paramyotonia congenita. Familial or individual factors other than the missense mutation per se influence the expression of the disease in sodium channel disorders.

K-aggravated myotonia mutations at residue G1306 differentially alter deactivation gating of human skeletal muscle sodium channels.

Fast inactivation and deactivation gating were compared between wild-type human voltage-gated skeletal muscle sodium channel (hNaV1.4) and potassium-aggravated myotonia (PAM) mutations G1306A, G1306E, and G1306V. Cell-attached macropatches were used to compare wild-type and PAM-gating properties in normal extracellular K+ (4 mM), decreased K+ (1 mM), and increased K+ (10 mM). G1306E/A increased the apparent valence of the conductance (g(V)) curve. Compared to hNaV1.4, the steady-state inactivation (h infinity) curve was depolarized for G1306E/A but hyperpolarized by G1306V, and this mutation increased apparent valence. G1306A/E slowed the rate of current rise towards peak activation. G1306V slowed open-state deactivation, inactivated-state deactivation, and recovery from fast inactivation. G1306A/E abbreviated open-state deactivation at negative commands. These mutants slowed open-state deactivation at more positive commands, at voltages for which fast inactivation might influence tail current decay. G1306E abbreviated recovery delay without affecting recovery rate. Low K+ increased peak current in hNaV1.4 and in G1306V. For G1306E, low K+ increased the rate of entry into fast inactivation, hyperpolarized the g(V) and h(infinity) curves, and increased recovery delay. Biophysical underpinnings of PAM caused by mutations of G1306 thus vary with the specific mutation, and hyperkalemic exacerbation of effects of mutations at this residue are not direct.

Human skeletal muscle sodium channelopathies

Ion channels are transmembrane proteins that allow ions to flow in or out of the cell. Sodium and potassium channel activation and inactivation are the basis of action potential's production and conduction. During the past 15 years, ion channels have been implicated in diseases that have come to be known as the channelopathies. Over 30 mutations of the muscle channel gene SCN4A, which encodes the muscle voltage-gated sodium channel, have been described and associated with neuromuscular disorders like hypo- and hyper-kalaemic periodic paralyses (hypoPP and hyperPP), paramyotonia congenita, sodium channel myotonias and congenital myasthenic syndrome. Different mutations within the same gene (SCN4A) cause distinct clinical disorders, while mutations in different channel genes may result in similar phenotypes. In addition, identical sodium channel mutations can result in different clinical phenotypes (hyperPP or paramyotonia) in different members of the same family, suggesting that the genetic background and perhaps other epigenetic factors may influence the clinical expression of a particular mutation. This article reviews the clinical features of the skeletal muscle sodium channel diseases and highlights the phenotypic or genetic overlap in these disorders.

Mexiletine block of wild-type and inactivation-deficient human skeletal muscle hNav1.4 Na+ channels.

Mexiletine is a class 1b antiarrhythmic drug used for ventricular arrhythmias but is also found to be effective for paramyotonia congenita, potassium-aggravated myotonia, long QT-3 syndrome, and neuropathic pain. This drug elicits tonic block of Na(+) channels when cells are stimulated infrequently and produces additional use-dependent block during repetitive pulses. We examined the state-dependent block by mexiletine in human skeletal muscle hNav1.4 wild-type and inactivation-deficient mutant Na(+) channels (hNav1.4-L443C/A444W) expressed in HEK293t cells with a beta1 subunit. The 50% inhibitory concentrations (IC(50)) for the inactivated-state block and the resting-state block of wild-type Na(+) channels by mexiletine were measured as 67.8 +/- 7.0 microm and 431.2 +/- 9.4 microm, respectively (n= 5). In contrast, the IC(50) for the block of open inactivation-deficient mutant channels at +30 mV by mexiletine was 3.3 +/- 0.1 microm (n= 5), which was within the therapeutic plasma concentration range (2.8-11 microm). Estimated on- and off-rates for the open-state block by mexiletine at +30 mV were 10.4 microm(-1) s(-1) and 54.4 s(-1), respectively. Use-dependent block by mexiletine was greater in inactivation-deficient mutant channels than in wild-type channels during repetitive pulses. Furthermore, the IC(50) values for the block of persistent late hNav1.4 currents in chloramine-T-pretreated cells by mexiletine was 7.5 +/- 0.8 microm (n= 5) at +30 mV. Our results together support the hypothesis that the in vivo efficacy of mexiletine is primarily due to the open-channel block of persistent late Na(+) currents, which may arise during various pathological conditions.

Impairment of slow inactivation as a common mechanism for periodic paralysis in DIIS4-S5.

Mutations in the human skeletal muscle sodium channels are associated with hyperKPP, hypoKPP, paramyotonia congenita, and potassium-aggravated myotonia. This article describes the clinical manifestations of a patient with hyperKPP carrying a mutation (L689I) occurring in the linker DIIS4-S5 and its functional expression in a mammalian system. To correlate the clinical manifestations of hyperkalemic periodic paralysis (hyperKPP) with the functional expression of a sodium channel mutation. The mutation was introduced into a mammalian expression vector and expressed in the human embryonic kidney 293 cells. The functional expression of the L689I and that of the wild-type channels was monitored using the whole cell voltage-clamp technique. There was no change in the kinetics of fast inactivation, and inactivation curves were indistinguishable from that of wild-type channels. However, the L689I mutation caused a hyperpolarizing shift in the voltage dependence of activation and the mutant channels showed an impaired slow inactivation process. In addition, the mutant channels have a larger persistent current at -40 mV where window current may occur. The L689I mutation has similar effects to the T704M mutation and causes hyperKPP in this family. Because both of these hyperKPP mutations cause episodic muscle weakness, and because patients harboring another mutation (I693T) also can have episodic weakness, it is hypothesized that mutations occurring in this region of the sodium channel may cause episodic weakness through an impaired slow inactivation process coupled with enhanced activation.

Gating of myotonic Na channel mutants defines the response to mexiletine and a potent derivative.

Myotonia and periodic paralysis caused by sodium channel mutations show variable responses to the anti-myotonic drug mexiletine. To investigate whether variability among sodium channel mutants results from differences in drug binding affinity or in channel gating. Whole-cell sodium currents (I(Na)) were recorded in tsA201 cells expressing human wild-type (WT) and mutant skeletal muscle sodium channels (A1156T, hyperkalemic periodic paralysis; R1448C, paramyotonia congenita; G1306E, potassium-aggravated myotonia). At a holding potential (hp) of -120 mV, mexiletine produced a tonic (TB, 0.33 Hz) and a use-dependent (UDB, 10 Hz) block of peak I(Na) with a potency following the order rank R1448C > WT approximately equal A1156T > G1306E. Yet, when assayed from an hp of -180 mV, TB and UDB by mexiletine were similar for the four channels. The different midpoints of channel availability curves found for the four channels track the half-maximum inhibitory value (IC50) measured at -120 mV. Thus differences in the partitioning of channels between the closed and fast-inactivated states underlie the different IC50 measured at a given potential. The mexiletine-derivative, Me7 (alpha-[(2-methylphenoxy)methyl]-benzenemethanamine), behaved similarly but was approximately 5 times more potent than mexiletine. Interestingly, the higher drug concentrations ameliorated the abnormally slower decay rate of myotonic I(Na). These results explain the basis of the apparent difference in block of mutant sodium channels by mexiletine and Me7, opening the way to a more rationale drug use and to design more potent drugs able to correct specifically the biophysical defect of the mutation in individual myotonic patients.

The Human Skeletal Muscle Na Channel Mutation R669H Associated with Hypokalemic Periodic Paralysis Enhances Slow Inactivation

Missense mutations of the human skeletal muscle voltage-gated Na channel (hSkM1) underlie a variety of diseases, including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita, and potassium-aggravated myotonia. Another disorder of sarcolemmal excitability, hypokalemic periodic paralysis (HypoPP), which is usually caused by missense mutations of the S4 voltage sensors of the L-type Ca channel, was associated recently in one family with a mutation in the outermost arginine of the IIS4 voltage sensor (R669H) of hSkM1 (Bulman et al., 1999). Intriguingly, an arginine-to-histidine mutation at the homologous position in the L-type Ca(2+) channel (R528H) is a common cause of HypoPP. We have studied the gating properties of the hSkM1-R669H mutant Na channel experimentally in human embryonic kidney cells and found that it has no significant effects on activation or fast inactivation but does cause an enhancement of slow inactivation. R669H channels exhibit an approximately 10 mV hyperpolarized shift in the voltage dependence of slow inactivation and a twofold to fivefold prolongation of recovery after prolonged depolarization. In contrast, slow inactivation is often disrupted in HyperPP-associated Na channel mutants. These results demonstrate that, in R669H-associated HypoPP, enhanced slow inactivation does not preclude, and may contribute to, prolonged attacks of weakness and add support to previous evidence implicating the IIS4 voltage sensor in slow-inactivation gating.

Inherited ion channel disorders.

The inherited ion channel disorders (channelopathies) are a group of disorders caused by mutations in genes encoding ion channels. Ion channel disorders can affect any tissue, but the majority affect skeletal muscle or the central nervous system. These disorders include skeletal muscle sodium channelopathies causing hyperkalaemic periodic paralysis, paramyotonia congenita and potassium-aggravated myotonia. Skeletal muscle calcium channelopathies can cause hypokalaemic periodic paralysis, malignant hyperthermia and central core disease. Skeletal muscle chloride channelopathies can cause Thomsen and Becker myotonia. A neuronal sodium channelopathy causes the generalised epilepsy febrile seizures plus syndrome. Neuronal potassium channelopathies can cause familial benign neonatal convulsions and episodic ataxia type 1. Finally, neuronal calcium channelopathies can cause episodic ataxia type 2, familial hemiplegic migraine and spinocerebellar ataxia type 6. The clinical features, aetiology and pathogenesis of inherited voltage-gated ion channel disorders affecting muscle and the central nervous system are reviewed here.

Lack of sodium channel mutation in an Italian family with paramyotonia congenita.

To conduct the genotype-phenotype correlation in a family in which several individuals share clinical and electrophysiologic features of paramyotonia congenita (PC). PC, hyperkalemic periodic paralysis (HyperPP), and potassium-aggravated myotonias form the group of hereditary sodium channelopathies. Each of these disorders is associated with different point mutations in SCN4A, the gene encoding the alpha-subunit of the adult human skeletal muscle sodium channel. However, in HyperPP families, evidence of a causative gene different from SCN4A has been found. We conducted direct clinical examination, electrophysiologic (EMG/electroneurographic) and cardiologic studies, as well as laboratory screening in several affected and nonaffected members of the family. We performed the genotype-phenotype correlation by microsatellite linkage and cDNA-mutation analyses of the SCN4A gene. Affected members in this family showed clinical and electrophysiologic features typical of PC. The disease phenotype segregated with the chromosomal region that includes the SCN4A gene. Analysis of the entire cDNA sequence of the SCN4A gene in the index case disclosed a G3826A transition, which results in the Val1276Ile substitution. However, PCR-single-stranded confirmation polymorphism and direct sequencing analysis of the segment coding for Val-1276 on genomic DNA confirmed the G3826A transition in the index case but was negative in 11 affected members of the family; however, neither mutations nor aberrant splicings causative of the PC phenotype in this family were found on SCN4A. The existence of a second gene different from SCN4A that can give rise to a clinical PC phenotype can be speculated upon.

Activation and Inactivation of the Voltage-Gated Sodium Channel: Role of Segment S5 Revealed by a Novel Hyperkalaemic Periodic Paralysis Mutation

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-aggravated myotonia are three autosomal dominant skeletal muscle disorders linked to the SCN4A gene encoding the alpha-subunit of the human voltage-sensitive sodium channel. To date, approximately 20 point mutations causing these disorders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at position 1495 located in the transmembrane segment S5 in the fourth homologous domain of the human alpha-subunit sodium channel. Introduction of the I1495F mutation into the wild-type channels disrupted the macroscopic current inactivation decay and shifted both steady-state activation and inactivation to the hyperpolarizing direction. The recovery from fast inactivation was slowed, and there was no effect on channel deactivation. Additionally, a significant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutation located in the cytoplasmic interface of the S5 segment of the second domain, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. These results, showing that the I1495F and T704M hyperkalaemic periodic paralysis mutations both have profound effects on channel activation and fast-slow inactivation, suggest that the S5 segment maybe in a location where fast and slow inactivation converge.

Effect of mexiletine on sea anemone toxin-induced non-inactivating sodium channels of rat skeletal muscle: a model of sodium channel myotonia.

The sea anemone toxin ATX II impairs skeletal muscle sodium channel inactivation, mimicking the persistent inward current observed in patients suffering from sodium channel myotonia. Mexiletine has beneficial effects on myotonia. To verify the efficiency of the drug on persistent inward current, we investigated the effect of 50 microM R(-)-mexiletine on sodium channels in cell-attached patches of rat skeletal muscle fibres, in the absence or presence of 2 microM ATX II. With the toxin, a proportion of channels displayed remarkable abnormal activity lasting the entire depolarisation, which resulted in a persistent inward current that represented up to 2.0% of the peak current. Mexiletine reduced by 75% the peak current elicited by depolarisation from -100 to -20 mV. This was due to the reduction by 60% of the maximal available peak current Imax and to the negative shift by -7 mV of steady-state inactivation. Mexiletine also greatly decreased the late current, but the effect was limited to 60% of reduction, comparable to that on Imax. Therefore mexiletine was able to block the ATX II-modified sodium channels, inhibiting the myotonia-producing persistent inward current.

Myopathic mutations affect differently the inactivation of the two gating modes of sodium channels.

Three groups of mutations of the alpha subunit of the rat skeletal muscle sodium channel (rSkM1), homologous to mutations linked to human muscle hereditary diseases, have been studied by heterologous expression in frog oocytes: S798F, G1299E, G1299V, and G1299A, linked with potassium-aggravated myotonia (PAM); T1306M, R1441C and R1441P, linked with paramyotonia congenita (PC); T698M and M1353V, linked with the hyperkalemic periodic paralysis (HyPP). Wild-type rSkM1 channels (WT) show two gating modes, M1 and M2, which differ mainly in the process of inactivation. The naturally most representative mode M1 is tenfold faster and develops at approximately 30 mV less depolarized potentials. A common feature of myopathy-linked mutants is an increase in the mode M2 probability, P(M2), but phenotype-specific alterations of voltage-dependence and kinetics of inactivation of both modes are also observed. The coexpression of the sodium channel beta1 subunit, which has been studied for WT and for the five best expressing mutants, generally caused a threefold reduction of P(M2) without changing the properties of the individual modes. This indicates that the mutations do not affect the alpha - beta1 interaction and that the phenotypic changes in P(M2) observed for the enhanced mode M2 behavior of the sole alpha subunits, although largely depressed in the native tissue, are likely to be the most important functional modification that causes the muscle hyperexcitability observed in all patients carrying the myotonic mutations. The interpretation of the more phenotype-specific changes revealed by our study is not obvious, but it may offer clues for understanding the different clinical manifestations of the diseases associated with the various mutations.

Defective fast inactivation recovery and deactivation account for sodium channel myotonia in the I1160V mutant

The skeletal muscle sodium channel mutant I1160V cosegregates with a disease phenotype producing myotonic discharges (observed as muscle stiffness) that are worsened by elevated K+ levels but unaffected by cooling. The I1160V alpha-subunit was co-expressed with the beta1-subunit in Xenopus oocytes. An electrophysiological characterization was undertaken to examine the underlying biophysical characteristics imposed by this mutation. Two abnormalities were found. 1) The voltage dependence of steady-state fast inactivation was reduced in I1160V, which resulted in faster rates of closed-state fast inactivation onset and recovery in I1160V compared with wild-type channels. 2) The rates of deactivation were slower in I1160V than in wild-type channels. Using a computer-simulated model, the combination of both defects elicited myotonic runs under conditions of elevated K+, consistent with the observed phenotype of the mutant.

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 α-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.

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.

S-6-2 Molecular basis of the periodic paralyses

The nondystrophic skeletal muscle channelopathies are a group of disorders caused by mutations of various voltage-gated ion channel genes, including nondystrophic myotonia and periodic paralysis.We identified patients with a diagnosis of muscle channelopathy from our neuromuscular database in a tertiary care pediatric center from 2005 to 2015. We then performed a retrospective review of their medical records for demographic characteristics, clinical features, investigations, treatment, and follow-up.Thirty-three patients were identified. Seventeen had nondystrophic myotonia. Seven of them had chloride channelopathy (four Becker disease and three Thomsen disease). Warm-up phenomenon and muscle hypertrophy were common clinical manifestations in this subgroup. Ten patients had sodium channelopathy (four paramyotonia congenita and six other sodium channel myotonia). Stiffness of the facial muscles was an important presenting symptom, and eyelid myotonia was a common clinical finding in this subgroup. The majority of these patients had electrical myotonia. Mexiletine was effective in controlling the symptoms in patients who had received treatment. Sixteen children had periodic paralysis (four hyperkalemic periodic paralysis, eight hypokalemic periodic paralysis, and four Andersen-Tawil syndrome). Acetazolamide was commonly used to prevent paralytic attacks and was found to be effective.Nondystrophic muscle channelopathies present with diverse clinical manifestations (myotonia, muscle hypertrophy, proximal weakness, swallowing difficulties, and periodic paralysis). Cardiac arrhythmias are potentially life threatening in Andersen-Tawil syndrome. Timely identification of these disorders is helpful for effective symptomatic management and genetic counseling.

Human sodium channel myotonia: slowed channel inactivation due to substitutions for a glycine within the III-IV linker.

1. Three families with a form of myotonia (muscle stiffness due to membrane hyperexcitability) clinically distinct from previously classified myotonias were examined. The severity of the disease greatly differed among the families. 2. Three dominant point mutations were discovered at the same nucleotide position of the SCN4A gene encoding the adult skeletal muscle Na+ channel alpha-subunit. They predict the substitution of either glutamic acid, valine or alanine for glycine1306, a highly conserved residue within the supposed inactivation gate. Additional SCN4A mutations were excluded. 3. Electrophysiological studies were performed on biopsied muscle specimens obtained for each mutation. Patch clamp recordings on sarcolemmal blebs revealed an increase in the time constant of fast Na+ channel inactivation, tau h, and in late channel openings as compared to normal controls. tau h was increased from 1.2 to 1.6-2.1 ms and the average late currents from 0.4 to 1-6% of the peak early current. 4. Intracellular recordings on resealed fibre segments revealed an abnormal tetrodotoxin-sensitive steady-state inward current, and repetitive action potentials. Since K+ and Cl- conductances were normal, only the increase in the number of non-inactivating Na+ channels has to be responsible for the membrane hyperexcitability. 5. Length, ramification and charge of the side-chains of the substitutions correlated well with the Na+ channel dysfunction and the severity of myotonia, with alanine as the most benign and glutamic acid as the substitution with a major steric effect. 6. Our electrophysiological and molecular genetic studies strongly suggest that these Na+ channel mutations cause myotonia. The naturally occurring mutants allowed us to gain further insight into the mechanism of Na+ channel inactivation.

A new method for selective diagnosis of the functional status of small nerve fibers

Nondystrophic myotonias are characterized by muscle stiffness triggered by voluntary movement. They are caused by mutations in either the CLCN1 gene in myotonia congenita or in the SCN4A gene in paramyotonia congenita and sodium channel myotonias. Clinical and electrophysiological phenotypes of these disorders have been well described. No concomitant mutations in both genes have been reported yet. We report five patients from three families showing myotonia with both chloride and sodium channel mutations. Their clinical and electrophysiological phenotypes did not fit with the phenotype known to be associated with the mutation initially found in SCN4A gene, which led us to screen and find an additional mutation in CLCN1 gene. Our electrophysiological and clinical observations suggest that heterozygous CLCN1 mutations can modify the clinical and electrophysiological expression of SCN4A mutation.

Topographic distribution of quantitative EEG effects of neuroactive drugs : W.G. Sannita, M. Cabri, A. Griss, C. Padovan and G. Rosadini (Genoa, Italy)

The nondystrophic skeletal muscle channelopathies are a group of disorders caused by mutations of various voltage-gated ion channel genes, including nondystrophic myotonia and periodic paralysis.We identified patients with a diagnosis of muscle channelopathy from our neuromuscular database in a tertiary care pediatric center from 2005 to 2015. We then performed a retrospective review of their medical records for demographic characteristics, clinical features, investigations, treatment, and follow-up.Thirty-three patients were identified. Seventeen had nondystrophic myotonia. Seven of them had chloride channelopathy (four Becker disease and three Thomsen disease). Warm-up phenomenon and muscle hypertrophy were common clinical manifestations in this subgroup. Ten patients had sodium channelopathy (four paramyotonia congenita and six other sodium channel myotonia). Stiffness of the facial muscles was an important presenting symptom, and eyelid myotonia was a common clinical finding in this subgroup. The majority of these patients had electrical myotonia. Mexiletine was effective in controlling the symptoms in patients who had received treatment. Sixteen children had periodic paralysis (four hyperkalemic periodic paralysis, eight hypokalemic periodic paralysis, and four Andersen-Tawil syndrome). Acetazolamide was commonly used to prevent paralytic attacks and was found to be effective.Nondystrophic muscle channelopathies present with diverse clinical manifestations (myotonia, muscle hypertrophy, proximal weakness, swallowing difficulties, and periodic paralysis). Cardiac arrhythmias are potentially life threatening in Andersen-Tawil syndrome. Timely identification of these disorders is helpful for effective symptomatic management and genetic counseling.