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1.  Non-dystrophic myotonia: prospective study of objective and patient reported outcomes 
Brain  2013;136(7):2189-2200.
Non-dystrophic myotonias are rare diseases caused by mutations in skeletal muscle chloride and sodium ion channels with considerable phenotypic overlap between diseases. Few prospective studies have evaluated the sensitivity of symptoms and signs of myotonia in a large cohort of patients. We performed a prospective observational study of 95 participants with definite or clinically suspected non-dystrophic myotonia recruited from six sites in the USA, UK and Canada between March 2006 and March 2009. We used the common infrastructure and data elements provided by the NIH-funded Rare Disease Clinical Research Network. Outcomes included a standardized symptom interview and physical exam; the Short Form-36 and the Individualized Neuromuscular Quality of Life instruments; electrophysiological short and prolonged exercise tests; manual muscle testing; and a modified get-up-and-go test. Thirty-two participants had chloride channel mutations, 34 had sodium channel mutations, nine had myotonic dystrophy type 2, one had myotonic dystrophy type 1, and 17 had no identified mutation. Phenotype comparisons were restricted to those with sodium channel mutations, chloride channel mutations, and myotonic dystrophy type 2. Muscle stiffness was the most prominent symptom overall, seen in 66.7% to 100% of participants. In comparison with chloride channel mutations, participants with sodium mutations had an earlier age of onset of stiffness (5 years versus 10 years), frequent eye closure myotonia (73.5% versus 25%), more impairment on the Individualized Neuromuscular Quality of Life summary score (20.0 versus 9.44), and paradoxical eye closure myotonia (50% versus 0%). Handgrip myotonia was seen in three-quarters of participants, with warm up of myotonia in 75% chloride channel mutations, but also 35.3% of sodium channel mutations. The short exercise test showed ≥10% decrement in the compound muscle action potential amplitude in 59.3% of chloride channel participants compared with 27.6% of sodium channel participants, which increased post-cooling to 57.6% in sodium channel mutations. In evaluation of patients with clinical and electrical myotonia, despite considerable phenotypic overlap, the presence of eye closure myotonia, paradoxical myotonia, and an increase in short exercise test sensitivity post-cooling suggest sodium channel mutations. Outcomes designed to measure stiffness or the electrophysiological correlates of stiffness may prove useful for future clinical trials, regardless of underlying mutation, and include patient-reported stiffness, bedside manoeuvres to evaluate myotonia, muscle specific quality of life instruments and short exercise testing.
PMCID: PMC3692030  PMID: 23771340
non-dystrophic myotonia; SCN4A; CLCN1; myotonia; paramyotonia
2.  Novel mutations in human and mouse SCN4A implicate AMPK in myotonia and periodic paralysis 
Brain  2014;137(12):3171-3185.
Corrochano Sanchez et al. identify a novel mutation (I588V) in SCN4A, which encodes the Nav1.4 voltage-gated sodium channel, in a patient with myotonia and periodic paralysis. By generating and characterizing a mouse model (‘draggen’) carrying the equivalent point mutation (I582V), they uncover novel pathological and metabolic features of SCN4A channelopathies.
Mutations in the skeletal muscle channel (SCN4A), encoding the Nav1.4 voltage-gated sodium channel, are causative of a variety of muscle channelopathies, including non-dystrophic myotonias and periodic paralysis. The effects of many of these mutations on channel function have been characterized both in vitro and in vivo. However, little is known about the consequences of SCN4A mutations downstream from their impact on the electrophysiology of the Nav1.4 channel. Here we report the discovery of a novel SCN4A mutation (c.1762A>G; p.I588V) in a patient with myotonia and periodic paralysis, located within the S1 segment of the second domain of the Nav1.4 channel. Using N-ethyl-N-nitrosourea mutagenesis, we generated and characterized a mouse model (named draggen), carrying the equivalent point mutation (c.1744A>G; p.I582V) to that found in the patient with periodic paralysis and myotonia. Draggen mice have myotonia and suffer from intermittent hind-limb immobility attacks. In-depth characterization of draggen mice uncovered novel systemic metabolic abnormalities in Scn4a mouse models and provided novel insights into disease mechanisms. We discovered metabolic alterations leading to lean mice, as well as abnormal AMP-activated protein kinase activation, which were associated with the immobility attacks and may provide a novel potential therapeutic target.
PMCID: PMC4240299  PMID: 25348630
SCN4A; mice; AMPK; periodic paralysis; myotonia
3.  Muscle channelopathies and electrophysiological approach 
Myotonic syndromes and periodic paralyses are rare disorders of skeletal muscle characterized mainly by muscle stiffness or episodic attacks of weakness. Familial forms are caused by mutation in genes coding for skeletal muscle voltage ionic channels. Familial periodic paralysis and nondystrophic myotonias are disorders of skeletal muscle excitability caused by mutations in genes coding for voltage-gated ion channels. These diseases are characterized by episodic failure of motor activity due to muscle weakness (paralysis) or stiffness (myotonia). Clinical studies have identified two forms of periodic paralyses: hypokalemic periodic paralysis (hypoKPP) and hyperkalemic periodic paralysis (hyperKPP), based on changes in serum potassium levels during the attacks, and three distinct forms of myotonias: paramyotonia congenita (PC), potassium-aggravated myotonia (PAM), and myotonia congenita (MC). PC and PAM have been linked to missense mutations in the SCN4A gene, which encodes α subunit of the voltage-gated sodium channel, whereas MC is caused by mutations in the chloride channel gene (CLCN1). Exercise is known to trigger, aggravate, or relieve symptoms. Therefore, exercise can be used as a functional test in electromyography to improve the diagnosis of these muscle disorders. Abnormal changes in the compound muscle action potential can be disclosed using different exercise tests. Five electromyographic (EMG) patterns (I-V) that may be used in clinical practice as guides for molecular diagnosis are discussed.
PMCID: PMC2781140  PMID: 19966974
Channelopathy; electromyographic; ion channel; myotonia; periodic paralysis
4.  Refined Exercise testing can aid DNA-based Diagnosis in Muscle Channelopathies 
Annals of neurology  2011;69(2):328-340.
To improve the accuracy of genotype prediction and guide genetic testing in patients with muscle channelopathies we applied and refined specialised electrophysiological exercise test parameters.
We studied 56 genetically confirmed patients and 65 controls using needle electromyography, the long exercise test, and short exercise tests at room temperature, after cooling, and rewarming.
Concordant amplitude-and-area decrements were more reliable than amplitude-only measurements when interpreting patterns of change during the short exercise tests. Concordant amplitude-and-area pattern I and pattern II decrements of >20% were 100% specific for PMC and MC respectively. When decrements at room temperature and after cooling were <20%, a repeat short exercise test after rewarming was useful in patients with myotonia congenita. Area measurements and rewarming distinguished true temperature sensitivity from amplitude reduction due to cold-induced slowing of muscle fibre conduction. In patients with negative short exercise tests, symptomatic eye closure myotonia predicted sodium channel myotonia over myotonia congenita. Distinctive ‘tornado-shaped’ neuromyotonia-like discharges may be seen in patients with paramyotonia congenita. In the long exercise test, area decrements from pre-exercise baseline were more sensitive than amplitude decrements-from-maximum-CMAP in patients with Andersen-Tawil syndrome. Possible ethnic differences in the normative data of the long exercise test argue for the use of appropriate ethnically-matched controls.
Concordant CMAP amplitude-and-area decrements of >20% allow more reliable interpretation of the short exercise tests and aid accurate DNA-based diagnosis. In patients with negative exercise tests, specific clinical features are helpful in differentiating sodium from chloride channel myotonia. A modified algorithm is suggested..
PMCID: PMC3051421  PMID: 21387378
5.  Preclinical evaluation of marketed sodium channel blockers in a rat model of myotonia discloses promising antimyotonic drugs 
Experimental Neurology  2014;255(100):96-102.
Although the sodium channel blocker mexiletine is considered the first-line drug in myotonia, some patients experiment adverse effects, while others do not gain any benefit. Other antimyotonic drugs are thus needed to offer mexiletine alternatives. In the present study, we used a previously-validated rat model of myotonia congenita to compare six marketed sodium channel blockers to mexiletine. Myotonia was induced in the rat by injection of anthracen-9-carboxylic acid, a muscle chloride channel blocker. The drugs were given orally and myotonia was evaluated by measuring the time of righting reflex. The drugs were also tested on sodium currents recorded in a cell line transfected with the human skeletal muscle sodium channel hNav1.4 using patch-clamp technique. In vivo, carbamazepine and propafenone showed antimyotonic activity at doses similar to mexiletine (ED50 close to 5 mg/kg); flecainide and orphenadrine showed greater potency (ED50 near 1 mg/kg); lubeluzole and riluzole were the more potent (ED50 near 0.1 mg/kg). The antimyotonic activity of drugs in vivo was linearly correlated with their potency in blocking hNav1.4 channels in vitro. Deviation was observed for propafenone and carbamazepine, likely due to pharmacokinetics and multiple targets. The comparison of the antimyotonic dose calculated in rats with the current clinical dose in humans strongly suggests that all the tested drugs may be used safely for the treatment of human myotonia. Considering the limits of mexiletine tolerability and the occurrence of non-responders, this study proposes an arsenal of alternative drugs, which may prove useful to increase the quality of life of individuals suffering from non-dystrophic myotonia. Further clinical trials are warranted to confirm these results.
•Seven sodium channel blockers show antimyotonic activity in a rat model of myotonia.•The ED50 value ranges from 0.1 (riluzole, lubeluzole) to 5 mg/kg (mexiletine, carbamazepine).•The drugs use-dependently block hNav1.4 channels in cells in myotonic-like conditions.•The IC50 values in vitro were well linearly correlated with the ED50 values in vivo.•The study discloses promising new therapeutic options for myotonic patients.
PMCID: PMC4004800  PMID: 24613829
Non-dystrophic myotonia; Sodium channel blockers; Mexiletine; Rat model; Patch-clamp; Over-excitability
6.  Prevalence study of genetically defined skeletal muscle channelopathies in England 
Neurology  2013;80(16):1472-1475.
To obtain minimum point prevalence rates for the skeletal muscle channelopathies and to evaluate the frequency distribution of mutations associated with these disorders.
Analysis of demographic, clinical, electrophysiologic, and genetic data of all patients assessed at our national specialist channelopathy service. Only patients living in the United Kingdom with a genetically defined diagnosis of nondystrophic myotonia or periodic paralysis were eligible for the study. Prevalence rates were estimated for England, December 2011.
A total of 665 patients fulfilled the inclusion criteria, of which 593 were living in England, giving a minimum point prevalence of 1.12/100,000 (95% confidence interval [CI] 1.03–1.21). Disease-specific prevalence figures were as follows: myotonia congenita 0.52/100,000 (95% CI 0.46–0.59), paramyotonia congenita 0.17/100,000 (95% CI 0.13–0.20), sodium channel myotonias 0.06/100,000 (95% CI 0.04–0.08), hyperkalemic periodic paralysis 0.17/100,000 (95% CI 0.13–0.20), hypokalemic periodic paralysis 0.13/100,000 (95% CI 0.10–0.17), and Andersen-Tawil syndrome (ATS) 0.08/100,000 (95% CI 0.05–0.10). In the whole sample (665 patients), 15 out of 104 different CLCN1 mutations accounted for 60% of all patients with myotonia congenita, 11 out of 22 SCN4A mutations for 86% of paramyotonia congenita/sodium channel myotonia pedigrees, and 3 out of 17 KCNJ2 mutations for 42% of ATS pedigrees.
We describe for the first time the overall prevalence of genetically defined skeletal muscle channelopathies in England. Despite the large variety of mutations observed in patients with nondystrophic myotonia and ATS, a limited number accounted for a large proportion of cases.
PMCID: PMC3662361  PMID: 23516313
7.  Muscle Channelopathies: the Nondystrophic Myotonias and Periodic Paralyses 
Continuum : Lifelong Learning in Neurology  2013;19(6 Muscle Disease):1598-1614.
Supplemental Digital Content is available in the text.
Purpose of Review
The muscle channelopathies are a group of rare inherited diseases caused by mutations in muscle ion channels. Mutations cause an increase or decrease in muscle membrane excitability, leading to a spectrum of related clinical disorders: the nondystrophic myotonias are characterized by delayed relaxation after muscle contraction, causing muscle stiffness and pain; the periodic paralyses are characterized by episodes of flaccid muscle paralysis. This review describes the clinical characteristics, molecular pathogenesis, and treatments of the nondystrophic myotonias and periodic paralyses.
Recent Findings
Advances have been made in both the treatment and our understanding of the molecular pathophysiology of muscle channelopathies: (1) a recent controlled trial showed that mexiletine was effective for reducing symptoms and signs of myotonia in nondystrophic myotonia; (2) the mechanisms by which hypokalemic periodic paralysis leads to a depolarized but unexcitable sarcolemma membrane have been traced to a novel gating pore current; and (3) an association was demonstrated between mutations in a potassium inward rectifier and patients with thyrotoxic periodic paralysis.
The muscle channelopathies are an expanding group of muscle diseases caused by mutations in sodium, chloride, potassium, and calcium ion channels that result in increased or decreased muscle membrane excitability. Recognizing patients with channelopathies and confirming the diagnosis is important, as treatment and management strategies differ based on mutation and clinical phenotype.
PMCID: PMC4234136  PMID: 24305449
8.  Mechanisms of a Human Skeletal Myotonia Produced by Mutation in the C-Terminus of NaV1.4: Is Ca2+ Regulation Defective? 
PLoS ONE  2013;8(12):e81063.
Mutations in the cytoplasmic tail (CT) of voltage gated sodium channels cause a spectrum of inherited diseases of cellular excitability, yet to date only one mutation in the CT of the human skeletal muscle voltage gated sodium channel (hNaV1.4F1705I) has been linked to cold aggravated myotonia. The functional effects of altered regulation of hNaV1.4F1705I are incompletely understood. The location of the hNaV1.4F1705I in the CT prompted us to examine the role of Ca2+ and calmodulin (CaM) regulation in the manifestations of myotonia. To study Na channel related mechanisms of myotonia we exploited the differences in rat and human NaV1.4 channel regulation by Ca2+ and CaM. hNaV1.4F1705I inactivation gating is Ca2+-sensitive compared to wild type hNaV1.4 which is Ca2+ insensitive and the mutant channel exhibits a depolarizing shift of the V1/2 of inactivation with CaM over expression. In contrast the same mutation in the rNaV1.4 channel background (rNaV1.4F1698I) eliminates Ca2+ sensitivity of gating without affecting the CaM over expression induced hyperpolarizing shift in steady-state inactivation. The differences in the Ca2+ sensitivity of gating between wild type and mutant human and rat NaV1.4 channels are in part mediated by a divergence in the amino acid sequence in the EF hand like (EFL) region of the CT. Thus the composition of the EFL region contributes to the species differences in Ca2+/CaM regulation of the mutant channels that produce myotonia. The myotonia mutation F1705I slows INa decay in a Ca2+-sensitive fashion. The combination of the altered voltage dependence and kinetics of INa decay contribute to the myotonic phenotype and may involve the Ca2+-sensing apparatus in the CT of NaV1.4.
PMCID: PMC3855693  PMID: 24324661
9.  Physiology and Pathophysiology of CLC-1: Mechanisms of a Chloride Channel Disease, Myotonia 
The CLC-1 chloride channel, a member of the CLC-channel/transporter family, plays important roles for the physiological functions of skeletal muscles. The opening of this chloride channel is voltage dependent and is also regulated by protons and chloride ions. Mutations of the gene encoding CLC-1 result in a genetic disease, myotonia congenita, which can be inherited as an autosmal dominant (Thomsen type) or an autosomal recessive (Becker type) pattern. These mutations are scattered throughout the entire protein sequence, and no clear relationship exists between the inheritance pattern of the mutation and the location of the mutation in the channel protein. The inheritance pattern of some but not all myotonia mutants can be explained by a working hypothesis that these mutations may exert a “dominant negative” effect on the gating function of the channel. However, other mutations may be due to different pathophysiological mechanisms, such as the defect of protein trafficking to membranes. Thus, the underlying mechanisms of myotonia are likely to be quite diverse, and elucidating the pathophysiology of myotonia mutations will require the understanding of multiple molecular/cellular mechanisms of CLC-1 channels in skeletal muscles, including molecular operation, protein synthesis, and membrane trafficking mechanisms.
PMCID: PMC3237021  PMID: 22187529
10.  Dominantly-Inherited Myotonia Congenita Resulting from a Mutation That Increases Open Probability of the Muscle Chloride Channel CLC-1 
Neuromolecular medicine  2012;14(4):328-337.
Myotonia congenita-inducing mutations in the muscle chloride channel CLC-1 normally result in reduced open probability (Po) of this channel. One well-accepted mechanism of the dominant inheritance of this disease involves a dominant-negative effect of the mutation on the function of the common gate of this homodimeric, double-barreled molecule. We report here a family with myotonia congenita characterized by muscle stiffness and clinical and electrophysiologic myotonic phenomena transmitted in an autosomal dominant pattern. DNA sequencing of DMPK and ZNF9 genes for myotonic muscular dystrophy types I and II was normal, whereas sequencing of CLC-1 encoding gene, CLCN1, identified a single heterozygous missense mutation, G233S. Patch-clamp analyses of this mutant CLC-1 channel in Xenopus oocytes revealed an increased Po of the channel’s fast gate, from ~ 0.4 in the wild type to > 0.9 in the mutant at −90 mV. In contrast, the mutant exhibits a minimal effect on the Po of the common gate. These results are consistent with the structural prediction that the mutation site is adjacent to the fast gate of the channel. Overall, the mutant could lead to a significantly reduced dynamic response of CLC-1 to membrane depolarization, from a 5-fold increase in chloride conductance in the wild type to a 2-fold increase in the mutant—this might result in slower membrane repolarization during an action potential. Since expression levels of the mutant and wild-type subunits in artificial model cell systems were unable to explain the disease symptoms, the mechanism leading to dominant inheritance in this family remains to be determined.
PMCID: PMC3508202  PMID: 22790975
myotonia congenita; muscle; chloride channel; CLCN1; dominant; gain of function
11.  Muscle Chloride Channel Dysfunction in Two Mouse Models of Myotonic Dystrophy 
Muscle degeneration and myotonia are clinical hallmarks of myotonic dystrophy type 1 (DM1), a multisystemic disorder caused by a CTG repeat expansion in the 3′ untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Transgenic mice engineered to express mRNA with expanded (CUG)250 repeats (HSALR mice) exhibit prominent myotonia and altered splicing of muscle chloride channel gene (Clcn1) transcripts. We used whole-cell patch clamp recordings and nonstationary noise analysis to compare and biophysically characterize the magnitude, kinetics, voltage dependence, and single channel properties of the skeletal muscle chloride channel (ClC-1) in individual flexor digitorum brevis (FDB) muscle fibers isolated from 1–3-wk-old wild-type and HSALR mice. The results indicate that peak ClC-1 current density at −140 mV is reduced >70% (−48.5 ± 3.6 and −14.0 ± 1.6 pA/pF, respectively) and the kinetics of channel deactivation increased in FDB fibers obtained from 18–20- d-old HSALR mice. Nonstationary noise analysis revealed that the reduction in ClC-1 current density in HSALR FDB fibers results from a large reduction in ClC-1 channel density (170 ± 21 and 58 ± 11 channels/pF in control and HSALR fibers, respectively) and a modest decrease in maximal channel open probability(0.91 ± 0.01 and 0.75 ± 0.03, respectively). Qualitatively similar results were observed for ClC-1 channel activity in knockout mice for muscleblind-like 1 (Mbnl1ΔE3/ΔE3), a second murine model of DM1 that exhibits prominent myotonia and altered Clcn1 splicing (Kanadia et al., 2003). These results support a molecular mechanism for myotonia in DM1 in which a reduction in both the number of functional sarcolemmal ClC-1 and maximal channel open probability, as well as an acceleration in the kinetics of channel deactivation, results from CUG repeat–containing mRNA molecules sequestering Mbnl1 proteins required for proper CLCN1 pre-mRNA splicing and chloride channel function.
PMCID: PMC2151606  PMID: 17158949
12.  A mutation in a rare type of intron in a sodium-channel gene results in aberrant splicing and causes myotonia 
Human mutation  2011;32(7):773-782.
Many mutations in the skeletal-muscle sodium-channel gene SCN4A have been associated with myotonia and/or periodic paralysis, but so far all of these mutations are located in exons. We found a patient with myotonia caused by a deletion/insertion located in intron 21 of SCN4A, which is an AT-AC type II intron. This is a rare class of introns that, despite having AT-AC boundaries, are spliced by the major or U2-type spliceosome. The patient's skeletal muscle expressed aberrantly spliced SCN4A mRNA isoforms generated by activation of cryptic splice sites. In addition, genetic suppression experiments using an SCN4A minigene showed that the mutant 5′ splice site has impaired binding to the U1 and U6 snRNPs, which are the cognate factors for recognition of U2-type 5′ splice sites. One of the aberrantly spliced isoforms encodes a channel with a 35-amino-acid insertion in the cytoplasmic loop between domains III and IV of Nav1.4. The mutant channel exhibited a marked disruption of fast inactivation, and a simulation in silico showed that the channel defect is consistent with the patient's myotonic symptoms. This is the first report of a disease-associated mutation in an AT-AC type II intron, and also the first intronic mutation in a voltage-gated ion channel gene showing a gain-of-function defect.
PMCID: PMC4109284  PMID: 21412952
skeletal muscle; myotonia; splicing; gain-of-function; simulation; channelopathy
13.  In vivo evaluation of antimyotonic efficacy of β-adrenergic drugs in a rat model of myotonia 
Neuropharmacology  2013;65(C):21-27.
The sodium channel blocker mexiletine is considered the first-line drug in myotonic syndromes, a group of muscle disorders characterized by membrane over-excitability. We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, block voltage-gated sodium channels in a manner reminiscent to mexiletine, whereas salbutamol and nadolol do not. We now developed a pharmacological rat model of myotonia congenita to perform in vivo preclinical test of antimyotonic drugs. Myotonia was induced by i.p. injection of 30 mg/kg of anthracene-9-carboxylic acid (9-AC), a muscle chloride channel blocker, and evaluated by measuring the time of righting reflex (TRR). The TRR was prolonged from <0.5 s in control conditions to a maximum of ∼4 s, thirty minutes after 9-AC injection, then gradually recovered in a few hours. Oral administration of mexiletine twenty minutes after 9-AC injection significantly hampered the TRR prolongation, with an half-maximum efficient dose (ED50) of 12 mg/kg. Both propranolol and clenbuterol produced a dose-dependent antimyotonic effect similar to mexiletine, with ED50 values close to 20 mg/kg. Antimyotonic effects of 40 mg/kg mexiletine and propranolol lasted for 2 h. We also demonstrated, using patch-clamp methods, that both propranolol enantiomers exerted a similar block of skeletal muscle hNav1.4 channels expressed in HEK293 cells. The two enantiomers (15 mg/kg) also showed a similar antimyotonic activity in vivo in the myotonic rat. Among the drugs tested, the R(+)-enantiomer of propranolol may merit further investigation in humans, because it exerts antimyotonic effect in the rat model, while lacking of significant activity on the β-adrenergic pathway. This study provides a new and useful in vivo preclinical model of myotonia congenita in order to individuate the most promising antimyotonic drugs to be tested in humans.
► An in vivo pharmacological model of myotonia congenita was developed in the rat using 9-AC i.p. injection. ► A preclinical screening of antimyotonic drugs was performed. ► Propranolol and clenbuterol exert antimyotonic activity comparable to mexiletine. ► Both propranolol enantiomers block skeletal muscle hNav1.4 sodium channels in vitro. ► Both propranolol enantiomers exert similar antimyotonic effect in vivo.
PMCID: PMC3546166  PMID: 23000075
Myotonia; Over-excitability; Propranolol; Mexiletine; In vivo rat model; hNav1.4; TRR, time of righting reflex; 9-AC, anthracene-9-carboxylic acid
14.  Mexiletine for Symptoms and Signs of Myotonia in Non-Dystrophic Myotonia: A Randomized Controlled Trial 
Non-dystrophic myotonias (NDM) are rare diseases caused by mutations in skeletal muscle ion channels. Patients experience delayed muscle relaxation causing functionally-limiting stiffness and pain. Mexiletine-induced sodium channel blockade reduced myotonia in case studies and one single blind trial. As is common in rare diseases, larger studies of safety and efficacy have not previously been considered feasible.
To determine the effects of mexiletine for symptoms and signs of myotonia in NDM.
Design, Setting, and Participation
Fifty-nine patients with NDM participated in a randomized, double-blind, placebo-controlled two-period crossover study conducted between December 23, 2008 and March 30, 2011 at 7 neuromuscular referral centers in 4 countries, as part of the NIH-funded Rare Disease Clinical Research Network.
Oral 200 mg mexiletine or placebo capsules three times daily for 4 weeks, followed by the opposite intervention for 4 weeks, with 1 week wash-out between periods.
Main Outcome Measures
Patient-reported stiffness recorded on an interactive voice response diary (IVR) was the primary endpoint (1 ‘minimal’ to 9 ‘worst ever experienced’). Secondary endpoints included IVR-reported changes in pain, weakness, and tiredness, clinical myotonia assessment, quantitative grip myotonia, Individualized Neuromuscular Quality of Life (INQoL, percent of maximal detrimental impact), SF-36, electrophysiological exercise testing, and needle EMG.
Mexiletine significantly improved patient-reported stiffness on the IVR. Because of a statistically significant interaction between treatment and period for this outcome, primary endpoint is presented by period (period 1 means were mexiletine 2.53 versus placebo 4.21, difference −1.68, 95% Confidence Interval [CI] −2.66, −0.706, P<0.001; period 2 means were mexiletine 1.60 versus placebo 5.27, difference −3.68, 95% CI −3.85, −0.139, P=0.04). Mexiletine improved the INQoL QOL score (mexiletine 14.0, placebo 16.7, difference −2.69, 95% CI −4.07, −1.30, P<0.001) and decreased handgrip myotonia on clinical exam (seconds: mexiletine 0.164, placebo 0.494, difference −0.330, 95% CI −0.633, −0.142, P<0.001). The most common adverse effect was gastrointestinal (9 mexiletine, 1 placebo). Two participants experienced transient cardiac effects that did not require stopping the study (1 placebo, 1 mexiletine). One serious adverse event was determined to be not study-related.
In this preliminary study of patients with NDM, the use of mexiletine compared with placebo resulted in improved patient-reported stiffness over 4 weeks of treatment, despite some concern about the maintenance of blinding.
Trial Registration identifier: NCT 00832000
PMCID: PMC3564227  PMID: 23032552
15.  Clinical Diversity of SCN4A-Mutation-Associated Skeletal Muscle Sodium Channelopathy 
Background and Purpose
Mutations of the skeletal muscle sodium channel gene SCN4A, which is located on chromosome 17q23-25, are associated with various neuromuscular disorders that are labeled collectively as skeletal muscle sodium channelopathy. These disorders include hyperkalemic periodic paralysis (HYPP), hypokalemic periodic paralysis, paramyotonia congenita (PMC), potassium-aggravated myotonia, and congenital myasthenic syndrome. This study analyzed the clinical and mutational spectra of skeletal muscle sodium channelopathy in Korean subjects.
Six unrelated Korean patients with periodic paralysis or nondystrophic myotonia associated with SCN4A mutations were included in the study. For the mutational analysis of SCN4A, we performed a full sequence analysis of the gene using the patients' DNA. We also analyzed the patients' clinical history, physical findings, laboratory tests, and responses to treatment.
We identified four different mutations (one of which was novel) in all of the patients examined. The novel heterozygous missense mutation, p.R225W, was found in one patient with mild nonpainful myotonia. Our patients exhibited various clinical phenotypes: pure myotonia in four, and PMC in one, and HYPP in one. The four patients with pure myotonia were initially diagnosed as having myotonia congenita (MC), but a previous analysis revealed no CLCN1 mutation.
Clinical differentiating between sodium-channel myotonia (SCM) and MC is not easy, and it is suggested that a mutational analysis of both SCN4A and CLCN1 is essential for the differential diagnosis of SCM and MC.
PMCID: PMC2806541  PMID: 20076800
Wordsaamyotonic disorders; familial periodic paralyses; SCN4A
16.  Phenotypic heterogeneity in skeletal muscle sodium channelopathies: A case report and literature review 
Skeletal muscle sodium channelopathies (SMSCs) including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita (PC), and sodium channel myotonia are caused by sodium channel gene (SCN4A) mutations, with altered sarcolemal excitability, and can present as episodes of skeletal muscle weakness, paralysis, and myotonia. We report a teenage boy, who presented with features of HyperPP, PC, myotonia congenita, and sodium channel myotonia. His electromyography (EMG) revealed myopathic changes, myotonia, and Fournier EMG pattern I, and posed a diagnostic challenge. Genetic analysis showed Thr704Met mutation in SCN4A gene. While with typical clinical phenotypes, the electromyographic patterns can be used to direct genetic testing, atypical phenotypes may pose diagnostic dilemmas. Clinicians dealing with neuromuscular disorders in children need to be aware of the unusual clinical presentations of SMSC, so that focused genetic testing can be carried out.
PMCID: PMC3783724  PMID: 24082935
Allelic disorder; myotonia; periodic paralysis; phenotype-genotype protocols; sodium channelopathy
17.  Skeletal Muscle Channelopathies: New insights into the periodic paralyses and nondystrophic myotonias 
Current opinion in neurology  2009;22(5):524-531.
Purpose of Review
To summarize advances in our understanding of the clinical phenotypes, genetics, and molecular pathophysiology of the periodic paralyses, the nondystrophic myotonias, and other muscle channelopathies.
Recent findings
The number of pathogenic mutations causing periodic paralysis, nondystrophic myotonias, and ryanodinopathies continues to grow with the advent of exon hierarchy analysis strategies for genetic screening and better understanding and recognition of disease phenotypes. Recent studies have expanded and clarified the role of gating pore current in channelopathy pathogenesis. It has been shown that the gating pore current can account for the molecular and phenotypic pathology observed in the muscle sodium channelopathies, and, given that homologous residues are affected in mutations of calcium channels, it is possible that pore leak represents a pathomechanism applicable to many channel diseases. Improvements in treatment of the muscle channelopathies are on the horizon. A randomized controlled trial has been initiated for the study of mexiletine in nondystrophic myotonias. The class IC anti-arrhythmia drug flecainide has been shown to depress ventricular ectopy and improve exercise capacity in patients with Andersen-Tawil syndrome.
Recent studies have expanded our understanding of gating pore current as a disease-causing mechanism in the muscle channelopathies and have allowed new correlations to be drawn between disease genotype and phenotype.
PMCID: PMC2763141  PMID: 19571750
channelopathies; periodic paralysis; myotonia; ryanodine receptor
18.  Novel Mutations in the CLCN1 Gene of Myotonia Congenita: 2 Case Reports 
Introduction: Myotonia Congenita is an inherited myotonia that is due to a mutation in the skeletal muscle chloride channel CLCN1. These mutations lead to reduced sarcolemmal chloride conductance, causing delayed muscle relaxation that is evident as clinical and electrical myotonia.
Methods: We report the clinical presentations of two individuals with Myotonia Congenita (MC).
Results: Patient 1 has been diagnosed with the recessive form of MC, known as the Becker variant, and Patient 2 has been diagnosed with the dominant form of MC, known as the Thomsen variant. In both patients, the diagnosis was made based on the clinical presentation, EMG and CLCN1 gene sequencing. Patient 1 also had a muscle biopsy.
Conclusions: Genetic testing in both patients reveals previously unidentified mutations in the CLCN1 gene specific to Myotonia Congenita. We report the salient clinical features of each patient and discuss the effects and common types of CLCN1 mutations and review the literature.
PMCID: PMC3584487  PMID: 23483815
Myotonia Congenita; Becker variant; Thomsen variant; CLCN1 mutation
19.  Functional characterization of ClC-1 mutations from patients affected by recessive myotonia congenita presenting with different clinical phenotypes☆ 
Experimental Neurology  2013;248(100):530-540.
Myotonia congenita (MC) is caused by loss-of-function mutations of the muscle ClC-1 chloride channel. Clinical manifestations include the variable association of myotonia and transitory weakness. We recently described a cohort of recessive MC patients showing, at a low rate repetitive nerves stimulation protocol, different values of compound muscle action potential (CMAP) transitory depression, which is considered the neurophysiologic counterpart of transitory weakness. From among this cohort, we studied the chloride currents generated by G190S (associated with pronounced transitory depression), F167L (little or no transitory depression), and A531V (variable transitory depression) hClC-1 mutants in transfected HEK293 cells using patch-clamp. While F167L had no effect on chloride currents, G190S dramatically shifts the voltage dependence of channel activation and A531V reduces channel expression. Such variability in molecular mechanisms observed in the hClC-1 mutants may help to explain the different clinical and neurophysiologic manifestations of each ClCN1 mutation. In addition we examined five different mutations found in compound heterozygosis with F167L, including the novel P558S, and we identified additional molecular defects. Finally, the G190S mutation appeared to impair acetazolamide effects on chloride currents in vitro.
•Myotonia congenita is a muscle disorder due to mutations in ClC-1 chloride channel.•Eight ClC-1 channel mutants were studied using patch-clamp technique.•Mutations induce a variety of molecular defects in ClC-1 channel function.•We discuss the relationship between genotype and clinical phenotype.
PMCID: PMC3781327  PMID: 23933576
Acetazolamide; Chloride channel mutation; ClC-1 chloride channel; Genotype–phenotype relationship; Myotonia congenita; Non-dystrophic myotonia; Patch-clamp; Transitory weakness
20.  A sodium channel knockin mutant (NaV1.4-R669H) mouse model of hypokalemic periodic paralysis  
The Journal of Clinical Investigation  2011;121(10):4082-4094.
Hypokalemic periodic paralysis (HypoPP) is an ion channelopathy of skeletal muscle characterized by attacks of muscle weakness associated with low serum K+. HypoPP results from a transient failure of muscle fiber excitability. Mutations in the genes encoding a calcium channel (CaV1.1) and a sodium channel (NaV1.4) have been identified in HypoPP families. Mutations of NaV1.4 give rise to a heterogeneous group of muscle disorders, with gain-of-function defects causing myotonia or hyperkalemic periodic paralysis. To address the question of specificity for the allele encoding the NaV1.4-R669H variant as a cause of HypoPP and to produce a model system in which to characterize functional defects of the mutant channel and susceptibility to paralysis, we generated knockin mice carrying the ortholog of the gene encoding the NaV1.4-R669H variant (referred to herein as R669H mice). Homozygous R669H mice had a robust HypoPP phenotype, with transient loss of muscle excitability and weakness in low-K+ challenge, insensitivity to high-K+ challenge, dominant inheritance, and absence of myotonia. Recovery was sensitive to the Na+/K+-ATPase pump inhibitor ouabain. Affected fibers had an anomalous inward current at hyperpolarized potentials, consistent with the proposal that a leaky gating pore in R669H channels triggers attacks, whereas a reduction in the amplitude of action potentials implies additional loss-of-function changes for the mutant NaV1.4 channels.
PMCID: PMC3195470  PMID: 21881211
21.  Double trouble in a patient with myotonia 
BMJ Case Reports  2013;2013:bcr2012008167.
Non-dystrophic myotonias (NDM) are characterised by muscle stiffness during voluntary movement owing to delayed skeletal muscle relaxation caused by mutations in the chloride (CLCN1) and sodium (SCN4A) skeletal muscle channel genes. Late onset acid maltase deficiency (AMD) is characterised by progressive respiratory and proximal muscle weakness; electrical but not clinical myotonia can be observed. Case report of a unique patient with concurrent NDM and AMD. We describe the clinical presentation and management of a patient with two rare neuromuscular disorders. This case illustrates the importance of reopening the differential diagnosis in patients who do not conform to the typical natural history of a specific disease.
PMCID: PMC3604294  PMID: 23417379
22.  Cold-Induced Defects of Sodium Channel Gating in Atypical Periodic Paralysis Plus Myotonia 
Neurology  2007;70(10):755-761.
Missense mutations of the skeletal muscle voltage-gated sodium channel (NaV1.4) are an established cause of several clinically distinct forms of periodic paralysis and myotonia. The mechanistic basis for the phenotypic variability of these allelic disorders of muscle excitability remains unknown. An atypical phenotype with cold-induced hypokalemic paralysis and myotonia at warm temperatures was reported to segregate with the P1158S mutation.
This study extends the functional characterization of the P1158S mutation and tests the specific hypothesis that impairment of Na channel slow inactivation is a common feature of periodic paralysis.
Mutant NaV1.4 channels (P1158S) were transiently expressed in HEK cells and characterized by voltage-clamp studies of Na currents.
Wildtype and P1158S channels displayed comparable behavior at 37 °C, but upon cooling to 25 °C mutant channels activated at more negative potentials and slow inactivation was destabilized.
Consistent with other NaV1.4 mutations associated with a paralytic phenotype, the P1158S mutation disrupts slow inactivation. The unique temperature sensitivity of the channel defect may contribute to the unusual clinical phenotype.
PMCID: PMC4094148  PMID: 17898326
NaV1.4; channelopathy; skeletal muscle; human
23.  Myotonia Congenita Mutation Enhances the Degradation of Human CLC-1 Chloride Channels 
PLoS ONE  2013;8(2):e55930.
Myotonia congenita is a hereditary muscle disorder caused by mutations in the human voltage-gated chloride (Cl−) channel CLC-1. Myotonia congenita can be inherited in an autosomal recessive (Becker type) or dominant (Thomsen type) fashion. One hypothesis for myotonia congenita is that the inheritance pattern of the disease is determined by the functional consequence of the mutation on the gating of CLC-1 channels. Several disease-related mutations, however, have been shown to yield functional CLC-1 channels with no detectable gating defects. In this study, we have functionally and biochemically characterized a myotonia mutant: A531V. Despite a gating property similar to that of wild-type (WT) channels, the mutant CLC-1 channel displayed a diminished whole-cell current density and a reduction in the total protein expression level. Our biochemical analyses further demonstrated that the reduced expression of A531V can be largely attributed to an enhanced proteasomal degradation as well as a defect in protein trafficking to surface membranes. Moreover, the A531V mutant protein also appeared to be associated with excessive endosomal-lysosomal degradation. Neither the reduced protein expression nor the diminished current density was rescued by incubating A531V-expressing cells at 27°C. These results demonstrate that the molecular pathophysiology of A531V does not involve anomalous channel gating, but rather a disruption of the balance between the synthesis and degradation of the CLC-1 channel protein.
PMCID: PMC3570542  PMID: 23424641
24.  Involvement of Helices at the Dimer Interface in ClC-1 Common Gating 
The Journal of General Physiology  2003;121(2):149-161.
ClC-1 is a dimeric, double-pored chloride channel that is present in skeletal muscle. Mutations of this channel can result in the condition myotonia, a muscle disorder involving increased muscle stiffness. It has been shown that the dominant form of myotonia often results from mutations that affect the so-called slow, or common, gating process of the ClC-1 channel. Mutations causing dominant myotonia are seen to cluster at the interface of the ClC-1 channel monomers. This study has investigated the role of the H, I, P, and Q helices, which lie on this interface, as well as the G helix, which is situated immediately behind the H and I helices, on ClC-1 gating. 11 mutant ClC-1 channels (T268M, C277S, C278S, S289A, T310M, S312A, V321S, T539A, S541A, M559T, and S572V) were produced using site-directed mutagenesis, and gating properties of these channels were investigated using electrophysiological techniques. Six of the seven mutations in G, H, and I, and two of the four mutations in P and Q, caused shifts of the ClC-1 open probability. In the majority of cases this was due to alterations in the common gating process, with only three of the mutants displaying any change in fast gating. Many of the mutant channels also showed alterations in the kinetics of the common gating process, particularly at positive potentials. The changes observed in common gating were caused by changes in the opening rate (e.g. T310M), the closing rate (e.g. C277S), or both rates. These results indicate that mutations in the helices forming the dimer interface are able to alter the ClC-1 common gating process by changing the energy of the open and/or closed channel states, and hence altering transition rates between these states.
PMCID: PMC2217322  PMID: 12566541
chloride channel; mutation; patch-clamping; myotonia
25.  A Novel Mutation in CLCN1 Associated with Feline Myotonia Congenita 
PLoS ONE  2014;9(10):e109926.
Myotonia congenita (MC) is a skeletal muscle channelopathy characterized by inability of the muscle to relax following voluntary contraction. Worldwide population prevalence in humans is 1∶100,000. Studies in mice, dogs, humans and goats confirmed myotonia associated with functional defects in chloride channels and mutations in a skeletal muscle chloride channel (CLCN1). CLCN1 encodes for the most abundant chloride channel in the skeletal muscle cell membrane. Five random bred cats from Winnipeg, Canada with MC were examined. All cats had a protruding tongue, limited range of jaw motion and drooling with prominent neck and proximal limb musculature. All cats had blepharospasm upon palpebral reflex testing and a short-strided gait. Electromyograms demonstrated myotonic discharges at a mean frequency of 300 Hz resembling the sound of a ‘swarm of bees’. Muscle histopathology showed hypertrophy of all fiber types. Direct sequencing of CLCN1 revealed a mutation disrupting a donor splice site downstream of exon 16 in only the affected cats. In vitro translation of the mutated protein predicted a premature truncation and partial lack of the highly conserved CBS1 (cystathionine β-synthase) domain critical for ion transport activity and one dimerization domain pivotal in channel formation. Genetic screening of the Winnipeg random bred population of the cats' origin identified carriers of the mutation. A genetic test for population screening is now available and carrier cats from the feral population can be identified.
PMCID: PMC4214686  PMID: 25356766

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