Sodium channel slow inactivation as a therapeutic target for myotonia congenita

BK channels promote action potential repolarization in skeletal muscle but contribute little to myotonia

Patients with myotonia congenita suffer from slowed relaxation of muscle (myotonia), due to hyperexcitability caused by loss-of-function mutations in the ClC-1 chloride channel. A recent study suggested that block of large-conductance voltage- and Ca2+- activated K+ channels (BK) may be effective as therapy. The mechanism underlying efficacy was suggested to be lessening of the depolarizing effect of build-up of K+ in t-tubules of muscle during repetitive firing. BK channels are widely expressed in the nervous system and have been shown to play a central role in regulation of excitability, but their contribution to muscle excitability has not been determined. We performed intracellular recordings as well as force measurements in both wild type and BK-/- mouse extensor digitorum longus muscles. Action potential width was increased in BK-/- muscle due to slowing of repolarization, consistent with the possibility K+ build-up in t-tubules is lessened by block of BK channels in myotonic muscle. However, there was no difference in the severity of myotonia triggered by block of muscle Cl- channels with 9-anthracenecarboxylic acid (9AC) in wild type and BK-/- muscle fibers. Further study revealed no difference in the interspike membrane potential during repetitive firing suggesting there was no reduction in K+ build-up in t-tubules of BK-/- muscle. Force recordings following block of muscle Cl- channels demonstrated little reduction in myotonia in BK-/- muscle. In contrast, the current standard of care, mexiletine, significantly reduced myotonia. Our data suggest BK channels regulate muscle excitability, but are not an attractive target for therapy of myotonia.

Reduced K+ build-up in t-tubules contributes to resistance of the diaphragm to myotonia

Patients with myotonia congenita suffer from slowed muscle relaxation caused by hyperexcitability. The diaphragm is only mildly affected in myotonia congenita; discovery of the mechanism underlying its resistance to myotonia could identify novel therapeutic targets. Intracellular recordings from two mouse models of myotonia congenita revealed the diaphragm had less myotonia than either the extensor digitorum longus (EDL) or the soleus muscles. A mechanism contributing to resistance of the diaphragm to myotonia was reduced depolarization of the interspike membrane potential during repetitive firing of action potentials, a process driven by build-up of K+ in small invaginations of muscle membrane known as t-tubules. We explored differences between diaphragm and EDL that might underlie reduction of K+ build-up in diaphragm t-tubules. Smaller size of diaphragm fibres, which promotes diffusion of K+ out of t-tubules, was identified as a contributor. Intracellular recording revealed slower repolarization of action potentials in diaphragm suggesting reduced Kv conductance. Higher resting membrane conductance was identified suggesting increased Kir conductance. Computer simulation found that a reduction of Kv conductance had little effect on K+ build-up whereas increased Kir conductance lessened build-up, although the effect was modest. Our data and computer simulation suggest opening of K+ channels during action potentials has little effect on K+ build-up whereas opening of K+ channels during the interspike interval slightly lessens K+ build-up. We conclude that activation of K+ channels may lessen myotonia by opposing depolarization to action potential threshold without worsening K+ build-up in t-tubules. KEY POINTS: In mouse models of the muscle disease myotonia congenita, the diaphragm has much less myotonia (muscle stiffness) than the extensor digitorum longus or soleus muscles. Identifying why the diaphragm is resistant to myotonia may help in developing novel therapy. We found the reason the diaphragm has less myotonia is that it is less prone to depolarization caused by K+ build-up in t-tubules during repetitive firing of action potentials. Smaller fibre size contributes to resistance to K+ build-up with differences in K+ currents playing little role. Our data suggest drugs that open K+ channels may be effective in treating myotonia as they may lessen excitability without worsening K+ build-up in t-tubules.

Preclinical study of the antimyotonic efficacy of safinamide in the myotonic mouse model

Mexiletine is the first choice drug in the treatment of non-dystrophic myotonias. However, 30% of patients experience little benefit from mexiletine due to poor tolerability, contraindications and limited efficacy likely based on pharmacogenetic profile. Safinamide inhibits neuronal voltage-gated sodium and calcium channels and shows anticonvulsant activity, in addition to a reversible monoamine oxidase-B inhibition. We evaluated the preclinical effects of safinamide in an animal model of Myotonia Congenita, the ADR (arrested development of righting response) mouse. In vitro studies were performed using the two intracellular microelectrodes technique in current clamp mode. We analyzed sarcolemma excitability in skeletal muscle fibers isolated from male and female ADR (adr/adr) and from Wild-Type (wt/wt) mice, before and after the application of safinamide and the reference compound mexiletine. In ADR mice, the maximum number of action potentials (N-spikes) elicited by a fixed current is higher with respect to that of WT mice. Myotonic muscles show an involuntary firing of action potential called after-discharges. A more potent activity of safinamide compared to mexiletine has been demonstrated in reducing N-spikes and the after-discharges in myotonic muscle fibers. The time of righting reflex (TRR) before and after administration of safinamide and mexiletine was evaluated in vivo in ADR mice. Safinamide was able to reduce the TRR in ADR mice to a greater extent than mexiletine. In conclusion, safinamide counteracted the abnormal muscle hyperexcitability in myotonic mice both in vitro and in vivo suggesting it as an effective drug to be indicated in Myotonia Congenita.

Periodic paralysis

Periodic paralysis is a rare, dominantly inherited disorder of skeletal muscle in which episodic attacks of weakness are caused by a transient impairment of fiber excitability. Attacks of weakness are often elicited by characteristic environmental triggers, which were the basis for clinically delineating subtypes of periodic paralysis and are an important distinction for optimal disease management. All forms of familial periodic paralysis are caused by mutations of ion channels, often selectively expressed in skeletal muscle, that destabilize the resting potential. The missense mutations usually alter channel function through gain-of-function changes rather than producing a complete loss-of-function null. The knowledge of which channel gene harbors a variant, whether that variant is expected to (or known to) alter function, and how altered function impairs fiber excitability aides in the interpretation of patient signs and symptoms, the interpretation of gene test results, and how to optimize therapeutic intervention for symptom management and improve quality of life.

Plateau potentials contribute to myotonia in mouse models of myotonia congenita

It has long been accepted that myotonia (muscle stiffness) in patients with muscle channelopathies is due to myotonic discharges (involuntary firing of action potentials). In a previous study, we identified a novel phenomenon in myotonic muscle: development of plateau potentials, transient depolarizations to near -35 mV lasting for seconds to minutes. In the current study we examined whether plateau potentials contribute to myotonia. A recessive genetic model (ClCadr mice) with complete loss of muscle chloride channel (ClC-1) function was used to model severe myotonia congenita with complete loss of ClC-1 function and a pharmacologic model using anthracene-9-carboxylic acid (9 AC) was used to model milder myotonia congenita with incomplete loss of ClC-1 function. Simultaneous measurements of action potentials and myoplasmic Ca2+ from individual muscle fibers were compared to recordings of whole muscle force generation. In ClCadr muscle both myotonia and plateau potentials lasted 10s of seconds to minutes. During plateau potentials lasting 1-2 min, there was a gradual transition from high to low intracellular Ca2+, suggesting a transition in individual fibers from myotonia to flaccid paralysis in severe myotonia congenita. In 9 AC-treated muscles, both myotonia and plateau potentials lasted only a few seconds and Ca2+ remained elevated during the plateau potentials, suggesting plateau potentials contribute to myotonia without causing weakness. We propose, that in myotonic muscle, there is a novel state in which there is contraction in the absence of action potentials. This discovery provides a mechanism to explain reports of patients with myotonia who suffer from electrically silent muscle contraction lasting minutes.

Drug repurposing in skeletal muscle ion channelopathies

Skeletal muscle ion channelopathies are rare genetic diseases mainly characterized by myotonia (muscle stiffness) or periodic paralysis (muscle weakness). Here, we reviewed the available therapeutic options in non-dystrophic myotonias (NDM) and periodic paralyses (PP), which consists essentially in drug repositioning to address stiffness or weakness attacks. Empirical use followed by successful randomized clinical trials eventually led to the orphan drug designation and marketing authorization granting of mexiletine for NDM and dichlorphenamide for PP. Yet, these treatments neither consider the genetic cause of the diseases nor address the individual variability in drug response. Thus, ongoing research aims at the identification of repurposed drugs alternative to mexiletine and dichlorphenamide to allow personalization of treatment. This review highlights how drug repurposing may represent an efficient strategy in rare diseases, allowing reduction of drug development time and costs in a context in which the return on investment may be particularly challenging.

Blockers of Skeletal Muscle Nav1.4 Channels: From Therapy of Myotonic Syndrome to Molecular Determinants of Pharmacological Action and Back

The voltage-gated sodium channels represent an important target for drug discovery since a large number of physiological processes are regulated by these channels. In several excitability disorders, including epilepsy, cardiac arrhythmias, chronic pain, and non-dystrophic myotonia, blockers of voltage-gated sodium channels are clinically used. Myotonia is a skeletal muscle condition characterized by the over-excitability of the sarcolemma, resulting in delayed relaxation after contraction and muscle stiffness. The therapeutic management of this disorder relies on mexiletine and other sodium channel blockers, which are not selective for the Na_v1.4 skeletal muscle sodium channel isoform. Hence, the importance of deepening the knowledge of molecular requirements for developing more potent and use-dependent drugs acting on Na_v1.4. Here, we review the available treatment options for non-dystrophic myotonia and the structure-activity relationship studies performed in our laboratory with a focus on new compounds with potential antimyotonic activity.

Clinical comparison and functional study of the L703P: a recurrent mutation in human SCN4A that causes sodium channel myotonia

The non-dystrophic myotonias are inherited skeletal muscle disorders characterized by skeletal muscle stiffness after voluntary contraction, without muscle atrophy. Based on their clinical features, non-dystrophic myotonias are classified into myotonia congenita, paramyotonia congenita, and sodium channel myotonia. Using whole-exome next-generation sequencing, we identified a L703P mutation (c.2108T>C, p.L703P) in SCN4A in a Chinese family diagnosed with non-dystrophic myotonias. The clinical findings of patients in this family included muscle stiffness and hypertrophy. The biophysical properties of wildtype and mutant channels were investigated using whole-cell patch clamp. L703P causes both gain-of-function and loss-of-function changes in Nav1.4 properties, including decreased current density, impaired recovery, enhanced activation and slow inactivation. Our study demonstrates that L703P is a pathogenic variant for myotonia, and provides additional electrophysiological information for understanding the pathogenic mechanism of SCN4A-associated channelopathies.

A novel mutation of the CLCN1 gene in a cat with myotonia congenita: Diagnosis and treatment

Case Description

A 10‐month‐old castrated male domestic longhair cat was evaluated for increasing frequency of episodic limb rigidity.

Clinical Findings

The cat presented for falling over and lying recumbent with its limbs in extension for several seconds when startled or excited. Upon examination, the cat had hypertrophied musculature, episodes of facial spasm, and a short‐strided, stiff gait.

Diagnostics

Electromyography (EMG) identified spontaneous discharges that waxed and waned in amplitude and frequency, consistent with myotonic discharges. A high impact 8‐base pair (bp) deletion across the end of exon 3 and intron 3 of the chloride voltage‐gated channel 1 (CLCN1) gene was identified using whole genome sequencing.

Treatment and Outcome

Phenytoin treatment was initiated at 3 mg/kg po q24 h and resulted in long‐term improvement.

Clinical Relevance

This novel mutation within the CLCN1 gene is a cause of myotonia congenita in cats and we report for the first time its successful treatment.

Open-label pilot study of ranolazine for cramps in amyotrophic lateral sclerosis

INTRODUCTION/AIMS

Neuronal hyperexcitability (manifested by cramps) plays a pathological role in amyotrophic lateral sclerosis (ALS), and drugs affecting it may help symptomatic management and slow disease progression. We aimed to determine safety and tolerability of two doses of ranolazine in patients with ALS and evaluate for preliminary evidence of drug-target engagement by assessing muscle cramp characteristics.

METHODS

We performed an open-label dose-ascending study of ranolazine in 14 individuals with ALS in two sequential cohorts: 500 mg (cohort 1) and 1000 mg (cohort 2) orally twice daily. Each had a 2-week run-in period, 4-week drug administration, and 6-week safety follow-up. Primary outcome was safety and tolerability. Exploratory measures included cramp frequency and severity, fasciculation frequency, cramp potential duration, ALS Functional Rating Scale---Revised score, and forced vital capacity.

RESULTS

Six and eight participants were enrolled in cohorts 1 and 2, respectively. There were no serious adverse events. Two subjects in cohort 2 discontinued the drug due to constipation. The most frequent drug-related adverse event was gastrointestinal (40%). Cramp frequency decreased by 54.8% (95% confidence interval [CI], 39%-70.8%) and severity decreased by 46.3% (95% CI, 29.5-63.3%), which appeared to be dose-dependent, with decreased awakening due to cramps. Other outcomes showed no change.

DISCUSSION

Ranolazine was well tolerated in ALS up to 2000 mg/day, with gastrointestinal side effects being the most frequent. Ranolazine reduced cramp frequency and severity, supporting its investigation for muscle cramps in a future placebo-controlled trial.

The Donnan-dominated resting state of skeletal muscle fibers contributes to resilience and longevity in dystrophic fibers

Duchenne muscular dystrophy (DMD) is an X-linked dystrophin-minus muscle-wasting disease. Ion homeostasis in skeletal muscle fibers underperforms as DMD progresses. But though DMD renders these excitable cells intolerant of exertion, sodium overloaded, depolarized, and spontaneously contractile, they can survive for several decades. We show computationally that underpinning this longevity is a strikingly frugal, robust Pump-Leak/Donnan (P-L/D) ion homeostatic process. Unlike neurons, which operate with a costly “Pump-Leak–dominated” ion homeostatic steady state, skeletal muscle fibers operate with a low-cost “Donnan-dominated” ion homeostatic steady state that combines a large chloride permeability with an exceptionally small sodium permeability. Simultaneously, this combination keeps fiber excitability low and minimizes pump expenditures. As mechanically active, long-lived multinucleate cells, skeletal muscle fibers have evolved to handle overexertion, sarcolemmal tears, ischemic bouts, etc.; the frugality of their Donnan dominated steady state lets them maintain the outsized pump reserves that make them resilient during these inevitable transient emergencies. Here, P-L/D model variants challenged with DMD-type insult/injury (low pump-strength, overstimulation, leaky Nav and cation channels) show how chronic “nonosmotic” sodium overload (observed in DMD patients) develops. Profoundly severe DMD ion homeostatic insult/injury causes spontaneous firing (and, consequently, unwanted excitation–contraction coupling) that elicits cytotoxic swelling. Therefore, boosting operational pump-strength and/or diminishing sodium and cation channel leaks should help extend DMD fiber longevity.

Ranolazine: An Old Drug with Emerging Potential; Lessons from Pre-Clinical and Clinical Investigations for Possible Repositioning

Ischemic heart disease is a significant public health problem with high mortality and morbidity. Extensive scientific investigations from basic sciences to clinics revealed multilevel alterations from metabolic imbalance, altered electrophysiology, and defective Ca²⁺/Na⁺ homeostasis leading to lethal arrhythmias. Despite the recent identification of numerous molecular targets with potential therapeutic interest, a pragmatic observation on the current pharmacological R&D output confirms the lack of new therapeutic offers to patients. By contrast, from recent trials, molecules initially developed for other fields of application have shown cardiovascular benefits, as illustrated with some anti-diabetic agents, regardless of the presence or absence of diabetes, emphasizing the clear advantage of “old” drug repositioning. Ranolazine is approved as an antianginal agent and has a favorable overall safety profile. This drug, developed initially as a metabolic modulator, was also identified as an inhibitor of the cardiac late Na⁺ current, although it also blocks other ionic currents, including the hERG/Ikr K⁺ current. The latter actions have been involved in this drug’s antiarrhythmic effects, both on supraventricular and ventricular arrhythmias (VA). However, despite initial enthusiasm and promising development in the cardiovascular field, ranolazine is only authorized as a second-line treatment in patients with chronic angina pectoris, notwithstanding its antiarrhythmic properties. A plausible reason for this is the apparent difficulty in linking the clinical benefits to the multiple molecular actions of this drug. Here, we review ranolazine’s experimental and clinical knowledge on cardiac metabolism and arrhythmias. We also highlight advances in understanding novel effects on neurons, the vascular system, skeletal muscles, blood sugar control, and cancer, which may open the way to reposition this “old” drug alone or in combination with other medications.

What Is in the Myopathy Literature?

ABSTRACT

This edition concentrates on inflammatory myopathies with reports of reclassification of polymyositis, cancer associations, evaluation of subclinical cardiac involvement, myositis-specific and -associated antibodies, and immune checkpoint inhibitor myositis. A number of reports address sporadic late-onset nemaline myopathy and point out its diagnostic difficulty and the importance of identifying an associated monoclonal gammopathy that is likely of clinical significance and may warrant aggressive immunotherapy. Finally, treatment of nondystrophic channelopathies is addressed.

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

Ion Channel Gene Mutations Causing Skeletal Muscle Disorders: Pathomechanisms and Opportunities for Therapy

Skeletal muscle ion channelopathies (SMICs) are a large heterogeneous group of rare genetic disorders caused by mutations in genes encoding ion channel subunits in the skeletal muscle mainly characterized by myotonia or periodic paralysis, potentially resulting in long-term disabilities. However, with the development of new molecular technologies, new genes and new phenotypes, including progressive myopathies, have been recently discovered, markedly increasing the complexity in the field. In this regard, new advances in SMICs show a less conventional role of ion channels in muscle cell division, proliferation, differentiation, and survival. Hence, SMICs represent an expanding and exciting field. Here, we review current knowledge of SMICs, with a description of their clinical phenotypes, cellular and molecular pathomechanisms, and available treatments.

Skeletal Muscle Metabolism: Origin or Prognostic Factor for Amyotrophic Lateral Sclerosis (ALS) Development?

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive and selective loss of motor neurons, amyotrophy and skeletal muscle paralysis usually leading to death due to respiratory failure. While generally considered an intrinsic motor neuron disease, data obtained in recent years, including our own, suggest that motor neuron protection is not sufficient to counter the disease. The dismantling of the neuromuscular junction is closely linked to chronic energy deficit found throughout the body. Metabolic (hypermetabolism and dyslipidemia) and mitochondrial alterations described in patients and murine models of ALS are associated with the development and progression of disease pathology and they appear long before motor neurons die. It is clear that these metabolic changes participate in the pathology of the disease. In this review, we summarize these changes seen throughout the course of the disease, and the subsequent impact of glucose–fatty acid oxidation imbalance on disease progression. We also highlight studies that show that correcting this loss of metabolic flexibility should now be considered a major goal for the treatment of ALS.

The mechanism underlying transient weakness in myotonia congenita

In addition to the hallmark muscle stiffness, patients with recessive myotonia congenita (Becker disease) experience debilitating bouts of transient weakness that remain poorly understood despite years of study. We performed intracellular recordings from muscle of both genetic and pharmacologic mouse models of Becker disease to identify the mechanism underlying transient weakness. Our recordings reveal transient depolarizations (plateau potentials) of the membrane potential to -25 to -35 mV in the genetic and pharmacologic models of Becker disease. Both Na+ and Ca2+ currents contribute to plateau potentials. Na+ persistent inward current (NaPIC) through NaV1.4 channels is the key trigger of plateau potentials and current through CaV1.1 Ca2+ channels contributes to the duration of the plateau. Inhibiting NaPIC with ranolazine prevents the development of plateau potentials and eliminates transient weakness in vivo. These data suggest that targeting NaPIC may be an effective treatment to prevent transient weakness in myotonia congenita.

Buprenorphine may be effective for treatment of paramyotonia congenita

INTRODUCTION/AIMS

Paramyotonia congenita (PMC) is a skeletal muscle sodium channelopathy characterized by paradoxical myotonia, cold sensitivity, and exercise/cold-induced paralysis. Treatment with sodium-channel-blocking antiarrhythmic agents may expose patients to a risk of arrhythmia or may be poorly tolerated or ineffective. In this study we explored the effectiveness of non-antiarrhythmic sodium-channel blockers in two patients with PMC.

METHODS

Earlier treatment with mexiletine was discontinued for gastrointestinal side effects in one of the patients and lack of clinical benefit in the other. One patient received lacosamide, ranolazine, and buprenorphine, and the other was given buprenorphine only. Drug efficacy was assessed by clinical scores, timed tests, and by long and short exercise tests.

RESULTS

In both patients, buprenorphine improved pain scores by at least 50%, stiffness and weakness levels, and handgrip/eyelid-opening times. The fall in compound muscle action potential (CMAP) during short exercise normalized in both patients at baseline, and improved after cooling. During long exercise, one patient showed an earlier recovery of CMAP, and the other patient had a less severe decrease (<60%). With buprenorphine, the fall in CMAP induced by cooling normalized in one patient (from -72% to -4%) and improved (from -49% to -37%) in the other patient.

DISCUSSION

Buprenorphine showed promising results for the treatment of exercise-induced paralysis and cold intolerance in the two patients assessed. The exercise test may be useful for quantitative assessment of treatment response. Further studies on a larger number of patients, under carefully controlled conditions, should be considered to address the effectiveness and long-term tolerability of this therapeutic option.

Targeted Therapies for Skeletal Muscle Ion Channelopathies: Systematic Review and Steps Towards Precision Medicine

BACKGROUND

Skeletal muscle ion channelopathies include non-dystrophic myotonias (NDM), periodic paralyses (PP), congenital myasthenic syndrome, and recently identified congenital myopathies. The treatment of these diseases is mainly symptomatic, aimed at reducing muscle excitability in NDM or modifying triggers of attacks in PP.

OBJECTIVE

This systematic review collected the evidences regarding effects of pharmacological treatment on muscle ion channelopathies, focusing on the possible link between treatments and genetic background.

METHODS

We searched databases for randomized clinical trials (RCT) and other human studies reporting pharmacological treatments. Preclinical studies were considered to gain further information regarding mutation-dependent drug effects. All steps were performed by two independent investigators, while two others critically reviewed the entire process.

RESULTS

For NMD, RCT showed therapeutic benefits of mexiletine and lamotrigine, while other human studies suggest some efficacy of various sodium channel blockers and of the carbonic anhydrase inhibitor (CAI) acetazolamide. Preclinical studies suggest that mutations may alter sensitivity of the channel to sodium channel blockers in vitro, which has been translated to humans in some cases. For hyperkalemic and hypokalemic PP, RCT showed efficacy of the CAI dichlorphenamide in preventing paralysis. However, hypokalemic PP patients carrying sodium channel mutations may have fewer benefits from CAI compared to those carrying calcium channel mutations. Few data are available for treatment of congenital myopathies.

CONCLUSIONS

These studies provided limited information about the response to treatments of individual mutations or groups of mutations. A major effort is needed to perform human studies for designing a mutation-driven precision medicine in muscle ion channelopathies.

Paxilline Prevents the Onset of Myotonic Stiffness in Pharmacologically Induced Myotonia: A Preclinical Investigation

Reduced Cl⁻ conductance causes inhibited muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. This represents the pathomechanism of myotonia congenita. Due to the prevailing data suggesting that an increased potassium level is a main contributor, we studied the effect of a modulator of a big conductance Ca²⁺- and voltage-activated K⁺ channels (BK) modulator on contraction and relaxation of slow- and high-twitch muscle specimen before and after the pharmacological induction of myotonia. Human and murine muscle specimens (wild-type and BK^−/−) were exposed to anthracene-9-carboxylic acid (9-AC) to inhibit CLC-1 chloride channels and to induce myotonia in-vitro. Functional effects of BK-channel activation and blockade were investigated by exposing slow-twitch (soleus) and fast-twitch (extensor digitorum longus) murine muscle specimens or human musculus vastus lateralis to an activator (NS1608) and a blocker (Paxilline), respectively. Muscle-twitch force and relaxation times (T_90/10) were monitored. Compared to wild type, fast-twitch muscle specimen of BK^−/− mice resulted in a significantly decreased T_90/10 in presence of 9-AC. Paxilline significantly shortened T_90/10 of murine slow- and fast-twitch muscles as well as human vastus lateralis muscle. Moreover, twitch force was significantly reduced after application of Paxilline in myotonic muscle. NS1608 had opposite effects to Paxilline and aggravated the onset of myotonic activity by prolongation of T_90/10. The currently used standard therapy for myotonia is, in some individuals, not very effective. This in vitro study demonstrated that a BK channel blocker lowers myotonic stiffness and thus highlights its potential therapeutic option in myotonia congenital (MC).

Guidelines on clinical presentation and management of nondystrophic myotonias

The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications.

Treatment Updates for Neuromuscular Channelopathies

PURPOSE OF REVIEW

This article aims to review the current and upcoming treatment options of primary muscle channelopathies including the non-dystrophic myotonias and periodic paralyses.

RECENT FINDINGS

The efficacy of mexiletine in the treatment of myotonia is now supported by two randomised placebo-controlled trials, one of which utilised a novel aggregated n-of-1 design. This has resulted in licencing of the drug via orphan drug status. There is also good evidence that mexiletine is well tolerated and safe in this patient group without the need for intensive monitoring. A range of alternative antimyotonic treatment options include lamotrigine, carbamazepine and ranolazine exist with variable evidence base. In vitro studies have shown insight into reasons for treatment failure of some medications with certain genotypes opening the era of mutation-specific therapy such as use of flecainide. In the periodic paralyses, the ability of MRI to distinguish between reversible oedema and irreversible fatty replacement makes it an increasingly useful tool to guide and assess pharmacological treatment. Unfortunately, the striking efficacy of bumetanide in hypokalaemic periodic paralysis animal models was not replicated in a recent pilot study in humans.

SUMMARY

The treatment of skeletal muscle channelopathies combines dietary and lifestyle advice together with pharmacological interventions. The rarity of these conditions remains a barrier for clinical studies but the example of the aggregated n-of-1 trial of mexiletine shows that innovative trial design can overcome these hurdles. Further research is required to test efficacy of drugs shown to have promising characteristics in preclinical experiments such as safinamide, riluzule and magnesium for myotonia or bumetanide for hypokalaemic periodic paralysis.

Preclinical pharmacological in vitro investigations on low chloride conductance myotonia: effects of potassium regulation

In myotonia, reduced Cl^- conductance of the mutated ClC-1 channels causes hindered muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. Repetitive contraction temporarily decreases myotonia, a phenomena called "warm up." The underlying mechanism for the reduction of hyperexcitability in warm-up is currently unknown. Since potassium displacement is known to reduce excitability in, for example, muscle fatigue, we characterized the role of potassium in native myotonia congenita (MC) muscle. Muscle specimens of ADR mice (an animal model for low gCl^- conductance myotonia) were exposed to increasing K⁺ concentrations. To characterize functional effects of potassium ion current, the muscle of ADR mice was exposed to agonists and antagonists of the big conductance Ca²⁺-activated K⁺ channel (BK) and the voltage-gated Kv7 channel. Effects were monitored by functional force and membrane potential measurements. By increasing [K⁺]₀ to 5 mM, the warm-up phenomena started earlier and at [K⁺]₀ 7 mM only weak myotonia was detected. The increase of [K⁺]₀ caused a sustained membrane depolarization accompanied with a reduction of myotonic bursts in ADR mice. Retigabine, a Kv7.2-Kv7.5 activator, dose-dependently reduced relaxation deficit of ADR myotonic muscle contraction and promoted the warm-up phenomena. In vitro results of this study suggest that increasing potassium conductivity via activation of voltage-gated potassium channels enhanced the warm-up phenomena, thereby offering a potential therapeutic treatment option for myotonia congenita.

TRPV4 Antagonism Prevents Mechanically Induced Myotonia

OBJECTIVE

Myotonia is caused by involuntary firing of skeletal muscle action potentials and causes debilitating stiffness. Current treatments are insufficiently efficacious and associated with side effects. Myotonia can be triggered by voluntary movement (electrically induced myotonia) or percussion (mechanically induced myotonia). Whether distinct molecular mechanisms underlie these triggers is unknown. Our goal was to identify ion channels involved in mechanically induced myotonia and to evaluate block of the channels involved as a novel approach to therapy.

METHODS

We developed a novel system to enable study of mechanically induced myotonia using both genetic and pharmacologic mouse models of myotonia congenita. We extended ex vivo studies of excitability to in vivo studies of muscle stiffness.

RESULTS

As previous work suggests activation of transient receptor potential vanilloid 4 (TRPV4) channels by mechanical stimuli in muscle, we examined the role of this cation channel. Mechanically induced myotonia was markedly suppressed in TRPV4-null muscles and in muscles treated with TRPV4 small molecule antagonists. The suppression of mechanically induced myotonia occurred without altering intrinsic muscle excitability, such that myotonia triggered by firing of action potentials (electrically induced myotonia) was unaffected. When injected intraperitoneally, TRPV4 antagonists lessened the severity of myotonia in vivo by approximately 80%.

INTERPRETATION

These data demonstrate that there are distinct molecular mechanisms triggering electrically induced and mechanically induced myotonia. Our data indicates that activation of TRPV4 during muscle contraction plays an important role in triggering myotonia in vivo. Elimination of mechanically induced myotonia by TRPV4 inhibition offers a new approach to treating myotonia. ANN NEUROL 2020;88:297-308.

Safinamide's potential in treating nondystrophic myotonias: Inhibition of skeletal muscle voltage-gated sodium channels and skeletal muscle hyperexcitability in vitro and in vivo

The antiarrhythmic sodium-channel blocker mexiletine is used to treat patients with myotonia. However, around 30% of patients do not benefit from mexiletine due to poor tolerability or suboptimal response. Safinamide is an add-on therapy to levodopa for Parkinson's disease. In addition to MAOB inhibition, safinamide inhibits neuronal sodium channels, conferring anticonvulsant activity in models of epilepsy. Here, we investigated the effects of safinamide on skeletal muscle hNa_v1.4 sodium channels and in models of myotonia, in-vitro and in-vivo. Using patch-clamp, we showed that safinamide reversibly inhibited sodium currents in HEK293T cells transfected with hNav1.4. At the holding potential (hp) of -120 mV, the half-maximum inhibitory concentrations (IC₅₀) were 160 and 33 μM at stimulation frequencies of 0.1 and 10 Hz, respectively. The calculated affinity constants of safinamide were dependent on channel state: 420 μM for closed channels and 9 μM for fast-inactivated channels. The p.F1586C mutation in hNav1.4 greatly impaired safinamide inhibition, suggesting that the drug binds to the local anesthetic receptor site in the channel pore. In a condition mimicking myotonia, i.e. hp. of -90 mV and 50-Hz stimulation, safinamide inhibited I_Na with an IC₅₀ of 6 μM, being two-fold more potent than mexiletine. Using the two-intracellular microelectrodes current-clamp method, action potential firing was recorded in vitro in rat skeletal muscle fibers in presence of the chloride channel blocker, 9-anthracene carboxylic acid (9-AC), to increase excitability. Safinamide counteracted muscle fiber hyperexcitability with an IC₅₀ of 13 μM. In vivo, oral safinamide was tested in the rat model of myotonia. In this model, intraperitoneal injection of 9-AC greatly increased the time of righting reflex (TRR) due to development of muscle stiffness. Safinamide counteracted 9-AC induced TRR increase with an ED₅₀ of 1.2 mg/kg, which is 7 times lower than that previously determined for mexiletine. In conclusion, safinamide is a potent voltage and frequency dependent blocker of skeletal muscle sodium channels. Accordingly, the drug was able to counteract abnormal muscle hyperexcitability induced by 9-AC, both in vitro and in vivo. Thus, this study suggests that safinamide may have potential in treating myotonia and warrants further preclinical and human studies to fully evaluate this possibility.

CLCN1 Molecular Characterization in 19 South-Italian Patients With Dominant and Recessive Type of Myotonia Congenita

Myotonia congenita is a genetic disease characterized by impaired muscle relaxation after forceful contraction (myotonia). It is caused by mutations in the CLCN1 gene, encoding the voltage-gated chloride channel of skeletal muscle, ClC-1. According to the pattern of inheritance, two distinct clinical forms have been described, Thomsen disease, inherited as an autosomal dominant trait and Becker disease inherited as an autosomal recessive trait. We report genetic and clinical data concerning 19 patients−13 familial and six isolated cases—all but one originating from the Campania Region, in southern Italy. Twelve patients (63.2%) present Becker type myotonia and 7 (36.8%) Thomsen type. Sex ratio M:F in Becker type is 6:6, while in Thomsen myotonia 4:3. The age of onset of the disease ranged from 2 to 15 years in Becker patients, and from 4 to 20 years in Thomsen. Overall 18 mutations were identified, 10 located in the coding part of the gene (exons 1, 3, 4, 5, 7, 8, 13, 15, 21, 22), and four in the intron part (introns 1, 2, 10, 18). All the exon mutations but two were missense mutations. Some of them, such as c.2551 G > A, c.817G > A and c.86A > C recurred more frequently. About 70% of mutations was inherited with an autosomal recessive pattern, two (c.86A and c.817G>A) with both mechanisms. Three novel mutations were identified, never described in the literature: p.Gly276Ser, p.Phe486Ser, and p.Gln812^*, associated with Becker phenotype. Furthermore, we identified three CLCN1 mutations—c.86A>C + c.2551G > A, c.313C > T + c.501C > G and 899G > A + c.2284+5C > T, two of them inherited in cis on the same allele, in three unrelated families. The concomitant occurrence of both clinical pictures—Thomsen and Becker—was observed in one family. Intra-familial phenotypic variability was observed in two families, one with Becker phenotype, and one with Thomsen disease. In the latter an incomplete penetrance was hypothesized.

Ranolazine: Multifaceted Role beyond Coronary Artery Disease, a Recent Perspective

Ranolazine is a piperazine derivative approved as an antianginal. Primarily used as a second-line antianginal in stable coronary artery disease. Ranolazine blocks the late Na + current and prevents the rise of cytosolic calcium. It decreases myocardial wall tension and improves coronary blood flow. Ranolazine is effective in atrial fibrillation (AF) as an adjunct to electrical or pharmacological cardioversion. It can be used in combination with amiodarone or dronedarone. It has also been used in AF arising after coronary artery bypass grafting surgery. Role of ranolazine is also being evaluated in pulmonary arterial hypertension, diastolic dysfunction, and chemotherapy-induced cardiotoxicity. Ranolazine has some anti-glycemic effect and has shown a reduction of hemoglobin A1c in multiple trials. The antianginal effect of ranolazine has also been seen to be more in patients with diabetes compared to those without diabetes. Ranolazine is being evaluated in patients with the peripheral arterial disease with intermittent claudication and hypertrophic cardiomyopathy. Pilot studies have shown that ranolazine may be beneficial in neurological conditions with myotonia. The evidence-base on the use of ranolazine in various conditions is rapidly increasing with results of further trials eagerly awaited. Accumulating evidence may see ranolazine in routine clinical use for many conditions beyond its traditional role as an antianginal.

Treatment of myotonia congenita with retigabine in mice

Patients with myotonia congenita suffer from muscle stiffness caused by muscle hyperexcitability. Although loss-of-function mutations in the ClC-1 muscle chloride channel have been known for 25 years to cause myotonia congenita, this discovery has led to little progress on development of therapy. Currently, treatment is primarily focused on reducing hyperexcitability by blocking Na⁺ current. However, other approaches such as increasing K⁺ currents might also be effective. For example, the K⁺ channel activator retigabine, which opens KCNQ channels, is effective in treating epilepsy because it causes hyperpolarization of the resting membrane potential in neurons. In this study, we found that retigabine greatly reduced the duration of myotonia in vitro. Detailed study of its mechanism of action revealed that retigabine had no effect on any of the traditional measures of muscle excitability such as resting potential, input resistance or the properties of single action potentials. Instead it appears to shorten myotonia by activating K⁺ current during trains of action potentials. Retigabine also greatly reduced the severity of myotonia in vivo, which was measured using a muscle force transducer. Despite its efficacy in vivo, retigabine did not improve motor performance of mice with myotonia congenita. There are a number of potential explanations for the lack of motor improvement in vivo including central nervous system side effects. Nonetheless, the striking effectiveness of retigabine on muscle itself suggests that activating potassium currents is an effective method to treat disorders of muscle hyperexcitability.

Elevation of extracellular osmolarity improves signs of myotonia congenita in vitro: a preclinical animal study

KEY POINTS

During myotonia congenita, reduced chloride (Cl- ) conductance results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Repetitive contraction of myotonic muscle decreases or even abolishes myotonic muscle stiffness, a phenomenon called 'warm up'. Pharmacological inhibition of low Cl- channels by anthracene-9-carboxylic acid in muscle from mice and ADR ('arrested development of righting response') muscle from mice showed a relaxation deficit under physiological conditions compared to wild-type muscle. At increased osmolarity up to 400 mosmol L-1 , the relaxation deficit of myotonic muscle almost reached that of control muscle. These effects were mediated by the cation and anion cotransporter, NKCC1, and anti-myotonic effects of hypertonicity were at least partly antagonized by the application of bumetanide.

ABSTRACT

Low chloride-conductance myotonia is caused by mutations in the skeletal muscle chloride (Cl- ) channel gene type 1 (CLCN1). Reduced Cl- conductance of the mutated channels results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Exercise decreases muscle stiffness, a phenomena called 'warm up'. To gain further insight into the patho-mechanism of impaired muscle stiffness and the warm-up phenomenon, we characterized the effects of increased osmolarity on myotonic function. Functional force and membrane potential measurements were performed on muscle specimens of ADR ('arrested development of righting response') mice (an animal model for low gCl- conductance myotonia) and pharmacologically-induced myotonia. Specimens were exposed to solutions of increasing osmolarity at the same time as force and membrane potentials were monitored. In the second set of experiments, ADR muscle and pharmacologically-induced myotonic muscle were exposed to an antagonist of NKCC1. Upon osmotic stress, ADR muscle was depolarized to a lesser extent than control wild-type muscle. High osmolarity diminished myotonia and facilitated the warm-up phenomenon as depicted by a faster muscle relaxation time (T90/10 ). Osmotic stress primarily resulted in the activation of the NKCC1. The inhibition of NKCC1 with bumetanide prevented the depolarization and reversed the anti-myotonic effect of high osmolarity. Increased osmolarity decreased signs of myotonia and facilitated the warm-up phenomenon in different in vitro models of myotonia. Activation of NKCC1 activity promotes warm-up and reduces the number of contractions required to achieve normal relaxation kinetics.

Open-label trial of ranolazine for the treatment of paramyotonia congenita

INTRODUCTION

Paramyotonia congenita (PMC) is a nondystrophic myotonic disorder that is believed to be caused by a defect in Na_v 1.4 sodium channel inactivation. Ranolazine, which acts by enhancing slow inactivation of sodium channels, has been proposed as a therapeutic option, but in vivo studies are lacking.

METHODS

We conducted an open-label, single-center trial of ranolazine to evaluate efficacy and tolerability in patients with PMC. Subjective symptoms of stiffness, weakness, and pain as well as clinical and electrical myotonia were evaluated. Baseline measures were compared with those after 4 weeks of treatment with ranolazine.

RESULTS

Ranolazine was tolerated well without any serious adverse events. Both subjective symptoms and clinical myotonia were significantly improved. Duration of myotonia was reduced according to electromyography, but this change was not statistically significant in all tested muscles.

DISCUSSION

Our findings support the use of ranolazine as a treatment for myotonia in PMC and suggest that a randomized, placebo-controlled trial is warranted. Muscle Nerve 59:240-243, 2019.

Myotonic Dystrophies: Targeting Therapies for Multisystem Disease

Myotonic dystrophy is an autosomal dominant muscular dystrophy not only associated with muscle weakness, atrophy, and myotonia but also prominent multisystem involvement. There are 2 similar, but distinct, forms of myotonic dystrophy; type 1 is caused by a CTG repeat expansion in the DMPK gene, and type 2 is caused by a CCTG repeat expansion in the CNBP gene. Type 1 is associated with distal limb, neck flexor, and bulbar weakness and results in different phenotypic subtypes with variable onset from congenital to very late-onset as well as variable signs and symptoms. The classically described adult-onset form is the most common. In contrast, myotonic dystrophy type 2 is adult-onset or late-onset, has proximal predominant muscle weakness, and generally has less severe multisystem involvement. In both forms of myotonic dystrophy, the best characterized disease mechanism is a RNA toxic gain-of-function during which RNA repeats form nuclear foci resulting in sequestration of RNA-binding proteins and, therefore, dysregulated splicing of premessenger RNA. There are currently no disease-modifying therapies, but clinical surveillance, preventative measures, and supportive treatments are used to reduce the impact of muscular impairment and other systemic involvement including cataracts, cardiac conduction abnormalities, fatigue, central nervous system dysfunction, respiratory weakness, dysphagia, and endocrine dysfunction. Exciting preclinical progress has been made in identifying a number of potential strategies including genome editing, small molecule therapeutics, and antisense oligonucleotide-based therapies to target the pathogenesis of type 1 and type 2 myotonic dystrophies at the DNA, RNA, or downstream target level.

The antimyotonic effect of lamotrigine in non-dystrophic myotonias: a double-blind randomized study

Mexiletine is the only drug with proven effect for treatment of non-dystrophic myotonia, but mexiletine is expensive, has limited availability and several side effects. There is therefore a need to identify other pharmacological compounds that can alleviate myotonia in non-dystrophic myotonias. Like mexiletine, lamotrigine is a sodium channel blocker, but unlike mexiletine, lamotrigine is available, inexpensive, and well tolerated. We investigated the potential of using lamotrigine for treatment of myotonia in patients with non-dystrophic myotonias. In this, randomized double-blind, placebo-controlled, two-period cross-over study, we included adult outpatients recruited from all of Denmark with clinical myotonia and genetically confirmed myotonia congenita and paramyotonia congenita for investigation at the Copenhagen Neuromuscular Center. A pharmacy produced the medication and placebo, and randomized patients in blocks of 10. Participants and investigators were all blinded to treatment until the end of the trial. In two 8-week periods, oral lamotrigine or placebo capsules were provided once daily, with increasing doses (from 25 mg, 50 mg, 150 mg to 300 mg) every second week. The primary outcome was a severity score of myotonia, the Myotonic Behaviour Scale ranging from asymptomatic (score 1) to invalidating myotonia (score 6), reported by the participants during Weeks 0 and 8 in each treatment period. Clinical myotonia was also measured and side effects were monitored. The study was registered at ClinicalTrials.gov (NCT02159963) and EudraCT (2013-003309-24). We included 26 patients (10 females, 16 males, age: 19-74 years) from 13 November 2013 to 6 July 2015. Twenty-two completed the entire study. One patient withdrew due to an allergic reaction to lamotrigine. Three patients withdrew for reasons not related to the trial intervention. The Myotonic Behaviour Scale at baseline was 3.2 ± 1.1, which changed after treatment with lamotrigine by 1.3 ± 0.2 scores (P < 0.001), but not with placebo (0.2 ± 0.1 scores, P = 0.4). The estimated effect size was 1.0 ± 0.2 (95% confidence interval = 0.5-1.5, P < 0.001, n = 22). The standardized effect size of lamotrigine was 1.5 (confidence interval: 1.2-1.8). Number needed to treat was 2.6 (P = 0.006, n = 26). No adverse or unsuspected event occurred. Common side effects occurred in both treatment groups; number needed to harm was 5.2 (P = 0.11, n = 26). Lamotrigine effectively reduced myotonia, emphasized by consistency between effects on patient-related outcomes and objective outcomes. The frequency of side effects was acceptable. Considering this and the high availability and low cost of the drug, we suggest that lamotrigine should be used as the first line of treatment for myotonia in treatment-naive patients with non-dystrophic myotonias.

Inhibiting persistent inward sodium currents prevents myotonia

OBJECTIVE

Patients with myotonia congenita have muscle hyperexcitability due to loss-of-function mutations in the ClC-1 chloride channel in skeletal muscle, which causes involuntary firing of muscle action potentials (myotonia), producing muscle stiffness. The excitatory events that trigger myotonic action potentials in the absence of stabilizing ClC-1 current are not fully understood. Our goal was to identify currents that trigger spontaneous firing of muscle in the setting of reduced ClC-1 current.

METHODS

In vitro intracellular current clamp and voltage clamp recordings were performed in muscle from a mouse model of myotonia congenita.

RESULTS

Intracellular recordings revealed a slow afterdepolarization (AfD) that triggers myotonic action potentials. The AfD is well explained by a tetrodotoxin-sensitive and voltage-dependent Na⁺ persistent inward current (NaPIC). Notably, this NaPIC undergoes slow inactivation over seconds, suggesting this may contribute to the end of myotonic runs. Highlighting the significance of this mechanism, we found that ranolazine and elevated serum divalent cations eliminate myotonia by inhibiting AfD and NaPIC.

INTERPRETATION

This work significantly changes our understanding of the mechanisms triggering myotonia. Our work suggests that the current focus of treating myotonia, blocking the transient Na⁺ current underlying action potentials, is an inefficient approach. We show that inhibiting NaPIC is paralleled by elimination of myotonia. We suggest the ideal myotonia therapy would selectively block NaPIC and spare the transient Na⁺ current. Ann Neurol 2017;82:385-395.

Depressed Synaptic Transmission and Reduced Vesicle Release Sites in Huntington's Disease Neuromuscular Junctions

Huntington's disease (HD) is a progressive and fatal degenerative disorder that results in debilitating cognitive and motor dysfunction. Most HD studies have focused on degeneration of the CNS. We previously discovered that skeletal muscle from transgenic R6/2 HD mice is hyperexcitable due to decreased chloride and potassium conductances. The progressive and early onset of these defects suggest a primary myopathy in HD. In this study, we examined the relationship between neuromuscular transmission and skeletal muscle hyperexcitability. We used an ex vivo preparation of the levator auris longus muscle from male and female late-stage R6/2 mice and age-matched wild-type controls. Immunostaining of the synapses and molecular analyses revealed no evidence of denervation. Physiologically, we recorded spontaneous miniature endplate currents (mEPCs) and nerve-evoked EPCs (eEPCs) under voltage-clamp, which, unlike current-clamp records, were independent of the changes in muscle membrane properties. We found a reduction in the number of vesicles released per action potential (quantal content) in R6/2 muscle, which analysis of eEPC variance and morphology indicate is caused by a reduction in the number of vesicle release sites (n) rather than a change in the probability of release (p_rel). Furthermore, analysis of high-frequency stimulation trains suggests an impairment in vesicle mobilization. The depressed neuromuscular transmission in R6/2 muscle may help compensate for the muscle hyperexcitability and contribute to motor impersistence.SIGNIFICANCE STATEMENT Recent evidence indicates that Huntington's disease (HD) is a multisystem disorder. Our examination of neuromuscular transmission in this study reveals defects in the motor nerve terminal that may compensate for the muscle hyperexcitability in HD. The technique we used eliminates the effects of the altered muscle membrane properties on synaptic currents and thus provides hitherto the most detailed analysis of synaptic transmission in HD. Clinically, the striking depression of neurotransmission we found may help explain the motor impersistence in HD patients. Therapies that target the highly accessible peripheral nerve and muscle system provide a promising new avenue to lessen the debilitating motor symptoms of HD.

Open-label trial of ranolazine for the treatment of myotonia congenita

OBJECTIVE

To determine open-label, pilot study whether ranolazine could improve signs and symptoms of myotonia and muscle stiffness in patients with myotonia congenita (MC).

METHODS

Thirteen participants were assessed at baseline and 2, 4, and 5 weeks. Ranolazine was started after baseline assessment (500 mg twice daily), increased as tolerated after week 2 (1,000 mg twice daily), and maintained until week 4. Outcomes included change from baseline to week 4 in self-reported severity of symptoms (stiffness, weakness, and pain), Timed Up and Go (TUG), hand grip and eyelid myotonia, and myotonia on EMG.

RESULTS

Self-reported severity of stiffness (p < 0.0001) and weakness (p < 0.01) was significantly improved compared with baseline. TUG and grip myotonia times were reduced (p = 0.03, p = 0.01). EMG of the abductor digiti minimi and tibialis anterior showed significantly reduced myotonia duration (p < 0.001, p < 0.01) at week 4. No participant discontinued ranolazine because of side effects.

CONCLUSIONS

Ranolazine appeared to be well tolerated over a period of 4 weeks in individuals with MC, and ranolazine resulted in improvement of signs and symptoms of muscle stiffness. The findings of this study suggest that ranolazine should be investigated in a larger controlled study.

CLASSIFICATION OF EVIDENCE

This study provides Class IV evidence that ranolazine improves myotonia in myotonia congenita.

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

The anti-convulsants lacosamide, lamotrigine, and rufinamide reduce myotonia in isolated human and rat skeletal muscle

INTRODUCTION

In myotonia congenita, loss of ClC-1 Cl^- channel function results in skeletal muscle hyperexcitability and myotonia. Anti-myotonic treatment has typically targeted the voltage-gated sodium channel in skeletal muscle (Nav1.4). In this study we explored whether 3 sodium channel-modulating anti-epileptics can reduce myotonia in isolated rat and human muscle.

METHODS

Dissected muscles were rendered myotonic by ClC-1 channel inhibition. The ability of the drugs to suppress myotonia was then assessed from subclinical to maximal clinical concentrations. Drug synergy was determined using isobole plots.

RESULTS

All drugs were capable of abolishing myotonia in both rat and human muscles. Lamotrigine and rufinamide completely suppressed myotonia at submaximal clinical concentrations, whereas lacosamide had to be raised above the maximal clinical concentration to suppress myotonia completely. A synergistic effect of lamotrigine and rufinamide was observed.

CONCLUSION

These findings suggest that lamotrigine and rufinamide could be considered for anti-myotonic treatment in myotonia congenita. Muscle Nerve 56: 136-142, 2017.

Increased sodium channel use-dependent inhibition by a new potent analogue of tocainide greatly enhances in vivo antimyotonic activity

Although the sodium channel blocker, mexiletine, is the first choice drug in myotonia, some myotonic patients remain unsatisfied due to contraindications, lack of tolerability, or incomplete response. More therapeutic options are thus needed for myotonic patients, which require clinical trials based on solid preclinical data. In previous structure-activity relationship studies, we identified two newly-synthesized derivatives of tocainide, To040 and To042, with greatly enhanced potency and use-dependent behavior in inhibiting sodium currents in frog skeletal muscle fibers. The current study was performed to verify their potential as antimyotonic agents. Patch-clamp experiments show that both compounds, especially To042, are greatly more potent and use-dependent blockers of human skeletal muscle hNav1.4 channels compared to tocainide and mexiletine. Reduced effects on F1586C hNav1.4 mutant suggest that the compounds bind to the local anesthetic receptor, but that the increased hindrance and lipophilia of the N-substituent may further strengthen drug-receptor interaction and use-dependence. Compared to mexiletine, To042 was 120 times more potent to block hNav1.4 channels in a myotonia-like cellular condition and 100 times more potent to improve muscle stiffness in vivo in a previously-validated rat model of myotonia. To explore toxicological profile, To042 was tested on hERG potassium currents, motor coordination using rotarod, and C2C12 cell line for cytotoxicity. All these experiments suggest a satisfactory therapeutic index for To042. This study shows that, owing to a huge use-dependent block of sodium channels, To042 is a promising candidate drug for myotonia and possibly other membrane excitability disorders, warranting further preclinical and human studies.

•

To040 and To042 are potent use-dependent hNav1.4 sodium channel blockers.

•

The compounds strengthen the molecular interaction at the local anesthetic receptor.

•

To042 is 120-fold more potent than mexiletine in vitro in myotonia-like conditions.

•

To042 is 100-fold more potent than mexiletine in vivo in a rat model of myotonia.

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To042 is a promising antimyotonic drug deserving further investigation.

Skeletal muscle sodium channelopathies

PURPOSE OF REVIEW

This is an update on skeletal muscle sodium channelopathies since knowledge in the field have dramatically increased in the past years.

RECENT FINDING

The relationship between two phenotypes and SCN4A has been confirmed with additional cases that remain extremely rare: severe neonatal episodic laryngospasm mimicking encephalopathy, which should be actively searched for since patients respond well to sodium channel blockers; congenital myasthenic syndromes, which have the particularity to be the first recessive Nav1.4 channelopathy. Deep DNA sequencing suggests the contribution of other ion channels in the clinical expressivity of sodium channelopathies, which may be one of the factors modulating the latter. The increased knowledge of channel molecular structure, the quantity of sodium channel blockers, and the availability of preclinical models would permit a most personalized choice of medication for patients suffering from these debilitating neuromuscular diseases.

SUMMARY

Advances in the understanding of the molecular structure of voltage-gated sodium channels, as well as availability of preclinical models, would lead to improved medical care of patients suffering from skeletal muscle, as well as other sodium channelopathies.

Therapeutic Approaches to Genetic Ion Channelopathies and Perspectives in Drug Discovery

In the human genome more than 400 genes encode ion channels, which are transmembrane proteins mediating ion fluxes across membranes. Being expressed in all cell types, they are involved in almost all physiological processes, including sense perception, neurotransmission, muscle contraction, secretion, immune response, cell proliferation, and differentiation. Due to the widespread tissue distribution of ion channels and their physiological functions, mutations in genes encoding ion channel subunits, or their interacting proteins, are responsible for inherited ion channelopathies. These diseases can range from common to very rare disorders and their severity can be mild, disabling, or life-threatening. In spite of this, ion channels are the primary target of only about 5% of the marketed drugs suggesting their potential in drug discovery. The current review summarizes the therapeutic management of the principal ion channelopathies of central and peripheral nervous system, heart, kidney, bone, skeletal muscle and pancreas, resulting from mutations in calcium, sodium, potassium, and chloride ion channels. For most channelopathies the therapy is mainly empirical and symptomatic, often limited by lack of efficacy and tolerability for a significant number of patients. Other channelopathies can exploit ion channel targeted drugs, such as marketed sodium channel blockers. Developing new and more specific therapeutic approaches is therefore required. To this aim, a major advancement in the pharmacotherapy of channelopathies has been the discovery that ion channel mutations lead to change in biophysics that can in turn specifically modify the sensitivity to drugs: this opens the way to a pharmacogenetics strategy, allowing the development of a personalized therapy with increased efficacy and reduced side effects. In addition, the identification of disease modifiers in ion channelopathies appears an alternative strategy to discover novel druggable targets.

Protein kinase C-dependent regulation of ClC-1 channels in active human muscle and its effect on fast and slow gating

KEY POINTS

Regulation of ion channel function during repeated firing of action potentials is commonly observed in excitable cells. Recently it was shown that muscle activity is associated with rapid, protein kinase C (PKC)-dependent ClC-1 Cl(-) channel inhibition in rodent muscle. While this PKC-dependent ClC-1 inhibition during muscle activity was shown to be important for the maintenance of contractile endurance in rat muscle it is unknown whether a similar regulation exists in human muscle. Also, the molecular mechanisms underlying the observed PKC-dependent ClC-1 inhibition are unclear. Here we present the first demonstration of ClC-1 inhibition in active human muscle fibres, and we determine the changes in ClC-1 gating that underlie the PKC-dependent ClC-1 inhibition in active muscle using human ClC-1 expressed in Xenopus oocytes. This activity-induced ClC-1 inhibition is suggested to represent a mechanism by which human muscle fibres maintain their excitability during sustained activity.

ABSTRACT

Repeated firing of action potentials (APs) is known to trigger rapid, protein kinase C (PKC)-dependent inhibition of ClC-1 Cl(-) ion channels in rodent muscle and this inhibition is important for contractile endurance. It is currently unknown whether similar regulation exists in human muscle, and the molecular mechanisms underlying PKC-dependent ClC-1 inhibition are unclear. This study first determined whether PKC-dependent ClC-1 inhibition exists in active human muscle, and second, it clarified how PKC alters the gating of human ClC-1 expressed in Xenopus oocytes. In human abdominal and intercostal muscles, repeated AP firing was associated with 30-60% reduction of ClC-1 function, which could be completely prevented by PKC inhibition (1 μm GF109203X). The role of the PKC-dependent ClC-1 inhibition was evaluated from rheobase currents before and after firing 1000 APs: while rheobase current was well maintained after activity under control conditions it rose dramatically if PKC-dependent ClC-1 inhibition had been prevented with the inhibitor. This demonstrates that the ClC-1 inhibition is important for maintenance of excitability in active human muscle fibres. Oocyte experiments showed that PKC activation lowered the overall open probability of ClC-1 in the voltage range relevant for AP initiation in muscle fibres. More detailed analysis of this reduction showed that PKC mostly affected the slow gate of ClC-1. Indeed, there was no effect of PKC activation in C277S mutated ClC-1 in which the slow gate is effectively locked open. It is concluded that regulation of excitability of active human muscle fibres relies on PKC-dependent ClC-1 inhibition via a gating mechanism.

Myotonic disorders: A review article

The myotonic disorders are a heterogeneous group of genetically determined diseases that are unified by the presence of myotonia, which is defined as failure of muscle relaxation after activation. The presentation of these disorders can range from asymptomatic electrical myotonia, as seen in some forms of myotonia congenita (MC), to severe disability with muscle weakness, cardiac conduction defects, and other systemic features as in myotonic dystrophy type I (DM1). In this review, we describe the clinical features and pathophysiology of the different myotonic disorders, their laboratory and electrophysiologic findings and briefly review the currently available treatments.

Genetic neurological channelopathies: molecular genetics and clinical phenotypes

Evidence accumulated over recent years has shown that genetic neurological channelopathies can cause many different neurological diseases. Presentations relating to the brain, spinal cord, peripheral nerve or muscle mean that channelopathies can impact on almost any area of neurological practice. Typically, neurological channelopathies are inherited in an autosomal dominant fashion and cause paroxysmal disturbances of neurological function, although the impairment of function can become fixed with time. These disorders are individually rare, but an accurate diagnosis is important as it has genetic counselling and often treatment implications. Furthermore, the study of less common ion channel mutation-related diseases has increased our understanding of pathomechanisms that is relevant to common neurological diseases such as migraine and epilepsy. Here, we review the molecular genetic and clinical features of inherited neurological channelopathies.