Non-dystrophic Myotonias Research Articles

G.P.136: Muscle channelopathies: Clinical and genetic features in a large cohort of Italian patients

Skeletal muscle channelopathies are rare diseases, including non-dystrophic myotonia and periodic paralysis, which are associated with a great inter- and intrafamilial phenotypic variability, making challenging genotype-phenotype correlations. Hence studies on large populations of patients are needed. We included patients referred to Carlo Besta Neurological Institute molecular laboratory with a clinical diagnosis of periodic paralysis or nondystrophic myotonia and mutated in CLCN1, SCN4A, KCNJ2 or CACNA1S. We investigated 301 patients, among which 195 (64.3%) patients mutated in CLCN1 gene, 74 (24.6%) in SCN4A, 28 (9.3%) in CACNA1S and 4 (1.3%) in KCNJ2. We found 22 novel mutations: 8 in CLCN1 gene, 13 in SCN4A and 1 in CACNA1S. All the mutations detected in SCN4A, CACNA1S and KCNJ2 genes were missense, except for an unreported 9-nucleotide deletion in SCN4A. On the contrary CLCN1mutations were missense in 131/195 (67.2%) patients and remaining cases showed nonsense, splice site or deletion mutations. Our study confirms genetic heterogeneity of muscle channelopathies, although a relatively small number of mutations is responsible for most of the cases.

Treatment and management of neuromuscular channelopathies.

Neuromuscular channelopathies are heterogeneous disorders with marked phenotypic and genotypic variability. These include non-dystrophic myotonia (NDM), periodic paralysis (PP), and congenital myasthenic syndrome (CMS). Their diverse clinical manifestations remain a challenge in diagnosis and management to this date. These disorders impact quality of life and cause lifelong disabling symptoms. Treatment options are few and not FDA-approved. This is largely due to a paucity of large, randomized clinical trials in these rare diseases. Challenges of conducting such trials include the rarity of these disorders and the genetic heterogeneity. Physicians rely on off-label use of drugs to treat muscle channelopathies to reduce morbidity and improve quality of life. Besides pharmacological treatment, dietary modifications, lifestyle changes, awareness of triggers, and genetic counseling also play an important role in long-term disease management. This article reviews the current management strategies for neuromuscular channelopathies.

Heterozygous CLCN1 mutations can modulate phenotype in sodium channel myotonia

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

The Course and Outcome of Pregnancy in Women with Nondystrophic Myotonias (P7.009)

The Course and Outcome of Pregnancy in Women with Nondystrophic Myotonias (P7.009)

Preclinical evaluation of marketed sodium channel blockers in a rat model of myotonia discloses promising antimyotonic drugs

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 5mg/kg); flecainide and orphenadrine showed greater potency (ED50 near 1mg/kg); lubeluzole and riluzole were the more potent (ED50 near 0.1mg/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.

Nondystrophic myotonia: Challenges and future directions

Non-dystrophic myotonias are rare diseases caused by mutations in skeletal muscle chloride and sodium ion channels with considerable phenotypic overlap between diseases. Common symptoms include muscle stiffness, transitory weakness, fatigue, and pain. Although seldom life-shortening, these myotonias cause life-time disability and affected individuals cannot perform many daily activities. A notable feature of the recessive form of chloride channelopathies is the presence of transient weakness. While there has been considerable progress in skeletal muscle channelopathies with regards to identifying biophysical abnormalities, the mechanism of transient weakness remains unclear. A recent study published in Experimental Neurology (Desaphy et al., 2013) explored this question further by comparing the biophysical properties of 3 chloride channel mutations associated with recessive myotonia congenita, with varying susceptibility to transient weakness. The authors identified a variety of functional defects in channel behavior among the 3 mutations, suggesting that this variability contributes to the differing phenotypes among chloride channelopathies. This commentary discusses nondystrophic myotonias, the results of Desaphy et al., and the treatment challenges in this rare disease.

Muscle Channelopathies

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. 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.

Frequency and causes of prevalence of p.Arg894* mutation in CLCN1 gene responsible for development of thomsen’s and becker’s myotonias in russian population

Thomsen's (TM) and Becker's (BM) Myotonias are nondystrophic myotonias. At present, 150 mutations in the CLCN1 gene, which results in the development of TM and BM, have been described. c.2680C > T (p.Arg894*) is the most common mutation. In the Northern Scandinavian countries, the population frequency of this mutation is 0.87%, while in the Russian Federation, it is equal to 1.2% (this study). Based on the results of a molecular-genetic analysis of CLCN1 gene in patients with nondystrophic myotonias, the calculated frequency of TM and BM in Russia is 1:8165 and 1:710, respectively. We have conducted haplotype analysis using microsatellite markers and intragene SNP, which has shown that the prevalence of p.Arg894* mutation in Russia results from the founder effect, and the time of its scattering is 3680 ± 1240 years.

P.12.12 Preclinical in vitro and in vivo evaluation of sodium channel blockers as possible alternative to mexiletine in the treatment of myotonia

The efficacy of mexiletine in the treatment of non-dystrophic myotonias was recently confirmed in a randomized, double-blind, placebo-controlled crossover trial (Statland et al., JAMA 2012). Nevertheless, not all myotonic patients can obtain benefits from mexiletine, either due to lack of therapeutic effects or occurrence of side effects limiting the compliance. In addition, mexiletine has been withdrawn from a number of European countries, leaving patients with an unmeet medical need. Thus there is a critical need to increase the arsenal of antimyotonic drugs. In this study, we tested the antimyotonic activity of a number of marketed sodium channel blockers in vivo, by using a rat model in which myotonia is pharmacologically induced by i.p. injection of 9-anthracene carboxylic acid (Desaphy et al., Neuropharmacology 2013). The myotonic state was assessed by measuring the time of righting reflex of rats after 9-AC injection and oral administration of drugs. The dose–response curves indicate an ED50 of 7.0 ± 1.4 mg/kg for mexiletine. Propafenone and carbamazepine were twofold, orphenadrine 7-fold, flecainide 10-fold, lubeluzole and riluzole 70-fold, more efficient that mexiletine in vivo. The in vivo antimyotonic activity was in accord with the relative activity of sodium channel blockade in vitro, determined on heterologously expressed skeletal muscle hNav1.4 channels with patch-clamp technique. Two exceptions regard carbamazepine, which was more efficient in vivo compared to in vitro, and propafenone, which was less efficient in vivo than in vitro. Such discrepancies are likely related to pharmacokinetic mechanisms. The comparison of antimyotonic doses in the rat with clinical doses used in human indications suggest that all the tested sodium channels might be used safely in myotonic patients, except for lubeluzole that may present some cardiac risk. Supported by Telethon-Italy (grant GGP10101) and Association Francaise contre les Myopathies (grant 1502).

Muscle MRI reveals distinct abnormalities in genetically proven non-dystrophic myotonias

We assessed the presence, frequency and pattern of MRI abnormalities in non-dystrophic myotonia patients. We reviewed T1-weighted and STIR (short-tau-inversion-recovery) 3T MRI sequences of lower limb muscles at thigh and calf level in 21 patients with genetically confirmed non-dystrophic myotonia: 11 with CLCN1 mutations and 10 with SCN4A mutations, and 19 healthy volunteers. The MRI examinations of all patients showed hyperintensity within muscles on either T1-weighted or STIR images. Mild extensive or marked T1-weighted changes were noted in 10/21 patients and no volunteers. Muscles in the thigh were equally likely to be affected but in the calf there was sparing of tibialis posterior. Oedema was common in calf musculature especially in the medial gastrocnemius with STIR hyperintensity observed in 18/21 patients. In 10/11 CLCN1 patients this included a previously unreported “central stripe”, also present in 3/10 SCN4A patients but no volunteers. Degree of fatty infiltration correlated with age (rho = 0.46, p < 0.05). Muscle MRI is frequently abnormal in non-dystrophic myotonia providing evidence of fatty infiltration and/or oedema. The pattern is distinct from other myotonic disorders; in particular the “central stripe” has not been reported in other conditions. Correlations with clinical parameters suggest a potential role for MRI as a biomarker.

Non-dystrophic myotonia: prospective study of objective and patient reported outcomes

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.

Prevalence study of genetically defined skeletal muscle channelopathies in England

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.

Measuring quality of life impairment in skeletal muscle channelopathies

Background and purposeFatigue and pain have been previously shown to be important determinants for decreasing quality of life (QoL) in one report in patients with non-dystrophic myotonia. The aims of our study were to assess QoL in skeletal muscle channelopathies (SMC) using INQoL (individualized QoL) and SF-36 questionnaires.MethodsWe administered INQoL and SF-36 to 66 Italian patients with SMC (26: periodic paralysis, 36: myotonia congenita and 4: Andersen-Tawil) and compared the results in 422 patients with myotonic dystrophies (DM1: 382; and DM2: 40).Results(i) INQoL index in SMC is similar to that in DMs (P = 0.79). (ii) Patients with myotonia congenita have the worst perception of QoL. (iii) Myotonia has the most detrimental effect on patients with myotonia congenita, followed by patients with DM2 and then by patients with DM1 and hyperkalemic periodic paralysis. (iv) Pain is a significant complaint in patients with myotonia congenita, hypokalemic periodic paralysis and DM2 but not in DM1. (v) Fatigue has a similar detrimental effect on all patient groups except for patients with hyperkalemic periodic paralysis in whom muscle weakness and myotonia more than fatigue affect QoL perception. (vi) Muscle symptoms considered in INQoL correlate with physical symptoms assessed by SF-36 (R from −0.34 to −0.76).ConclusionsQoL perception in patients with SMC is similar to that of patients with DMs, chronic multisystem disabling conditions. Our results provide information to target treatment and health care of these patients. The sensitivity of INQoL to changes in QoL in the SMC needs to be further explored in longitudinal studies.

Mexiletine for Treatment of Myotonia

Mexiletine for Treatment of Myotonia

Mexiletine for Symptoms and Signs of Myotonia in Nondystrophic Myotonia

Nondystrophic myotonias (NDMs) 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 small studies; however, 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 patients with NDMs. A randomized, double-blind, placebo-controlled 2-period crossover study at 7 neuromuscular referral centers in 4 countries of 59 patients with NDMs conducted between December 23, 2008, and March 30, 2011, as part of the National Institutes of Health-funded Rare Disease Clinical Research Network. Oral 200-mg mexiletine or placebo capsules 3 times daily for 4 weeks, followed by the opposite intervention for 4 weeks, with 1-week washout in between. Patient-reported severity score of stiffness recorded on an interactive voice response (IVR) diary (scale of 1 = minimal to 9 = worst ever experienced). Secondary end points included IVR-reported changes in pain, weakness, and tiredness; clinical myotonia assessment; quantitative measure of handgrip myotonia; and Individualized Neuromuscular Quality of Life summary quality of life score (INQOL-QOL, percentage of maximal detrimental impact). Mexiletine significantly improved patient-reported severity score stiffness on the IVR diary. Because of a statistically significant interaction between treatment and period for this outcome, primary end point is presented by period (period 1 means were 2.53 for mexiletine and 4.21 for placebo; difference, -1.68; 95% CI, -2.66 to -0.706; P < .001; period 2 means were 1.60 for mexiletine and 5.27 for placebo; difference, -3.68; 95% CI, -3.85 to -0.139; P = .04). Mexiletine improved the INQOL-QOL score (mexiletine, 14.0 vs placebo, 16.7; difference, -2.69; 95% CI, -4.07 to -1.30; P < .001) and decreased handgrip myotonia on clinical examination (mexiletine, 0.164 seconds vs placebo, 0.494 seconds; difference, -0.330; 95% CI, -0.633 to -0.142; P < .001). The most common adverse effect was gastrointestinal (9 mexiletine and 1 placebo). Two participants experienced transient cardiac effects that did not require stopping the study (1 in each group). One serious adverse event was determined to be not study related. In this preliminary study of patients with NDMs, 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. clinicaltrials.gov Identifier: NCT00832000.

Dramatic improvement of myotonia permanens with flecainide: a two-case report of a possible bench-to-bedside pharmacogenetics strategy

Dramatic improvement of myotonia permanens with flecainide: a two-case report of a possible bench-to-bedside pharmacogenetics strategy

G.P.101 New immunohistochemical method for improved myotonia and chloride channel mutation diagnostics

<h2>Abstract</h2> Most patients with myotonia have either congenital myotonia caused by mutated chloride or sodium channel, or progressive myotonic dystrophy (types 1 and 2). Currently, final diagnosis is frequently obtained by molecular genetic DNA testing. However, the increased use of genetic testing also results in many cases when the tests do not provide full clarification of the clinical disease. An immunohistochemical method using two different chloride channel protein specific antibodies for diagnostic purposes This method provided means to identify new mutations, to reclassify the W118G CLCN1 change as a moderately pathogenic mutation, and to clarify recessive Becker myotonia in cases when only one recessive mutation was identified by genetic testing. The developed immunohistochemical staining method for CLC-1 in muscle fibers proved to be a robust method for the detection of presence or loss of sarcolemmal CLC-1 protein on muscle sections. This in combination with gene sequencing is a powerful approach to achieving a final diagnosis of non-dystrophic myotonia.

A quantitative measure of handgrip myotonia in non‐dystrophic myotonia

Non-dystrophic myotonia (NDM) is characterized by myotonia without muscle wasting. A standardized quantitative myotonia assessment (QMA) is important for clinical trials. Myotonia was assessed in 91 individuals enrolled in a natural history study using a commercially available computerized handgrip myometer and automated software. Average peak force and 90% to 5% relaxation times were compared with historical normal controls studied with identical methods. Thirty subjects had chloride channel mutations, 31 had sodium channel mutations, 6 had DM2 mutations, and 24 had no identified mutation. Chloride channel mutations were associated with prolonged first handgrip relaxation times and warm-up on subsequent handgrips. Sodium channel mutations were associated with prolonged first handgrip relaxation times and paradoxical myotonia or warm-up, depending on underlying mutations. DM2 subjects had normal relaxation times but decreased peak force. Sample size estimates are provided for clinical trial planning. QMA is an automated, non-invasive technique for evaluating myotonia in NDM.

The spectrum of CLCN1 gene mutations in patients with nondystrophic Thomsen’s and Becker’s myotonias

Thomsen's and Becker's diseases are the most prevalent nondystrophic myotonias. Their frequency varies, according to different sources, from 1 : 100 000 to 1 : 10 000. Thomsen's myotonia is autosomal dominant, and Becker's myotonia is autosomal recessive. Both diseases result from mutations of the CLCN1 gene encoding chloride ion channels of skeletal muscles. Molecular genetic analysis of the CLCN1 gene has been performed in patients with diagnoses of nondystrophic Thomsen's and Becker's myotonias living in the Russian Federation. A sample of 79 unrelated probands with nondystrophic Thomsen's and Becker's myotonias and 44 their relatives has been formed in the Laboratory of DNA Diagnosis of the Medical Genetic Research Center of the Russian Academy of Medical Sciences. Forty CLCN1 gene mutations have been found in a total of 118 chromosomes of 66 probands, including 21 familial and 45 sporadic cases. About half the mutations detected (45%) have been found for the first time; they are not described in the SNP database (ncbi.nlm.nih.gov). The following mutations (substitutions) have been detected in more than one chromosome, accounting for a total of 59.3% of chromosomes with mutations: Glyl90Ser (5.9%), c.1437-1450del14 (9.3%), Ala493Glu (5.1%), Thr550Met (3.4%), Tyr686Stop (5.1%), and Arg894Stop (30.5%).

G.P.99 Molecular genetic diagnostics of myotonia congenita and analysis of the effects of mutations in the CLCN1 gene on the structure of the dimeric ClC1 chloride channel

Abstract Non-dystrophic myotonias are a group of skeletal muscle disorders associated with mutations in the CLCN1 and SCN4A genes. Mutations of CLCN1 result in either autosomal dominant (Thomsen) or autosomal recessive (Becker) myotonia congenita (MC). A subset of CLCN1 mutations have been found to cause both recessive and dominant myotonias, while mutations in the SCN4A gene are typically inherited as an autosomal dominant trait. The ClC1 protein is a homodimer with a separate ion pore within each monomer, and the varied inheritance pattern of myotonias appears to result from differential effects of a mutation on the channel dimer. Mutations causing recessive myotonia most likely affect properties of a mutant monomer, leaving the wild type monomer unaffected in the heterodimer, and on the other hand mutations causing dominant myotonia affect properties of both subunits in the wild type/mutant heterodimer. Our study addresses two points: (1) molecular genetic diagnostics of MC by tandem analysis of the CLCN1 and SCN4A genes and (2) homology modelling of the molecular structure of the dimeric ClC1 channel. In the first part, mutations associated with the disease were identified in 57 probands, 39 carrying mutations in CLCN1 and 18 in SCN4A. In the second part, we performed homology modelling of the dimeric ClC1 channel on the basis of known crystallographic structures. From this model, we predicted the aminoacids (AAs) which form the dimer interface and those which form the Cl− ion pathway. Further, using our model we performed searches for mutations of AAs forming the dimer interface or/and the Cl− ion pathway in our model which have been found in patients with MC to assess the correlation between the localisation of a mutation and the type of MC. This work was funded by the project “CEITEC – Central European Institute of Technology” (CZ.1.05/1.1.00/02.0068).