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.

Beyond the muscular involvement in non-dystrophic myotonias: The emerging role of neuromodulation.

BACKGROUND: The central nervous system involvement, in terms of a maladaptive sensory-motor plasticity, is well known in patients with dystrophic myotonias (DMs). To date, there are no data suggesting a central nervous system involvement in non-dystrophic myotonias (NDMs). OBJECTIVE: To investigate sensory-motor plasticity in patients with Myotonia Congenita (MC) and Paramyotonia Congenita (PMC) with or without mexiletine. METHODS: Twelve patients with a clinical, genetic, and electromyographic evidence of MC, fifteen with PMC, and 25 healthy controls (HC) were included in the study. TMS on both primary motor cortices (M1) and a rapid paired associative stimulation (rPAS) paradigm were carried out to assess M1 excitability and sensory-motor plasticity. RESULTS: patients showed a higher cortical excitability and a deterioration of the topographic specificity of rPAS aftereffects, as compared to HCs. There was no correlation among neurophysiological and clinical-demographic characteristics. Noteworthy, the patients who were under mexiletine showed a minor impairment of the topographic specificity of rPAS aftereffects as compared to those who did not take the drug. CONCLUSION: our findings could suggest the deterioration of cortical sensory-motor plasticity in patients with NDMs as a trait of the disease.

What the internist should know about hereditary muscle channelopathies.

OBJECTIVES: Non-dystrophic myotonia, periodic paralysis and, to a certain extent, myotonic dystrophies are rare hereditary skeletal muscle channelopathies, charactarized by myotonia or episodic muscle weakness. This review highlights the diagnostic challenges and treatment options. RESULTS: Some of these rare skeletal muscle disorders are associated with a broad range of systemic and nonspecific muscle symptoms. Consequently, patients are often referred to the internist before seeing a neurologist. This article provides clinical clues to better diagnose an tackle these unique disorders. CONCLUSION: A increased knowledge will reduce the diagnostic delay, improve monitoring and treatment, and might even prevent potentially life-threatening conditions as seen in DM.

The diagnosis and treatment of myotonic disorders.

Myotonia is a defining clinical symptom and sign common to a relatively small group of muscle diseases, including the myotonic dystrophies and the nondystrophic myotonic disorders. Myotonia can be observed on clinical examination, as can its electrical correlate, myotonic discharges, on electrodiagnostic testing. Research interest in the myotonic disorders continues to expand rapidly, which justifies a review of the scientific bases, clinical manifestations, and numerous therapeutic approaches associated with these disorders. We review the pathomechanisms of myotonia, the clinical features of the dystrophic and nondystrophic myotonic disorders, and the diagnostic approach and treatment options for patients with symptomatic myotonia.

Myotonic dystrophies type 1 and 2: anesthetic care.

SUMMARY: Myotonic dystrophy is classified as one of the myotonic syndromes although myotonia is only a minor characteristic of it. It is, in fact, also a multisystem disease with cardiac, digestive, ocular, and endocrine abnormalities. Two subgroups are currently identified with many similarities: DM1 refers to classic dystrophia myotonica (Steinert disease), while DM2, formerly called proximal myotonic myopathy has a later onset. The congenital form is present only in DM1. The genetic causes of DM1 and 2 are different but end up in a similar way of altering RNAm processing and splicing of other genes. The anesthetic risk is increased in case of DM1 type. This review summarizes current knowledge concerning the pathophysiology and anesthetic management of this disease in children and adults.

Clinical aspects, molecular pathomechanisms and management of myotonic dystrophies.

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) was described more than 100 years ago and is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) was identified only 18 years ago and is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. DM1 may present in four different forms: congenital, early childhood, adult onset and late-onset oligosymptomatic DM1. Congenital DM1 is the most severe form of DM characterized by extreme muscle weakness and mental retardation. In DM2 the clinical phenotype is extremely variable and there are no distinct clinical subgroups. Congenital and childhood-onset forms are not present in DM2 and, in contrast to DM1, myotonia may be absent even on EMG. Due to the lack of awareness of the disease among clinicians, DM2 remains largely underdiagnosed. The delay in receiving the correct diagnosis after onset of first symptoms is very long in DM: on average more than 5 years for DM1 and more than 14 years for DM2 patients. The long delay in the diagnosis of DM causes unnecessary problems for the patients to manage their lives and anguish with uncertainty of prognosis and treatment.

The myotonic dystrophies: molecular, clinical, and therapeutic challenges.

Myotonic dystrophy is the most common type of muscular dystrophy in adults and is characterised by progressive myopathy, myotonia, and multiorgan involvement. Two genetically distinct entities have been identified. Myotonic dystrophy type 1 (also known as Steinert's disease) was first described more than 100 years ago, whereas myotonic dystrophy type 2 was identified only 18 years ago, after genetic testing for type 1 disease could be applied. Both diseases are caused by autosomal dominant nucleotide repeat expansions. In patients with myotonic dystrophy type 1, a (CTG)(n) expansion is present in DMPK, whereas in patients with type 2 disease, there is a (CCTG)(n) expansion in CNBP. When transcribed into CUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in a spliceopathy of downstream effector genes. The prevailing paradigm therefore is that both disorders are toxic RNA diseases. However, research indicates several additional pathogenic effects take place with respect to protein translation and turnover. Despite clinical and genetic similarities, myotonic dystrophy type 1 and type 2 are distinct disorders requiring different diagnostic and management strategies.

Predictors of atrio-ventricular conduction disease, long-term outcomes in patients with myotonic dystrophy types I and II.

BACKGROUND: Patients with myotonic dystrophy (DM) have an annual mortality of approximately 3.5%, one-third of which is sudden cardiac death. The predictors of cardiac conduction disease in these patients are incompletely defined. METHODS: A single-center cohort study included 211 patients with DM type 1 (DM1) and 25 DM type 2 (DM2). A severe electrocardiogram (ECG) abnormality was defined as a PR interval of ≥240 ms or QRS duration of ≥120 ms. RESULTS: A severe ECG abnormality was found in 24% of DM1 patients and 17% of DM2 patients. Among DM1 patients, those with a severe ECG abnormality were older (41.6 ± 14.6 vs 35.4 ± 12.6 years) and more likely to have hypertension (13.2% vs 4.2%, P = 0.038), heart failure (4.4% vs 0%, P = 0.056), atrial arrhythmias (6.6% vs 0.7%, P < 0.001), a higher number of trinucleotide repeats (689 ± 451 vs 474 ± 322, P = 0.01), and a family history of sudden cardiac death (26.7% vs 5.6%, P < 0.001) or pacemaker implantation (20% vs 0.7%, P < 0.001). Pacemakers or defibrillators were implanted in 14% of all patients, including 65% of patients with severe ECG abnormalities. During 57 ± 46 months, 13 patients died (1.16% per year), including three patients who died suddenly, two of whom had normally functioning pacemakers. CONCLUSION: In DM1, atrio-ventricular conduction disease is associated with increasing age, concomitant cardiovascular disease, nucleotide repeat length, and family history. The systematic identification of conduction disease and aggressive use of prophylactic pacemakers is associated with low rate of sudden cardiac death.

The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment.

The non-dystrophic myotonias are an important group of skeletal muscle channelopathies electrophysiologically characterized by altered membrane excitability. Many distinct clinical phenotypes are now recognized and range in severity from severe neonatal myotonia with respiratory compromise through to milder late-onset myotonic muscle stiffness. Specific genetic mutations in the major skeletal muscle voltage gated chloride channel gene and in the voltage gated sodium channel gene are causative in most patients. Recent work has allowed more precise correlations between the genotype and the electrophysiological and clinical phenotype. The majority of patients with myotonia have either a primary or secondary loss of membrane chloride conductance predicted to result in reduction of the resting membrane potential. Causative mutations in the sodium channel gene result in an abnormal gain of sodium channel function that may show marked temperature dependence. Despite significant advances in the clinical, genetic and molecular pathophysiological understanding of these disorders, which we review here, there are important unresolved issues we address: (i) recent work suggests that specialized clinical neurophysiology can identify channel specific patterns and aid genetic diagnosis in many cases however, it is not yet clear if such techniques can be refined to predict the causative gene in all cases or even predict the precise genotype; (ii) although clinical experience indicates these patients can have significant progressive morbidity, the detailed natural history and determinants of morbidity have not been specifically studied in a prospective fashion; (iii) some patients develop myopathy, but its frequency, severity and possible response to treatment remains undetermined, furthermore, the pathophysiogical link between ion channel dysfunction and muscle degeneration is unknown; (iv) there is currently insufficient clinical trial evidence to recommend a standard treatment. Limited data suggest that sodium channel blocking agents have some efficacy. However, establishing the effectiveness of a therapy requires completion of multi-centre randomized controlled trials employing accurate outcome measures including reliable quantitation of myotonia. More specific pharmacological approaches are required and could include those which might preferentially reduce persistent muscle sodium currents or enhance the conductance of mutant chloride channels. Alternative strategies may be directed at preventing premature mutant channel degradation or correcting the mis-targeting of the mutant channels.

Myotonic disorders.

Myotonia reflects a state of muscle fiber hyperexcitability. Impaired transmembrane conductance of either chloride or sodium ions results in myotonia. Myotonic disorders include the myotonic dystrophies and nondystrophic myotonias. Mutations in the genes encoding chloride (ClC-1) or sodium (SCN4A) channels expressed exclusively in skeletal muscle cause nondystrophic myotonias. Genetic defects in the myotonic dystrophies do not involve ion channel or its regulator proteins. Recent research supports a novel RNA-mediated disease mechanism of myotonia in the myotonic dystrophies. Myotonic dystrophy Type 1 is caused by CTG repeat expansion in the 3' untranslated region in the Dystrophia Myotonica Protein Kinase (DMPK) gene. Myotonic dystrophy Type 2 is caused by CCTG repeat expansion in the first intron in Zinc Finger Protein 9 (ZNF9) gene. The expanded repeat is transcribed in RNA and forms discrete inclusions in nucleus in both types of myotonic dystrophies. Mutant RNA sequesters MBNL1, a splice regulator protein and depletes MBNL1 from the nucleoplasm. Loss of MBNL1 results in altered splicing of ClC-1 mRNA. Altered splice products do not encode functional ClC-1 protein. Subsequent loss of chloride conductance in muscle membrane causes myotonia in the myotonic dystrophies. The purpose of this review is to discuss the clinical presentation, recent advances in understanding the disease mechanism with particular emphasis on myotonic dystrophies and potential therapy options in myotonic disorders.

Treatment of neuromuscular channelopathies: current concepts and future prospects.

Our understanding of the molecular pathogenesis of the neuromuscular ion channelopathies has increased rapidly over the past two decades due to the identification of many of the genes whose mutation causes these diseases. These molecular discoveries have facilitated identification and classification of the hereditary periodic paralyses and the myotonias, and are likely to shed light on acquired ion channelopathies as well. Despite our better understanding of the pathogenesis of these disorders, current treatments are largely empirical and the evidence in favor of specific therapy largely anecdotal. For periodic paralysis, dichlorphenamide--a carbonic anhydrase inhibitor--has been shown in a controlled trial to prevent attacks for many patients with both hypokalemic and hypokalemic periodic paralysis. A second trial, comparing dichlorphenamide with acetazolamide versus placebo, is currently in progress. For myotonia, there is only anecdotal evidence for treatment, but a controlled trial of mexiletine versus placebo is currently being funded by a Food and Drug Administration-orphan products grant and is scheduled to begin in late 2008. In the future, mechanism-based approaches are likely to be developed. For example, exciting advances have already been made in one disorder, myotonic dystrophy-1 (DM-1). In a mouse model of DM-1, a morpholino antisense oligonucleuotide targeting the 3' splice site of CLCN1 exon 7a repaired the RNA splicing defect by promoting the production of full-length chloride channel transcripts. Abnormal chloride conductance was restored, and myotonia was abolished. Similar strategies hold potential for DM-2. The era of molecularly-based treatments is about to begin.

New classification and treatment for myotonic disorders.

Myotonia is repetitive firing of muscle action potentials causing prolonged muscle contractions even after mechanical stimulations to the muscles have ceased. Most common myotonic disorder is myotonic dystrophy which is now termed DM1, myotonic dystrophy type 1. In Japan, proximal myotonic myopathy, which is now called DM2 has not been reported. Both DM1 and DM2 have Cl channel abnormality which causes myotonia. Less commonly we encounter Thomsen's disease, and autosomal recessive generalized myotonia (Becker type) which also have a Cl channel abnormality. There are other myotonic disorders related to Na channelopathy which include three disorders: paramyotonia congenita, adynamia episodica hereditaria, and myotonia fluctuans. Myotonia has been treated by various Na channel blockers, mexiletine, phenytoin, and carbamazepine, but they were originally developed for cardiac arrhythmia, or seizure disorders and they have undesirable side effects, weakness. Comprehensive treatment includes myotonia control without reducing the strength, and care for systemic manifestations of DM1.