Afterdischarges in myotonic dystrophy type 1

OBJECTIVE

Electrodiagnostic testing is an important screening test for myotonic dystrophy type 1 (DM1). Although myotonic discharges are observed on electromyography in cases of DM1, it is difficult to distinguish DM1 from other myotonic disorders clinically. In the present study, afterdischarges, another type of pathological potential revealed by electrodiagnostic testing, were analyzed, and their role in distinguishing DM1 from other myotonic disorders was explored.

METHODS

Data from 33 patients with myotonic discharges on electromyography were analyzed retrospectively. According to gene testing, the patients were divided into DM1 (n = 20) and non-DM1 myotonia (n = 13) groups. Afterdischarges were investigated by retrospectively evaluating the electrodiagnostic findings of motor nerve conduction studies, F-waves, and repetitive nerve stimulations.

RESULTS

Afterdischarges were observed in 17 of the 20 patients with DM1, with an occurrence rate of approximately 85%. However, afterdischarges were absent in all patients with non-DM1 myotonia. There were significant differences in the occurrence rate between the two groups (P < 0.01).

CONCLUSION

Afterdischarges may serve as a suggestive role in clinical diagnosis of DM1. The discovery that DM1 can present with afterdischarges may pave a new way to study the pathogenesis of DM1.

A c.1775C > T Point Mutation of Sodium Channel Alfa Subunit Gene (SCN4A) in a Three-Generation Sardinian Family with Sodium Channel Myotonia

Background

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.

Methods

Next-generation sequencing including the CLCN1 and SCN4A genes was performed in patients with clinical neuromuscular disorders. Electromyography, Short Exercise Test, in vivo and in vitro electrophysiology, site-directed mutagenesis and heterologous expression were collected.

Results

A heterozygous point mutation (c.1775C > T, p.Thr592Ile) of muscle voltage-gated sodium channel α subunit gene (SCN4A) has been identified in five female patients over three generations, in a family with non-dystrophic myotonia. The muscle stiffness and myotonia involve mainly the face and hands, but also affect walking and running, appearing early after birth and presenting a clear cold sensitivity. Very hot temperatures, menstruation and pregnancy also exacerbate the symptoms; muscle pain and a warm-up phenomenon are variable features. Neither paralytic attacks nor post-exercise weakness has been reported. Muscle hypertrophy with cramp-like pain and increased stiffness developed during pregnancy. The symptoms were controlled with both mexiletine and acetazolamide. The Short Exercise Test after muscle cooling revealed two different patterns, with moderate absolute changes of compound muscle action potential amplitude.

Conclusions

The p.Thr592Ile mutation in the SCN4A gene identified in this Sardinian family was responsible of clinical phenotype of myotonia.

Diagnostics in skeletal muscle channelopathies

INTRODUCTION

Skeletal muscle channelopathies (SMCs) are a heterogenous group of disorders, caused by mutations in skeletal ion channels leading to abnormal muscle excitability, resulting in either delayed muscle relaxation (myotonia) which characterizes non-dystrophic myotonias (NDMs), or membrane transient inactivation, causing episodic weakness, typical of periodic paralyses (PPs).

AREAS COVERED

SMCs include myotonia congenita, paramyotonia congenita, and sodium-channel myotonia among NDMs, and hyper-normokalemic, hypokalemic, or late-onset periodic paralyses among PPs. When suspecting an SMC, a structured diagnostic approach is required. Detailed personal and family history and clinical examination are essential, while neurophysiological tests should confirm myotonia and rule out alternative diagnosis. Moreover, specific electrodiagnostic studies are important to further define the phenotype of de novo cases and drive molecular analyses together with clinical data. Definite diagnosis is achieved through genetic testing, either with Sanger sequencing or multigene next-generation sequencing panel. In still unsolved patients, more advanced techniques, as exome-variant sequencing or whole-genome sequencing, may be considered in expert centers.

EXPERT OPINION

The diagnostic approach to SMC is still mainly based on clinical data; moreover, definite diagnosis is sometimes complicated by the difficulty to establish a proper genotype-phenotype correlation. Lastly, further studies are needed to allow the genetic characterization of unsolved patients.

A complex CLCN1 variant associated with hereditary myotonia in a mixed-breed dog

Hereditary myotonia (HM) is characterized by delayed muscle relaxation after contraction as a result of a mutation in the CLCN1 gene. We describe here a complex CLCN1 variant in a mixed-breed dog with clinical and electromyographic signs of HM. Blood samples from the myotonic dog, as well as from his male littermate and parents, were analyzed via amplification of the 23 exons encoding CLCN1. After sequencing the CLCN1 gene, a complex variant was found in exon 6 c.[705T>G; 708del; 712_732del], resulting in a premature stop codon in exon 7 and a protein that was 717 amino acids shorter than the normal CLC protein. The myotonic dog was identified as homozygous recessive for the complex CLCN1 variant; its parents were heterozygous, and its male littermate was homozygous wild-type. Knowledge of the CLCN1 mutations responsible for the development of hereditary myotonia allows greater clarification of this condition.

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.

Muscle fat replacement and contractility in patients with skeletal muscle sodium channel disorders

Skeletal muscle sodium channel disorders give rise to episodic symptoms such as myotonia and/or periodic paralysis. Chronic symptoms with permanent weakness are not considered characteristic of the phenotypes. Muscle fat replacement represents irreversible damage that inevitably will impact on muscle strength. This study investigates muscle fat replacement and contractility in patients with pathogenic SCN4A variants compared to healthy controls. T1-weighted and 2-point Dixon MRI of the legs were conducted to assess fat replacement. Stationary dynamometry was used to assess muscle strength. Contractility was determined by maximal muscle contraction divided by cross-sectional muscle area. The average cross-sectional intramuscular fat fraction was greater in patients compared with controls by 2.5% in the calves (95% CI 0.74-4.29%, p = 0.007) and by 2.0% in the thighs (95% CI 0.75-3.2%, p = 0.003). Muscle contractility was less in patients vs. controls by 14-27% (p < 0.05). Despite greater fat fraction and less contractility, absolute strength was not significantly less. This study quantitatively documents greater fat fraction and additionally describes difference in muscle contractility in a large cohort of patients with skeletal muscle sodium channel disorders. The clinical impact of these abnormal findings is likely limited as muscle hypertrophy in the patients served to preserve absolute muscle strength. Subgroup analysis indicated significant difference in phenotype by genotype, however these findings lack statistical significance and serve as inspiration for future researchers to probe into the geno- phenotype relationship in these disorders.Trial registration: The study was registered at http://clinicaltrials.gov (identifier: NCT04808388).

Muscle Channelopathies

PURPOSE OF REVIEW

This article describes the clinical features, diagnosis, pathophysiology, and management of nondystrophic myotonia and periodic paralysis.

RECENT FINDINGS

An increasing awareness exists about the genotype-phenotype overlap in skeletal muscle channelopathies, and thus genetic testing is needed to make a definitive diagnosis. Electrodiagnostic testing in channelopathies is highly specialized with significant overlap in various mutation subtypes. Randomized clinical trials have now been conducted in these disorders with expanded treatment options for patients with muscle channelopathies.

SUMMARY

Skeletal muscle channelopathies are rare heterogeneous conditions characterized by lifelong symptoms that require a comprehensive management plan that includes pharmacologic and nonpharmacologic interventions. The significant variability in biophysical features of various mutations, coupled with the difficulties of performing clinical trials in rare diseases, makes it challenging to design and implement treatment trials for muscle channelopathies.

Sequence CLCN1 and SCN4A genes in patients with nondystrophic myotonia in Chinese people

BACKGROUND

This study aimed to characterize the genetic, pathological, and clinical alterations of 17 patients in China presenting with nondystrophic myotonia (NDM) and to analyze the relationship between genotype and clinical phenotype.

METHODS

CLCN1 and SCN4A genes in patients with clinical features and muscle pathology indicative of NDM were sequenced. Furthermore, KCNE3 and CACNA1S genes were assessed in patients with wild-type CLCN1 and SCN4A.

RESULTS

Patients may have accompanying atypical myopathy as well as muscle hypertrophy, secondary dystonia, and joint contracture as determined by needle electromyography. All the study participants were administered mexiletine in combination with carbamazepine and showed significant improvements in myotonia symptoms in response to this therapy. CLCN1 gene mutation was detected in 8 cases diagnosed with myotonia congenital using gene screening. The detected mutations included 5 missense, 2 nonsense, 1 deletion, and 2 insertions. Further gene analysis showed 4 mutations in the SCN4A gene in patients diagnosed with paramyotonia congenita.

CONCLUSIONS

Myotonia congenita and paramyotonia congenita are the predominant forms of NDM in China. NDM may be best diagnosed using genetic analysis in associated with clinical features.

Translating genetic and functional data into clinical practice: a series of 223 families with myotonia

High-throughput DNA sequencing is increasingly employed to diagnose single gene neurological and neuromuscular disorders. Large volumes of data present new challenges in data interpretation and its useful translation into clinical and genetic counselling for families. Even when a plausible gene is identified with confidence, interpretation of the clinical significance and inheritance pattern of variants can be challenging. We report our approach to evaluating variants in the skeletal muscle chloride channel ClC-1 identified in 223 probands with myotonia congenita as an example of these challenges. Sequencing of CLCN1, the gene that encodes CLC-1, is central to the diagnosis of myotonia congenita. However, interpreting the pathogenicity and inheritance pattern of novel variants is notoriously difficult as both dominant and recessive mutations are reported throughout the channel sequence, ClC-1 structure-function is poorly understood and significant intra- and interfamilial variability in phenotype is reported. Heterologous expression systems to study functional consequences of CIC-1 variants are widely reported to aid the assessment of pathogenicity and inheritance pattern. However, heterogeneity of reported analyses does not allow for the systematic correlation of available functional and genetic data. We report the systematic evaluation of 95 CIC-1 variants in 223 probands, the largest reported patient cohort, in which we apply standardized functional analyses and correlate this with clinical assessment and inheritance pattern. Such correlation is important to determine whether functional data improves the accuracy of variant interpretation and likely mode of inheritance. Our data provide an evidence-based approach that functional characterization of ClC-1 variants improves clinical interpretation of their pathogenicity and inheritance pattern, and serve as reference for 34 previously unreported and 28 previously uncharacterized CLCN1 variants. In addition, we identify novel pathogenic mechanisms and find that variants that alter voltage dependence of activation cluster in the first half of the transmembrane domains and variants that yield no currents cluster in the second half of the transmembrane domain. None of the variants in the intracellular domains were associated with dominant functional features or dominant inheritance pattern of myotonia congenita. Our data help provide an initial estimate of the anticipated inheritance pattern based on the location of a novel variant and shows that systematic functional characterization can significantly refine the assessment of risk of an associated inheritance pattern and consequently the clinical and genetic counselling.

Co-occurrence of DMPK expansion and CLCN1 mutation in a patient with myotonia

INTRODUCTION

Myotonic disorders are a group of diseases affecting the muscle, in different ways. Myotonic dystrophy type 1 (DM1) is related to (CTG)n expansion in the 3-untranslated region of the dystrophia myotonica protein kinase (DMPK) gene and is the most frequent and disabling form, causing muscular, visibility, respiratory, and cardiac impairment. Non-dystrophic myotonias (NDMs) affect the skeletal muscle alone. In particular, mutations in the chloride channel (CLCN1) gene cause myotonia congenita (MC), which can have autosomal dominant or recessive inheritance.

CASE REPORT

We describe a patient with a family history of asymptomatic or paucisymptomatic myotonia, who presented handgrip myotonia which sharply reduced after mexiletine administration. Molecular analysis showed both a paternally inherited DMPK expansion and a maternally inherited CLCN1 mutation.

CONCLUSIONS

Only one other similar case was reported so far; however, the segregation of the two mutations and the characteristics of the muscle were not studied. Since our patient lacked the classical phenotypical and muscle histopathological characteristics of DM1 and showed mild splicing alterations despite a pathogenic DMPK expansion and the nuclear accumulation of toxic RNA, we may speculate that the co-occurrence of a CLCN1 mutation could have attenuated the severity of DM1 phenotype.

Functional and Structural Characterization of ClC-1 and Nav1.4 Channels Resulting from CLCN1 and SCN4A Mutations Identified Alone and Coexisting in Myotonic Patients

Non-dystrophic myotonias have been linked to loss-of-function mutations in the ClC-1 chloride channel or gain-of-function mutations in the Na_v1.4 sodium channel. Here, we describe a family with members diagnosed with Thomsen’s disease. One novel mutation (p.W322*) in CLCN1 and one undescribed mutation (p.R1463H) in SCN4A are segregating in this family. The CLCN1-p.W322* was also found in an unrelated family, in compound heterozygosity with the known CLCN1-p.G355R mutation. One reported mutation, SCN4A-p.T1313M, was found in a third family. Both CLCN1 mutations exhibited loss-of-function: CLCN1-p.W322* probably leads to a non-viable truncated protein; for CLCN1-p.G355R, we predict structural damage, triggering important steric clashes. The SCN4A-p.R1463H produced a positive shift in the steady-state inactivation increasing window currents and a faster recovery from inactivation. These gain-of-function effects are probably due to a disruption of interaction R1463-D1356, which destabilizes the voltage sensor domain (VSD) IV and increases the flexibility of the S4-S5 linker. Finally, modelling suggested that the p.T1313M induces a strong decrease in protein flexibility on the III-IV linker. This study demonstrates that CLCN1-p.W322* and SCN4A-p.R1463H mutations can act alone or in combination as inducers of myotonia. Their co-segregation highlights the necessity for carrying out deep genetic analysis to provide accurate genetic counseling and management of patients.

A novel heteroplasmic mutation in mitochondrial tRNAArg gene associated with non-dystrophic myotonias

Non-dystrophic myotonias (NDM) are rare diseases caused by defects in skeletal muscle chloride and sodium ion channels. It is well established that high-energy consuming tissues such as muscular and nervous systems are exclusively dependent on the ATP generation by mitochondria. The mitochondrial dysfunction, which is caused by mitochondrial DNA mutations, played an important role in the pathogenesis of non-dystrophic myotonias. The purpose of this study is to identify mitochondrial tRNA mutations in non-dystrophic myotonias patients. In this study, 45 Iranian patients with non-dystrophic myotonia were investigated for intracellular ATP content and the mutation screening in all the mitochondrial tRNA genes by DNA sequencing. Our findings showed that lymphocyte intracellular ATP is significantly decreased in NDM patients compared with control subjects (p = 0.001). We found nine mutations in mitochondrial tRNA genes, including m.4454 T > C (in the TψC loop of tRNA^Met), m.5568 A > G (tRNA^Trp), m.5794 T > C (in the anticodon loop of tRNA^Cys), novel m.10438 A > T, and m.10462 T > C (in anticodon loop and ACC stem of tRNA^Arg), m.12308 A > G (tRNA^Leu(CUN)) and m.15907 A > G, m.15924 A > G, and m.15928 G > A (in the anticodon stem of tRNA^Thr) in 31 NDM patients. These results suggest that novel m.10438 A > T mutation is involved in NDM patients and reinforces the significant association between this mutation in mitochondrial tRNA^Arg Gene and NDM patients (p = 0.008).

A zebrafish model of nondystrophic myotonia with sodium channelopathy

Nondystrophic myotonias are disorders of Na⁺ (Na_v1.4 or SCN4A) and Cl^- (CLCN1) channels in skeletal muscles, and frequently show phenotype heterogeneity. The molecular mechanism underlying their pathophysiology and phenotype heterogeneity remains unclear. As zebrafish models have been recently exploited for studies of the pathophysiology and phenotype heterogeneity of various human genetic diseases, a zebrafish model may be useful for delineating nondystrophic myotonias. Here, we generated transgenic zebrafish expressing a human mutant allele of SCN4A, referred to as Tg(mylpfa:N440K), and needle electromyography revealed increased number of myotonic discharges and positive sharp waves in the muscles of Tg(mylpfa:N440K) than in controls. In addition, forced exercise test at a water temperature of 24 °C showed a decrease in the distance moved, time spent in and number of visits to the zone with stronger swimming resistance. Finally, a forced exercise test at a water temperature of 18 °C exhibited a higher number of dive-bombing periods and drifting-down behavior than in controls. These findings indicate that Tg(mylpfa:N440K) is a good vertebrate model of exercise- and cold-induced human nondystrophic myotonias. This zebrafish model may contribute to provide insight into the pathophysiology of myotonia in sodium channelopathy and could be used to explore a new therapeutic avenue.

Myasthenic congenital myopathy from recessive mutations at a single residue in NaV1.4

OBJECTIVE

To identify the genetic and physiologic basis for recessive myasthenic congenital myopathy in 2 families, suggestive of a channelopathy involving the sodium channel gene, SCN4A.

METHODS

A combination of whole exome sequencing and targeted mutation analysis, followed by voltage-clamp studies of mutant sodium channels expressed in fibroblasts (HEK cells) and Xenopus oocytes.

RESULTS

Missense mutations of the same residue in the skeletal muscle sodium channel, R1460 of NaV1.4, were identified in a family and a single patient of Finnish origin (p.R1460Q) and a proband in the United States (p.R1460W). Congenital hypotonia, breathing difficulties, bulbar weakness, and fatigability had recessive inheritance (homozygous p.R1460W or compound heterozygous p.R1460Q and p.R1059X), whereas carriers were either asymptomatic (p.R1460W) or had myotonia (p.R1460Q). Sodium currents conducted by mutant channels showed unusual mixed defects with both loss-of-function (reduced amplitude, hyperpolarized shift of inactivation) and gain-of-function (slower entry and faster recovery from inactivation) changes.

CONCLUSIONS

Novel mutations in families with myasthenic congenital myopathy have been identified at p.R1460 of the sodium channel. Recessive inheritance, with experimentally established loss-of-function, is a consistent feature of sodium channel based myasthenia, whereas the mixed gain of function for p.R1460 may also cause susceptibility to myotonia.

A Mixed Periodic Paralysis & Myotonia Mutant, P1158S, Imparts pH-Sensitivity in Skeletal Muscle Voltage-gated Sodium Channels

Skeletal muscle channelopathies, many of which are inherited as autosomal dominant mutations, include myotonia and periodic paralysis. Myotonia is defined by a delayed relaxation after muscular contraction, whereas periodic paralysis is defined by episodic attacks of weakness. One sub-type of periodic paralysis, known as hypokalemic periodic paralysis (hypoPP), is associated with low potassium levels. Interestingly, the P1158S missense mutant, located in the third domain S4-S5 linker of the "skeletal muscle", Nav1.4, has been implicated in causing both myotonia and hypoPP. A common trigger for these conditions is physical activity. We previously reported that Nav1.4 is relatively insensitive to changes in extracellular pH compared to Nav1.2 and Nav1.5. Given that intense exercise is often accompanied by blood acidosis, we decided to test whether changes in pH would push gating in P1158S towards either phenotype. Our results suggest that, unlike in WT-Nav1.4, low pH depolarizes the voltage-dependence of activation and steady-state fast inactivation, decreases current density, and increases late currents in P1185S. Thus, P1185S turns the normally pH-insensitive Nav1.4 into a proton-sensitive channel. Using action potential modeling we predict a pH-to-phenotype correlation in patients with P1158S. We conclude that activities which alter blood pH may trigger the noted phenotypes in P1158S patients.

The Dystrophic and Nondystrophic Myotonias

PURPOSE OF REVIEW

This article describes clinical and electrical myotonia and provides an update on the classification, diagnosis, and management of myotonic disorders.

RECENT FINDINGS

In the myotonic dystrophies, antisense oligonucleotides provide a general strategy to correct RNA gain of function and modulate the expression of CTG expanded repeats; they are currently being tested in a phase 1-2 randomized controlled trial in patients with adult-onset myotonic dystrophy type 1. New genetic mutations are continuously being identified in the nondystrophic myotonias involving sodium and chloride channels. This contributes to the difficulty in describing genotype-phenotype correlations as the same mutations can give rise to different phenotypes, and the same phenotypes can arise from different mutations. Pharmacologic therapy is moving toward mutation-targeted treatments.

SUMMARY

This article describes the clinical and diagnostic characteristics and management of the myotonic dystrophies and the nondystrophic myotonias. Clinical features of the congenital, juvenile, and classic adult forms of myotonic dystrophy type 1 are reviewed, and for the adult form, reference is made to the main diagnostic and follow-up tests for which general consensus exists. The different clinical presentations of myotonic dystrophy type 2 and its main differential diagnostic options are also discussed. The clinical spectrum of the sodium and chloride channelopathies is described, and clinical diagnostic clues to differentiate between these two groups are provided. Therapeutic options for patients with nondystrophic myotonias are also presented with reference to literature review and the author's personal experience.

A case of non-dystrophic myotonia with concomitant mutations in the SCN4A and CLCN1 genes

Non-dystrophic myotonias are caused by mutations of either the skeletal muscle chloride (CLCN1) or sodium channel (SCN4A) gene. They exhibit several distinct phenotypes, including myotonia congenita, paramyotonia congenita and sodium channel myotonia, and a genotype-phenotype correlation has been established. However, there are atypical cases that do not fit with the standard classification. We report a case of 27-year-old male who had non-dystrophic myotonia with periodic paralysis and two heterozygous mutations, E950K in CLCN1 and F1290L in SCN4A. His mother, who exhibited myotonia without paralytic attack, only harbored E950K, and no mutations were identified in his asymptomatic father. Therefore, the E950K mutation was presumed to be pathogenic, although it was reported as an extremely rare genetic variant. The proband experienced paralytic attacks that lasted for weeks and were less likely to be caused by CLCN1 mutation alone. Functional analysis of the F1290L mutant channel heterologously expressed in cultured cells revealed enhanced activation inducing membrane hyperexcitability. We therefore propose that the two mutations had additive effects on membrane excitability that resulted in more prominent myotonia in the proband. Our case stresses the value of performing genetic analysis of both CLCN1 and SCN4A genes for myotonic patients with an atypical phenotype.

[Myotonia and cardiac conduction defects in myotonic dystrophy and defect in ion channels]

Myotonic dystrophy (DM), the most common hereditary muscle disease in adults, is caused by the unstable genomic expansion of simple sequence repeats. This disease is characterized by myotonia and various multisystemic complications, most commonly those of the cardiac, endocrine, and central nervous systems. The cardiac abnormalities, especially cardiac conduction defects, significantly contribute to morbidity and mortality in DM patients. Therefore, understanding the pathophysiology of cardiac conduction defects in DM is important. The pathomechanism of DM has been thoroughly investigated. The mutant RNA transcripts containing the expanded repeat give rise to a toxic gain-of-function by perturbing splicing factors in the nucleus, leading to the misregulation of alternative pre-mRNA splicing. In particular, several studies, including ours, have shown that myotonia is caused by alternative splicing of the CLCN1 gene coding the voltage-gated chloride channel in skeletal muscle through an "RNA-dominant mechanism". Since the aberrantly spliced isoform does not seem to form a functional channel, the feature of skeletal muscle in DM can be interpreted as a "channelopathy" caused by reduced chloride channel protein. Similarly, we recently identified a misregulation of alternative splicing in an ion channel gene which is known to be responsible for arrhythmic disease showing Mendelian inheritance. Here, we review the cardiac manifestation and RNA-dominant mechanism of DM, and discuss the possible pathophysiology of cardiac conduction defects by referring to hereditary arrhythmic diseases, such as long QT syndrome and Brugada syndrome.

Muscle channelopathies and related diseases

Muscle channelopathies and related disorders are neuromuscular disorders predominantly of genetic origin which are caused by mutations in ion channels or genes that play a role in muscle excitability. They include different forms of periodic paralysis which are characterized by acute and reversible attacks of muscle weakness concomitant to changes in blood potassium levels. These disorders may also present as distinguishable myotonic syndromes (slowed muscle relaxation) which have in common lack of involvement of dystrophic changes of the muscle, in contrast to dystrophia myotonica. Recent advances have been made in the diagnosis of these different disorders, which require, in addition to a careful clinical evaluation, detailed EMG and molecular study. Although these diseases are rare, they deserve attention since patients may benefit from drugs which can dramatically improve their condition. Patients may have atypical presentations, sometimes life-threatening, which may delay a proper diagnosis, mostly in the first months of life. The creation of specialized reference centers in the Western world has greatly benefited the proper recognition of these neuromuscular diseases.

[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%).

Skeletal-muscle channelopathies: periodic paralysis and nondystrophic myotonias

PURPOSE OF REVIEW

To provide a current review of clinical phenotypes, genetics, molecular pathophysiology, and electro-diagnostic testing strategies of periodic paralysis and nondystrophic myotonias.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis and nondystrophic myotonias continues to increase. Important insight into the molecular pathogenesis of muscle sodium channelopathies has been revealed by the finding of 'leaky' closed sodium channels. Previously, alterations in sodium-channel activation or inactivation have been identified as important disease mechanisms. The recent discovery that substitutions of key arginine residues in the voltage-sensing segment of the channel may lead to a 'pore leak' when the channel is closed suggests a new mechanism. Since similar mutations exist in corresponding positions of other channels, this mechanism may apply to other channel diseases. The recognition of different electrophysiological patterns that are specific to muscle ion-channel genotypes will be useful in diagnosis and in guiding genetic testing. Recent studies demonstrate that magnetic resonance imaging may be used to detect intramuscular accumulation of sodium during episodes of weakness.

SUMMARY

Recent advances have refined our ability to make a precise molecular diagnosis in muscle channelopathies. The description of a pore leak with voltage-sensor mutations may represent a new disease mechanism.

Computerized hand grip myometry reliably measures myotonia and muscle strength in myotonic dystrophy (DM1)

The aim of this study was to develop a reliable, sensitive, quantitative measure of grip myotonia and strength and to determine whether CTG repeat length is correlated with grip myotonia and with muscle strength in myotonic dystrophy type 1 (DM1). Three maximum voluntary isometric contractions (MVICs) of the finger flexors (i.e., handgrip) were recorded on 2 successive days using a computerized handgrip myometer in 29 genetically confirmed DM1 patients and 17 normals. An automated computer program calculated MVIC peak force (PF) and relaxation times (RTs) along the declining (relaxation) phase of the force recordings at 90%, 75%, 50%, 10%, and 5% of PF. Patients also underwent quantitative strength testing (QST) manual muscle testing (MMT). The patients had longer grip RTs and lower PFs than normals. RT (90% to 5%) was above the normal mean +2.5 SD in 25 (86%) patients. In DM1, prolongation of RT was mainly in the terminal (50% to 5%), rather than the initial (90% to 50%) phase of relaxation. PFs and RTs for each patient were reproducible on consecutive days. RTs were positively correlated with leukocyte CTG repeat length, whereas measures of muscle strength, such as PF, QST, and MMT, were negatively correlated with repeat length. We conclude that computerized handgrip myometry provides a sensitive, reliable measure of myotonia and strength in DM1 and offers a method to assess natural history and response to treatment.

A novel mutation of the CLCN1 gene associated with myotonia hereditaria in an Australian cattle dog

BACKGROUND

Heritable myotonia is a genetic muscle disorder characterized by slow relaxation of skeletal muscles. The main clinical signs are skeletal muscle stiffness, especially after vigorous contraction, and muscle hypertrophy. Muscle stiffness may be enhanced by inactivity, and often is relieved by exercise. Myotonia can be inherited in an autosomal dominant or recessive manner (Thomsen- or Becker-type myotonia, respectively). In mice, goats, Miniature Schnauzer dogs, and most affected humans, the disorder is caused by mutations in CLCN1, which encodes the skeletal muscle voltage-gated chloride channel, Cl1C-1.

HYPOTHESIS

We hypothesized that an Australian Cattle Dog with generalized muscle stiffness and hypertrophy examined at the Ontario Veterinary College would have a mutation in the CLCN1 gene.

ANIMALS

A pure-bred Australian Cattle Dog from Ontario, Canada, was used.

METHODS

Based on clinical signs and electromyographic test results, a diagnosis of myotonia hereditaria was made, and a muscle biopsy was collected for genetic analysis.

RESULTS

Sequence data obtained from the affected dog confirmed that it was homozygous for a single base insertion in the CLCN1 coding sequence. This mutation would result in a truncated ClC-1 protein being expressed, which, based on molecular evidence from other studies, would result in functionally compromised chloride conduction in the skeletal muscles of the animal.

CONCLUSIONS AND CLINICAL IMPORTANCE

To the authors' knowledge, this report describes the Ist case of myotonia in an Australian Cattle Dog and represents the 1st non-Schnauzer canine myotonia to be genetically characterized. In addition, we developed a polymerase chain reaction-based genetic screen to detect heterozygotes with this mutation in the at-large Australian Cattle Dog population.

Myotonia-related mutations in the distal C-terminus of ClC-1 and ClC-0 chloride channels affect the structure of a poly-proline helix

Myotonia is a state of hyperexcitability of skeletal-muscle fibres. Mutations in the ClC-1 Cl- channel cause recessive and dominant forms of this disease. Mutations have been described throughout the protein-coding region, including three sequence variations (A885P, R894X and P932L) in a distal C-terminal stretch of residues [CTD (C-terminal domain) region] that are not conserved between CLC proteins. We show that surface expression of these mutants is reduced in Xenopus oocytes compared with wild-type ClC-1. Functional, biochemical and NMR spectroscopy studies revealed that the CTD region encompasses a segment conserved in most voltage-dependent CLC channels that folds with a secondary structure containing a short type II poly-proline helix. We found that the myotonia-causing mutation A885P disturbs this structure by extending the poly-proline helix. We hypothesize that this structural modification results in the observed alteration of the common gate that acts on both pores of the channel. We provide the first experimental investigation of structural changes resulting from myotonia-causing mutations.

[A girl with hereditary myotonia due to an exceptional sodium channel mutation]

A 22-month-old girl had cramps and stiffness of her muscles. After medical history, physical examination and an EMG, a short differential diagnosis based on the symptoms of myotonia was made. Initially, the symptoms were incorrectly assumed to be due to Becker's myotonia, an autosomal recessive condition caused by a mutation in the chloride channel. Molecular analysis did not show a defect in the chloride channel, but instead a defect in the sodium channel of the muscle fibre. Since defects in the sodium channel are responsible for several myotonic diseases, further analysis was necessary. Based on knowledge of the structure and mechanism of the sodium channel and study of literature on cases involving the identical mutation, the diagnosis 'potassium-aggravated myotonia' (PAM) was made. Re-evaluation of the patient showed that her symptoms fitted the diagnosis 'myotonia permanens', the severest form of PAM. She was treated with mexiletine. In myotonia several features can give direction to the diagnosis, including sensitivity to temperature and exercise, and family history. However, it is often necessary to use molecular analysis to be able to diagnose the disease correctly, make a prognosis and predict the risk of recurrence as well as to formulate a treatment plan.

Cold extends electromyography distinction between ion channel mutations causing myotonia

OBJECTIVE

Myotonias are inherited disorders of the skeletal muscle excitability. Nondystrophic forms are caused by mutations in genes coding for the muscle chloride or sodium channel. Myotonia is either relieved or worsened by repeated exercise and can merge into flaccid weakness during exposure to cold, according to causal mutations. We designed an easy electromyography (EMG) protocol combining repeated short exercise and cold as provocative tests to discriminate groups of mutations.

METHODS

Surface-recorded compound muscle action potential was used to monitor muscle electrical activity. The protocol was applied on 31 unaffected control subjects and on a large population of 54 patients with chloride or sodium channel mutations known to cause the different forms of myotonia.

RESULTS

In patients, repeated short exercise test at room temperature disclosed three distinct abnormal patterns of compound muscle action potential changes (I-III), which matched the clinical symptoms. Combining repeated exercise with cold exposure clarified the EMG patterns in a way that enabled a clear correlation between the electrophysiological and genetic defects.

INTERPRETATION

We hypothesize that segregation of mutations into the different EMG patterns depended on the underlying pathophysiological mechanisms. Results allow us to suggest EMG guidelines for the molecular diagnosis, which can be used in clinical practice.

Autosomal dominant monosymptomatic myotonia permanens

Myotonia permanens is associated with a G1306E mutation in the SCN4A gene. Two sporadic patients have been reported, but the clinical phenotype has not been fully characterized. The authors report a family in which the disease is autosomal dominantly inherited. The patients have severe myotonia, but the clinical picture is not qualitatively different from that seen in other nondystrophic myotonias.

Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy

RNA-mediated pathogenesis is a recently developed disease model that proposes that certain types of mutant genes produce toxic transcripts that inhibit the activities of specific proteins. This pathogenesis model was proposed first for the neuromuscular disease myotonic dystrophy (DM), which is associated with the expansion of structurally related (CTG)(n) and (CCTG)(n) microsatellites in two unrelated genes. At the RNA level, these expansions form stable hairpins that alter the pre-mRNA splicing activities of two antagonistic factor families, the MBNL and CELF proteins. It is unclear which altered activity is primarily responsible for disease pathogenesis and whether other factors and biochemical pathways are involved. Here, we show that overexpression of Mbnl1 in vivo mediated by transduction of skeletal muscle with a recombinant adeno-associated viral vector rescues disease-associated muscle hyperexcitability, or myotonia, in the HSA(LR) poly(CUG) mouse model for DM. Myotonia reversal occurs concurrently with restoration of the normal adult-splicing patterns of four pre-mRNAs that are misspliced during postnatal development in DM muscle. Our results support the hypothesis that the loss of MBNL1 activity is a primary pathogenic event in the development of RNA missplicing and myotonia in DM and provide a rationale for therapeutic strategies designed either to overexpress MBNL1 or inhibit MBNL1 interactions with CUG and CCUG repeat expansions.

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

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

Myotonia and muscle contractile properties in mice with SIX5 deficiency

Myotonic dystrophy (DM1) is an autosomal-dominant multisystem disease characterized by progressive skeletal muscle weakness, myotonia, cataracts, cardiac arrhythmias, mild mental retardation, and endocrinopathies. Heterozygous loss of SIX5 in mice causes cataracts and cardiac conduction disease, and homozygous loss also leads to sterility and decreased testicular mass, reminiscent of DM1 in humans. The effect of SIX5 deficiency in muscle is unknown. In this study, we found that muscle contractile properties, electromyographic insertional activity, and muscle histology were normal in SIX5 deficient mice. The implications of these findings for the pathogenesis of DM1 are discussed.

A C-terminal skeletal muscle sodium channel mutation associated with myotonia disrupts fast inactivation

Missense mutations in the skeletal muscle sodium channel alpha-subunit gene (SCN4A) are associated with a group of clinically overlapping diseases caused by alterations in the excitability of the sarcolemma. Sodium channel defects may increase excitability and cause myotonic stiffness or may render fibres transiently inexcitable to produce periodic paralysis. A patient with cold-aggravated myotonia did not harbour any of the common SCN4A mutations. We therefore screened all 24 exons by denaturing high-performance liquid chromatography, followed by direct sequencing. Two novel missense changes were found with predicted amino acid substitutions: T323M in the DIS5-S6 loop and F1705I in the intracellular C-terminus. The functional impact of these substitutions was assessed by recording whole-cell Na+ currents from transiently transfected HEK293 cells. T323M currents were indistinguishable from wild-type (WT). Fast inactivation was impaired for F1705I channels, as demonstrated by an 8.6-mV rightwards shift in voltage dependence and a two-fold slowing in the rate of inactivation. Recovery from fast inactivation was not altered, nor was there an increase in the persistent current after a 50- ms depolarization. Activation and slow inactivation were not appreciably affected. These data suggest that T323M is a benign polymorphism, whereas F1705I results in fast inactivation defects, which are often observed for myotonia. This is the first example of a C-terminal mutation in SCN4A associated with human disease. Like the cardiac disorders (long QT syndrome type 3 or Brugada syndrome) and generalized epilepsy with febrile seizures plus (GEFS+) associated with C-terminal mutations in other NaV channels, the primary effect of F1705I was a partial disruption of fast inactivation.

Treatment in myotonia and periodic paralysis

The myotonic disorders, including the myotonic dystrophies (myotonic dystrophy type 1, DM1; myotonic dystrophy type 2, DM2/PROMM/PDM), the muscle channelopathies or non-dystrophic myotonias (chloride, sodium, calcium and potassium channelopathies) are all characterized by myotonia and muscle weakness despite different pathophysiology involved in these disorders. Myotonia may affect the eye, facial and jaw muscles as well as the hands and legs. It may be painful and disabling. Muscle weakness may be episodical as in the paralytic attacks of the sodium and calcium channelopathies or culminate in permanent muscle weakness as in the calcium channelopathies and some sodium channelopathies associated to specific point mutations. The severity of myotonia may fluctuate in the myotonic dystrophies, but weakness is usually fixed, affecting neck flexors, facial and jaw muscles as well as proximal and distal muscles of the limbs. Despite the recent progress in molecular genetics the precise mechanisms responsible for myotonia and weakness are not fully understood and there is no standardized treatment strategy. We present a review of selected treatment trials in the myotonic disorders and the muscle channelopathies, and discuss our experience in the treatment of myotonia and muscle weakness, with reference to the limits and advantages of treatment trials in this field.

Late-onset mitochondrial disorder with electromyographic evidence of myotonia

We describe a patient with chronic progressive external ophthalmoplegia (CPEO) due to a deletion of mitochondrial DNA (mtDNA) who had electromyographic evidence of myotonic discharges. Myotonia has not previously been described in association with mitochondrial disease and this report extends the known phenotypic expression of these disorders.

A rare form of painful nondystrophic myotonia

OBJECTIVE

In this paper we report a painful nondystrophic myotonia which has not been previously described. Pain is a rare symptom in myotonia. We report a myotonic disorder in a 34-year-old woman and her 14-year-old daughter. Painful cramps occur during and after exercise in the mother, and both patients can demonstrate unusual contractions in the tongue. In the present study we try to evaluate the mechanisms behind the unique finding of trains of high amplitude of positive waves, not seen in the earlier known myotonic conditions.

METHODS

Clinical investigations and electromyography with single and dual channel recordings and muscle morphometry were performed.

RESULTS

The electromyographic recordings reveal positive waves, fibrillation potentials and myotonic discharges. In addition, extraordinary findings were made of trains of high frequency positive potentials with very high amplitudes and with conduction block along the muscle fibres.

CONCLUSIONS

In this new form of myotonia with likely dominant heredity, the specific finding of trains of high amplitude positive waves indicates ephaptic transmission within bundles of neighbouring muscle fibres.

Different flecainide sensitivity of hNav1.4 channels and myotonic mutants explained by state-dependent block

Flecainide, a class IC antiarrhythmic, was shown to improve myotonia caused by sodium channel mutations in situations where the class IB antiarrhythmic drug mexiletine was less efficient. Yet little is known about molecular interactions between flecainide and human skeletal muscle sodium (hNa(v)1.4) channels. Whole-cell sodium currents (I(Na)) were recorded in tsA201 cells expressing wild-type (WT) and mutant hNa(v)1.4 channels (R1448C, paramyotonia congenita; G1306E, potassium-aggravated myotonia). At a holding potential (HP) of -120 mV, flecainide use-dependently blocked WT and G1306E I(Na) equally but was more potent on R1448C channels. For WT, the extent of block depended on a holding voltage more negative than the activation threshold, being greater at -90 mV as compared to -120 and -180 mV. This behaviour was exacerbated by the R1448C mutation since block at -120 mV was greater than that at -180 mV. Thus flecainide can bind to inactivated sodium channels in the absence of channel opening. Nevertheless, all the channels showed the same closed-state affinity constant (K(R) approximately 480 microM) and the same inactivated-state affinity constant (K(I) approximately 18 microM). Simulations according to the modulated receptor hypothesis mimic the voltage-dependent block of WT and mutant channels by flecainide and mexiletine. All the results suggest similar blocking mechanisms for the two drugs. Yet, since flecainide exerts use-dependent block at lower frequency than mexiletine, it may exhibit greater benefit in all myotonic syndromes. Moreover, flecainide blocks hNa(v)1.4 channel mutants with a rightward shift of availability voltage dependence more specifically than mexiletine, owing to a lower K(R)/K(I) ratio. This study offers a pharmacogenetic strategy to better address treatment in individual myotonic patients.

Characterization of two new dominant ClC-1 channel mutations associated with myotonia

Voltage-gated ClC-1 chloride channels encoded by the CLCN1 gene have a major role in setting the membrane potential in skeletal muscle. More than 60 CLCN1 mutations have been associated with myotonia congenita. These mutations are traditionally classified as recessive (Becker's disease) or dominant (Thomsen's disease). In this study, we have electrophysiologically characterized two new dominant ClC-1 mutations, thereby elucidating the observed phenotype in patients. The two ClC-1 mutants M128V and E193K were identified, and the DNA was isolated from patients and subsequently expressed in Xenopus laevis oocytes for electrophysiological characterization. Both ClC-1 mutants, M128V and E193K, showed a large rightward shift in the current-voltage relationship. In addition, the activation kinetics were slowed in the ClC-1 M128V mutant, as compared to the wild-type ClC-1. Interestingly, ClC-1 E193K revealed a change in reversal potential compared to wild-type channels. This finding supports the notion that the E193 amino acid is an important determinant in the selectivity filter of the human ClC-1 channel. The electrophysiological behavior of both mutants demonstrates a severe reduction in ClC-1 channel conductance under physiologically relevant membrane potentials. These studies thereby explain the molecular background for the observed myotonia in patients.

A novel alteration of muscle chloride channel gating in myotonia levior

Mutations in the voltage-dependent skeletal muscle chloride channel, ClC-1, result in dominant or recessive myotonia congenita. The Q552R mutation causes a variant of dominant myotonia with a milder phenotype, myotonia levior. To characterise the functional properties of this mutation, homodimeric mutant and heterodimeric wild-type (WT) mutant channels were expressed in tsA201 cells and studied using the whole-cell recording technique. Q552R ClC-1 mutants formed functional channels with normal ion conduction but altered gating properties. Mutant channels were activated by membrane depolarisation, with a voltage dependence of activation that was shifted by more than +90 mV compared to WT channels. Q552R channels were also activated by hyperpolarisation, and this process was dependent upon the intracellular chloride concentration ([Cl(-)](i)). Together, these alterations resulted in a substantial reduction in the open probability at -85 mV at a physiological [Cl(-)](i). Heterodimeric WT-Q552R channels did not exhibit hyperpolarisation-activated gating transitions. As was the case for WT channels, activation occurred upon depolarisation, but the activation curve was shifted by 28 mV to more positive potentials. The functional properties of heterodimeric channels suggest a weakly dominant effect, a finding that correlates with the inheritance pattern and symptom profile of myotonia levior.

Skeletal muscle channelopathies

Ion channelopathies have common clinical features, recurrent patterns of mutations, and almost predictable mechanisms of pathogenesis. In skeletal muscle, disorders are associated with mutations in voltage-gated Na(+), K(+), Ca(2+), and Cl(-) channels leading to hypoexcitability, causing periodic paralysis and to hyperexcitabilty, resulting in myotonia or susceptibility to malignant hyperthermia.

Myotonia caused by mutations in the muscle chloride channel gene CLCN1

Pure non-syndromic, non-dystrophic myotonia in humans is caused by mutations in the genes coding for the skeletal muscle sodium channel (SCN5A) or the skeletal muscle chloride channel (CLCN1) with similar phenotypes. Chloride-channel myotonia can be dominant (Thomsen-type myotonia) or recessive (Becker-type myotonia). More than 60 myotonia-causing mutations in the CLCN1 gene have been identified, with only a few of them being dominant. A common phenotype of dominant mutations is a dominant negative effect of mutant subunits in mutant-WT heterodimers, causing a large shift of the steady-state open probability voltage-dependence towards more positive, unphysiological voltages. The study of the properties of disease causing mutations has helped in understanding the functional properties of the CLC-1 channel that is part of a nine-member gene family of chloride channels. The large body of knowledge obtained for CLC-1 may also help to better understand the other CLC channels, three of which are also involved in genetic diseases.

Mexiletine block of disease-associated mutations in S6 segments of the human skeletal muscle Na(+) channel

1. Over twenty different missense mutations in the alpha-subunit of the adult skeletal muscle Na(+) channel (hSkM1) have been identified as a cause of myotonia or periodic paralysis. We examined state-dependent mexiletine block for mutations involving the putative binding site in S6 segments (V445M, S804F, V1293I, V1589M and M1592V). Whole-cell Na(+) currents were measured from wild-type (WT) and mutant channels transiently expressed in HEK cells. 2. Use-dependent block (10 ms pulses to -10 mV, at 20 Hz) in 100 microM mexiletine was reduced modestly by mutations in IVS6 (V1589M, M1592V) and enhanced by the mutation in IS6 (V445M). For mutations in IIS6 (S804F) and IIIS6 (V1293I) use-dependent block was not statistically different from that of wild-type channels. 3. Resting-state block (10 ms pulses to -10 mV from -150 mV, at 0.1 Hz) of S6 mutants was comparable to that of WT (dissociation constant for resting channels, K(R) = 650 +/- 40 microM, n = 9). The S6 mutant with the greatest change in K(R) was V445M (K(R) = 794 +/- 45 microM, n = 5), but this difference was only marginally significant (P = 0.047). 4. A modified technique for estimating local anaesthetic affinity of inactivated channels was developed to reduce errors due to slow inactivation and to failure of drug binding to reach equilibrium. Mexiletine affinity for inactivated channels was reduced by mutations in IVS6 (V1589M: dissociation constant for the inactivated state (K(I)) = 44.7 microM; M1592V: K(I) = 40.0 microM) and increased by the mutation in IS6 (V445M: K(I) = 15.0 microM), compared to wild-type channels (K(I) = 28.3 microM). 5. We conclude that the disease-associated S6 mutations in domains I-IV cause at most a 2-fold change in inactivated state affinity and have even less of an effect on resting block. Model simulations show that the reduced use-dependent block of IVS6 mutants derives primarily from an increased off-rate at hyperpolarized potentials, whereas the enhanced use-dependent block of the IS6 mutant was due to a higher affinity for inactivated V445M channels.

Increased rigidity of the chiral centre of tocainide favours stereoselectivity and use-dependent block of skeletal muscle Na(+) channels enhancing the antimyotonic activity in vivo

1. Searching for the structural requirements improving the potency and the stereoselectivity of Na(+) channel blockers as antimyotonic agents, new derivatives of tocainide, in which the chiral carbon atom is constrained in a rigid alpha-proline or pyrrolo-imidazolic cycle, were synthesized as pure enantiomers. 2. Their ability to block Na(+) currents, elicited from -100 to -20 mV at 0.3 Hz (tonic block) and 2-10 Hz (use-dependent block) frequencies, was investigated in vitro on single fibres of frog semitendinosus muscle using the vaseline-gap voltage-clamp method. 3. The alpha-proline derivative, To5, was 5 and 21 fold more potent than tocainide in producing tonic and 10 Hz-use-dependent block, respectively. Compared to To5, the presence of one methyl group on the aminic (To6) or amidic (To7) nitrogen atom decreased use-dependence by 2- and 6-times, respectively. When methylene moieties were present on both nitrogen atoms (To8), both tonic and use-dependent block were reduced. 4. Contrarily to tocainide, all proline derivatives were stereoselective in relation to an increased rigidity. A further increase in the molecular rigidity as in pyrrolo-imidazolic derivatives markedly decreased the drug potency with respect to tocainide. 5. Antimyotonic activity, evaluated as the shortening of the time of righting reflexes of myotonic adr/adr mice upon acute drug in vivo administration was 3 fold more effective for R-To5 than for R-Tocainide. 6. Thus, constraining the chiral centre of tocainide in alpha-proline cycle leads to more potent and stereoselective use-dependent Na(+) channel blockers with improved therapeutic potential.

Mice transgenic for the human myotonic dystrophy region with expanded CTG repeats display muscular and brain abnormalities

The autosomal dominant mutation causing myotonic dystrophy (DM1) is a CTG repeat expansion in the 3'-UTR of the DM protein kinase (DMPK) gene. This multisystemic disorder includes myotonia, progressive weakness and wasting of skeletal muscle and extramuscular symptoms such as cataracts, testicular atrophy, endocrine and cognitive dysfunction. The mechanisms underlying its pathogenesis are complex. Recent reports have revealed that DMPK gene haploinsufficiency may account for cardiac conduction defects whereas cataracts may be due to haploinsufficiency of the neighboring gene, the DM-associated homeobox protein (DMAHP or SIX5) gene. Furthermore, mice expressing the CUG expansion in an unrelated mRNA develop myotonia and myopathy, consistent with an RNA gain of function. We demonstrated that transgenic mice carrying the CTG expansion in its human DM1 context (>45 kb) and producing abnormal DMPK mRNA with at least 300 CUG repeats, displayed clinical, histological, molecular and electrophysiological abnormalities in skeletal muscle consistent with those observed in DM1 patients. Like DM1 patients, these transgenic mice show abnormal tau expression in the brain. These results provide further evidence for the RNA trans-dominant effect of the CUG expansion, not only in muscle, but also in brain.

Two mutations in the IV/S4-S5 segment of the human skeletal muscle Na+ channel disrupt fast and enhance slow inactivation

Fast and slow inactivation (FI, SI) of the voltage-gated Na+ channel are two kinetically distinct and structurally dissociated processes. The voltage sensor IV/S4 and the intracellular IV/S4-S5 loop have been shown to play an important role in FI mediating the coupling between activation and inactivation. Two mutations in IV/S4-S5 of the human muscle Na+ channel, L1482C/A, disrupt FI by inducing a persistent Na+ current, shifting steady-state inactivation in the depolarizing direction and accelerating its recovery. These effects were more pronounced for L1482A. In contrast, SI of L1482C/A channels was enhanced showing a more complete SI and a 3-fold slowing of its recovery. Effects on SI were more pronounced for L1482C. The results indicate an important role of the IV/S4-S5 loop not only in FI but also in SI of the Na+ channel.

A new mutation in a family with cold-aggravated myotonia disrupts Na(+) channel inactivation

OBJECTIVE

To identify the molecular and physiologic abnormality in familial myotonia with cold sensitivity, hypertrophy, and no weakness.

BACKGROUND

Sodium channel mutations were previously identified as the cause of several allelic disorders with varying combinations of myotonia and periodic paralysis. A three-generation family with dominant myotonia aggravated by cooling, but no weakness, was screened for mutations in the skeletal muscle sodium channel alpha-subunit gene (SCN4A).

METHODS

Single-strand conformation polymorphism was used to screen all 24 exons of SCN4A and abnormal conformers were sequenced to confirm the presence of mutations. The functional consequence of a SCN4A mutation was explored by recording sodium currents from human embryonic kidney cells transiently transfected with an expression construct that was mutated to reproduce the genetic defect.

RESULTS

A three-generation Italian family with myotonia is presented, in which a novel SCN4A mutation (leucine 266 substituted by valine, L266V) is identified. This change removes only a single methylene group from the 1,836-amino-acid protein, and is present in a region of the protein previously not known to be critical for channel function (domain I transmembrane segment 5). Electrophysiologic studies of the L266V mutation showed defects in fast inactivation, consistent with other disease-causing SCN4A mutations studied to date. Slow inactivation was not impaired.

CONCLUSIONS

This novel mutation of the sodium channel indicates that a single carbon change in a transmembrane alpha-helix of domain I can alter channel inactivation and cause cold-sensitive myotonia.

Genetics, an alternative way to discover, characterize and understand ion channels

1. The conventional approach to understanding the structure and properties of ion channels has been to use physiological characterization. 2. Purification and molecular cloning of ion channel genes has enabled more detailed structure-function analyses to be undertaken. 3. An alternative approach to the identification of genes of pathophysiological importance has been the use of genetic linkage approaches and positional cloning or positional candidate analysis of ion channel genes. 4. Using genetic approaches, mutations have been described that cause inherited neurological disorders of neurons (e.g. epilepsy, migraine, deafness, ataxia and startle disease), skeletal muscle (myotonia, malignant hyperthermia, periodic paralysis and myasthenia) and cardiac muscle (long QT syndrome and ventricular fibrillation). 5. For each disease, gene structure-function analyses of the mutant alleles have provided further insights into the biology of ion channels. 6. The present brief review examines the methods used in genetic linkage studies and positional cloning of disease genes. Understanding how ion channel gene mutations give rise to dysfunctional channels will be important in defining and treating the episodic and chronic channelopathies.

From tonus to tonicity: physiology of CLC chloride channels

Chloride channels are involved in a multitude of physiologic processes ranging from basal cellular functions such as cell volume regulation and acidification of intracellular vesicles to more specialized mechanisms such as vectorial transepithelial transport and regulation of cellular excitability. This plethora of functions is accomplished by numerous functionally highly diverse chloride channels that are only partially identified at the molecular level. The CLC family of chloride channels comprises at present nine members in mammals that differ with respect to biophysical properties, cellular compartmentalization, and tissue distribution. Their common structural features include a predicted topology model with 10 to 12 transmembrane regions together with two C-terminal CBS domains. Loss of function mutations affecting three different members of the CLC channel family lead to three human inherited diseases : myotonia congenita, Dent's disease, and Bartter's syndrome. These diseases, together with the diabetes insipidus symptoms of a knockout mouse model, emphasize the physiologic relevance of this ion channel family.