Electrical myotonia in heterozygous carriers of recessive myotonia congenita

Inherited myopathy plus: Double-trouble from rare neuromuscular disorders

A rare disorder in the USA is one that affects <200,000 people, making inherited myopathies rare diseases. Increasing access to genetic testing has been instrumental for the diagnosis of inherited myopathies. Genetic findings, however, require clinical correlation due to variable phenotype, polygenic etiology of certain inherited disorders, and possible co-existing independent neuromuscular disorders. We searched the Mayo Clinic Rochester medical record (2004-2020) to identify adult patients carrying pathogenic variants or likely pathogenic variants in genes causative of myopathies and having a coexisting independent neuromuscular disorder classified as rare at https://rarediseases.info.nih.gov/. One additional patient was identified at Nationwide Children's hospital. Clinical and laboratory findings were reviewed. We identified 14 patients from 13 families fulfilling search criteria. Seven patients had a "double-trouble" inherited myopathy; two had an inherited myopathy with coexistent idiopathic myositis; three had an inherited myopathy with coexisting rare neuromuscular disorder of neurogenic type; a female DMD carrier had co-existing distal spinal muscular atrophy, which was featuring the clinical phenotype; and a patient with a MYH7 pathogenic variant had Sandhoff disease causing motor neuron disease. These cases highlight the relevance of correlating genetic findings, even when diagnostic, with clinical features, to allow precise diagnosis, optimal care, and accurate prognosis.

Thomsen or Becker myotonia? A novel autosomal recessive nonsense mutation in the CLCN1 gene associated with a mild phenotype

We describe a large Brazilian consanguineous kindred with 3 clinically affected patients with a Thomsen myotonia phenotype. They carry a novel homozygous nonsense mutation in the CLCN1 gene (K248X). None of the 6 heterozygote carriers show any sign of myotonia on clinical evaluation or electromyography. These findings confirm the autosomal recessive inheritance of the novel mutation in this family, as well as the occurrence of phenotypic variability in the autosomal recessive forms of myotonia.

Novel CLCN1 mutations and clinical features of Korean patients with myotonia congenita

Myotonia congenita (MC) is a form of nondystrophic myotonia caused by a mutation of CLCN1, which encodes human skeletal muscle chloride channel (CLC-1). We performed sequence analysis of all coding regions of CLCN1 in patients clinically diagnosed with MC, and identified 10 unrelated Korean patients harboring mutations. Detailed clinical analysis was performed in these patients to identify their clinical characteristics in relation to their genotypes. The CLCN1 mutational analyses revealed nine different point mutations. Of these, six (p.M128I, p.S189C, p.M373L, p.P480S, p.G523D, and p.M609K) were novel and could be unique among Koreans. While some features including predominant lower extremity involvement and normal to slightly elevated creatine kinase levels were consistently observed, general clinical features were highly variable in terms of age of onset, clinical severity, aggravating factors, and response to treatment. Our study is the first systematic study of MC in Korea, and shows its expanding clinical and genetic spectrums.

High frequency of co-segregating CLCN1 mutations among myotonic dystrophy type 2 patients from Finland and Germany

Based on previous reports the frequency of co-segregating recessive chloride channel (CLCN1) mutations in families with myotonic dystrophy type 2 (DM2) was suspected to be increased. We have studied the frequency of CLCN1 mutations in two separate patient and control cohorts from Germany and Finland, and for comparison in a German myotonic dystrophy type 1 (DM1) patient cohort. The frequency of heterozygous recessive chloride channel (CLCN1) mutations is disproportionally higher (5 %) in currently diagnosed DM2 patients compared to 1.6 % in the control population (p = 0.037), while the frequency in DM1 patients was the same as in the controls. Because the two genes segregate independently, the prevalence of CLCN1 mutations in the total DM2 patient population is, by definition, the same as in the control population. Our findings are, however, not based on the total DM2 population but on the currently diagnosed DM2 patients and indicate a selection bias in molecular diagnostic referrals. DM2 patients with co-segregating CLCN1 mutation have an increased likelihood to be referred for molecular diagnostic testing compared to DM2 patients without co-segregating CLCN1 mutation.

Non-genomic effects of sex hormones on CLC-1 may contribute to gender differences in myotonia congenita

Myotonia congenita is caused by mutations in the voltage-gated chloride channel ClC-1. It is more severe in men than women and often worsens during pregnancy, but the basis for these gender differences is not known. We show here that both testosterone and progesterone rapidly and reversibly inhibit wild-type ClC-1 channels expressed in Xenopus oocytes by causing a prominent rightward shift in the voltage dependence of their open probability. In contrast, 17beta-estradiol at similar concentrations causes only a small shift. Progesterone and testosterone also profoundly inhibit ClC-1 channels containing the mutation F297S associated with dominantly inherited myotonia congenita. The effects of sex hormones are likely to be non-genomic because of their speed of onset and reversibility. These results suggest a possible mechanism to explain how the severity of myotonia congenita can be modulated by sex hormones.

PATHOMECHANISMS IN CHANNELOPATHIES OF SKELETAL MUSCLE AND BRAIN

Ion channelopathies are a diverse array of human disorders caused by mutations in ion channel genes. This review focuses on the pathogenic mechanisms of channelopathies affecting skeletal muscle and brain arising from mutations of voltage-gated ion channels and fast ligand-gated ion channels expressed at the surface membrane. Derangements in channel function alter the electrical excitability of the cell and thereby increase susceptibility to transient symptomatic attacks including myasthenia, periodic paralysis, myotonic stiffness, seizures, headache, dyskinesia, or episodic ataxia. Although these disorders are rare, they stand out as exemplary cases for which disease pathogenesis can be traced from a point mutation to altered protein function, to altered cellular activity, and to clinical phenotype. The study of these disorders has provided insights on channel structure-function relations, the physiological roles of ion channels, and rational approaches toward therapeutic intervention for many disorders of cellular excitability.

Electrodiagnostic studies in the management and prognosis of neuromuscular disorders

Prognosis remains a neglected aspect of modern medical care and research, behind diagnosis and treatment. The very term "electrodiagnosis" implies as much. Despite this, much has been published regarding the use and benefit of electrodiagnostic techniques in assessing prognosis and assisting in management of patients after the diagnosis has been established. This information is often hidden or otherwise not emphasized. This review summarizes the literature regarding the use of such techniques for prognosis and management of disorders of lower motor neurons, peripheral nerves, neuromuscular transmission, and muscle.

Genetic disorders of neuromuscular ion channels

Ion channels are complex proteins that span the lipid bilayer of the cell membrane, where they orchestrate the electrical signals necessary for normal function of the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. The role of ion channel defects in the pathogenesis of numerous disorders, many of them neuromuscular, has become increasingly apparent over the last decade. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the study of expressed proteins at the level of single channel molecules possible. Understanding the molecular basis of ion channel function and dysfunction will facilitate both the accurate classification of these disorders and the rational development of specific therapeutic interventions. This review encompasses clinical, genetic, and pathophysiological aspects of ion channels disorders, focusing mainly on those with neuromuscular manifestations.

A novelCLCN1 mutation: P480T in a Japanese family with Thomsen's myotonia congenita

At least 50 disease-causing mutations in the skeletal muscle voltage-gated chloride channel gene (CLCN1), almost all of which originate from Caucasian families, have been identified. We investigated a Japanese family with Thomsen's myotonia congenita that included 16 affected individuals (8 men and 8 women) through five generations. Polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) screening of 11 members showed an aberrant conformer in exon 13 of CLCN1 complementary DNA (cDNA) in 8 affected and 1 unaffected members. By sequence analysis, we identified a C-to-A transition at nucleotide position 1438, resulting in a substitution of proline for threonine at amino acid position 480 (P480T), the same position of the original mutation (P480L) in Thomsen's disease. The P480T mutation was novel and absent in 100 normal controls. Seven of the 8 affected individuals were heterozygous; another, from affected parents, was homozygous. Clinically, myotonia in the homozygous patient was more severe than that in heterozygous patients, probably due to the gene dosage effect. On a long-train nerve-stimulation test at a rate of 3 Hz, M-wave responses in the homozygous patient showed marked decrement followed by recovery. In contrast, the heterozygous patients showed just a slight decrement or no changes, and none of 2 patients with myotonic muscular dystrophy or 2 normal controls revealed any decrement. Thus, the long-train nerve-stimulation test at a low stimulus frequency may be a useful tool to assess the disease-severity/genotype relationship in myotonia congenita.