Recent advances in understanding congenital myopathies.

By definition, congenital myopathy typically presents with skeletal muscle weakness and hypotonia at birth. Traditionally, congenital myopathy subtypes have been predominantly distinguished on the basis of the pathological hallmarks present on skeletal muscle biopsies. Many genes cause congenital myopathies when mutated, and a burst of new causative genes have been identified because of advances in gene sequencing technology. Recent discoveries include extending the disease phenotypes associated with previously identified genes and determining that genes formerly known to cause only dominant disease can also cause recessive disease. The more recently identified congenital myopathy genes account for only a small proportion of patients. Thus, the congenital myopathy genes remaining to be discovered are predicted to be extremely rare causes of disease, which greatly hampers their identification. Significant progress in the provision of molecular diagnoses brings important information and value to patients and their families, such as possible disease prognosis, better disease management, and informed reproductive choice, including carrier screening of parents. Additionally, from accurate genetic knowledge, rational treatment options can be hypothesised and subsequently evaluated in vitro and in animal models. A wide range of potential congenital myopathy therapies have been investigated on the basis of improved understanding of disease pathomechanisms, and some therapies are in clinical trials. Although large hurdles remain, promise exists for translating treatment benefits from preclinical models to patients with congenital myopathy, including harnessing proven successes for other genetic diseases.

Sodium Channelopathies of Skeletal Muscle.

The NaV1.4 sodium channel is highly expressed in skeletal muscle, where it carries almost all of the inward Na+ current that generates the action potential, but is not present at significant levels in other tissues. Consequently, mutations of SCN4A encoding NaV1.4 produce pure skeletal muscle phenotypes that now include six allelic disorders: sodium channel myotonia, paramyotonia congenita, hyperkalemic periodic paralysis, hypokalemic periodic paralysis, congenital myasthenia, and congenital myopathy with hypotonia. Mutation-specific alternations of NaV1.4 function explain the mechanistic basis for the diverse phenotypes and identify opportunities for strategic intervention to modify the burden of disease.

Digenic mutational inheritance of the integrin alpha 7 and the myosin heavy chain 7B genes causes congenital myopathy with left ventricular non-compact cardiomyopathy.

BACKGROUND: We report an Italian family in which the proband showed a severe phenotype characterized by the association of congenital fiber type disproportion (CFTD) with a left ventricular non-compaction cardiomyopathy (LVNC). This study was focused on the identification of the responsible gene/s. METHODS AND RESULTS: Using the whole-exome sequencing approach, we identified the proband homozygous missense mutations in two genes, the myosin heavy chain 7B (MYH7B) and the integrin alpha 7 (ITGA7). Both genes are expressed in heart and muscle tissues, and both mutations were predicted to be deleterious and were not found in the healthy population.The R890C mutation in the MYH7B gene segregated with the LVNC phenotype in the examined family. It was also found in one unrelated patient affected by LVNC, confirming a causative role in cardiomyopathy.The E882K mutation in the ITGA7 gene, a key component of the basal lamina of muscle fibers, was found only in the proband, suggesting a role in CFTD. CONCLUSIONS: This study identifies two novel disease genes. Mutation in MYH7B causes a classical LVNC phenotype, whereas mutation in ITGA7 causes CFTD. Both phenotypes represent alterations of skeletal and cardiac muscle maturation and are usually not severe. The severe phenotype of the proband is most likely due to a synergic effect of these two mutations.This study provides new insights into the genetics underlying Mendelian traits and demonstrates a role for digenic inheritance in complex phenotypes.

A novel N440K sodium channel mutation causes myotonia with exercise-induced weakness--exclusion of CLCN1 exon deletion/duplication by MLPA.

We report a 4-generation Turkish family with 10 affected members presenting with myotonia and potassium- and exercise-induced paralytic attacks. The clinical presentation was neither typical for the chloride channel myotonias Thomsen and Becker nor for the separate sodium channel myotonia entities potassium-aggravated myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis. It is best described by a combination of potassium-aggravated myotonia and hyperkalemic periodic paralysis. We excluded exonic chloride channel mutations including CLCN1 exon deletion/duplication by MLPA. Instead we identified a novel p.N440K sodium channel mutation that is located at the inner end of segment S6 of repeat I. We discuss the genotype phenotype relation.

Becker myotonia congenita associated with Wolff-Parkinson-White syndrome.

Becker's disease is a congenital myotonia with an autosomal recessive pattern of inheritance. Unlike myotonic dystrophy, it is not associated with cardiopathy. We present a 32-year-old man with Becker's disease diagnosed at age 11. He suffered episodes of loss of consciousness preceded by palpitations when he performed strenuous physical exercise. The electrocardiogram reveals a Wolff-Parkinson-White syndrome. Review of the literature in MEDLINE using the search terms of congenital myotonia or Becker's disease and arrhythmia or heart disease only found 2 articles on Thomsen disease but none on Becker's disease. Becker's myotonia congenita may exceptionally be associated with cardiopathy, thus patients with this disorder should have a cardiac evaluation.

Severe neonatal non-dystrophic myotonia secondary to a novel mutation of the voltage-gated sodium channel (SCN4A) gene.

We report on a patient with a severe, rare neonatal form of non-dystrophic myotonia. The patient presented with facial dysmorphism, muscle hypertrophy, severe constipation, psychomotor delay, and frequent cold-induced episodes of myotonia and muscle weakness leading to severe hypoxia and loss of consciousness. Muscle biopsy was non-specific and electromyography revealed intense generalized myotonia. The myotonic episodes improved after introducing oral mexiletine and maintaining room temperature at 28 degrees C. The patient died at 20 months of age following a bronchopulmonary infection. A previously undescribed de novo heterozygous c.3891C > A change, which predicts p.N1297K in the SCN4A gene. Mutations within the voltage-gated sodium channel alpha-subunit gene (SCN4A) have been described in association with several phenotypes including paramyotonia congenita, hyperkalemic or hypokalemic periodic paralysis, and potassium-aggravated myotonias. The cold-sensitive episodes of stiffness followed by weakness suggested the diagnosis of channelopathy in our patient. However, her neonatal onset, the triggering of severe episodes by exposure to modest decreases in temperature, involvement of respiratory muscles with prolonged apnea, early-onset muscle hypertrophy, psychomotor retardation, and fatal outcome are evocative of a distinct clinical subtype. Our observation expands the phenotypic spectrum of sodium channelopathies.

Functional consequences of chloride channel gene (CLCN1) mutations causing myotonia congenita.

OBJECTIVE: To determine the functional consequences of missense mutations within the skeletal muscle chloride channel gene CLCN1 that cause myotonia congenita. BACKGROUND: Myotonia congenita is a genetic muscle disease associated with abnormalities in the skeletal muscle voltage-gated chloride (ClC-1) channel. In order to understand the molecular basis of this inherited disease, it is important to determine the physiologic consequences of mutations found in patients affected by it. METHODS: The authors used a mammalian cell (human embryonic kidney 293) expression system and the whole-cell voltage-clamp technique to functionally express and physiologically characterize five CLCN1 mutations. RESULTS: The I329T mutation shifted the voltage dependence of open probability of ClC-1 channels to the right by 192 mV, and the R338Q mutation shifted it to the right by 38 mV. In addition, the I329T ClC-1 channels deactivated to a lesser extent than normal at negative potentials. The V165G, F167L, and F413C ClC-1 channels also shifted the voltage dependence of open probability, but only by +14 to +20 mV. CONCLUSIONS: The functional consequences of these mutations form the physiologic argument that these are disease-causing mutations and could lead to myotonia congenita by impairing the ability of the skeletal muscle voltage-gated chloride channels to maintain normal muscle excitability. Understanding of genetic and physiologic defects may ultimately lead to better diagnosis and treatment of patients with myotonia congenita.

Unusual causes of stiffness in two hockey players.

PURPOSE: To describe two cases of competitive hockey players whose performance was impaired by generalized painless muscle stiffness. They were each found to have a separate type of myotonia. CASE SUMMARY: The first hockey player experienced stiffness only when initiating activity after a period of inactivity. He had a family history of muscle stiffness, grip myotonia clinically, and electrophysiologic findings of myotonic discharges. He had myotonia congenita. The stiffness in the second case was episodic and occurred during bouts of intense physical activity. The legs, arms, and neck were affected. Myotonia fluctuans was the probable diagnosis. Both cases responded well to Mexiletine. DISCUSSION: The etiology, classification, and clinical presentation of myotonia are discussed. RELEVANCE: Because of the musculoskeletal nature of our practice, sport medicine clinicians should be aware of myotonic disorders so they can be recognized and appropriately treated.

Estudo do grupo sanguíneo ABO e do fator Rh em pacientes com distrofia miotônica e miotonia congênita / Study of the blood group ABO and the Rh factor in patients with myotonic dystrophy and congenital myotonia

Estudou-se o grupo sanguíneo ABO e o fator Rh de 23 doentes com distrofia miotônica (DM), originados de 11 famílias, e dois irmäos com miotonia congênita (MC). Entre os doentes com DM, 10 pertenciam ao grupo 0 e 13 ao A, todos eram Rh positivo. Os dois irmäos com MC pertenciam ao grupo 0, Rh positivo

Myotonic dystrophy.

Myotonic dystrophy (MYD) has been localized to chromosome 19. Like other hereditary diseases with a genetic locus assigned to chromosome 19, MYD is a genetically inherited autosomal dominant disease. It is speculated that such a genetic pattern may express the characteristics of chromosome 19. We previously proposed that abnormalities of bile acid metabolism play a pathogenetic role in MYD, in which deoxycholic acid acts as a maternal factor in association with the onset of congenital MYD. Increased gamma-glutamyl-transpeptidase (gamma GTP) (probably hydrophobic deoxycholate-solubilized gamma GTP instead of hydrophilic papain-solubilized gamma GTP) in MYD may be related to the abnormalities of bile acid metabolism. Muscle wasting in MYD may be explained by these abnormalities as well. The resemblances between common metabolism of bile acids and alcohol and between common changes in MYD and alcoholic myopathy may lend support to a hypothesis that 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 26-tetrol dehydrogenase and 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestan-26-al dehydrogenase activities are impaired in MYD.

Different effectiveness of tocainide and hydrochlorothiazide in paramyotonia congenita with hyperkalemic episodic paralysis.

We investigated the effectiveness of tocainide and hydrochlorothiazide on muscular symptoms in a patient with paramyotonia congenita and episodic attacks of hyperkalemic paralysis. Generalized weakness was evoked by exercise and potassium loading. Myotonia and weakness were evoked by local muscle cooling. Tocainide prevented myotonia and weakness induced by cooling, but failed to prevent hyperkalemic weakness. Hydrochlorothiazide prevented hyperkalemic weakness, but did not influence symptoms evoked by cooling. These results suggest that, in this disorder, two different mechanisms cause muscular weakness.

Schwartz-Jampel syndrome: a case report. Stimulatory effect of calcium and A23187 calcium ionophore for protein synthesis in muscle cell cultures.

This report describes a 3-year-old male patient with dwarfism, generalized muscular hypertrophy, stiffness, myotonia, multiple skeletal deformities and normal intelligence. Serum creatine kinase was twice elevated. EMG showed 'dive bomber' discharges and muscle biopsy revealed mild to moderate myopathic changes with variability in fiber size and 'moth-eaten' fibers. Multiple muscle cell cultures showed significantly lower values of total protein synthesis as determined by (3H)-leucine incorporation. The addition of calcium and/or A23187 calcium ionophore to cultures significantly stimulated total protein synthesis in contrast to the lack of effect of these drugs in control cultures. These findings suggest a dysfunction of muscle sarcolemma in Schwartz-Jampel syndrome.

Paramyotonia congenita (von Eulenburg) in Denmark.

Paramyotonia congenita (von Eulenburg) (PMC) is described in a Danish family in seven generations and 39 persons. PMC has not previously been described in Denmark and an epidemiological description of the family is given. The diagnostic difficulties versus hyperkalemic familial periodic paralysis with myotonia, adynamia episodica hereditaria and myotonia congenita (Thomsen) are discussed.

Generalized myopathy and cerebral malformations possibly related to an enteroviral infection.

A study was made on a case of generalized muscular hypotonus manifested at birth. Serological findings and epidemiological data suggested an association to a recently described enterovirus infection (enterovirus candidate 71) known to cause neurological disease in man. Autopsy revealed cerebral malformations and generalized myopathy compatible with a viral etiology of the disease.