Mechanism of muscle wasting in myotonic dystrophy

Myotonic dystrophy is associated with progressive muscular atrophy. In order to determine the mechanism of muscle wasting in this condition, we measured fractional mixed skeletal muscle protein synthesis in the postabsorptive state in 8 patients with myotonic dystrophy, and compared the results with those of 10 normal subjects. Fractional muscle protein synthesis was determined by measuring the increment of 13C leucine in mixed skeletal muscle protein obtained by needle biopsy from the quadriceps muscle during a primed-continuous infusion of L-(1-13C) leucine. We used plasma 13C alpha-ketoisocaproate (representing intracellular leucine labeling) as the precursor pool for the calculation of fractional muscle protein synthesis and leucine kinetics. Fractional muscle protein synthesis was depressed in the patients with myotonic dystrophy (28% decrease, p less than 0.02). Leucine flux, leucine oxidation, and the nonoxidative portion of leucine flux were not different between the patients with myotonic dystrophy and the normal control subjects. Muscle atrophy in myotonic dystrophy reflects a selective decrease in muscle protein synthesis without any similar decrease in nonmuscle protein synthesis. This decrease may result from an impaired end-organ response to anabolic hormones or substrates.     

Quantitative measurement of [3H]ouabain binding to human skeletal muscle cryostat sections

The etiopathogenesis of myotonic muscular dystrophy is thought to involve a basic defect in muscle membrane. Biochemical investigations of human muscle membrane have been hampered by difficulty in obtaining large quantities of muscle at biopsy for the preparation of sarcolemma. We have determined [3H]ouabain binding to normal and myotonic dystrophy human skeletal muscle by using cryostat sections. The binding increased with increase in number of tissue sections (protein) and in concentrations of [3H]ouabain, ATP and Na+. The binding of [3H]ouabain in myotonic dystrophy patients was 2-3 fold higher than in normal and disease controls. Kinetic analysis revealed that the increased binding of ouabain to myotonic tissue sections was independent of low-affinity sites directed by ATP and Na+. These findings provide further evidence for the involvement of membrane abnormalities in myotonic muscular dystrophy.     

Myotonic dystrophies type 1 and 2: a summary on current aspects

Myotonic dystrophies (DMs) encompass at least 2 forms: myotonic dystrophy type 1 and 2. In general, DMs are late-onset autosomal dominant disorders characterized by a variety of multisystemic features including myotonia, muscular dystrophy, cardiac conduction defects, dilated cardiomyopathy, posterior iridescent cataracts, frontal balding, insulin-resistance and disease-specific serological abnormalities such as gamma-glutamyltransferase and creatine kinase elevations, hyperglycemia, hypotestosteronism, and reduced immunoglobulin (Ig) G and IgM levels. Beyond the adult forms, in the classic DM1, a congenital form and an early-onset form is recognized. Here we summarize current aspects of the myotonic dystrophy pathogenesis and review the core features of both types of myotonic dystrophies, including the congenital DM1.     

Forearm 3-methylhistidine efflux in myotonic dystrophy

Myotonic dystrophy is associated with progressive muscular atrophy. To define the mechanism of muscle wasting in this disease, we studied myofibrillar proteolysis in vivo in 8 men moderately affected with myotonic dystrophy, and compared the results with those of 10 normal men. Myofibrillar proteolysis was estimated by measuring the 3-methylhistidine arteriovenous difference (A – V) and efflux (Q) across the forearm in the postabsorptive state. Plasma 3-methylhistidine concentrations were determined by high-performance liquid chromatography with postcolumn o-phthalaldehyde derivatization and fluorescence detection. Plasma flow to the forearm muscles (F) was estimated to represent 85% of total forearm plasma flow as determined by the indicator-dilution technique. Forearm 3-methylhistidine efflux was calculated as: Q = F(A – V). Mean muscle mass (24-hour creatinine excretion), lean body mass, and forearm volume were decreased in the patients with myotonic dystrophy, confirming the presence of muscle atrophy. Mean forearm 3-methylhistidine arteriovenous difference and efflux were not significantly different in the two groups. We conclude that myofibrillar protein degradation is not increased in myotonic dystrophy, even when measured in a muscle compartment selectively affected by wasting. Muscle atrophy in myotonic dystrophy is probably the result of defective anabolism rather than accelerated catabolism.     

Myotonic Dystrophies: Targeting Therapies for Multisystem Disease

Myotonic dystrophy is an autosomal dominant muscular dystrophy not only associated with muscle weakness, atrophy, and myotonia but also prominent multisystem involvement. There are 2 similar, but distinct, forms of myotonic dystrophy; type 1 is caused by a CTG repeat expansion in the DMPK gene, and type 2 is caused by a CCTG repeat expansion in the CNBP gene. Type 1 is associated with distal limb, neck flexor, and bulbar weakness and results in different phenotypic subtypes with variable onset from congenital to very late-onset as well as variable signs and symptoms. The classically described adult-onset form is the most common. In contrast, myotonic dystrophy type 2 is adult-onset or late-onset, has proximal predominant muscle weakness, and generally has less severe multisystem involvement. In both forms of myotonic dystrophy, the best characterized disease mechanism is a RNA toxic gain-of-function during which RNA repeats form nuclear foci resulting in sequestration of RNA-binding proteins and, therefore, dysregulated splicing of premessenger RNA. There are currently no disease-modifying therapies, but clinical surveillance, preventative measures, and supportive treatments are used to reduce the impact of muscular impairment and other systemic involvement including cataracts, cardiac conduction abnormalities, fatigue, central nervous system dysfunction, respiratory weakness, dysphagia, and endocrine dysfunction. Exciting preclinical progress has been made in identifying a number of potential strategies including genome editing, small molecule therapeutics, and antisense oligonucleotide-based therapies to target the pathogenesis of type 1 and type 2 myotonic dystrophies at the DNA, RNA, or downstream target level.     

Effects of age and gene dose on skeletal muscle sodium channel gating in mice deficient in myotonic dystrophy protein kinase

Myotonic muscular dystrophy (DM) is characterized by abnormal skeletal muscle Na channel gating and reduced levels of myotonic dystrophy protein kinase (DMPK). Electrophysiological measurements show that mice deficient in Dmpk have reduced Na currents in muscle. We now find that the Na channel expression level is normal in mouse muscle partially or completely deficient in Dmpk. Reduced current amplitudes are not changed by age or gene dose, and the reduction is not due to changes in macroscopic or microscopic gating kinetics. The mechanism of abnormal membrane excitability in DM may in part be silencing of muscle Na channels due to Dmpk deficiency.     

Gastroparesis in neonatal myotonic dystrophy

Myotonic dystrophy in the neonate is commonly accompanied by facial diplegia, generalized muscular hypotonia, talipes equinovarus, and muscular respiratory failure. The gastrointestinal manifestations of this disease include poor sucking, choking, regurgitation, aspiration, and swallowing difficulties. Gastroparesis can be a major contributor to the feeding difficulties experienced by these infants. We report on an infant with congenital myotonic dystrophy in whom a severe gastric motility problem was alleviated by metoclopramide therapy. This smooth muscle manifestation may be an important and potentially remediable source of morbidity in these infants.     

3例强直性肌营养不良的临床和病理报告

强直性肌营养不良症为一种少见的多系统受累的遗传性疾病。国内文献报告甚少，本文报告３例病人的临床特点，并对患肌显微结构和超微结构的改变进行了观察。认为遗传性肌营养不良性肌病有相似的病理过程。     

What we do not know about pregnancy in hereditary neuromuscular disorders

Only sparse information is available concerning the relationship between pregnancy and hereditary neuromuscular disorders. This review deals with several issues like the effects of such conditions on female fertility (myotonic dystrophy type 1 and mitochondrial disorders), on the risk to the fetus (myotonic dystrophy type 1 and Charcot-Marie-Tooth disease), on the ability to carry pregnancy and its complications (markedly increased preterm labor in myotonic dystrophies and spinal muscular atrophy), on the labor and its possible need for interventions (myotonic dystrophy type 1, facioscapulohumeral dystrophy and Charcot-Marie-Tooth disease). It also discusses the question of pregnancy effects on the course of the inherited neuromuscular disorders (myotonic dystrophies, spinal muscular atrophy, facioscapulohumeral dystrophy, Charcot-Marie-Tooth disease, congenital myopathy and limb-girdle muscular dystrophy). The aim of this critical review is to point at pregnancy-related problems that need further research.     

Possible role of apamin-sensitive K+ channels in myotonic dystrophy

Myotonic muscular dystrophy is a genetic disease characterized mainly by muscle atrophy and myotonia, a repetitive electrical activity of muscle. In the present study, the possible role of apamin-sensitive K⁺ channels in the genesis of myotonia was investigated. Apamin is a peptide from bee venom that specifically blocks small conductance Ca²⁺-activated K⁺ channels. The injection of a small amount of apamin (20–30 μl, 10 μmol/L) into the thenar muscle of myotonic dystrophy patients decreased the basal electrical activity during the electromyogram in the 6 patients studied. Myotonic discharges after muscle percussion were more difficult to trigger and of smaller intensity and duration. In 2 controls and in 2 patients with generalized myotonia, as well as in 1 patient with myotonia congenita (where the defect is in chloride channels), apamin had no effect. These results suggest that apamin-sensitive K⁺ channels participate in the mechanism that generates myotonia in myotonic dystrophy. © 1994 John Wiley & Sons, Inc.     

Myotonic dystrophy

Myotonic dystrophy is an autosomal dominant disorder that results in skeletal muscle weakness and wasting, myotonia, and numerous nonmuscular manifestations including frontal balding, cataracts, gonadal dysfunction, cardiac conduction abnormalities, respiratory insufficiency, and hypersomnolence. Although the gene defect in myotonic dystrophy has been mapped to chromosome 19, the exact metabolic abnormalities responsible for this disorder are unknown. Skeletal muscle has been found to be relatively insulin-resistant in myotonic dystrophy, and a decrease in the anabolic action of insulin on skeletal muscle may be related to muscle wasting in this disorder. Laboratory studies, including electromyography, electrocardiography, and muscle biopsy, are helpful in evaluating patients for this disorder, but the clinical aspects and a careful family history remain the mainstays of diagnosis. A number of management strategies preserve function and prevent complications in myotonic dystrophy.     

Myotonic disorders

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

Myotonic syndromes

PURPOSE OF REVIEW: To highlight recent advances in understanding the clinical manifestations and molecular genetics of myotonic syndromes, with particular emphasis on the myotonic dystrophies. RECENT FINDINGS: Myotonic syndromes include the non-dystrophic myotonias, caused by mutations in genes encoding the chloride or sodium channels that are specific to skeletal muscle, and the myotonic dystrophies. Previous studies have shown that myotonic dystrophy type 1 is caused by the expansion of a CTG repeat in the gene. Recently, it was discovered that myotonic dystrophy type 2 (proximal myotonic myopathy) is also caused by a DNA expansion mutation. In both types of myotonic dystrophy the expanded repeat is transcribed and the RNA produced from the mutant allele is retained in nuclear inclusions. Recent studies suggest that the mutant RNA has a toxic effect on muscle fibers by interfering with the essential functions of the myonucleus, such as RNA processing. SUMMARY: It now appears likely that myotonic dystrophy is the first instance of a genetic disease in which the harmful effect of a mutation involves the production of a pathogenic RNA. However, the exact mechanism is not understood, and it is unclear whether this RNA-mediated disease process is also responsible for the manifestations of myotonic dystrophy in non-muscle tissues.     

Das Krankheitsbild der myotonen Muskeldystrophie bei Patienten mit großer CTG-Tripletexpansion

Myotonic dystrophy is an autosomal dominant multisystem disorder involving muscle, brain, heart, eyes, and endocrine organs. The underlying mutation is an expanding trinucleotide CTG repeat in the 3'prime untranslated region of a serine-threonine kinase gene on chromosome 19q. A statistical correlation exists between the CTG copy number and the severity of the disease. Infants with severe congenital myotonic dystrophy have been shown to have on average a greater amplification of the CTG repeat than is seen in the non-congenital myotonic dystrophy population. However, not all patients with many CTG copies develop congenital myotonic dystrophy. We present 13 patients with more than 1500 CTG trinucleotide repeats and show their variable clinical course.     

Cognitive impairment in neuromuscular disorders

Several studies have suggested the presence of central nervous system involvement manifesting as cognitive impairment in diseases traditionally confined to the peripheral nervous system. The aim of this review is to highlight the character of clinical, genetic, neurofunctional, cognitive, and psychiatric deficits in neuromuscular disorders. A high correlation between cognitive features and cerebral protein expression or function is evident in Duchenne muscular dystrophy, myotonic dystrophy (Steinert disease), and mitochondrial encephalomyopathies; direct correlation between tissue-specific protein expression and cognitive deficits is still elusive in certain neuromuscular disorders presenting with or without a cerebral abnormality, such as congenital muscular dystrophies, congenital myopathies, amyotrophic lateral sclerosis, adult polyglucosan body disease, and limb-girdle muscular dystrophies. No clear cognitive deficits have been found in spinal muscular atrophy and facioscapulohumeral dystrophy.     

Vimentin and desmin in maturing skeletal muscle and developmental myopathies.

I studied vimentin and desmin immunoreactivities in the skeletal muscle of 30 human fetuses and children ranging from 8 weeks' gestation to 2 years of age, and in 45 infants and children and five adults with developmental neuromuscular diseases. Acridine orange-RNA fluorescence also identified regenerating myofibers in Duchenne muscular dystrophy and dermatomyositis for comparison with congenital myopathies. Vimentin and desmin are both strongly expressed in fetal myotubes and their immunohistochemical demonstration persists until 36 weeks' gestation. These cytoskeletal proteins are uniformly expressed in myofibers of neonates with X-linked recessive myotubular myopathy. Desmin but not vimentin is diffusely increased in infantile cases of myotonic dystrophy, in some cases of congenital muscle fiber-type disproportion, and in cerebrohepatorenal disease. In nemaline rod myopathy, desmin is focally increased in perinuclear zones and in regions of aggregated rods. The small myofibers in infantile spinal muscular atrophy show increased vimentin and desmin in the subsarcolemmal region. The demonstration of these intermediate filament proteins provides markers to enhance diagnostic precision in the interpretation of the infant muscle biopsy. Furthermore, persistently high fetal concentrations of vimentin/desmin may play a role in the pathogenesis of some developmental myopathies.     

Lack of relationship of hypogonadism to muscle wasting in myotonic dystrophy

Myotonic dystrophy is frequently associated with testicular atrophy. Since androgens may play a role in the maintenance of muscle mass, we have studied the levels of plasma testosterone and gonadotropins and of urinary 17-ketosteroids in 22 men with myotonic dystrophy, 36 normal men, and 16 men (control group) with muscle wasting. Results were correlated with muscle mass as estimated by creatinine excretion and total body potassium. Patients with myotonic dystrophy had significantly lower testosterone and higher gonadotropin levels than normal, and these changes were progressive in longitudinal studies. Testosterone levels were also lower than normal in disease control subjects. There was no correlation between low testosterone levels and diminished muscle mass in either myotonic dystrophy or disease control patients. The low plasma concentration of testosterone in men with myotonic dystrophy and other neuromuscular diseases does not appear to be directly related to their muscle wasting. This study does not exclude the possibility that an alteration in testosterone receptor or tissue effects may contribute to a loss of muscle tissue.     

Monovalent cation transport in myotonic dystrophy. Na-K pump ratio in erythrocytes.

Myotonic dystrophy is a dominantly-inherited disorder which affects skeletal muscle in combination with several other systems. Because of abnormalities in red blood cells, a universal membrane defect has been proposed as the primary disturbance. Erythrocyte cation pump ratios have also been reported to be abnormal. Hyperinsulinemia and glucose intolerance are present in a large number of patients. Since dramatic effects of insulin on membrane cation transport have been shown in several tissues, notably skeletal muscle, we wished first to confirm reports of altered pump ratio in these patients and then to evaluate the effects of insulin on cation fluxes. However, in our experiments myotonic dystrophy patients had normal pump ratios when compared with disease controls.     

Nuclear proteins and cell death in inherited neuromuscular disease

X-linked Emery-Dreifuss muscular dystrophy is caused by mutations in emerin, a novel nuclear membrane protein. Other major inherited neuromuscular diseases have now also been shown to involve proteins which localize and function at least partly in the cell nucleus. These include lamin A/C in autosomal dominant Emery-Dreifuss muscular dystrophy, SMN in spinal muscular atrophy, SIX5 in myotonic dystrophy, calpain3 in type 2A limb-girdle muscular dystrophy, PABP2 in oculopharyngeal dystrophy, androgen receptor in spinal and bulbar muscular atrophy and the ataxins in hereditary ataxias. This review compares the molecular basis for these various disorders and considers the role of cell death, including apoptosis, in their pathogenesis.     

Pentose phosphate pathway in neuromuscular diseases--evaluation of muscular glucose 6-phosphate dehydrogenase activity and RNA content]

There have been several reports concerning elevated glucose 6 phosphate dehydrogenase (G6PDH), the rate-limiting enzyme of pentose phosphate pathway (PPP), in experimental muscle disturbances. PPP produces ribose, a substrate of RNA, and NADPH which is a cofactor of fatty acid synthesis. PPP also has a role of by-path pathway of glycolysis. Then, we evaluated G6PDH activity and RNA content in biopsied quadriceps muscle. The subjects were muscles from 23 neurogenic amyotrophy, 54 myopathy including 19 progressive muscular dystrophy (PMD), and 10 controls whose muscle was obtained at orthopedic surgery. Neurogenic amyotrophy consisted of 12 amyotrophic lateral sclerosis (ALS), 4 spinal muscular atrophy and 7 peripheral nerve disorders. Myopathy were 3 Duchenne dystrophy, 2 congenital muscular dystrophy, 8 limb-girdle type dystrophy, 6 facio-scapular +-humeral muscular dystrophy, 6 myotonic dystrophy, 6 mitochondrial myopathy, 5 endocrinological myopathy, 3 hypokalemic myopathy, 8 polymyositis and 4 other inflammatory myopathy. The assays of G6PDH and RNA were performed after Glock's and Fleck's methods, respectively. The control values were 3.6 +/- 0.8 nmol formed NADPH/mg protein/min (M +/- SD) in G6PDH and 0.69 +/- 0.17 micrograms/mg non-collagen protein in RNA. Most cases of PMD, as well as some cases of ALS, hyperthyroidism, mitochondria hypokalemic myopathy, inflammatory myopathy showed increased values (beyond M + 2SD of control) both in G6PDH and RNA. There were significant positive correlations between G6PDH activity and RNA content in PMD and motor neuron disease. Myotonic dystrophy showed normal values in both G6PDH and RNA. Half number of cases of mitochondrial myopathy demonstrated increased G6PDH alone.(ABSTRACT TRUNCATED AT 250 WORDS)