The ABC's of limb-girdle muscular dystrophy: alpha-sarcoglycanopathy, Bethlem myopathy, calpainopathy and more

Rare disease clinical trials: Power in numbers

The limb-girdle muscular dystrophies (LGMDs) encompass a collection of genetic muscle diseases with proximal-predominant weakness of the limbs. Thirty-two of these disorders are named via the common nomenclature, including 8 autosomal-dominant (LGMD1A-H) and 24 autosomal-recessive (LGMD2A-X) disorders.(1) In addition, numerous other genetic muscle diseases, including Bethlem myopathy, dystrophinopathies, ryanodine receptor-associated myopathies, and many more, may clinically present with similar proximal-predominant weakness.(2) Therefore, current genetic testing panels targeting neuromuscular weakness frequently encompass >75 genes. These disorders are quite rare, each with minimum prevalence estimates of 0.01-0.60 cases per 100,000 persons.(3) LGMD2A (attributable to mutations in the gene for calpain-3) and LGMD2B (attributable to mutations in the gene for dysferlin) consistently are the 2 most prevalent LGMD subtypes in a variety of ethnic cohorts.

Young girl presenting with exercise-induced myoglobinuria

INTRODUCTION

The sarcoglycanopathies are a heterogeneous group of autosomal recessive limb-girdle muscular dystrophies that cause varying degrees of progressive proximal muscle weakness.

METHODS

We describe the case of a Caucasian girl who presented with exercise intolerance, myalgia, and dark urine. Onset of symptoms was at age 4, and she had myalgia with physical activity throughout childhood. Creatine kinase levels were as high as 18,000.

RESULTS

Immunostaining of a muscle biopsy showed mildly diminished alpha sarcoglycan staining, and SGCA gene sequencing revealed n.C229T; p.Arg77Cys (R77C) and n.C850T; p.Arg284Cys (R284C), which is associated with alpha sarcoglycanopathy.

CONCLUSIONS

This patient presented with exercise intolerance, myoglobinuria, and almost normal muscle strength into adolescence, which is uncommon in sarcoglycanopathies. This uncommon presentation should be kept in mind, so that early recognition and intervention may prevent future comorbidities and help preserve the quality of life. Muscle Nerve 54: 161-164, 2016.

Collagen VI related muscle disorders

Mutations in the genes encoding collagen VI (COL6A1, COL6A2, and COL6A3) cause Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD), two conditions which were previously believed to be completely separate entities. BM is a relatively mild dominantly inherited disorder characterised by proximal weakness and distal joint contractures. UCMD was originally described as an autosomal recessive condition causing severe muscle weakness with proximal joint contractures and distal hyperlaxity. Here we review the clinical phenotypes of BM and UCMD and their diagnosis and management, and provide an overview of the current knowledge of the pathogenesis of collagen VI related disorders.

Histopathological Characterization of the Skeletal Myopathy in rasH2 Mice Carrying Human Prototype c-Ha-ras Gene

A skeletal myopathy is found in approximately 100% of rasH2 mice. To confirm detailed features of the rasH2 skeletal myopathy, the biceps femoris, diaphragm, triceps brachii, gastrocnemial (types I and II fiber-mixed muscles) and soleus muscle (type I fiber-dominant muscle) obtained from male rasH2 and non-transgenic littermates aged 10-13 and 34 weeks were examined. Variations in the muscle fiber size, early-scattered degeneration/necrosis and regeneration of muscle fibers were detected in 10-13-week-old rasH2 mice. The severity of the above muscular lesions was more prominent in older rasH2 mice. These lesions were noted in the type II myofiber dominant muscles (biceps femoris, triceps brachii and gastrocnemial). NADH-TR stain clearly demonstrated a disorganized intermyofibrillar network and necrotic change in muscle fibers. No specific morphological changes, like rod structure or tubular aggregation seen in some types of myopathy, were noted in Gomori trichrome and NADH-TR stains in the rasH2 mouse like in many types of muscular dystrophy. Electronmicroscopically, occasional muscle fiber degeneration/regeneration, invaded phagocytic cells, indistinct Z-band suggesting excessive contraction and dilatation of the sarcoplasmic reticulum were observed. In summary, the skeletal myopathy occurring in rasH2 mice is consistent with muscular dystrophy characterized morphologically by progressive degeneration and regeneration of myofibers. The myopathy is confined to the type II myofiber predominant muscles and is not associated with any pathognomonic lesions. These characteristics will provide us with a useful model for research in muscular dystrophy of diverse myofibers.

Klinik und Genetik der Gliederg�rteldystrophien

Limb girdle muscular dystrophies (LGMDs) are a genetically heterogeneous group of primary myopathies involving progressive weakness and wasting of the muscles in the hip and shoulder girdles, with distal spread to the bulbar or respiratory musculature in rare cases. Depending on the mode of genetic transmission, six autosomal dominant forms (LGMD1A-F, 10-25%) and ten autosomal recessive forms (LGMD2A-J, 75-90%) are currently known. The prevalence of LGMDs is 0.8/100,000. These conditions are caused by mutations in genes encoding for myotilin (5q31, LGMD1A), lamin A/C (1q11-q21.2, LGMD1B), caveolin-3 (3p25, LGMD1C), unknown proteins (7q, LGMD1D, 6q23, LGMD1E, 7q32.1-32.2., LGMD1F), calpain-3 (15q15.1-21.1, LGMD2A), dysferlin (2p13.3-13.1, LGMD2B), gamma-sarcoglycan (13q12, LGMD2C), alpha-sarcoglycan, also known as adhalin (17q12-q21.3, LGMD2D), beta-sarcoglycan (4q12, LGMD2E), delta-sarcoglycan (5q33-q34, LGMD2F), telethonin (17q11-q12, LGMD2G), E3-ubiquitin ligase (9q31-q34.1, LGMD2H), fukutin-related protein (19q13.3, LGMD2I), and titin (2q31, LGMD2J). Cardiac involvement has been described for LGMD1B-E, LGMD2C-G, and LGMD2I. The time of onset varies between early childhood and middle age. There is no male or female preponderance. Disease progression and life expectancy vary widely, even among different members of the same family. The diagnosis is based primarily on DNA analysis. The history, clinical neurological examinations, blood chemistry investigations, electromyography, and muscle biopsy also provide information that is helpful for the diagnosis. No causal therapy is currently available.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Laminin alpha2 deficiency and muscular dystrophy; genotype-phenotype correlation in mutant mice

Deficiency of laminin alpha2 is the cause of one of the most severe muscular dystrophies in humans and other species. It is not yet clear how particular mutations in the laminin alpha2 chain gene affect protein expression, and how abnormal levels or structure of the protein affect disease. Animal models may be valuable for such genotype-phenotype analysis and for determining mechanism of disease as well as function of laminin. Here, we have analyzed protein expression in three lines of mice with mutations in the laminin alpha2 chain gene and in two lines of transgenic mice overexpressing the human laminin alpha2 chain gene in skeletal muscle. The dy(3K)/dy(3K) experimental mutant mice are completely deficient in laminin alpha2; the dy/dy spontaneous mutant mice have small amounts of apparently normal laminin; and the dy(W)/dy(W) mice express even smaller amounts of a truncated laminin alpha2, lacking domain VI. Interestingly, all mutants lack laminin alpha2 in peripheral nerve. We have demonstrated previously, that overexpression of the human laminin alpha2 in skeletal muscle in dy(2J)/dy(2J) and dy(W)/dy(W) mice under the control of a striated muscle-specific creatine kinase promoter substantially prevented the muscular dystrophy in these mice. However, dy(W)/dy(W) mice, expressing the human laminin alpha2 under the control of the striated muscle-specific portion of the desmin promoter, still developed muscular dystrophy. This failure to rescue is apparently because of insufficient production of laminin alpha2. This study provides additional evidence that the amount of laminin alpha2 is most critical for the prevention of muscular dystrophy. These data may thus be of significance for attempts to treat congenital muscular dystrophy in human patients.

Muscle pathology in dysferlin deficiency

Dysferlin deficiency is being increasingly recognized in limb-girdle dystrophy and distal myopathy but its role in the development of muscle pathology is still poorly understood. For this purpose, 26 muscle biopsies from 25 dysferlinopathy patients were analysed by routine histochemistry and by immunohistochemistry with eight different antibodies, and scored for inflammatory response and type of cell infiltrate, fibre degeneration and regeneration, fibre type composition and severity of histopathological changes. In cases with an advanced-stage dystrophic pattern we observed type 1 fibre predominance exceeding 80%, suggesting a selective loss of type 2 fibres or a conversion process. The extent of muscle fibre regeneration and degeneration in dysferlinopathy was intermediate between sarcoglycanopathy and Duchenne dystrophy or myositis, suggesting a rather aggressive course of the disease. An increased inflammatory response was observed in the majority of our patients (16/26), who also showed an active dystrophic pattern. Type and localization of cellular infiltrates suggest that inflammatory reaction is secondary to necrosis. Major histocompatibility complex (MHC) class I molecules were overexpressed in dysferlinopathy, mainly in association with fibre phagocytosis and regeneration; their occasional expression in non-necrotic fibres might represent a marker of ongoing necrosis. Muscle inflammation might be triggered by the structurally altered membrane consequent to dysferlin defect.

Functional characteristics of dystrophic skeletal muscle: insights from animal models

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders that show myofiber degeneration and regeneration. Identification of animal models of muscular dystrophy has been instrumental in research on the pathogenesis, pathophysiology, and treatment of these disorders. We review our understanding of the functional status of dystrophic skeletal muscle from selected animal models with a focus on 1) the mdx mouse model of Duchenne muscular dystrophy, 2) the Bio 14.6 delta-sarcoglycan-deficient hamster model of limb-girdle muscular dystrophy, and 3) transgenic null mutant murine lines of sarcoglycan (alpha, beta, delta, and gamma) deficiencies. Although biochemical data from these models suggest that the dystrophin-sarcoglycan-dystroglycan-laminin network is critical for structural integrity of the myofiber plasma membrane, emerging studies of muscle physiology suggest a more complex picture, with specific functional deficits varying considerably from muscle to muscle and model to model. It is likely that changes in muscle structure and function, downstream of the specific, primary biochemical deficiency, may alter muscle contractile properties.

Dystrophin-deficient muscular dystrophy in a Labrador retriever

Sex-linked muscular dystrophy associated with dystrophin deficiency has been reported in several breeds of dogs and is best characterized in the golden retriever. In this case report, a young, male Labrador retriever with dystrophin-deficient muscular dystrophy is presented. Clinical signs included generalized weakness, lingual hypertrophy, and dysphagia. Electromyographic abnormalities including complex repetitive discharges were present. Serum creatine kinase concentration was dramatically elevated. Histopathological changes within a muscle biopsy specimen confirmed a dystrophic myopathy, and dystrophin deficiency was demonstrated by immunohistochemical staining. While X-linked muscular dystrophy has not previously been reported in the Labrador retriever, a hereditary myopathy with an autosomal recessive mode of inheritance has been characterized. A correct diagnosis and classification of these two disorders are critical for breeders and owners since both the mode of inheritance and the prognosis differ.