Sarcolemmal proteins and the spectrum of limb-girdle muscular dystrophies

Anti-latent TGFβ binding protein 4 antibody improves muscle function and reduces muscle fibrosis in muscular dystrophy

Duchenne muscular dystrophy, like other muscular dystrophies, is a progressive disorder hallmarked by muscle degeneration, inflammation, and fibrosis. Latent transforming growth factor β (TGFβ) binding protein 4 (LTBP4) is an extracellular matrix protein found in muscle. LTBP4 sequesters and inhibits a precursor form of TGFβ. LTBP4 was originally identified from a genome-wide search for genetic modifiers of muscular dystrophy in mice, where there are two different alleles. The protective form of LTBP4, which contains an insertion of 12 amino acids in the protein’s hinge region, was linked to increased sequestration of latent TGFβ, enhanced muscle membrane stability, and reduced muscle fibrosis. The deleterious form of LTBP4 protein, lacking 12 amino acids, was more susceptible to proteolysis and promoted release of latent TGF-β, and together, these data underscored the functional role of LTBP4’s hinge. Here, we generated a monoclonal human anti-LTBP4 antibody directed toward LTBP4’s hinge region. In vitro, anti-LTBP4 bound LTBP4 protein and reduced LTBP4 proteolytic cleavage. In isolated myofibers, the LTBP4 antibody stabilized the sarcolemma from injury. In vivo, anti-LTBP4 treatment of dystrophic mice protected muscle against force loss induced by eccentric contraction. Anti-LTBP4 treatment also reduced muscle fibrosis and enhanced muscle force production, including in the diaphragm muscle, where respiratory function was improved. Moreover, the anti-LTBP4 in combination with prednisone, a standard of care for Duchenne muscular dystrophy, further enhanced muscle function and protected against injury in mdx mice. These data demonstrate the potential of anti-LTBP4 antibodies to treat muscular dystrophy.

Sarcoglycanopathies: an update

Sarcoglycanopathies are the most severe forms of autosomal recessive limb-girdle muscular dystrophies (LGMDs), constituting about 10-25% of LGMDs. The clinical phenotype is variable, but onset is usually in the first decade of life. Patients present muscle hypertrophy, elevated CK, variable muscle weaknesses, and progressive loss of ambulation. Four subtypes are known: LGMDR3, LGMDR4, LGMDR5 and LGMDR6, caused, respectively, by mutations in the SGCA, SGCB,SGCG and SGCD genes. Their four coded proteins, α-SG, ß-SG, λ-SG and δ-SG are part of the dystrophin-glycoprotein complex (DGC) present in muscle sarcolemma, which acts as a linker between the cytoskeleton of the muscle fiber and the extracellular matrix, providing mechanical support to the sarcolemma during myofiber contraction. Many different mutations have already been identified in all the sarcoglycan genes, with a predominance of some mutations in different populations. The diagnosis is currently based on the molecular screening for these mutations. Therapeutic approaches include the strategy of gene replacement mediated by a vector derived from adeno-associated virus (AAV). Pre-clinical studies have shown detectable levels of SG proteins in the muscle, and some improvement in the phenotype, in animal models. Therapeutic trials in humans are ongoing.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

Expression, purification, and structural analysis of the full-length human integral membrane protein γ-sarcoglycan

Mutation of the gene encoding γ-sarcoglycan (SGCG), an integral membrane protein responsible for maintaining the integrity of the muscle cell sarcolemma, results in Limb-Girdle Muscular Dystrophy (LGMD), a congenital disease with no current treatment options. This member of the sarcoglycan glycoprotein family is a vital component of the Dystrophin Complex, which together facilitate normal muscle function. However, very little is known about the structure and dynamics of these proteins, and of membrane glycoproteins in general. This is due to a number of factors, including their complexity, heterogeneity and highly-specific native environments. The expression, purification, and structural study of membrane proteins is further impeded by their hydrophobic nature and consequent propensity to aggregate in aqueous solutions. Here, we report the first successful expression and purification of milligram quantities of full-length recombinant SGCG, utilizing fusion protein-guided overexpression to inclusion bodies in Escherichia coli. Purification of SGCG from the fusion protein, TrpΔLE, was facilitated using chemical cleavage. Cleavage products were then isolated by size-exclusion chromatography. Successful purification of the protein was confirmed using SDS-PAGE and mass spectroscopy. Finally, solution nuclear magnetic resonance spectroscopy of uniformly ¹⁵N-labeled SGCG in detergent environments was performed, yielding the first spectra of the full-length membrane glycoprotein, SGCG. These results represent the initial structural studies of SGCG, laying the foundation for further investigation on the interaction and dynamics of other integral membrane proteins. More specifically, this data allows for opportunities in the future for enhanced treatment modalities and cures for LGMD.

Treatment with Recombinant Human MG53 Protein Increases Membrane Integrity in a Mouse Model of Limb Girdle Muscular Dystrophy 2B

Limb girdle muscular dystrophy type 2B (LGMD2B) and other dysferlinopathies are degenerative muscle diseases that result from mutations in the dysferlin gene and have limited treatment options. The dysferlin protein has been linked to multiple cellular functions including a Ca²⁺-dependent membrane repair process that reseals disruptions in the sarcolemmal membrane. Recombinant human MG53 protein (rhMG53) can increase the membrane repair process in multiple cell types both in vitro and in vivo. Here, we tested whether rhMG53 protein can improve membrane repair in a dysferlin-deficient mouse model of LGMD2B (B6.129-Dysf^tm1Kcam/J). We found that rhMG53 can increase the integrity of the sarcolemmal membrane of isolated muscle fibers and whole muscles in a Ca²⁺-independent fashion when assayed by a multi-photon laser wounding assay. Intraperitoneal injection of rhMG53 into mice before acute eccentric treadmill exercise can decrease the release of intracellular enzymes from skeletal muscle and decrease the entry of immunoglobulin G and Evans blue dye into muscle fibers in vivo. These results indicate that short-term rhMG53 treatment can ameliorate one of the underlying defects in dysferlin-deficient muscle by increasing sarcolemmal membrane integrity. We also provide evidence that rhMG53 protein increases membrane integrity independently of the canonical dysferlin-mediated, Ca²⁺-dependent pathway known to be important for sarcolemmal membrane repair.

Carrier frequency of the c.525delT mutation in the SGCG gene and estimated prevalence of limb girdle muscular dystrophy type 2C among the Moroccan population

Autosomal recessive limb-girdle muscular dystrophies (AR-LGMDs) are characterized by clinical and genetic heterogeneity. LGMD type 2C, or γ-sarcoglycanopathy, is the most frequent in North African populations as a result of the founder c.525delT mutation in the SGCG gene. Its epidemiology is poorly known in Morocco, and its prevalence among the Moroccan population has never been evaluated. This study screened 26 patients with a LGMD2C and 45 patients with an AR-LGMD phenotype for the c.525delT mutation. DNA extracted from umbilical cord blood samples of 250 newborns was tested for the same mutation. Molecular epidemiologic methods were used to calculate the frequency of heterozygotes for this mutation in Moroccan newborns and to estimate the prevalence of LGMD2C in the Moroccan population. The carrier frequency was estimated to be 1/250, which would imply that the prevalence of LGMD2C would be approximately 1/20,492 considering the effect of consanguinity. The homozygous c.525delT mutation was found in 65% of all patients with AR-LGMDs. These findings suggest that AR-LGMDs are prevalent in the Moroccan population and LGMD2C is one of the most common forms. This information might be useful for the development of diagnostic strategies on a large scale for better management of patients with AR-LGMD and genetic counseling of families.

ISPD gene mutations are a common cause of congenital and limb-girdle muscular dystrophies

Dystroglycanopathies are a clinically and genetically diverse group of recessively inherited conditions ranging from the most severe of the congenital muscular dystrophies, Walker-Warburg syndrome, to mild forms of adult-onset limb-girdle muscular dystrophy. Their hallmark is a reduction in the functional glycosylation of α-dystroglycan, which can be detected in muscle biopsies. An important part of this glycosylation is a unique O-mannosylation, essential for the interaction of α-dystroglycan with extracellular matrix proteins such as laminin-α2. Mutations in eight genes coding for proteins in the glycosylation pathway are responsible for ∼50% of dystroglycanopathy cases. Despite multiple efforts using traditional positional cloning, the causative genes for unsolved dystroglycanopathy cases have escaped discovery for several years. In a recent collaborative study, we discovered that loss-of-function recessive mutations in a novel gene, called isoprenoid synthase domain containing (ISPD), are a relatively common cause of Walker-Warburg syndrome. In this article, we report the involvement of the ISPD gene in milder dystroglycanopathy phenotypes ranging from congenital muscular dystrophy to limb-girdle muscular dystrophy and identified allelic ISPD variants in nine cases belonging to seven families. In two ambulant cases, there was evidence of structural brain involvement, whereas in seven, the clinical manifestation was restricted to a dystrophic skeletal muscle phenotype. Although the function of ISPD in mammals is not yet known, mutations in this gene clearly lead to a reduction in the functional glycosylation of α-dystroglycan, which not only causes the severe Walker-Warburg syndrome but is also a common cause of the milder forms of dystroglycanopathy.

Dystrophin and utrophin expression require sarcospan: loss of α7 integrin exacerbates a newly discovered muscle phenotype in sarcospan-null mice

Sarcospan (SSPN) is a core component of the major adhesion complexes in skeletal muscle, the dystrophin- and utrophin (Utr)-glycoprotein complexes (DGC and UGC). We performed a rigorous analysis of SSPN-null mice and discovered that loss of SSPN decreased DGC and UGC abundance, leading to impaired laminin-binding activity and susceptibility to eccentric contraction-induced injury in skeletal muscle. We show that loss of SSPN increased levels of α7β1 integrin. To genetically test whether integrin compensates for the loss of DGC and UGC function in SSPN-nulls, we generated mice lacking both SSPN and α7 integrin (DKO, double knockout). Muscle regeneration, sarcolemma integrity and fibrosis were exacerbated in DKO mice and were remarkably similar to muscle from Duchenne muscular dystrophy (DMD) patients, suggesting that secondary loss of integrin contributes significantly to pathogenesis. Expression of the DGC and UGC, laminin binding and Akt signaling were negatively impacted in DKO muscle, resulting in severely diminished specific force properties. We demonstrate that SSPN is a necessary component of dystrophin and Utr function and that SSPN modulation of integrin signaling is required for extracellular matrix attachment and muscle force development.

Sustained alpha-sarcoglycan gene expression after gene transfer in limb-girdle muscular dystrophy, type 2D

OBJECTIVE

The aim of this study was to attain long-lasting alpha-sarcoglycan gene expression in limb-girdle muscular dystrophy, type 2D (LGMD2D) subjects mediated by adeno-associated virus (AAV) gene transfer under control of a muscle specific promoter (tMCK).

METHODS

rAAV1.tMCK.hSGCA (3.25 × 10¹¹ vector genomes) was delivered to the extensor digitorum brevis muscle of 3 subjects with documented SGCA mutations via a double-blind, randomized, placebo controlled trial. Control sides received saline. The blind was not broken until the study was completed at 6 months and all results were reported to the oversight committee.

RESULTS

Persistent alpha-sarcoglycan gene expression was achieved for 6 months in 2 of 3 LGMD2D subjects. Markers for muscle fiber transduction other than alpha-sarcoglycan included expression of major histocompatibility complex I, increase in muscle fiber size, and restoration of the full sarcoglycan complex. Mononuclear inflammatory cells recruited to the site of gene transfer appeared to undergo programmed cell death, demonstrated by terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick-end labeling and caspase-3 staining. A patient failing gene transfer demonstrated an early rise in neutralizing antibody titers and T-cell immunity to AAV, validated by enzyme-linked immunospot on the second day after gene injection. This was in clear distinction to other participants with satisfactory gene expression.

INTERPRETATION

The findings of this gene replacement study in LGMD2D subjects have important implications not previously demonstrated in muscular dystrophy. Long-term, sustainable gene expression of alpha-sarcoglycan was observed following gene transfer mediated by AAV. The merit of a muscle-specific tMCK promoter, not previously used in a clinical trial, was evident, and the potential for reversal of disease was displayed.

Neuromuscular imaging in inherited muscle diseases

Driven by increasing numbers of newly identified genetic defects and new insights into the field of inherited muscle diseases, neuromuscular imaging in general and magnetic resonance imaging (MRI) in particular are increasingly being used to characterise the severity and pattern of muscle involvement. Although muscle biopsy is still the gold standard for the establishment of the definitive diagnosis, muscular imaging is an important diagnostic tool for the detection and quantification of dystrophic changes during the clinical workup of patients with hereditary muscle diseases. MRI is frequently used to describe muscle involvement patterns, which aids in narrowing of the differential diagnosis and distinguishing between dystrophic and non-dystrophic diseases. Recent work has demonstrated the usefulness of muscle imaging for the detection of specific congenital myopathies, mainly for the identification of the underlying genetic defect in core and centronuclear myopathies. Muscle imaging demonstrates characteristic patterns, which can be helpful for the differentiation of individual limb girdle muscular dystrophies. The aim of this review is to give a comprehensive overview of current methods and applications as well as future perspectives in the field of neuromuscular imaging in inherited muscle diseases. We also provide diagnostic algorithms that might guide us through the differential diagnosis in hereditary myopathies.

Restoration of gamma-sarcoglycan localization and mechanical signal transduction are independent in murine skeletal muscle

Limb girdle muscular dystrophy 2C is caused by mutations in the gamma-sarcoglycan gene (gsg) that results in loss of this protein, and disruption of the sarcoglycan (SG) complex. Signal transduction after mechanical perturbation is mediated, in part, through the SG complex and leads to phosphorylation of tyrosines on the intracellular portions of the sarcoglycans. This study tested if the Tyr(6) in the intracellular region of gamma-sarcoglycan protein (gamma-SG) was necessary for proper localization of the protein in skeletal muscle membranes or for the normal pattern of ERK1/2 phosphorylation after eccentric contractions. Viral mediated gene transfer of wild type gsg (WTgsg) and mutant gsg lacking Tyr(6) (Y6Agsg) was performed into the muscles of gsg(-/-) mice. Muscles were examined for production and stability of the gamma-SG, as well as the level of ERK1/2 phosphorylation before and after eccentric contraction. Sarcolemmal localization of gamma-SG was achieved regardless of which construct was expressed. However, only expression of WTgsg corrected the aberrant ERK1/2 phosphorylation associated with the absence of gamma-SG, whereas Y6Agsg failed to have any effect. This study shows that localization of gamma-SG does not require Tyr(6), but localization alone is insufficient for restoration of normal signal transduction patterns after mechanical perturbation.

Interventions for muscular dystrophy: molecular medicines entering the clinic

Muscular dystrophies are individually rare genetic disorders that cause much chronic disability, affecting young children and adults. In the past 20 years, more than 30 genetic types of muscular dystrophy have been defined. During this time, precise diagnosis, genetic counselling, and medical management have improved. These advances in medical practice have occurred while definitive therapies based on an improved knowledge of disease pathogenesis are awaited. A wide range of therapeutic options have been tested in animal models, and some are being tested in clinical trials. Various therapeutic targets are being investigated, from personalised medicines targeting specific mutations and drugs targeting cellular pathways to gene-based and cell-based therapies.

Caveolin regulates kv1.5 trafficking to cholesterol-rich membrane microdomains

The targeting of ion channels to cholesterol-rich membrane microdomains has emerged as a novel mechanism of ion channel localization. Previously, we reported that Kv1.5, a prominent cardiovascular K(+) channel alpha-subunit, localizes to caveolar microdomains. However, the mechanisms regulating Kv1.5 targeting and the functional significance of this localization are largely unknown. In this study, we demonstrate a role for caveolin in the trafficking of Kv1.5 to lipid raft microdomains where cholesterol modulates channel function. In cells lacking endogenous caveolin-1 or -3, the association of Kv1.5 with low-density, detergent-resistant membrane fractions requires coexpression with exogenous caveolin, which can form channel-caveolin complexes. Caveolin is not required for cell surface expression, however, and caveolin-trafficking mutants sequester Kv1.5, but not Kv2.1, in intracellular compartments, resulting in a loss of functional cell surface channel. Coexpression with wild type caveolin-1 does not alter Kv1.5 current density; rather, it induces depolarizing shifts in steady-state activation and inactivation. These shifts are analogous to those produced by elevation of membrane cholesterol. Together, these results show that caveolin modulates channel function by regulating trafficking to cholesterol-rich membrane microdomains.

Primary desminopathies

Mutations of the human desmin gene on chromosome 2q35 cause a familial or sporadic form of skeletal myopathy frequently associated with cardiac abnormalities. Skeletal and cardiac muscle from patients with primary desminopathies characteristically display cytoplasmic accumulation of desmin-immunoreactive material and myofibrillar changes. However, desmin-positive protein aggregates in conjunction with myofibrillar abnormalities are also the morphological hallmark of the large group of secondary desminopathies (synonyms: myofibrillar myopathies, desmin-related myopathies), which comprise sporadic and familial neuromuscular conditions of considerable clinical and genetic heterogeneity. Here, we will give an overview on the functional role of desmin in striated muscle as well as the main clinical, myopathological, genetic and patho-physiological aspects of primary desminopathies. Furthermore, we will discuss recent genetic and biochemical advances in distinguishing primary from secondary desminopathies.

The childhood limb-girdle muscular dystrophies

The heterogeneous childhood limb-girdle muscular dystrophies have originally been defined as a group of autosomal recessive and dominant diseases with progressive weakness and wasting of shoulder and pelvic-girdle muscles. Over the last 12 years, the underlying genetic defects for many of the diseases have been identified and insight into pathomechanisms of disease has been gained. At the same time, improved diagnostic techniques have allowed to extend the phenotypic spectrum for many of these devastating conditions, which showed that clinical symptoms and pathological findings are not restricted to skeletal muscles. Childhood limb-girdle muscular dystrophies are systemic diseases that often affect the musculoskeletal, respiratory, and cardiovascular system and that can go along with central nervous system involvement and gastrointestinal symptoms. The systemic nature of the diseases requires adequate management strategies that improve symptoms, longevity, and quality of life of the patients. As we are entering an era of translational research the need for precise molecular diagnoses, a thorough understanding of the natural history of the diseases and guidelines for standardized assessments of the patients become even more relevant. In this review, the best characterized childhood limb-girdle muscular dystrophies are discussed and their management aspects highlighted.

Impact of sarcoglycan complex on mechanical signal transduction in murine skeletal muscle

Loss of the dystrophin glycoprotein complex (DGC) or a subset of its components can lead to muscular dystrophy. However, the patterns of symptoms differ depending on which proteins are affected. Absence of dystrophin leads to loss of the entire DGC and is associated with susceptibility to contractile injury. In contrast, muscles lacking γ-sarcoglycan (γ-SG) display little mechanical fragility and still develop severe pathology. Animals lacking dystrophin or γ-SG were used to identify DGC components critical for sensing dynamic mechanical load. Extensor digitorum longus muscles from 7-wk-old normal (C57), dystrophin- null ( mdx), and γ-SG-null ( gsg−/−) mice were subjected to a series of eccentric contractions, after which ERK1/2 phosphorylation levels were determined. At rest, both dystrophic strains had significantly higher ERK1 phosphorylation, and gsg−/− muscle also had heightened ERK2 phosphorylation compared with wild-type controls. Eccentric contractions produced a significant and transient increase in ERK1/2 phosphorylation in normal muscle, whereas the mdx strain displayed no significant proportional change of ERK1/2 phosphorylation after eccentric contraction. Muscles from gsg−/− mice had no significant increase in ERK1 phosphorylation; however, ERK2 phosphorylation was more robust than in C57 controls. The reduction in mechanically induced ERK1 phosphorylation in gsg−/− muscle was not dependent on age or severity of phenotype, because muscle from both young and old (age 20 wk) animals exhibited a reduced response. Immunoprecipitation experiments revealed that γ-SG was phosphorylated in normal muscle after eccentric contractions, indicating that members of the DGC are modified in response to mechanical perturbation. This study provides evidence that the SGs are involved in the transduction of mechanical information in skeletal muscle, potentially unique from the entire DGC.

Limb-girdle muscular dystrophy in childhood

LGMD refers to a class of muscular dystrophies with onset in the proximal muscles. They are genetically heterogeneous, with both autosomal recessive and dominant forms. The autosomal recessive forms are more common and in general follow a more severe course compared to the dominant forms. It is important to reach a specific genetic diagnosis beyond making a group diagnosis of LGMD to provide adequate genetic counseling, to predict risks for the patient such as the development of cardiomyopathy, and to be able to take advantage of specific treatments when they become available. Establishing a specific diagnosis requires knowledge about the individual clinical features, expert analysis of the muscule biopsy, and the guided initiation of appropriate genetic testing.

Diagnostic value of muscle MRI in differentiating LGMD2I from other LGMDs

Mutations in the fukutin-related protein (FKRP) have recently been demonstrated to cause limb girdle muscular dystrophy type 2I (LGMD2I), one of the most common forms of the autosomal recessive LGMDs in Europe. We performed a systematic clinical and muscle MRI assessment in 6 LGMD2I patients and compared these findings with those of 14 patients with genetically confirmed diagnosis of other forms of autosomal recessive LGMDs or dystrophinopathies. All LGMD2I patients had a characteristic clinical phenotype with predominant weakness of hip flexion and adduction, knee flexion and ankle dorsiflexion. These findings were also mirrored on MRI of the lower extremities which demonstrated marked signal changes in the adductor muscles, the posterior thigh and posterior calf muscles. This characteristic clinical and MRI phenotype was also seen in LGMD2A. However, in LGMD2A there was a selective involvement of the medial gastrocnemius and soleus muscle in the lower legs which was not seen in LGMD2I. The pattern in LGMD2A and LGMD2I were clearly different from the one seen in alpha-sarcoglycanopathy and dystrophinopathy type Becker which showed marked signal abnormalities in the anterior thigh muscles. Our results indicate that muscular MRI is a powerful tool for differentiating LGMD2I from other forms of autosomal recessive LGMDs and dystrophinopathies.

Enrichment of the R77C α-sarcoglycan gene mutation in finnish LGMD2D patients

Limb-girdle muscular dystrophy 2D (LGMD2D) is caused by mutations in the alpha-sarcoglycan gene (SGCA). The most frequently reported mutation, 229CGC>TGC (R77C) in exon 3 of SGCA, results in the substitution of arginine by cysteine. We present here the clinical, immunohistochemical, and genetic data of 11 Finnish patients with LGMD2D caused by mutations in SGCA. Mutational analysis showed 10 patients homozygous and 1 compound heterozygous for R77C. A wide spectrum of SGCA mutations has been reported previously. Our results show an enrichment of R77C in Finland, further underlined by the observed carrier frequency of 1 per 150. According to the annual birth rate of approximately 60,000 in Finland, one LGMD2D patient with a homozygous mutation is expected to be born every 1 or 2 years on average. The presence of an ancient founder mutation is indicated by the fact that all patients shared a short common haplotype extending > or = 790 kilobases. Our results emphasize the need to include the SGCA gene R77C mutation test in routine DNA analyses of severe dystrophinopathy-like muscular dystrophies in Finland, and suggest that the applicability of this test in other populations should be studied as well.

Disruption of dystroglycan axis by β-dystroglycan processing in cardiomyopathic hamster muscle

Alpha-dystroglycan is a cell surface peripheral membrane protein which binds to the extracellular matrix, while beta-dystroglycan is a type I integral membrane protein which anchors alpha-dystroglycan to the cell membrane via the N-terminal extracellular domain. The complex composed of alpha- and beta-dystroglycan is called the dystroglycan complex. Although defects of the dystroglycan gene have not been identified as the primary causes of hereditary diseases in humans, secondary but significant abnormalities of the dystroglycan complex have been revealed in severe muscular dystrophies, including sarcoglycanopathy (LGMD2C, D, E and F). In this study, we investigated proteolytic processing of beta-dystroglycan and its effect on the extracellular matrix-cell membrane linkage in cardiomyopathic hamsters, the model animals of LGMD2F. Compared to normal controls, proteolytic processing of beta-dystroglycan was activated in the skeletal, cardiac and smooth muscles of cardiomyopathic hamsters and this resulted in the partial disruption of the dystroglycan complex in these tissues. These phenomena were observed from the early phase of muscle degeneration process. Our results suggest that proteolytic processing of beta-dystroglycan disrupts the extracellular matrix-cell membrane linkage via the dystroglycan complex and this may play a role in the molecular pathogenesis of muscle degeneration in cardiomyopathic hamsters.

On symptomatic heterozygous alpha-sarcoglycan gene mutation carriers

Mutations in the human alpha-sarcoglycan gene on chromosome 17q21.2 have been shown to cause a severe childhood autosomal recessive muscular dystrophy, a less severe limb girdle muscular dystrophy, exercise intolerance, or asymptomatic hyperCKemia. Here, we describe the clinical findings in a German family harboring a 371 T > C (Ile124Thr) missense mutation in the alpha-sarcoglycan gene. Whereas our index patient, an 11-year-old girl homozygous for this mutation, presented with a severe Duchenne-like phenotype, 7 out of 12 heterozygous mutation carriers from three generations showed mild to moderate scapular winging. In analogy to symptomatic female dystrophinopathy carriers, our results suggest that heterozygous alpha-sarcoglycan gene mutation carriers can be symptomatic with selective muscle weakness. This finding may be attributed to an additional negative variation in a yet unknown modifier gene essential to the function of the sarcoglycan complex in shoulder girdle muscles.

The myopathies

Extraordinary progress has been made in the recognition, understanding, and treatment of myopathy in the past several decades, aided by the application of molecular genetics, electrophysiology, muscle biopsy, and innovative therapies. The symptoms of muscle disease may vary among individual patients with acquired muscle disorders, as well as family members with inherited disorders. A careful history and examination is necessary to establish the symptoms, temporal progression, and distinctive areas of involvement, associated conditions, and suspected inheritance pattern in those patients in whom genetic cause is suspected. Selective laboratory testing is performed in symptomatic patients, which may include creatinine kinase level, electrodiagnostic studies, and, as warranted, examination of blood and muscle tissue for specific histopathological and genetic defects.