Sarcolemmopathy: muscular dystrophies with cell membrane defects

Facioscapulohumeral Disease as a myodevelopmental disease: Applying Ockham's razor to its various features

 Facioscapulohumeral muscular dystrophy (FSHD) is an exclusively human neuromuscular disease. In the last decades the cause of FSHD was identified: the loss of epigenetic repression of the D4Z4 repeat on chromosome 4q35 resulting in inappropriate transcription of DUX4. This is a consequence of a reduction of the array below 11 units (FSHD1) or of a mutation in methylating enzymes (FSHD2). Both require the presence of a 4qA allele and a specific centromeric SSLP haplotype. Muscles become involved in a rostro-caudally order with an extremely variable progression rate. Mild disease and non-penetrance in families with affected individuals is common. Furthermore, 2% of the Caucasian population carries the pathological haplotype without clinical features of FSHD.In order to explain the various features of FSHD we applied Ockham's Razor to all possible scenarios and removed unnecessary complexities. We postulate that early in embryogenesis a few cells escape epigenetic silencing of the D4Z4 repeat. Their number is assumed to be roughly inversely related to the residual D4Z4 repeat size. By asymmetric cell division, they produce a rostro-caudal and medio-lateral decreasing gradient of weakly D4Z4-repressed mesenchymal stem cells. The gradient tapers towards an end as each cell-division allows renewed epigenetic silencing. Over time, this spatial gradient translates into a temporal gradient based on a decreasing number of weakly silenced stem cells. These cells contribute to a mildly abnormal myofibrillar structure of the fetal muscles. They also form a downward tapering gradient of epigenetically weakly repressed satellite cells. When activated by mechanical trauma, these satellite cells de-differentiate and express DUX4. When fused to myofibrils they contribute to muscle cell death in various ways. Over time and dependent on how far the gradient reaches the FSHD phenotype becomes progressively manifest. We thus hypothesize FSHD to be a myodevelopmental disease with a lifelong attempt to restore DUX4 repression.

Mechanical regulation of oligodendrocyte biology

Oligodendrocytes (OL) are a subset of glial cells in the central nervous system (CNS) comprising the brain and spinal cord. The CNS environment is defined by complex biochemical and biophysical cues during development and response to injury or disease. In the last decade, significant progress has been made in understanding some of the key biophysical factors in the CNS that modulate OL biology, including their key role in myelination of neurons. Taken together, those studies offer translational implications for remyelination therapies, pharmacological research, identification of novel drug targets, and improvements in methods to generate human oligodendrocyte progenitor cells (OPCs) and OLs from donor stem cells in vitro. This review summarizes current knowledge of how various physical and mechanical cues affect OL biology and its implications for disease, therapeutic approaches, and generation of human OPCs and OLs.

Untangling the complexity of limb-girdle muscular dystrophies

The limb-girdle muscular dystrophies (LGMDs) are a group of genetically heterogeneous, autosomal inherited muscular dystrophies with a childhood to adult onset, manifesting with hip- and shoulder-girdle muscle weakness. When the term LGMD was first conceptualized in 1954, it was thought to be a single entity. Currently, there are 8 autosomal dominant (LGMD1A-1H) and 26 autosomal recessive (LGMD2A-2Z) variants according to the Online Mendelian Inheritance in Man database. In addition, there are other genetically identified muscular dystrophies with an LGMD phenotype not yet classified as LGMD. This highlights the entanglement of LGMDs, which represents an area in continuous expansion. Herein we aim to simplify the complexity of LGMDs by subgrouping them on the basis of the underlying defective protein and impaired function. Muscle Nerve 58: 167-177, 2018.

Myogenic Potential of Muscle Side and Main Population Cells after Intravenous Injection into Sub-lethally IrradiatedmdxMice

Muscle side population (SP) cells have demonstrated hematopoietic and myogenic activities in vivo upon intravenous (IV) injection into lethally irradiated mdx mice. In contrast, muscle main population (MP) cells were unable to rescue the bone marrow of lethally irradiated mice and, consequently, their in vivo myogenic potential could not be assessed using this method. In the current study, muscle SP or MP cells derived from syngeneic wild-type male mice were delivered to sub-lethally irradiated mdx female mice by single or serial IV injections. Recipient mice were euthanized 12 weeks after transplantation at which time the quadriceps and diaphragm muscles were analyzed for the presence of donor-derived cells. Mice injected with 104muscle SP cells or with 106MP cells appeared to have similar numbers of dystrophin-positive myofibers containing fused donor nuclei. Analysis of the remaining tissue via real-time quantitative PCR indicated that mice injected with muscle SP cells had a higher percentage of donor-derived Y-DNA in the quadriceps than mice injected with MP cells, suggesting that muscle SP cells may be enriched for progenitors able to engraft dystrophic skeletal muscles from the circulation. Although the overall engraftment did not reach therapeutically significant levels, these results indicate that further optimization of cell delivery techniques may lead to improved efficacy of cell-mediated therapy using muscle SP cells.

Influence of ovarian muscle contraction and oocyte growth on egg chamber elongation in Drosophila

Organs are formed from multiple cell types that make distinct contributions to their shape. The Drosophila egg chamber provides a tractable model to dissect such contributions during morphogenesis. Egg chambers consist of 16 germ cells (GCs) surrounded by a somatic epithelium. Initially spherical, these structures elongate as they mature. This morphogenesis is thought to occur through a 'molecular corset' mechanism, whereby structural elements within the epithelium become circumferentially organized perpendicular to the elongation axis and resist the expansive growth of the GCs to promote elongation. Whether this epithelial organization provides the hypothesized constraining force has been difficult to discern, however, and a role for GC growth has not been demonstrated. Here, we provide evidence for this mechanism by altering the contractile activity of the tubular muscle sheath that surrounds developing egg chambers. Muscle hypo-contraction indirectly reduces GC growth and shortens the egg, which demonstrates the necessity of GC growth for elongation. Conversely, muscle hyper-contraction enhances the elongation program. Although this is an abnormal function for this muscle, this observation suggests that a corset-like force from the egg chamber's exterior could promote its lengthening. These findings highlight how physical contributions from several cell types are integrated to shape an organ.

Gene Therapy for Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a relatively common inherited disorder caused by defective expression of the protein dystrophin. The most direct approach to treating this disease would be to restore dystrophin production in muscle. Recent progress has greatly increased the prospects for successful gene therapy of DMD, and here we summarize the most promising developments.

AREAS COVERED

Gene transfer using vectors derived from adeno-associated virus (AAV) has emerged as a promising method to restore dystrophin production in muscles bodywide, and represents a treatment option applicable to all DMD patients. Using information gleaned from PubMed searches of the literature, attendance at scientific conferences and results from our own lab, we provide an overview of the potential for gene therapy of DMD using AAV vectors including a summary of promising developments and issues that need to be resolved prior to large-scale therapeutic implementation.

EXPERT OPINION

Of the many approaches being pursued to treat DMD and BMD, gene therapy based on AAV-mediated delivery of microdystrophin is the most direct and promising method to treat the cause of the disorder. The major challenges to this approach are ensuring that microdystrophin can be delivered safely and efficiently without eliciting an immune response.

Isolation of sarcolemmal plasma membranes by mechanically skinning rat skeletal muscle fibers for phospholipid analysis

Membrane phospholipid (PL) composition has been shown to affect cellular function by altering membrane physical structure. The sarcolemma plasma membrane (SLpm) is integral to skeletal muscle function and health. Previous studies assessing SLpm PL composition have demonstrated contamination from transverse (t)-tubule, sarcoplasmic reticulum, and nuclear membranes. This study assessed the possibility of isolating SL by mechanically skinning skeletal muscle fiber segments for the analysis of SLpm PL composition. Mechanically skinned SLpm from rat extensor digitorum longus (EDL) muscle fibers underwent Western blot analysis to assess contamination from t-tubule, sarcoplasmic reticulum, nuclear and mitochondrial membranes. The results indicate that isolated SLpm had minimal nuclear and mitochondrial membrane contamination and was void of contamination from sarcoplasmic reticulum and t-tubule membranes. After performing both high-performance thin layer chromatography and gas chromatography, we found that the SLpm obtained by mechanical skinning had higher sphingomyelin and total fatty acid saturation and lower phosphatidylcholine when compared to previous literature. Thus, by avoiding the use of various chemical treatments and membrane fractionation, we present data that may truly represent the SLpm and future studies can use this technique to assess potential changes under various perturbations and disease conditions such as insulin resistance and muscular dystrophy.

Ca2+ overload and mitochondrial permeability transition pore activation in living delta-sarcoglycan-deficient cardiomyocytes

Muscular dystrophies are often associated with significant cardiac disease that can be the prominent feature associated with gene mutations in sarcoglycan. Cardiac cell death is a main feature of cardiomyopathy in sarcoglycan deficiency and may arise as a cardiomyocyte intrinsic process that remains unclear. Deficiency of delta-sarcoglycan (delta-SG) induces disruption of the dystrophin-associated glycoprotein complex, a known cause of membrane instability that may explain cardiomyocytes cytosolic Ca2+ increase. In this study we assessed the hypothesis that cytosolic Ca2+ increase triggers cardiomyocyte death through mitochondrial Ca2+ overload and dysfunction in the delta-SG-deficient CHF147 hamster. We showed that virtually all isolated CHF147 ventricular myocytes exhibited elevated cytosolic and mitochondrial Ca2+ levels by the use of the Fura-2 and Rhod-2 fluorescent probes. Observation of living cells with Mito-Tracker red lead to the conclusion that approximately 15% of isolated CHF147 cardiomyocytes had disorganized mitochondria. Transmission electron microscope imaging showed mitochondrial swelling associated with crest and membrane disruption. Analysis of the mitochondrial permeability transition pore (MPTP) activity using calcein revealed that mitochondria of CHF147 ventricular cells were twofold leakier than wild types, whereas reactive oxygen species production was unchanged. Bax, Bcl-2, and LC3 expression analysis by Western blot indicated that the intrinsic apoptosis and the cell death associated to autophagy pathways were not significantly activated in CHF147 hearts. Our results lead to conclusion that cardiomyocytes death in delta-SG-deficient animals is an intrinsic phenomenon, likely related to Ca2+-induced necrosis. In this process Ca2+ overload-induced MPTP activation and mitochondrial disorganization may have an important role.

Our trails and trials in the subsarcolemmal cytoskeleton network and muscular dystrophy researches in the dystrophin era

In 1987, about 150 years after the discovery of Duchenne muscular dystrophy (DMD), its responsible gene, the dystrophin gene, was cloned by Kunkel. This was a new substance. During these 20 odd years after the cloning, our understanding on dystrophin as a component of the subsarcolemmal cytoskeleton networks and on the pathomechanisms of and experimental therapeutics for DMD has been greatly enhanced. During this paradigm change, I was fortunately able to work as an active researcher on its frontiers for 12 years. After we discovered that dystrophin is located on the cell membrane in 1988, we studied the architecture of dystrophin and dystrophin-associated proteins (DAPs) complex in order to investigate the function of dystrophin and pathomechanism of DMD. During the conduct of these studies, we came to consider that the dystrophin-DAP complex serves to transmembranously connect the subsarcolemmal cytoskeleton networks and basal lamina to protect the lipid bilayer. It then became our working hypothesis that injury of the lipid bilayer upon muscle contraction is the cause of DMD. During this process, we predicted that subunits of the sarcoglycan (SG) complex are responsible for respective types of DMD-like muscular dystrophy with autosomal recessive inheritance. Our prediction was confirmed to be true by many researchers including ourselves. In this review, I will try to explain what we observed and how we considered concerning the architecture and function of the dystrophin-DAP complex, and the pathomechanisms of DMD and related muscular dystrophies.

Age-Related Dystrophin-Glycoprotein Complex Structure and Function in the Rat Extensor Digitorum Longus and Soleus Muscle

This study tested the hypothesis that age-related changes in the dystrophin-glycoprotein complex (DGC) may precede age-associated alterations in muscle morphology and function. Compared to those in adult (6 month) rats, extensor digitorum longus (EDL) and soleus muscle mass was decreased in old (30 month) and very old (36 month) Fischer 344/NNiaHSD x Brown Norway/BiNia rats. The amount of dystrophin, beta-dystroglycan, and alpha-sarcoglycan increased with aging in the EDL and decreased with aging in the soleus. alpha-Dystroglycan levels were increased with aging in both muscles and displayed evidence of altered glycosylation. Immunostaining for the presence of antibody infiltration and dystrophin following increased muscle stretch suggested that the aging in the soleus was characterized by diminished membrane integrity. Together, these data suggest that aging is associated with alterations in EDL and soleus DGC protein content and localization. These results may implicate the DGC as playing a role in age-associated skeletal muscle remodeling.

Biglycan binds to alpha- and gamma-sarcoglycan and regulates their expression during development

The dystrophin-associated protein complex (DAPC), which links the cytoskeleton to the extracellular matrix, is essential for muscle cell survival, and is defective in a wide range of muscular dystrophies. The DAPC contains two transmembrane subcomplexes-the dystroglycans and the sarcoglycans. Although several extracellular binding partners have been identified for the dystroglycans, none have been described for the sarcoglycan subcomplex. Here we show that the small leucine-rich repeat (LRR) proteoglycan biglycan binds to alpha- and gamma-sarcoglycan as judged by ligand blot overlay and co-immunoprecipitation assays. Our studies with biglycan-decorin chimeras show that alpha- and gamma-sarcoglycan bind to distinct sites on the polypeptide core of biglycan. Both biglycan proteoglycan as well as biglycan polypeptide lacking glycosaminoglycan (GAG) side chains are components of the dystrophin glycoprotein complex isolated from adult skeletal muscle membranes. Finally, our immunohistochemical and biochemical studies with biglycan null mice show that the expression of alpha- and gamma-sarcoglycan is selectively reduced in muscle from young (P14-P21) animals, while levels in adult muscle (> or = P35) are unchanged. We conclude that biglycan is a ligand for two members of the sarcoglycan complex and regulates their expression at discrete developmental ages.

Characterization of initiator and effector caspase expressions in dystrophinopathies

There is evidence that apoptotic cell death mechanisms contribute to muscle fiber loss in dystrophin-deficient muscle but there is little knowledge about the final degrading events of muscle fiber apoptosis. In muscle biopsy specimens from 14 patients with a dystrophinopathy (10 patients with DMD, two with Becker MD, two DMD carriers), expression of APAF-1 and caspase-9, upstream members of the apoptotic protease cascade, as well as of the downstream executioners caspase-2, -6 and -7, were studied by immunohistochemistry and Western blots. Besides predominant immunoreactivity in regenerating muscle fibers, which may contribute to apoptotic events during new muscle fiber formation, caspase-9, -6 and -7 displayed upregulation in non-regenerating, light microscopically intact but atrophic muscle fibers. Western blot analyses confirmed the upregulations. These findings indicate that, once activated, caspase-9 initiates a proteolytic, muscle fiber degrading cascade involving the downstream executioners caspase-6 and -7. However, lacking coexpression of APAF-1 suggests the existence of other pathways of caspase-9 activation than through the "apoptosome" in dystrophinopathies.

Autophagic vacuoles with sarcolemmal features delineate Danon disease and related myopathies

Among the autophagic vacuolar myopathies (AVMs), a subgroup is characterized pathologically by unusual autophagic vacuoles with sarcolemmal features (AVSF) and includes Danon disease and X-linked myopathy with excessive autophagy. The diagnostic importance and detailed morphologic features of AVSF in different AVMs have not been well established, and the mechanism of AVSF formation is not known. To address these issues, we have performed detailed histologic studies of myopathies with AVSF and other AVMs. In Danon disease and related AVMs, at the light microscopic level, autophagic vacuoles appeared to be accumulations of lysosomes, which, by electron microscopy consisted of clusters of autophagic vacuoles, indicative of autolysosomes. Some autolysosomes were surrounded by membranes with sarcolemmal proteins, acetylcholinesterase activity, and basal lamina. In Danon disease, the number of fibers with AVSF increased linearly with age while the number with autolysosomal accumulations decreased slightly, suggesting that AVSF are produced secondarily in response to autolysosomes. Most of the AVSF form enclosed spaces, indicating that the vacuolar membranes may be formed in situ rather than through sarcolemmal indentation. This unique intracytoplasmic membrane structure was not found in other AVMs. In conclusion, AVSF with acetylcholinesterase activity are autolysosomes surrounded by secondarily generated intracytoplasmic sarcolemma-like structure and delineates a subgroup of AVMs.

Diagnostic immunohistochemistry in neuromuscular disorders

Most neuromuscular disorders display only non-specific myopathological features in routine histological preparations. However, a number of proteins, including sarcolemmal, sarcomeric, and nuclear proteins as well as enzymes with defects responsible for neuromuscular disorders, have been identified during the past two decades, allowing a more specific and firm diagnosis of muscle diseases. Identification of protein defects relies predominantly on immunohistochemical preparations and on Western blot analysis. While immunohistochemistry is very useful in identifying abnormal expression of primary protein abnormalities in recessive conditions, it is less helpful in detecting primary defects in dominantly inherited disorders. Abnormal immunohistochemical expression patterns can be confirmed by Western blot analysis which may also be informative in dominant disorders, although its role has yet to be established. Besides identification of specific protein defects, immunohistochemistry is also helpful in the differentiation of inflammatory myopathies by subtyping cellular infiltrates and demonstrating up-regulation of subtle immunological parameters such as cell adhesion molecules. The role of immunohistochemistry in denervating disorders, however, remains controversial in the absence of a reliable marker of muscle fibre denervation. Nevertheless, as well as the diagnostic value of immunocytochemical analysis it may also widen understanding of muscle fibre pathology as well as help in the development of therapeutic strategies.

Tumour necrosis factor-mediated cell death pathways do not contribute to muscle fibre death in dystrophinopathies

There is evidence that apoptotic cell death mechanisms contribute to muscle fibre loss in dystrophinopathies, but little knowledge about the activators of the final degrading caspase cascade in muscle fibre apoptosis. As mitochondria-related activation of this caspase cascade, through e.g. APAF-1, could not be proven in dystrophin-deficient muscle, this study searches for other prospective candidates that may directly trigger apoptotic cell degradation by mitochondria-independent pathways involving the interaction of tumour necrosis factor-alpha (TNF-alpha) and TRAIL with death receptors and subsequent activation of caspase-8. The expression of TNF-alpha, TNF-R1, TRAIL, NF-(kappa)B and caspase-8 were studied in muscle biopsy specimens from 14 patients with a dystrophinopathy [10 Duchenne muscular dystrophies (DMD), 2 Becker MD, and 2 DMD carriers] by immunohistochemistry and Western blotting. In all types of dystrophinopathies, necrotic muscle fibres undergoing myophagocytosis displayed strong expression of TNF-alpha, TNF-R1, and TRAIL, which, however, was attributed to phagocytosing cells and not to the muscle fibres themselves. There was no up-regulation in normal-shaped or atrophic non-necrotic muscle fibres, or in intact muscle fibre segments adjacent to segmental necrosis and myophagocytosis. The expression profiles of caspase-8 and NF-(kappa)B resembled that of normal control muscle. There were likewise no significant differences in the Western blot analyses between normal control and dystrophin-deficient muscle. Based on these findings, a contribution of TNF-alpha or TRAIL-mediated cell death pathways to muscle fibre apoptosis or necrosis in dystrophinopathies could not be confirmed.

Cardiac involvement in muscular dystrophies: Molecular mechanisms

In this review, we draw attention to the multiple mechanisms responsible for the pathogenesis of cardiomyopathies in patients with muscular dystrophies. More than one single mechanism is likely to be involved in the development of skeletal and cardiac muscle pathology even when there is a single protein defect. The best example is dystrophin deficiency, in which increased sarcolemmal permeability following eccentric exercise, reduced force generation, and abnormal signaling are all likely to contribute to the progressive muscle damage observed. In other conditions, such as the sarcoglycanopathies, a protein deficiency both in the striated cardiomyocte and the vascular smooth muscle appears to play a significant role. An entirely different mechanism of disease is likely in defects of nuclear envelope proteins, although the precise pathogenesis of this group of conditions is still not clear. Differences between the organization of skeletal and cardiac muscle protein complex are also only starting to emerge and will very likely be the focus of future research.

Molecular and cell biology of the sarcoglycan complex

The original sarcoglycan (SG) complex has four subunits and comprises a subcomplex of the dystrophin-dystrophin-associated protein complex. Each SG gene has been shown to be responsible for limb-girdle muscular dystrophy, called sarcoglycanopathy (SGP). In this review, we detail the characteristics of the SG subunits, and the mechanism of the formation of the SG complex and various molecules associated with this complex. We discuss the molecular mechanisms of SGP based on studies mostly using SGP animal models. In addition, we describe other SG molecules, epsilon- and zeta-SGs, with special reference to their expression and roles in vascular smooth muscle, which are currently in dispute. We further consider the maternally imprinted nature of the epsilon-SG gene. Finally, we stress that the SG complex cannot work by itself and works in a larger complex system, called the transverse fixation system, which forms an array of molecules responsible for various muscular dystrophies.

Genetic compensation for sarcoglycan loss by integrin α7β1 in muscle

Disruption of the sarcoglycan complex leads to muscle membrane instability and muscular dystrophy in humans and mice. Through the dystrophin glycoprotein complex, sarcoglycan participates in connecting the internal cytoskeleton to the membrane and the extracellular matrix. Integrin α7β1 is also a transmembrane protein of skeletal and cardiac muscle that similarly links the cytoskeleton to the extracellular matrix. Mice lacking integrin α7 develop mild muscle degeneration, while sarcoglycan mutant mice display overt muscle degeneration and muscular dystrophy. In sarcoglycan-deficient muscle, integrin α7 protein was upregulated at the plasma membrane. To ascertain whether integrin α7 upregulation compensates for the loss of the transmembrane sarcoglycan linkage in sarcoglycan-deficient muscle, we generated mice lacking both integrin α7 and γ-sarcoglycan (gxi). These double-mutant gxi mice exhibit profound, rapid muscle degeneration leading to death before one month of age consistent with a weakened cellular attachment to the extracellular matrix. The regenerative capacity of gxi muscle was intact with increased embryonic myosin heavy chain expression, myofiber central nucleation and normal in vivo myoblast differentiation. Therefore, upregulation of integrin α7β1 compensates as a transmembrane muscle cell attachment for sarcoglycan consistent with overlapping roles for sarcoglycan and integrins in mediating cytoskeletal-membrane-extracellular matrix interaction.

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.

Pathological analysis of muscle hypertrophy and degeneration in muscular dystrophy in gamma-sarcoglycan-deficient mice

While calf muscle hypertrophy is a striking diagnostic finding in sarcoglycanopathy, as it is in Duchenne and Becker muscular dystrophies, its pathogenetic mechanism remains unknown. gamma-Sarcoglycan, one of the subunits of the sarcoglycan complex, is the protein responsible for gamma-sarcoglycanopathy. To elucidate the pathogenetic mechanisms of muscle hypertrophy and degeneration in muscular dystrophy, we utilized a mutant mouse as a model animal. In this study, we generated gamma-sarcoglycan-deficient (gsg-/-) mice by gene targeting. The gsg-/- mice described here, similar to the gsg-/- mice reported previously (J Cell Biol 142 (1998) 1279), demonstrated skeletal and cardiac muscle degeneration. The limb, shoulder, and pelvic muscles of the gsg-/- mice exhibited progressive muscle hypertrophy and weakness with age, and the findings were similar to those seen in other mouse models for limb-girdle and Duchenne muscular dystrophy. We found that the number of muscle fibers increased with age, and most of the fibers in the hypertrophic muscle were centrally nucleated regenerating fibers. Therefore, muscle hypertrophy of the gsg-/- mice may result from an increase of the number of muscle fibers and probable fiber branching and may not be due to the pseudohypertrophy caused by fibrous and fat tissue replacement, as has been long supposed in muscular dystrophy. The muscle pathology became more 'dystrophic' in mice over 1 year of age when there was a marked variation in fiber size with interstitial fibrosis.

Muscular dystrophies

PURPOSE OF REVIEW

Muscular dystrophy includes many genetically distinct disorders. The list of causative genes for muscular dystrophy has been expanding rapidly, including those for congenital muscular dystrophies.

RECENT FINDINGS

We review the newly identified causative genes and suggested molecular mechanisms, focusing on glycosylation abnormality of alpha-dystroglycan, collagen VI deficiency, four allelic diseases of caveolin-3 gene, and titin gene mutations.

SUMMARY

Several possible mechanisms causing muscular dystrophy were discussed. Defects in extracellular molecules have more significant effects resulting mainly in congenital muscular dystrophy, while intracellular molecular defects show milder effect on the phenotype. These hypotheses may provide a new paradigm in understanding the pathomechanism of muscular dystrophies.

Genomic organization and single-nucleotide polymorphism map of desmuslin, a novel intermediate filament protein on chromosome 15q26.3

BACKGROUND

Desmuslin is an alpha-dystrobrevin-interacting protein expressed primarily in heart and skeletal muscle. The desmuslin protein interacts with and is closely related to desmin, a protein encoded by a locus mutated in some forms of hereditary distal myopathy. As a muscle-specific intermediate filament protein, desmuslin is also a candidate for myopathies of unknown etiology.

RESULTS

The desmuslin gene was localized to chromosome 15q26.3 by electronic screening of the human DNA sequence database. Primer pairs were designed to amplify the 5 exons of the desmuslin gene in 11 overlapping DNA segments. The desmuslin gene was screened for mutations in 71 patients with various forms of myopathy for which there was no known cause. In this analysis, 10 common and 2 rare amino acid altering single-nucleotide polymorphisms were identified, all of which were seen in a control population of individuals thus making these unlikely causes of the phenotype. Interestingly, one of the single-nucleotide polymorphisms found in a patient resulted in a premature stop codon in the first exon. The nonsense mutation was also detected in the patient's unaffected father and one unaffected control; it was detected in 0.44% (2/454) of unrelated chromosomes and is therefore predicted to have a homozygous frequency of 0.002%.

CONCLUSION

No causative mutations were found in the desmuslin gene. However, the single-nucleotide polymorphisms mapped in this study represent a well-mapped group that can be used for disequilibrium studies of this region of chromosome 15q26.3.