The muscular dystrophy with myositis (mdm) mouse mutation disrupts a skeletal muscle-specific domain of titin

Muscular dystrophy with myositis (mdm) is a recessive mouse mutation that causes severe and progressive muscular degeneration. Here we report the identification of the mdm mutation as a complex rearrangement that includes a deletion and a LINE insertion in the titin (Ttn) gene. Mutant allele-specific splicing results in the deletion of 83 amino acids from the N2A region of TTN, a domain thought to bind calpain-3 (CAPN3) the product of the human limb-girdle muscular dystrophy type 2A (LGMD2A) gene. The Ttn(mdm) mutant mouse may serve as a model for human tibial muscular dystrophy, which maps to the TTN locus at 2q31 and shows a secondary reduction of CAPN3 similar to that observed in mdm skeletal muscle. This is the first demonstration that a mutation in Ttn is associated with muscular dystrophy and provides a novel animal model to test for functional interactions between TTN and CAPN3.     

Abnormal expression of a serine protease in human dystrophic muscle

The activities of serine protease in muscles from normal persons and from patients with progressive muscular and neuromuscular diseases have been determined. A significant increase in the level of serine protease was found in muscle of patients with Duchenne-type muscular dystrophy and with Becker-type muscular dystrophy, but the activity was not increased in muscle of a patient with amyotrophic lateral sclerosis.     

A rostrocaudal muscular dystrophy caused by a defect in choline kinase beta, the first enzyme in phosphatidylcholine biosynthesis

Muscular dystrophies include a diverse group of genetically heterogeneous disorders that together affect 1 in 2000 births worldwide. The diseases are characterized by progressive muscle weakness and wasting that lead to severe disability and often premature death. Rostrocaudal muscular dystrophy (rmd) is a new recessive mouse mutation that causes a rapidly progressive muscular dystrophy and a neonatal forelimb bone deformity. The rmd mutation is a 1.6-kb intragenic deletion within the choline kinase beta (Chkb) gene, resulting in a complete loss of CHKB protein and enzymatic activity. CHKB is one of two mammalian choline kinase (CHK) enzymes (alpha and beta) that catalyze the phosphorylation of choline to phosphocholine in the biosynthesis of the major membrane phospholipid phosphatidylcholine. While mutant rmd mice show a dramatic decrease of CHK activity in all tissues, the dystrophy is only evident in skeletal muscle tissues in an unusual rostral-to-caudal gradient. Minor membrane disruption similar to dysferlinopathies suggest that membrane fusion defects may underlie this dystrophy, because severe membrane disruptions are not evident as determined by creatine kinase levels, Evans Blue infiltration, and unaltered levels of proteins in the dystrophin-glycoprotein complex. The rmd mutant mouse offers the first demonstration of a defect in a phospholipid biosynthetic enzyme causing muscular dystrophy, representing a unique model for understanding mechanisms of muscle degeneration.     

The Cardiomyopathy of Muscular Dystrophy: Report of Two Cases with a Review of the Literature

About 50% of patients with progressive muscular dystrophy have a cardiomyopathy, manifested commonly by tachycardia, but also by arrythmias, refractory congestive heart failure and sudden death. Studies from the literature report manifold but nonspecific electrocardiographic changes in 41% to 85% of patients with progressive muscular dystrophy. The principal lesion is a diffuse myocardial fibrosis with minor degenerative changes in myocardial fibres unaccompanied by significant inflammation. The heart is enlarged and has a prominent deposit of epicardial fat. The myocardium is pale, coarse, flabby and friable, often showing gross evidence of scarring. The dilated chambers often contain mural thrombus.     

Muscular dystrophy of mink: a new animal model.

Muscular dystrophies comprise an important group of inherited disorders of man. Although the disease has been studied extensively, little is known about the underlying primary pathomechanisms. Consequently, treatment of patients is difficult and prognosis is poor. An animal model of muscular dystrophy is a useful research tool for approaching the basic problems of pathogenesis in muscle diseases. An inherited progressive muscular dystrophy of mink which resembles the amyotonic forms of human muscular dystrophy is currently under study. Clinically, the earliest sign is progressive muscular weakness and atrophy. Muscle enzyme activities in serum are usually elevated to pathologic levels. Urinary creatine/creatinine ratio is elevated. Pathologic changes are limited to skeletal muscle and are typical of those seen in amyotonic forms of human muscular dystrophy. These changes include variation in diameter size of muscle fibers, centralized nuclei, floccular and hyaline degeneration of scattered muscle fibers, increase in connective tissue in endomysial and perimysial areas, and regenerative attempts. Both type I and type II muscle fibers are involved in the disease process. Genetic studies indicate an autosomal recessive mode of inheritance. Although the primary defect in muscular dystrophy is traditionally thought to reside in skeletal muscle, recent studies have produced theories of primary involvement of other tissues and organ systems. These theories are presented and relationships to the traditional theory are discussed.     

Mutation of Large,which encodes a putative glycosyltransferase,in an animal model of muscular dystrophy

The myodystrophy (myd) mutation arose spontaneously and has an autosomal recessive mode of inheritance. Homozygous mutant mice display a severe, progressive muscular dystrophy. Using a positional cloning approach, we identified the causative mutation in myd as a deletion within the Large gene, which encodes a putative glycosyltransferase with two predicted catalytic domains. By immunoblotting, the alpha-subunit of dystroglycan, a key muscle membrane protein, is abnormal in myd mice. This aberrant protein might represent altered glycosylation of the protein and contribute to the muscular dystrophy phenotype. Our results are discussed in the light of recent reports describing mutations in other glycosyltransferase genes in several forms of human muscular dystrophy.     

Genetics and epigenetics of facio-scapulohumeral progressive (Landouzy-Dejerine) muscular dystrophy

Landouzy-Dejerine muscular dystrophy is a rare hereditary disease with prevalence of 0.9 to 1.4 in 100,000. Clinically the disease is characterized by weakness and atrophy of the facial and shoulder girdle muscles. It is caused by partial deletion of the 3.3-kb subtelomeric D4Z4 repeat on chromosome 4 (locus 4q35). This paper presents a critical review of the literature data and hypotheses explaining molecular mechanisms of progressive fascioscapulohumeral muscular dystrophy.     

Genetics and Epigenetics of Progressive Fascioscapulohumeral (Landouzy–Dejerine) Muscular Dystrophy

Landouzy–Dejerine muscular dystrophy is a rare hereditary disease with prevalence of 0.9 to 1.4 in 100000. Clinically the disease is characterized by weakness and atrophy of the facial and shoulder girdle muscles. It is caused by partial deletion of the 3.3-kb subtelomeric D4Z4 repeat on chromosome 4 (locus 4q35). This paper presents a critical review of the literature data and hypotheses explaining molecular mechanisms of progressive fascioscapulohumeral muscular dystrophy.     

Emery-Dreifuss muscular dystrophy

The muscular dystrophies are a group of disorders, genetically determined, with progressive degeneration of muscle(s), without central nervous nor peripheral nerve abnormalities. The Emery-Dreifuss muscular dystrophy is one of these. We report a case with typical features.     

Sparks, signals and shock absorbers: how dystrophin loss causes muscular dystrophy

The dystrophin-glycoprotein complex (DGC) can be considered as a specialized adhesion complex, linking the extracellular matrix to the actin cytoskeleton, primarily in muscle cells. Mutations in several components of the DGC lead to its partial or total loss, resulting in various forms of muscular dystrophy. These typically manifest as progressive wasting diseases with loss of muscle integrity. Debate is ongoing about the precise function of the DGC: initially a strictly mechanical role was proposed but it has been suggested that there is aberrant calcium handling in muscular dystrophy and, more recently, changes in MAP kinase and GTPase signalling have been implicated in the aetiology of the disease. Here, we discuss new and interesting developments in these aspects of DGC function and attempt to rationalize the mechanical, calcium and signalling hypotheses to provide a unifying hypothesis of the underlying process of muscular dystrophy.     

A Splice Site Mutation in Laminin-α2 Results in a Severe Muscular Dystrophy and Growth Abnormalities in Zebrafish

Congenital muscular dystrophy (CMD) is a clinically and genetically heterogeneous group of inherited muscle disorders. In patients, muscle weakness is usually present at or shortly after birth and is progressive in nature. Merosin deficient congenital muscular dystrophy (MDC1A) is a form of CMD caused by a defect in the laminin-α2 gene (LAMA2). Laminin-α2 is an extracellular matrix protein that interacts with the dystrophin-dystroglycan (DGC) complex in membranes providing stability to muscle fibers. In an N-ethyl-N-nitrosourea mutagenesis screen to develop zebrafish models of neuromuscular diseases, we identified a mutant fish that exhibits severe muscular dystrophy early in development. Genetic mapping identified a splice site mutation in the lama2 gene. This splice site is highly conserved in humans and this mutation results in mis-splicing of RNA and a loss of protein function. Homozygous lama2 mutant zebrafish, designated lama2(cl501/cl501), exhibited reduced motor function and progressive degeneration of skeletal muscles and died at 8-15 days post fertilization. The skeletal muscles exhibited damaged myosepta and detachment of myofibers in the affected fish. Laminin-α2 deficiency also resulted in growth defects in the brain and eye of the mutant fish. This laminin-α2 deficient mutant fish represents a novel disease model to develop therapies for modulating splicing defects in congenital muscular dystrophies and to restore the muscle function in human patients with CMD.     

Cross‐sectional serum metabolomic study of multiple forms of muscular dystrophy

Muscular dystrophies are characterized by a progressive loss of muscle tissue and/or muscle function. While metabolic alterations have been described in patients’‐derived muscle biopsies, non‐invasive readouts able to describe these alterations are needed in order to objectively monitor muscle condition and response to treatment targeting metabolic abnormalities. We used a metabolomic approach to study metabolites concentration in serum of patients affected by multiple forms of muscular dystrophy such as Duchenne and Becker muscular dystrophies, limb‐girdle muscular dystrophies type 2A and 2B, myotonic dystrophy type 1 and facioscapulohumeral muscular dystrophy. We show that 15 metabolites involved in energy production, amino acid metabolism, testosterone metabolism and response to treatment with glucocorticoids were differentially expressed between healthy controls and Duchenne patients. Five metabolites were also able to discriminate other forms of muscular dystrophy. In particular, creatinine and the creatine/creatinine ratio were significantly associated with Duchenne patients performance as assessed by the 6‐minute walk test and north star ambulatory assessment. The obtained results provide evidence that metabolomics analysis of serum samples can provide useful information regarding muscle condition and response to treatment, such as to glucocorticoids treatment.     

Linkage localization of facioscapulohumeral muscular dystrophy (FSHD) in 4q35.

Fasioscapulohumeral muscular dystrophy (FSHD) has recently been localized to 4q35. We have studied four families with FSHD. Linkage to the 4q35 probes D4S163, D4S139, and D4S171 was confirmed. We found no recombinants helpful in detailed localization of the FSHD gene. Two of our families include males with a rapidly progressive muscle disease that had been diagnosed, on the basis of clinical features, as Duchenne muscular dystrophy. One of these males is available for linkage study and shares the haplotype of his FSHD-affected aunt and cousin.     

Alpha sarcoglycan is required for FGF-dependent myogenic progenitor cell proliferation in vitro and in vivo

Mice deficient in α-sarcoglycan (Sgca-null mice) develop progressive muscular dystrophy and serve as a model for human limb girdle muscular dystrophy type 2D. Sgca-null mice suffer a more severe myopathy than that of mdx mice, the model for Duchenne muscular dystrophy. This is the opposite of what is observed in humans and the reason for this is unknown. In an attempt to understand the cellular basis of this severe muscular dystrophy, we isolated clonal populations of myogenic progenitor cells (MPCs), the resident postnatal muscle progenitors of dystrophic and wild-type mice. MPCs from Sgca-null mice generated much smaller clones than MPCs from wild-type or mdx dystrophic mice. Impaired proliferation of Sgca-null myogenic precursors was confirmed by single fiber analysis and this difference correlated with Sgca expression during MPC proliferation. In the absence of dystrophin and associated proteins, which are only expressed after differentiation, SGCA complexes with and stabilizes FGFR1. Deficiency of Sgca leads to an absence of FGFR1 expression at the membrane and impaired MPC proliferation in response to bFGF. The low proliferation rate of Sgca-null MPCs was rescued by transduction with Sgca-expressing lentiviral vectors. When transplanted into dystrophic muscle, Sgca-null MPCs exhibited reduced engraftment. The reduced proliferative ability of Sgca-null MPCs explains, at least in part, the severity of this muscular dystrophy and also why wild-type donor progenitor cells engraft efficiently and consequently ameliorate disease.     

Novel mutation in the fukutin gene in an Egyptian family with Fukuyama congenital muscular dystrophy and microcephaly

Fukuyama-type congenital muscular dystrophy (FCMD, MIM#253800) is an autosomal recessive disorder characterized by severe muscular dystrophy associated with brain malformations. FCMD is the second most common form of muscular dystrophy after Duchenne muscular dystrophy and one of the most common autosomal recessive diseases among the Japanese population, and yet few patients outside of Japan had been reported with this disorder. We report the first known Egyptian patient with FCMD, established by clinical features of generalized weakness, pseudohypertrophy of calf muscles, progressive joint contractures, severe scoliosis, elevated serum creatine kinase level, myopathic electrodiagnostic changes, brain MRI with cobblestone complex, and mutation in the fukutin gene. In addition, our patient displayed primary microcephaly, not previously reported associated with fukutin mutations. Our results expand the geographic and clinical spectrum of fukutin mutations.     

Mutation of Large,which encodes a putative glycosyltransferase,in an animal model of muscular dystrophy

The myodystrophy (myd) mutation arose spontaneously and has an autosomal recessive mode of inheritance. Homozygous mutant mice display a severe, progressive muscular dystrophy. Using a positional cloning approach, we identified the causative mutation in myd as a deletion within the Large gene, which encodes a putative glycosyltransferase with two predicted catalytic domains. By immunoblotting, the α-subunit of dystroglycan, a key muscle membrane protein, is abnormal in myd mice. This aberrant protein might represent altered glycosylation of the protein and contribute to the muscular dystrophy phenotype. Our results are discussed in the light of recent reports describing mutations in other glycosyltransferase genes in several forms of human muscular dystrophy.     

Hereditary progressive muscular dystrophies: serum myoglobin pattern in patients with different types of muscular dystrophies

Serum myoglobin (Mb) levels and creatine kinase (CK) activity were investigated in patients with different types of progressive muscular dystrophy and controls. The Mb levels were determined by radioimmunoassay and found to be significantly elevated in all patients under resting conditions. There was no correlation between Mb levels and CK activity. Physical exercise was followed by an increase in Mb levels and CK activity in patients and a minor variation in controls. Isoelectric focusing, electroblotting and application of a specific Mb antibody (rabbit anti-human Mb) make it possible to recognize marked differences between the Mb bands of patients and controls. All patients with progressive muscular dystrophy had an additional fourth Mb band (isoelectric point pH 6.3) in contrast to controls with three Mb bands.     

Correlation of Utrophin Levels with the Dystrophin Protein Complex and Muscle Fibre Regeneration in Duchenne and Becker Muscular Dystrophy Muscle Biopsies

Duchenne muscular dystrophy is a severe and currently incurable progressive neuromuscular condition, caused by mutations in the DMD gene that result in the inability to produce dystrophin. Lack of dystrophin leads to loss of muscle fibres and a reduction in muscle mass and function. There is evidence from dystrophin-deficient mouse models that increasing levels of utrophin at the muscle fibre sarcolemma by genetic or pharmacological means significantly reduces the muscular dystrophy pathology. In order to determine the efficacy of utrophin modulators in clinical trials, it is necessary to accurately measure utrophin levels and other biomarkers on a fibre by fibre basis within a biopsy section. Our aim was to develop robust and reproducible staining and imaging protocols to quantify sarcolemmal utrophin levels, sarcolemmal dystrophin complex members and numbers of regenerating fibres within a biopsy section. We quantified sarcolemmal utrophin in mature and regenerating fibres and the percentage of regenerating muscle fibres, in muscle biopsies from Duchenne, the milder Becker muscular dystrophy and controls. Fluorescent immunostaining followed by image analysis was performed to quantify utrophin intensity and β-dystrogylcan and ɣ –sarcoglycan intensity at the sarcolemma. Antibodies to fetal and developmental myosins were used to identify regenerating muscle fibres allowing the accurate calculation of percentage regeneration fibres in the biopsy. Our results indicate that muscle biopsies from Becker muscular dystrophy patients have fewer numbers of regenerating fibres and reduced utrophin intensity compared to muscle biopsies from Duchenne muscular dystrophy patients. Of particular interest, we show for the first time that the percentage of regenerating muscle fibres within the muscle biopsy correlate with the clinical severity of Becker and Duchenne muscular dystrophy patients from whom the biopsy was taken. The ongoing development of these tools to quantify sarcolemmal utrophin and muscle regeneration in muscle biopsies will be invaluable for assessing utrophin modulator activity in future clinical trials.     

Dysferlin and the plasma membrane repair in muscular dystrophy

Muscular dystrophy covers a group of genetically determined disorders that cause progressive weakness and wasting of the skeletal muscles. Dysferlin was identified as a gene mutated in limb-girdle muscular dystrophy (type 2B) and Miyoshi myopathy. The discovery of dysferlin revealed a new family of proteins, known as the ferlin family, which includes four different genes. Recent work suggests the function of dysferlin in membrane repair and demonstrates that defective membrane repair is a novel mechanism of muscle degeneration. These findings reveal the importance of a basic cellular function in skeletal muscle and a new class of muscular dystrophy where the defect lies in the maintenance, not the structure, of the plasma membrane. Here, we discuss the current knowledge of dysferlin function in the repair of the plasma membrane of the skeletal muscle cells.     

Recent advances in dystrophin research.

Evidence suggesting that dystrophin is a component of the membrane cytoskeleton of excitable cells continues to accumulate. Whereas the specific mechanisms leading to muscle pathology in Duchenne muscular dystrophy are still being debated it is apparent that the progressive weakness that occurs in this disease is the result of a chronic process that is initiated by dystrophin deficiency.