Dystrophin muscle enhancer 1 is implicated in the activation of non-muscle isoforms in the skeletal muscle of patients with X-linked dilated cardiomyopathy.

X-linked dilated cardiomyopathy (XLDC) is a dystrophinopathy characterized by severe cardiomyopathy with no skeletal muscle involvement. Several XLDC patients have been described with mutations that abolish dystrophin muscle (M) isoform expression. The absence of skeletal muscle degeneration normally associated with loss of dystrophin function was shown to be due to increased expression of brain (B) and cerebellar Purkinje (CP) isoforms of the gene exclusively in the skeletal muscle of these patients. This suggested that the B and CP promoters have an inherent capacity to function in skeletal muscle or that they are up-regulated by a skeletal muscle-specific enhancer unaffected by the mutations in these patients. In this work we have analyzed the deletion breakpoints of two XLDC patients with deletions removing the M promoter and exon 1, but not affecting the B and CP promoters. Despite the presence of several muscle-specific regulatory motifs, the B and CP promoters were found to be essentially inactive in muscle cell lines and primary cultures. As dystrophin muscle enhancer 1 (DME1), the only known muscle-specific enhancer within the dystrophin gene, is preserved in these patients, we tested its ability to up-regulate the B and CP promoters in muscle cells. B and CP promoter activity was significantly increased in the presence of DME1, and more importantly, activation was observed exclusively in cells presenting a skeletal muscle phenotype. These results point to a role for DME1 in the induction of B and CP isoform expression in the skeletal muscle of XLDC patients defective for M isoform expression.     

Mouse dystrophin enhancer preferentially targets lacZ expression in skeletal and cardiac muscle.

Duchenne muscular dystrophy is a muscle wasting disease that results from a dystrophin deficiency in skeletal and cardiac muscle. Studies concerning the regulatory elements that govern dystrophin gene expression in skeletal and/or cardiac muscle in both mouse and human have identified a promoter and an enhancer located in intron 1. In transgenic mice, the muscle promoter alone targets the expression of a lacZ reporter gene only to the right ventricle of the heart, suggesting the need for other regulatory elements to target skeletal muscle and the rest of the heart. Here we report that the mouse dystrophin enhancer from intron 1 can target the expression of a lacZ reporter gene in skeletal muscle as well as in other heart compartments of transgenic mice. Our results also suggest that sequences surrounding the mouse dystrophin enhancer may affect its function throughout mouse development.     

A point mutation in the 5' splice site of the dystrophin gene first intron responsible for X-linked dilated cardiomyopathy

X-linked dilated cardiomyopathy (XLDC) is a familial heart disease presenting in young males as a rapidly progressive congestive heart failure, without clinical signs of skeletal myopathy. This condition has recently been linked to the dystrophin gene in some families and deletions encompassing the genomic region coding for the first muscle exon have been detected. In order to identify the defect responsible for this disease at the molecular level and to understand the reasons for the selective heart involvement, a family with a severe form of XLDC was studied. In the affected members, no deletions of the dystrophin gene were observed. Analysis of the muscle promoter, first exon and intron regions revealed the presence of a single point mutation at the first exon-intron boundary, inactivating the universally conserved 5' splice site consensus sequence of the first intron. This mutation introduced a new restriction site for MseI, which cosegregates with the disease in the analyzed family. Expression of the major dystrophin mRNA isoforms (from the muscle-, brain- and Purkinje cell-promoters) was completely abolished in the myocardium, while the brain- and Purkinje cell- (but not the muscle-) isoforms were detectable in the skeletal muscle. Immunocytochemical studies with anti- dystrophin antibodies showed that the protein was reduced in quantity but normally distributed in the skeletal muscle, while it was undetectable in the cardiac muscle. These findings indicate that expression of the muscle dystrophin isoform is critical for myocardial function and suggest that selective heart involvement in dystrophin- linked dilated cardiomyopathy is related to the absence, in the heart, of a compensatory expression of dystrophin from alternative promoters.      

Long-term rescue of dystrophin expression and improvement in muscle pathology and function in dystrophic mdx mice by peptide-conjugated morpholino

Exon skipping is capable of correcting frameshift and nonsense mutations in Duchenne muscular dystrophy. Phase 2 clinical trials in the United Kingdom and the Netherlands have reported induction of dystrophin expression in muscle of Duchenne muscular dystrophy patients by systemic administration of both phosphorodiamidate morpholino oligomers (PMO) and 2'-O-methyl phosphorothioate. Peptide-conjugated phosphorodiamidate morpholino offers significantly higher efficiency than phosphorodiamidate morpholino, with the ability to induce near-normal levels of dystrophin, and restores function in both skeletal and cardiac muscle. We examined 1-year systemic efficacy of peptide-conjugated phosphorodiamidate morpholino targeting exon 23 in dystrophic mdx mice. The LD(50) of peptide-conjugated phosphorodiamidate morpholino was determined to be approximately 85 mg/kg. The half-life of dystrophin expression was approximately 2 months in skeletal muscle, but shorter in cardiac muscle. Biweekly injection of 6 mg/kg peptide-conjugated phosphorodiamidate morpholino produced >20% dystrophin expression in all skeletal muscles and ≤5% in cardiac muscle, with improvement in muscle function and pathology and reduction in levels of serum creatine kinase. Monthly injections of 30 mg/kg peptide-conjugated phosphorodiamidate morpholino restored dystrophin to >50% normal levels in skeletal muscle, and 15% in cardiac muscle. This was associated with greatly reduced serum creatine kinase levels, near-normal histology, and functional improvement of skeletal muscle. Our results demonstrate for the first time that regular 1-year administration of peptide-conjugated phosphorodiamidate morpholino can be safely applied to achieve significant therapeutic effects in an animal model.     

Alterations of dystrophin-associated glycoproteins in the heart lacking dystrophin or dystrophin and utrophin

Heart disease is a leading cause of death in patients with Duchenne muscular dystrophy (DMD). Patients with DMD lack the protein dystrophin, which is widely expressed in striated muscle. In skeletal muscle, the loss of dystrophin results in dramatically decreased expression of the dystrophin associated glycoprotein complex (DGC). Interestingly, in the heart the DGC is normally expressed without dystrophin; this has been attributed to presence of the dystrophin homologue utrophin. We demonstrate here that neither utrophin nor dystrophin are required for the expression of the cardiac DGC. However, alpha-dystroglycan (α-DG), a major component of the DGC, is differentially glycosylated in dystrophin-(mdx) and dystrophin-/utrophin-(dko) deficient mouse hearts. In both models the altered α-DG retains laminin binding activity, but has an altered localization at the sarcolemma. In hearts lacking both dystrophin and utrophin, the alterations in α-DG glycosylation are even more dramatic with changes in gel migration equivalent to 24 ± 3 kDa. These data show that the absence of dystrophin and utrophin alters the processing of α-DG; however it is not clear if these alterations are a consequence of the loss of a direct interaction with dystrophin/utrophin or results from an indirect response to the presence of severe pathology. Recently there have been great advances in our understanding of the glycosylation of α-DG regarding its role as a laminin receptor. Here we present data that alterations in glycosylation occur in the hearts of animal models of DMD, but these changes do not affect laminin binding. The physiological consequences of these alterations remain unknown, but may have significant implications for the development of therapies for DMD.     

Utrophin and dystrophin-associated glycoproteins in normal and dystrophin deficient cardiac muscle

In this study, various members of the dystrophin family (dystrophin, the short dystrophin product Dp 71, utrophin and DRP2), and different members of the dystrophin-associated glycoprotein (DAG) complex (beta-dystroglycan, alpha-, beta-, gamma- and delta-sarcoglycans) were localized in bovine cardiac muscle using a battery of specific antibodies. We have established that dystrophin is exclusively associated with beta-dystroglycan and both alpha- and delta-sarcoglycans in cardiac muscle cell membranes. In contrast, utrophin is a specific component of intercalated disks together with beta- and gamma-sarcoglycans, while beta-dystroglycan, alpha- and delta-sarcoglycans are not present. Dp 71 is mainly localized at the T tubule transverse area. In dystrophin deficient cardiac muscle, utrophin and beta-sarcoglycan were observed in intercalated disks and at the sarcolemma of each cardiocyte. Our results revealed that complexes of associated glycoproteins differ in cardiac muscle when associated with dystrophin or utrophin. Despite the described sequence homologies between dystrophin and utrophin, the present results indicate that these proteins have different roles in some specific cardiac cell areas.     

Prolonged dystrophin expression and functional correction of mdx mouse muscle following gene transfer with a helper-dependent (gutted) adenovirus-encoding murine dystrophin

Dystrophin gene transfer using helper-dependent adenoviruses (HDAd), which are deleted of all viral genes, is a promising option to treat muscles in Duchenne muscular dystrophy. We investigated the benefits of this approach by injecting the tibialis anterior (TA) muscle of neonatal and juvenile (4-6-week-old) dystrophin-deficient (mdx) mice with a fully deleted HDAd (HDCBDysM). This vector encoded two full-length murine dystrophin cDNAs regulated by the powerful cytomegalovirus enhancer/beta-actin promoter. At 10 days post-injection of neonatal muscles, 712 fibers (42% of the total number of TA fibers) were dystrophin-positive (dys+), a value that did not decrease for 6 months (the study duration). In treated juveniles, maximal transduction occurred at 30 days post-injection (414 dys+ fibers, 24% of the total number of TA fibers), but decreased by 51% after 6 months. All studied aspects of the pathology were improved in neonatally treated muscles: the percentage of dys+ fibers with centrally localized myonuclei remained low, localization of the dystrophin associated protein complex was restored at the plasma membrane, muscle hypertrophy was reduced, and maximal force-generating capacity and resistance to contraction-induced injuries were increased. The same pathological aspects were improved in the treated juveniles, except for reduction of muscle hypertrophy and maximal force-generating capacity. We demonstrated a strong humoral response against murine dystrophin in both animal groups, but mild inflammatory response occurred only in the treated juveniles. HDCBDysM is thus one of the most promising and efficient vectors for treating DMD by gene therapy.     

Direct retroviral-mediated transfer of a dystrophin minigene into mdx mouse muscle in vivo

At the cellular level, the primary pathology in Duchenne musculardystrophy (DMD) is caused by deficiency of the sarcolemmal-associatedprotein, dystrophin, in the striated musculature. Here we describethe somatic transfer and longterm expression of a human dystrophinminigene corresponding to a mild Becker muscular dystrophy (BMD)phenotype in skeletal muscle tissues of the dystrophin-deficientmdx mouse by direct retroviral transduction. Following a singleintramuscular injection of recombinant retrovirus, sarcolemmalexpression of dystrophin was observed in an average of     

Danon disease presenting with dilated cardiomyopathy and a complex phenotype

X-linked dilated cardiomyopathy (XLCM) was first described in 1987 and associated with dystrophin gene (DMD) mutations a decade later in one of the original two families. Here we report long-term follow-up of the second family (XLCM-2), for which a DMD mutation was never found. Analysis of the lysosome-associated membrane protein-2 (LAMP-2) gene detected a novel mutation, confirming a diagnosis of Danon disease. The broad phenotype in this family included dilated and hypertrophic cardiomyopathy, cardiac pre-excitation, skeletal myopathy with high serum creatine kinase, cognitive impairment (in males), and a pigmentary retinopathy in affected females. Cardiac biopsy specimens showed extensive vacuolar changes in an affected adult male. Remarkably, the skeletal muscle biopsy in a 13-month-old mutation-carrying male showed no vacuolization by standard histology. We conclude that XLCM may be the presenting sign of Danon disease and, in the presence of a familial history of HCM, pre-excitation, skeletal muscle involvement and retinal pigmentary dystrophy should prompt LAMP-2 clinical testing. Furthermore, the absence of vacuolar myopathy in biopsies from young patients may not exclude Danon disease. Supported by American Heart Association (0150453N), Muscular Dystrophy Association (PN0007-056) and NIH (1RO1HL69071-01 and 1K23Hl67915-01A1).     

Visualization of the subsarcolemmal cytoskeleton network of mouse skeletal muscle cells by en face views and application to immunoelectron localization of dystrophin

Summary The ultrastructural organization of the highly interconnected filamentous network underneath the sarcolemma as well as the role played by the muscle protein dystrophin within this cytoskeleton remain yet unclear. More accurate information has been obtained by using a method which provides three-dimensional en face views of large membrane areas applied to mouse cultured myotubes and isolated adult skeletal muscle fibres. Two levels have been distinguished in the cytoskeleton underlying the sarcolemma: the submembranous level, partly integrated into the membrane, and the cortical level, invading the proximal cytoplasmic space. Few differences have been found between the membrane cytoskeletons of myotubes issued from 14-day-old cultures and those of adult fibres. The comparison was done with cells where dystrophin is missing (mdx mouse muscle): surprisingly, the lack of dystrophin does not induce obvious or dramatic ultrastructural disorganization, either in the cortical cytoskeletal network or in the submembranous one. Immunogold labelling of either the central-rod or the C-terminal domain of dystrophin is not located among the cortical network. This study provides additional data on the spatial ordering of subsarcolemmal cytoskeletal elements: dystrophin does not appear as a filamentous structure entirely located among subsarcolemmal cytoskeleton but seems partly embedded in membranous material.     

抗肌萎缩蛋白基因非缺失/重复突变引起的Becker型肌营养不良症的研究

目的 探讨Becker型肌营养不良症(Becker muscular dystrophy,BMD)的基因突变类型,增加对抗肌萎缩蛋白基因非缺失/重复突变引起BMD的认识.方法 收集2例BMD患者的临床资料,应用多重连接依赖式探针扩增(multiplex ligation-dependent probe amplification assay,MLPA)方法对抗肌萎缩蛋白基因进行分析,并对其肌肉进行苏木素-伊红(hematoxylin-eosin,HE)染色、抗肌萎缩蛋白(dystrophin)染色及电镜检测.结果 2例患者经MLPA方法检测抗肌萎缩蛋白基因均呈非缺失/重复突变类型,肌肉活检光镜和电镜均呈肌营养不良改变.例1患者染色示肌膜dystrophin大部分呈不连续弱阳性,部分为阴性.例2患者染色示肌膜dystrophin-C为阴性,dystrophin-N为阳性.结论 对于抗肌萎缩蛋白基因非缺失/重复突变的临床症状较轻的患者,进行肌肉活检和抗肌萎缩蛋白免疫组化将有助于明确诊断BMD及判断其预后.

Abstract：

Objective To identify potential mutations in patients featuring Becker muscular dystrophy (BMD) and to enhance the understanding of non-deletion/duplication mutations of the dystrophin gene causing BMD. Methods linical data of two patients affected with BMD were collected. Potential mutations in the dystrophin gene were screened with multiplex ligation-dependent probe amplification assay (MLPA). Biopsied muscle samples were examined with HE staining, immnostaining with anti-dystrophin antibody, and electronic microscopy. Results MLPA assay suggested that both cases were probably due to non-deletion/duplication mutations of the dystrophin gene. Light and electronic microcopy of skeletal muscle biopsies confirmed dystrophic changes in both patients. For patient A, immunostaining showed non-contiguous weak staining for most parts of sarcolemma. For patient B, immunostaining showed positive result with N-terminal anti-dystrophin antibody and negative result with C-terminal anti-dystrophin antibody. Conclusion For patients with mild phenotypes but without dystrophin gene deletion/duplication, muscle biopsy and immunochemistry are helpful for diagnosis and prognosis.     

RNAi-mediated knockdown of dystrophin expression in adult mice does not lead to overt muscular dystrophy pathology

Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disorder caused by mutations in the dystrophin gene. DMD has a complex and as yet incompletely defined molecular pathophysiology. The peak of the pathology attributed to dystrophin deficiency happens between 3 and 8 weeks of age in mdx mice, the animal model of DMD. Accordingly, we hypothesized that the pathology observed with dystrophin deficiency may be developmentally regulated. Initially, we demonstrated that profound small interfering RNA-mediated dystrophin knockdown could be achieved in mouse primary muscle cultures. The use of adeno-associated virus vectors to express short-hairpin RNAs targeting dystrophin in skeletal muscle in vivo yielded a potent and specific dystrophin knockdown, but only after approximately 5 months, indicating the very long half-life of dystrophin. Interestingly, and in contrast to what is observed in congenital dystrophin deficiency, long-term ( approximately 1 year) dystrophin knockdown in adult mice did not result, per se, in overt dystrophic pathology or upregulation of utrophin. This supports our hypothesis and suggests new pathophysiology of the disease. Furthermore, taking into account the rather long half-life of dystrophin, and the notion that the development of pathology is age-dependent, it indicates that a single gene therapy approach before the onset of pathology might convey a long-term cure for DMD.