Dystrophin distribution in heterozygote MDX mice

The myonuclear domain in adult skeletal muscle fibres: past, present and future

Most cells in the body are mononuclear whereas skeletal muscle fibres are uniquely multinuclear. The nuclei of muscle fibres (myonuclei) are usually situated peripherally which complicates the equitable distribution of gene products. Myonuclear abundance can also change under conditions such as hypertrophy and atrophy. Specialised zones in muscle fibres have different functions and thus distinct synthetic demands from myonuclei. The complex structure and regulatory requirements of multinuclear muscle cells understandably led to the hypothesis that myonuclei govern defined 'domains' to maintain homeostasis and facilitate adaptation. The purpose of this review is to provide historical context for the myonuclear domain and evaluate its veracity with respect to mRNA and protein distribution resulting from myonuclear transcription. We synthesise insights from past and current in vitro and in vivo genetically modified models for studying the myonuclear domain under dynamic conditions. We also cover the most contemporary knowledge on mRNA and protein transport in muscle cells. Insights from emerging technologies such as single myonuclear RNA-sequencing further inform our discussion of the myonuclear domain. We broadly conclude: (1) the myonuclear domain can be flexible during muscle fibre growth and atrophy, (2) the mechanisms and role of myonuclear loss and motility deserve further consideration, (3) mRNA in muscle is actively transported via microtubules and locally restricted, but proteins may travel far from a myonucleus of origin and (4) myonuclear transcriptional specialisation extends beyond the classic neuromuscular and myotendinous populations. A deeper understanding of the myonuclear domain in muscle may promote effective therapies for ageing and disease.

Uniform sarcolemmal dystrophin expression is required to prevent extracellular microRNA release and improve dystrophic pathology

BACKGROUND

Duchenne muscular dystrophy (DMD) is a fatal muscle-wasting disorder caused by genetic loss of dystrophin protein. Extracellular microRNAs (ex-miRNAs) are putative, minimally invasive biomarkers of DMD. Specific ex-miRNAs (e.g. miR-1, miR-133a, miR-206, and miR-483) are highly up-regulated in the serum of DMD patients and dystrophic animal models and are restored to wild-type levels following exon skipping-mediated dystrophin rescue in mdx mice. As such, ex-miRNAs are promising pharmacodynamic biomarkers of exon skipping efficacy. Here, we aimed to determine the degree to which ex-miRNA levels reflect the underlying level of dystrophin protein expression in dystrophic muscle.

METHODS

Candidate ex-miRNA biomarker levels were investigated in mdx mice in which dystrophin was restored with peptide-PMO (PPMO) exon skipping conjugates and in mdx-Xist^Δhs mice that express variable amounts of dystrophin from birth as a consequence of skewed X-chromosome inactivation. miRNA profiling was performed in mdx-Xist^Δhs mice using the FirePlex methodology and key results validated by small RNA TaqMan RT-qPCR. The muscles from each animal model were further characterized by dystrophin western blot and immunofluorescence staining.

RESULTS

The restoration of ex-myomiR abundance observed following PPMO treatment was not recapitulated in the high dystrophin-expressing mdx-Xist^Δhs group, despite these animals expressing similar amounts of total dystrophin protein (~37% of wild-type levels). Instead, ex-miRNAs were present at high levels in mdx-Xist^Δhs mice regardless of dystrophin expression. PPMO-treated muscles exhibited a uniform pattern of dystrophin localization and were devoid of regenerating fibres, whereas mdx-Xist^Δhs muscles showed non-homogeneous dystrophin staining and sporadic regenerating foci.

CONCLUSIONS

Uniform dystrophin expression is required to prevent ex-miRNA release, stabilize myofiber turnover, and attenuate pathology in dystrophic muscle.

Connexin-43 reduction prevents muscle defects in a mouse model of manifesting Duchenne muscular dystrophy female carriers

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular disorder that affects males. However, 8% of female carriers are symptomatic and underrepresented in research due to the lack of animal models. We generated a symptomatic mouse model of DMD carriers via injection of mdx (murine DMD) embryonic stem cells (ESCs) into wild-type (WT) blastocysts (mdx/WT chimera). mdx/WT chimeras developed cardiomyopathic features and dystrophic skeletal muscle phenotypes including elevated mononuclear invasion, central nucleation, fibrosis and declined forelimb grip strength. The disease was accompanied by connexin-43 (Cx43) aberrantly enhanced in both cardiac and skeletal muscles and remodeled in the heart. Genetic reduction of Cx43-copy number in mdx/WT-Cx43(+/-) chimeras protected them from both cardiac and skeletal muscle fiber damage. In dystrophic skeletal muscle, Cx43 expression was not seen in the fibers but in adjacent F4/80+ mononuclear cells. Ethidium Bromide uptake in purified F4/80+/CD11b+ mdx macrophages revealed functional activity of Cx43, which was inhibited by administration of Gap19 peptide mimetic, a Cx43 hemichannel-specific inhibitor. Thus, we suggest that Cx43 reduction in symptomatic DMD carrier mice leads to prevention of Cx43 remodeling in the heart and prevention of aberrant Cx43 hemichannel activity in the skeletal muscle macrophages neighboring Cx43 non-expressing fibers.

Small Fractions of Muscular Dystrophy Embryonic Stem Cells Yield Severe Cardiac and Skeletal Muscle Defects in Adult Mouse Chimeras

Duchenne muscular dystrophy (DMD) is characterized by the loss of the protein dystrophin, leading to muscle fragility, progressive weakening, and susceptibility to mechanical stress. Although dystrophin-negative mdx mouse models have classically been used to study DMD, phenotypes appear mild compared to patients. As a result, characterization of muscle pathology, especially in the heart, has proven difficult. We report that injection of mdx embryonic stem cells (ESCs) into Wild Type blastocysts produces adult mouse chimeras with severe DMD phenotypes in the heart and skeletal muscle. Inflammation, regeneration and fibrosis are observed at the whole organ level, both in dystrophin-negative and dystrophin-positive portions of the chimeric tissues. Skeletal and cardiac muscle function are also decreased to mdx levels. In contrast to mdx heterozygous carriers, which show no significant phenotypes, these effects are even observed in chimeras with low levels of mdx ESC incorporation (10%-30%). Chimeric mice lack typical compensatory utrophin upregulation, and show pathological remodeling of Connexin-43. In addition, dystrophin-negative and dystrophin-positive isolated cardiomyocytes show augmented calcium response to mechanical stress, similar to mdx cells. These global effects highlight a novel role of mdx ESCs in triggering muscular dystrophy even when only low amounts are present. Stem Cells 2017;35:597-610.

Determining the role of skewed X-chromosome inactivation in developing muscle symptoms in carriers of Duchenne muscular dystrophy

Duchenne and Becker dystrophinopathies (DMD and BMD) are X-linked recessive disorders caused by mutations in the dystrophin gene that lead to absent or reduced expression of dystrophin in both skeletal and heart muscles. DMD/BMD female carriers are usually asymptomatic, although about 8 % may exhibit muscle or cardiac symptoms. Several mechanisms leading to a reduced dystrophin have been hypothesized to explain the clinical manifestations and, in particular, the role of the skewed XCI is questioned. In this review, the mechanism of XCI and its involvement in the phenotype of BMD/DMD carriers with both a normal karyotype or with X;autosome translocations with breakpoints at Xp21 (locus of the DMD gene) will be analyzed. We have previously observed that DMD carriers with moderate/severe muscle involvement, exhibit a moderate or extremely skewed XCI, in particular if presenting with an early onset of symptoms, while DMD carriers with mild muscle involvement present a random XCI. Moreover, we found that among 87.1 % of the carriers with X;autosome translocations involving the locus Xp21 who developed signs and symptoms of dystrophinopathy such as proximal muscle weakness, difficulty to run, jump and climb stairs, 95.2 % had a skewed XCI pattern in lymphocytes. These data support the hypothesis that skewed XCI is involved in the onset of phenotype in DMD carriers, the X chromosome carrying the normal DMD gene being preferentially inactivated and leading to a moderate-severe muscle involvement.

Blastocyst injection of wild type embryonic stem cells induces global corrections in mdx mice

Duchenne muscular dystrophy (DMD) is an incurable neuromuscular degenerative disease, caused by a mutation in the dystrophin gene. Mdx mice recapitulate DMD features. Here we show that injection of wild-type (WT) embryonic stem cells (ESCs) into mdx blastocysts produces mice with improved pathology and function. A small fraction of WT ESCs incorporates into the mdx mouse nonuniformly to upregulate protein levels of dystrophin in the skeletal muscle. The chimeric muscle shows reduced regeneration and restores dystrobrevin, a dystrophin-related protein, in areas with high and with low dystrophin content. WT ESC injection increases the amount of fat in the chimeras to reach WT levels. ESC injection without dystrophin does not prevent the appearance of phenotypes in the skeletal muscle or in the fat. Thus, dystrophin supplied by the ESCs reverses disease in mdx mice globally in a dose-dependent manner.

Variable dystrophin expression in different muscles of a Duchenne muscular dystrophy carrier

The majority of Duchenne muscular dystrophy (DMD) female carriers show dystrophin immunostaining abnormalities, although a significant proportion of clinically non-manifesting carriers are normal following this analysis. We had the opportunity to study dystrophin immunostaining in two different muscles, the vastus lateralis and the rectus abdominis of a possible DMD carrier. While the vastus showed normal dystrophin immunostaining, pathological staining was detected in her rectus abdominis. These findings seem to indicate that dystrophin expression can vary in different muscle groups of a DMD carrier. The implications of these findings in DMD carrier detection and possible dystrophin function are discussed.

Full-length dystrophin expression in half of the heart cells ameliorates beta-isoproterenol-induced cardiomyopathy in mdx mice

Gene therapy holds great promise for curing Duchenne muscular dystrophy (DMD), the most common fatal inherited childhood muscle disease. Success of DMD gene therapy depends upon functional improvement in both skeletal and cardiac muscle. Numerous gene transfer studies have been performed to correct skeletal muscle pathology, yet little is known about cardiomyopathy gene therapy. Since complete transduction of the entire heart is an impractical goal, it becomes critical to determine the minimal level of correction needed for successful DMD cardiomyopathy gene therapy. To address this question, we generated heterozygous mice that persistently expressed the full-length dystrophin gene in 50% of the cardiomyocytes of mdx mice, a model for DMD. We questioned whether dystrophin expression in half of the heart cells was sufficient to prevent stress-induced cardiomyopathy. Heart function of mdx mouse is normal in the absence of external stress. To determine the therapeutic effect, we challenged 3-month-old mice with beta-isoproterenol. Cardiomyocyte sarcolemma integrity was significantly impaired in mdx but not in heterozygous and C57Bl/10 mice. Importantly, in vivo closed-chest hemodynamic assays revealed normal left ventricular function in beta-isoproterenol-stimulated heterozygous mice. Since the expression profile in the heterozygous mice mimicked viral transduction, we conclude that gene therapy correction in 50% of the heart cells may be sufficient to treat cardiomyopathy in mdx mice. This finding may also apply to the gene therapy of other inherited cardiomyopathies.

Strategies for muscle-specific targeting of adenoviral gene transfer vectors

Currently, adenoviral transfer of therapeutic genes such as dystrophin is hampered by low transduction efficiency of adult skeletal muscle. This is largely due to the lack of appropriate virus attachment receptors on the myofiber surface. Recent studies in transgenic mice revealed that upregulation of Coxsackie- and adenovirus receptor improves gene transfer efficiency by approximately ten-fold. Conversely, the vector load that needed to be administered to achieve sufficient gene transfer could be lowered significantly. Reduced viral vector loads may help to control virally mediated toxicity and immunogenicity. To date, there are no drugs or methods known to increase Coxsackie- and adenovirus receptor expression in skeletal muscle that would be easily applicable in humans. However, alternative strategies such as vector retargeting are currently being investigated that may allow for an increase in binding of adenoviral vectors to skeletal muscle. Recent experiments have shown that directed mutagenesis of the adenoviral fiber knob allows for a significant reduction in Coxsackie- and adenovirus receptor binding and for introduction of a new binding domain. Therefore, vector retargeting towards efficient and specific infection of skeletal muscle may be achieved by directed genetic alteration of adenoviral capsid proteins.

Dynamic restoration of dystrophin to dystrophin-deficient myotubes

Dystrophin domains are observed in myoblast transplantation experiments and in muscle fibers after somatic reversion in human Duchenne and mdx mouse muscular dystrophy. However, the formation and evolution of dystrophin-positive domains are not well established. Using a muscle satellite cell coculture system, we examined the dynamic restoration of dystrophin expression in dystrophin-deficient myotubes. The dystrophin-positive domains around source nuclei were clearly identified in hybrid myotubes. The occurrence of dystrophin domains was higher in myotubes differentiated from cocultures with a low concentration of normal wild-type satellite cells in relation to dystrophin-deficient satellite cells. At higher seeding ratios, the domain feature of dystrophin expression was more transitory and decreased as myotubes differentiated over time in culture. The average domain size initially increased with the addition of new nuclei by fusion early after differentiation of cocultures. However, separating dystrophin-positive domains according to their number of dystrophin-expressing contributory nuclei showed that diffusion of dystrophin contributed to domain elongation, even in early myotubes and later without fusion of additional nuclei. Diffusion occurred for all domains of one to six wild-type nuclei, and the diffusion rate was higher in domains with larger numbers of nuclei. This dynamic domain feature of dystrophin expression was also related to restoring the organization of dystrophin-associated proteins and acetylcholine receptors to hybrid myotubes. Factors regulating domain formation and diffusion therefore are important considerations in the design of strategies for both myoblast transplantation and gene therapy of Duchenne muscular dystrophy.

Myelin mosaicism and brain plasticity in heterozygous females of a canine X-linked trait

The shaking (sh) pup, an animal model of Pelizaeus-Merzbacher disease, is characterized by severe central nervous system dysmyelination in affected males, and myelin mosaicism in some female heterozygotes as a result of X-linked inactivation. Heterozygous females develop a tremor of varying severity that usually disappears at 4 to 6 weeks, whereas male hemizygotes have severe, generalized tremor that persists throughout life. We have used these two myelin-deficient models to study the potential for recovery with time as reflected by brainstem auditory evoked responses (BAERs). At set time points, the state of myelination in the trapezoid body was studied microscopically. Sequential BAERs demonstrated consistently prolonged interpeak latencies during the period of gross tremor in heterozygotes, with the trend continuing to a lesser extent after tremor cessation. The random nature of X-linked inactivation resulted in variable myelin mosaicism that was reflected in variations in BAER changes within animals in the same litter. In most heterozygotes, the tremor resolved with time, the BAERs returned to near normal, and myelin mosaicism was lost. In contrast, in the affected males, the severity of tremor and lack of recovery was demonstrated by consistent abnormalities in BAER waves at all times studied, and severe and persistent myelin deficiency in the trapezoid body. These findings show that despite the normal tightly programmed temporal development of myelin in the brain in the heterozygous mosaic state, sufficient plasticity persists during the neonatal period for late-stage myelination to occur.

Efficient utrophin expression following adenovirus gene transfer in dystrophic muscle

Utrophin is a homologue of dystrophin, the protein whose absence is responsible for Duchenne muscular dystrophy (DMD). As a first step toward clarifying if adenovirus (AV)-mediated utrophin transfer is a possible option to treat DMD, we have constructed an AV expressing utrophin (AdCMV-Utr) and studied utrophin expression after intramuscular injection of mdx mice, the mouse DMD model. Overexpression of utrophin by AdCMV-Utr was marked and nontoxic. The recombinant utrophin was distributed homogeneously at the surface of the muscle fibers. Its expression was sufficient to restore the normal histochemical pattern of alpha-sarcoglycan and beta-dystroglycan at this site. These two proteins are members of the dystrophin associated protein complex whose distribution is greatly reduced at the surface of the DMD muscle. These data indicate that AV-mediated utrophin transfer is an efficient way of utrophin upregulation in muscle and has the potential of becoming a treatment for DMD.

Transplantation of myoblasts from a transgenic mouse overexpressing dystrophin prduced only a relatively small increase of dystrophin-positive membrane

Myoblast cultures from normal and Tg-MDA (transgenic mouse overexpressing dystrophin 50-fold) mice were transplanted into dystrophin-deficient mdx mouse muscles. Four weeks after transplantation, dystrophin-positive fibers were observed four times more frequently in cross sections of muscles injected with Tg-MDA. Myoblasts from Tg-MDA mice also expressing the beta-gal transgene (Tg-MDA/beta-gal) and myoblasts from beta-gal transgenic mice containing one normal dystrophin gene (normal/beta-gal) were also transplanted into mdx mouse muscles. Four weeks after transplantation, the fiber length positive for dystrophin (nuclear domain) was shorter (439 +/- 326 microm) than the beta-gal nuclear domain (1466 +/- 713 microm) of the same fiber when normal/beta-gal myoblasts were transplanted, but increased (1302 +/- 487 microm) when Tg-MDA/beta-gal myoblasts were used. These experiments show that despite the presence in Tg-MDA myoblasts of constructions which lead in vivo in transgenic mice to an overexpression of dystrophin 50-fold, the membrane area over which dystrophin was expressed was increased only threefold. This observation is also expected for vector-mediated gene therapy.

Genetic counseling of isolated carriers of Duchenne muscular dystrophy

It has recently become possible to detect female carriers of Duchenne muscular dystrophy with no affected male relative in the family. These "isolated carriers" represent about 10% of women with high serum creatine phosphokinase (CPK) levels and clinical evidence of a muscle disease. Most isolated carriers ascertained by clinical and/or CPK levels and diagnosed by dystrophin immunostaining of muscle biopsy show symptoms of a muscular dystrophy, and often carry the diagnosis of recessive "limb-girdle muscular dystrophy" prior to dystrophin analysis. It has been difficult to offer genetic counseling and prenatal diagnosis for Duchenne muscular dystrophy in the families of these isolated carriers, largely due to the difficulty in determining which of the dystrophin alleles segregating in the family harbors the mutation in the heterozygote. Here we report genetic counseling of three isolated carriers and their families. In two cases, prenatal diagnosis of at-risk pregnancies was conducted. We determined X inactivation patterns and inheritance of X chromosomes in each family, and used this information to define the at-risk dystrophin gene. In all three families, the mutation was a de novo event, two in the paternal germ-line, and one in the maternal germ-line. In each case we show that sibs of the heterozygous woman are at population risk, while pregnancies of each propositus are at high risk. Our results show that accurate genetic counseling and prenatal diagnosis can be offered to these families.

Yields of muscle from myogenic cells implanted into young and old mdx hosts

Implantation of normal muscle precursor cells (mpc) for treatment for inherited myopathies such as Duchenne muscular dystrophy is in clear need of improvement to become practicable, but few variables have been studied comparatively. Here, we report the first quantitative estimate of the effectiveness of implanting mpc into preirradiated muscles of young and old mice and into preirradiated and nonirradiated old muscles. Estimates were made of the amount of muscle formed by injection of 5 x 10(5) cells dissociated from neonatal normal mouse muscle into tibialis anterior muscles of the dystrophin-deficient mdx mouse. We show that normal mpc are incorporated slightly more efficiently into muscles of young than old host mice, to form some 10 mg of dystrophin-positive fibers. In older muscles, prior irradiation has little effect on the total yield of new muscle.

Lack of myoblasts migration between transplanted and host muscles of mdx and normal mice

Extensor digitorum longus muscles of normal mice (C57BL/10ScSn hereafter called C57) were orthotopically transplanted into dystrophin-deficient mice (mdx) and reciprocally, mdx Extensor digitorum longus muscles were transplanted into C57 mice. After an initial phase of degeneration, transplanted muscles regenerate nearly completely, as evaluated from the maximum isometric force of muscles isolated 60 days after the surgery. In other similar experiments, instead of isolating the grafted muscles, we excised the antero-external muscles of the leg, including the grafted muscle. Cryostat cross-sections at three levels along the muscles were immunostained with an anti-dystrophin antibody. No muscle cells of dystrophin-deficient muscles grafted into normal mice took the antibody except a few 'revertant' fibres, while all the muscle cells of the normal host were immunostained. Reciprocally, all the muscles cells of normal grafts were stained, whilst no antibody stained the cells of the surrounding muscles of the dystrophin-deficient host. These experiments show that very few if any of the myoblasts or muscle precursor cells, active during the regeneration of grafted muscle, migrate into the adjacent muscles. These results could be explained by the absence, in our work, of injuries of the grafted and adjacent host muscles epimysium and the absence of extensive inflammatory reactions. This lack of myoblast mobility suggest that when myoblast transfer is applied to muscle therapy, it will be necessary to inject myoblasts within each muscle to obtain an efficient treatment.

Is dystrophin always altered in Becker muscular dystrophy patients?

The differential diagnosis between autosomal recessive limb-girdle (LGMD) and X-linked Becker muscular dystrophy (BMD) is very important for genetic counseling. It has been hypothesized that all BMD patients would have dystrophin alterations and dystrophin analysis could identify the Xp21 MD. Qualitatively abnormal dystrophin is easily detectable, but it is generally associated with in-frame DNA deletions or duplications. In patients with no detectable DNA deletions, in which X-linked inheritance cannot be proved, dystrophin quantification is still the only available test for differential diagnosis. In order to assess the accuracy of dystrophin quantification test in delineating Becker patients, we analyzed dystrophin abundance in BMD patients with a positive history of X-linked inheritance and no DNA detectable mutation, as compared to patients from families with LGMD. We observed that patients from 2 among the 5 BMD families have nearly normal dystrophin, while alteration in dystrophin content was observed in patients from 2 among the 7 LGMD families studied (probably as a secondary effect of alteration in the whole dystrophin-glycoproteins complex). These results suggest that dystrophin quantification, as an isolated test is not helpful for differential diagnosis between BMD and LGMD.

Characterization and localization of a 77 kDa protein related to the dystrophin gene family

The Duchenne muscular dystrophy gene gives rise to transcripts of several lengths. These mRNAs differ in their coding content and tissue distribution. The 14 kb mRNA encodes dystrophin, a 427 kDa protein found in muscle and brain, and the short transcripts described encode DP71, a 77 kDa protein found in various organs. These short transcripts have many features common to the deduced primary structure of dystrophin, especially in the cysteine-rich specific C-terminal domains. The dystrophin C-terminal domain could be involved in membrane anchorage via the glycoprotein complex, but such a functional role for these short transcript products has yet to be demonstrated. Here we report the first isolation of a short transcript product from saponin-solubilized cardiac muscle membranes using alkaline buffer and affinity chromatography procedures. This molecule was found to be glycosylated and could be easily dissociated from cardiac muscle and other non-muscle tissues such as brain and liver. DP71-specific monoclonal antibody helped to identify this molecule as being related to the dystrophin gene family. Immunofluorescence analysis of bovine or chicken cardiac muscle showed a periodic distribution of DP71 in transverse T tubules and this protein was co-localized with the dystrophin glycoprotein complex in the Z-disk area.

Cell and gene therapy in Duchenne muscular dystrophy

Experiments in mice have supported the idea of treating Duchenne muscular dystrophy (DMD) by implanting normal muscle precursor cells into dystrophin-deficient muscles. However, similar experiments on DMD patients have had little success. Gene therapy for DMD, by introducing dystrophin constructs via retroviral or adenoviral vectors, has been shown to be possible in the mouse, but the efficiency and safety aspects of this technique will have to be carefully examined before similar experiments can be attempted in man. Direct injection of dystrophin cDNA constructs into mdx muscles has given rise to very low levels of dystrophin and this may be a possibility for the treatment of heart muscle.

Reciprocal expression of dystrophin and utrophin in muscles of Duchenne muscular dystrophy patients, female DMD-carriers and control subjects

We examined muscle biopsies from patients with Duchenne muscular dystrophy (DMD: 39 patients) and Becker muscular dystrophy (BMD: 11 patients), female DMD-carriers (4 patients), and control subjects (26 persons) for the expression of dystrophin and utrophin. Control subjects showed all fibers to be dystrophin-positive, while utrophin staining was negative or weak. On the other hand, muscles from DMD patients showed the inverse staining patterns: dystrophin was negative and utrophin staining strong. Thus, there was a reciprocal pattern of expression between dystrophin and utrophin. This reciprocal relationship was confirmed to some extent at the single-fiber level in female carriers of DMD showing a mosaic immunostaining of dystrophin. We consider that utrophin may have a function similar to that of dystrophin, and compensate to some extent for dystrophin deficiency in DMD.

Overexpression of dystrophin in transgenic mdx mice eliminates dystrophic symptoms without toxicity

Duchenne and Becker muscular dystrophy (DMD and BMD) are X-linked recessive diseases caused by defective expression of dystrophin. The mdx mouse, an animal model for DMD, has a mutation that eliminates expression of the 427K muscle and brain isoforms of dystrophin. Although these animals do not display overt muscle weakness or impaired movement, the diaphragm muscle of the mdx mouse is severely affected and shows progressive myofibre degeneration and fibrosis which closely resembles the human disease. Here we explore the feasibility of gene therapy for DMD by examining the potential of a full-length dystrophin transgene to correct dystrophic symptoms in mdx mice. We find that expression of dystrophin in muscles of transgenic mdx mice eliminates the morphological and immunohistological symptoms of muscular dystrophy. In addition, overexpression of dystrophin prevents the development of the abnormal mechanical properties associated with dystrophic muscle without causing deleterious side effects. Our results provide functional evidence for the feasibility of gene therapy for DMD.

Mdx transgenic mouse: restoration of recombinant dystrophin to the dystrophic muscle

We report the restoration of the 430-kD dystrophin in mdx, the mouse model of Duchenne muscular dystrophy, by expression of a single-copy recombinant dystrophin transgene. Muscle-specific expression was achieved using a creatine kinase promoter influenced by two enhancers. Immunostaining with anti-Xp21-coded dystrophin monoclonal antibodies showed that the recombinant dystrophin was localized to the muscle fiber membrane. However, there was variability in the level of dystrophin expression in various animals with aging, between fast and slow muscles, and within different regions of the same muscle. Curiously, recombinant dystrophin was relatively absent in the diaphragm muscle of these mdx transgenic animals. Our studies indicate that there is a direct correlation between the level of muscle fibers expressing recombinant dystrophin and the level of muscle fibers with peripheral nuclei, indicating an improvement in muscle pathology. These studies indicate that the regional expression of recombinant dystrophin in dystrophic muscle leads to regional restoration of normal muscle morphology.

Sarcolemmal distribution of abnormal dystrophin in Xp21 carriers

Some Becker muscular dystrophy carriers, related to patients with specific DNA deletions, demonstrate both normal and abnormally sized dystrophin bands through qualitative Western blot analysis. The purpose of the present investigation was to assess the sarcolemmal distribution of the altered dystrophin in such carriers. Fibres expressing the normal or deleted dystrophin were identified using specific antibodies which reacted with epitopes from within the deleted region. No negative fibres or patchy immunostaining could be seen when sections from four carriers were labelled with either antibodies (C-terminal and corresponding to the deleted region), although a significant amount of abnormal dystrophin was present in their muscle (as seen on blots). Thus, we were able to confirm that in a proportion of the myonuclei, the defective allele was present on the active X chromosome. Our results suggest that the two types of nuclei were randomly distributed, resulting in normal and abnormal dystrophin molecules which were so intimately mixed that dystrophin-incompetent fibres could not be distinguished in the skeletal muscle from the Xp21 carriers.

Diagnosis of DMD carrier status in a family with no known affected males

A 30-year-old woman and her two-year-old daughter were found by chance to have moderately raised serum creatine kinase (CK) levels. Since the mother was pregnant, the authors investigated the possibility that the two females were carriers of the common Duchenne muscular dystrophy (DMD) gene. No immunohistochemical abnormality was detected in the mother, but in the daughter a clear mosaic pattern of dystrophin positive and negative fibres was found, indicating carrier status for DMD. These data indicate that a diagnosis of DMD carrier status can be made even in families without a positive history for this disorder; therefore, immunocytochemical studies, using antidystrophin antibodies, should be performed on all females with raised CK levels, including the youngest.

Variability in clinical, genetic and protein abnormalities in manifesting carriers of Duchenne and Becker muscular dystrophy

We have analysed the results of clinical assessment, X-inactivation status, deletion screening and dystrophin analysis in eight manifesting carriers of Duchenne and Becker muscular dystrophy (DMD and BMD). Only two had a prior family history of X-linked muscle disease, all had normal karyotypes and none were twins. Presentation varied from 2 to 25 yr and progression varied from a DMD-like severity to a very mild BMD-like course. In one girl the initial symptoms were restricted to learning difficulties. Where methods for assessing X-inactivation were informative, three patients showed an abnormal pattern. However, in one patient, the obligate carrier daughter of a BMD patient who had presented at the age of 2 yr, X-inactivation appeared normal in lymphocytes and muscle. While dystrophin analysis seems to be reliable in identifying manifesting carriers of DMD and BMD, the relationship between X-inactivation status, dystrophin analysis and phenotype is not simple.

Challenges: Cell transplantation and gene therapy in muscular dystrophy

Duchenne's muscular dystrophy (DMD), which affects 1/3500 live male births, involves a progressive degeneration of skeletal and cardiac muscle, leading to early death. The protein dystrophin is lacking in DMD and present, but defective, in the allelic, less severe, Becker muscular dystrophy and is also missing in the mdx mouse. Experiments on the mdx mouse have suggested two possible therapies for these myopathies. Implantation of normal muscle precursor cells (mpc) into mdx skeletal muscle leads to the conversion of dystrophin-negative fibres to -positive, with consequent improvement in muscle histology. Direct injection of dystrophin cDNA into skeletal or cardiac muscle also gives rise to dystrophin-positive fibres. Although both appear promising, there are a number of questions to be answered and refinements to be made before either technique could be considered possible as treatments for myopathies in man.

X inactivation and dystrophin studies in a t(X;12) female: evidence for biochemical normalization in Duchenne muscular dystrophy carriers

A 4-year-old girl was identified with high creatine kinase (CK) values, and mild muscle weakness in a limb-girdle distribution. Results of dystrophin analysis of the muscle biopsy were consistent with a manifesting heterozygote for Duchenne muscular dystrophy. In peripheral lymphocytes she had a t(X;12) (p21.2;q24.33). Late DNA replication studies demonstrated inactivation of the normal X chromosome in 99.4% of cells. Dystrophin immunofluorescence showed 64% dystrophin-negative muscle fibers. Dystrophin content of muscle by immunoblot was approximately 5% of normal. The discordance between the percent of normal X inactivation and percent of dystrophin-negative cells may be explained by compensatory protection of dystrophin by rare nuclei with the normal X active in multi-nucleated muscle fibers with shared cytoplasm.

Normal dystrophin transcripts detected in Duchenne muscular dystrophy patients after myoblast transplantation

Gene delivery by transplantation of normal myoblasts has been proposed as a treatment of the primary defect, lack of the muscle protein dystrophin, that causes Duchenne muscular dystrophy (DMD), a lethal human muscle degenerative disorder. To test this possibility, we transplanted normal myoblasts from a father or an unaffected sibling into the muscle of eight boys with DMD, and assessed their production of dystrophin. Three patients with deletions in the dystrophin gene expressed normal dystrophin transcripts in muscle biopsy specimens taken from the transplant site one month after myoblast injection. Using the polymerase chain reaction we established that the dystrophin in these biopsies derived from donor myoblast DNA. These results show that transplanted myoblasts persist and produce dystrophin in muscle fibres of DMD patients.

A light and electron microscopic study of dystrophin localization at the mouse neuromuscular junction

Duchenne muscular dystrophy (DMD) is characterized by a lack of dystrophin expression. Dystrophin is a 420 Kd protein localized in the muscle sarcolemma that most likely provides stability to the muscle plasma membrane. Neuromuscular junctions (NMJs) were localized by revealing either the acetylcholine receptors (AChRs) with alpha-bungarotoxin coupled with cascade blue or by revealing desmin, a protein found in higher concentration at the NMJs using immunochemistry. An accumulation of dystrophin was observed in normal mice by immunoperoxidase labelling at NMJs identified with these markers. Dystrophin was pinpointed on the postjunctional folds of NMJs by electron microscopy and was more abundant on the postjunctional membrane than on the remaining muscle membrane. Our observations are similar to previous observations suggesting that dystrophin may interact with the AChRs.

Mosaic expression of dystrophin in the cerebellum of heterozygote dystrophic (mdx) mice

The monoclonal NCLDys1 revealed the presence of dystrophin in the Purkinje cells of normal mice but not of mdx mice and a mosaic staining in Purkinje cells of heterozygote mdx mice. Dystrophin was present in the soma and the dendrites of the dystrophin positive Purkinje cells and was absent in both regions of the dystrophin negative Purkinje cells. However, the polyclonal antibody d10 produced a uniform labeling of all Purkinje cells not only in the normal mice but also in mdx and heterozygote mdx mice. This staining was attributed to a reaction of this antibody not only with dystrophin but also with a different isoform of dystrophin or with a dystrophin related protein present even in mdx mice.

Dystrophin abnormality in progressive muscular dystrophy--a review article

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive disorder of muscle in children, with an incidence of approximately 1 in 3,300 male births. In about a third of affected boys, the disease is due to a new mutation, and most patients die in their early 20s. Over the last few years, the genetic, biochemical and histopathological basis of DMD has been elucidated greatly. In particular, the discovery of "dystrophin," the protein product of the DMD gene is truly an epoch-making success in the history of muscular dystrophy research. Dystrophin is now thought to be a cytoskeletal protein underlying the plasma membrane (known in muscle as the sarcolemma) of normal muscle fiber, and is undetectable or greatly reduced in DMD. In this review article, dystrophin in normal skeletal muscle and various neuromuscular diseases including DMD/BMD (Becker muscular dystrophy), and its carrier is discussed.

Characterisation of dystrophin in carriers of Duchenne muscular dystrophy

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, was studied in needle biopsy samples taken from the quadriceps muscle of 15 asymptomatic carriers of DMD (13 adults and 2 young girls) and one symptomatic adult carrier. Antibodies to N- and C-terminal regions of dystrophin were used for both Western blot analysis and immunocytochemistry and a monoclonal antibody to beta-spectrin used to assess membrane integrity. All asymptomatic adult carriers showed some abnormality in dystrophin immunostaining but very few negative fibres were present. A clear mosaic of dystrophin positive and negative fibres was seen only in the adult symptomatic carrier and the two young girls. On a Western blot, all carriers studied had dystrophin of normal molecular weight, but most had reduced abundance. In adult carriers, the amount of dystrophin relative to normal controls varied, but it was unrelated to age, serum creatine kinase (CK) levels or to the degree of pathology. Carriers with normal CK showed abnormalities in dystrophin expression. The dystrophin immunoblotting profile of the 2 young girls was very similar to that of their mothers, but the mosaic pattern of immunostaining was not apparent in the older carriers. In conclusion, dystrophin immunostaining and Western blot analysis of biopsy samples from asymptomatic carriers is often abnormal and they may be useful additional aids for establishing carrier status, particularly in younger girls.

Invited review: myoblast transfer: a possible therapy for inherited myopathies?

A potential therapeutic strategy for genetic diseases is to alter the genetic constitution of the affected tissues by means of grafts of normal precursor or stem cells. Over several years, evidence has accumulated to suggest that primary diseases of skeletal muscle, such as Duchenne muscular dystrophy, may be susceptible to this approach. This review makes a critical examination of such background evidence, and also of more recent data directly addressing the concept of therapy by means of grafts of normal myogenic cells. It is concluded that the data establish the principle that such grafts effect an alteration of the genetic constitution and phenotype of skeletal muscle and, therefore, might be used to alleviate recessively inherited myopathies. Several obstacles to the therapeutic application of this method to human disease are also identified; these seem to be problems of a technical nature rather than of basic principle, and none appears insuperable.

Is dystrophin present in the nerve terminal at the neuromuscular junction? An immunohistochemical study of the heterozygote dystrophic (mdx) mouse

Neuromuscular junctions (NMJs) were identified by revealing the presence of cholinergic receptors (AChR) with alpha-bungarotoxin coupled to the fluorescent dye cascade blue in 9- and 60-day-old normal and heterozygote mdx mice. Dystrophin was detected by an immunoperoxidase technique. All the muscle fibers of the normal animals observed in cross sections were immunoreactive for dystrophin and an accumulation of dystrophin was observed at all NMJs identified by alpha-bungarotoxin. In the 9-day-old mdx heterozygote animals, dystrophin positive, negative, and partially positive muscle cross sections were observed. Four different observations were made in these heterozygote animals on the coexistence of AChR and dystrophin. First, alpha-bungarotoxin sites (i.e., NMJs) were observed on dystrophin positive muscle fiber cross sections with an accumulation of dystrophin at these sites. Second, alpha-bungarotoxin sites were observed on dystrophin positive fibers without a dystrophin accumulation at NMJs. Third, there was a coexistence of alpha-bungarotoxin and dystrophin labelling at NMJs of muscle fibers with perimeters labelling negative for dystrophin. Fourth, NMJs, identified by alpha-bungarotoxin, were observed on muscle fibers negative for dystrophin even at the NMJ. These observations suggest that dystrophin is present not only in the muscle membrane but also in the presynaptic nerve terminals.

Decreased osmotic stability of dystrophin-less muscle cells from the mdx mouse

Human X-linked Duchenne and Becker muscular dystrophies are due to defects in dystrophin, the product of an exceptionally large gene. Although dystrophin has been characterized as a spectrin-like submembranous cytoskeletal protein, there is no experimental evidence for its function in the structural maintenance of muscle. Current hypotheses attribute necrosis of dystrophin-less fibres in situ to mechanical weakening of the outer membrane, to an excessive influx of Ca2+ ions, or to a combination of these two mechanism, possibly mediated by stretch-sensitive ion channels. Using hypo-osmotic shock to determine stress resistance and a mouse model (mdx) for the human disease, we show that functional dystrophin contributes to the stability of both cultured myotubes and isolated mature muscle fibres.

Dystrophin immunofluorescence pattern in manifesting and asymptomatic carriers of Duchenne's and Becker muscular dystrophies of different ages

In order to investigate if the same apparent decrease in dystrophin negative fibers with aging observed in mouse mdx female heterozygotes also occurs in carriers of the DMD and BMD gene, we have studied the muscle of 29 DMD carriers (19 adults and 10 young daughters of obligate carriers, including 3 manifesting carriers) and 5 adult asymptomatic heterozygotes for Becker dystrophy (BMD). All young DMD possible carriers and 11 of 24 adult DMB/BMD heterozygotes had increased serum enzymes activities. A population of dystrophin negative fibers, more evident with the use of the C-terminal antibody, was seen in the three manifesting and in a 9-yr-old possible DMD carrier. In the remaining females, a positive immunohistochemical pattern of dystrophin, which did not differ from normal controls, was observed. Our results suggest that: (1) the increased population of dystrophin negative fibers reported in young mdx female heterozygotes was not seen in young DMD carriers, aged 6-17 yr; and (2) abnormalities in dystrophin immunostaining are not easily observed and are more frequent in manifesting carriers, when the muscle is grossly altered.

Dystrophin-deficient mdx muscle fibers are preferentially vulnerable to necrosis induced by experimental lengthening contractions

Lengthening contractions were induced in the right anterior tibialis muscles (ATM) of anaesthetized normal and mdx (dystrophic) mice by supramaximal, nonfatiguing stimulation of the sciatic nerve for 180 min. In the left ATM of the same animals identical stimulation caused shortening contractions because of a prestimulation Achilles tenotomy. The prevalence of recently necrotic fibers was determined in all stimulated ATM by demonstrating the presence of IgG in the necrotic fibers using immunoperoxidase staining of cryostat sections. The results were compared to unstimulated normal and mdx ATM. A significantly higher rate of necrosis was demonstrated after lengthening contractions in the mdx ATM than normal ATM. Unstimulated normal and mdx ATM have either no or extremely infrequent necrotic fibers. We suggest that the enhanced vulnerability of mdx muscle fibers to lengthening contractions is related to the deficiency of dystrophin, which renders the sarcolemma more susceptible to suffer focal breaks. A similar situation may occur in Duchenne muscular dystrophy.

Normal myogenic cells from newborn mice restore normal histology to degenerating muscles of the mdx mouse

Dystrophin deficiency in skeletal muscle of the x-linked dystrophic (mdx) mouse can be partially remedied by implantation of normal muscle precursor cells (mpc) (Partridge, T. A., J. E. Morgan, G. R. Coulton, E. P. Hoffman, and L. M. Kunkel. 1989. Nature (Lond.). 337:176-179). However, it is difficult to determine whether this biochemical "rescue" results in any improvement in the structure or function of the treated muscle, because the vigorous regeneration of mdx muscle more than compensates for the degeneration (Coulton, G. R., N. A. Curtin, J. E. Morgan, and T. A. Partridge. 1988. Neuropathol. Appl. Neurobiol. 14:299-314). By using x-ray irradiation to prevent mpc proliferation, it is possible to study loss of mdx muscle fibers without the complicating effect of simultaneous fiber regeneration. Thus, improvements in fiber survival resulting from any potential therapy can be detected easily (Wakeford, S., D. J. Watt, and T. A. Patridge. 1990. Muscle & Nerve.) Here, we have implanted normal mpc, obtained from newborn mice, into such preirradiated mdx muscles, finding that it is far more extensively permeated and replaced by implanted mpc than is nonirradiated mdx muscle; this is evident both from analysis of glucose-6-phosphate isomerase isoenzyme markers and from immunoblots and immunostaining of dystrophin in the treated muscles. Incorporation of normal mpc markedly reduces the loss of muscle fibers and the deterioration of muscle structure which otherwise occurs in irradiated mdx muscles. Surprisingly, the regenerated fibers are largely peripherally nucleated, whereas regenerated mouse skeletal muscle fibers are normally centrally nucleated. We attribute this regeneration of apparently normal muscle to the tendency of newborn mouse mpc to recapitulate their neonatal ontogeny, even when grafted into 3-wk-old degenerating muscle.

Somatic reversion/suppression of the mouse mdx phenotype in vivo

The mdx mouse has a myopathy caused by dystrophin deficiency, and is therefore biochemically and genetically homologous to human Duchenne muscular dystrophy. While mdx mouse muscle shows no dystrophin by immunoblotting, a very small percentage of myofibers appear clearly dystrophin-positive by immunofluorescence microscopy. We have characterized these rare positive-staining fibers, and conclude that they are indeed expressing dystrophin despite a nonsense mutation within the dystrophin gene. Thus, the dystrophin-positive fibers probably represent somatic reversion or suppression of the mdx mutation. Cardiac muscle and skeletal muscle from mdx mice showed dramatically different patterns of dystrophin-positive cells. However, this difference is expected given the apparent clonal nature of the reversion/suppression events, the inability of cardiac muscle to regenerate, and other differences in the developmental programs of myofibers and cardiocytes. The prevalence of dystrophin-positive cells in mdx cardiac muscle was determined to be approximately 2 x 10(-5). The observed prevalence of dystrophin-positive cardiocytes in the mdx mouse is a possible estimate of the somatic reversion rate of the mdx mutation in vivo.