Dystrophin: a clinical perspective

Postanesthetic death in a cat with myopathy

There are few reports of naturally occurring muscular dystrophy in domestic animals. Herein, we describe a case of muscular dystrophy in a 4-year-old neutered male American domestic shorthair cat that died unexpectedly following anesthesia for an elective surgical procedure. Macroscopic muscular hypertrophy and histologic evidence of myofiber size variation, mineralization, myofiber degeneration, and necrosis were compatible with a diagnosis of muscular dystrophy. Extensive endomysial fibrosis was noted histologically in the diaphragm. A complete absence of dystrophin protein in Western blot confirmed the diagnosis of Duchenne muscular dystrophy. Immunofluorescence microscopy revealed reduced levels of dystrophin-associated proteins and an upregulation of utrophin at the sarcolemma. Anesthetic deaths can occur in dystrophin-deficient cats, and therefore muscular dystrophy and the associated cardiomyopathy should be considered in the differential diagnoses for perianesthetic death in cats.

Mini-dystrophin efficiently incorporates into the dystrophin protein complex in living cells

Dystrophin is a critical muscle cell structural protein which when deficient results in Duchenne muscular dystrophy. Recently miniature versions of the dystrophin gene have been constructed that ameliorate the pathology in mouse models. To characterize mini-dystrophin's incorporation into the dystrophin protein complex in living cells, two fusion proteins were constructed where mini-dystrophin is fused to the N- or C-terminus of an enhanced green fluorescent protein reporter gene. Both fusion proteins correctly localize at the plasma membrane in vitro and in vivo. Live cell microscopy establishes that mini-dystrophin translocates directly to the PM of differentiating muscle cells, within 4 h of expression. Latrunculin A treatment, actin and beta-dystroglycan binding domain deletion constructs, and co-immunoprecipitation assays demonstrate that mini-dystrophin is firmly anchored to the sarcolemma primarily through its connections to beta-dystroglycan, mimicking effects seen with wild type dystrophin. Furthermore, point mutations made within the putative beta-dystroglycan anchoring ZZ domain of mini-dystrophin result in an ablation of beta-dystroglycan binding and a nuclear translocation of the protein. These results demonstrate that mini-dystrophin is efficiently bound and incorporated into the dystrophin protein complex, via beta-dystroglycan in living cells, similarly to the full length dystrophin protein.

Diagnosis of dystrophinopathy by skin biopsy

We studied the expression of dystrophin in skin biopsy samples from 19 patients with neuromuscular diseases. Immunohistochemical procedures for dystrophin analyses were performed using monoclonal antibodies for three different domains. Arrector pili muscles, which are smooth muscles in the skin, expressed dystrophin in the patients with limb-girdle muscular dystrophy (5), facioscapulohumeral muscular dystrophy (1), and spinal muscular atrophy (3), and in normal controls (2). The C-terminus of dystrophin was slightly expressed in the patients with Duchenne muscular dystrophy, whereas the rod domain and N-terminus were absent. In one patient with Becker muscular dystrophy, the expression of dystrophin was reduced. The mosaic of dystrophin positive and negative smooth muscle fibers was observed in a manifesting carrier of Duchenne muscular dystrophy. Our results suggest that skin biopsy is very useful for the diagnosis of Duchenne/Becker muscular dystrophy and manifesting carrier of Duchenne muscular dystrophy, and can be performed even at an advanced stage of the disease.

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.

PCR and immunoblot analyses of dystrophin in Becker muscular dystrophy

The dystrophin gene was examined by PCR analysis in 30 Japanese patients with Becker muscular dystrophy (BMD). Fifteen PCR of these patients had exon deletion, generally, less than three exons. Muscle biopsies were also performed in 20 BMD patients (10 with sequence deletions, and 10 without detectable sequence deletions) in order to correlate PCR findings with immunoblot and immunostaining data. A patchy, heterogeneous membrane immunostaining pattern of reduced intensity was found, irrespective of the presence or absence of deletions. Immunoblotting studies demonstrated dystrophin of low molecular mass and quantity in BMD patients with deletion mutations, while a low quantity of dystrophin with an apparent wild type molecular mass was observed in nearly half the BMD patients without detectable deletions. However, these dystrophins were also found to have slightly abnormal molecular masses when the standard electrophoresis time was prolonged. This suggests that immunoblots and PCR data correlate well in patients with BMD. Additionally, it is suggested that immunoblot assays can detect abnormalities in dystrophin in the absence of detectable PCR deletions.

Independent localization of dystrophin N- and C-terminal regions to the sarcolemma of mdx mouse myofibres in vivo

Dystrophin has been proposed to associate with the skeletal muscle membrane by way of a glycoprotein complex that interacts with its C-terminal domains. Transfection of mdx mouse myotubes in culture or myofibres in vivo with recombinant genes encoding human dystrophin deletion mutants shows, however, that not only the C terminus of dystrophin but also its N-terminal actin-binding domain can locate independently to the muscle sarcolemma. This observation suggests that lack of sarcolemma-associated dystrophin in Duchenne muscular dystrophy (DMD) muscle may result from enhanced degradation of truncated mutation products rather than their inability per se to associate with the sarcolemma.

Immunohistochemical analysis of the distribution of MyoD1 in muscle biopsies of primary myopathies and neurogenic atrophy

The expression of the myogenic determination gene MyoD1 plays a primary role in the commitment of primitive mesenchymal cells to a striated muscle lineage and is down-regulated during later stages of differentiation. To determine the potential role of this gene in myopathic conditions, we examined its expression by means of immunohistochemical analysis, using a series of muscle biopsies from 14 patients with a variety of primary myopathies and neurogenic disorders. Utilizing the avidin-biotin-complex technique, cryostat sections were stained with monoclonal antibody 5.8 A, which we have previously described as having a high level of specificity for tumors with rhabdomyoblastic differentiation. Of special interest was the observation in 4 of 8 cases of neurogenic atrophy of varying levels of cytoplasmic positivity of muscle fibers, appearing to correlate with their degree of atrophy, in addition to weak nuclear staining. Muscle biopsies from 2 patients with Duchenne's muscular dystrophy and 2 patients with autoimmune inflammatory myopathies demonstrated various levels of nuclear positivity in scattered foci that appeared to correlate with areas of regeneration. A biopsy from a single case of neurogenic atrophy secondary to infantile spinal muscular atrophy (Werdnig-Hoffmann's disease) demonstrated diffuse but relatively weak staining of myofiber nuclei, in contrast to sections of normal striated muscle and muscle biopsies from patients with unexplained myoglobinuria, which exhibited only minimal amounts of staining. These data are compatible with observations that MyoD1 expression is related to electrical activity and muscle regeneration.

Localisation and characterisation of dystrophin in the central nervous system of controls and patients with Duchenne muscular dystrophy

The aim was to localise and characterise dystrophin in various human tissues, especially in the CNS. Immunoblotting and immunostaining studies were carried out with eight region-specific dystrophin antibodies. In necropsy tissue from controls, dystrophin was noted as a doublet in immunoblots of striated muscle, and as a single band in those of smooth muscle and the CNS. With immunostaining, punctate immunoreactivity was seen on the cell bodies and dendrites of the cerebral cortical neurons and cerebellar Purkinje cells. By contrast, dystrophin was not detected in any tissues, including the cerebrum and cerebellum, of patients with Duchenne muscular dystrophy who had an intellectual disturbance.

Dystrophin and dystrophin-related protein in the central nervous system of normal controls and Duchenne muscular dystrophy

To clarify the localization and characterization of dystrophin and dystrophin-related protein (DRP) in the human central nervous system (CNS), we carried out immunoblotting and immunostaining studies using three region-specific anti-dystrophin and one anti-DRP antibodies. With immunostaining, punctuate immuno-reactivity of dystrophin was seen along the cell bodies and dendrites of the cerebral cortical neurons and cerebellar Purkinje cells in the normal controls autopsied. By contrast, dystrophin was not detected at all in the CNS of Duchenne muscular dystrophy (DMD) patients with intellectual disturbance. Immunoreactivity of DRP was observed in the vascular walls of both normal and DMD brains, but not in the neuronal cells. Compensatory increase of DRP was not noted in DMD brains. This study suggests that in DMD the brain-type dystrophin originally present in neurons is absent and may be related to the intellectual disturbance.

Specific increase of genetic expression of parvalbumin in fast skeletal muscles of mdx mice

Parvalbumin mRNA was assayed by Northern blot analysis in muscles from normal and dystropic (mdx) mice. Its content was found to be specifically higher in mdx fast muscles than in control preparations. This suggests an increased expression of the protein in dystrophin-lacking fast fibres. A possible role in calcium homeostasis is discussed.

Upper gastrointestinal tract motility in children with progressive muscular dystrophy

Gastric emptying was evaluated in 15 children (mean age, 8.0 years) with progressive muscular dystrophy to detect early gastrointestinal smooth muscle involvement; 10 of the children also underwent esophageal manometry. Clinical evidence of skeletal muscle dysfunction was minimal in 14 of the 15 patients; 10 of them had no gastrointestinal symptoms. Gastric emptying studies were performed by using 500 muCi of technetium 99m-sulfur colloid bound to a scrambled egg, and scintigraphic measurements were made continuously for 60 to 90 minutes. Gastric emptying studies and manometric tracings were compared with those from 11 children (mean age, 8.4 years) without gastrointestinal or muscular disorders. Mean (+/- SD) percentage retention of gastric isotope was significantly greater in patients with muscular dystrophy than in control subjects. No differences were found between the two groups in distal esophageal motility or in upper and lower esophageal sphincter pressures or relaxation. Contraction amplitudes in the upper portion of the esophagus, however, were significantly lower in patients with myopathy than in control subjects. These data suggest that dysfunction of smooth muscle of the upper gastrointestinal tract is detectable in children with muscular dystrophy early in the course of the disease, even when gastrointestinal symptoms are absent and skeletal muscle symptoms are minimal.

Retroviral-mediated transfer of a dystrophin minigene into mdx mouse myoblasts in vitro

We have demonstrated expression of a 6.3 kb Becker muscular dystrophy (BMD) human dystrophin cDNA following retroviral-mediated transduction of cultured myoblasts from the dystrophin-deficient mdx mouse. The truncated dystrophin protein was localised to the sarcolemma of differentiated myotubes by antibodies against the C-terminus of the molecule, and produced an identical immunostaining pattern to that observed in control myotubes expressing normal endogenous dystrophin. These results indicate that retroviral-mediated gene transfer may be useful for experimental in vivo studies on the complementation of dystrophin gene mutations.

Human dystrophin expression in mdx mice after intramuscular injection of DNA constructs

Duchenne's muscular dystrophy (DMD), which affects one in 3,500 males, causes progressive myopathy of skeletal and cardiac muscles and premature death. One approach to treatment would be to introduce the normal dystrophin gene into diseased muscle cells. When pure plasmid DNA is injected into rodent skeletal or cardiac muscle, the cells express reporter genes. We now show that a 12-kilobase full-length human dystrophin complementary DNA gene and a 6.3-kilobase Becker-like gene can be expressed in cultured cells and in vivo. When the human dystrophin expression plasmids are injected intramuscularly into dystrophin-deficient mdx mice, the human dystrophin proteins are present in the cytoplasm and sarcolemma of approximately 1% of the myofibres. Myofibres expressing human dystrophin contain an increased proportion of peripheral nuclei. The results indicate that transfer of the dystrophin gene into the myofibres of DMD patients could be beneficial, but a larger number of genetically modified myofibres will be necessary for clinical efficacy.