Muscular dystrophy. Muscling in on gene therapy

Interventions for muscular dystrophy: molecular medicines entering the clinic

Muscular dystrophies are individually rare genetic disorders that cause much chronic disability, affecting young children and adults. In the past 20 years, more than 30 genetic types of muscular dystrophy have been defined. During this time, precise diagnosis, genetic counselling, and medical management have improved. These advances in medical practice have occurred while definitive therapies based on an improved knowledge of disease pathogenesis are awaited. A wide range of therapeutic options have been tested in animal models, and some are being tested in clinical trials. Various therapeutic targets are being investigated, from personalised medicines targeting specific mutations and drugs targeting cellular pathways to gene-based and cell-based therapies.

Transfer of full-length Dmd to the diaphragm muscle of Dmd(mdx/mdx) mice through systemic administration of plasmid DNA

Mutations in the gene encoding dystrophin, a large cytoskeletal protein in muscle, lead to Duchenne muscular dystrophy (DMD). Affected individuals often die of respiratory failure resulting primarily from diaphragm muscle degeneration. Here we report a new procedure to transfer the full-length dystrophin cDNA into the diaphragm muscle of Dmd(mdx/mdx) mice, which carry a mutation in the dystrophin gene (Dmd). Significant gene transfer was found after intravenous injection of naked plasmid DNA followed by a brief (eight second) occlusion of blood flow at the vena cava. This is the first demonstration of gene transfer into the diaphragm muscle through systemic administration of naked plasmid DNA. The approach has potential application for treatment of DMD.

Pluripotential mesenchymal cells repopulate bone marrow and retain osteogenic properties

Precursor cells, isolated from bone marrow, can develop into various cell types and may contribute to skeletal growth, remodeling, and repair. The D1 cell line was cloned from a multipotent mouse bone marrow stromal precursor and has osteogenic, chondrogenic, and adipogenic properties. The osteogenic phenotype of these precursor cells is relevant to the process of fracture healing and osteointegration of prosthetic implants. The D1 cells were labeled genetically using a replication incompetent retroviral vector encoding beta-galactosidase, an enzyme which is used as a marker. Labeled cells are readily identifiable by staining with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside and by flow cytometry, and retain the desired osteogenic characteristics in vivo as shown by von Kossa staining, alkaline phosphatase assay, an increase in cyclic adenosine monophosphate in response to parathyroid hormone, osteocalcin messenger ribonucleic acid production, and bone formation in diffusion chambers. In addition, the cells cloned from marrow stroma repopulate the marrow of host mice, persist for several weeks, and retain their osteogenic potential ex vivo. The data suggest that such cells may be used to replenish the number of osteoprogenitors in marrow, which appear to decrease with age, thereby leading to recovery from bone loss and improved bone growth and repair. Labeling these cells creates a model in which to study the potential of such cells to participate in fracture repair, ingrowth around prosthetic implants, treatment of osteoporosis, and to explore the possibility of gene delivery to correct mutations or defects in metabolism that are responsible for certain skeletal abnormalities.

Extracorporeal circulation as a new experimental pathway for myoblast implantation in mdx mice

The deficiency of dystrophin, a sarcolemmal associated protein, is responsible for Duchenne muscular dystrophy (DMD). Gene replacement is attractive as a potential therapy. In this article, we describe a new method for myoblast transplantation and expression of dystrophin in skeletal muscle tissue of dystrophin-deficient mdx mouse through iliac vessels extracorporeal circulation. We evaluated the extracorporeal circulation as an alternative route of delivering myoblasts to the target tissue. Two series of experiments were performed with the extracorporeal circulation. In a first series, L6 rat myoblasts, transfected with LacZ reporter gene, were perfused in limbs of 15 rats. In the second series, the muscle limbs of three 6-8-week-old mdx were perfused with myoblasts of donor C57BL10J mice. Before these perfusions, the right tibialis anterior (TA) muscle of the rats and mdx was injected three times at several sites with bupivacaine (BPVC) and hyaluronidase. The ability of injected cells to migrate in the host tissue was assessed in rats by technetium-99m cell labeling. No radioactivity was detected in the lungs, bowels, and liver of animals treated with extracorporeal circulation. The tissue integration and viability of the myoblasts were ultimately confirmed by genetic and histochemical analysis of LacZ reporter gene. Following a single extracorporeal perfusion of myoblasts from donor C57BL10J, sarcolemmal expression of dystrophin was observed in clusters of myofibers in tibialis anterior muscles previously treated with BPVC and hyaluronidase. Furthermore, large clusters of dystrophin-positive fibers were observed in muscles up to 21 days after repeated treatments. These clusters represented an average of 4.2% of the total muscle fibers. These results demonstrate that the extracorporeal circulation allows selective myoblast-mediated gene transfer to muscles, opening new perspectives in muscular dystrophy gene therapy.

Expression vectors encoding human growth hormone (hGH) controlled by human muscle-specific promoters: prospects for regulated production of hGH delivered by myoblast transfer or intravenous injection

We report here the construction of vectors that produce and secrete human growth hormone (hGH) in a muscle-specific manner. The promoter regions of the genes encoding human skeletal alpha-actin (HSA) and troponin I slow (HTnIs) were linked to the hGH-encoding gene. These vectors were designated pHSA2000GH and pHTnIs4200GH, respectively. The HSA and HTnIs promoters linked to the cat gene have previously been shown to be necessary and sufficient for developmentally regulated muscle-specific expression. Furthermore, these promoters function in a fibre-type-specific manner in transgenic animals. Transient and stable transfection analyses with pHSA2000GH and pHTnIs4200GH indicated that: (i) these vectors efficiently synthesized hGH in a muscle-specific manner; (ii) the myogenic master regulatory gene, myoD, a determinant of cell fate, trans-activated expression of hGH in pluripotential non-muscle cells; and (iii) these hGH expression vectors were developmentally regulated during myogenic differentiation. These regulated tissue/fibre-type-specific hGH-containing plasmids are suitable vectors for the delivery and stable production of GH in livestock and GH-deficient hosts by either transgenesis, myoblast transfer or liposome-mediated intravenous injection.

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.

Experimental challenge for the treatment of Duchenne muscular dystrophy using a vascularized free muscle graft

Following free vascularized normal muscle graft in mice, a study was made to determine whether dystrophin expression is possible in dystrophin-deficient muscles. In this study, dystrophic C57BL/10 ScSn-mdx mice were used as recipients and normal C57BL/10 ScSn mice as donors. A free vascularized quadriceps muscle 8.0 x 6.0 x 6.0 mm in size was orthotopically transplanted into a muscle defect produced in the recipient mouse. The diameter of the sutured vessels was about 0.4 mm. Transplantation was successful in 7 of 20 mice. At 12 weeks after the transplantation, the grafted muscle was examined by immunocytochemical stain using anti-dystrophin antibody. This study showed that dystrophin was expressed in the transplanted muscle but not in the adjacent recipient quadriceps muscle, suggesting that grafted donor cells with dystrophin failed to migrate into dystrophic muscle cells and fuse together. However, since the grafted normal skeletal muscle successfully survived and normal dystrophin was expressed in almost all the grafted muscle fibers, the possibility was suggested that the function of muscular dystrophy muscle can be compensated by complete replacement with a larger muscle.