Duchenne Muscular Dystrophy: Experimental Models on Physical Therapy

Neuromuscular disorders are a heterogeneous group of genetic diseases. Nowadays, more than 30 genetically defined forms are recognized and, in the last decade, mutations in several genes have been reported to result in the deficiency or loss of function of a variety of important muscle proteins (Shelton & Engvall, 2005). Defects in components of the dystrophin-glycoprotein complex (DGC) are known to be an important cause of different forms of muscular dystrophies (Ervasti & Campbell 1993; Yoshida & Ozawa 1990). Lack of dystrophin protein in muscle cells is characteristic of Duchenne muscular dystrophy (DMD), which is a progressive and fatal X-linked genetic disorder. Many animal models have been studied to identify an efficient treatment for this disease in humans. Two mammalian models of DMD have been widely used in preclinical trials to understand the pathogenesis of the disease and development of efficient therapeutic strategies for humans. Mdx-mouse is the most used animal model for DMD, followed by the Golden Retriever Muscular Dystrophy (GRMD) canine model. Mdx-mouse morphology displays some features of muscle degeneration, but the pathogenesis of the disease is comparatively mild. This model has a slightly shorter life spam as compared to wild-type controls (Banks & Chamberlain, 2008) and muscle degeneration is different from the one seen in DMD patients. An important degeneration and regeneration of muscle fibers is observed at a young age in the mdx-mouse (2 to 4 weeks), which results in the muscle morphological changes of centralized nuclei and heterogeneity of fiber size. Necrosis is also observed at this early age but decreases around sixty days. Loss of muscle tissue is slow and muscle weakness is not evident until later in life. Fibrosis, a marked feature of DMD muscle, is less pronounced in mdx-mouse, with the exception of diaphragm muscle (Hueber et al., 2008). Dystrophin deficiency has also been reported in cats as hypertrophic feline muscular dystrophy (HFMD), in which diaphragmatic hypertrophy is often fatal (Shelton & Engvall, 2005). Similar to mdx pathology, the skeletal muscle of the HFMD cats undergoes repeated cycles of degeneration and regeneration but does not develop the debilitating fibrosis that is