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Muscular dystrophy refers to not one but a group of muscle diseases These diseases have three features in common they are hereditary they are progressive and each causes a characteristic and selective pattern of weakness Duchenne muscular dystrophy DMD is the most prevalent and severe childhood form of this group of diseases Each form of muscular dystrophy is caused by a defect in a specific gene In 1986 scientists discovered exactly which piece of genetic material is missing in Duchenne muscular dystrophy patients They named it for Guillaume Benjamin Amand Duchenne1806-1875 a French neurologist who was one of the first doctors to discover and study the disease When functioning properly the Duchenne gene carries instructions for assembling a muscle protein known as dystrophin At about 2500000 nucleotides dystrophin is one of the largest genes known Dystrophin is largely responsible for reinforcing and stabilizing the sarcolemma Dystrophin associates with the muscle fiber sarcolemma by interacting with the actin microfilaments and with a transmembrane protein complex linked to the extracellular matrix This latter dystrophin-associated glycoprotein complex DAGC includes the extracellular proteoglycan Alpha-dystroglycan which binds to merosin in muscle fiber basal laminae as well as a number of other integral and cytoplasmic membrane proteins Alpha-dystroglycan Alpha- Beta- and Gamma- sarcoglycans see Figure 1 The DAGC provides a physical link and potentially a signaling pathway between the extracellular matrix and the internal scaffolding of the muscle cells Mutations in the Duchenne gene result in dystrophin deficiency which constitutes the pathogenic basis of DMD Dystrophin is either absent or severely deficient in a person with DMD When dystrophin is lost through gene mutation the muscle falls apart under the tension generated when it contracts Without dystrophin the muscle fibers also lose their ability to regenerate and are eventually replaced with adipose tissue and fibrous connective tissue see Fig 2 and Fig 3 The Duchenne gene has been located on the short arm of the X chromosome This means
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