Muscular Dystrophy

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 Duchenne(1806-1875), 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 2,500,000 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 that Duchenne muscular dystrophy is inherited as an X-linked recessive disease. Females have two X chromosomes, while males have one X chromosome and one Y chromosome. Therefore, in females, a normal X...

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...re out how get only one gene in the right cell, how to keep the gene from impairing any other functions and how to get the gene to produce the right amount of protein. Some researchers are trying a different approach; that of myoblast transfer therapy. This school of thought attempts to fuse healthy, immature muscle cells with dystrophic cells to make hybrid muscles that function normally. Unfortunately, initial tests have not fared very well because the injected myoblasts do not seem to travel very far from the injection site. The last type of research deals with a protein called utrophin. It is hoped that this protein could functionally take over for dystrophin. Scientists are trying to discover a chemical that will upregulate utrophin production without disrupting anything else in the body.

Until a cure is found 1 in approximately every 3,500 boys will inherit Duchenne muscular dystrophy and eventually die from it. The current treatments only place a temporary obstacle in front of the disease. It is hoped that advancements in the fields of science and medicine will help to speed along the discovery of a cure for this debilitating muscular disease.