Muscular Dystrophy
The term muscular dystrophy refers to any condition in which healthy muscle cells die and are replaced by fat and connective tissue. The result of this change is a weakening and wasting of muscles that progress over time. Eventually a person with this condition loses all control over his or her muscles, is no longer able to walk, and eventually dies of respiratory failure. Most patients do not live beyond the age of 30.
At least seven distinct forms of muscular dystrophy are known: Duchenne, facioscapulohumoral, limb-girdle, distal myopathy, ocular myopathy , myotonic, and Werdnig-Hoffman. All are hereditary disorders, although the genetic mechanisms by which they are transmitted differ from type to type.
The most common and most severe form of muscular dystrophy is named for a French neurologist, Guillaume B. A. Duchenne (1806-1875), who first described the disorder in 1861. Our current understanding of Duchenne muscular dystrophy is due in large part on the work of the American geneticist, Elizabeth Shull Russell (b.1913). In 1951, Russell accidentally observed Duchenne muscular dystrophy symptoms in a colony of mice with which she was working. Over a period of years, she was able to show that this form of muscular dystrophy is inherited as an X-linked recessive trait.
An important step forward in understanding and possibly treating Duchenne muscular dystrophy occurred in 1986 when scientists at Harvard Medical School discovered that the defective gene responsible for Duchenne muscular dystrophy is located on the short arm of the X chromosome. They found that the protein produced by the normal gene, dystrophin, is absent from the cells of Duchenne muscular dystrophy patients. One consequence of this discovery was a 1989 research project in which mice with a defective Duchenne muscular dystrophy gene were treated with immature muscle cells. The new muscle cells apparently contained correct copies of the gene and began producing dystrophin in normal amounts.
More recently, gene therapy has come to the forefront as a therapeutic approach for muscular dystrophy, especially since scientists have identified the mutated genes that can cause myotonic, Duchenne, Becker, some forms of limb-girdle, a form of congenital, and Emery-Dreifuss muscular dystrophies. For example, researchers have developed an artificial dystrophin gene to replace the absent protein in Duchenne muscular dystrophy, and research is ongoing into how to best deliver copies of these genes into muscle cells so that they work efficiently and continue to function.
One approach set for human clinical trials will involve injections of genes into the biceps muscle. In 1998, researchers also showed that genes containing the muscle protein sarcoglycan can correct defects in limb-girdle muscular dystrophy when the genes are injected into the leg muscles of hamsters. Since these studies are using a naturally occurring sarcoglycan protein, it can be inserted with minimal use of immunosuppressive drugs. The use of these drugs during gene therapy research using artificial proteins has proven to be a major obstacle to gene therapy because suppressing the immune system can make the patient susceptible to other diseases and immunosuppressive drugs have many potentially serious side effects. In addition to opening up a window for new therapies, research into the genetics of muscular dystrophy are helping scientists to better understand the disease. For example, because of recent insights into the genetic components of muscular dystrophy, researchers now know that Duchenne and Becker dystrophies are really the same disease distinguished only by variations in severity. In other cases, some types of muscular dystrophy thought to be one disease are now known to be different diseases caused by various genetic defects.
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