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Muscular dystrophies are a group of genetic conditions characterized by progressive muscle weakness and wasting. The Duchenne and Becker types of muscular dystrophy primarily affect the skeletal muscles, which are used for movement, and the muscles of the heart. Duchenne muscular dystrophy is the most common form of muscular dystrophy in children.
Duchenne and Becker muscular dystrophies have similar signs and symptoms and are caused by mutations in the same gene. The two conditions differ in their severity and age of onset. In people with Duchenne muscular dystrophy, muscle weakness tends to appear in early childhood and progress rapidly. Heart problems, which can become life-threatening, typically begin in the teenage years. The signs and symptoms of Becker muscular dystrophy are usually milder and exhibit a large range of variation. In general, they become apparent later in childhood or adolescence and progress at a much slower rate.
Duchenne and Becker muscular dystrophies together affect 1 in 3,500 to 5,000 male births. Between 400 and 600 boys in the United States are born with these conditions each year. Females are rarely affected by these forms of muscular dystrophy.
Mutations in the DMD gene cause muscular dystrophy, Duchenne and Becker types.
The DMD gene makes a protein called dystrophin. This protein helps stabilize and protect muscle fibers and may play a role in chemical signaling within cells. Mutations in the DMD gene alter the structure or function of dystrophin, or prevent any functional dystrophin from being produced. As a result, muscle fibers become damaged with repeated use. The damaged fibers weaken and die over time, leading to the muscle weakness and heart problems characteristic of Duchenne and Becker muscular dystrophies.
This condition is inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome) one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes) a mutation must be present in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
In about two thirds of cases, an affected male inherits the mutation from a mother who carries one altered copy of the DMD gene. The other one third of cases probably result from new mutations in the gene.
Females who carry one copy of a DMD mutation may have some signs and symptoms related to the condition (such as muscle weakness and cramping), but these are typically milder than the signs and symptoms seen in affected males.