Becker muscular dystrophy (BMD) is an inherited disorder caused by mutations in the gene for dystrophin. Mutations in the same gene can cause Duchenne muscular dystrophy (DMD). Lack of dystrophin causes muscle damage and progressive weakness, beginning in late childhood or adolescence. The course of the disease is slower and less predictable than that of DMD. In BMD, damage to the heart muscle has the most significant health consequences.
The dystrophin protein transfers the force of muscle contraction from the inside of the muscle cell outward to the cell membrane. Because it connects the center of the muscle cell to the periphery, the dystrophin protein is extremely long. One end is specialized for linking to the muscle interior, and the other end for linking to a variety of proteins at the cell membrane. The long middle section, called the rod domain, is taken up by a series of repeating units called spectrin repeats. The repeated spectrin units in the middle of the protein play an important role in linking the two ends, but the protein can still function (albeit not perfectly) with fewer of them than normal. Mutations that cause BMD decrease the number of these repeats, leading to muscle weakness.
In addition to its force-transfer role, dystrophin provides the scaffold for holding numerous molecules in place near the cell membrane. Loss of dystrophin displaces these molecules, with consequent disruptions in their functions. These include the signaling molecule nitric oxide synthase.
Gene-centered therapeutic strategies for BMD are directed toward replacing the mutant gene, or correcting the gene mutation. Other therapeutic strategies include introducing a similar protein, implanting new cells, reducing fibrosis, promoting muscle growth, and mitigating the cellular effects of loss of fully functional dystrophin.
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