Becker muscular dystrophy (BMD) is a progressive condition that causes muscle wasting (atrophy) and weakness. It is one of many types of muscular dystrophy and a genetic, or inheritable, condition predominantly affecting boys.
The symptoms of BMD are very similar to those of Duchenne muscular dystrophy (DMD) but tend to be milder and progress at a much slower pace.
Dystrophin and the DMD gene
BMD, like DMD, is caused by a mutation in the DMD gene, which encodes for a protein called dystrophin. Dystrophin is involved in protecting muscles from damage with each contraction. It acts as a glue to strengthen and keep the muscle cells together, as well as to anchor them to surrounding structures. This prevents damage as muscles contract and relax. Dystrophin is primarily found in skeletal muscles, which are responsible for movement, and in the heart or cardiac muscle.
BMD can develop as a result of mutations that cause reduced levels of dystrophin, or a smaller and less efficient dystrophin protein, to be produced. The less capacity of the mutated dystrophin protein results in compromised muscle integrity, leading to a slow accumulation of damage whenever the muscles are used. Over time, damaged muscle cells weaken and die resulting in muscular weakness.
More severe mutations, which result in little or no dystrophin being produced, are associated with DMD.
Types of BMD-causing mutations
Many different types of mutations have been identified in the DMD gene that can cause muscular dystrophy. Some examples are discussed here.
The DMD gene provides the instructions to make the dystrophin protein, and it is “read” in groups of three “letters” called “codons.” Each codon describes what next piece of the protein to add, or whether the protein is complete.
Genes are split into sections called exons and introns. Introns are removed as part of the protein-making process. One of the most common BMD-causing mutations involves one or more exons being deleted from the gene. The dystrophin protein will still be made but, because of the missing section, it will be slightly smaller and less efficient.
BMD can also be caused by smaller “point” mutations in the dystrophin gene that changes the DNA code so that it is read as a “stop” codon instead of the next part of the protein. This is called a “nonsense mutation,” and causes protein production to stop prematurely. The result is a shorter dystrophin protein that is, to some extent, still functional. The location of the wrongly introduced stop codon is important; if it is too close to the beginning of the gene, the resulting protein is too short to function and is destroyed by the cell. In this case, the patient has DMD.
A treatment called Translarna (ataluren) is designed to force the production of the dystrophin protein past the nonsense mutation, to continue making the rest of the protein. The treatment has been given conditional approval in Europe, but is not available in the U.S.
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