GENES AND PROTEINS
To understand dystrophin and DMD, it's necessary to know a little about genes and proteins. Genes are units of DNA, the genetic code of all life. Proteins are among the most important biological components in all forms of life. For example, they make up the structural components of our cells and therefore our tissues, perform the metabolic activities that go on in our cells, and carry out the functions of the immune system.
The code, or recipe, for each protein, is found in the DNA; each gene is a section of DNA that is the code for a specific protein. Sometimes, in a particular person, the code is wrong, like a word with too few letters or too many letters or the wrong letters. A misspelled word is analogous to a genetic mutation. If the code for a protein is wrong, the protein can be made improperly or not made at all.
In DMD, there's a mutation in the gene for dystrophin, which resides on the X chromosome in human cells. (see DMD is Genetic, But... for more about chromosomes.) In most children with this disorder, some dystrophin is probably made but is too abnormal to be of use to the muscles. This abnormal dystrophin isn't detected in the place where it normally would be found when a doctor examines a muscle biopsy (a small sample of muscle taken from the boy). For this reason, most doctors say that a child with DMD has no dystrophin in his muscles.
In a disorder that's closely related to DMD, known as Becker muscular dystrophy (BMD), some abnormal dystrophin is made that's close enough to normal to be of some use. When a doctor looks at a muscle biopsy from a boy with BMD, some dystrophin can be seen in the place where it's supposed to be. It seems clear that the presence of this dystrophin is the reason that boys with BMD have a much less severe disease course. (For more on BMD, see Becker MD — a Less Severe Disease.)
Sometimes, the dystrophin mutation is inherited from a parent (in X chromosome diseases, it's the mother who carries the defect but doesn't usually show symptoms), but often the mutation occurs for the first time in a boy with DMD. In these cases, there is no family history of the disorder, and testing of the mother reveals nothing.
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