DMD IS GENETIC, BUT. . .
Many parents, on learning their child has DMD, wonder how he could have gotten the disease. Often there's no family history of muscular dystrophy. You may have heard that DMD is passed from mother to son, but it's not that simple.
DMD is an inheritable disease, but that doesn't mean it's always inherited. Sometimes, mutations in genes occur without any mutation being present in the parents. In other cases, a mutation is inherited.
When a genetic disease is inherited, or "runs in the family," it's called familial. When it's not inherited, it's called sporadic. Either way, it's still a genetic disease, arising from a genetic mutation.
Genes are arranged on structures in the cell known as chromosomes. Each of us has 23 pairs of chromosomes -- 22 pairs of autosomes and one pair of sex chromosomes. The autosomes all have numbers (chromosome 1, 2, 3 and so forth), and the sex chromosomes have letters. Females have two X chromosomes, and males have an X and a Y chromosome.
Duchenne and Becker muscular dystrophies can be inherited in the X-linked recessive pattern, or can occur when the dystrophin gene spontaneously mutates.
Girls are luckier than boys when it comes to disease-causing mutations in genes on the X chromosome. Because they have two X chromosomes, they almost always have a normal "backup" gene on one X chromosome, even if they carry the mutated form of the gene on the other X chromosome. Males, in contrast, have only one X chromosome, their other sex chromosome being a Y. If their lone X chromosome contains a gene with a mutation, there's no "backup" gene to compensate for it, and they show the effects of the mutation.
The gene for dystrophin is on the X chromosome. In DMD and BMD, women can carry the mutation on one of their two X chromosomes, but they rarely show the disorder in any major way. A woman can, however, pass along to a son her X chromosome with the mutated dystrophin gene. The son, having no backup X, will show the disease. This is called an X-linked recessive inheritance pattern.
Nowadays, genetic testing for dystrophin mutations is quite precise. Girls can be tested to see whether they are carriers. Children suspected of having the disorder (including babies before birth) can likewise be tested.
A word of caution about carrier testing is in order, however. Some women have been reassured that they aren't carriers after they've had a genetic test of DNA taken from blood cells. (All our cells carry all genes; the differences in cells result from some genes being turned on and others being turned off in various cell types.) Usually, this kind of test is fine.
But, sometimes, a woman doesn't carry the genetic mutation in her blood cells, muscle cells or any of her own cells except her egg cells, and usually only some portion of these. This makes it very difficult to offer complete reassurance about carrier status to any woman who has previously had a son with DMD. It's possible that she still has more egg cells that contain the dystrophin mutation, and it would be very difficult to test for these by methods other than prenatal testing after an egg has been released and a baby has been conceived.
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