Cause of DMD
Until the 1980s, little was known about the cause of any of the forms of muscular dystrophy. In 1986, MDA-supported researchers identified a gene on the X chromosome that, when flawed (mutated), causes both Duchenne and Becker muscular dystrophies.
Genes contain codes, or recipes, for proteins, which are important biological components in all forms of life. In 1987, the protein associated with this gene was identified and named dystrophin.
DMD occurs because the mutated gene fails to produce virtually any functional dystrophin. (Individuals with Becker MD genetic mutations make dystrophin that is partially functional, which protects their muscles from degenerating as badly or as quickly as in DMD.)
Inheritance in DMD
DMD is inherited in an X-linked pattern, because the gene that can carry a DMD-causing mutation is on the X chromosome. Every boy inherits an X chromosome from his mother and a Y chromosome from his father, which is what makes him male. Girls get two X chromosomes, one from each parent.
Each son born to a woman with a dystrophin mutation on one of her two X chromosomes has a 50 percent chance of inheriting the flawed gene and having DMD. Each of her daughters has a 50 percent chance of inheriting the mutation and being a carrier. Carriers may not have any disease symptoms but can have a child with the mutation or the disease. DMD carriers are at risk for cardiomyopathy.
Although DMD often runs in a family, it’s possible for a family with no history of DMD to suddenly have a son with the disease. There are two possible explanations:
The genetic mutation leading to DMD may have existed in the females of a family for some generations without anyone knowing it. Perhaps no male children were born with the disease, or, even if a boy in an earlier generation was affected, relatives may not have known what disease he had.
The second possibility is that the child with DMD has a new genetic mutation that arose in one of his mother’s egg cells. Since this mutation isn’t in the mother’s blood cells, it’s impossible to detect by standard carrier testing.
If a mother gives birth to a child with DMD, there’s always the possibility that more than one of her egg cells has a dystrophin gene mutation, putting her at higher than average risk for passing the mutation to another child. And once the new mutation has been passed to a son or daughter, he or she can pass it to the next generation.
A man with DMD can’t pass the flawed gene to his sons because he gives a son a Y chromosome, not an X. But he’ll certainly pass it to his daughters, because each daughter inherits her father’s only X chromosome. They’ll then be carriers, and each of their sons will have a 50 percent chance of developing the disease and so on.