Duchenne Muscular Dystrophy is a fatal genetic disorder characterized by progressive muscle weakness. It occurs in 1 of every 3500 live male births (approximately 20,000 per year). It is caused by a mutation in a gene on the X-Chromosome that codes for dystrophin. Dystrophin is a muscle protein that helps hold the muscles together and helps to maintain muscle cell structure. This dystrophin gene is the largest gene in the human body. Boys with Duchenne have no dystrophin. Without dystrophin, muscle cells are damaged and replaced with fat and scar tissue. Eventually, those affected lose their ability to walk, their upper extremities become weaker. The heart and respiratory muscles are also affected leading to premature death, typically in the late 20's. Some men with DMD lives into their 30's and 40's, but others do not live to see young adulthood.
DMD is a sex-linked recessive disorder. It is usually passed from DMD carrier mothers to their sons. Mothers give each of her children an X-Chromosome. Fathers give their daughters an X Chromosome and they give their sons a Y-Chromosome. So girls have an XX-Chromosome and boys have an XY-Chromosome. If the mothers passed the gene onto her daughter, she will have an X from her father to fall back on. She will not have DMD, unless her other X has a problem. If the mother passed the gene to her son, he will have DMD. Mothers have a 25% chance of having a child with DMD. She can either have an unaffected daughter, carrier daughter, unaffected son, or a son with DMD. DMD can happen as a spontaneous mutation without any family history. DMD can skip several generations, making a family history unknown. Anyone can have a child with DMD.
Duchenne is diagnosed through a combination of several tests and clinical observation. One of the first tests is the CPK (creatinine prosphate kinase) blood test. When muscle damage occurs, CPK leaks out of the muscles into the blood stream. Normally levels range from 0-250. In boys with DMD, levels are ussually in the thousands, because their muscles are suffering severe damage. A level above 10,000 is typically found in DMD boys.
The next step in testing is genetic testing. This is a blood test that normally takes a month to receive results on. The test detects flaws in the dystrophin gene. Genetic testing does not detect every flaw in the dystrophin gene, but with a new method called SCAIP (Single Condition Amplification Internal Primer sequencing), 99% of flaws in the dystrophin gene can be detected.
Another test used for the diagnoses DMD is a muscle biopsy. In this test a small peice of muscle is removed from the boy's thigh and is studied. This test determines whether or not dystrophin is present. In DMD, dystrophin is absent. In a milder form of MD, called Becker Muscular Dystrophy (BMD), dystrophin is present but deficient.
The picture in the right hand column and some of the information above was obtained from the following website where you can find more information about DMD and BMD.
Parent Project Muscular Dystrophy: http://www.parentprojectmd.org