Duchenne muscular dystrophy (also known as Pseudohypertrophic or DMD) is the most common form of childhood muscular dystrophy. It is one of nine types of muscular dystrophy, a group of inherited, debilitating conditions that usually affect voluntary muscles. Muscular dystrophy causes the muscle fibers to become damaged. In some types of muscular dystrophy, heart muscles, respiratory system and organs are affected.
Duchenne muscular dystrophy is caused from a defective DMD (also known as dystrophin) gene. The DMD gene is the largest known human gene and is located on the X chromosome. The DMD gene provides instructions for making a protein called dystrophin. Dystrophin is a protein that helps keep muscle cells working effectively. It is found primarily in the skeletal and heart muscles. Mutations in the DMD gene prevent dystrophin from being produced, therefore, muscles become weak and damaged, including the heart muscle.
Duchenne muscular dystrophy is inherited in an X-linked recessive pattern. A disease is considered X-linked if the defective gene that causes the condition is located on the X chromosome. Duchenne muscular dystrophy is usually passed from mother to son through one of the mother's genes. The mother passes on the defective DMD gene which is located on the X chromosome. Boys inherit an X chromosome from their mother and a Y chromosome from their father. Girls inherit an X chromosome from their mother and an X chromosome from their father. The X-Y or X-X combination is what determines the sex of a child.
Males have one Y and one X chromosome; a defective copy of the X chromosome will likely cause muscular dystrophy. Females have two X chromosomes; a defective mutation must be present in both copies of the X gene to cause muscular dystrophy. If a female has one defective gene and one unaffected gene, they usually will not experience symptoms. The unaffected X gene usually will produce enough dystrophin to keep the muscles working effectively. However, there are a few females with one defective gene who experience mild muscle weakness and fatigue symptoms. These females usually exhibit very mild muscular weakness and usually do not have any other long-term complications.
The sons of female carriers (women who have one defective DMD gene) each have a 50% chance of having the disease. The daughters of female carriers each have a 50% chance of becoming carriers.
Approximately two thirds of all individuals with Duchenne muscular dystrophy inherit the defective gene from their mother. The other one third usually develops a new mutation of the DMD gene located on the X chromosome.
