The muscular dystrophies are a group of genetic and hereditary muscle diseases; characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue. In some forms of muscular dystrophy, cardiac and smooth muscles are affected. The muscular dystrophies are the most know hereditary diseases. more...
The most common form is Duchenne muscular dystrophy. This form is caused by mutations of the gene for the dystrophin protein. The dystrophin is the second largest gene in mammals.
The dystrophin gene is located on the X chromosome, thus making it a 'sex-linked' disorder. Accordingly, muscular dystrophies are much more common in males, as females have two copies of the X chromasome while males have only one.
How is it inherited? Duchenne muscular dystrophy is caused by an X-linked gene. This means that only boys are affected but that their mothers may be carriers. In almost half of all affected boys, the faulty gene has arisen by mutation in the boy himself and no other family member carries it. However, this may be difficult to prove and can be decided only after careful and expert assessment of the family.
In the remaining cases (somewhat over half of all cases), the mother carries the gene but is usually not herself affected by it. Such women are known as ‘carriers’. Each subsequent son of a carrier has a 50:50 chance of being affected and each daughter has a 50:50 chance of being a carrier herself. A small number of female carriers of the gene have a mild degree of muscle weakness themselves and are then known as ‘manifesting carriers’.
One of the most important things that needs to be done soon after the diagnosis of a boy with Duchenne muscular dystrophy is to seek genetic advice and appropriate tests for those family members who are at risk of being carriers.
The major types of muscular dystrophy include:
- Duchenne muscular dystrophy (OMIM 310200)
- Becker's muscular dystrophy (OMIM 300376)
- Congenital muscular dystrophy
- Distal muscular dystrophy
- Emery-Dreifuss muscular dystrophy (OMIM 181350, OMIM 310300, OMIM 604929)
- Facioscapulohumeral dystrophy (OMIM 158900, OMIM 158901)
- Fukuyama congenital muscular dystrophy (FCMD) (OMIM 253800)
- Limb-girdle muscular dystrophy (OMIM 159000, OMIM 159001, OMIM 253600, OMIM 253601, OMIM 253700, several others)
- Myotonic muscular dystrophy (OMIM 160900, OMIM 602668, OMIM 605377)
- Oculopharyngeal muscular dystrophy (OMIM 164300)
- Severe childhood autosomal recessive muscular dystrophy (OMIM 253700)
Duchenne MD is the most common form of muscular dystrophy affecting children, and myotonic muscular dystrophy is the most common form affecting adults. Muscular dystrophy can affect people of all ages. Although some forms first become apparent in infancy or childhood, others may not appear until middle age or later.
How common is it? About a 100 boys with Duchenne muscular dystrophy are born in the UK each year. There are about 1,500 known boys with the disorder living in the UK at any one time. For the general population the risk of having an affected child is about one in every 3,500 male births.
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