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Myotonic Muscular Dystrophy

Myotonic dystrophy is an inherited disorder of the muscles and other body systems. It is the most common form of muscular dystrophy that begins in adulthood.

This disorder is characterized by progressive muscle wasting and weakness, particularly in the lower legs, hands, neck, and face. People with myotonic dystrophy often have prolonged muscle tensing (myotonia) and are not able to relax certain muscles after use. For example, a person may be unable to release a grip on a doorknob or handle.

Other signs and symptoms of myotonic dystrophy include clouding of the lens of the eye (cataracts) and abnormalities of the electrical signals that control the heartbeat (cardiac conduction defects). In affected men, hormonal changes may lead to balding and an inability to father a child (infertility). The features of this disorder usually develop during a person's twenties or thirties, although they can occur at any age. The severity of the condition varies widely among affected people, even among members of the same family.

The two types of myotonic dystrophy are designated type 1 and type 2. The clinical features of type 2 myotonic dystrophy tend to be milder than those of type 1, and the two types are caused by mutations in different genes.

A variation of type 1 myotonic dystrophy, called congenital myotonic dystrophy, can be noted at birth. The signs and symptoms include generalized weakness, weak muscle tone (hypotonia), club foot, breathing problems, developmental delays, and mental retardation. In some cases, these medical problems are severe or life-threatening.

Causes

Mutations in the CNBP and DMPK genes cause myotonic dystrophy.

Myotonic dystrophy type 1 is caused by a mutation in the DMPK gene, while a mutation in the CNBP gene is responsible for the less common myotonic dystrophy type 2. Similar mutations in the DMPK and CNBP genes cause the two forms of myotonic dystrophy. In each case, a short segment of DNA is abnormally repeated many times, forming an unstable region in the gene. The mutated gene produces an altered version of messenger RNA, which is a molecular blueprint of the gene that is normally used for protein production. The abnormal messenger RNA forms clumps inside the cell that interfere with the production of many copies of proteins. These changes prevent cells in muscles and other tissues from functioning normally, leading to the signs and symptoms of myotonic dystrophy.

Inheritance Pattern

Both types of myotonic dystrophy are inherited in an autosomal dominant pattern.

 

Reference:

Genetics Home Reference, U. S. National Library of Medicine