Danon disease

Overview
Danon disease (or glycogen storage disease Type IIb) is a lysosomal storage disease that was first characterized in 1981 by Dr. M.J. Danon. Danon Disease is an X-linked dominant disorder that predominantly affects cardiac muscle. It affects both males and females, although males tend to see the majority of mental retardation and muscle weakness.

A mutated gene that codes a protein called LAMP2 causes Danon Disease. LAMP2 protein is a glycoprotein molecule comprised of a carbohydrate and a protein. It is typically found on the membrane of the lysosome. LAMP2 is deficient in individuals affected by Danon Disease but it is unknown how this deficiency disrupts the function of the lysosome.

Symptoms
Muscle weakness, heart disease, and mental retardation are three main symptoms associated with this disorder. Signs of muscle weakness and heart disease begin to manifest in early childhood or adolescence. Some affected individuals are unable to walk as muscular complications progress. Heart disease associated with Danon Disease involves heart arrhythmias and cardiomyopathy, or severe heart muscle disease. Visual problems may also occur.

In females, the symptoms of Danon Disease are less severe and manifest later than they do in affected males. Muscle weakness is present, but commonly less debilitating than it is in males. Heart disease, which may occur, will manifest in adulthood. Symptoms may include visual problems.

Inheritance Pattern
Danon Disease is an X-linked dominant disorder. It selectively and more often affects boys, since males have only one X chromosome. Females have two X chromosomes and therefore have an extra X to protect them from disorders of this inheritance nature, but cases of Danon Disease afflicting females have been reported. Danon Disease also commonly affects males more severely than it does females. Boys often develop symptoms in childhood or in adolescence. Symptoms may not appear in females until adolescence or adulthood.

Life Expectancy
Life expectancy for Danon Disease is 30 years of age or younger and result from cardiac failure.

Diagnosis and Testing
Several mutations have been found on the LAMP2 gene, and genetic analysis is a necessary diagnostic tool for confirming or disconfirming a diagnosis of Danon Disease. Prenatal genetic screening is available. A muscle tissue biopsy, which reveals large vacuoles containing elevated levels of glycogen, a kind of a sugar, used in conjunction with genetic screening may confirm a diagnosis.

Treatment
Medications to treat the heart disease are often prescribed and in severe cases, a heart transplant is often needed. Supportive treatment, such as physical therapy, may improve muscle strength and balance.