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Machado-Joseph Disease

What Causes It?

Machado-Joseph disease is classified as a disorder of movement, specifically a spinocerebellar ataxia. In these disorders, degeneration of cells in an area of the brain called the hindbrain leads to deficits in movement. The hindbrain includes the cerebellum (a bundle of tissue about the size of an apricot located at the back of the head), the brainstem, and the upper part of the spinal cord.
Machado-Joseph disease is an inherited, autosomal dominant disease, meaning that if a child inherits one copy of the defective gene from either parent, the child will develop symptoms of the disease. People with a defective gene have a 50 percent chance of passing the mutation on to their children.
Machado-Joseph disease belongs to a class of genetic disorders called triplet repeat diseases. The genetic mutation in triplet repeat diseases involves the extensive abnormal repetition of three letters of the DNA genetic code. In the case of Machado-Joseph disease (MJD), the code "CAG" is repeated within a gene located on chromosome 14q. The MJD gene produces a mutated protein called ataxin-3.
This protein accumulates in affected cells and forms intranuclear inclusion bodies, which are insoluble spheres located in the nucleus of the cell. These spheres interfere with the normal operation of the nucleus and cause the cell to degenerate and die.
One trait of Machado-Joseph disease and other triplet repeat diseases is a phenomenon called anticipation. In anticipation, children of affected parents tend to:
  • Develop symptoms of the disease much earlier in life
  • Have a faster progression of the disease
  • Experience more severe symptoms.
This is due to the tendency of the triplet repeat mutation to expand with the passing of genetic material to offspring. A longer expansion is associated with an earlier age at which the disease appears and a more severe form of the disease. However, it is impossible to predict the course of the disease for an individual based solely on the repeat length.
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