Spinocerebellar ataxia 3
Overview
Machado-Joseph disease (MJD)-also called spinocerebellar ataxia type 3-is a rare hereditary ataxia. (Ataxia is a general term meaning lack of muscle control.) The disease is characterized by clumsiness and weakness in the arms and legs, spasticity, a staggering lurching gait easily mistaken for drunkenness, difficulty with speech and swallowing, involuntary eye movements, double vision, and frequent urination. Some patients have dystonia (sustained muscle contractions that cause twisting of the body and limbs, repetitive movements, abnormal postures, and/or rigidity) or symptoms similar to those of Parkinson's disease. Others have twitching of the face or tongue, or peculiar bulging eyes.
Symptoms
MJD 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. MJD 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. MJD 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 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.
Causes
MJD 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. MJD 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. MJD 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 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.
Diagnosis
Physicians diagnose MJD by recognizing the symptoms of the disease and by taking a family history. They ask detailed questions about family members who show, or showed, symptoms of the disease, the kinds of symptoms these relatives had, the ages of disease onset, and the progression and severity of symptoms. A definitive diagnosis of MJD can only be made with a genetic test. Unfortunately, many legal and ethical considerations, such as loss of health insurance and employment discrimination, may discourage some individuals with symptoms from getting tested. For the same reasons, many physicians recommend against genetic testing for those individuals who have a family history of the disease but do not show symptoms. For more information on genetic testing and counseling, please consult the organizations listed in the section titled "Where can I get more information?"