Huntington's Disease (HD) is a dominant genetic disorder. Each child of a person who has HD has a 50% chance of inheriting the disease, and the disease does not skip a generation. HD is caused by a larger than normal CAG repeat in the Huntington gene. This larger than normal CAG repeat produces an abnormal protein that begins to kill brain cells when the person who has the gene reaches middle age
. The loss of these cells causes intense symptoms and eventually death.
HD was named after Dr. George Huntington
. In 1872, he was the first person to document an accurate description of the symptoms and course of the disease
. At the time he called it “hereditary chorea”. The Huntington gene was discovered in 1993 by the Huntington Study Group. The CAG repeats in the Huntington gene, which is located on chromosome 4, code for the protein huntingtin. The larger than normal number of CAGs in a person with HD causes the huntingtin protein to be abnormal, which leads to symptoms. Even though every cell in the body has the gene, only the cells in the brain seem to be affected.
A person has two alleles for every gene. One allele is inherited from the mother and one allele is inherited from the father. If either allele of the Huntington gene has the larger than normal CAG repeat, the person will have HD. If neither allele has the larger than normal CAG repeat, the person will not have HD and will not pass it on. When a person has children, only one allele of each gene is passed on. If a person with HD passes on the allele with the larger than normal CAG repeat, the child will have HD. If the allele with the normal CAG is passed on, the child will not have HD.
Early signs of HD are mood swings, irritability, depression, memory loss, and uncontrolled movements. As the disease progresses, it becomes more difficult to walk and speak. Also, memory and intellectual functions continue to decline.
A person who has a chance of inheriting HD can be tested any time after they reach the age of 21. Testing for the HD gene entails a blood sample genetic test that analyzes DNA for HD mutation by counting the number of CAG repeats in the HD gene region. A person will not get HD if their CAG repeats are lower than 30. If a person's CAG repeats are higher than 40 they will get HD. A person may or may not get HD if their CAG repeats are between 30-40. This is a gray area for researchers and they are still trying to understand why. A person isn't actually diagnosed with HD until they show symptoms.
Although there is no cure for HD, there are treatments for some symptoms. For involuntary movements, a person with HD is likely to be prescribed a neuroleptic such as Tetrabenazine, Reserpine, Trilaton (perhenazine), or Haldol. For depression, Fluoxetine, Sertraline Hydrochloride, or Nortriptyline are commonly used. Also, tranquilizers can be used to treat anxiety, and Lithium can be used for persons with severe mood swings. Speech therapy can also improve speech and swallowing for a person with HD. It is important that a person diagnosed with HD maintains a high calorie diet, not only to prevent weight loss, but it has also been shown to improve involuntary movement and behavioral problems.
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