What causes Huntington's disease?
Huntington's disease is a progressive disorder involving wasting (degeneration) of nerve cells in the brain. The disorder was first described in 1872 by George Huntington, an American physician. Huntington's disease is caused by a defect in the gene for a protein of unknown function called huntingtin. The defective gene contains 40 or more so-called "CAG repeats," compared to only 30 of these repeats in the normal huntingtin gene. C, A, and G are DNA nucleotides, the
building blocks of genes. The extra building blocks in the huntingtin gene cause the protein that is made from it to contain an extra section as well. It is currently thought that this extra protein section interacts with other proteins in brain cells where it occurs, and that this interaction ultimately leads to cell death.
Huntington's disease is inherited as a single faulty gene on chromosome #4. The HD gene is a dominant gene, meaning that only one copy of it is needed to develop the disease. HD affects both males and females. The gene may be inherited from either parent, who will also be affected by the disease. A parent with the HD gene has a 50% chance of passing it on to each offspring. The chances of passing on the HD gene are not affected by the results of previous pregnancies. There is a part of the gene that is repeated in multiple copies. The greater the number of repeats, the more likely it is that the person will develop symptoms and the greater the chance they will occur at a younger age. The disease may occur earlier and more severely in each succeeding affected generation because the number of repeats can increase.
Every child of a parent with the disorder has a 50% chance of inheriting Huntington's Disease. Symptoms do not usually appear until adulthood, typically between ages 35 and 50 years old but this depends on the number of repeats found in the gene so it may also appear in younger people. In children it may appear to be Parkinson's disease with rigidity, slow movements, and tremor.
There is progressive loss of mental function, including personality changes, and loss of cognitive functions such as judgment, and speech. Abnormal facial and body movements develop, including quick jerking movements. (The term chorea means "dance" and refers to the typical movements that develop.)