Huntington disease describes an autosomal dominant pattern of inheritance with high penetrance (a high proportion of persons with the gene develop the disease). The characteristic findings of Huntington disease are caused by loss of neurons (nerve cells) in the brain.
The disease is due a gene in chromosome band 4p16.3. The gene, called HD, contains an unstable repeating sequence of 3 nucleotide bases (CAG) in the DNA. Normal people have an average of 19 CAG repeats and at most 34 such repeats while virtually all patients with Huntington disease have more than 40.
The Huntington disease gene codes for a protein that has been named (confusingly) huntingtin whose function is unknown. The elevated numbers of CAG repeats in the Huntington disease gene lead to the production of an elongated huntingtin protein which appears to correlate with the loss of neurons in the disease.
Mood disturbance is usually the first symptom seen, with bipolar disorder-like mood swings that may include mania, depression, extreme irritability or angry outbursts, and psychosis. Other symptoms include chorea (restless, wiggling, turning movements), muscle stiffness and slowness of movement, and difficulties with memory and other cognitive processes. The HD gene is located on chromosome 4, and is an autosomal dominant gene. Only one copy need be inherited to cause the illness. Diagnosis is by genetic testing, and family members of people with Huntington's disease may also want to know if they carry the HD gene. At this time, there is no cure for HD, although medication may be used to control symptoms of the illness, such as mood swings and chorea.