Huntington Disease is an autosomal dominantly inherited neurodegenerative disease. The clinical features and age of onset are quite variable, but symptoms are most frequently detectable between 30-50 years of age.
HD is a seriously debilitating progressive neuro-psychiatric disorder of the central nervous system affecting as many as 1/10,000-1/20,000 people in the U.S.A. It results in cognitive impairment (e.g., loss of sensory-motor function, memory, and judgment), emotional/behavioral disturbance and loss of physical control, often over a period of ten to twenty years. Early physical symptoms may initially involve fidgeting, restlessness, changes in handwriting, or clumsiness. Symptoms will progress into more significant involuntary movements (chorea) of face, body and limbs, myoclonus, dystonia, incoordination, slurred speech (dysarthria) and difficulty with walking and swallowing (dysphagia). Cognitive, emotional and/or behavioral changes may precede, occur simultaneously, or follow the onset of physical symptoms. Such alterations may include disorganization, forgetfulness, poor judgment, and difficulty in decision-making. Personality changes may include depression, anger, denial, withdrawal, apathy, and impulsiveness. Delusions and psychotic behaviors are possible, but rare. With progression, judgment and reasoning become increasingly impaired, there is a global subcorticol dementia, and the affected individual eventually becomes physically and cognitively incapacitated, needing full time supervisory care. There is a great variability of progression, even within the same family. Some affected members may have marked physical problems while others are more significantly affected with cognitive and/or emotional disturbances.
Autosomal dominant inheritance implies the condition is caused by an alteration on one chromosome, which may be passed from an affected parent to an offspring. Each individual with an affected parent is said to be at risk because of a 50% chance of inheriting the gene. Men and women may be equally affected, with either parent passing on the condition to their offspring. Several studies reveal an earlier age of onset in offspring of affected fathers when compared to affected mothers. Presently, there is no cure for HD, although drugs are now available for reducing the severity of some emotional or psychiatric symptoms and chorea. The effectiveness of these medications varies from person to person.
In March 1993, a breakthrough scientific discovery led to the identification and location of the HD gene itself. It is now known that the HD gene is a CAG trinucleotide expanded sequence mapped to the short arm of chromosome 4. The presence of 40 or more CAG repeats results in a confirmed diagnosis of HD. Less then 30 CAG repeats rules out the expression of the gene for HD. There remains an area of less certainty and meitoic instability in the repeat region between 29-39 CAG repeats. It is estimated that 1-2% of the population would fall in this range. Direct analysis may be used for confirmatory, predictive or prenatal diagnosis, under the Protocol Guidelines.
The discovery of the gene brings hope to the goal of effective treatment. Scientists are directing basic and applied research into understanding the protein coded in this trinucleotide region. Once the gene function is understood, the hope would then be to interfere with the dysfunctional, pathological process.