Huntington's Disease (HD)
Huntington's Disease (HD) is a hereditary degenerative disorder of the central nervous system. It is inherited in an autosomal dominant fashion, meaning that each child of an affected parent, regardless of gender, has a 50% chance of inheriting the disease-causing gene. The prevalence of HD is estimated at 1/10,000 individuals in the United States, with approximately 5 at-risk individuals for every currently affected person. HD symptoms usually occur between ages 30-50, but symptoms have been seen in persons as young as 5 years or as old as 90 years. The early symptoms of HD vary and may be subtle enough to go undetected. These symptoms may include minor twitching, fidgeting, clumsiness, changes in gait, lapses in judgment and memory, and in some individuals, behavioral changes including depression and mood swings. Symptom progression is highly variable, and as the disease progresses, involuntary movements (chorea) become more pronounced, speech and swallowing difficulties often develop, and cognitive ability deteriorates. In the later stages of the disease, the affected individual is usually bedridden and totally dependent on others for all of his or her needs. The duration of symptoms may range from 10 to 25 years or more.
Direct genetic testing for Huntington disease (HD) has been available since 1993, allowing individuals at risk for the disorder to undergo pre-symptomatic testing. In individuals with HD, a CAG triple repeat (or trinucleotide), a part of individual's genetic code, is expanded beyond the normal number. Individuals without HD generally have less than 29 repeats. In people with Huntington's disease, the repeat is >40. An inconclusive range exists from 30-39. Although a weak correlation exists between the number of repeats and the age of onset, this correlation should not be used to estimate the age of onset. While the Huntington's disease gene discovery allows predictive testing for HD, there is still no cure for HD and no available treatment to delay its onset or to slow, stop or reverse the disease's progression. Counseling is important to help individuals to deal with personal, family, and ethical issues surrounding genetic testing for HD.
Generally, testing for HD falls into two categories: confirmatory testing of a symptomatic individual, and pre-symptomatic testing (e.g. an individual who does not exhibit symptoms but who is at-risk for HD, and for whom testing would be predictive). Occasionally, at-risk women who are pregnant may wish to take a prenatal test to permit the selective abortion of a fetus found to be a gene-carrier for HD. A non-disclosing prenatal test (a linkage test that requires multiple samples from relatives) is also an option for the at-risk individual who does not wish to know his/her own HD status.
Experience has shown that while many at-risk individuals indicate a desire to know their gene-carrier status, far fewer actually undergo testing. When confronted with the opportunity for testing, many find that the emotional toll or risks to confidentiality outweigh the benefits of learning their gene-carrier status. The decision to take a pre-symptomatic test for Huntington's disease should always be an informed, carefully considered and freely chosen personal decision. Individuals should not be coerced into testing, whether by a spouse, another family member, a physician, an insurance company or an employer. For those individuals who opt to undergo pre-symptomatic testing for HD, the medical team will make every effort to maintain the confidentiality of medical care.
Center for Genetic Medicine
676 North St. Clair Street, #1280
Department of Neurology Parkinson's Disease and Movement Disorders Center
Contact: Tanya Simuni, MD
675 North St Clair Street, Suite 20-100
Neurobehavior and Memory Health Clinic
Contact: Brenda Hanny-Pladdy, PhD
675 North St Clair Street, Suite 20-100