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An inherited disease that damages the brain, Huntington's disease affects about 30,000 people in the United States, according to the Family Caregiver Alliance. The National Human Genome Research Institute adds that 35,000 more people have some symptoms of Huntington's disease, and 75,000 people carry the Huntington's disease gene.
The symptoms of Huntington's disease occur when the neurons die in the brain, disrupting normal brain function. Patients with Huntington's disease have problems with movement. Early in the disease, patients have uncontrolled movements. As Huntington's disease progresses, patients have muscle stiffness. Huntington's disease also causes problems with cognitive function, such as difficulty multitasking, poor judgment and issues with short-term memory. Patients may have emotional problems, such as mild to severe depression.
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Huntington's Disease Gene
People get the disorder if they have the Huntington's disease gene: a mutation in the HTT gene. The National Human Genome Research Institute states that in 1993, the gene was discovered, which is located on chromosome 4. The HTT gene plays a role in the production of the huntingtin protein, which is involved with neurons - though its exact function is unclear.
The HTT mutation affects the CAG (cytosine, adenine, guanine) sequence of DNA. The Genetics Home Reference, a service of the United States National Library of Medicine, explains that the normal CAG repetition is 10 to 35 times in the gene; patients with Huntington's disease, however, have 36 to 120 CAG repeats. The extra CAG repeats cause abnormalities in the huntingtin protein, resulting in neuronal disfunctions..
The number of CAG repeats determines when a person will develop Huntington's disease. The Genetics Home Reference notes that people who have 40 to 50 repeats have adult-onset Huntington's disease, while people who have more than 60 repeats have early-onset Huntington's disease. The Family Caregiver Alliance states that Huntington's disease commonly starts between ages 30 and 50, though symptoms can appear as early as age 2.
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Inheritance of Huntington's Disease
The National Human Genome Research Institute states that the HTT mutation is a dominant gene, meaning that to get Huntington's disease, a patient only needs one copy of the gene in each cell. For example, if one parent has Huntington's disease, the children have a 50-50 risk of inheriting the disease.
However, as parents pass the Huntington's disease gene to their children, the number of CAG repeats may increase, according to the Genetics Home Reference. For example, people who have 27 to 35 repeats do not have the symptoms of Huntington's disease. But if the number of CAG repeats increase to 36 after being passed on to the child, then that child can develop Huntington's disease.
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Family Caregiver Alliance: Huntington's Disease (http://www.caregiver.org/caregiver/jsp/content_node.jsp?nodeid=574)
Genetics Home Reference: Huntington Disease (http://ghr.nlm.nih.gov/condition/huntington-disease)
National Human Genome Research Institute: Learning About Huntington's Disease (http://www.genome.gov/10001215)