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.