Huntington's Disease

Huntington's disease (HD) is unique among the other neurodegenerative diseases in that it is always inherited. HD is caused by abnormal huntingtin protein. The normal huntingtin protein contains multiple repeats of a sequence of three DNA bases, cytosine-adenine-guanine (CAG), which encodes the amino acid glutamine. When more than 36 repeats are present in the protein, stemming from a mutation in the huntingtin gene, an abnormal protein is produced. This mutant form of the protein is more prone to aggregation than versions of the protein with a normal number of repeats. Individuals with 36–40 CAG repeats may or may not develop symptoms of HD, while people with more than 40 repeats will develop the disorder during a normal lifetime. No case of HD has been diagnosed with a CAG repeat count fewer than 36.