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Near the start of the HTT gene is a short sequence — CAG — repeated over and over. Most people have fewer than 27 repeats. If the number grows past about 40, the protein made from the gene acquires a harmful new property and slowly kills brain cells. The more repeats, the earlier symptoms tend to begin.
HTT, at 4p16.3, encodes huntingtin, a large scaffolding protein with roles in vesicle transport and development. Expansion of the exon 1 CAG tract above roughly 40 repeats produces a polyglutamine tract conferring toxic gain of function, with an inverse relationship between repeat length and age of onset. Because wild-type huntingtin is essential, allele-selective strategies are preferred over total lowering.
Sources
- MedlinePlus Genetics, U.S. National Library of Medicine
HTT gene ↗