What is the repeat expansion associated with Huntington's Disease?

Huntington's Disease is caused by a specific type of mutation known as a repeat expansion in the HTT gene, which encodes for the protein huntingtin. The pathological feature of this mutation is the expansion of CAG trinucleotide repeats within this gene. Normally, the number of CAG repeats in the HTT gene ranges from about 10 to 35. However, in individuals with Huntington's Disease, this number can expand to 36 repeats or more, leading to the development of neurological symptoms associated with the disorder.

The CAG repeat is translated into a polyglutamine stretch in the huntingtin protein; as the number of repeats increases, the protein can misfold and aggregate, which is thought to play a critical role in the neurodegenerative process of the disease. Understanding the mechanism behind this repeat expansion is crucial for diagnosing Huntington's Disease and furthering research into potential therapies.