In Huntington disease, what trinucleotide sequence is expanded?

Huntington disease is caused by the expansion of a specific trinucleotide repeat sequence in the HTT gene located on chromosome 4. The pathogenic repeat consists of cytosine-adenine-guanine (CAG) sequences. In individuals with Huntington disease, the normal range of CAG repeats is typically between 10 to 35, but in affected individuals, the number of repeats can exceed 36 and may even reach over 100, leading to the degeneration of neurons in specific brain regions.

This expansion leads to the production of an abnormal form of the huntingtin protein, which aggregates and causes neuronal dysfunction and cell death. Recognizing the CAG repeat expansion is crucial for understanding the genetic basis and inheritance pattern of Huntington disease, as it follows an autosomal dominant pattern. The knowledge of this genetic alteration aids in diagnosis, genetic counseling, and research into potential therapies for the disease.