In human genetics, which condition is caused by an expanded CAG trinucleotide repeat in the HTT gene?

In human genetics, an expanded CAG trinucleotide repeat in the HTT gene causes Huntington’s disease.

The HTT gene is located on chromosome 4 and provides instructions for huntingtin, a protein found throughout the body. In Huntington’s disease, the CAG sequence is repeated too many times near the beginning of the gene. CAG encodes the amino acid glutamine, so the altered protein contains an unusually long polyglutamine segment.

Huntington’s disease is an autosomal dominant neurodegenerative disorder. A person with one expanded allele can develop the condition and has a 50% chance of passing that allele to each child. Symptoms commonly include involuntary movements, changes in thinking, and psychiatric effects, although onset and progression vary.

The repeat can expand when passed through a family, a phenomenon called anticipation. Expansion is especially associated with paternal transmission. A genetic test can identify the expanded repeat, but medical counseling is important because the result has major personal and family implications.