Huntington's disease is caused by an excessive number of repeats of which trinucleotide sequence?

• A. CGG repeats
• B. TTA repeats
• C. GCG repeats
• D. CAG repeats

Answer: (D)

Understanding trinucleotide repeat expansions helps explain Huntington's disease. In this condition, a sequence of three nucleotides, CAG, is repeated too many times in the HTT gene. The extra CAG repeats encode extra glutamine amino acids, creating a long polyglutamine tract in the huntingtin protein. This abnormal protein tends to misfold and form aggregates, damaging neurons—especially in the basal ganglia—leading to movement, cognitive, and psychiatric symptoms. The phenomenon of anticipation can occur as the number of repeats can increase in offspring, shortening the age of onset over generations. Other options point to different diseases: CGG repeats are associated with Fragile X syndrome, while the remaining repeats are not linked to Huntington's disease. Therefore, the repeat pattern in Huntington's disease is a CAG expansion.