What gene mutation is associated with Huntington's disease due to an excess of CAG repetitions?

Huntington's disease is specifically associated with mutations in the HTT gene, which encodes the huntingtin protein. The characteristic mutation that leads to the disease involves an expanded repeat of the CAG sequence within this gene. Normally, the CAG repeat is present in a limited number (typically less than 36 repeats), but in individuals with Huntington's disease, this sequence is expanded to 36 or more repetitions. The excess CAG repetitions lead to the production of an abnormal version of the huntingtin protein, which aggregates within neurons and ultimately contributes to the neurological symptoms associated with the disease.

The other options listed pertain to different genetic conditions or risk factors unrelated to Huntington's disease. For instance, APOE is linked to Alzheimer's disease, while BRCA1 and TP53 are associated with certain types of cancer. Therefore, the association of Huntington's disease with the HTT gene specifically due to CAG repeat expansions is what makes this choice the correct answer.