Pedigree — Huntington Disease Adult-Onset AD

A 12-year-old boy is brought to the pediatric neurology clinic with a 2-year history of progressive rigidity, cognitive decline, and seizures. His father was diagnosed with Huntington disease at age 42 and is now severely affected. Genetic testing in the boy reveals 68 CAG repeats in the HTT gene. The pedigree pattern marked **D** in the diagram shows anticipation in paternal transmission resulting in the juvenile form. Which of the following BEST explains why this patient's presentation differs from the typical adult-onset form of Huntington disease?