Spinocerebellar Ataxia Type 1 (ATXN1, AD)

A 38-year-old woman presents with 5 years of progressive gait instability, dysarthria, and clumsy hand movements. Family history reveals her father, paternal aunt, and paternal grandfather all developed similar symptoms in their 40s. Examination shows slowly progressive cerebellar ataxia with wide-based gait, bilateral appendicular dysmetria, dysdiadochokinesia, and scanning dysarthria, accompanied by hyperreflexia and extensor plantar responses. MRI brain demonstrates olivopontocerebellar atrophy. Genetic testing reveals a heterozygous CAG repeat expansion (52 repeats) in exon 8 of ATXN1 on **chromosome 6p23** (marked **A** in the diagram). Which of the following best describes the pathogenic mechanism of the mutation identified at this locus?