The association of CAG repeat length with clinical progression in Huntington disease.

OBJECTIVE: To determine whether the rate of clinical progression in Huntington disease (HD) is influenced by the size of the CAG expansion.

METHODS: The dataset consisted of 3,402 examinations of 512 subjects seen through the Baltimore Huntington's Disease Center. Subjects were seen for a mean of 6.64 visits, with mean follow-up of 6.74 years. Subjects were administered the Quantified Neurological Examination, with its subsets the Motor Impairment and Chorea Scores, the Mini-Mental State Examination, and the HD Activities of Daily Living (ADL) Scale.

RESULTS: In an analysis based on the Random Effects Model, CAG length was significantly associated with the rate of progression of all measures except chorea and ADL. There was a significant interaction term between CAG length and disease duration for all measures except chorea. Further graphical exploration of the data supported these linear models and suggested that subjects at the low end of the expanded CAG repeat range may experience a more benign late course.

CONCLUSIONS: CAG repeat length has a small effect on rate of progression that may be clinically important over time. Individuals with the shortest expansions appear to have the best prognosis. These effects of the CAG length may be relevant in the analysis of clinical trials.