Huntington's Disease with Small CAG Repeat Expansions

Abstract Background Carriers of small cytosine‐adenine‐guanine (CAG) repeats below 39 in the HTT gene are traditionally associated with milder Huntington's disease, but their clinical profile has not been extensively studied. Objective To study the phenotype of CAG 36‐38 repeat carriers. Methods We included 35 patients and premanifest carriers of CAG 36‐38 repeats. We compared clinical and neuropsychological profiles of 11 CAG 36‐38 patients with 11 matched CAG 40‐42 patients. In addition, we analyzed 243 CAG 36‐38 individuals from the ENROLL study to complete the phenotype description. Results Global cognitive efficiency and performance in different cognitive subdomains were similar in small CAG 36‐38 and typically CAG 40‐42 expanded individuals. Chorea as the first symptom was significantly less frequent for CAG 36‐38 patients ( P = 0.04) despite similar total motor scores at first visit. Total motor score at last visit was significantly lower in CAG 36‐38 carriers ( P = 0.003). The similar cognitive and different motor profile of CAG 36‐38 (n = 243) and CAG 40‐42 (n = 4675) carriers was confirmed in the ENROLL database. Additionally, clinicians were significantly less confident in diagnosing Huntington's disease ( P = 2.4e−8) and diagnosis happened significantly later in CAG 36‐38 ( P = 2.2e−6) despite a similar age at symptom onset ( P = 0.29). Conclusions We showed that small CAG 36‐38 expansion carriers had a similar cognitive profile to those with the more common CAG 40‐42 expansions. These individuals may evade molecular diagnosis because of the absence of chorea rather than because of a low penetrance of symptoms. This finding should encourage neurologists to consider Huntington's disease in cognitively impaired elderly patients without typical chorea and anticipate consequences for genetic counseling in their offspring. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.