Redefining the Pathogenic CAG Repeat Units Threshold in

Background and Objectives: Spinocerebellar ataxia type 6 (SCA6) is caused by expansion of CAG repeat units (RUs) in Methods: This was an observational study of patients with suspected spinocerebellar ataxia who underwent SCA6 genetic testing. We analyzed the relationship between Results: In total, 2,768 participants were enrolled. Family history positivity rates increased progressively above 19 RUs and plateaued at ≥23 RUs. Regression analysis of cases with ≥23 RUs showed that 96.20% of cases with ≥23 RUs, 90.67% of cases with 22 RUs, 91.15% of cases with 21 RUs, 61.54% of cases with 20 RUs, and 33.33% of cases with 19 RUs fell within the 95% prediction interval for AAO. However, no patients with ≤18 RUs were included. In the 21-22 RU group, OAs significantly influenced AAO, and ≥17 RUs had a significant effect. For ≥23 RUs, no significant OA effect was observed. Cases with 19-20 RUs showed a higher prevalence of OA with ≥19 RUs compared with cases with ≥23 RUs. Discussion: Our findings suggest that clinical manifestation within a typical lifespan likely requires at least 19 RUs. The 19-20 RU range represents an intermediate zone where OA may influence disease likelihood. For 21-22 RUs, OA significantly affects AAO, indicating a complex interplay between EA and OA. ≥23 RUs seem sufficient to cause disease onset within a typical lifespan, regardless of OA. These results provide a new paradigm for SCA6 diagnosis and genetic counseling, emphasizing the need for cautious interpretation of the intermediate RU range and consideration of OA.