Redefining the Pathogenic CAG Repeat Units Threshold in CACNA1A for Spinocerebellar Ataxia Type 6

Spinocerebellar ataxia type 6 (SCA6) is caused by expansion of CAG repeat units (RUs) in CACNA1A. While the pathologic threshold has been considered to be 20 or 21 RUs, the lower limit remains controversial. This study aimed to clarify the pathologic significance of RUs in SCA6, including the role of opposite alleles (OAs).

This was an observational study of patients with suspected spinocerebellar ataxia who underwent SCA6 genetic testing. We analyzed the relationship between CACNA1A RUs and age at onset (AAO). Family history positivity rates were examined for different RUs of the expanded allele (EA). Regression analyses were performed for AAO estimation based on the EA RUs. The influence of OAs on AAO was investigated, particularly in cases with 21-22 EA RUs.

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.

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.