A New Case Series Suggests That SCA48 (ATX/STUB1) Is Primarily a Monogenic Disorder

Background: Monoallelic, pathogenic STUB1 variants cause autosomal dominant cerebellar ataxia (ATX-STUB1/SCA48). Recently, a genetic interaction between STUB1 variants and intermediate or high-normal CAG/CAA repeats in TBP was suggested, indicating digenic inheritance or a disease-modifying role for TBP expansions. Objective: To determine the presence and impact of intermediate or high-normal TBP expansions in ataxic patients with heterozygous STUB1 variants. Methods: We describe 21 patients with ataxia carrying a heterozygous STUB1 variant and determined TBP repeat length. Results: A total of 15 of 21 patients (71%) carried a normal TBP_<40 allele, 4 (19%) carried an intermediate TBP_41–42 allele, and two carried a high-normal TBP₄₀ allele (9.5%). Five of six carriers (83%) of both STUB1 variants and TBP_40–42 alleles showed marked cognitive impairment. Conclusions: SCA48 is predominantly a monogenic disorder, because most patients carried an isolated, heterozygous STUB1 variant and presented with the typical combined phenotype of ataxia and cognitive dysfunction. Still, co-occurrence of TBP_41–42 or high-normal TBP₄₀ alleles was relatively frequent and associated with marked cognitive defects (28.5%), suggesting a modifying effect on clinical expression in some cases.