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The reduced penetrance of TBP intermediate alleles and the recently proposed possible digenic TBP/STUB1 inheritance raised questions on the possible mechanism involved opening a debate on the existence of SCA48 as a monogenic disorder. We here report clinical and genetic results of two apparently unrelated patients carrying the same STUB1 variant(c.244G > T;p.Asp82Tyr) with normal TBP alleles and a clinical picture fully resembling SCA48, including cerebellar ataxia, dysarthria and mild cognitive impairment. This report provides supportive evidence that this specific ataxia can also occur as a monogenic disease, considering classical TBP allelic ranges., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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 TBP 40 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 TBP 40 alleles was relatively frequent and associated with marked cognitive defects (28.5%), suggesting a modifying effect on clinical expression in some cases., (© 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.)