Exploring the Role of CHIP in the Pathogenesis of Digenic Spinocerebellar Ataxia Type 17

Spinocerebellar ataxia type 17 (SCA17) is a progressive neurodegenerative disease primarily affecting cerebellar Purkinje cells, which are vital for motor function. SCA17 is caused by the expansion of CAG/CAA repeats in the TATA-binding protein coding gene (TBP), leading to an extended polyglutamine (polyQ) tract and TBP nuclear aggregation. While expansions exceeding 47 repeats cause complete disease penetrance, incomplete penetrance is observed in individuals carrying an allele with 41-46 repeats. STUB1 gene contributes to this incomplete penetrance, acting as a disease modifier, and defining a digenic form of SCA17 (SCA17-DI).' STUB1 encodes E3 ubiquitin ligase CHIP (with a co-chaperone domain), and evidence suggests that CHIP may play a role in the degradation of TBP and prevent its nuclear aggregate accumulation. To investigate the pathomechanism underlying SCA17-DI, we developed iPSC lines from fibroblasts of individuals with distinct genetic backgrounds: a monogenic SCA17 patient, a healthy donor with an intermediate TBP allele, and a SCA17-DI patient carrying a STUB1 mutation. These iPSC lines, along with healthy control, are differentiated into neuronal precursor cells and Purkinje cell precursors using small molecules-based protocols. These models enable the study of TBP-CHIP interaction, as well as the impact of mutations associated with SCA17-DI on protein quality control in a relevant cellular context. To further explore the role of CHIP as SCA17-DI modifying factor, we also employ molecular approaches to directly assess how specific patient-derived STUB1 mutations affect the aggregation and degradation of polyQ-expanded transcription factors. This work was supported by Fondazione Cariplo - Italy (n. 2021-1544).