STUB1 mutants effect on TBP behavior in digenic spinocerebellar ataxia type 17

Spinocerebellar ataxias (SCAs) are hereditary, progressive, heterogenous neurodegenerative diseases (NDs). SCA type 17 (SCA17) presents an expansion of CAG nucleotide repeats of the TATA-box binding (TBP) gene that codes for an abnormally long polyglutamine (polyQ) tract in the N-terminal. This leads to reduced solubility and accumulation of the mutant TBP in neurons. Strikingly, TBP forms containing an intermediate polyQ tract (41-47/49 Qs) show incomplete penetrance (SCA17-DI). Mutations in STIP1 Homology And U-Box Containing Protein 1 (STUB1/CHIP), an E3 ubiquitin-ligase which participates in the protein quality control (PQC) system, plays a crucial role in SCA17-DI. Since TBP and STUB1 may be both involved in SCA17-DI we investigated their behaviour and interplay to better understand the underlying molecular mechanisms in disease. Our data show a punctate distribution and insoluble protein accumulation of elongated polyQ TBP, not observed in the wild-type or intermediate polyQ TBP expressing neurons. TBP accumulation is reverted by STUB1 over-expression suggesting that TBP degradation is mediated by STUB1. Since STUB1 plays a role in both ubiquitin proteasome system (UPS) and autophagy, we alternatively inhibited these pathways to study TBP behaviour. Autophagy inhibition did not modify STUB1 activity on TBP-Qs protein levels and aggregation. UPS inhibition blocked STUB1-mediated degradation on TBP-WT and TBP-Q54, while it did not affect TBP-Q43 protein levels suggesting that different pathways are responsible for STUB1-mediated TBP-Qs removal. Moreover, STUB1 SCA17-DI-linked mutations showed reduced activity on TBP clearance. Our data demonstrate that STUB1 mutations affect TBP biochemical behaviour in SCA17-DI. Therefore, we want to further investigate TBP and STUB1 interplay to better understand their pathological role. GRANTS: Fondazione Cariplo (2021-1544); National Center for Gene Therapy and Drugs based on RNA Technology (CN_00000041); Fondazione Regionale per la Ricerca Biomedica (FRRB_grant_Care4NeuroRare CP_20/2018 to F.T.).