CHN-1 knock-out impairs TBP proteasomal degradation leading to functional alteration of mechanosensory neurons

The human transcription factor TATA-box binding protein (TBP) gene is characterized by a polyQ-encoding CAG/CAA repeat whose expansion causes the autosomal dominant spinocerebellar ataxia type 17 (SCA17). Interestingly, while ≥47 polyQ repeats are fully-penetrant pathogenic alleles, 40-46 repeats show reduced penetrance and are thus defined intermediate alleles. We have recently described that affected patients with intermediate alleles carried a concurrent heterozygous pathogenic variant in STUB1 gene demonstrating that SCA17 is monogenic for TBP with ≥47 polyQ and digenic TBP/STUB1 for intermediate alleles (SCA17digenic). STUB1 encodes the C-terminus Hsp70-interacting protein (CHIP), an E3-ubiquitin ligase with co-chaperone activity. To unravel the nature of this interaction, we have generated Caenorhabditis elegans SCA17 models by pan-neuronal overexpression of the cDNA of human TBP alleles with 38 (wild-type, TBPWT), 43 (intermediate, TBPQ43), and 54 repeats (fully-penetrant, TBPQ54). We observed that only the fully-penetrant pathogenic polyQ expansion caused functional alteration of mechanosensory neurons. Instead, knockout of CHN-1 (chn-1(by155)), orthologue of human CHIP, compromised wild-type backward locomotion, consistent with the lower number of visible GABAergic motor neurons observed. Interestingly, the SCA17digenic model TBPQ43;chn-1(by155) showed an age-dependent defect in gentle touch response, similarly to TBPQ54 animals. Moreover, treatment of TBPQ43 animals with the proteasomal inhibitor MG132, but not with the lysosomal inhibitor chloroquine, impaired gentle touch response resulting in a defective phenotype similar to SCA17digenic model. These results support the hypothesis that CHIP/CHN-1 mediates TBP/TBP-1 proteasomal degradation. We have also confirmed, in single-copy integrated model of SCA17digenic, the functional alteration of mechanosensory neurons present in the overexpressed model, thus indicating that the phenotype observed is determined by TBP mutation rather than its overexpression. In conclusion, we have generated the first SCA17 and SCA17digenic C. elegans models that recapitulate disease pathogenicity. In particular, we observed that although intermediate polyQ expansion in TBP are not sufficient to cause an altered phenotype, CHN-1 absence impairs TBPQ43 proteasomal degradation triggering disease onset. Our model will be used to identify other players involved and new treatments able to rescue the altered phenotype.