Unraveling TBP/TBP-1 and CHIP/CHN-1 interaction in a C. elegans model of human digenic SCA17 disease: does it affect neuron function?

The 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). In particular, up to 39 repeats are wild-type, between 40 and 46 show reduced penetrance and are thus defined intermediate alleles, and ≥47 are fully penetrant pathogenic alleles. We have recently described that affected patients with intermediate alleles carried a concurrent heterozygous pathogenic variant in STUB1 gene. Therefore, we demonstrated 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. Mutations in STUB1 have been associated with the autosomal recessive spinocerebellar ataxia type 16 (SCAR16) and the autosomal dominant spinocerebellar ataxia type 48 (SCA48). Moreover, last year, STUB1 mutations have also been described as possible genetic modifiers in spinocerebellar ataxia type 8 (SCA8). In addition, CHIP has been found to act as a modifier in SCA1, SCA3, and Huntington disease in cellular and animal models. The molecular mechanism underlying the strong genetic interaction between CHIP and TBP in SCA17 is still unknown. To unravel the nature of this interaction, we took advantage of the nematode Caenorhabditis elegans whose genome harbors orthologs for both TBP and STUB1, named tbp-1 and chn-1, respectively. We have generated SCA17 animal 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 polyQ54 expansion caused defects in gentle touch response, suggesting an altered function of mechanosensory neurons. On the other hand, CHN-1 knockout (chn-1(by155)) 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 (TBPQ43;chn-1(by155)). These results support the hypothesis that CHIP/CHN-1 mediates TBP/TBP-1 degradation by proteasomal pathway. We therefore speculate that while intermediate polyQ expansions in TBP are not sufficient to result in an altered phenotype, the absence of CHN-1 impairs TBPQ43 proteasomal degradation triggering TBP accumulation and aggregation. Further studies are required to deeply define the interaction occurring between these two proteins and to identify other players in this interaction.