Application of innate immune receptor TRIM21 to target mutant huntingtin

Huntington's disease is caused by a genetically expanded CAG repeat encoding a poly-glutamine tract in exon 1 of the Huntington protein (HTT). The disease is dominantly inherited and is characterised histopathologically by aggregation of mutant HTT. There remains no cure and lingering uncertainty as to the molecular features that drive pathology. New methods are urgently required to enable dissection of the molecular pathogenesis and provide new routes to therapeutics. We have demonstrated the existence of intracellular innate immune pathways present in all cell types that can be leveraged for the degradation of intracellular protein aggregates. Driven by the intracellular antibody receptor and E3 ligase TRIM21, these pathways can destroy large aggregates of the neuronal protein Tau over the course of a few hours. As activation of TRIM21 is driven by clustering of its RING E3 ligase domain, this activity can be leveraged to provide selectivity for aggregates, leaving soluble monomeric protein untouched. Based on these findings in Tau, we believe there is an opportunity to develop HTT expansion-selective degraders (ie that specifically degrade proteins encoded by the mutant expanded allele, leaving the wild type proteins alone) as well as aggregate-selective selective degraders. Supported by data showing that TRIM21 is in principle capable of performing both these degradation modalities, we will develop a suite of genetically-encoded HTT-directed degraders. By targeting different epitopes on the HTT protein that are predicted to elicit TRIM21 activation when either expanded or aggregated, we expect to define a panel of degraders with diverse degradation characteristics. The panel will be extensively characterised in cellular models of HTT protein expansion and aggregation. As selective protein-level knockdown tools, the degraders will be exploited in a range of cellular models including iPSC derived neurons to assess the extent to which expansion and aggregation contribute to toxicity. Finally, we will test the ability of our degraders to elicit selective protein knockdown in a mouse model of Huntington's disease. The tools and data generated will provide new methods for the selective depletion of HTT protein and its aggregates. These are expected to be of high value to the research community in helping address the fundamental nature of which protein species contribute to disease progression and thus should be targeted therapeutically. The work will leave the team in an excellent position to extend the studies to address further research questions and assess the suitability of degraders for an AAV gene therapy approach.