Directing mutant huntingtin towards degradation: Strategies and therapeutic opportunities

Huntington's Disease (HD) is a progressive neurodegenerative disease that is caused by a CAG-repeat expansion in the exon 1 of the huntingtin (HTT) gene, which results in the formation of toxic mutant HTT (mHTT) exon 1 fragments with expanded polyglutamine (polyQ) stretch. These mHTT fragments are prone to misfolding and aggregation, causing cytotoxicity, particularly in the striatal neurons. The lack of disease-modifying treatments for HD emphasizes the need to develop new therapeutic strategies to prevent or reverse HD pathology. The reduction of the mHTT exon 1 fragment by facilitating its selective degradation may be a promising approach to prevent aggregation, thereby delaying or preventing the onset of HD. Since the ubiquitin-proteasome system (UPS) is not impaired in HD, enabling strategies to selectively target mHTT towards the UPS or autophagy would improve mHTT clearance. In this review, we describe three main strategies to modulate mHTT levels. First, we discuss different ubiquitin-modifying enzymes that have been associated with HTT and may act as possible therapeutic targets. Next, we focus on more targeted approaches for the selective degradation of mHTT with engineered small molecule compounds such as PROTACs and AUTOTACs. Finally, we summarize the current knowledge of HTT-specific intracellular antibodies (intrabodies) as potential candidates for gene therapy. Together, this review aims to assess different possible avenues for the treatment of HD.