Objective

Huntington’s disease is a lethal autosomal dominant neurodegenerative disorder resulting from a polyglutamine-encoding CAG expansion in the Huntingtin (HTT) gene. Histone deacetylase 4 (HDAC4) heterozygous knockout delayed mHTT aggregation in CNS tissue in the R6/2 HD mouse model, and ameliorated motor and CNS neurophysiological deficits and improved survival.

In addition to the direct HDAC4 inhibition approach, we have conjugated an exemplar of our lead series (selected from high-throughput screening against recombinant HDAC1-9 biochemical and cell-based HDAC Class I and IIa activity assays) to a VHL E3 Ubiquitin Ligase ligand in order to generate a proteolysis targeting chimera (PROTAC) tool to reduce total HDAC4 protein levels.

We validated that the PROTACs were direct Class IIa HDAC inhibitors in recombinant biochemical assays and retained the ability to inhibit Class IIa HDACs in a variety of mammalian cell lines. Surprisingly, these PROTACs acted as selective and potent degraders of HDAC4, with degradation DC₅₀ values in the low nM range in various cell types when tested by western blot and MSD assay for HDAC4 protein levels. Furthermore, our lead PROTAC molecule in human ES cell-derived neurons and mouse cortical neurons showed potent knockdown of HDAC4, in agreement with the data from the cell lines. We are now working towards proof of principle testing of these molecules in vivo in rodent model.