Discovery of a Novel Small-Molecule Targeting Selective Clearance of Mutant Huntingtin Fragments

Abstract

CAG-triplet repeat extension, translated into polyglutamines within the coding frame of otherwise unrelated gene products, causes 9 incurable neurodegenerative disorders, including Huntington's disease. Although an expansion in the CAG repeat length is the autosomal dominant mutation that causes the fully penetrant neurological phenotypes, the repeat length is inversely correlated with the age of onset. The precise molecular mechanism(s) of neurodegeneration remains elusive, but compelling evidence implicates the protein or its proteolytic fragments as the cause for the gain of novel pathological function(s). The authors sought to identify small molecules that target the selective clearance of polypeptides containing pathological polyglutamine extension. In a high-throughput chemical screen, they identified compounds that facilitate the clearance of a small huntingtin fragment with extended polyglutamines fused to green fluorescent protein reporter. Identified hits were validated in dose-response and toxicity tests. Compounds have been further tested in an assay for clearance of a larger huntingtin fragment, containing either pathological or normal polyglutamine repeats. In this assay, the authors identified compounds selectively targeting the clearance of mutant but not normal huntingtin fragments. These compounds were subjected to a functional assay, which yielded a lead compound that rescues cells from induced mutant polyglutamine toxicity.

Using Drosophila models of Huntington's disease as a translatable tool

“…Induced pluripotent stem cell (iPSC) derived neurons have therefore been generated from dermal fibroblasts, which display a HD like pathology when transplanted into rats (Jeon et al, 2012). Stably transfected cells expressing mutant Htt have also been used for high throughput drug screening to identify a number of compounds, which mitigate mutant Htt induced pathology (Coufal et al, 2007;Lazzeroni et al, 2013).…”

Discovery of Therapeutic Approaches for Polyglutamine Diseases: A Summary of Recent Efforts

“…With the purpose of identifying compounds promoting the clearance of the mutant polyQ protein in mammalian cells, Coufal et al. used the HTT gene with 103Qs in fusion with EGFP, placed under inducible control of an ecdysone control element in PC12 cells . These authors screened a library of 37,000 compounds, basing their screen on fast degradation of the Q130‐EGFP fusion proteins 24 hr after the inducer removal.…”

High Throughput Screening for Neurodegeneration and Complex Disease Phenotypes

“…In one study, Coufal el al. developed an assay that measures the level of a mutant htt-GFP fusion protein in a HTS mode and tested 16,000 compounds [40]. They identified one compound that selectively enhanced clearance of mutant htt, but without affecting the levels of wild type htt.…”