siRNA screen identifies QPCT as a druggable target for Huntington's disease

Abstract

Huntington’s disease (HD) is a currently incurable neurodegenerative condition caused by an abnormally expanded polyglutamine tract in huntingtin (HTT). We identified novel modifiers of mutant HTT toxicity by performing a large-scale “druggable genome” siRNA screen in human cultured cells, followed by hit validation in Drosophila. We focused on glutaminyl cyclase (QPCT), which had one of the strongest effects on mutant HTT-induced toxicity and aggregation in the cell-based siRNA screen, and which also rescued these phenotypes in Drosophila. We found that QPCT inhibition induced the levels of the molecular chaperone alpha B-crystallin and reduced the aggregation of diverse proteins. We generated novel QPCT inhibitors using in silico methods followed by in vitro screens, which rescued the HD-related phenotypes in cell, Drosophila and zebrafish HD models. Our data reveal a novel HD druggable target affecting mutant huntingtin aggregation, and provide proof-of-principle for a discovery pipeline from druggable genome screen to drug development.

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

“…Drosophila (Jimenez-Sanchez et al, 2015), warrants further investigation in mice. Other candidates from screens that have yet to be tested in mammalian systems are TCF, a protein involved in Wnt signaling, rpi, a metabolic enzyme, TDO, a member of the M a n u s c r i p t 13 kynurenine pathway, Tra1, a component of the HAT complex, and DJ-1α, an oxidationsensitive chaperone protein (Campesan et al, 2011;Dupont et al, 2012;Sajjad et al, 2014;Wang et al, 2012;Zhang et al, 2010).…”

“…In several studies, candidates found from primary screens in cultured mammalian or Drosophila cells were secondarily screened in Drosophila HD models. A siRNA screen for modifiers of toxicity and aggregation in a HEK293 cell line expressing Htt-138Q showed that silencing glutaminyl cyclase (QPCT) reduces both toxicity and aggregation, possibly by mediating an increase in αβ-crystallin chaperone levels (Jimenez-Sanchez et al, 2015). RNAi knockdown of QPCT rescued eye degeneration of Htt-48Q flies and reduced aggregation in the eyes of Htt-46Q::eGFP flies (Jimenez-Sanchez et al, 2015).…”

“…siRNA, RNAi, and small molecule screens have been performed in cell culture systems (Doumanis et al, 2009;Jimenez-Sanchez et al, 2015;Schulte et al, 2011;Zhang et al, 2010), generating a wealth of genes that modify mutant Htt toxicity (Table 2).…”

Huntington’s Disease: Mechanisms of Pathogenesis and Therapeutic Strategies

“…Molecular chaperones promote efficient folding and prevent aggregation (Hartl et al 2011), and increased levels of HSP40, HSP70, or HSP100 inhibit polyglutamine-induced protein aggregation and prevent its toxicity (Carmichael et al 2000;Jana et al 2000;Krobitsch and Lindquist 2000). A genomic screen recently identified that inhibition of glutaminyl-peptide cyclotransferase (QPCT), using small interference RNAs or small molecule inhibitors, leads to increased levels of the small heat shock protein (HSP) aB-crystallin, and consequently reduces aggregation and toxicity in several models of HD (Jimenez-Sanchez et al 2015).…”

Understanding Huntington's disease using Machine Learning Approaches

“…Such studies have so far been largely conducted on data derived from yeast-two hybrid protein interaction experiments. (Goehler et al, 2004;Riechers et al, 2016;Tourette et al, 2014) These studies have revealed important roles of Rho GTPase pathway (Tourette et al, 2014) and, caspase-6 interactors (Riechers et al, 2016) melanogaster and S.cerevesiae models of HD toxicity have enabled the identification of genetic modifiers of HD (Chatterjee et al, 2013;Faber et al, 2002;Giorgini et al, 2005;Imamura et al, 2016;Jimenez-Sanchez et al, 2015;Mason and Giorgini, 2011;Mason et al, 2013;Parker et al, 2004;Silva et al, 2011;Willingham, 2003;Yamamoto et al, 2006;Zhang et al, 2010). However, the complexity of the mechanisms of action of these genetic modifiers on their targets pose a serious challenge for deciphering the causative mechanisms behind the disease.…”