Abstract A2-57: Identification of genetic vulnerabilities within the proteostasis network of multiple myeloma
Authors: Acosta-Alvear Diego , Kampmann Martin , Cho Min Y. , Aftab Blake T. , Li Xiaokai , Gestwicki Jason E. , Shuman Marc A. , Weissman Jonathan S. , Walter Peter 
Published: November 2015

Abstract
Multiple myeloma (MM) remains an incurable disease afflicting more than 20,000 patients yearly in the US alone. Because of their secretory nature, MM cells rely on the proteostasis network and re-wire it to their advantage. We focused on two interconnected essential pathways within the proteostasis network to query genetic vulnerabilities yielding synthetic lethality. These pathways are the ubiquitin-proteasome system (UPS) and the unfolded protein response (UPR). Standard-of-care proteasome inhibitors, like bortezomib, remain non-curative in MM patients, suggesting the selection of genetic escape routes. The same rationale is applicable to the UPR, a conserved regulatory network overseeing the processing capacity of the endoplasmic reticulum (ER). Since proteasomal degradation of unfolded ER client proteins is accomplished after their dislocation and ubiquitination, we reasoned that common genetic escape routes might exist within the UPS and the UPR. Because single agents provide the driving force for acquired drug resistance and do not lead to long-term remission, we employed a systems-level shRNA screening method that systematically identified synthetic-lethal interactions that can enhance the therapeutic benefit of proteasome inhibition or modulation of the UPR. In the latter, we focused on the UPR branch overseen by the sensor kinase/nuclease IRE1 because it is thought to confer a survival advantage to MM. Applying chemical-genetics approaches using bortezomib or novel chemical inhibitors of IRE1, and an ultra-complex shRNA library we developed, we performed pooled shRNA screens in RPMI-8226 and MM1-S MM cells and identified genes whose diminished function impact the response to proteasome or IRE1 inhibition. Unlike proteasome inhibition, blocking IRE1 did not lead to considerable MM cell death, suggesting that IRE1 alone is not a fate determinant but a weakness that can be exploited through the identification of synthetic-lethal combinations. We applied the same rationale to proteasome inhibition and found synthetic-lethal pairs. Because some of these genes can be targeted pharmacologically, we explored the susceptibility of a panel of MM cells to combinations of bortezomib and drug-like molecules, including novel Hsp70 inhibitors. Retrospective analyses on publicly available gene expression datasets of MM patients treated with bortezomib indicated that several of the genes we found in our screen also predicted clinical outcomes. Together, our preliminary results indicate our approach is a powerful tool for the discovery of synthetic-lethal pairs that can be exploited in combination therapies.
Citation Format: Diego Acosta-Alvear, Martin Kampmann, Min Y. Cho, Blake T. Aftab, Xiaokai Li, Jason E. Gestwicki, Marc A. Shuman, Jonathan S. Weissman, Peter Walter. Identification of genetic vulnerabilities within the proteostasis network of multiple myeloma. [abstract]. In: Proceedings of the AACR Special Conference on Translation of the Cancer Genome; Feb 7-9, 2015; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(22 Suppl 1):Abstract nr A2-57.