Abstract 1614: Creation of malignant peripheral nerve sheath tumor models deficient for polycomb repressive complex 2 and identification of therapeutic vulnerabilities
Authors: Williams Kyle B. , Larsson Alex , Tibbits Justin , Moertel Christopher L. , Largaespada David A. 
Published: June 2022

Abstract
We constructed models of nerve sheath tumors that arise in Neurofibromatosis Type 1 (NF1) patients and conducted therapeutics discovery using synthetic lethal pharmacogenomic screens. Given both plexiform neurofibromas and malignant peripheral nerve sheath tumors (MPNSTs) arise within the Schwann cell lineage, we developed a drug discovery pipeline to identify targeted therapeutics for treating NF1-related neoplasia. Using CRISPR/Cas9, immortalized human Schwann cell lines were created lacking the NF1 gene or NF1 and components of Polycomb Repressive Complex 2 (PRC2), such as SUZ12. ~80% of all MPNST harbor loss of function mutations in PRC2 genes, which is highly suggestive that perturbation of epigenetic homeostasis plays a role in malignant transformation of neurofibromas.
Our models mimic the genetics of MPNSTs and allowed us to identify selectively lethal drugs which exploit vulnerabilities specific to these genetic drivers. We describe the identification of these novel drugs, extensive in vivo testing in models of MPNST, and mechanistic studies to explain why they act specifically on MPNSTs.
We identified compounds showing selective lethality towards NF1/SUZ12 double mutant cells. These include drugs affecting epigenetic homeostasis, such as HDAC inhibitors (HDACi). Moreover, many of these drugs showed strong synergy when tested in combination with a MEK inhibitor (MEKi) against NF1/SUZ12 deficient human Schwann and MPNST cell lines. We investigated the mechanism of this synergy using genetics, transcriptome, and proteome analysis. Interestingly, these mechanistic studies revealed possible emerging avenues of resistance the cells are utilizing to compensate for and survive therapeutic intervention. For instance, proteomics analysis of PRC2 deficient MPNST cell lines treated with combination therapy of MEKi and HDACi indicates activation of specific survival and stress response pathways. Targeting these pathways in conjunction with the therapeutics we have identified could prevent emergence of resistance and tumor escape.
Clinically interesting drug candidates were advanced and tested as single agents and in combination in multiple in vivo models of MPNST (including patient derived and cell line xenografts). The FDA approved drugs vorinostat (HDACi) and selumetinib (MEKi) have shown dramatic efficacy, with combination therapy exhibiting strong synergy in vivo. This includes durable responses in survival studies and the ability to dramatically shrink established tumors, with treated tumors showing markers of apoptosis.
The discovery of novel agents effective against several models of MPNST is exciting as there are currently no approved targeted therapies for MPNSTs. Our results implicate targeting of epigenetic homeostasis, in combination with MEKi, as a major vulnerability of MPNSTs deficient for PRC2 activity.
Citation Format: Kyle B. Williams, Alex Larsson, Justin Tibbits, Christopher L. Moertel, David A. Largaespada. Creation of malignant peripheral nerve sheath tumor models deficient for polycomb repressive complex 2 and identification of therapeutic vulnerabilities [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1614.