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Aberrant RAS/MAPK signaling is a common driver of oncogenesis that can be therapeutically targeted with clinically approved MEK inhibitors. Disease progression on single-agent MEK inhibitors is common, however, and combination therapies are typically required to achieve significant clinical benefit in advanced cancers. Here we focused on identifying MEK inhibitor-based combination therapies in neuroblastoma with mutations that activate the RAS/MAPK signaling pathway, which are rare at diagnosis but frequent in relapsed neuroblastoma. A genome-scale CRISPR-Cas9 functional genomic screen was deployed to identify genes that when knocked out sensitize RAS-mutant neuroblastoma to MEK inhibition. Loss of either CCNC or CDK8, two members of the mediator kinase module, sensitized neuroblastoma to MEK inhibition. Furthermore, small-molecule kinase inhibitors of CDK8 improved response to MEK inhibitors in vitro and in vivo in RAS-mutant neuroblastoma and other adult solid tumors. Transcriptional profiling revealed that loss of CDK8 or CCNC antagonized the transcriptional signature induced by MEK inhibition. When combined, loss of CDK8 or CCNC prevented the compensatory upregulation of progrowth gene expression induced by MEK inhibition. These findings propose a new therapeutic combination for RAS-mutant neuroblastoma and may have clinical relevance for other RAS-driven malignancies. Significance: Transcriptional adaptation to MEK inhibition is mediated by CDK8 and can be blocked by the addition of CDK8 inhibitors to improve response to MEK inhibitors in RAS-mutant neuroblastoma, a clinically challenging disease.

Mutations in the RAS/MAPK pathway are frequent drivers of oncogenesis. Clinically approved MEK inhibitors are effectively used to target RAS-pathway driven cancers in combination with RAF inhibitors in BRAF mutant cancers such as melanoma and non-small cell lung cancer. However, single agent treatment with MEK inhibitors is not typically sufficient for clinical benefit. Therefore, combination therapy approaches will be required for maximal efficacy in most advanced cancers. Here, we sought to identify MEK inhibitor-based combination therapy approaches in the setting of RAS-mutant neuroblastoma. Neuroblastoma is a cancer that arises in the developing peripheral nervous system. Children with low-risk disease frequently benefit from therapy, but 50-60% of children with high-risk disease will ultimately relapse, and relapsed disease is generally incurable. We performed genome-scale functional genomic drug modifier screens and identified mediator kinase module members CCNC and CDK8 as two genes that when lost sensitize RAS-mutant NBL to MEK inhibition. We used small molecule inhibitors of CDK8 and found that combined inhibition of MEK and CDK8 leads to an improved therapeutic response in vitro and in vivo in a xenograft model of neuroblastoma. Using transcriptional profiling, we established that CDK8 loss and MEK inhibition induce opposite transcriptional signatures. When combined, CDK8 loss prevents the compensatory upregulation of the pro-growth gene expression program induced by MEK inhibition, while the downregulation of MAPK signaling is maintained. Finally, we show that this combination is effective in a subset of other RAS-mutant cancers including pancreatic and lung cancers. Together these data suggest that the mediator kinase module is critical for the adaptive pro-survival transcriptional response induced by MEK inhibition in RAS-mutant neuroblastoma and other RAS-driven malignancies, and that the addition of a CDK8 inhibitor may improve clinical response to MEK inhibitors. Our data supports a model where agents with antagonistic transcriptional programs can have synergistic therapeutic effects when combined, which may be a broadly relevant approach in combination therapies. Citation Format: Clare F. Malone, Minjee Kim, Gabriela Alexe, Alexandra B. Forman, Amanda Robichaud, Amy Saur Conway, Amy Goodale, John M. Hatcher, Nathanael S. Gray, Federica Piccioni, Kimberly Stegmaier. Transcriptional antagonism by CDK8 inhibition improves therapeutic efficacy of MEK inhibitors [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 2010.

We developed an RNA sequencing-based pipeline to discover differentially expressed cell surface molecules in neuroblastoma that meet criteria for optimal immunotherapeutic target safety and efficacy. Here we show that GPC2 is a strong candidate immunotherapeutic target in this childhood cancer. We demonstrate high GPC2 expression in neuroblastoma due to MYCN transcriptional activation and/or somatic gain of the GPC2 locus. We confirm GPC2 to be highly expressed on most neuroblastomas, but not detectable at appreciable levels in normal childhood tissues. Additionally, we demonstrate that GPC2 is required for neuroblastoma proliferation. Finally, we develop a GPC2 directed antibody-drug conjugate that is potently cytotoxic to GPC2-expressing neuroblastoma cells. Collectively, these findings validate GPC2 as a non-mutated neuroblastoma oncoprotein and candidate immunotherapeutic target.

NF1 encodes a RAS GTPase-activating protein. Accordingly, aberrant RAS activation underlies the pathogenesis of NF1-mutant cancers. Nevertheless, it is unclear which RAS pathway components represent optimal therapeutic targets. Here, we identify mTORC1 as the key PI3K effector in NF1-mutant nervous system malignancies and conversely show that mTORC2 and AKT are dispensable. However, we find that tumor regression requires sustained inhibition of both mTORC1 and MEK. Transcriptional profiling studies were therefore used to establish a signature of effective mTORC1–MEK inhibition in vivo. We unexpectedly found that the glucose transporter GLUT1 was potently suppressed, but only when both pathways were inhibited. Moreover, unlike VHL- and LKB1-mutant cancers, reduction of 18F-FDG uptake required the suppression of both mTORC1 and MEK. Together, these studies identify optimal and suboptimal therapeutic targets in NF1-mutant malignancies and define a noninvasive means of measuring combined mTORC1–MEK inhibition in vivo, which can be readily incorporated into clinical trials. Significance: This work demonstrates that mTORC1 and MEK are key therapeutic targets in NF1-mutant cancers and establishes a noninvasive biomarker of effective, combined target inhibition that can be evaluated in clinical trials. Cancer Discov; 4(9); 1062–73. ©2014 AACR. This article is highlighted in the In This Issue feature, p. 973

Luminal breast cancers are typically estrogen receptor–positive and generally have the best prognosis. However, a subset of luminal tumors, namely luminal B cancers, frequently metastasize and recur. Unfortunately, the causal events that drive their progression are unknown, and therefore it is difficult to identify individuals who are likely to relapse and should receive escalated treatment. Here, we identify a bifunctional RasGAP tumor suppressor whose expression is lost in almost 50% of luminal B tumors. Moreover, we show that two RasGAP genes are concomitantly suppressed in the most aggressive luminal malignancies. Importantly, these genes cooperatively regulate two major oncogenic pathways, RAS and NF-κB, through distinct domains, and when inactivated drive the metastasis of luminal tumors in vivo. Finally, although the cooperative effects on RAS drive invasion, NF-κB activation triggers epithelial-to-mesenchymal transition and is required for metastasis. Collectively, these studies reveal important mechanistic insight into the pathogenesis of luminal B tumors and provide functionally relevant prognostic biomarkers that may guide treatment decisions. Significance: The lack of insight into mechanisms that underlie the aggressive behavior of luminal B breast cancers impairs treatment decisions and therapeutic advances. Here, we show that two RasGAP tumor suppressors are concomitantly suppressed in aggressive luminal B tumors and demonstrate that they drive metastasis by activating RAS and NF-κB. Cancer Discov; 7(2); 202–17. ©2016 AACR. See related commentary by Sears and Gray, p. 131. This article is highlighted in the In This Issue feature, p. 115

Pediatric cancers, such as neuroblastoma, have a relative lack of oncogenic mutations and the known drivers of these cancers are largely transcription factors, a target class notoriously difficult to “drug.” In order to identify novel therapeutic targets for high-risk, MYCN-amplified neuroblastoma, we employed functional genomic screening using a CRISPR-Cas9 approach. We generated genome-scale CRISPR dependency data to identify a candidate gene list of 197 putative genetic dependencies in neuroblastoma. We next created a focused sgRNA library targeting these genes and performed time-course dropout screens using CRISPR and CRISPRi and Annexin-V-based positive-selection cell death screens in four MYCN-amplified neuroblastoma cell lines. We also screened this library in vivo in two xenograft models of MYCN-amplified neuroblastoma. At the intersection of these screens, we identified the nuclear export factor NXT1 as a top candidate for therapeutic development in neuroblastoma. NXT1 scores as a strong dependency using both CRISPR and CRISPRi, is strongly enriched in our Annexin-V based positive selection cell death screen and scores in our in vivo screen. We have validated that NXT1 is indeed a genetic dependency in neuroblastoma in low-throughput and that NXT1 loss induces apoptosis in these cell lines. We were next interested in identifying biomarkers of dependency on NXT1, and by integrating RNAseq data with CRISPR dependency data, we identified low expression of NXT2, a paralog of NXT1, as the top predictive feature of NXT1 dependency. Importantly, while NXT1 itself is not currently druggable, inhibitors against another nuclear export protein, CRM1, are currently in Phase II clinical trials, demonstrating this class of proteins has the potential to be effectively drugged. Together, we show that CRISPR-Cas9 functional screens can be used to identify new therapeutic targets, particularly in diseases that lack “druggable” oncogenic drivers, such as many pediatric cancers. Citation Format: Clare F. Malone, Neekesh V. Dharia, Guillaume Kugener, Brenton Paolella, Michael Rothberg, Mai Abdusamad, Alfredo Gonzalez, Nancy Dumont, Scott Younger, David Root, Francisca Vazquez, Kimberly Stegmaier. CRISPR-Cas9 screens identify the nuclear export factor NXT1 as a novel therapeutic target in MYCN-amplified neuroblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2878.

Many children with metastatic or recurrent pediatric solid tumors continue to have poor survival, and there is an immense need to identify novel therapeutic approaches. Moreover, these cancers typically have simple genomes with limited known druggable molecular events. In order to discover new vulnerabilities in pediatric solid tumors, we have performed genome-scale CRISPR-Cas9 loss-of-function screening and deep “omic” characterization in over 60 pediatric cancer cell lines to date, including neuroblastoma, medulloblastoma, Ewing sarcoma, malignant rhabdoid tumor and rhabdomyosarcoma lines, to begin defining a pediatric cancer dependency map. Global analyses of the pediatric dependency landscape have identified emerging classes of pediatric cancers, including epigenetic-driven, aberrant transcription factor-driven and receptor tyrosine kinase-driven malignancies. For example, the preferential dependencies identified in a subset of neuroblastoma, which has aberrantly high expression of the transcription factor MYCN, are highly enriched for an interconnected network of genes annotated to have transcription factor activity. In addition to the global evaluation, we have developed methods and tools for prioritizing targets for further validation within a cancer type. These tools computationally integrate the pediatric dependency data across multiple datasets to identify categories of genetic dependencies that are especially strong hits or enriched hits in a specific pediatric malignancy. As an example, the intersection of MYCN-amplified neuroblastoma specific dependencies and H3-lysine 27 acetylation (H3K27ac) profiling across MYCN-amplified neuroblastoma allowed us to identify a transcriptional core regulatory circuit (CRC) that may drive the malignant state. Furthermore, targeting transcription with the BRD4 inhibitor JQ1 and CDK7 inhibitor THZ1 caused synergistic killing of neuroblastoma cells suggesting a novel therapeutic approach to treating this disease. Thus, defining a comprehensive pediatric cancer dependency map and developing the methods and tools to prioritize vulnerabilities in different cancer types will allow us to discover both novel biology and new therapeutic opportunities in childhood malignancies. Citation Format: Neekesh V. Dharia, Clare Malone, Amanda Balboni Iniguez, Lillian Guenther, Liying Chen, Gabriela Alexe, Adam D. Durbin, Mark W. Zimmerman, Andrew Hong, Pratiti Bandopadhayay, Mariella G. Filbin, Thomas Howard, Brenton Paolella, Iris Fung, Josephine Lee, Phil Montgomery, John M. Krill-Burger, Brian J. Abraham, Jennifer Roth, David E. Root, Richard A. Young, A. Thomas Look, Rameen Beroukhim, Jesse S. Boehm, William C. Hahn, Todd R. Golub, Aviad Tsherniak, Francisca Vazquez, Kimberly Stegmaier. Defining a pediatric cancer dependency map [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2352.

SummaryRas-driven tumors are often refractory to conventional therapies. Here we identify a promising targeted therapeutic strategy for two Ras-driven cancers: Nf1-deficient malignancies and Kras/p53 mutant lung cancer. We show that agents that enhance proteotoxic stress, including the HSP90 inhibitor IPI-504, induce tumor regression in aggressive mouse models, but only when combined with rapamycin. These agents synergize by promoting irresolvable ER stress, resulting in catastrophic ER and mitochondrial damage. This process is fueled by oxidative stress, which is caused by IPI-504-dependent production of reactive oxygen species, and the rapamycin-dependent suppression of glutathione, an important endogenous antioxidant. Notably, the mechanism by which these agents cooperate reveals a therapeutic paradigm that can be expanded to develop additional combinations.