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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.

NXT1 is a selective and lethal dependency in neuroblastoma

NXT2 expression is not associated with neuroblastoma disease subtypes

NXT2 expression is associated with NXT1 dependency

CRISPR guide sequences

Differential NXT2 expression in neuroblastoma cell lines

CRISPR screens prioritize rapidly lethal in vivo relevant dependencies

NXT2 loss leads to loss of NXF1

Cancer dependency maps, which use CRISPR/Cas9 depletion screens to profile the landscape of genetic dependencies in hundreds of cancer cell lines, have identified context-specific dependencies that could be therapeutically exploited. An ideal therapy is both lethal and precise, but these depletion screens cannot readily distinguish between gene effects that are cytostatic or cytotoxic. Here, we use a diverse panel of functional genomic screening assays to identify NXT1 as a selective and rapidly lethal in vivo relevant genetic dependency in MYCN-amplified neuroblastoma. NXT1 heterodimerizes with NXF1, and together they form the principal mRNA nuclear export machinery. We describe a previously unrecognized mechanism of synthetic lethality between NXT1 and its paralog NXT2: their common essential binding partner NXF1 is lost only in the absence of both. We propose a potential therapeutic strategy for tumor-selective elimination of a protein that, if targeted directly, is expected to cause widespread toxicity.Significance:We provide a framework for identifying new therapeutic targets from functional genomic screens. We nominate NXT1 as a selective lethal target in neuroblastoma and propose a therapeutic approach where the essential protein NXF1 can be selectively eliminated in tumor cells by exploiting the NXT1–NXT2 paralog relationship.See related commentary by Wang and Abdel-Wahab, p. 2129.This article is highlighted in the In This Issue feature, p. 2113

Supplemental Figures S1-S6 and Legends

Supplementary Table S1 STARS Scores

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.

Supplementary Table S3 Gene Sets

Cancer dependency maps, which use CRISPR/Cas9 depletion screens to profile the landscape of genetic dependencies in hundreds of cancer cell lines, have identified context-specific dependencies that could be therapeutically exploited. An ideal therapy is both lethal and precise, but these depletion screens cannot readily distinguish between gene effects that are cytostatic or cytotoxic. Here, we use a diverse panel of functional genomic screening assays to identify NXT1 as a selective and rapidly lethal in vivo relevant genetic dependency in MYCN-amplified neuroblastoma. NXT1 heterodimerizes with NXF1, and together they form the principal mRNA nuclear export machinery. We describe a previously unrecognized mechanism of synthetic lethality between NXT1 and its paralog NXT2: their common essential binding partner NXF1 is lost only in the absence of both. We propose a potential therapeutic strategy for tumor-selective elimination of a protein that, if targeted directly, is expected to cause widespread toxicity. Significance: We provide a framework for identifying new therapeutic targets from functional genomic screens. We nominate NXT1 as a selective lethal target in neuroblastoma and propose a therapeutic approach where the essential protein NXF1 can be selectively eliminated in tumor cells by exploiting the NXT1–NXT2 paralog relationship. See related commentary by Wang and Abdel-Wahab, p. 2129. This article is highlighted in the In This Issue feature, p. 2113

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.