LAUSR

Interferon-gamma (IFNg) is critical for the success of immune checkpoint blockade therapy. Disruption of IFNg signaling makes tumors resistant to anti-PD-1/L1 treatment primarily by evasion of the growth inhibition (GI) induced by this cytokine. However, the pathways mediating IFNg-induced GI are largely unknown. We used two independent approaches to delineate the IFNg GI pathway, a chemical genomics screen and a whole genome targeting CRISPR/Cas9 screen in an IFNg GI-sensitive patient-derived melanoma (PDM) cell line. Drug screening revealed that treatment with RAF and ERK inhibitors disrupted IFNg GI in PDM cells. For the CRISPR screen, gene-level analysis showed that the magnitude of enrichment for guide RNAs targeting ERK2 in IFNg-treated cells was comparable to proximal IFNg signaling proteins like JAK1, STAT1, IFNGR2, and JAK2 (>26-fold enrichment for ERK2, p = 2.28e-07). Thus, results from both these experiments converged upon ERK as the primary downstream target of IFNg essential for GI induction. We validated the involvement of ERK by detecting a sustained increase in phospho-ERK1/2 levels following IFNg treatment. In live imaging experiments, we found that blocking ERK activity with the ERK inhibitor Ulixertinib blocks the induction of cell death after IFNg treatment in 17 of 23 (~74%) IFNg-sensitive PDM lines covering all the MAPK mutant and triple wildtype molecular subtypes of melanoma. Decreasing and increasing ERK activity levels using shRNA and overexpression approaches also led to a decrease and increase in IFNg-mediated GI, respectively. We have identified NOXA and DR5 as likely mediators of cell death through the IFNg-ERK GI pathway by comparing gene expression profiles of control and IFNg-treated PDM cells. Finally, phorbol 12-myristate 13-acetate (PMA), a known ERK activator, synergizes with IFNg treatment to induce increased GI in PDM cells (Bliss synergy score 16.87, p = 1.83e-123). In summary, the IFNg-ERK signaling axis mediates cell death downstream of IFNg treatment in melanoma cells. This pathway is active in all melanoma subtypes, making it an attractive target to enhance IFNg GI in tumor cells. Our results provide a new understanding of the IFNg GI pathway that will also be crucial to define mechanisms of GI resistance in tumor cells. Citation Format: Ameya S. Champhekar, Rachel Heymans, Cynthia Gonzalez, Guillem Turon Font, Daniel Karin, June Lee, Robert Damoiseaux, Antoni Ribas. CRISPR and drug screens identify ERK as the mediator of IFNg-induced melanoma growth inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 674.