ERK signaling plays an essential role in the IFNg-mediated apoptosis of melanoma cells. [CRISPR screen]

Interferon-gamma (IFNg) exerts potent growth inhibitory (antiproliferative/pro-apoptotic) effects on a wide range of tumors, including melanoma. While several terminal effectors of cell cycle arrest and apoptosis have been identified, a definitive account of the signaling pathway leading to these events is lacking. We set up a chemical genomics screen and a whole genome targeting CRISPR/Cas9 screen in a growth inhibition (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, the gene-level analysis showed that the magnitude of enrichment for guide RNAs targeting ERK2 in IFNg-treated cells was comparable to core IFNg signaling proteins like IFNGR2, JAK1, JAK2, and STAT1. We validated the involvement of ERK by detecting a sustained increase in phospho-ERK1/2 (p-ERK) levels following IFNg treatment. In live imaging experiments, we found that an ERK inhibitor (ERKi) blocks the induction of cell death in 17 of 23 (~74%) IFNg-sensitive PDM lines covering all the MAPK mutant and triple wildtype molecular subtypes. Through gene expression analysis, we found that IFNg-ERK signaling triggered the expression of stress response genes and subsequent upregulation of DR5 and NOXA, which were both critical for the induction of cell death. In summary, the IFNg-ERK signaling axis mediates cell death downstream of IFNg treatment in melanoma cells. Our results provide a new understanding of the IFNg-mediated GI pathway that will also be crucial to define mechanisms of GI resistance in tumor cells. Overall design: Cas9-expressing M238 melanoma clonal line was transduced with either the full human GeCKO v2 or Brunello sgRNA libraries at an MOI =0.3. Transduced cells were selected with Puromycin for 7 days and treated with or without IFNg. Cells were harvested after 10-14 days and gDNA was isolated for library preparation. Human GeCKO library was used to perform one screen, while the Brunello library was used in two screens.