LYMPHOID NEOPLASIA RNA interference screening identi fi es lenalidomide sensitizers in multiple myeloma , including RSK 2

• High-throughput RNAi screening identified lenalidomide sensitizer genes, including RSK2, RAB, peroxisome, and potassium channel family members. • Knockdown or inhibition of RSK2 synergized with lenalidomide to induce myeloma cytotoxicity and downregulation of interferon regulatory factor 4 and MYC. To identify molecular targets that modify sensitivity to lenalidomide, we measured proliferation inmultiplemyeloma (MM) cells transfectedwith 27 968 small interferingRNAs in the presence of increasing concentrations of drug and identified 63 genes that enhance activity of lenalidomide upon silencing. Ribosomal protein S6 kinase (RPS6KA3orRSK2) was the most potent sensitizer. Other notable gene targets included 5 RAB family members, 3 potassium channel proteins, and 2 peroxisome familymembers. Single genes of interest included I-k-B kinase-a (CHUK), and a phosphorylation dependent transcription factor (CREB1), which associatewith RSK2 to regulate several signaling pathways. RSK2 knockdown induced cytotoxicity across a panel of MM cell lines and consistently increased sensitivity to lenalidomide. Accordingly, 3 small molecular inhibitors of RSK2 demonstrated synergy with lenalidomide cytotoxicity in MM cells even in the presence of stromal contact. Both RSK2 knockdown and small molecule inhibition downregulate interferon regulatory factor 4 andMYC, and provides an explanation for the synergy between lenalidomide and RSK2 inhibition. Interestingly, RSK2 inhibition also sensitized MM cells to bortezomib, melphalan, and dexamethasone, but did not downregulate Ikaros or influence lenalidomide-mediated downregulation of tumor necrosis factor-a or increase lenalidomide-induced IL-2 upregulation. In summary, inhibition of RSK2may prove a broadly useful adjunct to MM therapy. (Blood. 2015;125(3):483-491)