PROJECT SUMMARY Amplification of 1q21 defines one of the most common multiple myeloma (MM) subtypes with an adverse prognosis. The 1q21 amplicon contains many genes, and while it is unlikely that all contribute to the pathobiology of high-risk MM, the critical genes that do drive this high-risk phenotype have not yet been fully clarified. Identifying such genes and their contributions to this phenotype would enable the development of new and effective targeted therapy strategies for high-risk MM and thus improve their survival outcomes. In this application, we propose to investigate the biological and molecular mechanisms behind the 1q21 amplification's contribution to high-risk MM with the ultimate goal of obtaining a list of validated therapeutic targets to inform the design of novel translational clinical trials for this subgroup of patients. In preliminary functional genomic studies, we identified five 1q21 essential genes whose loss results in MM cell death and/or growth inhibition. Our initial mechanistic studies of one of these genes, ILF2 (interleukin enhancer binding factor 2), suggest that the gene is involved in the regulation of the DNA damage response and mediates drug resistance to genotoxic agents in a dose-dependent manner, which may explain why 1q21 MM patients benefit less from high-dose chemotherapy than non?1q21 MM patients do. On the basis of these data, we hypothesize that the five 1q21 gene candidates we have identified make essential contributions to the high-risk MM phenotype and that the preclinical validation of these critical 1q21 vulnerabilities will yield novel therapeutic targets for these patients. To test these hypotheses, we will pursue the following three specific aims: 1) To dissect the molecular mechanisms underlying 1q21 gene candidates' contributions to the high-risk MM phenotype, we will subject these candidates to in vitro functional validation studies using a panel of genomically characterized human MM cell lines, primary MM samples, and xenograft models. 2) To identify 1q21 gene candidates' roles in MM pathogenesis in vivo, we will employ a novel, physiologically relevant genetic approach that targets these genes' expression in the germinal center cells, considered to be the cell-of- origin of MM plasma cells. 3) To determine the feasibility of therapeutically targeting ILF2, we will collaborate with IONIS Pharmaceuticals to develop antisense oligonucleotides targeting ILF2 and functionally validate their effectiveness both alone and in combination with DNA-damaging agents in inhibiting MM growth/progression in preclinical xenograft mouse models of disseminated MM. We anticipate that the proposed study will not only expand our understanding of 1q21 genes' contributions to MM pathobiology but also inform the development of new targeted approaches to improve the outcomes of MM patients who have high-risk disease that is refractory to our approved therapeutic agents.