Highlights 28 • NF-kB and MYC co-activation originates (pre)plasmablast-like cancer 29 • NF-kB/MYC + renders cancer cells addicted to IL6 for survival and phenotypic stability 30 • NF-kB/MYC + cancers are alike a fraction of human plasmablastic lymphoma 31 • t(8;14)[MYC-IGH] multiple myeloma is linked to a NF-kB/MYC co-activation signature 32 33 2 Summary 34 NF-kB and MYC are found co-deregulated in human B and plasma-cell cancers. In physiology, NF-kB is 35 necessary for terminal B-to-plasma cell differentiation, whereas MYC repression is required. It is thus 36 unclear if NF-kB/MYC co-deregulation is developmentally compatible in carcinogenesis and/or impacts 37 cancer cell differentiation state, possibly uncovering unique sensitivities. Using a mouse system to trace 38 cell lineage and oncogene activation we found that NF-kB/MYC co-deregulation originated cancers with a 39 plasmablast-like phenotype, alike human plasmablastic-lymphoma and was linked to t(8;14)[MYC-40 IGH] multiple myeloma. Notably, in contrast to NF-kB or MYC activation alone, co-deregulation rendered 41 cells addicted to IL6 for survival and phenotypic stability. We propose that conflicting oncogene-driven 42 differentiation pressures can be accommodated at a cost in poorly-differentiated cancers. 43 44 Significance 45 Our studies improve the understanding of cancer pathogenesis by demonstrating that co-deregulation of 46 NF-kB and MYC synergize in forming a cancer with a poorly-differentiated state. The cancers in the mouse 47 system share features with human Plasmablastic lymphoma that has a dismal prognosis and no standard of 48 care, and with t(8;14)[MYC-IGH] Multiple myeloma, which is in overall resistant to standard therapy. 49 Notably, we found that NF-kB and MYC co-deregulation uniquely render cells sensitive to IL6 deprivation, 50 providing a road-map for patient selection. Because of the similarity of the cancers arising in the compound 51 mutant mouse model with that of human Plasmablastic lymphoma and t(8;14)[MYC-IGH] Multiple 52 myeloma, this model could serve in preclinical testing to investigate novel therapies for these hard-to-treat 53 diseases. 54 55 Keywords 56 NF-kB, MYC, IL6, B-cell, plasma-cell, plasmablast, B-cell terminal differentiation, Diffuse large B-cell 57 lymphoma, plasmablastic lymphoma, multiple myeloma, phenotypic stability 58 59 60 61 62 63 64 65 66 67 90 Tornatore et al., 2014). 91 92 The knowledge that NF-kB directly induces the expression of genes essential in B-to-plasma cell 93 differentiation and that ABC-DLBCL cancer cells despite being mature B-cells display features of 94 plasmacytic differentiation suggest that a block of B-to-plasma cell is required in the pathogenesis of ABC-95 DLBCL. Consistently, the activity of BLIMP1, key for B-to-plasma cell differentiation, is lost exclusively 96 in DLBCL of the ABC subtype through BLIMP1 genetic aberrations (∼30% of cases), and indirectly by 97 deregulated BCL6 expression from chromosomal translocations (∼26% of cases) (Mandelbaum et al.,98 2010; Pasqualuc...