The precise molecular mechanism of action and targets through which thalidomide and related immunomodulatory drugs (IMiDs) exert their antitumor effects remains unclear. We investigated the role of cereblon (CRBN), a primary teratogenic target of thalidomide, in the antimyeloma activity of IMiDs. CRBN depletion is initially cytotoxic to human myeloma cells, but surviving cells with stable CRBN depletion become highly resistant to both lenalidomide and pomalidomide, but not to the unrelated drugs bortezomib, dexamethasone, and melphalan. Acquired deletion of CRBN was found to be the primary genetic event differentiating isogenic MM1.S cell lines cultured to be sensitive or resistant to lenalidomide and pomalidomide.

No abstract

• The incidence of mutations within the MAPK pathway, the CRBN pathway, and TP53 is significantly increased in drug-refractory MM.• Mutations in CRBN might contribute to IMiD resistance in drug-refractory MM.In this study, targeted sequencing to screen 50 multidrug refractory multiple myeloma (rMM) patients was performed by using the Multiple Myeloma Mutation Panel. Patients were pretreated with both immunomodulatory drugs (IMiDs) and proteasome inhibitors (PIs), and 88%, 78%, and 68% were refractory to an IMiD, a PI, or both, respectively. The majority of patients had progressive (82%) or refractory (78%) disease immediately before sampling, with 43% being IMiD refractory and 46% being PI refractory in the most recent line of therapy. Compared with newly diagnosed MM, an increased prevalence of mutations in the Ras pathway genes KRAS, NRAS, and/or BRAF (72%), as well as TP53 (26%), CRBN (12%), and CRBN pathway genes (10%) was observed. Longitudinal analyses performed in 3 patients with CRBN mutations at time of IMiD resistance confirmed that these mutations were undetectable at earlier, IMiD-sensitive time points. Furthermore, the functional introduction of these mutations in MM cells conferred lenalidomide resistance in vitro. These data indicate a differential genetic landscape in rMM associated with drug response. (Blood. 2016;128(9):1226-1233

Cereblon (CRBN) mediates immunomodulatory drug (IMiD) action in multiple myeloma (MM). We demonstrate here that no patient with very low CRBN expression responded to IMiD plus dexamethasone therapy. In 53 refractory MM patients treated with pomalidomide and dexamethasone, CRBN levels predict for decreased response rates and significant differences in PFS (3.0 vs. 8.9 months, p < 0.001) and OS (9.1 vs. 27.2 months, p = 0.01) (lowest quartile vs. highest three quartiles). While higher CRBN levels can serve as a surrogate for low risk disease, our study demonstrates that low CRBN expression can predict resistance to IMiD monotherapy and is a predictive biomarker for survival outcomes.

The molecular target(s) cooperating with proteasome inhibition in multiple myeloma (MM) remain unknown. We therefore measured proliferation in MM cells transfected with 13 984 small interfering RNAs in the absence or presence of increasing concentrations of bortezomib. We identified 37 genes, which when silenced, are not directly cytotoxic but do synergistically potentiate the growth inhibitory effects of bortezomib. To focus on bortezomib sensitizers, genes that also sensitized MM to melphalan were excluded. When suppressed, the strongest bortezomib sensitizers were the proteasome subunits PSMA5, PSMB2, PSMB3, and PSMB7 providing internal validation, but others included BAZ1B, CDK5, CDC42SE2, MDM4, NME7, RAB8B, TFE3, TNFAIP3, TNK1, TOP1, VAMP2, and YY1. The strongest hit CDK5 also featured prominently in pathway analysis of primary screen data. Cyclin IntroductionBortezomib is a potent and selective proteasome inhibitor used in the treatment of multiple myeloma (MM) and low-grade lymphoma patients. Bortezomib produces significant clinical responses in both newly diagnosed and advanced MM, but only 40% of patients respond to the single agent, 1,2 and the majority of these patients become resistant over time. The mechanism of antimyeloma activity of bortezomib is therefore a subject of intense study with inhibition of the proteasome, autophagy, and activation of the unfolded protein stress response pathway apparently critical. A downstream consequence of proteasome inhibition relevant to MM is blockade of nuclear factor B (NF-B) activity, which may partly mediate bortezomib activity in MM because activating mutations in the NF-B pathway were identified in at least 17% of MM patients and 41% of human myeloma cell lines and appear to mediate accelerated growth and survival of malignant plasma cells. [3][4][5] However, the 35% to 50% response rate to bortezomib cannot be totally interpreted by NF-B abnormality, suggesting that other specific molecular targets, resistance mechanisms, or perhaps unique pharmacokinetics are inherent in patients. Furthermore, resistance to bortezomib therapy often develops even in initially sensitive patients; and although certain mechanisms such as mutations in proteasome subunits have been postulated, 6 the underlying mechanism defining this nonresponsiveness is largely unknown. Understanding the cooperating mechanisms of sensitivity to proteasome inhibition will not only allow more targeted use of proteasome inhibitors but should also make it possible to rationally design synergistic drug combinations and predict patient response to therapy.To begin to address these issues, a druggable genome RNAi screen was used to identify modifiers of bortezomib sensitivity in human MM cells. Through this high-throughput screen, we identified a panel of genes whose loss of expression enhances bortezomib sensitivity (sensitizer). Using shRNA expression systems and a small-molecule inhibitor, we have further validated one of the most potent bortezomib sensitizer genes as cyclin-dependent kinase 5 (...