Abstract 4631: SCFFbxo7 ubiquitin ligase targets cereblon to limit the anti-myeloma activity of IMiDs

Multiple myeloma (MM) patients have witnessed significantly improved survival after the approved treatment with the proteasome inhibitor bortezomib and immunomodulatory drugs (IMiDs) including thalidomide and its derivatives lenalidomide and pomalidomide. IMiDs directly bind cereblon (CRBN), an adaptor of the Cul4-DDB1 (CRL4) ubiquitin ligase, and activate CRL4CRBN to target B cell-specific transcription factors IKZF1 and IKZF3 for ubiquitination. IKZF1 and IKZF3 are essential for MM cell survival and their proteasomal degradation induced by IMiDs provides insights into how these drugs work. Paradoxically, IMiDs achieve better clinical response in MM patients when combined with bortezomib, which blocks IKZF1 and IKZF3 turnover and would in principle counteract IMiDs therapy. We performed a CRISPR-based genome-wide knockout screening for regulators of pomalidomide sensitivity in MM cells, and found that CRBN is inherently unstable and its level is controlled by the COP9 signalosome (CSN) to sustain IMiDs cytotoxicity. When unbound to CRL4, CRBN is recruited by the F-box protein Fbxo7 to the Cul1-Skp1 (SCF) ubiquitin ligase for ubiquitination and degradation, a process suppressed by CSN-mediated de-neddylation of Cul1. Deletion of SCFFbxo7 components, or treatment with bortezomib or neddylation inhibitor MLN4924 stabilizes CRBN, and IMiDs further promote the loading of CRBN to CRL4 by inhibiting SCFFbxo7-targeted CRBN degradation, leading to enhanced turnover of IKZF1 and IKZF3 in MM cells and synergistic inhibition of MM cell proliferation. Our findings unravel a dual role of IMiDs in activating CRL4CRBN E3 ligase in MM cells and provide a framework to select patients most susceptible to bortezomib and IMiDs combination therapy. Citation Format: Jiye Liu, Wenrong Zhou, Yan Feng, Tianyu Song, Shaobing Gao, Junfeng Wu, Zhenggang Peng, Yong Cang. SCFFbxo7 ubiquitin ligase targets cereblon to limit the anti-myeloma activity of IMiDs. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4631.