A genome-scale CRISPR-Cas9 screening in myeloma cells identifies regulators of immunomodulatory drug sensitivity

Immunomodulatory drugs (IMiDs) including lenalidomide and pomalidomide bind cereblon (CRBN) and activate the CRL4 CRBN ubiquitin ligase to trigger proteasomal degradation of the essential transcription factors IKZF1 and IKZF3 and multiple myeloma (MM) cytotoxicity. We have shown that CRBN is also targeted for degradation by SCF Fbxo7 ubiquitin ligase. In the current study, we explored the mechanisms underlying sensitivity of MM cells to IMiDs using genome-wide CRISPR-Cas9 screening. We validate that CSN9 signalosome complex, a deactivator of Cullin-RING ubiquitin ligase, inhibits SCF Fbxo7 E3 ligase-mediated CRBN degradation, thereby conferring sensitivity to IMiDs; conversely, loss of function of CSN9 signalosome activates SCF Fbxo7 complex, thereby enhancing degradation of CRBN and conferring IMiD resistance. Finally, we show that pretreatment with either proteasome inhibitors or NEDD8 activating enzyme (NAE) inhibitors can abrogate degradation and maintain levels of CRBN, thereby enhancing sensitivity to IMiDs. These studies therefore demonstrate that CSN9 signalosome complex regulates sensitivity to IMiDs by modulating CRBN expression.