CDK4/6 Inhibition Reverses MEIS2 Suppression of CRL4<CRBN/SUP> to Enhance Immunomodulatory Drug Therapy in Multiple Myeloma

MEIS2 was identified biochemically as a substrate of cereblon (CRBN), a receptor of the CRL4CRBN E3 ubiquitin ligase required for the anti-myeloma activity of immunomodulatory drugs (IMiD)s and CELMoDs. However, its function in myeloma is unknown. We discovered that MEIS2 is aberrantly expressed in bone marrow myeloma cells (BMMC)s, and that high MEIS2, CDK4 or CDK6 and low CRBN expression predisposes patients to inferior overall survival in IMiD therapy. Inhibition of CDK4/6 (CDK4/6i) reprograms BMMCs for IMiD vulnerability ex vivo that mimics patients response to IMiDs. Mechanistically, CDK4/6i both rapidly accelerates the displacement of MEIS2 from CRBN by IMiD and destabilizes the MEIS2 protein while increasing the CRBN protein in cooperation with IMiD and CELMoD. This enhances CRL4CRBN ubiquitination of IKZF3 and IKZF1 for degradation that exacerbates the loss of IRF4 to relieve IRF7 for induction of the interferon response, culminating in TRAIL-mediated apoptosis. Additionally, MEIS2 promotes BCMA expression and antagonizes repression of BCMA by IMiD and CELMoD for survival of myeloma cells. Thus, CDK4/6i reverses MEIS2 inhibition of CRL4CRBN in cooperation with IMiD and CELMoD, and mitigates MEIS2-mediated BCMA signaling for survival, suggesting targeting MEIS2 and CDK4/6 as a new strategy to advance immunomodulatory drug therapy in multiple myeloma.