Abstract 2300: CDK4/6 inhibition triggers dissociation of MEIS2 from CRL4CRBN E3 ligase to induce interferon response and reprogram myeloma for IMiD vulnerability
Authors: Huang Xiangao , Jayabalan David , Liberto Maurizio Di , Chen Zhengming , Schinzel Anna , Ely Scott A. , Rossi Adriana , Coleman Morton , Lane Joseph M. , Hahn William C. , Mark Tomer M. , Niesvizky Ruben , Chen-Kiang Selina 
Published: August 2018

Abstract
Inhibition of CDK4/6 has emerged as an effective therapy in human cancer. Palbociclib, the first selective CDK4/6 inhibitor (CDK4/6i), is FDA-approved for treatment of metastatic breast cancer, and abemaciclib has been fast tracked for approval as a front line breast cancer treatment. Recent evidence in mouse models of breast cancer further indicates that CDK4/6 inhibition triggers anti-tumor immunity, but the precise tumor intrinsic mechanism that mediates CDK4/6 inhibition for a clinical response remains to be defined. To address this, we inhibited CDK4/6 with palbociclib in sequential combination with immunomodulary drugs (IMiD)s in primary bone marrow myeloma cells (BMMCs), using normal bone marrow plasma cells as a control. IMiDs, lenalidomide (Len) and pomalidomide (Pom), are standard of care for multiple myeloma (MM) and lymphoma. Each rarely achieves complete remission despite a high overall response rate and durability. However, each markedly potentiates the clinical efficacy of diverse partners by unknown mechanisms. Cereblon (CRBN), a substrate receptor of the CUL4-ROC1-DDB1-CRBN (CRL4CRBN) E3 ubiquitin ligase, is requited for IMiD's anti-tumor activity. Binding of Len or Pom to CRBN accelerates the recruitment of transcription factors IKZF1/3 to CRL4CRBN for ubiquitination and degradation in MM cells. One of the targets of IKZF1/3 is IRF4, which is required for survival of MM cells and reduced after exposure to Len or Pom. Moreover, MEIS2, a homeobox transcription factor was identified to be an substrate of CRBN in crystal structure and by biochemical screen, implicating a role for MEIS2 in modulates IMiD's anti-tumor activity. However, evidence for MEIS2 expression in myeloma cells is lacking. Previously, we have shown that prolonged early G1 arrest (pG1) induced by sustained CDK4/6 inhibition reprogrammed cancer cells for vulnerability to diverse clinically relevant agents ex vivo and in animal models. To address if CDK4 inhibition reprograms MM cells for IMiD vulnerability, first we show that the intrinsic IMiD sensitivity in primary BMMCs ex vivo is a major determinant for the clinical response to IMiD therapy in myeloma patients. Second, we discovered that MEIS2 is aberrantly expressed and associated with CRBN in BMMCs. Third, MEIS2 is regulated by the cell cycle in primary MM cells and required for the survival of MM cells in part by maintaining IRF4 expression. Finally, CDK4/6 inhibition liberates CRBN from association with MEIS2 in pG1. This rapidly accelerates Len-mediated ubiquitination and degradation of IKZF1/3, loss of IRF4, de-repression of IRF7 and induction of interferon (IFN) responsive genes that culminated in TRAIL-mediated apoptosis. Dissociation of MEIS2 from CRBN thus represents a noel mechanism by which CDK4/6 inhibition reprograms myeloma for vulnerability to IMiDs through induction anti-tumor IFN response.
Citation Format: Xiangao Huang, David Jayabalan, Maurizio Di Liberto, Zhengming Chen, Anna Schinzel, Scott A. Ely, Adriana Rossi, Morton Coleman, Joseph M. Lane, William C. Hahn, Tomer M. Mark, Ruben Niesvizky, Selina Chen-Kiang. CDK4/6 inhibition triggers dissociation of MEIS2 from CRL4CRBN E3 ligase to induce interferon response and reprogram myeloma for IMiD vulnerability [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2300.