Abstract 5489: Targeting MYC in multiple myeloma by BET protein inhibition
Authors: Riggs Daniel L. , Chesi Marta , Bergsagel P Leif 
Published: May 2015

Abstract
The numerous and diverse genetic lesions in multiple myeloma (MM) make it difficult to discern those genetic abnormalities that may be suitable therapeutic targets. We have observed that the MYC locus is frequently rearranged in MM (about half of both patients and cell lines), suggesting that MYC dysregulation is a common feature of this disease. To evaluate MYC suppression as a therapeutic strategy in MM, we treated human myeloma cell lines with CPI203, a potent and selective inhibitor of bromodomain and extra-terminal (BET) proteins. The bromodomain inhibitor CPI203 has potency and selectivity similar to that of JQ1, but has improved pharmacokinetics for in vivo studies. From our collection of MM cell lines we have identified sensitive and resistant cell lines and characterized their response to CPI203 in detail. A number of conclusions have emerged from this work. First, response correlates with MYC protein levels after 3 days of treatment. MYC levels are significantly suppressed in sensitive lines and much less affected in resistant lines, supporting the hypothesis that MYC suppression alone determines cell fate in MM cell lines. In sensitive cell lines treatment with CPI203 caused a rapid (within 90 min.) decrease in MYC transcription that is consistent with the response being an intrinsic property of the cell lines. Inhibition of MYC transcription was followed by eventual cell cycle arrest and limited apoptosis in these sensitive cell lines. Both sensitive and resistant cell lines exhibited induction of Histone2H2BE gene in response to CPI203 treatment, indicating that drug efflux pumps alone do not mediate resistance. Surprisingly, several of the resistant cell lines exhibited an initial phase of sensitivity to BET inhibition. Upon drug addition, MYC transcription and MYC protein levels are repressed 2- to 5-fold, followed by a gradual recovery of MYC message and protein levels to the initial levels. In this second phase the cells are truly insensitive to BET inhibition: the drug remains active during this incubation and addition of fresh medium with drug does not affect MYC transcription. Furthermore, many aspects of the BET inhibition gene expression profile remain constant throughout this transition between sensitivity and resistance. These results suggest that the recovery phase is not a simple reversal of BET inhibition but rather the emergence of an alternative-signaling pathway that activates MYC expression in the presence of BET inhibition. Presently we are analyzing gene expression data to identify genes and signaling pathways responsible for this “induced resistance”. The identification of such pathways will help us better understand the molecular basis of the response to BET inhibition, which in turn might enable us to identify predictive biomarkers of the therapeutic response.
Citation Format: Daniel L. Riggs, Marta Chesi, P Leif Bergsagel. Targeting MYC in multiple myeloma by BET protein inhibition. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5489. doi:10.1158/1538-7445.AM2014-5489