Abstract B17: Dexamethasone synthetic lethal screens uncover two potential combination therapies for the treatment of multiple myeloma
Authors: Roulston Anne , Bernier Cynthia , Nguyen Mai , Ertel Franziska , Robert Francis , Bellomo Maryanne , Huang Xian Fang , Bramoullé Alexandre , Gravel Michel , Sebag Michael , Pelletier Jerry , Shore Gordon 
Published: August 2014

Abstract
Multiple myeloma (MM) is an incurable malignancy of plasma B lymphoid cells that occurs in the bone marrow. Treatment usually involves autologous stem cell transplantation followed by various combinations of chemotherapies all of which include dexamethasone (DEX) in first-line as well as end-stage therapies. DEX is a steroid hormone analog that binds to the glucocorticoid nuclear receptor and acts by differentially regulating the expression of genes involved in inflammation, and cell death. Despite the utility of glucocorticoids, resistance is common and MM remains an incurable disease.
Using a pooled shRNA screening approach, MCL1 was identified as a gene whose knock-down confers increased sensitivity to dexamethasone. MCL-1 is an anti-apoptotic Bcl-2 family member often over-expressed in multiple cancers including MM, and has been linked with resistance to chemotherapy. Obatoclax is a pan BCL-2 family inhibitor that potently disrupts MCL-1/BAK interactions and is highly cytotoxic to MCL-1 dependent cell lines. The combination of dexamethasone with obatoclax results in a synergistic inhibition of MM cell line growth. The synergy is most pronounced when cells are pre-treated for 48h with dexamethasone suggesting the involvement of gene regulation in the combination therapy. BIM, a pro-apoptotic MCL-1 binding partner, is induced in multiple myeloma cells in vitro and in vivo in the Vk*myc murine MM model following dexamethasone treatment. BIM induction is important for BAK and BAX oligomerization at the mitochondrial membrane initiating the cell death response and is required for dexamethasone mediated cell killing of MM cells. Obatoclax is effective as a single agent in the Vk*myc model and is currently being evaluated in combination with dexamethasone.
In a second genome-wide siRNA screen in MM cells, knockdown of Aurora kinase B (AURKB) was identified as a dexamethasone sensitizer. The AURKB specific inhibitor, AZD1152 synergizes with dexamethasone in several cell lines, when treatment with AZD1152 precedes that of dexamethasone. A KMS-11 cell line overexpressing BCL-2 can inhibit the synergy between AZD1152 and dexamethasone, demonstrating the involvement of the BCL-2 family of apoptotic regulators in cell death mediated by the combination. The synergy may be in part mediated through TP53 as knockdown of TP53 with siRNAs eliminates AZD1152-mediated cell death. SiRNA-mediated knockdown experiments show that two TP53 regulated genes and pro-apoptotic members of the BCL-2 family, NOXA and PUMA, are also involved in the mechanism of cell death. PUMA, like BIM, is an activator of BAK/BAX mediated outer mitochondrial memebrane permeabilization (MOMP) and NOXA is a sensitizer protein that can displace the activator proteins from MCL-1/BCL-2 inhibitors thus sensitizing cells to MOMP. The AZD1152/DEX combination is currently being tested in the Vk*myc murine model of MM.
These results propose two potential new combination therapies that operate through a mechanism involving DEX mediated BIM induction and by modulation of BCL-2 family rheostat. Obatoclax inhibits the function of anti-apoptotic MCL-1 activity and AZD1152 leads to induction of NOXA and PUMA protein, sensitizing the cells to DEX mediated cell death.
Citation Format: Anne Roulston, Cynthia Bernier, Mai Nguyen, Franziska Ertel, Francis Robert, Maryanne Bellomo, Xian Fang Huang, Alexandre Bramoullé, Michel Gravel, Michael Sebag, Jerry Pelletier, Gordon Shore. Dexamethasone synthetic lethal screens uncover two potential combination therapies for the treatment of multiple myeloma. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Synthetic Lethal Approaches to Cancer Vulnerabilities; May 17-20, 2013; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(5 Suppl):Abstract nr B17.