Abstract 29: Characterization and treatment of a novel adoptive transfer model of Sf3b1mut/Atmdel chronic lymphocytic leukemia
Authors: Hacken Elisa ten , Wang Lili , Regis Fara Faye , Thomas Michael , Deng Jing , Baranowski Kaitlyn , Cartun Zachary , Buonamici Silvia , Neuberg Donna , Letai Anthony , Carrasco Ruben , Wu Catherine J. 
Published: August 2018

Abstract
The spliceosome component SF3B1 is among the most frequently mutated genes in CLL (Landau et al. Nature 2015), and its mutations are associated with transcriptomic changes in numerous pathways involved in the regulation of CLL-cell survival (Wang et al. Cancer Cell 2016). Across studies, the presence of SF3B1 mutations has been associated with poorer prognosis, and expansion of SF3B1-mutated subclones has been associated with resistance to current CLL therapies. Therefore, therapeutic options that selectively interfere with spliceosome components may represent an attractive therapeutic strategy for selected CLL patients.
We recently developed a novel mouse model of human CLL, based on the B-cell restricted coexpression of mutated Sf3b1 and deletion of Atm (Sf3b1mut/Atmdel), two commonly co-occurring genetic alterations in CLL. One of the main advantages of this model is its transplantability in either immunocompetent C57BL/6 or immunocompromised NSG hosts, with the engrafted mice rapidly succumbing to disease (3-4 months for C57BL/6, 4-6 weeks for NSG), thus providing a time frame for informative treatment studies. Sf3b1mut/Atmdelcells express unmutated IGHV B cell receptors (BCR), they show high BCR signaling responsiveness, and express high levels of the CD49d integrin and the chemokine receptor CXCR4, all phenotypic and functional features associated with aggressive human CLL. Sf3b1mut/Atmdel also express high levels of the aberrantly spliced isoforms of Tnpo3, Aurke, and Narfl1 genes, indicative of alternative splicing dysregulation.
We took advantage of the spliceosome modulator E7107, to test the effects of splicing inhibition in NSG mice challenged intravenously (i.v.) with 5 x 106 Sf3b1mut/Atmdel splenocytes. Fifteen days post-transplant, 40-50% circulating B220+CD5+Igk+ cells were detectable in the peripheral blood (PB) of recipient mice by flow cytometry. Mice were then randomized and treated i.v. for 5 days with either 4 mg/kg E7107 or vehicle control (3 controls/4 treated), then immediately euthanized. Three hours after the first E7107 dose, we observed significant reduction in Slc25a19 and Dph2 mature-RNA levels, two known targets of E7107, as analyzed by RT-PCR of PB cells collected from the treated animals. After the 5-day treatment, we observed a significant reduction in leukemia burden in the PB, bone marrow (BM), peritoneum (PLC), and spleen (SP) of the treated animals, associated to the presence of apoptotic AnnexinV+ Sf3b1mut/Atmdel cells. Interestingly, MCL1 protein and RNA levels were significantly reduced after the 5-day E7107 administration, as tested by RT-PCR and Western blot analysis of splenocytes harvested at sacrifice. Depletion of MCL1 total protein levels was also observed after 12 hours of in vitro treatment of Sf3b1mut/Atmdel cells with up to 1 μM E7107.
In conclusion, we propose a novel and valuable preclinical platform of Sf3b1mut/Atmdel CLL, with phenotypic and functional features similar to human CLL. We demonstrate that splicing modulation by E7107 treatment can reduce leukemia burden, not only in the PB, but also in the SP, BM, and PLC of these mice, by inducing CLL-cell apoptosis. Apoptosis induction is associated with depletion of MCL1 protein, an important antiapoptotic factor in CLL, which is commonly upregulated by microenvironmental interactions in lymphoid organs, and frequently associated with treatment resistance. Studies aimed at further evaluating E7107 selectivity towards Sf3b1mut as compared to Sf3b1wt cells are under way, along with combination treatments with standard or novel CLL therapeutics. These studies may provide the basis for personalized treatment strategies for patients harboring SF3B1 mutations, and novel therapeutic opportunities for refractory CLL patients carrying SF3B1mut subclones.
Citation Format: Elisa ten Hacken, Lili Wang, Fara Faye Regis, Michael Thomas, Jing Deng, Kaitlyn Baranowski, Zachary Cartun, Silvia Buonamici, Donna Neuberg, Anthony Letai, Ruben Carrasco, Catherine J. Wu. Characterization and treatment of a novel adoptive transfer model of Sf3b1mut/Atmdel chronic lymphocytic leukemia [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr 29.