Abstract 5093: Bcr-Abl leads to mast cell differentiation and promotes degranulation of cells derived from a chronic myeloid leukemia mouse model in a Gab2 dependent manner

Chronic myeloid leukemia (CML) is driven by the fusion kinase Bcr-Abl, caused by a chromosomal translocation between chromosome 9 and 22. This tyrosine kinase organizes its own signaling network with various proteins, especially the docking protein Gab2. By being recruited to Bcr-Abl via Grb2, Gab2 plays a critical role in this network. We recently applied a transgenic approach to investigate the role of Gab2 in Bcr-Abl driven disease. Using Gab2 knock-out mice, we analyzed the in vivo role of Gab2 in a chronic-phase CML mouse model in which a tetracycline regulated Bcr-Abl transgene is expressed in hematopoietic stem cells in their native microenvironment. We demonstrated that Gab2 deficiency impairs disease development in a steady-state in vivo setting. In the course of this work, we also identified an abnormal number of mast cells infiltrating the kidneys of Bcr-Abl expressing Gab2 proficient mice, a phenotype associated with an inflammatory urothelium and hydronephrosis. Interestingly, this phenotype was completely absent in Gab2 deficient mice. Therefore, we aimed to analyze the role of mast cells in the CML mouse model in more detail. First, we performed bone marrow transplantations, using Bcr-Abl positive donor mice and C57/BL6N mice as recipients. Also in these mice, we observed high mast cell counts in the bone marrow and developed hydronephrosis in some cases, which demonstrates that these alterations were induced by cell-autonomous properties of the Bcr-Abl positive donor cells. Next, we were interested whether these cells are Bcr-Abl transformed mast cell precursors or reactive mast cells resulting from secondary effects of the leukemic disease. Therefore, we isolated bone marrow cells from Bcr-Abl transgenic mice and analyzed them for Bcr-Abl activity and mast cell properties. Strikingly, Bcr-Abl positive progenitors differentiated into mast cells under cytokine free conditions. Next, we assessed mast cell functionality of these cells by degranulation assays measuring β-hexosaminidase activity and cytokine release. Importantly, Bcr-Abl positive cells were more sensitive towards antigen stimulation and displayed a stronger degranulation und higher levels of secreted cytokines compared to Bcr-Abl negative controls. This suggests that Bcr-Abl positive mast cells could be responsible for the inflammatory urothelium and the hydronephrosis which we observed in our CML mouse model. In line with our previous results, Gab2 deficient cells from Bcr-Abl transgenic mice showed no elevated degranulation. In summary, we could show that Bcr-Abl can drive the expansion of mast cells. In addition, Bcr-Abl leads to stronger degranulation in a Gab2 dependent manner. This data and our previous work on Gab2 invite for the further evaluation of Gab2 as a biomarker and as a valuable target in the treatment of CML and, possibly, systemic mastocytosis. Citation Format: Julia Ellermann, Franziska Maria Uhl, Martin Kohler, Julia Huber, Robert Zeiser, Steffen Koschmieder, Konrad Aumann, Tilman Brummer, Sebastian Halbach. Bcr-Abl leads to mast cell differentiation and promotes degranulation of cells derived from a chronic myeloid leukemia mouse model in a Gab2 dependent manner [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 5093.