Oncogenic gain-of function mutations in JAK2 are frequently detected in various leukemias and myeloproliferative disorders. Some of these activating mutations have recently been identified in and strongly associate with a subset of B-progenitor acute lymphoblastic leukemia (pre B-ALL) and Down syndrome ALL (DS-ALL) shown to overexpress cytokine receptor-like factor 2 (CRLF2). This novel association suggests that mutant JAK2 and CRLF2 cooperate to transform B-progenitor cells and therefore inhibition of CRLF2/JAK2 signaling and its downstream effectors may represent a new therapeutic approach for this subset of B-ALL patients.
Initial in vitro studies on two pre B-ALL cell lines, MUTZ-5 and MHH-CALL4, with known activating JAK2 mutation and CRLF2 overexpression exhibited moderate sensitivity to the JAK2 inhibitor TG101209. In order to identify more efficacious therapeutic targets, we investigated JAK2 signaling mechanisms in these cells and observed constitutive activation of STAT5 and high expression levels of the pro-survival protein, Bcl-xL. We therefore hypothesized that MUTZ-5 cells would be sensitive to the Bcl-2/Bcl-xL antagonist ABT-737. Surprisingly, these cells were extremely resistant to ABT-737, suggesting that the expression of Bcl-xL alone might not be decisive for MUTZ-5 cell survival. However, combined treatment with ABT-737 and TG101209 synergistically induced apoptosis in these cells. Pathway and protein expression analysis will be used to unravel the mechanisms underlying this synergistic action.
Moreover, this project also aims to determine if the MAPK/ERK and PI3K/AKT pathways are important for pre B-ALL cell proliferation and cell survival. Inhibitors specific for the respective signaling molecules will be used to treat pre-B ALL cells in vitro, and concomitant cell cycle- and cell death-analysis will be performed. Furthermore, we have successfully established two transgenic mice models to perform future in vivo studies in; one overexpressing the human CRLF2 receptor, and the other which overexpress the murine CRLF2 receptor in a B-cell specific manner.