The FLT3 Tyrosine Kinase in Leukemia Deciphering the Downstream Signaling Events and Drug-Escape Mechanisms

Abstract: Acute myeloid leukemia (AML) is a severe disease, which originates in blood-forming cells. Although major advances in understanding the biology of AML, the majority of patients eventually succumb to the disease. The tyrosine kinase receptor FLT3 has become an attractive therapeutic target AML for two major reasons; 1) It is one of the most frequently mutated genes in AML (about 30%). 2) Most of these mutations (FLT3-ITDs) correlate with an increased risk of relapse and poor overall survival. Small targeting inhibitors towards FLT3 have been designed and evaluated in clinical trials. However, the experiences from clinical trials are that drug resistance develops in a substantial number of patients. To overcome these resistance-associated problems it its important to improve the understanding of how FLT3 mutations function and how they respond to targeting drugs. This was addressed in this thesis by elucidating FLT3-ITD cell transformation mechanisms, identifying key downstream target molecules of mutated FLT3 and exploring the effect of various targeting inhibitors. The major finding of my thesis is that FLT3-targeting drugs elicit apoptosis through a FOXO3a-dependent upregulation of proapoptotic BH3-only protein Bim via inactivation of the PI3K/AKT signaling pathway. Furthermore, we have identified an interesting apoptotic mechanism, linked to increased ROS levels caused by expressing hyperactivated AKT in hematopoietic stem cells and bone marrow progenitor cells from FLT3-ITD transgenic mice. These findings are interesting from a therapeutic point of view. We have also shown that canertinib, an inhibitor of the ERBB receptor family, targets mutated FLT3 in vitro and in vivo. The irreversible binding mechanism of canertinib, as well as its multikinase activity, is attractive features. Overall, the results presented herein could provide basis for future directions in treatment of FLT3 mutant positive AML patients. Finally, we studied nine different FLT3-ITD mutations ranging in length from 6-33 amino acids. Data from this study suggest that different FLT3-ITDs may induce distinct degrees of transformation and that they respond differentially to FLT3-targeting drugs. These differences were not associated with size of the duplication but rather the mutational composition. In conclusion, this thesis explores the biologic features of FLT3 mutations and therapeutic targeting opportunities.