Role of FLT3 in Acute Myeloid Leukemia: Molecular mechanisms and Therapeutic opportunities

Abstract: Acute myeloid leukemia (AML) is a highly heterogeneous blood disease which is characterized by different mutations and chromosomal rearrangements. Nearly 60% of genetic alterations have been found in AML patients involve in signaling pathways including signaling of tyrosine kinase receptor FLT3. FLT3 mutations emerged as one of the most common mutations in AML which represent around 35% of all AML cases, making it an attractive therapeutic target in AML. Among these mutations, FLT3-ITD is associated with a high risk of relapse and poor prognosis. Although several FLT3 inhibitors have been developed and showed promising results in clinical trials, many patients develop drug resistance shortly after treatment starts and display poor outcome. Therefore, understanding how FLT3 signaling pathways are regulated is increasingly needed in order to identify new drugs targeting the oncogenic FLT3 and to overcome resistance. In this thesis, we have addressed the importance of associating proteins in regulating FLT3 signaling as well as identified novel therapeutic targets to overcome FLT3-related resistance. We identified SLAP2 and ABL2 as potent interaction partners of FLT3 through their SH2 domain. Our results show that SLAP2 suppresses FLT3 downstream signaling pathways including AKT, ERK, p38 and STAT5 and facilitates FLT3 degradation through enhancing ubiquitination while ABL2 expression does not alter FLT3 stability or ubiquitination but partially suppresses FLT3 downstream signaling through the PI3K/AKT pathway. In contrast to the case of many kinases, we have found that the activation loop of FLT3 is not essential for its activation. Rather, we found that phosphorylated activation loop Y842 serves as a binding site of SHP2, which is required for FLT3-induced activation of RAS/ERK pathway. Our results suggest that SLAP2 and ABL2 regulate FLT3 signaling and modulation of SLAP2 expression levels or targeting ABL2 could potentially synergize with FLT3 inhibitors to treat FLT3-ITD positive AML. Furthermore, Y842 is found to be critical for FLT3-mediated RAS/ERK signaling and cellular transformation.Using a panel of kinase inhibitors, we found ALK inhibitor AZD3463 selectively inhibited the activation and downstream signaling of FLT3-ITD and did not affect the wild-type FLT3 (FLT3-WT). These findings are interesting from a therapeutic point of view since FLT3-WT is essential for normal hematopoiesis process. Moreover, we showed that AZD3463 effectively overcame the secondary resistance to sorafenib in FLT3-ITD positive AML cells. Thus, this suggests that AZD3463 is a promising inhibitor to target FLT3-ITD positive AML. In conclusion, this thesis explores the mechanisms of regulating FLT3 signaling and therapeutic targeting opportunities.