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OBJECTIVES: Internal tandem duplications (ITDs) of the Fms-like tyrosine kinase 3 (FLT3) represent the most frequent molecular aberrations in acute myeloid leukemia (AML) and are associated with an inferior prognosis. The pattern of downstream activation by this constitutively activated receptor tyrosine kinase is influenced by the localization of FLT3-ITD depending on its glycosylation status. Different pharmacological approaches can affect FLT3-ITD-driven oncogenic pathways by the modulation of FLT3-ITD localization. AIMS: The objective of this study was to investigate the effects of N-glycosylation inhibitors (tunicamycin or 2-deoxy-D-glucose) or the histone deacetylase inhibitor valproic acid (VPA) on FLT3-ITD localization and downstream activity. We sought to determine the potential differences between the distinct FLT3-ITD variants, particularly concerning their susceptibility towards combined treatment by addressing either N-glycosylation and the heat shock protein 90 (HSP90) by 17-AAG, or by targeting the PI3K/AKT/mTOR pathway by rapamycin after treatment with VPA. METHODS: Murine Ba/F3 leukemia cell lines were stably transfected with distinct FLT3-ITD variants resulting in IL3-independent growth. These Ba/F3 FLT3-ITD cell lines or FLT3-ITD-expressing human MOLM13 cells were exposed to tunicamycin, 2-deoxy-D-glucose or VPA, and 17-AAG or rapamycin, and characterized in terms of downstream signaling by immunoblotting. FLT3 surface expression, apoptosis, and metabolic activity were analyzed by flow cytometry or an MTS assay. Proteome analysis by liquid chromatography–tandem mass spectrometry was performed to assess differential protein expression. RESULTS: The susceptibility of FLT3-ITD-expressing cells to 17-AAG after pre-treatment with tunicamycin or 2-deoxy-D-glucose was demonstrated. Importantly, in Ba/F3 cells that were stably expressing distinct FLT3-ITD variants that were located either in the juxtamembrane domain (JMD) or in the tyrosine kinase 1 domain (TKD1), response to the sequential treatments with tunicamycin and 17-AAG varied between individual FLT3-ITD motifs without dependence on the localization of the ITD. In all of the FLT3-ITD cell lines that were investigated, incubation with tunicamycin was accompanied by intracellular retention of FLT3-ITD due to the inhibition of glycosylation. In contrast, treatment of Ba/F3-FLT3-ITD cells with VPA was associated with a significant increase of FLT3-ITD surface expression depending on FLT3 protein synthesis. The allocation of FLT3 to different cellular compartments that was induced by tunicamycin, 2-deoxy-D-glucose, or VPA resulted in the activation of distinct downstream signaling pathways. Whole proteome analyses of Ba/F3 FLT3-ITD cells revealed up-regulation of the relevant chaperone proteins (e.g., calreticulin, calnexin, HSP90beta1) that are directly involved in the stabilization of FLT3-ITD or in its retention in the ER compartment. CONCLUSION: The allocation of FLT3-ITD to different cellular compartments and targeting distinct downstream signaling pathways by combined treatment with N-glycosylation and HSP90 inhibitors or VPA and rapamycin might represent new therapeutic strategies to overcome resistance towards tyrosine kinase inhibitors in FLT3-ITD-positive AML. The treatment approaches addressing N-glycosylation of FLT3-ITD appear to depend on patient-specific FLT3-ITD sequences, potentially affecting the efficacy of such pharmacological strategies. [ABSTRACT FROM AUTHOR]

Background: Nucleophosmin (NPM1) and Flt3 internal tandem duplications (Flt3-ITD mutations) represent the most frequent molecular aberrations in patients with acute myeloid leukemia (AML). While NPM1 mutations are associated with favourable prognosis in younger AML patients, Flt3-ITD mutations reflect an unfavourable prognostic factor in these patients. So far, especially NPM1 mutations have not yet been evaluated exclusively in older patients. Patients and methods: We retrospectively analysed the prevalence of NPM1 and Flt3-ITD mutations and its association with complete remission (CR), and survival in 99 elderly patients (median age 71 yr, range 60–85 yr) newly diagnosed for AML. Primary treatment approach was curative in 54, and palliative in 38 patients, while seven patients received best supportive care only. The mean follow-up of surviving patients was 600 d. Results: Sixty-seven patients were tested negative for NPM1 and Flt3-ITD mutations (group 1), 16 patients carried only a NPM1 mutation (group 2) and nine patients had only a Flt3-ITD mutation (group 3) while additional seven patients were positive for both aberrations (group 4). We can demonstrate a significant higher rate of CR comparing wildtype vs. NPM1 positive patients (40.5% for group 1 vs. 80.0% for group 2, P = 0.03) for patients receiving curative therapy. Interestingly, there is no significant difference in overall survival between group 1 and group 2 (Log-rank test P = 0.22, median 440 d vs. 1125 d). In contrast, patients carrying a Flt3-ITD mutation had a significant worse overall survival compared to wildtype patients ( P = 0.03, median 210 d for group 3 + 4 vs. 634 d for group 1 + 2) while no difference of CR rate could be observed (42.8% vs. 48.9%, P = 0.91). Conclusion: As elderly but medically fit patients with AML carrying a NPM1 mutation have a high CR rate, age itself should not be a barrier for induction treatment. However, new therapeutic concepts of postremission therapy (e.g. allogeneic stem cell transplantation after dose-reduced conditioning) should be considered for these patients in first CR. [ABSTRACT FROM AUTHOR]

FLT3 activating mutations can be detected in about 35% of acute myeloid leukemia (AML). FLT3 internal tandem duplications (FLT3-ITD) represent the majority of FLT3 mutations (25 - 30%) while FLT3-TKD (tyrosine kinase domain) mutations can be found in about 7% of AML patients. In this study, we addressed the question whether especially primary AML cells carrying FLT3-ITD mutations show differences in terms of their protein expression pattern compared to FLT3 wild-type blasts. We investigated bone marrow samples that were isolated at diagnosis from 36 AML patients expressing either FLT3 wild-type (n = 16) or an activating FLT3 mutation (FLT3-ITD, n = 15; FLT3-TKD, n = 5). Proteomic analysis was performed by means of surface enhanced laser desorption/ionization-time of flight (SELDI-TOF) mass spectrometry which has shown its high efficiency in finding biomarkers in solid tumors. Here, we demonstrate that a large series of proteins is differently expressed in primary AML blasts harboring FLT3-ITD mutations. Furthermore, there are also significant differences of the protein expression profile between FLT3-ITD and FLT3-TKD mutations. Interestingly, further analysis of FLT3-ITD positive AML according to its response to the induction chemotherapy demonstrates putative prognostic markers for this subgroup of AML. We suggest that SELDI-TOF mass spectrometry represents a promising tool of proteomic analysis of AML that might help to establish new prognostic markers in AML. [ABSTRACT FROM AUTHOR]

Abstract: We present our first experiences with determination of minimal residual disease (MRD) based on patient specific Flt3-ITD (internal tandem duplication) mutations. We analysed MRD status of 11 AML patients in a retrospective investigation and its potential impact on the follow up of these patients. In five out of six patients with a positive Flt3-ITD based MRD status a relapse of AML was observed in the follow up while one patient lacks a clinical relapse so far. In contrast, four out of five patients with a negative MRD status remain free of disease. One of these patients relapsed with a switch of FAB subtype including loss of Flt3-ITD mutation. Furthermore, in one patient we could identify a Flt3-ITT (internal tandem triplication mutation). [Copyright &y& Elsevier]

Treatment of FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD)-positive acute myeloid leukemia (AML) remains a challenge despite the development of novel FLT3-directed tyrosine kinase inhibitors (TKI); the relapse rate is still high even after allogeneic stem cell transplantation. In the era of next-generation FLT3-inhibitors, such as midostaurin and gilteritinib, we still observe primary and secondary resistance to TKI both in monotherapy and in combination with chemotherapy. Moreover, remissions are frequently short-lived even in the presence of continuous treatment with next-generation FLT3 inhibitors. In this comprehensive review, we focus on molecular mechanisms underlying the development of resistance to relevant FLT3 inhibitors and elucidate how this knowledge might help to develop new concepts for improving the response to FLT3-inhibitors and reducing the development of resistance in AML. Tailored treatment approaches that address additional molecular targets beyond FLT3 could overcome resistance and facilitate molecular responses in AML. [ABSTRACT FROM AUTHOR]

Abstract: TNF-α has pleiotropic effects on cell survival and apoptosis. The E3 ubiquitin ligase TRAF2 plays a crucial role for TNF-α mediated signaling since NF-κB activation by TNF-α is at least partially mediated by TRAF2. The objective of this study was to investigate whether TNF-α can induce apoptosis in FLT3-ITD-positive AML cells and to elucidate the influence of TRAF2. Stable lentiviral mediated down-regulation of TRAF2 resulted in a decrease of phosphorylation of the anti-apoptotic protein AKT and its downstream target GSK-3β. Induction of apoptosis and impaired proliferation after TNF-α exposure were observed. Co-treatment of FLT3-ITD-positive cells with the specific FLT3 inhibitor AC220 was able to overcome TNF-α resistance. Taken together, we conclude that TRAF2 plays an important role in signal transduction and survival of AML cells. [Copyright &y& Elsevier]