Disease relevance of FLT3

• Certain classes of FLT3 mutations cause constitutively activated forms of the receptor that are found in significant numbers of patients with acute myelogenous leukemia (AML) [1].
• Biologic and clinical significance of the FLT3 transcript level in acute myeloid leukemia [2].
• FLT3 mutations in childhood acute lymphoblastic leukemia [3].
• The FLT3 receptor tyrosine kinase is highly expressed in most acute leukemias and frequently mutated in acute myeloid leukemia (AML) [4].
• These results indicate that FLT3 mutations are secondary events in leukemogenesis, are unstable, and thus should be used cautiously for the detection of minimal residual disease [5].
• Exploring the mechanisms by which these FLT3 alterations lead to dysregulated proliferation should provide a better understanding of the molecular pathogenesis of acute myeloid leukemia (AML) and may provide insights into potential therapeutic interventions [6].

High impact information on FLT3

• These studies demonstrate the importance of the Flt3 receptor tyrosine kinase as the earliest marker of hematopoietic cell fate commitment in that erythrocyte and megakaryocyte potentials are lost first as HSCs differentiate to lymphocyte progenitors [7].
• The FLT3/FLK2 receptor tyrosine kinase is closely related to two receptors, c-Kit and c-Fms, which function with their respective ligands, Kit ligand and macrophage colony-stimulating factor to control differentiation of haematopoietic and non-haematopoietic cells [8].
• Ligand for FLT3/FLK2 receptor tyrosine kinase regulates growth of haematopoietic stem cells and is encoded by variant RNAs [8].
• We have purified to homogeneity and partially sequenced a soluble form of the FLT3/FLK2 ligand produced by mouse thymic stromal cells [8].
• Repression of Flt3 by Pax5 is crucial for B-cell lineage commitment [9].

Chemical compound and disease context of FLT3

• FLT3 mutations, either internal tandem duplications (ITDs) or aspartate residue 835 (D835) point mutations, are present in approximately one third of patients with acute myeloid leukemia (AML) and have been associated with an increased relapse rate [5].
• An innovative phase I clinical study demonstrates inhibition of FLT3 phosphorylation by SU11248 in acute myeloid leukemia patients [10].
• The combination of 17-AAG with FLT3 kinase inhibitors can enhance targeted therapy in leukemias with FLT3 mutations, such as MLL fusion gene leukemias [11].
• Establishment of a Stroma-Dependent Human Acute Myelomonocytic Leukemia Cell Line, NAMO-2, with FLT3 Tandem Duplication [12].
• Because of the correlation between FLT3 internal tandem duplication (ITD) mutations and poor prognosis in acute myelogenous leukemia (AML), we conducted a phase 1 trial of tandutinib in 40 patients with either AML or high-risk myelodysplastic syndrome (MDS) [13].

Biological context of FLT3

• FLT3 is a type III receptor tyrosine kinase that is thought to play a key role in hematopoiesis [1].
• FLT3 is constitutively activated either by an internal tandem duplication (ITD) or by a point mutation (PM) in 17% to 24% of pediatric AML cases [14].
• SU11248 inhibits FLT3-driven phosphorylation and induces apoptosis in vitro [15].
• FLT3/ITD-positive patients were significantly older and had higher percentages of normal cytogenetic findings or French-American-British (FAB) classification M1/M2 and lower percentages of 11q23 abnormalities or FAB M5 [16].
• Introduction of these dual ITD-TKD, but not single D835N or Y842H FLT3 mutants, in Ba/F3 cells restored the FLT3 inhibitor resistant phenotype [17].

Anatomical context of FLT3

• Activating mutations in FLT3, including internal tandem duplication (ITD) in the juxtamembrane domain, transform hematopoietic cell lines to factor independent growth [18].
• Internal tandem duplications of the FLT3 gene are present in leukemia stem cells [19].
• We determined the antileukemic activity of CEP-701, a potent and selective FLT3 inhibitor, in 8 ALL cell lines and 39 bone marrow samples obtained at diagnosis from infants and children with various subtypes of ALL [20].
• Both mutations were associated with higher white blood cell (WBC) count at presentation; 75% of the patients with WBC counts of 10 x 10(9)/L or greater had mutant FLT3 [21].
• FLT3 is a receptor tyrosine kinase involved in the proliferation and differentiation of hematopoietic stem cells [16].

Associations of FLT3 with chemical compounds

• SU11248 is a novel FLT3 tyrosine kinase inhibitor with potent activity in vitro and in vivo [15].
• SU5416 is a small molecule RTK inhibitor (RTKI) of VEGFR-2, c-kit, and both wild-type and mutant FLT3 [22].
• The Expand Long Template PCR System was used to determine the allelic location of internal tandem duplication of FLT3 (FLT3/ITD) and Asp(835) mutations [23].
• The current study evaluated the single and combined effects of 17-AAG and GTP14564, and the role of FLT3 in their inhibitory effects [11].
• Complete remission was observed following treatment with daunorubicin and cytosine arabinoside, but after 37 months the patient relapsed with t-AML of subtype M3 with a t(15;17) and the same FLT3/ITD was still present [24].
• The potency of Sunitinib against FLT3 K663Q was similar to its potency against FLT3 ITD mutations [25].

Physical interactions of FLT3

• We found that activation of FLT3 in human AML inhibits CCAAT/enhancer binding protein alpha (C/EBPalpha) function by ERK1/2-mediated phosphorylation, which may explain the differentiation block of leukemic blasts [26].
• JNK1 co-immunoprecipitated from such lysates with p85-cbl-crkII/L and bound to Crk species SH3 domain in pull-down assay. siRNA-mediated depletion of Flt3 or of cbl, the adaptor at the nexus of this signaling group, inhibited JNK activity on substrate c-jun [27].
• Identification of Y589 and Y599 in the juxtamembrane domain of Flt3 as ligand-induced autophosphorylation sites involved in binding of Src family kinases and the protein tyrosine phosphatase SHP2 [28].
• A chimeric cytokine, progenipoietin-1 (ProGP-1), containing the G-CSF and FL receptor agonists binds both the G-CSF receptor and FLT-3 [29].

Enzymatic interactions of FLT3

• Immunoprecipitation and immunoblotting analyses demonstrated that FLT3/ITDs were constitutively phosphorylated in the absence of FL [30].

Regulatory relationships of FLT3

• In addition, SU11248 inhibits FLT3-induced VEGF production [15].
• In some cases, FLT3 was expressed at levels equivalent to GAPDH in the absence of genomic amplification [31].
• Autocrine stimulation of wild-type (WT) FLT3 by coexpressed FLT3 ligand (FL) occurs in many other cases [14].
• In this study, we examined the possible role of SHP-1 in regulating FLT3 signaling [32].
• These observations suggest that reduced AML1 activities predispose cells to the acquisition of the activating FLT3 mutation as a secondary event leading to full transformation in AML M0 [33].

Other interactions of FLT3

• NPM1 mutations were associated with normal karyotype and with internal tandem duplication (ITD) and D835 mutations in FLT3, but not with other mutations [34].
• We studied 203 patients with PML-RARA-positive APL; 43% of the patients had an FLT3 mutation (65 internal tandem duplications [ITDs], 19 D835/I836, 4 ITD+D835/I836) [21].
• Gene expression scores of FLT3, MeisI, and CD44 for samples with MLL rearrangements were particularly high compared with those for other ALL samples [35].
• In acute myeloid leukemia (AML), constitutive activation of the FLT3 receptor tyrosine kinase, either by internal tandem duplications (FLT3-ITD) of the juxtamembrane region or by point mutations in the second tyrosine kinase domain (FLT3-TKD), as well as point mutations of the NRAS gene (NRAS-PM) are among the most frequent somatic gene mutations [36].
• Further analyses of 23 of these cases uncovered one FLT3 internal tandem duplication and five TP53 mutations [37].

Analytical, diagnostic and therapeutic context of FLT3

• Patients with a high mutant/wt ratio (ie, greater than 0.78) had significantly shorter overall and disease-free survival, whereas survival in patients with ratios below 0.78 did not differ from those without FLT3 aberrations [38].
• Our results indicate the need for clinical trials to test the efficacy of drugs that inhibit the FLT3 RTK in this subset of patients with T-ALL [39].
• To determine if FLT3 protein was also overexpressed, proteins were extracted from leukemic BM samples and screened by Western blotting with anti-FLT3 antisera [40].
• Primary AML blasts bearing FLT3-Y842C mutations showed constitutive FLT3 and signal transducer and activator of transcription 5 (STAT-5) tyrosine phosphorylation [41].
• FLT3/ITD-positive patients had lower remission induction rates (70% vs 88%; P =.01) and lower 5-year probability rates of event-free survival (pEF) (29% vs 46%; P =.0046) and overall survival (32% vs 58%; P =.037) [16].

References