Acute myeloid leukaemia with FLT3 gene mutations of both internal tandem duplication and point mutation type

FLT3 gene mutations, either internal tandem duplication or point mutation type, are common in acute myeloid leukaemia (AML). We describe 21 AML cases with both types of gene mutations, so-called dual mutations, representing approximately 1% of all cases. Most newly diagnosed AML with FLT3 dual mutations had monocytic differentiation and a normal karyotype. Over the disease course, changes in FLT3 mutation status were seen in 89% of cases, and were associated with cytogenetic changes. We conclude that FLT3 dual mutations occur rarely in AML, and appear to be related to clonal evolution.

Mutations in KIT and RAS are frequent events in pediatric core-binding factor acute myeloid leukemia

Activating mutations in RAS and receptor tyrosine kinases such as KIT and FLT3 are hypothesized to cooperate with chimeric transcription factors in the pathogenesis of acute myeloid leukemia (AML). To test this hypothesis, we genotyped 150 pediatric AML samples for mutations in KIT (exons 8, 17), NRAS and KRAS (exons 1, 2) and FLT3/ITD. This is the largest cohort of pediatric AML patients reported thus far screened for all four mutations. Of the children with AML, 40% had a mutation in KIT (11.3%), RAS (18%) or FLT3/ITD (11.1%), and 70% of cases of core-binding factor (CBF) leukemia were associated with a mutation of KIT or RAS. Mutations in RAS or FLT3/ITD were frequently found in association with a normal karyotype. Patients with a FLT3/ITD mutation had a significantly worse clinical outcome. However, the presence of a KIT or RAS mutation did not significantly influence clinical outcome. We demonstrate that KIT exon 8 mutations result in constitutive ligand-independent kinase activation that can be inhibited by clinically relevant concentrations of imatinib. Our results demonstrate that abnormalities of signal transduction pathways are frequent in pediatric AML. Future clinical studies are needed to determine whether selective targeting of these abnormalities will improve treatment results.

Human leukemias with mutated FLT3 kinase are synergistically sensitive to FLT3 and Hsp90 inhibitors: the key role of the STAT5 signal transduction pathway

17-allylamino-17-demethoxygeldanamycin (17-AAG), an inhibitor of the molecular chaperone heat shock protein 90, results in cell type-specific inhibition of proliferation of leukemic cells. GTP14564 is a tyrosine kinase inhibitor actively against FLT3. The current study evaluated the single and combined effects of 17-AAG and GTP14564, and the role of FLT3 in their inhibitory effects. The importance of FLT3 mutations was demonstrated using small interfering RNA (siRNA) targeted to FLT3. Similar to FLT3 siRNA, GTP14564 inhibited FLT3 internal tandem duplication (ITD) cells (MV4;11) and FLT3 amplified wild-type cells (SEMK2-M1), but not wild-type FLT3 cells (RS4;11). However, when RS4;11 cells were stimulated with FLT3-ligand, phosphorylation of STAT5 and GTP14564 inhibition were observed. Responses to GTP14564 in all cell types were directly related to the level of STAT5 phosphorylation in the cells. We observed synergistic effects of combined 17-AAG and GTP14564 in cell lines with FLT3-ITD and amplified wild-type FLT3. Combined treatment with 17-AAG and GTP14564 reduced the levels of p-FLT3 and p-STAT5, enhanced G0/G1 arrest and apoptosis in FLT3-ITD and amplified wild-type FLT3. The combination of 17-AAG with FLT3 kinase inhibitors can enhance targeted therapy in leukemias with FLT3 mutations, such as MLL fusion gene leukemias.

Nikolaou K, Vrachnis N. Neurotrophin-3 and FLT3 tyrosine kinase receptor in perinatal life

Our aim is to determine—in 30 healthy full-term infants and their mothers—circulating levels of neurotrophin-3 (NT-3) (important for antenatal and postnatal brain development and implicated in the immune response) and FLT3 tyrosine kinase receptor (FLT3) (controlling hematopoiesis and found in the nervous tissue), in the fetal and neonatal life. NT-3 levels, in contrast to FLT3 ones, increased significantly on the fourth postnatal day in relation to the low levels found in the mother, fetus, and day 1 neonate (P = .03, respectively). Maternal and umbilical NT3 levels positively correlated with respective FLT3 levels (P = .003 and P = .03). Circulating NT-3 levels increased in early neonatal life, possibly due to exposure to various stimuli soon after birth. FLT3 levels do not seem to behave accordingly, although these two substances probably synergize.

Delineation and molecular characterization of acute myeloid leukemia patients with coduplication of FLT3 and MLL

Partial tandem (PTD) and internal tandem duplications (ITD) of the MLL or FLT3 genes respectively, have been demonstrated in acute myeloid leukemia (AML). While occurrence of each of these PTD/ITD seem to confer an unfavorable prognosis, the literature contains only sparse information of the occurrence and the prognosis of simultaneous PTD/ITD of these genes. We have therefore attempted to determine the presence and its consequence in AML and with the further aim of characterizing such patients with respect to other genetic aberrations and to prototype variables in this disease. We analyzed blast cells from 250 adult patients treated at the same institution during a 15-year period for FLT3 ITD and MLL PTD and the duplications were found in 24% and 4%, respectively. The four co-duplicated cases (2%) did not differ with respect to sex, age, FAB-type, or immunophenotype, promoter methylation of p15, E-cadherin (CDH1), Estrogen receptor, MDR1, expression of apoptosis-related or multidrug resistance-related genes, though a trend toward decreased gene expression of MDR1 was observed. Two of the patients had a normal karyotypic analysis, while the remaining two showed aberrations in chromosome 11, one with trisomy 11 and the other with a der (11). The extensive molecular characterization of FLT3/MLL coduplicated patients presented here indicates that, even though they do not differ molecularly from the groups of patients with single ITDs, their prognosis and overall survival is universally poor. More patients are needed to determine whether coduplication has independent clinical implications compared to patients with single ITD/PTD.

AML1B transcriptional repressor function is impaired by the Flt3-internal tandem duplication

Fms-like tyrosine kinase 3 (Flt3) is a type III receptor tyrosine kinase. The internal tandem duplication (ITD) of the juxtamembrane region of this receptor is the most prevalent mutation in acute myeloid leukaemia (AML). The silencing mediator of retinoic and thyroid hormone receptors (SMRT) co-repressor recruits histone deacetylases (HDAC) and mediates transcriptional repression by interacting with various transcription factors. We recently reported that Flt3-ITD interferes with the transcriptional and biological action of promyelocytic leukaemia zinc finger transcriptional repressor by dissociating it from SMRT. In this study, we aimed to clarify whether the repressional activity of other well-known oncoproteins, such as AML1/Runx1 (AML1), is also affected by Flt3-ITD. We verified that the repression activity of AML1B, the isoform of AML1, is dependent on HDAC activity by using HDAC inbitor trichostatin A in GAL4 reporter assays. Mammalian two-hybrid assays demonstrated that this protein interacts with SMRT. Furthermore, this AML1B-SMRT interaction was disrupted by the overexpression of Flt3-ITD, leading to the reduction of AML1B repression activity. Additionally, we showed AML1B repression target, p21 (WAF1/CIP1), was aberrantly expressed in Flt3-ITD stably expressed BaF3 cells. Taken together, Flt3-ITD disrupts transcriptional repressor functions resulting in aberrant gene regulation in leukaemic cells.

RAS mutation in acute myeloid leukemia is associated with distinct cytogenetic subgroups but does not influence outcome in patients younger than 60 years

The pathogenesis of acute myeloid leukemia (AML) involves the cooperation of mutations promoting proliferation/survival and those impairing differentiation. The RAS pathway has been implicated as a key component of the proliferative drive in AML. We have screened AML patients, predominantly younger than 60 years and treated within 2 clinical trials, for NRAS (n = 1106), KRAS (n = 739), and HRAS (n = 200) hot-spot mutations using denaturing high-performance liquid chromatography or restriction fragment length polymorphism (RFLP) analysis. NRAS mutations were confirmed in 11% of patients (126/1106) and KRAS mutations in 5% (39/739). No HRAS mutations were detected in 200 randomly selected samples. Codons most frequently mutated were N12 (43%), N13 (21%), and K12 (21%). KRAS mutations were relatively overrepresented in French-American-British (FAB) type M4 (P < .001). NRAS mutation was over-represented in the t(3;5)(q21 approximately 25;q31 approximately q35) subgroup (P < .001) and underrepresented in t(15;17)(q22;q21) (P < .001). KRAS mutation was overrepresented in inv(16)(p13q22) (P = .004). Twenty-three percent of KRAS mutations were within the inv(16) subgroup. RAS mutation and FLT3 ITD were rarely coexistent (14/768; P < .001). Median percentage of RAS mutant allele assayed by quantitative RFLP analysis was 28% (N12), 19% (N13), 25% (N61), and 21% (K12). RAS mutation did not influence clinical outcome (overall/disease-free survival, complete remission, relapse rate) either for the entire cohort or within cytogenetic risk groups.

Mutations in the receptor tyrosine kinase pathway are associated with clinical outcome in patients with acute myeloblastic leukemia harboring t(8;21)(q22;q22)

AML1-MTG8 generated by t(8;21) contributes to leukemic transformation, but additional events are required for full leukemogenesis. We examined whether mutations in the receptor tyrosine kinase (RTK) pathway could be the genetic events that cause acute myeloblastic leukemia (AML) harboring t(8;21). Mutations in the second tyrosine kinase domain, juxtamembrane (JM) domain and exon 8 of the C-KIT gene were observed in 10, one and three of 37 AML patients with t(8;21), respectively. Three patients showed an internal tandem duplication in the JM domain of the FLT3 gene. One patient had a mutation in the K-Ras gene at codon 12. As the occurrence of these mutations was mutually exclusive, a total of 18 (49%) patients showed mutations in the RTK pathway. These results suggest that activating mutations in the RTK pathway play a role in part as an additional event leading to the development of t(8;21) AML. The 6-year cumulative incidence of relapse in patients with RTK pathway mutations was 79.8%, compared with 13.5% in patients lacking such mutations (P=0.0029). Furthermore, the 6-year relapse-free survival in patients with mutations was 18% compared to 60% in those without mutations (P=0.0340), indicating that RTK mutations are associated with the clinical outcome in t(8;21) AML.

Simultaneous detection of NPM1 and FLT3-ITD mutations by capillary electrophoresis in acute myeloid leukemia

Mutations in the Nucleophosmin (NPM1) gene have been recently described to occur in about one-third of acute myeloid leukemias (AML) and represent the most frequent genetic alteration currently known in this subset. These mutations generate an elongated NPM1 protein that localizes aberrantly in the cytoplasm. In analogy with Flt3 alterations, NPM1 mutations are mostly detectable in AML with normal karyotype and their recognition may be relevant to identify distinct response to treatment. Hence, in addition to conventional karyotyping and RT-PCR of fusion genes, combined analysis of both Flt3 and NPM1 mutations will be increasingly relevant in the genetic diagnosis work-up of AML. We developed a multiplex RT-PCR assay followed by capillary electrophoresis to simultaneously analyze NPM1 and Flt3 gene alterations (NFmPCR assay). The assay was validated in leukemic cell RNAs extracted from 38 AML patients, which had been previously characterized for Flt3 status by conventional RT-PCR. Direct sequencing of NPM1 RT-PCR products was carried out in 15 cases to verify results obtained by capillary electrophoresis. Both NPM1 sequencing and conventional RT-PCR Flt3 results showed 100% concordance with the results of the NFmPCR assay. We suggest that this assay may be introduced in routine analysis of genetic alterations in AML.

Minimal residual disease based on patient specific Flt3-ITD and -ITT mutations in acute myeloid leukemia

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).

The FLT3 Internal Tandem Duplication Mutation Prevents Apoptosis in Interleukin-3-Deprived BaF3 Cells Due to Protein Kinase A and Ribosomal S6 Kinase 1–Mediated BAD Phosphorylation at Serine 112

Internal tandem duplication (ITD) mutations in the FLT3 tyrosine kinase have been detected in approximately 20% of acute myeloid leukemia (AML) patients. Patients harboring FLT3/ITD mutations have a relatively poor prognosis. FLT3/ITD results in constitutive autophosphorylation of the receptor and factor-independent survival. Previous studies have shown that FLT3/ITD activates the signal transducers and activators of transcription 5 (STAT5), p42/p44 mitogen-activated protein kinase [MAPK; extracellular signal-regulated kinase (ERK) 1/2], and phosphatidylinositol 3-kinase/Akt pathways. We herein provide biochemical and biological evidence that ribosomal S6 kinase 1 (RSK1) and protein kinase A (PKA) are the two principal kinases that mediate the antiapoptotic function of FLT3/ITD via phosphorylation of BAD at Ser112. Inhibiting both MAPK kinase (MEK)/ERK and PKA pathways by a combination of U0126 (10 micromol/L) and H-89 (5 micromol/L) reduced most of BAD phosphorylation at Ser112 and induced apoptosis to a level comparable with that induced by FLT3 inhibitor AG1296 (5 micromol/L) in BaF3/FLT3/ITD cells. RNA interference of RSK1 or PKA catalytic subunit reduced BAD phosphorylation and induced apoptosis. The MEK inhibitor U0126 and/or the PKA inhibitor H-89 greatly enhanced the efficacy of the FLT3 inhibitor AG1296, suggesting that combining FLT3/ITD downstream pathway inhibition with FLT3 inhibitors may be a viable therapeutic strategy for AML caused by a FLT3/ITD mutation.

Cutting edge: generation of splenic CD8+ and CD8- dendritic cell equivalents in Fms-like tyrosine kinase 3 ligand bone marrow cultures

We demonstrate that functional and phenotypic equivalents of mouse splenic CD8(+) and CD8(-) conventional dendritic cell (cDC) subsets can be generated in vitro when bone marrow is cultured with fms-like tyrosine kinase 3 (flt3) ligand. In addition to CD45RA(high) plasmacytoid DC, two distinct CD24(high) and CD11b(high) cDC subsets were present, and these subsets showed equivalent properties to splenic CD8(+) and CD8(-) cDC, respectively, in the following: 1) surface expression of CD11b, CD24, and signal regulatory protein-alpha; 2) developmental dependence on, and mRNA expression of, IFN regulatory factor-8; 3) mRNA expression of TLRs and chemokine receptors; 4) production of IL-12 p40/70, IFN-alpha, MIP-1alpha, and RANTES in response to TLR ligands; 5) expression of cystatin C; and 6) cross-presentation of exogenous Ag to CD8 T cells. Furthermore, despite lacking surface CD8 expression, the CD24(high) subset contained CD8 mRNA and up-regulated surface expression when transferred into mice. This culture system allows access to bona fide counterparts of the splenic DC subsets.

The AML1-ETO fusion gene and the FLT3 length mutation collaborate in inducing acute leukemia in mice

The molecular characterization of leukemia has demonstrated that genetic alterations in the leukemic clone frequently fall into 2 classes, those affecting transcription factors (e.g., AML1-ETO) and mutations affecting genes involved in signal transduction (e.g., activating mutations of FLT3 and KIT). This finding has favored a model of leukemogenesis in which the collaboration of these 2 classes of genetic alterations is necessary for the malignant transformation of hematopoietic progenitor cells. The model is supported by experimental data indicating that AML1-ETO and FLT3 length mutation (FLT3-LM), 2 of the most frequent genetic alterations in AML, are both insufficient on their own to cause leukemia in animal models. Here we report that AML1-ETO collaborates with FLT3-LM in inducing acute leukemia in a murine BM transplantation model. Moreover, in a series of 135 patients with AML1-ETO-positive AML, the most frequently identified class of additional mutations affected genes involved in signal transduction pathways including FLT3-LM or mutations of KIT and NRAS. These data support the concept of oncogenic cooperation between AML1-ETO and a class of activating mutations, recurrently found in patients with t(8;21), and provide a rationale for therapies targeting signal transduction pathways in AML1-ETO-positive leukemias.

Acute myeloid leukemia bearing cytoplasmic nucleophosmin (NPMc+ AML) shows a distinct gene expression profile characterized by up-regulation of genes involved in stem-cell maintenance

Approximately one third of acute myeloid leukemias (AMLs) are characterized by aberrant cytoplasmic localization of nucleophosmin (NPMc+ AML), consequent to mutations in the NPM putative nucleolar localization signal. These events are mutually exclusive with the major AML-associated chromosomal rearrangements, and are frequently associated with normal karyotype, FLT3 mutations, and multilineage involvement. We report the gene expression profiles of 78 de novo AMLs (72 with normal karyotype; 6 without major chromosomal abnormalities) that were characterized for the subcellular localization and mutation status of NPM. Unsupervised clustering clearly separated NPMc+ from NPMc– AMLs, regardless of the presence of FLT3 mutations or non–major chromosomal rearrangements, supporting the concept that NPMc+ AML represents a distinct entity. The molecular signature of NPMc+ AML includes up-regulation of several genes putatively involved in the maintenance of a stem-cell phenotype, suggesting that NPMc+ AML may derive from a multipotent hematopoietic progenitor.

Systemic mobilization of antigen presenting cells, with a chimeric Flt-3 and G-CSF receptor agonist, during immunization of with HIV-1 antigens is insufficient to modulate immune responses or vaccine efficacy

In order to improve the efficacy of current vaccine candidates against HIV/AIDS, we sought to strengthen the induction of immune responses via simultaneous in vivo mobilization of dendritic cells using a chimeric Flt-3 and G-CSF receptor agonists (ProGP). We investigated ProGP treatment in combination with two DNA immunizations encoding HIV-Env89.6, SIV-Gag proteins to increase the priming of immune responses. Administration of this Flt-3/G-CSF chimera elicited marked increases in numbers of both plasmacytoid and myeloid dendritic cells. However, there was no increase seen in T-cell responses either directly following the DNA immunization or after further boosting with MVA vectors expressing HIV-Env89.6p, SIV-Gag. After challenge with SHIV89.6p all animals became infected and no differences were seen between the ProGP treated versus the control group with regard to plasma virus load or CD4 T-cell count. We conclude that besides mobilization of dendritic cells additional stimuli to induce dendritic cell maturation may be needed for avid boosting of antigen specific immune activation.

Specific detection of Flt3 point mutations by highly sensitive real-time polymerase chain reaction in acute myeloid leukemia

Among activating class III receptor tyrosine kinase (Flt3) mutations, internal tandem duplications of Flt3 (Flt3-ITD) are detected in about 25% of patients with acute myeloid leukemia (AML). In contrast, mutations within the tyrosine kinase domain of Flt3 (Flt3-TKD mutations) are less frequent (approximately 7%), and there are only limited data on the frequency of recently demonstrated activating Flt3 point mutation at codon 592 (Flt3-V592A mutation). We evaluated a new approach for rapid screening of Flt3-TKD and Flt3-V592A mutations using the fluorescence resonance energy transfer (FRET) principle in a group of 122 patients. Based on individual Flt3-TKD mutations, we designed patient-specific primers to perform a highly sensitive polymerase chain reaction (PCR) assay for rapid detection of minimal residual disease (MRD). We also used a model system with MonoMac-6 cells carrying the Flt3-V592A mutation to establish a mutation-specific real-time PCR approach also for this molecular aberration. We identified 9 cases (8%) of Flt3-TKD mutations (5 cases of mutation D835Y, 3 cases of mutation D835H, and 1 case of mutation Del836), and no cases of Flt3-V592A mutation. Screening for Flt3-TKD mutations with fluorescent probes is equivalent to conventional screening using standard PCR followed by EcoRV restriction. We present a real-time PCR protocol that can be used for MRD analyses based on individual Flt3-TKD mutations. Examples of MRD analyses are presented for all 3 subtypes of Flt3-TKD mutation identified in this study. In summary, we demonstrate new methodological approaches for rapid screening of Flt3 point mutations and for detection of MRD based on patient-specific Flt3-TKD mutations.

Emerging treatments in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is the most common form of leukaemia in young adults. Although 75-85% of patients will achieve complete remission after induction chemotherapy, the long-term survival is still < 50% at 5 years. Chemotherapy has increased in intensity in recent years and is perceived to have reached the limit of toxicity. Allogeneic bone marrow transplantation, which is undoubtedly the most effective way to prevent relapse, may not add substantial survival benefits. Several new pharmacological approaches to the treatment of AML are now becoming available, with various molecular targets identified, including the farnesylation of RAS family proteins and tyrosine kinases involved in signal transduction and epigenetic methylation. More selective delivery of chemotherapeutic agents is also feasible using humanised monoclonal antibodies, with the intriguing possibility of increasing treatment delivery without increasing the toxicity. However, despite the progress in the rational design of drugs in disorders such as chronic myeloid leukaemia, AML lacks a single specific pathognomic genetic event to act as a drug target. This review discusses the drugs presently under investigation in Phase II or Phase III trials in AML.

CHIR-258: A Potent Inhibitor of FLT3 Kinase in Experimental Tumor Xenograft Models of Human Acute Myelogenous Leukemia

PURPOSE

Fms-like tyrosine kinase 3 (FLT3) encodes a receptor tyrosine kinase (RTK) for which activating mutations have been identified in a proportion of acute myelogenous leukemia (AML) patients and associated with poor clinical prognosis. Given the relevance of FLT3 mutations in AML, we investigated the activity of CHIR-258, an orally active, multitargeted small molecule, with potent activity against FLT3 kinase and class III, IV, and V RTKs involved in endothelial and tumor cell proliferation in AML models.

EXPERIMENTAL DESIGN

CHIR-258 was tested on two human leukemic cell lines in vitro and in vivo with differing FLT3 mutational status [MV4;11 cells express FLT3 internal tandem duplications (ITD) versus RS4;11 cells with wild-type (WT) FLT3].

RESULTS

Antiproliferative activity of CHIR-258 against MV4;11 was approximately 24-fold greater compared with RS4;11, indicating more potent inhibition against cells with constitutively activated FLT3 ITD. Dose-dependent down modulation of receptor phosphorylation and downstream signaling [signal transducer and activator of transcription 5 (STAT5) and extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase] in MV4;11 cells with CHIR-258 confirmed the molecular mechanism of action. Target modulation of phospho-FLT3, phospho-STAT5, and phospho-ERK in MV4;11 tumors was achieved at biologically active doses of CHIR-258. Tumor regressions and eradication of AML cells from the bone marrow were shown in s.c. and bone marrow engraftment leukemic xenograft models. Tumor responses were characterized by decreased cellular proliferation and positive immunohistochemical staining for active caspase-3 and cleaved poly(ADP-ribose) polymerase, suggesting cell death was mediated in part via apoptosis.

CONCLUSIONS

Our data indicate that CHIR-258 may be an effective therapy in FLT3-associated AML and warrants clinical trials.

Morphologic, cytogenetic, and molecular abnormalities in therapy-related acute promyelocytic leukemia

We describe 17 cases of therapy-related acute promyelocytic leukemia (tAPL). Treatment for the initial neoplasms (mostly carcinomas and non-Hodgkin lymphomas) included radiation and chemotherapy in 11 patients, radiation in 3, and chemotherapy in 3. The interval between the initial neoplasm and tAPL ranged from 17 to 166 months (median, 40 months). Morphologically, all 13 cases with available bone marrow aspirate smears showed tAPL. Dyserythropoiesis or dysmegakaryopoiesis was identified in 11 cases. In 2 cases, too few nonneoplastic cells and, in all cases, too few maturing granulocytes were present to assess for dysplasia. Conventional cytogenetics or fluorescence in situ hybridization (FISH) showed the t(15;17)(q22;q21) in all cases; 6 as a sole abnormality, 9 with additional abnormalities, and 2 assessed only by FISH. Reverse transcription-polymerase chain reaction (PCR) studies showed PML/RARa in 13 cases (8 short form, 5 long form). Mutations of the flt3 gene were detected by PCR in 5 (42%) of 12 cases. We conclude that dysplastic features, secondary cytogenetic abnormalities, and flt3 mutations are common in tAPL.

Impact of FLT3 mutations and promyelocytic leukaemia-breakpoint on clinical characteristics and prognosis in acute promyelocytic leukaemia

In the present study 170 newly diagnosed acute promyelocytic leukaemia patients (M3: n = 121; M3v: n = 49) were molecularly characterised with respect to PML breakpoint and additional molecular mutations. In total, 83 patients were positive for bcr1 (49%), five for bcr2 (3%) and 82 for bcr3 (48%). Bcr3 was more frequent in M3v (65.3%) compared with M3 (41.3%) (P = 0.005). Cases with bcr3 showed a significantly higher white blood cell count (median: 3.65 x 10(9)/l vs. 1.59 x 10(9)/l, P = 0.003), as well as a higher PML-RARAABL expression ratio (14.8% vs. 72.7%, P < 0.005) compared with bcr1. FLT3-length-mutations were detected more frequently together with bcr3 compared with bcr1 (56.5% vs. 19.4%, P < 0.001) and in M3v compared with M3 (64.5% vs. 24.1%, P < 0.005). FLT3 tyrosine kinase mutations were found in eight cases (6.4%) and were distributed equally within the total group. Analysis for further mutations revealed no MLL-PTD and KIT mutations and only two cases of 99 analysed (2%) with NRAS mutations. FLT3-mutations were detected in 62 of 139 cases (44.6%) and associated with a significant lower overall survival (P = 0.0339). In addition, cases with bcr3 showed a tendency for a worse event-free survival (P = 0.0795) compared with the bcr1 group.

Rapid and sensitive detection of internal tandem duplication and activating loop mutations of FLT3

Mutations of the FLT3 gene, a receptor tyrosine kinase, are the most frequent genetic alteration reported in acute myeloid leukaemia, with internal tandem duplications (ITD) or mutations within the activating loop (AL) reported at a frequency of around 24% and 6%, respectively. ITD mutations have associated with a poor prognosis. In this study we have used polymerase chain reaction (PCR), combined with restriction enzyme digestion for the detection of AL mutations, with the DNA products separated on the Agilent 2100 Bioanalyser using a DNA-500 kit. This analysis enabled the rapid identification of mutations in FLT3, approximate sizing of the ITD, an estimate of the proportion of mutant RNA and in some cases, specific heteroduplex patterns associated with triplet deletions. Our data shows that approximately 16% of the patients examined had an ITD mutation and over 13% had a mutation in the AL including triplet deletions involving codons 835/836 and point mutations in codon D839. Based on the sensitivity and speed of the bioanalyser, we suggest that this method is invaluable and provides an improvement to the current use of agarose gels for the analysis of FLT3 PCR products.

FLT3 inhibition in t(4;11)+ adult acute lymphoid leukaemia

The present study was designed to investigate, in t(4;11)+ adult lymphoid leukaemia (ALL) blast cells, the pathogenetic role of the FLT3 protein, its level of mRNA and protein expression, the degree of constitutive phosphorylation, the possible presence of mutations of the sequence, the capacity of signal transduction and the potential therapeutic role of specific inhibitors. We evaluated nine adult ALL patients carrying this translocation. The increased FLT3 mRNA levels, determined by oligonucleotide microarray analysis, was in agreement with the increased protein expression evaluated by Western blot. The protein was constitutively phosphorylated in all cases analysed. Polymerase chain reaction detected no internal tandem duplication or point mutations. The signal transduction apparatus, after stimulation with the specific ligand, was preserved. We then investigated the effect of specific FLT3 inhibition on signal transduction and survival. The PKC412 inhibitor specifically inhibited ligand-induced phosphorylation; the same inhibitor reduced the survival of leukaemic cells when compared with untreated cells. These data indicate that the FLT3 protein might play a role in this subgroup of ALL with a particularly poor prognosis. Specific inhibition of the kinase receptor must be hypothesised as an innovative therapeutic tool for t(4;11)+ ALL patients.

Identification of Ki23819, a highly potent inhibitor of kinase activity of mutant FLT3 receptor tyrosine kinase

Constitutively active internal tandem duplication (ITD) in the juxtamembrane domain of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase, is the most common molecular defect associated with acute myeloid leukemia. Its presence confers a poor outcome in patients with acute myeloid leukemia who receive conventional chemotherapy. FLT3-ITD has therefore been considered to be an attractive molecular target for a novel therapeutic modality. We describe here the identification and characterization of Ki23819 as a novel FLT3 inhibitor. Ki23819 suppressed proliferation and induced apoptosis of FLT3-ITD-expressing human leukemia cell lines. The growth-inhibitory effect of Ki23819 on MV4-11 cells was superior to that of SU11248, another FLT3 inhibitor (IC(50)<1 vs 3-10 nM). Ki23819 inhibited the autophosphorylation of FLT3-ITD more efficiently than that of wild-type FLT3. FLT3-ITD-dependent activation of the downstream signaling proteins ERK and STAT5 was also inhibited within similar concentration ranges. Thus, Ki23819 is a potent in vitro inhibitor of FLT3.