Activating mutation of D835 within the activation loop of FLT3 in human hematologic malignancies

Activating mutations of RTK/ras signal transduction pathway in pediatric acute myeloid leukemia

Activating mutations of receptor tyrosine kinases (RTKs) and their downstream affectors are common in acute myeloid leukemia (AML). We performed mutational analysis of FLT3, c-kit, c-fms, vascular endothelial growth factor (VEGF) receptors (Flt-1, KDR [kinase domain receptor]), and ras genes in a group of 91 pediatric patients with AML treated on Children's Cancer Group clinical trial CCG-2891. Forty-six percent of patients had activating mutations of FLT3 (24.5%), c-kit (3%), or ras (21%) genes. Mutation-positive patients had a higher median diagnostic white blood cell (WBC) count (71.5 vs 19.6 x 10(9)/L; P =.005) and lower complete remission rate (55% versus 76%; P =.046) than mutation-negative patients. The Kaplan-Meier estimate of overall survival (OS) for patients with and without an activating mutation was 34% versus 57%, respectively (P =.035). However, within this group, patients with FLT3/ALM (activation loop mutation) had good outcomes (OS, 86%). Exclusion of the FLT3/ALM from analysis decreased the OS for the remaining mutation-positive patients to 26% (P =.003). Ten of the 23 mutation-positive and 11 of the 34 mutation-negative patients received an allogeneic bone marrow transplant (BMT) in first complete remission (CR). In the mutation-positive group, the disease-free survival (DFS) for the allogeneic BMT recipients was 72% versus 23% for the 13 patients who received chemotherapy or autologous BMT (P =.01). DFS for the mutation-free patients with and without allogeneic BM transplantation was 55% and 40%, respectively (P =.38). Activating mutations in the RTK/ras signaling pathway are common in pediatric AML, and their presence may identify a population at higher risk of poor outcome who may benefit from allogeneic BM transplantation.

Flt-3 and c-kit mutation studies in a spectrum of chronic myeloid disorders including systemic mast cell disease

We screened 115 patients with chronic myeloid disorders (CMD) for known flt-3 and c-kit mutations in both the juxtamembrane (JM) and the activation loop (AL) domains. None of the patients displayed flt-3 (JM or AL) or c-kit JM mutations. However, the c-kit AL (D816V) mutation was detected in 5 of 16 patients with systemic mast cell disease (SMCD) but in none of the remaining 99 patients with other CMD. In SMCD, the presence of D816V mutation was significantly associated with advanced age, an aggressive clinical course, increased bone marrow mast cell content, and chronic myelomonocytic leukemia.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

Survival of acute myeloid leukemia cells requires PI3 kinase activation

The mechanisms that regulate the growth and survival of acute myeloid leukemia (AML) cells are largely unknown. We hypothesized that constitutive activation of phosphatidyl-inositide 3 kinase (PI3 kinase) could regulate survival in primary cells from patients with AML. Here we demonstrate that Akt, a critical substrate of PI3 kinase, is activated in AML blasts. In a short-term culture system, most AML patient samples showed a dose-dependent decrease in survival after incubation with the PI3 kinase inhibitor LY294002. This decrease in survival was partially due to the induction of apoptosis. Furthermore, we have shown that p70 S6 kinase and 4EBP-1, downstream mediators of Akt signaling, also are phosphorylated in AML blasts. Phosphorylation of these proteins is inhibited by the mTOR inhibitor RAD001. Incubation of AML blasts with RAD001 induces only a small decrease in survival of the cells; however, when combined with Ara-C, RAD001 enhances the toxicity of Ara-C. These results demonstrate that constitutive activation of the PI3 kinase pathway is necessary for the survival of AML blasts and that targeting of this pathway with pharmacologic inhibitors may be of clinical benefit in treatment of AML.

Sensitivity toward tyrosine kinase inhibitors varies between different activating mutations of the FLT3 receptor

Activating mutations of FLT3 have been detected in patients with acute myeloid leukemia (AML). Two distinct types of FLT3 mutations are most common: internal tandem duplication (ITD) of sequences coding for the juxtamembrane domain and point mutations at codon 835 (Asp835) within the kinase domain. Both types of mutations constitutively activate the tyrosine kinase activity of FLT3 in experimental systems and result in factor-independent proliferation of Ba/F3 and 32D cells. Recently, novel mutations within the activation loop were identified in patients with AML: deletion of isoleucine 836 (Ile836del) and an exchange of isoleucine 836 to methionine plus an arginine insertion (Ile836Met+Arg). To examine whether the Ile836 mutations result in constitutive activation of the FLT3 receptor, we introduced both mutant FLT3 cDNAs transiently into HEK 293 cells. Both mutant FLT3 receptors were constitutively autophosphorylated in the absence of ligand and kinase activity led to constitutive activation of downstream signaling cascades as determined by activation of the STAT5 (signal transducer and activator of transcription 5) pathway. When stably expressed in the growth factor-dependent cell lines Ba/F3 and 32D, both deletion and insertion mutants led to factor-independent proliferation, indicating that both mutants have transforming capabilities. We then examined the sensitivity of the FLT3 ITD, FLT3 Asp835Tyr, and the novel FLT3 receptor mutants toward the kinase inhibitors AG1296, PKC412, and SU5614. We show that these FLT3 kinase inhibitors have distinct inhibitory potencies against different activating FLT3 receptor mutants. These results suggest that it may be useful to determine the exact kind of FLT3 mutation when applying receptor kinase inhibitors in clinical trials.

Cooperation of cytokine signaling with chimeric transcription factors in leukemogenesis: PML-retinoic acid receptor alpha blocks growth factor-mediated differentiation

We utilized a mouse model of acute promyelocytic leukemia (APL) to investigate how aberrant activation of cytokine signaling pathways interacts with chimeric transcription factors to generate acute myeloid leukemia. Expression in mice of the APL-associated fusion, PML-RARA, initially has only modest effects on myelopoiesis. Whereas treatment of control animals with interleukin-3 (IL-3) resulted in expanded myelopoiesis without a block in differentiation, PML-RARA abrogated differentiation that normally characterizes the response to IL-3. Retroviral transduction of bone marrow with an IL-3-expressing retrovirus revealed that IL-3 and promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) combined to generate a lethal leukemia-like syndrome in <21 days. We also observed that a constitutively activated mutant IL-3 receptor, beta(c)V449E, cooperated with PML-RARalpha in leukemogenesis, whereas a different activated mutant, beta(c)I374N, did not. Analysis of additional mutations introduced into beta(c)V449E showed that, although tyrosine phosphorylation of beta(c) is necessary for cooperation, the Src homology 2 domain-containing transforming protein binding site is dispensable. Our results indicate that chimeric transcription factors can block the differentiative effects of growth factors. This combination can be potently leukemogenic, but the particular manner in which these types of mutations interact determines the ability of such combinations to generate acute myeloid leukemia.

Prognostic implications of the presence of FLT3 mutations in patients with acute myeloid leukemia

Several studies have shown that mutations in the FLT3 gene are common events in AML, with approximately one third of adult patients harbouring either an internal tandem duplication in the juxtramembrane domain or a D835 mutation in the kinase domain. The majority of studies in pediatric and adult AML have shown that FLT3 mutations are powerful prognostic factors predicting for increased relapse risk and adverse overall survival. Some reports have suggested that loss of the wild type allele might be associated with an even worse prognosis. Changes in the pattern of FLT3 mutations between disease presentation and relapse restrict their value as a marker of minimal residual disease, and have significant implications for therapy. The optimum treatment for patients with FLT3 mutations remains unknown and large prospective studies are warranted to evaluate the efficacy of various treatment modalities such as bone marrow transplantation and targeted therapy with tyrosine kinase inhibitors.

Rapamycin inhibits IL-4--induced dendritic cell maturation in vitro and dendritic cell mobilization and function in vivo

Rapamycin (RAPA) is a potent immunosuppressive macrolide hitherto believed to mediate its action primarily via suppression of lymphocyte responses to interleukin 2 (IL-2) and other growth factors. We show here that this view is incomplete and provide evidence that RAPA suppresses the functional activation of dendritic cells (DCs) both in vitro and in vivo. In vitro, RAPA inhibits IL-4-dependent maturation and T-cell stimulatory activity of murine bone marrow-derived DCs. These effects are associated with posttranscriptional down-regulation of both subunits of the IL-4 receptor complex (CD124, CD132) and are mediated via binding of RAPA to its intracellular receptor FK506-binding protein 12 (FKBP12). In vivo, RAPA impairs steady-state DC generation and fms-like tyrosine 3 kinase ligand (Flt3L)-induced DC mobilization. In addition, in vivo administration of RAPA impairs DC costimulatory molecule up-regulation, production of proinflammatory cytokines, and T-cell allostimulatory capacity. These novel findings have implications for RAPA-based therapy of chronic DC-triggered autoimmune diseases, transplant rejection, and hematologic malignancies with activating Flt3 mutations.

FLT3-TKD mutation in childhood acute myeloid leukemia

Mutations of receptor tyrosine kinases are implicated in the constitutive activation and development of human hematologic malignancies. An internal tandem duplication (ITD) of the juxtamembrane domain-coding sequence of the FLT3 gene (FLT3-ITD) is found in 20-25% of adult acute myeloid leukemia (AML) and at a lower frequency in childhood AML. FLT3-ITD is associated with leukocytosis and a poor prognosis, especially in patients with normal karyotype. Recently, there have been three reports on point mutations at codon 835 of the FLT3 gene (D835 mutations) in adult AML. These mutations are located in the activation loop of the second tyrosine kinase domain (TKD) of FLT3 (FLT3-TKD). The clinical and prognostic relevance of the TKD mutations is less clear. To the best of our knowledge, there has been no report to describe FLT3-TKD mutations in childhood AML. In this pediatric series, FLT3-TKD mutations occurred in three of 91 patients (3.3%), an incidence significantly lower than that of FLT3-ITD (14 of 91 patients, 15.4%) in the same cohort of patients. None of them had both FLT3-TKD and FLT3-ITD mutations. Sequence analysis showed one each of D835 Y, D835 V, and D835 H. Of the three patients carrying FLT3-TKD, two had AML-M3 with one each of L- and V-type PML-RARalpha, and another one had AML-M2 with AML1-ETO. None of our patients with FLT3-TKD had leukocytosis at diagnosis. At bone marrow relapse, one of the four patients examined acquired FLT3-ITD mutation and none gained FLT3-TKD mutation.

Detection of FLT3 internal tandem duplication and D835 mutations by a multiplex polymerase chain reaction and capillary electrophoresis assay

FLT3 is a receptor tyrosine kinase that is expressed on early hematopoietic progenitor cells and plays an important role in stem cell survival and differentiation. Two different types of functionally important FLT3 mutations have been identified. Internal tandem duplication mutations arise from duplications of the juxtamembrane portion of the gene and result in constitutive activation of the FLT3 protein. This alteration has been identified in approximately 20% to 30% of patients with acute myelogenous leukemia and appears to be associated with a worse prognosis. The second type of FLT3 mutation, missense mutations at aspartic acid residue 835, occurs in approximately 7.0% of acute myelogenous leukemia cases. These mutations also appear to be activating and to portend a worse prognosis. Identification of FLT3 mutations is important because it provides prognostic information and may play a pivotal role in determining appropriate treatment options. We have developed an assay to identify both internal tandem duplication and D835 FLT3 mutations in a single multiplex polymerase chain reaction. After amplification, the polymerase chain reaction products are analyzed by capillary electrophoresis for length mutations and resistance to EcoRV digestion. Here we describe the performance characteristics of the assay, assay validation, and our clinical experience using this assay to analyze 147 clinical specimens.

SU11248 is a novel FLT3 tyrosine kinase inhibitor with potent activity in vitro and in vivo

FLT3 (fms-related tyrosine kinase/Flk2/Stk-2) is a receptor tyrosine kinase (RTK) primarily expressed on hematopoietic cells. In blasts from acute myelogenous leukemia (AML) patients, 2 classes of FLT3 activating mutations have been identified: internal tandem duplication (ITD) mutations in the juxtamembrane domain (25%-30% of patients) and point mutations in the kinase domain activation loop (7%-8% of patients). FLT3-ITD mutations are the most common molecular defect identified in AML and have been shown to be an independent prognostic factor for decreased survival. FLT3-ITD is therefore an attractive molecular target for therapy. SU11248 is a recently described selective inhibitor with selectivity for split kinase domain RTKs, including platelet-derived growth factor receptors, vascular endothelial growth factor receptors, and KIT. We show that SU11248 also has potent activity against wild-type FLT3 (FLT3-WT), FLT3-ITD, and FLT3 activation loop (FLT3-Asp835) mutants in phosphorylation assays. SU11248 inhibits FLT3-driven phosphorylation and induces apoptosis in vitro. In addition, SU11248 inhibits FLT3-induced VEGF production. The in vivo efficacy of SU11248 was investigated in 2 FLT3-ITD models: a subcutaneous tumor xenograft model and a bone marrow engraftment model. We show that SU11248 (20 mg/kg/d) dramatically regresses FLT3-ITD tumors in the subcutaneous tumor xenograft model and prolongs survival in the bone marrow engraftment model. Pharmacokinetic and pharmacodynamic analysis in subcutaneous tumors showed that a single administration of an efficacious drug dose potently inhibits FLT3-ITD phosphorylation for up to 16 hours following a single dose. These results suggest that further exploration of SU11248 activity in AML patients is warranted.

A model of APL with FLT3 mutation is responsive to retinoic acid and a receptor tyrosine kinase inhibitor, SU11657

The PML-RAR alpha fusion protein is central to the pathogenesis of acute promyelocytic leukemia (APL). Expression of this protein in transgenic mice initiates myeloid leukemias with features of human APL, but only after a long latency (8.5 months in MRP8 PML-RARA mice). Thus, additional changes contribute to leukemic transformation. Activating mutations of the FLT3 receptor tyrosine kinase are common in human acute myeloid leukemias and are frequent in human APL. To assess how activating mutations of FLT3 contribute to APL pathogenesis and impact therapy, we used retroviral transduction to introduce an activated allele of FLT3 into control and MRP8 PML-RARA transgenic bone marrow. Activated FLT3 cooperated with PML-RAR alpha to induce leukemias in 62 to 299 days (median latency, 105 days). In contrast to the leukemias that arose spontaneously in MRP8 PML-RARA mice, the activated FLT3/PML-RAR alpha leukemias were characterized by leukocytosis, similar to human APL with FLT3 mutations. Cytogenetic analysis revealed clonal karyotypic abnormalities, which may contribute to pathogenesis or progression. SU11657, a selective, oral, multitargeted tyrosine kinase inhibitor that targets FLT3, cooperated with all-trans retinoic acid to rapidly cause regression of leukemia. Our results suggest that the acquisition of FLT3 mutations by cells with a pre-existing t(15;17) is a frequent pathway to the development of APL. Our findings also indicate that APL patients with FLT3 mutations may benefit from combination therapy with all-trans retinoic acid plus an FLT3 inhibitor.

Suppression of myeloid transcription factors and induction of STAT response genes by AML-specific Flt3 mutations

The receptor tyrosine kinase Flt3 is expressed and functionally important in early myeloid progenitor cells and in the majority of acute myeloid leukemia (AML) blasts. Internal tandem duplications (ITDs) in the juxtamembrane domain of the receptor occur in 25% of AML cases. Previously, we have shown that these mutations activate the receptor and induce leukemic transformation. In this study, we performed genome-wide parallel expression analyses of 32Dcl3 cells stably transfected with either wild-type or 3 different ITD isoforms of Flt3. Comparison of microarray expression analyses revealed that 767 of 6586 genes differed in expression between FLT3-WT- and FLT3-ITD-expressing cell lines. The target genes of mutationally activated Flt3 resembled more closely those of the interleukin 3 (IL-3) receptor than those of ligand-activated Flt3. The serine-threonine kinase Pim-2 was up-regulated on the mRNA and the protein level in Flt3-ITD-expressing cells. Further experiments indicated that Pim-2 function was important for clonal growth of 32D cells. Several genes repressed by the mutations were found to be involved in myeloid gene regulation. Pu.1 and C/EBPalpha, both induced by ligand-activation of wild-type Flt3, were suppressed in their expression and function by the Flt3 mutations. In conclusion, internal tandem duplication mutations of Flt3 activate transcriptional programs that partially mimic IL-3 activity. Interestingly, other parts of the transcriptional program involve novel, IL-3-independent pathways that antagonize differentiation-inducing effects of wild-type Flt3. The identification of the transcriptional program induced by ITD mutations should ease the development of specific therapies.

Tyrosine kinases as targets in cancer therapy - successes and failures

Protein kinases play a crucial role in signal transduction and also in cellular proliferation, differentiation and various regulatory mechanisms. The inhibition of growth-related kinases, especially tyrosine kinases, might therefore provide new therapies for diseases such as cancer. Due to the enormous progress that has been made in the past few years in the identification of the human genome, in molecular and cell biology technologies, in structural biology and in bioinformatics, the number of receptor and non-receptor tyrosine kinases that have been identified as valuable molecular targets has greatly increased. Currently, more than 20 different tyrosine kinase targets are under evaluation in drug discovery projects in oncology. The progress made in the crystallisation of protein kinases, in most cases complexed with ATP-site-directed inhibitors, has confirmed that the ATPbinding domain of tyrosine kinases is an attractive target for rational drug design; more than 20 ATP-competitive, low molecular weight inhibitors are in various phases of clinical evaluation. Meanwhile, clinical proof-of-concept (POC) has been achieved with several antibodies and small molecules targeted against tyrosine kinases. With Herceptin, Glivec and Iressa (registered in Japan), the first kinase drugs have entered the market. This review describes the preclinical and clinical status of low molecular weight drugs targeted against different tyrosine kinases (e.g., epidermal growth factor receptor [EGFR], vascular endothelial growth factor receptor [VEGFR], platelet-derived growth factor receptor [PDGFR], Kit, Fms-like tyrosine kinase [Flt]-3), briefly describes new targets, and provides a critical analysis of the current situation in the area of tyrosine kinase inhibitors.

t(10;16)(q22;p13) and MORF-CREBBP fusion is a recurrent event in acute myeloid leukemia

Recently, it was shown that t(10;16)(q22;p13) fuses the MORF and CREBBP genes in a case of childhood acute myeloid leukemia (AML) M5a, with a complex karyotype containing other rearrangements. Here, we report a new case with the MORF-CREBBP fusion in an 84-year-old patient diagnosed with AML M5b, in which the t(10;16)(q22;p13) was the only cytogenetic aberration. This supports that this is a recurrent pathogenic translocation in AML.

Molecular targets in acute myelogenous leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, most patients will either die of relapse or die from the complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed. Recently, a small molecular inhibitor (STI571 or Gleevec) that targets the BCR-ABL gene was found to have a dramatic clinical effect in patients with chronic myelogenous leukemia (CML). These results have encouraged investigators to search for additional small molecular inhibitors and other targeted therapies that may be applicable to other forms of leukemia. In this review, we examine some of the signaling pathways that are aberrantly regulated in AML, focusing on the tyrosine kinase/RAS/MAP kinase and JAK/STAT pathways. After reviewing these two pathways, we explore some of the targeted therapies directed at these pathways that are under development for AML, many of which are already in clinical trials.

The protein tyrosine kinase inhibitor SU5614 inhibits FLT3 and induces growth arrest and apoptosis in AML-derived cell lines expressing a constitutively activated FLT3

Activating mutations of the protein tyrosine kinase (PTK) FLT3 can be found in approximately 30% of patients with acute myeloid leukemia (AML), thereby representing the most frequent single genetic alteration in AML. These mutations occur in the juxtamembrane (FLT3 length mutations; FLT3-LMs) and the second tyrosine kinase domain of FLT3-TKD and confer interleukin 3 (IL-3)-independent growth to Ba/F3 cells. In the mouse bone marrow transplantation model, FLT3-LMs induce a myeloproliferative syndrome stressing their transforming activity in vivo. In this study, we analyzed the pro-proliferative and antiapoptotic potential of FLT3 in FLT3-LM/TKD-mutation-transformed Ba/F3 cells and AML-derived cell lines. The PTK inhibitor SU5614 has inhibitory activity for FLT3 and selectively induces growth arrest, apoptosis, and cell cycle arrest in Ba/F3 and AML cell lines expressing a constitutively activated FLT3. In addition, the compound reverts the antiapoptotic and pro-proliferative activity of FLT3 ligand (FL) in FL-dependent cells. No cytotoxic activity of SU5614 was found in leukemic cell lines that express a nonactivated FLT3 or no FLT3 protein. At the biochemical level, SU5614 down-regulated the activity of the hyperphosphorylated FLT3 receptor and its downstream targets, signal transducer and activator of (STAT) 3, STAT5, and mitogen-activated protein kinase (MAPK), and the STAT5 target genes BCL-X(L) and p21. Our results show that SU5614 is a PTK inhibitor of FLT3 and has antiproliferative and proapoptotic activity in AML-derived cell lines that endogenously express an activated FLT3 receptor. The selective and potent cytotoxicity of FLT3 PTK inhibitors support a clinical strategy of targeting FLT3 as a new molecular treatment option for patients with FLT3-LM/TKD-mutation(+) AML.

Effect of tyrosine kinase inhibitor STI571 on the kinase activity of wild-type and various mutated c-kit receptors found in mast cell neoplasms

Systemic mastocytosis (SM) is a rare disease caused by an abnormal mast cell accumulation in various tissues. Two classes of constitutive activating c-kit mutations are found in SM. The most frequent class occurs in the catalytic pocket coding region with substitutions at codon 816 and the other in the intracellular juxtamembrane coding region. Therefore, kinase inhibitors that block mutated c-kit activity might be used as therapeutic agents in SM. Here, we show that STI571 inhibits both wild-type and juxtamembrane mutant c-kit kinase activity, but has no effect on the activity of the D816 V mutant. Accordingly, STI571 selectively decreases the survival of normal mast cell and of mast cell lines either with juxtamembrane c-kit mutations, but not that of tumoral mast cell from patient with SM or of mast cell lines with the D816 V mutation. Therefore, STI571 is not a good candidate to treat SM and specific kinase inhibitors should be designed to inhibit constitutive activating mutations at codon 816.

Inhibition of FLT3 in MLL. Validation of a therapeutic target identified by gene expression based classification

We recently found that MLL-rearranged acute lymphoblastic leukemias (MLL) have a unique gene expression profile including high level expression of the receptor tyrosine kinase FLT3. We hypothesized that FLT3 might be a therapeutic target in MLL and found that 5 of 30 MLLs contain mutations in the activation loop of FLT3 that result in constitutive activation. Three are a newly described deletion of I836 and the others are D835 mutations. The recently described FLT3 inhibitor PKC412 proved cytotoxic to Ba/F3 cells dependent upon activated FLT3 containing either mutation. PKC412 is also differentially cytotoxic to leukemia cells with MLL translocations and FLT3 that is activated by either overexpression of the wild-type receptor or mutation. Finally, we developed a mouse model of MLL and used bioluminescent imaging to determine that PKC412 is active against MLL in vivo.

Necessity of tyrosine 719 and phosphatidylinositol 3'-kinase-mediated signal pathway in constitutive activation and oncogenic potential of c-kit receptor tyrosine kinase with the Asp814Val mutation

Substitution of valine (Val) for aspartic acid (Asp) at codon 814 constitutively activates murine c-kit receptor tyrosine kinase (KIT), and Asp816Val mutation, corresponding to murine Asp814Val mutation, is found in patients with mastocytosis and acute myelocytic leukemia. However, the signal transduction pathways responsible for oncogenesis by the Asp814Val mutant (KIT(Val814)) are not fully understood. To examine the oncogenic signal transduction of KIT(Val814), we converted 20 tyrosine (Tyr) residues to phenylalanine (Phe) in the cytoplasmic domain of KIT(Val814) or deleted the C-terminal region containing 2 other tyrosine residues (Del). Among various KIT(Val814)- derived mutants, KIT(Val814-Tyr719Phe) and KIT(Val814-Del) severely impaired receptor tyrosine phosphorylation and association with the p85 subunit of phosphatidylinositol 3'-kinase (p85 (PI3-K)). Moreover, KIT(Val814-Tyr719Phe) and KIT(Val814-Del) failed to induce ligand-independent growth in Ba/F3 cells, indicating that Tyr719, the binding site for p85(PI3-K), and the C-terminal region are indispensable for factor-independent growth by KIT(Val814). Although the C-terminal region was also required for ligand-dependent growth by wild-type KIT (KIT(WT)), the Tyr719Phe substitution had negligible effects on ligand-dependent growth by KIT(WT). Furthermore, dominant-negative PI3-K significantly inhibited ligand-independent growth by KIT(Val814). These results demonstrate that Tyr719 is crucial for constitutive activation of KIT(Val814), but not for the ligand-induced activation of KIT(WT), and that the downstream signaling of PI3-K plays an important role in ligand-independent growth and tumorigenicity by KIT(Val814), thereby suggesting that KIT(Val814) is a unique activating mutation that leads to a distinguishable function from the effects of KIT(WT).

Mutational analysis of class III receptor tyrosine kinases (C-KIT, C-FMS, FLT3) in idiopathic myelofibrosis

Genomic DNA from patients with idiopathic myelofibrosis (IMF) was screened by polymerase chain reaction (PCR) and conformation sensitive gel electrophoresis (CSGE) for mutations in the C-KIT gene (60 patients), as well as the C-FMS and FLT3 genes (40 patients). Intronic primers were used to amplify the entire coding region of both the C-KIT and C-FMS genes, and selected regions of the FLT3 gene. CSGE and direct DNA sequencing detected all previously reported as well as several novel polymorphisms in each of the genes. A novel c-fms exon 9 mutation (Gly413Ser) was detected in two patients. Its functional significance remains to be determined. The c-kit mutation Asp52Asn, previously described in two of six IMF patients in Japan, was not detected in this study. In addition, the reported c-fms mutations involving codons 301 and 969 were not identified. Therefore, in contrast to acute myeloid leukaemia, mutations in RTKs class III do not appear to play a significant pathogenetic role in idiopathic myelofibrosis.

PDGFRA activating mutations in gastrointestinal stromal tumors

Most gastrointestinal stromal tumors (GISTs) have activating mutations in the KIT receptor tyrosine kinase, and most patients with GISTs respond well to Gleevec, which inhibits KIT kinase activity. Here we show that approximately 35% (14 of 40) of GISTs lacking KIT mutations have intragenic activation mutations in the related receptor tyrosine kinase, platelet-derived growth factor receptor alpha (PDGFRA). Tumors expressing KIT or PDGFRA oncoproteins were indistinguishable with respect to activation of downstream signaling intermediates and cytogenetic changes associated with tumor progression. Thus, KIT and PDGFRA mutations appear to be alternative and mutually exclusive oncogenic mechanisms in GISTs.

FLT3 mutations in acute myeloid leukemia cell lines

Internal tandem duplications (ITD) and D835 point mutations of the receptor tyrosine kinase (RTK) FLT3 are found in a high proportion of cases with acute myeloid leukemia (AML). These genetic aberrations may lead to the constitutive activation of the receptor, thus providing the molecular basis for a persisting growth stimulus. We have screened 69 AML-derived cell lines for FLT3 mutations. Four of these cell lines showed ITD of the FLT3 gene, none carried a D835 point mutation. Two cell lines (MUTZ-11 and MV4-11) expressed exclusively the mutated allele, the other two cell lines (MOLM-13 and PL-21) displayed a mutated and the wild-type version of the gene. Although mutationally activated FLT3 is supposed to substitute for the stimulatory signal of a growth factor, one of these cell lines (MUTZ-11) was strictly cytokine-dependent. FLT3 transcripts were found in all four cell lines, but the constitutively phosphorylated receptor protein was clearly detectable only in cell line MV4-11, possibly explaining why MUTZ-11 cells were growth-factor dependent. Thus, not all FLT3 ITD-positive cells express high levels of the active receptor protein, a finding that might be of relevance for a possible future application of a kinase inhibitor as therapeutic agent. It had been described that STAT-5 phosphorylation was part of the FLT3 signalling chain and that STAT-5 molecules were constitutively phosphorylated in FLT3 ITD-positive cells. Although we observed the constitutive phosphorylation of STAT-5 molecules in FLT3-mutant cells, FLT3 ligand (FL) did not induce STAT-5 phosphorylation in FLT3 wild-type cells. These results suggest that the signalling mechanisms of the mutated FL receptor differ at least to some extent from those conferred by wild-type FLT3. In conclusion, (1) not all cells with FLT3 ITD express significant amounts of the mutated receptor protein; (2) signals downstream from wild-type and mutant FLT3 receptors are not 100% identical; and (3) MV4-11 represents a model cell line for FLT3 ITD signalling.

Update in childhood acute myeloid leukemia: recent developments in the molecular basis of disease and novel therapies

Childhood acute myeloid leukemia is a heterogeneous group of disorders that remains challenging to treat. There are multiple common genetic alterations in childhood acute myeloid leukemia. These include chromosomal translocations affecting RUNX1-CBFbeta, RARalpha, and MLL. There are known activating mutations in the genes for the receptor tyrosine kinases FLT3, KIT, and FMS. As these abnormalities are better understood, they are providing important insights into the pathogenesis of disease as well as information about prognosis. Although intensive chemotherapy remains the mainstay of acute myeloid leukemia therapy, long-term cure rates with chemotherapy alone remain approximately 50%, creating an urgent need for better therapies. Multiple avenues are being explored in the design of new treatments for pediatric acute myeloid leukemia. Targeted therapies include targeted antibody therapy; inhibitors of FLT3, KIT, and farnesyltransferase; diphtheria toxin conjugated to the granulocyte-macrophage colony-stimulating factor; and antisense oligonucleotides. Another area of interest is chromatin remodeling and differentiation therapy, including agents such as all- retinoic acid, arsenic trioxide, and inhibitors of DNA methylation and histone deacetylation. There are also ongoing trials of antiangiogenesis agents. Another avenue for novel therapies is immunotherapy with agents such as interleukin-2 and tumor vaccines. This article reviews recent advances in understanding of the molecular basis for childhood acute myeloid leukemia and the design of novel therapies for the treatment of childhood acute myeloid leukemia.

Role of constitutively activated protein tyrosine kinases in malignant myeloproliferative disorders: an update

Modern molecular technology helped identify more than 10 protein tyrosine kinases related to myeloid malignancies, which allowed the development of small molecule inhibitors targeting deregulated protein tyrosine kinase activity. Protein tyrosine kinase deregulation can occur as a consequence of fusion gene formation because of chromosomal translocations, or as distinct gain-of-function point mutations. Although the tyrosine kinase inhibitor imatinib mesylate (Gleevec) targeting the ABL protein tyrosine kinase has revolutionized current chronic myeloid leukemia therapy, it became rapidly evident that overcoming the multiple cellular resistance mechanisms will be very challenging. To develop efficient therapeutic alternatives, one must understand the complex signal transduction mechanisms involved in transformation by deregulated protein tyrosine kinases. This article reviews the most recently identified molecular mechanisms involved in cell transformation by the BCR/ABL protein tyrosine kinase fusion and presents new members of the increasing family of deregulated protein tyrosine kinases involved in myeloproliferative disorders. In addition, the article discusses new, promising small molecule protein tyrosine kinase inhibitors and the molecular mechanism that may lead to resistance to these drugs. Finally, the article highlights putative alternative strategies that could be used to block signal transduction pathways of deregulated protein tyrosine kinase activity.

Acute Myeloid Leukemia and Acute Promyelocytic Leukemia

The therapeutic approach to the patient with acute myeloid leukemia (AML) currently evolves toward new frontiers. This is particularly apparent from the entree of high-throughput diagnostic technologies and the identification of prognostic and therapeutic targets, the introduction of therapies in genetically defined subgroups of AML, as well as the influx of investigational approaches and novel drugs into the pipeline of clinical trials that target pathogenetic mechanisms of the disease.

In Section I, Dr. Bob Löwenberg reviews current issues in the clinical practice of the management of adults with AML, including those of older age. Dr. Löwenberg describes upcoming possibilities for predicting prognosis in defined subsets by molecular markers and reviews experimental strategies to improve remission induction and postinduction treatment.

In Section II, Dr. James Griffin reviews the mechanisms that lead to activation of tyrosine kinases by mutations in AML, the consequences of that activation for the cell, and the opportunities for targeted therapy and discusses some examples of developing novel drugs (tyrosine kinase inhibitors) and their effectiveness in AML (FLT3).

In Section III, Dr. Martin Tallman describes the evaluation and management of patients with acute promyelocytic leukemia, a notable example of therapeutic progress in a molecularly defined entity of leukemia. Dr. Tallman focuses on the molecular genetics of APL, current curative treatment strategies and approaches for patients with relapsed and refractory disease. In addition, areas of controversy regarding treatment are addressed.

Prognostic significance of activating FLT3 mutations in younger adults (16 to 60 years) with acute myeloid leukemia and normal cytogenetics: a study of the AML Study Group Ulm

To assess the prognostic relevance of activating mutations of the FLT3 gene in homogeneously treated adults 16 to 60 years of age with acute myeloid leukemia (AML) and normal cytogenetics, pretreatment samples from 224 patients entered into 2 consecutive multicenter treatment trials were analyzed for FLT3 internal tandem duplications (ITDs) and Asp835 mutations. Treatment included intensive double-induction therapy and postremission therapy with high cumulative doses of high-dose cytarabine. ITDs were detected in 32% of the patients and were related to de novo AML and to high white blood cell (WBC) counts, percentages of peripheral blood (PB) and bone marrow (BM) blasts, and serum lactate dehydrogenase levels. Asp835 mutations were present in 14% of the patients and were associated with WBC counts and percentages of PB and BM blasts that were higher than those of patients without FLT3 mutations. With a median follow-up of 34 months, remission duration and overall survival (OS) were significantly shorter for patients with Asp835 mutations or an ITD than for those without FLT3 mutations (P =.03 and P =.0004, respectively). These results were attributable mainly to the negative prognostic effect of FLT3 ITDs. On multivariate analysis, mutant FLT3 was an independent marker affecting remission duration and OS (hazard ratio, 2.35 and 2.11, respectively). Fluorescence in situ hybridization did not detect monoallelic FLT3 deletions in ITD-positive patients. FLT3 mutations identify a subset of young AML patients with normal cytogenetics who do not benefit from intensive chemotherapy, including double-induction and postremission therapy with high-dose cytarabine.

Genetics of myeloid leukemias

Human leukemias are typified by acquired recurring chromosomal translocations. Cloning of these translocation breakpoints has provided important insights into pathogenesis of disease as well as novel therapeutic approaches. Chronic myelogenous leukemias (CML) are caused by constitutively activated tyrosine kinases, such as BCR/ABL, that confer a proliferative and survival advantage to hematopoietic progenitors but do not affect differentiation. These activated kinases are validated targets for therapy with selective tyrosine kinase inhibitors, a paradigm that may have broad applications in treatment of hematologic malignancies as well as solid tumors. Chromosomal translocations in acute myeloid leukemias (AML) most often result in loss-of-function mutations in transcription factors that are required for normal hematopoietic development. These latter mutations, however, are not sufficient to cause AML. The available evidence indicates that activating mutations in the hematopoietic tyrosine kinases FLT3 and c-KIT, and in N-RAS and K-RAS, confer proliferative advantage to hematopoietic progenitors and cooperate with loss-of-function mutations in hematopoietic transcription factors to cause an acute leukemia phenotype characterized by proliferation and impaired differentiation. The data supporting this hypothesis and the clinical and therapeutic implications of these observations are reviewed.

Acute leukemia: a pediatric perspective

The spectrum of hematological malignancies differs markedly between children and adults. Moreover, diseases such as acute lymphoblastic leukemia, acute myeloid leukemia, and myelodysplastic syndrome also demonstrate distinct biologic features and responses to treatment between these populations. In this review, we summarize our current understanding of the molecular pathology of acute leukemia and myelodysplastic syndrome, emphasizing areas in which studies in pediatric patients are providing unique insights into the hematopoietic malignancies of adults.

Alterations of the FLT3 gene in acute promyelocytic leukemia: association with diagnostic characteristics and analysis of clinical outcome in patients treated with the Italian AIDA protocol

Alterations in the FLT3 gene, including internal tandem duplications (ITDs) and D835 mutations occur frequently in acute myelogenous leukemia. We investigated the prevalence and clinico-biological correlations of FLT3 ITDs and D835 mutations in 90 patients with acute promyelocytic leukemia (APL) receiving the AIDA protocol. Twenty patients in which both presentation and relapse material was available were analyzed sequentially. Thirty-three patients (37%) harbored the ITD, and seven (7.7%) the D835 mutation in blasts obtained at diagnosis. Presence of ITDs was strongly associated with high WBC count (P = 0.0001), M3 variant (P = 0.0004), and the short (BCR3) PML/RARalpha isoform (P = 0.003). There was no difference in response to induction in the two ITD+ve and ITD-ve groups, while a trend towards inferior outcome was observed for ITD+ve cases when analyzing disease-free survival (DFS) and relapse risk (RR). These differences, however, did not reach statistical significance. Sequential studies showed variable patterns in diagnostic and relapse material, ie ITD (-ve/-ve, +ve/+ve, +ve/-ve, -ve/+ve) and D835 (-ve/-ve, +ve/-ve, -ve/+ve). Our results indicate that FLT3 alterations are associated in APL with more aggressive clinical features and suggest that these lesions may not play a major role in leukemia progression.

A new and recurrent activating length mutation in exon 20 of the FLT3 gene in acute myeloid leukemia

Activating length mutations in the juxtamembrane (JM) domain of the FLT3 gene (FLT3-LM) and mutations in the catalytic domain (FLT3D835/836) of this receptor tyrosine kinase represent the most frequent genetic alterations in acute myeloid leukemia (AML). Here, we describe a 6-bp insertion in the activation loop of FLT3 between codons 840 and 841 of FLT3 (FLT3-840GS) in 2 unrelated patients with AML. Screening for other activating mutations of FLT3, KIT, and NRAS showed no further genetic alterations in patients carrying the FLT3-840GS. In functional analyses we could show that this mutant is hyperphosphorylated on tyrosine and confers interleukin 3-independent growth to Ba/F3 cells, which can be inhibited by a specific FLT3 protein tyrosine kinase (PTK) inhibitor. Our results show for the first time that in addition to known mutations in the JM and the catalytic domain, further activating length mutations exist in the FLT3 gene.

SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase

Internal tandem duplication (ITD) in the juxtamembrane portion of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase (RTK), is the most common molecular defect associated with acute myeloid leukemia (AML). The high prevalence of this activating mutation makes it a potential target for molecularly based therapy. Indolinone tyrosine kinase inhibitors have known activity against KIT, another member of the type III RTK family. Given the conserved homology between members of this family, we postulated that the activity of some KIT inhibitors would extend to FLT3. We used various leukemic cell lines (BaF3, MV 4-11, RS 4;11) to test the activity of indolinone compounds against the FLT3 kinase activity of both wild-type (WT) and ITD isoforms. Both SU5416 and SU5614 were capable of inhibiting autophosphorylation of ITD and WT FLT3 (SU5416 concentration that inhibits 50% [IC(50)], 100 nM; and SU5614 IC(50) 10 nM). FLT3-dependent activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 5 (STAT5) was also inhibited by treatment in the same concentration ranges. FLT3 inhibition by SU5416 and SU5614 resulted in reduced proliferation (IC(50), 250 nM and 100 nM, respectively) and induction of apoptosis of FLT3 ITD-positive leukemic cell lines. Treatment of these cells with an alternative growth factor (granulocyte-macrophage colony-stimulating factor [GM-CSF]) restored MAPK signaling and cellular proliferation, demonstrating specificity of the observed inhibitory effects. We conclude that SU5416 and SU5614 are potent inhibitors of FLT3. Our finding that inhibition of FLT3 induces apoptosis of leukemic cells supports the feasibility of targeting FLT3 as a novel treatment strategy for AML.

Studies of FLT3 mutations in paired presentation and relapse samples from patients with acute myeloid leukemia: implications for the role of FLT3 mutations in leukemogenesis, minimal residual disease detection, and possible therapy with FLT3 inhibitors

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. We have studied FLT3 mutations in paired presentation and relapse samples to ascertain the biology of these mutations and to evaluate whether they can be used as markers of minimal residual disease. At diagnosis, 24 patients were wild-type FLT3, and 4 acquired a FLT3 mutation at relapse (2 D835(+), 2 ITD(+)), with a further patient acquiring an ITD at second relapse. Of 20 patients positive at diagnosis (18 ITD(+), 2 D835(+)), 5 who were all originally ITD(+) had no detectable mutation at relapse, as determined by a sensitive radioactive polymerase chain reaction. One of these patients had acquired an N-Ras mutation not detectable at presentation. Furthermore, another patient had a completely different ITD at relapse, which could not be detected in the presentation sample. 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.

Signaling revisited in acute promyelocytic leukemia

Although transcription factors are still the main focus to understanding leukemogenesis, recent results strongly suggest that alteration of a receptor and/or subsequent signaling plays a critical and co-operative role in the pathogenesis of acute myeloid leukemia (AML). The t(15;17) translocation, found in 95% of APL, encodes a PML-RARalpha fusion protein. A main model proposed for acute promyelocytic leukemia (APL) is that PML-RARalpha exerts its oncogenic effects by repressing retinoic acid-inducible genes critical to myeloid differentiation. Dysregulation of these genes may result in abnormal signaling, thereby freeing pre-leukemic cells from controls which normally induce the onset of differentiation. It is also likely that treatment of APL cells by retinoic acid induces de novo up-regulation of the same genes which are dominantly repressed by PML-RARalpha and whose expression is required for reactivation of the differentiation program. Identification of such genes together with the signaling pathways interrupted at the early stages of leukemia transformation and reactivated during retinoic acid-induced differentiation in APL cells will contribute to the development of new molecular targets for treatment of leukemia.

Inhibition of the transforming activity of FLT3 internal tandem duplication mutants from AML patients by a tyrosine kinase inhibitor

FLT3 is a receptor tyrosine kinase that may play a role in a significant proportion of leukemias. In addition to being aberrantly expressed in acute leukemias, activating mutations of the FLT3 gene have been found in patients with AML, myelodysplastic syndrome (MDS) and more rarely, ALL. Internal tandem duplications (ITDs) of the FLT3 gene have been detected in 17-34% of patients with AML and portend a poor prognosis for these patients. FLT3 receptors containing ITD mutations (FLT3/ITDs) are constitutively activated in the absence of FLT3 ligand (FL) stimulation leading to the activation of downstream signaling proteins, including ERK and STAT 5. FLT3 activity, therefore, is a logical target for therapeutic intervention. AG1296 is a tyrosine kinase inhibitor of the tyrphostin class that shows inhibitory activity for wild-type FLT3, in addition to the PDGF and c-KIT receptors. We examined the inhibitory effects of AG1296 on FLT3/ITDs isolated from AML patients in the IL-3-dependent cell line, Ba/F3, as well as in primary leukemia samples from AML patients. Immunoprecipitation and immunoblotting analyses demonstrated that FLT3/ITDs were constitutively phosphorylated in the absence of FL. The auto-phosphorylation of FLT3/ITDs was inhibited by AG1296 with an IC(50) of approximately 1 microM. FLT3/ITDs were associated with constitutive phosphorylation of ERK, STAT 5A, STAT 5B, CBL, VAV and SHP2 in Ba/F3 cells. The phosphorylation of these downstream signaling molecules was suppressed in a dose-responsive fashion by AG1296. AG1296 inhibited IL-3 independent growth and induced apoptosis in Ba/F3 cells transformed by FLT3/ITDs. AG1296 also inhibited FLT3 auto-phosphorylation, and induced a cytotoxic effect, in primary AML cells. These findings suggest that inhibiting the activity of FLT3 may have a therapeutic value in some leukemias expressing FLT3/ITDs.

Internal tandem duplication of FLT3 in relapsed acute myeloid leukemia: a comparative analysis of bone marrow samples from 108 adult patients at diagnosis and relapse

Analysis of internal tandem duplications of FLT3 (FLT3/ITD) was performed on bone marrow samples obtained at diagnosis and relapse from 108 adult patients with de novo acute myeloid leukemia (AML) to determine the role of this mutation in leukemic relapse. Eighty-three patients had wild-type FLT3 at both diagnosis and relapse, 16 had FLT3/ITD at both stages, whereas 8 had acquired the mutation and 1 had lost it at relapse. Using Genescan analysis, we found that FLT3/ITD levels at first relapse were significantly higher than those at diagnosis (mean +/- SE, 40.5% +/- 4.8% versus 17.9% +/- 3.6%, P <.001). The increase in mutation levels at relapse as compared with diagnosis did not correlate with the difference in blast cell percentages at both stages (P =.777). A hemizygous deletion of wild-type FLT3 was found in 4 patients at relapse compared to none at diagnosis. Nine of the 11 patients carrying a single mutation at diagnosis relapsed with an identical mutation. All 6 patients with more than one FLT3/ITD mutation at diagnosis showed changes in mutation patterns and levels at first relapse; however, each patient retained at least one mutation in the relapse sample. The changes of mutation patterns had implications for the monitoring of minimal residual disease. Our results suggest that FLT3/ITD may contribute as the initial transforming event in AML, and relapse can reflect the selection and outgrowth of a mutant clone or evolution of a new clone harboring this mutation.

Human AML cells in NOD/SCID mice: engraftment potential and gene expression

Most cases of human acute myeloid leukemia (AML) engraft in irradiated non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice. Intravenous transfer of as few as 10(5) human AML cells resulted in engraftment. Cases with poor prognosis clinical features, including FLT3 mutations, tended to engraft efficiently. Nevertheless, AML cells obtained from patients at relapse did not engraft more efficiently than cells obtained from the same patients at initial diagnosis. One passage of human AML cells in NOD/SCID mice did not appear to select for increased virulence, as measured by serial transplantation efficiency. Finally, cDNA microarray analyses indicated that approximately 95% of genes were expressed at similar levels in human AML cells immunopurified after growth in mice, as compared to cells assessed directly from patients. Thus, the growth of human AML cells in NOD/SCID mice could yield large numbers of human AML cells for direct experimental use and could also function as a renewable, potentially unlimited source of leukemia cells, via serial transplantation.

The roles of FLT3 in hematopoiesis and leukemia

FLT3 is a receptor tyrosine kinase expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. The ligand for FLT3 is expressed by marrow stromal cells and other cells and synergizes with other growth factors to stimulate proliferation of stem cells, progenitor cells, dendritic cells, and natural killer cells. Mutations of FLT3 have been detected in about 30% of patients with acute myelogenous leukemia and a small number of patients with acute lymphocytic leukemia or myelodysplastic syndrome. Patients with FLT3 mutations tend to have a poor prognosis. The mutations most often involve small tandem duplications of amino acids within the juxtamembrane domain of the receptor and result in constitutive tyrosine kinase activity. Expression of a mutant FLT3 receptor in murine marrow cells results in a lethal myeloproliferative syndrome and preliminary studies suggest that mutant FLT3 cooperates with other leukemia oncogenes to confer a more aggressive phenotype. Taken together, these results suggest that FLT3 is an attractive therapeutic target for kinase inhibitors or other approaches for patients with mutations of this gene.

Prognostic significance of FLT3 internal tandem repeat in patients with de novo acute myeloid leukemia treated with reinforced courses of chemotherapy

FLT3 internal tandem duplications (FLT3-ITDs) are present in nearly 25% of patients with AML and have been associated with poor response to conventional therapy and poor outcome. We retrospectively evaluated the effect of reinforced courses of chemotherapy on the prognostic value of FLT3-ITDs in 159 AML patients prospectively enrolled in the ALFA-9000 trial, which randomly compared three reinforced induction regimens (standard 3+7 including high-dose daunorubicin, double induction, and timed-sequential therapy). FLT3-ITD was present in 40/159 (25%) blast samples and associated with high WBC (P = 0.002) and cytogenetics (P < 0.001) with a higher incidence (35%) in patients with a normal karyotype. There was no difference in CR rate between FLT3-wt and FLT3-ITD patients (80% vs 78%). Relapse-free survival (RFS) was similar in both groups (5-year RFS, 33% vs 32%; P = 0.41), even after adjustment for age, sex, WBC, cytogenetics, and treatment arm. A trend to a worse survival was observed in the FLT3-ITD group (estimated 5-year OS, 23% vs 37%; P = 0.09), mainly in patients with a normal karyotype. This was associated with a dramatic outcome in relapsing FLT3-ITD patients (estimated 3-year post-relapse survival, 0% vs 27%; P = 0.04). These results suggest that the bad prognosis associated with FLT3-ITDs in AML might be partly overcome using reinforced chemotherapy. Early detection of FLT3 mutations might thus be useful to intensify induction as well as post-remission therapy in FLT3-ITD patients.

MLL-ENL cooperates with SCF to transform primary avian multipotent cells

The MLL gene is targeted by chromosomal translocations, which give rise to heterologous MLL fusion proteins and are associated with distinct types of acute lymphoid and myeloid leukaemia. To determine how MLL fusion proteins alter the proliferation and/or differentiation of primary haematopoietic progenitors, we introduced the MLL-AF9 and MLL-ENL fusion proteins into primary chicken bone marrow cells. Both fusion proteins caused the sustained outgrowth of immature haematopoietic cells, which was strictly dependent on stem cell factor (SCF). The renewing cells have a long in vitro lifespan exceeding the Hayflick limit of avian cells. Analysis of clonal cultures identified the renewing cells as immature, multipotent progenitors, expressing erythroid, myeloid, lymphoid and stem cell surface markers. Employing a two-step commitment/differentiation protocol involving the controlled withdrawal of SCF, the MLL-ENL-transformed progenitors could be induced to terminal erythroid or myeloid differentiation. Finally, in cooperation with the weakly leukaemogenic receptor tyrosine kinase v-Sea, the MLL-ENL fusion protein gave rise to multilineage leukaemia in chicks, suggesting that other activated, receptor tyrosine kinases can substitute for ligand-activated c-Kit in vivo.

Bis(1H-2-indolyl)-1-methanones as inhibitors of the hematopoietic tyrosine kinase Flt3

Aberrant expression and activating mutations of the class III receptor tyrosine kinase Flt3 (Flk-2, STK-1) have been linked to poor prognosis in acute myeloid leukemia (AML). Inhibitors of Flt3 tyrosine kinase activity are, therefore, of interest as potential therapeutic compounds. We previously described bis(1H-2-indolyl)-1-methanones as a novel class of selective inhibitors for platelet-derived growth factor receptors (PDGFR). Several bis(1H-2-indolyl)-1-methanone derivatives, represented by the compounds D-64406 and D-65476, are also potent inhibitors of Flt3. They inhibit proliferation of TEL-Flt3-transfected BA/F3 cells with IC(50) values of 0.2-0.3 microM in the absence of IL-3 but >10 microM in the presence of IL-3. Ligand-stimulated autophosphorylation of Flt3 in EOL-1 cells and corresponding downstream activation of Akt/PKB are effectively inhibited by bis(1H-2-indolyl)-1-methanones whereas autophosphorylation of c-Kit/SCF receptor or c-Fms/CSF-1 receptor is less sensitive or insensitive, respectively. Flt3 kinase purified by different methods is potently inhibited in vitro, demonstrating a direct mechanism of inhibition. 32D cells, expressing a constitutively active Flt3 variant with internal tandem duplication are greatly sensitized to radiation-induced apoptosis in the presence of D-64406 or D-65476 in the absence but not in the presence of IL-3. Thus, bis(1H-2-indolyl)-1-methanones are potential candidates for the treatment of Flt3-driven leukemias.

FLT3 in human hematologic malignancies

FLT3, a member of the receptor tyrosine kinase (RTK) class III, is preferentially expressed on the surface of a high proportion of acute myeloid leukemia (AML) and B-lineage acute lymphocytic leukemia (ALL) cells in addition to hematopoietic stem cells, brain, placenta and liver. An interaction of FLT3 and its ligand has been shown to play an important role in the survival, proliferation and differentiation of not only normal hematopoetic cells but also leukemia cells. Mutations of the FLT3 gene was first reported as an internal tandem duplication (ITD) of the juxtamembrane (JM) domain-coding sequence, subsequently as a missense mutation of D835 within a kinase domain. ITD- and D835-mutations are essentially found in AML and their frequencies are approximately 20 and 6% of adults with AML, respectively. Thus, mutation of the FLT3 gene is so far the most frequent genetic alteration reported to be involved in AML. Several large-scale studies in well-documented patients published to date have demonstrated that ITD-mutation is strongly associated with leukocytosis and a poor prognosis. In this review, we summarize the clinical and biological significance of FLT3-mutations and discuss the possibility of targeting FLT3 kinase for the treatment of leukemia.

Selective apoptosis of tandemly duplicated FLT3-transformed leukemia cells by Hsp90 inhibitors

An internal tandem duplication of the juxtamembrane (JM) domain of FLT3, a family of ligand-activated receptor tyrosine kinases, has been found in 20% of cases of acute myeloid leukemia (AML), and this mutation is correlated with leukocytosis and a poor prognosis. As a therapeutic approach, we previously reported that herbimycin A (HA) inhibited the growth of tandemly duplicated FLT3 (TDFLT3)-transformed cells (Leukemia 2000; 14: 374). Here, we have investigated the mechanism behind the cytotoxicity of HA, an ansamycin derivative which is now known to target Hsp90. The treatment with HA or another Hsp90 inhibitor, radicicol, induced selective apoptosis in TDFLT3-transformed 32D cells (TDFLT3/32D). The tyrosine-phosphorylation of TDFLT3 was inhibited by HA, whereas FLT3 ligand-induced phosphorylation of wild-type FLT3 (WtFLT3) was not. The downstream signal molecules MAPK, Akt and STAT5a were also dephosphorylated by HA in TDFLT3/32D. Immunoprecipitation analysis showed that TDFLT3 but not WtFLT3 formed a complex with Hsp90, and that the HA treatment dissociated TDFLT3 from the Hsp90 chaperone complex. These findings imply that targeting of Hsp90 will facilitate the development of anti-TDFLT3 therapy, and that Hsp90 is closely involved in the oncogenic activation of FLT3.

Acute myeloid leukemia

Through the hard work of a large number of investigators, the biology of acute myeloid leukemia (AML) is becoming increasingly well understood, and as a consequence, new therapeutic targets have been identified and new model systems have been developed for testing novel therapies. How these new therapies can be most effectively studied in the clinic and whether they will ultimately improve cure rates are questions of enormous importance. In this article, Dr. Jacob Rowe presents a summary of the current state-of-the-art therapy for adult AML. His contribution emphasizes the fact that AML is not a single disease, but a number of related diseases each distinguished by unique cytogenetic markers which in turn help determine the most appropriate treatment. Dr. Jerald Radich continues on this theme, emphasizing how these cytogenetic abnormalities, as well as other mutations, give rise to abnormal signal transduction and how these abnormal pathways may represent ideal targets for the development of new therapeutics. A third contribution by Dr. Frederick Appelbaum describes how AML might be made the target of immunologic attack. Specifically, strategies using antibody-based or cell-based immunotherapies are described including the use of unmodified antibodies, drug conjugates, radioimmunoconjugates, non-ablative allogeneic transplantation, T cell adoptive immunotherapy and AML vaccines. Finally, Dr. John Dick provides a review of the development of the NOD/SCID mouse model of human AML emphasizing both what it has taught us about the biology of the disease as well as how it can be used to test new therapies. Taken together, these reviews are meant to help us understand more about where we are in the treatment of AML, where we can go and how we might get there.

Progress and controversies in the treatment of pediatric acute myelogenous leukemia

The outcome for children and adolescents with acute myelogenous leukemia (AML) has substantially improved during the past several decades, such that nearly 50% of these patients can be cured. A significant part of this improvement has occurred over the past 10 years, during which time, dose intensification has played a much greater role in therapeutic strategies. Although dose intensification increased the cure rate for pediatric patients with AML, there has also been increased treatment-related morbidity and mortality. Further, despite such toxicity, the primary cause of death is still leukemia. This article outlines some of the different strategies leading to our current treatments and presents several questions and controversies. These questions lead to what future therapeutic options and approaches will be pursued.

Role of FLT3 in leukemia

FLT3 is the most frequently mutated gene in cases of acute myelogenous leukemia (AML). About 30 to 35% of patients have either internal tandem duplications (ITDs) in the juxtamembrane domain or mutations in the activating loop of FLT3. FLT3 mutations occur in a broad spectrum of FAB subtypes in adult and pediatric AML and are particularly common in acute promyelocytic leukemia (APL). FLT3 mutations confer a poor prognosis in most retrospective studies. The consequence of either FLT3-ITD or activating loop mutations, which occur predominantly at position D835, is constitutive activation of the tyrosine kinase; FLT3 mutants confer factor-independent growth to Ba/F3 and 32D cells and activate similar transduction pathways as the native receptor in response to ligand, including the STAT, RAS/mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3; kinase (PI3K)/AKT pathways. Injection of FLT3-ITD transformed cells, such as Ba/F3 or 32D, into syngeneic recipient mice results in a leukemia-like syndrome, and expression in primary murine bone marrow cells in a retroviral transduction assay results in a myeloproliferative disorder. Mutations that abrogate FLT3 kinase activity result in loss of transforming properties in these assays. Further, FLT3-selective inhibitors impair transformation of primary AML cells that harbor these mutations, and also inhibit FLT3 transformed hematopoietic cell lines, and leukemias induced by activated FLT3 mutants in murine models. Collectively, these data indicate that FLT3 may be a viable therapeutic target for treatment of AML.

Genomic approaches to the pathogenesis and treatment of acute lymphoblastic leukemias

During the past 2 years, gene expression-profiling technology has been used to characterize multiple tumor types. These studies represent the initial attempts to understand cancer at the whole genome level and have given us our first glimpse into what can be learned from such analysis. This technology is beginning to be applied to the study of human leukemias. Recent studies focusing on lymphoblastic leukemia have provided insight into classification and pathogenesis.

Analysis of FLT3 length mutations in 1003 patients with acute myeloid leukemia: correlation to cytogenetics, FAB subtype, and prognosis in the AMLCG study and usefulness as a marker for the detection of minimal residual disease

FLT3 length mutation (FLT3-LM) is a molecular marker potentially useful for the characterization of acute myeloid leukemia (AML). To evaluate the distribution of FLT3-LM within biologic subgroups, we screened 1003 patients with AML at diagnosis for this mutation. FLT3-LM was found in 234 (23.5%) of all patients and thus is the most frequent mutation in AML described so far. Of all positive patients, 165 (70.5%) revealed a normal karyotype. Of the 69 patients with chromosome aberrations, 24 (34.8%) had a t(15;17). The mutation was rare in AML with t(8;21), inv(16) 11q23 rearrangements, and complex karyotypes. FLT3-LM was not distributed equally within different French-American-British (FAB) subtypes and was correlated with a high peripheral blood count in FAB M1, M2, and M4 (P <.0001). In addition, the median age of patients with the mutation was lower (54.9 vs 57.6 years; P =.043), and, at a ratio of 1.36:1 (P =.023), the mutation was more frequent in females than in males. Within the AMLCG study, FLT3-LM was of intermediate prognostic significance. The complete remission rate of 70.3% in patients with FLT3-LM was similar to that (70.4%) in patients without FLT3-LM. Overall survival was not different between patients with or without FLT3-LM. In contrast, patients with FLT3-LM had a significantly shorter event-free survival (7.4 vs 12.6 months; P =.0072) because of a higher relapse rate. Besides the importance of FLT3-LM for biologic and clinical characterization of AML, we show its value as a marker for disease monitoring based on 120 follow-up samples of 34 patients.