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

FLT3 D835/I836 mutations are associated with poor disease-free survival and a distinct gene-expression signature among younger adults with de novo cytogenetically normal acute myeloid leukemia lacking FLT3 internal tandem duplications

The prognostic relevance of FLT3 D835/I836 mutations (FLT3-TKD) in cytogenetically normal acute myeloid leukemia (CN-AML) remains to be established. After excluding patients with FLT3 internal tandem duplications, we compared treatment outcome of 16 de novo CN-AML patients with FLT3-TKD with that of 123 patients with wild-type FLT3 (FLT3-WT), less than 60 years of age and similarly treated on Cancer and Leukemia Group B protocols. All FLT3-TKD(+) patients and 85% of FLT3-WT patients achieved a complete remission (P = .13). Disease-free survival (DFS) of FLT3-TKD(+) patients was worse than DFS of FLT3-WT patients (P = .01; estimated 3-year DFS rates, 31% vs 60%, respectively). In a multivariable analysis, FLT3-TKD was associated with worse DFS (P = .02) independent of NPM1 status and percentage of bone marrow blasts. To gain further biologic insights, a gene-expression signature differentiating FLT3-TKD(+) from FLT3-WT patients was identified. The signature (333 probe sets) included overexpression of VNN1, C3AR1, PTPN6, and multiple other genes involved in monocarboxylate transport activity, and underexpression of genes involved in signal transduction regulation. These associations with outcome, other prognostic markers, and the elucidated expression signature enhance our understanding of FLT3-TKD-associated biology and may lead to development of novel therapies that improve clinical outcome of CN-AML patients with FLT3-TKD.

FLT3 mutations confer enhanced proliferation and survival properties to multipotent progenitors in a murine model of chronic myelomonocytic leukemia

Despite their known transforming properties, the effects of leukemogenic FLT3-ITD mutations on hematopoietic stem and multipotent progenitor cells and on hematopoietic differentiation are not well understood. We report a mouse model harboring an ITD in the murine Flt3 locus that develops myeloproliferative disease resembling CMML and further identified FLT3-ITD mutations in a subset of human CMML. These findings correlated with an increase in number, cell cycling, and survival of multipotent stem and progenitor cells in an ITD dose-dependent manner in animals that exhibited alterations within their myeloid progenitor compartments and a block in normal B cell development. This model provides insights into the consequences of constitutive signaling by an oncogenic tyrosine kinase on hematopoietic progenitor quiescence, function, and cell fate.

Sensitivity toward sorafenib and sunitinib varies between different activating and drug-resistant FLT3-ITD mutations

OBJECTIVE

Activating mutations in FLT3 are known to be a frequent transforming event in acute myeloid leukemia. Small molecule-inhibitor therapy targeting the FLT3 kinase is, therefore, an attractive strategy. FLT3 kinase inhibitors, such as PKC412, have already entered clinical trials. Even though results are encouraging, emergence of primary and secondary resistance does occur in the majority of patients. Thus, it will be crucial to carefully characterize the activity of every single compound against different activating and resistance FLT3-internal tandem duplication (ITD) mutations. Here we tested the efficacy of sunitinib and sorafenib to inhibit primary FLT3 activating mutations (ITD and D835Y) and of secondary resistance mutations.

METHODS

Ba/F3 cell lines stably expressing oncogenic FLT3 mutations were used to calculate cellular IC(50) values for sunitinib and sorafenib using cell proliferation assays. Differential IC(50) values for sorafenib toward FLT3-ITD and FLT3-D835Y were confirmed by Western blotting. Cell death was measured by propidium-iodide staining and flow cytometry.

RESULTS

Sorafenib inhibits FLT3-ITD more potent than FLT3-D835Y, while sunitinib is equally effective against both mutant forms of FLT3. Importantly, sensitivity toward sorafenib and sunitinib varies between the different secondary FLT3-ITD resistance mutations.

CONCLUSIONS

These results establish sensitivity profiles for the FLT3 inhibitors sunitinib and sorafenib. This may help to develop rational treatment strategies for acute myeloid leukemia with these compounds.

Potentiation of antileukemic therapies by Smac mimetic, LBW242: effects on mutant FLT3-expressing cells

Members of the inhibitor of apoptosis protein (IAP) family play a role in mediating apoptosis. Studies suggest that these proteins may be a viable target in leukemia because they have been found to be variably expressed in acute leukemias and are associated with chemosensitivity, chemoresistance, disease progression, remission, and patient survival. Another promising therapeutic target, FLT3, is mutated in about one third of acute myelogenous leukemia (AML) patients; promising results have recently been achieved in clinical trials investigating the effects of the protein tyrosine kinase inhibitor PKC412 on AML patients harboring mutations in the FLT3 protein. Of growing concern, however, is the development of drug resistance resulting from the emergence of point mutations in targeted tyrosine kinases used for treatment of acute leukemia patients. One approach to overriding resistance is to combine structurally unrelated inhibitors and/or inhibitors of different signaling pathways. The proapoptotic IAP inhibitor, LBW242, was shown in proliferation studies done in vitro to enhance the killing of PKC412-sensitive and PKC412-resistant cell lines expressing mutant FLT3 when combined with either PKC412 or standard cytotoxic agents (doxorubicin and Ara-c). In addition, in an in vivo imaging assay using bioluminescence as a measure of tumor burden, a total of 12 male NCr-nude mice were treated for 10 days with p.o. administration of vehicle, LBW242 (50 mg/kg/day), PKC412 (40 mg/kg/day), or a combination of LBW242 and PKC412; the lowest tumor burden was observed in the drug combination group. Finally, the combination of LBW242 and PKC412 was sufficient to override stromal-mediated viability signaling conferring resistance to PKC412.

Evolution of FLT3-ITD and D835 activating point mutations in relapsing acute myeloid leukemia and response to salvage therapy

Internal tandem duplications (ITDs) of the juxtamembrane region of the FLT3 tyrosine kinase receptor are the most frequent genetic alterations in acute myeloid leukemia (AML). The presence of this mutation has been recognized as an independent poor prognostic factor. In this study, we compared the FLT3 mutational status between diagnosis and subsequent relapses in 31 patients with AML. At diagnosis, seven patients were identified to contain FLT3-ITD mutations and one patient to harbor the D835 mutation. Five patients remained FLT3-ITD positive throughout the disease course (+/+). Three patients lost the FLT3 gene mutation at first (one FLT3-ITD, one D835 mutation), or second relapse (one FLT3-ITD) (+/-). One additional patient lost a small FLT3-ITD positive clone at relapse and at the same time gained an apparently unrelated FLT3-ITD positive clone. One patient without FLT3 mutation at diagnosis relapsed with an FLT3-ITD mutation (-/+). A shorter median duration of first remission (6 months versus 11.5 months) and a higher relapse rate after salvage therapy (e.g. allogeneic peripheral blood stem cell transplantation) resulted in a lower leukemia-free survival in the FLT3 mutated group (11% versus 31%). The loss of a clone with a mutation in the FLT3 gene at relapse did not improve the prognosis.

FLT3 regulates beta-catenin tyrosine phosphorylation, nuclear localization, and transcriptional activity in acute myeloid leukemia cells

Deregulated accumulation of nuclear beta-catenin enhances transcription of beta-catenin target genes and promotes malignant transformation. Recently, acute myeloid leukemia (AML) cells with activating mutations of FMS-like tyrosine kinase-3 (FLT3) were reported to display elevated beta-catenin-dependent nuclear signaling. Tyrosine phosphorylation of beta-catenin has been shown to promote its nuclear localization. Here, we examined the causal relationship between FLT3 activity and beta-catenin nuclear localization. Compared to cells with wild-type FLT3 (FLT3-WT), cells with the FLT3 internal tandem duplication (FLT3-ITD) and tyrosine kinase domain mutation (FLT3-TKD) had elevated levels of tyrosine-phosphorylated beta-catenin. Although beta-catenin was localized mainly in the cytoplasm in FLT3-WT cells, it was primarily nuclear in FLT3-ITD cells. Treatment with FLT3 kinase inhibitors or FLT3 silencing with RNAi decreased beta-catenin tyrosine phosphorylation and nuclear localization. Conversely, treatment of FLT3-WT cells with FLT3 ligand increased tyrosine phosphorylation and nuclear accumulation of beta-catenin. Endogenous beta-catenin co-immunoprecipitated with endogenous activated FLT3, and recombinant activated FLT3 directly phosphorylated recombinant beta-catenin. Finally, FLT3 inhibitor decreased tyrosine phosphorylation of beta-catenin in leukemia cells obtained from FLT3-ITD-positive AML patients. These data demonstrate that FLT3 activation induces beta-catenin tyrosine phosphorylation and nuclear localization, and thus suggest a mechanism for the association of FLT3 activation and beta-catenin oncogeneic signaling in AML.

A molecular brake in the kinase hinge region regulates the activity of receptor tyrosine kinases

Activating mutations in the tyrosine kinase domain of receptor tyrosine kinases (RTKs) cause cancer and skeletal disorders. Comparison of the crystal structures of unphosphorylated and phosphorylated wild-type FGFR2 kinase domains with those of seven unphosphorylated pathogenic mutants reveals an autoinhibitory "molecular brake" mediated by a triad of residues in the kinase hinge region of all FGFRs. Structural analysis shows that many other RTKs, including PDGFRs, VEGFRs, KIT, CSF1R, FLT3, TEK, and TIE, are also subject to regulation by this brake. Pathogenic mutations activate FGFRs and other RTKs by disengaging the brake either directly or indirectly.

Uniform sensitivity of FLT3 activation loop mutants to the tyrosine kinase inhibitor midostaurin

Small molecule inhibitors that target fms-like tyrosine kinase 3 (FLT3)-activating mutations have potential in the treatment of leukemias. However, certain mutations can simultaneously activate the tyrosine kinase, and confer resistance to small molecule inhibitors. We therefore tested the sensitivity of 8 FLT3 activation loop mutants to midostaurin. Each mutant conferred IL-3 factor-independent proliferation to Ba/F3 cells, and each resulted in the constitutive activation of FLT3 and its targets, signal transducer and activator of transcription 5 (STAT5) and extracellular stimuli-responsive kinase (ERK). For each mutant tested, midostaurin inhibited cell growth and phosphorylation of FLT3, STAT5, and ERK. In contrast, midostaruin did not inhibit Ba/F3 cells stably transduced with FLT3-internal tandem duplications containing a G697R mutation that confers resistance to midostaurin, demonstrating that midostaurin inhibition of FLT3 activation loop mutants was not due to off-target effects. We conclude that midostaurin is a potent inhibitor of a spectrum of FLT3 activation loop mutations, and that acute myeloid leukemia patients with such mutations are potential candidates for clinical trials involving midostaurin.

FLT3 tyrosine kinase domain mutations are biologically distinct from and have a significantly more favorable prognosis than FLT3 internal tandem duplications in patients with acute myeloid leukemia

The prognostic impact of tyrosine kinase domain (TKD) mutations of the fms-like tyrosine kinase-3 (FLT3) gene in acute myeloid leukemia (AML) is currently uncertain. To resolve this issue we screened 1107 young adult nonacute promyelocytic leukemia AML patients with known FLT3 internal tandem duplication (ITD) status for FLT3/TKDs; they were detected in 127 (11%) cases. Mutations were associated with a high white cell count (P =.006) and patients with inv(16) (P = .005) but were infrequent in patients with adverse cytogenetics and secondary AML. Overall survival (OS) at 5 years was 53% and 37% for FLT3/TKD mutant and wild-type patients respectively (odds ratio, 0.72; 95% confidence interval, 0.58 to 0.89; P = .002). For both the cumulative incidence of relapse and OS the difference in outcome between FLT3/ITDs and FLT3/TKDs was highly significant (P < .001). In multivariate analysis, impact of FLT3/TKDs on OS when including all mutant-positive patients was not significant, but patients with high-level mutations (more than 25% mutant) had a significantly improved outcome (P = .004). The novel finding that biologically distinct activating mutations of the same gene can be associated with markedly different clinical outcomes has implications for risk stratification and therapy and is significant to the understanding of chemoresistance in AML.

Identification of TSC-22 as a potential tumor suppressor that is upregulated by Flt3-D835V but not Flt3-ITD

Transforming growth factor-beta (TGF-beta)-stimulated clone-22 (TSC-22) was originally isolated as a TGF-beta-inducible gene. In this study, we identified TSC-22 as a potential leukemia suppressor. Two types of FMS-like tyrosine kinase-3 (Flt3) mutations are frequently found in acute myeloid leukemia: Flt3-ITD harboring an internal tandem duplication in the juxtamembrane domain associated with poor prognosis and Flt3-TKD harboring a point mutation in the kinase domain. Comparison of gene expression profiles between Flt3-ITD- and Flt3-TKD-transduced Ba/F3 cells revealed that constitutive activation of Flt3 by Flt3-TKD, but not Flt3-ITD, upregulated the expression of TSC-22. Importantly, treatment with an Flt3 inhibitor PKC412 or an Flt3 small interfering RNA decreased the expression level of TSC-22 in Flt3-TKD-transduced cells. Forced expression of TSC-22 suppressed the growth and accelerated the differentiation of several leukemia cell lines into monocytes, in particular, in combination with differentiation-inducing reagents. On the other hand, a dominant-negative form of TSC-22 accelerated the growth of Flt3-TKD-transduced 32Dcl.3 cells. Collectively, these results suggest that TSC-22 is a possible target of leukemia therapy.

Pediatric oncology

Intensive use of cytotoxic agents in multimodality therapeutic regimens has resulted in almost 80% five-year disease-free survival and cure in the majority of childhood cancer patients. However, such success has come at the expense of severe acute or delayed toxicities and an increased occurrence of secondary cancers. With an increasing understanding of the genetic changes that underlie transformation in childhood cancer, rational approaches using agents that target these transforming events are being developed. Current and future strategies in developing tumor-selective therapy using inhibitors of signaling pathways dysregulated in leukemias (FLT3, NOTCH1) and solid/brain tumors (ErbB1-4, IGF-IR, PTCH1), and the challenges in developing less toxic, but equally effective treatments in pediatric oncology are presented.

A Novel FLT3 Inhibitor FI-700 Selectively Suppresses the Growth of Leukemia Cells with FLT3 Mutations

PURPOSE

The aim of this study was to evaluate the antileukemia activity of a novel FLT3 kinase inhibitor, FI-700.

EXPERIMENTAL DESIGN

The antileukemia activity of FI-700 was evaluated in human leukemia cell lines, mutant or wild-type (Wt)-FLT3-expressing mouse myeloid precursor cell line, 32D and primary acute myeloid leukemia cells, and in xenograft or syngeneic mouse leukemia models.

RESULTS

FI-700 showed a potent IC(50) value against FLT3 kinase at 20 nmol/L in an in vitro kinase assay. FI-700 showed selective growth inhibition against mutant FLT3-expressing leukemia cell lines and primary acute myeloid leukemia cells, whereas it did not affect the FLT3 ligand (FL)-driven growth of Wt-FLT3-expressing cells. These antileukemia activities were induced by the significant dephosphorylations of mutant FLT3 and STAT5, which resulted in G(1) arrest of the cell cycle. Oral administration of FI-700 induced the regression of tumors in a s.c. tumor xenograft model and increased the survival of mice in an i.v. transplanted model. Furthermore, FI-700 treatment eradicated FLT3/ITD-expressing leukemia cells, both in the peripheral blood and in the bone marrow. In this experiment, the depletion of FLT3/ITD-expressing cells by FI-700 was more significant than that of Ara-C, whereas bone marrow suppression by FI-700 was lower than that by Ara-C.

CONCLUSIONS

FI-700 is a novel and potent FLT3 inhibitor with promising antileukemia activity.

FLT3 inhibition in acute myeloid leukaemia

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that appears to play a significant role in leukaemogenesis. Activating mutations of FLT3 are present in approximately one-third of acute myeloid leukaemia patients and are associated with adverse clinical outcome, while many non-mutated cases also show evidence of FLT3 activation. FLT3 thus represents a potentially exciting molecular therapeutic target. A number of small-molecule tyrosine kinase inhibitors with anti-FLT3 activity have been developed and several of these compounds have entered early phase clinical trials where clinical anti-leukaemic activity has been demonstrated. The depth and duration of clinical responses to FLT3 inhibitor monotherapy have been modest, however, and a number of mechanisms by which blasts may acquire resistance have been proposed. Based on preclinical evidence of synergy with conventional chemotherapy, several combination trials are now underway. FLT3 inhibition may also be effective used in combination with other molecularly targeted agents, in postchemotherapy stem-cell-directed maintenance therapy and in MLL-rearranged infant acute lymphoblastic leukaemia.

Targeting receptor tyrosine kinase signaling in acute myeloid leukemia

Acute myeloid leukemia (AML) is a quickly progressing, heterogeneous clonal disorder of hematopoietic progenitor cells. Significant progress in understanding the pathogenesis of AML has been achieved in the past few years. Two major types of genetic events are thought to give rise to leukemic transformation: alterations in the activity of transcription factors controlling hematopoietic differentiation and activation of components of receptor tyrosine kinase (RTK) signaling pathways. This has led to the development of promising new therapeutic strategies for the disease. In this article, we will discuss recent developments in the field of molecularly targeted therapies for AML, which involve RTKs such as FMS-like tyrosine kinase 3 (Flt3), c-Kit and signal transduction via the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways. Initial results imply that targeting RTKs is a very promising approach for AML and that other receptors, such as the insulin-like growth factor receptor (IGF-IR), could also represent new targets in the future.

Arginine 595 is duplicated in patients with acute leukemias carrying internal tandem duplications of FLT3 and modulates its transforming potential

FLT3–internal tandem duplications (FLT3-ITDs) comprise a heterogeneous group of mutations in patients with acute leukemias that are prognostically important. To characterize the mechanism of transformation by FLT3-ITDs, we sequenced the juxtamembrane region (JM) of FLT3 from 284 patients with acute leukemias. The length of FLT3-ITDs varied from 2 to 42 amino acids (AAs) with a median of 17 AAs. The analysis of duplicated AAs showed that in the majority of patients, the duplications localize between AAs 591 to 599 (YVDFREYEY). Arginine 595 (R595) within this region is duplicated in 77% of patients. Single duplication of R595 in FLT3 conferred factor-independent growth to Ba/F3 cells and activated STAT5. Moreover, deletion or substitution of the duplicated R595 in 2 FLT3-ITD constructs as well as the deletion of wild-type R595 in FLT3-ITD substantially reduced the transforming potential and STAT5 activation, pointing to a critical role of the positive charge of R595 in stabilizing the active confirmation of FLT3-ITDs. Deletion of R595 in FLT3-WT nearly abrogated the ligand-dependent activation of FLT3-WT. Our data provide important insights into the molecular mechanism of transformation by FLT3-ITDs and show that duplication of R595 is important for the leukemic potential of FLT3-ITDs.

HLA-DR-negative AML (M1 and M2): FLT3 mutations (ITD and D835) and cell-surface antigen expression

FLT3 mutations and cell-surface antigen were investigated in 29 DR-negative (DR(-)) M1/M2 AML samples in comparison with 30 DR-positive (DR(+)) M1/M2 AML samples. FLT3-ITD was detected in 59.3% and D835 was detected in 7.4% of the samples. The incidence of FLT3-ITD was higher in the DR(-) group (59.3%) than in the DR(+) group (17.9%; P=0.002). The DR(-) status was associated with the CD34(-) (82.8%), CD7(-) (92.9%) and CD45RO(+) status (76%). Our results indicated that FLT3 mutation is the most common gene alteration found in the DR(-) M1/M2 AML. These results are important for further characterizing this phenotypic AML entity.

Importance of early detection and follow-up of FLT3 mutations in patients with acute myeloid leukemia

Mutations in the fms-like tyrosine kinase 3 (FLT3) gene, such as internal tandem duplication (FLT3/ITD) in the juxtamembrane domain and point mutations in the tyrosine kinase domain, are the most common abnormalities in acute myeloid leukemia (AML). FLT3/ITD and FLT3/D835 mutations were analyzed in 113 Serbian adult AML patients using polymerase chain reaction. Twenty patients were found to be FLT3/ITD positive (17.7%). The mutations occurred most frequently in M5 and M0 subtypes of AML. They were mainly associated with the normal karyotype. All patients harboring FLT3/ITD had a higher number of white blood cells than patients without it (p = 0.027). FLT3/ITD mutations were associated with lower complete remission (CR) rate (chi (2 )= 5.706; p = 0.017) and shorter overall survival (OS; Log rank = 8.76; p = 0.0031). As for disease-free survival, the difference between FLT3/ITD-positive and FLT3/ITD-negative patients was not statistically significant (Log rank = 0.78; p = 0.3764). In multivariate analysis, the presence of FLT3/ITD mutations was the most significant prognostic factor for both OS and CR rate (p = 0.0287; relative risk = 1.73; 95% CI = 1.06-2.82). However, in the group of patients with the intermediate-risk karyotype, the mere presence of FLT3/ITD was not associated with inferior clinical outcome. FLT3/D835 point mutation was found in four patients (3.5%) only. Follow-up of the FLT3/ITD-positive patients revealed stability of this mutation during the course of the disease. However, changes in the pattern of FLT3/D835 mutations in initial and relapsed AML were observed. Our results indicate an association of FLT3/ITD with the adverse outcome in AML patients treated with standard induction chemotherapy. Because FLT3/ITD mutation is a target for specific therapeutic inhibition, its early detection could be helpful in clinical practice.

4-Amino-6-piperazin-1-yl-pyrimidine-5-carbaldehyde oximes as potent FLT-3 inhibitors

A series of 4-amino-6-piperazin-1-yl-pyrimidine-5-carbaldehyde oximes has been discovered and developed as potent FLT3 tyrosine kinase inhibitors. The series exhibited potent antiproliferative activity against both an FLT3 ITD-mutated human leukemic cell line as well as a wild-type FLT3 BaF(3) expressed cell line. The structure-activity relationship of this class of compounds is described.

Prognostic factors and risk-based therapy in pediatric acute myeloid leukemia

Acute myeloid leukemia (AML) has posed significant therapeutic challenges to pediatric oncologists. Despite intensive therapy, half of the children with AML relapse and die from their disease. Efforts to identify risk factors in AML are directed toward defining populations who may benefit from alternative therapies. Patients at lower risk for relapse may benefit from treatment de-escalation, sparing them adverse side effects. Management of high-risk patients may prove more difficult, as the nearly myeloablative nature of AML therapy leaves little room for therapy escalation short of stem cell transplantation. This review evaluates prognostic factors in pediatric AML and discusses the feasibility of using these factors in risk-adapted therapy regimens.

Molecular mechanisms of drug resistance in acute myeloid leukaemia

Resistance to chemotherapy in acute myeloid leukaemia is a major obstacle to a successful outcome for many patients. Often, there is resistance against a broad range of drugs due to multiple, simultaneously active processes. These mechanisms include effects on drug influx and efflux, drug activation/inactivation, DNA repair mechanisms, altered response of end targets, an altered haematopoietic microenvironment and dysfunctional apoptotic pathways. This article reviews the factors that determine leukaemic cell chemosensitivity and discusses the potential for rationally guided therapy.

Inhibition of Flt3-activating mutations does not prevent constitutive activation of ERK/Akt/STAT pathways in some AML cells: a possible cause for the limited effectiveness of monotherapy with small-molecule inhibitors

The Flt3 receptor tyrosine kinase is a critical mediator in the pathogenesis of acute myeloid leukaemia (AML). Flt3-activating mutations have been associated with poor prognosis and decreased overall survival of AML patients, thus Flt3 constitutes an ideal target for drug treatment of such disease. Unfortunately, the monotherapy with small-molecule tyrosine kinase inhibitors in clinical trials shows that remission is not permanent, presumably by resistance of Flt3 mutants to inhibitors. An alternative approach for treatment is based on the cooperation between Flt3 and additional intracellular pathways for AML transformation in some patients. Thus, the inhibition of both Flt3 and such pathways may be exploited for successful treatment of the disease. We investigated the importance of Flt3-activating mutations for the constitutive activation of intracellular pathways in primary AML cells, and their effect on cell survival. We found that the main compounds involved in the differentiation, proliferation and survival of AML (MAPK/AKT/STAT) were constitutively activated. However, only four samples showed internal tandem duplications (ITDs) for Flt3. Surprisingly, contrary to previous reports, we found that inhibition of ITD/Flt3 activity did not prevent the phosphorylation of ERK, STAT5 or Akt in some primary AML cells. In parallel, we found that in these cells, Flt3 and ERK or Akt cooperate to regulate cell survival. Our results support the hypothesis that the optimal therapeutic treatment of AML may require not only the oncogenic tyrosine kinase, but also the appropriate combination of different specific inhibitors, thus providing a more effective approach to reverse leukaemogenesis. Thus, we propose that each AML patient should have an individually tailored combination treatment.

Distribution analysis of nonsynonymous polymorphisms within the human kinase gene family

The human kinase gene family is composed of 518 genes that are involved in a diverse spectrum of physiological functions. They are also implicated in a number of diseases and encompass 10% of current drug targets. Contemporary, high-throughput sequencing efforts have identified a rich source of naturally occurring single nucleotide polymorphisms (SNPs) in kinases, a subset of which occur in the coding region of genes (cSNPs) and result in a change in the encoded amino acid sequence (nonsynonymous coding SNP; nscSNPs). What fraction of this naturally occurring variation underlies human disease is largely unknown (uDC), and much of it is assumed not to be disease causing (DC). We pursued a comprehensive computational analysis of the distribution of 1463 nscSNPs and 999 DC nscSNPs within the kinase gene family and have found that DCs are overrepresentated in the kinase catalytic domain and in receptor structures. In addition, the frequencies with which specific amino acid changes occur differ between the DCs and the uDCs, implying different biological characteristics for the two sets of human polymorphisms. Our results provide insights into the sequence and structural phenomena associated with naturally occurring kinase nscSNPs that contribute to human diseases.

Trisomy 13 is strongly associated with AML1/RUNX1 mutations and increased FLT3 expression in acute myeloid leukemia

AML1/RUNX1 is implicated in leukemogenesis on the basis of the AML1-ETO fusion transcript as well as somatic mutations in its DNA-binding domain. Somatic mutations in RUNX1 are preferentially detected in acute myeloid leukemia (AML) M0, myeloid malignancies with acquired trisomy 21, and certain myelodysplastic syndrome (MDS) cases. By correlating the presence of RUNX1 mutations with cytogenetic and molecular aberration in a large cohort of AML M0 (N = 90) at diagnosis, we detected RUNX1 mutations in 46% of cases, with all trisomy 13 cases (n = 18) being affected. No mutations of NRAS or KIT were detected in the RUNX1-mutated group and FLT3 mutations were equally distributed between RUNX1-mutated and unmutated samples. Likewise, a high incidence of RUNX1 mutations (80%) was detected in cases with trisomy 13 from other French-American-British (FAB) subgroups (n = 20). As FLT3 is localized on chromosome 13, we hypothesized that RUNX1 mutations might cooperate with trisomy 13 in leukemogenesis by increasing FLT3 transcript levels. Quantitation of FLT3 transcript levels revealed a highly significant (P < .001) about 5-fold increase in AML with RUNX1 mutations and trisomy 13 compared with samples without trisomy 13. The results of the present study indicate that in the absence of FLT3 mutations, FLT3 overexpression might be a mechanism for FLT3 activation, which cooperates with RUNX1 mutations in leukemogenesis.

Acute myeloid leukemia with the 8q22;21q22 translocation: secondary mutational events and alternative t(8;21) transcripts

Nonrandom and somatically acquired chromosomal translocations can be identified in nearly 50% of human acute myeloid leukemias. One common chromosomal translocation in this disease is the 8q22;21q22 translocation. It involves the AML1 (RUNX1) gene on chromosome 21 and the ETO (MTG8, RUNX1T1) gene on chromosome 8 generating the AML1-ETO fusion proteins. In this review, we survey recent advances made involving secondary mutational events and alternative t(8;21) transcripts in relation to understanding AML1-ETO leukemogenesis.

Constitutive activation of Flt3 and STAT5A enhances self-renewal and alters differentiation of hematopoietic stem cells

OBJECTIVE

To model human leukemogenesis by transduction of human hematopoietic stem cells (HSC) with genes associated with leukemia and expressed in leukemic stem cells.

METHODS

Constitutive activation of Flt3 (Flt3-ITD) has been reported in 25 to 30% of patients with acute myeloid leukemia (AML). Retroviral vectors expressing constitutively activated Flt3 and STAT5A were used to transduce human cord blood CD34(+) cells and HSC cell self-renewal and differentiation were evaluated.

RESULTS

We have demonstrated that retroviral transduction of Flt3 mutations into CD34(+) cells enhanced HSC self-renewal as measured in vitro in competitive stromal coculture and limiting-dilution week-2 cobblestone (CAFC) assays. Enhanced erythropoiesis and decreased myelopoiesis were noted together with strong activation of STAT5A. Consequently, transduction studies were undertaken with a constitutively active mutant of STAT5A (STAT5A[1( *)6]) and here also a marked, selective expansion of transduced CD34(+) cells was noted, with a massive increase in self-renewing CAFC detectable at both 2 and 5 weeks of stromal coculture. Differentiation was biased to erythropoiesis, including erythropoietin independence, with myeloid maturation inhibition. The observed phenotypic changes correlated with differential gene expression, with a number of genes differentially regulated by both the Flt3 and STAT5A mutants. These included upregulation of genes involved in erythropoiesis and downregulation of genes involved in myelopoiesis. The phenotype of week-2 self-renewing CAFC also characterized primary Flt3-ITD(+) AML bone marrow samples. Isolation of leukemic stem cells (LSC) with a CD34(+), CD38(-), HLA-DR(-) phenotype was undertaken with Flt3-ITD(+) AML samples resulting in co-purification of early CAFC. Gene expression of LSC relative to the bulk leukemic population revealed upregulation of homeobox genes (HOXA9, HOXA5) implicated in leukemogenesis, and hepatic leukemia factor (HLF) involved in stem cell proliferation.

CONCLUSION

Myeloid leukemogenesis is a multi-stage process that can involve constitutively activated receptors and downstream pathways involving STAT5, HOX genes, and HLF.

ABC transporter expression in hematopoietic stem cells and the role in AML drug resistance

ATP-binding cassette (ABC) transporters are known to play an important role in human physiology, toxicology, pharmacology, and numerous disorders including acute myeloid leukemia (AML). In AML only a few cells have properties allowing for ongoing proliferation and for expansion of this malignant disorder. These very primitive cells, referred to as leukemic stem cells, reside mostly in a quiescent cell cycle state. These cells have the capacity of self-renewal and are likely characterized by a high expression of a number of ABC transporters. In addition, over-expression of certain ABC transporters in leukemic cells has been associated with poor treatment outcome in AML patients. Therefore, to be able to improve diagnostics and therapies for AML patients, it may be important to better characterize this quiescent stem cell population. Particularly knowledge of the biology of highly expressed ABC transporters in these primitive leukemic cells might provide new insights to improve therapeutic options. This review provides an overview about ABC transporters and AML in general and particularly of the ABC transporters involved in multidrug resistance and cholesterol metabolism in primitive normal and leukemic cells.

Insight into the molecular pathogenesis of myeloid malignancies

PURPOSE OF REVIEW

Molecular mutations play an increasing role for classification, prognostication, and therapeutic strategies in acute myeloid leukemia and myelodysplastic syndrome. Due to the rapid expansion of known molecular markers, this paper aims to outline some of the recent progress to improve understanding of the pathogenesis in these myeloid malignancies.

RECENT FINDINGS

Novel concepts conceive myelodysplastic syndrome and acute myeloid leukemia as endpoints of a continuous process of leukemogenesis, which is characterized by the interaction of mutations interfering with transcription and differentiation with activating mutations enhancing proliferation. The detection of novel molecular mutations such as NPM1 widened the spectrum of molecular markers in acute myeloid leukemia. Finally, attention focusses on detailed subtyping of already known molecular markers.

SUMMARY

The fast progress in the molecular characterization of acute myeloid leukemia and myelodysplastic syndrome in recent years provides the basis for an optimization of therapeutic concepts. The introduction of new methods such as gene expression profiling catalyzes this process.

Diagnostic pathways in acute leukemias: a proposal for a multimodal approach

Acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) each represent a heterogeneous complex of disorders, which result from diverse mechanisms of leukemogenesis. Modern therapeutic concepts are based on individual risk stratification at diagnosis and during follow-up. For some leukemia subtypes such as AML M3/M3v with t(15;17)/PML-RARA or Philadelphia-positive ALL targeted therapy options are available. Thus, optimal therapeutic conditions are based on exact classification of the acute leukemia subtype at diagnosis and are guided by exact and sensitive quantification of minimal residual disease during complete hematologic remission. Today, a multimodal diagnostic approach combining cytomorphology, multiparameter flow cytometry, chromosome banding analysis, accompanied by diverse fluorescence in situ hybridization techniques, and molecular analyses is needed to meet these requirements. As the diagnostic process becomes more demanding with respect to experience of personnel, time, and costs due to the expansion of methods, algorithms, which guide the diagnostic procedure from basic to more specific methods and which lead finally to a synopsis of the respective results, are essential for modern diagnostics and therapeutic concepts.

Mouse models of acute promyelocytic leukemia

Mouse models of acute promyelocytic leukemia have been generated through transgenic, knock-in, retroviral, and xenograft strategies. These models have been used to elucidate mechanisms underlying leukemogenesis. Among the areas investigated are the role of reciprocal fusions; effects of target cells, expression levels, and mouse strains; cooperating events; and restrictive and permissive factors. These models have also been used to gain insight into the effects of the immune system on leukemic cells and the mechanism of response to retinoic acid. Furthermore, preclinical studies utilizing these mice have advanced therapy for myeloid leukemia.

Transgenic mice expressing Tel-FLT3, a constitutively activated form of FLT3, develop myeloproliferative disease

Evidence is continuing to accumulate that the FMS-like tyrosine kinase 3 (FLT3) receptor plays an important role in acute leukemias. Acute myeloid leukemia patients often express constitutive active mutant forms of the receptor in their leukemic cells. A t(12;13)(p13;q12) translocation between Tel and the FLT3 receptor was recently described in a patient with myeloproliferative disease (MPD). Here a Tel-FLT3 construct mimicking this fusion protein was used to generate transgenic mice. The fusion protein was previously found to constitutively activate FLT3 signaling and transform Ba/F3 cells. Expression of the fusion protein in the transgenic mice was found in all tissues assayed including spleen, bone marrow (BM), thymus and liver. These mice developed splenomegaly and had a high incidence of MPD with extramedullary hematopoiesis in the liver and lymph nodes. Spleens also had increased dendritic and natural killer cell populations. In vitro analysis of the hematopoietic progenitor cells derived from Tel-FLT3 transgenic mice showed a significant increase in the number of CFU-GM in the BM, and CFU-GM, BFU-E and CFU-GEMM in the spleen. BM also showed significant increases of in vivo CFU-S colonies. Thus, transgenic mice expressing constitutively activated Tel-FLT3 develop MPD with a long latency and also result in the expansion of the hematopoietic stem/progenitor cells.

The antitumor effects of sunitinib (formerly SU11248) against a variety of human hematologic malignancies: enhancement of growth inhibition via inhibition of mammalian target of rapamycin signaling

We studied antitumor effects of receptor tyrosine kinase inhibitor sunitinib (formerly SU11248) against a variety of hematologic malignancies including the following leukemias: eosinophilic (EOL-1), acute myeloid (THP-1, U937, Kasumi-1), biphenotypic (MV4-11), acute lymphoblastic (NALL-1, Jurkat, BALL-2, PALL-1, PALL-2), blast crisis of chronic myeloid (KU812, Kcl-22, K562), and adult T-cell (MT-1, MT-2, MT-4), as well as non-Hodgkin's lymphoma (KS-1, Dauji, Akata) and multiple myeloma (U266). Thymidine uptake studies showed that sunitinib was active against EOL-1, MV4-11, and Kasumi-1 cells, which possessed activating mutations of the PDGFRalpha, FLT-3, and c-KIT genes, respectively, with IC(50)s of <30 nmol/L. In addition, sunitinib inhibited the proliferation of freshly isolated leukemia cells from patients possessing mutations in FLT3 gene. Annexin V staining showed that sunitinib induced apoptosis of these cells. Sunitinib inhibited phosphorylation of FLT3 and PDGFRalpha in conjunction with blockade of mammalian target of rapamycin signaling in MV4-11 and EOL-1 cells, respectively. Interestingly, rapamycin analogue RAD001 enhanced the ability of sunitinib to inhibit the proliferation of leukemia cells and down-regulate levels of mammalian target of rapamycin effectors p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 in these cells. Taken together, sunitinib may be useful for treatment of individuals with leukemias possessing activation mutation of tyrosine kinase, and the combination of sunitinib and RAD001 represents a promising novel treatment strategy.

Mutant Transcription Factors and Tyrosine Kinases as Therapeutic Targets for Leukemias: From Acute Promyelocytic Leukemia to Chronic Myeloid Leukemia and Beyond

Mutations in transcription factors (TFs) and protein tyrosine kinases (PTKs), which result in inhibition of differentiation/apoptosis or enhanced proliferative/survival advantage of hematopoietic stem/progenitor cells, are two classes of the most frequently detected genetic abnormalities in leukemias. The critical roles for mutant TFs and/or PTKs to play in leukemogenesis, and the absence of mutant TFs/PTKs in normal hematopoietic cells, suggest that the two types of aberrant molecules may serve as ideal therapeutic targets. The great success of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) in treating acute promyelocytic leukemia through modulation of the causative PML-RARalpha oncoprotein represents the first two paradigms of mutant TFs-targeting therapeutic strategies for leukemia. More recently, tyrosine kinase inhibitor STI-571/Imatinib mesylate/Gleevec in the treatment of Breakpoint Cluster Region-Abelson (BCR-ABL) positive leukemia elicits paradigm of mutant PTKs as ideal antileukemia targets. Thus to further improve clinical outcome of leukemia patients, elucidation of pathogenesis of leukemia, screening for oncoprotein-targeting small molecules, as well as rationally designed combination of drugs with potential synergy are of importance.

Implication of the molecular characterization of acute myeloid leukemia

The identification of molecular genetic alterations such as gene mutations or deregulated gene expression in acute myeloid leukemia (AML) has greatly advanced our understanding of leukemogenesis. These markers now allow us to unravel the enormous heterogeneity seen within cytogenetically defined subgroups of AML. Furthermore, the molecular alterations are providing targets for molecular therapies. In this article, major molecular findings of prognostic and predictive significance are reviewed, with an emphasis on the discussion of gene mutations found in two major AML subgroups, cytogenetically normal and core-binding factor AML.

PKC 412 small-molecule tyrosine kinase inhibitor

BACKGROUND

PKC412 is a kinase inhibitor that blocks protein kinase C (PKC), vascular endothelial growth factor receptors, platelet-derived growth factor receptor FLT3, and other class III receptor tyrosine kinases. The enthusiasm for this compound is based on its inhibitory effect even in the case of FLT3 mutations. The aim of this study was to analyze the role of FLT3 in pancreatic cancer and to study the biological activity of combined inhibition of neovascularization and mitogenesis in this disease.

METHODS

FLT3 expression was analyzed in 18 pancreatic cancer specimens by real-time quantitative polymerase chain reaction (RTQ-PCR) and immunohistochemistry. Sixteen pancreatic cancer cell lines were screened for ITD and D835 point mutations of the FLT3 gene. MTT assays and anchorage-independent growth assays were used to study cell growth. Flow cytometry was used for cell cycle analysis and apoptosis quantification. In vivo AsPC-1 and HPAF-II cells were used for orthotopic tumor modeling. Immunohistochemistry was used to quantify tumor angiogenesis.

RESULTS

FLT3 expression is down-regulated in pancreatic cancer. Activating FLT3 mutations (ITD, D835) were not detectable in any of the pancreatic cancer cell lines. Cell growth was significantly inhibited as cell-cycle progression was reduced and programmed cell death increased. In vivo PKC412 therapy resulted in a significant inhibition of orthotopic tumor growth with abrogation of tumor angiogenesis.

CONCLUSIONS

These data highlight that PKC412 may be a new compound in target therapy of inoperable pancreatic cancer patients and suggest a potential role for the combined use of broad spectrum kinase inhibitors in the management of these patients.

Point mutations of protein kinases and individualised cancer therapy

The treatment of cancer is rapidly changing, with an increasing focus on converting our improved understanding of the molecular basis of disease into clinical benefit for patients. Protein kinases that are mutated in cancer represent attractive targets, as they may result in cellular dependency on the mutant kinase or its associated pathway for survival, a condition known as 'oncogene addiction'. Early clinical experiences have demonstrated dramatic clinical benefit of targeting oncogenic mutations in diseases that have been largely resistant to traditional cytotoxic chemotherapy. Further, mutational activation of kinases can indicate which patients are likely to respond to targeted therapeutics. However, these experiences have also illuminated a number of critical challenges that will have to be addressed in the development of effective drugs across different cancers, to fully realise the potential of individualised molecular therapy. This review utilises examples of genetic activation of kinases to illustrate many of the lessons learned, as well as those yet to be implemented.

New molecular therapy targets in acute myeloid leukemia

Despite improvements to acute myelogenous leukemia (AML) therapy during the last 25 years, the majority of patients still succumb to the disease. Thus, there remains an urgent need for further improvements in this field. The present chapter focuses on exciting areas of research in the field of AML therapy, including promising results with regards to recent improvements in our understanding of angiogenesis, tyrosine kinase signaling, farnesylation, cell cycling, modulation of gene expression, protein degradation, modulation of intracellular proteins, apoptosis, metabolism, and the possible retargeting of oncogenic proteins.

Soluble phosphorylated fms-like tyrosine kinase III. FLT3 protein in patients with acute myeloid leukemia (AML)

FLT3 ligand (FL) has a significant role in the proliferation and differentiation of hematopoietic cells. Mutations in the FLT3 receptor gene have been reported in 30% of patients with AML. We investigated whether abnormal phosphorylation of FLT3 may be more common in AML. We evaluated FLT3 protein and its phosphorylation in the plasma from 85 patients with AML, 16 patients with myelodysplastic syndrome (MDS) and 5 patients with acute lymphoblastic leukemia (ALL). There were no significant differences in the level of plasma FLT3 protein level in the different diseases (p=0.57). AML patients had a significantly higher level of phospho-FLT3:FLT3 ratio (p=0.02). FLT3-ITD and FLT3 point mutations were present in 27 (32%) of the AML patients. Phosphorylated FLT3 was significantly higher in the plasma from patients with FLT3 mutation (p=0.002). Overall, there was no correlation between survival and the plasma level of FLT3 protein or its phosphorylated form. However, amongst the patients without FLT3 mutations, those with a higher level of phosphorylated FLT3 had a significantly shorter duration of remission (p=0.04). Other mechanisms may be responsible for abnormal phosphorylation of FLT3 and inhibitors of FLT3 should also be investigated in patients without mutations.

Acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a heterogeneous clonal disorder of haemopoietic progenitor cells and the most common malignant myeloid disorder in adults. The median age at presentation for patients with AML is 70 years. In the past few years, research in molecular biology has been instrumental in deciphering the pathogenesis of the disease. Genetic defects are thought to be the most important factors in determining the response to chemotherapy and outcome. Whereas significant progress has been made in the treatment of younger adults, the prospects for elderly patients have remained dismal, with median survival times of only a few months. This difference is related to comorbidities associated with ageing and to disease biology. Current efforts in clinical research focus on the assessment of targeted therapies. Such new approaches will probably lead to an increase in the cure rate.

A neoepitope generated by an FLT3 internal tandem duplication (FLT3-ITD) is recognized by leukemia-reactive autologous CD8+ T cells

The FLT3 receptor tyrosine kinase is expressed in more than 90% of acute myelogeneous leukemias (AMLs), up to 30% of which carry an internal tandem duplication (ITD) within the FLT3 gene. Although varying duplication sites exist, most FLT3-ITDs affect a single protein domain. We analyzed the FLT3-ITD of an AML patient for encoding HLA class I–restricted immunogenic peptides. One of the tested peptides (YVDFREYEYY) induced in vitro autologous T-cell responses restricted by HLA-A*0101 that were also detectable ex vivo. These peptide-reactive T cells recognized targets transfected with the patient's FLT3-ITD, but not wild-type FLT3, and recognized the patient's AML cells. Our results demonstrate that AML leukemic blasts can in principle process and present immunogenic FLT3-ITD neoepitopes. Therefore, FLT3-ITD represents a potential candidate target antigen for the immunotherapy of AML.

The effects of lestaurtinib (CEP701) and PKC412 on primary AML blasts: the induction of cytotoxicity varies with dependence on FLT3 signaling in both FLT3-mutated and wild-type cases

The receptor tyrosine kinase FLT3 is a promising molecular therapeutic target in acute myeloid leukemia (AML). Activating mutations of FLT3 are present in approximately one-third of patients, while many nonmutants show evidence of FLT3 activation, which appears to play a significant role in leukemogenesis. We studied the effects of lestaurtinib (CEP701) and PKC412, 2 small molecule inhibitors of FLT3, on 65 diagnostic AML blast samples. Both agents induced concentration-dependent cytotoxicity in most cases, although responses to PKC412 required higher drug concentrations. Cytotoxic responses were highly heterogeneous and were only weakly associated with FLT3 mutation status and FLT3 expression. Importantly, lestaurtinib induced cytotoxicity in a synergistic fashion with cytarabine, particularly in FLT3 mutant samples. Both lestaurtinib and PKC412 caused inhibition of FLT3 phosphorylation in all samples. Translation of FLT3 inhibition into cytotoxicity was influenced by the degree of residual FLT3 phosphorylation remaining and correlated with deactivation of STAT5 and MAP kinase. FLT3 mutant and wild-type cases both varied considerably in their dependence on FLT3 signaling for survival. These findings support the continued clinical assessment of FLT3 inhibitors in combination with cytotoxic chemotherapy: Entry to future clinical trials should include FLT3 wild-type patients and should remain unrestricted by FLT3 expression level.

Newly identified c-KIT receptor tyrosine kinase ITD in childhood AML induces ligand-independent growth and is responsive to a synergistic effect of imatinib and rapamycin

Activating mutations of c-KIT lead to ligand-independent growth. Internal tandem duplications (ITDs) of exon 11, which encodes the juxtamembrane domain (JMD), are constitutively activating mutations found in 7% of gastrointestinal stromal tumors (GISTs) but have not been described in childhood acute myeloid leukemia (AML). DNA and cDNA from 60 children with AML were screened by polymerase chain reaction (PCR) for mutations of the JMD. A complex ITD (kit cITD) involving exon 11 and exon 12 was identified with a relative frequency of 7% (4/60). The human kit cITDs were inserted into the murine c-Kit backbone and expressed in Ba/F3 cells. KIT cITD induced factorindependent growth and apoptosis resistance, and exhibited constitutive autophosphorylation. KIT cITD constitutively activated the PI3K/AKT pathway and phosphorylated STAT1, STAT3, STAT5, and SHP-2. Imatinib (IM) or rapamycin (Rap) led to complete inhibition of growth, with IC50 values at nanomolar levels. IM and Rap synergistically inhibited growth and surmounted KIT cITD-induced apoptosis resistance. IM but not LY294002 inhibited phosphorylation of STAT3 and STAT5, suggesting aberrant cross talk between PI3K- and STAT-activating pathways. The findings presented may have immediate therapeutic impact for a subgroup of childhood AML-expressing c-KIT mutations.

Emerging Flt3 kinase inhibitors in the treatment of leukaemia

Acute myeloid leukaemia (AML) is characterised by the infiltration of the bone marrow with highly proliferative leukaemic cells that stop to differentiate at different stages of myeloid development and carry survival advantages. Conventionally, AML is treated with aggressive cytotoxic therapy, in eligible patients followed by allogeneic bone marrow transplantation. However, despite this aggressive treatment, many patients relapse and eventually die from the disease. Activating mutations in the coding sequence of the receptor tyrosine kinase Flt3 are found in leukaemic blasts from approximately 30% of AML patients. The mutations have been described to severely alter the signalling properties of this receptor and to have transforming activity in cell-line models and in primary mouse bone marrow. The prognosis of patients harbouring the most common Flt3 mutations tends to be worse than that of comparable patients without the mutations. Thus, Flt3 seems a promising target for therapeutic intervention. Several small molecules that inhibit Flt3 kinase activity are being evaluated for the treatment of AML in clinical trials. This review article discusses the signal transduction and biological function of Flt3 and its mutations in normal and malignant haematopoiesis and recent progress in drug development aiming at the inhibition of Flt3 kinases.

Constitutively activated FLT3 phosphorylates BAD partially through pim-1

Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 (Fms-like tyrosine kinase 3) play an important role in leukaemogenesis and their presence is associated with a poor prognosis in acute myeloid leukaemia (AML). Examining the anti- and proapoptotic proteins in constitutively activated FLT3 signalling in BaF3/ITD and MV4-11 cells, we found that the level of Bcl-2 antagonist of cell death (BAD) phosphorylation was greatly decreased in response to FLT3 inhibition. Both Ser-112 and Ser-136 of BAD are rapidly dephosphorylated after treatment with the FLT3 inhibitor CEP-701 in BaF3/ITD and MV4-11 cells. In confirmation of the cell line data, BAD was highly phosphorylated in both constitutively activated wild-type and mutant FLT3 primary AML samples, and rapidly dephosphorylated after treatment of the primary samples with CEP-701. Upstream proteins known to phosphorylate BAD include Akt, extracellular signal-regulated kinase/mitogen-activated protein kinase (Erk/ MAPK), Pim-1 and Pim-2. We and other groups have shown that constitutively activated FLT3 induces multiple signalling pathways, including phosphatidylinositol 3-kinase (PI3K)/Akt, Erk/MAPK and Janus kinase/signal transducers and activators of transcription (Jak/STAT). Thus, BAD may be a nexus point upon which these multiple signalling pathways converge in FLT3-mediated cell survival. In support of this, siRNA knockdown of BAD expression in MV4-11 cells conferred resistance to CEP-701-mediated apoptosis. Our data suggests that Pim-1 is one of the principal kinases mediating the anti-apoptotic function of FLT3/ITD signalling via the phosphorylation of BAD.

DNA repair contributes to the drug-resistant phenotype of primary acute myeloid leukaemia cells with FLT3 internal tandem duplications and is reversed by the FLT3 inhibitor PKC412

The presence of internal tandem duplications (ITD) mutations in the FMS-like tyrosine kinase 3 (FLT3) receptor influences the risk of relapse in acute myeloid leukaemia (AML). We have investigated DNA repair in FLT3-ITD and wild-type (WT) cells. Using the comet assay, we have demonstrated that the FLT3 inhibitor PKC412 significantly inhibits repair of DNA damage in the MV4-11-FLT3-ITD cell line and FLT3-ITD patient samples but not in the HL-60-FLT3-WT cell line or FLT3-WT patient samples. Following the discovery that transcript levels of the DNA repair gene RAD51 are significantly correlated with FLT3 transcript levels in FLT3-ITD patients, we further investigated the role of RAD51 in FLT3-ITD-AML. The reduction in DNA repair in PKC412-treated FLT3-ITD cells was shown to be associated with downregulation of RAD51 mRNA and protein expression and correlates with the maintenance of phosphorylated H2AX levels, implying that PKC412 inhibits the homologous recombination double-strand break repair pathway in FLT3-ITD cells. Using FLT3-short interfering RNA (siRNA), we also demonstrated that genetic silencing of FLT3 results in RAD51 downregulation in FLT3-ITD cells but not in FLT3-WT cells. This work suggests that the use of FLT3 inhibitors such as PKC412 may reverse the drug-resistant phenotype of FLT3-ITD-AML cells by inhibiting repair of chemotherapy-induced genotoxic damage and thereby reduce the risk of disease relapse.

Prolonged exposure to FLT3 inhibitors leads to resistance via activation of parallel signaling pathways

Continuous treatment of malignancies with tyrosine kinase inhibitors (TKIs) may select for resistant clones (ie, imatinib mesylate). To study resistance to TKIs targeting FLT3, a receptor tyrosine kinase that is frequently mutated in acute myelogenous leukemia (AML), we developed resistant human cell lines through prolonged coculture with FLT3 TKIs. FLT3 TKI-resistant cell lines and primary samples still exhibit inhibition of FLT3 phosphorylation on FLT3 TKI treatment. However, FLT3 TKI-resistant cell lines and primary samples often show continued activation of downstream PI3K/Akt and/or Ras/MEK/MAPK signaling pathways as well as continued expression of genes involved in FLT3-mediated cellular transformation. Inhibition of these signaling pathways restores partial sensitivity to FLT3 TKIs. Mutational screening of FLT3 TKI-resistant cell lines revealed activating N-Ras mutations in 2 cell lines that were not present in the parental FLT3 TKI-sensitive cell line. Taken together, these data indicate that FLT3 TKI-resistant cells most frequently become FLT3 independent because of activation of parallel signaling pathways that provide compensatory survival/proliferation signals when FLT3 is inhibited. Anti-FLT3 mAb treatment was still cytotoxic to FLT3 TKI-resistant clones. An approach combining FLT3 TKIs with anti-FLT3 antibodies and/or inhibitors of important pathways downstream of FLT3 may reduce the chances of developing resistance.

FLT3/D835 mutation and inversion of chromosome 16 in leukemic transformation of myelofibrosis

We present an atypical case of myelofibrosis developing into secondary leukemia FAB subtype M4, with inversion of chromosome 16, FLT3/D835 point mutation and diffuse osteolytic lesions accompanied by elevated TNF-alpha. The simultaneous occurrence of these mutations reflects the progressive association of genetic lesions developing into secondary leukemia with a relatively benign course.

Signal transduction therapy in haematological malignancies: identification and targeting of tyrosine kinases

Tyrosine kinases play key roles in cell proliferation, survival and differentiation. Their aberrant activation, caused either by the formation of fusion genes by chromosome translocation or by intragenic changes, such as point mutations or internal duplications, is of major importance in the development of many haematological malignancies. An understanding of the mechanisms by which BCR-ABL contributes to the pathogenesis of chronic myeloid leukaemia led to the development of imatinib, the first of several tyrosine kinase inhibitors to enter clinical trials. Although the development of resistance has been problematic, particularly in aggressive disease, the development of novel inhibitors and combination with other forms of therapy shows promise.

Cytokines in the differentiation therapy of leukemia: from laboratory investigations to clinical applications

Differentiation therapy of leukemia is the treatment of leukemia cells with biological or chemical agents that induce the terminal differentiation of the cancer cells. It is regarded as a novel and targeted approach to leukemia treatment, based on our better understanding of the hematopoietic process and the mechanisms of its deregulation during leukemogenesis. Clinically, differentiation therapy has been most successful in acute promyelocytic leukemia using all-trans-retinoic acid as the inducer, either alone or in combination with chemotherapy. This review presents evidence that a number of hematopoietic cytokines play important roles in both normal and aberrant hematopoietic processes. In vitro laboratory investigations in the past two decades using well-characterized myeloid leukemic cell lines and primary blast cells from leukemia patients have revealed that many hematopoietic cytokines can trigger lineage-specific differentiation of leukemia cells, which may have important implications in the clinical setting. Moreover, our current understanding of cytokine interactions and the molecular mechanisms of cytokine-induced leukemic cell differentiation will be discussed in the light of recent findings. Finally, ways in which laboratory research on cytokines in the differentiation therapy of leukemia can lead to the improved design of protocols for future clinical applications to leukemia therapy will also be addressed.