Variable prognostic value of FLT3 internal tandem duplications in patients with de novo AML and a normal karyotype, t(15;17), t(8;21) or inv(16)

INTRODUCTION

Internal tandem duplication of the FLT3 gene (FLT3/ITD) has been linked to poor outcome in acute myeloid leukemia (AML). However, the prognostic value of FLT3/ITD in various cytogenetic risk groups is still a matter of debate. The aim of this study was to evaluate the prognostic significance in patients with de novo AML and a normal karyotype or a t(15;17), t(8;21) or inv(16) (good risk group).

PATIENTS AND METHODS

Diagnostic samples of 100 patients were investigated by single-step PCR of exons 11 and 12 of the FLT3 gene in a single center retrospective analysis. Subgroups included 53 patients with normal karyotype, 21 patients with t(15;17), 9 patients with t(8;21) and 17 patients with inv(16).

RESULTS

FLT3/ITD was found in 26 out of 100 patients: 30% of patients with a normal karyotype and 38% of t(15;17) patients tested positive. The complete remission (CR) rates for the ITD(+) or ITD(-) groups were 50 vs 76% in normal karyotypes, and 100 vs 53% in t(15;17) patients, while the relapse rates were 75 vs 25% in normal karyotypes and 50 vs 42% in t(15;17) patients. Overall survival (OS) and disease free survival (DFS) at 5 years were significantly different in patients with normal cytogenetics: ITD(+) vs ITD(-): OS 6 vs 28% (P<0.003); DFS 13 vs 41% (P<0.02) Interestingly, FLT3/ITD had no significant effect on the outcome of t(15;17) patients: ITD(+) vs ITD(-): OS 85 vs 53% (P=0.056), DFS 45 vs 60% (P=0.6) at 50 months.

CONCLUSIONS

These data suggest a high prognostic value of FLT3/ITD in patients with normal cytogenetics. However, we find no evidence that FLT3/ITD is a predictive marker for patients with t(15;17).

FLT3 mutations in the activation loop of tyrosine kinase domain are frequently found in infant ALL with MLL rearrangements and pediatric ALL with hyperdiploidy

Point mutations of D835/I836 of the FLT3 gene have been reported in adult acute myeloid leukemia (AML), but not in pediatric AML or acute lymphoblastic leukemia (ALL). FLT3-D835/I836 mutations were found in 6 (5.4%) of 112 children with ALL older than 1 year and in 8 (16.0%) of 50 infants with ALL. Missense mutations were found in 11 patients, 3-base pair deletions in 2 patients, and a deletion/insertion in 1 patient. Remarkably, FLT3-D835/I836 mutations were found in 8 (18.2%) of 44 infants with ALL with MLL rearrangements and in 4 (21.5%) of 19 patients with hyperdiploid ALL, but they were not found in any patients older than 1 year who had TEL-AML1 (n = 11), E2APBX1 (n = 4), or BCR-ABL (n = 6) fusion genes. Although infant ALL patients with mutations had poorer prognoses than did those without mutations, pediatric ALL patients with mutations who were older than 1 year had good prognoses. We also found FLT3-D835 mutations in 2 of 11 leukemic cell lines with MLL rearrangements. FLT3 was highly phosphorylated in these cell lines with FLT3-D835 mutations, leading to constitutive activation of downstream targets such as signal transducer and activator of transcription 5 (STAT5) without FLT3 ligand stimulation. These results suggested that FLT3-D835/I836 mutations are one of the second genetic events in infant ALL with MLL rearrangements or pediatric ALL with hyperdiploidy.

Internal tandem duplication and Asp835 mutations of the FMS-like tyrosine kinase 3 (FLT3) gene in acute promyelocytic leukemia

BACKGROUND

The clinical relevance of mutations of the FMS-like tyrosine kinase 3 (FLT3) gene in specific cytogenetic subgroups is not clear. The authors examined internal tandem duplication (ITD) and Asp835 mutations of FLT3 in patients with acute promyelocytic leukemia (APL) to determine the incidence of these mutations and to analyze the results for correlations with clinicohematologic features and outcome.

METHODS

Bone marrow samples from 107 patients with APL were analyzed. Isoforms of PML-RAR alpha were identified using a reverse transcription-polymerase chain reaction assay. A standard polymerase chain reaction (PCR) assay was used to detect FLT3/ITD mutations. Asp835 mutations were analyzed by PCR amplification of exon 20 followed by EcoRV digestion. All aberrant PCR products subsequently were sequenced.

RESULTS

Twenty-two patients had FLT3/ITD mutations: 9 of 63 patients with L-type PML/RAR alpha, 13 of 34 patients with S-type PML/RAR alpha, and 0 of 10 patients with V-type PML/RAR alpha (P = 0.005). The incidence of FLT3/ITD mutations was significantly higher in patients with S-type PML/RAR alpha than in patients with L-type PML/RAR alpha or V-type PML/RAR alpha. Twenty patients had Asp835 mutations (L-type PML/RAR alpha: n = 11; S-type PML/RAR alpha: n = 8; V-type PML/RAR alpha: n = 1). The frequency of Asp835 mutations was not significantly different among patients with different PML/RAR alpha isoforms (P = 0.582). Three patients had both ITD and Asp835 mutations. The microgranular variant (M3v) form of leukemia was found to be associated with a higher frequency of ITD (P = 0.002) but not with a higher frequency of Asp835 mutations (P = 1.000); analysis of clinicohematologic variables revealed no significant differences in FLT3 mutation incidence among other patient subgroups. There was no significant difference in complete remission rate, overall survival, or event-free survival between patients with ITDs and those without ITDs or between patients with Asp835 mutations and those without Asp835 mutations.

CONCLUSIONS

The current study found that ITD or Asp835 mutations of the FLT3 gene were present in 36.4% of patients with APL; however, these mutations had no prognostic impact. FLT3/ITD frequently was associated with S-type PML/RAR alpha and with the M3v form of leukemia.

FLT3 and MLL intragenic abnormalities in AML reflect a common category of genotoxic stress

MLL rearrangements in acute myeloid leukemia (AML) include translocations and intragenic abnormalities such as internal duplication and breakage induced by topoisomerase II inhibitors. In adult AML, FLT3 internal tandem duplications (ITDs) are more common in cases with MLL intragenic abnormalities (33%) than those with MLL translocation (8%). Mutation/deletion involving FLT3 D835 are found in more than 20% of cases with MLL intragenic abnormalities compared with 10% of AML with MLL translocation and 5% of adult AML with normal MLL status. Real-time quantification of FLT3 in 141 cases of AML showed that all cases with FLT3 D835 express high level transcripts, whereas FLT3-ITD AML can be divided into cases with high-level FLT3 expression, which belong essentially to the monocytic lineage, and those with relatively low-level expression, which predominantly demonstrate PML-RARA and DEK-CAN. FLT3 abnormalities in CBF leukemias with AML1-ETO or CBFbeta-MYH11 were virtually restricted to cases with variant CBFbeta-MYH11 fusion transcripts and/or atypical morphology. These data suggest that the FLT3 and MLL loci demonstrate similar susceptibility to agents that modify chromatin configuration, including topoisomerase II inhibitors and abnormalities involving PML and DEK, with consequent errors in DNA repair. Variant CBFbeta-MYH11 fusions and bcr3 PML-RARA may also be initiated by similar mechanisms.

FLT3-activating mutations in acute promyelocytic leukaemia: a rationale for risk-adapted therapy with FLT3 inhibitors

Our understanding of the genetic basis of acute myeloid leukaemias has been enhanced through cloning of recurring chromosomal translocation breakpoints. However, the remarkable observation, more than a decade ago, that all-trans retinoic acid (ATRA) induced remission in patients with t(15;17) acute promyelocytic leukaemia (APL) was a driving force in the subsequent cloning and characterization of the PML-RARalpha fusion that is causally implicated in the pathogenesis of this disease. Major improvements in treatment and outcome of APL patients have been made since that time by incorporating ATRA in conventional chemotherapy but 30% of APL patients still succumb to complications of their disease or their therapy. Recent information that the haematopoietic receptor tyrosine kinase FLT3 is mutated in about 30% of APL patients suggests strategies for further improving treatment and outcome in this subset of APL patients using small-molecule inhibitors of FLT3. The role of FLT3 mutations in APL and other AML will be discussed.

Oncogenic receptor tyrosine kinase in leukemia

Growth, survival and differentiation of hematopoietic cells are regulated by the interaction between hematopoietic growth factors and their receptors. While the defect in this interaction results in an insufficient hematopoiesis, the aberrantly elevated activation leads to the transformation of hematopoietic cells. The constitutive active mutations of receptor tyrosine kinase, such as c-Kit platelet-derived growth factor receptor (PDGFR) or fins-like tyrosine kinase 3 (Flt3), play a major role in the development of hematopoietic neoplasia. The constitutive activation is provoked by several mechanisms, such as making fusion genes by chromosomal translocations, or various mutations involving regulatory regions of the receptor. The chromosomal translocation brings the receptor intracytoplasmic domain juxtaposed to an unrelated molecule which has dimerization or multimerization motif, resulting in the constitutive dimerization of the receptor. The missense, insertion or deletion mutations in the regulatory regions, such as juxtamembrane domain, activation loop and extracellular domain, cause constitutive activation by releasing the respective auto-inhibitory functions of each regulatory region. Constitutive active receptors generate different signals quantitatively and qualitatively from wild type receptor, which mediate the oncogenic phenotype. Given the frequent involvement of constitutive active receptor tyrosine kinase in hematopoietic malignancies, targeted inhibitions of active tyrosine kinase and downstream aberrant signaling are rapidly developing novel therapeutic modality with much promise.

The angioregulatory phenotype of native human acute myelogenous leukemia cells: influence of karyotype, Flt3 abnormalities and differentiation status

INTRODUCTION

The cytogenetic abnormalities and the response to induction therapy have been regarded as the most important prognostic parameters in acute myelogenous leukemia (AML) patients. Recent studies have demonstrated that internal tandem duplications and specific D-835 point mutations of the Flt3 gene, as well as the angioregulatory phenotype represent additional adverse prognostic factors. The aim of the study was to investigate possible associations between genetic abnormalities, differentiation status and angioregulatory phenotype in native human AML blasts.

METHOD

Native AML blasts derived from consecutive patients were cultured in vitro and concentrations of angioregulatory molecules determined in the supernatants.

RESULTS

Most patients released at least two different angioregulatory mediators. Pro-angiogenic interleukin 8 (IL8) was released at relatively high levels for most patients, many of these patients showed additional release of pro-angiogenic vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF). High release of anti-angiogenic IL12 was associated with high release of pro-angiogenic IL8 and VEGF. Furthermore, patients with D-835 mutations showed increased IL12 release, whereas patients with normal karyotype had decreased HGF release. Myelomonocytic differentiation was associated with IL18 release and CD34 expression with low IL12 release.

CONCLUSION

Our results suggest that native human AML blasts have a pro-angiogenic phenotype. Although the investigated genetic abnormalities are associated with variation in the in vitro release of angioregulators, these differences are relatively small and do not quantitatively involve the most important IL8 release. It therefore seems unlikely that this phenotypic variation can explain the prognostic impact of the genetic abnormalities.

The role of FLT3 in haematopoietic malignancies

Normal haematopoietic cells use complex systems to control proliferation, differentiation and cell death. The control of proliferation is, in part, accomplished through the ligand-induced stimulation of receptor tyrosine kinases, which signal to downstream effectors through the RAS pathway. Recently, mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, which encodes a receptor tyrosine kinase, have been found to be the most common genetic lesion in acute myeloid leukaemia (AML), occurring in approximately 25% of cases. Exploring the mechanism by which these FLT3 mutations cause uncontrolled proliferation might lead to a better understanding of how cells become cancerous and provide insights for the development of new drugs.

FLT3: ITDoes matter in leukemia

FMS-like tyrosine kinase-3 (FLT3), a receptor tyrosine kinase, is important for the development of the hematopoietic and immune systems. Activating mutations of FLT3 are now recognized as the most common molecular abnormality in acute myeloid leukemia, and FLT3 mutations may play a role in other hematologic malignancies as well. The poor prognosis of patients harboring these mutations renders FLT3 an obvious target of therapy. This review summarizes the data on the molecular biology and clinical impact of FLT3 mutations, as well as the therapeutic potential of several small-molecule FLT3 inhibitors currently in development.

Flow cytometric measurement of phosphorylated STAT5 in AML: lack of specific association with FLT3 internal tandem duplications

STAT5 phosphorylation has been noted in 69-95% of AML cases by Western blotting. We used flow cytometry to measure phosphorylated STAT5 on a semi-quantitative scale. The method was validated on K562 cells, which constitutively express phosphorylated STAT5, but lose this when BCR-abl tyrosine kinase activity is blocked by STI571. Phosphorylated STAT5 was found to measure 2.22+/-0.09 relative fluorescence units (RFU) falling to 0.925+/-0.005RFU in the presence of STI571. Phosphorylated STAT5 expression was 0.99 to 2.09RFU in 28 primary AML samples. There was no logical cut-off point between positive and negative fluorescence. FLT3 internal tandem duplications, found in 11/28 samples, were not significantly associated with the level of phosphorylated STAT5 expression. We conclude that STAT5 phosphorylation can be measured sensitively by flow cytometry in AML and that its expression should not be dichotomised as present or absent.

Selective cytotoxic mechanism of GTP-14564, a novel tyrosine kinase inhibitor in leukemia cells expressing a constitutively active Fms-like tyrosine kinase 3 (FLT3)

The receptor tyrosine kinase FLT3 is constitutively activated by an internal tandem duplication (ITD) mutation within the juxtamembrane domain in 20-30% of patients with acute myeloid leukemia. In this study, we identified GTP-14564 as a specific kinase inhibitor for ITD-FLT3 and investigated the molecular basis of its specificity. GTP-14564 inhibited the growth of interleukin-3-independent Ba/F3 expressing ITD-FLT3 at 1 microM, whereas a 30-fold higher concentration of GTP-14564 was required to inhibit FLT3 ligand-dependent growth of Ba/F3 expressing wild type FLT3 (wt-FLT3). However, this inhibitor suppressed the kinase activities of wt-FLT3 and ITD-FLT3 equally, suggesting that the signaling pathways for proliferation differ between wt-FLT3 and ITD-FLT3. Analysis of downstream targets of FLT3 using GTP-14564 revealed STAT5 activation to be essential for growth signaling of ITD-FLT3. In contrast, wt-FLT3 appeared to mainly use the MAPK pathway rather than the STAT5 pathway to transmit a proliferative signal. Further analysis demonstrated that the first two tyrosines in an ITD were critical for STAT5 activation and growth induction but that all of the tyrosines in the juxtamembrane region were dispensable in terms of the proliferation signals of wt-FLT3. These results indicate that an ITD mutation in FLT3 elicits an aberrant STAT5 activation that results in increased sensitivity to GTP-14564. Thus, FLT3-targeted inhibition is an attractive approach, with the potential for selective cytotoxicity, to the treatment of ITD-FLT3-positive acute myeloid leukemia.

Two distinct gene expression signatures in pediatric acute lymphoblastic leukemia with MLL rearrangements

Acute lymphoblastic leukemia (ALL) with 11q23 translocations is usually associated with MLL gene rearrangement, but little is known about its leukemogenesis. We analyzed the gene expression profiles of pediatric ALL samples according to their translocations. Using oligonucleotide microarray analysis, we identified distinct expression profiles for 23 ALL samples with 11q23 translocations, including t(4;11) (n = 15), t(11;19) (n = 6), and t(5;11) (n = 2), compared with 9 ALL samples with other translocations, including t(12;21) (n = 6) and t(1;19) (n = 3). Gene expression scores of FLT3, MeisI, and CD44 for samples with MLL rearrangements were particularly high compared with those for other ALL samples. Statistical analysis of the gene expression profiles for the 21 ALL samples with MLL rearrangements at diagnosis revealed two subgroups that exclusively correlated with prognosis but not with any other clinico-pathological factor. The transcription factors CBF2 and CDP were highly expressed in the poor and good prognosis subgroups, respectively. In addition, their downstream target genes were differentially expressed. These findings provide new insights into the biological mechanisms of leukemogenesis and prognosis for pediatric ALL with MLL rearrangements.

Human CD34+ hematopoietic stem cells capable of multilineage engrafting NOD/SCID mice express flt3: distinct flt3 and c-kit expression and response patterns on mouse and candidate human hematopoietic stem cells

The cytokine tyrosine kinase receptors c-kit and flt3 are expressed and function in early mouse and human hematopoiesis. Through its ability to promote ex vivo expansion and oncoretroviral transduction of primitive human hematopoietic progenitors, the flt3 ligand (FL) has emerged as a key stimulator of candidate human hematopoietic stem cells (HSCs). However, recent studies in the mouse suggest that though it is present on short-term repopulating cells, flt3 is not expressed on bone marrow long-term reconstituting HSCs, the ultimate target for the development of cell replacement and gene therapy. Herein we demonstrate that though only a fraction of human adult bone marrow and cord blood CD34+long-term culture-initiating cells (LTC-ICs) express flt3, most cord blood lymphomyeloid HSCs capable of in vivo reconstituting nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice are flt3+. The striking difference in flt3 and c-kit expression on mouse and candidate human HSCs translated into a corresponding difference in flt3 and c-kit function because FL was more efficient than SCF at supporting the survival of candidate human HSCs. In contrast, SCF is far superior to FL as a viability factor for mouse HSCs. Thus, the present data provide compelling evidence for a contrasting expression and response pattern of flt3 and c-kit on mouse and human HSCs.

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.

Flt3 ligand regulates dendritic cell development from Flt3+ lymphoid and myeloid-committed progenitors to Flt3+ dendritic cells in vivo

Stimulation of Flt3 receptor tyrosine kinase through its cognate ligand expands early hematopoietic progenitor and dendritic cells (DCs) in humans and mice. The exact developmental stages at which hematopoietic progenitors express Flt3, are responsive to its ligand, and subsequently develop to DCs, are not known. Here we show that common lymphoid and common myeloid progenitors, as well as steady state DCs in thymus, spleen, and epidermis, express Flt3. The receptor is down-regulated once definitive B cell, T cell, and megakaryocyte/erythrocyte commitment occurs, and Flt3 is not detectable on other steady state hematopoietic cell populations. Upon in vivo Flt3 ligand (Flt3L) administration, Flt3+ progenitor cells and their progeny DCs are expanded, whereas Flt3- downstream progenitors are not, or are only slightly increased. Transplantation of common lymphoid and common myeloid progenitors and subsequent Flt3L injection increases progeny DCs of both precursor populations. These findings provide a definitive map of Flt3 expression in the hematopoietic hierarchy and directly demonstrate that Flt3L can drive DC development along both the lymphoid and myeloid developmental pathways from Flt3+ progenitors to Flt3+ DCs.

Prognostic significance of FLT3 ITD and D835 mutations in AML patients

Both ITD and D835 mutations of the fms-like tyrosine kinase (FLT3) gene cause constitutive activation of the receptor, in the absence of ligand. We have examined a cohort of 91 patients, AML (80) and MDS (11), to determine the prevalence of these mutations and any correlations between the two mutations and disease prognosis. FLT3/ITD (ITD+) or D835 mutations (D835+) were not detected in MDS patients examined. However, 10% (8/80) and 7.5% (6/80) of AML patients were ITD+ and D835+, respectively. ITD+ patients have a higher rate of relapse than patients with wild-type (WT) FLT3. Median overall survival was 4.6 months (range 0.6-36.2) for ITD+ and 19.85 months (range 0.2-197.5) for WT patients (P=0.0066), and disease-free survival (DFS) was also worse for ITD+ patients than FLT3/WT patients (P=0.047). FLT3/ITD is also a significant prognostic marker for overall survival (OS) and DFS in patients in the standard karyotype group (P=0.0040, 0.0365, respectively). ITD is more prevalent in patients in the standard karyotype category (7/41, 17.1%) as compared to patients in the poor-risk category (1/32, 3.1%). Similar to ITD, D835 mutations were found to be more frequent in patients with standard-risk rather than poor-risk cytogenetic category. WBC count (mean 63.8 x 10(9)/l) was significantly higher in ITD+ patients than patients with D835 mutations (mean 34.8 x 10(9)/l) and WT patients (mean 26.4 x 10(9)/l) (P=0.004). D835 mutants did not appear to have a worse median OS or DFS compared with the WT group. We conclude that FLT3/ITD mutations may be an important prognostic marker in AML, especially in the standard/good risk karyotype groups, where it may allow risk-directed therapy.

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.

Comparative analysis of MLL partial tandem duplication and FLT3 internal tandem duplication mutations in 956 adult patients with acute myeloid leukemia

Partial tandem duplication (PTD) of the MLL gene and internal tandem duplication (ITD) of the juxtamembrane region of the FLT3 receptor tyrosine kinase gene have been described in acute myeloid leukemia (AML) patients, preferentially in those with normal cytogenetics. These alterations have been associated with a poor prognosis. In our study, we analyzed the prevalence and the potential prognostic impact of these aberrations in a large unselected and well-defined cohort of 956 patients with AML. Results were correlated with cytogenetic data and clinical outcome. MLL PTD was detected by RT-PCR, subsequent nucleotide sequencing, and Southern blotting. The overall incidence was found to be 5.0% (48/956), whereas FLT3 ITD was detected in 19.2% (184/956). Sixteen cases were positive for both alterations. The rate of MLL PTD in FLT3 ITD positive patients was significantly higher than that in FLT3 ITD negative patients [16/184 (8.7%); 32/772 (4.1%); P = 0.025]. However, both aberrations were highly increased in patients with normal karyotype (MLL PTD 35/431, P = 0.004; FLT3 ITD 132/334, P < 0.001). When restricted to this subgroup, the rate of MLL PTD in patients with FLT3 mutations was not significantly increased. No statistically significant differences were detected between patients positive for MLL PTD and patients negative for MLL PTD in the rate of complete remissions or the overall survival, although we did see a significantly shorter disease-free survival in patients age 60 or younger. In conclusion, although there is an overlap in the mutational spectrum in AML with FLT3 ITD and MLL PTD mutations, our data do not support a common mechanistic basis. Although associated with inferior disease-free survival, the results of this study do not unequivocally support the notion that MLL PTD mutations represent an independent prognostic factor.

FLT3 mutations do not influence MDR-1 gene expression in acute myeloid leukemia

BACKGROUND

A worse outcome for patients carrying FLT3 mutations or expressing MDR1 gene has been reported. This study was designed to verify the possible co-expression of mutated-FLT3 and MDR1 genes and to evaluate their independent prognostic role.

PATIENTS AND METHODS

Sixty-one AML patients were tested for MDR1-mRNA levels by real-time PCR and for FLT3 mutations by PCR and Genescan analysis.

RESULTS

FLT3 mutations were detected in 31% of patients, more frequently in patients < 60 years and at standard cytogenetic risk. Among 92% of patients MDR1-positive, 50% expressed high levels of mRNA. No significant correlation was found between FLT3 mutations and high levels of MDR1. FLT3 or MDR1 mutations failed to show any role concerning the CR achievement and OS. On the contrary, a significant disadvantage for DFS was found in the group of patients carrying FLT3/ITD.

CONCLUSION

The results reported above show that FLT3 mutations and MDR1 expression represent two independent clinical prognostic factors for AML patients.

Gene expression signatures in MLL-rearranged T-lineage and B-precursor acute leukemias: dominance of HOX dysregulation

Rearrangements of the MLL locus, located on human chromosome 11q23, are frequent in both infant and therapy-related leukemias. Gene expression analysis of MLL-rearranged B-precursor acute lymphoblastic leukemias (MLL B-ALLs) has identified these cases as a unique subtype of leukemia, characterized by the expression of genes associated with both lymphoid and myeloid hematopoietic lineages. Here we show that MLL fusions also generate a distinct genetic subtype of T-lineage ALL (MLL T-ALL), in which leukemic cells are characterized by an early arrest in thymocyte differentiation, with suggestive evidence of commitment to the gammadelta lineage. Interestingly, multiple genes linked to cell proliferation (eg, PCNA, MYC, CDK2, and POLA) were down-regulated in MLL-fusion samples, relative to those transformed by other T-ALL oncogenes (P <.000 001, Fisher exact test). Overall, MLL T-ALL cases consistently demonstrated increased levels of expression of a subset of major HOX genes--HOXA9, HOXA10, and HOXC6--and the MEIS1 HOX coregulator (P <.008, one-sided Wilcoxon test), a pattern of gene expression that was reiterated in MLL B-ALLs. However, expression of myeloid lineage genes, previously reported in MLL B-ALLs, was not identified in T-lineage cases with this abnormality, suggesting that myeloid gene dysregulation is dispensable in leukemic transformation mediated by MLL fusion proteins. Our findings implicate dysregulation of HOX gene family members as a dominant mechanism of leukemic transformation induced by chimeric MLL oncogenes.

Overexpression and constitutive activation of FLT3 induces STAT5 activation in primary acute myeloid leukemia blast cells

PURPOSE

Activating length mutations in the juxtamembrane domain (FLT3-LM) and mutations in the tyrosine kinase domain (FLT3-TKD) of FLT3 represent the most frequent genetic alterations in acute myeloid leukemia (AML). However, the functional role of active FLT3 mutants in primary AML blast cells is not well characterized.

EXPERIMENTAL DESIGN

We analyzed the transforming potential and the signaling of FLT3-ITD mutants in Ba/F3 cells and in primary AML blasts.

RESULTS

FLT3-ITD mutants induce an autophosphorylation of the receptor, interleukin 3-independent growth in Ba/F3 cells, and a strong STAT5 and mitogen-activated protein kinase (MAPK) activation. In contrast to the FLT3-ITD mutants, the ligand-stimulated FLT3-WT receptor was unable to transduce a fully proliferative response in Ba/F3 and monocytic OCI-AML5 cells. The ligand-stimulated FLT3-WT receptor activated AKT and MAPK, but not STAT5. In primary blast cells from 60 patients with AML, FLT3 was expressed in 91.9% of patients carrying a FLT3-LM/TKD mutation compared with 77.8% in FLT3-LM/TKD-negative patients. STAT3 and STAT5 were constitutively activated in 76 and 63% of patients, respectively. In accordance with the results in Ba/F3 cells, a high FLT3 expression and the presence of a FLT3-LM was strongly associated with the STAT5 but not with the STAT3 activation in primary AML blast cells. Moreover, the constitutive tyrosine phosphorylation of STAT5 was efficiently down-regulated by a FLT3 protein tyrosine kinase inhibitor in AML cells expressing an active FLT3 mutant.

CONCLUSIONS

Active FLT3 receptor mutants have transforming potential in hematopoietic cells and induce a strong activation of STAT5 in primary AML cells. The FLT3-STAT5 pathway contributes to the malignant phenotype and represents a promising molecular therapeutic target structure in AML.

Immunophenotypic findings in acute myeloid leukemia with FLT3 internal tandem duplication

BACKGROUND AND OBJECTIVES

The biological characteristics and the prognostic significance of the internal tandem duplication of the FLT3 (FLT3/ITD) were investigated in a series of de novo acute myeloid leukemia (AML) patients. One hundred and fifty-six adult patients with AML were included in the study. FLT3/ITD was detected in 41 (26%) patients (FLT3/ITD(+)).

DESIGN AND METHODS

The main differences observed between the groups with and without FLT3/ITD: a higher leukocyte count, a raised percentage of a normal karyotype and a more frequent M5 FAB diagnosis in the FLT3/ITD(+) patients. As regards the immunophenotype characteristics the FLT3/ITD(+) group very often expressed monocytic markers (CD36 and CD11b) and less commonly immature markers (CD34 and CD117). A promyelocytic-like immunophenotype pattern was also detected in a minority of these patients(4/36).

RESULTS

The FLT3/ITD(+) patients had a shorter overall survival, a shorter event-free survival and a higher probability of relapse. Minimal residual disease (MRD) was investigated in the FLT3/ITD(+) patients using flow cytometry. This technique had a sensitivity of 62% and a specificity of 83% in relapse prediction. Minimal residual disease analysis was hampered by the low number of patients with detectable aberrant immunophenotype.

INTERPRETATION AND CONCLUSIONS

A high frequency of changes in the phenotype and/or genotype pattern between diagnosis and relapse was detected (5/6). FLT3/ITD is a frequent molecular lesion in de novo adult AML and seems to be associated with a monocytic differentiation, a high leukocyte count and a poor prognosis. Immunophenotype and genotype patterns observed at relapse suggest that the FLT3/ITD(+) blasts may be genetically unstable and prone to clonal evolution. FLT3/ITD may not be a suitable target for minimal residual disease studies.

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.