Acute promyelocytic leukemia: where does it stem from?

Eradication of leukemia stem cells as a new goal of therapy in leukemia

Leukemias have traditionally been classified and treated on the basis of phenotypic characteristics, such as morphology and cell-surface markers, and, more recently, cytogenetic aberrations. These classification systems are flawed because they do not take into account cellular function. The leukemia cell population is functionally heterogeneous: it consists of leukemia stem cells (LSC) and mature leukemia cells that differentiate abnormally to varying extents. Like normal hematopoietic stem cells, LSCs are quiescent and have self-renewal and clonogenic capacity. Because they are quiescent, LSCs do not respond to cell cycle-specific cytotoxic agents used to treat leukemia and so contribute to treatment failure. These cells may undergo mutations and epigenetic changes, further leading to drug resistance and relapse. Recent data suggest that mature leukemia cells may acquire LSC characteristics, thereby evading chemotherapeutic treatment and sustaining the disease. Ongoing research is likely to reveal the molecular mechanisms responsible for LSC characteristics and lead to novel strategies for eradicating leukemia.

Epithelial tissue architecture protects against cancer

We consider the design of colon crypts from the point of view of minimizing the likelihood of generation of cancerous mutations. A stochastic mathematical model (a finite branching process) is developed and fully analyzed. It is found that depending on the mutation rates, different designs are evolutionarily advantageous. If the mutation rates associated with stem cells are a lot higher than the mutation rates of daughter cells, then few stem cells per crypt is the evolutionarily optimal strategy. If the mutation rates of stem cells are of the same order of magnitude or lower than those for daughter cells, then having as many stem cells per crypt as possible is the desirable design. We also found that the optimal evolutionary strategy may work very well to protect the organism from cancer in the young age, but the same strategy becomes detrimental as the organism ages. It pushes the onset of cancer back in time, but it results in an elevated cancer initiation rates as the organism gets older. Our model quantifies the idea that cancer and aging are the two sides of one coin.

Antisense c-myc fragments induce normal differentiation cycles in HL-60 cells

BACKGROUND

We have investigated the potential for using antisense technology as a means of delivering treatment for acute myeloblastic leukaemia (FAB-M2) by gene therapy.

MATERIALS AND METHODS

A test recombinant adenovirus vector was constructed containing human c-myc antisense fragments to study the effects of altering c-myc overexpression in the human HL-60 cell line. Control vector contained the human LacZ gene. Transfection efficiency in HL-60 cells was determined using control vector in the presence of protamine sulphate and multiplicity of infection of 100. Morphological and mechanistic changes were assessed using immunohistochemical analysis, flow cytometry and reverse transcription-polymerase chain reaction.

RESULTS

Transfection efficiency of control vector was 79.8% and morphological differences were observed after 72 h in culture. The rate of proliferation of HL-60 cells infected with test vector was inhibited by 73% compared with control following 6 days in culture. Normal terminal differentiation leading to apoptosis was only evident in test vector infected cells. Peak apoptosis (34.7%) was detected at day 6 and cell cycle arrest at days 2, 4 and 6. Expression of c-fos protein was significantly increased in test vector treated cells with a noticeable down-regulation of c-myc expression.

CONCLUSIONS

These data suggest that transfection of a human HL-60 cell line with vector containing c-myc antisense fragments could inhibit proliferation, but induce differentiation and apoptosis. Thus, we believe that further study of this construct is warranted as a potential gene therapy reagent for treatment of acute myeloblastic leukaemia.

Dicer-dependent turnover of intergenic transcripts from the human beta-globin gene cluster

The widespread occurrence of intergenic transcription in eukaryotes is increasingly evident. Intergenic transcription in the beta-globin gene cluster has been described in murine and human cells, and models for a role in gene and chromatin activation have been proposed. In this study, we analyze intergenic transcription and the chromatin state throughout the human beta-globin gene cluster and find that the data are not consistent with such activation-linked models. Thus, intergenic transcript levels correlate with neither chromatin activation nor globin gene expression. Instead, we find that intergenic transcripts of the beta-globin gene cluster are specifically upregulated in Dicer-deficient cells. This is accompanied by a shift towards more activated chromatin as indicated by changes in histone tail modifications. Our results strongly implicate RNA interference (RNAi)-related mechanisms in regulating intergenic transcription in the human beta-globin gene cluster and further suggest that RNAi-dependent chromatin silencing in vertebrates is not restricted to the centromeres.

High stem cell frequency in acute myeloid leukemia at diagnosis predicts high minimal residual disease and poor survival

PURPOSE

In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34(+)CD38(-) stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated.

EXPERIMENTAL DESIGN

First, the leukemogenic potential of unpurified CD34(+)CD38(-) cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34(+)CD38(-) compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients.

RESULTS

In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34(+)CD38(-) stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34(+) percentage showed no such correlations.

CONCLUSIONS

Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34(+)CD38(-) population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.

Interpreting epithelial cancer biology in the context of stem cells: tumor properties and therapeutic implications

Over 90% of all human neoplasia is derived from epithelia. Significant progress has been made in the identification of stem cells of many epithelia. In general, epithelial stem cells lack differentiation markers, have superior in vivo and in vitro proliferative potential, form clusters in association with a specialized mesenchymal environment (the 'niche'), are located in well-protected and nourished sites, and are slow-cycling and thus can be experimentally identified as 'label-retaining cells'. Stem cells may divide symmetrically giving rise to two identical stem cell progeny. Any stem cells in the niche, which defines the size of the stem cell pool, may be randomly expelled from the niche due to population pressure (the stochastic model). Alternatively, a stem cell may divide asymmetrically yielding one stem cell and one non-stem cell that is destined to exit from the stem cell niche (asymmetric division model). Stem cells separated from their niche lose their stemness, although such a loss may be reversible, becoming 'transit-amplifying cells' that are rapidly proliferating but have a more limited proliferative potential, and can give rise to terminally differentiated cells. The identification of the stem cell subpopulation in a normal epithelium leads to a better understanding of many previously enigmatic properties of an epithelium including the preferential sites of carcinoma formation, as exemplified by the almost exclusive association of corneal epithelial carcinoma with the limbus, the corneal epithelial stem cell zone. Being long-term residents in an epithelium, stem cells are uniquely susceptible to the accumulation of multiple, oncogenic changes giving rise to tumors. The application of the stem cell concept can explain many important carcinoma features including the clonal origin and heterogeneity of tumors, the occasional formation of tumors from the transit amplifying cells or progenitor cells, the formation of precancerous 'patches' and 'fields', the mesenchymal influence on carcinoma formation and behavior, and the plasticity of tumor cells. While the concept of cancer stem cells is extremely useful and it is generally assumed that such cells are derived from normal stem cells, more work is needed to identify and characterize epithelial cancer stem cells, to address their precise relationship with normal stem cells, to study their markers and their proliferative and differentiation properties and to design new therapies that can overcome their unusual resistance to chemotherapy and other conventional tumor modalities.

Gatekeeper pathways and cellular background in the pathogenesis and therapy of AML

Acute myelogenous leukemia (AML) is characterized by the accumulation of immature cells due to disturbed differentiation and proliferation of the myeloid lineage. Genetic alterations affecting transcription factors and receptor tyrosine kinases have been identified in AML and causally linked to the disease. The goal of this review is to address the role of the different genetic alterations in self-renewal and proliferation and to discuss the cellular background in which these events occur during the pathogenesis of AML. Data from AML samples, clinical studies and mouse models for AML will be used to support the different theories regarding the leukemogenesis of AML. Finally, this review wants to highlight the implication of these findings for the therapy of AML.

Relationship between FLT3 mutation status, biologic characteristics, and response to targeted therapy in acute promyelocytic leukemia

The prognostic significance of FLT3 mutations in acute promyelocytic leukemia (APL) is not firmly established and is of particular interest given the opportunities for targeted therapies using FLT3 inhibitors. We studied 203 patients with PML-RARA-positive APL; 43% of the patients had an FLT3 mutation (65 internal tandem duplications [ITDs], 19 D835/I836, 4 ITD+D835/I836). Both mutations were associated with higher white blood cell (WBC) count at presentation; 75% of the patients with WBC counts of 10 x 10(9)/L or greater had mutant FLT3. FLT3/ITDs were correlated with M3v subtype (P < .001), bcr3 PML breakpoint (P < .001), and expression of reciprocal RARA-PML transcripts (P = .01). Microarray analysis revealed differences in expression profiles among patients with FLT3/ITD, D835/I836, and wild-type FLT3. Patients with mutant FLT3 had a higher rate of induction death (19% vs 9%; P = .04, but no significant difference in relapse risk (28% vs 23%; P = .5) or overall survival (59% vs 67%; P = .2) at 5 years. In in vitro differentiation assays using primary APL blasts (n = 6), the FLT3 inhibitor CEP-701 had a greater effect on cell survival/proliferation in FLT3/ITD+ cells, but this inhibition was reduced in the presence of ATRA. Furthermore, in the presence of CEP-701, ATRA-induced differentiation was reduced in FLT3/ITD+ cells. These data carry implications for the use of FLT3 inhibitors as frontline therapy for APL.

Identification of B-cell translocation gene 1 as a biomarker for monitoring the remission of acute myeloid leukemia

Acute myeloid leukemia (AML) is a biologically heterogeneous disease of the hematopoietic system characterized by a clonal accumulation of immature blast cells in bone marrow. We used a proteomic approach based on two-dimensional electrophoresis and mass spectrometry to search for biomarkers related to the complete remission (CR) state of AML patients. We detected one AML-related protein, which was identified as the B-cell translocation gene 1 (BTG1) protein that belongs to anti-proliferative protein family. In the CR state of AML-M2 and M3 patients (by French-American-British subtype classification), the BTG1 protein was upregulated in bone marrow mononuclear cells. It was also expressed robustly in normal bone marrow mononuclear cells. In addition, the BTG1 levels in AML-M2 patients in a non-remission state after therapy did not increase as they did before therapy. Overexpression of BTG1 mRNA was also observed in the CR state of all-trans-retinoic acid (ATRA)-treated AML-M3 patients and ATRA-treated HL-60 cells. Taken together, these results suggest that BTG1 may play a role in the differentiation process of myeloid cells and can therefore be used as a potential treatment-related biomarker for monitoring the remission status of AML-M2 and M3 patients.

Identification of cell lineages involved by t(15;17) in acute promyelocytic leukemia by combined fluorescence activated cell sorting and FISH

Bone marrow cells from five patients with acute promyelocytic leukemia (APL) with t(15;17) were studied by a combination of fluorescence activated cell sorting and fluorescence in situ hybridization (FISH) to establish the cell lineage involvement of t(15;17). Interphase FISH demonstrated that the fusion gene (PML/RARA) was present in almost all abnormal promyelocytes. In one case, the translocation was demonstrated in both CD34+ and CD34- APL cells. The t(15;17) abnormality was not detectable in erythroblasts nor in T- or B-lymphoid cells. These results suggest that lymphocytes and erythroblasts are not clonally involved in APL, and that malignant transformation in some cases of APL may occur at the level of CD34+ cells.

AML M3 and AML M3 variant each have a distinct gene expression signature but also share patterns different from other genetically defined AML subtypes

Acute promyelocytic leukemia (APL) with t(15;17) appears in two phenotypes: AML M3, with abnormal promyelocytes showing heavy granulation and bundles of Auer rods, and AML M3 variant (M3v), with non- or hypogranular cytoplasm and a bilobed nucleus. We investigated the global gene expression profiles of 35 APL patients (19 AML M3, 16 AML M3v) by using high-density DNA-oligonucleotide microarrays. First, an unsupervised approach clearly separated APL samples from other AMLs characterized genetically as t(8;21) (n = 35), inv(16) (n = 35), or t(11q23)/MLL (n = 35) or as having a normal karyotype (n = 50). Second, we found genes with functional relevance for blood coagulation that were differentially expressed between APL and other AMLs. Furthermore, a supervised pairwise comparison between M3 and M3v revealed differential expression of genes that encode for biological functions and pathways such as granulation and maturation of hematologic cells, explaining morphologic and clinical differences. Discrimination between M3 and M3v based on gene signatures showed a median classification accuracy of 90% by use of 10-fold CV and support vector machines. Additional molecular mutations such as FLT3-LM, which were significantly more frequent in M3v than in M3 (P < 0.0001), may partly contribute to the different phenotypes. However, linear regression analysis demonstrated that genes differentially expressed between M3 and M3v did not correlate with FLT3-LM.

Diosgenin dose-dependent apoptosis and differentiation induction in human erythroleukemia cell line and sedimentation field-flow fractionation monitoring

To limit or stop cancer spreading, one of the most prevalent strategies is to induce cancer cell death. Differentiation therapy and apoptosis induction are two ways to achieve this goal. Sedimentation field-flow fractionation (SdFFF) has been described as an effective tool for cell separation, respecting integrity and viability. Because SdFFF takes advantage of intrinsic properties of eluted cells (size, density, shape), we studied the capacity of SdFFF to monitor specific biophysical modifications that occurred during cellular apoptosis or differentiation induction. Then, we used, as an in vitro cellular model of apoptosis and differentiation, diosgenin dose-dependent induction in the polyvalent human erythroleukemia cell line. Two other chemicals were used: phorbol myristate acetate (differentiation inducer) and staurosporine (apoptosis inducer). Our results demonstrated a correlation between SdFFF elution profile changes and induction of effective biological processes. Thus, after acquisition of a reference profile, SdFFF could be used alone to follow chemically induced biological events, suggesting many different applications such as testing series of molecules, evaluation of new cellular/biological models used in different life science fields, or sorting purified populations with the aim of better understanding mechanisms of induced cellular events.

Multiple platelet defects identified by flow cytometry at diagnosis in acute myeloid leukaemia

Summary Previous findings of megakaryocytic hypogranulation and dysmegakaryocytopoietic features in acute myeloid leukaemia (AML) strongly indicate defects in platelet production. The bleeding tendency of these patients may result from dysregulated platelet production, resulting in thrombocytopenia as well as qualitative platelet defects. The present study examined platelet function at diagnosis in 50 AML patients by whole blood flow cytometry. Following in vitro platelet agonist stimulation, platelet activation markers were analysed and compared with 20 healthy individuals. To detect recent in vivo platelet activation, plasma soluble P-selectin (sP-selectin) was measured. Flow cytometric analysis of platelet activation markers demonstrated reduced CD62P [35.6 vs. 118.5 x 10(3) molecules of equivalent soluble fluorochrome (MESF); P < 0.0001], CD63 (11.3 vs. 50.7 x 10(3) MESF; P < 0.0001), and PAC-1 (41.5 vs. 90.5%; P = 0.0001) while reductions in CD42b were abnormal (45.6 vs. 70%; P < 0.0001). sP-selectin levels were similar in patients and healthy controls (0.04 vs. 0.27 fg/platelet; P = 0.84). The presented data indicate that AML pathogenesis may result in multiple platelet defects, involving adhesion, aggregation, and secretion and demonstrate that flow cytometry is a feasible method for platelet function analysis in patients with thrombocytopenia.

Concepts of human leukemic development

Two fundamental problems in cancer research are identification of the normal cell within which cancer initiates and identification of the cell type capable of sustaining the growth of the neoplastic clone. There is overwhelming evidence that virtually all cancers are clonal and represent the progeny of a single cell. What is less clear for most cancers is which cells within the tumor clone possess tumorigenic or 'cancer stem cell' (CSC) properties and are capable of maintaining tumor growth. The concept that only a subpopulation of rare CSC is responsible for maintenance of the neoplasm emerged nearly 50 years ago. Testing of this hypothesis is most advanced for the hematopoietic system due to the establishment of functional in vitro and in vivo assays for stem and progenitor cells at all stages of development. This body of work led to conclusive proof for CSC with the identification and purification of leukemic stem cells capable of repopulating NOD/SCID mice. This review will focus on the historical development of the CSC hypothesis, the mechanisms necessary to subvert normal developmental programs, and the identification of the cell in which these leukemogenic events first occur.

Role of the TEL-AML1 fusion gene in the molecular pathogenesis of childhood acute lymphoblastic leukaemia

Balanced chromosomal translocations are frequently associated with haematopoietic neoplasms and often involve genes that encode transcription factors, which play critical roles in normal haematopoiesis. Fusion oncoproteins that arise from chimeric genes generated by such translocations are usually stable and consistent molecular markers for a given disease subtype and contribute to the leukaemogenic processes. The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in paediatric cancer, occurring in approximately 25% of common (c) B-cell precursor acute lymphoblastic leukaemia (cALL) cases. The rearrangement results in the in-frame fusion of the 5' region of the ETS-related gene, TEL (ETV6), to almost the entire AML1 (RUNX1) locus and is associated with favourable prognosis following conventional therapeutic strategies. We discuss here the prenatal origins of the TEL/AML1 translocation as an initiating mutation, the role of TEL-AML1 in cellular transformation and the molecular mechanisms by which the chimeric protein imposes altered patterns of gene expression.