Primitive interleukin 3 null hematopoietic cells transduced with BCR-ABL show accelerated loss after culture of factor-independence in vitro and leukemogenic activity in vivo

Pyruvate anaplerosis is a targetable vulnerability in persistent leukaemic stem cells

Deregulated oxidative metabolism is a hallmark of leukaemia. While tyrosine kinase inhibitors (TKIs) such as imatinib have increased survival of chronic myeloid leukaemia (CML) patients, they fail to eradicate disease-initiating leukemic stem cells (LSCs). Whether TKI-treated CML LSCs remain metabolically deregulated is unknown. Using clinically and physiologically relevant assays, we generate multi-omics datasets that offer unique insight into metabolic adaptation and nutrient fate in patient-derived CML LSCs. We demonstrate that LSCs have increased pyruvate anaplerosis, mediated by increased mitochondrial pyruvate carrier 1/2 (MPC1/2) levels and pyruvate carboxylase (PC) activity, in comparison to normal counterparts. While imatinib reverses BCR::ABL1-mediated LSC metabolic reprogramming, stable isotope-assisted metabolomics reveals that deregulated pyruvate anaplerosis is not affected by imatinib. Encouragingly, genetic ablation of pyruvate anaplerosis sensitises CML cells to imatinib. Finally, we demonstrate that MSDC-0160, a clinical orally-available MPC1/2 inhibitor, inhibits pyruvate anaplerosis and targets imatinib-resistant CML LSCs in robust pre-clinical CML models. Collectively these results highlight pyruvate anaplerosis as a persistent and therapeutically targetable vulnerability in imatinib-treated CML patient-derived samples.

Signal-transducing adapter protein-1 is required for maintenance of leukemic stem cells in CML

The family of signal-transducing adapter proteins (STAPs) has been reported to be involved in a variety of intracellular signaling pathways and implicated as transcriptional factors. We previously cloned STAP-2 as a c-Fms interacting protein and explored its effects on chronic myeloid leukemia (CML) leukemogenesis. STAP-2 binds to BCR-ABL, upregulates BCR-ABL phosphorylation, and activates its downstream molecules. In this study, we evaluated the role of STAP-1, another member of the STAP family, in CML pathogenesis. We found that the expression of STAP-1 is aberrantly upregulated in CML stem cells (LSCs) in patients’ bone marrow. Using experimental model mice, deletion of STAP-1 prolonged the survival of CML mice with inducing apoptosis of LSCs. The impaired phosphorylation status of STAT5 by STAP-1 ablation leads to downregulation of antiapoptotic genes, Bcl-2 and Bcl-xL. Interestingly, transcriptome analyses indicated that STAP-1 affects several signaling pathways related to BCR-ABL, JAK2, and PPARγ. This adapter protein directly binds to not only BCR-ABL, but also STAT5 proteins, showing synergistic effects of STAP-1 inhibition and BCR-ABL or JAK2 tyrosine kinase inhibition. Our results identified STAP-1 as a regulator of CML LSCs and suggested it to be a potential therapeutic target for CML.

Tissue "Hypoxia" and the Maintenance of Leukemia Stem Cells

The relationship of the homing of normal hematopoietic stem cells (HSC) in the bone marrow to specific environmental conditions, referred to as the stem cell niche (SCN), has been intensively studied over the last three decades. These conditions include the action of a number of molecular and cellular players, as well as critical levels of nutrients, oxygen and glucose in particular, involved in energy production. These factors are likely to act also in leukemias, due to the strict analogy between the hierarchical structure of normal hematopoietic cell populations and that of leukemia cell populations. This led to propose that leukemic growth is fostered by cells endowed with stem cell properties, the leukemia stem cells (LSC), a concept readily extended to comprise the cancer stem cells (CSC) of solid tumors. Two alternative routes have been proposed for CSC generation, that is, the oncogenic staminalization (acquisition of self-renewal) of a normal progenitor cell (the "CSC in normal progenitor cell" model) and the oncogenic transformation of a normal (self-renewing) stem cell (the "CSC in normal stem cell" model). The latter mechanism, in the hematological context, makes LSC derive from HSC, suggesting that LSC share SCN homing with HSC. This chapter is focused on the availability of oxygen and glucose in the regulation of LSC maintenance within the SCN. In this respect, the most critical aspect in view of the outcome of therapy is the long-term maintenance of the LSC subset capable to sustain minimal residual disease and the related risk of relapse of disease.

Imatinib-mesylate enhances the maintenance of chronic myeloid leukemia stem cell potential in the absence of glucose

The introduction of BCR/Abl tyrosine kinase inhibitors (TKI), such as imatinib-mesylate (IM), has revolutioned the treatment of chronic myeloid leukemia (CML). However, although extremely effective in inducing CML remission, IM is unable to eliminate leukemia stem cells (LSC). This is largely due to the suppression of BCR/Abl protein, driven by the reduction of energy supply due to oxygen or glucose shortage, in stem cell niches of bone marrow. Here, we investigated whether, in K562 and KCL22 CML cell cultures, glucose shortage induces refractoriness of stem cell potential to IM. In the absence of glucose, IM, while maintaining its detrimental effect on CML cell bulk, actually enhanced colony formation ability and stem cell potential. This was paralleled by an increased expression of the Nanog and Sox-2 stem cell markers. These evidences stress further the importance of developing strategies alternative to TKI capable to target LSC of CML.

Clonal competition in BcrAbl-driven leukemia: how transplantations can accelerate clonal conversion

Background

Clonal competition in cancer describes the process in which the progeny of a cell clone supersedes or succumbs to other competing clones due to differences in their functional characteristics, mostly based on subsequently acquired mutations. Even though the patterns of those mutations are well explored in many tumors, the dynamical process of clonal selection is underexposed.

Methods

We studied the dynamics of clonal competition in a BcrAbl-induced leukemia using a γ-retroviral vector library encoding the oncogene in conjunction with genetic barcodes. To this end, we studied the growth dynamics of transduced cells on the clonal level both in vitro and in vivo in transplanted mice.

Results

While we detected moderate changes in clonal abundancies in vitro, we observed monoclonal leukemias in 6/30 mice after transplantation, which intriguingly were caused by only two different BcrAbl clones. To analyze the success of these clones, we applied a mathematical model of hematopoietic tissue maintenance, which indicated that a differential engraftment capacity of these two dominant clones provides a possible explanation of our observations. These findings were further supported by additional transplantation experiments and increased BcrAbl transcript levels in both clones.

Conclusion

Our findings show that clonal competition is not an absolute process based on mutations, but highly dependent on selection mechanisms in a given environmental context.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-017-0668-x) contains supplementary material, which is available to authorized users.

New Mouse Models to Investigate the Efficacy of Drug Combinations in Human Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) comprises a simple and effective paradigm for generating new insights into the cellular origin, pathogenesis, and treatment of many types of human cancer. In particular, mouse models of CML have greatly facilitated the understanding of the underlying molecular mechanisms and pathogenesis of this disease and have led to the identification of new drug targets that in some cases offer the possibility of functional cure. There are currently three established CML mouse models: the BCR-ABL transgenic model, the BCR-ABL retroviral transduction/transplantation model, and the xenotransplant immunodeficient model. Each has its own unique advantages and disadvantages. Depending on the question of interest, some models may be more appropriate than others. In this chapter, we describe a newly developed xenotransplant mouse model to determine the efficacy of novel therapeutic agents, either alone or in combination. The model facilitates the evaluation of the frequency of leukemic stem cells with long-term leukemia-initiating activity, a critical subcellular population that causes disease relapse and progression, through the utilization of primary CD34(+) CML stem/progenitor cells obtained from CML patients at diagnosis and prior to drug treatment. We have also investigated the effectiveness of new combination treatment strategies designed to prevent the development of leukemia in vivo using BCR-ABL (+) blast crisis cells as a model system. These types of in vivo studies are important for the prediction of individual patient responses to drug therapy, and have the potential to facilitate the design of personalized combination therapy strategies.

IL-33 signaling contributes to the pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are characterized by the clonal expansion of one or more myeloid cell lineage. In most cases, proliferation of the malignant clone is ascribed to defined genetic alterations. MPNs are also associated with aberrant expression and activity of multiple cytokines; however, the mechanisms by which these cytokines contribute to disease pathogenesis are poorly understood. Here, we reveal a non-redundant role for steady-state IL-33 in supporting dysregulated myelopoiesis in a murine model of MPN. Genetic ablation of the IL-33 signaling pathway was sufficient and necessary to restore normal hematopoiesis and abrogate MPN-like disease in animals lacking the inositol phosphatase SHIP. Stromal cell-derived IL-33 stimulated the secretion of cytokines and growth factors by myeloid and non-hematopoietic cells of the BM, resulting in myeloproliferation in SHIP-deficient animals. Additionally, in the transgenic JAK2V617F model, the onset of MPN was delayed in animals lacking IL-33 in radio-resistant cells. In human BM, we detected increased numbers of IL-33-expressing cells, specifically in biopsies from MPN patients. Exogenous IL-33 promoted cytokine production and colony formation by primary CD34+ MPN stem/progenitor cells from patients. Moreover, IL-33 improved the survival of JAK2V617F-positive cell lines. Together, these data indicate a central role for IL-33 signaling in the pathogenesis of MPNs.

The inducible tissue-specific expression of the human IL-3/GM-CSF locus is controlled by a complex array of developmentally regulated enhancers

The closely linked human IL-3 and GM-CSF genes are tightly regulated and are expressed in activated T cells and mast cells. In this study, we used transgenic mice to study the developmental regulation of this locus and to identify DNA elements required for its correct activity in vivo. Because these two genes are separated by a CTCF-dependent insulator, and the GM-CSF gene is regulated primarily by its own upstream enhancer, the main objective in this study was to identify regions of the locus required for correct IL-3 gene expression. We initially found that the previously identified proximal upstream IL-3 enhancers were insufficient to account for the in vivo activity of the IL-3 gene. However, an extended analysis of DNase I-hypersensitive sites (DHSs) spanning the entire upstream IL-3 intergenic region revealed the existence of a complex cluster of both constitutive and inducible DHSs spanning the -34- to -40-kb region. The tissue specificity of these DHSs mirrored the activity of the IL-3 gene, and included a highly inducible cyclosporin A-sensitive enhancer at -37 kb that increased IL-3 promoter activity 40-fold. Significantly, inclusion of this region enabled correct in vivo regulation of IL-3 gene expression in T cells, mast cells, and myeloid progenitor cells.

Ex vivo expansion of normal and chronic myeloid leukemic stem cells without functional alteration using a NUP98HOXA10homeodomain fusion gene

HOX genes have been implicated as regulators of normal and leukemic stem cell functionality, but the extent to which these activities are linked is poorly understood. Previous studies revealed that transduction of primitive mouse hematopoietic cells with a NUP98HOXA10homeodomain (NA10HD) fusion gene enables a subsequent rapid and marked expansion in vitro of hematopoietic stem cell numbers without causing their transformation or deregulated expansion in vivo. To determine whether forced expression of NA10HD in primitive human cells would have a similar effect, we compared the number of long-term culture-initiating cells (LTC-ICs) present in cultures of lenti-NA10HD versus control virus-transduced CD34(+) cells originally isolated from human cord blood and chronic phase (CP) chronic myeloid leukemia (CML) patients. We found that NA10HD greatly increases outputs of both normal and Ph(+)/BCR-ABL(+) LTC-ICs, and this effect is particularly pronounced in cultures containing growth factor-producing feeders. Interestingly, NA10HD did not affect the initial cell cycle kinetics of the transduced cells nor their subsequent differentiation. Moreover, immunodeficient mice repopulated with NA10HD-transduced CP-CML cells for more than 8 months showed no evidence of altered behavior. Thus, NA10HD provides a novel tool to enhance both normal and CP-CML stem cell expansion in vitro, without apparently altering other properties.

Deficiency of SATB1 expression in Sezary cells causes apoptosis resistance by regulating FasL/CD95L transcription

Sézary syndrome (SS) is an aggressive subtype of cutaneous T-cell lymphoma that is characterized by circulating leukemic Sézary cells. The accumulation of these malignant cells has been shown to be the result of the resistance to apoptosis, in particular, activation-induced cell death. However, the mechanism of apoptosis resistance remains unknown. By characterizing the gene transcription profiles of purified CD4(+)CD7(-) Sézary cells from patients with SS and cultured Sézary cells, it was found that Sézary cells are deficient in the expression of special AT-rich region binding protein 1 (SATB1), a key regulator of T-cell development and maturation. Retrovirus-mediated gene transduction revealed that SATB1 restoration in cultured Sézary cells (Hut78) triggered spontaneous cell death and sensitized Hut78 cells to activation-induced cell death, with associated activation of caspase 8 and caspase 3. Furthermore, endogenous expression of FasL in Sézary cells was increased in transcriptional and translational levels on restoration of SATB1 expression in cultured Sézary cells. These results suggest that deficiency in SATB1 expression in Sézary cells plays an important role in SS pathogenesis by causing apoptosis resistance. Thus, restoration of SATB1 expression may represent a potential molecular targeted therapy for SS, which does not have a cure at present.

BCR-ABL but not JAK2 V617F inhibits erythropoiesis through the Ras signal by inducing p21CIP1/WAF1

BCR-ABL is a causative tyrosine kinase (TK) of chronic myelogenous leukemia (CML). In CML patients, although myeloid cells are remarkably proliferating, erythroid cells are rather decreased and anemia is commonly observed. This phenotype is quite different from that observed in polycythemia vera (PV) caused by JAK2 V617F, whereas both oncogenic TKs activate common downstream molecules at the level of hematopoietic stem cells (HSCs). To clarify this mechanism, we investigated the effects of BCR-ABL and JAK2 V617F on erythropoiesis. Enforced expression of BCR-ABL but not of JAK2 V617F in murine LSK (Lineage(-)Sca-1(hi)CD117(hi)) cells inhibited the development of erythroid cells. Among several signaling molecules downstream of BCR-ABL, an active mutant of N-Ras (N-RasE12) but not of STAT5 or phosphatidylinositol 3-kinase (PI3-K) inhibited erythropoiesis, while N-RasE12 enhanced the development of myeloid cells. BCR-ABL activated Ras signal more intensely than JAK2 V617F, and inhibition of Ras by manumycin A, a farnesyltransferase inhibitor, ameliorated erythroid colony formation of CML cells. As for the mechanisms of Ras-induced suppression of erythropoiesis, we found that GATA-1, an erythroid-specific transcription factor, blocked Ras-mediated mitogenic signaling at the level of MEK through the direct interaction. Furthermore, enforced expression of N-RasE12 in LSK cells derived from p53-, p16(INK4a)/p19(ARF)-, and p21(CIP1/WAF1)-null/wild-type mice revealed that suppressed erythroid cell growth by N-RasE12 was restored only by p21(CIP1/WAF1) deficiency, indicating that a cyclin-dependent kinase (CDK) inhibitor, p21(CIP1/WAF1), plays crucial roles in Ras-induced suppression of erythropoiesis. These data would, at least partly, explain why respective oncogenic TKs cause different disease phenotypes.

AHI-1 interacts with BCR-ABL and modulates BCR-ABL transforming activity and imatinib response of CML stem/progenitor cells

Chronic myeloid leukemia (CML) represents the first human malignancy successfully treated with a tyrosine kinase inhibitor (TKI; imatinib). However, early relapses and the emergence of imatinib-resistant disease are problematic. Evidence suggests that imatinib and other inhibitors may not effectively eradicate leukemic stem/progenitor cells, and that combination therapy directed to complimentary targets may improve treatment. Abelson helper integration site 1 (Ahi-1)/AHI-1 is a novel oncogene that is highly deregulated in CML stem/progenitor cells where levels of BCR-ABL transcripts are also elevated. Here, we demonstrate that overexpression of Ahi-1/AHI-1 in murine and human hematopoietic cells confer growth advantages in vitro and induce leukemia in vivo, enhancing effects of BCR-ABL. Conversely, RNAi-mediated suppression of AHI-1 in BCR-ABL–transduced lin⁻CD34⁺ human cord blood cells and primary CML stem/progenitor cells reduces their growth autonomy in vitro. Interestingly, coexpression of Ahi-1 in BCR-ABL–inducible cells reverses growth deficiencies exhibited by BCR-ABL down-regulation and is associated with sustained phosphorylation of BCR-ABL and enhanced activation of JAK2–STAT5. Moreover, we identified an AHI-1–BCR-ABL–JAK2 interaction complex and found that modulation of AHI-1 expression regulates phosphorylation of BCR-ABL and JAK2–STAT5 in CML cells. Importantly, this complex mediates TKI response/resistance of CML stem/progenitor cells. These studies implicate AHI-1 as a potential therapeutic target downstream of BCR-ABL in CML.

Instability of BCR-ABL gene in primary and cultured chronic myeloid leukemia stem cells

BACKGROUND

Imatinib mesylate treatment causes remissions in a majority of patients with chronic myeloid leukemia (CML), but relapses are an increasing problem. We hypothesized that imatinib-resistant leukemic cells emerge from CML stem cells that acquire BCR-ABL gene mutations even before exposure to BCR-ABL-targeted agents such as imatinib.

METHODS

Lineage-negative (i.e., immature) CD34+ CD38- CML stem cell-enriched populations were isolated from five patients with chronic phase CML samples by fluorescence-activated cell sorting. To identify BCR-ABL gene mutations, complementary DNAs (cDNAs) prepared from purified CML stem cells were subjected to allele-specific amplification using primers corresponding to 16 kinase domain mutations, with normal bone marrow cells serving as negative controls. We also cloned and directly sequenced BCR-ABL cDNAs prepared from freshly isolated CML stem cells and from their progeny generated after 3-5 weeks of culture.

RESULTS

In 20%-33% of cDNA preparations from freshly isolated CML stem cell-enriched populations, both allele-specific amplification and direct sequencing methods revealed mutations in sequences corresponding to the BCR-ABL kinase domain. Mutations were not observed in cDNA sequences encoding the c-ABL kinase domain that were obtained from similar types of primitive normal cells. More than 70 different BCR-ABL mutations (including frameshift mutations and premature stop codons) were identified in the progeny of cultured CML stem cells. Analysis of individual clones derived from the cultured cells demonstrated that new BCR-ABL mutations were produced.

CONCLUSIONS

Primary CML stem cells display instability of the BCR-ABL fusion gene both in vivo and in vitro. Thus, patients may possess leukemic stem cells with BCR-ABL kinase mutations before initiation of BCR-ABL-targeted therapies and would likely be predisposed to develop resistance to these agents.

The challenges of targeting chronic myeloid leukemia stem cells

Chronic myeloid leukemia (CML) is sustained by a clonally amplified population of Bcr Abl-positive pluripotent stem cells. Persistence of a large, functionally intact yet suppressed residual normal hematopoietic stem cell population in most patients with CML has made it possible to aim at the development of curative therapies. However, achieving this goal requires the identification of agents that will eradicate the leukemic stem cell population. Several potent Bcr-Abl-targeted drugs have now been introduced into clinical practice with remarkable effects. Nevertheless, accumulating data indicate that the leukemic CML stem cells in patients with chronic phase CML are less responsive to these agents than the bulk of the neoplastic cells. In this article, we review emerging evidence that CML stem cells have a number of unusual properties that underlie their relative insensitivity to treatment, including those that specifically target the Bcr-Abl oncoprotein. The biology of the neoplastic stem cells in patients with CML is clearly important to the future attainment of cures and might also prove a paradigm relevant to other types of malignancies that are sustained by transformed stem cell populations.

Chronic myeloid leukemia stem cells possess multiple unique features of resistance to BCR-ABL targeted therapies

The leukemic stem cells in patients with chronic myeloid leukemia (CML) are well known to be clinically resistant to conventional chemotherapy and may also be relatively resistant to BCR-ABL-targeted drugs. Here we show that the lesser effect of imatinib mesylate (IM) on the 3-week output of cells produced in vitro from lin(-)CD34(+)CD38(-) CML (stem) cells compared with cultures initiated with the CD38(+) subset of lin(-)CD34(+) cells is markedly enhanced (>10-fold) when conditions of reduced growth factor stimulation are used. Quantitative analysis of genes expressed in these different CML subsets revealed a differentiation-associated decrease in IL-3 and G-CSF transcripts, a much more profound decrease in expression of BCR-ABL than predicted by changes in BCR expression, decreasing expression of ABCB1/MDR and ABCG2 and increasing expression of OCT1. p210(BCR-ABL) and kinase activity were also higher in the lin(-)CD34(+)CD38(-) cells and formal evidence that increasing BCR-ABL expression decreases IM sensitivity was obtained from experiments with a cell line model. Nevertheless, within the entire CD34(+) subset of CML cells, BCR-ABL expression was not strongly affected by changes in cell cycle status. Taken together, these results provide the first evidence of multiple mechanisms of innate IM resistance in primitive and quiescent CML cells.

Evidence for an oncogenic role of AHI-1 in Sezary syndrome, a leukemic variant of human cutaneous T-cell lymphomas

Ahi-1 (Abelson helper integration site 1) is a novel gene frequently activated by provirus insertional mutagenesis in murine leukemias and lymphomas. Its involvement in human leukemogenesis is demonstrated by gross perturbations in its expression in human leukemia cells, particularly in cutaneous T-cell lymphoma cell lines where increases in AHI-1 transcripts of 40-fold are seen. To test directly whether deregulated expression of AHI-1 contributes to their transformed properties, knockdown of AHI-1 expression in Hut78 cells, a cell line derived from a patient with Sezary syndrome (SS), was performed using retroviral-mediated RNA interference. Retroviral-mediated suppression specifically inhibited expression of AHI-1 and its isoforms in transduced cells by 80% and also reduced autocrine production of interleukin (IL)-2, IL-4 and tumor necrosis factor-alpha (TNFalpha) by up to 85%. It further significantly reduced their growth factor independence in vitro and the ability to produce tumors in immunodeficient mice. Interestingly, aberrant expression of AHI-1, particularly truncated isoforms, was present in CD4+CD7- Sezary cells from some patients with SS. Elevated expression of IL-2 and TNFalpha was also found in these cells. These findings provide strong evidence of the oncogenic activity of AHI-1 in human leukemogenesis and demonstrate that its deregulation may contribute to the development of SS.

Isolation and characterization of dominant and recessive IL-3-independent hematopoietic transformants

Retroviral integration mutagenesis and treatment with the frameshift mutagen ICR191 were used to transform v-H-ras expressing PB-3c cells to interleukin-3 (IL-3) independence. Six clones displayed viral integrations into the 3' region of the IL-3 gene thus acting post-transcriptionally by disrupting the AU-rich instability element. Two clones contained reverse orientation integration into the raf-1 gene revealing an enhancer insertion mechanism. Growth by this mechanism was sensitive to the Raf-1 inhibitor BAY 43-9006 and the Mek inhibitor U0126. Following treatment with ICR191, IL-3-independent clones were recovered and studied by cell fusion. With 21/22 clones, IL-3 independence resulted from a recessive mechanism as cellular hybrids with parental cells reverted to IL-3 dependence. Recessive clone D2c displayed increased phospho-Erk1/2 levels and was growth sensitive to U0126, but not to BAY43-9006. The single dominant clone, D5a, showed no signs of mitogen-activated protein kinases pathway activation but displayed constitutive phosphorylation of Stat5. We conclude that PB-3c has several options to acquire IL-3 growth autonomy involving transcriptional or post-transcriptional mechanisms affecting the distal regulators Erk or Stat5. The reported panel of independent dominant and recessive transformants should provide a useful tool for inhibitor profiling.

Expression of activated M-Ras in hemopoietic stem cells initiates leukemogenic transformation, immortalization and preferential generation of mast cells

Cultures of purified hemopoietic stem cells transduced with an activated mutant of M-Ras contained abnormal cells that, despite the presence of only low levels of growth factors, generated large, dense colonies of macrophages and blast cells. Cells from these colonies survived and grew continuously in the absence of growth factors and generated clonal cell-lines that were mainly composed of well-differentiated mast cells, with a low frequency of undifferentiated cells. When transplanted into sublethally irradiated syngeneic mice, four out of four such clones gave rise to a systemic mastocytosis and mast-cell leukemia. However, the donor clones also generated low percentages of cells with the morphological and cell-surface characteristics of erythrocytes, granulocytes, monocytes and T- and B-lymphocytes. These data indicate that signals downstream of activated M-Ras are sufficient to transform hemopoietic stem cells, and while preserving their capacity to generate other cell-lineages in vivo, result in preferential generation of mast cells.

Different subsets of primary chronic myeloid leukemia stem cells engraft immunodeficient mice and produce a model of the human disease

Xenograft models of chronic phase human chronic myeloid leukemia (CML) have been difficult to develop because of the persistence of normal hematopoietic stem cells in most chronic phase CML patients and the lack of methods to selectively isolate the rarer CML stem cells. To circumvent this problem, we first identified nine patients' samples in which the long-term culture-initiating cells were predominantly leukemic and then transplanted cells from these samples into sublethally irradiated NOD/SCID and NOD/SCID-beta2microglobulin-/- mice. This resulted in the consistent and durable (>5 months) repopulation of both host genotypes with similar numbers of BCR-ABL+/Ph+ cells. The regenerated leukemic cells included an initial, transient population derived from CD34+CD38+ cells as well as more sustained populations derived from CD34+CD38- progenitors, indicative of a hierarchy of transplantable leukemic cells. Analysis of the phenotypes produced revealed a reduced output of B-lineage cells, enhanced myelopoiesis with excessive production of erythroid and megakaropoietic cells and the generation of primitive (CD34+) leukemic cells displaying an autocrine IL-3 and G-CSF phenotype, all characteristics of primary CML cells. These findings demonstrate the validity of this xenograft model of chronic phase human CML, which should enable future investigation of disease pathogenesis and new approaches to therapy.

BCR-ABL-transduced human cord blood cells produce abnormal populations in immunodeficient mice

In this study, we describe the successful use of a gene transfer approach to demonstrate the ability of forced BCR-ABL expression to deregulate the growth and differentiation of primitive naive human hematopoietic cells after their transplantation into immunodeficient mice. Human CD34+ cord blood cells were exposed to an MSCV retrovirus containing a BCR-ABL-IRES-GFP (P210) cassette and then injected immediately into sublethally irradiated nonobese diabetic-severe combined immunodeficiency (NOD/SCID) or NOD/SCID-beta2microglobulin-/- mice. P210- and control-transduced (GFP+) human hematopoietic cells were produced in the bone marrow of the mice at similar levels until termination of the experiments 5-6 months later. However, the P210-transduced cells produced a markedly different spectrum of progeny, with an increased ratio of myeloid to B-lymphoid cells and a frequently prolonged increase in erythroid and megakaryocytic cells. After 5 months, several of the mice transplanted with P210-transduced cells developed an increased WBC count and/or splenomegaly due to an expansion of the human GFP+ population. These findings demonstrate that forced expression of BCR-ABL in primitive transplantable human hematopoietic cells is sufficient to cause a rapid and persistent deregulation of their growth and differentiation in vivo with occasional evidence after several months of progression to an early stage of disease.

Animal models of chronic myelogenous leukemia

Of the current mouse chronic myelogenous leukemia (CML) models,the murine bone marrow (BM) transduction and transplantation model most efficiently mimics many of the central features of human CML. In this model, lethally irradiated mice are reconstituted with primary murine BM cells transduced with a P210BCR/ABL retrovirus. All recipient mice develop a fatal peripheral blood and BM granulocytosis and splenomegaly, a disease termed the murine CML-like myeloproliferative disorder. This model has been used to establish the causative role of Bcr/Abl in CML, identify those signaling pathways and regions of Bcr/Abl critical for leukemogenesis, and explore the limitations of targeted CML therapy. Future refinements in this CML mouse model will make it a more effective tool for studying imatinib-resistant CML, reproducing chronic- and blastic-phase human CML, and performing CML progenitor studies.

Growth autonomy and lineage switching in BCR-ABL-transduced human cord blood cells depend on different functional domains of BCR-ABL

The tyrosine kinase activity of p210BCR-ABL is essential to its leukemogenic potential, but the role of other functional domains in primary human hematopoietic cells has not been previously investigated. Here we show that infection of normal human CD34+ cord blood (CB) cells with a retroviral vector encoding p210BCR-ABL rapidly activates a factor-independent phenotype and autocrine interleukin-3/granulocyte colony-stimulating factor/erythropoietin production in the transduced cells. These changes are characteristic of primitive chronic myeloid leukemic (CML) cells and are important to the leukemogenicity of BCR-ABL-transduced murine hematopoietic stem cells. When BCR-ABL-transduced human CB cells were incubated with imatinib mesylate, an inhibitor of the p210BCR-ABL kinase, or when human CB cells were transduced with a BCR-ABL cDNA lacking the SH2 domain (p210DeltaSH2), factor independence was significantly reduced. In contrast, deletion of the SH2 domain had little impact on the p210BCR-ABL kinase-dependent promotion of erythropoietic differentiation also seen immediately following the BCR-ABL transduction of primitive human CB cells, but not in naturally occurring CML. Thus, p210BCR-ABL has distinct biological effects in primary human hematopoietic cells, which variably mimic features of human CML, and activation of these changes can show different dependencies on the integrity of the SH1 and SH2 domains of p210BCR-ABL.

Elevated expression of a subset of interferon inducible genes in primary bone marrow cells expressing p185 Bcr-Abl versus p210 Bcr-Abl by DNA microarray analysis

p185 Bcr-Abl has a more aggressive biological/clinical leukemia phenotype than p210 Bcr-Abl. In this study, we examined differential gene expression using microarrays to determine if upregulation or downregulation of specific genes may explain the distinct phenotypes produced by the two Bcr-Abl forms. RNA was collected from mouse bone marrow mononuclear cells expressing equivalent levels of p185 or p210, and the RNAs were subjected to microarray analysis. Significant differences in gene expression were observed on hierarchical clustering. A group of interferon-gamma-inducible genes, including those encoding a family of 47 kDa GTPases, were significantly increased in p185 versus p210. This family of GTPases has previously been implicated in interferon-gamma-induced resistance against intracellular pathogens, however their exact cellular functions are unknown. Our data suggest that their increased expression may contribute to the biological/clinical phenotype associated with p185.

Deregulated expression in Ph+ human leukemias of AHI-1, a gene activated by insertional mutagenesis in mouse models of leukemia

Ahi-1/AHI-1 (Abelson helper integration site-1) encodes a family of protein isoforms containing one Src homology 3 (SH3) domain and multiple tryptophan-aspartic acid 40 (WD40)-repeat domains. The function of these proteins is unknown, but involvement in leukemogenesis has been suggested by the high frequency of Ahi-1 mutations seen in certain virus-induced murine leukemias. Here we show that in both mice and humans, Ahi-1/AHI-1 expression is highest in the most primitive hematopoietic cells with specific patterns of down-regulation in different lineages. Cells from patients with chronic myeloid leukemia (CML; n = 28) show elevated AHI-1 transcripts in all disease phases and, in chronic phase, in the leukemic cells at all stages of differentiation, including quiescent (G(0)) CD34(+) cells as well as terminally differentiating cells. In the most primitive lin(-)CD34(+)CD38(-) CML cells, transcripts for the 2 shorter isoforms of AHI-1 are also increased. Although 15 of 16 human lymphoid and myeloid leukemic cell lines showed aberrant control of AHI-1 expression, this was not seen in blasts obtained directly from patients with acute Philadelphia chromosome-negative (Ph(-)) leukemia (n = 15). Taken together, our results suggest that down-regulation of AHI-1 expression is an important conserved step in primitive normal hematopoietic cell differentiation and that perturbations in AHI-1 expression may contribute to the development of specific types of human leukemia.

Imatinib Mesylate Resistance Through BCR-ABL Independence in Chronic Myelogenous Leukemia

Imatinib mesylate (IM) binds to the BCR-ABL protein, inhibiting its kinase activity and effectively controlling diseases driven by this kinase. IM resistance has been associated with kinase mutations or increased BCR-ABL expression. However, disease progression may be mediated by other mechanisms that render tumor cells independent of BCR-ABL. To demonstrate this potential, IM-resistant cells were found in chronic myelogenous leukemia patients with continuous BCR-ABL gene expression but undetectable BCR-ABL protein expression. These cells were unresponsive to IM and acquired BCR-ABL-independent signaling characteristics. IM resistance in some patients may be mediated through loss of kinase target dependence.

Interleukin-3 receptor in acute leukemia

Recent studies indicate that abnormalities of the interleukin-3 receptor (IL-3R) are frequently observed in acute myeloid leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the evidences indicating that the IL-3R represents one of the target molecules involved in the stimulation of proliferation of AMLs, and the overexpression of the IL-3Ralpha chain may represent one of the mechanisms contributing to the development of a highly malignant leukemic phenotype. Furthermore, there is evidence that the IL-3Ralpha is a marker of leukemic stem cells, at variance with normal stem cells that are IL-3Ralpha-. Finally, the IL-3R may represent an important target for the development of new antileukemic drugs.

Evidence for a positive role of SHIP in the BCR-ABL-mediated transformation of primitive murine hematopoietic cells and in human chronic myeloid leukemia

Previous studies suggested that the SH2-containing inositol-5-phosphatase (SHIP) may play a tumor suppressor-like function in BCR-ABL-mediated leukemogenesis. To investigate this possibility, we first developed a new assay for quantitating transplantable multilineage leukemia-initiating cells (L-ICs) in hematopoietic stem cell (HSC)-enriched mouse bone marrow (BM) cells transduced with a BCR-ABL-GFP (green fluorescent protein) retrovirus. The frequency of L-ICs (1 of 430 Sca-1+lin- cells) was 7-fold lower than the frequency of HSCs in the Sca-1+lin- subset transduced with a control virus (1 of 65 cells). Forced BCRABL expression was also accompanied by a loss of regular HSC activity consistent with the acquisition of an increased probability of differentiation. Interestingly, the frequency and in vivo behavior of wild-type (+/+) and SHIP-/- L-ICs were indistinguishable, and in vitro, Sca-1+lin- BCR-ABL-transduced SHIP-/- cells showed a modestly reduced factor independence. Comparison of different populations of cells from patients with chronic myeloid leukemia (CML) in chronic phase and normal human BM showed that the reduced expression of full-length SHIP proteins seen in the more mature (CD34-lin+) leukemic cells is not mirrored in the more primitive (CD34+lin-) leukemic cells. Thus, SHIP expression appears to be differently altered in the early and late stages of differentiation of BCR-ABL-transformed cells, underscoring the importance of the cellular context in which its mechanistic effects are analyzed.

IL-3 receptor signaling is dispensable for BCR-ABL-induced myeloproliferative disease

BCR-ABL expression led to a dramatic up-regulation of the IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor receptor beta common (IL-3Rbetac) and IL-3 receptor beta (IL-3Rbeta) chains in murine embryonic stem cell-derived hematopoietic cells coincident with an expansion of multipotent progenitors and myeloid elements. This up-regulation required BCR-ABL tyrosine kinase activity and led to IL-3Rbetac/beta chain tyrosine phosphorylation in the absence of detectable IL-3 production. These results suggested that cytokine-independent IL-3 receptor activation could be a dominant signaling component in BCR-ABL-induced leukemogenesis. To unambiguously define the significance of IL-3 receptor-dependent signaling in BCR-ABL-induced leukemogenesis, BCR-ABL-transduced bone marrow cells deficient in either IL-3Rbetac chain or both IL-3Rbetac/beta chain expression were examined for their ability in generating myeloproliferative disease (MPD). These BCR-ABL-expressing knockout cells were capable of generating MPD similar to control cells, demonstrating that IL-3 receptor activation is not essential for BCR-ABL-induced MPD. However, the IL-3Rbetac/beta chain could act as a cofactor in BCR-ABL-induced leukemogenesis by activation of its many known oncogenic signaling pathways.

New models to investigate mechanisms of disease genesis from primitive BCR-ABL(+) hematopoietic cells

Three years ago we described a novel autocrine IL-3/G-CSF mechanism active in the leukemic CD34(+) cells from chronic myeloid leukemia (CML) patients in chronic phase (PNAS 96: 12804-12809, [1999]). We also showed that exposure of the most primitive CD34(+) cells from normal human bone marrow to excess IL-3 stimulates not only the division of these cells but also their differentiation. In contrast, both IL-3 and G-CSF cause an expansion of the more mature types of normal CD34(+) progenitors. These findings suggested that the autocrine IL-3/G-CSF mechanism active in CML stem cells can compromise their self-renewal in spite of increasing their proliferative activity, which, in turn, might explain the paradoxically slow rate of expansion of this compartment over time in patients with latent disease. To investigate this hypothesis, we have begun to characterize the numbers and types of cells generated from chronic phase CML patients' cells transplanted into adult immunodeficient mice or fetal sheep, and also from transplants of primitive murine and human hematopoietic cells transduced with a retroviral BCR-ABL vector. Our findings to date using these models reinforce the importance of the autocrine IL-3/G-CSF mechanism in the development of CML. BCR-ABL appears to directly activate IL-3 and G-CSF production in primitive hematopoietic cells and this is important to their transplantable leukemogenic activity. However, the development in vivo of an overt leukemia from primitive BCR-ABL(+) hematopoietic cells can be very delayed. These models thus offer new opportunities for analyzing the molecular events that underlie the pathogenesis of human CML and the future testing of new therapeutic approaches.