Modeling the initiation and progression of human acute leukemia in mice

Molecular mechanisms involved in the progression of myelodysplastic syndrome

Myelodysplastic syndromes (MDS) are a heterogeneous group of diseases characterized by ineffective hematopoiesis presenting with peripheral cytopenias in combination with a hyperplastic bone marrow. MDS patients have an increased risk of disease evolution to acute leukemia. Strong efforts have been made to gain further insights into the pathobiology of MDS. Development and progression of MDS to acute myeloid leukemia is suggested to be a multistep alteration to hematopoietic stem cells consisting of class I and class II alterations: the former targeting genes that are involved in signal transduction (e.g., FLT3, RAS and KIT), whereas the latter affect transcription factors (e.g., RUNX, RARA, EVI1 and WT1). These alterations consist of not only genomic mutations but also epigenetic aberrations, which can lead to reversible gene silencing. However, whether numerical and structural alterations of chromosomes and/or single genes or epigenetic changes represent the initiating event or, more likely, secondary events remains part of the discussion. Accumulation of such defects may finally cause the leukemic transformation of MDS.

MLL-AF9-induced leukemogenesis requires coexpression of the wild-type Mll allele

Oncogenic fusion proteins are capable of initiating tumorigenesis, but the role of their wild-type counterparts in this process is poorly understood. The mixed lineage leukemia (MLL) gene undergoes chromosomal translocations, resulting in the formation of oncogenic MLL fusion proteins (MLL-FPs). Here, we show that menin recruits both wild-type MLL and oncogenic MLL-AF9 fusion protein to the loci of HOX genes to activate their transcription. Wild-type MLL not only catalyzes histone methylation at key target genes but also controls distinct MLL-AF9-induced histone methylation. Notably, the wild-type Mll allele is required for MLL-AF9-induced leukemogenesis and maintenance of MLL-AF9-transformed cells. These findings suggest an essential cooperation between an oncogene and its wild-type counterpart in MLL-AF9-induced leukemogenesis.

Instructive role of the vascular niche in promoting tumour growth and tissue repair by angiocrine factors

The precise mechanisms whereby anti-angiogenesis therapy blocks tumour growth or causes vascular toxicity are unknown. We propose that endothelial cells establish a vascular niche that promotes tumour growth and tissue repair not only by delivering nutrients and O2 but also through an 'angiocrine' mechanism by producing stem and progenitor cell-active trophogens. Identification of endothelial-derived instructive angiocrine factors will allow direct tumour targeting, while diminishing the unwanted side effects associated with the use of anti-angiogenic agents.

Extrinsic signals determine myeloid-erythroid lineage switch in MN1 leukemia

OBJECTIVE

Transcriptional control of hematopoietic lineage fate relies on the integration of many intra- and extracellular signals. To test whether the microenvironment impacts on leukemic phenotype, we exploited the MN1 model of acute myeloid leukemia under defined genetically modified microenvironmental conditions.

MATERIALS AND METHODS

The requirement of both FLT3 and c-Kit signaling for MN1 leukemias was investigated using retroviral infection of bone marrow cells from wild-type, c-Kit-mutated (W41), and Flt3-ligand knockout cells, and bone marrow transplantation into wild-type, c-Kit-mutated, or Flt3-ligand knockout mice.

RESULTS

Genetic disruption of both FLT3 and c-Kit signaling in the MN1-leukemia model was dispensable for MN1-induced leukemogenesis. However, it induced a switch from myeloid to erythroid phenotype that was preserved, when FLT3 signaling was restored by secondary transplantation of leukemic cells into wild-type recipients.

CONCLUSIONS

Our findings underscore the importance of microenvironmental signals for lineage choice in leukemia and identify signals that are important in myeloid-erythroid lineage decisions.

Leukemia-initiating cells from some acute myeloid leukemia patients with mutated nucleophosmin reside in the CD34(-) fraction

Leukemia-initiating cells (LICs) in acute myeloid leukemia (AML) are believed to be restricted to the CD34(+) fraction. However, one of the most frequently mutated genes in AML is nucleophosmin (NPM), and this is associated with low CD34 expression. We, therefore, investigated whether NPM-mutated AMLs have LICs restricted to the CD34(+) fraction. We transplanted sorted fractions of primary NPM-mutated AML into immunodeficient mice to establish which fractions initiate leukemia. Approximately one-half of cases had LICs exclusively within the CD34(-) fraction, whereas the CD34(+) fraction contained normal multilineage hematopoietic repopulating cells. Most of the remaining cases had LICs in both CD34(+) and CD34(-) fractions. When samples were sorted based on CD34 and CD38 expression, multiple fractions initiated leukemia in primary and secondary recipients. The data indicate that the phenotype of LICs is more heterogeneous than previously realized and can vary even within a single sample. This feature of LICs may make them particularly difficult to eradicate using therapies targeted against surface antigens.

Oncogenic RAS enables DNA damage- and p53-dependent differentiation of acute myeloid leukemia cells in response to chemotherapy

Acute myeloid leukemia (AML) is a clonal disease originating from myeloid progenitor cells with a heterogeneous genetic background. High-dose cytarabine is used as the standard consolidation chemotherapy. Oncogenic RAS mutations are frequently observed in AML, and are associated with beneficial response to cytarabine. Why AML-patients with oncogenic RAS benefit most from high-dose cytarabine post-remission therapy is not well understood. Here we used bone marrow cells expressing a conditional MLL-ENL-ER oncogene to investigate the interaction of oncogenic RAS and chemotherapeutic agents. We show that oncogenic RAS synergizes with cytotoxic agents such as cytarabine in activation of DNA damage checkpoints, resulting in a p53-dependent genetic program that reduces clonogenicity and increases myeloid differentiation. Our data can explain the beneficial effects observed for AML patients with oncogenic RAS treated with higher dosages of cytarabine and suggest that induction of p53-dependent differentiation, e.g. by interfering with Mdm2-mediated degradation, may be a rational approach to increase cure rate in response to chemotherapy. The data also support the notion that the therapeutic success of cytotoxic drugs may depend on their ability to promote the differentiation of tumor-initiating cells.

Tumour-initiating cells: challenges and opportunities for anticancer drug discovery

The hypothesis that cancer is driven by tumour-initiating cells (popularly known as cancer stem cells) has recently attracted a great deal of attention, owing to the promise of a novel cellular target for the treatment of haematopoietic and solid malignancies. Furthermore, it seems that tumour-initiating cells might be resistant to many conventional cancer therapies, which might explain the limitations of these agents in curing human malignancies. Although much work is still needed to identify and characterize tumour-initiating cells, efforts are now being directed towards identifying therapeutic strategies that could target these cells. This Review considers recent advances in the cancer stem cell field, focusing on the challenges and opportunities for anticancer drug discovery.

Heterogeneity in cancer: cancer stem cells versus clonal evolution

The identification and characterization of cancer stem cells might lead to more effective treatments for some cancers by focusing therapy on the most malignant cells. To achieve this goal it will be necessary to determine which cancers follow a cancer stem cell model and which do not, to address technical issues related to tumorigenesis assays, and to test the extent to which cancer cell heterogeneity arises from genetic versus epigenetic differences.

Identification of tumor-initiating cells in a highly aggressive brain tumor using promoter activity of nucleostemin

Controversy remains over whether the cancer stem cell (CSC) theory applies to all tumors. To determine whether cells within a highly aggressive solid tumor are stochastically or hierarchically organized, we combined a reporter system where the nucleostemin (NS) promoter drives GFP expression (termed NS-GFP) with a mouse brain tumor model induced by retroviral Ras expression on a p16(Ink4a)/p19(Arf)-deficient background. The NS-GFP system allowed us to monitor the differentiation process of normal neural stem/precursor cells by analyzing GFP fluorescence intensity. In tumor-bearing mice, despite the very high frequency of tumorigenic cells, we successfully identified the NS-GFP(+) cells as tumor-initiating cells (T-ICs). The clonal studies conclusively established that phenotypical heterogeneity can exist among the cells comprising a genetically homogeneous tumor, suggesting that this aggressive brain tumor follows the CSC model. Detailed analyses of the NS-GFP(+) brain tumor cells revealed that T-ICs showed activation of the receptor tyrosine kinase c-Met, which functions in tumor invasiveness. Thus, the NS-GFP system provides a powerful tool to elucidate stem cell biology in normal and malignant tissues.

Cell-cell interaction networks regulate blood stem and progenitor cell fate

Communication networks between cells and tissues are necessary for homeostasis in multicellular organisms. Intercellular (between cell) communication networks are particularly relevant in stem cell biology, as stem cell fate decisions (self-renewal, proliferation, lineage specification) are tightly regulated based on physiological demand. We have developed a novel mathematical model of blood stem cell development incorporating cell-level kinetic parameters as functions of secreted molecule-mediated intercellular networks. By relation to quantitative cellular assays, our model is capable of predictively simulating many disparate features of both normal and malignant hematopoiesis, relating internal parameters and microenvironmental variables to measurable cell fate outcomes. Through integrated in silico and experimental analyses, we show that blood stem and progenitor cell fate is regulated by cell–cell feedback, and can be controlled non-cell autonomously by dynamically perturbing intercellular signalling. We extend this concept by demonstrating that variability in the secretion rates of the intercellular regulators is sufficient to explain heterogeneity in culture outputs, and that loss of responsiveness to cell–cell feedback signalling is both necessary and sufficient to induce leukemic transformation in silico.

Chelation of intracellular iron with the antifungal agent ciclopirox olamine induces cell death in leukemia and myeloma cells

Off-patent drugs with previously unrecognized anticancer activity could be rapidly repurposed for this new indication. To identify such compounds, we conducted 2 independent cell-based chemical screens and identified the antimicrobial ciclopirox olamine (CPX) in both screens. CPX decreased cell growth and viability of malignant leukemia, myeloma, and solid tumor cell lines as well as primary AML patient samples at low-micromolar concentrations that appear pharmacologically achievable. Furthermore, oral CPX decreased tumor weight and volume in 3 mouse models of leukemia by up to 65% compared with control without evidence of weight loss or gross organ toxicity. In addition, oral CPX prevented the engraftment of primary AML cells in nonobese diabetic/severe combined immunodeficiency mouse models, thereby establishing its ability to target leukemia stem cells. Mechanistically, CPX bound intracellular iron, and this intracellular iron chelation was functionally important for its cytotoxicity. By electron paramagnetic resonance, CPX inhibited the iron-dependent enzyme ribonucleotide reductase at concentrations associated with cell death. Thus, in summary, CPX has previously unrecognized anticancer activity at concentrations that are pharmacologically achievable. Therefore, CPX could be rapidly repurposed for the treatment of malignancies, including leukemia and myeloma.

Model systems for examining effects of leukemia-associated oncogenes in primary human CD34+ cells via retroviral transduction

The use of primary human cells to model cancer initiation and progression is now within the grasp of investigators. It has been nearly a decade since the first defined genetic elements were introduced into primary human epithelial and fibroblast cells to model oncogenesis. This approach has now been extended to the hematopoietic system, with the first described experimental transformation of primary human hematopoietic cells. Human cell model systems will lead to a better understanding of the species and cell type specific signals necessary for oncogenic initiation and progression, and will allow investigators to interrogate the cancer stem cell hypothesis using a well-defined hierarchical system that has been studied for decades. The molecular and biochemical link between self-renewal and differentiation can now be experimentally approached using primary human cells. In addition, the models that result from these experiments are likely to generate highly relevant systems for use in identification and validation of potential therapeutic targets as well as testing of small molecule therapeutics. We describe here the methodologies and reagents that are used to examine the effects of leukemia fusion protein expression on primary human hematopoietic cells, both in vitro and in vivo.

The leukemic stem cell niche: current concepts and therapeutic opportunities

The genetic events that contribute to the pathogenesis of acute myeloid leukemia are among the best characterized of all human malignancies. However, with notable exceptions such as acute promyelocytic leukemia, significant improvements in outcome based on these insights have not been forthcoming. Acute myeloid leukemia is a paradigm of cancer stem (or leukemia initiating) cells with hierarchy analogous to that seen in hematopoiesis. Normal hematopoiesis requires complex bidirectional interactions between the bone marrow microenvironment (or niche) and hematopoietic stem cells (HSCs). These interactions are critical for the maintenance of normal HSC quiescence and perturbations can influence HSC self-renewal. Leukemia stem cells (LSCs), which also possess limitless self-renewal, may hijack these homeostatic mechanisms, take refuge within the sanctuary of the niche during chemotherapy, and consequently contribute to eventual disease relapse. We will discuss the emerging evidence supporting the importance of the bone marrow microenvironment in LSC survival and consider the physiologic interactions of HSCs and the niche that inform our understanding of microenvironment support of LSCs. Finally, we will discuss approaches for the rational development of therapies that target the microenvironment.

The MLLgnant consequences of reverting to an embryonic transcriptional program

In this issue of Cell Stem Cell, Somervaille et al. (2009) have demonstrated convincingly that reversion to an embryonic transcriptional program through defective MLL gene expression contributes to the generation of myeloid leukemia stem cells.

Leukemia stem cells and human acute lymphoblastic leukemia

Leukemias and other cancers have been proposed to contain a subpopulation of cells that display characteristics of stem cells and maintain tumor growth. The fact that most anticancer therapy is directed against the bulk of the tumor, and possibly spares the cancer stem cells, may lie at the heart of treatment failures with conventional modalities. Leukemia stem cells are fairly well described for acute myeloid leukemia (AML), but their existence and relevance for acute lymphoblastic leukemia (ALL) is less clear. Several reports describe subpopulations with primitive phenotypes in clinical ALL samples. However, it has also been suggested that the majority of leukemic subfractions can propagate leukemia in the appropriate experimental setting, and that their hierarchical organization is less strict than in AML. In addition, it is uncertain whether cancer stem cells arise from malignant transformation of a tissue-specific stem cell, or from committed progenitors or differentiated cells that re-acquire a stem cell-like program. In common childhood ALL, current evidence points towards the cell of origin being a committed lymphoid progenitor. In this review, we highlight recent findings relating to the question of leukemia stem cells in ALL.

Aberrant chromatin at genes encoding stem cell regulators in human mixed-lineage leukemia

Mixed-lineage leukemia (MLL) fusion proteins are potent inducers of leukemia, but how these proteins generate aberrant gene expression programs is poorly understood. Here we show that the MLL-AF4 fusion protein occupies developmental regulatory genes important for hematopoietic stem cell identity and self-renewal in human leukemia cells. These MLL-AF4-bound regions have grossly altered chromatin structure, with histone modifications catalyzed by trithorax group proteins and DOT1 extending across large domains. Our results define direct targets of the MLL fusion protein, reveal the global role of epigenetic misregulation in leukemia, and identify new targets for therapeutic intervention in cancer.

Use of humanized severe combined immunodeficient mice for human vaccine development

The severe combined immunodeficient (SCID) mouse has no adaptive immunity, lacking mature T and B cells in the peripheral blood or the lymphoid organs. It has been used extensively in biomedical research as a valuable translational model for xeno-engraftment of human tissues and cells. This review focuses on the engraftment of human peripheral blood cells and tissues in SCID mice, as well as in the newly established and more permissive SCID mice deficient in the IL-2 receptor gamma-chain. Human immune responses could be elicited and assessed in these humanized SCID mice upon vaccination or sensitization with allogeneic tissues. A translational model is proposed to attain preclinical data for testing human vaccines.

Stem cell concepts renew cancer research

Although uncontrolled proliferation is a distinguishing property of a tumor as a whole, the individual cells that make up the tumor exhibit considerable variation in many properties, including morphology, proliferation kinetics, and the ability to initiate tumor growth in transplant assays. Understanding the molecular and cellular basis of this heterogeneity has important implications in the design of therapeutic strategies. The mechanistic basis of tumor heterogeneity has been uncertain; however, there is now strong evidence that cancer is a cellular hierarchy with cancer stem cells at the apex. This review provides a historical overview of the influence of hematology on the development of stem cell concepts and their linkage to cancer.

Is the focus moving toward a combination of targeted drugs?

The concept of combining targeted agents for the treatment of acute myeloid leukemia (AML) is a relatively new but potentially promising area of investigation. A number of targeted agents may have limited single-agent activity but could show significant promise when used in conjunction with other types of similar compounds. Combinations of targeted agents may effectively interrupt multiple pathways in either a linear or parallel fashion. There are currently numerous combination regimens under investigation at either the preclinical or clinical levels, including histone deacetylase (HDAC) and CDK inhibitors; HDAC and proteasome inhibitors; HDAC and NF-kappaB (IKKbeta) inhibitors; CHK1 and MEK1/2 inhibitors; and BCL-2 antagonists and CDK inhibitors. Although combinations of targeted agents will not displace conventional cytotoxic regimens in AML or related disorders in the foreseeable future, these combinations clearly warrant further attention.

Murine models of human acute myeloid leukemia

Primary human AML cells can be isolated and studied in vitro, but many experimental questions can only be addressed using in vivo models. In particular, tractable animal models are needed to test novel therapies. The genetic complexity of human AML poses significant challenges for the generation of reliable animal models. The hematopoietic systems of both zebrafish ( Danio rerio) and Drosophila have been well characterized ( reviewed in [5, 31]) . Both organisms are well suited to forward genetics mutagenesis screens. Although this approach has been useful for identification of mutants with hematopoietic phenotypes ( e.g., cloche), the impact on cancer biology and hematopoietic malignancies in particular has been limited. A zebrafish model of acute lymphoblastic leukemia has been generated [37] and Drosophila models have shed light on the biology of epithelial tumors ( reviewed in [60]). Nonetheless, in vivo modeling of human AML relies most heavily on mice. Most cellular, molecular, and developmental features of the hematopoietic system are well conserved across mammalian species. The availability of the human and mouse genome sequences and the capability of manipulating the mouse genome make mice the most valuable model organism for AML research. Mice have additional practical value because they have a short reproductive cycle and are relatively inexpensive to house.

Shwachman-Diamond syndrome: implications for understanding the molecular basis of leukaemia

Inherited bone marrow failure syndromes provide extremely useful genetic models for understanding leukaemogenesis because the initial genetic defect can be identified and the risk of leukaemia is very high. Shwachman-Diamond syndrome is one of the most common inherited bone marrow failure syndromes and an example of such a model. Here, I describe the malignant features of Shwachman-Diamond syndrome and discuss the potential molecular mechanisms that can lead to leukaemia.

Acute myeloid leukemia induced by MLL-ENL is cured by oncogene ablation despite acquisition of complex genetic abnormalities

Chromosomal translocations involving 11q23 are frequent in infant acute leukemia and give rise to the formation of MLL fusion genes. The mechanism of leukemic transformation by these fusions has been the subject of numerous investigations. However, the dependence of acute leukemia on MLL fusion activity in vivo and the efficacy of targeting this activity to eliminate disease have not been established. We have developed a model for conditional expression of MLL-ENL in hematopoietic progenitor cells, in which expression of the fusion oncogene is turned off by doxycycline. Conditionally immortalized myeloblast cells derived from these progenitors were found to induce leukemia in vivo. Leukemic cells isolated from primary recipient mice were shown to have acquired additional genetic abnormalities and, when transplanted into secondary recipients, induced leukemia with shortened latencies. However, the leukemic cells remained dependent on MLL-ENL expression in vitro and in vivo, and its ablation resulted in regression of established leukemias. This study demonstrates that even genetically complex leukemias can be reversed on inactivation of the initiating MLL fusion and has important implications for the design of novel leukemia therapies.

Transforming human blood stem and progenitor cells: a new way forward in leukemia modeling

MLL-AF9 (MA9) is a leukemia fusion gene formed upon translocation of the AF9 gene on chromosome 9 and the MLL gene on chromosome 11. MA9 is commonly found in acute myeloid leukemia (AML) and occasionally in acute lymphoid leukemia and is associated with intermediate to poor outcome. The specific signaling pathways downstream of MA9 are still poorly understood. We have recently described a model system whereby we expressed the MA9 fusion gene in human CD34(+) Umbilical Cord Blood (UCB) cells and showed that these cells transformed to acute myeloid or lymphoid leukemia when injected into immunodeficient mice. The Mixed Lineage Leukemia (MLL) oncogenes are unique in this model system in that they promote full transformation of primary human blood cells, while all other leukemia-associated oncogenes tested thus far have induced only partial phenotypes. Here we provide an update on the use of this system for modeling human leukemia and its potential application for therapeutic testing of novel compounds to treat the disease. We focus specifically on the Rho family of small guanosine triphosphatases (GTPases) as potential therapeutic targets, which we have implicated in the pathogenesis of AML associated with MA9 expression.

Reconstructing the disease model and epigenetic networks for MLL-AF4 leukemia

The lack of a proper animal model has impeded understanding of the molecular mechanism of leukemia associated with the MLL-AF4 fusion. In this issue of Cancer Cell, Krivtsov et al. report a much-improved murine Mll-AF4 model and propose a molecular link with H3K79 methylation mediated by the histone methyltransferase DOT1L.

Cancer stem cells in solid tumours: accumulating evidence and unresolved questions

Solid tumours are an enormous cancer burden and a major therapeutic challenge. The cancer stem cell (CSC) hypothesis provides an attractive cellular mechanism to account for the therapeutic refractoriness and dormant behaviour exhibited by many of these tumours. There is increasing evidence that diverse solid tumours are hierarchically organized and sustained by a distinct subpopulation of CSCs. Direct evidence for the CSC hypothesis has recently emerged from mouse models of epithelial tumorigenesis, although alternative models of heterogeneity also seem to apply. The clinical relevance of CSCs remains a fundamental issue but preliminary findings indicate that specific targeting may be possible.

Cancer stem cells in multiple myeloma

Several key observations providing evidence for the cancer stem cell hypothesis and insights into the unique biology of these cells have come from the study of multiple myeloma. These include evidence that cancer cells may be functionally heterogeneous in spite of their genetic homogeneity and that malignant progenitors share many biological features with normal adult stem cells including drug resistance and regulatory processes governing self-renewal. We review studies that have examined clonogenic cells in multiple myeloma, highlight controversies regarding the cell of origin in multiple myeloma, and discuss potential targeting strategies.

Acquisition of granule neuron precursor identity is a critical determinant of progenitor cell competence to form Shh-induced medulloblastoma

Whether the brain tumor medulloblastoma originates from stem cells or restricted progenitor cells is unclear. To investigate this, we activated oncogenic Hedgehog (Hh) signaling in multipotent and lineage-restricted central nervous system (CNS) progenitors. We observed that normal unipotent cerebellar granule neuron precursors (CGNPs) derive from hGFAP(+) and Olig2(+) rhombic lip progenitors. Hh activation in a spectrum of early- and late-stage CNS progenitors generated similar medulloblastomas, but not other brain cancers, indicating that acquisition of CGNP identity is essential for tumorigenesis. We show in human and mouse medulloblastoma that cells expressing the glia-associated markers Gfap and Olig2 are neoplastic and retain features of embryonic-type granule lineage progenitors. Thus, oncogenic Hh signaling promotes medulloblastoma from lineage-restricted granule cell progenitors.

New insights into cord blood stem cell transplantation

PURPOSE OF REVIEW

To review the available clinical and biological advances of umbilical cord blood allogeneic stem cell transplantation in pediatric and adult patients.

RECENT FINDINGS

Recent large international studies suggested that allogeneic umbilical cord blood transplantation may potentially emerge as the frontline stem cell source for pediatric patients with hematopoietic malignancies because of its ability to confer superior overall and relapse-free survival compared with matched marrow stem cells. In adults, umbilical cord blood transplantation, double umbilical cord blood units and nonmyeloablative engraftment strategies have attracted further attention in clinical practice with the advantages of possible stronger graft-versus-leukemia effect and expanding transplantation indications. Additional advances in the basic biology of umbilical cord blood also appear very promising in development of enhanced engraftment approaches for limiting hematopoietic stem cell numbers or expansion of repopulating cells.

SUMMARY

Umbilical cord blood is a valuable alternative source of hematopoietic stem cells for patients that require allogeneic transplantation in the absence of readily available human leukocyte antigen matched marrow or blood hematopoietic stem cells. The current advances in clinical and biological research will further expand its application in pediatric and adult hematopoietic stem cells transplantation for treating hematologic disorders.

Exploring the role of cancer stem cells in radioresistance

Radiobiological research over the past decades has provided evidence that cancer stem cell content and the intrinsic radiosensitivity of cancer stem cells varies between tumours, thereby affecting their radiocurability. Translation of this knowledge into predictive tests for the clinic has so far been hampered by the lack of methods to discriminate between stem cells and non-stem cells. New technologies allow isolation of cells expressing specific surface markers that are differentially expressed in tumour cell subpopulations that are enriched for cancer stem cells. Combining these techniques with functional radiobiological assays holds the potential to elucidate the role of cancer stem cells in radioresistance in individual tumours, and to use this knowledge for the development of predictive markers for optimization of radiotherapy.

Anti-CD38 antibody-mediated clearance of human repopulating cells masks the heterogeneity of leukemia-initiating cells

Immunodeficient mice are increasingly used to assay human hematopoietic repopulating cells as well as leukemia-initiating cells. One method commonly used to isolate these rare cells is to sort cells stained with fluorochrome-conjugated antibodies into fractions, then transplant the different fractions into immunodeficient mice to test their repopulating ability. The antibodies are generally treated as being neutral in terms of their effects on the experiment. Human repopulating cells are thought to express CD34 and lack CD38. Here we present evidence that anti-CD38 antibodies have a profound inhibitory effect on engraftment of cord blood and leukemia cells. We show that this effect is Fc-mediated and can be overcome by treating mice with immunosuppressive antibodies. When this inhibitory effect is prevented, we demonstrate that the CD34(+)CD38(+) fraction of certain acute myeloid leukemia samples contains all, or at least most, leukemia-initiating cell capacity. This study highlights the potential pitfall of antibody-mediated clearance of repopulating cells and is important for any groups working with this model. More importantly, the work suggests that there is greater variation in the phenotypes of leukemia-initiating cells than previously suggested.

MLL-AF9 leukemia stem cells: hardwired or taking cues from the microenvironment?

MLL rearrangements in humans lead to both acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL). While AML has been successfully produced in mice, modeling ALL has been more difficult. In this issue of Cancer Cell, Wei et al. (2008) describe generation of AML, ALL, and biphenotypic leukemia by manipulating the cytokine milieu of human progenitor cells expressing MLL-AF9. They demonstrate that both multipotent and lineage-restricted progenitors are targeted by MLL-AF9 fusion proteins and that Rac signaling is crucial for survival. This study demonstrates the heterogeneity of MLL-AF9 leukemic stem cells and the importance of the microenvironment in determining lineage outcome.

Microenvironment determines lineage fate in a human model of MLL-AF9 leukemia

Faithful modeling of mixed-lineage leukemia in murine cells has been difficult to achieve. We show that expression of MLL-AF9 in human CD34+ cells induces acute myeloid, lymphoid, or mixed-lineage leukemia in immunodeficient mice. Some leukemia stem cells (LSC) were multipotent and could be lineage directed by altering either the growth factors or the recipient strain of mouse, highlighting the importance of microenvironmental cues. Other LSC were strictly lineage committed, demonstrating the heterogeneity of the stem cell compartment in MLL disease. Targeting the Rac signaling pathway by pharmacologic or genetic means resulted in rapid and specific apoptosis of MLL-AF9 cells, suggesting that the Rac signaling pathway may be a valid therapeutic target in MLL-rearranged AML.

Occurrence of cancer at multiple sites: towards distinguishing multigenesis from metastasis

Background

Occurrence of tumors at multiple sites is a hallmark of malignant cancers and contributes to the high mortality of cancers. The formation of multi-site cancers (MSCs) has conventionally been regarded as a result of hematogenous metastasis. However, some MSCs may appear as unusual in the sense of vascular dissemination pattern and therefore be explained by alternative metastasis models or even by non-metastatic independent formation mechanisms.

Results

Through literature review and incorporation of recent advance in understanding aging and development, we identified two alternative mechanisms for the independent formation of MSCs: 1) formation of separate tumors from cancer-initiating cells (CICs) mutated at an early stage of development and then diverging as to their physical locations upon further development, 2) formation of separate tumors from different CICs that contain mutations in some convergent ways. Either of these processes does not require long-distance migration and/or vascular dissemination of cancer cells from a primary site to a secondary site. Thus, we classify the formation of these MSCs from indigenous CICs (iCICs) into a new mechanistic category of tumor formation – multigenesis.

Conclusion

A multigenesis view on multi-site cancer (MSCs) may offer explanations for some "unusual metastasis" and has important implications for designing expanded strategies for the diagnosis and treatment of cancers.

Reviewers

This article was reviewed by Carlo C. Maley nominated by Laura F. Landweber and Razvan T. Radulescu nominated by David R. Kaplan. For the full reviews, please go to the Reviewers' comments section.

Epidermal stem cells in skin homeostasis and cutaneous carcinomas

Skin and squamous epithelia regulate water and heat homeostasis and constitute our first barrier of protection against pathogens. Cells from the outermost layer of the skin, the cornified envelope (stratum corneum), are constantly being shed, imposing a constant demand for replenishment to maintain homeostasis. Hair follicles and sebaceous glands provide protective hair growth and skin sebum, and continuously undergo cycles of growth and regression. The outstanding ability of the epidermis, hair follicles and sebaceous glands to self-renew relies on a population of adult stem cells that are maintained throughout our life span. In this review we will provide an overview of our current knowledge about epidermal stem cells, and some of the molecular mechanisms that identify them and dictate their behaviour. We will also summarise our view on the possible link between adult epidermal stem cells and cancer stem cells within skin and squamous neoplasias. The potential of epidermal stem cells in regenerative medicine and for designing targeted antitumoral therapies will be discussed.

The treatment of adults with acute lymphoblastic leukemia

Despite the relatively low incidence of acute lymphoblastic leukemia (ALL) in adults, large national and international collaborations have recently improved our understanding of how to treat ALL in adults. This article documents and examines the current evidence base for a "state of the art" therapy in both Philadelphia chromosome-negative and -positive adult ALL. The article comments upon areas of therapeutic debate, such as the role of bone marrow transplantation. In particular, the controversial subject of whether the superior outcome seen in younger patients is predicated on disease biology or therapeutic strategy is examined closely. Promising approaches under development are also discussed.

Comment on "Tumor growth need not be driven by rare cancer stem cells"

Kelly et al. (Brevia, 20 July 2007, p. 337) questioned xenotransplant experiments supporting the cancer stem cell (CSC) hypothesis because they found a high frequency of leukemia-initiating cells (L-IC) in some transgenic mouse models. However, the CSC hypothesis depends on prospective purification of cells with tumor-initiating capacity, irrespective of frequency. Moreover, we found similar L-IC frequencies in genetically comparable leukemias using syngeneic or xenogeneic models.

Hematopoietic developmental pathways: on cellular basis

To elucidate the molecular mechanisms underlying normal and malignant hematopoietic development, it is critical to identify developmental intermediates for each lineage downstream of hematopoietic stem cells. Recent advances in prospective isolation of hematopoietic stem and progenitor cells, and efficient xenogeneic transplantation systems have provided a detailed developmental map in both mouse and human hematopoiesis, demonstrating that surface phenotypes of mouse stem-progenitor cells and their human counterparts are considerably different. Here, we summarize the phenotype and functional properties and their differences of hematopoietic stem and progenitor cell populations between mouse and human.

MLL translocations, histone modifications and leukaemia stem-cell development

Translocations that involve the mixed lineage leukaemia (MLL) gene identify a unique group of acute leukaemias, and often predict a poor prognosis. The MLL gene encodes a DNA-binding protein that methylates histone H3 lysine 4 (H3K4), and positively regulates gene expression including multiple Hox genes. Leukaemogenic MLL translocations encode MLL fusion proteins that have lost H3K4 methyltransferase activity. A key feature of MLL fusion proteins is their ability to efficiently transform haematopoietic cells into leukaemia stem cells. The link between a chromatin modulator and leukaemia stem cells provides support for epigenetic landscapes as an important part of leukaemia and normal stem-cell development.

Molecular heterogeneity of breast carcinomas and the cancer stem cell hypothesis

Human breast cancers are heterogeneous, both in their pathology and in their molecular profiles. This suggests the hypothesis that breast cancers can initiate in different cell types, either breast epithelial stem cells or their progeny (transit amplifying cells or committed differentiated cells). In this respect, breast cancer could be viewed as being similar to haematological malignancies for which an analogous model has been proposed. Drawing such parallels might help to unravel the molecular nature of the initiating events in breast cancer and might have substantial clinical implications.