A primitive hematopoietic cell is the target for the leukemic transformation in human Philadelphia-positive acute lymphoblastic leukemia

Disruption of lineage integrity as a precursor to breast tumor initiation

Increase in lineage infidelity and/or imbalance is frequently observed around the earliest stage of breast tumor initiation. In response to disruption of homeostasis, differentiated cells can partially lose their identity and gain cellular plasticity, a process involving epigenome landscape remodeling. This increase of cellular plasticity may promote the malignant transformation of breast tumors and fuel their heterogeneity. Here, we review recent studies that have yield insights into important regulators of lineage integrity and mechanisms that trigger mammary epithelial lineage derail, and evaluate their impacts on breast tumor development.

Vienna Cancer Stem Cell Club (VCSCC): 20 year jubilee and future perspectives

INTRODUCTION

The Vienna Cancer Stem Cell Club (VCSCC) was launched by a group of scientists in Vienna in 2002.

AREAS COVERED

Major aims of the VCSCC are to support research on cancer stem cells (CSC) in hematopoietic malignancies and to translate CSC-related markers and targets into clinical application. A primary focus of research in the VCSCC is the leukemic stem cell (LSC). Between 2013 and 2021, members of the VCSCC established a special research program on myeloproliferative neoplasms and since 2008, members of the VCSCC run the Ludwig Boltzmann Institute for Hematology and Oncology. In all these years, the VCSCC provided a robust intellectual platform for translational hematology and LSC research in Vienna. Furthermore, the VCSCC interacts with several national and international study groups and societies in the field. Representatives of the VCSCC also organized a number of international meetings and conferences on neoplastic stem cells, including LSC, in the past 15 years, and contributed to the definition and classification of CSC/LSC and related pre-malignant and malignant conditions.

EXPERT OPINION

The VCSCC will continue to advance the field and to develop LSC-detecting and LSC-eradicating concepts through which diagnosis, prognostication, and therapy of blood cancer patients should improve.

Phenotypic and genotypic infidelity in B-lineage neoplasms, including transdifferentiation following targeted therapy: Report from the 2021 SH/EAHP Workshop

OBJECTIVES

Session 2 of the 2021 Society for Hematopathology and European Association for Haematopathology Workshop collected examples of lineage infidelity and transdifferentiation in B-lineage neoplasms, including after targeted therapy.

METHODS

Twenty cases were submitted. Whole-exome sequencing and genome-wide RNA expression analysis were available on a limited subsample.

RESULTS

A diagnosis of B-cell acute lymphoblastic leukemia (B-ALL) was rendered on at least 1 biopsy from 13 patients. There was 1 case of acute myeloid leukemia (AML); the remaining 6 cases were mature B-cell neoplasms. Targeted therapy was administered in 7 cases of B-ALL and 4 cases of mature B-cell neoplasms. Six cases of B-ALL underwent lineage switch to AML or mixed-phenotype acute leukemia at relapse, 5 of which had rearranged KMT2A. Changes in maturational state without lineage switch were observed in 2 cases. Examples of de novo aberrant T-cell antigen expression (n = 2) were seen among the mature B-cell lymphoma cohort, and their presence correlated with alterations in tumor cell gene expression patterns.

CONCLUSIONS

This cohort of cases enabled us to illustrate, discuss, and review current concepts of lineage switch and aberrant antigen expression in a variety of B-cell neoplasms and draw attention to the role targeted therapies may have in predisposing neoplasms to transdifferentiation as well as other, less expected changes in maturational status.

Comparison of Predator-Prey Model and Hawk-Dove Game for Modelling Leukemia

Game theory is an excellent mathematical tool to describe the interaction between the immune system and cancerous leukocytes (c.leu). The feature of cancerous leukocytes to differentiate and mutate to give rise to leukemia is in the domain of ecological models as well. In this work, the dynamic of leukemia is described and compared by two models: firstly by a simple probabilistic mathematical model using the zero-sum two player game of Hawk and Dove, and secondly by Leslie Predator Prey model of ecology. The main goal of this study is to compare the results of both models and then discuss the treatment of leukemia i.e., Hematopoietic Stem cell transplant with the best model among them. Hawk and Dove model also describes the cell to cell interaction of cancerous leukocytes and healthy leukocytes (leu) after diagnoses and the condition of the patient before and after treatments. In this work, Hematopoietic Stem cell transplant is discussed by using concepts of a zero-sum three player game. Also, both models will be characterized by determining the stability properties, identifying basins of attraction, and locating the equilibrium points to see, at what extent the patient's survival is possible with leukemia in its body. Results for both models will be presented graphically.

A distinct subpopulation of leukemia initiating cells in acute precursor B lymphoblastic leukemia: quiescent phenotype and unique transcriptomic profile

In leukemia, a distinct subpopulation of cancer-initiating cells called leukemia stem cells (LSCs) is believed to drive population expansion and tumor growth. Failing to eliminate LSCs may result in disease relapse regardless of the amount of non-LSCs destroyed. The first step in targeting and eliminating LSCs is to identify and characterize them. Acute precursor B lymphoblastic leukemia (B-ALL) cells derived from patients were incubated with fluorescent glucose analog 2-(N-(7-Nitrobenz-2-oxa-1, 3-diazol-4-yl) Amino)-2-Deoxyglucose (NBDG) and sorted based on NBDG uptake. Cell subpopulations defined by glucose uptake were then serially transplanted into mice and evaluated for leukemia initiating capacity. Gene expression profiles of these cells were characterized using RNA-Sequencing (RNA-Seq). A distinct population of NBDG-low cells was identified in patient B-ALL samples. These cells are a small population (1.92% of the entire leukemia population), have lower HLA expression, and are smaller in size (4.0 to 7.0 μm) than the rest of the leukemia population. All mice transplanted with NBDG-low cells developed leukemia between 5 and 14 weeks, while those transplanted with NBDG-high cells did not develop leukemia (p ≤ 0.0001-0.002). Serial transplantation of the NBDG-low mouse model resulted in successful leukemia development. NBDG-medium (NBDG-med) populations also developed leukemia. Interestingly, comprehensive molecular characterization of NBDG-low and NBDG-med cells from patient-derived xenograft (PDX) models using RNA-Seq revealed a distinct profile of 2,162 differentially-expressed transcripts (DETs) (p<0.05) with 70.6% down-regulated in NBDG-low cells. Hierarchical clustering of DETs showed distinct segregation of NBDG-low from NBDG-med and NBDG-high groups with marked transcription expression alterations in the NBDG-low group consistent with cancer survival. In conclusion, A unique subpopulation of cells with low glucose uptake (NBDG-low) in B-ALL was discovered. These cells, despite their quiescence characteristics, once transplanted in mice, showed potent leukemia initiating capacity. Although NBDG-med cells also initiated leukemia, gene expression profiling revealed a distinct signature that clearly distinguishes NBDG-low cells from NBDG-med and the rest of the leukemia populations. These results suggest that NBDG-low cells may represent quiescent LSCs. These cells can be activated in the appropriate environment in vivo, showing leukemia initiating capacity. Our study provides insight into the biologic mechanisms of B-ALL initiation and survival.

Oncogene-Induced Reprogramming in Acute Lymphoblastic Leukemia: Towards Targeted Therapy of Leukemia-Initiating Cells

Simple Summary

Acute lymphoblastic leukemia is a heterogeneous disease characterized by a diversity of genetic alterations, following a sophisticated and controversial organization. In this review, we present and discuss the concepts exploring the cellular, molecular and functional heterogeneity of leukemic cells. We also review the emerging evidence indicating that cell plasticity and oncogene-induced reprogramming should be considered at the biological and clinical levels as critical mechanisms for identifying and targeting leukemia-initiating cells.

Abstract

Our understanding of the hierarchical structure of acute leukemia has yet to be fully translated into therapeutic approaches. Indeed, chemotherapy still has to take into account the possibility that leukemia-initiating cells may have a distinct chemosensitivity profile compared to the bulk of the tumor, and therefore are spared by the current treatment, causing the relapse of the disease. Therefore, the identification of the cell-of-origin of leukemia remains a longstanding question and an exciting challenge in cancer research of the last few decades. With a particular focus on acute lymphoblastic leukemia, we present in this review the previous and current concepts exploring the phenotypic, genetic and functional heterogeneity in patients. We also discuss the benefits of using engineered mouse models to explore the early steps of leukemia development and to identify the biological mechanisms driving the emergence of leukemia-initiating cells. Finally, we describe the major prospects for the discovery of new therapeutic strategies that specifically target their aberrant stem cell-like functions.

The evolution of leukaemia from pre-leukaemic and leukaemic stem cells

Haematopoietic stem and progenitor cells (HSPCs) are defined as unspecialized cells that give rise to more differentiated cells. In a similar way, leukaemic stem and progenitor cells (LSPCs) are defined as unspecialized leukaemic cells, which can give rise to more differentiated cells. Leukaemic cells carry leukaemic mutations/variants and have clear differentiation abnormalities. Pre-leukaemic HSPCs (PreL-HSPCs) carry pre-leukaemic mutations/variants (pLMs) and are capable of producing mature functional cells, which will carry the same variants. Under the roof of LSPCs, one can find a broad range of cell types genetic and disease phenotypes. Present-day knowledge suggests that this phenotypic heterogeneity is the result of interactions between the cell of origin, the genetic background and the microenvironment background. The combination of these attributes will define the LSPC phenotype, frequency, differentiation capacity and evolutionary trajectory. Importantly, as LSPCs are leukaemia-initiating cells that sustain clinical remission and are the source of relapse, an improved understanding of LSPCs phenotype would offer better clinical opportunities for the treatment and hopefully prevention of human leukaemia. The current review will focus on LSPCs attributes in the context of human haematologic malignancies.

Targeting Leukemia-Initiating Cells in Acute Lymphoblastic Leukemia

The concept that different leukemias are developmentally distinct and, like in normal hematopoiesis, generated by restricted populations of cells named leukemia-initiating cells (LIC), is becoming more established. These cancer stem-like cells have been assumed to have unique properties, including the capability of self-renewing and giving rise to "differentiated" or non-LICs that make up the whole tumor. Cell populations enriched with LIC activity have been characterized in different hematopoietic malignancies, including human acute lymphoblastic leukemia (ALL). Related studies have also demonstrated that LICs are functionally distinct from bulk cells and modulated by distinct molecular signaling pathways and epigenetic mechanisms. Here we review several biological and clinical aspects related to LICs in ALL, including (i) immunophenotypic characterization of LIC-enriched subsets in human and mouse models of ALL, (ii) emerging therapeutics against regulatory signaling pathways involved in LIC progression and maintenance in T- and B-cell leukemias, (iii) novel epigenetic and age-related mechanisms of LIC propagation, and (iv) ongoing efforts in immunotherapy to eradicate LIC-enriched cell subsets in relapsed and refractory ALL cases. Current conventional treatments do not efficiently eliminate LICs. Therefore, innovative therapeutics that exclusively target LICs hold great promise for developing an effective cure for ALL.

Leukemia Stem Cells: Concept and Implications

Only 10 years ago, the existence of cancer stem cells (CSCs) was still hotly debated. Even today, when their presence in most tumor types has been clearly demonstrated, all the consequences of their existence are far from being realized neither in the clinic nor, very often, in basic and translational cancer research. The existence of CSCs supposes a true change of paradigm in our understanding of cancer, but it will only have a real impact when we will properly assimilate its implications and apply these insights to both cancer research and cancer treatment. In this primer to the topic of leukemia stem cells (LSCs) our aim is to highlight with broad brushstrokes the most relevant of their properties, how these characteristics led to their identification, and the implications that the existence of LSCs has for the research and fight against leukemia.

The lysine methyltransferase SMYD2 is required for normal lymphocyte development and survival of hematopoietic leukemias

The 5 membered SET and MYND Domain-containing lysine methyltransferase (SMYD) family plays pivotal roles in development and proliferation. Initially characterized within the cardiovascular system, one such member, SMYD2, has been implicated as an oncogene in leukemias deriving from flawed hematopoietic stem cell (HSC) differentiation. We show here that conditional SMYD2 loss disrupts hematopoiesis at and downstream of the HSC via both apoptotic loss and transcriptional deregulation of HSC proliferation and disruption of Wnt-β-Catenin signaling. Yet previously documented SMYD2 cell cycle targets were unscathed. Turning our analysis to human leukemias, we observed that SMYD2 is highly expressed in CML, MLLr-B-ALL, AML, T-ALL and B-ALL leukemias and its levels in B-ALL correlate with poor survival. SMYD2 knockdown results in apoptotic death and loss of anchorage-independent transformation of each of these hematopoietic leukemias. These data provide an underlying mechanism by which SMYD2 acts during normal hematopoiesis and as a proto-oncogene in leukemia.

Leukemia stem cell immunophenotyping tool for diagnostic, prognosis, and therapeutics

The existence of cancer stem cells is debatable in numerous solid tumors, yet in leukemia, there is compelling evidence of this cell population. Leukemic stem cells (LSCs) are altered cells in which accumulating genetic and/or epigenetic alterations occur, resulting in the transition between the normal, preleukemic, and leukemic status. These cells do not follow the normal differentiation program; they are arrested in a primitive state but with high proliferation potential, generating undifferentiated blast accumulation and a lack of a mature cell population. The identification of LSCs might guide stem cell biology research and provide key points of distinction between these cells and their normal counterparts. The identification and characterization of the main features of LSCs can be useful as tools for diagnosis and treatment. In this context, the aim of the present review was to connect immunophenotype data in the main types of leukemia to further guide technical improvements.

Immune-Based Therapies in Acute Leukemia

Treatment resistance remains a leading cause of acute leukemia-related deaths. Thus, there is an unmet need to develop novel approaches to improve outcome. New immune-based therapies with chimeric antigen receptor (CAR) T cells, bi-specific T cell engagers (BiTEs), and immune checkpoint blockers (ICBs) have emerged as effective treatment options for chemoresistant B cell acute lymphoblastic leukemia (B-ALL) and acute myeloid leukemia (AML). However, many patients show resistance to these immune-based approaches. This review describes crucial lessons learned from immune-based approaches targeting high-risk B-ALL and AML, such as the leukemia-intrinsic (e.g., target antigen loss, tumor heterogeneity) and -extrinsic (e.g., immunosuppressive microenvironment) mechanisms that drive treatment resistance, and discusses alternative approaches to enhance the effectiveness of these immune-based treatment regimens.

The cell of origin and the leukemia stem cell in acute myeloid leukemia

There is experimental and observational evidence that the cells of the leukemic clone in acute myeloid leukemia (AML) have different phenotypes even though they share the same somatic mutations. The organization of the malignant clone in AML has many similarities to normal hematopoiesis, with leukemia stem cells (LSCs) that sustain leukemia and give rise to more differentiated cells. LSCs, similar to normal hematopoietic stem cells (HSCs), are those cells that are able to give rise to a new leukemic clone when transplanted into a recipient. The cell of origin of leukemia (COL) is defined as the normal cell that is able to transform into a leukemia cell. Current evidence suggests that the COL is distinct from the LSC. Here, we will review the current knowledge about LSCs and the COL in AML.

Leukemia propagating cells in Philadelphia chromosome-positive ALL: a resistant phenotype with an adverse prognosis

Background

Targeted therapy has revolutionized the management of Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL); however, relapse still occurs because of the presence of quiescent stem cells, termed leukemia propagating cells (LPCs). This study aimed to assess the phenotypic diversity of LPCs in adult patients with Ph+ B-Acute ALL (B-ALL) and to assess its prognostic impact.

Methods

Seventy adults with newly diagnosed Ph+ B-ALL were recruited at the Mansoura Oncology Center. Multiparameter flow cytometry studies of mononuclear blast cells for cluster of differentiation (CD)34, CD38, and CD58 were performed.

Results

Seventeen patients had blasts with the pattern of LPCs (CD34+CD38-CD58-), while 53 cases had other diverse phenotypic patterns. The rate of complete response was significantly lower in patients with the LPC phenotype (47% vs. 81%, P=0.006). The median time to achieve a complete response was prolonged in patients with the CD34+CD38-CD58- phenotype (48 vs. 32 days, P=0.016). The three-year overall survival was significantly lower in patients with the CD34+CD38-CD58- phenotype (37% vs. 55% respectively, P=0.028). Multivariate analysis showed that the CD34+CD38- CD58- phenotype was an independent risk factor for overall survival.

Conclusion

The presence of CD34+CD38-CD58- LPCs at diagnosis allows rapid identification of higher risk patients. Risk stratification of these patients is needed to further guide therapy and develop effective LPCs-targeted therapy to improve treatment outcome.

Phenotyping and Target Expression Profiling of CD34+/CD38- and CD34+/CD38+ Stem- and Progenitor cells in Acute Lymphoblastic Leukemia

Leukemic stem cells (LSCs) are an emerging target of curative anti-leukemia therapy. In acute lymphoblastic leukemia (ALL), LSCs frequently express CD34 and often lack CD38. However, little is known about markers and targets expressed in ALL LSCs. We have examined marker- and target expression profiles in CD34⁺/CD38⁻ LSCs in patients with Ph⁺ ALL (n = 22) and Ph⁻ ALL (n = 27) by multi-color flow cytometry and qPCR. ALL LSCs expressed CD19 (B4), CD44 (Pgp-1), CD123 (IL-3RA), and CD184 (CXCR4) in all patients tested. Moreover, in various subgroups of patients, LSCs also displayed CD20 (MS4A1) (10/41 = 24%), CD22 (12/20 = 60%), CD33 (Siglec-3) (20/48 = 42%), CD52 (CAMPATH-1) (17/40 = 43%), IL-1RAP (13/29 = 45%), and/or CD135 (FLT3) (4/20 = 20%). CD25 (IL-2RA) and CD26 (DPPIV) were expressed on LSCs in Ph⁺ ALL exhibiting BCR/ABL1_p210, whereas in Ph⁺ ALL with BCR/ABL1_p190, LSCs variably expressed CD25 but did not express CD26. In Ph⁻ ALL, CD34⁺/CD38⁻ LSCs expressed IL-1RAP in 6/18 patients (33%), but did not express CD25 or CD26. Normal stem cells stained negative for CD25, CD26 and IL-1RAP, and expressed only low amounts of CD52. In xenotransplantation experiments, CD34⁺/CD38⁻ and CD34⁺/CD38⁺ cells engrafted NSG mice after 12–20 weeks, and targeting with antibodies against CD33 and CD52 resulted in reduced engraftment. Together, LSCs in Ph⁺ and Ph⁻ ALL display unique marker- and target expression profiles. In Ph⁺ ALL with BCR/ABL1_p210, the LSC-phenotype closely resembles the marker-profile of CD34⁺/CD38⁻ LSCs in chronic myeloid leukemia, confirming the close biologic relationship of these neoplasms. Targeting of LSCs with specific antibodies or related immunotherapies may facilitate LSC eradication in ALL.

Pathogenesis of ETV6/RUNX1-positive childhood acute lymphoblastic leukemia and mechanisms underlying its relapse

ETV6/RUNX1 (E/R) is the most common fusion gene in childhood acute lymphoblastic leukemia (ALL). Multiple lines of evidence imply a “two-hit” model for the molecular pathogenesis of E/R-positive ALL, whereby E/R rearrangement is followed by a series of secondary mutations that trigger overt leukemia. The cellular framework in which E/R arises and the maintenance of a pre-leukemic condition by E/R are fundamental to the mechanism that underlies leukemogenesis. Accordingly, a variety of studies have focused on the relationship between the clones giving rise to the primary and recurrent E/R-positive ALL. We review here the most recent insights into the pathogenic mechanisms underlying E/R-positive ALL, as well as the molecular abnormalities prevailing at relapse.

Leukemia-propagating cells demonstrate distinctive gene expression profiles compared with other cell fractions from patients with de novo Philadelphia chromosome-positive ALL

Relapse remains one of the major obstacles in Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph⁺ALL) even after allogeneic hematopoietic stem cell transplantation. The persistence of leukemia-propagating cells (LPCs) may lead to the recurrence of Ph⁺ALL. Using a xenograft assay, LPCs enrichment in the CD34⁺CD38^-CD58^- fraction in Ph⁺ALL was recently identified. A further cohort study indicated that the LPCs phenotype at diagnosis was an independent risk factor for relapse of Ph⁺ALL. However, little is known about the potential molecular mechanism of LPCs-mediated relapse. Therefore, the gene expression profiles of the sorted LPCs and other cell fractions from patients with de novo Ph⁺ALL were investigated using RNA sequencing (RNA-Seq). Most of the differentially expressed genes between the LPCs and other cell fractions were related to the regulation of the cell cycle and metabolism, as identified by the gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. Consistent with the RNA-Seq results, the mRNA levels of cell cycle-related genes, such as cyclin-dependent kinase 4, were significantly lower in the LPCs fraction than in other cell fractions. Moreover, the proportion of quiescent cells in LPCs was significantly higher than in other cell fractions. In summary, distinctive gene expression profiles and clusters, which were mostly related to the regulation of the cell cycle and metabolism, were demonstrated between LPCs and other cell fractions from patients with de novo Ph⁺ALL. Therefore, it would be beneficial to develop novel LPCs-based therapeutic strategies for Ph⁺ALL patients.

Cellular Pliancy and the Multistep Process of Tumorigenesis

Completion of early stages of tumorigenesis relies on the dynamic interplay between the initiating oncogenic event and the cellular context. Here, we review recent findings indicating that each differentiation stage within a defined cellular lineage is associated with a unique susceptibility to malignant transformation when subjected to a specific oncogenic insult. This emerging notion, named cellular pliancy, provides a rationale for the short delay in the development of pediatric cancers of prenatal origin. It also highlights the critical role of cellular reprogramming in early steps of malignant transformation of adult differentiated cells and its impact on the natural history of tumorigenesis.

CD34 and CD38 are prognostic biomarkers for acute B lymphoblastic leukemia

CD34 and CD38 proteins are usually used as surface markers to identify HSCs and Leukemic stem cells. However, there have been cases that lacked CD34 or CD38 protein but still had leukemia initiating capacity in B-ALL suggesting the restrictive of these two markers. CD34 and CD38 expression were detected in most B-ALL and can serve as a specific biomarker for the prognosis of this subset of leukemia. Lack of CD34 or high CD38 expression is associated with favorable prognosis.

Modeling of Chronic Myeloid Leukemia: An Overview of In Vivo Murine and Human Xenograft Models

Over the past years, a wide variety of in vivo mouse models have been generated in order to unravel the molecular pathology of Chronic Myeloid Leukemia (CML) and to develop and improve therapeutic approaches. These models range from (conditional) transgenic models, knock-in models, and murine bone marrow retroviral transduction models followed by transplantation. With the advancement of immunodeficient xenograft models, it has become possible to use human stem/progenitor cells for in vivo studies as well as cells directly derived from CML patients. These models not only mimic CML but also have been instrumental in uncovering various fundamental mechanisms of CML disease progression and tyrosine kinase inhibitor (TKI) resistance. With the availability of iPSC technology, it has become feasible to derive, maintain, and expand CML subclones that are at least genetically identical to those in patients. The following review provides an overview of all murine as well as human xenograft models for CML established till date.

Could Vitamin D Analogues Be Used to Target Leukemia Stem Cells?

Leukemic stem cells (LSCs) are defined as cells that possess the ability to self-renew and give rise to the differentiated cancer cells that comprise the tumor. These LSCs seem to show chemo-resistance and radio-resistance leading to the failure of conventional cancer therapies. Current therapies are directed at the fast growing tumor mass leaving the LSC fraction untouched. Eliminating LSCs, the root of cancer origin and recurrence, is considered to be a hopeful approach to improve survival or even to cure cancer patients. In order to achieve this, the characterization of LSCs is a prerequisite in order to develop LSC-based therapies to eliminate them. Here we review if vitamin D analogues may allow an avenue to target the LSCs.

Genetically engineered mouse models of human B-cell precursor leukemias

B-cell precursor acute lymphoblastic leukemias (pB-ALLs) are the most frequent type of malignancies of the childhood, and also affect an important proportion of adult patients. In spite of their apparent homogeneity, pB-ALL comprises a group of diseases very different both clinically and pathologically, and with very diverse outcomes as a consequence of their biology, and underlying molecular alterations. Their understanding (as a prerequisite for their cure) will require a sustained multidisciplinary effort from professionals coming from many different fields. Among all the available tools for pB-ALL research, the use of animal models stands, as of today, as the most powerful approach, not only for the understanding of the origin and evolution of the disease, but also for the development of new therapies. In this review we go over the most relevant (historically, technically or biologically) genetically engineered mouse models (GEMMs) of human pB-ALLs that have been generated over the last 20 years. Our final aim is to outline the most relevant guidelines that should be followed to generate an “ideal” animal model that could become a standard for the study of human pB-ALL leukemia, and which could be shared among research groups and drug development companies in order to unify criteria for studies like drug testing, analysis of the influence of environmental risk factors, or studying the role of both low-penetrance mutations and cancer susceptibility alterations.

Stem Cell Hierarchy and Clonal Evolution in Acute Lymphoblastic Leukemia

Cancer is characterized by a remarkable intertumoral, intratumoral, and cellular heterogeneity that might be explained by the cancer stem cell (CSC) and/or the clonal evolution models. CSCs have the ability to generate all different cells of a tumor and to reinitiate the disease after remission. In the clonal evolution model, a consecutive accumulation of mutations starting in a single cell results in competitive growth of subclones with divergent fitness in either a linear or a branching succession. Acute lymphoblastic leukemia (ALL) is a highly malignant cancer of the lymphoid system in the bone marrow with a dismal prognosis after relapse. However, stabile phenotypes and functional data of CSCs in ALL, the so-called leukemia-initiating cells (LICs), are highly controversial and the question remains whether there is evidence for their existence. This review discusses the concepts of CSCs and clonal evolution in respect to LICs mainly in B-ALL and sheds light onto the technical controversies in LIC isolation and evaluation. These aspects are important for the development of strategies to eradicate cells with LIC capacity. Common properties of LICs within different subclones need to be defined for future ALL diagnostics, treatment, and disease monitoring to improve the patients' outcome in ALL.

Cell surface markers of cancer stem cells: diagnostic macromolecules and targets for drug delivery

The recognition that the persistence of cancer stem cells (CSCs) in patients following chemotherapy can result in disease relapse underscores the necessity to develop therapeutics against those cells. CSCs display a unique repertoire of cell surface macromolecules, which have proven essential for their characterization and isolation. Additionally, CSC-specific cell surface macromolecules or markers provide targets for the development of specific agents to destroy them. In this review, we compiled those cell surface molecules that have been validated as CSC markers for many common blood and solid tumors. We describe the unique chemical and structural features of the most common cell surface markers, as well as recent efforts to deliver chemotherapeutic agents into CSCs by targeting those macromolecules.

Xenograft models for normal and malignant stem cells

The model systems available for studying human hematopoiesis, malignant hematopoiesis, and hematopoietic stem cell (HSC) function in vivo have improved dramatically over the last decade, primarily due to improvements in xenograft mouse strains. Several recent reviews have focused on the historic development of immunodeficient mice over the last 2 decades, as well as their use in understanding human HSC and leukemia stem cell (LSC) biology and function in the context of a humanized mouse. However, in the intervening time since these reviews, a number of new mouse models, technical approaches, and scientific advances have been made. In this review, we update the reader on the newest and best models and approaches available for studying human malignant and normal HSCs in immunodeficient mice, including newly developed mice for use in chemotherapy testing and improved techniques for humanizing mice without laborious purification of HSC. We also review some relevant scientific findings from xenograft studies and highlight the continued limitations that confront researchers working with human HSC and LSC in vivo.

Cancer stem cells in basic science and in translational oncology: can we translate into clinical application?

Since their description and identification in leukemias and solid tumors, cancer stem cells (CSC) have been the subject of intensive research in translational oncology. Indeed, recent advances have led to the identification of CSC markers, CSC targets, and the preclinical and clinical evaluation of the CSC-eradicating (curative) potential of various drugs. However, although diverse CSC markers and targets have been identified, several questions remain, such as the origin and evolution of CSC, mechanisms underlying resistance of CSC against various targeted drugs, and the biochemical basis and function of stroma cell-CSC interactions in the so-called ‘stem cell niche.’ Additional aspects that have to be taken into account when considering CSC elimination as primary treatment-goal are the genomic plasticity and extensive subclone formation of CSC. Notably, various cell fractions with different combinations of molecular aberrations and varying proliferative potential may display CSC function in a given neoplasm, and the related molecular complexity of the genome in CSC subsets is considered to contribute essentially to disease evolution and acquired drug resistance. In the current article, we discuss new developments in the field of CSC research and whether these new concepts can be exploited in clinical practice in the future.

Lymphoid progenitor cells from childhood acute lymphoblastic leukemia are functionally deficient and express high levels of the transcriptional repressor Gfi-1

Acute lymphoblastic leukemia (ALL) is the most frequent malignancy of childhood. Substantial progress on understanding the cell hierarchy within ALL bone marrow (BM) has been recorded in the last few years, suggesting that both primitive cell fractions and committed lymphoid blasts with immature stem cell-like properties contain leukemia-initiating cells. Nevertheless, the biology of the early progenitors that initiate the lymphoid program remains elusive. The aim of the present study was to investigate the ability of lymphoid progenitors from B-cell precursor ALL BM to proliferate and undergo multilineage differentiation. By phenotype analyses, in vitro proliferation assays, and controlled culture systems, the lymphoid differentiation potentials were evaluated in BM primitive populations from B-cell precursor ALL pediatric patients. When compared to their normal counterparts, functional stem and progenitor cell contents were substantially reduced in ALL BM. Moreover, neither B nor NK or dendritic lymphoid-cell populations developed recurrently from highly purified ALL-lymphoid progenitors, and their proliferation and cell cycle status revealed limited proliferative capacity. Interestingly, a number of quiescence-associated transcription factors were elevated, including the transcriptional repressor Gfi-1, which was highly expressed in primitive CD34⁺ cells. Together, our findings reveal major functional defects in the primitive hematopoietic component of ALL BM. A possible contribution of high levels of Gfi-1 expression in the regulation of the stem/progenitor cell biology is suggested.

Pharmacogenomic considerations of xenograft mouse models of acute leukemia

The use of combination chemotherapy to cure acute lymphoblastic leukemia in children and acute myeloid leukemia in adults emerged for acute myeloid leukemia in the 1960s and for acute lymphoblastic leukemia in the 1980s as a paradigm for curing any disseminated cancer. This article summarizes recent developments and considerations in the use of acute leukemia xenografts established in immunodeficient mice to elucidate the genetic and genomic basis of acute leukemia pathogenesis and treatment response.

Cancer Stem Cells: A Review of Potential Clinical Applications

Context.—Cancer stem cells (CSCs) comprise a minor cell population in a tumor; however, they possess self-renewal capacity and are responsible for tumor recurrence and the emerging issue of tumor resistance. Despite recent advances in the study of pathogenesis and mechanisms of CSC-mediated disease recurrence and multidrug resistance, many questions remain unanswered.

Objectives.—To provide an overview of CSC theory and to describe major methods of CSC detection and isolation, with the emphasis on those techniques that are potentially relevant in clinical laboratory practice. Particular attention is given to CSC markers, such as cancer testis antigens, which could become promising targets in the development of immunotherapy in settings of minimal residual disease.

Data Sources.—The review is based on analysis of peer-reviewed literature cited in PubMed, as well as preliminary results of studies conducted in our laboratory.

Conclusions.—Despite a lack of consensus in the scientific community on research methodology, CSCs have demonstrated significant potential as therapeutic targets in the treatment of cancer. Further research of CSC biology and markers will eventually lead to the development of novel therapeutic approaches for targeting these cells to treat resistant and recurrent tumors and minimal residual disease.

Expression of CD133 in acute leukemia

There have been conflicting results regarding a correlation between CD133 expression and disease outcome. To assess CD133 expression in patients with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) and to evaluate its correlation with the different clinical and laboratory data as well as its relation to disease outcome, the present study included 60 newly diagnosed acute leukemic patients; 30 ALL patients with a male to female ratio of 1.5:1 and their ages ranged from 9 months to 48 years, and 30 AML patients with a male to female ratio of 1:1 and their ages ranged from 17 to 66 years. Flow cytometric assessment of CD133 expression was performed on blast cells. In ALL, no correlations were elicited between CD133 expression and some monoclonal antibodies, but in AML group, there was a significant positive correlation between CD133 and HLA-DR, CD3, CD7 and TDT, CD13 and CD34. In ALL group, patients with negative CD133 expression achieved complete remission more than patients with positive CD133 expression. In AML group, there was no statistically significant association found between positive CD133 expression and treatment outcome. The Kaplan-Meier curve illustrated a high significant negative correlation between CD133 expression and the overall survival of the AML patients. CD133 expression is an independent prognostic factor in acute leukemia, especially ALL patients and its expression could characterize a group of acute leukemic patients with higher resistance to standard chemotherapy and relapse. CD133 expression was highly associated with poor prognosis in acute leukemic patients.

Prospective cohort study of the circadian rhythm pattern in allogeneic sibling donors undergoing standard granulocyte colony-stimulating factor mobilization

Introduction

Prior in vivo murine studies suggest circadian oscillations for hematopoietic stem cell release, which are maintained following administration of granulocyte colony-stimulating factor (G-CSF) or plerixafor. Furthermore, retrospective data analysis of healthy donors who underwent G-CSF-induced mobilization demonstrated significantly increased CD34⁺ cell yields when collected in the afternoon compared with the morning.

Methods

A prospective study was conducted to directly examine the number of peripheral blood CD34⁺ and CD34⁺CD38^– progenitor/stem cells at baseline and then every 6 hours for 24 hours on days 4 to 5 of G-CSF (10 μg/kg/day in the morning) mobilization in 11 allogeneic donors. Data were analyzed using mixed-model analysis of repeated measures.

Results

Whereas we observed a significant increase in CD34⁺ cell counts toward the evening, counts were then sustained on the morning of day 5. The correlation between CD34⁺CD38^– cell counts and the less defined CD34⁺ populations was weak.

Conclusions

Our results suggest that the pharmacodynamic activity and timing of G-CSF may alter endogenous progenitor rhythms. Donor age, medical history, and medications may also impact circadian rhythm. Further studies should examine the circadian rhythm at the peak of G-CSF mobilization and should consider potential confounders such as the time of G-CSF administration and the age of the subjects.

Parthenolide eliminates leukemia-initiating cell populations and improves survival in xenografts of childhood acute lymphoblastic leukemia

Approximately 20% of children with acute lymphoblastic leukemia (ALL) relapse because of failure to eradicate the disease. Current drug efficacy studies focus on reducing leukemia cell burden. However, if drugs have limited effects on leukemia-initiating cells (LICs), then these cells may expand and eventually cause relapse. Parthenolide (PTL) has been shown to cause apoptosis of LIC in acute myeloid leukemia. In the present study, we assessed the effects of PTL on LIC populations in childhood ALL. Apoptosis assays demonstrated that PTL was effective against bulk B- and T-ALL cells, whereas the CD34(+)/CD19(-), CD34(+)/CD7(-), and CD34(-) subpopulations were more resistant. However, functional analyses revealed that PTL treatment prevented engraftment of multiple LIC populations in NOD/LtSz-scid IL-2Rγ(c)-null mice. PTL treatment of mice with established leukemias from low- and high-risk patients resulted in survival and restoration of normal murine hemopoiesis. In only 3 cases, disease progression was significantly slowed in mice engrafted with CD34(+)/CD19(-) or CD34(+)/CD7(-) and CD34(-) cells, but was not prevented, demonstrating that individual LIC populations within patients have different responses to therapy. These observations indicate that PTL may have therapeutic potential in childhood ALL and provide a basis for developing effective therapies that eradicate all LIC populations to prevent disease progression and reduce relapse.

Complexity of cancer stem cells

Heterogeneity of tumor tissue has been accounted for in recent years by a hierarchy-based model in which cancer stem cells (CSCs) have the ability both to self-renew and to give rise to differentiated tumor cells and are responsible for the overall organization of a tumor. Research into CSCs has progressed rapidly and concomitantly with recent advances in the biology of normal tissue stem cells, resulting in the identification of CSCs in a wide range of human tumors. Studies of mouse models of human cancer have provided further insight into the characteristics of CSCs as well as a basis for the development of novel therapies targeted to these cells. However, recent studies have revealed complexities, such as plasticity of stem cell properties and clonal diversity of CSCs, in certain tumor types that have led to revision of the original CSC model. In this review, we summarize the history of the discovery and characterization of CSCs, as well as address recent advances that have revealed the complexity of these cells and their therapeutic implications.

Acute B lymphoblastic leukaemia-propagating cells are present at high frequency in diverse lymphoblast populations

Leukaemia-propagating cells are more frequent in high-risk acute B lymphoblastic leukaemia than in many malignancies that follow a hierarchical cancer stem cell model. It is unclear whether this characteristic can be more universally applied to patients from non-'high-risk' sub-groups and across a broad range of cellular immunophenotypes. Here, we demonstrate in a wide range of primary patient samples and patient samples previously passaged through mice that leukaemia-propagating cells are found in all populations defined by high or low expression of the lymphoid differentiation markers CD10, CD20 or CD34. The frequency of leukaemia-propagating cells and their engraftment kinetics do not differ between these populations. Transcriptomic analysis of CD34(high) and CD34(low) blasts establishes their difference and their similarity to comparable normal progenitors at different stages of B-cell development. However, consistent with the functional similarity of these populations, expression signatures characteristic of leukaemia propagating cells in acute myeloid leukaemia fail to distinguish between the different populations. Together, these findings suggest that there is no stem cell hierarchy in acute B lymphoblastic leukaemia.

Contribution of GATA1 dysfunction to multi-step leukemogenesis

In mammals, hematopoietic homeostasis is maintained by a fine-tuned balance among the self-renewal, proliferation, differentiation and survival of hematopoietic stem cells and their progenies. Each process is also supported by the delicate balance of the expression of multiple genes specific to each process. GATA1 is a transcription factor that comprehensively regulates the genes that are important for the development of erythroid and megakaryocytic cells. Accumulating evidence supports the notion that defects in GATA1 function are intimately linked to hematopoietic disorders. In particular, the somatic mutation of the GATA1 gene, which leads to the production of N-terminally truncated GATA1, contributes to the genesis of transient myeloproliferative disorder and acute megakaryoblastic leukemia in infants with Down syndrome. Similarly, a mutation in the GATA1 regulatory region that reduces GATA1 expression is involved in the onset of erythroid leukemia in mice. In both cases, the accumulation of immature progenitor cells caused by GATA1 dysregulation underlies the pathogenesis of the leukemia. This review provides a summary of multi-step leukemogenesis with a focus on GATA1 dysfunction.

Oridonin in combination with imatinib exerts synergetic anti-leukemia effect in Ph+ acute lymphoblastic leukemia cells in vitro by inhibiting activation of LYN/mTOR signaling pathway

Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL) is triggered by constitutively activated BCR-ABL and SRC family tyrosine kinases.They account for the activations of multiple growth-signaling pathways, including Raf/MEK/ERK, Akt/mTOR and STAT5 pathways. The BCR-ABL tyrosine kinase inhibitor imatinib is the standard treatment for Ph+ leukemia and plays efficacious role in CML. However, imatinib has few inhibitory effects on SRC tyrosine kinase with response rate of Ph+ ALL lower and relapse more frequent and quicker compared with CML. Previous studies showed that oridonin inhibits proliferation and induces apoptosis in many tumor cells. However, the anticancer activity and mechanism of oridonin in Ph+ ALL is unknown. To investigate the anticancer activity of oridonin, we examined its role in constitutively activated Akt/mTOR, Raf/MEK/ERK, STAT5 and SRC pathway, mRNA level of bcr/abl gene, cell viability and apoptosis in Ph+ ALL SUP-B15 cells. Furthermore, we detected synergetic effect of oridonin plus imatinib. Our results showed that oridonin inhibiting activations of LYN (one of SRC family kinases) and ABL and their downstream Akt/mTOR, Raf/MEK/ERK and STAT5 pathways, downregulated Bcl-2 but upregulated Bax protein and then induced apoptosis in Ph+ ALL cells. Oridonin plus imatinib exerted synergetic effects by overcoming imatinib defect of upregulating Akt/mTOR and LYN signaling. Additionally, we examined the effect of oridonin on the signaling pathways in the primary specimens from Ph+ ALL patients. Our data showed that oridonin remarkably suppressed activations of Akt/mTOR, Raf/MEK and STAT5 pathway in these primary specimens and oridonin with imatinib exerted synergetic suppressive effects on mTOR, STAT5 and LYN signaling in one imatinib resistant patient specimen. Additional evaluation of oridonin as a potential therapeutic agent for Ph+ ALL seems warranted.

Philadelphia chromosome-positive leukemia stem cells in acute lymphoblastic leukemia and tyrosine kinase inhibitor therapy

Leukemia stem cells (LSCs), which constitute a minority of the tumor bulk, are functionally defined on the basis of their ability to transfer leukemia into an immunodeficient recipient animal. The presence of LSCs has been demonstrated in acute lymphoblastic leukemia (ALL), of which ALL with Philadelphia chromosome-positive (Ph(+)). The use of imatinib, a tyrosine kinase inhibitor (TKI), as part of front-line treatment and in combination with cytotoxic agents, has greatly improved the proportions of complete response and molecular remission and the overall outcome in adults with newly diagnosed Ph(+) ALL. New challenges have emerged with respect to induction of resistance to imatinib via Abelson tyrosine kinase mutations. An important recent addition to the arsenal against Ph(+) leukemias in general was the development of novel TKIs, such as nilotinib and dasatinib. However, in vitro experiments have suggested that TKIs have an antiproliferative but not an antiapoptotic or cytotoxic effect on the most primitive ALL stem cells. None of the TKIs in clinical use target the LSC. Second generation TKI dasatinib has been shown to have a more profound effect on the stem cell compartment but the drug was still unable to kill the most primitive LSCs. Allogeneic stem cell transplantation (SCT) remains the only curative treatment available for these patients. Several mechanisms were proposed to explain the resistance of LSCs to TKIs in addition to mutations. Hence, TKIs may be used as a bridge to SCT rather than monotherapy or combination with standard chemotherapy. Better understanding the biology of Ph(+) ALL will open new avenues for effective management. In this review, we highlight recent findings relating to the question of LSCs in Ph(+) ALL.

Tumor stem cells: A new approach for tumor therapy (Review)

Recent studies have demonstrated the existence of a minority of tumor cells possessing the stem cell properties of self-renewal and differentiation in leukemia and several solid tumors. However, these cells do not possess the normal regulatory mechanisms of stem cells. Following transplantation, they are capable of initiating tumorigenesis and are therefore known as 'tumor stem cells'. Cellular origin analysis of tumor stem cells has resulted in three hypotheses: Embryonal rest hypothesis, anaplasia and maturation arrest. Several signaling pathways which are involved in carcinogenesis, including Wnt/β-catenin, Notch and Oct-4 signaling pathways are crucial in normal stem cell self-renewal decisions, suggesting that breakdown in the regulation of self-renewal may be a key event in the development of tumors. Thus, tumors can be regarded as an abnormal organ in which stem cells have escaped from the normal constraints on self-renewal, thus, leading to abnormally differentiated tumor cells that lose the ability to form tumors. This new model for maligancies has significance for clinical research and treatment.

Early lymphoid development and microenvironmental cues in B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B-ALL) is a hematological disorder characterized by malignant and uncontrolled proliferation of B-lymphoid precursor cells in bone marrow. Over the last few years remarkable advances have been made in identifying genetic aberrations, patterns of abnormal transcriptional activity controlling early fate decisions and environmental cues that may influence leukemic development. In this review we focus on the structure of the early lymphoid system and the current knowledge about cell composition and function of the hematopoietic microenvironment that might control progenitor cell activity and lead to differentiation, proliferation and survival of developing B leukemic precursors. Learning the biology of special leukemic niches is central to understanding the pathogenesis of B-ALL and for the development of novel therapies.

5T4 oncofetal antigen is expressed in high risk of relapse childhood pre-B acute lymphoblastic leukemia and is associated with a more invasive and chemotactic phenotype

Although the overall prognosis in childhood acute lymphoblastic leukemia (ALL) is good, outcome after relapse is poor. Recurrence is frequently characterized by the occurrence of disease at extramedullary sites, such as the central nervous system and testes. Subpopulations of blasts able to migrate to such areas may have a survival advantage and give rise to disease recurrence. Gene expression profiling of 85 diagnostic pre-B-ALL bone marrow samples revealed higher 5T4 oncofetal antigen transcript levels in cytogenetic high-risk subgroups of patients (P<0.001). Flow cytometric analysis determined that bone marrow from relapse patients have a significantly higher percentage of 5T4-positive leukemic blasts than healthy donors (P=0.005). The high-risk Sup-B15 pre-B-ALL line showed heterogeneity in 5T4 expression, and the derived, 5T4(+) (Sup5T4) and 5T4(-) (Sup) subline cells, displayed differential spread to the omentum and ovaries following intraperitoneal inoculation of immunocompromised mice. Consistent with this, Sup5T4 compared with Sup cells show increased invasion in vitro concordant with increased LFA-1 and VLA-4 integrin expression, adhesion to extracellular matrix and secretion of matrix metalloproteases (MMP-2/-9). We also show that 5T4-positive Sup-B15 cells are susceptible to 5T4-specific superantigen antibody-dependent cellular toxicity providing support for targeted immunotherapy in high-risk pre-B-ALL.