Expression of BCR/ABL and BCL-2 in myeloid progenitors leads to myeloid leukemias

BH3 mimetics and TKI combined therapy for Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) was considered for a long time one of the most hostile leukemia that was incurable for most of the patients, predominantly due to the extreme resistance to chemotherapy. Part of the resistance to cell death (apoptosis) is the result of increased levels of anti-apoptotic and decreased levels of pro-apoptotic member of the BCL-2 family induced by the BCR-ABL1 oncoprotein. BCR-ABL1 is a constitutively active tyrosine kinase responsible for initiating multiple and oncogenic signaling pathways. With the development of specific BCR-ABL1 tyrosine kinase inhibitors (TKIs) CML became a much more tractable disease. Nevertheless, TKIs do not cure CML patients and a substantial number of them develop intolerance or become resistant to the treatment. Therefore, novel anti-cancer strategies must be developed to treat CML patients independently or in combination with TKIs. Here, we will discuss the mechanisms of BCR-ABL1-dependent and -independent resistance to TKIs and the use of BH3-mimetics as a potential tool to fight CML.

Lupane Triterpene Derivatives Improve Antiproliferative Effect on Leukemia Cells through Apoptosis Induction

Leukemia is one of the most frequent types of cancer. No effective treatment currently exists, driving a search for new compounds. Simple structural modifications were made to novel triterpenes isolated from Phoradendron wattii. Of the three resulting derivatives, 3α-methoxy-24-hydroxylup-20(29)-en-28-oic acid (T1m) caused a decrease in the median inhibitory concentration (IC₅₀) on the K562 cell line. Its mode of action was apparently apoptosis, ROS generation, and loss of mitochondrial membrane potential (MMP). Molecular docking analysis showed T1m to produce lower binding energies than its precursor for the Bcl-2 and EGFR proteins. Small, simple, and viable modifications to triterpenes can improve their activity against leukemia cell lines. T1m is a potentially promising element for future research. Clarifying the targets in its mode of action will improve its applicability.

Harnessing Macrophages through the Blockage of CD47: Implications for Acute Myeloid Leukemia

CD47 is a surface membrane protein expressed by all normal tissues. It is the so-called "don't eat me signal" because it protects the cells against phagocytosis. The CD47 interacts with the signal regulatory protein alpha (SIRPα) on the surface of macrophages, leading to downstream inhibitory signaling that dampens phagocytic capacity. Since macrophages exert immune surveillance against cancers, cancer cells overexpress CD47 to defend themselves against phagocytosis. Acute myeloid leukemia (AML) is a cancer of hematopoietic stem/progenitor cells (HSPC), and similar to other types of cancers, leukemic blasts show enhanced levels of CD47. In patients with AML, CD47 has been associated with a higher disease burden and poor overall survival. Blockage of CD47-SIRPα signaling leads to improved phagocytosis of AML cells and better overall survival in xenograft models. However, the introduction of a pro-phagocytic signal is needed to induce greater phagocytic capacity. These pro-phagocytic signals can be either Fc receptor stimulants (such as monoclonal antibodies) or natural pro-phagocytic molecules (such as calreticulin). Based on these pre-clinical findings, various clinical trials investigating the blockade of CD47-SIRPα interaction have been designed as monotherapy and in combination with other anti-leukemic agents. In this review, we will discuss CD47 biology, highlight its implications for AML pathophysiology, and explore the potential clinical translation of disrupting CD47-SIRPα to treat patients with AML.

Treatment-Free Remission in Chronic Myeloid Leukemia and New Approaches by Targeting Leukemia Stem Cells

The therapeutic landscape for chronic myeloid leukemia (CML) has improved significantly with the approval of tyrosine kinase inhibitors (TKIs) for therapeutic use. Most patients with optimal responses to TKIs can have a normal life expectancy. Treatment-free remission (TFR) after discontinuing TKI has increasingly become a new goal for CML treatment. However, TKI only "control" CML, and relapse after discontinuation has become a key factor hindering patient access to attempt TFR. In this study, we reviewed studies on TKI discontinuation, including both first and second-generation TKI. We also reviewed predictors of relapse, new monitoring methods, and strategies targeting leukemic stem cells.

Molecular, Cytogenetic, and Hematological Analysis of Chronic Myeloid Leukemia Patients and Discovery of Two Novel Translocations

Chronic myeloid leukemia (CML) is a disease of hematopoietic stem cells and is caused by the balanced translocations among the long arms of chromosomes 9 and 22, which are called the Philadelphia (Ph) chromosome. In this study, 131 CML patients were enrolled. Complete blood cell count was performed at the time of diagnosis for all the patients. Cytogenetic (karyotyping) examination using bone marrow samples was conducted on 76 CML patients for the confirmation of Ph-positive (9;22)(q34;q11) standard translocation, complex variant translocation, and additional chromosome abnormalities. FISH was performed on 38 patients for diagnostic purposes and on 39 patients for monitoring purposes. Twenty-two samples of CML patients were evaluated by reverse transcriptase PCR and real-time PCR for the patients who failed to respond against imatinib mesylate. In this study, 72 (54.96%) were males and 59 (45.03%) were females with a median age of 38.5 years. CBC values in the diagnosis process showed that 75 patients had high values of WBC being >100 × 103/μl, while 71 (58.01) patients exhibited reduced values of hemoglobin, i.e., <10.00 mg/dl, and high values of PLTs > 100 were observed in 40 (30.53%) patients. Cytogenetic results show that standard translocation was developed in 63 (82.89%), development of complex variant translocations in 4 (5.32%), additional chromosomal abnormalities (ACAs) in 3 (3.94%), and ACAs together with complex variant translocations in 1 (1.31%) patient. At the time of diagnosis, 61 (92.95%) patients were in the chronic phase, 4 (5.63%) were in the accelerated phase, and only 1 (1.40%) was in the blast crisis. Out of twenty-two patients, only 6 CML patients who were shifted from imatinib mesylate to nilotinib showed BCR-ABL-positive amplification. However, only 7 out of twenty-one patients exhibit BCR-ABL gene values ≥ 1 after three months of follow-up when analyzed by the quantitative real-time PCR. In conclusion, we found a novel five-way translocation 46XX,t(1;2;2;17;9;22)(p36.3,q21;q11.2,q21,q34,q11.2) and a novel four-way complex variant translocation 48XY,+8(8;17)(9;22),+der(22)(q11.2;q23)(q34;q11.2) in the accelerated phase.

The impact of fusion genes on cancer stem cells and drug resistance

With ever increasing evidences on the role of fusion genes as the oncogenic protagonists in myriad cancers, it's time to explore if fusion genes can be the next generational drug targets in meeting the current demands of higher drug efficacy. Eliminating cancer stem cells (CSC) has become the current focus; however, we have reached a standstill in drug development owing to the lack of effective strategies to eradicate CSC. We believe that fusion genes could be the novel targets to overcome this limitation. The intriguing feature of fusion genes is that it dominantly impacts every aspect of CSC including self-renewal, differentiation, lineage commitment, tumorigenicity and stemness. Given the clinical success of fusion gene-based drugs in hematological cancers, our attempt to target fusion genes in eradicating CSC can be rewarding. As fusion genes are expressed explicitly in cancer cells, eradicating CSC by targeting fusion genes provides yet an another advantage of negligible patient side effects since normal cells remain unaffected by the drug. We hereby delineate the latest evidences on how fusion genes regulate CSC and drug resistance.

The application of BH3 mimetics in myeloid leukemias

Execution of the intrinsic apoptotic pathway is controlled by the BCL-2 proteins at the level of the mitochondrial outer membrane (MOM). This family of proteins consists of prosurvival (e.g., BCL-2, MCL-1) and proapoptotic (e.g., BIM, BAD, HRK) members, the functional balance of which dictates the activation of BAX and BAK. Once activated, BAX/BAK form pores in the MOM, resulting in cytochrome c release from the mitochondrial intermembrane space, leading to apoptosome formation, caspase activation, and cleavage of intracellular targets. This pathway is induced by cellular stress including DNA damage, cytokine and growth factor withdrawal, and chemotherapy/drug treatment. A well-documented defense of leukemia cells is to shift the balance of the BCL-2 family in favor of the prosurvival proteins to protect against such intra- and extracellular stimuli. Small molecule inhibitors targeting the prosurvival proteins, named 'BH3 mimetics', have come to the fore in recent years to treat hematological malignancies, both as single agents and in combination with standard-of-care therapies. The most significant example of these is the BCL-2-specific inhibitor venetoclax, given in combination with standard-of-care therapies with great success in AML in clinical trials. As the number and variety of available BH3 mimetics increases, and investigations into applying these novel inhibitors to treat myeloid leukemias continue apace the need to evaluate where we currently stand in this rapidly expanding field is clear.

Increased level of long non coding RNA H19 is correlated with the downregulation of miR-326 and BCL-2 genes in pediatric acute lymphoblastic leukemia, a possible hallmark for leukemogenesis

Long non-coding RNAs (lncRNAs) and their role in competitive endogenous RNA (ceRNA) networks have emerged as fundamental debates in the biological processes of initiation and progression of cancer. This study aimed to identify and measure the expression levels of relevant ceRNA regulatory genes contributing to acute lymphoblastic leukemia (ALL). lncRNA H19 and BCL-2 mRNA were chosen based on in silico studies and their interactions with miR-326. Subsequently, the aforementioned coding/non-coding gene expression profiles were measured using qRT-PCR in 50 bone marrow samples, including 33 cases with pediatric ALL and 17 controls with no evidence of malignancy. lncRNA H19 was identified as an oncogenic factor which was noticeably increased in the newly diagnosed patients (P = 0.0019, AUC = 0.84) and negatively associated with miR-326 (r = -0.6, P = 0.02). Furthermore, a negative correlation was introduced between the transcriptional levels of miR-326 and the anti-apoptotic BCL-2 gene (r = -0.6, P = 0.028). The novel experimental and bioinformatic results achieved in this study may provide new insights into the molecular leukemogenesis of pediatric ALL.

Polycythemia vera: the current status of preclinical models and therapeutic targets

INTRODUCTION

Polycythemia vera (PV) is the most common myeloproliferative neoplasm (MPN). PV is characterized by erythrocytosis, leukocytosis, thrombocytosis, increased hematocrit, and hemoglobin in the peripheral blood. Splenomegaly and myelofibrosis often occur in PV patients. Almost all PV patients harbor a mutation in the JAK2 gene, mainly represented by the JAK2^V617F point mutation.

AREAS COVERED

This article examines the recent in vitro and in vivo available models of PV and moreover, it offers insights on emerging biomarkers and therapeutic targets. The evidence from mouse models, resembling a PV-like phenotype generated by different technical approaches, is discussed. The authors searched PubMed, books, and clinicaltrials.gov for original and review articles and drugs development status including the terms Myeloproliferative Neoplasms, Polycythemia Vera, erythrocytosis, hematocrit, splenomegaly, bone marrow fibrosis, JAK2^V617F, Hematopoietic Stem Cells, MPN cytoreductive therapy, JAK2 inhibitor, histone deacetylase inhibitor, PV-like phenotype, JAK2^V617F BMT, transgenic JAK2^V617F mouse, JAK2 physiologic promoter.

EXPERT OPINION

Preclinical models of PV are valuable tools for enabling an understanding of the pathophysiology and the molecular mechanisms of the disease. These models provide new biological insights on the contribution of concomitant mutations and the efficacy of novel drugs in a 'more faithful' setting. This may facilitate an enhanced understanding of pathogenetic mechanisms and targeted therapy.

Cancer stem cells and nanotechnological approaches for eradication

Cancer stem cells (CSCs) are currently known as the main cause of tumor recurrence. After chemotherapy is completed, CSCs proliferate and then differentiate to generate new tumor tissues. Similar to normal stem cells, this non-uniformly distributed cell population in the tumor tissue has self-renewal capacity and is responsible for survival of the tumor and difference in its genetic and metabolic characteristics. Followed by gene instability in CSCs, new phenotypic markers are aberrantly expressed in CSCs subpopulation. Hence, some of the surface markers and metabolic pathways that are upregulated in CSCs may be applied as specific targets for development of diagnostic and therapeutic approaches. In this review article, the distinctive properties of CSCs including signal pathways implicated in self-renewal and surface markers were discussed. Moreover, targeting CSCs based on their specific properties using nanodrugs was reviewed.

Targeting BCL2 in Chronic Lymphocytic Leukemia and Other Hematologic Malignancies

Apoptosis, the process of programmed cell death, occurs normally during development and aging. Members of the B-cell lymphoma 2 (BCL2) family of proteins are central regulators of apoptosis, and resistance to apoptosis is one of the hallmarks of cancer. Targeting the apoptotic pathway via BCL2 inhibitors has been considered a promising treatment strategy in the past decade. Initial efforts with small molecule BH3 mimetics such as ABT-737 and ABT-263 (navitoclax) pioneered the development of the first-in-class Food and Drug Administration (FDA)-approved oral BCL2 inhibitor, venetoclax. Venetoclax was approved for the treatment of chronic lymphocytic leukemia and acute myeloid leukemia, and is now being studied in a number of hematologic malignancies. Several other inhibitors targeting different BCL2 family members are now in early stages of development.

Differential gene expression changes and their implication on the disease progression in patients with Chronic Myeloid Leukemia

The molecular mechanisms responsible for disease progression of CML are not conclusive. The main functional changes associated with disease evolution in CML was high proliferation rate, decreased apoptosis, blockade of differentiation, and strong resistance to chemotherapeutic agents. The current study analyzed the relative expressional profiles of genes related with proliferation, apoptosis, differentiation, and resistance to chemotherapeutic agents such as c-MYC, BAD, BCL-2, C/EBPα/-β and ABCB1 respectively in different clinical stages of CML by SYBR Green I quantitative real-time (qRT) PCR. We selected a total of 183 CML patients and 30 healthy control samples. The study populations were classified into four groups, including de novo CML-CP (50/183), CML-AP (32/183), CML-BC (51/183) and Imatinib Mesylate or IM resistant CML-CP (50/183) groups. qRT PCR analysis revealed that significant overexpression of c-MYC, ABCB1 and BCL-2 was observed in advanced phases and IM resistant CP of CML compared to healthy controls. Likewise, the mean expression level of BAD, C/EBPα/-β genes were found to be significantly down regulated. Present study concluded that the complex interplay of several candidate genes like overexpression of c-MYC, ABCB1, BCL-2 and down regulation of BAD, C/EBPα/-β played a significant role in the disease evolution and development of drug resistant in CML.

Non ABL-directed inhibitors as alternative treatment strategies for chronic myeloid leukemia

The introduction of ABL Tyrosine Kinase Inhibitors (TKIs) has significantly improved the outcome of Chronic Myeloid Leukemia (CML) patients that, in large part, achieve satisfactory hematological, cytogenetic and molecular remissions. However, approximately 15-20% fail to obtain optimal responses according to the current European Leukemia Network recommendation because of drug intolerance or resistance.Moreover, a plethora of evidence suggests that Leukemic Stem Cells (LSCs) show BCR-ABL1-independent survival. Hence, they are unresponsive to TKIs, leading to disease relapse if pharmacological treatment is discontinued.All together, these biological events generate a subpopulation of CML patients in need of alternative therapeutic strategies to overcome TKI resistance or to eradicate LSCs in order to allow cure of the disease.In this review we update the role of "non ABL-directed inhibitors" targeting signaling pathways downstream of the BCR-ABL1 oncoprotein and describe immunological approaches activating specific T cell responses against CML cells.

Disrupting the CD47-SIRPα anti-phagocytic axis by a humanized anti-CD47 antibody is an efficacious treatment for malignant pediatric brain tumors

Morbidity and mortality associated with pediatric malignant primary brain tumors remain high in the absence of effective therapies. Macrophage-mediated phagocytosis of tumor cells via blockade of the anti-phagocytic CD47-SIRPα interaction using anti-CD47 antibodies has shown promise in preclinical xenografts of various human malignancies. We demonstrate the effect of a humanized anti-CD47 antibody, Hu5F9-G4, on five aggressive and etiologically distinct pediatric brain tumors: group 3 medulloblastoma (primary and metastatic), atypical teratoid rhabdoid tumor, primitive neuroectodermal tumor, pediatric glioblastoma, and diffuse intrinsic pontine glioma. Hu5F9-G4 demonstrated therapeutic efficacy in vitro and in vivo in patient-derived orthotopic xenograft models. Intraventricular administration of Hu5F9-G4 further enhanced its activity against disseminated medulloblastoma leptomeningeal disease. Notably, Hu5F9-G4 showed minimal activity against normal human neural cells in vitro and in vivo, a phenomenon reiterated in an immunocompetent allograft glioma model. Thus, Hu5F9-G4 is a potentially safe and effective therapeutic agent for managing multiple pediatric central nervous system malignancies.

How One Thing Led to Another

I started research in high school, experimenting on immunological tolerance to transplantation antigens. This led to studies of the thymus as the site of maturation of T cells, which led to the discovery, isolation, and clinical transplantation of purified hematopoietic stem cells (HSCs). The induction of immune tolerance with HSCs has led to isolation of other tissue-specific stem cells for regenerative medicine. Our studies of circulating competing germline stem cells in colonial protochordates led us to document competing HSCs. In human acute myelogenous leukemia we showed that all preleukemic mutations occur in HSCs, and determined their order; the final mutations occur in a multipotent progenitor derived from the preleukemic HSC clone. With these, we discovered that CD47 is an upregulated gene in all human cancers and is a "don't eat me" signal; blocking it with antibodies leads to cancer cell phagocytosis. CD47 is the first known gene common to all cancers and is a target for cancer immunotherapy.

An overview of chronic myeloid leukemia and its animal models

Chronic myeloid leukemia (CML) is a form of leukemia characterized by the presence of clonal bone marrow stem cells with the proliferation of mature granulocytes (neutrophils, eosinophils, and basophils) and their precursors. CML is a type of myeloproliferative disease associated with a characteristic chromosomal translocation called the Philadelphia (Ph) chromosome or t (9;22) translocation (BCR-ABL). CML is now usually treated with targeted drugs called tyrosine kinase inhibitors (TKIs). The mechanism and natural history of CML is still unclear. Here, we summarize the present CML animal disease models and compare them with each other. Meanwhile, we propose that it is a very wise choice to establish zebrafish (Danio rerio) CML model mimics clinical CML. This model could be used to learn more about the mechanism of CML, and to aid in the development of new drugs to treat CML.

BCL2 gene polymorphisms and splicing variants in chronic myeloid leukemia

Recent data suggest that constitutional genetic variation in the antiapoptotic BCL2 gene could be associated with the susceptibility to develop chronic myeloid leukemia (CML) and the clinical outcome in several hematological malignancies. The present study examines whether BCL2 single nucleotide polymorphisms (SNPs) predispose to CML or may potentially influence the disease characteristics at diagnosis. Notably, no association was observed between the four candidate BCL2 SNPs and the risk of developing CML. Instead, the 4777C>A (rs2279115) and the 5735A>G (rs1801018) SNPs were significantly associated with the disease risk profile as determined by the Sokal score. We found that such polymorphisms correlated with the expression of BCL2 alternative splicing transcripts (BCL2-α, BCL2-β) in healthy donors, but not in CML patients, although the relative levels of BCL2 mRNA splicing variants were shown to change during the clinical course of CML. Our findings suggest that BCL2 polymorphisms could influence the clinical features of CML patients at diagnosis. However, the pathogenic mechanisms involved in such association remain to be ascertained.

A personalised medicine approach for ponatinib-resistant chronic myeloid leukaemia

BACKGROUND

Chronic myeloid leukaemia (CML) is characterised by the presence of a fusion driver oncogene, BCR-ABL1, which is a constitutive tyrosine kinase. Tyrosine kinase inhibitors (TKIs) are the central treatment strategy for CML patients and have significantly improved survival rates, but the T315I mutation in the kinase domain of BCR-ABL1 confers resistance to all clinically approved TKIs, except ponatinib. However, compound mutations can mediate resistance even to ponatinib and remain a clinical challenge in CML therapy. Here, we investigated a ponatinib-resistant CML patient through whole-genome sequencing (WGS) to identify the cause of resistance and to find alternative therapeutic targets.

PATIENTS AND METHODS

We carried out WGS on a ponatinib-resistant CML patient and demonstrated an effective combination therapy against the primary CML cells derived from this patient in vitro.

RESULTS

Our findings demonstrate the emergence of compound mutations in the BCR-ABL1 kinase domain following ponatinib treatment, and chromosomal structural variation data predicted amplification of BCL2. The primary CD34(+) CML cells from this patient showed increased sensitivity to the combination of ponatinib and ABT-263, a BCL2 inhibitor with a negligible effect against the normal CD34(+) cells.

CONCLUSION

Our results show the potential of personalised medicine approaches in TKI-resistant CML patients and provide a strategy that could improve clinical outcomes for these patients.

Detection of BCR/ABL Fusion Gene by Hematological and Cytogenetical Analysis in Chronic Myeloid Leukemia Patients in Quetta, Pakistan

BACKGROUND

Chronic myeloid leukemia (CML) is a myeloproliferative disorder of pluripotent stem cells, caused by reciprocal translocation between the long arms of chromosomes 9 and 22, t(9;22)(q34;q11), known as the Philadelphia chromosome.

MATERIALS AND METHODS

A total of 51 CML patients were recruited in this study. Complete blood counts of all CML patients were performed to find out their total leukocytes, hemoglobin and platelets. FISH was performed for the detection of BCR-ABL fusion and cryptogenic tests using bone marrow samples were performed for the conformation of Ph (9;22)(q34;q11) and variant translocation mechanisms.

RESULTS

In cytogenetic analysis we observed that out of 51 CML patients 40 (88.9%) were Ph positive and 4 (8.88%) had Ph negative chromosomes. Mean values of WBC 134.5 103/μl, hemoglobin 10.44 mg/dl, and platelets 288.6 103/μl were observed in this study.

CONCLUSIONS

In this study, Ph positive translocation between chromosome (9:22)(q34;q11) were observed in 40 (88.9%) CML patients.

[Generation and identification of P210(T315I-BCR/ABL) transgenic mice]

OBJECTIVE

To construct the P210(T315I-BCR/ABL) transgenic mice model.

METHODS

The transgenic vector in which the P210(T315I-BCR/ABL) gene and eGFP gene was derived by APN/CD13 promoter was constructed and microinjected into the single-cell fertilized eggs of C57 mice. Transgene integration was conformed by PCR genotyping and P210(T315I-BCR/ABL) expression levels was evaluated by RT-PCR. The CML phenotype was confirmed by blood routine examination, Wright's staining for peripheral blood and bone marrow smears, HE staining for organs of transgenic mice.

RESULTS

Three transgenic mice lines with high expression of P210(T315I-BCR/ABL) gene and eGFP gene was selected. Compared with the wild type mice, the levels of WBC, platelet and neutrophil granulocyte of transgenic mice began to increase gradually at 2 months, and increase to 23.9×10⁹/L, 4 136×10⁹/L, and 74.6% respectively at 6 months. The remarkable hyperplasia of granulocytes was seen in the peripheral blood and bone marrow smears with splenomegaly infiltrated by leukemic cells.

CONCLUSION

The P210(T315I-BCR/ABL) transgenic mice was constructed and provided a model to explore the mechanism of T315I CML and screen out the drug for T315 CML patient.

Application of stem cells in targeted therapy of breast cancer: a systematic review

BACKGROUND

The aim of this systematic review was to investigate whether stem cells could be effectively applied in targeted therapy of breast cancer.

MATERIAL AND METHOD

A systematic literature search was performed for original articles published from January 2007 until May 2012.

RESULTS

Nine studies met the inclusion criteria for phase I or II clinical trials, of which three used stem cells as vehicles, two trials used autologous hematopoetic stem cells and in four trials cancer stem cells were targeted. Mesenchymal stem cells (MSCs) were applied as cellular vehicles to transfer therapeutic agents. Cell therapy with MSC can successfully target resistant cancers. Cancer stem cells were selectively targeted via a proteasome-dependent suicide gene leading to tumor regression. Wnt/β-catenin signaling pathway has been also evidenced to be an attractive CSC-target.

CONCLUSIONS

This systematic review focused on two different concepts of stem cells and breast cancer marking a turning point in the trials that applied stem cells as cellular vehicles for targeted delivery therapy as well as CSC-targeted therapies. Applying stem cells as targeted therapy could be an effective therapeutic approach for treatment of breast cancer in the clinic and in therapeutic marketing; however this needs to be confirmed with further clinical investigations.

Re-evaluating the role of BCR/ABL in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) requires the BCR/ABL tyrosine kinase for disease onset and maintenance. As a result, CML can be successfully treated with tyrosine kinase inhibitors (TKIs) such as imatinib. Most patients are maintained in a disease-suppressed state on daily TKI therapy for several years and in many cases this treatment prevents progression to the blast phase. If the TKI is discontinued, CML redevelops in 95% of patients as a result of persisting leukemia initiating cells (LICs). There are several hypotheses that describe the potential mechanism(s) responsible for LIC persistence in CML, but supporting evidence is limited. Furthermore, of the few patients who discontinue TKI therapy and are "cured" (i.e., in treatment-free remission), most have residual BCR/ABL-expressing cells in their hematopoietic tissues. There are also healthy individuals without a CML diagnosis who express the BCR/ABL mutation in a fraction of their hematopoietic cells. Finally, mice that express BCR/ABL from the Bcr locus as a knockin mutation do not develop CML. These mice have lower BCR/ABL levels than retroviral or transgenic models of BCR/ABL that do develop CML. Understanding why mice with BCR/ABL expressed from the Bcr locus and some people that express BCR/ABL are not afflicted with CML will provide insights into therapies to prevent or cure this disease.

Comparison of hematopoietic cancer stem cells with normal stem cells leads to discovery of novel differentially expressed SSRs

Tandem repeat expansion in the transcriptomics level has been considered as one of the underlying causes of different cancers. Cancer stem cells are a small portion of cancer cells within the main neoplasm and can remain alive during chemotherapy and re-induce tumor growth. The EST-SSR background of cancer stem cells and possible roles of expressed SSRs in altering normal stem cells to cancer ones have not been investigated yet. Here, SSR distributions in hematopoietic normal and cancer stem cells were compared based on the expressed EST-SSR. One hundred eighty nine and 223 EST-SSRs were identified in cancer and normal stem cells, respectively. The EST-SSR expression pattern was significantly different between normal and cancer stem cells. The frequencies of AC/GT and TA/TA EST-SSRs were about 10% higher in cancer than normal stem cells. Remarkably, the number of triplets in cancer stem cells was 1.5 times higher than that in normal stem cells. GAT EST-SSR was frequent in cancer stem cells, but, conversely, normal stem cells did not express GAT EST-SSR. We suggest this EST-SSR as a novel triplet in cancer stem cell induction. Translating EST-SSRs to amino acids demonstrated that Asp and Ile were more abundant in cancer stem cells compared to normal stem cells. Finally, Gene Ontology (GO) enrichment analysis was carried out on genes containing triplet SSRs and showed that SSRs intentionally visit some specific GO classes. Interestingly, a NF-kappa (nuclear factor-kB) binding transcription factor was significantly hit by SSR instability which is a hallmark for leukemia stem cells. NF-kappa is an over represented transcription factor during cancer progression. It seems that there is a crosstalk between the NF-kB transcription factor and expressed GAT tandem repeat which negatively regulate apoptosis. In addition to better understanding of tumorigenesis, the findings of this study offer new DNA markers for diagnostic purposes and identifying at risk populations. In addition, a new approach for gene discovery in cancer by target analysis of differentially expressed EST-SSRs between cancer and normal stem cells is presented here.

Glioblastoma cancer stem cells: Biomarker and therapeutic advances

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor in humans. It accounts for fifty-two percent of primary brain malignancies in the United States and twenty percent of all primary intracranial tumors. Despite the current standard therapies of maximal safe surgical resection followed by temozolomide and radiotherapy, the median patient survival is still less than 2 years due to inevitable tumor recurrence. Glioblastoma cancer stem cells (GSCs) are a subgroup of tumor cells that are radiation and chemotherapy resistant and likely contribute to rapid tumor recurrence. In order to gain a better understanding of the many GBM-associated mutations, analysis of the GBM cancer genome is on-going; however, innovative strategies to target GSCs and overcome tumor resistance are needed to improve patient survival. Cancer stem cell biology studies reveal basic understandings of GSC resistance patterns and therapeutic responses. Membrane proteomics using phage and yeast display libraries provides a method to identify novel antibodies and surface antigens to better recognize, isolate, and target GSCs. Altogether, basic GBM and GSC genetics and proteomics studies combined with strategies to discover GSC-targeting agents could lead to novel treatments that significantly improve patient survival and quality of life.

Transcriptional activation of hypoxia-inducible factor-1 (HIF-1) in myeloid cells promotes angiogenesis through VEGF and S100A8

Emerging evidence indicates that myeloid cells are essential for promoting new blood vessel formation by secreting various angiogenic factors. Given that hypoxia-inducible factor (HIF) is a critical regulator for angiogenesis, we questioned whether HIF in myeloid cells also plays a role in promoting angiogenesis. To address this question, we generated a unique strain of myeloid-specific knockout mice targeting HIF pathways using human S100A8 as a myeloid-specific promoter. We observed that mutant mice where HIF-1 is transcriptionally activated in myeloid cells (by deletion of the von Hippel-Lindau gene) resulted in erythema, enhanced neovascularization in matrigel plugs, and increased production of vascular endothelial growth factor (VEGF) in the bone marrow, all of which were completely abrogated by either genetic or pharmacological inactivation of HIF-1. We further found that monocytes were the major effector producing VEGF and S100A8 proteins driving neovascularization in matrigel. Moreover, by using a mouse model of hindlimb ischemia we observed significantly improved blood flow in mice intramuscularly injected with HIF-1-activated monocytes. This study therefore demonstrates that HIF-1 activation in myeloid cells promotes angiogenesis through VEGF and S100A8 and that this may become an attractive therapeutic strategy to treat diseases with vascular defects.

Toward a therapeutic reduction of imatinib refractory myeloproliferative neoplasm-initiating cells

Myeloproliferative neoplasms (MPNs) such as chronic myelogenous (CML) and chronic myelomonocytic leukemias (CMML) are frequently induced by tyrosine kinase oncogenes. Although these MPNs are sensitive to tyrosine kinase inhibitors such as imatinib, patients often relapse upon withdrawal of therapy. We used a model of MPN, which is induced by co-expression of the oncoproteins HIP1/PDGFβR (H/P) and AML1/ETO (A/E) from their endogenous loci, to examine the mechanisms of disease development and recurrence following imatinib withdrawal. Although the MPN displayed a full hematologic response to imatinib, 100% of the diseased mice relapsed upon drug withdrawal. MPN persistence was not due to imatinib resistance mutations in the H/P oncogene or massive gene expression changes. Within one week of imatinib treatment, more than 98% of gene expression changes induced by the oncogenes in isolated hematopoietic stem and progenitor cells (LSKs) normalized. Supplementation of imatinib with G-CSF or arsenic trioxide reduced MPN-initiating cell frequencies and the combination of imatinib with arsenic trioxide cured a large fraction of mice with MPNs. In contrast, no mice in the imatinib-treated control cohorts were cured. These data suggest that treatment with a combination of arsenic trioxide and imatinib can eliminate refractory MPN-initiating cells and reduce disease relapse.

NPMc+ cooperates with Flt3/ITD mutations to cause acute leukemia recapitulating human disease

Cytoplasmic nucleophosmin (NPMc(+)) mutations and FMS-like tyrosine kinase 3 (FLT3) internal tandem duplication (ITD) mutations are two of the most common known molecular alterations in acute myeloid leukemia (AML); they frequently occur together, suggesting cooperative leukemogenesis. To explore the specific relationship between NPMc+ and FLT3/ITD in vivo, we crossed Flt3/ITD knock-in mice with transgenic NPMc+ mice. Mice with both mutations develop a transplantable leukemia of either myeloid or lymphoid lineage, definitively demonstrating cooperation between Flt3/ITD and NPMc+. In mice with myeloid leukemia, functionally significant loss of heterozygosity of the wild-type Flt3 allele is common, similar to what is observed in human FLT3/ITD+ AML, providing further in vivo evidence of the importance of loss of wild-type FLT3 in leukemic initiation and progression. Additionally, in vitro clonogenic assays reveal that the combination of Flt3/ITD and NPMc+ mutations causes a profound monocytic expansion, in excess of that seen with either mutation alone consistent with the predominance of myelomonocytic phenotype in human FLT3/ITD+/NPMc+ AML. This in vivo model of Flt3/ITD+/NPMc+ leukemia closely recapitulates human disease and will therefore serve as a tool for the investigation of the biology of this common disease entity.

Expression of BCR/ABL p210 from a knockin allele enhances bone marrow engraftment without inducing neoplasia

Chronic myeloid leukemia (CML) and some acute lymphoblastic leukemias are characterized by the t(9;22) chromosome, which encodes the BCR/ABL oncogene. Multiple mouse models of CML express BCR/ABL at high levels from non-Bcr promoters, resulting in the development of leukemias. In contrast, a significant fraction of healthy humans have been found to have BCR/ABL-positive hematopoietic cells. To bridge the gap between the information derived from current mouse models and nonleukemic humans with the BCR/ABL oncogene, we generated a knockin model with BCR/ABL p210 expressed from the Bcr locus. Unlike previous models, expression of BCR/ABL from the knockin allele did not induce leukemia. BCR/ABL mutant cells did exhibit favorable bone marrow engraftment compared to control cells. These data suggest that BCR/ABL expression alone is insufficient to induce disease. This model allows for inducible spatial and temporal control of BCR/ABL expression for analysis of early steps in the pathogenesis of BCR/ABL-expressing leukemias.

Bcl-xL anti-apoptotic network is dispensable for development and maintenance of CML but is required for disease progression where it represents a new therapeutic target

The dismal outcome of blast crisis chronic myelogenous leukemia (CML-BC) patients underscores the need for a better understanding of the mechanisms responsible for the development of drug resistance. Altered expression of the anti-apoptoticBcl-xL has been correlated with BCR-ABL leukemogenesis; however, its involvement in the pathogenesis and evolution of CML has not been formally demonstrated yet. Thus, we generated an inducible mouse model in which simultaneous expression of p210-BCR-ABL1 and deletion of bcl-x occurs within hematopoietic stem and progenitor cells. Absence of Bcl-xL did not affect development of the chronic phase-like myeloproliferative disease, but none of the deficient mice progressed to an advanced phenotype, suggesting the importance of Bcl-xL in survival of progressing early progenitor cells. Indeed, pharmacological antagonism of Bcl-xL, with ABT-263, combined with PP242-induced activation of BAD markedly augmented apoptosis of CML-BC cell lines and primary CD34(+) progenitors but not those from healthy donors, regardless of drug resistance induced by bone marrow stromal cell-generated signals. Moreover, studies in which BAD or Bcl-xL expression was molecularly altered strongly support their involvement in ABT-263/PP242-induced apoptosis of CML-BC progenitors. Thus, suppression of the antiapoptotic potential of Bcl-xL together with BAD activation represents an effective pharmacological approach for patients undergoing blastic transformation.

An antiapoptotic Bcl-2 family protein index predicts the response of leukaemic cells to the pan-Bcl-2 inhibitor S1

BACKGROUND

Bcl-2-like members have been found to be inherently overexpressed in many types of haematologic malignancies. The small-molecule S1 is a BH3 mimetic and a triple inhibitor of Bcl-2, Mcl-1 and Bcl-XL.

METHODS

The lethal dose 50 (LD(50)) values of S1 in five leukaemic cell lines and 41 newly diagnosed leukaemia samples were tested. The levels of Bcl-2 family members and phosphorylated Bcl-2 were semiquantitatively measured by western blotting. The interactions between Bcl-2 family members were tested by co-immunoprecipitation. The correlation between the LD(50) and expression levels of Bcl-2 family members, alone or in combination, was analysed.

RESULTS

S1 exhibited variable sensitivity with LD(50) values ranging >2 logs in both established and primary leukaemic cells. The ratio of pBcl-2/(Bcl-2+Mcl-1) could predict the S1 response. Furthermore, we demonstrated that pBcl-2 antagonised S1 by sequestering the Bak and Bim proteins that were released from Mcl-1, andpBcl-2/Bak, pBcl-2/Bax and pBcl-2/Bim complexes cannot be disrupted by S1.

CONCLUSION

A predictive index was obtained for the novel BH3 mimetic S1. The shift of proapoptotic proteins from being complexed with Mcl-1 to being complexed with pBcl-2 was revealed for the first time, which is the mechanism underlying the index value described herein.

Cancer Stem Cells: A Moving Target

Even though the number of anti-cancer drugs entering clinical trials and approved by the FDA has increased in recent years, many cancer patients still experience poor survival outcome. The main explanation for such a dismal prognosis is that current therapies might leave behind a population of cancer cells with the capacity for long-term self-renewal, so-called cancer stem cells (CSCs), from which most tumors are believed to be derived and fueled. CSCs might favor local and distant recurrence even many years after initial treatment, thus representing a potential target for therapies aimed at improving clinical outcome. In this review, we will address the CSC hypothesis with a particular emphasis on its current paradigms and debates, and discuss several mechanisms of CSC resistance to conventional therapies.

A Pan-BCL2 inhibitor renders bone-marrow-resident human leukemia stem cells sensitive to tyrosine kinase inhibition

Leukemia stem cells (LSCs) play a pivotal role in the resistance of chronic myeloid leukemia (CML) to tyrosine kinase inhibitors (TKIs) and its progression to blast crisis (BC), in part, through the alternative splicing of self-renewal and survival genes. To elucidate splice-isoform regulators of human BC LSC maintenance, we performed whole-transcriptome RNA sequencing, splice-isoform-specific quantitative RT-PCR (qRT-PCR), nanoproteomics, stromal coculture, and BC LSC xenotransplantation analyses. Cumulatively, these studies show that the alternative splicing of multiple prosurvival BCL2 family genes promotes malignant transformation of myeloid progenitors into BC LSCS that are quiescent in the marrow niche and that contribute to therapeutic resistance. Notably, sabutoclax, a pan-BCL2 inhibitor, renders marrow-niche-resident BC LSCs sensitive to TKIs at doses that spare normal progenitors. These findings underscore the importance of alternative BCL2 family splice-isoform expression in BC LSC maintenance and suggest that the combinatorial inhibition of prosurvival BCL2 family proteins and BCR-ABL may eliminate dormant LSCs and obviate resistance.

Myeloproliferative neoplasm animal models

Myeloproliferative neoplasm (MPN) animal models accurately re-capitulate human disease in mice and have been an important tool for the study of MPN biology and therapy. Transplantation of BCR-ABL transduced bone marrow into irradiated syngeneic mice established the field of MPN animal modeling. Genetically engineered MPN animal models have enabled detailed characterization of the effects of specific MPN-associated genetic abnormalities on hematopoietic stem and progenitor cells (HSPCs). Xenograft models have allowed the study of primary human MPN-propagating cells in vivo. JAK2V617F, the most common molecular abnormality in BCR-ABL negative MPN, has been extensively studied using retroviral, transgenic, knock-in and xenograft models.

Inflammation linking EMT and cancer stem cells

Similar to actors changing costumes during a performance, cancer cells undergo many rapid changes during the process of tumor metastasis, including epithelial-mesenchymal transition (EMT), acquisition of cancer stem cells (CSCs) properties, and mesenchymal-epithelial transition (MET). Such changes allow the tumor to compete with the normal microenvironment to overcome anti-tumorigenic pressures. Then, once tissue homeostasis is lost, the altered microenvironment, like that accompanying inflammation, can itself become a potent tumor promoter. This review will discuss the changes that cancer cells undergo in converting from EMT to CSCs in an inflammation microenvironment, to understand the mechanisms behind invasion and metastasis and provide insights into prevention of metastasis.

Distinct roles for long-term hematopoietic stem cells and erythroid precursor cells in a murine model of Jak2V617F-mediated polycythemia vera

In the current model of the pathogenesis of polycythemia vera (PV), the JAK2V617F mutation arises in hematopoietic stem cells (HSCs) that maintain the disease, while erythroid precursor populations expand, resulting in excessive red blood cell production. We examined the role of these specific cell populations using a conditional Jak2V617F knockin murine model. We demonstrate that the most immature long-term (LT) HSCs are solely responsible for initiating and maintaining the disease in vivo and that Jak2V617F mutant LT-HSCs dominate hematopoiesis over time. When we induced Jak2V617F expression in erythropoietin receptor expressing precursor cells, the mice developed elevated hematocrit, expanded erythroid precursors, and suppressed erythropoietin levels. However, the disease phenotype was significantly attenuated compared with mice expressing Jak2V617F in LT-HSCs. In addition to developing a PV phenotype, all mice transplanted with Jak2V617F LT-HSCs underwent myelofibrotic transformation over time. These findings recapitulate the development of post-PV myelofibrosis in human myeloproliferative neoplasms. In aggregate, these results demonstrate the distinct roles of LT-HSCs and erythroid precursors in the pathogenesis of PV.

Murine retroviral bone marrow transplantation models for the study of human myeloproliferative disorders

Human myeloproliferative diseases are common hematologic disorders characterized by clonal overproduction of maturing myeloid or erythroid cells, often caused by expression of a mutant, dysregulated tyrosine kinase (TK). These diseases can be accurately modeled in laboratory mice by the retroviral transfer of a mutant TK gene into murine hematopoietic stem and progenitor cells, followed by transplantation of these cells into irradiated recipient mice. This yields a model system for analyzing the molecular pathophysiology of these conditions and provides a platform for testing therapies, particularly molecularly targeted new chemical entities (NCEs). The Basic Protocol in this unit describes the preparation of mouse bone marrow cells to express the relevant human oncogene before transplanting them into irradiated recipient mice. An alternate protocol describes a similar technique that allows specific induction of lymphoproliferative disease by some TKs. Support protocols for generating and titering retroviral stocks are also included.

Diverging fates of cells of origin in acute and chronic leukaemia

The large difference in phenotypes among tumour populations may stem from the stochastic origin of tumours from distinct cells – tumour cells are assumed to retain the phenotypes of the cells from which they derive. Yet, functional studies addressing the cellular origin of leukaemia are lacking. Here we show that the cells of origin of both, BCR/ABL-induced chronic myeloid (CML) and B-cell acute lymphoid leukaemia (B-ALL), resemble long-term haematopoietic stem cells (LT-HSCs). During disease-maintenance, CML LT-HSCs persist to function as cancer stem cells (CSCs) that maintain leukaemia and require signalling by the transcription factor STAT5. In contrast, B-ALL LT-HSCs differentiate into CSCs that correspond to pro-B cells. This transition step requires a transient IL-7 signal and is lost in IL-7Rα-deficient cells. Thus, in BCR/ABLp185⁺ B-ALL and BCR/ABLp210⁺ CML, the final phenotype of the tumour as well as the abundance of CSCs is dictated by diverging differentiation fates of their common cells of origin.

The Role of BCL2 Family of Apoptosis Regulator Proteins in Acute and Chronic Leukemias

The disturbance of apoptosis molecular signaling pathways is involved in carcinogenesis. BCL2 family of proteins is the hallmark of apoptosis regulation. In the last decade, new members of BCL2 gene family were discovered and cloned and were found to be differentially expressed in many types of cancer. BCL2 protein family, through its role in regulation of apoptotic pathways, is possibly related to cancer pathophysiology and resistance to conventional chemotherapy. It is well known that leukemias are haematopoietic malignancies characterized by biological diversity, varied cytogenetics, different immunophenotype profiles, and diverse outcome. Current research focuses on the prognostic impact and specific role of these proteins in the pathogenesis of leukemias. The understanding of the molecular pathways that participate in the biology of leukemias may lead to the design of new therapies which may improve patients' survival. In the present paper, we describe current knowledge on the role of BCL2 apoptosis regulator proteins in acute and chronic leukemias.

Small GTPase RAB45-mediated p38 activation in apoptosis of chronic myeloid leukemia progenitor cells

Chronic myelogenous leukemia (CML) is characterized by a reciprocal chromosomal translocation (9;22) that generates the Bcr-Abl fusion gene. BCR-ABL transforming activity is mediated by critical downstream signaling pathways that are aberrantly activated by tyrosine kinases. However, the mechanisms of BCR-ABL anti-apoptotic effects and the signaling pathways by which BCR-ABL influences apoptosis in BCR-ABL-expressing cells are poorly defined. In this study, we found that treatment with ABL kinase inhibitors or depletion of BCR-ABL induced the expression of RAB45 messenger RNA and protein and induced apoptosis via reduction of mitochondrial membrane potential and p38 activation in CML cell lines and BCR-ABL(+) progenitor cells from CML patients. Overexpressed RAB45 induced the activation of caspases-3 and -9 and reduced the expression of Survivin, XIAP, c-IAP1 and c-IAP2 in CML cells. Moreover, in colony-forming cells derived from CML-aldehyde dehydrogenase(hi)/CD34(+) cells, treatment with ABL kinase inhibitors induced RAB45 expression and reduced mitochondrial membrane potential, resulting in inhibited colony formation of Bcr-Abl(+) progenitor cells. The overexpression of RAB45 significantly decreased colony numbers and induced apoptosis through the activation of caspases-3 and -9. Furthermore, the overexpression of RAB45 increased the phosphorylation levels of p38, resulting in the induction of apoptosis and inhibition of proliferation of CML progenitor cells. Our results identify a new signaling molecule involved in BCR-ABL modulation of apoptosis and suggest that RAB45 induction strategies may have therapeutic utility in patients with CML.

Mouse models as tools to understand and study BCR-ABL1 diseases

Mouse models of human malignancy have greatly enhanced our understanding of disease pathophysiology and have led to novel therapeutic approaches, some with extraordinary success, one such example being inhibition of the BCR-ABL1 oncogene in chronic myeloid leukaemia (CML). Here, we review aspects of the biology of CML that have been uncovered at least in part through the generation and analysis of retroviral and transgenic mouse models of BCR-ABL1 disease. It can be expected that these models will also serve as important tools in the future, especially in the rational design of strategies to eradicate leukemic stem cells and potentially cure CML as well as other cancers.

Cancer stem cells and cancer therapy

Cancer stem cells (CSCs) are a subpopulation of tumour cells that possess the stem cell properties of self-renewal and differentiation. Stem cells might be the target cells responsible for malignant transformation, and tumour formation may be a disorder of stem cell self-renewal pathway. Epigenetic alterations and mutations of genes involved in signal transmissions may promote the formation of CSCs. These cells have been identified in many solid tumours including breast, brain, lung, prostate, testis, ovary, colon, skin, liver, and also in acute myeloid leukaemia. The CSC theory clarifies not only the issue of tumour initiation, development, metastasis and relapse, but also the ineffectiveness of conventional cancer therapies. Treatments directed against the bulk of the cancer cells may produce striking responses but they are unlikely to result in long-term remissions if the rare CSCs are not targeted. In this review, we consider the properties of CSCs and possible strategies for controlling the viability and tumourigenecity of these cells, including therapeutic models for selective elimination of CSCs and induction of their proper differentiation.

Cell Fusion, Drug Resistance and Recurrence CSCs

Cancer stem cells (CSCs) are a rare population of cancer cells exhibiting stem cell properties, such as self-renewal, differentiation and tissue restoration. Beside the initiation of the primary tumor, CSCs have also been associated with metastasis formation and cancer relapses. In the context of cancer relapses, we have recently postulated the existence of so-called recurrence CSCs (rCSCs). These specific CSC subtype will initiate relapses exhibiting an "oncogenic resistance" phenotype, which are characterized by a markedly increased malignancy concomitant with a drug resistance towards first line therapy. In the present chapter we will discuss the necessity of rCSCs as a distinct CSC subtype and that cell fusion could be one mechanism how rCSCs could originate.

Cancer stem cells in head and neck squamous cell carcinoma

Accumulating evidence suggests that self-renewal and differentiation capabilities reside only in a subpopulation of tumor cells, termed cancer stem cells (CSCs), whereas the remaining tumor cell population lacks the ability to initiate tumor development or support continued tumor growth. In head and neck squamous cell carcinoma (HNSCC), as with other malignancies, cancer stem cells have been increasingly shown to have an integral role in tumor initiation, disease progression, metastasis and treatment resistance. In this paper we summarize the current knowledge of the role of CSCs in HNSCC and discuss the therapeutic implications and future directions of this field.

The cancer stem cell paradigm: a new understanding of tumor development and treatment

IMPORTANCE OF THE FIELD

Cancer is the second leading cause of death in the United States, and therefore remains a central focus of modern medical research. Accumulating evidence supports a 'cancer stem cell' (CSC) model - where cancer growth and/or recurrence is driven by a small subset of tumor cells that exhibit properties similar to stem cells. This model may provide a conceptual framework for developing more effective cancer therapies that target cells propelling cancer growth.

AREAS COVERED IN THIS REVIEW

We review evidence supporting the CSC model and associated implications for understanding cancer biology and developing novel therapeutic strategies. Current controversies and unanswered questions of the CSC model are also discussed.

WHAT THE READER WILL GAIN

This review aims to describe how the CSC model is key to developing novel treatments and discusses associated shortcomings and unanswered questions.

TAKE HOME MESSAGE

A fresh look at cancer biology and treatment is needed for many incurable cancers to improve clinical prognosis for patients. The CSC model posits a hierarchy in cancer where only a subset of cells drive malignancy, and if features of this model are correct, has implications for development of novel and hopefully more successful approaches to cancer therapy.

Chronic myeloid leukemia: mechanisms of blastic transformation

The BCR-ABL1 oncoprotein transforms pluripotent HSCs and initiates chronic myeloid leukemia (CML). Patients with early phase (also known as chronic phase [CP]) disease usually respond to treatment with ABL tyrosine kinase inhibitors (TKIs), although some patients who respond initially later become resistant. In most patients, TKIs reduce the leukemia cell load substantially, but the cells from which the leukemia cells are derived during CP (so-called leukemia stem cells [LSCs]) are intrinsically insensitive to TKIs and survive long term. LSCs or their progeny can acquire additional genetic and/or epigenetic changes that cause the leukemia to transform from CP to a more advanced phase, which has been subclassified as either accelerated phase or blastic phase disease. The latter responds poorly to treatment and is usually fatal. Here, we discuss what is known about the molecular mechanisms leading to blastic transformation of CML and propose some novel therapeutic approaches.

Immature B-cell progenitors survive oncogenic stress and efficiently initiate Ph+ B-acute lymphoblastic leukemia

Philadelphia chromosome-positive (Ph(+)) B-acute lymphoblastic leukemia (B-ALL) can initiate in committed B-cell progenitors. However, the stages of B-cell differentiation in which disease can initiate and the efficiency with which this occurs are unclear. We now demonstrate that B-cell progenitors, up to and including the pro-B cell, efficiently initiate Ph(+) B-ALL. However, cells at the pre-B-cell stage of development did not initiate disease. We show that this difference in leukemia initiating potential is due to the level at which the Arf tumor suppressor gene is induced in specific stages of B lymphopoiesis. Whereas immature B-cell progenitors survive the relatively low levels of Arf that are induced after oncogene expression, pre-B cells express the tumor suppressor gene at high levels and undergo massive apoptosis. These data demonstrate that the molecular events that control Ph(+) B-ALL initiation and tumor suppression in the B-cell lineage are developmentally regulated.

Hes1 immortalizes committed progenitors and plays a role in blast crisis transition in chronic myelogenous leukemia

Hairy enhancer of split 1 (Hes1) is a basic helix-loop-helix transcriptional repressor that affects differentiation and often helps maintain cells in an immature state in various tissues. Here we show that retroviral expression of Hes1 immortalizes common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs) in the presence of interleukin-3, conferring permanent replating capability on these cells. Whereas these cells did not develop myeloproliferative neoplasms when intravenously administered to irradiated mice, the combination of Hes1 and BCR-ABL in CMPs and GMPs caused acute leukemia resembling blast crisis of chronic myelogenous leukemia (CML), resulting in rapid death of the recipient mice. On the other hand, BCR-ABL alone caused CML-like disease when expressed in c-Kit-positive, Sca-1-positive, and lineage-negative hematopoietic stem cells (KSLs), but not committed progenitors CMPs or GMPs, as previously reported. Leukemic cells derived from Hes1 and BCR-ABL-expressing CMPs and GMPs were more immature than those derived from BCR-ABL-expressing KSLs. Intriguingly, Hes1 was highly expressed in 8 of 20 patients with CML in blast crisis, but not in the chronic phase, and dominant negative Hes1 retarded the growth of some CML cell lines expressing Hes1. These results suggest that Hes1 is a key molecule in blast crisis transition in CML.

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.

FLT3-ITD up-regulates MCL-1 to promote survival of stem cells in acute myeloid leukemia via FLT3-ITD-specific STAT5 activation

Myeloid cell leukemia-1 (MCL-1) is an essential survival factor for hematopoiesis. In humans, hematopoietic stem cells (HSCs) express MCL-1 at the highest level in response to FMS-like tyrosine kinase-3 (FLT3) signaling. We here show that this FLT3-dependent stem cell maintenance system also plays a critical role in survival of leukemic stem cells (LSCs) in acute myeloid leukemia (AML). The CD34(+)CD38(-) LSC fraction expresses high levels of FLT3 as well as MCL-1, even compared with normal HSCs. Treatment with FLT3 ligand induced further MCL-1 up-regulation in LSCs in all AML cases tested. Interestingly, the group of samples expressing the highest levels of MCL-1 constituted AML with FLT3-internal tandem duplications (ITD). In FLT3-ITD AML cell lines, cells expressed a high level of MCL-1, and an inhibition of MCL-1 induced their apoptotic cell death. A tyrosine kinase inhibitor suppressed MCL-1 expression, and induced apoptosis that was reversed by the enforced MCL-1 expression. Finally, transduction of FLT3-ITD into HSCs strongly activated MCL-1 expression through its signal transducer and activator of transcription 5 (STAT5)-docking domains. This effect was completely abrogated when STAT5 activation was blocked. Thus, the acquisition of FLT3-ITD ensures LSC survival by up-regulating MCL-1 via constitutive STAT5 activation that is independent of wild-type FLT3 signaling.