c-Met inhibition in a HOXA9/Meis1 model of CN-AML

Real-Time Characterization of Clonal Fate Decisions in Complex Leukemia Samples by Fluorescent Genetic Barcoding

Clonal heterogeneity in acute myeloid leukemia (AML) forms the basis for treatment failure and relapse. Attempts to decipher clonal evolution and clonal competition primarily depend on deep sequencing approaches. However, this prevents the experimental confirmation of the identified disease-relevant traits on the same cell material. Here, we describe the development and application of a complex fluorescent genetic barcoding (cFGB) lentiviral vector system for the labeling and subsequent multiplex tracking of up to 48 viable AML clones by flow cytometry. This approach allowed the visualization of longitudinal changes in the in vitro growth behavior of multiplexed color-coded AML clones for up to 137 days. Functional studies of flow cytometry-enriched clones documented their stably inherited increase in competitiveness, despite the absence of growth-promoting mutations in exome sequencing data. Transplantation of aliquots of a color-coded AML cell mix into mice revealed the initial engraftment of similar clones and their subsequent differential distribution in the animals over time. Targeted RNA-sequencing of paired pre-malignant and de novo expanded clones linked gene sets associated with Myc-targets, embryonic stem cells, and RAS signaling to the foundation of clonal expansion. These results demonstrate the potency of cFGB-mediated clonal tracking for the deconvolution of verifiable driver-mechanisms underlying clonal selection in leukemia.

Targeting Tyrosine Kinases in Acute Myeloid Leukemia: Why, Who and How?

Acute myeloid leukemia (AML) is a myeloid malignancy carrying a heterogeneous molecular panel of mutations participating in the blockade of differentiation and the increased proliferation of myeloid hematopoietic stem and progenitor cells. The historical "3 + 7" treatment (cytarabine and daunorubicin) is currently challenged by new therapeutic strategies, including drugs depending on the molecular landscape of AML. This panel of mutations makes it possible to combine some of these new treatments with conventional chemotherapy. For example, the FLT3 receptor is overexpressed or mutated in 80% or 30% of AML, respectively. Such anomalies have led to the development of targeted therapies using tyrosine kinase inhibitors (TKIs). In this review, we document the history of TKI targeting, FLT3 and several other tyrosine kinases involved in dysregulated signaling pathways.

Lentiviral Fluorescent Genetic Barcoding for Multiplex Fate Tracking of Leukemic Cells

Tracking the behavior of leukemic samples both in vitro and in vivo plays an increasingly large role in efforts to better understand the leukemogenic processes and the effects of potential new therapies. Such work can be accelerated and made more efficient by methodologies enabling the characterization of leukemia samples in multiplex assays. We recently developed three sets of lentiviral fluorescent genetic barcoding (FGB) vectors that create 26, 14, and 6 unique immunophenotyping-compatible color codes from GFP-, yellow fluorescent protein (YFP)-, and monomeric kusabira orange 2 (mKO2)-derived fluorescent proteins. These vectors allow for labeling and tracking of individual color-coded cell populations in mixed samples by real-time flow cytometry. Using the prototypical Hoxa9/Meis1 murine model of acute myeloid leukemia, we describe the application of the 6xFGB vector system for assessing leukemic cell characteristics in multiplex assays. By transplanting color-coded cell mixes, we investigated the competitive growth behavior of individual color-coded populations, determined leukemia-initiating cell frequencies, and assessed the dose-dependent potential of cells exposed to the histone deacetylase inhibitor Entinostat for bone marrow homing. Thus, FGB provides a useful tool for the multiplex characterization of leukemia samples in a wide variety of applications with a concomitant reduction in workload, processing times, and mouse utilization.

PBX3 and MEIS1 Cooperate in Hematopoietic Cells to Drive Acute Myeloid Leukemias Characterized by a Core Transcriptome of the MLL-Rearranged Disease

Overexpression of HOXA/MEIS1/PBX3 homeobox genes is the hallmark of mixed lineage leukemia (MLL)-rearranged acute myeloid leukemia (AML). HOXA9 and MEIS1 are considered to be the most critical targets of MLL fusions and their coexpression rapidly induces AML. MEIS1 and PBX3 are not individually able to transform cells and were therefore hypothesized to function as cofactors of HOXA9. However, in this study, we demonstrate that coexpression of PBX3 and MEIS1 (PBX3/MEIS1), without ectopic expression of a HOX gene, is sufficient for transformation of normal mouse hematopoietic stem/progenitor cells in vitro. Moreover, PBX3/MEIS1 overexpression also caused AML in vivo, with a leukemic latency similar to that caused by forced expression of MLL-AF9, the most common form of MLL fusions. Furthermore, gene expression profiling of hematopoietic cells demonstrated that PBX3/MEIS1 overexpression, but not HOXA9/MEIS1, HOXA9/PBX3, or HOXA9 overexpression, recapitulated the MLL-fusion-mediated core transcriptome, particularly upregulation of the endogenous Hoxa genes. Disruption of the binding between MEIS1 and PBX3 diminished PBX3/MEIS1-mediated cell transformation and HOX gene upregulation. Collectively, our studies strongly implicate the PBX3/MEIS1 interaction as a driver of cell transformation and leukemogenesis, and suggest that this axis may play a critical role in the regulation of the core transcriptional programs activated in MLL-rearranged and HOX-overexpressing AML. Therefore, targeting the MEIS1/PBX3 interaction may represent a promising therapeutic strategy to treat these AML subtypes.

Cytokine Regulation of Microenvironmental Cells in Myeloproliferative Neoplasms

The term myeloproliferative neoplasms (MPN) refers to a heterogeneous group of diseases including not only polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), but also chronic myeloid leukemia (CML), and systemic mastocytosis (SM). Despite the clinical and biological differences between these diseases, common pathophysiological mechanisms have been identified in MPN. First, aberrant tyrosine kinase signaling due to somatic mutations in certain driver genes is common to these MPN. Second, alterations of the bone marrow microenvironment are found in all MPN types and have been implicated in the pathogenesis of the diseases. Finally, elevated levels of proinflammatory and microenvironment-regulating cytokines are commonly found in all MPN-variants. In this paper, we review the effects of MPN-related oncogenes on cytokine expression and release and describe common as well as distinct pathogenetic mechanisms underlying microenvironmental changes in various MPN. Furthermore, targeting of the microenvironment in MPN is discussed. Such novel therapies may enhance the efficacy and may overcome resistance to established tyrosine kinase inhibitor treatment in these patients. Nevertheless, additional basic studies on the complex interplay of neoplastic and stromal cells are required in order to optimize targeting strategies and to translate these concepts into clinical application.

Knockdown of homeobox A5 by small hairpin RNA inhibits proliferation and enhances cytarabine chemosensitivity of acute myeloid leukemia cells

Homeobox genes encode transcription factors that are essential for embryonic morphogenesis and differentiation. Transcription factors containing the highly conserved homeobox motif show considerable promise as potential regulators of hematopoietic maturation events. Previous studies have suggested that the increased expression levels of homeobox (HOX)A genes was correlated with the cytogenetic findings associated with poor prognosis in acute myeloid leukemia and mixed lineage leukemia. The aim of the present study was to investigate the role of HOXA5 in leukemia. The U937 human leukemia cell line was transfected with a HOXA5‑targeted short hairpin RNA (shRNA) to determine the effects of downregulation of the HOXA5 on proliferation, apoptosis, cell cycle distribution and chemoresistance in leukemia cells. Reverse transcription‑quantitative polymerase chain reaction and western blot analyses demonstrated that the mRNA and protein expression levels of HOXA5 were markedly suppressed following transfection with an shRNA‑containing vector. Knockdown of HOXA5 significantly inhibited cell proliferation, as determined by Cell Counting kit‑8 assay. Flow cytometry revealed that reduced HOXA5 expression levels resulted in cell cycle arrest at the G1 phase, and induced apoptosis. In addition, western blot analysis demonstrated that HOXA5 knockdown increased the expression levels of caspase‑3, and reduced the expression levels of survivin in the U937 cells. Furthermore, knockdown of HOXA5 in the U937 cells enhanced their chemosensitivity to cytarabine. The results of the present study suggested that downregulation of HOXA5 by shRNA may trigger apoptosis and overcome drug resistance in leukemia cells. Therefore, HOXA5 may serve as a potential target for developing novel therapeutic strategies for leukemia.

The HOX-Apoptosis Regulatory Interplay in Development and Disease

Apoptosis is a cellular suicide program, which is on the one hand used to remove superfluous cells thereby promoting tissue or organ morphogenesis. On the other hand, the programmed killing of cells is also critical when potentially harmful cells emerge in a developing or adult organism thereby endangering survival. Due to its critical role apoptosis is tightly controlled, however so far, its regulation on the transcriptional level is less studied and understood. Hox genes, a highly conserved gene family encoding homeodomain transcription factors, have crucial roles in development. One of their prominent functions is to shape animal body plans by eliciting different developmental programs along the anterior-posterior axis. To this end, Hox proteins transcriptionally regulate numerous processes in a coordinated manner, including cell-type specification, differentiation, motility, proliferation as well as apoptosis. In this review, we will focus on how Hox proteins control organismal morphology and function by regulating the apoptotic machinery. We will first focus on well-established paradigms of Hox-apoptosis interactions and summarize how Hox transcription factors control morphological outputs and differentially shape tissues along the anterior-posterior axis by fine-tuning apoptosis in a healthy organism. We will then discuss the consequences when this interaction is disturbed and will conclude with some ideas and concepts emerging from these studies.

Role of Hox genes in stem cell differentiation

Hox genes are an evolutionary highly conserved gene family. They determine the anterior-posterior body axis in bilateral organisms and influence the developmental fate of cells. Embryonic stem cells are usually devoid of any Hox gene expression, but these transcription factors are activated in varying spatial and temporal patterns defining the development of various body regions. In the adult body, Hox genes are among others responsible for driving the differentiation of tissue stem cells towards their respective lineages in order to repair and maintain the correct function of tissues and organs. Due to their involvement in the embryonic and adult body, they have been suggested to be useable for improving stem cell differentiations in vitro and in vivo. In many studies Hox genes have been found as driving factors in stem cell differentiation towards adipogenesis, in lineages involved in bone and joint formation, mainly chondrogenesis and osteogenesis, in cardiovascular lineages including endothelial and smooth muscle cell differentiations, and in neurogenesis. As life expectancy is rising, the demand for tissue reconstruction continues to increase. Stem cells have become an increasingly popular choice for creating therapies in regenerative medicine due to their self-renewal and differentiation potential. Especially mesenchymal stem cells are used more and more frequently due to their easy handling and accessibility, combined with a low tumorgenicity and little ethical concerns. This review therefore intends to summarize to date known correlations between natural Hox gene expression patterns in body tissues and during the differentiation of various stem cells towards their respective lineages with a major focus on mesenchymal stem cell differentiations. This overview shall help to understand the complex interactions of Hox genes and differentiation processes all over the body as well as in vitro for further improvement of stem cell treatments in future regenerative medicine approaches.

The Hepatocyte Growth Factor (HGF)/Met Axis: A Neglected Target in the Treatment of Chronic Myeloproliferative Neoplasms?

Met is the receptor of hepatocyte growth factor (HGF), a cytoprotective cytokine. Disturbing the equilibrium between Met and its ligand may lead to inappropriate cell survival, accumulation of genetic abnormalities and eventually, malignancy. Abnormal activation of the HGF/Met axis is established in solid tumours and in chronic haematological malignancies, including myeloma, acute myeloid leukaemia, chronic myelogenous leukaemia (CML), and myeloproliferative neoplasms (MPNs). The molecular mechanisms potentially responsible for the abnormal activation of HGF/Met pathways are described and discussed. Importantly, inCML and in MPNs, the production of HGF is independent of Bcr-Abl and JAK2V617F, the main molecular markers of these diseases. In vitro studies showed that blocking HGF/Met function with neutralizing antibodies or Met inhibitors significantly impairs the growth of JAK2V617F-mutated cells. With personalised medicine and curative treatment in view, blocking activation of HGF/Met could be a useful addition in the treatment of CML and MPNs for those patients with high HGF/MET expression not controlled by current treatments (Bcr-Abl inhibitors in CML; phlebotomy, hydroxurea, JAK inhibitors in MPNs).