Defective Notch activation in microenvironment leads to myeloproliferative disease

Granulocyte-Macrophage-Colony-Stimulating-Factor Combined with Prostaglandin E1 Create Dendritic Cells of Leukemic Origin from AML Patients' Whole Blood and Whole Bone Marrow That Mediate Antileukemic Processes after Mixed Lymphocyte Culture

Although several (chemotherapeutic) protocols to treat acute myeloid leukemia (AML) are available, high rates of relapses in successfully treated patients occur. Strategies to stabilize remissions are greatly needed. The combination of the (clinically approved) immune-modulatory compounds Granulocyte-Macrophage-Colony-Stimulating-Factor (GM-CSF) and Prostaglandine E1 (PGE-1) (Kit-M) converts myeloid blasts into dendritic cells of leukemic origin (DCleu). After stimulation with DCleu ex vivo, leukemia-specific antileukemic immune cells are activated. Therefore, Kit-M treatment may be an attractive immunotherapeutic tool to treat patients with myeloid leukemia. Kit-M-mediated antileukemic effects on whole bone marrow (WBM) were evaluated and compared to whole blood (WB) to evaluate the potential effects of Kit-M on both compartments. WB and WBM samples from 17 AML patients at first diagnosis, in persisting disease and at relapse after allogeneic stem cell transplantation (SCT) were treated in parallel with Kit-M to generate DC/DCleu. Untreated samples served as controls. After a mixed lymphocyte culture enriched with patients' T cells (MLC), the leukemia-specific antileukemic effects were assessed through the degranulation- (CD107a+ T cells), the intracellular IFNγ production- and the cytotoxicity fluorolysis assay. Quantification of cell subtypes was performed via flow cytometry. In both WB and WBM significantly higher frequencies of (mature) DCleu were generated without induction of blast proliferation in Kit-M-treated samples compared to control. After MLC with Kit-M-treated vs. not pretreated WB or WBM, frequencies of (leukemia-specific) immunoreactive cells (e.g., non-naive, effector-, memory-, CD3+β7+ T cells, NK- cells) were (significantly) increased, whereas leukemia-specific regulatory T cells (Treg, CD152+ T cells) were (significantly) decreased. The cytotoxicity fluorolysis assay showed a significantly improved blast lysis in Kit-M-treated WB and WBM compared to control. A parallel comparison of WB and WBM samples revealed no significant differences in frequencies of DCleu, (leukemia-specific) immunoreactive cells and achieved antileukemic processes. Kit-M was shown to have comparable effects on WB and WBM samples regarding the generation of DCleu and activation of (antileukemic) immune cells after MLC. This was true for samples before or after SCT. In summary, a potential Kit-M in vivo treatment could lead to antileukemic effects in WB as well as WBM in vivo and to stabilization of the disease or remission in patients before or after SCT. A clinical trial is currently being planned.

Host factor TIMP1 sustains long-lasting myeloid-biased hematopoiesis after severe infection

Infection is able to promote innate immunity by enhancing a long-term myeloid output even after the inciting infectious agent has been cleared. However, the mechanisms underlying such a regulation are not fully understood. Using a mouse polymicrobial peritonitis (sepsis) model, we show that severe infection leads to increased, sustained myelopoiesis after the infection is resolved. In post-infection mice, the tissue inhibitor of metalloproteinases 1 (TIMP1) is constitutively upregulated. TIMP1 antagonizes the function of ADAM10, an essential cleavage enzyme for the activation of the Notch signaling pathway, which suppresses myelopoiesis. While TIMP1 is dispensable for myelopoiesis under the steady state, increased TIMP1 enhances myelopoiesis after infection. Thus, our data establish TIMP1 as a molecular reporter of past infection in the host, sustaining hyper myelopoiesis and serving as a potential therapeutic target for modulating HSPC cell fate.

A Question of Frame: The Role of the Bone Marrow Stromal Niche in Myeloid Malignancies

Until a few years ago, the onset of acute myeloid leukemia (AML) was entirely ascribed to genetic lesions in hematopoietic stem cells. These mutations generate leukemic stem cells, which are known to be the main ones responsible for chemoresistance and relapse. However, in the last years, increasing evidence demonstrated that dynamic interplay between leukemic cells and bone marrow (BM) niche is of paramount relevance in the pathogenesis of myeloid malignancies, including AML. Specifically, BM stromal niche components, such as mesenchymal stromal cells (MSCs) and their osteoblastic cell derivatives, play a key role not only in supporting normal hematopoiesis but also in the manifestation and progression of myeloid malignancies. Here, we reviewed recent clinical and experimental findings about how genetic and functional alterations in MSCs and osteolineage progeny can contribute to leukemogenesis and how leukemic cells in turn generate a corrupted niche able to support myeloid neoplasms. Moreover, we discussed how the newest single-cell technologies may help dissect the interactions between BM stromal cells and malignant hematopoiesis. The deep comprehension of the tangled relationship between stroma and AML blasts and their modulation during disease progression may have a valuable impact on the development of new microenvironment-directed therapeutic strategies, potentially useful for a wide cohort of patients.

An Updated Overview of the Role of CYP450 during Xenobiotic Metabolization in Regulating the Acute Myeloid Leukemia Microenvironment

Oxidative stress is associated with several acute and chronic disorders, including hematological malignancies such as acute myeloid leukemia, the most prevalent acute leukemia in adults. Xenobiotics are usually harmless compounds that may be detrimental, such as pharmaceuticals, environmental pollutants, cosmetics, and even food additives. The storage of xenobiotics can serve as a defense mechanism or a means of bioaccumulation, leading to adverse effects. During the absorption, metabolism, and cellular excretion of xenobiotics, three steps may be distinguished: (i) inflow by transporter enzymes, (ii) phases I and II, and (iii) phase III. Phase I enzymes, such as those in the cytochrome P450 superfamily, catalyze the conversion of xenobiotics into more polar compounds, contributing to an elevated acute myeloid leukemia risk. Furthermore, genetic polymorphism influences the variability and susceptibility of related myeloid neoplasms, infant leukemias associated with mixed-lineage leukemia (MLL) gene rearrangements, and a subset of de novo acute myeloid leukemia. Recent research has shown a sustained interest in determining the regulators of cytochrome P450, family 2, subfamily E, member 1 (CYP2E1) expression and activity as an emerging field that requires further investigation in acute myeloid leukemia evolution. Therefore, this review suggests that CYP2E1 and its mutations can be a therapeutic or diagnostic target in acute myeloid leukemia.

Bone Marrow Microenvironment as a Source of New Drug Targets for the Treatment of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a heterogeneous disease with one of the worst survival rates of all cancers. The bone marrow microenvironment is increasingly being recognised as an important mediator of AML chemoresistance and relapse, supporting leukaemia stem cell survival through interactions among stromal, haematopoietic progenitor and leukaemic cells. Traditional therapies targeting leukaemic cells have failed to improve long term survival rates, and as such, the bone marrow niche has become a promising new source of potential therapeutic targets, particularly for relapsed and refractory AML. This review briefly discusses the role of the bone marrow microenvironment in AML development and progression, and as a source of novel therapeutic targets for AML. The main focus of this review is on drugs that modulate/target this bone marrow microenvironment and have been examined in in vivo models or clinically.

Pathogenic Mechanisms in Acute Myeloid Leukemia

OPINION STATEMENT

Acute myeloid leukemia (AML) is the most common form of leukemia in adults, leading to the highest number of annual leukemia-associated deaths in the USA. Although most AML patients initially enter remission following induction therapy, most eventually relapse, underscoring the unmet need for more effective therapies. In recent years, novel high-throughput sequencing techniques, and mouse and human models of disease have increased our understanding of the molecular mechanisms that lead to AML. Leukemogenic mechanisms can be broadly classified into two types-cell-intrinsic and cell-extrinsic. Cell-intrinsic mechanisms include an array of genetic and epigenetic alterations that lead to dysregulated gene expression and function in hematopoietic stem/progenitor cells, leading to their increased fitness and ultimately, malignant transformation. Extrinsic mechanisms include both hematopoietic and non-hematopoietic stromal components of the leukemic microenvironment that interact with pre-leukemic and leukemic clones to promote their survival, self-renewal, and/or resistance to therapy. Through the individual and concerted action of these factors, pre-leukemic clones acquire the changes necessary for leukemic transformation. In addition, following therapy, specific leukemic clones are selected for that eventually re-initiate disease. Improving our understanding of these cell-intrinsic and cell-extrinsic mechanisms will provide novel opportunities to treat AML as well as prevent the development of disease.

Regulation of Malignant Myeloid Leukemia by Mesenchymal Stem Cells

Bone marrow microenvironment (BMM) has been proven to have benefits for both normal hematopoietic stem cell niche and pathological leukemic stem cell niche. In fact, the pathological leukemia microenvironment reprograms bone marrow niche cells, especially mesenchymal stem cells for leukemia progression, chemoresistance and relapse. The growth and differentiation of MSCs are modulated by leukemia stem cells. Moreover, chromatin abnormality of mesenchymal stem cells is sufficient for leukemia initiation. Here, we summarize the detailed relationship between MSC and leukemia. MSCs can actively and passively regulate the progression of myelogenous leukemia through cell-to-cell contact, cytokine-receptor interaction, and exosome communication. These behaviors benefit LSCs proliferation and survival and inhibit physiological hematopoiesis. Finally, we describe the recent advances in therapy targeting MSC hoping to provide new perspectives and therapeutic strategies for leukemia.

Bone Marrow Aging and the Leukaemia-Induced Senescence of Mesenchymal Stem/Stromal Cells: Exploring Similarities

Bone marrow aging is associated with multiple cellular dysfunctions, including perturbed haematopoiesis, the propensity to haematological transformation, and the maintenance of leukaemia. It has been shown that instructive signals from different leukemic cells are delivered to stromal cells to remodel the bone marrow into a supportive leukemic niche. In particular, cellular senescence, a physiological program with both beneficial and deleterious effects on the health of the organisms, may be responsible for the increased incidence of haematological malignancies in the elderly and for the survival of diverse leukemic cells. Here, we will review the connection between BM aging and cellular senescence and the role that these processes play in leukaemia progression. Specifically, we discuss the role of mesenchymal stem cells as a central component of the supportive niche. Due to the specificity of the genetic defects present in leukaemia, one would think that bone marrow alterations would also have particular changes, making it difficult to envisage a shared therapeutic use. We have tried to summarize the coincident features present in BM stromal cells during aging and senescence and in two different leukaemias, acute myeloid leukaemia, with high frequency in the elderly, and B-acute lymphoblastic leukaemia, mainly a childhood disease. We propose that mesenchymal stem cells are similarly affected in these different leukaemias, and that the changes that we observed in terms of cellular function, redox balance, genetics and epigenetics, soluble factor repertoire and stemness are equivalent to those occurring during BM aging and cellular senescence. These coincident features may be used to explore strategies useful to treat various haematological malignancies.

Bone Marrow Niches of Hematopoietic Stem and Progenitor Cells

The mammalian hematopoietic system is remarkably efficient in meeting an organism’s vital needs, yet is highly sensitive and exquisitely regulated. Much of the organismal control over hematopoiesis comes from the regulation of hematopoietic stem cells (HSCs) by specific microenvironments called niches in bone marrow (BM), where HSCs reside. The experimental studies of the last two decades using the most sophisticated and advanced techniques have provided important data on the identity of the niche cells controlling HSCs functions and some mechanisms underlying niche-HSC interactions. In this review we discuss various aspects of organization and functioning of the HSC cell niche in bone marrow. In particular, we review the anatomy of BM niches, various cell types composing the niche, niches for more differentiated cells, metabolism of HSCs in relation to the niche, niche aging, leukemic transformation of the niche, and the current state of HSC niche modeling in vitro.

A CIBERSORTx-based immune cell scoring system could independently predict the prognosis of patients with myelodysplastic syndromes

Aside from cell intrinsic factors such as genetic alterations, immune dysregulation in the bone marrow (BM) microenvironment plays a role in the development and progression of myelodysplastic syndromes (MDS). However, the prognostic implications of various immune cells in patients with MDS remain unclear. We adopted CIBERSORTx to estimate the relative fractions of 22 subtypes of immune cells in the BM of 316 patients with MDS and correlated the results with clinical outcomes. A lower fraction of unpolarized M0 macrophages and higher fractions of M2 macrophages and eosinophils were significantly associated with inferior survival. An immune cell scoring system (ICSS) was constructed based on the proportion of these 3 immune cells in the BM. The ICSS high-risk patients had higher BM blast counts, higher frequencies of poor-risk cytogenetics, and more NPM1, TP53, and WT1 mutations than intermediate- and low-risk patients. The ICSS could stratify patients with MDS into 3 risk groups with distinct leukemia-free survival and overall survival among the total cohort and in the subgroups of patients with lower and higher disease risk based on the revised International Prognostic Scoring System (IPSS-R). The prognostic significance of ICSS was also validated in another independent cohort. Multivariable analysis revealed that ICSS independently predicted prognosis, regardless of age, IPSS-R, and mutation status. Bioinformatic analysis demonstrated a significant correlation between high-risk ICSS and nuclear factor κB signaling, oxidative stress, and leukemic stem cell signature pathways. Further studies investigating the mechanistic insight into the crosstalk between stem cells and immune cells are warranted.

Restoration of aged hematopoietic cells by their young counterparts through instructive microvesicles release

This study addresses the potential to reverse age-associated morbidity by establishing methods to restore the aged hematopoietic system. Parabiotic animal models indicated that young secretome could restore aged tissues, leading us to establish a heterochronic transwell system with aged mobilized peripheral blood (MPB), co-cultured with young MPB or umbilical cord blood (UCB) cells. Functional studies and omics approaches indicate that the miRNA cargo of microvesicles (MVs) restores the aged hematopoietic system. The in vitro findings were validated in immune deficient (NSG) mice carrying an aged hematopoietic system, improving aged hallmarks such as increased lymphoid:myeloid ratio, decreased inflammation and cellular senescence. Elevated MYC and E2F pathways, and decreased p53 were key to hematopoietic restoration. These processes require four restorative miRs that target the genes for transcription/differentiation, namely PAX and phosphatase PPMIF. These miRs when introduced in aged cells were sufficient to restore the aged hematopoietic system in NSG mice. The aged MPBs were the drivers of their own restoration, as evidenced by the changes from distinct baseline miR profiles in MPBs and UCB to comparable expressions after exposure to aged MPBs. Restorative natural killer cells eliminated dormant breast cancer cells in vivo, indicating the broad relevance of this cellular paradigm - preventing and reversing age-associated disorders such as clearance of early malignancies and enhanced responses to vaccine and infection.

Bone marrow niches in myelodysplastic syndromes

Genetic and epigenetic lesions within hematopoietic cell populations drive diverse hematological malignancies. Myelodysplastic syndromes (MDS) are a group of myeloid neoplasms affecting the hematopoietic stem cells characterized by recurrent genetic abnormalities, myelodysplasia (a pathological definition of abnormal bone marrow structure), ineffective hematopoiesis resulting in blood cytopenia, and a propensity to evolve into acute myelogenous leukemia. Although there is evidence that the accumulation of a set of genetic mutations is an essential event in MDS, there is an increased appreciation of the contribution of specific microenvironments, niches, in the pathogenesis of MDS and response to treatment. In physiologic hematopoiesis, niches are critical functional units that maintain hematopoietic stem and progenitor cells and regulate their maturation into mature blood cells. In MDS and other hematological malignancies, altered bone marrow niches can promote the survival and expansion of mutant hematopoietic clones and provide a shield from therapy. In this review, we focus on our understanding of the composition and function of hematopoietic niches and their role in the evolution of myeloid malignancies, with an emphasis on MDS.

Leukemic Stem Cells: From Leukemic Niche Biology to Treatment Opportunities

Acute myeloid leukemia (AML) is one of the most common types of leukemia in adults. While complete remission can be obtained with intensive chemotherapy in young and fit patients, relapse is frequent and prognosis remains poor. Leukemic cells are thought to arise from a pool of leukemic stem cells (LSCs) which sit at the top of the hierarchy. Since their discovery, more than 30 years ago, LSCs have been a topic of intense research and their identification paved the way for cancer stem cell research. LSCs are defined by their ability to self-renew, to engraft into recipient mice and to give rise to leukemia. Compared to healthy hematopoietic stem cells (HSCs), LSCs display specific mutations, epigenetic modifications, and a specific metabolic profile. LSCs are usually considered resistant to chemotherapy and are therefore the drivers of relapse. Similar to their HSC counterpart, LSCs reside in a highly specialized microenvironment referred to as the “niche”. Bidirectional interactions between leukemic cells and the microenvironment favor leukemic progression at the expense of healthy hematopoiesis. Within the niche, LSCs are thought to be protected from genotoxic insults. Improvement in our understanding of LSC gene expression profile and phenotype has led to the development of prognosis signatures and the identification of potential therapeutic targets. In this review, we will discuss LSC biology in the context of their specific microenvironment and how a better understanding of LSC niche biology could pave the way for new therapies that target AML.

Mesenchymal Stromal Cells as a Cellular Target in Myeloid Malignancy: Chances and Challenges in the Genome Editing of Stromal Alterations

The contribution of bone marrow stromal cells to the pathogenesis and therapy response of myeloid malignancies has gained significant attention over the last decade. Evidence suggests that the bone marrow stroma should not be neglected in the design of novel, targeted-therapies. In terms of gene-editing, the focus of gene therapies has mainly been on correcting mutations in hematopoietic cells. Here, we outline why alterations in the stroma should also be taken into consideration in the design of novel therapeutic strategies but also outline the challenges in specifically targeting mesenchymal stromal cells in myeloid malignancies caused by somatic and germline mutations.

From the niche to malignant hematopoiesis and back: reciprocal interactions between leukemia and the bone marrow microenvironment

The bone marrow microenvironment (BMME) regulates hematopoiesis through a complex network of cellular and molecular components. Hematologic malignancies reside within, and extensively interact with, the same BMME. These interactions consequently alter both malignant and benign hematopoiesis in multiple ways, and can encompass initiation of malignancy, support of malignant progression, resistance to chemotherapy, and loss of normal hematopoiesis. Herein, we will review supporting studies for interactions of the BMME with hematologic malignancies and discuss challenges still facing this exciting field of research. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Oncogenic KrasG12D Activation in the Nonhematopoietic Bone Marrow Microenvironment Causes Myelodysplastic Syndrome in Mice

The bone marrow microenvironment (BMME) is key player in regulation and maintenance of hematopoiesis. Oncogenic RAS mutations, causing constitutive activation of multiple tumor-promoting pathways, are frequently found in human cancer. So far in hematologic malignancies, RAS mutations have only been reported to occur in hematopoietic cells. In this study, we investigated the effect of oncogenic Kras expression in the BMME in a chimeric mouse model. We observed that an activating mutation of Kras in the nonhematopoietic system leads to a phenotype resembling myelodysplastic syndrome (MDS) characterized by peripheral cytopenia, marked dysplasia within the myeloid lineage as well as impaired proliferation and differentiation capacity of hematopoietic stem and progenitor cells. The phenotypic changes could be reverted when the BM was re-isolated and transferred into healthy recipients, indicating that the Kras^G12D -activation in the nonhematopoietic BMME was essential for the MDS phenotype. Gene expression analysis of sorted nonhematopoietic BM niche cells from Kras^G12D mice revealed upregulation of multiple inflammation-related genes including IL1-superfamily members (Il1α, Il1β, Il1f9) and the NLPR3 inflammasome. Thus, pro-inflammatory IL1-signaling in the BMME may contribute to MDS development. Our findings show that a single genetic change in the nonhematopoietic BMME can cause an MDS phenotype. Oncogenic Kras activation leads to pro-inflammatory signaling in the BMME which impairs HSPCs function. IMPLICATIONS: These findings may help to identify new therapeutic targets for MDS.

Cell interactions in the bone marrow microenvironment affecting myeloid malignancies

The bone marrow is a complex tissue in which heterogeneous populations of stromal cells interact with hematopoietic cells to dynamically respond to organismal needs in defense, hemostasis, and oxygen delivery. Physiologic challenges modify stromal/hematopoietic cell interactions to generate changes in blood cell production. When either stroma or hematopoietic cells are impaired, the system distorts. The distortions associated with myeloid malignancy are reviewed here and may provide opportunities for therapeutic intervention.

Adamantinomatous craniopharyngioma as a model to understand paracrine and senescence-induced tumourigenesis

Cellular senescence is a process that can prevent tumour development in a cell autonomous manner by imposing a stable cell cycle arrest after oncogene activation. Paradoxically, senescence can also promote tumour growth cell non-autonomously by creating a permissive tumour microenvironment that fuels tumour initiation, progression to malignancy and metastasis. In a pituitary tumour known as adamantinomatous craniopharyngioma (ACP), cells that carry oncogenic β-catenin mutations and overactivate the WNT signalling pathway form cell clusters that become senescent and activate a senescence-associated secretory phenotype (SASP). Research in mouse models of ACP has provided insights into the function of the senescent cell clusters and revealed a critical role for SASP-mediated activities in paracrine tumour initiation. In this review, we first discuss this research on ACP and subsequently explore the theme of paracrine tumourigenesis in other tumour models available in the literature. Evidence is accumulating supporting the notion that paracrine signalling brought about by senescent cells may underlie tumourigenesis across different tumours and cancer models.

The Dynamic Interface Between the Bone Marrow Vascular Niche and Hematopoietic Stem Cells in Myeloid Malignancy

Hematopoietic stem cells interact with bone marrow niches, including highly specialized blood vessels. Recent studies have revealed the phenotypic and functional heterogeneity of bone marrow endothelial cells. This has facilitated the analysis of the vascular microenvironment in steady state and malignant hematopoiesis. In this review, we provide an overview of the bone marrow microenvironment, focusing on refined analyses of the marrow vascular compartment performed in mouse studies. We also discuss the emerging role of the vascular niche in “inflamm-aging” and clonal hematopoiesis, and how the endothelial microenvironment influences, supports and interacts with hematopoietic cells in acute myeloid leukemia and myelodysplastic syndromes, as exemplar states of malignant myelopoiesis. Finally, we provide an overview of strategies for modulating these bidirectional interactions to therapeutic effect in myeloid malignancies.

Vascular Notch Signaling in Stress Hematopoiesis

Canonical Notch signaling is one of the most conserved signaling cascades. It regulates cell proliferation, cell differentiation, and cell fate maintenance in a variety of biological systems during development and cancer (Fortini, 2009; Kopan and Ilagan, 2009; Andersson et al., 2011; Ntziachristos et al., 2014). For the hematopoietic system, during embryonic development, Notch1 is essential for the emergence of hematopoietic stem cells (HSCs) at the aorta-gornado-mesonephro regions of the dorsal aorta. At adult stage, Notch receptors and Notch targets are expressed at different levels in diverse hematopoietic cell types and influence lineage choices. For example, Notch specifies T cell lineage over B cells. However, there has been a long-lasting debate on whether Notch signaling is required for the maintenance of adult HSCs, utilizing transgenic animals inactivating different components of the Notch signaling pathway in HSCs or niche cells. The aims of the current mini-review are to summarize the evidence that disapproves or supports such hypothesis and point at imperative questions waiting to be addressed; hence, some of the seemingly contradictory findings could be reconciled. We need to better delineate the Notch signaling events using biochemical assays to identify direct Notch targets within HSCs or niche cells in specific biological context. More importantly, we call for more elaborate studies that pertain to whether niche cell type (vascular endothelial cells or other stromal cell)-specific Notch ligands regulate the differentiation of T cells in solid tumors during the progression of T-lymphoblastic lymphoma (T-ALL) or chronic myelomonocytic leukemia (CMML). We believe that the investigation of vascular endothelial cells' or other stromal cell types' interaction with hematopoietic cells during homeostasis and stress can offer insights toward specific and effective Notch-related therapeutics.

The Hematopoietic Microenvironment in Myeloproliferative Neoplasms: The Interplay Between Nature (Stem Cells) and Nurture (the Niche)

Hematopoietic stem cells (HSCs) rely on instructive cues from the marrow microenvironment for their maintenance and function. Accumulating evidence indicates that the survival and proliferation of hematopoietic neoplasms are dependent not only on cell-intrinsic, genetic mutations, and other molecular alterations present within neoplastic stem cells, but also on the ability of the surrounding microenvironmental cells to nurture and promote the malignancy. It is anticipated that a better understanding of the molecular and cellular events responsible for these microenvironmental features of neoplastic hematopoiesis will lead to improved treatment for patients. This review will focus on the myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), in which an acquired signaling kinase mutation (JAK2V617F) plays a central, pathogenetic role in 50-100% of patients with these disorders. Evidence is presented that the development of an MPN requires both an abnormal, mutation-bearing (i.e., neoplastic) HSC and an abnormal, mutation-bearing microenvironment.

Oncogenic and Tumor-Suppressive Functions of NOTCH Signaling in Glioma

Although the role of NOTCH signaling has been extensively studied in health and disease, many questions still remain unresolved. Being crucial for tissue homeostasis, NOTCH signaling is also implicated in multiple cancers by either promoting or suppressing tumor development. In this review we illustrate the context-dependent role of NOTCH signaling during tumorigenesis with a particular focus on gliomas, the most frequent and aggressive brain tumors in adults. For a long time, NOTCH has been considered an oncogene in glioma mainly by virtue of its neural stem cell-promoting activity. However, the recent identification of NOTCH-inactivating mutations in some glioma patients has challenged this notion, prompting a re-examination of the function of NOTCH in brain tumor subtypes. We discuss recent findings that might help to reconcile the controversial role of NOTCH signaling in this disease, and pose outstanding questions that still remain to be addressed.

Myelodysplastic syndromes: moving towards personalized management

The myelodysplastic syndromes (MDS) share their origin in the hematopoietic stem cell but have otherwise very heterogeneous biological and genetic characteristics. Clinical features are dominated by cytopenia and a substantial risk for progression to acute myeloid leukemia. According to the World Health Organization, MDS is defined by cytopenia, bone marrow dysplasia and certain karyotypic abnormalities. The understanding of disease pathogenesis has undergone major development with the implementation of next-generation sequencing and a closer integration of morphology, cytogenetics and molecular genetics is currently paving the way for improved classification and prognostication. True precision medicine is still in the future for MDS and the development of novel therapeutic compounds with a propensity to markedly change patients' outcome lags behind that for many other blood cancers. Treatment of higher-risk MDS is dominated by monotherapy with hypomethylating agents but novel combinations are currently being evaluated in clinical trials. Agents that stimulate erythropoiesis continue to be first-line treatment for the anemia of lower-risk MDS but luspatercept has shown promise as second-line therapy for sideroblastic MDS and lenalidomide is an established second-line treatment for del(5q) lower-risk MDS. The only potentially curative option for MDS is hematopoietic stem cell transplantation, until recently associated with a relatively high risk of transplant-related mortality and relapse. However, recent studies show increased cure rates due to better tools to target the malignant clone with less toxicity. This review provides a comprehensive overview of the current status of the clinical evaluation, biology and therapeutic interventions for this spectrum of disorders.

Longitudinal Cytokine Profiling Identifies GRO-α and EGF as Potential Biomarkers of Disease Progression in Essential Thrombocythemia

Myeloproliferative neoplasms (MPNs) are characterized by deregulation of mature blood cell production and increased risk of myelofibrosis (MF) and leukemic transformation. Numerous driver mutations have been identified but substantial disease heterogeneity remains unexplained, implying the involvement of additional as yet unidentified factors. The inflammatory microenvironment has recently attracted attention as a crucial factor in MPN biology, in particular whether inflammatory cytokines and chemokines contribute to disease establishment or progression. Here we present a large-scale study of serum cytokine profiles in more than 400 MPN patients and identify an essential thrombocythemia (ET)-specific inflammatory cytokine signature consisting of Eotaxin, GRO-α, and EGF. Levels of 2 of these markers (GRO-α and EGF) in ET patients were associated with disease transformation in initial sample collection (GRO-α) or longitudinal sampling (EGF). In ET patients with extensive genomic profiling data (n = 183) cytokine levels added significant prognostic value for predicting transformation from ET to MF. Furthermore, CD56+CD14+ pro-inflammatory monocytes were identified as a novel source of increased GRO-α levels. These data implicate the immune cell microenvironment as a significant player in ET disease evolution and illustrate the utility of cytokines as potential biomarkers for reaching beyond genomic classification for disease stratification and monitoring.

New Insights of Human Parvovirus B19 in Modulating Erythroid Progenitor Cell Differentiation

Human parvovirus B19 (B19), a human pathogen of the erythroparvovirus genus, is responsible for a variety of diseases. B19 cause less symptoms in healthy individuals, also cause acute and chronic anemia in immunodeficiency patients. Transient aplastic crisis and pure red cell aplasia are two kinds of anemic hemogram, respectively, in acute and chronic B19 infection phase, especially occurring in patients with a shortened red cell survival or with immunodeficiency. In addition, B19-infected pregnant women may cause hydrops fetalis or fetal loss. B19 possesses high affinity to bone marrow and fetal liver due to its extremely restricted cytotoxicity to erythroid progenitor cells (EPCs) mediated by viral proteins. The nonstructural protein NS1 is considered to be the major pathogenic factor, which has been shown to inhibit the differentiation and maturation of EPCs through inducing viral DNA damage responses and cell cycle arrest. The time phase property of NS1 activity during DNA replication and conformity to transient change of hemogram are suggestive of its role in regulating differentiation of hematopoietic cells, which is not completely understood. In this review, we summarized the bridge between B19 NS1 and Notch signaling pathway or transcriptional factors GATA, which play an important role in erythroid cell proliferation and differentiation, to provide a new insight of the potential mechanism of B19-induced differential inhibition of EPCs.

Bone marrow niches in haematological malignancies

Haematological malignancies were previously thought to be driven solely by genetic or epigenetic lesions within haematopoietic cells. However, the niches that maintain and regulate daily production of blood and immune cells are now increasingly being recognized as having an important role in the pathogenesis and chemoresistance of haematological malignancies. Within haematopoietic cells, the accumulation of a small number of recurrent mutations initiates malignancy. Concomitantly, specific alterations of the niches, which support haematopoietic stem cells and their progeny, can act as predisposition events, facilitating mutant haematopoietic cell survival and expansion as well as contributing to malignancy progression and providing protection of malignant cells from chemotherapy, ultimately leading to relapse. In this Perspective, we summarize our current understanding of the composition and function of the specialized haematopoietic niches of the bone marrow during health and disease. We discuss disease mechanisms (rather than malignancy subtypes) to provide a comprehensive description of key niche-associated pathways that are shared across multiple haematological malignancies. These mechanisms include primary driver mutations in bone marrow niche cells, changes associated with increased hypoxia, angiogenesis and inflammation as well as metabolic reprogramming by stromal niche cells. Consequently, remodelling of bone marrow niches can facilitate immune evasion and activation of survival pathways favouring malignant haematopoietic cell maintenance, defence against excessive reactive oxygen species and protection from chemotherapy. Lastly, we suggest guidelines for the handling and biobanking of patient samples and analysis of the niche to ensure that basic research identifying therapeutic targets can be more efficiently translated to the clinic. The hope is that integrating knowledge of how bone marrow niches contribute to haematological disease predisposition, initiation, progression and response to therapy into future clinical practice will likely improve the treatment of these disorders.

The haematopoietic stem cell niche: a new player in cardiovascular disease?

Haematopoiesis, the process of blood production, can be altered during the initiation or progression of many diseases. Cardiovascular disease (CVD) has been shown to be heavily influenced by changes to the haematopoietic system, including the types and abundance of immune cells produced. It is now well established that innate immune cells are increased in people with CVD, and the mechanisms contributing to this can be vastly different depending on the risk factors or comorbidities present. Many of these changes begin at the level of the haematopoietic stem and progenitor cells (HSPCs) that reside in the bone marrow (BM). In general, the HSPCs and downstream myeloid progenitors are expanded via increased proliferation in the setting of atherosclerotic CVD. However, HSPCs can also be encouraged to leave the BM and colonise extramedullary sites (i.e. the spleen). Within the BM, HSPCs reside in specialized microenvironments, often referred to as a niche. To date in depth studies assessing the damage or dysregulation that occurs in the BM niche in varying CVDs are scarce. In this review, we provide a general overview of the complex components and interactions within the BM niche and how they influence the function of HSPCs. Additionally, we discuss the main findings regarding changes in the HSPC niche that influence the progression of CVD. We hypothesize that understanding the influence of the BM niche in CVD will aid in delineating new pathways for therapeutic interventions.

Haematopoietic stem cell activity and interactions with the niche

The haematopoietic stem cell (HSC) microenvironment in the bone marrow, termed the niche, ensures haematopoietic homeostasis by controlling the proliferation, self-renewal, differentiation and migration of HSCs and progenitor cells at steady state and in response to emergencies and injury. Improved methods for HSC isolation, driven by advances in single-cell and molecular technologies, have led to a better understanding of their behaviour, heterogeneity and lineage fate and of the niche cells and signals that regulate their function. Niche regulatory signals can be in the form of cell-bound or secreted factors and other local physical cues. A combination of technological advances in bone marrow imaging and genetic manipulation of crucial regulatory factors has enabled the identification of several candidate cell types regulating the niche, including both non-haematopoietic (for example, perivascular mesenchymal stem and endothelial cells) and HSC-derived (for example, megakaryocytes, macrophages and regulatory T cells), with better topographical understanding of HSC localization in the bone marrow. Here, we review advances in our understanding of HSC regulation by niches during homeostasis, ageing and cancer, and we discuss their implications for the development of therapies to rejuvenate aged HSCs or niches or to disrupt self-reinforcing malignant niches.

The Bone Marrow Niche - The Tumor Microenvironment That Ensures Leukemia Progression

The human body requires a constant delivery of fresh blood cells that are needed to maintain body homeostasis. Hematopoiesis is the process that drives the formation of new blood cells from a single stem cell. This is a complex, orchestrated and tightly regulated process that occurs within the bone marrow. When such process is faulty or deregulated, leukemia arises, develops and thrives by subverting normal hematopoiesis and availing the supplies of this rich milieu.In this book chapter we will describe and characterize the bone marrow microenvironment and its key importance for leukemia expansion. The several components of the bone marrow niche, their interaction with the leukemic cells and the cellular pathways activated within the malignant cells will be emphasized. Finally, novel therapeutic strategies to target this sibling interaction will also be discussed.

Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.

Mathematical analysis of the Cancitis model and the role of inflammation in blood cancer progression

Recently, a tight coupling has been observed between inflammation and blood cancer such as the Myeloproliferative Neoplasms (MPNs). A mechanism based six-dimensional model-the Cancitis model-describing the progression of blood cancer coupled to the inflammatory system is analyzed. An analytical investigation provides criteria for the existence of physiological steady states, trivial, hematopoietic, malignant and co-existing steady states. The classification of steady states is explicitly done in terms of the inflammatory stimuli. Several parameters are crucial in determining the attracting steady state(s). In particular, increasing inflammatory stimuli may transform a healthy state into a malignant state under certain circumstances. In contrast for the co-existing steady state, increasing inflammatory stimuli may reduce the malignant cell burden. The model provides an overview of the possible dynamics which may inform clinical practice such as whether to use inflammatory inhibitors during treatment.

Stem and progenitor cell microenvironment for bone regeneration and repair

Stem cells reside in their native microenvironment, which provides dynamic physical and chemical cues essential to their survival, proliferation and function. A typical cell-based therapeutic approach requires the mesenchymal stem cells (MSC) to depart their native microenvironment, transplant to in-vivo environment, differentiate toward multiple lineages and participate in bone formation. The long-term survival, function and fate of MSC are dependent on the microenvironment in which they are transplanted. Transplantation of morselized autologous bone, which contains both stem cells and their native microenvironment, results in a good clinical outcome. However, implantation of bone graft substitutes does not provide the complete and dynamic microenvironment for MSC. Current bone graft therapeutics may need to be improved further to provide an optimal engineered MSC microenvironment.

Ally to adversary: mesenchymal stem cells and their transformation in leukaemia

Mesenchymal stem cells (MSC) are the key regulators of hematopoiesis. Owing to their dynamic nature; MSC differentiate into various lineages that further constitute the niche which are required for maintenance of the hematopoietic stem cells (HSC). A plethora of growth factors and cytokines secreted by MSC are essential for regulating the homeostasis within the niche in terms of cycling and quiescence of HSC. Additionally, there is a strong evidence suggesting the role of MSC in transformation of the niche to favour survival of leukemic cells. Regulation of HSC by MSC via BMP, Wnt, Notch and Sonic Hedgehog signalling has been well elaborated, however the modulation of MSC by HSC/LSC is yet unresolved. The cross talk between the HSC and MSC via paracrine or autocrine mechanisms is essential for the transformation. There are some reports implicating cell adhesion molecules, growth factors and cytokines; in modulation of MSC function and differentiation. The role of exosome mediated modulation has also been reported in the context of MSC transformation however, much needs to be done to understand this phenomenon in the present context. Similarly, the role of circulating nucleic acids, a well-studied molecular phenomenon in other tumours, requires attention in their potential role in crosstalk between MSC and HSC. This review underlines the current understanding of the physiological and pathophysiological roles of MSC and its transformation in diseased state, laying stress on developing further understanding of MSC regulation for development of the latter as therapeutic targets.

The role of cell adhesion in hematopoiesis and leukemogenesis

The cells of the bone marrow microenvironment are emerging as important contributors and regulators of normal hematopoiesis. This microenvironment is perturbed during leukemogenesis, and evidence points toward a bidirectional communication between leukemia cells and the normal cells of the bone marrow, mediated by direct cell-cell contact as well as soluble factors. These interactions are increasingly appreciated to play a role in leukemogenesis and possibly in resistance to chemotherapy. In fact, several compounds that specifically target the bone marrow microenvironment, including inhibitors of cell adhesion, are being tested as adjuncts to leukemia therapy.

Notch Signaling in Nestin-Expressing Cells in the Bone Marrow Maintains Erythropoiesis via Macrophage Integrity

Notch signaling plays pivotal roles in both hematopoietic stem/progenitor and their niche cells. Myeloproliferative phenotypes are induced by disruption of Notch signaling in nonhematopoietic bone marrow (BM) cells. Nestin-expressing cells in the BM reportedly represent a component of the hematopoietic stem cell niche. We established mice in which rare Nestin-expressing cells in the BM were marked by green fluorescent protein, and Notch signaling was conditionally disrupted in these cells specifically. We observed impairment of erythropoiesis in the BM accompanying splenomegaly with BM hematopoietic programs in other lineages undisturbed. Transplantation experiments revealed that the microenvironmental rather than the hematopoietic cells were attributable to these phenotypes. We further found that the erythroid-island-forming ability of BM central macrophages was compromised along with the transcriptional upregulation of interleukin-6. Various Inflammatory conditions hamper BM erythropoiesis, which often accompanies extramedullary hematopoiesis. The mouse model demonstrated here may be of relevance to this common pathophysiologic condition. Stem Cells 2019;37:924-936.

Histone methylation regulator PTIP is required to maintain normal and leukemic bone marrow niches

The bone is essential for locomotion, calcium storage, and harboring the hematopoietic stem cells (HSCs) that supply the body with mature blood cells throughout life. HSCs reside at the interface of the bone and bone marrow (BM), where active bone remodeling takes place. Although the cellular components of the BM niche have been characterized, little is known about its epigenetic regulation. Here we find that the histone methylation regulator PTIP (Pax interaction with transcription-activation domain protein-1) is required to maintain the integrity of the BM niche by promoting osteoclast differentiation. PTIP directly promotes chromatin changes required for the expression of Pparγ (peroxisome proliferator-activated receptor-γ), a transcription factor essential for osteoclastogenesis. PTIP deletion leads to a drastic reduction of HSCs in the BM and induces extramedullary hematopoiesis. Furthermore, exposure of acute myeloid leukemia cells to a PTIP-deficient BM microenvironment leads to a reduction in leukemia-initiating cells and increased survival upon transplantation. Taken together, our data identify PTIP as an epigenetic regulator of osteoclastogenesis that is required for the integrity of the BM niche to sustain both normal hematopoiesis and leukemia.

Hematopoiesis and microenvironment in hematological malignancies

Adult hematopoietic stem cells (HSCs) and progenitors (HPCs) reside in the bone marrow, a highly orchestrated architecture. In the bone marrow, the process of how HSCs exert self-renewal and differentiation is tightly regulated by the surrounding microenvironment, or niche. Recent advances in imaging technologies and numerous knockout or knockin mouse models have greatly improved our understanding of the organization of the bone marrow niche. This niche compartment includes a complex network of mesenchymal stem cells (MSC), osteolineage cells, endothelial cells (arterioles and sinusoids), sympathetic nerves, nonmyelinating Schwann cells and megakaryocytes. In addition, different types of mediators, such as cytokines/chemokines, reactive oxygen species (ROS) and exosomes play a pivotal role in regulating the function of hematopoietic cells. Therefore, the niche components and the hematopoietic system make up an ecological environment that maintains the homeostasis and responds to stress, damage or disease conditions. On the other hand, the niche compartment can become a traitor that can do harm to normal hematopoietic cells under pathological conditions. Studies on the diseased bone marrow niche have only recently begun to appear in the extant literature. In this short review, we discuss the most recent advances regarding the behaviors of normal hematopoietic cells and their niche alterations in hematological malignancies.

Germline mutations in the bone marrow microenvironment and dysregulated hematopoiesis

The relationship between the hematopoietic stem cell (HSC) population and its surrounding bone marrow microenvironment is a rapidly evolving area of research. Normal HSC processes rely heavily on a complex communication network involving various marrow niches. Although leukemogenesis largely results from abnormal genetic activity within the leukemia stem cell itself, mounting evidence indicates a significant contributory role played by marrow niche dysregulation. Furthermore, numerous instances of activating or inactivating germline mutations within marrow microenvironment cells have been shown to be sufficient for development of myelodysplastic syndrome, myeloproliferative neoplasm, and acute myeloid leukemia, even in the context of wild-type HSCs. Recent evidence suggests that targeting aberrant chemokine production from germline-mutated marrow stromal cells can potentially reverse the process of leukemogenesis. This elaborate interplay between the HSC population and the marrow microenvironment allows for a number of unique clinical possibilities in efforts to induce remission, enhance chemosensitivity, manage relapsed disease, and prevent leukemia development, both in de novo and germline mutation-associated leukemias, including the use of targeted cytokine/chemokine inhibitors, immune checkpoint blockade, CXCR4/CXCL12 axis antagonists, and combined allogeneic HSC and mesenchymal stem cell transplantation. In this review, we discuss the pathways underlying normal and abnormal bone marrow niche functioning, the relationship between germline mutations in the stem cell microenvironment and dysregulated hematopoiesis, and future clinical perspectives that may be particularly applicable to prevention and treatment of germline-associated leukemias.

Niche TWIST1 is critical for maintaining normal hematopoiesis and impeding leukemia progression

The bone marrow microenvironment regulates normal and malignant hematopoiesis, but the underlying molecular mechanisms remain poorly defined. Using a chimeric mice model, we demonstrate that Twist1 deletion in the bone marrow microenvironment results in alteration of multiple niche cells as well as downregulated expression of major hematopoietic stem cell supportive factors. The perturbed microenvironment reduces hematopoietic stem cell homing and retention, impairs hematopoietic stem cell self-renewal and induces myeloid skewing. Nevertheless, it accelerates the progression of MLL-AF9 leukemia, which is partially mediated by Jagged-2-dependent Notch signaling. Our data provide the first demonstration of a pivotal role of TWIST1 in favoring normal hematopoiesis and hampering leukemia development. They also bring new insights into the role of the bone marrow niche in driving the development of acute myeloid leukemia, and suggest possible new avenues, exploiting the niche, to improve leukemia treatments.

Where Hematopoietic Stem Cells Live: The Bone Marrow Niche

Hematopoietic stem cells (HSCs) can sustain the production of blood throughout one's lifetime. However, for proper self-renewal of its own population and differentiation to blood, the HSC requires a specialized microenvironment called the "niche." Recent Advances: Recent studies using novel mouse models have shed new light on the cellular architecture and function of the HSC niche. Here, we review the different cells that constitute the HSC niche and the molecular mechanisms that underlie HSC and niche interaction. We discuss the evidence and potential features that distinguish the HSC niche from other microenvironments in the bone marrow. The relevance of the niche in malignant transformation of the HSCs and harboring cancer metastasis to the bone is also outlined. In addition, we address how the niche may regulate reactive oxygen species levels surrounding the HSCs. Critical Issues and Future Directions: We propose future directions and remaining challenges in investigating the niche of HSCs. We discuss how a better understanding of the HSC niche may help in restoring an aged hematopoietic system, fighting against malignancies, and transplanting purified HSCs safely and effectively into patients. Antioxid. Redox Signal. 00, 000-000.

The Bone Marrow Microenvironment in Health and Myeloid Malignancy

Hematopoietic stem cells (HSCs) interact dynamically with an intricate network of cells in the bone marrow (BM) microenvironment or niche. These interactions provide instructive cues that influence the production and lineage determination of different types of blood cells and maintenance of HSC quiescence. They also contribute to hematopoietic deregulation and hematological myeloid malignancies. Alterations in the BM niche are commonly observed in myeloid malignancies and contribute to the aberrant function of myelodysplastic and leukemia-initiating stem cells. In this work, we review how different components of the BM niche affect normal hematopoiesis, the molecular signals that govern this interaction, and how genetic changes in stromal cells or alterations in remodeled malignant BM niches contribute to myeloid malignancies. Understanding the intricacies between normal and malignant niches and their modulation may provide insights into developing novel therapeutics for blood disorders.

Regulation of Malignant Hematopoiesis by Bone Marrow Microenvironment

Hematopoietic stem cells (HSCs) that give rise to all kinds of hematopoietic lineage cells on various demands throughout life are maintained in a specialized microenvironment called “niche” in the bone marrow (BM). Defining niche cells and unveiling its function have been the subject of intense study, and it is becoming increasingly clear how niche cells regulate HSCs in normal hematopoiesis. Leukemia stem cells (LSCs), which are able to produce leukemic cells and maintain leukemic clones, are assumed to share common features with healthy HSCs. Accumulating evidence suggests that LSCs reside in a specialized BM microenvironment; moreover, LSCs could control and rebuild the microenvironment to enhance their progression and survival. This article discusses the recent advances in our knowledge of the microenvironment supporting malignant hematopoiesis, including LSC niche.

The interplay of leukemia cells and the bone marrow microenvironment

The interplay of cancer cells and surrounding stroma is critical in disease progression. This is particularly evident in hematological malignancies that infiltrate the bone marrow and peripheral lymphoid organs. Despite clear evidence for the existence of these interactions, the precise repercussions on the growth of leukemic cells are poorly understood. Recent development of novel imaging technology and preclinical disease models has advanced our comprehension of leukemia-microenvironment crosstalk and has potential implications for development of novel treatment options.

Targeting the bone marrow microenvironment in acute leukemia

Despite individual differences between certain leukemias, the overall survival rate in acute leukemia remains low at approximately 40%. Novel therapeutics, including targeted therapies like tyrosine kinase inhibitors, have been incorporated into treatment regimens, but most have failed at eradicating leukemic stem cells (LSCs). The causes of disease relapse, progression, and resistance to chemotherapy are as yet not entirely clear but thought to be linked to protection in the bone marrow microenvironment (BMM). In this review, we summarize current knowledge on the BMM in acute leukemias and examine the ongoing efforts to target the BMM, which include treatment strategies targeting (a) leukemia-BMM interactions, (b) leukemia-cell intrinsic pathways influenced by the BMM, and (c) direct BMM targeting strategies. It is likely that the future ploy against leukemia will involve these and other innovative strategies designed to eradicate the last remaining warrior - the LSC.

Notch signaling: its roles and therapeutic potential in hematological malignancies

Notch is a highly conserved signaling system that allows neighboring cells to communicate, thereby controlling their differentiation, proliferation and apoptosis, with the outcome of its activation being highly dependent on signal strength and cell type. As such, there is growing evidence that disturbances in physiological Notch signaling contribute to cancer development and growth through various mechanisms. Notch was first reported to contribute to tumorigenesis in the early 90s, through identification of the involvement of the Notch1 gene in the chromosomal translocation t(7;9)(q34;q34.3), found in a small subset of T-cell acute lymphoblastic leukemia. Since then, Notch mutations and aberrant Notch signaling have been reported in numerous other precursor and mature hematological malignancies, of both myeloid and lymphoid origin, as well as many epithelial tumor types. Of note, Notch has been reported to have both oncogenic and tumor suppressor roles, dependent on the cancer cell type. In this review, we will first give a general description of the Notch signaling pathway, and its physiologic role in hematopoiesis. Next, we will review the role of aberrant Notch signaling in several hematological malignancies. Finally, we will discuss current and potential future therapeutic approaches targeting this pathway.

The microenvironment in myelodysplastic syndromes: Niche-mediated disease initiation and progression

Myelodysplastic syndromes (MDSs) are clonal disorders of hematopoietic stem and progenitor cells and represent the most common cause of acquired marrow failure. Hallmarked by ineffective hematopoiesis, dysplastic marrow, and risk of transformation to acute leukemia, MDS remains a poorly treated disease. Although identification of hematopoietic aberrations in human MDS has contributed significantly to our understanding of MDS pathogenesis, evidence now identify the bone marrow microenvironment (BMME) as another key contributor to disease initiation and progression. With improved understanding of the BMME, we are beginning to refine the role of the hematopoietic niche in MDS. Despite genetic diversity in MDS, interaction between MDS and the BMME appears to be a common disease feature and therefore represents an appealing therapeutic target. Further understanding of the interdependent relationship between MDS and its niche is needed to delineate the mechanisms underlying hematopoietic failure and how the microenvironment can be targeted clinically. This review provides an overview of data from human MDS and murine models supporting a role for BMME dysfunction at several steps of disease pathogenesis. Although no models or human studies so far have combined all of these findings, we review current data identifying BMME involvement in each step of MDS pathogenesis organized to reflect the chronology of BMME contribution as the normal hematopoietic system becomes myelodysplastic and MDS progresses to marrow failure and transformation. Although microenvironmental heterogeneity and dysfunction certainly add complexity to this syndrome, data are already demonstrating that targeting microenvironmental signals may represent novel therapeutic strategies for MDS treatment.