Polarization of tumor-associated macrophages: a novel strategy for vascular normalization and antitumor immunity

TET2 encodes an enzyme that hydroxylates methylcytosine and is frequently targeted by loss-of-function mutations in myelodysplasia, myeloproliferative disorders, and acute myeloid leukemia. In this issue of Cancer Cell, two studies show that inactivation of Tet2 enhances hematopoietic stem cell self renewal and promotes the development of myeloproliferative disorders.     

TET2 inactivation results in pleiotropic hematopoietic abnormalities in mouse and is a recurrent event during human lymphomagenesis

Loss-of-function mutations affecting one or both copies of the Ten-Eleven-translocation (TET)2 gene have been described in various human myeloid malignancies. We report that inactivation of Tet2 in mouse perturbs both early and late steps of hematopoiesis including myeloid and lymphoid differentiation in a cell-autonomous manner, endows the cells with competitive advantage, and eventually leads to the development of malignancies. We subsequently observed TET2 mutations in human lymphoid disorders. TET2 mutations could be detected in immature progenitors endowed with myeloid colony-forming potential. Our results show that the mutations present in lymphoid tumor cells may occur at both early and later steps of lymphoid development and indicate that impairment of TET2 function or/and expression predisposes to the development of hematological malignancies.     

Leukemic IDH1 and IDH2 mutations result in a hypermethylation phenotype, disrupt TET2 function, and impair hematopoietic differentiation

Cancer-associated IDH mutations are characterized by neomorphic enzyme activity and resultant 2-hydroxyglutarate (2HG) production. Mutational and epigenetic profiling of a large acute myeloid leukemia (AML) patient cohort revealed that IDH1/2-mutant AMLs display global DNA hypermethylation and a specific hypermethylation signature. Furthermore, expression of 2HG-producing IDH alleles in cells induced global DNA hypermethylation. In the AML cohort, IDH1/2 mutations were mutually exclusive with mutations in the α-ketoglutarate-dependent enzyme TET2, and TET2 loss-of-function mutations were associated with similar epigenetic defects as IDH1/2 mutants. Consistent with these genetic and epigenetic data, expression of IDH mutants impaired TET2 catalytic function in cells. Finally, either expression of mutant IDH1/2 or Tet2 depletion impaired hematopoietic differentiation and increased stem/progenitor cell marker expression, suggesting a shared proleukemogenic effect.     

TET2 as an epigenetic master regulator for normal and malignant hematopoiesis

DNA methylation is one of the critical epigenetic modifications regulating various cellular processes such as differentiation or proliferation, and its dysregulation leads to disordered stem cell function or cellular transformation. The ten‐eleven translocation (TET) gene family, initially found as a chromosomal translocation partner in leukemia, turned out to be a key enzyme for DNA demethylation. TET genes hydroxylate 5‐methylcytosine to 5‐hydroxymethylcytosine, which is then converted to unmodified cytosine through multiple mechanisms. Somatic mutations of the TET2 gene were reported in a variety of human hematological malignancies such as leukemia, myelodysplastic syndrome, and malignant lymphoma, suggesting a critical role for TET2 in hematopoiesis. The importance of the TET‐mediated cytosine demethylation pathway is also underscored by a recurrent mutation of isocitrate dehydrogenase 1 (IDH1) and IDH2 in hematological malignancies, whose mutation inhibits TET function through a novel oncometabolite, 2‐hydroxyglutarate. Studies using mouse models revealed that TET2 is critical for the function of hematopoietic stem cells, and disruption of TET2 results in the expansion of multipotent as well as myeloid progenitors, leading to the accumulation of premalignant clones. In addition to cytosine demethylation, TET proteins are involved in chromatin modifications and other cellular processes through the interaction with O‐linked β‐N‐acetylglucosamine transferase. In summary, TET2 is a critical regulator for hematopoietic stem cell homeostasis whose functional impairment leads to hematological malignancies. Future studies will uncover the whole picture of epigenetic and signaling networks wired with TET2, which will help to develop ways to intervene in cellular pathways dysregulated by TET2 mutations.     

Role of TET2 Mutations in Myeloproliferative Neoplasms

Recently, 5-hydroxymethylcytosine (5-hmC), the 6th base of DNA, was discovered as the product of the hydroxylation of 5-methylcytosine (5-mC) by the ten-eleven translocation (TET) oncogene family members. One of them, TET oncogene family member 2 (TET2), is mutated in a variety of myeloid malignancies, including in 15% of myeloproliferative neoplasms (MPNs). Recent studies tried to go further into the biological and epigenetic function of TET2 protein and 5-hmC marks in the pathogenesis of myeloid malignancies. Although its precise function remains partially unknown, TET2 appears to be an important regulator of hematopoietic stem cell biology. In both mouse and human cells, its inactivation leads to a dramatic deregulation of hematopoiesis that ultimately triggers blood malignancies. Understanding this leukemogenic process will provide tools to develop new epigenetic therapies against blood cancers.     

Tet2 deficiency in immune cells exacerbates tumor progression by increasing angiogenesis in a lung cancer model

Immune cells harboring somatic mutations reportedly infiltrate cancer tissues in patients with solid cancers and accompanying clonal hematopoiesis. Loss-of-function TET2 mutations are frequently observed in clonal hematopoiesis in solid cancers. Here, using a mouse lung cancer model, we evaluated the activity of Tet2-deficient immune cells in tumor tissues. Myeloid-specific Tet2 deficiency enhanced tumor growth in mice relative to that seen in controls. Single-cell sequencing analysis of immune cells infiltrating tumors showed relatively high expression of S100a8/S100a9 in Tet2-deficient myeloid subclusters. In turn, treatment with S100a8/S100a9 promoted Vegfa production by cancer cells, leading to a marked increase in the tumor vasculature in Tet2-deficient mice relative to controls. Finally, treatment of Tet2-deficient mice with an antibody against Emmprin, a known S100a8/S100a9 receptor, suppressed tumor growth. These data suggest that immune cells derived from TET2-mutated clonal hematopoiesis exacerbate lung cancer progression by promoting tumor angiogenesis and may provide a novel therapeutic target for lung cancer patients with TET2-mutated clonal hematopoiesis.     

Lymphoid clonal hematopoiesis: implications for malignancy,immunity, and treatment

Clonal hematopoiesis (CH) is the age-related expansion of hematopoietic stem cell clones caused by the acquisition of somatic point mutations or mosaic chromosomal alterations (mCAs). Clonal hematopoiesis caused by somatic mutations has primarily been associated with increased risk of myeloid malignancies, while mCAs have been associated with increased risk of lymphoid malignancies. A recent study by Niroula et al. challenged this paradigm by finding a distinct subset of somatic mutations and mCAs that are associated with increased risk of lymphoid malignancy. CH driven by these mutations is termed lymphoid clonal hematopoiesis (L-CH). Unlike myeloid clonal hematopoiesis (M-CH), L-CH has the potential to originate at both stem cells and partially or fully differentiated progeny stages of maturation. In this review, we explore the definition of L-CH in the context of lymphocyte maturation and lymphoid malignancy precursor disorders, the evidence for L-CH in late-onset autoimmunity and immunodeficiency, and the development of therapy-related L-CH following chemotherapy or hematopoietic stem cell transplantation.Subject terms: Cancer genetics, Haematological cancer     

Detection of TET2 Mutation in Patients with De Novo Acute Myeloid Leukemia: A Mutation Analysis of 51 Iranian Patients

Acute myeloid leukemia (AML) is a heterogeneous clonal disease that is considered to originate from hematopoietic stem cells, which are characterized by impaired myelopoiesis and blast proliferation. TET oncogene family member 2 (TET2) mutations are frequent in myeloid malignancies and several studies have assessed the clinical importance of TET2 mutations. However, its frequency ratio has not yet been fully clarified. Method: Hence, our study was aimed to analyze TET2mut in patients with de-novo AML and their association with clinical, molecular characteristics and Nucleophosmin 1 (NPM1), Fms-like tyrosine kinase 3 (FLT3), CCAAT Enhancer Binding Protein Alpha (CEBPA) and Wilms’ tumor protein (WT1) gene expression. Fifty-one Iranian patients were screened by polymerase chain reaction (PCR) and direct sequencing to evaluate TET2 mutations frequency. Results: Out of all patients, 10 mutations in 8 patients (15.6%) were detected and closely associated with higher age and higher hemoglobin levels (p-value <0.05). Although FLT3, NPM1 and CEBPA gene expression did not show any significant correlation with TET2mut, cytogenetically normal acute myeloid leukemia (CN-AML)  patients appear to bear TET2mut more frequently with lower platelet counts. Monocyte-lineages leukemia has seemed to be more linked with TET2mut in these patients. Conclusion: Our study suggests the frequency of TET2mut in our study (15.6%) is in line with previous studies and reveals the critical role of TET2 in myeloid transformation, especially in leukemia with monocytic subtypes.     

Identification of hematopoietic stem/progenitor cells: strength and drawbacks of functional assays

A major challenge in hematopoiesis is to conceive assays that could bring useful insights into experimental and clinical hematology. This means identifying separately the various classes of hematopoietic progenitors that are produced sequentially during the progression from stem cells to differentiated functional cells. Standardized short-term colony assays easily quantify lineage-committed myeloid precursors, but identification of primitive cells, which have both the ability to repopulate durably myeloid and lymphoid lineages and perhaps to self-renew, still depends on in vivo assays. Whatever the assay, two important requisites have to be solved: one is the definition of appropriate read-outs that will depend solely on the function of these cells, and the second is to evaluate precisely their numbers and proliferative potential in quantitative assays. When evaluating hematopoiesis, three parameters have to be taken into account: (1) the lack of reliable correlation between the phenotype of a given cell and its function. This is especially problematic in post-transplantation situations where cells from transplanted animals are analysed; (2) functionally heterogeneous cells are identified in a single assay; and (3) ontogeny-related changes in hematopoietic cell proliferation and self-renewal that, in human beings, hampers the exploration of adult stem cells. Nevertheless, years of progress in the manipulation of hematopoietic stem cells have recently resulted in the purification of a cell subset that repopulates irradiated recipients with absolute efficiency.     

Autocrine transforming growth factor-beta regulation of hematopoiesis: many outcomes that depend on the context

Transforming growth factor-beta (TGF-beta) is a pleiotropic regulator of all stages of hematopoieis. The three mammalian isoforms (TGF-beta1, 2 and 3) have distinct but overlapping effects on hematopoiesis. Depending on the differentiation stage of the target cell, the local environment and the concentration and isoform of TGF-beta, in vivo or in vitro, TGF-beta can be pro- or antiproliferative, pro- or antiapoptotic, pro- or antidifferentiative and can inhibit or increase terminally differentiated cell function. TGF-beta is a major regulator of stem cell quiescence, at least in vitro. TGF-beta can act directly or indirectly through effects on the bone marrow microenvironment. In addition, paracrine and autocrine actions of TGF-beta have overlapping but distinct regulatory effects on hematopoietic stem/progenitor cells. Since TGF-beta can act in numerous steps in the hematopoietic cascade, loss of function mutations in hematopoeitic stem cells (HSC) have different effects on hematopoiesis than transient blockade of autocrine TGF-beta1. Transient neutralization of autocrine TGF-beta in HSC has therapeutic potential. In myeloid and erythroid leukemic cells, autocrine TGF-beta1 and/or its Smad signals controls the ability of these cells to respond to various differentiation inducers, suggesting that this pathway plays a role in determining the cell fate of leukemic cells.     

骨髓增生异常综合征最新研究进展:第54届美国血液学会年会报道

 【摘要】　骨髓增生异常综合征（MDS）是一组起源于造血干／祖细胞水平损伤而产生的获得性克隆性疾病,以无效造血和易转化为急性白血病为特点。第54届美国血液学会（ASH）报道了大量MDS发病相关分子机制（如ASXL1、Dido1突变等）以及其与患者预后之间的关系的一系列报道。治疗方面,分子机制的进展使临床新药不断出现,但造血干细胞移植仍是目前MDS的唯一根治方法,大会也报道了造血干细胞移植的最新进展。     

Telomere length dynamics in normal hematopoiesis and in disease states characterized by increased stem cell turnover.

Telomeres both reflect and limit the replicative lifespan of normal somatic cells. Immature sub-populations of human CD34+38- hematopoietic stem cell (HSC) can be identified in vitro based on their growth kinetics and telomere length. Fluorescence in situ hybridization and flow cytometry (flow-FISH) has been used to characterize telomere length dynamics as a surrogate marker for HSC turnover in vivo. Investigations in normal steady-state hematopoiesis provided the basis for follow-up studies in model scenarios characterized by increased HSC turnover. Disorders with underlying malignant transformation of HSC (e.g., chronic myeloid leukemia (CML)) can be discriminated from disease states with increased HSC turnover rates secondary to depletion of the stem cell compartment, for example, as in defined bone marrow failure syndromes. In some of these model scenarios, the degree of telomere shortening can be correlated with disease duration, disease stage and severity as well as with response to disease-modifying treatment strategies. Whether increased telomere shortening represents a causal link between HSC turnover, replicative senescence and/or the induction of genetic instability in acquired HSC disorders remains to be shown. However, data from congenital disorders, like dyskeratosis congenita (DKC), suggest that disturbed telomere maintenance may play a role for replicative exhaustion of the HSC pool in vivo.     

Clonal hematopoiesis and associated diseases: A review of recent findings

Recent genome-wide studies have revealed that aging or chronic inflammation can cause clonal expansion of cells in normal tissues. Clonal hematopoiesis has been the most intensively studied form of clonal expansion in the last decade. Clonal hematopoiesis of indeterminate potential (CHIP) is an age-related phenomenon observed in elderly individuals with no history of hematological malignancy. The most frequently mutated genes in CHIP are DNMT3A, TET2, and ASXL1, which are associated with initiation of leukemia. Importantly, CHIP has been the focus of a number of studies because it is an independent risk factor for myeloid malignancy, cardiovascular disease (CVD), and all-cause mortality. Animal models recapitulating human CHIP revealed that CHIP-associated mutations alter the number and function of hematopoietic stem and progenitor cells (HSPCs) and promote leukemic transformation. Moreover, chronic inflammation caused by infection or aging confers a fitness advantage to the CHIP-associated mutant HSPCs. Myeloid cells, such as macrophages with a CHIP-associated mutation, accelerate chronic inflammation and are associated with increased levels of inflammatory cytokines. This positive feedback loop between CHIP and chronic inflammation promotes development of atherosclerosis and chronic heart failure and thereby increases the risk for CVD. Notably, HSPCs with a CHIP-associated mutation may alter not only innate but also acquired immune cells. This suggests that CHIP is involved in the development of solid cancers or immune disorders, such as aplastic anemia. In this review, we provide an overview of recent findings on CHIP. We also discuss potential interventions for treating CHIP and preventing myeloid transformation and CVD progression.     

骨髓增生异常综合征最新研究进展

骨髓增生异常综合征（MDS）是一组由于造血干/祖细胞水平损伤而产生的获得性克隆性疾病,以无效造血和易转化成急性白血病为特点。近年来其分子发病机制、临床新药及新的联合用药、免疫治疗、造血干细胞移植方面都发展迅速。文章结合第57届美国血液学会年会相关报道,着重就近两年关于 MDS 研究的几个亮点进行综述。     

The forgotten myeloproliferative disorder: myeloid metaplasia

Myelofibrosis with myeloid metaplasia is a hematologic disorder currently classified with polycythemia vera and essential thrombocythemia as a chronic myeloproliferative disease. The median age at diagnosis is 60 years, and more than 90% of patients are diagnosed after age 40 years. Clinical manifestations include massive splenomegaly, progressive anemia, profound constitutional symptoms, and extramedullary hematopoiesis. The diagnosis is confirmed by bone marrow examination after other causes of myelofibrosis are ruled out. Median survival is 5 years and causes of death include leukemic transformation. Prognosis is adversely affected by the presence of anemia (hemoglobin <10 g/dl), leukopenia or leukocytosis (white blood cells >30,000/ micro l), circulating blasts, and hypercatabolic symptoms. Conventional treatment is palliative and does not improve survival. In this regard, androgen preparations, corticosteroids, and erythropoietin are useful for the treatment of disease-associated anemia. Symptomatic splenomegaly is best managed by cytoreductive therapy or surgical removal. Radiation therapy is most useful in the treatment of nonhepatosplenic extramedullary hematopoiesis. New treatment approaches include the use of thalidomide alone or in combination with prednisone and hematopoietic stem cell transplantation.     

Retinoic acid induction of CD38 antigen expression on normal and leukemic human myeloid cells: relationship with cell differentiation

Differentiation in the hematopoietic system involves, among other changes, altered expression of antigens, including the CD34 and CD38 surface antigens. In normal hematopoiesis, the most immature stem cells have the CD34+CD34 -phenotype. In acute myeloid leukemia (AML), although blasts from most patients are CD38+, some are CD38 -. AML blasts are blocked at early stages of differentiation; in some leukemic cells this block can be overcome by a variety of agents, including retinoids, that induce maturation into macrophages and granulocytes both in vitro and in vivo . Retinoids can also induce CD38 expression. In the present study, we investigated the relationship between induction of CD38 expression and induction of myeloid differentiation by retinoic acid (RA) in normal and leukemic human hematopoietic cells. In the promyelocytic (PML) CD34 -cell lines, HL60 and CB-1, as well as in normal CD34+CD34 -hematopietic progenitor cells RA induced both CD38 expression as well as morphological and functional myeloid differentiation that resulted in loss of self-renewal. In contrast, in the myeloblastic CD34+ leukemic cell lines, ML-1 and KG-1a, as well as in primary cultures of cells derived from CD34+-AML (M 0 and M 1 ) patients, RA caused an increase in CD38+ that was not associated with significant differentiation. Yet, long exposure of ML-1, but not KG-1, cells to RA resulted in loss of self-renewal. The results suggest that while in normal hematopoietic cells and in PML CD34 -cells induction of CD38 antigen expression by RA results in terminal differentiation along the myeloid lineage, in early myeloblastic leukemic CD34+ cells, induction of CD38 and differentiation are not functionally related. Since, several lines of evidence suggest that the CD38 -cells are the targets of leukemic transformation, transition of these cells into CD38+ phenotype by RA or other drugs may have therapeutic effect, either alone or in conjunction with cytotoxic drugs, regardless the ability of the cells to undergo differentiation.