Loss of TET2 has dual roles in murine myeloproliferative neoplasms: disease sustainer and disease accelerator

Increased CXCL10 (IP-10) is associated with advanced myeloproliferative neoplasms and its loss dampens erythrocytosis in mouse models

Key studies in pre-leukemic disorders have linked increases in pro-inflammatory cytokines with accelerated phases of the disease, but the precise role of the cellular microenvironment in disease initiation and evolution remains poorly understood. In myeloproliferative neoplasms (MPNs), higher levels of specific cytokines have been previously correlated with increased disease severity (tumor necrosis factor-alpha [TNF-α], interferon gamma-induced protein-10 [IP-10 or CXCL10]) and decreased survival (interleukin 8 [IL-8]). Whereas TNF-α and IL-8 have been studied by numerous groups, there is a relative paucity of studies on IP-10 (CXCL10). Here we explore the relationship of IP-10 levels with detailed genomic and clinical data and undertake a complementary cytokine screen alongside functional assays in a wide range of MPN mouse models. Similar to patients, levels of IP-10 were increased in mice with more severe disease phenotypes (e.g., JAK2V617F/V617F TET2-/- double-mutant mice) compared with those with less severe phenotypes (e.g., CALRdel52 or JAK2+/V617F mice) and wild-type (WT) littermate controls. Although exposure to IP-10 did not directly alter proliferation or survival in single hematopoietic stem cells (HSCs) in vitro, IP-10-/- mice transplanted with disease-initiating HSCs developed an MPN phenotype more slowly, suggesting that the effect of IP-10 loss was noncell-autonomous. To explore the broader effects of IP-10 loss, we crossed IP-10-/- mice into a series of MPN mouse models and showed that its loss reduces the erythrocytosis observed in mice with the most severe phenotype. Together, these data point to a potential role for blocking IP-10 activity in the management of MPNs.

Jak2V617F Reversible Activation Shows Its Essential Requirement in Myeloproliferative Neoplasms

Gain-of-function mutations activating JAK/STAT signaling are seen in the majority of patients with myeloproliferative neoplasms (MPN), most commonly JAK2V617F. Although clinically approved JAK inhibitors improve symptoms and outcomes in MPNs, remissions are rare, and mutant allele burden does not substantively change with chronic therapy. We hypothesized this is due to limitations of current JAK inhibitors to potently and specifically abrogate mutant JAK2 signaling. We therefore developed a conditionally inducible mouse model allowing for sequential activation, and then inactivation, of Jak2V617F from its endogenous locus using a combined Dre-rox/Cre-lox dual-recombinase system. Jak2V617F deletion abrogates MPN features, induces depletion of mutant-specific hematopoietic stem/progenitor cells, and extends overall survival to an extent not observed with pharmacologic JAK inhibition, including when cooccurring with somatic Tet2 loss. Our data suggest JAK2V617F represents the best therapeutic target in MPNs and demonstrate the therapeutic relevance of a dual-recombinase system to assess mutant-specific oncogenic dependencies in vivo.

SIGNIFICANCE

Current JAK inhibitors to treat myeloproliferative neoplasms are ineffective at eradicating mutant cells. We developed an endogenously expressed Jak2V617F dual-recombinase knock-in/knock-out model to investigate Jak2V617F oncogenic reversion in vivo. Jak2V617F deletion abrogates MPN features and depletes disease-sustaining MPN stem cells, suggesting improved Jak2V617F targeting offers the potential for greater therapeutic efficacy. See related commentary by Celik and Challen, p. 701. This article is featured in Selected Articles from This Issue, p. 695.

Noncoding rules of survival: epigenetic regulation of normal and malignant hematopoiesis

Hematopoiesis is an essential process for organismal development and homeostasis. Epigenetic regulation of gene expression is critical for stem cell self-renewal and differentiation in normal hematopoiesis. Increasing evidence shows that disrupting the balance between self-renewal and cell fate decisions can give rise to hematological diseases such as bone marrow failure and leukemia. Consequently, next-generation sequencing studies have identified various aberrations in histone modifications, DNA methylation, RNA splicing, and RNA modifications in hematologic diseases. Favorable outcomes after targeting epigenetic regulators during disease states have further emphasized their importance in hematological malignancy. However, these targeted therapies are only effective in some patients, suggesting that further research is needed to decipher the complexity of epigenetic regulation during hematopoiesis. In this review, an update on the impact of the epigenome on normal hematopoiesis, disease initiation and progression, and current therapeutic advancements will be discussed.

Mutations, inflammation and phenotype of myeloproliferative neoplasms

Knowledge on the myeloproliferative neoplasms (MPNs) - polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF) - has accumulated since the discovery of the JAK/STAT-activating mutations associated with MPNs: JAK2V617F, observed in PV, ET and PMF; and the MPL and CALR mutations, found in ET and PMF. The intriguing lack of disease specificity of these mutations, and of the chronic inflammation associated with MPNs, triggered a quest for finding what precisely determines that MPN patients develop a PV, ET or PMF phenoptype. The mechanisms of action of MPN-driving mutations, and concomitant mutations (ASXL1, DNMT3A, TET2, others), have been extensively studied, as well as the role played by these mutations in inflammation, and several pathogenic models have been proposed. In parallel, different types of drugs have been tested in MPNs (JAK inhibitors, interferons, hydroxyurea, anagrelide, azacytidine, combinations of those), some acting on both JAK2 and inflammation. Yet MPNs remain incurable diseases. This review aims to present current, detailed knowledge on the pathogenic mechanisms specifically associated with PV, ET or PMF that may pave the way for the development of novel, curative therapies.

PELI1 and EGFR cooperate to promote breast cancer metastasis

Pellino-1 (PELI1) is an E3 ubiquitin ligase acting as a key regulator for the inflammation and autoimmunity via the ubiquitination of the substrate proteins. There is increasing evidence to support that PELI1 functions as an oncoprotein in tumorigenesis and metastasis. However, the molecular mechanism underlying the high expression and oncogenic roles of PELI1 in cancers remains limited. Herein, we revealed a novel regulation mechanism by which PELI1 and EGFR cooperate to promote breast cancer metastasis. EGFR is positively correlated with PELI1 expression in breast cancers, and its activation led to the phosphorylation of PELI1 at Tyr154 and Thr264, which subsequently activated its E3 ubiquitin ligase. Simultaneously, PELI1 physically interacted with and enhanced the stability of EGFR via the K63-linked polyubiquitination in reverse. The co-inhibition of the PELI1-EGFR showed synergetic effect to repress breast cancer metastasis. Furthermore, we identified a compound S62 as a small molecule disruptor of PELI1/EGFR that effectively repressed breast cancer metastasis. Our study not only uncovered the emerging roles of PELI1/EGFR interaction in the progression of breast cancer, but also provided an effective strategy for the inhibition of metastasis in breast cancer.

Epigenetic Modification of Cytosines in Hematopoietic Differentiation and Malignant Transformation

The mammalian DNA methylation landscape is established and maintained by the combined activities of the two key epigenetic modifiers, DNA methyltransferases (DNMT) and Ten-eleven-translocation (TET) enzymes. Once DNMTs produce 5-methylcytosine (5mC), TET proteins fine-tune the DNA methylation status by consecutively oxidizing 5mC to 5-hydroxymethylcytosine (5hmC) and further oxidized derivatives. The 5mC and oxidized methylcytosines are essential for the maintenance of cellular identity and function during differentiation. Cytosine modifications with DNMT and TET enzymes exert pleiotropic effects on various aspects of hematopoiesis, including self-renewal of hematopoietic stem/progenitor cells (HSPCs), lineage determination, differentiation, and function. Under pathological conditions, these enzymes are frequently dysregulated, leading to loss of function. In particular, the loss of DNMT3A and TET2 function is conspicuous in diverse hematological disorders, including myeloid and lymphoid malignancies, and causally related to clonal hematopoiesis and malignant transformation. Here, we update recent advances in understanding how the maintenance of DNA methylation homeostasis by DNMT and TET proteins influences normal hematopoiesis and malignant transformation, highlighting the potential impact of DNMT3A and TET2 dysregulation on clonal dominance and evolution of pre-leukemic stem cells to full-blown malignancies. Clarification of the normal and pathological functions of DNA-modifying epigenetic regulators will be crucial to future innovations in epigenetic therapies for treating hematological disorders.

Role of TET dioxygenases in the regulation of both normal and pathological hematopoiesis

The family of ten-eleven translocation dioxygenases (TETs) consists of TET1, TET2, and TET3. Although all TETs are expressed in hematopoietic tissues, only TET2 is commonly found to be mutated in age-related clonal hematopoiesis and hematopoietic malignancies. TET2 mutation causes abnormal epigenetic landscape changes and results in multiple stages of lineage commitment/differentiation defects as well as genetic instability in hematopoietic stem/progenitor cells (HSPCs). TET2 mutations are founder mutations (first hits) in approximately 40–50% of cases of TET2-mutant (TET2^MT) hematopoietic malignancies and are later hits in the remaining cases. In both situations, TET2^MT collaborates with co-occurring mutations to promote malignant transformation. In TET2^MT tumor cells, TET1 and TET3 partially compensate for TET2 activity and contribute to the pathogenesis of TET2^MT hematopoietic malignancies. Here we summarize the most recent research on TETs in regulating of both normal and pathogenic hematopoiesis. We review the concomitant mutations and aberrant signals in TET2^MT malignancies. We also discuss the molecular mechanisms by which concomitant mutations and aberrant signals determine lineage commitment in HSPCs and the identity of hematopoietic malignancies. Finally, we discuss potential strategies to treat TET2^MT hematopoietic malignancies, including reverting the methylation state of TET2 target genes and targeting the concomitant mutations and aberrant signals.

Mutational landscape of blast phase myeloproliferative neoplasms (MPN-BP) and antecedent MPN

Myeloproliferative neoplasms (MPN) have an inherent tendency to evolve to the blast phase (BP), characterized by ≥20% myeloblasts in the blood or bone marrow. MPN-BP portends a dismal prognosis and currently, effective treatment modalities are scarce, except for allogeneic hematopoietic stem cell transplantation (allo-HSCT) in selected patients, particularly those who achieve complete/partial remission. The mutational landscape of MPN-BP differs from de novo acute myeloid leukemia (AML) in several key aspects, such as significantly lower frequencies of FLT3 and DNMT3A mutations, and higher incidence of IDH1/2 and TP53 in MPN-BP. Herein, we comprehensively review the impact of the three signaling driver mutations (JAK2 V617F, CALR exon 9 indels, MPL W515K/L) that constitutively activate the JAK/STAT pathway, and of the other somatic non-driver mutations (epigenetic, mRNA splicing, transcriptional regulators, and mutations in signal transduction genes) that cooperatively or independently promote MPN progression and leukemic transformation. The MPN subtype, harboring two or more high-molecular risk (HMR) mutations (epigenetic regulators and mRNA splicing factors) and "triple-negative" PMF are among the critical factors that increase risk of leukemic transformation and shorten survival. Primary myelofibrosis (PMF) is the most aggressive MPN; and polycythemia vera (PV) and essential thrombocythemia (ET) are relatively indolent subtypes. In PV and ET, mutations in splicing factor genes are associated with progression to myelofibrosis (MF), and in ET, TP53 mutations predict risk for leukemic transformation. The advent of targeted next-generation sequencing and improved prognostic scoring systems for PMF inform decisions regarding allo-HSCT. The emergence of treatments targeting mutant enzymes (e.g., IDH1/2 inhibitors) or epigenetic pathways (BET and LSD1 inhibitors) along with new insights into the mechanisms of leukemogenesis will hopefully lead the way to superior management strategies and outcomes of MPN-BP patients.

Philadelphia-positive acute lymphoblastic leukemia in a case of MPL p.(W515L) variant essential thrombocythemia: case report and literature review

Acute lymphoblastic leukemia (ALL) arising in preexisting myeloproliferative neoplasms (MPN) is rare with historical cases unable to differentiate between concomitant malignancies or leukemic transformation. Here, we report a case of patient with Philadelphia positive B lymphoblastic leukemia (Ph+ALL) who developed from MPL-mutated essential thrombocythemia (ET) 13 years after initial presentation. Molecular studies showed the discrepancy between the high percentage of lymphocyte blasts (91%) and the low MPL p.(W515L) variant allele frequency (2.59%) at diagnosis in the bone marrow, indicating that the Ph+ALL clone did not originate from the ET clone carrying the MPL p.(W515L) variant. After the treatment of a new tyrosine kinase inhibitor flumatinib and prednisolone, cytogenetic and molecular remission had been achieved rapidly and followed by the recovery of original ET manifestation. Although relapsed eventually, this is still a very rare case of simultaneous presence of two cytogenetics abnormalities and evolution of MPL p.(W515L) variant ET to Ph+ALL and may provide evidence to illustrate the clonal relationship of MPN and post-MPN ALL.

Molecular pathogenesis of the myeloproliferative neoplasms

The Philadelphia negative myeloproliferative neoplasms (MPN) compromise a heterogeneous group of clonal myeloid stem cell disorders comprising polycythaemia vera, essential thrombocythaemia and primary myelofibrosis. Despite distinct clinical entities, these disorders are linked by morphological similarities and propensity to thrombotic complications and leukaemic transformation. Current therapeutic options are limited in disease-modifying activity with a focus on the prevention of thrombus formation. Constitutive activation of the JAK/STAT signalling pathway is a hallmark of pathogenesis across the disease spectrum with driving mutations in JAK2, CALR and MPL identified in the majority of patients. Co-occurring somatic mutations in genes associated with epigenetic regulation, transcriptional control and splicing of RNA are variably but recurrently identified across the MPN disease spectrum, whilst epigenetic contributors to disease are increasingly recognised. The prognostic implications of one MPN diagnosis may significantly limit life expectancy, whilst another may have limited impact depending on the disease phenotype, genotype and other external factors. The genetic and clinical similarities and differences in these disorders have provided a unique opportunity to understand the relative contributions to MPN, myeloid and cancer biology generally from specific genetic and epigenetic changes. This review provides a comprehensive overview of the molecular pathophysiology of MPN exploring the role of driver mutations, co-occurring mutations, dysregulation of intrinsic cell signalling, epigenetic regulation and genetic predisposing factors highlighting important areas for future consideration.

The Prevalence of TET2 Gene Mutations in Patients with BCR-ABL-Negative Myeloproliferative Neoplasms (MPN): A Systematic Review and Meta-Analysis

Multiple recurrent somatic mutations have recently been identified in association with myeloproliferative neoplasms (MPN). This meta-analysis aims to assess the pooled prevalence of TET2 gene mutations among patients with MPN. Six databases (PubMed, Scopus, ScienceDirect, Google Scholar, Web of Science and Embase) were searched for relevant studies from inception till September 2020, without language restrictions. The eligibility criteria included BCR-ABL-negative MPN adults with TET2 gene mutations. A random-effects model was used to estimate the pooled prevalence with 95% confidence intervals (CIs). Subgroup analyses explored results among different continents and countries, WHO diagnostic criteria, screening methods and types of MF. Quality assessment was undertaken using the Joanna Briggs Institute critical appraisal tool. The study was registered with PROSPERO (CRD42020212223). Thirty-five studies were included (n = 5121, 47.1% female). Overall, the pooled prevalence of TET2 gene mutations in MPN patients was 15.5% (95% CI: 12.1-19.0%, I2 = 94%). Regional differences explained a substantial amount of heterogeneity. The prevalence of TET2 gene mutations among the three subtypes PV, ET and MF were 16.8%, 9.8% and 15.7%, respectively. The quality of the included studies was determined to be moderate-high among 83% of the included studies. Among patients with BCR-ABL-negative MPN, the overall prevalence of TET2 gene mutations was 15.5%.

Recent Advances in the Use of Molecular Analyses to Inform the Diagnosis and Prognosis of Patients with Polycythaemia Vera

Genetic studies in the past decade have improved our understanding of the molecular basis of the BCR-ABL1-negative myeloproliferative neoplasm (MPN) polycythaemia vera (PV). Such breakthroughs include the discovery of the JAK2V617F driver mutation in approximately 95% of patients with PV, as well as some very rare cases of familial hereditary MPN caused by inherited germline mutations. Patients with PV often progress to fibrosis or acute myeloid leukaemia, both associated with very poor clinical outcome. Moreover, thrombosis and major bleeding are the principal causes of morbidity and mortality. As a result of increasingly available and economical next-generation sequencing technologies, mutational studies have revealed the prognostic relevance of a few somatic mutations in terms of thrombotic risk and risk of transformation, helping to improve the risk stratification of patients with PV. Finally, knowledge of the molecular basis of PV has helped identify targets for directed therapy. The constitutive activation of the tyrosine kinase JAK2 is targeted by ruxolitinib, a JAK1/JAK2 tyrosine kinase inhibitor for PV patients who are resistant or intolerant to cytoreductive treatment with hydroxyurea. Other molecular mechanisms have also been revealed, and numerous agents are in various stages of development. Here, we will provide an update of the recent published literature on how molecular testing can improve the diagnosis and prognosis of patients with PV and present recent advances that may have prognostic value in the near future.

Lessons from mouse models of MPN

Over the past decades, a variety of MPN mouse models have been developed to express in HSC the main mutations identified in patients: JAK2^V617F, CALRdel52 or ins5 and MPL^W515L. These models mimic quite faithfully human PV or ET with their natural evolutions into MF and their hemostasis complications, demonstrating the driver function of these mutations in MPN. Here, we review these models and show how they have improved our general understanding of MPN regarding (1) the mechanisms of fibrosis, thrombosis/hemorrhages and disease initiation, (2) the roles of additional mutations and signaling pathways in disease progression and (3) the preclinical development of novel therapies. We also address controversial results between these models and remind how these models may differ from human MPN onset and also how basically mice are not humans, encouraging caution when one draw lessons from mice to humans. Furthermore, the contribution of germline genetic predisposition, HSC and niche aging, metabolic, oxidative, replicative or genotoxic stress, inflammation, immune escape and additional mutations need to be considered in further investigations to encompass the full complexity of human MPN in mice.

Epigenetic Dysregulation of Myeloproliferative Neoplasms

This article reviews the genetic data on epigenetic modifying mutations in myeloproliferative neoplasms and their clinical implications, preclinical studies exploring our current understanding of how mutations in epigenetic modifying proteins cooperate with myeloproliferative neoplasms drivers to promote disease progression, and recent advances in novel therapeutics supporting the role of targeting epigenetic pathways to treat fibrotic progression.

Impact of Integrated Genetic Information on Diagnosis and Prognostication for Myeloproliferative Neoplasms in the Next-Generation Sequencing Era

Since next-generation sequencing has been widely used in clinical laboratories, the diagnosis and risk stratification of hematologic malignancies are greatly dependent on genetic aberrations. In this study, we analyzed the genomic landscapes of 200 patients with myeloproliferative neoplasms (MPNs) and evaluated the impact of the genomic landscape on diagnosis and risk stratification. Mutations in JAK2, CALR and MPL were detected in 76.4% of MPNs. The proportion of patients with clonal genetic markers increased up to 86.4% when all detectable genetic aberrations were included. Significant co-occurring genetic aberrations potentially associated with phenotype and/or disease progression, including those in JAK2/SF3B1 and TP53/del(13q), del(5q), −7/del(7q) and complex karyotypes, were detected. We also identified genetic aberrations associated with patient outcomes: TP53 and −7/del(7q) were associated with an inferior chance of survival, RUNX1, TP53 and IDH1/2 were associated with leukemic transformation and SF3B1, IDH1/2, ASXL1 and del(20q) were associated with fibrotic progression. We compared risk stratification systems and found that mutation-enhanced prognostic scoring systems could identify lower risk polycythemia vera, essential thrombocythemia and higher risk primary myelofibrosis. Furthermore, the new risk stratification systems showed a better predictive capacity for patient outcome. These results collectively indicate that integrated genetic information can enhance diagnosis and prognostication in patients with myeloproliferative neoplasms.

Murine Modeling of Myeloproliferative Neoplasms

Myeloproliferative neoplasms, such as polycythemia vera, essential thrombocythemia, and primary myelofibrosis, are bone marrow disorders that result in the overproduction of mature clonal myeloid elements. Identification of recurrent genetic mutations has been described and aid in diagnosis and prognostic determination. Mouse models of these mutations have confirmed the biologic significance of these mutations in myeloproliferative neoplasm disease biology and provided greater insights on the pathways that are dysregulated with each mutation. The models are useful tools that have led to preclinical testing and provided data as validation for future myeloproliferative neoplasm clinical trials.

Mouse models of myeloproliferative neoplasms for pre-clinical testing of novel therapeutic agents

Myeloproliferative neoplasms (MPN), are clonal hematopoietic stem cell (HSC) disorders driven by gain-of-function mutations in JAK2 (JAK2-V617F), CALR or MPL genes. MPN treatment options currently mainly consist of cytoreductive therapy with hydroxyurea and JAK2 inhibitors such as ruxolitinib and fedratinib. Pegylated interferon-alpha can induce complete molecular remission (CMR) in some MPN patients when applied at early stages of disease. The ultimate goal of modern MPN treatment is to develop novel therapies that specifically target mutant HSCs in MPN and consistently induce CMR. Basic research has identified a growing number of candidate drugs with promising effects in vitro. A first step on the way to developing these compounds into drugs approved for treatment of MPN patients often consists of examining the effects in vivo using pre-clinical mouse models of MPN. Here we review the current state of MPN mouse models and the experimental setup for their optimal use in drug testing. In addition to novel compounds, combinatorial therapeutic approaches are often considered for the treatment of MPN. Optimized and validated mouse models can provide an efficient way to rapidly assess and select the most promising combinations and thereby contribute to accelerating the development of novel therapies of MPN.

Metabolic Vulnerabilities and Epigenetic Dysregulation in Myeloproliferative Neoplasms

The Janus kinase 2 (JAK2)-driven myeloproliferative neoplasms (MPNs) are associated with clonal myelopoiesis, elevated risk of death due to thrombotic complications, and transformation to acute myeloid leukemia (AML). JAK2 inhibitors improve the quality of life for MPN patients, but these approved therapeutics do not readily reduce the natural course of disease or antagonize the neoplastic clone. An understanding of the molecular and cellular changes requisite for MPN development and progression are needed to develop improved therapies. Recently, murine MPN models were demonstrated to exhibit metabolic vulnerabilities due to a high dependence on glucose. Neoplastic hematopoietic progenitor cells in these mice express elevated levels of glycolytic enzymes and exhibit enhanced levels of glycolysis and oxidative phosphorylation, and the disease phenotype of these MPN model mice is antagonized by glycolytic inhibition. While all MPN-driving mutations lead to aberrant JAK2 activation, these mutations often co-exist with mutations in genes that encode epigenetic regulators, including loss of function mutations known to enhance MPN progression. In this perspective we discuss how altered activity of epigenetic regulators (e.g., methylation and acetylation) in MPN-driving stem and progenitor cells may alter cellular metabolism and contribute to the MPN phenotype and progression of disease. Specific metabolic changes associated with epigenetic deregulation may identify patient populations that exhibit specific metabolic vulnerabilities that are absent in normal hematopoietic cells, and thus provide a potential basis for the development of more effective personalized therapeutic approaches.

TET2: A cornerstone in normal and malignant hematopoiesis

Regulation of genome‐wide DNA methylation is fundamental for a variety of biological processes such as mammalian development, stem cell function, cellular proliferation/differentiation, and oncogenesis. Among the regulators of DNA methylation, ten‐eleven translocation 2 (TET2) is one of the most frequently mutated genes in clonal hematopoiesis of indeterminate potential and in various hematological malignancies, underscoring a pivotal role for TET2 in blood homeostasis and hematopoietic transformation. TET2 oxidizes methylated cytosines to further modify cytosines, which behave as intermediates in active/passive DNA demethylation processes. TET2 itself associates with histone modifiers, thereby regulating histone modifications and expression of target genes. A number of studies have reported pleiotropic effects of TET2 on hematopoietic stem cell self‐renewal, hematopoietic differentiation, genome instability and inflammatory response. Recent single‐cell genomics studies have identified gene promoters as well as transcription factor binding sites as TET2‐targeted genetic loci in which disruption of DNA methylation can fundamentally modify hematopoietic differentiation and promote leukemogenesis. TET2 mutations show convergent cooperativity with other disease alleles in signaling molecules, epigenetic modifiers, and spliceosome factors in hematopoietic transformation. Future studies focusing on the molecular basis of stem cell and immune regulation by TET2 loss will further deepen our understanding of the entire landscape of pathophysiology and molecular vulnerabilities of TET2‐mutated hematological malignancies.

Germline TET2 loss of function causes childhood immunodeficiency and lymphoma

Molecular dissection of inborn errors of immunity can help to elucidate the nonredundant functions of individual genes. We studied 3 children with an immune dysregulation syndrome of susceptibility to infection, lymphadenopathy, hepatosplenomegaly, developmental delay, autoimmunity, and lymphoma of B-cell (n = 2) or T-cell (n = 1) origin. All 3 showed early autologous T-cell reconstitution following allogeneic hematopoietic stem cell transplantation. By whole-exome sequencing, we identified rare homozygous germline missense or nonsense variants in a known epigenetic regulator of gene expression: ten-eleven translocation methylcytosine dioxygenase 2 (TET2). Mutated TET2 protein was absent or enzymatically defective for 5-hydroxymethylating activity, resulting in whole-blood DNA hypermethylation. Circulating T cells showed an abnormal immunophenotype including expanded double-negative, but depleted follicular helper, T-cell compartments and impaired Fas-dependent apoptosis in 2 of 3 patients. Moreover, TET2-deficient B cells showed defective class-switch recombination. The hematopoietic potential of patient-derived induced pluripotent stem cells was skewed toward the myeloid lineage. These are the first reported cases of autosomal-recessive germline TET2 deficiency in humans, causing clinically significant immunodeficiency and an autoimmune lymphoproliferative syndrome with marked predisposition to lymphoma. This disease phenotype demonstrates the broad role of TET2 within the human immune system.

Use of Interferon Alfa in the Treatment of Myeloproliferative Neoplasms: Perspectives and Review of the Literature

Interferon alfa was first used in the treatment of myeloproliferative neoplasms (MPNs) over 30 years ago. However, its initial use was hampered by its side effect profile and lack of official regulatory approval for MPN treatment. Recently, there has been renewed interest in the use of interferon in MPNs, given its potential disease-modifying effects, with associated molecular and histopathological responses. The development of pegylated formulations and, more recently, ropeginterferon alfa-2b has resulted in improved tolerability and further expansion of interferon's use. We review the evolving clinical use of interferon in essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis (MF). We discuss interferon's place in MPN treatment in the context of the most recent clinical trial results evaluating interferon and its pegylated formulations, and its role in special populations such as young and pregnant MPN patients. Interferon has re-emerged as an important option in MPN patients, with future studies seeking to re-establish its place in the existing treatment algorithm for MPN, and potentially expanding its use for novel indications and combination therapies.

Calreticulin haploinsufficiency augments stem cell activity and is required for onset of myeloproliferative neoplasms in mice

Mutations in JAK2, myeloproliferative leukemia virus (MPL), or calreticulin (CALR) occur in hematopoietic stem cells (HSCs) and are detected in more than 80% of patients with myeloproliferative neoplasms (MPNs). They are thought to play a driver role in MPN pathogenesis via autosomal activation of the JAK-STAT signaling cascade. Mutant CALR binds to MPL, activates downstream MPL signaling cascades, and induces essential thrombocythemia in mice. However, embryonic lethality of Calr-deficient mice precludes determination of a role for CALR in hematopoiesis. To clarify the role of CALR in normal hematopoiesis and MPN pathogenesis, we generated hematopoietic cell-specific Calr-deficient mice. CALR deficiency had little effect on the leukocyte count, hemoglobin levels, or platelet count in peripheral blood. However, Calr-deficient mice showed some hematopoietic properties of MPN, including decreased erythropoiesis and increased myeloid progenitor cells in the bone marrow and extramedullary hematopoiesis in the spleen. Transplantation experiments revealed that Calr haploinsufficiency promoted the self-renewal capacity of HSCs. We generated CALRdel52 mutant transgenic mice with Calr haploinsufficiency as a model that mimics human MPN patients and found that Calr haploinsufficiency restored the self-renewal capacity of HSCs damaged by CALR mutations. Only recipient mice transplanted with Lineage-Sca1+c-kit+ cells harboring both CALR mutation and Calr haploinsufficiency developed MPN in competitive conditions, showing that CALR haploinsufficiency was necessary for the onset of CALR-mutated MPNs.

Neoplastic fibrocytes play an essential role in bone marrow fibrosis in Jak2V617F-induced primary myelofibrosis mice

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm (MPN) characterized by clonal myeloproliferation, progressive bone marrow (BM) fibrosis, splenomegaly, and anemia. BM fibrosis was previously thought to be a reactive phenomenon induced by mesenchymal stromal cells that are stimulated by the overproduction of cytokines such as transforming growth factor (TGF)-β1. However, the involvement of neoplastic fibrocytes in BM fibrosis was recently reported. In this study, we showed that the vast majority of collagen- and fibronectin-producing cells in the BM and spleens of Jak2V617F-induced myelofibrosis (MF) mice were fibrocytes derived from neoplastic hematopoietic cells. Neoplastic monocyte depletion eliminated collagen- and fibronectin-producing fibrocytes in BM and spleen, and ameliorated most characteristic MF features in Jak2V617F transgenic mice, including BM fibrosis, anemia, and splenomegaly, while had little effect on the elevated numbers of megakaryocytes and stem cells in BM, and leukothrombocytosis in peripheral blood. TGF-β1, which was produced by hematopoietic cells including fibrocytes, promoted the differentiation of neoplastic monocytes to fibrocytes, and elevated plasma TGF-β1 levels were normalized by monocyte depletion. Collectively, our data suggest that neoplastic fibrocytes are the major contributor to BM fibrosis in PMF, and TGF-β1 is required for their differentiation.

The Molecular Genetics of Myeloproliferative Neoplasms

Activated JAK-STAT signaling is central to the pathogenesis of BCR-ABL-negative myeloproliferative neoplasms (MPNs) and occurs as a result of MPN phenotypic driver mutations in JAK2, CALR, or MPL The spectrum of concomitant somatic mutations in other genes has now largely been defined in MPNs. With the integration of targeted next-generation sequencing (NGS) panels into clinical practice, the clinical significance of concomitant mutations in MPNs has become clearer. In this review, we describe the consequences of concomitant mutations in the most frequently mutated classes of genes in MPNs: (1) DNA methylation pathways, (2) chromatin modification, (3) RNA splicing, (4) signaling pathways, (5) transcription factors, and (6) DNA damage response/stress signaling. The increased use of molecular genetics for early risk stratification of patients brings the possibility of earlier intervention to prevent disease progression in MPNs. However, additional studies are required to decipher underlying molecular mechanisms and effectively target them.

miR-543 regulates the epigenetic landscape of myelofibrosis by targeting TET1 and TET2

Myelofibrosis (MF) is a myeloproliferative neoplasm characterized by cytopenia and extramedullary hematopoiesis, resulting in splenomegaly. Multiple pathological mechanisms (e.g., circulating cytokines and genetic alterations, such as JAKV617F mutation) have been implicated in the etiology of MF, but the molecular mechanism causing resistance to JAK2V617F inhibitor therapy remains unknown. Among MF patients who were treated with the JAK inhibitor ruxolitinib, we compared noncoding RNA profiles of ruxolitinib therapy responders versus nonresponders and found miR-543 was significantly upregulated in nonresponders. We validated these findings by reverse transcription-quantitative PCR. in this same cohort, in 2 additional independent MF patient cohorts from the United States and Romania, and in a JAK2V617F mouse model of MF. Both in vitro and in vivo models were used to determine the underlying molecular mechanism of miR-543 in MF. Here, we demonstrate that miR-543 targets the dioxygenases ten-eleven translocation 1 (TET1) and 2 (TET2) in patients and in vitro, causing increased levels of global 5-methylcytosine, while decreasing the acetylation of histone 3, STAT3, and tumor protein p53. Mechanistically, we found that activation of STAT3 by JAKs epigenetically controls miR-543 expression via binding the promoter region of miR-543. Furthermore, miR-543 upregulation promotes the expression of genes related to drug metabolism, including CYP3A4, which is involved in ruxolitinib metabolism. Our findings suggest miR-543 as a potentially novel biomarker for the prognosis of MF patients with a high risk of treatment resistance and as a potentially new target for the development of new treatment options.

The role of driver mutations in myeloproliferative neoplasms: insights from mouse models

High frequency of JAK2V617F or CALR exon 9 mutations is a main molecular feature of myeloproliferative neoplasms (MPNs). Analysis of mouse models driven by these mutations suggests that they are a direct cause of MPNs and that the expression levels of the mutated genes define the disease phenotype. The function of MPN-initiating cells has also been elucidated by these mouse models. Such mouse models also play an important role in modeling disease to investigate the effects and action mechanisms of therapeutic drugs, such as JAK2 inhibitors and interferon α, against MPNs. The mutation landscape of hematological tumors has already been clarified by next-generation sequencing technology, and the importance of functional analysis of mutant genes in vivo should increase further in the future.

Experimental Modeling of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPN) are genetically very complex and heterogeneous diseases in which the acquisition of a somatic driver mutation triggers three main myeloid cytokine receptors, and phenotypically expresses as polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF). The course of the diseases may be influenced by germline predispositions, modifying mutations, their order of acquisition and environmental factors such as aging and inflammation. Deciphering these contributory elements, their mutual interrelationships, and their contribution to MPN pathogenesis brings important insights into the diseases. Animal models (mainly mouse and zebrafish) have already significantly contributed to understanding the role of several acquired and germline mutations in MPN oncogenic signaling. Novel technologies such as induced pluripotent stem cells (iPSCs) and precise genome editing (using CRISPR/Cas9) contribute to the emerging understanding of MPN pathogenesis and clonal architecture, and form a convenient platform for evaluating drug efficacy. In this overview, the genetic landscape of MPN is briefly described, with an attempt to cover the main discoveries of the last 15 years. Mouse and zebrafish models of the driver mutations are discussed and followed by a review of recent progress in modeling MPN with patient-derived iPSCs and CRISPR/Cas9 gene editing.

Molecular heterogeneity unravelled by single-cell transcriptomics in patients with essential thrombocythaemia

Significant phenotypic heterogeneity exists in patients with all subtypes of myeloproliferative neoplasms (MPN), including essential thrombocythaemia (ET). Single-cell RNA sequencing (scRNA-Seq) holds the promise of unravelling the biology of MPN at an unprecedented level of resolution. Herein we employed this approach to dissect the transcriptomes in the CD34⁺ cells from the peripheral blood of seven previously untreated ET patients and one healthy adult. The mutational profiles in these patients were as follows: JAK2 V617F in two, CALR in three (one type I and two type II) and triple-negative (TN) in two. Our results reveal substantial heterogeneity within this enrolled cohort of patients. Activation of JAK/STAT signalling was recognized in discrepant progenitor lineages among different samples. Significantly disparate molecular profiling was identified in the comparison between ET patients and the control, between patients with different driver mutations (JAK2 V617F and CALR exon 9 indel), and even between patients harbouring the same driver. Intra-individual clonal diversity was also found in the CD34⁺ progenitor population of a patient, possibly indicating the presence of multiple clones in this case. Estimation of subpopulation size based on cellular immunophenotyping suggested differentiation bias in all analysed samples. Furthermore, combining the transcriptomic information with data from targeted sequencing enabled us to unravel key somatic mutations that are molecularly relevant. To conclude, we demonstrated that scRNA-Seq extended our knowledge of clonal diversity and inter-individual heterogeneity in patients with ET. The obtained information could potentially leapfrog our efforts in the elucidation of the pathogenesis of the disease.

Myelofibrosis in 2019: moving beyond JAK2 inhibition

Myelofibrosis (MF) is a myeloproliferative neoplasm characterized by ineffective clonal hematopoiesis, splenomegaly, bone marrow fibrosis, and the propensity for transformation to acute myeloid leukemia. The discovery of mutations in JAK2, CALR, and MPL have uncovered activated JAK-STAT signaling as a primary driver of MF, supporting a rationale for JAK inhibition. However, JAK inhibition alone is insufficient for long-term remission and offers modest, if any, disease-modifying effects. Given this, there is great interest in identifying mechanisms that cooperate with JAK-STAT signaling to predict disease progression and rationally guide the development of novel therapies. This review outlines the latest discoveries in the biology of MF, discusses current clinical management of patients with MF, and summarizes the ongoing clinical trials that hope to change the landscape of MF treatment.

Leukemic Transformation of Myeloproliferative Neoplasms: Therapeutic and Genomic Considerations

PURPOSE OF REVIEW

Although BCR-ABL1-negative myeloproliferative neoplasms (MPN) are chronic, clonal hematopoietic stem cell (HSC) disorders marked by proliferation of one or more myeloid lineages, a substantial proportion of patients transform to acute myeloid leukemia. Leukemic transformation (LT) from a pre-existing MPN carries a dismal prognosis. Here, we review recent genetic, biological, and clinical data regarding LT.

RECENT FINDINGS

In the last decade, DNA sequencing has revolutionized our understanding of the genomic landscape of LT. Mutations in TP53, ASXL1, EZH2, IDH1/2, and SRSF2 are significantly associated with increased risk of LT of MPNs. Preclinical modeling of these mutations is underway and has yielded important biological insights, some of which have therapeutic implications. Recent progress has led to the identification of recurrent genomic alterations in patients with LT. This has allowed mechanistic and therapeutic insight into the process of LT. In turn, this may lead to more mechanism-based therapeutic strategies that may improve patient outcomes.

Clonal approaches to understanding the impact of mutations on hematologic disease development

Interrogation of hematopoietic tissue at the clonal level has a rich history spanning over 50 years, and has provided critical insights into both normal and malignant hematopoiesis. Characterization of chromosomes identified some of the first genetic links to cancer with the discovery of chromosomal translocations in association with many hematological neoplasms. The unique accessibility of hematopoietic tissue and the ability to clonally expand hematopoietic progenitors in vitro has provided fundamental insights into the cellular hierarchy of normal hematopoiesis, as well as the functional impact of driver mutations in disease. Transplantation assays in murine models have enabled cellular assessment of the functional consequences of somatic mutations in vivo. Most recently, next-generation sequencing-based assays have shown great promise in allowing multi-"omic" characterization of single cells. Here, we review how clonal approaches have advanced our understanding of disease development, focusing on the acquisition of somatic mutations, clonal selection, driver mutation cooperation, and tumor evolution.

Perspectives on interferon-alpha in the treatment of polycythemia vera and related myeloproliferative neoplasms: minimal residual disease and cure?

The first clinical trials of the safety and efficacy of interferon-alpha2 (IFN-alpha2) were performed about 30 years ago. Since then, several single-arm studies have convincingly demonstrated that IFN-alpha2 is a highly potent anti-cancer agent in several cancer types but unfortunately not being explored sufficiently due to a high toxicity profile when using non-pegylated IFN-alpha2 or high dosages or due to competitive drugs, that for clinicians at first glance might look more attractive. Within the hematological malignancies, IFN-alpha2 has only recently been revived in patients with the Philadelphia-negative myeloproliferative neoplasms-essential thrombocytosis, polycythemia vera, and myelofibrosis (MPNs)-and in patients with chronic myelogenous leukemia (CML) in combination with tyrosine kinase inhibitors. In this review, we tell the IFN story in MPNs from the very beginning in the 1980s up to 2018 and describe the perspectives for IFN-alpha2 treatment of MPNs in the future. The mechanisms of actions are discussed and the impact of chronic inflammation as the driving force for clonal expansion and disease progression in MPNs is discussed in the context of combination therapies with potent anti-inflammatory agents, such as the JAK1-2 inhibitors (licensed only ruxolitinib) and statins as well. Interferon-alpha2 being the cornerstone treatment in MPNs and having the potential of inducing minimal residual disease (MRD) with normalization of the bone marrow and low-JAK2V617F allele burden, we believe that combination therapy with ruxolitinib may be even more efficacious and hopefully revert disease progression in many more patients to enter the path towards MRD. In patients with advanced and transforming disease towards leukemic transformation or having transformed to acute myeloid leukemia, "triple therapy" is proposed as a novel treatment modality to be tested in clinical trials combining IFN-alpha2, DNA-hypomethylator, and ruxolitinib. The rationale for this "triple therapy" is given, including the fact that even in AML, IFN-alpha2 as monotherapy may revert disease progression. We envisage a new and bright future with many more patients with MPNs obtaining MRD on the above therapies. From this stage-and even before-vaccination strategies may open a new horizon with cure being the goal for some patients.

Heterogeneity in myeloproliferative neoplasms: Causes and consequences

Myeloproliferative neoplasms (MPNs) are haematopoietic stem cell-derived clonal disorders characterised by proliferation of some or all myeloid lineages, depending on the subtype. MPNs are classically categorized into three disease subgroups; essential thrombocythaemia (ET), polycythaemia vera (PV) and primary myelofibrosis (PMF). The majority (>85%) of patients carry a disease-initiating or driver mutation, the most prevalent occurring in the janus kinase 2 gene (JAK2 V617F), followed by calreticulin (CALR) and myeloproliferative leukaemia virus (MPL) genes. Although these diseases are characterised by shared clinical, pathological and molecular features, one of the most challenging aspects of these disorders is the diverse clinical features which occur in each disease type, with marked variability in risks of disease complications and progression to leukaemia. A remarkable aspect of MPN biology is that the JAK2 V617F mutation, often occurring in the absence of additional mutations, generates a spectrum of phenotypes from asymptomatic ET through to aggressive MF, associated with a poor outcome. The mechanisms promoting MPN heterogeneity remain incompletely understood, but contributing factors are broad and include patient characteristics (gender, age, comorbidities and environmental exposures), additional somatic mutations, target disease-initiating cell, bone marrow microenvironment and germline genetic associations. In this review, we will address these in detail and discuss their role in heterogeneity of MPN disease phenotypes. Tailoring patient management according to the multiple different factors that influence disease phenotype may prove to be the most effective approach to modify the natural history of the disease and ultimately improve outcomes for patients.

Cardiovascular disease in chronic myelomonocytic leukemia: do monocytosis and chronic inflammation predispose to accelerated atherosclerosis?

Patients with chronic myelomonocytic leukemia (CMML) have monocytosis and likely a state of chronic inflammation. Both have been associated with an increased risk of atherosclerosis. The aim of the study was to test the hypothesis that CMML patients are at increased risk of developing cardiovascular disease (CVD) due to persistent monocytosis and sustained chronic inflammation. In a retrospective cohort study, we assessed hazards for cardiovascular events after diagnosis in 112 CMML patients and 231 chronic lymphocytic leukemia (CLL) patients. Analyses were carried out on restricted cohorts (CMML = 84, CLL = 186), excluding patients with a prior history of CVD, as well as on unrestricted cohorts. In the restricted cohorts, a significant effect of cardiovascular event occurrence did not remain after adjustment (HR 2.49, 95% CI 0.94-6.60). In unrestricted cohorts, we found a more than twofold increased rate of cardiovascular events in CMML (HR 2.34, 95% CI 1.05-5.20). Our results indicate an increased risk of CVD after the diagnosis in CMML patients.

Single-cell approaches identify the molecular network driving malignant hematopoietic stem cell self-renewal

Recent advances in single-cell technologies have permitted the investigation of heterogeneous cell populations at previously unattainable resolution. Here we apply such approaches to resolve the molecular mechanisms driving disease in mouse hematopoietic stem cells (HSCs), using JAK2V617F mutant myeloproliferative neoplasms (MPNs) as a model. Single-cell gene expression and functional assays identified a subset of JAK2V617F mutant HSCs that display defective self-renewal. This defect is rescued at the single HSC level by crossing JAK2V617F mice with mice lacking TET2, the most commonly comutated gene in patients with MPN. Single-cell gene expression profiling of JAK2V617F-mutant HSCs revealed a loss of specific regulator genes, some of which were restored to normal levels in single TET2/JAK2 mutant HSCs. Of these, Bmi1 and, to a lesser extent, Pbx1 and Meis1 overexpression in JAK2-mutant HSCs could drive a disease phenotype and retain durable stem cell self-renewal in functional assays. Together, these single-cell approaches refine the molecules involved in clonal expansion of MPNs and have broad implications for deconstructing the molecular network of normal and malignant stem cells.

Reduced expression but not deficiency of GFI1 causes a fatal myeloproliferative disease in mice

Growth factor independent 1 (Gfi1) controls myeloid differentiation by regulating gene expression and limits the activation of p53 by facilitating its de-methylation at Lysine 372. In human myeloid leukemia, low GFI1 levels correlate with an inferior prognosis. Here, we show that knockdown (KD) of Gfi1 in mice causes a fatal myeloproliferative disease (MPN) that could progress to leukemia after additional mutations. Both KO and KD mice accumulate myeloid cells that show signs of metabolic stress and high levels of reactive oxygen species. However, only KO cells have elevated levels of Lysine 372 methylated p53. This suggests that in contrast to absence of GFI1, KD of GFI1 leads to the accumulation of myeloid cells because sufficient amount of GFI1 is present to impede p53-mediated cell death, leading to a fatal MPN. The combination of myeloid accumulation and the ability to counteract p53 activity under metabolic stress could explain the role of reduced GF1 expression in human myeloid leukemia.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

Integrated genomic analysis identifies deregulated JAK/STAT-MYC-biosynthesis axis in aggressive NK-cell leukemia

Aggressive NK-cell leukemia (ANKL) is a rare form of NK cell neoplasm that is more prevalent among people from Asia and Central and South America. Patients usually die within days to months, even after receiving prompt therapeutic management. Here we performed the first comprehensive study of ANKL by integrating whole genome, transcriptome and targeted sequencing, cytokine array as well as functional assays. Mutations in the JAK-STAT pathway were identified in 48% (14/29) of ANKL patients, while the extracellular STAT3 stimulator IL10 was elevated by an average of 56-fold (P < 0.0001) in the plasma of all patients examined. Additional frequently mutated genes included TP53 (34%), TET2 (28%), CREBBP (21%) and MLL2 (21%). Patient NK leukemia cells showed prominent activation of STAT3 phosphorylation, MYC expression and transcriptional activities in multiple metabolic pathways. Functionally, STAT3 activation and MYC expression were critical for the proliferation and survival of ANKL cells. STAT signaling regulated the MYC transcription program, and both STAT signaling and MYC transcription were required to maintain the activation of nucleotide synthesis and glycolysis. Collectively, the JAK-STAT pathway represents a major target for genomic alterations and IL10 stimulation in ANKL. This newly discovered JAK/STAT-MYC-biosynthesis axis may provide opportunities for the development of novel therapeutic strategies in treating this subtype of leukemia.

Impact of DNA methylation programming on normal and pre-leukemic hematopoiesis

Epigenome regulation is a critical mechanism that governs cell identity, lineage specification and developmental cell fates. With the advent of low-input and single-cell technologies as well as sophisticated cell labeling techniques, our understanding of transcriptional and epigenetic regulation of hematopoiesis is currently undergoing dramatic changes. Increasingly, evidence suggests that the epigenome conformation acts as a critical decision-making mechanism that instructs self-renewal, differentiation and developmental fates of hematopoietic progenitor cells. When dysregulated, this leads to the evolution of disease states such as leukemia. Indeed, aberrations in DNA methylation, histone modifications and genome architecture are characteristic features of many hematopoietic neoplasms in which epigenetic enzymes are frequently mutated. Sequencing studies and characterization of the epigenetic landscape in lymphomas, leukemias and in aged healthy individuals with clonal hematopoiesis have been indispensible to identify epigenetic regulators that play a role in transformation or pre-disposition to hematopoietic malignancies. In this review, we outline the current view of the hematopoietic system and the epigenetic mechanisms regulating hematopoiesis under homeostatic conditions, with a particular focus on the role of DNA methylation in this process. We will also summarize the current knowledge on the mechanisms underlying dysregulated DNA methylation in hematologic malignancies and how this contributes to our understanding of the physiological functions of epigenetic regulators in hematopoiesis.

Mutations in myeloproliferative neoplasms - their significance and clinical use

INTRODUCTION

Clonal hematologic diseases of the blood such as polycythemia vera, essential thrombocythemia and primary myelofibrosis belong to the BCR-ABL negative Myeloproliferative Neoplasms (MPN). These diseases are characterized by clonal expansion of hematopoietic precursor cells followed by increased production of differentiated cells of the myeloid lineage. Initiation of clonal hematopoiesis, formation of a clinical phenotype as well as disease progression form part of MPN disease evolution. The disease is driven by acquired somatic mutations in critical pathways such as cytokine signaling, epigenetic regulation, RNA splicing, and transcription factor signaling. Areas covered: The following review aims to provide an overview of the mutational landscape of MPN, the impact of these mutations in MPN pathogenesis as well as their prognostic value. Finally, a summary of how these mutations are being used or could potentially be used for the treatment of MPN patients is presented. Expert commentary: The genetic landscape of MPN patients has been successfully dissected within the past years with the advent of new sequencing technologies. Integrating the genetic information within a clinical setting is already benefitting patients in terms of disease monitoring and prognostic information of disease progression but will be further intensified within the next years.

Pathogenesis of Myeloproliferative Disorders

Myeloproliferative neoplasms (MPNs) are a set of chronic hematopoietic neoplasms with overlapping clinical and molecular features. Recent years have witnessed considerable advances in our understanding of their pathogenetic basis. Due to their protracted clinical course, the evolution to advanced hematological malignancies, and the accessibility of neoplastic tissue, the study of MPNs has provided a window into the earliest stages of tumorigenesis. With the discovery of mutations in CALR, the majority of MPN patients now bear an identifiable marker of clonal disease; however, the mechanism by which mutated CALR perturbs megakaryopoiesis is currently unresolved. We are beginning to understand better the role of JAK2(V617F) homozygosity, the function of comutations in epigenetic regulators and spliceosome components, and how these mutations cooperate with JAK2(V617F) to modulate MPN phenotype.

Overview of Transgenic Mouse Models of Myeloproliferative Neoplasms (MPNs)

Myeloproliferative neoplasms (MPNs) are a class of hematologic diseases characterized by aberrant proliferation of one or more myeloid lineages and progressive bone marrow fibrosis. In 2005, seminal work by multiple groups identified the JAK2V617F mutation in a significant fraction of MPN patients. Since that time, murine models of JAK2V617F have greatly enhanced the understanding of the role of aberrant JAK-STAT signaling in MPN pathogenesis and have provided an in vivo pre-clinical platform that can be used to develop novel therapies. From early retroviral transduction models to transgenics, and ultimately conditional knock-ins, murine models have established that JAK2V617F alone can induce an MPN-like syndrome in vivo. However, additional mutations co-occur with JAK2V617F in MPNs, often in proteins involved in epigenetic regulation that can dramatically influence disease outcomes. In vivo modeling of these mutations in the context of JAK2V617F has provided additional insights into the role of epigenetic dysregulation in augmenting MPN hematopoiesis. In this overview, early murine model development of JAK2V617F is described, with an analysis of its effects on the hematopoietic stem/progenitor cell niche and interactions with downstream signaling elements. This is followed by a description of more recent in vivo models developed for evaluating the effect of concomitant mutations in epigenetic modifiers on MPN maintenance and progression. Mouse models of other driver mutations in MPNs, including primarily calreticulin (CALR) and Tpo-receptor (MPL), which occur in a significant percentage of MPN patients with wild-type JAK2, are also briefly reviewed. © 2017 by John Wiley & Sons, Inc.

Hmga2 collaborates with JAK2V617F in the development of myeloproliferative neoplasms

High-mobility group AT-hook 2 (HMGA2) is crucial for the self-renewal of fetal hematopoietic stem cells (HSCs) but is downregulated in adult HSCs via repression by MIRlet-7 and the polycomb-recessive complex 2 (PRC2) including EZH2. The HMGA2 messenger RNA (mRNA) level is often elevated in patients with myelofibrosis that exhibits an advanced myeloproliferative neoplasm (MPN) subtype, and deletion of Ezh2 promotes the progression of severe myelofibrosis in JAK2^V617F mice with upregulation of several oncogenes such as Hmga2. However, the direct role of HMGA2 in the pathogenesis of MPNs remains unknown. To clarify the impact of HMGA2 on MPNs carrying the driver mutation, we generated ΔHmga2/JAK2^V617F mice overexpressing Hmga2 due to deletion of the 3' untranslated region. Compared with JAK2^V617F mice, ΔHmga2/JAK2^V617F mice exhibited more severe leukocytosis, anemia and splenomegaly, and shortened survival, whereas severity of myelofibrosis was comparable. ΔHmga2/JAK2^V617F cells showed a greater repopulating ability that reproduced the severe MPN compared with JAK2^V617F cells in serial bone marrow transplants, indicating that Hmga2 promotes MPN progression at the HSC level. Hmga2 also enhanced apoptosis of JAK2^V617F erythroblasts that may worsen anemia. Relative to JAK2^V617F hematopoietic stem and progenitor cells (HSPCs), over 30% of genes upregulated in ΔHmga2/JAK2^V617F HSPCs overlapped with those derepressed by Ezh2 loss in JAK2^V617F/Ezh2^Δ/Δ HSPCs, suggesting that Hmga2 may facilitate upregulation of Ezh2 targets. Correspondingly, deletion of Hmga2 ameliorated anemia and splenomegaly in JAK2^V617F/Ezh2^Δ/wild-type mice, and MIRlet-7 suppression and PRC2 mutations correlated with the elevated HMGA2 mRNA levels in patients with MPNs, especially myelofibrosis. These findings suggest the crucial role of HMGA2 in MPN progression.

Clinical characteristics and whole exome/transcriptome sequencing of coexisting chronic myeloid leukemia and myelofibrosis

Myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell (HSC) disorders that can be classified on the basis of genetic, clinical, phenotypic features. Genetic lesions such as JAK2 mutations and BCR-ABL translocation are often mutually exclusive in MPN patients and lead to essential thrombocythemia, polycythemia vera, or myelofibrosis or chronic myeloid leukemia, respectively. Nevertheless, coexistence of these genetic aberrations in the same patient has been reported. Whether these aberrations occur in the same stem cell or a different cell is unclear, but an unstable genome in the HSCs seems to be the common antecedent. In an effort to characterize the underlying genetic events that might contribute to the appearance of more than one MPN in a patient, we studied neoplastic cells from patients with dual MPNs by next-generation sequencing. We observed that most patients with two MPNs harbored mutations in genes known to contribute to clonal hematopoiesis through altered epigenetic regulation such as TET2, ASXL1/2, SRSF2, and IDH2 at varying frequencies (1%-47%). In addition, we found that some patients also harbored oncogenic mutations in N/KRAS, TP53, BRAF, EZH2, and GNAS at low frequencies, which probably represent clonal evolution. These findings support the hypothesis that hematopoietic cells from MPN patients harbor multiple genetic aberrations, some of which can contribute to clonal dominance. Acquiring mutations in JAK2/CALR/MPL or the BCR-ABL translocation probably drive the oncogenic phenotype towards a specific MPN. Further, we propose that the acquisition of BCR-ABL in these patients is frequently a secondary event resulting from an unstable genome.

Epigenetic dysregulation of hematopoietic stem cells and preleukemic state

Recent genetic analyses have revealed that premalignant somatic mutations in hematopoietic cells are common in older people without an evidence of hematologic malignancies, leading to clonal hematopoietic expansion. This phenomenon has been termed clonal hematopoiesis of indeterminate potential (CHIP). Frequency of such clonal somatic mutations increases with age: in 5-10% of people older than 70 years and around 20% of people older than 90 years. The most commonly mutated genes found in individuals with CHIP were epigenetic regulators, including DNA methyltransferase 3A (DNMT3A), Ten-eleven-translocation 2 (TET2), and Additional sex combs-like 1 (ASXL1), which are also recurrently mutated in myeloid malignancies. Recent functional studies have uncovered pleiotropic effect of mutations in DNMT3A, TET2, and ASXL1 in hematopoietic stem cell regulation and leukemic transformation. Of note, CHIP is associated with an increased risk of hematologic malignancy and all-cause mortality, albeit the annual risk of leukemic transformation was relatively low (0.5-1%). These findings suggest that clonal hematopoiesis per se may not be sufficient to engender preleukemic state. Further studies are required to decipher the exact mechanism by which preleukemic stem cells originate and transform into a full-blown leukemic state.

Myeloproliferative neoplasm stem cells

Myeloproliferative neoplasms (MPNs) arise in the hematopoietic stem cell (HSC) compartment as a result of the acquisition of somatic mutations in a single HSC that provides a selective advantage to mutant HSC over normal HSC and promotes myeloid differentiation to engender a myeloproliferative phenotype. This population of somatically mutated HSC, which initiates and sustains MPNs, is termed MPN stem cells. In >95% of cases, mutations that drive the development of an MPN phenotype occur in a mutually exclusive manner in 1 of 3 genes: JAK2, CALR, or MPL The thrombopoietin receptor, MPL, is the key cytokine receptor in MPN development, and these mutations all activate MPL-JAK-STAT signaling in MPN stem cells. Despite common biological features, MPNs display diverse disease phenotypes as a result of both constitutional and acquired factors that influence MPN stem cells, and likely also as a result of heterogeneity in the HSC in which MPN-initiating mutations arise. As the MPN clone expands, it exerts cell-extrinsic effects on components of the bone marrow niche that can favor the survival and expansion of MPN stem cells over normal HSC, further sustaining and driving malignant hematopoiesis. Although developed as targeted therapies for MPNs, current JAK2 inhibitors do not preferentially target MPN stem cells, and as a result, rarely induce molecular remissions in MPN patients. As the understanding of the molecular mechanisms underlying the clonal dominance of MPN stem cells advances, this will help facilitate the development of therapies that preferentially target MPN stem cells over normal HSC.

The Amelioration of Myelofibrosis with Thrombocytopenia by a JAK1/2 Inhibitor, Ruxolitinib, in a Post-polycythemia Vera Myelofibrosis Patient with a JAK2 Exon 12 Mutation

Less than 5% of patients with polycythemia vera (PV) show JAK2 exon 12 mutations. Although PV patients with JAK2 exon 12 mutations are known to develop post-PV myelofibrosis (MF) as well as PV with JAK2V617F, the role of JAK inhibitors in post-PV MF patients with JAK2 exon 12 mutations remains unknown. We describe how treatment with a JAK1/2 inhibitor, ruxolitinib, led to the rapid amelioration of marrow fibrosis, erythrocytosis and thrombocytopenia in a 77-year-old man with post-PV MF who carried a JAK2 exon 12 mutation (JAK2H538QK539L). This case suggests that ruxolitinib is a treatment option for post-PV MF in patients with thrombocytopenia or JAK2 exon 12 mutations.

Genetic basis and molecular pathophysiology of classical myeloproliferative neoplasms

The genetic landscape of classical myeloproliferative neoplasm (MPN) is in large part elucidated. The MPN-restricted driver mutations, including those in JAK2, calreticulin (CALR), and myeloproliferative leukemia virus (MPL), abnormally activate the cytokine receptor/JAK2 pathway and their downstream effectors, more particularly the STATs. The most frequent mutation, JAK2V617F, activates the 3 main myeloid cytokine receptors (erythropoietin receptor, granulocyte colony-stimulating factor receptor, and MPL) whereas CALR or MPL mutants are restricted to MPL activation. This explains why JAK2V617F is associated with polycythemia vera, essential thrombocythemia (ET), and primary myelofibrosis (PMF) whereas CALR and MPL mutants are found in ET and PMF. Other mutations in genes involved in epigenetic regulation, splicing, and signaling cooperate with the 3 MPN drivers and play a key role in the PMF pathogenesis. Mutations in epigenetic regulators TET2 and DNMT3A are involved in disease initiation and may precede the acquisition of JAK2V617F. Other mutations in epigenetic regulators such as EZH2 and ASXL1 also play a role in disease initiation and disease progression. Mutations in the splicing machinery are predominantly found in PMF and are implicated in the development of anemia or pancytopenia. Both heterogeneity of classical MPNs and prognosis are determined by a specific genomic landscape, that is, type of MPN driver mutations, association with other mutations, and their order of acquisition. However, factors other than somatic mutations play an important role in disease initiation as well as disease progression such as germ line predisposition, inflammation, and aging. Delineation of these environmental factors will be important to better understand the precise pathogenesis of MPN.