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

Modeling myeloproliferative neoplasms: From mutations to mouse models and back again

Myeloproliferative neoplasms (MPNs) are defined according to the 2008 World Health Organization (WHO) classification and the recent 2016 revision. Over the years, several genetic lesions have been associated with the development of MPNs, with important consequences for identifying unique biomarkers associated with specific neoplasms and for developing targeted therapies. Defining the genotype-phenotype relationship in MPNs is essential to identify driver somatic mutations that promote MPN development and maintenance in order to develop curative targeted therapies. While studies with human samples can identify putative driver mutations, murine models are mandatory to demonstrate the causative role of mutations and for pre-clinical testing of specific therapeutic interventions. This review focuses on MPN mouse models specifically developed to assess the pathogenetic roles of gene mutations found in human patients, as well as murine MPN-like phenotypes identified in genetically modified mice.

Dysregulation of TET2 in hematologic malignancies

The TET dioxygenases, TET1, TET2, and TET3, catalyze transfer of an oxygen atom to the methyl group of 5-methylcytocine (5-mC), converting it to 5-hydroxymethylcytocine (5-hmC). Among the genes encoding these enzymes, ten-eleven translocation 2 (TET2) is frequently mutated somatically in both myeloid and lymphoid malignancies. Because these TET2 mutations result in the impairment of the dioxygenase activity of TET2, it is thought that these mutations interfere with 5-mC to 5-hmC conversion. There is ample evidence indicating that TET2 mutations are a driver of tumorigenesis in blood cells and that TET2 mutations are often acquired at the hematopoietic stem/early progenitor cell stage. In addition, TET2 is the second-most frequently mutated gene in clonal hematopoiesis in individuals with no apparent blood cancers, suggesting that while TET2 mutations alone are insufficient to cause hematologic malignancy, they represent an early event during tumorigenesis. A number of questions, including the precise target genome regions of TET2, and the importance of the balance of 5-mC and 5-hmC in the regulatory regions in transcriptional control, remain.

Calreticulin mutant mice develop essential thrombocythemia that is ameliorated by the JAK inhibitor ruxolitinib

Mutations of calreticulin (CALR) are detected in 25–30% of patients with essential thrombocythemia (ET) or primary myelofibrosis and cause frameshifts that result in proteins with a novel C-terminal. We demonstrate that CALR mutations activated signal transducer and activator of transcription 5 (STAT5) in 293T cells in the presence of thrombopoietin receptor (MPL). Human megakaryocytic CMK11-5 cells and erythroleukemic F-36P-MPL cells with knocked-in CALR mutations showed increased growth and acquisition of cytokine-independent growth, respectively, accompanied by STAT5 phosphorylation. Transgenic mice expressing a human CALR mutation with a 52 bp deletion (CALRdel52-transgenic mice (TG)) developed ET, with an increase in platelet count, but not hemoglobin level or white blood cell count, in association with an increase in bone marrow (BM) mature megakaryocytes. CALRdel52 BM cells did not drive away wild-type (WT) BM cells in in vivo competitive serial transplantation assays, suggesting that the self-renewal capacity of CALRdel52 hematopoietic stem cells (HSCs) was comparable to that of WT HSCs. Therapy with the Janus kinase (JAK) inhibitor ruxolitinib ameliorated the thrombocytosis in TG mice and attenuated the increase in number of BM megakaryocytes and HSCs. Taken together, our study provides a model showing that the C-terminal of mutant CALR activated JAK-STAT signaling specifically downstream of MPL and may have a central role in CALR-induced myeloproliferative neoplasms.

Oncogenic Drivers in Myeloproliferative Neoplasms: From JAK2 to Calreticulin Mutations

During the past 10 years, major progress has been accomplished with the discovery of activating mutations that are associated with the vast majority of BCR-ABL negative human myeloproliferative neoplasms (MPNs). The identification in 2005 of JAK2 V617F triggered great interest in the JAK2-STAT5/STAT3 pathway. Discovery in 2006 of mutants of thrombopoietin receptor (TPO-R/MPL) and later on of mutants in negative regulators of JAK-STAT pathway led to the notion that persistent JAK2 activation is a hallmark of MPNs. In 2013, mutations in the gene coding for the chaperone calreticulin were reported in 20-30% of essential thrombocythemia and primary myelofibrosis patients. Here, we will address the question: what do we know about calreticulin that could help us understand its role in MPNs? In addition to oncogenic driver mutations, certain MPNs also exhibit epigenetic mutations. Targeting of both oncogenic drivers and epigenetic defects could be required for effective therapy.

Role of TET enzymes in DNA methylation, development, and cancer

Ten eleven translocation (TET) genes, and especially TET2, are frequently mutated in various cancers, but how the TET proteins contribute to the onset and maintenance of these malignancies is largely unknown. In this review, Rasmussen and Helin highlight recent advances in understanding the physiological function of the TET proteins and their role in regulating DNA methylation and transcription.

The pattern of DNA methylation at cytosine bases in the genome is tightly linked to gene expression, and DNA methylation abnormalities are often observed in diseases. The ten eleven translocation (TET) enzymes oxidize 5-methylcytosines (5mCs) and promote locus-specific reversal of DNA methylation. TET genes, and especially TET2, are frequently mutated in various cancers, but how the TET proteins contribute to prevent the onset and maintenance of these malignancies is largely unknown. Here, we highlight recent advances in understanding the physiological function of the TET proteins and their role in regulating DNA methylation and transcription. In addition, we discuss some of the key outstanding questions in the field.

The loss of Ezh2 drives the pathogenesis of myelofibrosis and sensitizes tumor-initiating cells to bromodomain inhibition

Loss of Ezh2 in the presence of activating mutation in JAK2 (JAK2^V617F) cooperatively alters transcriptional programs of hematopoiesis, activates specific oncogenes, and promotes the development of myelofibrosis.

EZH2 is a component of polycomb repressive complex 2 (PRC2) and functions as an H3K27 methyltransferase. Loss-of-function mutations in EZH2 are associated with poorer outcomes in patients with myeloproliferative neoplasms (MPNs), particularly those with primary myelofibrosis (MF [PMF]). To determine how EZH2 insufficiency is involved in the pathogenesis of PMF, we generated mice compound for an Ezh2 conditional deletion and activating mutation in JAK2 (JAK2V617F) present in patients with PMF. The deletion of Ezh2 in JAK2^V617F mice markedly promoted the development of MF, indicating a tumor suppressor function for EZH2 in PMF. The loss of Ezh2 in JAK2^V617F hematopoietic cells caused significant reductions in H3K27 trimethylation (H3K27me3) levels, resulting in an epigenetic switch to H3K27 acetylation (H3K27ac). These epigenetic switches were closely associated with the activation of PRC2 target genes including Hmga2, an oncogene implicated in the pathogenesis of PMF. The treatment of JAK2^V617F/Ezh2-null mice with a bromodomain inhibitor significantly attenuated H3K27ac levels at the promoter regions of PRC2 targets and down-regulated their expression, leading to the abrogation of MF-initiating cells. Therefore, an EZH2 insufficiency not only cooperated with active JAK2 to induce MF, but also conferred an oncogenic addiction to the H3K27ac modification in MF-initiating cells that was capable of being restored by bromodomain inhibition.

Low Ten-eleven-translocation 2 (TET2) transcript level is independent of TET2 mutation in patients with myeloid neoplasms

BACKGROUND

New sequencing technologies have enabled the identification of mutations in Ten-eleven-translocation 2 (TET2), an enzyme that catalyzes the conversion of 5-methylcytosine into 5-hydroxymethylcytosine (5-hmC) in myeloid neoplasms. We have recently identified reduced TET2 mRNA expression in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), which is associated with a poor overall survival in MDS. We herein aimed to investigate TET2 mutations and their impact on TET2 expression in a cohort of patients with myeloid neoplasms, including MDS and AML patients.

FINDINGS

TET2 mutations were observed in 8 out of 19 patients (42 %) with myeloid neoplasms. The TET2 expression profile was similar between in wild type and in TET2 mutated patients.

CONCLUSION

Our results suggest that TET2 expression is reduced in MDS/AML patients, independently of mutational status.

DNMT3A(R882H) mutant and Tet2 inactivation cooperate in the deregulation of DNA methylation control to induce lymphoid malignancies in mice

TEN-ELEVEN-TRANSLOCATION-2 (TET2) and DNA-METHYLTRANSFERASE-3A (DNMT3A), both encoding proteins involved in regulating DNA methylation, are mutated in hematological malignancies affecting both myeloid and lymphoid lineages. We previously reported an association of TET2 and DNMT3A mutations in progenitors of patients with angioimmunoblastic T-cell lymphomas (AITL). Here, we report on the cooperative effect of Tet2-inactivation and DNMT3A mutation affecting arginine 882 (DNMT3A^R882H) using a murine bone marrow transplantation assay. Five out of 18 primary recipients developed hematological malignancies with one mouse developing an AITL-like disease, 2 mice presenting acute myeloid leukemia (AML)-like and 2 others T cell acute lymphoblastic leukemia (T-ALL)-like diseases within 6 months following transplantation. Serial transplantations of DNMT3A^R882H Tet2^−/− progenitors led to a differentiation bias toward the T-cell compartment, eventually leading to AITL-like disease in 9/12 serially transplanted recipients. Expression profiling suggested that DNMT3A^R882H Tet2^−/− T-ALLs resemble those of NOTCH1 mutant. Methylation analysis of DNMT3A^R882H Tet2^−/− T-ALLs showed a global increase in DNA methylation affecting tumor suppressor genes and local hypomethylation affecting genes involved in the Notch pathway. Our data confirm the transformation potential of DNMT3A^R882H Tet2^−/− progenitors and represent the first cooperative model in mice involving Tet2-inactivation driving lymphoid malignancies.

Pathogenesis of myeloproliferative neoplasms

Major progress has been recently made in understanding the molecular pathogenesis of myeloproliferative neoplasms (MPN). Mutations in one of four genes-JAK2, MPL, CALR, and CSF3R-can be found in the vast majority of patients with MPN and represent driver mutations that can induce the MPN phenotype. Hyperactive JAK/STAT signaling appears to be the common denominator of MPN, even in patients with CALR mutations and the so-called "triple-negative" MPN, where the driver gene mutation is still unknown. Mutations in epigenetic regulators, transcription factors, and signaling components modify the course of the disease and can contribute to disease initiation and/or progression. The central role of JAK2 in MPN allowed development of small molecular inhibitors that are in clinical use and are active in almost all patients with MPN. Advances in understanding the mechanism of JAK2 activation open new perspectives of developing the next generation of inhibitors that will be selective for the mutated forms of JAK2.

Gene expression profiling of loss of TET2 and/or JAK2V617F mutant hematopoietic stem cells from mouse models of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are clinically characterized by the chronic overproduction of differentiated peripheral blood cells and the gradual expansion of malignant intramedullary/extramedullary hematopoiesis. In MPNs mutations in JAK2 MPL or CALR are detected mutually exclusive in more than 90% of cases [1], [2]. Mutations in them lead to the abnormal activation of JAK/STAT signaling and the autonomous growth of differentiated cells therefore they are considered as “driver” gene mutations. In addition to the above driver gene mutations mutations in epigenetic regulators such as TET2 DNMT3A ASXL1 EZH2 or IDH1/2 are detected in about 5%–30% of cases respectively [3]. Mutations in TET2 DNMT3A EZH2 or IDH1/2 commonly confer the increased self-renewal capacity on normal hematopoietic stem cells (HSCs) but they do not lead to the autonomous growth of differentiated cells and only exhibit subtle clinical phenotypes [[4], [6], [7], [8],5]. It was unclear how mutations in such epigenetic regulators influenced abnormal HSCs with driver gene mutations how they influenced the disease phenotype or whether a single driver gene mutation was sufficient for the initiation of human MPNs. Therefore we focused on JAK2V617F and loss of TET2—the former as a representative of driver gene mutations and the latter as a representative of mutations in epigenetic regulators—and examined the influence of single or double mutations on HSCs (Lineage⁻Sca-1⁺c-Kit⁺ cells (LSKs)) by functional analyses and microarray whole-genome expression analyses [9]. Gene expression profiling showed that the HSC fingerprint genes [10] was statistically equally enriched in TET2-knockdown-LSKs but negatively enriched in JAK2V617F–LSKs compared to that in wild-type-LSKs. Double-mutant-LSKs showed the same tendency as JAK2V617F–LSKs in terms of their HSC fingerprint genes but the expression of individual genes differed between the two groups. Among 245 HSC fingerprint genes 100 were more highly expressed in double-mutant-LSKs than in JAK2V617F–LSKs. These altered gene expressions might partly explain the mechanisms of initiation and progression of MPNs which was observed in the functional analyses [9]. Here we describe gene expression profiles deposited at the Gene Expression Omnibus (GEO) under the accession number GSE62302 including experimental methods and quality control analyses.

TET2 loss, a rescue of JAK2V617F HSCs

In this issue of Blood, Chen et al and Kameda et al demonstrate that Tet2 loss has 2 effects in Jak2V617F mice: it increases both the severity of the myeloproliferative disorders and the self-renewal properties of the Jak2V617F hematopoietic stem cells (HSCs).

Novel recurrent mutations in ethanolamine kinase 1 (ETNK1) gene in systemic mastocytosis with eosinophilia and chronic myelomonocytic leukemia

Although KITD816V occurs universally in adult systemic mastocytosis (SM), the clinical heterogeneity of SM suggests presence of additional phenotype-patterning mutations. Because up to 25% of SM patients have KITD816V-positive eosinophilia, we undertook whole-exome sequencing in a patient with aggressive SM with eosinophilia to identify novel genetic alterations. We conducted sequencing of purified eosinophils (clone/tumor sample), with T-lymphocytes as the matched control/non-tumor sample. In addition to KITD816V, we identified a somatic missense mutation in ethanolamine kinase 1 (ETNK1N244S) that was not present in 50 healthy controls. Targeted resequencing of 290 patients showed ETNK1 mutations to be distributed as follows: (i) SM (n=82; 6% mutated); (ii) chronic myelomonocytic leukemia (CMML; n=29; 14% mutated); (iii) idiopathic hypereosinophilia (n=137; <1% mutated); (iv) primary myelofibrosis (n=32; 0% mutated); and (v) others (n=10; 0% mutated). Of the 82 SM cases, 25 had significant eosinophilia; of these 20% carried ETNK1 mutations. The ten mutations (N244S=6, N244T=1, N244K=1, G245A=2) targeted two contiguous amino acids in the ETNK1 kinase domain, and are predicted to be functionally disruptive. In summary, we identified novel somatic missense ETNK1 mutations that were most frequent in SM with eosinophilia and CMML; this suggests a potential pathogenetic role for dysregulated cytidine diphosphate-ethanolamine pathway metabolites in these diseases.