JAK2V617F-mediated phosphorylation of PRMT5 downregulates its methyltransferase activity and promotes myeloproliferation

Focus on the epigenome in the myeloproliferative neoplasms

The discovery of mutations activating JAK-STAT signaling in the majority of patients with myeloproliferative neoplasms (MPNs) led to identification of tyrosine kinase activation as a predominant mechanism driving MPN pathogenesis. Despite this, the existence of additional genetic events that modify the MPN phenotype, predate JAK2 mutations, and/or contribute to leukemic transformation of MPNs has been suggested. Recently, mutations in several epigenetic modifiers have been described in patients with MPNs, including mutations in ASXL1, DNMT3A, EZH2, IDH1, IDH2, and TET2. Moreover, the mutant JAK2 itself has been shown recently to affect histone posttranslational modifications directly. Here we review the biological and clinical implications of epigenetic alterations in the pathogenesis of MPNs.

Protein arginine methylation of non-histone proteins and its role in diseases

Protein arginine methyltransferases (PRMTs) are a family of enzymes that can methylate arginine residues on histones and other proteins. PRMTs play a crucial role in influencing various cellular functions, including cellular development and tumorigenesis. Arginine methylation by PRMTs is found on both nuclear and cytoplasmic proteins. Recently, there is increasing evidence regarding post-translational modifications of non-histone proteins by PRMTs, illustrating the previously unknown importance of PRMTs in the regulation of various cellular functions by post-translational modifications. In this review, we present the recent developments in the regulation of non-histone proteins by PRMTs.

Immunoaffinity enrichment and mass spectrometry analysis of protein methylation

Protein methylation is a common posttranslational modification that mostly occurs on arginine and lysine residues. Arginine methylation has been reported to regulate RNA processing, gene transcription, DNA damage repair, protein translocation, and signal transduction. Lysine methylation is best known to regulate histone function and is involved in epigenetic regulation of gene transcription. To better study protein methylation, we have developed highly specific antibodies against monomethyl arginine; asymmetric dimethyl arginine; and monomethyl, dimethyl, and trimethyl lysine motifs. These antibodies were used to perform immunoaffinity purification of methyl peptides followed by LC-MS/MS analysis to identify and quantify arginine and lysine methylation sites in several model studies. Overall, we identified over 1000 arginine methylation sites in human cell line and mouse tissues, and ∼160 lysine methylation sites in human cell line HCT116. The number of methylation sites identified in this study exceeds those found in the literature to date. Detailed analysis of arginine-methylated proteins observed in mouse brain compared with those found in mouse embryo shows a tissue-specific distribution of arginine methylation, and extends the types of proteins that are known to be arginine methylated to include many new protein types. Many arginine-methylated proteins that we identified from the brain, including receptors, ion channels, transporters, and vesicle proteins, are involved in synaptic transmission, whereas the most abundant methylated proteins identified from mouse embryo are transcriptional regulators and RNA processing proteins.

PRMT5 is upregulated in malignant and metastatic melanoma and regulates expression of MITF and p27(Kip1.)

Protein arginine methyltransferase-5 (PRMT5) is a Type II arginine methyltransferase that regulates various cellular functions. We hypothesized that PRMT5 plays a role in regulating the growth of human melanoma cells. Immunohistochemical analysis indicated significant upregulation of PRMT5 in human melanocytic nevi, malignant melanomas and metastatic melanomas as compared to normal epidermis. Furthermore, nuclear PRMT5 was significantly decreased in metastatic melanomas as compared to primary cutaneous melanomas. In human metastatic melanoma cell lines, PRMT5 was predominantly cytoplasmic, and associated with its enzymatic cofactor Mep50, but not STAT3 or cyclin D1. However, histologic examination of tumor xenografts from athymic mice revealed heterogeneous nuclear and cytoplasmic PRMT5 expression. Depletion of PRMT5 via siRNA inhibited proliferation in a subset of melanoma cell lines, while it accelerated growth of others. Loss of PRMT5 also led to reduced expression of MITF (microphthalmia-associated transcription factor), a melanocyte-lineage specific oncogene, and increased expression of the cell cycle regulator p27^Kip1. These results are the first to report elevated PRMT5 expression in human melanoma specimens and indicate this protein may regulate MITF and p27^Kip1 expression in human melanoma cells.

Automethylation of protein arginine methyltransferase 8 (PRMT8) regulates activity by impeding S-adenosylmethionine sensitivity

Protein arginine methyltransferase (PRMT) 8 is unique among the PRMTs, as it has a highly restricted tissue expression pattern and an N terminus that contains two automethylation sites and a myristoylation site. PRMTs catalyze the transfer of a methyl group from S-adenosylmethionine (AdoMet) to a peptidylarginine on a protein substrate. Currently, the physiological roles, regulation, and cellular substrates of PRMT8 are poorly understood. However, a thorough understanding of PRMT8 kinetics should provide insights into each of these areas, thereby enhancing our understanding of this unique enzyme. In this study, we determined how automethylation regulates the enzymatic activity of PRMT8. We found that preventing automethylation with lysine mutations (preserving the positive charge of the residue) increased the turnover rate and decreased the Km of AdoMet but did not affect the Km of the protein substrate. In contrast, mimicking automethylation with phenylalanine (i.e. mimicking the increased hydrophobicity) decreased the turnover rate. The inhibitory effect of the PRMT8 N terminus could be transferred to PRMT1 by creating a chimeric protein containing the N terminus of PRMT8 fused to PRMT1. Thus, automethylation of the N terminus likely regulates PRMT8 activity by decreasing the affinity of the enzyme for AdoMet.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.

Emerging roles for chromatin as a signal integration and storage platform

Cells of a multicellular organism, all containing nearly identical genetic information, respond to differentiation cues in variable ways. In addition, cells are plastic, able to execute their specialized function while maintaining the ability to adapt to environmental changes. This is achieved through multiple mechanisms, including the direct regulation of chromatin-based processes in response to stimuli. How signal transduction pathways directly communicate with chromatin to change the epigenetic landscape is poorly understood. The preponderance of covalent modifications on histone tails coupled with a relatively small number of functional outputs raises the possibility that chromatin acts as a site of signal integration and storage.

Molecular pathways: Jak/STAT pathway: mutations, inhibitors, and resistance

Aberrant activation of the JAK/STAT pathway has been reported in a variety of disease states, including inflammatory conditions, hematologic malignancies, and solid tumors. For instance, a large proportion of patients with myeloproliferative neoplasms (MPN) carry the acquired gain-of-function JAK2 V617F somatic mutation. This knowledge has dramatically improved our understanding of the pathogenesis of MPNs and has facilitated the development of therapeutics capable of suppressing the constitutive activation of the JAK/STAT pathway, now recognized as a common underlying biologic abnormality in MPNs. Ruxolitinib is an oral JAK1 and JAK2 inhibitor that has recently been approved for the treatment of myelofibrosis and has been tested against other hematologic malignancies. A series of agents with different specificities against different members of the JAK family of proteins is currently undergoing evaluation in clinical trials for patients with MPNs, lymphoma, and solid tumors such as breast or pancreatic cancer. Despite the significant clinical activity exhibited by these agents in myelofibrosis, some patients fail to respond or progress during JAK kinase inhibitor therapy. Recent reports have shed light into the mechanisms of resistance to JAK inhibitor therapy. Several approaches hold promise to overcome such resistance.

Menin epigenetically represses Hedgehog signaling in MEN1 tumor syndrome

Multiple endocrine neoplasia type 1 (MEN1) is an inherited tumor syndrome that includes susceptibility to pancreatic islet tumors. This syndrome results from mutations in the MEN1 gene, encoding menin. Although menin acts as an oncogenic cofactor for mixed lineage leukemia (MLL) fusion protein-mediated histone H3 lysine 4 methylation, the precise basis for how menin suppresses gene expression and proliferation of pancreatic beta cells remains poorly understood. Here, we show that menin ablation enhances Hedgehog signaling, a proproliferative and oncogenic pathway, in murine pancreatic islets. Menin directly interacts with protein arginine methyltransferase 5 (PRMT5), a negative regulator of gene transcription. Menin recruits PRMT5 to the promoter of the Gas1 gene, a crucial factor for binding of Sonic Hedgehog (Shh) ligand to its receptor PTCH1 and subsequent activation of the Hedgehog signaling pathway, increases repressive histone arginine symmetric dimethylation (H4R3m2s), and suppresses Gas1 expression. Notably, MEN1 disease-related menin mutants have reduced binding to PRMT5, and fail to impart the repressive H4R3m2s mark at the Gas1 promoter, resulting in its elevated expression. Pharmacologic inhibition of Hedgehog signaling significantly reduces proliferation of insulinoma cells, and expression of Hedgehog signaling targets including Ptch1, in MEN1 tumors of mice. These findings uncover a novel link between menin and Hedgehog signaling whereby menin/PRMT5 epigenetically suppresses Hedgehog signaling, revealing it as a target for treating MEN1 tumors.

Protein arginine methyltransferase 1 interacts with and activates p38α to facilitate erythroid differentiation

Protein arginine methylation is emerging as a pivotal posttranslational modification involved in regulating various cellular processes; however, its role in erythropoiesis is still elusive. Erythropoiesis generates circulating red blood cells which are vital for body activity. Deficiency in erythroid differentiation causes anemia which compromises the quality of life. Despite extensive studies, the molecular events regulating erythropoiesis are not fully understood. This study showed that the increase in protein arginine methyltransferase 1 (PRMT1) levels, via transfection or protein transduction, significantly promoted erythroid differentiation in the bipotent human K562 cell line as well as in human primary hematopoietic progenitor CD34(+) cells. PRMT1 expression enhanced the production of hemoglobin and the erythroid surface marker glycophorin A, and also up-regulated several key transcription factors, GATA1, NF-E2 and EKLF, which are critical for lineage-specific differentiation. The shRNA-mediated knockdown of PRMT1 suppressed erythroid differentiation. The methyltransferase activity-deficient PRMT1G80R mutant failed to stimulate differentiation, indicating the requirement of arginine methylation of target proteins. Our results further showed that a specific isoform of p38 MAPK, p38α, promoted erythroid differentiation, whereas p38β did not play a role. The stimulation of erythroid differentiation by PRMT1 was diminished in p38α- but not p38β-knockdown cells. PRMT1 appeared to act upstream of p38α, since expression of p38α still promoted erythroid differentiation in PRMT1-knockdown cells, and expression of PRMT1 enhanced the activation of p38 MAPK. Importantly, we showed for the first time that PRMT1 was associated with p38α in cells by co-immunoprecipitation and that PRMT1 directly methylated p38α in in vitro methylation assays. Taken together, our findings unveil a link between PRMT1 and p38α in regulating the erythroid differentiation program and provide evidence suggesting a novel regulatory mechanism for p38α through arginine methylation.

Structure of the arginine methyltransferase PRMT5-MEP50 reveals a mechanism for substrate specificity

The arginine methyltransferase PRMT5-MEP50 is required for embryogenesis and is misregulated in many cancers. PRMT5 targets a wide variety of substrates, including histone proteins involved in specifying an epigenetic code. However, the mechanism by which PRMT5 utilizes MEP50 to discriminate substrates and to specifically methylate target arginines is unclear. To test a model in which MEP50 is critical for substrate recognition and orientation, we determined the crystal structure of Xenopus laevis PRMT5-MEP50 complexed with S-adenosylhomocysteine (SAH). PRMT5-MEP50 forms an unusual tetramer of heterodimers with substantial surface negative charge. MEP50 is required for PRMT5-catalyzed histone H2A and H4 methyltransferase activity and binds substrates independently. The PRMT5 catalytic site is oriented towards the cross-dimer paired MEP50. Histone peptide arrays and solution assays demonstrate that PRMT5-MEP50 activity is inhibited by substrate phosphorylation and enhanced by substrate acetylation. Electron microscopy and reconstruction showed substrate centered on MEP50. These data support a mechanism in which MEP50 binds substrate and stimulates PRMT5 activity modulated by substrate post-translational modifications.

Intrinsic resistance to JAK2 inhibition in myelofibrosis

PURPOSE

Recent results have shown that myeloproliferative neoplasms (MPN) are strongly associated with constitutive activation of the Janus-activated kinase (JAK)2 tyrosine kinase. However, JAK2 inhibitors currently approved or under development for treating myeloproliferative neoplasms do not selectively deplete the malignant clone, and the inhibition of activity of the drug target (JAK2) has not been rigorously evaluated in the clinical studies. Therefore, in this study we developed an in vitro assay to gain insight into how effectively JAK2 activity is inhibited in the samples of patients.

EXPERIMENTAL DESIGN

We treated primary cells from normal donors and patients with MPN with JAK2 inhibitors and measured phosphorylation of downstream targets STAT5 and STAT3 by flow cytometry. Obtained results were next correlated with JAK2 V617F allele burden and plasma cytokine level.

RESULTS

We observed a dose-dependent decrease in pSTAT5 and pSTAT3 in ex vivo treated granulocytes. However, phosphorylation of STAT3 and STAT5 in cells from patients with myelofibrosis was significantly less inhibited when compared with cells from patients with polycythemia vera, essential thrombocythemia, and normal donors. Sensitivity to inhibition did not correlate with JAK2 V617F clonal burden. Mixing studies using plasma from patients with myelofibrosis did not transfer resistance to sensitive cells. Likewise, no single cytokine measured seemed to account for the observed pattern of resistance.

CONCLUSIONS

Taken together, these observations suggest that there are cell intrinsic mechanisms that define a priori resistance to JAK2 inhibition in myelofibrosis, and the lesion is localized upstream of STAT3 and STAT5.

Histone-modifying enzymes: their role in the pathogenesis of acute leukemia and their therapeutic potential

Histone-modifying enzymes have recently been shown to play a central role in the regulation of both normal and malignant hematopoiesis. Post-translational modifications of histones and non-histone proteins underlies a regulatory complexity affecting numerous processes including transcriptional regulation, RNA processing and DNA damage response. Insights into the functions of these enzymes as well as their role in the epigenetic alterations found in leukemia will guide the development of novel therapeutic approaches. This review discusses examples of the proteins that have been implicated in the pathogenesis of leukemia, that may serve as potential therapeutic targets.

Overexpression of PRMT5 promotes tumor cell growth and is associated with poor disease prognosis in epithelial ovarian cancer

PRMT5 has been reported to be involved in the processes of tumor progression at various steps. The aim of this study was to examine the role of PRMT5 in epithelial ovarian cancer (EOC). In this study, PRMT5 and Ki-67 expression were examined by immunohistochemistry (IHC) in cohorts of normal, benign, and cancerous ovarian tissues. PRMT5 overexpression was observed in 83.1% (98/118) of EOCs, and it was significantly associated with serous type, poor differentiation, advanced tumor stage, lymph node invasion, presence of residual tumor, and high expression of Ki-67 (p<0.05, respectively). Moreover, overexpression of PRMT5 was an independent prognostic marker for decreased overall survival and progression-free survival in univariate survival analysis and multivariate Cox regression analysis. In ovarian cancer cell lines A2780 and SKOV3, PRMT5 knockdown by siRNA inhibited cell growth/proliferation and induced apoptosis via upregulation of E2F-1. These results suggest that overexpression of PRMT5 correlates with an aggressive malignant phenotype and may constitute a novel prognostic factor for EOC. Thus, PRMT5 may represent a clinically effective new target for therapy of ovarian cancer.

Protein arginine methyltransferases and cancer

There are nine protein arginine methyltransferases (PRMTs) encoded in mammalian genomes, the protein products of which catalyse three types of arginine methylation--monomethylation and two types of dimethylation. Protein arginine methylation is an abundant modification that has been implicated in signal transduction, gene transcription, DNA repair and mRNA splicing, among others. Studies have only recently linked this modification to carcinogenesis and metastasis. Sequencing studies have not generally found alterations to the PRMTs; however, overexpression of these enzymes is often associated with various cancers, which might make some of them viable targets for therapeutic strategies.

Mechanisms of epigenetic regulation of leukemia onset and progression

Over the past decade, it has become clear that both genetics and epigenetics play pivotal roles in cancer onset and progression. The importance of epigenetic regulation in proper maintenance of cellular state is highlighted by the frequent mutation of chromatin modulating factors across cancer subtypes. Identification of these mutations has created an interest in designing drugs that target enzymes involved in DNA methylation and posttranslational modification of histones. In this review, we discuss recurrent genetic alterations to epigenetic modulators in both myeloid and lymphoid leukemias. Furthermore, we review how these perturbations contribute to leukemogenesis and impact disease outcome and treatment efficacy. Finally, we discuss how the recent advances in our understanding of chromatin biology may impact treatment of leukemia.

Genetic and epigenetic alterations of myeloproliferative disorders

The classical BCR-ABL negative myeloproliferative neoplasms (MPN) polycythemia vera, essential thrombocythemia, and primary myelofibrosis are clonal hematopoietic disorders characterized by excessive production of terminally differentiated myeloid cells. In MPN patients, the disease can progress to secondary myelofibrosis or acute myeloid leukemia. Clonal hematopoiesis, disease phenotype, and progression are caused by somatically acquired genetic lesions of genes involved in cytokine signaling, RNA splicing, as well as epigenetic regulation. This review provides an overview of point mutations and cytogenetic lesions associated with MPN and addresses the role of these somatic lesions in MPN disease progression.

Small-molecule inhibitors in myeloproliferative neoplasms: are we aiming for the right targets?

The ATP-binding pocket of the kinase domain of JAK2 is the major target of the present treatment of myeloproliferative neoplasms. Several inhibitors of JAK2 that are ATP competitive have been developed, but they do not discriminate between wild-type and mutant JAK2. These inhibitors have been used in myelofibrosis and, for the first time, treatment induced a reduction in spleen size and in constitutional symptoms. However, no dramatic effects on BM fibrosis, allele burden, or peripheral blast numbers were observed. These data indicate that other avenues should be explored that would either target mutant molecules (JAKs or receptors) more specifically and spare wild-type JAK2 or that would address other pathways that contribute to the malignant proliferation. Future success in treating myeloproliferative neoplasms will depend on advances of the understanding of JAK-STAT signaling and also on a better understanding of the disease pathogenesis, especially the role that mutants in spliceosome factors and epigenetic regulators play in the phenotype of the disease and the precise mechanism of fibrosis development.

Hematopoietic stem cells, progenitor cells and leukemic stem cells in adult myeloproliferative neoplasms

The understanding of myeloproliferative neoplasms has changed dramatically since Dameshek proposed his classification over 50 years ago. Our knowledge of the types of cells which constitute the hematopoietic system and of how they are regulated has also appreciated significantly over this time. This review relates what is currently known about the acquired genetic mutations associated with adult myeloproliferative neoplasms to how they lead to the hematopoietic perturbations of myeloproliferative disease. There is a particular focus on how stem and progenitor cell compartments are affected by BCR-ABL1 and JAK2V617F mutations, and the particular issue of resistance of leukemic stem cells to conventional and targeted therapies.

Crystal structure of the human PRMT5:MEP50 complex

Protein arginine methyltransferases (PRMTs) play important roles in several cellular processes, including signaling, gene regulation, and transport of proteins and nucleic acids, to impact growth, differentiation, proliferation, and development. PRMT5 symmetrically di-methylates the two-terminal ω-guanidino nitrogens of arginine residues on substrate proteins. PRMT5 acts as part of a multimeric complex in concert with a variety of partner proteins that regulate its function and specificity. A core component of these complexes is the WD40 protein MEP50/WDR77/p44, which mediates interactions with binding partners and substrates. We have determined the crystal structure of human PRMT5 in complex with MEP50 (methylosome protein 50), bound to an S-adenosylmethionine analog and a peptide substrate derived from histone H4. The structure of the surprising hetero-octameric complex reveals the close interaction between the seven-bladed β-propeller MEP50 and the N-terminal domain of PRMT5, and delineates the structural elements of substrate recognition.

Three distinct patterns of histone H3Y41 phosphorylation mark active genes

The JAK2 tyrosine kinase is a critical mediator of cytokine-induced signaling. It plays a role in the nucleus, where it regulates transcription by phosphorylating histone H3 at tyrosine 41 (H3Y41ph). We used chromatin immunoprecipitation coupled to massively parallel DNA sequencing (ChIP-seq) to define the genome-wide pattern of H3Y41ph in human erythroid leukemia cells. Our results indicate that H3Y41ph is located at three distinct sites: (1) at a subset of active promoters, where it overlaps with H3K4me3, (2) at distal cis-regulatory elements, where it coincides with the binding of STAT5, and (3) throughout the transcribed regions of active, tissue-specific hematopoietic genes. Together, these data extend our understanding of this conserved and essential signaling pathway and provide insight into the mechanisms by which extracellular stimuli may lead to the coordinated regulation of transcription.

The role of mutations in epigenetic regulators in myeloid malignancies

Recent genomic studies have identified novel recurrent somatic mutations in patients with myeloid malignancies, including myeloproliferative neoplasms (MPNs), myelodysplastic syndrome (MDS) and acute myeloid leukaemia (AML). In some cases these mutations occur in genes with known roles in regulating chromatin and/or methylation states in haematopoietic progenitors, and in other cases genetic and functional studies have elucidated a role for specific mutations in altering epigenetic patterning in myeloid malignancies. In this Review we discuss recent genetic and functional data implicating mutations in epigenetic modifiers, including tet methylcytosine dioxygenase 2 (TET2), isocitrate dehydrogenase 1 (IDH1), IDH2, additional sex combs-like 1 (ASXL1), enhancer of zeste homologue 2 (EZH2) and DNA methyltransferase 3A (DNMT3A), in the pathogenesis of MPN, MDS and AML, and discuss how this knowledge is leading to novel clinical, biological and therapeutic insights.

Epigenetic dysregulation of secreted frizzled-related proteins in myeloproliferative neoplasms complements the JAK2V617F-mutation

Background

Secreted frizzled-related proteins (SFRPs) are antagonists of the Wnt signaling pathway, which plays a central role in stem cell maintenance and differentiation of stem cells and hematopoietic progenitors. Epigenetic downregulation of SFRPs by promoter hypermethylation has been described to be involved in the pathogenesis of hematopoietic malignancies. There is an association between aberrant Wnt signaling and the established cancer stem cell concept. In contrast to BCR-ABL1-positive chronic myeloid leukemia CML, BCR-ABL1-negative myeloproliferative neoplasms (Ph^-MPN) are characterized by the frequent occurrence of an autoactivating mutation in the JAK2 tyrosine kinase (JAK2V617F) or other mutations in the JAK-STAT pathway. However, pathogenetic mechanisms of JAK2 mutated or unmutated Ph^-MPN remain not completely understood. We determined the promoter methylation status of SFRP-1, -2, -4, and -5 in 57 MPN patient samples by methylation-specific polymerase chain reaction (PCR) (MSP). JAK2V617F was assessed by allele-specific PCR.

Results

Aberrant methylation among primary MPN samples was 4% for SFRP-1, 25% for SFRP-2, 2% for SFRP-4, and 0% for SFRP-5. Hypermethylation of SFRP-2, which was the most frequently hypermethylated gene in our study, could not be correlated to any specific MPN subtype. However, we detected a significant correlation between SFRP-2 methylation and presence of a JAK2V617F mutation (P = 0.008). None of the 10 CML samples showed any SFRP-methylation.

Conclusions

Our data indicate that epigenetic dysregulation of the Wnt signaling pathway is a common event in MPN with aberrant methylation of at least one SFRP being detected in 25% of the primary patient samples and in 30% if only accounting for Ph^-MPN. A significant correlation between SFRP-2 methylation and presence of JAK2V617F in our data supports the hypothesis that epigenetic dysregulation may be a complementary mechanism to genetic aberrations. Aberrant methylation of crucial stem cell maintenance genes seems to contribute to disease pathogenesis in Ph^-MPN.

Disordered signaling in myeloproliferative neoplasms

The human myeloproliferative neoplasms (MPN) have long been associated with abnormal responses to cytokines and activation of signaling pathways, although the exact molecular mechanisms underlying these observations were unknown. This situation altered with the discovery of the JAK2 V617F, which presaged the ongoing description of further mutations predicted to activate canonical signaling pathways in MPN. This article covers the nature of these mutations and summarizes functional experiments in model systems and in human MPN cells to define the signaling pathways altered and how these drive and determine the MPN cellular phenotype. Also discussed are recently described, novel noncanonical signaling pathways to chromatin predicted to alter gene transcription more directly and to also contribute to the MPN phenotype.

Protein arginine methyltransferase 5 is a potential oncoprotein that upregulates G1 cyclins/cyclin-dependent kinases and the phosphoinositide 3-kinase/AKT signaling cascade

Increasing evidence suggests that PRMT5, a protein arginine methyltransferase, is involved in tumorigenesis. However, no systematic research has demonstrated the cell-transforming activity of PRMT5. We investigated the involvement of PRMT5 in tumor formation. First, we showed that PRMT5 was associated with many human cancers, through statistical analysis of microarray data in the NCBI GEO database. Overexpression of ectopic PRMT5 per se or its specific shRNA enhanced or reduced cell growth under conditions of normal or low concentrations of serum, low cell density, and poor cell attachment. A stable clone that expressed exogenous PRMT5 formed tumors in nude mice, which demonstrated that PRMT5 is a potential oncoprotein. PRMT5 accelerated cell cycle progression through G1 phase and modulated regulators of G1; for example, it upregulated cyclin-dependent kinase (CDK) 4, CDK6, and cyclins D1, D2 and E1, and inactivated retinoblastoma protein (Rb). Moreover, PRMT5 activated phosphoinositide 3-kinase (PI3K)/AKT and suppressed c-Jun N-terminal kinase (JNK)/c-Jun signaling cascades. However, only inhibition of PI3K activity, and not overexpression of JNK, blocked PRMT5-induced cell proliferation. Further analysis of PRMT5 expression in 64 samples of human lung cancer tissues by microarray and western blot analysis revealed a tight association of PRMT5 with lung cancer. Knockdown of PRMT5 retarded cell growth of lung cancer cell lines A549 and H1299. In conclusion, to the best of our knowledge, we have characterized the cell-transforming activity of PRMT5 and delineated its underlying mechanisms for the first time.

New pieces of a puzzle: the current biological picture of MPN

Over the last years, we have witnessed significant improvement in our ability to elucidate the genetic events, which contribute to the pathogenesis of acute and chronic leukemias, and also in patients with myeloproliferative neoplasms (MPN). However, despite significant insight into the role of specific mutations, including the JAK2V617F mutation, in MPN pathogenesis, the precise mechanisms by which specific disease alleles contribute to leukemic transformation in MPN remain elusive. Here we review recent studies aimed at understanding the role of downstream signaling pathways in MPN initiation and phenotype, and discuss how these studies have begun to lead to novel insights with biologic, clinical, and therapeutic relevance.

Negative regulation of JAK2 by H3K9 methyltransferase G9a in leukemia

Histone methylation at specific lysine residues is a crucial regulatory process in transcriptional regulation. Using chromatin immunoprecipitation with microarray technology (ChIP-chip) analysis, we found that the H3K9-me2 target gene JAK2 was an important factor during differentiation of the HL-60 promyelocytic leukemia cell line by all-trans-retinoic acid (ATRA) treatment. Here, we report that the H3K9 methyltransferase G9a negatively regulated JAK2 transcription in histone methyltransferase activity and in a YY1-dependent manner during ATRA-mediated leukemia cell differentiation. We found that G9a knockdown repressed ATRA-mediated HL-60 cell differentiation. We demonstrated that G9a interacts with YY1 and is recruited to the JAK2 promoter along with corepressors, including histone deacetylase, that induced H3K9-me2. Repression of JAK2 transcription by G9a decreased H3Y41 phosphorylation and promoted inhibition of the recently identified JAK2-H3Y41P-HP1α pathway-mediated leukemogenesis.

Myelofibrosis 2012: it's complicated

Major advances in myeloproliferative neoplasms in the last decade have cast light on their complexity. The identification of JAK2 (V617F) briefly promised a unifying mechanism of pathogenesis with a single pathway that could be efficiently targeted. Instead, there have been major advances in understanding acquired and background genetic and epigenetic contributors to this group of disorders, with refined risk prediction models and experimental therapeutics that have provided a more nuanced model of disease. In aggregate these observations likely explain the heterogeneity of these disorders and their generally unpredictable response to therapy. Molecular studies, beginning with the identification of JAK2 (V617F), have led to a concept of MPN subtypes existing on a continuum, and additional discoveries such as TET2 and EZH2 mutations have provided the molecular underpinnings to begin to explain overlapping phenotypes in myeloid malignancies more generally. In many ways the pace of molecular discovery is outstripping our ability to integrate these observations into clinical care, both in terms of molecular diagnostics and medical decision making. This review will attempt to summarize, within a clinical context, our evolving understanding of myeloproliferative neoplasms. It focuses on biology, histopathology, prognostic scoring systems, stem cell transplantation as well as selected clinical/preclinical therapeutic observations.

Protein arginine methyltransferases (PRMTs) as therapeutic targets

INTRODUCTION

Protein arginine methyltransferases (PRMTs) add one or two monomethyl groups to the guanidino nitrogen atoms of arginine residues, resulting in epigenetic modification of histones or changes of protein-protein interactions, which in turn leads to the regulation of a variety of biological functions, including transcriptional activation/repression, signal transduction, cell differentiation, and embryonic development. As dysregulation of PRMTs has been observed in diverse types of cancers and modulation of their levels affects cancer cell growth, these enzymes are considered to be potential therapeutic targets.

AREAS COVERED

In this review, we examined recent advances in our understanding of the regulatory mechanisms of PRMT activity and the biological roles of PRMTs in embryonic stem cell, Wnt/β-catenin signaling, and cancer development.

EXPERT OPINION

The roles of PRMTs have been fairly well established, but further studies are required to determine how PRMTs are regulated by cellular signaling pathways in vivo. Since the usage of adult stem cells is under intense scrutiny by society, identification of the roles of PRMTs in adult stem cells is expected in the near future. Although small molecules specific to PRMTs with high potency in vitro have been identified, development of small molecules that can regulate the activity of PRMTs in vivo is urgently required for therapeutic purposes.

Regulation of p27 (Kip1) by mitogen-induced tyrosine phosphorylation

Extracellular mitogen signal transduction is initiated by ligand binding to specific receptors of target cells. This causes a cellular response that frequently triggers the activation of tyrosine kinases. Non-receptor kinases like Src and Lyn can directly phosphorylate the Cdk inhibitor protein p27 (Kip1) . Tyrosine phosphorylation can cause impaired Cdk-inhibitory activity and decreased stability of p27. In addition to these non-receptor tyrosine kinases, the receptor-associated tyrosine kinase Janus kinase 2 (JAK2) was recently identified to phosphorylate p27. JAK2 becomes activated through binding of various cytokines and growth factors to their corresponding receptors and can directly bind and selectively phosphorylate tyrosine residue 88 (Y88) of the Cdk inhibitor p27. This impairs Cdk inhibition by p27 and promotes its ubiquitin-dependent proteasomal degradation. Via this mechanism, JAK2 can link cytokine and growth factor initiated signal transduction to p27 regulation, whereas oncogenes like JAK2V617F or BCR-Abl can use this mechanism to inactivate the Cdk inhibitor.

Emerging epigenetic targets and therapies in cancer medicine

Abnormalities in the epigenetic regulation of chromatin structure and function can lead to aberrant gene expression and cancer development. Consequently, epigenetic therapies aim to restore normal chromatin modification patterns through the inhibition of various components of the epigenetic machinery. Histone deacetylase and DNA methyltransferase inhibitors represent the first putative epigenetic therapies; however, these agents have pleiotropic effects and it remains unclear how they lead to therapeutic responses. More recently, drugs that inhibit histone methyltransferases were developed, perhaps representing more specific agents. We review emerging epigenetic targets in cancer and present recent models of promising epigenetic therapies.

SIGNIFICANCE

The use of DNA methyltransferase and histone deacetylase inhibitors in patients has validated the use of drugs targeted to epigenetic enzymes and strengthened the need for development of additional therapies. In this review, we summarize recently discovered epigenetic abnormalities, their implications for cancer, and the approaches taken for discovering small-molecule inhibitors targeting various properties of the epigenetic machinery.

Disordered epigenetic regulation in the pathophysiology of myeloproliferative neoplasms

The discovery of mutations activating JAK-STAT signaling in the majority of patients with myeloproliferative neoplasms (MPNs) led to identification of tyrosine kinase activation as the common predominant mechanism driving MPN pathogenesis. Nevertheless, the existence of additional genetic events that modify the MPN phenotype, predate JAK2 mutations, or contribute to leukemic transformation of MPNs was suspected. Recent advances in genomic technologies have led to the discovery of mutations in a number of epigenetic modifiers in patients with MPNs, including mutations in TET2, ASXL1, IDH1, IDH2, and EZH2. In addition to mutation, alterations in the expression or activity of chromatin-modifying/reading proteins PRMT5 and L3MBTL1 have been found to be important in MPN development. Moreover, the JAK2 mutation itself recently has been shown to directly affect histone post-translational modifications. This article reviews the clinical and functional implications of epigenetic alterations in the pathogenesis of MPNs.

Biology and significance of the JAK/STAT signalling pathways

Since its discovery two decades ago, the activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway by numerous cytokines and growth factors has resulted in it becoming one of the most well-studied intracellular signalling networks. The field has progressed from the identification of the individual components to high-resolution crystal structures of both JAK and STAT, and an understanding of the complexities of the molecular activation and deactivation cycle which results in a diverse, yet highly specific and regulated pattern of transcriptional responses. While there is still more to learn, we now appreciate how disruption and deregulation of this pathway can result in clinical disease and look forward to adoption of the next generation of JAK inhibitors in routine clinical treatment.

HOXA9 methylation by PRMT5 is essential for endothelial cell expression of leukocyte adhesion molecules

The induction of proinflammatory proteins in stimulated endothelial cells (EC) requires activation of multiple transcription programs. The homeobox transcription factor HOXA9 has an important regulatory role in cytokine induction of the EC-leukocyte adhesion molecules (ELAM) E-selectin and vascular cell adhesion molecule 1 (VCAM-1). However, the mechanism underlying stimulus-dependent activation of HOXA9 is completely unknown. Here, we elucidate the molecular mechanism of HOXA9 activation by tumor necrosis factor alpha (TNF-α) and show an unexpected requirement for arginine methylation by protein arginine methyltransferase 5 (PRMT5). PRMT5 was identified as a TNF-α-dependent binding partner of HOXA9 by mass spectrometry. Small interfering RNA (siRNA)-mediated depletion of PRMT5 abrogated stimulus-dependent HOXA9 methylation with concomitant loss in E-selectin or VCAM-1 induction. Chromatin immunoprecipitation analysis revealed that PRMT5 is recruited to the E-selectin promoter following transient HOXA9 binding to its cognate recognition sequence. PRMT5 induces symmetric dimethylation of Arg140 on HOXA9, an event essential for E-selectin induction. In summary, PRMT5 is a critical coactivator component in a newly defined, HOXA9-containing transcription complex. Moreover, stimulus-dependent methylation of HOXA9 is essential for ELAM expression during the EC inflammatory response.

Targeting the interleukin-6/Jak/stat pathway in human malignancies

The Janus kinase/signal transducer and activator of transcription (Jak/Stat) pathway was discovered 20 years ago as a mediator of cytokine signaling. Since this time, more than 2,500 articles have been published demonstrating the importance of this pathway in virtually all malignancies. Although there are dozens of cytokines and cytokine receptors, four Jaks, and seven Stats, it seems that interleukin-6-mediated activation of Stat3 is a principal pathway implicated in promoting tumorigenesis. This transcription factor regulates the expression of numerous critical mediators of tumor formation and metastatic progression. This review will examine the relative importance and function of this pathway in nonmalignant conditions as well as malignancies (including tumor intrinsic and extrinsic), the influence of other Stats, the development of inhibitors to this pathway, and the potential role of inhibitors in controlling or eradicating cancers.

Epigenetic mechanisms in tumorigenesis, tumor cell heterogeneity and drug resistance

Resistance of cancer cells to chemotherapeutics and emerging targeted drugs is a devastating problem in the treatment of cancer patients. Multiple mechanisms contribute to drug resistance such as increased drug efflux, altered drug metabolism, secondary mutations in drug targets, and activation of downstream or parallel signal transduction pathways. The rapid kinetics, the reversibility of acquired drug resistance and the absence of genetic mutations suggest an epigenetic basis for drug insensitivity. Similar to the cellular variance seen in the human body, epigenetic mechanisms, through reversible histone modifications and DNA methylation patterns, generate a variety of transcriptional states resulting in a dynamic heterogeneous tumor cell population. Consequently, epigenomes favoring survival in the presence of a drug by aberrant transcription of drug transporters, DNA-repair enzymes and pro-apoptotic factors render cytotoxic and targeted drugs ineffective and allow selection of rare drug-resistant tumor cells. Recent advances in charting cancer genomes indeed strongly indicate a role for epigenetic regulators in driving cancer, which may result in the acquisition of additional (epi)genetic modifications leading to drug resistance. These observations have important clinical consequences as they provide an opportunity for "epigenetic drugs" to change reversible drug-resistance-associated epigenomes to prevent or reverse non-responsiveness to anti-cancer drugs.

Targeting JAK2 in the therapy of myeloproliferative neoplasms

INTRODUCTION

Myeloproliferative neoplasms (MPNs) are a group of stem cell diseases, including polycythemia vera, essential thrombocythemia and primary myelofibrosis. Currently, there is no curative therapy for these diseases other than bone marrow transplant; therefore there is an apparent need for palliative treatment. MPNs are frequently associated with activating mutations in JAK2; small-molecule drugs targeting this molecule have entered clinical trials.

AREAS COVERED

In this review novel JAK2 inhibitors are discussed and alternative approaches to inhibiting their transforming potential are highlighted. Current clinical approaches do not only aim at blocking JAK2 activity, but also at reducing its stability and expression are highlighted, including inhibition of heat shock protein 90 (HSP90) and deacetylases (DAC) have the potential to significantly enhance the efficacy of JAK2 inhibitors.

EXPERT OPINION

Preliminary results from clinical trials indicate the feasibility and efficacy of JAK2-targeted approaches. However, JAK2 inhibitor treatment is limited by dose-dependent toxicity and combination treatment might be required. The discovery of JAK2 mutations that cause secondary resistance in vitro would further highlight the need for the development of next-generation JAK2 inhibitors and novel synergistic approaches.

Human protein arginine methyltransferase 7 (PRMT7) is a type III enzyme forming ω-NG-monomethylated arginine residues

Full-length human protein arginine methyltransferase 7 (PRMT7) expressed as a fusion protein in Escherichia coli was initially found to generate only ω-N(G)-monomethylated arginine residues in small peptides, suggesting that it is a type III enzyme. A later study, however, characterized fusion proteins of PRMT7 expressed in bacterial and mammalian cells as a type II/type I enzyme, capable of producing symmetrically dimethylated arginine (type II activity) as well as small amounts of asymmetric dimethylarginine (type I activity). We have sought to clarify the enzymatic activity of human PRMT7. We analyzed the in vitro methylation products of a glutathione S-transferase (GST)-PRMT7 fusion protein with robust activity using a variety of arginine-containing synthetic peptides and protein substrates, including a GST fusion with the N-terminal domain of fibrillarin (GST-GAR), myelin basic protein, and recombinant human histones H2A, H2B, H3, and H4. Regardless of the methylation reaction conditions (incubation time, reaction volume, and substrate concentration), we found that PRMT7 only produces ω-N(G)-monomethylarginine with these substrates. In control experiments, we showed that mammalian GST-PRMT1 and Myc-PRMT5 were, unlike PRMT7, able to dimethylate both peptide P-SmD3 and SmB/D3 to give the expected asymmetric and symmetric products, respectively. These experiments show that PRMT7 is indeed a type III human methyltransferase capable of forming only ω-N(G)-monomethylarginine, not asymmetric ω-N(G),N(G)-dimethylarginine or symmetric ω-N(G),N(G')-dimethylarginine, under the conditions tested.

Protein arginine methyltransferase 5 (PRMT5) signaling suppresses protein kinase Cδ- and p38δ-dependent signaling and keratinocyte differentiation

PKCδ is a key regulator of keratinocyte differentiation that activates p38δ phosphorylation leading to increased differentiation as measured by an increased expression of the structural protein involucrin. Our previous studies suggest that p38δ exists in association with protein partners. A major goal is to identify these partners and understand their role in regulating keratinocyte differentiation. In this study we use affinity purification and mass spectrometry to identify protein arginine methyltransferase 5 (PRMT5) as part of the p38δ signaling complex. PRMT5 is an arginine methyltransferase that symmetrically dimethylates arginine residues on target proteins to alter target protein function. We show that PRMT5 knockdown is associated with increased p38δ phosphorylation, suggesting that PRMT5 impacts the p38δ signaling complex. At a functional level we show that PRMT5 inhibits the PKCδ- or 12-O-tetradecanoylphorbol-13-acetate-dependent increase in human involucrin expression, and PRMT5 dimethylates proteins in the p38δ complex. Moreover, PKCδ expression reduces the PRMT5 level, suggesting that PKCδ activates differentiation in part by reducing PRMT5 level. These studies indicate antagonism between the PKCδ and PRMT5 signaling in control of keratinocyte differentiation.

Protein arginine methyltransferase Prmt5-Mep50 methylates histones H2A and H4 and the histone chaperone nucleoplasmin in Xenopus laevis eggs

Histone proteins carry information contained in post-translational modifications. Eukaryotic cells utilize this histone code to regulate the usage of the underlying DNA. In the maturing oocytes and eggs of the frog Xenopus laevis, histones are synthesized in bulk in preparation for deposition during the rapid early developmental cell cycles. During this key developmental time frame, embryonic pluripotent chromatin is established. In the egg, non-chromatin-bound histones are complexed with storage chaperone proteins, including nucleoplasmin. Here we describe the identification and characterization of a complex of the protein arginine methyltransferase 5 (Prmt5) and the methylosome protein 50 (Mep50) isolated from Xenopus eggs that specifically methylates predeposition histones H2A/H2A.X-F and H4 and the histone chaperone nucleoplasmin on a conserved motif (GRGXK). We demonstrate that nucleoplasmin (Npm), an exceedingly abundant maternally deposited protein, is a potent substrate for Prmt5-Mep50 and is monomethylated and symmetrically dimethylated at Arg-187. Furthermore, Npm modulates Prmt5-Mep50 activity directed toward histones, consistent with a regulatory role for Npm in vivo. We show that H2A and nucleoplasmin methylation appears late in oogenesis and is most abundant in the laid egg. We hypothesize that these very abundant arginine methylations are constrained to pre-mid blastula transition events in the embryo and therefore may be involved in the global transcriptional repression found in this developmental time frame.