New mutations of MPL in primitive myelofibrosis: only the MPL W515 mutations promote a G1/S-phase transition

A review on the functional characteristics of the c-Myeloproliferative Leukaemia (c-MPL) gene and its isoforms

The c-MPL-TPO axis regulates hematopoiesis by activating various signalling cascades, including JAK/STAT, MAPK/ERK, and PIK3/AKT. Here, we have summarized how TPO is regulated by c-MPL and, how mutations in the c-MPL regulate hematopoiesis. We also focus on its non-hematological regulatory role in diseases like Unstable Angina and pathways like DNA damage repair, skeletal homeostasis, & apoptotic regulation of neurons/HSCs at the embryonic state. We discuss the therapeutic efficiency of c-MPL and, its potential to be developed as a bio-marker for detecting metastasis and development of chemo-resistance in various cancers, justifying the multifaceted nature of c-MPL. We have also highlighted the importance of c-MPL isoforms and their stoichiometry in controlling the HSC quiescent and proliferative state. The regulation of the ratio of different isoforms through gene-therapy can open future therapeutic avenues. A systematic understanding of c-MPL-isoforms would undoubtedly take one step closer to facilitating c-MPL from basic-research towards translational medicine.

Assessment of intestinal status in MPLW515L mutant myeloproliferative neoplasms mice model

The MPLW515L mutation is a prevalent genetic mutation in patients with myeloproliferative neoplasms (MPN), and utilizing this mutation in mice model can provide important insights into the disease. However, the relationship between intestinal homeostasis and MPN mice model remains elusive. In this study, we utilized a retroviral vector to transfect hematopoietic stem cells with the MPLW515L mutation, creating mutated MPN mice model to investigate their intestinal status. Our results revealed that the MPLW515L in MPN mice model aggravated inflammation in the intestines, decreased the levels of tight junction proteins and receptors for bacteria metabolites. Additionally, there was increased activation of the caspase1/IL-1β signaling pathway and a significant reduction in phos-p38 levels in the intestinal tissue in MPN mice. The MPLW515L mutation also led to up-expression of anti-microbial genes in the intestinal tract. Though the mutation had no impact on the alpha diversity and dominant bacterial taxa, it did influence the rare bacterial taxa/sub-communities and consequently impacted intestinal homeostasis. Our findings demonstrate the significance of MPLW515L mice model for studying MPN disease and highlight the mutation's influence on intestinal homeostasis, including inflammation, activation of the IL-1β signaling pathway, and the composition of gut microbial communities.

Simultaneous detection of JAK2, CALR, and MPL mutations and quantitation of JAK2 V617F allele burden in myeloproliferative neoplasms using the quenching probe-Tm method in i-densy IS-5320

INTRODUCTION

Accurate detection of myeloproliferative neoplasms (MPN)-associated gene mutations is necessary to correctly diagnose MPN. However, conventional gene testing has various limitations, including the requirement of skilled technicians, cumbersome experimental procedures, and turnaround time of several days. The gene analyzer i-densy IS-5320 allows gene testing using the quenching probe-Tm method. Specifically, pretreatment of samples including DNA extraction, amplification and detection of genes, and analysis of results are performed in a fully automatic manner after samples and test reagents are added into this system, which is compact and can be easily installed in a laboratory. The aim of this study is to investigate the sensitivity and specificity associated with the simultaneous detection of MPN-associated gene mutations.

METHODS

We conducted an analysis of MPN-associated genes using i-densy IS-5320. We analyzed 384 samples (171 JAK2 V617F mutations, 10 JAK2 exon12 mutations, 104 CALR mutations, and 26 MPL mutations) that had been examined using conventional approaches such as allele-specific polymerase chain reaction (PCR), droplet digital PCR, and the direct sequencing method.

RESULTS

The detection accuracy of JAK2 V617F, JAK2 exon 12, CALR, and MPL was 100.0% (383/383), 99.7% (383/384), 100.0% (370/370), and 99.7% (377/378), respectively. There was a strong positive correlation between the JAK2 V617F allele burden measured using conventional methods and i-densy IS-5320 (r = .989).

CONCLUSION

Overall, i-densy IS-5320 exhibited good accuracy in terms of analyzing MPN-associated genes; thus, it can serve as a replacement for conventional methods of MPN-associated gene testing.

The MPL mutation

Myeloproliferative neoplasms (MPN) patients share driver mutations in JAK2, MPL or CALR genes leading to the activation of the thrombopoietin receptor (TPOR) and downstream signaling pathways. JAK2 mutation drives all the three major entities of MPN (Polycythemia Vera, Essential Thrombocythemia and Primary Myelofibrosis) through the constitutive activation of TPOR, erythropoietin (EPOR) and colony stimulating factor 3 receptor (CSF3R) signaling. MPL is a proto-oncogene encoding for TPOR, the hematopoietic growth factor receptor of myeloid stem cells. MPL mutants induce the stable dimerization of TPOR that in turn activate JAK2 and the thrombopoietin pathway. The thrombopoietin pathway plays an important role in the development of megakaryocytes and platelets as well as the self-renewal of hematopoietic stem cells. Little wonder therefore that mutations of MPL result in thrombocytosis, leading to an abnormal MPL trafficking or receptor activation. Finally, some extremely rare germline genetic variants in MPL can induce MPN-like hereditary disease. Against this molecular background, TPOR is a key actor in the MPN development and MPL mutations are of major relevance to fully elucidate the molecular mechanisms underlying the clinical manifestations of MPN and to arrange novel therapeutic strategies aiming to disrupt the dysegulated signaling cascade. This chapter will focus on the role MPL in the pathogenesis of MPN and in familial thrombocytosis and will review these different subtypes of somatic and germline genetic variants by dissecting how they impact clinical phenotype.

MAEA is an E3 ubiquitin ligase promoting autophagy and maintenance of haematopoietic stem cells

Haematopoietic stem cells (HSCs) tightly regulate their quiescence, proliferation, and differentiation to generate blood cells during the entire lifetime. The mechanisms by which these critical activities are balanced are still unclear. Here, we report that Macrophage-Erythroblast Attacher (MAEA, also known as EMP), a receptor thus far only identified in erythroblastic island, is a membrane-associated E3 ubiquitin ligase subunit essential for HSC maintenance and lymphoid potential. Maea is highly expressed in HSCs and its deletion in mice severely impairs HSC quiescence and leads to a lethal myeloproliferative syndrome. Mechanistically, we have found that the surface expression of several haematopoietic cytokine receptors (e.g. MPL, FLT3) is stabilised in the absence of Maea, thereby prolonging their intracellular signalling. This is associated with impaired autophagy flux in HSCs but not in mature haematopoietic cells. Administration of receptor kinase inhibitor or autophagy-inducing compounds rescues the functional defects of Maea-deficient HSCs. Our results suggest that MAEA provides E3 ubiquitin ligase activity, guarding HSC function by restricting cytokine receptor signalling via autophagy.

MOLECULAR GENETIC ABNORMALITIES IN THE GENOME OF PATIENTS WITH Ph-NEGATIVE MYELOPROLIFERATIVE NEOPLASIA AFFECTED BY IONIZING RADIATION AS A RESULT OF THE CHORNOBYL NUCLEAR ACCIDENT

OBJECTIVE

to determine the frequency of major somatic mutations in the JAK2, MPL and CALR genes in the genomeof patients with Ph-negative myeloproliferative neoplasms that occur in individuals who have been exposed to ionizing radiation as a result of the Chornobyl accident.

MATERIALS AND METHODS

Molecular genetic analysis of genomic DNA samples isolated from blood was performed in90 patients with Ph-negative myeloproliferative neoplasia (MPN) with a history of radiation exposure and 191patients with spontaneous MPN utilizing allele-specific polymerase chain reaction (PCR).

RESULTS

The presence of major mutations in the genes JAK2, CALR and MPL was revealed in patients with MPN witha history of radiation exposure with a frequency 58.9 % (53 of 90), 12.2 % (11 of 90), and 0 % respectively, and without exposure with frequency 75.4 % (144 of 191), 3.1 % (6 out of 191) and 1.6 % (3 out of 191) respectively.Mutations JAK2 V617F in patients with spontaneous MPN were observed in each clinical form: polycythemia vera (PV),essential thrombocythemia (ET) and primary myelofibrosis (PMF). CALR mutations were detected exclusively inpatients with PMF and ET, significantly more often in groups with a radiation exposure history (18.9 % and 33.3 %,vs. 4.2 % and 6.5 %) than without one. At the same time, the occurence of MPL mutations was determined only inpatients with spontaneous MPN in 1.6 % of casees. Triple negative mutation status of genes JAK2, MPL and CALR prevailed in the group of patients with MPN with a history of radiation exposure and was 27.8 %, against 16.2 % inpatients without radiation exposure (p = 0.05).

CONCLUSIONS

Genomic research of patients with Ph-negative MPN revealed features of molecular genetic damage inthose patients who were exposed to IR as a result of the Chornobyl accident and those with spontaneous MPN. Thedata obtained by determining of JAK2, MPL and CALR genes mutational status in the genome of patients with MPN isnecessary to expand the understanding of the mechanism of leukogenesis, especially caused by radiation.

Prefibrotic Myelofibrosis Presenting with Multiple Cerebral Embolic Infarcts and the Rare MPL W515S Mutation

Acquired, activating mutations of MPL W515 are recognised driver mutations of the myeloproliferative neoplasms (MPN), namely, essential thrombocythemia and primary myelofibrosis. The most common mutation at this codon is W515L with several other mutations also described at a lower frequency. Of these less common mutations, MPL W515S has only been reported sporadically with limited information on clinicopathological associations. We describe the case of an elderly man with persistent thrombocytosis presenting with an ischemic cerebral event. Bone marrow biopsy showed evidence of prefibrotic myelofibrosis with targeted sequencing demonstrating the presence of the rare MPL W515S mutation. Thrombolytic and cytoreductive therapies resulted in a favorable outcome and follow-up. This case provides additional, necessary, and phenotypic data for the rare MPN-associated MPL W515S mutation.

Two activating mutations of MPL in triple-negative myeloproliferative neoplasms

MPLW515K or W515L mutation plays an important role in the pathogenesis of myeloproliferative neoplasms (MPNs) through signaling molecules of the cytokine receptor axis. Besides MPLW515K or W515L, more than 30 atypical MPL mutations have been reported in patients who are negative for JAK2V617F, MPLW515K/L, and CALR mutations. Here, we aimed to identify the disease-causing mutations in the triple-negative case of ET. We described two MPL mutations in patients diagnosed with ET by target sequencing the hotspot mutation region of MPL gene. The MPLA497-L498ins4 is an insertion mutation detected recurrently in ET patients, and the MPLW515RQ516E is a novel double-point mutation found in an ET patient. Functional studies of MPLA497-L498ins4 and MPLW515RQ516E revealed that they are gain-of-function mutations. Mutants of MPLA497-L498ins4 and MPLW515RQ516E promoted autonomous proliferation on Ba/F3 cells in the absence of IL-3. Autonomous activation of TPO-R without ligand TPO was observed in MPLA497-L498ins4 and MPLW515RQ516E mutants. Lower percentage of cells in G1 phase and higher percentage of cells in S phase of two atypical MPL mutants were detected after culturing without any cytokines. These two atypical MPL mutations also presented increase in phosphorylation of signaling proteins including JAK2/STAT, PI3K/AKT, and MAPK/RAS. In summary, the MPLA497-L498ins4 and MPLW515RQ516E are gain-of-function mutations which may be novel driving factors participating in the pathogenesis of triple-negative MPN.

MPL Y252H anMd PL F126fs mutations in essential thrombocythemia: Case series and review of literature

Essential thrombocythemia (ET) is a clonal bone marrow disease, characterized by increased production of platelets along with other clinical and bone marrow findings. Most patients with ET will have a somatic mutation in one of the known gene locations of JAK2, CALR, or MPL that can upregulate the JAK-STAT pathway. MPL mutation is present in 5% of cases with the most common mutations being W515L and W515K. In this report we describe 2 cases of patients with clinical and laboratory picture of ET. One patient carried MPLY252H mutation which is previously unreported in the adult population but has been shown to be a gain-of-function mutation. The other patient carried MPL F126fs mutation which is not known to be of clinical importance and has not been previously reported.

Characteristics of myeloproliferative neoplasms in patients exposed to ionizing radiation following the Chernobyl nuclear accident

Myeloproliferative neoplasms (MPNs) driver mutations are usually found in JAK2, MPL, and CALR genes; however, 10%-15% of cases are triple negative (TN). A previous study showed lower rate of JAK2 V617F in primary myelofibrosis patients exposed to low doses of ionizing radiation (IR) from Chernobyl accident. To examine distinct driver mutations, we enrolled 281 Ukrainian IR-exposed and unexposed MPN patients. Genomic DNA was obtained from peripheral blood leukocytes. JAK2 V617F, MPL W515, types 1- and 2-like CALR mutations were identified by Sanger Sequencing and real time polymerase chain reaction. Chromosomal alterations were assessed by oligo-SNP microarray platform. Additional genetic variants were identified by whole exome and targeted sequencing. Statistical significance was evaluated by Fisher's exact test and Wilcoxon's rank sum test (R, version 3.4.2). IR-exposed MPN patients exhibited a different genetic profile vs unexposed: lower rate of JAK2 V617F (58.4% vs 75.4%, P = .0077), higher rate of type 1-like CALR mutation (12.2% vs 3.1%, P = .0056), higher rate of TN cases (27.8% vs 16.2%, P = .0366), higher rate of potentially pathogenic sequence variants (mean numbers: 4.8 vs 3.1, P = .0242). Furthermore, we identified several potential drivers specific to IR-exposed TN MPN patients: ATM p.S1691R with copy-neutral loss of heterozygosity at 11q; EZH2 p.D659G at 7q and SUZ12 p.V71 M at 17q with copy number loss. Thus, IR-exposed MPN patients represent a group with distinct genomic characteristics worthy of further study.

Molecular Markers and Prognosis of Myelofibrosis in the Genomic Era: A Meta-analysis

Molecular markers are important in guiding treatment and predicting outcome in the genomic era. Meta-analysis of molecular markers in myelofibrosis through a search of PubMed and Medline through October 31, 2017 was performed. Markers with more than 3 studies that compared overall survival (OS) and leukemia-free survival (LFS) were analyzed. A total of 16 studies were included. Hazard ratios (HRs) for OS were as follows: IDH 2.65 (95% confidence interval [CI], 1.66-4.21), SRSF2 2.12 (95% CI, 1.18-3.79), high-risk myeloma 2.11 (95% CI, 1.70-2.61), ASXL1 1.92 (95% CI, 1.60-2.32), EZH2 1.88 (95% CI, 1.32-2.67), JAK2 1.41 (95% CI, 1.04-1.93) in the univariate analysis and 1.49 (95% CI, 0.42-5.30) in the multivariate analysis. LFS of JAK2 and SRSF2 had HRs of 1.81 (95% CI, 0.42-5.30) and 0.36 (95% CI, 0.02-6.48), respectively. In conclusion, mutations in IDH, SRSF2, and ASXL1 had worse prognosis in OS with HRs around 2. JAK2 and SRSF2 mutation were not associated with increased leukemia transformation. The adverse effect of triple-negative, which was often compared with CALR mutation, needs to be explored.

Epigenetic changes in myelofibrosis: Distinct methylation changes in the myeloid compartments and in cases with ASXL1 mutations

This is the first study to compare genome-wide DNA methylation profiles of sorted blood cells from myelofibrosis (MF) patients and healthy controls. We found that differentially methylated CpG sites located to genes involved in 'cancer' and 'embryonic development' in MF CD34+ cells, in 'inflammatory disease' in MF mononuclear cells, and in 'immunological diseases' in MF granulocytes. Only few differentially methylated CpG sites were common among the three cell populations. Mutations in the epigenetic regulators ASXL1 (47%) and TET2 (20%) were not associated with a specific DNA methylation pattern using an unsupervised approach. However, in a supervised analysis of ASXL1 mutated versus wild-type cases, differentially methylated CpG sites were enriched in regions marked by histone H3K4me1, histone H3K27me3, and the bivalent histone mark H3K27me3 + H3K4me3 in human CD34+ cells. Hypermethylation of selected CpG sites was confirmed in a separate validation cohort of 30 MF patients by pyrosequencing. Altogether, we show that individual MF cell populations have distinct differentially methylated genes relative to their normal counterparts, which likely contribute to the phenotypic characteristics of MF. Furthermore, differentially methylated CpG sites in ASXL1 mutated MF cases are found in regulatory regions that could be associated with aberrant gene expression of ASXL1 target genes.

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.

Megakaryocytes in Myeloproliferative Neoplasms Have Unique Somatic Mutations

Myeloproliferative neoplasms (MPNs) are a group of related clonal hemopoietic stem cell disorders associated with hyperproliferation of myeloid cells. They are driven by mutations in the hemopoietic stem cell, most notably JAK2^V617F, CALR, and MPL. Clinically, they have the propensity to progress to myelofibrosis and transform to acute myeloid leukemia. Megakaryocytic hyperplasia with abnormal features are characteristic, and it is thought that these cells stimulate and drive fibrotic progression. The biological defects underpinning this remain to be explained. In this study we examined the megakaryocyte genome in 12 patients with MPNs to determine whether there are somatic variants and whether there is any association with marrow fibrosis. We performed targeted next-generation sequencing for 120 genes associated with myeloid neoplasms on megakaryocytes isolated from aspirated bone marrow. Ten of the 12 patients had genomic defects in megakaryocytes that were not present in nonmegakaryocytic hemopoietic marrow cells from the same patient. The greatest allelic burden was in patients with increased reticulin deposition. The megakaryocyte-unique mutations were predominantly in genes that regulate chromatin remodeling, chromosome alignment, and stability. These findings show that genomic abnormalities are present in megakaryocytes in MPNs and that these appear to be associated with progression to bone marrow fibrosis.

Genetic Alterations of the Thrombopoietin/MPL/JAK2 Axis Impacting Megakaryopoiesis

Megakaryopoiesis is an original and complex cell process which leads to the formation of platelets. The homeostatic production of platelets is mainly regulated and controlled by thrombopoietin (TPO) and the TPO receptor (MPL)/JAK2 axis. Therefore, any hereditary or acquired abnormality affecting this signaling axis can result in thrombocytosis or thrombocytopenia. Thrombocytosis can be due to genetic alterations that affect either the intrinsic MPL signaling through gain-of-function (GOF) activity (MPL, JAK2, CALR) and loss-of-function (LOF) activity of negative regulators (CBL, LNK) or the extrinsic MPL signaling by THPO GOF mutations leading to increased TPO synthesis. Alternatively, thrombocytosis may paradoxically result from mutations of MPL leading to an abnormal MPL trafficking, inducing increased TPO levels by alteration of its clearance. In contrast, thrombocytopenia can also result from LOF THPO or MPL mutations, which cause a complete defect in MPL trafficking to the cell membrane, impaired MPL signaling or stability, defects in the TPO/MPL interaction, or an absence of TPO production.

Chasing down the triple-negative myeloproliferative neoplasms: Implications for molecular diagnostics

The majority of patients with classical myeloproliferative neoplasms (MPN) of polycythemia vera, essential thrombocythemia, and primary myelofibrosis harbor distinct disease-driving mutations within the JAK2, CALR, or MPL genes. The term triple-negative has been recently applied to those MPN without evidence of these consistent mutations, prompting whole or targeted exome sequencing approaches to determine the driver mutational status of this subgroup. These strategies have identified numerous novel mutations that occur in alternative exons of both JAK2 and MPL, the majority of which result in functional activation. Current molecular diagnostic approaches may possess insufficient coverage to detect these alternative mutations, prompting further consideration of targeted exon sequencing into routine diagnostic practice. How to incorporate these illuminating findings into the expanding molecular diagnostic algorithm for MPN requires continual attention.

Protective role of R381Q (rs11209026) polymorphism in IL-23R gene in immune-mediated diseases: A comprehensive review

Interleukin-23 (IL-23) is a regulator of cellular immune responses involved in controlling infection and autoimmune diseases. Strong evidence has shown that IL-23 plays a role in the maintenance of immune responses by influencing the proliferation and survival of IL-17-producing T-helper (TH)-17 cells. The critical role of the IL-23/TH17 axis in immune-mediated diseases has emerged from different studies. It has also been seen that polymorphisms in the IL-23 receptor (IL-23R) gene might influence IL-23 responses. Interestingly, a functional single nucleotide polymorphism (SNP) in the IL-23 receptor gene (IL-23R; rs11209026, 1142 G wild-type A reduced function, Arg381Gln, R381Q) seems to confer a measure of protection against development of inflammatory bowel disease (IBD; Crohn's disease, ulcerative colitis), ankylosing spondylitis, rheumatoid arthritis, psoriasis, thyroiditis, recurrent spontaneous abortion and asthma, suggesting that a perturbation in the IL-23 signaling pathway is likely to be relevant to the pathophysiology of these diseases. The aim of this review was to provide an evaluation of what is currently known about the protective role of R381Q variant in IL-23R gene in immune-based diseases.

Presence of atypical thrombopoietin receptor (MPL) mutations in triple-negative essential thrombocythemia patients

Enrichment of atypical MPL mutations in essential thrombocythemia. MPLS204P and MPLY591N mutants are weak gain-of-function mutants.

Myeloproliferative neoplasms: Current molecular biology and genetics

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by increased production of mature blood cells. Philadelphia chromosome-negative MPNs (Ph-MPNs) consist of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). A number of stem cell derived mutations have been identified in the past 10 years. These findings showed that JAK2V617F, as a diagnostic marker involving JAK2 exon 14 with a high frequency, is the best molecular characterization of Ph-MPNs. Somatic mutations in an endoplasmic reticulum chaperone, named calreticulin (CALR), is the second most common mutation in patients with ET and PMF after JAK2 V617F mutation. Discovery of CALR mutations led to the increased molecular diagnostic of ET and PMF up to 90%. It has been shown that JAK2V617F is not the unique event in disease pathogenesis. Some other genes' location such as TET oncogene family member 2 (TET2), additional sex combs-like 1 (ASXL1), casitas B-lineage lymphoma proto-oncogene (CBL), isocitrate dehydrogenase 1/2 (IDH1/IDH2), IKAROS family zinc finger 1 (IKZF1), DNA methyltransferase 3A (DNMT3A), suppressor of cytokine signaling (SOCS), enhancer of zeste homolog 2 (EZH2), tumor protein p53 (TP53), runt-related transcription factor 1 (RUNX1) and high mobility group AT-hook 2 (HMGA2) have also identified to be involved in MPNs phenotypes. Here, current molecular biology and genetic mechanisms involved in MNPs with a focus on the aforementioned factors is presented.

Inflammation as a Keystone of Bone Marrow Stroma Alterations in Primary Myelofibrosis

Primary myelofibrosis (PMF) is a clonal myeloproliferative neoplasm where severity as well as treatment complexity is mainly attributed to a long lasting disease and presence of bone marrow stroma alterations as evidenced by myelofibrosis, neoangiogenesis, and osteosclerosis. While recent understanding of mutations role in hematopoietic cells provides an explanation for pathological myeloproliferation, functional involvement of stromal cells in the disease pathogenesis remains poorly understood. The current dogma is that stromal changes are secondary to the cytokine “storm” produced by the hematopoietic clone cells. However, despite therapies targeting the myeloproliferation-sustaining clones, PMF is still regarded as an incurable disease except for patients, who are successful recipients of allogeneic stem cell transplantation. Although the clinical benefits of these inhibitors have been correlated with a marked reduction in serum proinflammatory cytokines produced by the hematopoietic clones, further demonstrating the importance of inflammation in the pathological process, these treatments do not address the role of the altered bone marrow stroma in the pathological process. In this review, we propose hypotheses suggesting that the stroma is inflammatory-imprinted by clonal hematopoietic cells up to a point where it becomes “independent” of hematopoietic cell stimulation, resulting in an inflammatory vicious circle requiring combined stroma targeted therapies.

Genomic diversity in myeloproliferative neoplasms: focus on myelofibrosis

The classical myeloproliferative neoplasms (MPNs) are a group of clonal diseases comprising essential thrombocythaemia (ET), polycythaemia vera (PV) and primary myelofibrosis (PMF). PMF is the rarest disease sub type and has been challenging to address due to the lack of a specific genetic marker, inadequate risk identification models and a highly variable clinical course. Continuous efforts have over time, seen the inclusion of cytogenetic information in prognostic scoring models that have resulted in improved risk stratification models providing further rationale for therapeutic management. Technological advances using single nucleotide polymorphism arrays increased the detection of known and novel MPN related changes and variant detection by massively parallel sequencing provided a large scale screening tool for the multitude of somatic gene mutations that have more recently been described in MPN. Some of these mutations show an association with specific cytogenetic changes or phenotypes. While PMF occurs mainly in adults, it has also been described in paediatric cases and shows distinct histopathological, genetic and clinical features in comparison. This review provides an overview of the genomics landscape of PMF and current developments in MPN therapy.

The secret life of a megakaryocyte: emerging roles in bone marrow homeostasis control

Megakaryocytes are rare cells found in the bone marrow, responsible for the everyday production and release of millions of platelets into the bloodstream. Since the discovery and cloning, in 1994, of their principal humoral factor, thrombopoietin, and its receptor c-Mpl, many efforts have been directed to define the mechanisms underlying an efficient platelet production. However, more recently different studies have pointed out new roles for megakaryocytes as regulators of bone marrow homeostasis and physiology. In this review we discuss the interaction and the reciprocal regulation of megakaryocytes with the different cellular and extracellular components of the bone marrow environment. Finally, we provide evidence that these processes may concur to the reconstitution of the bone marrow environment after injury and their deregulation may lead to the development of a series of inherited or acquired pathologies.

Frequencies, clinical characteristics, and outcome of somatic CALR mutations in JAK2-unmutated essential thrombocythemia

Calreticulin (CALR) mutations were recently identified in patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF) devoid of JAK2 and MPL mutations. We evaluated the clinical, laboratory, and molecular features of a Taiwanese population of patients with ET. Among 147 ET patients, CALR mutations were detected in 33 (22.5 %), JAK2V617F in 94 (63.9 %), and MPL mutations in 4 (2.7 %). Sixteen (10.9 %) patients were negative for all three mutations (CALR, JAK2V617F, and MPL; triple negative). Interestingly, one patient with the type 2 CALR mutation also harbored a low allele burden (0.025 %) of JAK2V617F mutation. Furthermore, we found a novel CALR mutation, with the resultant protein sharing an identical amino acid sequence to the type 6 CALR mutant. Compared to those with JAK2 mutation, CALR-mutated ET patients were characterized by younger age, lower leukocyte count, higher platelet count, and decreased risk of thrombosis. CALR mutations had a favorable impact on thrombosis-free survival (TFS) for ET patients, whereas the respective TFS outcomes were similarly poorer in JAK2-mutated ET and PV patients. Multivariate analysis confirmed that younger age (<60 years), presence of CALR mutations, and a lower platelet count (<1,000 × 10(9)/L) were independently associated with a longer TFS in ET patients. The current study demonstrates that CALR mutations characterize a special group of ET patients with unique phenotypes that are not discrepant from those seen in Western countries.

The evolving genomic landscape of myeloproliferative neoplasms

Our understanding of the genetic basis of the Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) has moved forward at a staggering pace over the last decade. With the discoveries of underlying mutations in JAK2, MPL, and, most recently, calreticulin (CALR), that together account for ∼90% of patients with MPNs, these conditions are now among the best characterized of hematological malignancies. While JAK-STAT pathway activation has been shown to be central to the pathogenesis of the MPN phenotype, the mechanism by which mutant CALR alters cellular function to result in myeloid proliferation remains unclear. Other mutations in several epigenetic modifiers, such as ASXL1, DNMT3a, TET2, EZH2, IDH1, and IDH2, as well as in genes involved in mRNA splicing, such as SF3B1 and U2AF2, have also been described in recent years in patients with MPNs, and evidence is emerging as to how these may be contributing to disease biology. From a therapeutic perspective, the discovery of aberrations in JAK2 has rapidly translated into the successful clinical use of JAK inhibitors in MPNs. Mutant calreticulin has the potential to be a tumor-specific therapeutic target because the mutations generate a novel protein C-terminus. In this chapter, we detail the genomic alterations that underlie MPNs, with a focus on the recent discovery of mutations in CALR, and explore the clinical and biological relevance of the altered genomic landscape in MPNs.

Molecular genetic biomarkers in myeloid malignancies

CONTEXT

Recent studies using massively parallel sequencing technologies, so-called next-generation sequencing, have uncovered numerous recurrent, single-gene variants or mutations across the spectrum of myeloid malignancies.

OBJECTIVES

To review the recent advances in the understanding of the molecular basis of myeloid neoplasms, including their significance for diagnostic and prognostic purposes and the possible implications for the development of novel therapeutic strategies.

DATA SOURCES

Literature review.

CONCLUSIONS

The recurrent mutations found in myeloid malignancies fall into distinct functional categories. These include (1) cell signaling factors, (2) transcription factors, (3) regulators of the cell cycle, (4) regulators of DNA methylation, (5) regulators of histone modification, (6) RNA-splicing factors, and (7) components of the cohesin complex. As the clinical significance of these mutations and mutation combinations is established, testing for their presence is likely to become a routine part of the diagnostic workup. This review will attempt to establish a framework for understanding these mutations in the context of myeloproliferative neoplasms, myelodysplastic syndromes, and acute myeloid leukemia.

Detection of MPLW515L/K mutations and determination of allele frequencies with a single-tube PCR assay

A gain-of-function mutation in the myeloproliferative leukemia virus (MPL) gene, which encodes the thrombopoietin receptor, has been identified in patients with essential thrombocythemia and primary myelofibrosis, subgroups of classic myeloproliferative neoplasms (MPNs). The presence of MPL gene mutations is a critical diagnostic criterion for these diseases. Here, we developed a rapid, simple, and cost-effective method of detecting two major MPL mutations, MPLW515L/K, in a single PCR assay; we termed this method DARMS (dual amplification refractory mutation system)-PCR. DARMS-PCR is designed to produce three different PCR products corresponding to MPLW515L, MPLW515K, and all MPL alleles. The amplicons are later detected and quantified using a capillary sequencer to determine the relative frequencies of the mutant and wild-type alleles. Applying DARMS-PCR to human specimens, we successfully identified MPL mutations in MPN patients, with the exception of patients bearing mutant allele frequencies below the detection limit (5%) of this method. The MPL mutant allele frequencies determined using DARMS-PCR correlated strongly with the values determined using deep sequencing. Thus, we demonstrated the potential of DARMS-PCR to detect MPL mutations and determine the allele frequencies in a timely and cost-effective manner.

Granulocyte colony-stimulating factor receptor mutations in myeloid malignancy

Granulocyte colony-stimulating factor is a cytokine able to stimulate both myelopoiesis and hematopoietic stem cell mobilization, which has seen it used extensively in the clinic to aid hematopoietic recovery. It acts specifically via the homodimeric granulocyte colony-stimulating factor receptor (G-CSFR), which is principally expressed on the surface of myeloid and hematopoietic progenitor cells. A number of pathogenic mutations have now been identified in CSF3R, the gene encoding G-CSFR. These fall into distinct classes, each of which is associated with a particular spectrum of myeloid disorders, including malignancy. This review details the various CSF3R mutations, their mechanisms of action, and contribution to disease, as well as discussing the clinical implications of such mutations.

A multiplex snapback primer system for the enrichment and detection of JAK2 V617F and MPL W515L/K mutations in Philadelphia-negative myeloproliferative neoplasms

A multiplex snapback primer system was developed for the simultaneous detection of JAK2 V617F and MPL W515L/K mutations in Philadelphia chromosome- (Ph-) negative myeloproliferative neoplasms (MPNs). The multiplex system comprises two snapback versus limiting primer sets for JAK2 and MPL mutation enrichment and detection, respectively. Linear-After exponential (LATE) PCR strategy was employed for the primer design to maximize the amplification efficiency of the system. Low ionic strength buffer and rapid PCR protocol allowed for selective amplification of the mutant alleles. Amplification products were analyzed by melting curve analysis for mutation identification. The multiplex system archived 0.1% mutation load sensitivity and <5% coefficient of variation inter-/intra-assay reproducibility. 120 clinical samples were tested by the multiplex snapback primer assay, and verified with amplification refractory system (ARMS), quantitative PCR (qPCR) and Sanger sequencing method. The multiplex system, with a favored versatility, provided the molecular diagnosis of Ph-negative MPNs with a suitable implement and simplified the genetic test process.

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.

Acquired copy-neutral loss of heterozygosity of chromosome 1p as a molecular event associated with marrow fibrosis in MPL-mutated myeloproliferative neoplasms

We studied mutations of MPL exon 10 in patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF), first investigating a cohort of 892 consecutive patients. MPL mutation scanning was performed on granulocyte genomic DNA by using a high-resolution melt assay, and the mutant allele burden was evaluated by using deep sequencing. Somatic mutations of MPL, all but one involving codon W515, were detected in 26/661 (4%) patients with ET, 10/187 (5%) with PMF, and 7/44 (16%) patients with post-ET myelofibrosis. Comparison of JAK2 (V617F)-mutated and MPL-mutated patients showed only minor phenotypic differences. In an extended group of 62 MPL-mutated patients, the granulocyte mutant allele burden ranged from 1% to 95% and was significantly higher in patients with PMF or post-ET myelofibrosis compared with those with ET. Patients with higher mutation burdens had evidence of acquired copy-neutral loss of heterozygosity (CN-LOH) of chromosome 1p in granulocytes, consistent with a transition from heterozygosity to homozygosity for the MPL mutation in clonal cells. A significant association was found between MPL-mutant allele burden greater than 50% and marrow fibrosis. These observations suggest that acquired CN-LOH of chromosome 1p involving the MPL location may represent a molecular mechanism of fibrotic transformation in MPL-mutated myeloproliferative neoplasms.

JAK2 V617F, MPL W515L and JAK2 Exon 12 Mutations in Chinese Patients with Primary Myelofibrosis

OBJECTIVE

JAK2 V617F, MPL W515L and JAK2 exon 12 mutations are novel acquired mutations that induce constitutive cytokine-independent activation of the JAK-STAT pathway in myeloproliferative disorders (MPD). The discovery of these mutations provides novel mechanism for activation of signal transduction in hematopoietic malignancies. This research was to investigate their prevalence in Chinese patients with primary myelofibrosis (PMF).

METHODS

We introduced allele-specific PCR (AS-PCR) combined with sequence analysis to simultaneously screen JAK2 V617F, MPL W515L and JAK2 exon 12 mutations in 30 patients with PMF.

RESULTS

Fifteen PMF patients (50.0%) carried JAK2 V617F mutation, and only two JAK2 V617F-negative patients (6.7%) harbored MPL W515L mutation. None had JAK2 exon 12 mutations. Furthermore, these three mutations were not detected in 50 healthy controls.

CONCLUSION

MPL W515L and JAK2 V617F mutations existed in PMF patients but JAK2 exon 12 mutations not. JAK2 V617F and MPL W515L and mutations might contribute to the primary molecular pathogenesis in patients with PMF.

Different mutations of the human c-mpl gene indicate distinct haematopoietic diseases

The human c-mpl gene (MPL) plays an important role in the development of megakaryocytes and platelets as well as the self-renewal of haematopoietic stem cells. However, numerous MPL mutations have been identified in haematopoietic diseases. These mutations alter the normal regulatory mechanisms and lead to autonomous activation or signalling deficiencies. In this review, we summarise 59 different MPL mutations and classify these mutations into four different groups according to the associated diseases and mutation rates. Using this classification, we clearly distinguish four diverse types of MPL mutations and obtain a deep understand of their clinical significance. This will prove to be useful for both disease diagnosis and the design of individual therapy regimens based on the type of MPL mutations.

Nonfamilial, MPL S505N-Mutated Essential Thrombocythaemia

Mutations of MPL are present in a significant proportion of patients with the myeloproliferative neoplasms (MPN), primary myelofibrosis (PMF), and essential thrombocythaemia (ET). The most frequent of these mutations, W515L and W515K, occur in exon 10 of MPL, which encodes the receptor for thrombopoietin. Another exon 10 mutation, MPL S505N, has been shown to be a founder mutation in several pedigrees with familial thrombocythaemia where it is associated with a high thrombotic risk, splenomegaly and progression to bone marrow fibrosis. Rare cases of sporadic, nonfamilial, MPL S505N MPN have been documented, but the presenting laboratory and clinical features have not been described in detail. The diagnosis and clinical course of a case of MPL S505N-positive MPN are presented with diagnostic features and treatment response resembling typical ET but with evidence of increasing bone marrow fibrosis. Further MPN cases possessing this genotype require reporting in order to ascertain whether any particular morphological or clinical features, if present, determine clinical course and aid the refinement of therapeutic options.

COLD-PCR and innovative microarray substrates for detecting and genotyping MPL exon 10 W515 substitutions

BACKGROUND

Myeloproliferative neoplasms (MPNs) include polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). Somatic mutations in exon 10 of the MPL (myeloproliferative leukemia virus oncogene) gene, mainly substitutions encoding W515 variants, have recently been described in a minority of patients with ET or PMF. We optimized analytically sensitive methods for detecting and genotyping MPL variants.

METHODS

We used DNA previously isolated from circulating granulocytes of 60 patients with MPN that had previously been analyzed by high-resolution melting (HRM), direct sequencing, and the TaqMan allelic-discrimination assay. We developed conditions for enriching tumor mutant alleles with COLD-PCR (coamplification at lower denaturation temperature PCR) and coupled it with direct sequencing. Assays were designed for identifying MPL W515 substitutions with full COLD-PCR protocols. In parallel, we used innovative microarray substrates to develop assays for evaluating the mutant burden in granulocyte cells.

RESULTS

Mutations that were present at very low levels in patients who had previously been scored as having an MPL variant by HRM and as wild type by direct sequencing were successfully identified in granulocyte DNA. Notably, the microarray approach displayed analytical sensitivities of 0.1% to 5% mutant allele, depending on the particular mutation. This analytical sensitivity is similar to that obtained with COLD-PCR. The assay requires no enrichment strategy and allows both the characterization of each variant allele and the evaluation of its proportion in every patient.

CONCLUSIONS

These procedures, which are transferable to clinical diagnostic laboratories, can be used for detecting very low proportions of minority mutant alleles that cannot be identified by other, conventional methods.

Genotype-phenotype interactions in the myeloproliferative neoplasms

The chronic myeloproliferative neoplasms (MPNs) are clonal disorders characterized by overproduction of mature myeloid cells. They share associations with molecular abnormalities such as the JAK2V617F mutation but are distinguished by important phenotypic differences. This review first considers the factors that may influence phenotype in JAK2-mutated MPNs, especially polycythemia vera (PV) and essential thrombocythemia (ET), and then discusses the mutations implicated in JAK2-negative MPNs such as in MPL and epigenetic regulators. Current evidence supports a model where ET and PV are disorders of relatively low genetic complexity, whereas evolution to myelofibrosis or blast-phase disease reflects accumulation of a higher mutation burden.

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.

The cell cycle regulator CDC25A is a target for JAK2V617F oncogene

The JAK2(V617F) mutation is present in the majority of patients with polycythemia vera and one-half of those with essential thrombocythemia and primary myelofibrosis. JAK2(V617F) is a gain-of-function mutation resulting in constitutive JAK2 signaling involved in the pathogenesis of these diseases. JAK2(V617F) has been shown to promote S-phase entry. Here, we demonstrate that the CDC25A phosphatase, a key regulator of the G1/S cell-cycle transition, is constitutively overexpressed in JAK2(V617F)-positive cell lines, JAK2-mutated patient CD36(+) progenitors, and in vitro-differentiated proerythroblasts. Accordingly, CDC25A is overexpressed in BM and spleen of Jak2(V617F) knock-in mice compared with wild-type littermates. By using murine FDC-P1-EPOR and human HEL and SET-2 cell lines, we found that JAK2(V617F)-induced CDC25A up-regulation was caused neither by increased CDC25A transcription or stability nor by the involvement of its upstream regulators Akt and MAPK. Instead, our results suggest that CDC25A is regulated at the translational level through STAT5 and the translational initiation factor eIF2α. CDC25A inhibition reduces the clonogenic and proliferative potential of JAK2(V617F)-expressing cell lines and erythroid progenitors while moderately affecting normal erythroid differentiation. These results suggest that CDC25A deregulation may be involved in hematopoietic cells expansion in JAK2(V617F) patients, making this protein an attracting potential therapeutic target.

Normal and malignant megakaryopoiesis

Megakaryopoiesis is the process by which bone marrow progenitor cells develop into mature megakaryocytes (MKs), which in turn produce platelets required for normal haemostasis. Over the past decade, molecular mechanisms that contribute to MK development and differentiation have begun to be elucidated. In this review, we provide an overview of megakaryopoiesis and summarise the latest developments in this field. Specially, we focus on polyploidisation, a unique form of the cell cycle that allows MKs to increase their DNA content, and the genes that regulate this process. In addition, because MKs have an important role in the pathogenesis of acute megakaryocytic leukaemia and a subset of myeloproliferative neoplasms, including essential thrombocythemia and primary myelofibrosis, we discuss the biology and genetics of these disorders. We anticipate that an increased understanding of normal MK differentiation will provide new insights into novel therapeutic approaches that will directly benefit patients.

Functional studies on the IBD susceptibility gene IL23R implicate reduced receptor function in the protective genetic variant R381Q

Genome-wide association studies (GWAS) in several populations have demonstrated significant association of the IL23R gene with IBD (Crohn's disease (CD) and ulcerative colitis (UC)) and psoriasis, suggesting that perturbation of the IL-23 signaling pathway is relevant to the pathophysiology of these diseases. One particular variant, R381Q (rs11209026), confers strong protection against development of CD. We investigated the effects of this variant in primary T cells from healthy donors carrying IL23R(R381) and IL23R(Q381) haplotypes. Using a proprietary anti-IL23R antibody, ELISA, flow cytometry, phosphoflow and real-time RT-PCR methods, we examined IL23R expression and STAT3 phosphorylation and activation in response to IL-23. IL23R(Q381) was associated with reduced STAT3 phosphorylation upon stimulation with IL-23 and decreased number of IL-23 responsive T-cells. We also observed slightly reduced levels of proinflammatory cytokine secretion in IL23R(Q381) positive donors. Our study shows conclusively that IL23R(Q381) is a loss-of-function allele, further strengthening the implication from GWAS results that the IL-23 pathway is pathogenic in human disease. This data provides an explanation for the protective role of R381Q in CD and may lead to the development of improved therapeutics for autoimmune disorders like CD.