Differential association of calreticulin type 1 and type 2 mutations with myelofibrosis and essential thrombocytemia: relevance for disease evolution

BCR::ABL1-negative myeloproliferative neoplasms in the era of next-generation sequencing

The classical BCR::ABL1-negative myeloproliferative neoplasms such as polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF) are clonal diseases with the presence of characteristic "driver mutations" in one of the genes: JAK2, CALR, or MPL. The search for mutations in these three genes is required for the diagnosis of MPNs. Nevertheless, the progress that has been made in the field of molecular genetics has opened a new era in medicine. The search for additional mutations in MPNs is helpful in assessing the risk stratification, disease progression, transformation to acute myeloid leukemia (AML), or choosing the right treatment. In some cases, advanced technologies are needed to find a clonal marker of the disease and establish a diagnosis. This review focuses on how the use of new technologies like next-generation sequencing (NGS) helps in the diagnosis of BCR::ABL1-negative myeloproliferative neoplasms.

Clinical features and current treatment status of essential thrombocythemia in older adults: a multicenter real-world study in China

Approximately half of patients diagnosed with essential thrombocythemia (ET) are older adults (aged ≥ 60 years), but to date, little is known about the clinical and molecular characteristics of older patients diagnosed according to the 2016 World Health Organization criteria. We retrospectively collected clinical and molecular data from 282 older (≥ 60 years) and 621 younger ET patients (18-59 years) in China from March 1, 2012 to November 1, 2021 and summarized the clinical characteristics and treatment of these older ET patients. Compared to younger patients, older patients had a higher incidence of the JAK2V617F mutation (P = 0.001), a lower incidence of CALR mutations (P = 0.033) and a higher rate of epigenetic mutations (P < 0.001), TP53 mutations (P = 0.005), and RNA splicing mutations (P < 0.001). Older patients had not only a higher incidence of thrombosis but also a higher incidence of bleeding events. Furthermore, older patients had a significantly higher mortality rate after disease progression (P = 0.050) or after thrombotic events (P = 0.013). Risk factors for thrombosis or prognosis were significantly different between older patients and the entire ET cohort. In older patients, non-driver mutations contributed significantly to thrombotic complications and a poor prognosis, while the JAK2V617F mutation was a risk factor for overall survival but not for thrombotic events. The application of interferon in older ET patients was not inferior to that of hydroxyurea in terms of efficacy and safety. Older patients presented unique characteristics different from those of younger patients, which could provide new information for formulating more appropriate treatment and follow-up strategies.

Mutation-Driven S100A8 Overexpression Confers Aberrant Phenotypes in Type 1 CALR-Mutated MPN

Numerous pathogenic CALR exon 9 mutations have been identified in myeloproliferative neoplasms (MPN), with type 1 (52bp deletion; CALRDEL) and type 2 (5bp insertion; CALRINS) being the most prevalent. Despite the universal pathobiology of MPN driven by various CALR mutants, it is unclear why different CALR mutations result in diverse clinical phenotypes. Through RNA sequencing followed by validation at the protein and mRNA levels, we found that S100A8 was specifically enriched in CALRDEL but not in CALRINS MPN-model cells. The expression of S100a8 could be regulated by STAT3 based on luciferase reporter assay complemented with inhibitor treatment. Pyrosequencing demonstrated relative hypomethylation in two CpG sites within the potential pSTAT3-targeting S100a8 promoter region in CALRDEL cells as compared to CALRINS cells, suggesting that distinct epigenetic alteration could factor into the divergent S100A8 levels in these cells. The functional analysis confirmed that S100A8 non-redundantly contributed to accelerated cellular proliferation and reduced apoptosis in CALRDEL cells. Clinical validation showed significantly enhanced S100A8 expression in CALRDEL-mutated MPN patients compared to CALRINS-mutated cases, and thrombocytosis was less prominent in those with S100A8 upregulation. This study provides indispensable insights into how different CALR mutations discrepantly drive the expression of specific genes that contributes to unique phenotypes in MPN.

Cytological Diagnosis of Classic Myeloproliferative Neoplasms at the Age of Molecular Biology

Myeloproliferative neoplasms (MPN) are clonal hematopoietic stem cell-derived disorders characterized by uncontrolled proliferation of differentiated myeloid cells. Two main groups of MPN, BCR::ABL1-positive (Chronic Myeloid Leukemia) and BCR::ABL1-negative (Polycythemia Vera, Essential Thrombocytosis, Primary Myelofibrosis) are distinguished. For many years, cytomorphologic and histologic features were the only proof of MPN and attempted to distinguish the different entities of the subgroup BCR::ABL1-negative MPN. World Health Organization (WHO) classification of myeloid neoplasms evolves over the years and increasingly considers molecular abnormalities to prove the clonal hematopoiesis. In addition to morphological clues, the detection of JAK2, MPL and CALR mutations are considered driver events belonging to the major diagnostic criteria of BCR::ABL1-negative MPN. This highlights the preponderant place of molecular features in the MPN diagnosis. Moreover, the advent of next-generation sequencing (NGS) allowed the identification of additional somatic mutations involved in clonal hematopoiesis and playing a role in the prognosis of MPN. Nowadays, careful cytomorphology and molecular biology are inseparable and complementary to provide a specific diagnosis and to permit the best follow-up of these diseases.

Structural and Dynamic Differences between Calreticulin Mutants Associated with Essential Thrombocythemia

Essential thrombocythemia (ET) is a blood cancer. ET is characterized by an overproduction of platelets that can lead to thrombosis formation. Platelet overproduction occurs in megakaryocytes through a signaling pathway that could involve JAK2, MPL, or CALR proteins. CALR mutations are associated with 25–30% of ET patients; CALR variants must be dimerized to induce ET. We classified these variants into five classes named A to E; classes A and B are the most frequent classes in patients with ET. The dynamic properties of these five classes using structural models of CALR’s C-domain were analyzed using molecular dynamics simulations. Classes A, B, and C are associated with frameshifts in the C-domain. Their dimers can be stable only if a disulfide bond is formed; otherwise, the two monomers repulse each other. Classes D and E cannot be stable as dimers due to the absence of disulfide bonds. Class E and wild-type CALR have similar dynamic properties. These results suggest that the disulfide bond newly formed in classes A, B, and C may be essential for the pathogenicity of these variants. They also underline that class E cannot be directly related to ET but corresponds to human polymorphisms.

Clinical and molecular characteristics of forty Chinese children with essential thrombocythemia: A single-center, retrospective analysis

Due to the infrequency of essential thrombocythemia (ET) in children, little is known about its pathophysiological mechanism. To learn about the clinical and molecular features of Chinese children with ET, we retrospectively analysed 40 children with ET in a single center from 2015-2021. More than half of the children (51.3%, 20/39) were asymptomatic at diagnosis. Nearly half of the children (48.7%, 19/39) had microvascular symptoms, including headache, dizziness, stomachache, and paresthesia. Only two cases experienced vascular events. The proportion of children with typical "driver gene mutations" (i.e., JAK2 p.V617F, CALR exon 9, or MPL exon 10 mutation) was low (12.5%, 5/40). The equivalent ratio of children carried atypical driver gene mutations; however, 30 (75%) patients did not harbour driver gene mutations. Children carrying JAK2 p.V617F had lower platelet count (938 × 10⁹ /L vs. 1654 × 10⁹ /L, p = 0.031) compared to those without driver gene mutations. Cases harbouring typical driver mutations had higher median WBC counts than those without driver gene mutations (15.14 × 10⁹ /L vs. 8.01 × 10⁹ /L, p = 0.015). Compared to those without driver gene mutations, cases carrying typical and atypical driver gene mutations were both younger (median ages were 12, 6, and 7 years old, respectively; p = 0.023). The most prevalent non-driver gene mutations and those mutations with prognostic significance in adult counterparts were less common in children with ET compared to adults with ET.

The different variant allele frequencies of type I/type II mutations and the distinct molecular landscapes in CALR-mutant essential thrombocythaemia and primary myelofibrosis

OBJECTIVE

Calreticulin (CALR) mutations have been identified as driver mutations in a quarter of patients with essential thrombocythaemia (ET) and primary myelofibrosis (PMF), which are subgroups of myeloproliferative neoplasms (MPNs). A 52-bp deletion (type I mutation) and a 5-bp insertion (type II mutation) are the most frequent variants. To better understand the impact of different CALR mutant variants, with or without nondriver mutations, on the clinical subtypes of MPN needs further investigation.

METHODS

The clinical characteristics, laboratory parameters and genetic mutation statuses were analysed in a cohort of 77 MPN patients with CALR mutations (ET = 24, prePMF = 33, and overt PMF = 20). Targeted NGS using a 38-gene panel was performed to evaluate the variant allele frequency (VAF) of CALR type I/type II mutations and assess the molecular landscape of nondriver gene mutations.

RESULTS

A lower VAF of type I vs. type II was observed in CALR-mutant ET, prePMF and overt PMF, and a higher frequency of type I vs. type II was found in CALR-mutant overt PMF. Additional somatic mutations were indicated to be useful in understanding the pathogenesis of MPN. In this study, the mutation landscape was more complex in overt PMF than in ET or in prePMF. Mutations in epigenetic regulators (ASXL1, EZH2 and TET2) were more common in overt PMF.

CONCLUSIONS

The two different subtypes of CALR mutations may have different impacts on MPN. A lower VAF of CALR type I indicates a greater contribution to disease progression in MPN, and increased nondriver mutations may be important in myelofibrosis progression.

Effects of CALR-Mutant Type and Burden on the Phenotype of Myeloproliferative Neoplasms

Somatic CALR mutations occur in approximately 70% of patients with JAK2 V617F-negative essential thrombocythemia (ET) and primary myelofibrosis (PMF). We evaluated the effects of the CALR mutant type and burden on the phenotype of CALR-mutated myeloproliferative neoplasms (MPN). Of the 510 patients with suspected or diagnosed MPN, all 49 patients detected with CALR mutations were diagnosed with ET (n = 32) or PMF (n = 17). The CALR mutant burden was significantly higher in PMF than in ET (45% vs. 34%), and type 1-like and type 2-like mutations were detected in 49% and 51% patients, respectively. Patients with MPN and type 2-like mutation showed a significantly higher median platelet count than those with type 1-like mutation. Particularly, patients with ET and type 2-like mutation had no thrombotic events, despite higher platelet counts. The effect of CALR mutant burden differed depending on the mutant type. A higher mutant burden tended to be associated with a cytopenic phenotype (i.e., lower hemoglobin levels and platelet counts) in patients with the type 1-like mutation and a proliferative hematological phenotype (i.e., higher platelet and neutrophil counts) in patients with the type 2-like mutation. This study suggests that the disease phenotype of MPN may be altered through CALR mutant burden and mutant type.

Impact of molecular profiling on the management of patients with myelofibrosis

Myelofibrosis (MF) is a chronic myeloproliferative neoplasm (MPN) characterized by a highly heterogeneous clinical course, which can be complicated by severe constitutional symptoms, massive splenomegaly, progressive bone marrow failure, cardiovascular events, and development of acute leukemia. Constitutive signaling through the JAK-STAT pathway plays a fundamental role in its pathogenesis, generally due to activating mutations of JAK2, CALR and MPL genes (i.e., the MPN driver mutations), present in most MF patients. Next Generation Sequencing (NGS) panel testing has shown that additional somatic mutations can already be detected at the time of diagnosis in more than half of patients, and that they accumulate along the disease course. These mutations, mostly affecting epigenetic modifiers or spliceosome components, may cooperate with MPN drivers to favor clonal dominance or influence the clinical phenotype, and some, such as high molecular risk mutations, correlate with a more aggressive clinical course with poor treatment response. The current main role of molecular profiling in clinical practice is prognostication, principally for selecting high-risk patients who may be candidates for transplantation, the only curative treatment for MF to date. To this end, contemporary prognostic models incorporating molecular data are useful tools to discriminate different risk categories. Aside from certain clinical situations, decisions regarding medical treatment are not based on patient molecular profiling, yet this approach may become more relevant in novel treatment strategies, such as the use of vaccines against the mutant forms of JAK2 or CALR, or drugs directed against actionable molecular targets.

CALR type 1 mutations are associated with an increased incidence of myelofibrosis in young male patients

Background

Calreticulin (CALR) mutations are commonly identified in patients with essential thrombocythaemia or myelofibrosis. CALR type 1 mutations are known to have a higher overall incidence in males but little is known about the risks of mutation subtypes on myelofibrotic change across patient age and sex.

Aims

To identify differences in the incidence of myelofibrotic change within subgroups of patients with CALR type 1 mutations.

Methods

All patients with a positive CALR exon 9 mutation identified within our unit between February 2016 and September 2020 were reviewed with note taken of patient sex, age at diagnosis, initial MPN diagnosis, and subsequent disease transformation.

Results

In our cohort, young male patients with CALR type 1 mutations were shown to be at significantly increased risk of myelofibrosis compared to age matched female patients.

Conclusions

Male patients have a worse myeloproliferative neoplasm phenotype than female patients with it occurring at a younger age and being more myelofibrotic in nature. Further investigation is needed into the reasons for this variability.

Molecular Pathogenesis of Myeloproliferative Neoplasms: From Molecular Landscape to Therapeutic Implications

Despite distinct clinical entities, the myeloproliferative neoplasms (MPN) share morphological similarities, propensity to thrombotic events and leukemic evolution, and a complex molecular pathogenesis. Well-known driver mutations, JAK2, MPL and CALR, determining constitutive activation of JAK-STAT signaling pathway are the hallmark of MPN pathogenesis. Recent data in MPN patients identified the presence of co-occurrence somatic mutations associated with epigenetic regulation, messenger RNA splicing, transcriptional mechanism, signal transduction, and DNA repair mechanism. The integration of genetic information within clinical setting is already improving patient management in terms of disease monitoring and prognostic information on disease progression. Even the current therapeutic approaches are limited in disease-modifying activity, the expanding insight into the genetic basis of MPN poses novel candidates for targeted therapeutic approaches. This review aims to explore the molecular landscape of MPN, providing a comprehensive overview of the role of drive mutations and additional mutations, their impact on pathogenesis as well as their prognostic value, and how they may have future implications in therapeutic management.

Progression in Ph-Chromosome-Negative Myeloproliferative Neoplasms: An Overview on Pathologic Issues and Molecular Determinants

Progression in Ph-chromosome-negative myeloproliferative neoplasms (MPN) develops with variable incidence and time sequence in essential thrombocythemia, polycythemia vera, and primary myelofibrosis. These diseases show different clinic-pathologic features and outcomes despite sharing deregulated JAK/STAT signaling due to mutations in either the Janus kinase 2 or myeloproliferative leukemia or CALReticulin genes, which are the primary drivers of the diseases, as well as defined diagnostic criteria and biomarkers in most cases. Progression is defined by the development or worsening of marrow fibrosis or the progressive increase in the marrow blast percentage. Progression is often related to additional genetic aberrations, although some can already be detected during the chronic phase. Detailed scoring systems for clinical usage that are mostly applied in patients with primary myelofibrosis have been defined, and the most recent ones include cytogenetic and molecular parameters with prognostic significance. Additional different clinic-pathologic changes have been reported that may occur during the course of the disease and that are, at present, classified as WHO-defined types of progression, although they likely represent such an event. The present review is meant to provide an updated overview on progression in Ph-chromosome-negative MPN, with a major focus on the pathologic side.

Progression in Ph-Chromosome-Negative Myeloproliferative Neoplasms: An Overview on Pathologic Issues and Molecular Determinants

Progression in Ph-chromosome-negative myeloproliferative neoplasms (MPN) develops with variable incidence and time sequence in essential thrombocythemia, polycythemia vera, and primary myelofibrosis. These diseases show different clinic-pathologic features and outcomes despite sharing deregulated JAK/STAT signaling due to mutations in either the Janus kinase 2 or myeloproliferative leukemia or CALReticulin genes, which are the primary drivers of the diseases, as well as defined diagnostic criteria and biomarkers in most cases. Progression is defined by the development or worsening of marrow fibrosis or the progressive increase in the marrow blast percentage. Progression is often related to additional genetic aberrations, although some can already be detected during the chronic phase. Detailed scoring systems for clinical usage that are mostly applied in patients with primary myelofibrosis have been defined, and the most recent ones include cytogenetic and molecular parameters with prognostic significance. Additional different clinic-pathologic changes have been reported that may occur during the course of the disease and that are, at present, classified as WHO-defined types of progression, although they likely represent such an event. The present review is meant to provide an updated overview on progression in Ph-chromosome-negative MPN, with a major focus on the pathologic side.

Classical Philadelphia-negative myeloproliferative neoplasms (MPNs): A continuum of different disease entities

Classical Philadelphia-negative myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell-derived disorders characterized by uncontrolled proliferation of differentiated myeloid cells and close pathobiologic and clinical features. According to the 2016 World Health Organization (WHO) classification, MPNs include polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). The 2016 revision aimed in particular at strengthening the distinction between masked PV and JAK2-mutated ET, and between prefibrotic/early (pre-PMF) and overt PMF. Clinical manifestations in MPNs include constitutional symptoms, microvascular disorders, thrombosis and bleeding, splenomegaly secondary to extramedullary hematopoiesis, cytopenia-related symptoms, and progression to overt MF and acute leukemia. A dysregulation of the JAK/STAT pathway is the unifying mechanistic hallmark of MPNs, and is guided by somatic mutations in driver genes including JAK2, CALR and MPL. Additional mutations in myeloid neoplasm-associated genes have been also identified, with established prognostic relevance, particularly in PMF. Prognostication of MPN patients relies on disease-specific clinical models. The increasing knowledge of MPN biology led to the development of integrated clinical and molecular prognostic scores that allow a more refined stratification. Recently, the therapeutic landscape of MPNs has been revolutionized by the introduction of potent, selective JAK inhibitors (ruxolitinib, fedratinib), that proved effective in controlling disease-related symptoms and splenomegaly, yet leaving unmet critical needs, owing the lack of disease-modifying activity. In this review, we will deal with molecular, clinical, and therapeutic aspects of the three classical MPNs aiming at highlighting either shared characteristics, that overall define a continuum within a single disease family, and uniqueness, at the same time.

Integration of Molecular Information in Risk Assessment of Patients with Myeloproliferative Neoplasms

Philadelphia chromosome-negative myeloproliferative neoplasms (MPN) are clonal disorders of a hematopoietic stem cell, characterized by an abnormal proliferation of largely mature cells driven by mutations in JAK2, CALR, and MPL. All these mutations lead to a constitutive activation of the JAK-STAT signaling, which represents a target for therapy. Beyond driver ones, most patients, especially with myelofibrosis, harbor mutations in an array of "myeloid neoplasm-associated" genes that encode for proteins involved in chromatin modification and DNA methylation, RNA splicing, transcription regulation, and oncogenes. These additional mutations often arise in the context of clonal hematopoiesis of indeterminate potential (CHIP). The extensive characterization of the pathologic genome associated with MPN highlighted selected driver and non-driver mutations for their clinical informativeness. First, driver mutations are enlisted in the WHO classification as major diagnostic criteria and may be used for monitoring of residual disease after transplantation and response to treatment. Second, mutation profile can be used, eventually in combination with cytogenetic, histopathologic, hematologic, and clinical variables, to risk stratify patients regarding thrombosis, overall survival, and rate of transformation to secondary leukemia. This review outlines the molecular landscape of MPN and critically interprets current information for their potential impact on patient management.

Synoptic Diagnostics of Myeloproliferative Neoplasms: Morphology and Molecular Genetics

Simple Summary

The diagnosis of myeloproliferative neoplasms requires assessment of a combination of clinical, morphological, immunophenotypic and genetic features, and this integrated, multimodal approach forms the basis for precise classification. Evaluation includes cell counts and morphology in the peripheral blood, bone marrow aspiration and trephine biopsy, and may encompass flow cytometry for specific questions. Diagnosis nowadays is completed by targeted molecular analysis for the detection of recurrent driver and, optionally, disease-modifying mutations. According to the current World Health Organization classification, all myeloproliferative disorders require assessment of molecular features to support the diagnosis or confirm a molecularly defined entity. This requires a structured molecular analysis workflow tailored for a rapid and cost-effective diagnosis. The review focuses on the morphological and molecular features of Ph-negative myeloproliferative neoplasms and their differential diagnoses, addresses open questions of classification, and emphasizes the enduring role of histopathological assessment in the molecular era.

Abstract

The diagnosis of a myeloid neoplasm relies on a combination of clinical, morphological, immunophenotypic and genetic features, and an integrated, multimodality approach is needed for precise classification. The basic diagnostics of myeloid neoplasms still rely on cell counts and morphology of peripheral blood and bone marrow aspirate, flow cytometry, cytogenetics and bone marrow trephine biopsy, but particularly in the setting of Ph− myeloproliferative neoplasms (MPN), the trephine biopsy has a crucial role. Nowadays, molecular studies are of great importance in confirming or refining a diagnosis and providing prognostic information. All myeloid neoplasms of chronic evolution included in this review, nowadays feature the presence or absence of specific genetic markers in their diagnostic criteria according to the current WHO classification, underlining the importance of molecular studies. Crucial differential diagnoses of Ph− MPN are the category of myeloid/lymphoid neoplasms with eosinophilia and gene rearrangement of PDGFRA, PDGFRB or FGFR1, or with PCM1-JAK2, and myelodysplastic/myeloproliferative neoplasms (MDS/MPN). This review focuses on morphological, immunophenotypical and molecular features of BCR-ABL1-negative MPN and their differential diagnoses. Furthermore, areas of difficulties and open questions in their classification are addressed, and the persistent role of morphology in the area of molecular medicine is discussed.

Calreticulin mutations in myeloproliferative neoplasms

Calreticulin (CALR) is a chaperone present in the endoplasmic reticulum, which is involved in the quality control of N-glycosylated proteins and storage of calcium ions. In 2013, the C-terminal mutation in CALR was identified in half of the patients with essential thrombocythemia and primary myelofibrosis who did not have a JAK2 or MPL mutation. The results of 8 years of intensive research are changing the clinical practice associated with treating myeloproliferative neoplasms (MPNs). The presence or absence of CALR mutations and their mutation types already provide important information for diagnosis and treatment decision making. In addition, the interaction with the thrombopoietin receptor MPL, which is the main mechanism of transformation by CALR mutation, and the expression of the mutant protein on the cell surface have a great potential as targets for molecular-targeted drugs and immunotherapy. This chapter presents recent findings on the clinical significance of the CALR mutation and the molecular basis by which this mutation drives MPNs.

Induced Pluripotent Stem Cells Enable Disease Modeling and Drug Screening in Calreticulin del52 and ins5 Myeloproliferative Neoplasms

Mutations in the calreticulin (CALR) gene are seen in about 30% of essential thrombocythemia and primary myelofibrosis patients. To address the contribution of the human CALR mutants to the pathogenesis of myeloproliferative neoplasms (MPNs) in an endogenous context, we modeled the CALRdel52 and CALRins5 mutants by induced pluripotent stem cell (iPSC) technology using CD34⁺ progenitors from 4 patients. We describe here the generation of several clones of iPSC carrying heterozygous CALRdel52 or CALRins5 mutations. We showed that CALRdel52 induces a stronger increase in progenitors than CALRins5 and that both CALRdel52 and CALRins5 mutants favor an expansion of the megakaryocytic lineage. Moreover, we found that both CALRdel52 and CALRins5 mutants rendered colony forming unit–megakaryocyte (CFU-MK) independent from thrombopoietin (TPO), and promoted a mild constitutive activation level of signal transducer and activator of transcription 3 in megakaryocytes. Unexpectedly, a mild increase in the sensitivity of colony forming unit-granulocyte (CFU-G) to granulocyte-colony stimulating factor was also observed in iPSC CALRdel52 and CALRins5 compared with control iPSC. Moreover, CALRdel52-induced megakaryocytic spontaneous growth is more dependent on Janus kinase 2/phosphoinositide 3-kinase/extracellular signal-regulated kinase than TPO-mediated growth and opens a therapeutic window for treatments in CALR-mutated MPN. The iPSC models described here represent an interesting platform for testing newly developed inhibitors. Altogether, this study shows that CALR-mutated iPSC recapitulate MPN phenotypes in vitro and may be used for drug screening.

Disease modifying agents of myeloproliferative neoplasms: a review

The identification of driver mutations in Janus kinase (JAK) 2, calreticulin (CALR), and myeloproliferative leukemia (MPL) has contributed to a better understanding of disease pathogenesis by highlighting the importance of JAK signal transducer and activator of transcription (STAT) signaling in classical myeloproliferative neoplasms (MPNs). This has led to the therapeutic use of novel targeted treatments, such as JAK2 inhibitors. More recently, with the development of next-generation sequencing, additional somatic mutations, which are not restricted to MPNs, have been elucidated. Treatment decisions for MPN patients are influenced by the MPN subtype, symptom burden, and risk classification. Although prevention of vascular events is the main objective of therapy for essential thrombocythemia (ET) and polycythemia vera (PV) patients, disease-modifying drugs are needed to eradicate clonal hematopoiesis and prevent progression to more aggressive myeloid neoplasms. JAK inhibitors are a valuable therapeutic strategy for patients with myelofibrosis (MF) who have splenomegaly and/or disease-related symptoms, but intolerance, refractory, resistance, and disease progression still present challenges. Currently, allogeneic stem cell transplantation remains the only curative treatment for MF, but it is typically limited by age-related comorbidities and high treatment-related mortality. Therefore, a better understanding of the molecular pathogenesis and potential new therapies with the aim of modifying the natural history of the disease is important. In this article, I review the current understanding of the molecular basis of MPNs and clinical studies on potential disease-modifying agents.

Lessons from mouse models of MPN

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

The Contemporary Approach to CALR-Positive Myeloproliferative Neoplasms

CALR mutations are a revolutionary discovery and represent an important hallmark of myeloproliferative neoplasms (MPN), especially essential thrombocythemia and primary myelofibrosis. To date, several CALR mutations were identified, with only frameshift mutations linked to the diseased phenotype. It is of diagnostic and prognostic importance to properly define the type of CALR mutation and subclassify it according to its structural similarities to the classical mutations, a 52-bp deletion (type 1 mutation) and a 5-bp insertion (type 2 mutation), using a statistical approximation algorithm (AGADIR). Today, the knowledge on the pathogenesis of CALR-positive MPN is expanding and several cellular mechanisms have been recognized that finally cause a clonal hematopoietic expansion. In this review, we discuss the current basis of the cellular effects of CALR mutants and the understanding of its implementation in the current diagnostic laboratorial and medical practice. Different methods of CALR detection are explained and a diagnostic algorithm is shown that aids in the approach to CALR-positive MPN. Finally, contemporary methods joining artificial intelligence in accordance with molecular-genetic biomarkers in the approach to MPN are presented.

A Broad Overview of Signaling in Ph-Negative Classic Myeloproliferative Neoplasms

Simple Summary

There is growing evidence that Ph-negative myeloproliferative neoplasms are disorders in which multiple signaling pathways are significantly disturbed. The heterogeneous phenotypes observed among patients have highlighted the importance of having a comprehensive knowledge of the molecular mechanisms behind these diseases. This review aims to show a broad overview of the signaling involved in myeloproliferative neoplasms (MPNs) and other processes that can modify them, which could be helpful to better understand these diseases and develop more effective targeted treatments.

Abstract

Ph-negative myeloproliferative neoplasms (polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF)) are infrequent blood cancers characterized by signaling aberrations. Shortly after the discovery of the somatic mutations in JAK2, MPL, and CALR that cause these diseases, researchers extensively studied the aberrant functions of their mutant products. In all three cases, the main pathogenic mechanism appears to be the constitutive activation of JAK2/STAT signaling and JAK2-related pathways (MAPK/ERK, PI3K/AKT). However, some other non-canonical aberrant mechanisms derived from mutant JAK2 and CALR have also been described. Moreover, additional somatic mutations have been identified in other genes that affect epigenetic regulation, tumor suppression, transcription regulation, splicing and other signaling pathways, leading to the modification of some disease features and adding a layer of complexity to their molecular pathogenesis. All of these factors have highlighted the wide variety of cellular processes and pathways involved in the pathogenesis of MPNs. This review presents an overview of the complex signaling behind these diseases which could explain, at least in part, their phenotypic heterogeneity.

Combination of myeloproliferative neoplasm driver gene activation with mutations of splice factor or epigenetic modifier genes increases risk of rapid blastic progression

OBJECTIVES

Myeloproliferative neoplasms (MPN) comprising polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF) follow a bi-phasic course of disease with fibrotic and/or blastic progression. At presentation in the chronic phase, currently there are only insufficient tools to predict the risk of progression in individual cases.

METHODS

In this study, chronic phase MPN (16 PMF, 11 PV, and 11 MPN unclassified) with blastic transformation during course of disease (n = 38, median follow-up 5.3 years) were analyzed by high-throughput sequencing. MPN cases with a comparable follow-up period and without evidence of blast increase served as control (n = 63, median follow-up 5.8 years).

RESULTS

Frequent ARCH/CHIP-associated mutations (TET2, ASXL1, DNMT3A) found at presentation were not significantly associated with blastic transformation. By contrast, mutations of SRSF2, U2AF1, and IDH1/2 at first presentation were frequently observed in the progression cohort (13/38, 34.2%) and were completely missing in the control group without blast transformation during follow-up (P = .0007 for SRSF2; P = .0063 for U2AF1 and IDH1/2).

CONCLUSION

Unlike frequent ARCH/CHIP alterations (TET2, ASXL1, DNMT3A), mutations in SRSF2, IDH1/2, and U2AF1 when manifest already at first presentation provide an independent risk factor for rapid blast transformation of MPN.

CALR-ETdb, the database of calreticulin variants diversity in essential thrombocythemia

Essential thrombocythemia (ET) is a blood cancer defined by a strong increase of platelet numbers. A quarter of patients suffering from ET show mutations in the last exon of calreticulin (CALR) gene. Two variants named type 1 and type 2 represent 85% of these patients. However, a large number of other variants have been determined. In this study, we have compiled variants taken from COSMIC database and literature leading to 155 different variants. This large number of variants allowed redefining 5 new classes extending the classification of type 1-like and type 2-like to a finer description. These analyses showed that last class, named E, corresponding to more than 10% of CALR variants seemed not attached to ET. Structural properties analyzed showed that CALR variants associated to ET have common features. All the compiled and refined information had been included into a freely dedicated database CALR-ETdb (https://www.dsimb.inserm.fr/CALR-ET).

Impact of Mutational Profile on the Management of Myeloproliferative Neoplasms: A Short Review of the Emerging Data

Philadelphia-chromosome negative myeloproliferative neoplasms (MPN) are a heterogeneous group of clonal hematopoietic stem cell disorders characterized by an increased risk of thrombosis and progression to acute myeloid leukemia. MPN are associated with driver mutations in JAK2, CALR and MPL which are crucial for the diagnosis and lead to a constitutive activation of the JAK-STAT signaling, independent of cytokine regulation. Moreover, most patients have concomitant mutations in genes involved in DNA methylation, chromatin modification, messenger RNA splicing, transcription regulation and signal transduction. These additional mutations may arise before, in the context of clonal hematopoiesis of indeterminate potential (CHIP), or after the acquisition of the driver mutation. The clinical phenotype of MPN results from complex interactions between mutations and host factors. The increased application of next-generation sequencing (NGS) techniques to a large series of patients with MPN has expanded the knowledge of mutational landscape and contributed to define the clinical significance of mutations. This molecular information is being increasingly used to refine diagnosis, risk stratification, monitoring of residual disease and response to treatment. ASXL1, SRSF2, EZH2, IDH1/IDH2 and U2AF1 mutations are associated with a more advanced disease and reduced overall survival in primary myelofibrosis (PMF), whereas spliceosome mutations in Polycythemia vera (PV) and essential thrombocythemia (ET) adversely affect both overall (SF3B1, SRSF2 in ET and SRSF2 in PV) and myelofibrosis-free (U2AF1, SF3B1 in ET) survival. This review discusses current knowledge of the molecular landscape of MPN, and how the availability of those molecular information may impact patient management.

The Genetic Basis of Primary Myelofibrosis and Its Clinical Relevance

Among classical BCR-ABL-negative myeloproliferative neoplasms (MPN), primary myelofibrosis (PMF) is the most aggressive subtype from a clinical standpoint, posing a great challenge to clinicians. Whilst the biological consequences of the three MPN driver gene mutations (JAK2, CALR, and MPL) have been well described, recent data has shed light on the complex and dynamic structure of PMF, that involves competing disease subclones, sequentially acquired genomic events, mostly in genes that are recurrently mutated in several myeloid neoplasms and in clonal hematopoiesis, and biological interactions between clonal hematopoietic stem cells and abnormal bone marrow niches. These observations may contribute to explain the wide heterogeneity in patients' clinical presentation and prognosis, and support the recent effort to include molecular information in prognostic scoring systems used for therapeutic decision-making, leading to promising clinical translation. In this review, we aim to address the topic of PMF molecular genetics, focusing on four questions: (1) what is the role of mutations on disease pathogenesis? (2) what is their impact on patients' clinical phenotype? (3) how do we integrate gene mutations in the risk stratification process? (4) how do we take advantage of molecular genetics when it comes to treatment decisions?

Calreticulin del52 and ins5 knock-in mice recapitulate different myeloproliferative phenotypes observed in patients with MPN

Somatic mutations in the calreticulin (CALR) gene are associated with approximately 30% of essential thrombocythemia (ET) and primary myelofibrosis (PMF). CALR mutations, including the two most frequent 52 bp deletion (del52) and 5 bp insertion (ins5), induce a frameshift to the same alternative reading frame generating new C-terminal tails. In patients, del52 and ins5 induce two phenotypically distinct myeloproliferative neoplasms (MPNs). They are equally found in ET, but del52 is more frequent in PMF. We generated heterozygous and homozygous conditional inducible knock-in (KI) mice expressing a chimeric murine CALR del52 or ins5 with the human mutated C-terminal tail to investigate their pathogenic effects on hematopoiesis. Del52 induces greater phenotypic changes than ins5 including thrombocytosis, leukocytosis, splenomegaly, bone marrow hypocellularity, megakaryocytic lineage amplification, expansion and competitive advantage of the hematopoietic stem cell compartment. Homozygosity amplifies these features, suggesting a distinct contribution of homozygous clones to human MPNs. Moreover, homozygous del52 KI mice display features of a penetrant myelofibrosis-like disorder with extramedullary hematopoiesis linked to splenomegaly, megakaryocyte hyperplasia and the presence of reticulin fibers. Overall, modeling del52 and ins5 mutations in mice successfully recapitulates the differences in phenotypes observed in patients.

Calreticulin del52 and ins5 mutations induce two phenotypically distinct myeloproliferative neoplasms in patients. Here the authors show that modeling these mutations in knock-in mice recapitulate the two diseases and highlight how they impact the different hematopoietic compartments.

Murine Models of Myelofibrosis

Myelofibrosis (MF) is subtype of myeloproliferative neoplasm (MPN) characterized by a relatively poor prognosis in patients. Understanding the factors that drive MF pathogenesis is crucial to identifying novel therapeutic approaches with the potential to improve patient care. Driver mutations in three main genes (janus kinase 2 (JAK2), calreticulin (CALR), and myeloproliferative leukemia virus oncogene (MPL)) are recurrently mutated in MPN and are sufficient to engender MPN using animal models. Interestingly, animal studies have shown that the underlying molecular mutation and the acquisition of additional genetic lesions is associated with MF outcome and transition from early stage MPN such as essential thrombocythemia (ET) and polycythemia vera (PV) to secondary MF. In this issue, we review murine models that have contributed to a better characterization of MF pathobiology and identification of new therapeutic opportunities in MPN.

Different impact of calreticulin mutations on human hematopoiesis in myeloproliferative neoplasms

Mutations of calreticulin (CALRm) define a subtype of myeloproliferative neoplasms (MPN). We studied the biological and genetic features of CALR-mutated essential thrombocythemia and myelofibrosis patients. In most cases, CALRm were found in granulocytes, monocytes, B and NK cells, but also in T cells. However, the type 1 CALRm spreads more easily than the type 2 CALRm in lymphoid cells. The CALRm were also associated with an early clonal dominance at the level of hematopoietic stem and progenitor cells (HSPC) with no significant increase during granulo/monocytic differentiation in most cases. Moreover, we found that half of type 2 CALRm patients harbors some homozygous progenitors. Those patients were associated with a higher clonal dominance during granulo/monocytic differentiation than patients with only heterozygous type 2 CALRm progenitors. When associated mutations were present, CALRm were the first genetic event suggesting that they are both the initiating and phenotypic event. In blood, type 1 CALRm led to a greater increased number of all types of progenitors compared with the type 2 CALRm. However, both types of CALRm induced an increase in megakaryocytic progenitors associated with a ruxolitinib-sensitive independent growth and with a mild constitutive signaling in megakaryocytes. At the transcriptional level, type 1 CALRm seems to deregulate more pathways than the type 2 CALRm in megakaryocytes. Altogether, our results show that CALRm modify both the HSPC and megakaryocyte biology with a stronger effect for type 1 than for type 2 CALRm.

Mutant calreticulin in myeloproliferative neoplasms

Recurrent mutations in calreticulin are present in ∼20% of patients with myeloproliferative neoplasms (MPNs). Since its discovery in 2013, we now have a more precise understanding of how mutant CALR, an endoplasmic reticulum chaperone protein, activates the JAK/STAT signaling pathway via a pathogenic binding interaction with the thrombopoietin receptor MPL to induce MPNs. In this Spotlight article, we review the current understanding of the biology underpinning mutant CALR-driven MPNs, discuss clinical implications, and highlight future therapeutic approaches.

Mutant Calreticulin in the Myeloproliferative Neoplasms

Mutations in the gene for calreticulin (CALR) were identified in the myeloproliferative neoplasms (MPNs) essential thrombocythaemia (ET) and primary myelofibrosis (MF) in 2013; in combination with previously described mutations in JAK2 and MPL, driver mutations have now been described for the majority of MPN patients. In subsequent years, researchers have begun to unravel the mechanisms by which mutant CALR drives transformation and to understand their clinical implications. Mutant CALR activates the thrombopoietin receptor (MPL), causing constitutive activation of Janus kinase 2 (JAK2) signaling and cytokine independent growth in vitro. Mouse models show increased numbers of hematopoietic stem cells (HSCs) and overproduction of megakaryocytic lineage cells with associated thrombocytosis. In the clinic, detection of CALR mutations has been embedded in World Health Organization and other international diagnostic guidelines. Distinct clinical and laboratory associations of CALR mutations have been identified together with their prognostic significance, with CALR mutant patients showing increased overall survival. The discovery and subsequent study of CALR mutations have illuminated novel aspects of megakaryopoiesis and raised the possibility of new therapeutic approaches.

Recent insights regarding the molecular basis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are a heterogeneous group of clonal disorders characterized by the overproduction of mature blood cells that have an increased risk of thrombosis and progression to acute myeloid leukemia. Next-generation sequencing studies have provided key insights regarding the molecular mechanisms of MPNs. MPN driver mutations in genes associated with the JAK-STAT pathway include JAK2 V617F, JAK2 exon 12 mutations and mutations in MPL, CALR, and CSF3R. Cooperating driver genes are also frequently detected and also mutated in other myeloid neoplasms; these driver genes are involved in epigenetic methylation, messenger RNA splicing, transcription regulation, and signal transduction. In addition, other genetic factors such as germline predisposition, order of mutation acquisition, and variant allele frequency also influence disease initiation and progression. This review summarizes the current understanding of the genetic basis of MPN, and demonstrates how molecular pathophysiology can improve both our understanding of MPN heterogeneity and clinical practice.

Mutation profile in Indian primary myelofibrosis patients and its clinical implications

Background:

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm (MPN) characterized by abnormal proliferation of megakaryocytes, bone marrow fibrosis, and extramedullary hematopoiesis. We did mutation profile of 50 patients of PMF and tried to correlate it with initial clinical presentation of these patients.

Materials and Methods:

All new and follow up patients who were diagnosed as PMF based on WHO 2016 definition of PMF were included. Mutation profile of these patients including JAK2 V617F, JAK2 exon 12, CALR and MPL mutations was done and all clinical, demographic and laboratory details were recorded.

Results:

Total 50 patients were enrolled out of which 29 were males and 21 were females. Out of these patients, 32 (64%) were JAK2 positive, 13 (26%) were CALR positive, 1 (2%) were MPL positive and 4 (8%) were triple negative. As compared to JAK2+ve patients and triple negative group, CALR positive patients were younger, had lower total leucocyte count, larger spleen size, lower dynamic international prognostic scoring system (DIPSS) score and higher grade of fibrosis of marrow.

Conclusion:

This study depicts that incidence of JAK2 and CALR mutations in Indian PMF patients is fairly similar to that in rest of the world. CALR positive patients have better clinical parameters at presentation and have better prognosis as compared to JAK2 positive patients.

Unfolding the Role of Calreticulin in Myeloproliferative Neoplasm Pathogenesis

In 2013, two seminal studies identified gain-of-function mutations in the Calreticulin (CALR) gene in a subset of JAK2/MPL-negative myeloproliferative neoplasm (MPN) patients. CALR is an endoplasmic reticulum (ER) chaperone protein that normally binds misfolded proteins in the ER and prevents their export to the Golgi and had never previously been reported mutated in cancer or to be associated with hematologic disorders. Further investigation determined that mutated CALR is able to achieve oncogenic transformation primarily through constitutive activation of the MPL-JAK-STAT signaling axis. Here we review our current understanding of the role of CALR mutations in MPN pathogenesis and how these insights can lead to innovative therapeutics approaches.

Detection of CALR Mutations Using High Resolution Melting Curve Analysis (HRM-A); Application on a Large Cohort of Greek ET and MF Patients

Background and Objectives

Somatic mutations in the calreticulin gene (CALR) are detected in approximately 70% of patients with essential thrombocythemia (ET) and primary or secondary myelofibrosis (MF), lacking the JAK2 and MPL mutations. To determine the prevalence of CALR frameshift mutations in a population of MPN patients of Greek origin, we developed a rapid low-budget PCR-based assay and screened samples from 5 tertiary Haematology units. This is a first of its kind report of the Greek patient population that also disclosed novel CALR mutants.

Methods

MPN patient samples were collected from different clinical units and screened for JAK2 and MPL mutations after informed consent was obtained. Negative samples were analyzed for the presence of CALR mutations. To this end, we developed a modified post Real Time PCR High-Resolution Melting Curve analysis (HRM-A) protocol. Samples were subsequently confirmed by Sanger sequencing.

Results

Using this protocol we screened 173 MPN, JAK2 and MPL mutation negative, patients of Greek origin, of whom 117 (67.63%) displayed a CALR exon nine mutation. More specifically, mutations were detected in 90 out of 130 (69.23%) essential thrombocythaemia cases (ET), in 18 out of 33 (54.55%) primary myelofibrosis patients (pMF) and in 9 out of 10 (90%) cases of myelofibrosis secondary to ET (post-ET sMF). False positive results were not detected. The limit of detection (LoD) of our protocol was 2%. Furthermore, our study revealed six rare novel mutations which are to be added in the COSMIC database.

Conclusions

Overall, our method could rapidly and cost-effectively detect the mutation status in a representative cohort of Greek patients; the mutation make-up in our group was not different from what has been published for other national groups.

JAK2 (and other genes) be nimble with MPN diagnosis, prognosis, and therapy

Now that the spectrum of somatic mutations that initiate, propagate, and drive the progression of myeloproliferative neoplasms (MPNs) has largely been defined, recent efforts have focused on integrating this information into clinical decision making. In this regard, the greatest progress has been made in myelofibrosis, in which high-molecular-risk mutations have been identified and incorporated into prognostic models to help guide treatment decisions. In this chapter, we focus on advances in 4 main areas: (1) What are the MPN phenotypic driver mutations? (2) What constitutes high molecular risk in MPN (focusing on ASXL1)? (3) How do we risk-stratify patients with MPN? And (4) What is the significance of molecular genetics for MPN treatment? Although substantial progress has been made, we still have an incomplete understanding of the molecular basis for phenotypic diversity in MPN, and few rationally designed therapeutic approaches to target high-risk mutations are available. Ongoing research efforts in these areas are critical to understanding the biological consequences of genetic heterogeneity in MPN and to improving outcomes for patients.

Heterogeneity in myeloproliferative neoplasms: Causes and consequences

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

Rare type 1-like and type 2-like calreticulin mutants induce similar myeloproliferative neoplasms as prevalent type 1 and 2 mutants in mice

Frameshift mutations in the calreticulin (CALR) gene are present in 30% of essential thrombocythemia and myelofibrosis patients. The two most frequent mutations are CALR del52 (type 1, approximately 60%) and CALR ins5 (type 2, around 30%), but many other rarer mutations exist accounting each for less than 2% of all CALR mutations. Most of them are structurally classified as type 1-like and type 2-like CALR mutations according to the absence or presence of a residual wild-type calcium-binding motif and the modification of the alpha-helix structure. Yet, several key questions remain unanswered, especially the reason of such low frequencies of these other mutations. In an attempt to investigate specific pathogenic differences between type 1-like and type 2-like CALR mutations and del52 and ins5, we modeled two type 1-like (del34 and del46) and one type 2-like (del19) mutations in cell lines and in mice. All CALR mutants constitutively activate JAK2 and STAT5/3/1 in a similar way in the presence of the thrombopoietin receptor (MPL) and induced cytokine-independent cell growth but to a lesser extent with rare mutants over time. This correlates with reduced expression levels of rare CALR mutants compared to del52 and ins5. Lethally irradiated mice that were engrafted with bone marrow transduced with the different CALR mutations developed thrombocytosis, but to a much lesser extent with ins5 and the type 2-like CALR mutation. In contrast to type 2-like mice, type 1-like mice developed marked myelofibrosis and splenomegaly 10 months after engraftment. Similar to del52, type 1-like CALR mutations induced an expansion at an early stage of hematopoiesis compared to ins5 and type 2-like mutation. Thus, type 1-like and type 2-like CALR mutants structurally and functionally resemble del52 and ins5 mutants, respectively.

Driver mutations and prognosis in primary myelofibrosis: Mayo-Careggi MPN alliance study of 1,095 patients

The 2013 discovery of calreticulin (CALR) mutations in myeloproliferative neoplasms was attended by their association with longer survival in primary myelofibrosis (PMF). Subsequent studies have suggested prognostic distinction between type 1/like and type 2/like CALR mutations and detrimental effect from triple-negative mutational status. Among 709 Mayo Clinic patients with PMF, 467 (66%) harbored JAK2, 112 (16%) CALR type 1/like, 24 (3.4%) CALR type 2/like, 38 (5.4%) MPL mutations and 68 (10%) were triple-negative. Survival was longer with type 1/like CALR, compared to JAK2 (HR 2.6, 95% CI 1.9-3.5), type 2/like CALR (HR 2.5, 95% CI 1.4-4.5), MPL (HR 1.8, 95% CI 1.1-2.9) and triple-negative mutational status (HR 2.4, 95% CI 1.6-3.6), but otherwise similar between the non-type 1/like CALR mutational states (P = .41). In multivariable analysis, the absence of type 1/like CALR (P < .001; HR 2, 95% CI 1.4-2.7), presence of ASXL1/SRSF2 mutations (P < .001; HR 1.9, 95% CI 1.5-2.4) and DIPSS-plus (P < .001) were each predictive of inferior survival. Furthermore, among 210 patients with ASXL1/SRSF2 mutations, survival was significantly longer in the presence vs. absence of type 1/like CALR mutations (median 5.8 vs. 2.9 years; P < .001). Triple-negative status did not disclose additional prognostic information for overall or leukemia-free survival. The observations regarding the prognostic distinction between CALR mutation variants were validated in an external cohort of 386 patients from the University of Florence Careggi hospital. We conclude that type 1/like CALR mutations in PMF not only predict superior survival, but also partially amend the detrimental effect of high molecular risk mutations.

The clinical and prognostic relevance of driver mutations in 203 Taiwanese patients with primary myelofibrosis

AIMS

We investigated the clinical and prognostic relevance of the mutational status of driver genes with allele burden and endogenous erythroid colony (EEC) growth in 203 Taiwanese patients with primary myelofibrosis (PMF).

METHODS

Pyrosequencing was used to detect JAK2^V617F mutational status and measure allele burden, while MPL (exon 10) mutations were analysed by PCR assay and then by direct sequencing. CALR exon 9 mutations were first screened for length changes by GeneScan followed by sequencing. The allele burden of the mutated CALR gene was measured by pyrosequencing. The EEC assay was conducted using a serum-free culture system.

RESULTS

The frequencies of the three driver mutations and triple-negative status were similarly distributed between pre-PMF and overt PMF patients, except that pre-PMF patients had a higher incidence of CALR type 2/type-2 like mutations and a lower JAK2^V617F allele burden. EEC growth and CALR mutations conferred favourable overall survival (OS). A lower JAK2^V617F allele burden and grade 3 bone marrow fibrosis were associated with shorter OS and decreased leukaemia-free survival (LFS). Type 2/type 2-like CAL mutations were associated with better LFS compared with type1/type 1-like mutations. Patients with triple-negative mutation status had significantly worse OS and LFS. The allele burden of CALR mutations remained unchanged, while some JAK2^V617F mutations showed clonal expansion in patients during secondary acute myeloid leukaemia transformation.

CONCLUSIONS

Our study showed that EEC growth, a higher JAK2^V617F allele burden and CALR mutations, especially type 2, were independent predictors for better outcomes in PMF. The allele burden of CALR mutations remained stable, but the allele burden of JAK2^V617Fmutations was variable during leukaemia transformation.

JAK-STAT signaling in the therapeutic landscape of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are a group of disorders defined by clonal proliferation of mature myeloid cells with overlapping clinical features. The driver mutations of these disorders, namely JAK2 (Janus Kinase), MPL (Myeloproliferative Leukaemia Virus) and CALR (Calreticulin) upregulate JAK-STAT signaling with increase in downstream transcription and gene expression. Epigenetic mutations are prevalent in MPNs but their interplay with aberrant JAK-STAT signaling is not known. This understanding lead to development of first targeted treatment in MPN; ruxolitinib for primary myelofibrosis. This has shown clinical benefit in overall survival and symptoms improvement but has yet to show significant disease modifying effects. This review will focus on contemporaneous understanding of altered JAK-STAT signaling in MPN and targeted treatments in clinical practice.

Extension of 2016 World Health Organization (WHO) Classification into a New Set of Clinical, Laboratory, Molecular, and Pathological Criteria for the Diagnosis of Myeloproliferative Neoplasms: From Dameshek to Vainchenker, Green, and Kralovics

Improved Clinical, Laboratory, Molecular, and Pathological (CLMP) 2017 criteria for myeloproliferative neoplasms (MPN) define the JAK2V617F trilinear MPNs as a broad continuum of essential thrombocythaemia (ET), polycythaemia vera (PV), masked PV, and post-ET or post-PV myelofibrosis (MF). Normal versus increased erythrocyte counts (5.8×1012/L) on top of bone marrow histology separate JAK2V617F ET and prodromal PV from early and classical PV. Bone marrow histology of the JAK2V617F trilinear MPNs show variable degrees of normocellular megakaryocytic, erythrocytic megakaryocytic and erythrocytic megakaryocytic granulocytic (EMG) myeloproliferation, peripheral cytoses, and splenomegaly related to JAK2V617F allele burden. MPL515 thrombocythaemia displays predominantly normocellular megakaryocytic proliferation. CALR thrombocythaemia intially presents with megakaryocytic followed by dual granulocytic and megakaryocytic myeloproliferation without features of PV. The megakaryocytes are large, mature, and pleomorphic with hyperlobulated nuclei in JAK2V617F ET and prodromal, classical, and masked PV. The megakaryocytes are large to giant with hyperlobulated staghorn-like nuclei in MPL515 thrombocythaemia. The megakaryocytes are densely clustered, large, and immature dysmorphic with bulky (bulbous) hyperchromatic nuclei in CALR thrombocythaemia and MF.