Mutant calreticulin knockin mice develop thrombocytosis and myelofibrosis without a stem cell self-renewal advantage

New approaches to standard of care in early-phase myeloproliferative neoplasms: can interferon-α alter the natural history of the disease?

The classical BCR::ABL-negative myeloproliferative neoplasms (MPN) include polycythemia vera, essential thrombocythemia, and primary myelofibrosis. They are acquired clonal disorders of hematopoietic stem cells leading to hyperplasia of one or several myeloid lineages. MPN are caused by three main recurrent mutations, JAK2V617F and mutations in the calreticulin (CALR) and thrombopoietin receptor (MPL) genes. Here, we review the general diagnosis, the complications, and the management of MPN. Second, we explain the physiopathology of the natural disease development and its regulation, which contributes to MPN heterogeneity. Thirdly, we describe the new paradigm of MPN development highlighting the early origin of driver mutations, decades before the onset of symptoms, and the consequence of early detection of MPN cases in the general population for prompt diagnosis and better medical management. Finally, we present interferon-α therapy as a potential, early disease-modifying drug after reporting its good hematologic and molecular efficacies in polycythemia vera, essential thrombocythemia, and early myelofibrosis in clinical trials as well as its mechanism of action in pre-clinical studies. As a result, we may expect that, in the future, MPN patients will be diagnosed very early during the course of disease and that new selective therapies under development, such as interferon-α, JAK2V617F inhibitors and CALRmut monoclonal antibodies, will be able to intercept the mutated clones.

Clonal hematopoiesis and hematological malignancy

Clonal hematopoiesis (CH), the expansion of hematopoietic stem cells and their progeny driven by somatic mutations in leukemia-associated genes, is a common phenomenon that rises in prevalence with advancing age to affect most people older than 70 years. CH remains subclinical in most carriers, but, in a minority, it progresses to a myeloid neoplasm, such as acute myeloid leukemia, myelodysplastic syndrome, or myeloproliferative neoplasm. Over the last decade, advances in our understanding of CH, its molecular landscape, and the risks associated with different driver gene mutations have culminated in recent developments that allow for a more precise estimation of myeloid neoplasia risk in CH carriers. In turn, this is leading to the development of translational and clinical programs to intercept and prevent CH from developing into myeloid neoplasia. Here, we give an overview of the spectrum of CH driver mutations, what is known about their pathophysiology, and how this informs the risk of incident myeloid malignancy.

Circulating platelets modulate oligodendrocyte progenitor cell differentiation during remyelination

Revealing unknown cues that regulate oligodendrocyte progenitor cell (OPC) function in remyelination is important to optimise the development of regenerative therapies for multiple sclerosis (MS). Platelets are present in chronic non-remyelinated lesions of MS and an increase in circulating platelets has been described in experimental autoimmune encephalomyelitis (EAE) mice, an animal model for MS. However, the contribution of platelets to remyelination remains unexplored. Here we show platelet aggregation in proximity to OPCs in areas of experimental demyelination. Partial depletion of circulating platelets impaired OPC differentiation and remyelination, without altering blood-brain barrier stability and neuroinflammation. Transient exposure to platelets enhanced OPC differentiation in vitro, whereas sustained exposure suppressed this effect. In a mouse model of thrombocytosis (Calr+/-), there was a sustained increase in platelet aggregation together with a reduction of newly-generated oligodendrocytes following toxin-induced demyelination. These findings reveal a complex bimodal contribution of platelet to remyelination and provide insights into remyelination failure in MS.

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

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

Neutrophil-specific expression of JAK2-V617F or CALRmut induces distinct inflammatory profiles in myeloproliferative neoplasia

BACKGROUND

Neutrophils play a crucial role in inflammation and in the increased thrombotic risk in myeloproliferative neoplasms (MPNs). We have investigated how neutrophil-specific expression of JAK2-V617F or CALRdel re-programs the functions of neutrophils.

METHODS

Ly6G-Cre JAK2-V617F and Ly6G-Cre CALRdel mice were generated. MPN parameters as blood counts, splenomegaly and bone marrow histology were compared to wild-type mice. Megakaryocyte differentiation was investigated using lineage-negative bone marrow cells upon in vitro incubation with TPO/IL-1β. Cytokine concentrations in serum of mice were determined by Mouse Cytokine Array. IL-1α expression in various hematopoietic cell populations was determined by intracellular FACS analysis. RNA-seq to analyse gene expression of inflammatory cytokines was performed in isolated neutrophils from JAK2-V617F and CALR-mutated mice and patients. Bioenergetics of neutrophils were recorded on a Seahorse extracellular flux analyzer. Cell motility of neutrophils was monitored in vitro (time lapse microscopy), and in vivo (two-photon microscopy) upon creating an inflammatory environment. Cell adhesion to integrins, E-selectin and P-selection was investigated in-vitro. Statistical analysis was carried out using GraphPad Prism. Data are shown as mean ± SEM. Unpaired, two-tailed t-tests were applied.

RESULTS

Strikingly, neutrophil-specific expression of JAK2-V617F, but not CALRdel, was sufficient to induce pro-inflammatory cytokines including IL-1 in serum of mice. RNA-seq analysis in neutrophils from JAK2-V617F mice and patients revealed a distinct inflammatory chemokine signature which was not expressed in CALR-mutant neutrophils. In addition, IL-1 response genes were significantly enriched in neutrophils of JAK2-V617F patients as compared to CALR-mutant patients. Thus, JAK2-V617F positive neutrophils, but not CALR-mutant neutrophils, are pathogenic drivers of inflammation in MPN. In line with this, expression of JAK2-V617F or CALRdel elicited a significant difference in the metabolic phenotype of neutrophils, suggesting a stronger inflammatory activity of JAK2-V617F cells. Furthermore, JAK2-V617F, but not CALRdel, induced a VLA4 integrin-mediated adhesive phenotype in neutrophils. This resulted in reduced neutrophil migration in vitro and in an inflamed vessel. This mechanism may contribute to the increased thrombotic risk of JAK2-V617F patients compared to CALR-mutant individuals.

CONCLUSIONS

Taken together, our findings highlight genotype-specific differences in MPN-neutrophils that have implications for the differential pathophysiology of JAK2-V617F versus CALR-mutant disease.

Calreticulin: Endoplasmic reticulum Ca2+ gatekeeper

Endoplasmic reticulum (ER) luminal Ca2+ is vital for the function of the ER and regulates many cellular processes. Calreticulin is a highly conserved, ER-resident Ca2+ binding protein and lectin-like chaperone. Over four decades of studying calreticulin demonstrate that this protein plays a crucial role in maintaining Ca2+ supply under different physiological conditions, in managing access to Ca2+ and how Ca2+ is used depending on the environmental events and in making sure that Ca2+ is not misused. Calreticulin plays a role of ER luminal Ca2+ sensor to manage Ca2+-dependent ER luminal events including maintaining interaction with its partners, Ca2+ handling molecules, substrates and stress sensors. The protein is strategically positioned in the lumen of the ER from where the protein manages access to and distribution of Ca2+ for many cellular Ca2+-signalling events. The importance of calreticulin Ca2+ pool extends beyond the ER and includes influence of cellular processes involved in many aspects of cellular pathophysiology. Abnormal handling of the ER Ca2+ contributes to many pathologies from heart failure to neurodegeneration and metabolic diseases.

Molecular Genetic Profile of Myelofibrosis: Implications in the Diagnosis, Prognosis, and Treatment Advancements

Myelofibrosis (MF) is an essential element of primary myelofibrosis, whereas secondary MF may develop in the advanced stages of other myeloid neoplasms, especially polycythemia vera and essential thrombocythemia. Over the last two decades, advances in molecular diagnostic techniques, particularly the integration of next-generation sequencing in clinical laboratories, have revolutionized the diagnosis, classification, and clinical decision making of myelofibrosis. Driver mutations involving JAK2, CALR, and MPL induce hyperactivity in the JAK-STAT signaling pathway, which plays a central role in cell survival and proliferation. Approximately 80% of myelofibrosis cases harbor additional mutations, frequently in the genes responsible for epigenetic regulation and RNA splicing. Detecting these mutations is crucial for diagnosing myeloproliferative neoplasms (MPNs), especially in cases where no mutations are present in the three driver genes (triple-negative MPNs). While fibrosis in the bone marrow results from the disturbance of inflammatory cytokines, it is fundamentally associated with mutation-driven hematopoiesis. The mutation profile and order of acquiring diverse mutations influence the MPN phenotype. Mutation profiling reveals clonal diversity in MF, offering insights into the clonal evolution of neoplastic progression. Prognostic prediction plays a pivotal role in guiding the treatment of myelofibrosis. Mutation profiles and cytogenetic abnormalities have been integrated into advanced prognostic scoring systems and personalized risk stratification for MF. Presently, JAK inhibitors are part of the standard of care for MF, with newer generations developed for enhanced efficacy and reduced adverse effects. However, only a minority of patients have achieved a significant molecular-level response. Clinical trials exploring innovative approaches, such as combining hypomethylation agents that target epigenetic regulators, drugs proven effective in myelodysplastic syndrome, or immune and inflammatory modulators with JAK inhibitors, have demonstrated promising results. These combinations may be more effective in patients with high-risk mutations and complex mutation profiles. Expanding mutation profiling studies with more sensitive and specific molecular methods, as well as sequencing a broader spectrum of genes in clinical patients, may reveal molecular mechanisms in cases currently lacking detectable driver mutations, provide a better understanding of the association between genetic alterations and clinical phenotypes, and offer valuable information to advance personalized treatment protocols to improve long-term survival and eradicate mutant clones with the hope of curing MF.

Preleukemic single-cell landscapes reveal mutation-specific mechanisms and gene programs predictive of AML patient outcomes

Acute myeloid leukemia (AML) and myeloid neoplasms develop through acquisition of somatic mutations that confer mutation-specific fitness advantages to hematopoietic stem and progenitor cells. However, our understanding of mutational effects remains limited to the resolution attainable within immunophenotypically and clinically accessible bulk cell populations. To decipher heterogeneous cellular fitness to preleukemic mutational perturbations, we performed single-cell RNA sequencing of eight different mouse models with driver mutations of myeloid malignancies, generating 269,048 single-cell profiles. Our analysis infers mutation-driven perturbations in cell abundance, cellular lineage fate, cellular metabolism, and gene expression at the continuous resolution, pinpointing cell populations with transcriptional alterations associated with differentiation bias. We further develop an 11-gene scoring system (Stem11) on the basis of preleukemic transcriptional signatures that predicts AML patient outcomes. Our results demonstrate that a single-cell-resolution deep characterization of preleukemic biology has the potential to enhance our understanding of AML heterogeneity and inform more effective risk stratification strategies.

Exploring the Molecular Landscape of Myelofibrosis, with a Focus on Ras and Mitogen-Activated Protein (MAP) Kinase Signaling

Myelofibrosis (MF) is a clonal myeloproliferative neoplasm (MPN) characterized clinically by cytopenias, fatigue, and splenomegaly stemming from extramedullary hematopoiesis. MF commonly arises from mutations in JAK2, MPL, and CALR, which manifests as hyperactive Jak/Stat signaling. Triple-negative MF is diagnosed in the absence of JAK2, MPL, and CALR but when clinical, morphologic criteria are met and other mutation(s) is/are present, including ASXL1, EZH2, and SRSF2. While the clinical and classic molecular features of MF are well-established, emerging evidence indicates that additional mutations, specifically within the Ras/MAP Kinase signaling pathway, are present and may play important role in disease pathogenesis and treatment response. KRAS and NRAS mutations alone are reportedly present in up to 15 and 14% of patients with MF (respectively), and other mutations predicted to activate Ras signaling, such as CBL, NF1, BRAF, and PTPN11, collectively exist in as much as 21% of patients. Investigations into the prevalence of RAS and related pathway mutations in MF and the mechanisms by which they contribute to its pathogenesis are critical in better understanding this condition and ultimately in the identification of novel therapeutic targets.

Different niches for stem cells carrying the same oncogenic driver affect pathogenesis and therapy response in myeloproliferative neoplasms

Aging facilitates the expansion of hematopoietic stem cells (HSCs) carrying clonal hematopoiesis-related somatic mutations and the development of myeloid malignancies, such as myeloproliferative neoplasms (MPNs). While cooperating mutations can cause transformation, it is unclear whether distinct bone marrow (BM) HSC-niches can influence the growth and therapy response of HSCs carrying the same oncogenic driver. Here we found different BM niches for HSCs in MPN subtypes. JAK-STAT signaling differentially regulates CDC42-dependent HSC polarity, niche interaction and mutant cell expansion. Asymmetric HSC distribution causes differential BM niche remodeling: sinusoidal dilation in polycythemia vera and endosteal niche expansion in essential thrombocythemia. MPN development accelerates in a prematurely aged BM microenvironment, suggesting that the specialized niche can modulate mutant cell expansion. Finally, dissimilar HSC-niche interactions underpin variable clinical response to JAK inhibitor. Therefore, HSC-niche interactions influence the expansion rate and therapy response of cells carrying the same clonal hematopoiesis oncogenic driver.

Effects of calreticulin mutations on cell transformation and immunity

Myeloproliferative neoplasms (MPNs) are cancers involving dysregulated production and function of myeloid lineage hematopoietic cells. Among MPNs, Essential thrombocythemia (ET), Polycythemia Vera (PV) and Myelofibrosis (MF), are driven by mutations that activate the JAK-STAT signalling pathway. Somatic mutations of calreticulin (CRT), an endoplasmic reticulum (ER)-localized lectin chaperone, are driver mutations in approximately 25% of ET and 35% of MF patients. The MPN-linked mutant CRT proteins have novel frameshifted carboxy-domain sequences and lack an ER retention motif, resulting in their secretion. Wild type CRT is a regulator of ER calcium homeostasis and plays a key role in the assembly of major histocompatibility complex (MHC) class I molecules, which are the ligands for antigen receptors of CD8+ T cells. Mutant CRT-linked oncogenesis results from the dysregulation of calcium signalling in cells and the formation of stable complexes of mutant CRT with myeloproliferative leukemia (MPL) protein, followed by downstream activation of the JAK-STAT signalling pathway. The intricate participation of CRT in ER protein folding, calcium homeostasis and immunity suggests the involvement of multiple mechanisms of mutant CRT-linked oncogenesis. In this review, we highlight recent findings related to the role of MPN-linked CRT mutations in the dysregulation of calcium homeostasis, MPL activation and immunity.

Human gene-engineered calreticulin mutant stem cells recapitulate MPN hallmarks and identify targetable vulnerabilities

Calreticulin (CALR) mutations present the main oncogenic drivers in JAK2 wildtype (WT) myeloproliferative neoplasms (MPN), including essential thrombocythemia and myelofibrosis, where mutant (MUT) CALR is increasingly recognized as a suitable mutation-specific drug target. However, our current understanding of its mechanism-of-action is derived from mouse models or immortalized cell lines, where cross-species differences, ectopic over-expression and lack of disease penetrance are hampering translational research. Here, we describe the first human gene-engineered model of CALR MUT MPN using a CRISPR/Cas9 and adeno-associated viral vector-mediated knock-in strategy in primary human hematopoietic stem and progenitor cells (HSPCs) to establish a reproducible and trackable phenotype in vitro and in xenografted mice. Our humanized model recapitulates many disease hallmarks: thrombopoietin-independent megakaryopoiesis, myeloid-lineage skewing, splenomegaly, bone marrow fibrosis, and expansion of megakaryocyte-primed CD41⁺ progenitors. Strikingly, introduction of CALR mutations enforced early reprogramming of human HSPCs and the induction of an endoplasmic reticulum stress response. The observed compensatory upregulation of chaperones revealed novel mutation-specific vulnerabilities with preferential sensitivity of CALR mutant cells to inhibition of the BiP chaperone and the proteasome. Overall, our humanized model improves purely murine models and provides a readily usable basis for testing of novel therapeutic strategies in a human setting.

CALR-mutated cells are vulnerable to combined inhibition of the proteasome and the endoplasmic reticulum stress response

Cancer is driven by somatic mutations that provide a fitness advantage. While targeted therapies often focus on the mutated gene or its direct downstream effectors, imbalances brought on by cell-state alterations may also confer unique vulnerabilities. In myeloproliferative neoplasms (MPN), somatic mutations in the calreticulin (CALR) gene are disease-initiating through aberrant binding of mutant CALR to the thrombopoietin receptor MPL and ligand-independent activation of JAK-STAT signaling. Despite these mechanistic insights into the pathogenesis of CALR-mutant MPN, there are currently no mutant CALR-selective therapies available. Here, we identified differential upregulation of unfolded proteins, the proteasome and the ER stress response in CALR-mutant hematopoietic stem cells (HSCs) and megakaryocyte progenitors. We further found that combined pharmacological inhibition of the proteasome and IRE1-XBP1 axis of the ER stress response preferentially targets Calr-mutated HSCs and megakaryocytic-lineage cells over wild-type cells in vivo, resulting in an amelioration of the MPN phenotype. In serial transplantation assays following combined proteasome/IRE1 inhibition for six weeks, we did not find preferential depletion of Calr-mutant long-term HSCs. Together, these findings leverage altered proteostasis in Calr-mutant MPN to identify combinatorial dependencies that may be targeted for therapeutic benefit and suggest that eradicating disease-propagating Calr-mutant LT-HSCs may require more sustained treatment.

Mesenchymal stem cells in fibrotic diseases-the two sides of the same coin

Fibrosis is caused by extensive deposition of extracellular matrix (ECM) components, which play a crucial role in injury repair. Fibrosis attributes to ~45% of all deaths worldwide. The molecular pathology of different fibrotic diseases varies, and a number of bioactive factors are involved in the pathogenic process. Mesenchymal stem cells (MSCs) are a type of multipotent stem cells that have promising therapeutic effects in the treatment of different diseases. Current updates of fibrotic pathogenesis reveal that residential MSCs may differentiate into myofibroblasts which lead to the fibrosis development. However, preclinical and clinical trials with autologous or allogeneic MSCs infusion demonstrate that MSCs can relieve the fibrotic diseases by modulating inflammation, regenerating damaged tissues, remodeling the ECMs, and modulating the death of stressed cells after implantation. A variety of animal models were developed to study the mechanisms behind different fibrotic tissues and test the preclinical efficacy of MSC therapy in these diseases. Furthermore, MSCs have been used for treating liver cirrhosis and pulmonary fibrosis patients in several clinical trials, leading to satisfactory clinical efficacy without severe adverse events. This review discusses the two opposite roles of residential MSCs and external MSCs in fibrotic diseases, and summarizes the current perspective of therapeutic mechanism of MSCs in fibrosis, through both laboratory study and clinical trials.

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.

Molecular Pathogenesis of Myeloproliferative Neoplasms

PURPOSE OF REVIEW

Myeloproliferative neoplasms (MPNs) are chronic hematological malignancies characterized by increased proliferation of MPN stem and myeloid progenitor cells with or without bone marrow fibrosis that typically lead to increased peripheral blood cell counts. The genetic and cytogenetic alterations that initiate and drive the development of MPNs have largely been defined, and we summarize these here.

RECENT FINDINGS

In recent years, advances in understanding the pathogenesis of MPNs have defined a long-preclinical phase in JAK2-mutant MPN, identified genetic loci associated with MPN predisposition and uncovered mechanistic insights in CALR-mutant MPN. The integration of molecular genetics into prognostic risk models is well-established in myelofibrosis and ongoing studies are interrogating the prognostic implications of concomitant mutations in ET and PV. Despite all these advances, the field is deficient in clonally selective therapies to effectively target the MPN clone at any stage of disease, from pre-clinical to advanced. Although the biological understanding of the pathogenesis of MPNs has progressed quickly, substantial knowledge gaps remain, including in the molecular mechanisms underlying MPN progression and myelofibrotic transformation. An ongoing goal for the MPN field is to translate advances in biological understanding to improved treatments for patients.

STAT1 is essential for HSC function and maintains MHCIIhi stem cells that resist myeloablation and neoplastic expansion

Adult hematopoietic stem cells (HSCs) are predominantly quiescent and can be activated in response to acute stress such as infection or cytotoxic insults. STAT1 is a pivotal downstream mediator of interferon (IFN) signaling and is required for IFN-induced HSC proliferation, but little is known about the role of STAT1 in regulating homeostatic hematopoietic stem/progenitor cells (HSPCs). Here, we show that loss of STAT1 altered the steady state HSPC landscape, impaired HSC function in transplantation assays, delayed blood cell regeneration following myeloablation, and disrupted molecular programs that protect HSCs, including control of quiescence. Our results also reveal STAT1-dependent functional HSC heterogeneity. A previously unrecognized subset of homeostatic HSCs with elevated major histocompatibility complex class II (MHCII) expression (MHCIIhi) displayed molecular features of reduced cycling and apoptosis and was refractory to 5-fluorouracil-induced myeloablation. Conversely, MHCIIlo HSCs displayed increased megakaryocytic potential and were preferentially expanded in CALR mutant mice with thrombocytosis. Similar to mice, high MHCII expression is a feature of human HSCs residing in a deeper quiescent state. Our results therefore position STAT1 at the interface of stem cell heterogeneity and the interplay between stem cells and the adaptive immune system, areas of broad interest in the wider stem cell field.

Whole-genome CRISPR screening identifies N-glycosylation as a genetic and therapeutic vulnerability in CALR-mutant MPNs

Calreticulin (CALR) mutations are frequent, disease-initiating events in myeloproliferative neoplasms (MPNs). Although the biological mechanism by which CALR mutations cause MPNs has been elucidated, there currently are no clonally selective therapies for CALR-mutant MPNs. To identify unique genetic dependencies in CALR-mutant MPNs, we performed a whole-genome clustered regularly interspaced short palindromic repeats (CRISPR) knockout depletion screen in mutant CALR-transformed hematopoietic cells. We found that genes in the N-glycosylation pathway (among others) were differentially depleted in mutant CALR-transformed cells as compared with control cells. Using a focused pharmacological in vitro screen targeting unique vulnerabilities uncovered in the CRISPR screen, we found that chemical inhibition of N-glycosylation impaired the growth of mutant CALR-transformed cells, through a reduction in MPL cell surface expression. We treated Calr-mutant knockin mice with the N-glycosylation inhibitor 2-deoxy-glucose (2-DG) and found a preferential sensitivity of Calr-mutant cells to 2-DG as compared with wild-type cells and normalization of key MPNs disease features. To validate our findings in primary human cells, we performed megakaryocyte colony-forming unit (CFU-MK) assays. We found that N-glycosylation inhibition significantly reduced CFU-MK formation in patient-derived CALR-mutant bone marrow as compared with bone marrow derived from healthy donors. In aggregate, our findings advance the development of clonally selective treatments for CALR-mutant MPNs.

Targeting human CALR-mutated MPN progenitors with a neoepitope-directed monoclonal antibody

Calreticulin (CALR) is recurrently mutated in myelofibrosis via a frameshift that removes an endoplasmic reticulum retention signal, creating a neoepitope potentially targetable by immunotherapeutic approaches. We developed a specific rat monoclonal IgG2α antibody, 4D7, directed against the common sequence encoded by both insertion and deletion mutations. 4D7 selectively bound to cells co-expressing mutant CALR and thrombopoietin receptor (TpoR) and blocked JAK-STAT signalling, TPO-independent proliferation and megakaryocyte differentiation of mutant CALR myelofibrosis progenitors by disrupting the binding of CALR dimers to TpoR. Importantly, 4D7 inhibited proliferation of patient samples with both insertion and deletion CALR mutations but not JAK2 V617F and prolonged survival in xenografted bone marrow models of mutant CALR-dependent myeloproliferation. Together, our data demonstrate a novel therapeutic approach to target a problematic disease driven by a recurrent somatic mutation that would normally be considered undruggable.

CALR frameshift mutations in MPN patient-derived iPSCs accelerate maturation of megakaryocytes

Calreticulin (CALR) mutations are driver mutations in myeloproliferative neoplasms (MPNs), leading to activation of the thrombopoietin receptor and causing abnormal megakaryopoiesis. Here, we generated patient-derived CALRins5- or CALRdel52-positive induced pluripotent stem cells (iPSCs) to establish an MPN disease model for molecular and mechanistic studies. We demonstrated myeloperoxidase deficiency in granulocytic cells derived from homozygous CALR mutant iPSCs, rescued by repairing the mutation using CRISPR/Cas9. iPSC-derived megakaryocytes showed characteristics of primary megakaryocytes such as formation of demarcation membrane system and cytoplasmic pro-platelet protrusions. Importantly, CALR mutations led to enhanced megakaryopoiesis and accelerated megakaryocytic development in a thrombopoietin-independent manner. Mechanistically, our study identified differentially regulated pathways in mutated versus unmutated megakaryocytes, such as hypoxia signaling, which represents a potential target for therapeutic intervention. Altogether, we demonstrate key aspects of mutated CALR-driven pathogenesis dependent on its zygosity, and found novel therapeutic targets, making our model a valuable tool for clinical drug screening in MPNs.

•

CALR-mutated iPSCs allow efficient modeling of human MPN disease

•

CRISPR-mediated repair of CALR mutations rescues normal iPSC function

•

Megakaryopoiesis in CALR-mutated iPSCs is hyperplastic and accelerated

•

Transcriptome screen of mutated megakaryocytes identifies novel therapeutic options

Hematopoietic stem and progenitor cells outside the bone marrow: where, when, and why

Bone marrow (BM) is the primary site of adult blood production, hosting the majority of all hematopoietic stem and progenitor cells (HSPCs). Rare HSPCs are also found outside of the BM at steady state. In times of large hematopoietic demand or BM failure, substantial production of mature blood cells from HSPCs can occur in a number of tissues, in a process termed extramedullary hematopoiesis (EMH). Over the past decades, our understanding of BM hematopoiesis has advanced drastically. In contrast there has been very little focus on the study of extramedullary HSPC pools and their contributions to blood production. Here we summarize what is currently known about extramedullary HSPCs and EMH in mice and humans. We describe the evidence of existing extramedullary HSPC pools at steady state, then discuss their role in the hematopoietic stress response. We highlight that although EMH in humans is much less pronounced and likely physiologically distinct to that in mice, it can be informative about premalignant and malignant changes. Finally, we reflect on the open questions in the field and on whether a better understanding of EMH, particularly in humans, may have relevant clinical implications for hematological and nonhematological disorders.

From Metcalf to myeloproliferative neoplasms - a personal journey

The human myeloproliferative neoplasms constitute a biologically fascinating group of chronic myeloid malignancies. This perspective outlines how a postdoctoral fellowship working in Don Metcalf's unit proved a formative and immensely enjoyable experience for my family and me. It laid the foundation for a subsequent body of work over three decades that revealed the genetic basis of these diseases, defined how these genetic alterations subvert normal hematopoiesis, altered clinical practice, and provided insights of broad biological relevance for cancer and cytokine signaling.

Alterations of Signaling Pathways in Essential Thrombocythemia with Calreticulin Mutation

Purpose

Though mutations of the calreticulin (CALR) gene have been identified in essential thrombocythemia patients, the detailed mechanisms for CALR mutations have not been completely clarified. Our study is aimed at characterizing alteration of protein expression in ET patients with mutated CALR^del52 and further recognizing possible involvement of signaling pathways associated with CALR mutations.

Patients and Methods

Protein pathway array was performed to analyze the expression levels of proteins involved in various signaling pathways in peripheral blood neutrophils from 18 ET patients with mutated CALR^del52, 20 ET patients with JAK2^V617F mutation and 20 controls.

Results

We found 20 proteins differentially expressed in ET patients with mutated CALR^del52 compared with healthy controls. These proteins were associated with molecular mechanisms of cancer in ingenuity pathways analysis (IPA) network. We identified top ten canonical pathways which including apoptotic pathways and cellular cytokine pathways might participate in pathogenesis of ET with mutated CALR^del52. Additionally, there were 8 proteins found to be dysregulated differently between ET patients with mutated CALR^del52 and those with JAK2^V617F mutation. These proteins might be related to the unique signaling pathways activated by CALR^del52 mutation which were different to JAK/STATs pathway by JAK2^V617F mutation.

Conclusion

Our study demonstrated that numerous alterations of signaling proteins and pathways in ET patients with mutated CALR^del52. These findings could help to gain insights into the pathological mechanisms of ET.

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.

Hematopoietic expression of a chimeric murine-human CALR oncoprotein allows the assessment of anti-CALR antibody immunotherapies in vivo

Myeloproliferative neoplasms (MPNs) are characterized by a pathologic expansion of myeloid lineages. Mutations in JAK2, CALR and MPL genes are known to be three prominent MPN disease drivers. Mutant CALR (mutCALR) is an oncoprotein that interacts with and activates the thrombopoietin receptor (MPL) and represents an attractive target for targeted therapy of CALR mutated MPN. We generated a transgenic murine model with conditional expression of the human mutant exon 9 (del52) from the murine endogenous Calr locus. These mice develop essential thrombocythemia like phenotype with marked thrombocytosis and megakaryocytosis. The disease exacerbates with age showing prominent signs of splenomegaly and anemia. The disease is transplantable and mutCALR stem cells show proliferative advantage when compared to wild type stem cells. Transcriptome profiling of hematopoietic stem cells revealed oncogenic and inflammatory gene expression signatures. To demonstrate the applicability of the transgenic animals for immunotherapy, we treated mice with monoclonal antibody raised against the human mutCALR. The antibody treatment lowered platelet and stem cell counts in mutant mice. Secretion of mutCALR did not constitute a significant antibody sink. This animal model not only recapitulates human MPN but also serves as a relevant model for testing immunotherapeutic strategies targeting epitopes of the human mutCALR.

Activated IL-6 signaling contributes to the pathogenesis of, and is a novel therapeutic target for, CALR-mutated MPNs

Calreticulin (CALR), an endoplasmic reticulum-associated chaperone, is frequently mutated in myeloproliferative neoplasms (MPNs). Mutated CALR promotes downstream JAK2/STAT5 signaling through interaction with, and activation of, the thrombopoietin receptor (MPL). Here, we provide evidence of a novel mechanism contributing to CALR-mutated MPNs, represented by abnormal activation of the interleukin 6 (IL-6)-signaling pathway. We found that UT7 and UT7/mpl cells, engineered by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) to express the CALR type 1-like (DEL) mutation, acquired cytokine independence and were primed to the megakaryocyte (Mk) lineage. Levels of IL-6 messenger RNA (mRNA), extracellular-released IL-6, membrane-associated glycoprotein 130 (gp130), and IL-6 receptor (IL-6R), phosphorylated JAK1 and STAT3 (p-JAK1 and p-STAT3), and IL-6 promoter region occupancy by STAT3 all resulted in increased CALR DEL cells in the absence of MPL stimulation. Wild-type, but not mutated, CALR physically interacted with gp130 and IL-6R, downregulating their expression on the cell membrane. Agents targeting gp130 (SC-144), IL-6R (tocilizumab [TCZ]), and cell-released IL-6 reduced proliferation of CALR DEL as well as CALR knockout cells, supporting a mutated CALR loss-of-function model. CD34+ cells from CALR-mutated patients showed increased levels of IL-6 mRNA and p-STAT3, and colony-forming unit-Mk growth was inhibited by either SC144 or TCZ, as well as an IL-6 antibody, supporting cell-autonomous activation of the IL-6 pathway. Targeting IL-6 signaling also reduced colony formation by CD34+ cells of JAK2V617F-mutated patients. The combination of TCZ and ruxolitinib was synergistic at very low nanomolar concentrations. Overall, our results suggest that target inhibition of IL-6 signaling may have therapeutic potential in CALR, and possibly JAK2V617F, mutated MPNs.

Precision medicine in myeloid malignancies

Myeloid malignancies have always been at the forefront of an improved understanding of the molecular pathogenesis of cancer. In accordance, over the last years, basic research focusing on the aberrations underlying malignant transformation of myeloid cells has provided the basis for precision medicine approaches and subsequently has led to the development of powerful therapeutic strategies. In this review article, we will recapitulate what has happened since in the 1980s the use of all-trans retinoic acid (ATRA), as a first targeted cancer therapy, has changed one of the deadliest leukemia subtypes, acute promyelocytic leukemia (APL), into one that can be cured without classical chemotherapy today. Similarly, imatinib, the first molecularly designed cancer therapy, has revolutionized the management of chronic myeloid leukemia (CML). Thus, targeted treatment approaches have become the paradigm for myeloid malignancy, but many questions still remain unanswered, especially how identical mutations can be associated with different phenotypes. This might be linked to the impact of the cell of origin, gene-gene interactions, or the tumor microenvironment including the immune system. Continuous research in the field of myeloid neoplasia has started to unravel the molecular pathways that are not only crucial for initial treatment response, but also resistance of leukemia cells under therapy. Ongoing studies focusing on leukemia cell vulnerabilities do already point to novel (targetable) "Achilles heels" that can further improve myeloid cancer therapy.

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.

The Thrombopoietin Receptor, MPL, Is a Therapeutic Target of Opportunity in the MPN

The myeloproliferative neoplasms, polycythemia vera, essential thrombocytosis and primary myelofibrosis share driver mutations that either activate the thrombopoietin receptor, MPL, or indirectly activate it through mutations in the gene for JAK2, its cognate tyrosine kinase. Paradoxically, although the myeloproliferative neoplasms are classified as neoplasms because they are clonal hematopoietic stem cell disorders, the mutations affecting MPL or JAK2 are gain-of-function, resulting in increased production of normal erythrocytes, myeloid cells and platelets. Constitutive JAK2 activation provides the basis for the shared clinical features of the myeloproliferative neoplasms. A second molecular abnormality, impaired posttranslational processing of MPL is also shared by these disorders but has not received the recognition it deserves. This abnormality is important because MPL is the only hematopoietic growth factor receptor expressed in hematopoietic stem cells; because MPL is a proto-oncogene; because impaired MPL processing results in chronic elevation of plasma thrombopoietin, and since these diseases involve normal hematopoietic stem cells, they have proven resistant to therapies used in other myeloid neoplasms. We hypothesize that MPL offers a selective therapeutic target in the myeloproliferative neoplasms since impaired MPL processing is unique to the involved stem cells, while MPL is required for hematopoietic stem cell survival and quiescent in their bone marrow niches. In this review, we will discuss myeloproliferative neoplasm hematopoietic stem cell pathophysiology in the context of the behavior of MPL and its ligand thrombopoietin and the ability of thrombopoietin gene deletion to abrogate the disease phenotype in vivo in a JAK2 V617 transgenic mouse model of PV.

The Role of Megakaryocytes in Myelofibrosis

Megakaryocytes give rise to platelets, which have a wide variety of functions in coagulation, immune response, inflammation, and tissue repair. Dysregulation of megakaryocytes is a key feature of in the myeloproliferative neoplasms, especially myelofibrosis. Megakaryocytes are among the main drivers of myelofibrosis by promoting myeloproliferation and bone marrow fibrosis. In vivo targeting of megakaryocytes by genetic and pharmacologic approaches ameliorates the disease, underscoring the important role of megakaryocytes in myeloproliferative neoplasms. Here we review the current knowledge of the function of megakaryocytes in the JAK2, CALR, and MPL-mutant myeloproliferative neoplasms.

Murine Modeling of Myeloproliferative Neoplasms

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

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

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

Roles of Calreticulin in Protein Folding, Immunity, Calcium Signaling and Cell Transformation

The endoplasmic reticulum (ER) is an organelle that mediates the proper folding and assembly of proteins destined for the cell surface, the extracellular space and subcellular compartments such as the lysosomes. The ER contains a wide range of molecular chaperones to handle the folding requirements of a diverse set of proteins that traffic through this compartment. The lectin-like chaperones calreticulin and calnexin are an important class of structurally-related chaperones relevant for the folding and assembly of many N-linked glycoproteins. Despite the conserved mechanism of action of these two chaperones in nascent protein recognition and folding, calreticulin has unique functions in cellular calcium signaling and in the immune response. The ER-related functions of calreticulin in the assembly of major histocompatibility complex (MHC) class I molecules are well-studied and provide many insights into the modes of substrate and co-chaperone recognition by calreticulin. Calreticulin is also detectable on the cell surface under some conditions, where it induces the phagocytosis of apoptotic cells. Furthermore, mutations of calreticulin induce cell transformation in myeloproliferative neoplasms (MPN). Studies of the functions of the mutant calreticulin in cell transformation and immunity have provided many insights into the normal biology of calreticulin, which are discussed.

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?

The stem/progenitor landscape is reshaped in a mouse model of essential thrombocythemia and causes excess megakaryocyte production

Frameshift mutations in CALR (calreticulin) are associated with essential thrombocythemia (ET), but the stages at and mechanisms by which mutant CALR drives transformation remain incompletely defined. Here, we use single-cell approaches to examine the hematopoietic stem/progenitor cell landscape in a mouse model of mutant CALR-driven ET. We identify a trajectory linking hematopoietic stem cells (HSCs) with megakaryocytes and prospectively identify a previously unknown intermediate population that is overrepresented in the disease state. We also show that mutant CALR drives transformation primarily from the earliest stem cell compartment, with some contribution from megakaryocyte progenitors. Last, relative to wild-type HSCs, mutant CALR HSCs show increases in JAK-STAT signaling, the unfolded protein response, cell cycle, and a previously undescribed up-regulation of cholesterol biosynthesis. Overall, we have identified a novel megakaryocyte-biased cell population that is increased in a mouse model of ET and described transcriptomic changes linking CALR mutations to increased HSC proliferation and megakaryopoiesis.

[Pathological characteristics of megakaryocytes in myeloproliferative neoplasms and their correlation with driver gene mutations]

Objective: To investigate the pathological characteristics of megakaryocytes in myeloproliferative neoplasms(MPN)and their correlations with driver gene mutations. Methods: Trephine specimens administered for 160 patients with MPN from February 2012 to October 2017 were reevaluated according to the World Health Organization(WHO)'s(2016)diagnostic criteria. Results: This cohort of patients included 72(45.0%)men, with the median age of 59(range, 13-87)years, comprising 39 with polycythemia vera(PV), 33 with essential thrombocythemia(ET), 37 with prefibrotic/early-primary myelofibrosis(pre-PMF), 37 with overt PMF, 1 with post-ET MF, 2 with post-PV MF, and 11 with MPN-unclassifiable(MPN-U)after the re-diagnosis. With PV, ET, pre-PMF, and overt PMF changes, proportions of dense clusters, hypolobulated nuclei, and naked nuclei of megakaryocytes gradually increased, whereas erythropoiesis gradually decreased. Proportions of reticulin, collagen, and osteosclerosis grades of ≥1 also increased. Dense clusters, hypolobulated nuclei, and naked nuclei of megakaryocytes were negatively correlated with erythropoiesis and positively correlated with granulopoiesis and fibrosis. In patients with pre- and overt PMF, dense clusters and naked nuclei of megakaryocytes were positively correlated with fibrosis. Patients with JAK2V617F MPN had significantly increased erythropoiesis(P=0.022). Patients with CALR-mutated MPN were characterized by increased loose and dense clusters; paratrabecular distribution and naked nuclei of megakaryocytes(P=0.055, P=0.002, P=0.018, P=0.008); and increased reticulin, collagen, and osteosclerosis(P=0.003, P<0.001, P=0.001). In patients with pre- and overt PMF, patients with JAK2V617F had increased cellularity(P=0.037). CALR-mutated patients had increased dense clusters and giant sizes of megakaryocytes, collagen, and osteosclerosis(P=0.055, P=0.059, P=0.011, P=0.046). Conclusion: Megakaryocytes showed abnormal MPN morphology and distribution, which were related to fibrosis. CALR mutation was probably associated with abnormal morphology and distribution of megakaryocytes and fibrosis.

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.

Clonal Hematopoiesis and Mutations of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are associated with the fewest number of mutations among known cancers. The mutations propelling these malignancies are phenotypic drivers providing an important implement for diagnosis, treatment response monitoring, and gaining insight into the disease biology. The phenotypic drivers of Philadelphia chromosome negative MPN include mutations in JAK2, CALR, and MPL. The most prevalent driver mutation JAK2V617F can cause disease entities such as essential thrombocythemia (ET) and polycythemia vera (PV). The divergent development is considered to be influenced by the acquisition order of the phenotypic driver mutation relative to other MPN-related mutations such as TET2 and DNMT3A. Advances in molecular biology revealed emergence of clonal hematopoiesis (CH) to be inevitable with aging and associated with risk factors beyond the development of blood cancers. In addition to its well-established role in thrombosis, the JAK2V617F mutation is particularly connected to the risk of developing cardiovascular disease (CVD), a pertinent issue, as deep molecular screening has revealed the prevalence of the mutation to be much higher in the background population than previously anticipated. Recent findings suggest a profound under-diagnosis of MPNs, and considering the impact of CVD on society, this calls for early detection of phenotypic driver mutations and clinical intervention.

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

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

Genomic heterogeneity in myeloproliferative neoplasms and applications to clinical practice

The myeloproliferative neoplasms (MPN) polycythaemia vera, essential thrombocythaemia and primary myelofibrosis are chronic myeloid disorders associated most often with mutations in JAK2, MPL and CALR, and in some patients with additional acquired genomic lesions. Whilst the molecular mechanisms downstream of these mutations are now clearer, it is apparent that clinical phenotype in MPN is a product of complex interactions, acting between individual mutations, between disease subclones, and between the tumour and background host factors. In this review we first discuss MPN phenotypic driver mutations and the factors that interact with them to influence phenotype. We consider the importance of ongoing studies of clonal haematopoiesis, which may inform a better understanding of why MPN develop in specific individuals. We then consider how best to deploy genomic testing in a clinical environment and the challenges as well as opportunities that may arise from more routine, comprehensive genomic analysis of patients with MPN.

Mechanism underlying the development of myeloproliferative neoplasms through mutant calreticulin

Deregulation of cytokine signaling is frequently associated with various pathological conditions, including malignancies. In patients with myeloproliferative neoplasms (MPNs), recurrent somatic mutations in the calreticulin (CALR) gene, which encodes a molecular chaperone that resides in the endoplasmic reticulum, have been reported. Studies have defined mutant CALR as an oncogene promoting the development of MPN, and deciphered a novel molecular mechanism by which mutant CALR constitutively activates thrombopoietin receptor MPL and its downstream molecules to induce cellular transformation. The mechanism of interaction and activation of MPL by mutant CALR is unique, not only due to the latter forming a homomultimeric complex through a novel mutant‐specific sequence generated by frameshift mutation, but also for its ability to interact with immature asparagine‐linked glycan for eventual engagement with immature MPL in the endoplasmic reticulum. The complex formed between mutant CALR and MPL is then transported to the cell surface, where it induces constitutive activation of downstream kinase JAK2 bound to MPL. Refined structural and cell biological studies can provide an in‐depth understanding of this unusual mechanism of receptor activation by a mutant molecular chaperone. Mutant CALR is also involved in modulation of the immune response, transcription, and intracellular homeostasis, which could contribute to the development of MPN. In the present article, we comprehensively review the current understanding of the underlying molecular mechanisms for mutant molecular chaperone‐induced cellular transformation.

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.

Enhanced engraftment of human myelofibrosis stem and progenitor cells in MISTRG mice

The engraftment potential of myeloproliferative neoplasms in immunodeficient mice is low. We hypothesized that the physiological expression of human cytokines (macrophage colony-stimulating factor, interleukin-3, granulocyte-macrophage colony-stimulating factor, and thrombopoietin) combined with human signal regulatory protein α expression in Rag2-/-Il2rγ-/- (MISTRG) mice might provide a supportive microenvironment for the development and maintenance of hematopoietic stem and progenitor cells (HSPC) from patients with primary, post-polycythemia or post-essential thrombocythemia myelofibrosis (MF). We show that MISTRG mice, in contrast to standard immunodeficient NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ and Rag2-/-Il2rγ-/- mice, supported engraftment of all patient samples investigated independent of MF disease stage or risk category. Moreover, MISTRG mice exhibited significantly higher human MF engraftment levels in the bone marrow, peripheral blood, and spleen and supported secondary repopulation. Bone marrow fibrosis development was limited to 3 of 14 patient samples investigated in MISTRG mice. Disease-driving mutations were identified in all xenografts, and targeted sequencing revealed maintenance of the primary patient sample clonal composition in 7 of 8 cases. Treatment of engrafted mice with the current standard-of-care Janus kinase inhibitor ruxolitinib led to a reduction in human chimerism. In conclusion, the established MF patient-derived xenograft model supports robust engraftment of MF HSPCs and maintains the genetic complexity observed in patients. The model is suited for further testing of novel therapeutic agents to expedite their transition into clinical trials.

Thrombopoietin is required for full phenotype expression in a JAK2V617F transgenic mouse model of polycythemia vera

The myeloproliferative neoplasms, polycythemia vera, essential thrombocytosis and primary myelofibrosis are hematopoietic stem cell disorders and share driver mutations that either directly activate the thrombopoietin receptor, MPL, or activate it indirectly through gain-of-function mutations in the gene for JAK2, its cognate tyrosine kinase. Paradoxically, MPL surface expression in hematopoietic stem cells is also reduced in the myeloproliferative neoplasms due to abnormal post-translational glycosylation and premature destruction of JAK2, suggesting that the myeloproliferative neoplasms are disorders of MPL processing since MPL is the only hematopoietic growth factor receptor in hematopoietic stem cells. To examine this possibility, we genetically manipulated MPL expression and maturation in a JAK2^V617F transgenic mouse model of polycythemia vera. Elimination of MPL expression completely abrogated the polycythemia vera phenotype in this JAK2^V617F transgenic mouse model, which could only be partially restored by expression of one MPL allele. Most importantly, elimination of thrombopoietin gene expression abrogated the polycythemia vera phenotype in this JAK2^V617F transgenic mouse model, which could be completely restored by expression of a single thrombopoietin allele. These data indicate that polycythemia vera is in part a thrombopoietin-dependent disorder and that targeting the MPL-thrombopoietin axis could be an effective, nonmyelotoxic therapeutic strategy in this disorder.

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.