Thrombopoietin receptor activation by myeloproliferative neoplasm associated calreticulin mutants

Isolation of T cell receptors targeting recurrent neoantigens in hematological malignancies

Mutation-derived neoantigens represent an important class of tumour-specific, tumour rejection antigens, and are attractive targets for TCR gene therapy of cancer. The majority of such mutations are patient-specific and targeting these requires a fully personalized approach. However, some mutations are found recurrently among cancer patients, and represent potential targets for neoantigen-specific TCR gene therapy that is more widely applicable. Therefore, we have investigated if some cancer mutations found recurrently in hematological malignancies encode immunogenic neoantigens presented by common European Caucasoid HLA class I alleles and can form targets for TCR gene therapy. We initially focused on identifying HLA class I neoepitopes derived from calreticulin (CALR) exon 9 mutations, found in ~ 80% of JAK2wt myeloproliferative neoplasms (MPN). Based on MHC class I peptide predictions, a number of peptides derived from mutant CALR (mCALR) were predicted to bind to HLA-A*03:01 and HLA-B*07:02. However, using mass spectrometry and ex vivo pMHC multimer staining of PBMC from MPN patients with CALR exon 9 mutations, we found no evidence that these peptides were naturally processed and presented on the surface of mCALR-expressing target cells. We next developed a protocol utilizing pMHC multimers to isolate CD8⁺ T cells from healthy human donor PBMC that are specific for mCALR and additional putative neoepitopes found recurrently in hematological malignancies. Using this approach, CD8⁺ T cells specific for HLA-A*03:01- and HLA-B*07:02-presented mCALR peptides and an HLA-A*11:01-presented mutant FBXW7 (mFBXW7) peptide were successfully isolated. TCRs isolated from mCALR-specific CD8⁺ T cell populations were not able to recognize target cells engineered to express mCALR. In contrast, mFBXW7-specific CD8⁺ T cells were able to recognize target cells engineered to express mFBXW7. In conclusion, while we found no evidence for mCALR derived neoepitope presentation in the context of the HLA class I alleles studied, our data suggests that the recurrent pR465H mutation in FBXW7 may encode an HLA-A*11:01 presented neoepitope, and warrants further investigation as a target for T cell based immunotherapy of cancer.

Homomultimerization of mutant calreticulin is a prerequisite for MPL binding and activation

Studies have previously shown that mutant calreticulin (CALR), found in a subset of patients with myeloproliferative neoplasms (MPNs), interacts with and subsequently promotes the activation of the thrombopoietin receptor (MPL). However, the molecular mechanism behind the activity of mutant CALR remains unknown. Here we show that mutant, but not wild-type, CALR interacts to form a homomultimeric complex. This intermolecular interaction among mutant CALR proteins depends on their carboxyl-terminal domain, which is generated by a unique frameshift mutation found in patients with MPN. With a competition assay, we demonstrated that the formation of mutant CALR homomultimers is required for the binding and activation of MPL. Since association with MPL is required for the oncogenicity of mutant CALR, we propose a model in which the constitutive activation of the MPL downstream pathway by mutant CALR multimers induces the development of MPN. This study provides a potential novel therapeutic strategy against mutant CALR-dependent tumorigenesis via targeting the intermolecular interaction among mutant CALR proteins.

Tumor-associated calreticulin variants functionally compromise the peptide loading complex and impair its recruitment of MHC-I

Major histocompatibility complex-I-β₂m dimers (MHC-I) bind peptides derived from intracellular proteins, enabling the immune system to distinguish between normal cells and those expressing pathogen-derived or mutant proteins. The peptides bind to MHC-I in the endoplasmic reticulum (ER), and this binding is facilitated by the peptide loading complex (PLC), which contains calreticulin (CRT). CRT associates with MHC-I via a conserved glycan present on MHC-I and recruits it to the PLC for peptide binding. Somatic frameshift mutations in CRT (CRT-FS) drive the proliferation of a subset of myeloproliferative neoplasms, which are chronic blood tumors. All CRT-FS proteins have a C-terminal sequence lacking the normal ER-retention signal and possessing a net negative charge rather than the normal positive charge. We characterized the effect of CRT-FS on antigen presentation by MHC-I in human cells. Our results indicate that CRT-FS cannot mediate CRT's peptide loading function in the PLC. Cells lacking CRT exhibited reduced surface MHC-I levels, consistent with reduced binding of high-affinity peptides, and this was not reversed by CRT-FS expression. CRT-FS was secreted and not detectably associated with the PLC, leading to poor MHC-I recruitment, although CRT-FS could still associate with MHC-I in a glycan-dependent manner. The addition of an ER-retention sequence to CRT-FS restored its association with the PLC but did not rescue MHC-I recruitment or its surface expression, indicating that the CRT-FS mutants functionally compromise the PLC. MHC-I down-regulation permits tumor cells to evade immune surveillance, and these findings may therefore be relevant for designing effective immunotherapies for managing myeloproliferative neoplasms.

Increased B cell activation is present in JAK2V617F-mutated, CALR-mutated and triple-negative essential thrombocythemia

Essential thrombocythemia (ET) is a BCL-ABL1-negative myeloproliferative neoplasm. We have reported that increased activated B cells can facilitate platelet production mediated by cytokines regardless JAK2 mutational status in ET. Recently, calreticulin (CALR) mutations were discovered in ~30% JAK2/MPL-unmutated ET and primary myelofibrosis. Here we sought to screen for CALR mutations and to evaluate B cell immune profiles in a cohort of adult Taiwanese ET patients. B cell populations, granulocytes/monocytes membrane-bound B cell-activating factor (mBAFF) levels, B cells toll-like receptor 4 (TLR4) expression and intracellular levels of interleukin (IL)-1β/IL-6 and the expression of CD69, CD80, and CD86 were quantified by flow cytometry. Serum BAFF concentration was measured by ELISA. 48 healthy adults were used for comparison. 19 (35.2%) of 54 ET patients harbored 8 types of CALR exon 9 mutations including 4 (7.4%) patients with concomitant JAK2V617F mutations. Compared to JAK2V617F mutation, CALR mutations correlated with younger age at diagnosis (p=0.04), higher platelet count (p=0.004), lower hemoglobin level (p=0.013) and lower leukocyte count (p=0.013). Multivariate analysis adjusted for age, sex, follow-up period and hematological parameters confirmed that increased activated B cells were universally present in JAK2-mutated, CALR-mutated and triple-negative ET patients when compared to healthy adults. JAK2- and CALR-mutated ET have significantly higher fraction of B cells with TLR4 expression when compared to triple-negative ET (p=0.019 and 0.02, respectively). CALR-mutated ET had significantly higher number of CD69-positive activated B cells when compared to triple-negative ET (p=0.035). In conclusion, increased B cell activation is present in ET patients across different mutational subgroups.

Recent advances in the genomics and therapy of BCR/ABL1-positive and -negative chronic myeloproliferative neoplasms

This review is based on the presentations and deliberations at the 7th John Goldman Chronic Myeloid Leukemia (CML) and Myeloproliferative Neoplasms (MPN) Colloquium which took place in Estoril, Portugal on the 15th October 2017, and the 11th post-ASH International Workshop on CML and MPN which took place on the 6th-7th December 2016, immediately after the 58th American Society of Hematology Annual Meeting. Rather than present a resume of the proceedings, we have elected to address some of the topical translational research and clinically relevant topics in greater detail. We address recent updates in the genetics and epigenetics of MPN, the mechanisms of transformation by mutant calreticulin, advances in the biology and therapy of systemic mastocytosis, clinical updates on JAK2 inhibitors and other therapeutic approaches for patients with MPNs, cardiovascular toxicity related to tyrosine kinase inhibitors and the concept of treatment-free remission for patients with CML.

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.

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

Somatic mutations in the endoplasmic reticulum chaperone calreticulin (CALR) are detected in approximately 40% of patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF). Multiple different mutations have been reported, but all result in a +1-bp frameshift and generate a novel protein C terminus. In this study, we generated a conditional mouse knockin model of the most common CALR mutation, a 52-bp deletion. The mutant novel human C-terminal sequence is integrated into the otherwise intact mouse CALR gene and results in mutant CALR expression under the control of the endogenous mouse locus. CALRdel/+ mice develop a transplantable ET-like disease with marked thrombocytosis, which is associated with increased and morphologically abnormal megakaryocytes and increased numbers of phenotypically defined hematopoietic stem cells (HSCs). Homozygous CALRdel/del mice developed extreme thrombocytosis accompanied by features of MF, including leukocytosis, reduced hematocrit, splenomegaly, and increased bone marrow reticulin. CALRdel/+ HSCs were more proliferative in vitro, but neither CALRdel/+ nor CALRdel/del displayed a competitive transplantation advantage in primary or secondary recipient mice. These results demonstrate the consequences of heterozygous and homozygous CALR mutations and provide a powerful model for dissecting the pathogenesis of CALR-mutant ET and PMF.

Calreticulin Affects Hematopoietic Stem/Progenitor Cell Fate by Impacting Erythroid and Megakaryocytic Differentiation

Calreticulin (CALR) is a chaperone protein that localizes primarily to the endoplasmic reticulum (ER) lumen where it is responsible for the control of proper folding of neo-synthesized glycoproteins and the retention of calcium. Recently, mutations affecting exon 9 of the CALR gene have been described in approximately 40% of patients with myeloproliferative neoplasms (MPNs). Although the role of mutated CALR in the development of MPNs has begun to be clarified, there are still no data available on the function of wild-type (WT) CALR during physiological hematopoiesis. To shed light on the role of WT CALR during normal hematopoiesis, we performed gene silencing and overexpression experiments in hematopoietic stem progenitor cells (HSPCs). Our results showed that CALR overexpression is able to affect physiological hematopoiesis by enhancing both erythroid and megakaryocytic (MK) differentiation. In agreement with overexpression data, CALR silencing caused a significant decrease in both erythroid and MK differentiation of human HSPCs. Gene expression profiling (GEP) analysis showed that CALR is able to affect the expression of several genes involved in HSPC differentiation toward both the erythroid and MK lineages. Moreover, GEP data also highlighted the modulation of several genes involved in ER stress response, unfolded protein response (UPR), and DNA repair, and of several genes already described to play a role in MPN development, such as proinflammatory cytokines and hematological neoplasm-related markers. Altogether, our data unraveled a new and unexpected role for CALR in the regulation of normal hematopoietic differentiation. Moreover, by showing the impact of CALR on the expression of genes involved in several biological processes already described in cellular transformation, our data strongly suggest a more complex role for CALR in MPN development that goes beyond the activation of the THPO receptor and involves ER stress response, UPR, and DNA repair.

JAK inhibitors for the treatment of myeloproliferative neoplasms and other disorders

JAK inhibitors have been developed following the discovery of the JAK2V617F in 2005 as the driver mutation of the majority of non- BCR-ABL1 myeloproliferative neoplasms (MPNs). Subsequently, the search for JAK2 inhibitors continued with the discovery that the other driver mutations ( CALR and MPL) also exhibited persistent JAK2 activation. Several type I ATP-competitive JAK inhibitors with different specificities were assessed in clinical trials and exhibited minimal hematologic toxicity. Interestingly, these JAK inhibitors display potent anti-inflammatory activity. Thus, JAK inhibitors targeting preferentially JAK1 and JAK3 have been developed to treat inflammation, autoimmune diseases, and graft-versus-host disease. Ten years after the beginning of clinical trials, only two drugs have been approved by the US Food and Drug Administration: one JAK2/JAK1 inhibitor (ruxolitinib) in intermediate-2 and high-risk myelofibrosis and hydroxyurea-resistant or -intolerant polycythemia vera and one JAK1/JAK3 inhibitor (tofacitinib) in methotrexate-resistant rheumatoid arthritis. The non-approved compounds exhibited many off-target effects leading to neurological and gastrointestinal toxicities, as seen in clinical trials for MPNs. Ruxolitinib is a well-tolerated drug with mostly anti-inflammatory properties. Despite a weak effect on the cause of the disease itself in MPNs, it improves the clinical state of patients and increases survival in myelofibrosis. This limited effect is related to the fact that ruxolitinib, like the other type I JAK2 inhibitors, inhibits equally mutated and wild-type JAK2 (JAK2WT) and also the JAK2 oncogenic activation. Thus, other approaches need to be developed and could be based on either (1) the development of new inhibitors specifically targeting JAK2V617F or (2) the combination of the actual JAK2 inhibitors with other therapies, in particular with molecules targeting pathways downstream of JAK2 activation or the stability of JAK2 molecule. In contrast, the strong anti-inflammatory effects of the JAK inhibitors appear as a very promising therapeutic approach for many inflammatory and auto-immune diseases.

Crucial factors of the inflammatory microenvironment (IL-1β/TNF-α/TIMP-1) promote the maintenance of the malignant hemopoietic clone of myelofibrosis: an in vitro study

Along with molecular abnormalities (mutations in JAK2, Calreticulin (CALR) and MPL genes), chronic inflammation is the major hallmark of Myelofibrosis (MF). Here, we investigated the in vitro effects of crucial factors of the inflammatory microenvironment (Interleukin (IL)-1β, Tumor Necrosis Factor (TNF)-α, Tissue Inhibitor of Metalloproteinases (TIMP)-1 and ATP) on the functional behaviour of MF-derived circulating CD34⁺ cells.

We found that, regardless mutation status, IL-1β or TNF-α increases the survival of MF-derived CD34⁺ cells. In addition, along with stimulation of cell cycle progression to the S-phase, IL-1β or TNF-α ± TIMP-1 significantly stimulate(s) the in vitro clonogenic ability of CD34⁺ cells from JAK2^V617 mutated patients. Whereas in the JAK2^V617F mutated group, the addition of IL-1β or TNF-α + TIMP-1 decreased the erythroid compartment of the CALR mutated patients. Megakaryocyte progenitors were stimulated by IL-1β (JAK2^V617F mutated patients only) and inhibited by TNF-α. IL-1β + TNF-α + C-X-C motif chemokine 12 (CXCL12) ± TIMP-1 highly stimulates the in vitro migration of MF-derived CD34+ cells. Interestingly, after migration toward IL-1β + TNF-α + CXCL12 ± TIMP-1, CD34⁺ cells from JAK2^V617F mutated patients show increased clonogenic ability.

Here we demonstrate that the interplay of these inflammatory factors promotes and selects the circulating MF-derived CD34⁺ cells with higher proliferative activity, clonogenic potential and migration ability. Targeting these micro-environmental interactions may be a clinically relevant approach.

Phenotypic Screening for Inhibitors of a Mutant Thrombopoietin Receptor

An inhibitor for the thrombopoietin receptor (TpoR) would be more specific for the treatment of myeloproliferative neoplasms (MPNs) due to constitutively active mutant TpoR compared to the current treatment approach of inhibiting Janus kinase 2 (JAK2). We describe a cell-based high-throughput phenotypic screening approach to identify inhibitors for constitutively active mutant TpoR. A stepwise elimination process is used to differentiate generally cytotoxic compounds from compounds that specifically inhibit growth of cells expressing wild-type TpoR and/or mutant TpoR. We have systematically optimized the phenotypic screening assay and documented in this chapter critical parameters for a successful phenotypic screen, such as cell growth and seeding optimization, plate reproducibility and uniformity studies, and an assay robustness analysis with a pilot screen.

SOHO State-of-the-Art Update and Next Questions: MPN

The discovery of the activating Janus kinase (JAK)2V617F mutation in 2005 in most patients with the classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPN) spurred intense interest in research into these disorders, culminating in the identification of activating mutations in MPL in 2006 and indels in the gene encoding calreticulin (CALR) in 2013, thus providing additional mechanistic explanations for the universal activation of JAK-signal transducer and activator of transcription (JAK-STAT) observed in these conditions, and the success of the JAK1/2 inhibitor ruxolitinib, which first received regulatory approval in 2011. The field has continued to advance rapidly since then, and the past 2 years have witnessed important changes to the classification of MPN and diagnostic criteria for polycythemia vera (PV), novel insights into the mechanisms of bone marrow fibrosis in primary myelofibrosis (PMF), increasing appreciation of the biologic differences between essential thrombocythemia (ET), prefibrotic and overt PMF, and between primary and post-PV/ET myelofibrosis (MF). Additionally, the mechanisms through which mutant CALR drives JAK-STAT pathway activation and oncogenic transformation are now better understood. Although mastocytosis is no longer included under the broad heading of MPN in the 2016 revision to the World Health Organization classification, an important milestone in mastocytosis research was reached in 2017 with the regulatory approval of midostaurin for patients with advanced systemic mastocytosis (AdvSM). In this article, we review the major recent developments in the areas of PV, ET, and MF, and also briefly summarize the literature on midostaurin and other KIT inhibitors for patients with AdvSM.

Defining the requirements for the pathogenic interaction between mutant calreticulin and MPL in MPN

Mutations in calreticulin (CALR) are phenotypic drivers in the pathogenesis of myeloproliferative neoplasms. Mechanistic studies have demonstrated that mutant CALR binds to the thrombopoietin receptor MPL, and that the positive electrostatic charge of the mutant CALR C terminus is required for mutant CALR-mediated activation of JAK-STAT signaling. Here we demonstrate that although binding between mutant CALR and MPL is required for mutant CALR to transform hematopoietic cells; binding alone is insufficient for cytokine independent growth. We further show that the threshold of positive charge in the mutant CALR C terminus influences both binding of mutant CALR to MPL and activation of MPL signaling. We find that mutant CALR binds to the extracellular domain of MPL and that 3 tyrosine residues within the intracellular domain of MPL are required to activate signaling. With respect to mutant CALR function, we show that its lectin-dependent function is required for binding to MPL and for cytokine independent growth, whereas its chaperone and polypeptide-binding functionalities are dispensable. Together, our findings provide additional insights into the mechanism of the pathogenic mutant CALR-MPL interaction in myeloproliferative neoplasms.

CALR mutational status identifies different disease subtypes of essential thrombocythemia showing distinct expression profiles

Polycythemia vera (PV) and essential thrombocythemia (ET) are Philadelphia-negative myeloproliferative neoplasms (MPNs) characterized by erythrocytosis and thrombocytosis, respectively. Approximately 95% of PV and 50–70% of ET patients harbor the V617F mutation in the exon 14 of JAK2 gene, while about 20–30% of ET patients carry CALRins5 or CALRdel52 mutations. These ET CALR-mutated subjects show higher platelet count and lower thrombotic risk compared to JAK2-mutated patients. Here, we showed that CALR-mutated and JAK2V617F-positive CD34+ cells display different gene and miRNA expression profiles. Indeed, we highlighted several pathways differentially activated between JAK2V617F- and CALR-mutated progenitors, i.e., mTOR, MAPK/PI3K, and MYC pathways. Furthermore, we unveiled that the expression of several genes involved in DNA repair, chromatin remodeling, splicing, and chromatid cohesion are decreased in CALR-mutated cells. According to the low risk of thrombosis in CALR-mutated patients, we also found the downregulation of several genes involved in thrombin signaling and platelet activation. As a whole, these data support the model that CALR-mutated ET could be considered as a distinct disease entity from JAK2V617F-positive MPNs and may provide the molecular basis supporting the different clinical features of these patients.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

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.

Calreticulin: Challenges Posed by the Intrinsically Disordered Nature of Calreticulin to the Study of Its Function

Calreticulin is a Ca²⁺-binding chaperone protein, which resides mainly in the endoplasmic reticulum but also found in other cellular compartments including the plasma membrane. In addition to Ca²⁺, calreticulin binds and regulates almost all proteins and most of the mRNAs deciding their intracellular fate. The potential functions of calreticulin are so numerous that identification of all of them is becoming a nightmare. Still the recent discovery that patients affected by the Philadelphia-negative myeloproliferative disorders essential thrombocytemia or primary myelofibrosis not harboring JAK2 mutations carry instead calreticulin mutations disrupting its C-terminal domain has highlighted the clinical need to gain a deeper understanding of the biological activity of this protein. However, by contrast with other proteins, such as enzymes or transcription factors, the biological functions of which are strictly defined by a stable spatial structure imprinted by their amino acid sequence, calreticulin contains intrinsically disordered regions, the structure of which represents a highly dynamic conformational ensemble characterized by constant changes between several metastable conformations in response to a variety of environmental cues. This article will illustrate the Theory of calreticulin as an intrinsically disordered protein and discuss the Hypothesis that the dynamic conformational changes to which calreticulin may be subjected by environmental cues, by promoting or restricting the exposure of its active sites, may affect its function under normal and pathological conditions.

Endoplasmic reticulum in health and disease: the 12th International Calreticulin Workshop, Delphi, Greece

Starting from 1994, every 2 years, an international workshop is organized focused on calreticulin and other endoplasmic reticulum chaperones. In 2017, the workshop took place at Delphi Greece. Participants from North and South America, Europe, Asia and Australia presented their recent data and discussed them extensively with their colleagues. Presentations dealt with structural aspects of calreticulin and calnexin, the role of Ca²⁺ in cellular signalling and in autophagy, the endoplasmic reticulum stress and the unfolded protein response, the role of calreticulin in immune responses. Several presentations focused on the role of calreticulin and other ER chaperones in a variety of disease states, including haemophilia, obesity, diabetes, Sjogren's syndrome, Chagas diseases, multiple sclerosis, amyotrophic lateral sclerosis, neurological malignancies (especially glioblastoma), haematological malignancies (especially essential thrombocythemia and myelofibrosis), lung adenocarcinoma, renal pathology with emphasis in fibrosis and drug toxicity. In addition, the role of calreticulin and calnexin in growth and wound healing was discussed, as well as the possible use of extracellular calreticulin as a marker for certain diseases. It was agreed that the 13th International Calreticulin Workshop will be organized in 2019 in Montreal, Quebec, Canada.

miR-494-3p overexpression promotes megakaryocytopoiesis in primary myelofibrosis hematopoietic stem/progenitor cells by targeting SOCS6

Primary myelofibrosis (PMF) is a chronic Philadelphia-negative myeloproliferative neoplasm characterized by hematopoietic stem cell-derived clonal myeloproliferation, involving especially the megakaryocyte lineage. To better characterize how the altered expression of microRNAs might contribute to PMF pathogenesis, we have previously performed the integrative analysis of gene and microRNA expression profiles of PMF hematopoietic stem/progenitor cells (HSPCs), which allowed us to identify miR-494-3p as the upregulated microRNA predicted to target the highest number of downregulated mRNAs.To elucidate the role of miR-494-3p in hematopoietic differentiation, in the present study we demonstrated that miR-494-3p enforced expression in normal HSPCs promotes megakaryocytopoiesis. Gene expression profiling upon miR-494-3p overexpression allowed the identification of genes commonly downregulated both after microRNA overexpression and in PMF CD34+ cells. Among them, suppressor of cytokine signaling 6 (SOCS6) was confirmed to be a miR-494-3p target by luciferase assay. Western blot analysis showed reduced level of SOCS6 protein as well as STAT3 activation in miR-494-3p overexpressing cells. Furthermore, transient inhibition of SOCS6 expression in HSPCs demonstrated that SOCS6 silencing stimulates megakaryocytopoiesis, mimicking the phenotypic effects observed upon miR-494-3p overexpression. Finally, to disclose the contribution of miR-494-3p upregulation to PMF pathogenesis, we performed inhibition experiments in PMF HSPCs, which showed that miR-494-3p silencing led to SOCS6 upregulation and impaired megakaryocyte differentiation.Taken together, our results describe for the first time the role of miR-494-3p during normal HSPC differentiation and suggest that its increased expression, and the subsequent downregulation of its target SOCS6, might contribute to the megakaryocyte hyperplasia commonly observed in PMF patients.

Mutations in myeloproliferative neoplasms - their significance and clinical use

INTRODUCTION

Clonal hematologic diseases of the blood such as polycythemia vera, essential thrombocythemia and primary myelofibrosis belong to the BCR-ABL negative Myeloproliferative Neoplasms (MPN). These diseases are characterized by clonal expansion of hematopoietic precursor cells followed by increased production of differentiated cells of the myeloid lineage. Initiation of clonal hematopoiesis, formation of a clinical phenotype as well as disease progression form part of MPN disease evolution. The disease is driven by acquired somatic mutations in critical pathways such as cytokine signaling, epigenetic regulation, RNA splicing, and transcription factor signaling. Areas covered: The following review aims to provide an overview of the mutational landscape of MPN, the impact of these mutations in MPN pathogenesis as well as their prognostic value. Finally, a summary of how these mutations are being used or could potentially be used for the treatment of MPN patients is presented. Expert commentary: The genetic landscape of MPN patients has been successfully dissected within the past years with the advent of new sequencing technologies. Integrating the genetic information within a clinical setting is already benefitting patients in terms of disease monitoring and prognostic information of disease progression but will be further intensified within the next years.

Gene editing rescue of a novel MPL mutant associated with congenital amegakaryocytic thrombocytopenia

Thrombopoietin (Tpo) and its receptor (Mpl) are the principal regulators of early and late thrombopoiesis and hematopoietic stem cell maintenance. Mutations in MPL can drastically impair its function and be a contributing factor in multiple hematologic malignancies, including congenital amegakaryocytic thrombocytopenia (CAMT). CAMT is characterized by severe thrombocytopenia at birth, which progresses to bone marrow failure and pancytopenia. Here we report unique familial cases of CAMT that presented with a previously unreported MPL mutation: T814C (W272R) in the background of the activating MPL G117T (K39N or Baltimore) mutation. Confocal microscopy, proliferation and surface biotinylation assays, co-immunoprecipitation, and western blotting analysis were used to elucidate the function and trafficking of Mpl mutants. Results showed that Mpl protein bearing the W272R mutation, alone or together with the K39N mutation, lacks detectable surface expression while being strongly colocalized with the endoplasmic reticulum (ER) marker calreticulin. Both WT and K39N-mutated Mpl were found to be signaling competent, but single or double mutants bearing W272R were unresponsive to Tpo. Function of the deficient Mpl receptor could be rescued by using 2 separate approaches: (1) GRASP55 overexpression, which partially restored Tpo-induced signaling of mutant Mpl by activating an autophagy-dependent secretory pathway and thus forcing ER-trapped immature receptors to traffic to the cell surface; and (2) CRISPR-Cas9 gene editing used to repair MPL T814C mutation in transfected cell lines and primary umbilical cord blood-derived CD34⁺ cells. We demonstrate proof of principle for rescue of mutant Mpl function by using gene editing of primary hematopoietic stem cells, which indicates direct therapeutic applications for CAMT patients.

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.

Novel molecular mechanism of cellular transformation by a mutant molecular chaperone in myeloproliferative neoplasms

Deregulation of the cytokine‐receptor signaling pathway plays a significant role in tumorigenesis. Such deregulation is frequently caused by alterations in the genes involved in the signaling pathway. At the end of 2013, recurrent somatic mutations in the calreticulin (CALR) gene that encodes a molecular chaperone were identified in a subset of patients with Philadelphia‐chromosome negative myeloproliferative neoplasms (MPN). The present review focuses on the role of CALR mutations in the oncogenic transformations observed in MPN. All the CALR mutations were found to generate a + 1 frameshift in the reading frame on exon 9, which encodes the carboxy (C)‐terminus end of CALR, and thus conferred a common mutant‐specific sequence in all the CALR mutants. The mutant CALR (but not the wild‐type) constitutively activates the thrombopoietin (TPO) receptor, myeloproliferative leukemia protein (MPL), even in the absence of TPO to induce cellular transformation. Preferential interaction between the mutant CALR and MPL is achieved by a presumptive conformational change induced by the mutant‐specific C‐terminus domain, which allows N‐domain binding to MPL. Even though mutant CALR is expressed on the cell surface and is secreted out of cells, it only presents autocrine capacity for MPL activation. These findings define a novel molecular mechanism by which the mutant molecular chaperone constitutively activates the cytokine receptor to induce cellular transformation.

JAK2 inhibitors for myeloproliferative neoplasms: what is next?

Since its approval in 2011, the Janus kinase 1/2 (JAK1/2) inhibitor ruxolitinib has evolved to become the centerpiece of therapy for myelofibrosis (MF), and its use in patients with hydroxyurea resistant or intolerant polycythemia vera (PV) is steadily increasing. Several other JAK2 inhibitors have entered clinical testing, but none have been approved and many have been discontinued. Importantly, the activity of these agents is not restricted to patients with JAK2 V617F or exon 12 mutations. Although JAK2 inhibitors provide substantial clinical benefit, their disease-modifying activity is limited, and rational combinations with other targeted agents are needed, particularly in MF, in which survival is short. Many such combinations are being explored, as are other novel agents, some of which could successfully be combined with JAK2 inhibitors in the future. In addition, new JAK2 inhibitors with the potential for less myelosuppression continue to be investigated. Given the proven safety and efficacy of ruxolitinib, it is likely that ruxolitinib-based combinations will be a major way forward in drug development for MF. If approved, less myelosuppressive JAK2 inhibitors such as pacritinib or NS-018 could prove to be very useful additions to the therapeutic armamentarium in MF. In PV, inhibitors of histone deacetylases and human double minute 2 have activity, but their role, if any, in the future treatment algorithm is uncertain, given the availability of ruxolitinib and renewed interest in interferons. Ruxolitinib is in late-phase clinical trials in essential thrombocythemia, in which it could fill an important void for patients with troublesome symptoms.

Involvement of MAF/SPP1 axis in the development of bone marrow fibrosis in PMF patients

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by hyperplastic megakaryopoiesis and myelofibrosis. We recently described the upregulation of MAF (v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog) in PMF CD34+ hematopoietic progenitor cells (HPCs) compared to healthy donor. Here we demonstrated that MAF is also upregulated in PMF compared with the essential thrombocytemia (ET) and polycytemia vera (PV) HPCs. MAF overexpression and knockdown experiments shed some light into the role of MAF in PMF pathogenesis, by demonstrating that MAF favors the megakaryocyte and monocyte/macrophage commitment of HPCs and leads to the increased expression of proinflammatory and profibrotic mediators. Among them, we focused our further studies on SPP1 and LGALS3. We assessed SPP1 and LGALS3 protein levels in 115 PMF, 47 ET and 24 PV patients plasma samples and we found that SPP1 plasma levels are significantly higher in PMF compared with ET and PV patients. Furthermore, in vitro assays demonstrated that SPP1 promotes fibroblasts and mesenchymal stromal cells proliferation and collagen production. Strikingly, clinical correlation analyses uncovered that higher SPP1 plasma levels in PMF patients correlate with a more severe fibrosis degree and a shorter overall survival. Collectively our data unveil that MAF overexpression contributes to PMF pathogenesis by driving the deranged production of the profibrotic mediator SPP1.

The calreticulin (CALR) exon 9 mutations are promising targets for cancer immune therapy

The calreticulin (CALR) exon 9 mutations are found in ∼30% of patients with essential thrombocythemia and primary myelofibrosis. Recently, we reported spontaneous immune responses against the CALR mutations. Here, we describe that CALR-mutant (CALRmut)-specific T cells are able to specifically recognize CALRmut cells. First, we established a T-cell culture specific for a CALRmut epitope. These specific T cells were able to recognize several epitopes in the CALRmut C terminus. Next, we established a CALRmut-specific CD4⁺ T-cell clone by limiting dilution. These CD4⁺ T cells recognized autologous CALRmut monocytes and hematopoietic stem cells, and T-cell recognition of target cells was dependent on the presence of CALR. Furthermore, we showed that the CALRmut response was human leukocyte antigen (HLA)-DR restricted. Finally, we demonstrated that the CALRmut-specific CD4⁺ T cells, despite their phenotype, were cytotoxic to autologous CALRmut cells, and that the cytotoxicity was mediated by degranulation of the T cells. In conclusion, the CALR exon 9 mutations are targets for specific T cells and thus are promising targets for cancer immune therapy such as peptide vaccination in patients harboring CALR exon 9 mutations.

Epigenetics in Myeloproliferative Neoplasms

A decade on from the description of JAK2 V617F, the MPNs are circumscribed by an increasingly intricate landscape. There is now evidence that they are likely the result of combined genetic dysregulation, with several mutated genes involved in the regulation of epigenetic mechanisms. Epigenetic changes are not due to a change in the DNA sequence but are reversible modifications that dictate the way in which genes may be expressed (or silenced). Among the epigenetic mechanisms, DNA methylation is probably the best described. Currently known MPN‐associated mutations now include JAK2, MPL, LNK, CBL, CALR, TET2, ASXL1, IDH1, IDH2, IKZF1 and EZH2. Enhancing our knowledge about the mutation profile of patients may allow them to be stratified into risk groups which would aid clinical decision making. Ongoing work will answer whether the use of epigenetic therapies as alterative pathway targets in combination with JAK inhibitors may be more effective than single agent treatment.

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.

Anti-Platelet Factor 4/Heparin Antibody Formation Occurs Endogenously and at Unexpected High Frequency in Polycythemia Vera

Background

Myeloproliferative neoplasms (MPN) encounter thromboses due to multiple known risk factors. Heparin-induced thrombocytopenia (HIT) is a thrombotic syndrome mediated by anti-platelet factor 4 (PF4)/heparin antibodies with undetermined significance for thrombosis in MPN. We hypothesized that anti-PF4/heparin Ab might occur in MPN and promote thrombosis.

Methods

Anti-PF4/heparin antibodies were analyzed in 127 MPN patients including 76 PV and 51 ET. Screening, validation testing, and isotype testing of anti-PF4/heparin Ab were correlated with disease characteristics.

Results

Anti-PF4/heparin antibodies were detected in 21% of PV and 12% of ET versus 0.3–3% in heparin-exposed patients. Validation testing confirmed anti-PF4/heparin immunoglobulins in 15% of PV and 10% of ET. Isotype testing detected 9.2% IgG and 5.3% IgM in PV and exclusively IgM in ET. IgG-positive PV patients encountered thromboses in 57.1% suggesting anti-PF4/heparin IgG may contribute to higher risk for thrombosis in MPN. Overall, 45% of PV patients experienced thromboses with 11.8% positive for anti-PF4/heparin IgG versus 7.1% in PV without thrombosis.

Conclusion

Anti-PF4/heparin antibodies occur endogenously and more frequently in MPN than upon heparin exposure. Thrombotic risk increases in anti-PF4/heparin IgG-positive PV reflecting potential implications and calling for larger, confirmatory cohorts. Anti-PF4/heparin IgG should be assessed upon thrombosis in PV to facilitate avoidance of heparin in anti-PF4/heparin IgG-positive PV.

Leptin-receptor-expressing bone marrow stromal cells are myofibroblasts in primary myelofibrosis

Bone marrow fibrosis is a critical component of primary myelofibrosis (PMF). However, the origin of the myofibroblasts that drive fibrosis is unknown. Using genetic fate mapping we found that bone marrow leptin receptor (Lepr)-expressing mesenchymal stromal lineage cells expanded extensively and were the fibrogenic cells in PMF. These stromal cells downregulated the expression of key haematopoietic-stem-cell-supporting factors and upregulated genes associated with fibrosis and osteogenesis, indicating fibrogenic conversion. Administration of imatinib or conditional deletion of platelet-derived growth factor receptor a (Pdgfra) from Lepr+ stromal cells suppressed their expansion and ameliorated bone marrow fibrosis. Conversely, activation of the PDGFRA pathway in bone marrow Lepr+ cells led to expansion of these cells and extramedullary haematopoiesis, features of PMF. Our data identify Lepr+ stromal lineage cells as the origin of myofibroblasts in PMF and suggest that targeting PDGFRA signalling could be an effective way to treat bone marrow fibrosis.

Calreticulin in Essential Thrombocythemia: StressINg OUT the Megakaryocyte Nucleus

Calreticulin (CALR) is a multifaceted protein primarily involved in intracellular protein control processes. The identification of CALR mutations in essential thrombocythemia (ET) and primary myelofibrosis that are mutually exclusive with the JAK2 V617F mutation has stirred an intensive research interest about the molecular functions of CALR and its mutants in myeloproliferative neoplasms (MPNs) and its diagnostic/prognostic value. The recently characterized protein–protein interaction of CALR mutants and MPL receptor has advanced our knowledge on the functional role of CALR mutants in thrombocythemia but it has also uncovered limitations of the current established research models. Human cell lines and mouse models provide useful information but they lack the advantages provided by ex vivo primary cultures of physiologically relevant to the disease cell types [i.e., megakaryocytes (MKs), platelets]. The results from gene expression and chromatin occupancy analysis have focused on the JAK-STAT pathway activated in both JAK2 V617F- and CALR-mutated MPN patient groups, although a more complete analysis is needed to be performed in MKs. Stress related processes seem to be affected in CALR mutant ET-MKs, but the precise mechanism is not known yet. Herein, we describe a culture method for human MKs from peripheral blood progenitors, which could help further toward an unbiased characterization of the role of CALR in ET and MK differentiation.

Assessing the thrombotic risk of patients with essential thrombocythemia in the genomic era

The molecular characterization of myeloproliferative neoplasms, including essential thrombocythemia (ET), has enabled deeper understanding of their pathogenesis. A driver lesion, namely, Janus kinase (JAK)2V617F, calreticulin (CALR) or myeloproliferative leukemia (MPL) gene mutation can be identified in the vast majority of patients. Each of these mutations is associated with distinct clinical features and may modulate the patients' clinical course, risk of complications, including vascular events, and survival. JAK2V617F appears to be a risk-modifying mutation and has been shown to increase the likelihood of thrombotic events in patients with ET across studies. As such, it has been included in prognostic models and its presence may influence treatment decisions. The association of CALR and MPL mutations with the incidence of vascular events has been less clear. Even more limited information is available on the contribution of additional non-driver lesions to the thrombotic risk. In this review we discuss the available evidence on the role of recurrent mutations in the risk of thrombotic complications in patients with ET and how these mutations weigh into modern prognostic scores.

Mechanisms of Resistance to JAK2 Inhibitors in Myeloproliferative Neoplasms

Myeloproliferative neoplasms are driven by activated JAK2 signaling due to somatic mutations in JAK2, the thrombopoietin receptor MPL or the chaperone calreticulin in hematopoietic stem/progenitor cells. JAK2 inhibitors have been developed, but despite clinical benefits, they do not signficantly reduce the mutant clone. Loss of response to JAK2 inhibitors occurs and several mechanisms of resistance, genetic and functional, have been identified. Resistance mutations have not been reported in MPN patients suggesting incomplete target inhibition. Alternative targeting of JAK2 by HSP90 inhibitors or type II JAK2 inhibition overcomes resistance to current JAK2 inhibitors. Additional combined therapy approaches are currently being evaluated.

Somatic mutations of calreticulin in myeloproliferative neoplasms

Recurrent somatic mutations in calreticulin (CALR) gene that encodes a molecular chaperone residing in the endoplasmic reticulum were identified in 2013 in a subset of patients with myeloproliferative neoplasms (MPNs). All of these mutations found in patients were either small insertion or deletion in a narrow region on exon 9 of CALR gene, and caused +1 frameshift in the reading frame for the translation of the carboxyl-terminus of CALR. Because of this unique feature, the CALR mutation is believed to be a gain-of-function mutation. However, there was essentially no rationale model to implicate the involvement of mutant CALR in the pathogenesis of MPN or other malignancies. Based on the recent findings, this review summarizes a novel molecular mechanism by which this mutant molecular chaperone constitutively activates the cytokine receptor to induce cellular transformation in MPNs.

Recommendations for molecular testing in classical Ph1-neg myeloproliferative disorders-A consensus project of the Italian Society of Hematology

The discovery that Philadelphia-negative classical myeloproliferative neoplasms (MPNs) present with several molecular abnormalities, including the mostly represented JAK2V617F mutation, opened new horizons in the diagnosis, prognosis, and monitoring of these disorders. However, the great strides in the knowledge on molecular genetics need parallel progresses on the best approach to methods for detecting and reporting disease-associated mutations, and to shape the most effective and rationale testing pathway in the diagnosis, prognosis and monitoring of MPNs. The MPN taskforce of the Italian Society of Hematology (SIE) assessed the scientific literature and composed a framework of the best, possibly evidence-based, recommendations for optimal molecular methods as well as insights about the applicability and interpretation of those tests in the clinical practice, and clinical decision for testing MPNs patients. The issues dealt with: source of samples and nucleic acid template, the most appropriate molecular abnormalities and related detection methods required for diagnosis, prognosis, and monitoring of MPNs, how to report a diagnostic molecular test, calibration and quality control. For each of these issues, practice recommendations were provided.

A novel signalling screen demonstrates that CALR mutations activate essential MAPK signalling and facilitate megakaryocyte differentiation

Most myeloproliferative neoplasm (MPN) patients lacking JAK2 mutations harbour somatic CALR mutations that are thought to activate cytokine signalling although the mechanism is unclear. To identify kinases important for survival of CALR-mutant cells, we developed a novel strategy (KISMET) that utilizes the full range of kinase selectivity data available from each inhibitor and thus takes advantage of off-target noise that limits conventional small-interfering RNA or inhibitor screens. KISMET successfully identified known essential kinases in haematopoietic and non-haematopoietic cell lines and identified the mitogen activated protein kinase (MAPK) pathway as required for growth of the CALR-mutated MARIMO cells. Expression of mutant CALR in murine or human haematopoietic cell lines was accompanied by myeloproliferative leukemia protein (MPL)-dependent activation of MAPK signalling, and MPN patients with CALR mutations showed increased MAPK activity in CD34 cells, platelets and megakaryocytes. Although CALR mutations resulted in protein instability and proteosomal degradation, mutant CALR was able to enhance megakaryopoiesis and pro-platelet production from human CD34+ progenitors. These data link aberrant MAPK activation to the MPN phenotype and identify it as a potential therapeutic target in CALR-mutant positive MPNs.

Splanchnic vein thrombosis in myeloproliferative neoplasms: pathophysiology and molecular mechanisms of disease

Myeloproliferative neoplasms (MPNs) are the most common underlying prothrombotic disorder found in patients with splanchnic vein thrombosis (SVT). Clinical risk factors for MPN-associated SVTs include younger age, female sex, concomitant hypercoagulable disorders, and the JAK2 V617F mutation. These risk factors are distinct from those associated with arterial or deep venous thrombosis (DVT) in MPN patients, suggesting disparate disease mechanisms. The pathophysiology of SVT is thought to derive from local interactions between activated blood cells and the unique splanchnic endothelial environment. Other mutations commonly found in MPNs, including CALR and MPL, are rare in MPN-associated SVT. The purpose of this article is to review the clinical and molecular risk factors for MPN-associated SVT, with particular focus on the possible mechanisms of SVT formation in MPN patients.

The Calreticulin control of human stress erythropoiesis is impaired by JAK2V617F in polycythemia vera

Calreticulin (CALR) is a Ca²⁺-binding protein that shuttles among cellular compartments with proteins bound to its N/P domains. The knowledge that activation of the human erythropoietin receptor induces Ca²⁺ fluxes prompted us to investigate the role of CALR in human erythropoiesis. As shown by Western blot analysis, erythroblasts generated in vitro from normal sources and JAK2V617F polycythemia vera (PV) patients expressed robust levels of CALR. However, Ca²⁺ regulated CALR conformation only in normal cells. Normal erythroblasts expressed mostly the N-terminal domain of CALR (N-CALR) on their cell surface (as shown by flow cytometry) and C-terminal domain (C-CALR) in their cytoplasm (as shown by confocal microscopy) and expression of both epitopes decreased with maturation. In the proerythroblast (proEry) cytoplasm, C-CALR was associated with the glucocorticoid receptor (GR), which initiated the stress response. In these cells, Ca²⁺ deprivation and inhibition of nuclear export increased GR nuclear localization while decreasing cytoplasmic detection of C-CALR and C-CALR/GR association and proliferation in response to the GR agonist dexamethasone (Dex). C-CALR/GR association and Dex responsiveness were instead increased by Ca²⁺ and erythropoietin. In contrast, JAK2V617F proErys expressed normal cell-surface levels of N-CALR but barely detectable cytoplasmic levels of C-CALR. These cells contained GR mainly in the nucleus and were Dex unresponsive. Ruxolitinib rescued cytoplasmic detection of C-CALR, C-CALR/GR association, and Dex responsiveness in JAK2V617F proErys and its effects were antagonized by nuclear export and Ca²⁺ flux inhibitors. These results indicates that Ca²⁺-induced conformational changes of CALR regulate nuclear export of GR in normal erythroblasts and that JAK2V617F deregulates this function in PV.

Genomics of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are a group of related clonal hematologic disorders characterized by excess accumulation of one or more myeloid cell lineages and a tendency to transform to acute myeloid leukemia. Deregulated JAK2 signaling has emerged as the central phenotypic driver of BCR -ABL1-negative MPNs and a unifying therapeutic target. In addition, MPNs show unexpected layers of genetic complexity, with multiple abnormalities associated with disease progression, interactions between inherited factors and phenotype driver mutations, and effects related to the order in which mutations are acquired. Although morphology and clinical laboratory analysis continue to play an important role in defining these conditions, genomic analysis is providing a platform for better disease definition, more accurate diagnosis, direction of therapy, and refined prognostication. There is an emerging consensus with regard to many prognostic factors, but there is a clear need to synthesize genomic findings into robust, clinically actionable and widely accepted scoring systems as well as the need to standardize the laboratory methodologies that are used.