Risk of Bleeding in Essential Thrombocythemia Patients with Extreme Thrombocytosis.

About 25% of essential thrombocythemia (ET) patients present with extreme thrombocytosis (ExT), defined as having a platelet count ≥1000 x 10^9/L. ExT patients may have an increased bleeding risk associated with acquired von Willebrand syndrome. We retrospectively analyzed the risk of bleeding and thrombosis in ExT vs. non-ExT ET patients at Dana Farber Cancer Institute and Massachusetts General Hospital from 2014-2022 to inform treatment decisions. We abstracted the first major bleed, clinically relevant non-major bleed (CRNMB), and thrombotic event from medical records. We identified 128 (28%) ExT patients and 323 (72%) non-ExT patients. Cumulative incidence of bleeding was not different in ExT vs. non-ExT patients (21% vs. 13%, p=0.28 for major bleed; 16% vs. 15%, p=0.50 for CRNMB). Very low and low thrombotic risk ExT patients were more likely to be cytoreduced compared to very low and low risk non-ExT patients (69% vs. 50%, p=0.060 for very low risk; 83% vs. 53%, p=0.0059 for low risk). However, we found no differences in bleeding between ExT and non-ExT patients when restricting the risk of bleed from diagnosis to cytoreduction start date (28% vs. 19%, p=0.29 for major bleed; 24% vs. 22%, p=0.75 for CRNMB). Cumulative incidence of thrombosis was also not different between ExT and non-ExT patients (28% vs. 25%, p=0.98). This suggests that cytoreduction may not be necessary to reduce bleeding risk based only on a platelet count of 1 million. We identified novel risk factors for bleeding in ET patients including diabetes mellitus and the DNMT3A mutation.

Mesenchymal Stromal Cells Facilitate Neutrophil-Trained Immunity by Reprogramming Hematopoietic Stem Cells.

Novel therapeutics are urgently needed to prevent opportunistic infections in immunocompromised individuals undergoing cancer treatments or other immune-suppressive therapies. Trained immunity is a promising strategy to reduce this burden of disease. We previously demonstrated that mesenchymal stromal cells (MSCs) preconditioned with a class A CpG oligodeoxynucleotide (CpG-ODN), a Toll-like receptor 9 (TLR9) agonist, can augment emergency granulopoiesis in a murine model of neutropenic sepsis. Here, we used a chimeric mouse model to demonstrate that MSCs secrete paracrine factors that act on lineage-negative c-kit+ hematopoietic stem cells (HSCs), leaving them "poised" to enhance emergency granulopoiesis months after transplantation. Chimeric mice developed from HSCs exposed to conditioned media from MSCs and CpG-ODN-preconditioned MSCs showed significantly higher bacterial clearance and increased neutrophil granulopoiesis following lung infection than control mice. By Cleavage Under Targets and Release Using Nuclease (CUT&RUN) chromatin sequencing, we identified that MSC-conditioned media leaves H3K4me3 histone marks in HSCs at genes involved in myelopoiesis and in signaling persistence by the mTOR pathway. Both soluble factors and extracellular vesicles from MSCs mediated these effects on HSCs and proteomic analysis by mass spectrometry revealed soluble calreticulin as a potential mediator. In summary, this study demonstrates that trained immunity can be mediated by paracrine factors from MSCs to induce neutrophil-trained immunity by reprogramming HSCs for long-lasting functional changes in neutrophil-mediated antimicrobial immunity.

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.

Mechanical checkpoint regulates monocyte differentiation in fibrotic niches.

Myelofibrosis is a progressive bone marrow malignancy associated with monocytosis, and is believed to promote the pathological remodelling of the extracellular matrix. Here we show that the mechanical properties of myelofibrosis, namely the liquid-to-solid properties (viscoelasticity) of the bone marrow, contribute to aberrant differentiation of monocytes. Human monocytes cultured in stiff, elastic hydrogels show proinflammatory polarization and differentiation towards dendritic cells, as opposed to those cultured in a viscoelastic matrix. This mechanically induced cell differentiation is blocked by inhibiting a myeloid-specific isoform of phosphoinositide 3-kinase, PI3K-γ. We further show that murine bone marrow with myelofibrosis has a significantly increased stiffness and unveil a positive correlation between myelofibrosis grading and viscoelasticity. Treatment with a PI3K-γ inhibitor in vivo reduced frequencies of monocyte and dendritic cell populations in murine bone marrow with myelofibrosis. Moreover, transcriptional changes driven by viscoelasticity are consistent with transcriptional profiles of myeloid cells in other human fibrotic diseases. These results demonstrate that a fibrotic bone marrow niche can physically promote a proinflammatory microenvironment.

Calreticulin mutant myeloproliferative neoplasms induce MHC-I skewing, which can be overcome by an optimized peptide cancer vaccine.

The majority of JAK2V617F-negative myeloproliferative neoplasms (MPNs) have disease-initiating frameshift mutations in calreticulin (CALR), resulting in a common carboxyl-terminal mutant fragment (CALRMUT), representing an attractive source of neoantigens for cancer vaccines. However, studies have shown that CALRMUT-specific T cells are rare in patients with CALRMUT MPN for unknown reasons. We examined class I major histocompatibility complex (MHC-I) allele frequencies in patients with CALRMUT MPN from two independent cohorts. We observed that MHC-I alleles that present CALRMUT neoepitopes with high affinity are underrepresented in patients with CALRMUT MPN. We speculated that this was due to an increased chance of immune-mediated tumor rejection by individuals expressing one of these MHC-I alleles such that the disease never clinically manifested. As a consequence of this MHC-I allele restriction, we reasoned that patients with CALRMUT MPN would not efficiently respond to a CALRMUT fragment cancer vaccine but would when immunized with a modified CALRMUT heteroclitic peptide vaccine approach. We found that heteroclitic CALRMUT peptides specifically designed for the MHC-I alleles of patients with CALRMUT MPN efficiently elicited a CALRMUT cross-reactive CD8+ T cell response in human peripheral blood samples but not to the matched weakly immunogenic CALRMUT native peptides. We corroborated this effect in vivo in mice and observed that C57BL/6J mice can mount a CD8+ T cell response to the CALRMUT fragment upon immunization with a CALRMUT heteroclitic, but not native, peptide. Together, our data emphasize the therapeutic potential of heteroclitic peptide-based cancer vaccines in patients with CALRMUT MPN.

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.

Augmenting emergency granulopoiesis with CpG conditioned mesenchymal stromal cells in murine neutropenic sepsis.

Patients with immune deficiencies from cancers and associated treatments represent a growing population within the intensive care unit with increased risk of morbidity and mortality from sepsis. Mesenchymal stromal cells (MSCs) are an integral part of the hematopoietic niche and express toll-like receptors, making them candidate cells to sense and translate pathogenic signals into an innate immune response. In this study, we demonstrate that MSCs administered therapeutically in a murine model of radiation-associated neutropenia have dual actions to confer a survival benefit in Pseudomonas aeruginosa pneumo-sepsis that is not from improved bacterial clearance. First, MSCs augment the neutrophil response to infection, an effect that is enhanced when MSCs are preconditioned with CpG oligodeoxynucleotide, a toll-like receptor 9 agonist. Using cytometry by time of flight, we identified proliferating neutrophils (Ly6GlowKi-67+) as the main expanded cell population within the bone marrow. Further analysis revealed that CpG-MSCs expand a lineage restricted progenitor population (Lin-Sca1+C-kit+CD150-CD48+) in the bone marrow, which corresponded to a doubling in the myeloid proliferation and differentiation potential in response to infection compared with control. Despite increased neutrophils, no reduction in organ bacterial count was observed between experimental groups. However, the second effect exerted by CpG-MSCs is to attenuate organ damage, particularly in the lungs. Neutrophils obtained from irradiated mice and cocultured with CpG-MSCs had decreased neutrophil extracellular trap formation, which was associated with decreased citrullinated H3 staining in the lungs of mice given CpG-MSCs in vivo. Thus, this preclinical study provides evidence for the therapeutic potential of MSCs in neutropenic sepsis.

The Molecular Genetics of Myeloproliferative Neoplasms.

Activated JAK-STAT signaling is central to the pathogenesis of BCR-ABL-negative myeloproliferative neoplasms (MPNs) and occurs as a result of MPN phenotypic driver mutations in JAK2, CALR, or MPL The spectrum of concomitant somatic mutations in other genes has now largely been defined in MPNs. With the integration of targeted next-generation sequencing (NGS) panels into clinical practice, the clinical significance of concomitant mutations in MPNs has become clearer. In this review, we describe the consequences of concomitant mutations in the most frequently mutated classes of genes in MPNs: (1) DNA methylation pathways, (2) chromatin modification, (3) RNA splicing, (4) signaling pathways, (5) transcription factors, and (6) DNA damage response/stress signaling. The increased use of molecular genetics for early risk stratification of patients brings the possibility of earlier intervention to prevent disease progression in MPNs. However, additional studies are required to decipher underlying molecular mechanisms and effectively target them.

Molecular mechanisms of bleeding disorderassociated GFI1BQ287* mutation and its affected pathways in megakaryocytes and platelets.

Dominant-negative mutations in the transcription factor Growth Factor Independence-1B (GFI1B), such as GFI1BQ287*, cause a bleeding disorder characterized by a plethora of megakaryocyte and platelet abnormalities. The deregulated molecular mechanisms and pathways are unknown. Here we show that both normal and Q287* mutant GFI1B interacted most strongly with the lysine specific demethylase-1 - REST corepressor - histone deacetylase (LSD1-RCOR-HDAC) complex in megakaryoblasts. Sequestration of this complex by GFI1BQ287* and chemical separation of GFI1B from LSD1 induced abnormalities in normal megakaryocytes comparable to those seen in patients. Megakaryocytes derived from GFI1BQ287*-induced pluripotent stem cells also phenocopied abnormalities seen in patients. Proteome studies on normal and mutant-induced pluripotent stem cell-derived megakaryocytes identified a multitude of deregulated pathways downstream of GFI1BQ287* including cell division and interferon signaling. Proteome studies on platelets from GFI1BQ287* patients showed reduced expression of proteins implicated in platelet function, and elevated expression of proteins normally downregulated during megakaryocyte differentiation. Thus, GFI1B and LSD1 regulate a broad developmental program during megakaryopoiesis, and GFI1BQ287* deregulates this program through LSD1-RCOR-HDAC sequestering.

The kinases IKBKE and TBK1 regulate MYC-dependent survival pathways through YB-1 in AML and are targets for therapy.

To identify novel therapeutic targets in acute myeloid leukemia (AML), we examined kinase expression patterns in primary AML samples. We found that the serine/threonine kinase IKBKE, a noncanonical IkB kinase, is expressed at higher levels in myeloid leukemia cells compared with normal hematopoietic cells. Inhibiting IKBKE, or its close homolog TANK-binding kinase 1 (TBK1), by either short hairpin RNA knockdown or pharmacological compounds, induces apoptosis and reduces the viability of AML cells. Using gene expression profiling and gene set enrichment analysis, we found that IKBKE/TBK1-sensitive AML cells typically possess an MYC oncogenic signature. Consistent with this finding, the MYC oncoprotein was significantly downregulated upon IKBKE/TBK1 inhibition. Using proteomic analysis, we found that the oncogenic gene regulator YB-1 was activated by IKBKE/TBK1 through phosphorylation, and that YB-1 binds to the MYC promoter to enhance MYC gene transcription. Momelotinib (CYT387), a pharmacological inhibitor of IKBKE/TBK1, inhibits MYC expression, reduces viability and clonogenicity of primary AML cells, and demonstrates efficacy in a murine model of AML. Together, these data identify IKBKE/TBK1 as a promising therapeutic target in AML.

Introducing high-throughput sequencing into mainstream genetic diagnosis practice in inherited platelet disorders.

Inherited platelet disorders are a heterogeneous group of rare diseases, caused by inherited defects in platelet production and/or function. Their genetic diagnosis would benefit clinical care, prognosis and preventative treatments. Until recently, this diagnosis has usually been performed via Sanger sequencing of a limited number of candidate genes. High-throughput sequencing is revolutionizing the genetic diagnosis of diseases, including bleeding disorders. We have designed a novel high-throughput sequencing platform to investigate the unknown molecular pathology in a cohort of 82 patients with inherited platelet disorders. Thirty-four (41.5%) patients presented with a phenotype strongly indicative of a particular type of platelet disorder. The other patients had clinical bleeding indicative of platelet dysfunction, but with no identifiable features. The high-throughput sequencing test enabled a molecular diagnosis in 70% of these patients. This sensitivity increased to 90% among patients suspected of having a defined platelet disorder. We found 57 different candidate variants in 28 genes, of which 70% had not previously been described. Following consensus guidelines, we qualified 68.4% and 26.3% of the candidate variants as being pathogenic and likely pathogenic, respectively. In addition to establishing definitive diagnoses of well-known inherited platelet disorders, high-throughput sequencing also identified rarer disorders such as sitosterolemia, filamin and actinin deficiencies, and G protein-coupled receptor defects. This included disease-causing variants in DIAPH1 (n=2) and RASGRP2 (n=3). Our study reinforces the feasibility of introducing high-throughput sequencing technology into the mainstream laboratory for the genetic diagnostic practice in inherited platelet disorders.

A dominant-negative GFI1B mutation in the gray platelet syndrome.

The gray platelet syndrome is a hereditary, usually autosomal recessive bleeding disorder caused by a deficiency of alpha granules in platelets. We detected a nonsense mutation in the gene encoding the transcription factor GFI1B (growth factor independent 1B) that causes autosomal dominant gray platelet syndrome. Both gray platelets and megakaryocytes had abnormal marker expression. In addition, the megakaryocytes had dysplastic features, and they were abnormally distributed in the bone marrow. The GFI1B mutant protein inhibited nonmutant GFI1B transcriptional activity in a dominant-negative manner. Our studies show that GFI1B, in addition to being causally related to the gray platelet syndrome, is key to megakaryocyte and platelet development.