Blood

Patients with immune thrombocytopenia (ITP) remain a challenge to diagnose, manage, and predict bleeding risk. A comprehensive assessment of platelet function may aid clinical management. This study assessed platelet parameters to predict bleeding in ITP. Blood from 103 clinically-annotated cases with isolated thrombocytopenia or 123 healthy donors was evaluated. In the ITP cohort, 75/110 encounters (68%) had platelet counts below 50 x 10/L. Platelet surface proteins, reticulated platelets, and activation were quantified using flow cytometry. Soluble receptor fragments, citrullinated histone-DNA (CitH3-DNA) complexes, and thrombopoietin (TPO) were quantified by ELISA. Whole blood clotting and platelet contribution to clot formation were evaluated using viscoelastography. Elevated levels of glycoprotein (GP) VI (p=0.0012), Trem-like transcript-1 (TLT-1) (p=0.0248), platelet-bound immunoglobulin (Ig) G (p<0.0029), CitH3-DNA complexes (p=0.0022), TPO (p<0.0001), and reduced platelet contribution to clot formation (p<0.0001), were observed in primary ITP patients with bleeding and bruising symptoms. A multivariable analysis revealed that measuring platelet indices strengthened a predictive bleeding model over platelet count alone (78.1% vs. 70.48%). Symptomatic ITP patients have measurable platelet dysfunction and quantifiable differences on platelet surface, increased evidence of NETosis and elevated TPO levels. Identifying biomarkers for ITP outcomes can define subsets of disease with clinical relevance. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=142 SRC="FIGDIR/small/25331619v1_ufig1.gif" ALT="Figure 1"> View larger version (48K): org.highwire.dtl.DTLVardef@aff1e8org.highwire.dtl.DTLVardef@132c615org.highwire.dtl.DTLVardef@6aa5eeorg.highwire.dtl.DTLVardef@1b3ec12_HPS_FORMAT_FIGEXP M_FIG Visual abstract Key biomarkers and assays evaluated in this study. The top left panel depicts flow cytometry-based approaches performed in whole blood samples for studying platelet surface proteins, including integrins, glycoproteins, and activation markers, as well as the assessment of pathways regulating integrin IIb3 activation. The right panel highlights ELISA-based detection of soluble fragments (sTLT-1, sGPVI) released from activated platelets, along with serum TPO and NET formation in plasma and serum samples. The bottom panel illustrates ROTEM analysis in normal clot formation, ITP patients with thrombocytopenia and its ability to detect platelet dysfunction. C_FIG

Platelets are key players in hemostasis and thrombosis. Essential thrombocythemia (ET) is a myeloproliferative neoplasm (MPN) in which the JAK2 V617F, MPL W515K/L, and CALR mutations determine differences in clinical phenotype, in particular the thrombotic risk and the risk of myelofibrosis. Here, we examined the proteome of platelets in ET by mass spectrometry (MS) in combination with functional assays to gain insights into platelet activation in ET. MS analysis revealed a different proteome in ET platelets with stoichiometric differences in mitochondrial proteins compared with normal platelets. The tricarboxylic acid cycle enzymes (TCA) were in general downregulated in ET platelets while glycolysis enzymes were upregulated changing modes in energy production. Acetyl salicylic acid (ASA) treatment increased levels of TCA enzymes in controls and restored them only partially in JAK2 V617F platelets. Interestingly, membrane CD36 was higher in CALR Type1 implicating lipid transport and fatty acid oxidation in platelet lifespan. Aggregation levels specifically in JAK2 V617F platelets were similar or lower to healthy controls while activation markers i.e. CD62P were higher in untreated CALR Type2 than controls and the rest of ET. In summary, analysis of platelet proteome in ET implicates mitochondrial activity in platelet activation and also identified differences between JAK2 V617F and CALR patients. Our study suggests that metabolic finetuning can be critical in the control of platelet reactivity. Key pointsO_LITCA cycle enzymes are downregulated in ET platelets. ASA treatment leads to a partial correction in JAK2 V617F platelets. C_LIO_LICD62P expression and aggregation levels of untreated CALR Type2 are higher than JAK2 V617F platelets. C_LI Visual Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=79 SRC="FIGDIR/small/25323560v1_ufig1.gif" ALT="Figure 1"> View larger version (11K): org.highwire.dtl.DTLVardef@1e2e8ccorg.highwire.dtl.DTLVardef@21c3b0org.highwire.dtl.DTLVardef@b69467org.highwire.dtl.DTLVardef@c907ff_HPS_FORMAT_FIGEXP M_FIG C_FIG

Genetic predisposition to venous thrombosis may impact COVID-19 infection and its sequelae. Participants in the ongoing prospective cohort study, Million Veteran Program (MVP), who were tested for COVID-19, with European ancestry, were evaluated for associations with polygenic venous thromboembolic risk, Factor V Leiden mutation (FVL) (rs6025) and prothrombin gene 3 -UTR mutation (F2 G20210A)(rs1799963), and their interactions. Logistic regression models assessed genetic associations with VTE diagnosis, COVID-19 (positive) testing rates and outcome severity (modified WHO criteria), and post-test conditions, adjusting for outpatient anticoagulation medication usage, age, sex, and genetic principal components. 108,437 out of 464,961 European American MVP participants were tested for COVID-19 with 9786 (9%) positive. PRS(VTE), FVL, F2 G20210A were not significantly associated with the propensity of being tested for COVID-19. PRS(VTE) was significantly associated with a positive COVID-19 test in F5 wild type (WT) individuals (OR 1.05; 95% CI [1.02-1.07]), but not in FVL carriers (0.97, [0.91-1.94]). There was no association with severe outcome for FVL, F2 G20210A or PRS(VTE). Outpatient anticoagulation usage in the two years prior to testing was associated with worse clinical outcomes. PRS(VTE) was associated with prevalent VTE diagnosis among both FVL carriers or F5 wild type individuals as well as incident VTE in the two years prior to testing. Increased genetic propensity for VTE in the MVP was associated with increased COVID-19 positive testing rates, suggesting a role of coagulation in the initial steps of COVID-19 infection. Key PointsO_LIIncreased genetic predisposition to venous thrombosis is associated with increased COVID-19 positive testing rates. C_LIO_LIPRS for VTE further risk stratifies factor V Leiden carriers regarding their VTE risk. C_LI

BackgroundAnti-Platelet Factor 4 (PF4) IgG antibodies that activate platelets via Fc{gamma}RIIa have been shown to be an important part of the pathophysiology of vaccine-induced immune thrombocytopenia and thrombosis (VITT). There is now extensive literature on its presentation and initial management. There is no literature however on what happens to these patients following discharge. MethodsWe collected clinical data and samples from seven patients presenting with VITT and followed them up for 82-145 days. We also collected clinical samples from them at last follow-up. Testing for anti-PF4/heparin antibodies was done using an anti-PF4/heparin enzymatic immunoassay. Flow Cytometry was used to look at Fc{gamma}RIIa levels on patient platelets. Light Transmission Aggregometry with patient serum and healthy donor / patient platelets was used to analyse platelet responsiveness, in the presence and absence of PF4. FindingsAll patients were discharged on direct oral anticoagulants. Two patients remain completely symptom free, three have ongoing headaches, two have residual neurological deficits. Two patients developed mild thrombocytopenia and worsening headache (but without cerebral venous sinus thrombosis) and were retreated, one of these with rituximab. All patients, except the one treated with rituximab, had similar anti-PF4 antibody titres at 80-120 days to their levels at diagnosis. Platelets from patients at follow-up had normal levels of Fc{gamma}RIIa and had normal responses to thrombin and collagen-related-peptide. Patient serum from diagnosis strongly activated healthy donor platelets in the presence of PF4. Serum from follow-up was much weaker at stimulating platelets, even in the presence of PF4. InterpretationThis study shows that despite similar PF4 antibody titres at diagnosis and during follow-up, there are further differences in patient serum, that are not apparent from currently used testing, that result in lower levels of platelet activation during the follow-up period. Further understanding of these factors are important in order to assess duration of anticoagulation for these patients. FundingThis work was supported by an Accelerator Grant (AA/18/2/34218) from the British Heart Foundation (BHF) and by a National Institute for Health Research (NIHR) grant. Key pointsO_LIPF4 antibody titres do not reduce up to 4-months post ChAdOx1 nCoV-19 in patients with VITT C_LIO_LIDespite similar PF4 antibody titres, diagnostic serum is more potent at activating platelets in the presence of PF4 than follow-up serum. C_LI

BackgroundGermline RUNX1 haplodeficiency (RHD) is associated with thrombocytopenia, platelet dysfunction and predisposition to myeloid malignancies. Platelet expression profiling of a RHD patient showed decreased F13A1, encoding for the A subunit of factor XIII, a transglutaminase that cross-links fibrin and induces clot stabilization. FXIII-A is synthesized by hematopoietic cells, megakaryocytes and monocytes. AimsTo understand RUNX1 regulation of F13A1 expression in platelet/megakaryocyte and the mechanisms and consequences of decreased F13A1 in RHD. MethodsWe performed studies in platelets, HEL cells and human CD34+ cell-derived megakaryocytes including on clot contraction in cells following small inhibitor (si)RNA knockdown (KD) of RUNX1 or F13A1. ResultsPlatelet F13A1 mRNA and protein were decreased in our index patient and in two siblings from an unrelated family with RHD. Platelet-driven clot contraction was decreased in the patient and affected daughter. Promoter studies in HEL cells showed that RUNX1 regulates F13A1 transcription; RUNX1 overexpression increased and (si)RNA RUNX1 KD reduced F13A1 promoter activity and protein. Following RUNX1 or F13A1 KD clot contraction by HEL cells was decreased as were FXIII-A surface expression, myosin light chain phosphorylation and PAC1 binding upon activation. F13A1 expression and clot contraction were impaired on RUNX1 downregulation in human megakaryocytes. ConclusionsRUNX1 regulates platelet-megakaryocyte F13A1 expression, which is decreased in RHD, reflecting regulation of a coagulation protein by a hematopoietic transcription factor. Platelet and megakaryocyte clot contraction is decreased in RHD, related to multiple impaired mechanisms including F13A1 expression, myosin phosphorylation and IIb{beta}3 activation. Scientific category - Platelets and thrombopoiesis EssentialsO_LIRUNX1 regulates expression of FXIII-A chain (F13A1) in megakaryocytes (MK) and platelets. C_LIO_LIPlatelet and MK F13A1 expression and clot contraction are decreased in RUNX1 deficiency. C_LIO_LIMK clot contraction, myosin phosphorylation and PAC1-binding are impaired in F13A1 deficiency. C_LIO_LIDefective clot contraction in RHD arises from defects in multiple platelet-MK mechanisms. C_LI

Rare variants outside the classical coagulation cascade might cause rare inherited thrombosis. We aimed to identify the variant(s) causing venous thromboembolism (VTE) in a family with multiple relatives affected with unprovoked VTE and no thrombophilia defects. We identified by whole exome sequencing an extremely rare Arg to Gln variant (R89Q) in the Microtubule Associated Serine/Threonine Kinase 2 (MAST2) gene that segregates with VTE in the family. Free-tissue factor pathway inhibitor (f-TFPI) plasma levels were significantly decreased in affected family members compared to healthy relatives. Conversely, plasminogen activator inhibitor-1 (PAI-1) levels were significantly higher in affected members than in healthy relatives. RNA sequencing analysis of RNA interference experimental data conducted in endothelial cells revealed that, of the 13,387 detected expressed genes, 2,354 have their level of expression modified by MAST2 knockdown, including SERPINE1 coding for PAI-1 and TFPI. In HEK293 cells overexpressing the MAST2R289Q variant, TFPI and SERPINE1 promoter activities were respectively lower and higher than in cells overexpressing the MAST2 wild type allele. This study identifies a novel thrombophilia-causing R89Q variant in the MAST2 gene that is here proposed as a new molecular player in the etiology of VTE by interfering with hemostatic balance of endothelial cells. Author SummaryVenous thromboembolism (VTE) is a multifactorial disease in which the genetic burden. We here present the case of a French family with multiple relatives affected with unprovoked VTE (i.e. that occurred in the absence of clinical risk factors) in which no thrombophilia defects had been identified. Adopting a whole exome sequencing approach, we identified an extremely rare variant located in the Microtubule-associated serine/threonine-protein kinase-2 (MAST2) gene that perfectly segregates with the VTE phenotype and that interferes with hemostatic balance of endothelial cells. Our results pave the way for adding MAST2 to the list of genes to be sequenced and looked for in thrombophilia families with unprovoked VTE.

Platelet -granules have numerous proteins, some synthesized by megakaryocytes (MK) and others not synthesized but incorporated by endocytosis, an incompletely understood process in platelets/MK. Germline RUNX1 haplodeficiency, referred to as familial platelet defect with predisposition to myeloid malignancies (FPDMM), is associated with thrombocytopenia, platelet dysfunction and granule deficiencies. In previous studies, we found that platelet albumin, fibrinogen and IgG levels were decreased in a FPDMM patient. We now show that platelet endocytosis of fluorescent-labeled albumin, fibrinogen and IgG is decreased in the patient and his daughter with FPDMM. In megakaryocytic human erythroleukemia (HEL) cells, siRNA RUNX1 knockdown (KD) increased uptake of these proteins over 24 hours compared to control cells, with increases in caveolin-1 and flotillin-1 (two independent regulators of clathrin-independent endocytosis), LAMP2 (a lysosomal marker), RAB11 (a marker of recycling endosomes) and IFITM3. Caveolin-1 downregulation in RUNX1-deficient HEL cells abrogated the increased uptake of albumin, but not fibrinogen. Albumin, but not fibrinogen, partially colocalized with caveolin-1. RUNX1 knockdown increased colocalization of albumin with flotillin and of fibrinogen with RAB11 suggesting altered trafficking of both. The increased albumin and fibrinogen uptake and levels of caveolin-1, flotillin-1, LAMP2 and IFITM3 were recapitulated by shRNA RUNX1 knockdown in CD34+-derived MK. These studies provide the first evidence that in RUNX1-haplodeficiency platelet endocytosis of albumin and fibrinogen is impaired and that megakaryocytes have enhanced endocytosis with defective trafficking leading to loss of these proteins by distinct mechanisms. They provide new insights into mechanisms governing endocytosis and -granule deficiencies in RUNX1-haplodeficiency. Key pointsO_LIPlatelet content and endocytosis of -granule proteins, albumin, fibrinogen and IgG, are decreased in germline RUNX1 haplodeficiency. C_LIO_LIIn RUNX1-deficient HEL cells and primary MK endocytosis is enhanced with defective trafficking leading to decreased protein levels. C_LI

Cancer-associated venous thromboembolism (CAT) is a well-described complication of cancer and a leading cause of death in cancer patients. The purpose of this study was to assess potential associations of molecular signatures with CAT, including tumor-specific mutations and the presence of clonal hematopoiesis. We analyzed deep-coverage targeted DNA-sequencing data of >14,000 solid tumor samples using the MSK-IMPACT platform to identify somatic alterations associated with VTE. Endpoint was defined as the first instance of cancer-associated pulmonary embolism and/or proximal/distal lower extremity deep vein thrombosis. Cause-specific Cox proportional hazards regression was used, adjusting for pertinent clinical covariates. Of 11,695 evaluable individuals, 72% had metastatic disease at time of IMPACT. Tumor-specific mutations in KRAS (HR=1.34 [1.09-1.64]; adjusted p=0.08), STK11 (HR=2.12 [1.55-2.89]; adjusted p<0.001), KEAP1 (HR=1.84 [1.21-2.79]; adjusted p=0.07), CTNNB1 (HR=1.73 [1.15-2.60]; adjusted p=0.09), CDKN2B (HR= 1.45 [1.13-1.85], adjusted p=0.07) and MET (HR=1.83 [1.15-2.92]; adjusted p=0.09) were associated with a significantly increased risk of CAT independent of tumor type. Mutations in SETD2 were associated with a decreased risk of CAT (HR=0.35 [0.16-0.79], adjusted p=0.09). The presence of clonal hematopoiesis was not associated with an increased VTE rate. This is the first large-scale analysis to elucidate tumor-specific genomic events associated with CAT. Somatic tumor mutations of STK11, KRAS, CTNNB1, KEAP1, CDKN2B and MET were associated with an increased risk of VTE in solid tumor patients. Further analysis is needed to validate these findings and identify additional molecular signatures unique to individual tumor types. Key PointsO_LITumor mutations in STK11, KRAS, CTNNB1, KEAP1, CDKN2B, MET and SETD2 modulate the risk of cancer-associated thrombosis. C_LIO_LIThe presence of clonal hematopoiesis does not affect the risk of cancer-associated thrombosis. C_LI

BackgroundBlood donation saves lives. Provided they are in good health, male volunteers can donate as often as six times per year from the age of 18 into their late sixties. The burden of blood donation is very unevenly distributed, with a small minority of altruistic individuals providing this critical resource. While the consequences of persistent iron depletion in blood donors have been studied in the context of cancer and coronary heart disease, potential effects of the erythropoietic stress from repetitive large-volume phlebotomy remain unexplored. We sought to investigate if and how repeated blood donations affect the clonal composition of the hematopoietic stem and progenitor cell (HSPC) compartment. Methods105 healthy, male individuals with an extensive blood donation history (median of 120 donations per donor; median age of 66 yrs.) were screened for the presence of clonal hematopoiesis (CH) using a sequencing panel covering 141 genes commonly mutated in human myeloid neoplasms. The control cohort consisted of 103 healthy, male donors with a median of 5 donations per donor and a median age of 63. Donors positive for CH were subsequently studied longitudinally. The pathogenicity of detected variants was compared using established scoring systems. Finally, to assess the functional consequences of blood-donation induced CH, selected CH mutations were introduced by CRISPR-mediated editing into HSPCs from human cord blood (CB) or bone marrow (BM). The effect of these mutations was tested under different stress stimuli using functional ex vivo long-term culture initiating cells (LTC-IC) assays. ResultsCompared to the control cohort, frequent donors were significantly more likely to have mutations in genes encoding for epigenetic modifiers (44.7 vs. 22.3 %), most specifically in the two genes most commonly mutated in CH, DNMT3A and TET2 (35.2 vs. 20.3 %). However, no difference in the variant allele frequency (VAF) of detected mutations was found between the groups. Longitudinal analysis revealed that the majority of the mutations remained at a stable VAF over an observation period of approximately one year. Three DNMT3A variants from the frequent donor cohort were introduced into healthy HSPCs and functionally analyzed: All expanded in response to EPO, but none responded to LPS or IFN{gamma} stimulation. This contrasted with the leukemogenic DNMT3A R882H mutation, which did not expand in the presence of EPO but instead responded strongly to inflammatory stimuli. ConclusionsFrequent whole blood donation is associated with a higher prevalence of CH driven by mutations in genes encoding for epigenetic modifiers, with DNMT3A and TET2 being the most common. This increased CH prevalence is not associated with a higher pathogenicity of the associated variants and is likely a result of the selection of clones with improved responsiveness to EPO under the condition of bleeding stress. Our data show that even highly frequent blood donations over many years is not increasing the risk for malignant clones further underscoring the safety of repetitive blood donations. To our knowledge, this is the first CH study analyzing a cohort of individuals known for their superior health and survival, able to donate blood until advanced age. Thus, our analysis possibly identified mutations associated with beneficial outcomes, rather than a disease condition, such as mutations in DNMT3A that mediated the improved expansion of HSPCs in EPO enriched environments. Our data support the notion of ongoing Darwinian evolution in humans at the somatic stem cell level and present EPO as one of the environmental factors to which HSPCs with specific mutations may respond with superior fitness.

Transplant-associated thrombotic microangiopathy (TA-TMA) is a severe endothelial complication following allogeneic hematopoietic stem-cell transplantation (allo-HSCT), associated with high mortality when not promptly diagnosed and treated. This study aimed to delineate the genetic landscape associated with TA-TMA and assess its impact on clinical outcomes. We retrospectively analyzed 1069 allo-HSCT recipients between January 2016 and May 2020, identifying 131 patients who met diagnostic criteria for TA-TMA (incidence rate: 12.25%). Genomic DNA sequencing was performed targeting 17 complement- related genes, identifying 74 genetic variants in 58 TA-TMA patients, including seven large deletions within the CFH-CFHR5 locus. Survival analysis indicated significantly poorer outcomes for TA-TMA patients compared to non-TMA patients (24.4% vs 62.4% survival at maximum follow-up, p = 0.025). However, cumulative incidence curves revealed no significant difference in TA-TMA onset between genetic variant carriers and non-carriers. These findings underscore the complexity of TA-TMA pathogenesis, suggesting that genetic predisposition alone is insufficient without additional endothelial insults. The limited predictive value of individual markers highlights the need for integrated biomarker strategies. Future research should focus on refining risk stratification models incorporating comprehensive genetic profiles, dynamic biomarkers, and longitudinal clinical parameters to enable earlier identification and targeted interventions, thereby improving post-transplant survival outcomes.

Fanconi anemia (FA) is an inherited disorder classically characterized by childhood-onset bone marrow dysfunction and lifelong cancer predisposition. FA is caused by pathogenic variants in any one of 23 genes identified so far. Of these, FANCA is the most frequently mutated and accounts for disease in two-thirds of all patients with FA. The spectrum of FANCA pathogenic variants (mutations) is broad, and genotype-phenotype correlation is often unclear. Here we describe the natural history of cytopenias associated with FANCA pathogenic variants in 139 individuals diagnosed in 1995 or later. We followed blood cell counts over time and correlated these with the classification of patient mutation subtypes. Most participants experienced age-related declines in hematologic parameters beginning in early childhood. Platelets underwent the earliest decline, reaching platelet count below 50K/l at a median age of 8.2 years. The erythrocyte lineage was the most stable with hemoglobin below 8 g/dl identified at a median age of 10.7 years. Androgen therapy delayed the blood count decline. The presence of at least one predicted hypomorphic pathogenic variant in the FANCA gene significantly slowed the progression of the hematologic abnormalities. This study sheds light on the importance of mutation type in predicting the severity of hematological manifestations in FA. Furthermore, it serves as a historical comparative cohort for emerging therapies aimed at altering hematological disease progression in FANCA-deficient FA patients. Key PointsO_LIThrombocytopenia and neutropenia are the earliest and most reliable indicators of hematologic decline in FANCA-deficient patients C_LIO_LIPresence of two FANCA variants with predicted complete loss of function leads to earlier onset and faster progression of disease C_LI

BackgroundBloom Syndrome (BSyn) is an autosomal recessive disorder caused by biallelic germline variants in BLM, which functions to maintain genomic stability. BSyn patients have poor growth, immune defects, insulin resistance, and a significantly increased risk of malignancies, most commonly hematologic. The malignancy risk in carriers of pathogenic variants in BLM (BLM variant carriers) remains understudied. Clonal hematopoiesis of indeterminate potential (CHIP) is defined by presence of somatic mutations in leukemia-related genes in blood of individuals without leukemia and is associated with increased risk of leukemia. We hypothesize that somatic mutations driving clonal expansion may be an underlying mechanism leading to increased cancer risk in BSyn patients and BLM variant carriers. MethodsTo determine whether de novo or somatic variation is increased in BSyn patients or carriers, we performed and analyzed exome sequencing on BSyn and control trios. ResultsWe discovered that both BSyn patients and carriers had increased numbers of low-frequency, putative somatic variants in CHIP genes compared to controls. Furthermore, BLM variant carriers had increased numbers of somatic variants in DNA methylation genes compared to controls. There was no statistical difference in the numbers of de novo variants in BSyn probands compared to control probands. ConclusionOur findings of increased CHIP in BSyn probands and carriers suggest that one or two germline pathogenic variants in BLM could be sufficient to increase the risk of clonal hematopoiesis. These findings warrant further studies in larger cohorts to determine the significance of CHIP as a potential biomarker of aging, cancer, cardiovascular disease, morbidity and mortality.

BACKGROUNDPlatelet factor 4-polyanion enzyme-linked immunosorbent assays (ELISAs) are considered highly sensitive for diagnosing heparin-induced thrombocytopenia (HIT), such that current practice guidelines recommend use of ELISA-negative results to exclude HIT. Once HIT is ruled out, alternative, non-heparin-based anticoagulant treatments are ceased, and heparin reintroduction frequently occurs. METHODSAntigen-based and PF4-dependent functional testing were used to study PF4/polyvinyl sulfonate ELISA-negative platelet-activating antibodies in HIT-suspected patients and mice immunized with PF4/heparin. RESULTSThree patients with clinical presentations consistent with HIT tested negative in an ELISA using PF4-polyvinylsulfonate (PF4/PVS), an antigenic target very commonly used for HIT antibody detection. All three patients demonstrated PF4-dependent platelet activation in functional testing that was sensitive to blockade of platelet Fc{gamma}RIIa receptors and inhibited by high concentrations of heparin, consistent with pathogenic HIT antibodies. Functional testing-based screening of 500 ELISA-negative patients identified three patients whose sera activated platelets in a PF4- and Fc{gamma}RIIa-dependent manner, and had clinical histories consistent with HIT. Five of the six ELISA-negative HIT patients were re-exposed to heparin, which precipitated a decrease in platelet counts in all re-exposed patients, and one patient developed a new thrombus. To advance the study of ELISA-negative HIT antibodies, mice were immunized with PF4/heparin, and functional and antigenic assays were simultaneously used to successfully identify an ELISA-negative, PF4-dependent platelet-activating murine monoclonal antibody that recapitulated the serological characteristics of ELISA-negative HIT patients. CONCLUSIONSRecognition of ELISA-negative HIT is critical to avoid harm due to the cessation of alternative anticoagulation therapy and re-exposure of these patients to heparin.

Heparin-induced thrombocytopenia (HIT) is a potentially life-threatening disorder characterized by antibodies to Platelet Factor 4 (PF4)-polyanion complexes which cause thrombocytopenia and thrombosis. Currently used technically-simple frontline assays such as the PF4-polyanion enzyme-linked immunosorbent assays (ELISAs) lack specificity, and more accurate functional assays such as the serotonin release assay (SRA) and PF4-dependent P-selectin expression assay (PEA) have long turnaround times due to technical complexity and availability only in the reference laboratory setting. There is a critical need for accurate near-patient functional testing to guide patient management, but a key barrier to attaining this goal is the short-term viability of platelets. Here, we detail a process of platelet cryopreservation that renders them viable for at least one year and show that PF4-treated cryopreserved platelets, when coupled with ELISA-based measurement of thrombospondin-1 release (a platelet -granule protein), detects pathogenic HIT antibodies with high accuracy. Furthermore, testing of a cohort of non-pathogenic HIT antibodies that were strongly reactive in PF4/polyanion ELISA but negative in functional assays demonstrated negative results in the thrombospondin-1 release assay, confirming high specificity of this technique. These findings have the potential to transform the diagnostic testing paradigm in HIT by making in-hospital functional testing available for rapid and accurate diagnosis.

Gray platelet syndrome (GPS) is a rare recessive disorder caused by variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet [a]-granules, splenomegaly and bone marrow (BM) fibrosis. Due to its rarity, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathological features, we performed a detailed clinical genotypic and phenotypic study of 47 GPS patients. We identified 33 new causal variants in NBEAL2. Our GPS patient cohort exhibited known phenotypes, including macro-thrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. We also observed novel clinical phenotypes; these include reduced leukocyte counts and increased presence of autoimmune disease and positive autoantibodies. There were widespread differences in the transcriptome and proteome of GPS platelets, neutrophils, monocytes, and CD4-lymphocytes. Proteins less abundant in these cells were enriched for constituents of granules, supporting a role for Nbeal2 in the function of these organelles across a wide range of blood cells. Proteomic analysis of GPS plasma showed increased levels of proteins associated with inflammation and immune response. One quarter of plasma proteins increased in GPS are known to be synthesized outside of hematopoietic cells, predominantly in the liver. In summary, our data demonstrate that, in addition to the well-described platelet defects in GPS, there are also immune defects. The abnormal immune cells may be the drivers of systemic abnormalities, such as autoimmune disease.

Clonal hematopoiesis of indeterminate potential (CHIP) is associated with increased mortality and malignancy risk, yet the determinants of clonal expansion remain poorly understood. We performed sequencing at >4,000x depth of coverage for CHIP mutations in 6,986 postmenopausal women from the Womens Health Initiative at two timepoints approximately 15 years apart. Among 3,685 mutations detected at baseline (VAF [&ge;] 0.5%), 50% progressed to CHIP (VAF [&ge;] 2%) at follow-up. We confirmed that clonal expansion is highly dependent on initial clone size and CHIP driver gene, with SF3B1 and JAK2 mutations exhibiting the fastest growth rate. We identified germline variants in TERT, IL6R, TCL1A, and MSI2 that modulate clonal expansion rate. Measured baseline leukocyte telomere length showed differential effects on incident CHIP risk, with shorter baseline leukocyte telomere length predisposing to incident PPM1D mutations and longer baseline leukocyte telomere length favoring incident DNMT3A mutations. We discovered that the IL6R missense variant p.Asp358Ala specifically impairs TET2 clonal expansion, supported by direct measurements of soluble interleukin-6 receptor and interleukin-6. Faster clonal growth rate was associated with increased risk of cytopenia, leukemia, and all-cause mortality. Notably, CHIP clonal expansion rate mediated 34.4% and 43.7% of the Clonal Hematopoiesis Risk Scores predictive value for leukemia and all-cause mortality, respectively. These findings reveal key biological determinants of CHIP progression and suggest that incorporating growth rate measurements could enhance risk stratification.

BACKGROUNDPatients with refractory thrombocytopenia (RT) are not sensitive to conventional therapies, such as platelet transfusions and thrombopoietin receptor agonists (TPO-RAs). The persistently high risk of life-threatening hemorrhage in this population highlights the urgent need for novel therapeutic strategies. Megakaryocyte (MK)-based therapies have emerged as a promising alternative, as MKs are the natural precursor cells responsible for platelet production. However, whether allogeneic MK therapy can improve platelet counts and function in patients with RT remains unclear. METHODSWe evaluated HLA-mismatched allogeneic MK therapy in 10 patients with RT following allogeneic hematopoietic stem-cell transplantation (allo-HSCT). All patients exhibited no response after at least one month of continuous treatment with TPO-RAs or other thrombopoiesis promoting therapies. MKs were expanded ex vivo and administered in a single infusion at one of three doses (1x106, 5x106, or 1x107 MKs per kilogram of body weight). Safety and efficacy were closely monitored. RESULTSMK infusion had minimal impact on inflammatory cytokine levels and coagulation parameters of patients. Among the 10 patients treated, 8 (80%) demonstrated a clinical response; including 3 complete response, and 5 partial response. Clinical improvement was observed within 28 days after infusion across all dose levels. CONCLUSIONSAmong 10 patients with RT, 8 responded to MK therapy without experiencing major toxic effects. (ClinicalTrials.gov number, NCT06534255.)

Previous studies demonstrated that splicing factor mutations are recurrent events in hematopoietic malignancies with both clinical and functional implications. However, their aberrant splicing patterns in acute myeloid leukaemia remain largely unexplored. In this study we characterized mutations in SRSF2, U2AF1 and SF3B1, the most commonly mutated splicing factors. In our clinical analysis of 2678 patients, splicing factor mutations showed inferior relapse-free and overall survival, however, these mutations did not represent independent prognostic markers. RNA-sequencing of 246 and independent validation in 177 patients revealed an isoform expression profile highly characteristic for each individual mutation, with several isoforms showing a strong dysregulation. By establishing a custom differential splice junction usage pipeline we accurately detected aberrant splicing in splicing factor mutated samples. Mutated samples were characterized predominantly by decreased junction usage. A large proportion of differentially used junctions were novel. Targets of splicing dysregulation included several genes with a known role in leukaemia. In SRSF2(P95H) mutants we further explored the possibility of a cascading effect through the dysregulation of the splicing pathway. We conclude that splicing factor mutations do not represent independent prognostic markers. However, they do have genome-wide consequences on gene splicing leading to dysregulated isoform expression of several genes.

PurposeHCT is vital for treating hematological malignancies, relying on HLA matching between unrelated patient-donor pairs to significantly reduce adverse outcomes. Recent studies recognize the potential impact of HLA-DPB1 mismatches on HCT outcomes. Multiple approaches focus on finding better-tolerated HLA-DPB1 mismatches. Additionally, recent studies suggest matching at noncoding HLA sequence may improve HCT outcomes. This study aims to evaluate different approaches for categorizing DPB1 mismatches in patient-donor pairs and explore the potential impact of noncoding mismatches (available in class I HLA genes) on clinical outcomes. MethodsA retrospective study of 5,106 HCT pairs using Cox proportional hazards models, weighted by a machine learning algorithm, evaluates the impact of particular combinations of HLA-DPB1 mismatches in the context of noncoding HLA class I mismatches on outcomes of HCT. HLA-DPB1 mismatch criteria included T-cell epitope permissive/non-permissive mismatches, expression markers, and evolutionary clade mismatches. ResultsTwo HLA-DPB1 mismatches, using multiple criteria, lead to significant hazards of acute graft versus host disease grades 2-4, in the T cell replete group. When HLA-DPB1 mismatches occurred across evolutionary clades (DP2 allele/low-expression patient vs DP5 allele/high-expression in the donor), the deplete group showed significant hazards for treatment-related mortality (TRM) (HR=1.94, p-value=8.9x10-7) and overall survival (OS) (HR=1.67, p-value=1.3x10-5) for additive effects of noncoding mismatches with two HLA-DPB1 mismatches. ConclusionTwo HLA-DPB1 mismatches remain to predict worse outcomes. However, noncoding mismatches in HLA class I genes confer elevated hazards of TRM and OS in conjunction with mismatches across evolutionary branches of HLA-DPB1. Therefore, noncoding mismatches may inform donor selection in the presence of HLA-DPB1 mismatches and improve HCT outcomes, emphasizing the utility of comprehensive sequencing of HLA alleles in HCT settings.

The pathophysiology of COVID-19 associated thrombosis seems to be multifactorial, involving interplay between cellular and plasmatic elements of the hemostasis. We hypothesized that COVID-19 is accompanied by platelet apoptosis with subsequent alteration of the coagulation system. We investigated depolarization of mitochondrial inner transmembrane potential ({Delta}{Psi}m), cytosolic calcium (Ca2+) concentration, and phosphatidylserine (PS) externalization by flow cytometry. Platelets from intensive care unit (ICU) COVID-19 patients (n=21) showed higher {Delta}{Psi}m depolarization, cytosolic Ca2+ concentration and PS externalization, compared to healthy controls (n=18) and COVID-19 non-ICU patients (n=4). Moreover significant higher cytosolic Ca2+ concentration and PS was observed compared to septic ICU control group (ICU control). In ICU control group (n=5; ICU non-COVID-19) cytosolic Ca2+ concentration and PS externalization was comparable to healthy control, with an increase {Delta}{Psi}m depolarization. Sera from ICU COVID-19 13 patients induced significant increase in apoptosis markers ({Delta}{Psi}m depolarization, cytosolic Ca2+ concentration and PS externalization). compared to healthy volunteer and septic ICU control. Interestingly, immunoglobulin G (IgG) fractions from COVID-19 patients induced an Fc gamma receptor IIA dependent platelet apoptosis ({Delta}{Psi}m depolarization, cytosolic Ca2+ concentration and PS externalization). Enhanced PS externalization in platelets from ICU COVID-19 patients was associated with increased sequential organ failure assessment (SOFA) score (r=0.5635) and DDimer (r=0.4473). Most importantly, patients with thrombosis had significantly higher PS externalization compared to those without. The strong correlations between apoptosis markers and increased D-Dimer levels as well as the incidence of thrombosis may indicate that antibody-mediated platelet apoptosis potentially contributes to sustained increased thromboembolic risk in ICU COVID-19 patients. Key pointsO_LISevere COVID-19 is associated with increased antibody-mediated platelet apoptosis. C_LIO_LIPlatelet apoptosis in severe COVID-19 is correlated with D-Dimer and higher incidence of thromboembolisms. C_LI