Relationship between plasma tissue Factor Pathway Inhibitor (TFPI) levels, thrombin generation and clinical risk of bleeding in patients with severe haemophilia A or B.

INTRODUCTION: Bleeding severity in severe haemophilic patients, with low thrombin generation (TG) capacity, can vary widely between patients, possibly reflecting differences in tissue factor pathway inhibitor (TFPI) level. AIM: To compare free TFPI (fTFPI) levels in patients with severe haemophilia A (sHA) and severe haemophilia B (sHB) and to investigate in these patients as a whole the relationships between bleeding and TG potential, between TG potential and fTFPI level and between fTFPI level and bleeding tendency. METHODS: Data on bleeding episodes retrospectively recorded during follow-up visits over 5-10 years were collected and used to calculate the annualised joint bleeding rate (AJBR). fTFPI levels and basal TG parameters were determined in platelet-poor plasma (PPP) and platelet-rich plasma (PRP) using calibrated automated tomography (CAT). RESULTS: Mean fTFPI levels did not differ significantly between sHA (n = 34) and sHB (n = 19) patients. Mean values of endogenous thrombin potential (ETP) and thrombin peak (peak) in PPP and PRP were two-fold higher when fTFPI levels < 9.4 versus > 14.3 ng/mL. In patients treated on demand, ETP and peak in PRP were doubled when AJBR was ≤ 4.9 $ \le 4.9$ , AJBR being halved in patients with a low fTFPI level (9.4 ng/mL). In patients on factor prophylaxis, no association was found between TG parameters and either fTFPI level or AJBR. CONCLUSION: In patients treated on demand, bleeding tendency was influenced by fTFPI levels, which in turn affected basal TG potential. In patients on prophylaxis, bleeding tendency is probably determined primarily by the intensity of this treatment.

Association of laboratory test results with the bleeding history in patients with inherited platelet function disorders (the Bleeding Assesment Tool - LABoratory tests substudy): communication from the Platelet Physiology ISTH-SSC.

Background: In hemophilia and von Willebrand disease, the degree of alteration of laboratory assays correlates with bleeding manifestations. Few studies have assessed the predictive value for bleeding of laboratory assays in patients with inherited platelet function disorders (IPFDs). Objectives: To assess whether there is an association between platelet function assay results and bleeding history, as evaluated by the International Society on Thrombosis and Haemostasis (ISTH) bleeding assessment tool (BAT). Methods: Centers participating in the international ISTH-BAT validation study were asked to provide results of the diagnostic assays employed for the patients they enrolled, and the association with the individual patients' bleeding score (BS) was assessed. Results: Sixty-eight patients with 14 different IPFDs were included. Maximal amplitude of platelet aggregation was significantly lower in patients with a pathologic BS and correlated inversely with the BS, a finding largely driven by the subgroup of patients with Glanzmann thrombasthenia and CalDAG-GEFI deficiency; after their exclusion, TRAP-induced aggregation remained significantly lower in patients with a pathologic BS. Bleeding time was significantly more prolonged in patients with a high BS than in those with a normal BS (27.1 ± 6.2 minutes vs 15.1 ± 10.6 minutes; P < .01). Reduced α-granule content was significantly more common among patients with a pathologic BS than among those with a normal BS (80% vs 20%; P < .05). Receiver operating characteristic curve analysis revealed a significant discriminative ability of all the aforementioned tests for pathologic BS (P < .001), also after exclusion of patients with Glanzmann thrombasthenia and CalDAG-GEFI deficiency. Conclusion: This study shows that altered platelet laboratory assay results are associated with an abnormal ISTH-BAT BS in IPFD.

Emergency management of patients with Glanzmann thrombasthenia: consensus recommendations from the French reference center for inherited platelet disorders.

Glanzmann thrombasthenia (GT) is a genetic bleeding disorder characterised by severely reduced/absent platelet aggregation in response to multiple physiological agonists. The severity of bleeding in GT varies markedly, as does the emergency situations and complications encountered in patients. A number of emergency situations may occur in the context of GT, including spontaneous or provoked bleeding, such as surgery or childbirth. While general management principles apply in each of these settings, specific considerations are essential for the management of GT to avoid escalating minor bleeding events. These recommendations have been developed from a literature review and consensus from experts of the French Network for Inherited Platelet Disorders, the French Society of Emergency Medicine, representatives of patients' associations, and Orphanet to aid decision making and optimise clinical care by non-GT expert health professionals who encounter emergency situations in patients with GT.

Single-cell analysis of megakaryopoiesis in peripheral CD34+ cells: insights into ETV6-related thrombocytopenia.

BACKGROUND: Germline mutations in the ETV6 transcription factor gene are responsible for familial thrombocytopenia and leukemia predisposition syndrome. Although previous studies have shown that ETV6 plays an important role in megakaryocyte (MK) maturation and platelet formation, the mechanisms by which ETV6 dysfunction promotes thrombocytopenia remain unclear. OBJECTIVES: To decipher the transcriptional mechanisms and gene regulatory network linking ETV6 germline mutations and thrombocytopenia. METHODS: Presuming that ETV6 mutations result in selective effects at a particular cell stage, we applied single-cell RNA sequencing to understand gene expression changes during megakaryopoiesis in peripheral CD34+ cells from healthy controls and patients with ETV6-related thrombocytopenia. RESULTS: Analysis of gene expression and regulon activity revealed distinct clusters partitioned into 7 major cell stages: hematopoietic stem/progenitor cells, common-myeloid progenitors (CMPs), MK-primed CMPs, granulocyte-monocyte progenitors, MK-erythroid progenitors (MEPs), progenitor MKs/mature MKs, and platelet-like particles. We observed a differentiation trajectory in which MEPs developed directly from hematopoietic stem/progenitor cells and bypassed the CMP stage. ETV6 deficiency led to the development of aberrant cells as early as the MEP stage, which intensified at the progenitor MK/mature MK stage, with a highly deregulated core "ribosome biogenesis" pathway. Indeed, increased translation levels have been documented in patient CD34+-derived MKs with overexpression of ribosomal protein S6 and phosphorylated ribosomal protein S6 in both CD34+-derived MKs and platelets. Treatment of patient MKs with the ribosomal biogenesis inhibitor CX-5461 resulted in an increase in platelet-like particles. CONCLUSION: These findings provide novel insight into both megakaryopoiesis and the link among ETV6, translation, and platelet production.

Reinvestigation of unidentified causative variants in FXI-deficient patients: Focus on gene segment deletions.

INTRODUCTION: Data on failure to identify the molecular mechanism underlying FXI deficiency by Sanger analysis and the contribution of gene segment deletions are almost inexistent. AIMS AND METHODS: Prospective and retrospective analysis was conducted on FXI-deficient patients' DNA via Next Generation Sequencing (NGS), or Sanger sequencing and Multiplex Probe Ligation-dependent Assay (MLPA) to detect cryptic causative gene variants or gene segment deletions. RESULTS: Sanger analysis or NGS enabled us to identify six severe and one partial (median activity 41 IU/dl) FXI deficient index cases with deletions encompassing exons 11-15, the whole gene, or both. After Sanger sequencing, retrospective evaluation using MLPA detected seven additional deletion cases in apparently homozygous cases in non-consanguineous families, or in previously unsolved FXI-deficiency cases. Among the 504 index cases with a complete genetic investigation (Sanger/MLPA, or NGS), 23 remained unsolved (no abnormality found [n = 14] or rare intronic variants currently under investigation, [n = 9]). In the 481 solved cases (95% efficiency), we identified F11 gene-deleted patients (14 cases; 2.9%). Among these, whole gene deletion accounted for four heterozygous cases, exons 11-15 deletion for five heterozygous and three homozygous ones, while compound heterozygous deletion and isolated exon 12 deletion accounted for one case each. CONCLUSION: Given the high incidence of deletions in our population (2.9%), MLPA (or NGS with a reliable bioinformatic pipeline) should be systematically performed for unsolved FXI deficiencies or apparently homozygous cases in non-consanguineous families.

Screening platelet function in blood donors.

BACKGROUND: Transfusion of defective platelets could contribute to the inefficiency of platelet transfusion in preventing or stopping bleeding. STUDY DESIGN AND METHODS: This single-center prospective study aimed to determine the prevalence of functional platelet abnormalities in a population of blood donors with a clinical history of bleeding diathesis or with history of hematoma (>4 cm) during blood donation. Donors with positive bleeding screening questionnaire were referred to the reference center for rare platelet diseases at La Timone University Hospital (Marseille) to confirm the bleeding tendency using a more extensive bleeding questionnaire (MCMDMscore) and to assess hemostasis, including a comprehensive platelet analysis. RESULTS: One hundred and ninety-five donors identified based on a history of hematoma and 2434 blood donors were included in the study. Eighty-eight donors (3.6%) had a bleeding score indicating a potential bleeding disorder. Five donors with a history of hematoma (2.5%) and 15 (17%) donors with a confirmed bleeding score underwent hemostatic analysis, including two men and 18 women with average age of 33.9 years. Minor hemostatic abnormalities were observed in three donors. Two donors exhibited accelerated fibrinolysis with reduced euglobulin lysis time and increased D-dimer levels in serum. Two donors had a platelet granule defect, without identification of genetic abnormality. CONCLUSION: The bleeding questionnaire proved to be a valuable tool to screen blood donors for potential platelet defects. Platelet dysfunction was rare in the blood donor population assessed. Additional studies are necessary to understand the clinical impact that the transfusion of platelets with qualitative defects has on recipients.

GATA1 pathogenic variants disrupt MYH10 silencing during megakaryopoiesis.

BACKGROUND: GATA1 is an essential transcription factor for both polyploidization and megakaryocyte (MK) differentiation. The polyploidization defect observed in GATA1 variant carriers is not well understood. OBJECTIVE: To extensively phenotype two pedigrees displaying different variants in the GATA1 gene and determine if GATA1 controls MYH10 expression levels, a key modulator of MK polyploidization. METHOD: A total of 146 unrelated propositi with constitutional thrombocytopenia were screened on a multigene panel. We described the genotype-phenotype correlation in GATA1 variant carriers and investigated the effect of these novel variants on MYH10 transcription using luciferase constructs. RESULTS: The clinical profile associated with the p.L268M variant localized in the C terminal zinc finger was unusual in that the patient displayed bleeding and severe platelet aggregation defects without early-onset thrombocytopenia. p.N206I localized in the N terminal zinc finger was associated, on the other hand, with severe thrombocytopenia (15G/L) in early life. High MYH10 levels were evidenced in platelets of GATA1 variant carriers. Analysis of MKs anti-GATA1 chromatin immunoprecipitation-sequencing data revealed two GATA1 binding sites, located in the 3' untranslated region and in intron 8 of the MYH10 gene. Luciferase reporter assays showed their respective role in the regulation of MYH10 gene expression. Both GATA1 variants significantly alter intron 8 driven MYH10 transcription. CONCLUSION: The discovery of an association between MYH10 and GATA1 is a novel one. Overall, this study suggests that impaired MYH10 silencing via an intronic regulatory element is the most likely cause of GATA1-related polyploidization defect.

Recombinant Factor VIII Fc Fusion Protein (rFVIIIFc) in Real Life: One-Year Clinical and Economic Outcomes.

BACKGROUND: Recombinant factor VIII Fc fusion protein (rFVIIIFc) is the first extended half-life (EHL) recombinant clotting factor with marketing authorization; it has been available in France since October 2016. However, data and literature about rFVIIIFc in clinical practice are scarce. OBJECTIVE: We propose a 1-year clinical and economic outcome evaluation in patients with hemophilia A taking into consideration treatment adherence. PATIENTS AND METHODS: We reviewed the diaries of all patients treated with rFVIIIFc at Marseille Hemophilia Center for 1 year. All the data were related to the patients' infusion (i.e., annual number of infusions, weekly dose/kg, and annual consumption) and bleeding reports. The clotting factor costs were considered, whereas additional costs (e.g., infusion devices and nurse intervention) were neglected. RESULTS: A total of 34 patients were evaluated. Their median age was 18 years (IQR = 18). Treatment adherence was observed in 62% for FVIII and 66% for rFVIIIFc. The analysis revealed a negligible decrease in the annual clotting factor consumption following the switch (- 2%, p = 0.7339). These data were combined with a significant reduction in the annual number of infusion (- 22.5%, median = 138.5, IQR = 65.8 for FVIII; median = 105, IQR = 24 for rFVIIIFc, p < 0.0001) and bleeding (- 50%, median = 5, IQR = 7.5 for FVIII; median = 1, IQR = 4 for rFVIIIFc, p < 0.0001). With regard to the cost, a decreasing trend was observed (- 8%, p = 0.1300). CONCLUSION: The analysis in a real-life setting revealed that the input of switches toward rFVIIIFc in different treatment (age of patients and regimen) patterns seems to corroborate previous studies. The results suggest that switches have a beneficial effect in terms of efficacy, clotting factor consumption, and cost.

The ISTH bleeding assessment tool as predictor of bleeding events in inherited platelet disorders: Communication from the ISTH SSC Subcommittee on Platelet Physiology.

BACKGROUND: The ISTH Bleeding Assessment Tool (ISTH-BAT) has been validated for clinical screening of suspected von Willebrand disease (VWD) and for bleeding prediction. Recently it has been validated for subjects with inherited platelet disorders (IPD) (BAT-VAL study). OBJECTIVES: To determine whether the ISTH-BAT bleeding score (BS) predicts subsequent bleeding events requiring treatment in IPD patients. METHODS: Patients with IPD, type 1 VWD (VWD-1) and age- and sex-matched healthy controls enrolled in the BAT-VAL study were prospectively followed-up for 2 years and bleeding episodes requiring treatment were recorded. RESULTS: Of the 1098 subjects initially enrolled, 955 were followed-up and 124 suffered hemorrhages during follow-up, 60% of whom had inherited platelet function disorders (IPFD). Total number of events was significantly higher in IPFD (n = 235) than VWD-1 (n = 52) or inherited thrombocytopenia (IT; n = 20). Events requiring transfusions were 66% in IPFD, 5.7% in VWD-1, and 3% in IT. Baseline BS was significantly higher in IPFD patients with a bleeding event at follow-up than in those without (p < .01) and the percentage of subjects suffering a bleeding event increased proportionally to baseline BS quartile. A significant association between the BS and the chance of suffering severe bleeding was found in the overall, IPFD, and VWD-1 populations. Similar results were obtained for the pediatric population. CONCLUSIONS: Inherited platelet function disorder patients with high BS at enrollment are more likely to suffer from bleeding events requiring treatment at follow-up. Moreover, the higher the baseline BS quartile the greater the incidence of subsequent events, suggesting that independently from diagnosis a high BS is associated with a greater risk of subsequent hemorrhage.

Long-Term Antithrombotic Treatments Prescribed for Cardiovascular Diseases in Patients with Hemophilia: Results from the French Registry.

Cardiovascular diseases (CVDs) are a major issue in aging patients with hemophilia (PWHs). Antithrombotic agents are widely used in the general population for CVD treatment, but this recommendation is not fully applicable to PWHs. To improve treatment strategies, a prospective case-control study (COCHE) that analyzed CVD management and follow-up (2 years/patient) in PWHs was performed in France from 2011 to 2018. In total, 68 PWHs (median age: 65 years [39-89]; 48 mild, 10 moderate, and 10 severe hemophilia) were included (n = 50 with acute coronary syndrome, n = 17 with atrial fibrillation, n = 1 with both). They were matched with 68 control PWHs without antithrombotic treatment. In our series, bleeding was significantly influenced by (1) hemophilia severity, with a mean annualized bleeding ratio significantly higher in COCHE patients than in controls with basal clotting factor level up to 20%, (2) antihemorrhagic regimen (on-demand vs. prophylaxis) in severe (hazard ratio [HR] = 16.69 [95% confidence interval, CI: 8.2-47.26]; p < 0.0001) and moderate hemophilia (HR = 42.43 [95% CI: 1.86-966.1]; p = 0.0028), (3) type of antithrombotic treatment in mild hemophilia, with a significantly higher risk of bleeding in COCHE patients than in controls for dual-pathway therapy (HR = 15.64 [95% CI: 1.57-115.8]; p = 0.019), anticoagulant drugs alone (HR = 9.91 [95% CI: 1.34-73.47]; p = 0.0248), dual antiplatelet therapy (HR = 5.31 [95% CI: 1.23-22.92]; p = 0.0252), and single antiplatelet therapy (HR = 3.76 [95% CI: 1.13-12.55]; p = 0.0313); and (4) HAS-BLED score ≥3 (odds ratio [OR] = 33 [95% CI: 1.43-761.2]; p = 0.0065). Gastrointestinal bleeding was also significantly higher in COCHE patients than in controls (OR = 15 [95% CI: 1.84-268]; p = 0.0141). The COCHE study confirmed that antithrombotic treatments in PWHs are associated with increased bleeding rates in function of hemophilia-specific factors and also of known factors in the general population.

Gastrointestinal bleeding from angiodysplasia in von Willebrand disease: Improved diagnosis and outcome prediction using videocapsule on top of conventional endoscopy.

BACKGROUND: Despite a high prevalence of angiodysplasia, no specific guidelines are available for the modalities of endoscopic exploration of gastrointestinal (GI) bleeding in von Willebrand disease (VWD). Whether VWD patients could benefit from video capsule endoscopy (VCE) looking for angiodysplasia eligible to endoscopic treatment or at high risk of bleeding is unknown. OBJECTIVES: To assess the diagnostic efficacy for angiodysplasia and the prognostic value of VCE on top of conventional endoscopy in VWD patients with GI bleeding. PATIENTS/METHODS: A survey was sent to the 30 centers of the French-network on inherited bleeding disorders to identify VWD patients referred for endoscopic exploration of GI bleeding from January 2015 to December 2017. Data obtained included patient characteristics, VWD phenotype/genotype, GI bleeding pattern, results of endoscopic investigations, and medical management applied including endoscopic therapy. We assessed by Kaplan-Meier analysis the recurrence-free survival after the first GI bleeding event according to endoscopic categorization and, in patients with angiodysplasia, to the presence of small-bowel localizations on VCE exploration. RESULTS: GI bleeding source localization was significantly improved when including VCE exploration (P < .01), even in patients without history of angiodysplasia (P < .05). Patients with angiodysplasia had more GI bleeding recurrences (P < .01). A lower recurrence-free survival was observed in patients with angiodysplasia (log-rank test, P = .02), and especially when lesions were located in the small bowel (log-rank test, P < .01), even after endoscopic treatment with argon plasma coagulation (log-rank test, P < .01). CONCLUSION: VCE should be more systematically used in VWD patients with unexplained or recurrent GI bleeding looking for angiodysplasia eligible to endoscopic treatment or at high risk of relapse.

[Heavy menstrual bleeding in teenage girls and women with inherited bleeding disorders]. / Ménométrorragies de l'adolescente et de la jeune femme ayant un trouble héréditaire de l'hémostase.

Menometrorrhagia is a frequent bleeding symptom in young women, and may be related to an inherited bleeding disorder. If there is no gynecological etiology, hemostasis tests are required. The early medical management of these teenage girls is important, especially when a bleeding disorder is known. The bleeding risk of the first periods may then be anticipated. Afterwards, the objective of the treatment is to keep the bleeding symptoms under control: anti-fibrinolytic treatment, specific replacement therapy for bleeding disorder and hormonal treatment. This management requires a multidisciplinary medical team, mainly hematologist and gynecologist, all along the genital lifespan, from the first periods to the desire for pregnancy.

MÉNOMÉTRORRAGIES DE L'ADOLESCENTE ET DE LA JEUNE FEMME AYANT UN TROUBLE HÉRÉDITAIRE DE L'HÉMOSTASE Les ménométrorragies constituent une symptomatologie hémorragique fréquente de la jeune femme. Elles peuvent être en lien avec un trouble de l'hémostase, qu'il faut savoir dépister en l'absence de cause gynéco-obstétricale. La prise en charge précoce des jeunes filles est essentielle, notamment quand le diagnostic d'anomalie de l'hémostase est établi. Il est important d'annoncer le risque hémorragique potentiel pour permettre d'anticiper les premières règles. Ensuite, la prise en charge consiste à maîtriser l'abondance des règles : traitement antifibrinolytique, traitement substitutif spécifique du trouble de l'hémostase et traitement hormonal. Cette prise en charge doit être multidisciplinaire, associant l'hématologue et le gynécologue, des premières règles de l'adolescente au désir de grossesse des jeunes femmes.

[Clinical management of patients with hemophilia A in nephrology: Diagnostic and therapeutic challenges illustrated by the cases of 2 patients]. / Prise en charge néphrologique des patients hémophiles A : difficultés diagnostiques et thérapeutiques illustrées par le cas de 2 patients.

Hemophilia A is an X-linked genetic hemorrhagic disorder characterized by a factor VIII deficiency. The availability of secured substitution products has led to a dramatic improvement of life expectancy in hemophiliac patients. Nowadays, adult hemophiliac patients may develop Chronic Kidney Disease (CKD) resulting from age-related comorbidities (hypertension, obesity, diabetes). In addition, the high prevalence of viral infections in this population exposes patients to an increased risk of CKD. The risk of hemorrhage in hemophiliac patients is a challenge for their clinical management, both for diagnostic procedures (kidney biopsy in particular) and for renal replacement therapy (dialysis or renal transplantation) when it is needed. This work provides an update of the literature data concerning the management of hemophiliac patients in nephrology, illustrated by the cases of two patients.

Determinants of adherence and consequences of the transition from adolescence to adulthood among young people with severe haemophilia (TRANSHEMO): study protocol for a multicentric French national observational cross-sectional study.

INTRODUCTION: Severe haemophilia is a rare disease characterised by spontaneous bleeding from early childhood, which may lead to various complications, especially in joints. It is nowadays possible to avoid these complications thanks to substitutive therapies for which the issue of adherence is major. The transition from adolescence to adulthood in young people with severe haemophilia is a critical period as it is associated with a high risk of lack of adherence to healthcare, which might have serious consequences on daily activities and on quality of life. METHODS AND ANALYSIS: We present the protocol for a cross-sectional, observational, multicentric study to assess the differences between adolescents and young adults with severe haemophilia in France through the transition process, especially on adherence to healthcare. This study is based on a mixed methods design, with two complementary and consecutive phases, comparing data from a group of adolescents (aged 14-17 years) with those from a group of young adults (aged 20-29 years). The quantitative phase focuses on the determinants (medical, organisational, sociodemographic and social and psychosocial and behavioural factors) of adherence to healthcare (considered as a marker of the success of transition). The qualitative phase explores participants' views in more depth to explain and refine the results from the quantitative phase. Eligible patients are contacted by the various Haemophilia Treatment Centres participating in the French national registry FranceCoag. ETHICS AND DISSEMINATION: The study was approved by the French Ethics Committee and by the French National Agency for Medicines and Health Products Safety (number: 2016-A01034-47). Study findings will be disseminated to the scientific and medical community in peer-reviewed journals and presented at scientific meetings. Results will be popularised to be communicated via the French association for people with haemophilia to participants and to the general public. TRIAL REGISTRATION NUMBER: NCT02866526; Pre-results.

Macrothrombocytopenia and dense granule deficiency associated with FLI1 variants: ultrastructural and pathogenic features.

Congenital macrothrombocytopenia is a family of rare diseases, of which a significant fraction remains to be genetically characterized. To analyze cases of unexplained thrombocytopenia, 27 individuals from a patient cohort of the Bleeding and Thrombosis Exploration Center of the University Hospital of Marseille were recruited for a high-throughput gene sequencing study. This strategy led to the identification of two novel FLI1 variants (c.1010G>A and c.1033A>G) responsible for macrothrombocytopenia. The FLI1 variant carriers' platelets exhibited a defect in aggregation induced by low-dose adenosine diphosphate (ADP), collagen and thrombin receptor-activating peptide (TRAP), a defect in adenosine triphosphate (ATP) secretion, a reduced mepacrine uptake and release and a reduced CD63 expression upon TRAP stimulation. Precise ultrastructural analysis of platelet content was performed using transmission electron microscopy and focused ion beam scanning electron microscopy. Remarkably, dense granules were nearly absent in the carriers' platelets, presumably due to a biogenesis defect. Additionally, 25-29% of the platelets displayed giant α-granules, while a smaller proportion displayed vacuoles (7-9%) and autophagosome-like structures (0-3%). In vitro study of megakaryocytes derived from circulating CD34+ cells of the carriers revealed a maturation defect and reduced proplatelet formation potential. The study of the FLI1 variants revealed a significant reduction in protein nuclear accumulation and transcriptional activity properties. Intraplatelet flow cytometry efficiently detected the biomarker MYH10 in FLI1 variant carriers. Overall, this study provides new insights into the phenotype, pathophysiology and diagnosis of FLI1 variant-associated thrombocytopenia.

Germline variants in ETV6 underlie reduced platelet formation, platelet dysfunction and increased levels of circulating CD34+ progenitors.

Variants in ETV6, which encodes a transcription repressor of the E26 transformation-specific family, have recently been reported to be responsible for inherited thrombocytopenia and hematologic malignancy. We sequenced the DNA from cases with unexplained dominant thrombocytopenia and identified six likely pathogenic variants in ETV6, of which five are novel. We observed low repressive activity of all tested ETV6 variants, and variants located in the E26 transformation-specific binding domain (encoding p.A377T, p.Y401N) led to reduced binding to corepressors. We also observed a large expansion of megakaryocyte colony-forming units derived from variant carriers and reduced proplatelet formation with abnormal cytoskeletal organization. The defect in proplatelet formation was also observed in control CD34+ cell-derived megakaryocytes transduced with lentiviral particles encoding mutant ETV6. Reduced expression levels of key regulators of the actin cytoskeleton CDC42 and RHOA were measured. Moreover, changes in the actin structures are typically accompanied by a rounder platelet shape with a highly heterogeneous size, decreased platelet arachidonic response, and spreading and retarded clot retraction in ETV6 deficient platelets. Elevated numbers of circulating CD34+ cells were found in p.P214L and p.Y401N carriers, and two patients from different families suffered from refractory anemia with excess blasts, while one patient from a third family was successfully treated for acute myeloid leukemia. Overall, our study provides novel insights into the role of ETV6 as a driver of cytoskeletal regulatory gene expression during platelet production, and the impact of variants resulting in platelets with altered size, shape and function and potentially also in changes in circulating progenitor levels.