Glanzmann Thrombasthenia 10 Years Later: Progress Made and Future Directions

Glanzmann thrombasthenia (GT) is the most common inherited platelet disorder (IPD) with mucocutaneous bleeding and a failure of platelets to aggregate when stimulated. The molecular cause is insufficient or defective αIIbβ3, an integrin encoded by the ITGA2B and ITGB3 genes. On activation αIIbβ3 undergoes conformational changes and binds fibrinogen (Fg) and other proteins to join platelets in the aggregate. The application of next-generation sequencing (NGS) to patients with IPDs has accelerated genotyping for GT; progress accompanied by improved mutation curation. The evaluation by NGS of variants in other hemostasis and vascular genes is a major step toward understanding why bleeding varies so much between patients. The recently discovered role for glycoprotein VI in thrombus formation, through its binding to fibrin and surface-bound Fg, may offer a mechanosensitive back-up for αIIbβ3, especially at sites of inflammation. The setting up of national networks for IPDs and GT is improving patient care. Hematopoietic stem cell therapy provides a long-term cure for severe cases; however, prophylaxis by monoclonal antibodies designed to accelerate fibrin formation at injured sites in the vasculature is a promising development. Gene therapy using lentil-virus vectors remains a future option with CRISPR/Cas9 technologies offering a promising alternative route.

APOLD1 loss causes endothelial dysfunction involving cell junctions, cytoskeletal architecture, and Weibel-Palade bodies, while disrupting hemostasis

Vascular homeostasis is impaired in various diseases thereby contributing to the progression of their underlying pathologies. The endothelial immediate early gene Apolipoprotein L domain-containing 1 (APOLD1) helps to regulate endothelial function. However, its precise role in endothelial cell biology remains unclear. We have localized APOLD1 to endothelial cell contacts and to Weibel-Palade bodies (WPB) where it associates with von Willebrand factor (VWF) tubules. Silencing of APOLD1 in primary human endothelial cells disrupted the cell junction-cytoskeletal interface, thereby altering endothelial permeability accompanied by spontaneous release of WPB contents. This resulted in an increased presence of WPB cargoes, notably VWF and angiopoietin-2 in the extracellular medium. Autophagy flux, previously recognized as an essential mechanism for the regulated release of WPB, was impaired in the absence of APOLD1. In addition, we report APOLD1 as a candidate gene for a novel inherited bleeding disorder across three generations of a large family in which an atypical bleeding diathesis was associated with episodic impaired microcirculation. A dominant heterozygous nonsense APOLD1:p.R49* variant segregated to affected family members. Compromised vascular integrity resulting from an excess of plasma angiopoietin-2, and locally impaired availability of VWF may explain the unusual clinical profile of APOLD1:p.R49* patients. In summary, our findings identify APOLD1 as an important regulator of vascular homeostasis and raise the need to consider testing of endothelial cell function in patients with inherited bleeding disorders without apparent platelet or coagulation defects.

Healing through the lens of immunothrombosis: Biology-inspired, evolution-tailored, and human-engineered biomimetic therapies

Evolution, from invertebrates to mammals, has yielded and shaped immunoclotting as a defense and repair response against trauma and infection. This mosaic of immediate and local wound-sealing and pathogen-killing mechanisms results in survival, restoration of homeostasis, and tissue repair. In mammals, immunoclotting has been complemented with the neuroendocrine system, platelets, and contact system among other embellishments, adding layers of complexity through interconnecting blood-born proteolytic cascades, blood cells, and the neuroendocrine system. In doing so, immunothrombosis endows humans with survival advantages, but entails vulnerabilities in the current unprecedented and increasingly challenging environment. Immunothrombosis and tissue repair appear to go hand in hand with common mechanisms mediating both processes, a fact that is underlined by recent advances that are deciphering the mechanisms of the repair process and of the biochemical pathways that underpins coagulation, hemostasis and thrombosis. This review is intended to frame both the universal aspects of tissue repair and the therapeutic use of autologous fibrin matrix as a biology-as-a-drug approach in the context of the evolutionary changes in coagulation and hemostasis. In addition, we will try to shed some light on the molecular mechanisms underlying the use of the autologous fibrin matrix as a biology-inspired, evolution-tailored, and human-engineered biomimetic therapy.

Inherited platelet diseases with normal platelet count: phenotypes, genotypes and diagnostic strategy

Inherited platelet disorders resulting from platelet function defects and a normal platelet count cause a moderate or severe bleeding diathesis. Since the description of Glanzmann thrombasthenia resulting from defects of ITGA2B and ITGB3, new inherited platelet disorders have been discovered, facilitated by the use of high throughput sequencing and genomic analyses. Defects of RASGRP2 and FERMT3 responsible for severe bleeding syndromes and integrin activation have illustrated the critical role of signaling molecules. Important are mutations of P2RY12 encoding the major ADP receptor causal for an inherited platelet disorder with inheritance characteristics that depend on the variant identified. Interestingly, variants of GP6 encoding the major subunit of the collagen receptor GPVI/FcRassociate only with mild bleeding. The numbers of genes involved in dense granule defects including Hermansky-Pudlak and Chediak Higashi syndromes continue to progress and are updated. The ANO6 gene encoding a Ca2+-activated ion channel required for phospholipid scrambling is responsible for the rare Scott syndrome and decreased procoagulant activity. A novel EPHB2 defect in a familial bleeding syndrome demonstrates a role for this tyrosine kinase receptor independent of the classical model of its interaction with ephrins. Such advances highlight the large diversity of variants affecting platelet function but not their production, despite the difficulties in establishing a clear phenotype when few families are affected. They have provided insights into essential pathways of platelet function and have been at the origin of new and improved therapies for ischemic disease. Nevertheless, many patients remain without a diagnosis and requiring new strategies that are now discussed.

Novel manifestations of immune dysregulation and granule defects in gray platelet syndrome

Gray platelet syndrome (GPS) is a rare recessive disorder caused by biallelic variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet α-granules, splenomegaly, and bone marrow (BM) fibrosis. Due to the rarity of GPS, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathologic features, we performed a detailed clinical genotypic and phenotypic study of 47 patients with GPS and identified 32 new etiologic variants in NBEAL2. The GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. Novel clinical phenotypes were also observed, including 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 show that, in addition to the well-described platelet defects in GPS, there are immune defects. The abnormal immune cells may be the drivers of systemic abnormalities such as autoimmune disease.

Absence of bleeding upon dual antiplatelet therapy in a patient with a immune GPVI deficiency

Acquired deficiencies in platelet glycoprotein VI are rare and have not been found associated with other defects. Here we report the case of a 64-year old male patient presenting an immune GPVI deficiency associated to a mutation in the alpha-actinin gene and who has been treated with dual anti platelet therapy without bleeding.Introduction: Glycoprotein (GP) VI, a pluripotent receptor interacting with collagen and fibrin(ogen) is responsible for thrombus formation, growth and stability (1-4). It is co-expressed with the Fc receptor γ (FcRγ) chain (5). GPVI is not critical for haemostasis since subjects with a GPVI deficiency usually present low or even no bleeding tendency (6, 7). Acquired GPVI deficiency due to antibody-induced GPVI depletion is the most frequent finding. At least 10 patients have been described with an acquired GPVI deficiency, most often associated to immune thrombocytopenia, moderate bleeding and impaired collagen-induced platelet aggregation (7). Several mechanisms leading to the GPVI deficiency are proposed including antibody-triggered GPVI internalization and/or shedding of the extracellular domain (8, 9). We report the case of a patient presenting an acquired GPVI deficiency different from those previously described: (i) he is male whereas all previous cases were female, (ii) he is heterozygous for a mutation in α (alpha)-actinin-1 gene and (iii) he was treated with dual antiplatelet therapy with no haemorrhagic manifestation.

Inherited thrombocytopenias: history, advances and perspectives

Over the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes have gradually been defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then, since 1948, when Bernard-Soulier syndrome was first described, inherited thrombocytopenia became a fascinating example of Mendelian disease. The platelet count is often severely decreased and platelet size variable; associated platelet function defects frequently aggravate bleeding. Macrothrombocytopenia with variable proportions of enlarged platelets is common. The number of circulating platelets will depend on platelet production, consumption and lifespan. The bulk of macrothrombocytopenias arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early megakaryocyte development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping has allowed their early classification. It quickly became apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematologic malignancies. In the last decade, the application of next-generation sequencing, including whole exome sequencing, and the use of gene platforms for rapid testing have greatly accelerated the discovery of causal genes and extended the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use next-generation sequencing in first-step screening and to define bleeding risk and treatment better.

A Glanzmann thrombasthenia family associated with a TUBB1-related macrothrombocytopenia

BACKGROUND

Macrothrombocytopenia (MTP) is a rare but enigmatic complication of Glanzmann thrombasthenia (GT), an inherited bleeding disorder caused by the absence of platelet aggregation due to deficiencies of the αIIbβ3 integrin.

OBJECTIVES

We report a family with type I GT and a prolonged bleeding time but unusually associated with congenital mild thrombocytopenia and platelet size heterogeneity with giant forms.

METHODS AND RESULTS

Sanger sequencing of DNA from the propositus identified 2 heterozygous ITGB3 gene mutations: p.P189S and p.C210S both of which prevent αIIbβ3 expression and are causative of GT but without explaining the presence of enlarged platelets. High-throughput screening led to the detection of a predicted disease-causing heterozygous mutation in the TUBB1 gene: p.G146R, encoding β1-tubulin, a component of the platelet cytoskeleton and a gene where mutations are a known cause of MTP.

CONCLUSIONS

Family screening confirmed that this rare phenotype results from oligogenic inheritance while suggesting that the GT phenotype dominates clinically.

Antithrombotic prophylaxis for surgery-associated venous thromboembolism risk in patients with inherited platelet disorders. The SPATA-DVT Study

Major surgery is associated with an increased risk of venous thromboembolism (VTE), thus the application of mechanical or pharmacologic prophylaxis is recommended. The incidence of VTE in patients with inherited platelet disorders (IPD) undergoing surgical procedures is unknown and no information on the current use and safety of thromboprophylaxis, particularly of low-molecular-weight-heparin in these patients is available. Here we explored the approach to thromboprophylaxis and thrombotic outcomes in IPD patients undergoing surgery at VTE-risk participating in the multicenter SPATA study. We evaluated 210 surgical procedures carried out in 155 patients with well-defined forms of IPD (VTE-risk: 31% high, 28.6% intermediate, 25.2% low, 15.2% very low). The use of thromboprophylaxis was low (23.3% of procedures), with higher prevalence in orthopedic and gynecological surgeries, and was related to VTE-risk. The most frequently employed thromboprophylaxis was mechanical and appeared to be effective, as no patients developed thrombosis, including patients belonging to the highest VTE-risk classes. Low-molecular-weight-heparin use was low (10.5%) and it did not influence the incidence of post-surgical bleeding or of antihemorrhagic prohemostatic interventions use. Two thromboembolic events were registered, both occurring after high VTE-risk procedures in patients who did not receive thromboprophylaxis (4.7%). Our findings suggest that VTE incidence is low in patients with IPD undergoing surgery at VTE-risk and that it is predicted by the Caprini score. Mechanical thromboprophylaxis may be of benefit in patients with IPD undergoing invasive procedures at VTE-risk and low-molecular-weight-heparin should be considered for major surgery.

Correction of Severe Myelofibrosis, Impaired Platelet Functions and Abnormalities in a Patient with Gray Platelet Syndrome Successfully Treated by Stem Cell Transplantation

Gray platelet syndrome (GPS) is an inherited disorder. Patients harboring GPS have thrombocytopenia with large platelets lacking α-granules. A long-term complication is myelofibrosis with pancytopenia. Hematopoietic stem cell transplant (HSCT) could be a curative treatment. We report a male GPS patient with severe pancytopenia, splenomegaly and a secondary myelofibrosis needing red blood cells transfusion. He received an HSCT from a 10/10 matched HLA-unrelated donor after a myeloablative conditioning regimen. Transfusion independence occurred at day+21, with a documented neutrophil engraftment. At day+ 180, we added ruxolitinib to cyclosporine and steroids for a moderate chronic graft versus host disease (GVHD) and persistent splenomegaly. At day+240 GVHD was controlled and splenomegaly reduced. Complete donor chimesrism was documented in blood and marrow and platelets functions and morphology normalized. At day+ 720, the spleen size normalized and there was no evidence of marrow fibrosis on the biopsy. In GPS, HSCT may be a curative treatment in selected patients with pancytopenia and myelofibrosis.

Bi-allelic Loss-of-Function CACNA1B Mutations in Progressive Epilepsy-Dyskinesia

The occurrence of non-epileptic hyperkinetic movements in the context of developmental epileptic encephalopathies is an increasingly recognized phenomenon. Identification of causative mutations provides an important insight into common pathogenic mechanisms that cause both seizures and abnormal motor control. We report bi-allelic loss-of-function CACNA1B variants in six children from three unrelated families whose affected members present with a complex and progressive neurological syndrome. All affected individuals presented with epileptic encephalopathy, severe neurodevelopmental delay (often with regression), and a hyperkinetic movement disorder. Additional neurological features included postnatal microcephaly and hypotonia. Five children died in childhood or adolescence (mean age of death: 9 years), mainly as a result of secondary respiratory complications. CACNA1B encodes the pore-forming subunit of the pre-synaptic neuronal voltage-gated calcium channel Ca_v2.2/N-type, crucial for SNARE-mediated neurotransmission, particularly in the early postnatal period. Bi-allelic loss-of-function variants in CACNA1B are predicted to cause disruption of Ca²⁺ influx, leading to impaired synaptic neurotransmission. The resultant effect on neuronal function is likely to be important in the development of involuntary movements and epilepsy. Overall, our findings provide further evidence for the key role of Ca_v2.2 in normal human neurodevelopment.

PAICA: A Method for Characterizing Platelet-Associated Antibodies - Its Application to the Study of Idiopathic Thrombocytopenic Purpura and to the Detection of Platelet-bound c7E3

In idiopathic thrombocytopenic purpura (ITP), autoantibodies reacting with antigens on the platelet membrane bring about accelerated platelet destruction. We now report PAICA (“Platelet-Associated IgG Characterization Assay”), a method for detecting autoantibodies bound to specific membrane glycoproteins in total platelet lysates. This monoclonal antibody (MAb) capture assay takes into account the fact that antibodies on circulating platelets may be translocated to internal pools as well as being on the surface. A total of twenty ITP patients were examined by PAICA, and the results compared with those obtained by measuring (i) serum antibodies bound to paraformaldehyde-fixed control platelets by ELISA, (ii) IgG bound to the surface of the patient’s own platelets by flow cytometry (PSIgG), (iii) total platelet-associated IgG (PAIgG) by ELISA and (iv) serum antibodies reacting with control platelets by MAIPA (“Monoclonal Antibody-specific Immobilization of Platelet Antigens”). Of twelve patients with elevated PAIgG, nine had increased PSIgG yet eleven reacted positively in PAICA. Of these, eight possessed antibodies directed against GP Ilb-IIIa, two against GP Ib-IX and one patient possessed antibodies directed against GP Ilb-IIIa and GP Ia-IIa respectively. Only seven of the patients possessed serum antibodies detectable by MAIPA. PAICA was also able to detect platelet-associated c7E3 (the chimeric form of Fab fragments of the MAb 7E3) following its infusion during antithrombotic therapy, when it proved more sensitive over a seven-day period than a MAIPA assay adapted for assessing surface-bound antibody. We propose that PAICA provides added sensitivity to the detection of platelet-associated antibodies in immune thrombocytopenias or following therapy with humanized MAbs.

Management of pregnancy for a patient with the new syndromic macrothrombocytopenia, DIAPH1-related disease

The number of genes involved in the identification of macrothrombocytopenia (MTP) is growing but the clinical consequences for the affected patients are not well determined. Here, we report the management of the bleeding risk for a patient with the newly reported and rare DIAPH1-related disease during surgery for infertility and then during her subsequent pregnancy. The R1213* DIAPH1 variant responsible for a mild bleeding syndrome in six families was considered a potential risk factor for our patient. Preliminary laparoscopic surgery was followed by neosalpingostomy to open the obstructed fallopian tube that was followed by an ectopic pregnancy requiring further surgery, tranexamic acid was used on each occasion and no bleeding complications were observed. A second pregnancy proceeded to term; the mother's platelet count was controlled throughout the gestation period and remained close to her basal values. No bleeding occurred at delivery or during the postpartum period. In conclusion, with strict repeated assessments of blood parameters and maintenance of the platelet count, the bleeding risk in pregnancy in DIAPH1-related disease can be successfully controlled.

Comprehensive Cancer-Predisposition Gene Testing in an Adult Multiple Primary Tumor Series Shows a Broad Range of Deleterious Variants and Atypical Tumor Phenotypes

Multiple primary tumors (MPTs) affect a substantial proportion of cancer survivors and can result from various causes, including inherited predisposition. Currently, germline genetic testing of MPT-affected individuals for variants in cancer-predisposition genes (CPGs) is mostly targeted by tumor type. We ascertained pre-assessed MPT individuals (with at least two primary tumors by age 60 years or at least three by 70 years) from genetics centers and performed whole-genome sequencing (WGS) on 460 individuals from 440 families. Despite previous negative genetic assessment and molecular investigations, pathogenic variants in moderate- and high-risk CPGs were detected in 67/440 (15.2%) probands. WGS detected variants that would not be (or were not) detected by targeted resequencing strategies, including low-frequency structural variants (6/440 [1.4%] probands). In most individuals with a germline variant assessed as pathogenic or likely pathogenic (P/LP), at least one of their tumor types was characteristic of variants in the relevant CPG. However, in 29 probands (42.2% of those with a P/LP variant), the tumor phenotype appeared discordant. The frequency of individuals with truncating or splice-site CPG variants and at least one discordant tumor type was significantly higher than in a control population (χ2 = 43.642; p ≤ 0.0001). 2/67 (3%) probands with P/LP variants had evidence of multiple inherited neoplasia allele syndrome (MINAS) with deleterious variants in two CPGs. Together with variant detection rates from a previous series of similarly ascertained MPT-affected individuals, the present results suggest that first-line comprehensive CPG analysis in an MPT cohort referred to clinical genetics services would detect a deleterious variant in about a third of individuals.

Platelet function and microparticle levels in atrial fibrillation: Changes during the acute episode

BACKGROUND

Thrombotic risk constitutes a major complication of atrial fibrillation (AF). Platelets and microparticles (MPs) are important for hemostasis and thrombosis, however their participation during AF is not well known. The aim of this study was to characterize platelet function and MPs procoagulant and fibrinolytic activity in AF patients and to determine the effects of an acute-AF episode.

METHODS

Blood was collected from paroxysmal (21) and persistent (16) AF patients referred for AF catheter ablation. Ten patients in sinus rhythm for 10days were induced in AF allowing comparisons of left atrium samples before and after induction. Platelet aggregation with ADP, TRAP, collagen, and ristocetin was studied. Platelet surface expression of PAR-1, αIIbβ3, GPIb and P-selectin were evaluated by flow cytometry, and MPs-associated procoagulant and fibrinolytic activity levels were determined by functional assays.

RESULTS

A specific reduction in platelet aggregation to TRAP, activating the thrombin receptor PAR-1, was found in all AF patients. No differences in platelet receptor expression were found. Yet, after acute-induced AF, the platelet response was improved. Furthermore, a significant decrease of left atrium tissue factor-dependent procoagulant activity of MPs was observed.

CONCLUSION

Acute episodes of AF results in a decrease in MPs-associated tissue factor activity, possibly corresponding to consumption, which in turn favors coagulation and the local production of thrombin. A decreased platelet basal aggregation to TRAP may result from PAR1 desensitization, whereas the improved response after an induced episode of AF suggests activation of coagulation and PAR1 re-sensitization.

Mutations of the integrin αIIb/β3 intracytoplasmic salt bridge cause macrothrombocytopenia and enlarged platelet α-granules

Rare gain-of-function mutations within the ITGA2B or ITGB3 genes have been recognized to cause macrothrombocytopenia (MTP). Here we report three new families with autosomal dominant (AD) MTP, two harboring the same mutation of ITGA2B, αIIbR995W, and a third family with an ITGB3 mutation, β3D723H. In silico analysis shows how the two mutated amino acids directly modify the salt bridge linking the intra-cytoplasmic part of αIIb to β3 of the integrin αIIbβ3. For all affected patients, the bleeding syndrome and MTP was mild to moderate. Platelet aggregation tended to be reduced but not absent. Electron microscopy associated with a morphometric analysis revealed large round platelets; a feature being the presence of abnormal large α-granules with some giant forms showing signs of fusion. Analysis of the maturation and development of megakaryocytes reveal no defect in their early maturation but abnormal proplatelet formation was observed with increased size of the tips. Interestingly, this study revealed that in addition to the classical phenotype of patients with αIIbβ3 intracytoplasmic mutations there is an abnormal maturation of α-granules. It is now necessary to determine if this feature is a characteristic of all mutations disturbing the αIIb R995/β3 D723 salt bridge.

Marked bleeding diathesis in patients with platelet dysfunction due to a novel mutation in RASGRP2, encoding CalDAG-GEFI (p.Gly305Asp)

Congenital platelet function disorders are often the result of defects in critical signal transduction pathways required for platelet adhesion and clot formation. Mutations affecting RASGRP2, the gene encoding the Rap GTPase activator, CalDAG-GEFI, give rise to a novel, and rare, group of platelet signal transduction abnormalities. We here report platelet function studies for two brothers (P1 and P2) expressing a novel variant of RASGRP2, CalDAG-GEFI(p.Gly305Asp). P1 and P2 have a lifelong history of bleeding with severe epistaxis successfully treated with platelet transfusions or rFVIIa. Other bleedings include extended hemorrhage from minor wounds. Platelet counts and plasma coagulation were normal, as was αIIbβ3 and GPIb expression on the platelet surface. Aggregation of patients' platelets was significantly impaired in response to select agonists including ADP, epinephrine, collagen, and calcium ionophore A23187. Integrin αIIbβ3 activation and granule release were also impaired. CalDAG-GEFI protein expression was markedly reduced but not absent. Homology modeling places the Gly305Asp substitution at the GEF-Rap1 interface, suggesting that the mutant protein has very limited catalytic activity. In summary, we here describe a novel mutation in RASGRP2 that affects both expression and function of CalDAG-GEFI and that causes impaired platelet adhesive function and significant bleeding in humans.

Ultrastructural Analysis of Megakaryocytes in GPV Knockout Mice

Lesions in the genes for GPIbα, GPIbβ or GPIX result in a bleeding diathesis, the Bernard-Soulier syndrome (BSS), which associates a platelet adhesion defect with thrombocytopenia, giant platelets and abnormal megakaryocytes (MK). The role of GPV, also absent in BSS, was recently addressed by gene targeting in mice. While a negative modulator function for GPV on thrombin-induced platelet responses was found in one model, the absence of GP V had no effect on GPIb-IX expression or platelet adhesion. Our study extends previous results and reports that electron microscopy of bone marrow from the GPV knockout mice revealed a normal MK ultrastructure and development of the demarcation membrane system (DMS). There was a usual presence of MK fragments in the bone marrow vascular sinus. Immunogold labelling of MK from the knockout mice showed a normal distribution of GPIb-IX in the DMS and on the cell surface. The distribution of fibrinogen, vWF and P-selectin was unchanged with, interestingly, P-selectin also localised within the DMS in both situations. Thus GPV is not crucial to MK development and platelet production, consistent with the fact that no mutation in the GPV gene has as yet been described in BSS.

Paradoxical Platelet Activation Was not Observed on Dissociation of Abciximab from GPIIb-IIIa Complexes

The ability of abciximab to bind and dissociate from platelets raises the question of the conformational state of GPIIb-IIIa complexes losing abciximab and the risk of paradoxical drug-induced platelet activation. Platelets incubated with abciximab and mixed in vitro with c7E3 Fab-free platelets lost the drug to the new platelets giving a single platelet population with a unimodal abciximab distribution within 17 h. Prelabeling the receiving platelets with phycoerythrin-labeled anti-GPIb monoclonal antibody (MoAb), permitted their identification by flow cytometry. Binding of PAC-1 and AP6, two MoAbs specific for activated GPIIb-IIIa, was then assessed to both losing and receiving platelet populations during transfer of abciximab. The subpopulation losing c7E3 Fab failed to show increased binding of these MoAbs. However, PAC-1 binding increased in both subpopulations after addition of ADP. Thus GPIIb-IIIa complexes are not in an activated state after dissociation of abciximab unless there is an additional source of activation.

Analysis of the Amino Acid Requirement for a Normal αIIbβ3 Maturation at αIIbGlu324 Commonly Mutated in Glanzmann Thrombasthenia

Glanzmann thrombasthenia is an inherited bleeding disorder arising from quantitative or qualitative defects of the αIIbβ3 integrin of platelets. Here, we report that PCR-SSCP analysis and DNA sequencing revealed a homozygous single base pair substitution in exon 12 of the αIIb gene leading to a Glu324 (E) to Lys (K) substitution in the αIIb subunit in a patient with Type I disease. As this mutation is found on at least 3 continents, the codon for Glu324 may be a mutational hotspot of the disease. To better understand this mutation, we analyzed the effect of substituting E324 with A324, L324, D324, Q324, N324, S324, as well as K324, looking at both αIIbβ3 maturation and cell surface expression in transiently transfected Cos-7 cells. The maturation state of the receptor clearly correlated with the level of cell membrane expression. Maturation efficiency was dependent on the electric charge as well as the size of the side chain of the amino acid present in what is a highly conserved N-terminal position in the third β-strand of blade 5 of the αIIbβ-propeller.

A Novel Hemizygous Bernard-Soulier Syndrome (BSS) Mutation in the Amino Terminal Domain of Glycoprotein (GP)Ibβ- Platelet Characterization and Transfection Studies

Glycoprotein (GP) Ib-V-IX is a unique adhesion receptor complex on platelets. Mutations in GPIbα, Ibβ, and IX can lead to the rare bleeding disorder, Bernard-Soulier Syndrome (BSS). Here, we report a novel hemizygous variant of BSS in which Pro29 in one GPIbβ allele is substituted by a Leu (GPIbβ:P29L). Fluoresence in situ hybridisation revealed that the 22q11 locus was deleted from the homologous chromosome. The pedigree was determined and revealed inheritance of the GPIbβ:P29L allele from the father. Flow cytometry with a range of antibodies detected no expression of GPIb-V-IX on the surface of the patient’s platelets. Western blotting revealed an absence of GPIbα and GPIbβ from platelet lysates. Co-expression of GPIbβ:P29L with normal GPIbα and GPIX in a heterologous cell system confirmed that the mutant subunit did not support surface expression of the complex. Interestingly, residual expression of GPIbβ:P29L anchored in the plasma membrane alone was now seen. This novel BSS mutation expressed in heterologous cells is in agreement with recent in vitro evidence that the correct conformation of the amino terminal region of GPIbβ is required for normal expression of the intact receptor complex.

Two Different β3 Cysteine Substitutions Alter αIIbβ3 Maturation and Result in Glanzmann Thrombasthenia

We report the defects responsible for Glanzmann thrombasthenia in two patients showing traces of abnormally migrating platelet β3 in immunoblotting. Using PCR-SSCP and direct sequencing, we identified a novel homozygous mutation in exon 10 of the β3 gene of patient 1 which gave a C457 to Y amino acid substitution. A C542 to R substitution in β3 of patient 2 was previously reported by us. These cysteines are present in EGF-domains 1 and 3 respectively of β3. We therefore constructed mutants carrying substitutions on cysteine residues in each of the first three EGF domains of β3, C457, C495 and C542 respectively. Transient expression of these mutants in COS-7 cells, including the C542 and C547 double mutant, proved that disulfide disruption directly affects cell surface expression of the integrin. We then showed by metabolic (35S) labeling and Endo-H glycosidase treatment that these substitutions strongly affected complex maturation within the cell.

Abnormal Tyrosine Phosphorylation Linked to a Defective Interaction between ADP and Its Receptor on Platelets

ADP, a primary stimulus of platelets, binds to one or more populations of receptors on the platelet surface. These receptors are linked to discrete activation pathways. Both G proteins and tyrosine kinases have been implicated in the cellular responses to this agonist. We have studied a patient with a congenital abnormality of ADP-induced platelet aggregation in an effort to gain information on the signalling pathways used by ADP. Immunoblotting with a broadly reactive rabbit antibody recognizing the GTP-binding domain of G protein α-subunits, and with rabbit antibodies specific for Giαl-3, and Gα12 all showed normal reactivity when tested against the patient‘s platelets. The phosphorylation of proteins was studied using an anti-phosphotyrosine MoAb (4G10) and platelets stimulated in a platelet aggregometer with ADP, a thromboxane A2 mimetic (IBOP), TRAP-14-mer peptide and α-thrombin. With normal platelets, a time-dependent phosphorylation of several bands in the 60 to 130 kDa mol. wt. range was observed with all agonists. For the patient, minimal aggregation and little or no phosphorylation of proteins of 80-85 kDa (cortactin), 100-105 kDa and 125-130 kDa were seen in response to ADP. The aggregation and phosphorylation responses were slightly modified in the presence of low doses of thrombin but were normal with high doses. Aggregation and tyrosine phosphorylation were virtually absent with IBOP, a finding reproduced when normal platelets were incubated with IBOP and the CP/CPK ADP scavenging system, thereby underlining the role of ADP in the response to IBOP. Our results show that the ADP receptor pathway deficient in the patient is linked to a selective tyrosine phosphorylation response.

Use of Recombinant Factor VIIa in 3 Patients with Inherited Type I Glanzmann’s Thrombasthenia Undergoing Invasive Procedures

The treatment of bleeds in Glanzmann’s thrombasthenia is a challenging issue, especially when repeated platelet transfusions have induced anti-glycoprotein (GP) IIb-IIIa or anti-HLA allo-immunisation. In an attempt to find an alternative treatment regimen, we used recombinant factor VIIa (rFVIIa, NovoSeven®, Novo Nordisk, Denmark) as first-line therapy in 3 patients with Glanzmann’s thrombasthenia and anti-GPIIb-IIIa iso-antibodies who were scheduled for invasive procedures. The administration of an initial bolus dose of rFVIIa (70–110 µg/kg) was immediately followed by continuous infusion at the rate of 9-30 µg/kg/h for 3–15 days. The treatment resulted in an excellent clinical efficacy and tolerance in 2 cases. In the third patient, whereas efficacy was excellent at the surgical site, pharyngonasal bleeds of traumatic origin persisted for 10 days, and a severe thromboembolic complication occurred 5 days after discontinuation of rFVIIa. Complementary studies are needed for patients with congenital platelet disorders in order to evaluate the safety and the potential therapeutic place of rFVIIa treatment.

Delayed immunologic thrombocytopenia induced by abciximab

Abciximab is an anti-GPIIb-IIIa drug widely used to prevent thrombotic complications during percutaneous coronary intervention. We now report on the immunologic origin of thrombocytopenia developing between 7 and 12 days after the onset of abciximab infusion. Antibodies directed against abciximabcoated platelets were located in 5 patients with delayed thrombocytopenia, just as they were present in a patient whose platelet count fell within a few hours after receiving the drug. Abciximab-dependent IgG antibody was revealed in serum using control platelets in the monoclonal antibody immobilization of platelet antigens assay (MAIPA) performed with SZ22, a MoAb to GPIIb. The presence of IgG antibodies specific for platelets sensitized with abciximab was confirmed by flow cytometry. They were not located in 13 patients receiving abciximab but whose platelet counts remained stable. For three patients, antibodies were transient and their presence related to the extent of the thrombocytopenia. Surprisingly, antibodycontaining plasma from three patients induced abciximabdependent activation and aggregation of normal platelets, a finding confirmed by electron microscopy. Immunogold labeling revealed that abciximab was associated with platelets in the aggregate, suggesting that its inhibitory effect was overcome by the platelet stimulation. In summary, these results show that abciximab-dependent thrombocytopenia can be delayed and potentially prothrombotic.

Type 2N von Willebrand disease due to compound heterozygosity for R854Q and a novel R763G mutation at the cleavage site of von Willebrand factor propeptide

Type 2N von Willebrand disease (VWD) is characterized by a markedly decreased affinity of von Willebrand factor (VWF) for factorVIII (FVIII) and is caused by mutations in the D’ or D3 domain of mature VWF. We now report a French patient with an atypical 2N VWD phenotype associating FVIII deficiency with plasmaVWF unable to bind FVIII (undetectableVWF:FVIIIB) but with an abnormal multimeric profile. This patient is heterozygous for both the frequent R854Q type 2NVWD mutation and a novel R763G mutation at the cleavage site betweenVWF propeptide and mature VWF. Four children of the patient displayed moderately decreased VWF:FVIIIB of plasma VWF and were heterozygous for either the R763G or the R854Q mutation. Children with the R763G mutation displayed the same abnormal multimeric profile as their father. Recombinant VWF (rVWF) expression studies performed in COS-7 cells showed that the R763G mutation subtly affects its multimeric profile and dramatically impairs its FVIII binding function. Furthermore, the characteristics of hybrid G763/Q854 rVWF resulting from cotransfection experiments were in agreement with the type 2N VWD diagnosis of the patient. We conclude that R763G is a new type 2N VWD mutation located in the VWF propeptide which alters the proteolytic processing of VWF and consequently its binding to FVIII.

Rare homozygous status of P43 β1-tubulin polymorphism causes alterations in platelet ultrastructure

β1-tubulin is the main constituent of the platelet marginal band and studies with deficient mice showed that it maintains discoid shape and it is required for normal platelet formation. TUBB1 Q43P polymorphism is associated with decreased β1-tubulin expression, diminished platelet reactivity, and partial loss of discoid shape in heterozygous carriers. However, to date no studies have been carried out on homozygous PP individuals. Our study included 19 subjects genotyped for TUBB1 Q43P polymorphism (4 QQ, 4 QP, and 2 PP). The two PP individuals were recruited after genotyping of 2073 individuals. Biochemical, microscopy, and molecular studies were performed. Real-time PCR showed ∼40% decrease in TUBB1 mRNA in the two PP individuals compared to four QQ subjects. Western blot analysis confirmed this reduction. Electron microscopy revealed a majority of normal discoid platelets in PP individuals, although platelets with loose, re-orientated or invaginated protofilaments, and an over-developed open canalicular system were observed. Such abnormalities were not observed in QQ subjects. Morphometric analyses showed no differences between PP and QQ individuals. Immunofluorescence confirmed the presence of a normal marginal band in a majority of platelets from PP subjects. Interestingly, both PP subjects had a 40% lower platelet count than QP and QQ. TUBB1 Q43P polymorphism in homozygosity mildly affects platelet ultrastructure and our data further suggest that high levels of β1-tubulin might not be critical to sustain platelet discoid shape.

Congenital disorders associated with platelet dysfunctions

Genetic defects of the megakaryocyte lineage give rise to bleeding syndromes of varying severity. Blood platelets are unable to fulfill their hemostatic function of preventing blood loss on vessel injury. Spontaneous bleeding is mostly mucocutaneous in nature. Most studied are deficiencies of glycoprotein (GP) mediators of adhesion (Bernard-Soulier syndrome) and aggregation (Glanzmann thrombasthenia) which concern the GPIb-IX-V complex and the integrin αIIbβ3, respectively. Defects of primary receptors for stimuli include the P2Y12 ADP receptor pathology. Agonist-specific deficiencies in the platelet aggregation response and abnormalities of signaling pathways are common and lead to trauma-related bleeding. Inherited defects of secretion from storage organelles, of ATP production, and of the generation of procoagulant activity are also encountered. In some disorders, such as the Chediak-Higashi, Hermansky-Pudlak, Wiskott-Aldrich and Scott syndromes, the molecular lesion extends to other cells. In familial thrombocytopenia (FT), platelets are produced in insufficient numbers to assure haemostasis. Some of these disorders affect platelet morphology and give rise to the so-called ‘giant platelet’ syndromes (MYH9-related diseases) with changes in megakaryocyte maturation within the bone marrow and premature release of platelets. Diseases of platelet production may extend to other cells and in some cases interfere with development. Transfusion of platelets remains the most common treatment of severe bleeding, management with desmopressin is common for mild disorders. Substitute therapies are available including rFVIIa and the potential use of TPO analogues for FT. Stem cell or bone marrow transplanation is being used for severe diseases while gene therapy may be on the horizon.

Thrombocytopenia after abciximab use results from different mechanisms

Our study concerns thrombocytopenia in patients with acute ischaemic coronary artery disease receiving antiplatelet drugs to the αIIbβ3 inte-grin (GPIIb/IIIa). We have screened for drug-dependent antibodies (DDAB) in 18 patients who suffered a fall of > 50% in platelet count (9 patients had a nadir of <50,000 platelets/μl) after receiving abciximab and related results to clinical outcome. Serum or plasma was screened for DDAB using (i) a direct ELISA against purified αIIbβ3, αIIbβ3-abciximab complexes or abciximab alone, (ii) control platelets and flow cytometry and (iii) monoclonal antibody immobilisation of platelet antigens. DDAB were found for 11 patients, with αIIbβ3 ELISA the most sensitive test. Progressive platelet consumption linked with haemoglobin loss and/or use of intra-aortic balloon pumping, another potential cause of a fall in platelet count, was also evaluated. DDAB were identified that recognised αIIbβ3 associated with abciximab and/ or abciximab alone. Screening of both progressive and delayed thrombocytopenia (appearing after 5 to 11 days) suggested that antibodies against abciximab preceded those recognising neo-epitopes on αIIbβ3, with a time-dependent broadening of antibody specificities. Higher titres were seen after second abciximab use. Five antibodies were platelet-activating. In conclusion, the mechanisms responsible for this complication of anti-αIIbβ3 therapy are multiple and often associated with a complex immune response.

Diagnosis of inherited platelet disorders on a blood smear: a tool to facilitate worldwide diagnosis of platelet disorders

Essentials There are many hereditary platelet disorders (HPD) but diagnosing these is challenging. We provide a method to diagnose several HPDs using standard blood smears requiring < 100 µL blood. By this approach, the underlying cause of HPD was characterized in ~25-30% of referred individuals. The method facilitates diagnosis of HPD for patients of all ages around the world.

SUMMARY

Background Many hereditary thrombocytopenias and/or platelet function disorders have been identified, but diagnosis of these conditions remains challenging. Diagnostic laboratory techniques are available only in a few specialized centers and, using fresh blood, often require the patient to travel long distances. For the same reasons, patients living in developing countries usually have limited access to diagnosis. Further, the required amount of blood is often prohibitive for pediatric patients. Objectives By a collaborative international approach of four centers, we aimed to overcome these limitations by developing a method using blood smears prepared from less than 100 μL blood, for a systematic diagnostic approach to characterize the platelet phenotype. Methods We applied immunofluorescence labelling (performed centrally) to standard air-dried peripheral blood smears (prepared locally, shipped by regular mail), using antibodies specific for proteins known to be affected in specific hereditary platelet disorders. Results By immunofluorescence labelling of blood smears we characterized the underlying cause in 877/3217 (27%) patients with suspected hereditary platelet disorders (HPD). Currently about 50 genetic causes for HPD are identified. Among those, the blood smear method was especially helpful to identify MYH9 disorders/MYH9-related disease, biallelic Bernard-Soulier syndrome, Glanzmann thrombasthenia and gray platelet syndrome. Diagnosis could be established for GATA1 macrothrombocytopenia, GFI1B macrothrombocytopenia, ß1-tubulin macrothrombocytopenia, filamin A-related thrombocytopenia and Wiskott-Aldrich syndrome. Conclusion Combining basic and widely available preanalytical methods with the immunomorphological techniques presented here, allows detailed characterization of the platelet phenotype. This supports genetic testing and facilitates diagnosis of hereditary platelet disorders for patients of all ages around the world.

Phenotype analysis and clinical management in a large family with a novel truncating mutation in RASGRP2, the CalDAG-GEFI encoding gene

BACKGROUND

Genetic variants in the RASGRP2 gene encoding calcium and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) represent a new inherited bleeding disorder linked to major defects of platelet aggregation and activation of αIIbβ3 integrin. They are of major interest as CalDAG-GEFI is receiving attention as a potential target for antiplatelet therapy for prevention and treatment of cardiovascular disorders including arterial thrombosis and atherosclerosis.

OBJECTIVES

To better understand the phenotypical and clinical profiles of patients with CalDAG-GEFI deficiency.

PATIENTS

We report a five-generation family with a novel truncating CalDAG-GEFI mutation detailing clinical management and phenotypic variability.

RESULTS

Patients IV.6 & IV.4 manifested with episodes of serious mucocutanous bleeding or bleeding after surgery not responding to platelet transfusion but responding well to recombinant Factor VIIa infusions. Their blood counts and coagulation parameters were normal but platelet aggregation to ADP and collagen was defective. Further work-up confirmed normal levels of αIIb and β3 in their platelets but decreased αIIbβ3 function. DNA analysis by whole exome sequencing within the BRIDGE-BPD consortium (Cambridge, UK), allowed us to highlight a homozygous c.1490delT predicted to give rise to a p.F497Sfs*22 truncating mutation near to the C-terminal domain of CalDAG-GEFI. Sanger sequencing confirmed that both patients were homozygous for the c.1490delT and 3 out of 4 close family members were heterozygous.

CONCLUSIONS

A long-term prospective study is warranted for full clinical exploration of CalDAG-GEFI to understand the bleeding phenotyes and their management.

A Cdc42/RhoA regulatory circuit downstream of glycoprotein Ib guides transendothelial platelet biogenesis

Blood platelets are produced by large bone marrow (BM) precursor cells, megakaryocytes (MKs), which extend cytoplasmic protrusions (proplatelets) into BM sinusoids. The molecular cues that control MK polarization towards sinusoids and limit transendothelial crossing to proplatelets remain unknown. Here, we show that the small GTPases Cdc42 and RhoA act as a regulatory circuit downstream of the MK-specific mechanoreceptor GPIb to coordinate polarized transendothelial platelet biogenesis. Functional deficiency of either GPIb or Cdc42 impairs transendothelial proplatelet formation. In the absence of RhoA, increased Cdc42 activity and MK hyperpolarization triggers GPIb-dependent transmigration of entire MKs into BM sinusoids. These findings position Cdc42 (go-signal) and RhoA (stop-signal) at the centre of a molecular checkpoint downstream of GPIb that controls transendothelial platelet biogenesis. Our results may open new avenues for the treatment of platelet production disorders and help to explain the thrombocytopenia in patients with Bernard-Soulier syndrome, a bleeding disorder caused by defects in GPIb-IX-V.

Gain-of-Function Mutation in Filamin A Potentiates Platelet Integrin αIIbβ3 Activation

OBJECTIVE

Dominant mutations of the X-linked filamin A (FLNA) gene are responsible for filaminopathies A, which are rare disorders including brain periventricular nodular heterotopia, congenital intestinal pseudo-obstruction, cardiac valves or skeleton malformations, and often macrothrombocytopenia.

APPROACH AND RESULTS

We studied a male patient with periventricular nodular heterotopia and congenital intestinal pseudo-obstruction, his unique X-linked FLNA allele carrying a stop codon mutation resulting in a 100-amino acid-long FLNa C-terminal extension (NP_001447.2: p.Ter2648SerextTer101). Platelet counts were normal, with few enlarged platelets. FLNa was detectable in all platelets but at 30% of control levels. Surprisingly, all platelet functions were significantly upregulated, including platelet aggregation and secretion, as induced by ADP, collagen, or von Willebrand factor in the presence of ristocetin, as well as thrombus formation in blood flow on a collagen or on a von Willebrand factor matrix. Most importantly, patient platelets stimulated with ADP exhibited a marked increase in α_IIbβ₃ integrin activation and a parallel increase in talin recruitment to β₃, contrasting with normal Rap1 activation. These results are consistent with the mutant FLNa affecting the last step of α_IIbβ₃ activation. Overexpression of mutant FLNa in the HEL megakaryocytic cell line correlated with an increase (compared with wild-type FLNa) in PMA-induced fibrinogen binding to and in talin and kindlin-3 recruitment by α_IIbβ₃.

CONCLUSIONS

Altogether, our results are consistent with a less binding of mutant FLNa to β₃ and the facilitated recruitment of talin by β₃ on platelet stimulation, explaining the increased α_IIbβ₃ activation and the ensuing gain-of-platelet functions.

Bleeding risk of surgery and its prevention in patients with inherited platelet disorders

Excessive bleeding at surgery is a feared complication in patients with inherited platelet disorders. However, very few studies have evaluated the frequency of surgical bleeding in these hemorrhagic disorders. We performed a worldwide, multicentric, retrospective study to assess the bleeding complications of surgery, the preventive and therapeutic approaches adopted, and their efficacy in patients with inherited platelet disorders: the Surgery in Platelet disorders And Therapeutic Approach (SPATA) study. We rated the outcome of 829 surgical procedures carried out in 423 patients with well-defined forms of inherited platelet disorders: 238 inherited platelet function disorders and 185 inherited platelet number disorders. Frequency of surgical bleeding was high in patients with inherited platelet disorders (19.7%), with a significantly higher bleeding incidence in inherited platelet function disorders (24.8%) than in inherited platelet number disorders (13.4%). The frequency of bleeding varied according to the type of inherited platelet disorder, with biallelic Bernard Soulier syndrome having the highest occurrence (44.4%). Frequency of bleeding was predicted by a pre-operative World Health Organization bleeding score of 2 or higher. Some types of surgery were associated with a higher bleeding incidence, like cardiovascular and urological surgery. The use of pre-operative pro-hemostatic treatments was associated with a lower bleeding frequency in patients with inherited platelet function disorders but not in inherited platelet number disorders. Desmopressin, alone or with antifibrinolytic agents, was the preventive treatment associated with the lowest bleedings. Platelet transfusions were used more frequently in patients at higher bleeding risk. Surgical bleeding risk in inherited platelet disorders is substantial, especially in inherited platelet function disorders, and bleeding history, type of disorder, type of surgery and female sex are associated with higher bleeding frequency. Prophylactic pre-operative pro-hemostatic treatments appear to be required and are associated with a lower bleeding incidence.

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.

Mutations in tropomyosin 4 underlie a rare form of human macrothrombocytopenia

Platelets are anuclear cells that are essential for blood clotting. They are produced by large polyploid precursor cells called megakaryocytes. Previous genome-wide association studies in nearly 70,000 individuals indicated that single nucleotide variants (SNVs) in the gene encoding the actin cytoskeletal regulator tropomyosin 4 (TPM4) exert an effect on the count and volume of platelets. Platelet number and volume are independent risk factors for heart attack and stroke. Here, we have identified 2 unrelated families in the BRIDGE Bleeding and Platelet Disorders (BPD) collection who carry a TPM4 variant that causes truncation of the TPM4 protein and segregates with macrothrombocytopenia, a disorder characterized by low platelet count. N-Ethyl-N-nitrosourea–induced (ENU-induced) missense mutations in Tpm4 or targeted inactivation of the Tpm4 locus led to gene dosage–dependent macrothrombocytopenia in mice. All other blood cell counts in Tpm4-deficient mice were normal. Insufficient TPM4 expression in human and mouse megakaryocytes resulted in a defect in the terminal stages of platelet production and had a mild effect on platelet function. Together, our findings demonstrate a nonredundant role for TPM4 in platelet biogenesis in humans and mice and reveal that truncating variants in TPM4 cause a previously undescribed dominant Mendelian platelet disorder.

Rare variants in GP1BB are responsible for autosomal dominant macrothrombocytopenia

The von Willebrand receptor complex, which is composed of the glycoproteins Ibα, Ibβ, GPV, and GPIX, plays an essential role in the earliest steps in hemostasis. During the last 4 decades, it has become apparent that loss of function of any 1 of 3 of the genes encoding these glycoproteins (namely, GP1BA, GP1BB, and GP9) leads to autosomal recessive macrothrombocytopenia complicated by bleeding. A small number of variants in GP1BA have been reported to cause a milder and dominant form of macrothrombocytopenia, but only 2 tentative reports exist of such a variant in GP1BB By analyzing data from a collection of more than 1000 genome-sequenced patients with a rare bleeding and/or platelet disorder, we have identified a significant association between rare monoallelic variants in GP1BB and macrothrombocytopenia. To strengthen our findings, we sought further cases in 2 additional collections in the United Kingdom and Japan. Across 18 families exhibiting phenotypes consistent with autosomal dominant inheritance of macrothrombocytopenia, we report on 27 affected cases carrying 1 of 9 rare variants in GP1BB.

Should any genetic defect affecting α-granules in platelets be classified as gray platelet syndrome?

There is much current interest in the role of the platelet storage pool of α-granule proteins both in hemostasis and non-hemostatic events. As well as in the arrest of bleeding, the secreted proteins participate in wound healing, inflammation, and innate immunity while in pathology they may be actors in arterial thrombosis and atherosclerosis as well as cancer and metastasis. For a long time, gray platelet syndrome (GPS) has been regarded as the classic inherited platelet disorder caused by an absence of α-granules and their contents. While NBEAL2 is the major source of mutations in GPS, other gene variants may give rise to significant α-granule deficiencies in platelets. These include GATA1, VPS33B, or VIPAS39 in the arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome and now GFI1B. Nevertheless, many phenotypic differences are associated with mutations in these genes. This critical review was aimed to assess genotype/phenotype variability in disorders of platelet α-granule biogenesis and to urge caution in grouping all genetic defects of α-granules as GPS. Am. J. Hematol. 91:714-718, 2016. © 2016 Wiley Periodicals, Inc.

Clopidogrel: An Antithrombotic Drug Acting on the ADP-dependent Activation Pathway of Human Platelets

The aim of the study was to determine the effect of clopidogrel on adenosine diphosphate (ADP)- induced platelet activation in human volunteers. Platelets from human volunteers before and after a 7-day treatment with clopidogrel (75 mg/kg), were tested for their sensi tivity to ADP by measuring ADP-induced aggregation, adenylyl cyclase downregulation, and [3H]-2-MeS-ADP binding. Platelet membrane glycoprotein (GP IIb-IIIa; GP Ib, GMP-140) expression was measured by flow cy tometry using fluorescent-labeled antibodies or fibrino gen. After oral administration to human volunteers (75 mg/day for 7 days), clopidogrel, a novel ADP-selective antiplatelet agent, inhibited ADP-induced aggregation of platelets ex vivo. This effect was irreversible in nature, and no activity could be detected in the plasma of treated subjects. Although clopidogrel did not modify ADP- induced shape change, it prevented the inhibitory effect of ADP (but not that of epinephrine) on the prostoglandin- E1(PGE1)-induced increase in platelet cAMP. The num ber of binding sites for [3H]-2-MeS-ADP, a stable ana logue of ADP that labels ADP binding sites linked to the inhibition of stimulated adenylyl cyclase, was reduced from 525 ± 62 sites/cell in the controls to 32 ± 5 sites/cell after treatment with clopidogrel (p < 0.001). This effect occurred with no consistent change in the binding affinity of [3H]-2-MeS-ADP, indicating that inhibition of platelet functions by clopidogrel was mainly due to a selective and irreversible reduction of ADP binding sites on plate lets. Flow cytometry experiments showed that clopi dogrel selectively inhibited ADP-inducing binding of fi brinogen to platelets. This effect occurred through a ma jor reduction of the ADP-induced activation of the GP IIb-IIIa complex. These findings therefore indicate that clopidogrel downregulates platelet responses via a selec tive and direct interaction with the ADP receptors, me diating the inhibition of stimulated adenylyl cyclase ac tivity in human platelets.

Haematological spectrum and genotype-phenotype correlations in nine unrelated families with RUNX1 mutations from the French network on inherited platelet disorders

Background

Less than 50 patients with FPD/AML (OMIM 601309) have been reported as of today and there may an underestimation. The purpose of this study was to describe the natural history, the haematological features and the genotype-phenotype correlations of this entity in order to, first, screen it better and earlier, before leukaemia occurrence and secondly to optimize appropriate monitoring and treatment, in particular when familial stem cell transplantation is considered.

Methods

We have investigated 41 carriers of RUNX1 alteration belonging to nine unrelated French families with FPD/AML and two syndromic patients, registered in the French network on rare platelet disorders from 2005 to 2015.

Results

Five missense, one non-sense, three frameshift mutations and two large deletions involving several genes including RUNX1 were evidenced. The history of familial leukaemia was suggestive of FPD/AML in seven pedigrees, whereas an autosomal dominant pattern of lifelong thrombocytopenia was the clinical presentation of two. Additional syndromic features characterized two large sporadic deletions. Bleeding tendency was mild and thrombocytopenia moderate (>50 x10⁹/L), with normal platelet volume. A functional platelet defect consistent with a δ-granule release defect was found in ten patients regardless of the type of RUNX1 alteration. The incidence of haematological malignancies was higher when the mutated RUNX1 allele was likely to cause a dominant negative effect (19/34) in comparison with loss of function alleles (3/9). A normal platelet count does not rule out the diagnosis of FPD/AML, since the platelet count was found normal for three mutated subjects, a feature that has a direct impact in the search for a related donor in case of allogeneic haematopoietic stem cell transplantation.

Conclusions

Platelet dysfunction suggestive of defective δ-granule release could be of values for the diagnosis of FPD/AML particularly when the clinical presentation is an autosomal dominant thrombocytopenia with normal platelet size in the absence of familial malignancies. The genotype-phenotype correlations might be helpful in genetic counselling and appropriate optimal therapeutic management.

A diagnostic approach that may help to discriminate inherited thrombocytopenia from chronic immune thrombocytopenia in adult patients

Inherited thrombocytopenia (IT) is a heterogeneous group of rare diseases that are often confused with immune thrombocytopenia (ITP). The objective of this study was to supply clinicobiological elements that allow a distinction to be drawn between IT and chronic ITP. We then compared 23 adult patients with IT and 9 patients with chronic ITP. Our study revealed six discriminating criteria: (i) an age of discovery <34 years: positive predictive value (PPV) = 88.2% [63.6; 98.5], (ii) a family history of thrombocytopenia: PPV = 100.0% [82.4; 100.0], (iii) a personal history of bleeding: PPV = 100% [76.8; 100.0], (iv) a mean platelet volume >11 fL: PPV = 93.3% [68.1; 99.8], (v) an excess of giant platelets on blood smear: 100.0% [76.8; 100.0], and (vi) a percentage >44% of platelets with a surface area >4 µm(2) in electron microscopy: PPV = 83.3% [58.6; 96.4]. If at least three of these criteria were combined, it was possible to distinguish IT from chronic ITP with 91.3% [72.0; 98.9] sensitivity and PPV = 100.0% [66.4; 100.0] specificity. The secondary objective of this study was to assess the prevalence of potential IT diagnosis in patients with chronic thrombocytopenia of uncertain origin. Applying our diagnostic approach to a series of 20 cases allowed us to estimate that 40% of them could be suffering from IT. Finally, our diagnostic approach may help to correctly distinguish IT from chronic ITP, particularly in the context of macrothrombocytopenia.