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 αIIbß3, an integrin encoded by the ITGA2B and ITGB3 genes. On activation αIIbß3 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 αIIbß3, 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.

Platelet glycoprotein VI promotes metastasis through interaction with cancer cell-derived galectin-3.

Increasing evidence suggests that platelets play a predominant role in colon and breast cancer metastasis, but the underlying molecular mechanisms remain elusive. Glycoprotein VI (GPVI) is a platelet-specific receptor for collagen and fibrin that triggers platelet activation through immunoreceptor tyrosine-based activation motif (ITAM) signaling and thereby regulates diverse functions, including platelet adhesion, aggregation, and procoagulant activity. GPVI has been proposed as a safe antithrombotic target, because its inhibition is protective in models of arterial thrombosis, with only minor effects on hemostasis. In this study, the genetic deficiency of platelet GPVI in mice decreased experimental and spontaneous metastasis of colon and breast cancer cells. Similar results were obtained with mice lacking the spleen-tyrosine kinase Syk in platelets, an essential component of the ITAM-signaling cascade. In vitro and in vivo analyses supported that mouse, as well as human GPVI, had platelet adhesion to colon and breast cancer cells. Using a CRISPR/Cas9-based gene knockout approach, we identified galectin-3 as the major counterreceptor of GPVI on tumor cells. In vivo studies demonstrated that the interplay between platelet GPVI and tumor cell-expressed galectin-3 uses ITAM-signaling components in platelets and favors the extravasation of tumor cells. Finally, we showed that JAQ1 F(ab')2-mediated inhibition of GPVI efficiently impairs platelet-tumor cell interaction and tumor metastasis. Our study revealed a new mechanism by which platelets promote the metastasis of colon and breast cancer cells and suggests that GPVI represents a promising target for antimetastatic therapies.

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/FcRγ associate 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.

A mutation of the human EPHB2 gene leads to a major platelet functional defect.

The ephrin transmembrane receptor family of tyrosine kinases is involved in platelet function. We report the first EPHB2 variant affecting platelets in 2 siblings (P1 and P2) from a consanguineous family with recurrent bleeding and normal platelet counts. Whole-exome sequencing identified a c.2233C>T variant (missense p.R745C) of the EPHB2 gene. P1 and P2 were homozygous for this variant, while their asymptomatic parents were heterozygous. The p.R745C variant within the tyrosine kinase domain was associated with defects in platelet aggregation, αIIbß3 activation, and granule secretion induced by G-protein-coupled receptor (GPCR) agonists and convulxin, as well as in thrombus formation on collagen under flow. In contrast, clot retraction, flow-dependent platelet adhesion, and spreading on fibrinogen were only mildly affected, indicating limited effects on αIIbß3 outside-in signaling. Most importantly, Lyn, Syk, and FcRγ phosphorylation, the initial steps in glycoprotein VI (GPVI) platelet signaling were drastically impaired in the absence of platelet-platelet contact, indicating a positive role for EPHB2 in GPVI activation. Likewise platelet activation by PAR4-AP showed defective Src activation, as opposed to normal protein kinase C activity and Ca2+ mobilization. Overexpression of wild-type and R745C EPHB2 variant in RBL-2H3 (rat basophilic leukemia) cells stably expressing human GPVI confirmed that EPHB2 R745C mutation impaired EPHB2 autophosphorylation but had no effect on ephrin ligand-induced EPHB2 clustering, suggesting it did not interfere with EPHB2-ephrin-mediated cell-to-cell contact. In conclusion, this novel inherited platelet disorder affecting EPHB2 demonstrates this tyrosine kinase receptor plays an important role in platelet function through crosstalk with GPVI and GPCR signaling.

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 gain-of-function variant in DIAPH1 causes dominant macrothrombocytopenia and hearing loss.

Macrothrombocytopenia (MTP) is a heterogeneous group of disorders characterized by enlarged and reduced numbers of circulating platelets, sometimes resulting in abnormal bleeding. In most MTP, this phenotype arises because of altered regulation of platelet formation from megakaryocytes (MKs). We report the identification of DIAPH1, which encodes the Rho-effector diaphanous-related formin 1 (DIAPH1), as a candidate gene for MTP using exome sequencing, ontological phenotyping, and similarity regression. We describe 2 unrelated pedigrees with MTP and sensorineural hearing loss that segregate with a DIAPH1 R1213* variant predicting partial truncation of the DIAPH1 diaphanous autoregulatory domain. The R1213* variant was linked to reduced proplatelet formation from cultured MKs, cell clustering, and abnormal cortical filamentous actin. Similarly, in platelets, there was increased filamentous actin and stable microtubules, indicating constitutive activation of DIAPH1. Overexpression of DIAPH1 R1213* in cells reproduced the cytoskeletal alterations found in platelets. Our description of a novel disorder of platelet formation and hearing loss extends the repertoire of DIAPH1-related disease and provides new insight into the autoregulation of DIAPH1 activity.

Personal reflections on the early contributions of Gus Born to platelet research.

Professor GVR Born, Gus to his friends, was one of the great pioneers of platelet research. My early memories of him have enabled me to look back at his early years in Oxford and London. A brilliant and generous man with always the time to discuss and advise he was instrumental in deciphering the principle stages of the aggregation of blood platelets by ADP, a path aided by his development and use of the platelet aggregometer. He applied his knowledge to the real time analysis of platelet and leukocyte involvement in thrombus formation in animal models and to the development of atherosclerosis and thrombosis and their pharmacological inhibition. What follows is a personal account of the major steps in this early work and of the actors involved.

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 αIIbß3 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 αIIbß3 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.

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.

Linkage disequilibrium amongst ITGA2B and ITGB3 gene variants in patients with Glanzmann thrombasthenia confirms that most disease-causing mutations are recent.

We recently reported mutation analysis of the largest cohort of Glanzmann thrombasthenia (GT) patients so far examined. Sanger sequencing of coding regions, splice sites, upstream and downstream regions of the ITGA2B and ITGB3 genes identified 78 causal genetic variants (55 novel); 4 large deletions or duplications were also detected. We have now analysed the expression of non-causal gene polymorphisms in the sequenced regions of both genes in selected members of this cohort. We identified 10 mostly silent variants in ITGA2B and 37 in ITGB3; all were present in control donor databases. Three non-synonymous single nucleotide polymorphisms present were human platelet alloantigen (HPA) variants. A series of haplogroups, often including HPA-3b in ITGA2B, repeated with little variation across unrelated families of wide geographical origins and with different GT-causing mutations whether in ITGA2B or ITGB3. In contrast, a deleterious heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B shared a common ITGA2B haplogroup composed of at least five gene polymorphisms and re-occurred in seven European families with no known family relationships. Our results highlight the value of gene polymorphism analysis in GT and are consistent with the bulk of disease-causing mutations in GT being of recent origin.

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.

Expanding the Mutation Spectrum Affecting αIIbß3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort.

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbß3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbß3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbß3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and ß3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Procoagulant platelets form an α-granule protein-covered "cap" on their surface that promotes their attachment to aggregates.

Strongly activated "coated" platelets are characterized by increased phosphatidylserine (PS) surface expression, α-granule protein retention, and lack of active integrin αIIbß3. To study how they are incorporated into thrombi despite a lack of free activated integrin, we investigated the structure, function, and formation of the α-granule protein "coat." Confocal microscopy revealed that fibrin(ogen) and thrombospondin colocalized as "cap," a single patch on the PS-positive platelet surface. In aggregates, the cap was located at the point of attachment of the PS-positive platelets. Without fibrin(ogen) retention, their ability to be incorporated in aggregates was drastically reduced. The surface fibrin(ogen) was strongly decreased in the presence of a fibrin polymerization inhibitor GPRP and also in platelets from a patient with dysfibrinogenemia and a fibrinogen polymerization defect. In contrast, a fibrinogen-clotting protease ancistron increased the amount of fibrin(ogen) and thrombospondin on the surface of the PS-positive platelets stimulated with collagen-related peptide. Transglutaminases are also involved in fibrin(ogen) retention. However, platelets from patients with factor XIII deficiency had normal retention, and a pan-transglutaminase inhibitor T101 had only a modest inhibitory effect. Fibrin(ogen) retention was normal in Bernard-Soulier syndrome and kindlin-3 deficiency, but not in Glanzmann thrombasthenia lacking the platelet pool of fibrinogen and αIIbß3. These data show that the fibrin(ogen)-covered cap, predominantly formed as a result of fibrin polymerization, is a critical mechanism that allows coated (or rather "capped") platelets to become incorporated into thrombi despite their lack of active integrins.

Congenital platelet disorders and understanding of platelet function.

Genetic defects of platelets constitute rare diseases that include bleeding syndromes of autosomal dominant, recessive or X-linked inheritance. They affect platelet production, resulting in a low circulating platelet count and changes in platelet morphology, platelet function, or a combination of both with altered megakaryopoiesis and a defective platelet response. As a result, blood platelets fail to fulfil their haemostatic function. Most studied of the platelet function disorders are deficiencies of glycoprotein mediators of adhesion and aggregation while defects of primary receptors for stimuli include the P2Y12 ADP receptor. Studies on inherited defects of (i) secretion from storage organelles (dense granules, α-granules), (ii) the platelet cytoskeleton and (iii) the generation of pro-coagulant activity have identified genes indirectly controlling the functional response. Signalling pathway defects leading to agonist-specific modifications of platelet aggregation are the current target of exome-sequencing strategies. We now review recent advances in the molecular characterization of platelet function defects.

Platelet membrane glycoproteins: a historical review.

The search for the components of the platelet surface that mediate platelet adhesion and platelet aggregation began for earnest in the late 1960s when electron microscopy demonstrated the presence of a carbohydrate-rich, negatively charged outer coat that was called the "glycocalyx." Progressively, electrophoretic procedures were developed that identified the major membrane glycoproteins (GP) that constitute this layer. Studies on inherited disorders of platelets then permitted the designation of the major effectors of platelet function. This began with the discovery in Paris that platelets of patients with Glanzmann thrombasthenia, an inherited disorder of platelet aggregation, lacked two major GP. Subsequent studies established the role for the GPIIb-IIIa complex (now known as integrin αIIbß3) in binding fibrinogen and other adhesive proteins on activated platelets and the formation of the protein bridges that join platelets together in the platelet aggregate. This was quickly followed by the observation that platelets of patients with the Bernard-Soulier syndrome, with macrothrombocytopenia and a distinct disorder of platelet adhesion, lacked the carbohydrate-rich, negatively charged, GPIb. It was shown that GPIb, through its interaction with von Willebrand factor, mediated platelet attachment to injured sites in the vessel wall. What follows is a personal reflection on the studies that were performed in the early pioneering days.

Glanzmann thrombasthenia: state of the art and future directions.

Glanzmann thrombasthenia (GT) is the principal inherited disease of platelets and the most commonly encountered disorder of an integrin. GT is characterized by spontaneous mucocutaneous bleeding and an exaggerated response to trauma caused by platelets that fail to aggregate when stimulated by physiologic agonists. GT is caused by quantitative or qualitative deficiencies of αIIbß3, an integrin coded by the ITGA2B and ITGB3 genes and which by binding fibrinogen and other adhesive proteins joins platelets together in the aggregate. Widespread genotyping has revealed that mutations spread across both genes, yet the reason for the extensive variation in both the severity and intensity of bleeding between affected individuals remains poorly understood. Furthermore, although genetic defects of ITGB3 affect other tissues with ß3 present as αvß3 (the vitronectin receptor), the bleeding phenotype continues to dominate. Here, we look in detail at mutations that affect (i) the ß-propeller region of the αIIb head domain and (ii) the membrane proximal disulfide-rich epidermal growth factor (EGF) domains of ß3 and which often result in spontaneous integrin activation. We also examine deep vein thrombosis as an unexpected complication of GT and look at curative procedures for the diseases, including allogeneic stem cell transfer and the potential for gene therapy.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models.

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbß3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for ß3 has a wide tissue distribution when present as αvß3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of ß3-deficiency or nonfunction are being increasingly described.