Dominant inheritance of a novel integrin beta3 mutation associated with a hereditary macrothrombocytopenia and platelet dysfunction in two Italian families

Gestational diabetes mellitus is associated with in vivo platelet activation and platelet hyperreactivity

BACKGROUND

Gestational diabetes mellitus is associated with obstetrical and long-term cardiovascular complications. Although platelet hyperresponsiveness in type-2 diabetes mellitus has been well characterized and has been shown to play a crucial role in cardiovascular complications, this aspect has been little studied in gestational diabetes mellitus.

OBJECTIVE

We aimed to evaluate platelet reactivity, in vivo platelet activation, and endothelial function in gestational diabetes mellitus in comparison with normal pregnancy.

STUDY DESIGN

This was a prospective, case-control study of 23 women with gestational diabetes mellitus and 23 healthy pregnant women who were studied at 26 to 28 and 34 to 36 weeks of gestation and at 8 weeks postpartum. Platelet reactivity and in vivo platelet activation, including light transmission aggregometry, PFA-100, platelet activation antigen expression, platelet adhesion under flow, platelet nitric oxide and reactive oxygen species production, and endothelial dysfunction markers, were assessed.

RESULTS

The study of platelet function showed a condition of platelet hyperreactivity in cases with gestational diabetes mellitus when compared with healthy pregnant women at enrollment, which was further enhanced at the end of pregnancy and tended to decrease 2 months after delivery, although it still remained higher in gestational diabetes mellitus. In vivo platelet activation was also evident in gestational diabetes mellitus, especially at the end of pregnancy, in part persisting up to 8 weeks after delivery. Finally, women with gestational diabetes mellitus showed defective platelet nitric oxide production and endothelial dysfunction when compared with healthy pregnancies.

CONCLUSION

Our data showed that gestational diabetes mellitus generates a condition of platelet hyperreactivity that in part persists up to 2 months after delivery. Impaired platelet sensitivity to nitric oxide and reduced platelet and endothelial nitric oxide production may contribute to the platelet hyperreactivity condition. Platelet hyperreactivity may play a role in the long-term cardiovascular complications of gestational diabetes mellitus women.

Talin variant P229S compromises integrin activation and associates with multifaceted clinical symptoms

Adhesion of cells to the extracellular matrix (ECM) must be exquisitely coordinated to enable development and tissue homeostasis. Cell-ECM interactions are regulated by multiple signalling pathways that coordinate the activation state of the integrin family of ECM receptors. The protein talin is pivotal in this process, and talin's simultaneous interactions with the cytoplasmic tails of the integrins and the plasma membrane are essential to enable robust, dynamic control of integrin activation and cell-ECM adhesion. Here, we report the identification of a de novo heterozygous c.685C>T (p.Pro229Ser) variant in the TLN1 gene from a patient with a complex phenotype. The mutation is located in the talin head region at the interface between the F2 and F3 domains. The characterization of this novel p.P229S talin variant reveals the disruption of adhesion dynamics that result from disturbance of the F2-F3 domain interface in the talin head. Using biophysical, computational and cell biological techniques, we find that the variant perturbs the synergy between the integrin-binding F3 and the membrane-binding F2 domains, compromising integrin activation, adhesion and cell migration. Whilst this remains a variant of uncertain significance, it is probable that the dysregulation of adhesion dynamics we observe in cells contributes to the multifaceted clinical symptoms of the patient and may provide insight into the multitude of cellular processes dependent on talin-mediated adhesion dynamics.

Genetics of inherited thrombocytopenias

The inherited thrombocytopenia syndromes are a group of disorders characterized primarily by quantitative defects in platelet number, though with a variety demonstrating qualitative defects and/or extrahematopoietic findings. Through collaborative international efforts applying next-generation sequencing approaches, the list of genetic syndromes that cause thrombocytopenia has expanded significantly in recent years, now with over 40 genes implicated. In this review, we focus on what is known about the genetic etiology of inherited thrombocytopenia syndromes and how the field has worked to validate new genetic discoveries. We highlight the important role for the clinician in identifying a germline genetic diagnosis and strategies for identifying novel causes through research-based endeavors.

A p.Arg127Gln variant in GPIbα LRR5 allosterically enhances affinity for VWF: a novel form of platelet-type VWD

We identified a novel GP1BA variant in the LRR5 domain of GPIbα (p.Arg127Gln) in a patient with a mild PT-VWD phenotype.

GOF variants in the LRR of GPIbα alter the dynamics of the C-terminal disulfide loop generating a conformation with high affinity for VWF.

Gain-of-function (GOF) variants in GP1BA cause platelet-type von Willebrand disease (PT-VWD), a rare inherited autosomal dominant bleeding disorder characterized by enhanced platelet GPIbα to von Willebrand factor (VWF) interaction, and thrombocytopenia. To date, only 6 variants causing PT-VWD have been described, 5 in the C-terminal disulfide loop of the VWF-binding domain of GPIbα and 1 in the macroglycopeptide. GOF GP1BA variants generate a high-affinity conformation of the C-terminal disulfide loop with a consequent allosteric conformational change on another region of GPIbα, the leucine-rich-repeat (LRR) domain. We identified a novel GP1BA variant (p.Arg127Gln) affecting the LRR5 domain of GPIbα in a boy with easy bruising and laboratory test results suggestive of PT-VWD. We thus aimed to investigate the impact of the p.Arg127Gln variant on GPIbα affinity for VWF and GPIbα structure. Chinese hamster ovary cells expressing p.Arg127Gln GPIbα showed increased binding of VWF induced by ristocetin and enhanced tethering on immobilized VWF as compared with cells expressing wild-type GPIbα. Surface plasmon resonance confirmed that p.Arg127Gln enhances the binding affinity of GPIbα for VWF. Hydrogen‐deuterium exchange mass spectrometry showed that p.Arg127Gln of LRR, while having little effect on the dynamics of the LRR locally, enhances the conformational dynamics of the GPIbα C-terminal disulfide loop structure. Our data demonstrate for the first time that GOF variants outside the GPIbα C-terminal disulfide loop may be pathogenic and that aminoacidic changes in the LRR may cause allosterically conformational changes in the C-terminal disulfide loop of GPIbα, inducing a conformation with high affinity for VWF.

Advances in understanding the pathogenesis of hereditary macrothrombocytopenia

Low platelet count, or thrombocytopenia, is a common haematological abnormality, with a wide differential diagnosis, which may represent a clinically significant underlying pathology. Macrothrombocytopenia, the presence of large platelets in combination with thrombocytopenia, can be acquired or hereditary and indicative of a complex disorder. In this review, we discuss the interpretation of platelet count and volume measured by automated haematology analysers and highlight some important technical considerations relevant to the analysis of blood samples with macrothrombocytopenia. We review how large cohorts, such as the UK Biobank and INTERVAL studies, have enabled an accurate description of the distribution and co-variation of platelet parameters in adult populations. We discuss how genome-wide association studies have identified hundreds of genetic associations with platelet count and mean platelet volume, which in aggregate can explain large fractions of phenotypic variance, consistent with a complex genetic architecture and polygenic inheritance. Finally, we describe the large genetic diagnostic and discovery programmes, which, simultaneously to genome-wide association studies, have expanded the repertoire of genes and variants associated with extreme platelet phenotypes. These have advanced our understanding of the pathogenesis of hereditary macrothrombocytopenia and support a future clinical diagnostic strategy that utilises genotype alongside clinical and laboratory phenotype data.

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

Glanzmann thrombasthenia (GT) is the most widely studied inherited disease of platelet function. Platelets fail to aggregate due to a defect in platelet-to-platelet attachment. The hemostatic plug fails to form and a moderate to severe bleeding diathesis results. Classically of autosomal recessive inheritance, GT is caused by defects within the ITGA2B and ITGB3 genes that encode the αIIbβ3 integrin expressed at high density on the platelet surface and also in intracellular pools. Activated αIIbβ3 acts as a receptor for fibrinogen and other adhesive proteins that hold platelets together in a thrombus. Over 50 years of careful clinical and biological investigation have provided important advances that have improved not only the quality of life of the patients but which have also contributed to an understanding of how αIIbβ3 functions. Despite major improvements in our knowledge of GT and its genetic causes, extensive biological and clinical variability with respect to the severity and intensity of bleeding remains poorly understood. I now scan the repertoire of ITGA2B and ITGB3 gene defects and highlight the wide genetic and biological heterogeneity within the type II and variant subgroups especially with regard to bleeding, clot retraction, the internal platelet Fg storage pool and the nature of the mutations causing the disease. I underline the continued importance of gene profiling and biological studies and emphasize the multifactorial etiology of the clinical expression of the disease. This is done in a manner to provide guidelines for future studies and future treatments of a disease that has not only aided research on rare diseases but also contributed to advances in antithrombotic therapy.

Glanzmann Thrombasthenia: Perspectives from Clinical Practice on Accurate Diagnosis and Optimal Treatment Strategies

Glanzmann thrombasthenia (GT) is a rare autosomal recessive disorder of fibrinogen-mediated platelet aggregation due to a quantitative or qualitative deficit of the α_IIbβ₃ integrin at the platelet surface membrane resulting from mutation(s) in ITGA2B and/or ITGB3. Patients tend to present in early childhood with easy bruising and mucocutaneous bleeding. The diagnostic process requires consideration of more common disorders of haemostasis and coagulation prior to confirming the disorder with platelet light transmission aggregation, flow cytometry of CD41 and CD61 expression, and/or exon sequencing of ITGA2B and ITGB3. Antifibrinolytic therapy, recombinant activated factor VII, and platelet transfusions are the mainstay of therapy, although the latter may trigger formation of anti-platelet antibodies in GT patients and inadvertent platelet-refractory disease. The management of these patients therefore remains complex, particularly in the context of trauma, labour and delivery, and perioperative care. Bone marrow transplantation remains the sole curative option, although the venue of gene therapy is being increasingly explored as a future alternative for definitive treatment of GT.

Learning the Ropes of Platelet Count Regulation: Inherited Thrombocytopenias

Inherited thrombocytopenias (IT) are a group of hereditary disorders characterized by a reduced platelet count sometimes associated with abnormal platelet function, which can lead to bleeding but also to syndromic manifestations and predispositions to other disorders. Currently at least 41 disorders caused by mutations in 42 different genes have been described. The pathogenic mechanisms of many forms of IT have been identified as well as the gene variants implicated in megakaryocyte maturation or platelet formation and clearance, while for several of them the pathogenic mechanism is still unknown. A range of therapeutic approaches are now available to improve survival and quality of life of patients with IT; it is thus important to recognize an IT and establish a precise diagnosis. ITs may be difficult to diagnose and an initial accurate clinical evaluation is mandatory. A combination of clinical and traditional laboratory approaches together with advanced sequencing techniques provide the highest rate of diagnostic success. Despite advancement in the diagnosis of IT, around 50% of patients still do not receive a diagnosis, therefore further research in the field of ITs is warranted to further improve patient care.

αIIbβ3 variants in ten families with autosomal dominant macrothrombocytopenia: Expanding the mutational and clinical spectrum

Background

Rare pathogenic variants in either the ITGA2B or ITGB3 genes have been linked to autosomal dominant macrothrombocytopenia associated with abnormal platelet production and function, deserving the designation of Glanzmann Thrombasthenia-Like Syndrome (GTLS) or ITGA2B/ITGB3-related thrombocytopenia.

Objectives

To describe a series of patients with familial macrothrombocytopenia and decreased expression of αIIbβ3 integrin due to defects in the ITGA2B or ITGB3 genes.

Methods

We reviewed the clinical and laboratory records of 10 Portuguese families with GTLS (33 patients and 11 unaffected relatives), including the functional and genetic defects.

Results

Patients had absent to moderate bleeding, macrothrombocytopenia, low αIIbβ3 expression, impaired platelet aggregation/ATP release to physiological agonists and low expression of activation-induced binding sites on αIIbβ3 (PAC-1) and receptor-induced binding sites on its ligand (bound fibrinogen), upon stimulation with TRAP-6 and ADP. Evidence for constitutive αIIbβ3 activation, occurred in 2 out of 9 patients from 8 families studied, but also in 2 out of 12 healthy controls. We identified 7 missense variants: 3 in ITGA2B (5 families), and 4 in ITGB3 (5 families). Three variants (αIIb: p.Arg1026Trp and p.Arg1026Gln and β3: p.Asp749His) were previously reported. The remaining (αIIb: p.Gly1007Val and β3: p.Thr746Pro, p.His748Pro and p.Arg760Cys) are new, expanding the αIIbβ3 defects associated with GTLS. The integration of the clinical and laboratory data allowed the identification of two GTLS subgroups, with distinct disease severity.

Conclusions

Previously reported ITGA2B and ITGB3 variants related to thrombocytopenia were clustered in a confined region of the membrane-proximal cytoplasmic domains, the inner membrane clasp. For the first time, variants are reported at the outer membrane clasp, at the transmembrane domain of αIIb, and at the membrane distal cytoplasmic domains of β3. This is the largest single-center series of inherited macrothrombocytopenia associated with αIIbβ3 variants published to date.

Pathogenic and likely pathogenic variants in at least five genes account for approximately 3% of mild isolated nonsyndromic thrombocytopenia

BACKGROUND

Thrombocytopenia has a variety of different etiologies, both acquired and hereditary. Inherited thrombocytopenia may be associated with other symptoms (syndromic forms) or may be strictly isolated. To date, only about half of all the familial forms of thrombocytopenia have been accounted for in terms of well-defined genetic abnormalities. However, data are limited on the nature and frequency of the underlying causative genetic variants in individuals with mild isolated nonsyndromic thrombocytopenia.

STUDY DESIGN AND METHODS

Thirteen known or candidate genes for isolated thrombocytopenia were included in a gene panel analysis in which targeted next-generation sequencing was performed on 448 French blood donors with mild isolated nonsyndromic thrombocytopenia.

RESULTS

A total of 68 rare variants, including missense, splice site, frameshift, nonsense, and in-frame variants (all heterozygous) were identified in 11 of the 13 genes screened. Twenty-nine percent (N = 20) of the variants detected were absent from both the French Exome Project and gnomAD exome databases. Using stringent criteria and an unbiased approach, we classified seven predicted loss-of-function variants (three in ITGA2B and four in TUBB1) and four missense variants (one in GP1BA, two in ITGB3 and one in ACTN1) as being pathogenic or likely pathogenic. Altogether, they were found in 13 members (approx. 3%) of our studied cohort.

CONCLUSION

We present the results of gene panel sequencing of known and candidate thrombocytopenia genes in mild isolated nonsyndromic thrombocytopenia. Pathogenic and likely pathogenic variants in five known thrombocytopenia genes were identified, accounting for approximately 3% of individuals with the condition.

Platelet immunophenotyping in health and inherited bleeding disorders, a review and practical hints

Inherited platelet function disorders are rare hemorrhagic diseases. The gold standard for their exploration is optical aggregometry; however, investigations by flow cytometry (FCM) are being increasingly used. In this review, the physiology of platelets is first recalled, setting the stage for the compartments of platelets that can be apprehended by specific and appropriate labeling. As this requires some pre-analytical precautions and specific analytical settings, a second part focuses on these characteristic aspects, based on literature and on the authors' experience in the field, for qualitative or quantitative explorations. Membrane labeling with antibodies to CD42a or CD41, respectively, useful to assess the genetic-related defects of Glanzmann thrombocytopenia and Bernard Soulier syndrome are then described. Platelet degranulation disorders are detailed in the next section, as they can be explored, upon platelet activation, by measuring the expression of surface P-Selectin (CD62P) or CD63. Mepacrin uptake and release after activation is another test allowing to explore the function of dense granules. Finally, the flip-flop anomaly related to Scott syndrome is depicted. Tables summarizing possible FCM assays, and characteristic histograms are provided as reference for flow laboratories interested in developing platelet exploration.

Knock-in mice bearing constitutively active αIIb(R990W) mutation develop macrothrombocytopenia with severe platelet dysfunction

BACKGROUND

To date, several mutations that induce constitutive activation of integrin αIIbβ3 have been identified in congenital macrothrombocytopenia. Of these, αIIb(R995W) is the most prevalent mutation observed in Japanese patients with αIIbβ3-related congenital macrothrombocytopenia.

OBJECTIVE AND METHODS

The present study aimed to explore the effects of constitutive activation of the αIIb(R995W) mutation on platelet production, morphology, and function. We generated αIIb(R990W) knock-in (KI) mice corresponding to human αIIb(R995W).

RESULTS

Platelet counts of heterozygous (hetero) and homozygous (homo) KI mice were decreased by ~10% and ~25% relative to those of wild-type (WT) mice, respectively, with increase in platelet size. Decrease in absolute reticulated platelet numbers in steady state, delayed recovery from thrombocytopenia induced by anti-platelet antibody and impaired response to exogenous thrombopoietin administration suggested impaired platelet production in KI mice. WT and KI mice showed no significant differences in the number of megakaryocytes and ploidy of megakaryocytes, whereas proplatelet formation was significantly impaired in homo mice. We observed a slight but significant reduction in platelet lifespan in homo mice. The homo mice showed dramatic reduction in αIIbβ3 expression in platelets, which was accompanied by severe in vivo and in vitro platelet dysfunction.

CONCLUSION

The αIIb(R990W) KI mice developed macrothrombocytopenia, which was primarily attributed to impaired proplatelet formation. In addition, homo KI mice showed marked downregulation in αIIbβ3 expression in platelets with severe impaired platelet function, similar to Glanzmann thrombasthenia.

Eltrombopag for the treatment of inherited thrombocytopenias: a phase II clinical trial

Patients with inherited thrombocytopenias often require platelet transfusions to raise their platelet count before surgery or other invasive procedures; moreover, subjects with clinically significant spontaneous bleeding may benefit from an enduring improvement of thrombocytopenia. The hypothesis that thrombopoietin-mimetics can increase platelet count in inherited thrombocytopenias is appealing, but evidence is scarce. We conducted a prospective, phase II clinical trial to investigate the efficacy of the oral thrombopoietin-mimetic eltrombopag in different forms of inherited thrombocytopenia. We enrolled 24 patients affected by MYH9-related disease, ANKRD26-related thrombocytopenia, X-linked thrombocytopenia/ Wiskott-Aldrich syndrome, monoallelic Bernard-Soulier syndrome, or ITGB3-related thrombocytopenia. The average pre-treatment platelet count was 40.4 ×10⁹/L. Patients received a 3- to 6-week course of eltrombopag in a dose-escalated manner. Of 23 patients evaluable for response, 11 (47.8%) achieved a major response (platelet count >100 ×10⁹/L), ten (43.5%) had a minor response (platelet count at least twice the baseline value), and two patients (8.7%) did not respond. The average increase of platelet count compared to baseline was 64.5 ×10⁹/L (P<0.001). Four patients with clinically significant spontaneous bleeding entered a program of long-term eltrombopag administration (16 additional weeks): all of them obtained remission of mucosal hemorrhages, with the remission persisting throughout the treatment period. Treatment was globally well tolerated: five patients reported mild adverse events and one patient a moderate adverse event. In conclusion, eltrombopag was safe and effective in increasing platelet count and reducing bleeding symptoms in different forms of inherited thrombocytopenia. Despite these encouraging results, caution is recommended when using thrombopoietinmimetics in inherited thrombocytopenias predisposing to leukemia. ClinicalTrials.gov identifier: NCT02422394.

Genome-wide linkage analysis and whole-exome sequencing identifies an ITGA2B mutation in a family with thrombocytopenia

Hereditary thrombocytopenias can be subclassified based on mode of inheritance and platelet size. Here we report a family with autosomal dominant (AD) thrombocytopenia with normal platelet size. Linkage analysis and whole exome sequencing identified the R1026W substitution in ITGA2B as the causative defect. The same mutation has been previously reported in 7 Japanese families/patients with AD thrombocytopenia, but all of these patients had macrothrombocytopenia. This is the first report of a family with AD thrombocytopenia with normal platelet size resulting from mutation in ITGA2B. ITGA2B mutations should therefore be included in the differential diagnosis of this latter disorder.

Newly Identified HNP-F from Human Neutrophil Peptide-1 Promotes Hemostasis

Active hemostatic agents can play a crucial role in saving patients' lives during surgery. Active hemostats have several advantages including utilization of natural blood coagulation and biocompatibility. Among them, although human neutrophil peptide-1 (HNP-1) has been previously reported with the hemostatic mechanism, which part of HNP-1 facilitates the hemostatic activity is not known. Here, a partial peptide (HNP-F) promoting hemostasis, originating from HNP-1, has been newly identified by the blood coagulation ability test. HNP-F shows the best hemostatic effect between the anterior half and posterior half of peptides. Moreover, microscopic images show platelet aggregation and an increase in the concentration of platelet factor 4, and the scanning electron microscope image of platelets support platelet activation by HNP-F. Thromboelastography indicates decreased clotting time and increased physical properties of blood clotting. Mouse liver experiments demonstrate improved hemostatic effect by treatment of peptide solution. Cell viability and hemolysis assays confirm the HNP-F's biosafety. It is hypothesized that the surface charge and structure of HNP-F could be favorable to interact with fibrinogen or thrombospondin-1. Collectively, because HNP-F as an active peptide hemostat has many advantages, it could be expected to become a potent hemostatic biomaterial, additive or pharmaceutical candidate for various hemostatic applications.

Challenges on the diagnostic approach of inherited platelet function disorders: Is a paradigm change necessary?

Inherited platelet function disorders (IPFD) have been assessed for more than 50 years by aggregation- and secretion-based tests. Several decision trees are available intending to standardize the investigation of IPFD. A large variability of approaches is still in use among the laboratories across the world. In spite of costly and lengthy laboratory evaluation, the results have been found inconclusive or negative in a significant part of patients having bleeding manifestations. Molecular investigation of newly identified IPFD has recently contributed to a better understanding of the complexity of platelet function. Once considered "classic" IPFDs, Glanzmann thrombasthenia and Bernard-Soulier syndrome have each had their pathophysiology reassessed and their diagnosis made more precise and informative. Megakaryopoiesis, platelet formation, and function have been found tightly interlinked, with several genes being involved in both inherited thrombocytopenias and impaired platelet function. Moreover, genetic approaches have moved from being used as confirmatory diagnostic tests to being tools for identification of genetic variants associated with bleeding disorders, even in the absence of a clear phenotype in functional testing. In this study, we aim to address some limits of the conventional tests used for the diagnosis of IPFD, and to highlight the potential contribution of recent molecular tools and opportunities to rethink the way we should approach the investigation of IPFD.

Molecular basis of inherited thrombocytopenias: an update

PURPOSE OF REVIEW

Inherited thrombocytopenias are a heterogeneous group of diseases caused by mutations in many genes. They account for approximately only 50% of cases, suggesting that novel genes have yet to be identified for a comprehensive understanding of platelet biogenesis defects. This review provides an update of the last year of discoveries on inherited thrombocytopenias focusing on the molecular basis and potential pathogenic mechanisms affecting megakaryopoiesis and platelet production.

RECENT FINDINGS

Most of the novel discoveries are related to identification of mutations in novel inherited thrombocytopenia genes using a next-generation sequencing approach. They include MECOM, DIAPH1, TRPM7, SRC, FYB, and SLFN14, playing different roles in megakaryopoiesis and platelet production. Moreover, it is worth mentioning data on hypomorphic mutations of FLI1 and the association of single nucleotide polymorphisms, such as that identified in ACTN1, with thrombocytopenia.

SUMMARY

Thanks to the application of next-generation sequencing, the number of inherited thrombocytopenia genes is going to increase rapidly. Considering the wide genetic heterogeneity (more than 30 genes), these technologies can also be used for diagnostic purpose. Whatever is the aim, extreme caution should be taken in interpreting data, as inherited thrombocytopenias are mainly autosomal dominant diseases caused by variants of apparent unknown significance.

High-throughput sequencing approaches for diagnosing hereditary bleeding and platelet disorders

Hereditary bleeding and platelet disorders (BPDs) are characterized by marked genetic heterogeneity, far greater than previously appreciated. The list of genes involved in the regulation of megakaryopoiesis, platelet formation, platelet function and bleeding has been growing rapidly since the introduction of high-throughput sequencing (HTS) approaches in research. Thanks to the gradual adoption of HTS in diagnostic practice, these discoveries are improving the diagnostic yield for BPD patients, who may or may not present with bleeding problems and often have other clinical symptoms unrelated to the blood system. However, it was previously found that screening for all known etiologies gives a diagnostic yield of over 90% when the phenotype closely matches a known BPD but drops to 10% when the phenotype is indicative of a novel disorder. Thus, further research is needed to identify currently unknown etiologies for BPDs. Novel genes are likely to be found to be implicated in BPDs. New modes of inheritance, including digenic inheritance, are likely to play a role in some cases. Additionally, identifying and interpreting pathogenic variants outside exons is a looming challenge that can only be tackled with an improved understanding of the regulatory landscape of relevant cell types and with the transition from targeted sequencing to whole-genome sequencing in the clinic.

An intracytoplasmic β3 Leu718 deletion in a patient with a novel platelet phenotype

A novel heterozygous ITGB3 Leu718del shows loss of synchronization between the intracytoplasmic tail of β3 with that of αIIb.Decreased activation of αIIbβ3 accompanies enlarged platelets that contain giant granules and give a poor aggregation response.

[New perspectives on the role of αIIbβ3 integrin in defective megakaryopoiesis]

In recent years, the understanding of the molecular mechanisms involved in platelet production (megakaryopoiesis) has extremely increased, thanks to the study of genetic diseases causing inherited thrombocytopenia. Among the wide variety of transmembrane receptors covering the platelet membrane, αIIbβ3 integrin is the major one, allowing platelets to aggregate upon the occurrence of vascular breach. Platelet counts are usually normal in patients with αIIbβ3 deficiency, suggesting that its role for normal platelet production and morphology is very limited. However, recently, new clinical observations of genetic diseases provided evidence against this hypothesis, bringing new data on the role of αIIbβ3 integrin in defective megakaryopoiesis.

[Diagnosis of inherited thrombocytopenia]

Inherited thrombocytopenias are rare, heterogenous and probably under-diagnosed because often classified as autoimmune thrombocytopenia. About 20 genes were described responsible for these thrombocytopenias. Precise diagnosis is necessary because the prognosis is different and some of them can evolve into hemopathies. First of all, it is important to gather a body of evidence to orientate towards an inherited cause: presence of the thrombocytopenia since childhood and of other family cases is a strong argument. Secondly, it is difficult to target the genetic investigations that settle the precise diagnosis. Genetic variants responsible for inherited thrombocytopenias affect different stage during megakaryocytopoiesis and cause thrombocytopenias with distinct characteristics. Presence of extra-hematological features, platelets' size measurement and evaluation of bone marrow megakaryocyte morphology when it is possible allow a primary orientation. We propose a diagnostic approach considering extra-hematological features, mode of inheritance, morphology, molecular and functional platelets' studies and bone marrow megakaryocyte morphology in order to better target genetic study. Nevertheless, despite this approach, some inherited thrombocytopenias remain still unexplained and could benefit from new methods of new generation sequencing in the future.

Cytoskeletal perturbation leads to platelet dysfunction and thrombocytopenia in variant forms of Glanzmann thrombasthenia

Several patients have been reported to have variant dominant forms of Glanzmann thrombasthenia, associated with macrothrombocytopenia and caused by gain-of-function mutations of ITGB3 or ITGA2B leading to reduced surface expression and constitutive activation of integrin αIIbβ3. The mechanisms leading to a bleeding phenotype of these patients have never been addressed. The aim of this study was to unravel the mechanism by which ITGB3 mutations causing activation of αIIbβ3 lead to platelet dysfunction and macrothrombocytopenia. Using platelets from two patients carrying the β3 del647-686 mutation and Chinese hamster ovary cells expressing different αIIbβ3-activating mutations, we showed that reduced surface expression of αIIbβ3 is due to receptor internalization. Moreover, we demonstrated that permanent triggering of αIIbβ3-mediated outside-in signaling causes an impairment of cytoskeletal reorganization arresting actin turnover at the stage of polymerization. The induction of actin polymerization by jasplakinolide, a natural toxin that promotes actin nucleation and prevents depolymerization of stress fibers, in control platelets produced an impairment of platelet function similar to that of patients with variant forms of dominant Glanzmann thrombasthenia. del647-686β3-transduced murine megakaryocytes generated proplatelets with a reduced number of large tips and asymmetric barbell-proplatelets, suggesting that impaired cytoskeletal rearrangement is the cause of macrothrombocytopenia. These data show that impaired cytoskeletal remodeling caused by a constitutively activated αIIbβ3 is the main effector of platelet dysfunction and macrothrombocytopenia, and thus of bleeding, in variant forms of dominant Glanzmann thrombasthenia.

The Ras/Rap GTPase activating protein RASA3: from gene structure to in vivo functions

RASA3 (or GTPase Activating Protein III, R-Ras GTPase-activating protein, GAP1(IP4BP)) is a GTPase activating protein of the GAP1 subfamily which targets Ras and Rap1. RASA3 was originally purified from pig platelet membranes through its intrinsic ability to bind inositol 1,3,4,5-tetrakisphosphate (I(1,3,4,5)P4) with high affinity, hence its first name GAP1(IP4BP) (for GAP1 subfamily member which binds I(1,3,4,5)P4). RASA3 was thus the first I(1,3,4,5)P4 receptor identified and cloned. The in vitro and in vivo functions of RASA3 remained somewhat elusive for a long time. However, recently, using genetically-modified mice and cells derived from these mice, the function of RASA3 during megakaryopoiesis, megakaryocyte adhesion and migration as well as integrin signaling has been reported. The goal of this review is thus to summarize and comment recent and less recent data in the literature on RASA3, in particular on the in vivo function of this specific GAP1 subfamily member.

Diagnosis of suspected inherited platelet function disorders: results of a worldwide survey

BACKGROUND

Diagnosis of inherited platelet function disorders (IPFDs) is important for appropriate management and to improve epidemiologic and clinical knowledge. However, there remains a lack of consensus on the diagnostic approach.

OBJECTIVES

To gain knowledge on the current practices for the diagnosis of IPFD worldwide.

METHODS

A 67-item questionnaire was distributed to the ISTH members and to the members of several national hemostasis and thrombosis societies.

RESULTS

A total of 202 laboratories from 37 countries participated in the survey. The most frequent criterion to define patients with a suspected IPFD was a history of mucocutaneous bleeding and no acquired cause, but heterogeneity on the identification criteria was evident. Only 64.5% of respondents performed a direct clinical interview. On average, each laboratory studied 72 patients per year. The most commonly used laboratory equipment were the light-transmission aggregometer, the Platelet Function Analyzer-100, and the flow cytometer. Screening tests were platelet count, peripheral blood smear, light-transmission aggregometry, and Platelet Function Analyzer-100. Second-step tests were flow cytometry, molecular genetic analysis, and electron microscopy. Methodologies varied widely. In total, ~ 14,000 patients were investigated yearly and 60% turned out to not have a defect. Of the remaining 40%, only 8.7% received a diagnosis at a molecular level.

CONCLUSIONS

Many laboratories worldwide are involved in the diagnosis of IPFD. A large fraction of the patients studied remain without a diagnosis. A high variability in the diagnostic approaches is evident.

Rasa3 controls megakaryocyte Rap1 activation, integrin signaling and differentiation into proplatelet

Rasa3 is a GTPase activating protein of the GAP1 family which targets Ras and Rap1. Ubiquitous Rasa3 catalytic inactivation in mouse results in early embryonic lethality. Here, we show that Rasa3 catalytic inactivation in mouse hematopoietic cells results in a lethal syndrome characterized by severe defects during megakaryopoiesis, thrombocytopenia and a predisposition to develop preleukemia. The main objective of this study was to define the cellular and the molecular mechanisms of terminal megakaryopoiesis alterations. We found that Rasa3 catalytic inactivation altered megakaryocyte development, adherence, migration, actin cytoskeleton organization and differentiation into proplatelet forming megakaryocytes. These megakaryocyte alterations were associated with an increased active Rap1 level and a constitutive integrin activation. Thus, these mice presented a severe thrombocytopenia, bleeding and anemia associated with an increased percentage of megakaryocytes in the bone marrow, bone marrow fibrosis, extramedular hematopoiesis, splenomegaly and premature death. Altogether, our results indicate that Rasa3 catalytic activity controls Rap1 activation and integrin signaling during megakaryocyte differentiation in mouse.

Megakaryocytes are the bone marrow cellular precursors of circulating blood platelets and give rise to nascent platelets by forming branching filaments called proplatelets. Terminal differentiation of round megakaryocytes into branched proplatelet forming megakaryocytes is a complex cytoskeletal-driven process which is affected in rare human familial thrombocytopenias. Interactions of megakaryocytes with extracellular matrix proteins are essential in this process since constitutive megakaryocyte integrin activity caused by specific mutations in ITGA2B or ITGB3 genes encoding for extracellular matrix protein receptors may result in abnormal adherent megakaryocytes, defect in proplatelet formation and thrombocytopenia. Here, we show that Rasa3, a GTPase activating protein of the GAP1 family, controls Rap1 activation and integrin signaling during megakaryocyte differentiation. We found that Rasa3 catalytic inactivation in mice altered megakaryocyte development, adherence, migration, actin cytoskeleton organization and differentiation into proplatelet. Thus, these mice presented a severe thrombocytopenia, bleeding and anemia.

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 αIIbβ3, 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 β3 present as αvβ3 (the vitronectin receptor), the bleeding phenotype continues to dominate. Here, we look in detail at mutations that affect (i) the β-propeller region of the αIIb head domain and (ii) the membrane proximal disulfide-rich epidermal growth factor (EGF) domains of β3 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.

Platelet size for distinguishing between inherited thrombocytopenias and immune thrombocytopenia: a multicentric, real life study

The most frequent forms of inherited thrombocytopenia (IT) are characterized by platelet size abnormalities and it has been suggested that this parameter is useful for their differentiation from immune thrombocytopenia (ITP). Recently, a monocentric study identified cut-off values for mean platelet volume (MPV) and mean platelet diameter (MPD) with good diagnostic accuracy in this respect. To validate these cut-off values in a different and larger case series of patients, we enrolled 130 subjects with ITP and 113 with IT in six different centres. The platelet count and MPV was each measured by the instrument routinely used in each institution. In some centres, platelet count was also measured by optical microscopy. MPD was evaluated centrally by image analysis of peripheral blood films. The previously identified cut-off value for MPV had 91% specificity in distinguishing ITP from inherited macrothrombocytopenias (mono and biallelic Bernard-Soulier, MYH9-related disease), while its sensitivity was greatly variable depending on the instrument used. With an appropriate instrument, specificity was 83%. The diagnostic accuracy of MPD was lower than that obtained with MPV. We concluded that MPV is a useful parameter for differentiating ITP from IT provided that it is measured by appropriate cell counters.

Demonstration of novel gain-of-function mutations of αIIbβ3: association with macrothrombocytopenia and glanzmann thrombasthenia-like phenotype

Integrin αIIbβ3 is indispensable for normal hemostasis, but its role for thrombopoiesis is still controversial. Recently, αIIb and β3 mutations have been identified in patients with congenital macrothrombocytopenia. We analyzed three unrelated Japanese families with congenital macrothrombocytopenia. Expression and activation state of αIIbβ3 in platelets was examined by flow cytometry and immunoblotting. Sequence of whole coding region and exon-intron boundaries of ITGA2B and ITGB3 genes was performed. The effects of mutations on αIIbβ3 activation state and phosphorylation of FAK were analyzed in transfected cells. We newly identified three mutations: two mutations in highly conserved Gly-Phe-Phe-Lys-Arg sequence in juxtamembrane region of αIIb, p.Gly991Cys and p.Phe993del, and one donor site mutation of intron 13 of ITGB3 leading to 40 amino acids deletion, p.(Asp621_Glu660del), in the membrane proximal β-tail domain of β3. One patient, who showed Glanzmann thrombasthenia-like marked reduction in surface αIIbβ3 expression (3-11% of normal control), was a compound heterozygote with ITGA2B p.Gly991Cys and a novel nonsense mutation, ITGA2B p.Arg422*. All three mutations, ITGA2B p.Gly991Cys, ITGA2B p.Phe993del, and ITGB3 p.(Asp621_Glu660del), led to highly activated conformation of αIIbβ3 and spontaneous tyrosine phosphorylation of FAK in transfected cells. These results suggest that gain-of-function mutations around membrane region of αIIbβ3 lead to abnormal platelet number and morphology with impaired surface αIIbβ3 expression.

Inherited thrombocytopenias: the evolving spectrum

The chapter of inherited thrombocytopenias has expanded greatly over the last decade and many "new" forms deriving from mutations in "new" genes have been identified. Nevertheless, nearly half of patients remain without a definite diagnosis because their illnesses have not yet been described. The diagnostic approach to these diseases can still take advantage of the algorithm proposed by the Italian Platelet Study Group in 2003, although an update is required to include the recently described disorders. So far, transfusions of platelet concentrates have represented the main tool for preventing or treating bleedings, while haematopoietic stem cell transplantation has been reserved for patients with very severe forms. However, recent disclosure that an oral thrombopoietin mimetic is effective in increasing platelet count in patients with MYH9-related thrombocytopenia opened new therapeutic perspectives. This review summarizes the general aspects of inherited thrombocytopenias and describes in more detail MYH9-related diseases (encompassing four thrombocytopenias previously recognized as separate diseases) and the recently described ANKRD26-related thrombocytopenia, which are among the most frequent forms of inherited thrombocytopenia.

Does size matter in platelet production?

Platelet (PLT) production represents the final stage of megakaryocyte (MK) development. During differentiation, bone marrow MKs extend and release long, branched proPLTs into sinusoidal blood vessels, which undergo repeated abscissions to yield circulating PLTs. Circular-prePLTs are dynamic intermediate structures in this sequence that have the capacity to reversibly convert into barbell-proPLTs and may be related to "young PLTs" and "large PLTs" of both inherited and acquired macrothrombocytopenias. Conversion is regulated by the diameter and thickness of the peripheral microtubule coil, and PLTs are capable of enlarging in culture to generate barbell-proPLTs that divide to yield 2 smaller PLT products. Because PLT number and size are inversely proportional, this raises the question: do macrothrombocytopenias represent a failure in the intermediate stages of PLT production? This review aims to bring together and contextualize our current understanding of terminal PLT production against the backdrop of human macrothrombocytopenias to establish how "large PLTs" observed in both conditions are similar, how they are different, and what they can teach us about PLT formation. A better understanding of the cytoskeletal mechanisms that regulate PLT formation and determine PLT size offers the promise of improved therapies for clinical disorders of PLT production and an important source of PLTs for infusion.

Alteration of liver enzymes is a feature of the MYH9-related disease syndrome

Background

MYH9-related disease (MYH9-RD) is a rare autosomal dominant genetic syndrome characterized by congenital thrombocytopenia associated with the risk of developing progressive nephropathy, sensorineural deafness, and presenile cataract. During the collection of a large case-series of patients with MYH9-RD we noticed several cases with unexplained elevation of liver enzymes. Our aim was to evaluate if the alteration of liver tests is a feature of the MYH9-RD and to define its clinical significance.

Methods and Findings

Data concerning liver tests, prospectively recorded in the Italian Registry for MYH9-RD, were collected and compared with those of three control populations: patients with autoimmune thrombocytopenia, patients with inherited thrombocytopenias other than MYH9-RD, and the participants to a large epidemiologic survey in an Italian geographic isolate. Thirty-eight of 75 evaluable MYH9-RD patients (50.7%) showed an elevation of ALT and/or AST, and 17 of 63 (27.0%) an increase of GGT. The increases ranged from 1.9±0.7 to 2.7±1.6 fold the upper normal limit. The prevalence of liver test alterations was significantly higher in MYH9-RD patients than in each of the control populations, with odds ratios ranging from 8.2 (95% CIs 2.2–44.8) to 24.7 (14.8–40.8). Clinical follow-up and more detailed liver studies of a subset of patients, including ultrasound liver scan, liver elastography and liver biopsy in one case, did not show any significant structural damage or evolution towards liver insufficiency.

Conclusions

Elevation of liver enzymes is a frequent and previously unrecognized feature of the MYH9-RD syndrome; however, this defect does not appear to have poor prognostic value.

Outside-in signalling generated by a constitutively activated integrin αIIbβ3 impairs proplatelet formation in human megakaryocytes

BACKGROUND

The interaction of megakaryocytes with matrix proteins of the osteoblastic and vascular niche is essential for megakaryocyte maturation and proplatelet formation. Fibrinogen is present in the vascular niche and the fibrinogen receptor α(IIb)β(3) is abundantly expressed on megakaryocytes, however the role of the interaction between fibrinogen and α(IIb)β(3) in proplatelet formation in humans is not yet fully understood. We have recently reported a novel congenital macrothrombocytopenia associated with a heterozygous mutation of the β(3) subunit of α(IIb)β(3). The origin of thrombocytopenia in this condition remains unclear and this may represent an interesting natural model to get further insight into the role of the megakaryocyte fibrinogen receptor in megakaryopoiesis.

METHODOLOGY/PRINCIPAL FINDINGS

Patients' peripheral blood CD45+ cells in culture were differentiated into primary megakaryocytes and their maturation, spreading on different extracellular matrix proteins, and proplatelet formation were analyzed. Megakaryocyte maturation was normal but proplatelet formation was severely impaired, with tips decreased in number and larger in size than those of controls. Moreover, megakaryocyte spreading on fibrinogen was abnormal, with 50% of spread cells showing disordered actin distribution and more evident focal adhesion points than stress fibres. Integrin α(IIb)β(3) expression was reduced but the receptor was constitutively activated and a sustained, and substrate-independent, activation of proteins of the outside-in signalling was observed. In addition, platelet maturation from preplatelets was impaired.

CONCLUSIONS/SIGNIFICANCE

Our data show that constitutive activation of α(IIb)β(3)-mediated outside-in signalling in human megakaryocytes negatively influences proplatelet formation, leading to macrothombocytopenia.

Development and validation of competition binding assays for affinity to the extracellular matrix receptors, α(v)β(3) and α(IIb)β(3) integrin

The RGD (Arg-Gly-Asp) binding integrins α(v)β(3) and α(IIb)β(3) are integral components of various pathological and physiological processes, including tumor angiogenesis, osteoclast function, and thrombus formation. Because of this, there is interest in identifying novel compounds and proteins binding to these receptors as well as investigating the mechanism of these interactions. In this article, we describe the development and validation of competition binding assays for determining the affinity of test compounds to α(v)β(3) and α(IIb)β(3) integrin. Assays were successfully developed for each receptor, and the affinity of known compounds was comparable to published results. However, the inability of binding between α(IIb)β(3) integrin and the labeled echistatin protein ligand to reach equilibrium resulted in an assay that did not meet the assumptions of the competition binding model. Nevertheless, there was good agreement between this assay and known literature values, and intra- and interassay variability was acceptable. Binding by conformation-specific antibodies provided evidence that solid-phase bound α(IIb)β(3) receptor was in an activated conformation. This study also demonstrated that current models and methods for determining receptor affinity are simplistic and fail to account for common receptor-ligand interactions such as nondissociable interactions and varying receptor activation states.

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, αIIbβ3, 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 β3 has a wide tissue distribution when present as αvβ3 (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 β3-deficiency or nonfunction are being increasingly described.

Advances in our understanding of the molecular basis of disorders of platelet function

Genetic defects of platelet function give rise to mucocutaneous bleeding of varying severity because platelets fail to fulfil their haemostatic role after vessel injury. Abnormalities of pathways involving glycoprotein (GP) mediators of adhesion (Bernard-Soulier syndrome, platelet-type von Willebrand disease) and aggregation (Glanzmann thrombasthenia) are the most studied and affect the GPIb-IX-V complex and integrin αIIbβ3, respectively. Leukocyte adhesion deficiency-III combines Glanzmann thrombasthenia with infections and defects of kindlin-3, a mediator of integrin activation. Agonist-specific deficiencies in platelet aggregation relate to mutations of primary receptors for ADP (P2Y(12)), thromboxane A(2) (TXA2R) and collagen (GPVI); however, selective abnormalities of intracellular signalling pathways remain better understood in mouse models. Defects of secretion from δ-granules are accompanied by pigment defects in the Hermansky-Pudlak and Chediak-Higashi syndromes; they concern multiple genes and protein complexes involved in secretory organelle biogenesis and function. Quebec syndrome is linked to a tandem duplication of the urokinase plasminogen activator (PLAU) gene while locus assignment to chromosome 3p has advanced the search for the gene(s) responsible for α-granule deficiency in the gray platelet syndrome. Defects of α-granule biosynthesis also involve germline VPS33B mutations in the ARC (arthrogryposis, renal dysfunction and cholestasis) syndrome. A mutation in transmembrane protein 16F (TMEM16F) has been linked to a defective procoagulant activity and phosphatidylserine expression in the Scott syndrome. Cytoskeletal dysfunction (with platelet anisotrophy) occurs not only in the Wiskott-Aldrich syndrome but also in filamin A deficiency or MYH9-related disease while GATA1 mutations or RUNX1 haploinsufficiency can affect expression of multiple platelet proteins.

Heterozygous ITGA2B R995W mutation inducing constitutive activation of the αIIbβ3 receptor affects proplatelet formation and causes congenital macrothrombocytopenia

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. αIIbβ3 has not been implicated in these conditions. We identified a novel, conserved heterozygous ITGA2B R995W mutation in 4 unrelated families. The surface expression of platelet αIIbβ3 was decreased to 50% to 70% of control. There was spontaneous PAC-1 and fibrinogen binding to resting platelets without CD62p expression. The activation state of αIIbβ3 in 293T cells was higher for αIIb-W995 than for β3-H723 but was weaker than for β3-N562. FAK was spontaneously phosphorylated in αIIb-W995/β3-transfected 293T cells. These results indicate that αIIb-W995/β3 has a constitutive, activated conformation but does not induce platelet activation. αIIb-W995/β3-transfected CHO cells developed membrane ruffling and abnormal cytoplasmic protrusions. The increased size and decreased number of proplatelet tips in αIIb-W995/β3-transduced mouse fetal liver-derived megakaryocytes indicate defective pro-platelet formation. We propose that activating mutations in ITGA2B and ITGB3 represent the etiology of a subset of congenital macrothrombocytopenias.