Bernard-Soulier Syndrome

Bernard-Soulier syndrome caused by two novel heterozygous GP1BA gene mutations: a case report and literature review

BACKGROUND

Bernard-Soulier syndrome (BSS) is a rare inherited macrothrombocytopenia, usually autosomal recessive, which is characterized by prolonged bleeding, thrombocytopenia, and abnormally large platelets.

METHODS

For more than 6 years, we misdiagnosed a patient with BSS without an obvious bleeding tendency as having idiopathic thrombocytopenia purpura (ITP), prior to obtaining a genetic analysis. On admission, routine hematology showed a platelet count of 30 × 109/L and mean platelet volume (MPV) of 14.0 fL.

RESULTS

Whole-exome sequencing revealed two likely pathogenic heterozygous mutations (c.95_101del and c.1012del) in GP1BA. Flow cytometry analysis of platelet membrane glycoproteins indicated that the expression of GP1b was 0.28% of the normal level. Platelet aggregation tests indicated that platelet aggregation was inhibited by ristocetin- (1.7%), ADP- (14.5%), and arachidonic acid- (5.6%) induced platelet aggregation. A literature review identified reports on 53 mutations in the GP1BA gene in 253 patients, 29 mutations in the GP1BB gene in 90 patients, and 32 mutations in the GP9 gene in 114 patients.

CONCLUSION

This case report describes two novel gene mutation sites that have not been reported previously, enriching understanding of the GP1BA mutation spectrum.

Binding Mechanism between Platelet Glycoprotein and Cyclic Peptide Elucidated by McMD-Based Dynamic Docking

The cyclic peptide OS1 (amino acid sequence: CTERMALHNLC), which has a disulfide bond between both termini cysteine residues, inhibits complex formation between the platelet glycoprotein Ibα (GPIbα) and the von Willebrand factor (vWF) by forming a complex with GPIbα. To study the binding mechanism between GPIbα and OS1 and, therefore, the inhibition mechanism of the protein-protein GPIbα-vWF complex, we have applied our multicanonical molecular dynamics (McMD)-based dynamic docking protocol starting from the unbound state of the peptide. Our simulations have reproduced the experimental complex structure, although the top-ranking structure was an intermediary one, where the peptide was bound in the same location as in the experimental structure; however, the β-switch of GPIbα attained a different conformation. Our analysis showed that subsequent refolding of the β-switch results in a more stable binding configuration, although the transition to the native configuration appears to take some time, during which OS1 could dissociate. Our results show that conformational changes in the β-switch are crucial for successful binding of OS1. Furthermore, we identified several allosteric binding sites of GPIbα that might also interfere with vWF binding, and optimization of the peptide to target these allosteric sites might lead to a more effective inhibitor, as these are not dependent on the β-switch conformation.

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

Blood donor biobank as a resource in personalised biomedical genetic research

Health questionnaires and donation criteria result in accumulation of highly selected individuals in a blood donor population. To understand better the usefulness of a blood donor-based biobank in personalised disease-associated genetic studies, and for possible personalised blood donation policies, we evaluated the occurrence and distributions of common and rare disease-associated genetic variants in Finnish Blood Service Biobank. We analysed among 31,880 blood donors the occurrence and geographical distribution of (i) 53 rare Finnish-enriched disease-associated variants, (ii) mutations assumed to influence blood donation: four Bernard-Soulier syndrome and two hemochromatosis mutations, (iii) type I diabetes risk genotype HLA-DQ2/DQ8. In addition, we analysed the level of consanguinity in Blood Service Biobank. 80.3% of blood donors carried at least one (range 0-9 per donor) of the rare variants, many in homozygous form, as well. Donors carrying multiple rare variants were enriched in Eastern Finland. Haemochromatosis mutation HFE C282Y homozygosity was 43.8% higher than expected, whereas mutations leading to Bernard-Soulier thrombocytopenia were rare. The frequency of HLA-DQ2/DQ8 genotype was slightly lower than expected. First-degree consanguinity was higher in Blood Service Biobank than in the general population. We demonstrate that despite donor selection, the Blood Service Biobank is a valuable resource for personalised medical research and for genotype-selected samples from unaffected individuals. The geographical genetic substructure of Finland enables efficient recruitment of donors carrying rare variants. Furthermore, we show that blood donor biobank material can be utilised for personalised blood donation policies.

Primary Hemostasis Disorders as a Cause of Heavy Menstrual Bleeding in Women of Reproductive Age

Heavy menstrual bleeding (HMB) is a common clinical condition affecting adolescent and adult women and compromising their quality of life. Primary hemostasis disorders, affecting platelet plug formation, can be the underlying cause of HMB. They comprise a heterogeneous group of diseases with Von Willebrand disease (VWD) being the most commonly diagnosed; other disorders in this group that have been linked to HMB include (a) Glanzmann thrombasthenia, (b) Bernard-Soulier syndrome, (c) Hermansky-Pudlak syndrome, (d) immune thrombocytopenia (ITP), and (e) Ehlers-Danlos syndromes (EDS) and hypermobility spectrum disorders (HSD). Diagnosing these diseases can be challenging, as the basic laboratory investigations can be within the normal range. Thus, identification of specific clinical features and a thorough hematologic workup can be very important, providing the correct diagnosis. Proper diagnosis of the underlying disorder is important, as management may vary accordingly. Although disease-specific management guidelines exist for some of these disorders such as VWD and ITP, due to the rarity of most primary hemostasis disorders, the best approach for the management of HMB in these women remains elusive. The goal of this study was to create an informative, comprehensive review of the primary hemostasis disorders that have been linked to HMB. This study provides a summary of the basic published information regarding epidemiology, pathophysiology, clinical phenotype, diagnosis, and treatment of HMB in those diseases and serves as a reference guide for further reading.

Why platelet mechanotransduction matters for hemostasis and thrombosis

Mechanotransduction is the ability of cells to "feel" or sense their mechanical microenvironment and integrate and convert these physical stimuli into adaptive biochemical cellular responses. This phenomenon is vital for the physiology of numerous nucleated cell types to affect their various cellular processes. As the main drivers of hemostasis and clot retraction, platelets also possess this ability to sense the dynamic mechanical microenvironments of circulation and convert those signals into biological responses integral to clot formation. Like other cell types, platelets leverage their "hands" or receptors/integrins to mechanotransduce important signals in responding to vascular injury to achieve hemostasis. The clinical relevance of cellular mechanics and mechanotransduction is imperative as pathologic alterations or aberrant mechanotransduction in platelets has been shown to lead to bleeding and thrombosis. As such, the aim of this review is to provide an overview of the most recent research related to platelet mechanotransduction, from platelet generation to platelet activation, within the hemodynamic environment and clot contraction at the site of vascular injury, thereby covering the entire "life cycle" of platelets. Additionally, we describe the key mechanoreceptors in platelets and discuss the new biophysical techniques that have enabled the field to understand how platelets sense and respond to their mechanical microenvironment via those receptors. Finally, the clinical significance and importance of continued exploration of platelet mechanotransduction have been discussed as the key to better understanding of both thrombotic and bleeding disorders lies in a more complete mechanistic understanding of platelet function by way of mechanotransduction.

A mathematical model for assessing shear induced bleeding risk

PURPOSE

The objective of this study is to develop a bleeding risk model for assessing device-induced bleeding risk in patients supported with blood contact medical devices (BCMDs).

METHODS

The mathematical model for evaluating bleeding risk considers the effects of shear stress on von Willebrand factor (vWF) unfolding, high molecular weight multimers-vWF (HMWM-vWF) degradation, platelet activation and receptor shedding and platelet-vWF binding ability. Functions of the effect of shear stress on the above factors are fitted/employed and solved by the Eulerian transport equation. An axial flow-through Couette device and two clinical VADs which are HeartWare Ventricular Assist Device (HVAD) and HeartMate II (HM II) blood pump were employed to perform the simulation to evaluate platelet receptor shedding (GPIbα and GPIIb/IIIa), loss of HWMW-vWF, platelet-vWF binding ability and bleeding risk for validating the accuracy of our model.

RESULTS

The platelet-vWF binding ability after being subjected to high shear region in the axial flow-through Couette device predicted by our bleeding model was highly consistent with reported experimental data. As indicated by our CFD simulation results in the axial flow-through Couette device, it can find that an increase in shear stress led to a decrease in the adhesion ability of platelets on vWF, while the binding ability of vWF with platelets first increase and then decrease as shear stress elevates gradually beyond a threshold. The factor of exposure time can enhance the effect of shear stress. Additionally, the shear-induced bleeding risk predicted by our model increases with increasing shear stress and exposure time in an axial flow-through Couette device. As indicated by our numerical model, the bleeding risk in HVAD was higher than HMII, which is highly consistent with the meta-analysis based on clinical statistics. Our simulation investigations in these two clinical VADs also found that HVAD caused a higher rate of platelet receptor shedding and lower damage to HWMW-vWF than HeartMate II. The high shear stress generated in the narrow and turbulent regions of both VADs was the underlying cause of device-induced bleeding.

CONCLUSION

In this study, the shear-induced bleeding risk predicted by our bleeding model in axial flow-through Couette device and two clinical VADs is consistent or highly correlated with experimental and clinical findings, which proves the accuracy of our bleeding model. Our bleeding model can be used to aid the development of new BCMDs with improved functional characteristics and biocompatibility, and help to reduce risk of device-induced adverse events in patients.

Comprehensive analysis of platelet glycoprotein Ibα ectodomain glycosylation

BACKGROUND

Platelet glycoprotein (GP) Ibα is the major ligand-binding subunit of the GPIb-IX-V complex that binds von Willebrand factor. GPIbα is heavily glycosylated, and its glycans have been proposed to play key roles in platelet clearance, von Willebrand factor binding, and as target antigens in immune thrombocytopenia syndromes. Despite its importance in platelet biology, the glycosylation profile of GPIbα is not well characterized.

OBJECTIVES

The aim of this study was to comprehensively analyze GPIbα amino acid sites of glycosylation (glycosites) and glycan structures.

METHODS

GPIbα ectodomain that was recombinantly expressed or that was purified from human platelets was analyzed by Western blot, mass spectrometry glycomics, and mass spectrometry glycopeptide analysis to define glycosites and the structures of the attached glycans.

RESULTS

We identified a diverse repertoire of N- and O-glycans, including sialoglycans, Tn antigen, T antigen, and ABO(H) blood group antigens. In the analysis of the recombinant protein, we identified 62 unique O-glycosites. In the analysis of the endogenous protein purified from platelets, we identified 48 unique O-glycosites and 1 N-glycosite. The GPIbα mucin domain is densely O-glycosylated. Glycosites are also located within the macroglycopeptide domain and mechanosensory domain.

CONCLUSIONS

This comprehensive analysis of GPIbα glycosylation lays the foundation for further studies to determine the functional and structural roles of GPIbα glycans.

Experience of kidney transplantation to a patient with Bernard Soulier syndrome: A case report

Renal transplantation could be a challenging operation in patients with haemorrhagic diathesis, with predictable difficulties or even with unpredictable hurdles. Bernard Soulier Syndrome (BSS) is one of the ethiologies of the thrombocytopenia and it is a rare hereditary disease associated with defects of the platelet glycoprotein complex glycoprotein Ib/V/IX and characterized by large platelets, thrombocytopenia, and severe bleeding symptoms. Here, we present a challenging renal transplantation in BSS.

Platelet-Neutrophil Association in NETs-Rich Areas in the Retrieved AIS Patient Thrombi

Histological structure of thrombi is a strong determinant of the outcome of vascular recanalization therapy, the only treatment option for acute ischemic stroke (AIS) patients. A total of 21 AIS patients from this study after undergoing non-enhanced CT scan and multimodal MRI were treated with mechanical stent-based and manual aspiration thrombectomy, and thromboembolic retrieved from a cerebral artery. Complementary histopathological and imaging analyses were performed to understand their composition with a specific focus on fibrin, von Willebrand factor, and neutrophil extracellular traps (NETs). Though distinct RBC-rich and platelet-rich areas were found, AIS patient thrombi were overwhelmingly platelet-rich, with 90% of thrombi containing <40% total RBC-rich contents (1.5 to 37%). Structurally, RBC-rich areas were simple, consisting of tightly packed RBCs in thin fibrin meshwork with sparsely populated nucleated cells and lacked any substantial von Willebrand factor (VWF). Platelet-rich areas were structurally more complex with thick fibrin meshwork associated with VWF. Plenty of leukocytes populated the platelet-rich areas, particularly in the periphery and border areas between platelet-rich and RBC-rich areas. Platelet-rich areas showed abundant activated neutrophils (myeloperoxidase+ and neutrophil-elastase+) containing citrullinated histone-decorated DNA. Citrullinated histone-decorated DNA also accumulated extracellularly, pointing to NETosis by the activated neutrophils. Notably, NETs-containing areas showed strong reactivity to VWF, platelets, and high-mobility group box 1 (HMGB1), signifying a close interplay between these components.

Characterization of zebrafish gp1ba mutant and modelling Bernard Soulier syndrome

The aim of this study is to model classical Bernard Soulier Syndrome in the zebrafish by targeting Gp1ba. We obtained gp1ba mutant embryos from Zebrafish International Resource Center and grew them to adulthood. The tail clips from these fish were used to prepare DNA and sequenced to identify heterozygotes. They were then bred to obtain homozygotes. The mutation was confirmed by DNA sequencing as a termination codon UAA in place of AAA codon at position 886 in the gp1ba transcript. Thus, at the Pro-295, the Gp1ba protein could be terminated. The blood from gp1ba homozygous and heterozygous mutants showed decreased ristocetin-mediated agglutination in the whole blood agglutination assay. The gp1ba heterozygous and homozygous larvae were subjected to a laser-assisted arterial thrombosis assay, and the results showed the prolonged occlusion in the caudal artery. These results suggested that the gp1ba mutant had a bleeding phenotype. The blood smears from the adult gp1ba, heterozygous and homozygous mutants, showed macrothrombocytes, similar to the human GP1BA deficiency that showed giant platelets. The bleeding assay on these heterozygous and homozygous mutants showed greater bleeding than wildtype, confirming the above findings. Taken together, the characterization of gp1ba zebrafish mutant suggested an autosomal dominant mode of inheritance. The zebrafish gp1ba mutant models classical Bernard Soulier Syndrome and could be used for reversing this phenotype to identify novel factors by the genome-wide piggyback knockdown method.

Murine models of glycoprotein Ib-IX

The utility of mouse models to dissect the molecular basis of hemostasis and thrombosis is now well established. The anucleate properties of circulating blood platelet and their specialized release from mature megakaryocytes makes the use of in vivo models all the more informative and powerful. Indeed, they are powerful but there do exist limitations. Here, we review the contributions of mouse models to the pathogenesis of the Bernard-Soulier syndrome, their use in platelet-specific gene expression, the recent development of mice expressing both human GPIb-IX and human von Willebrand factor (VWF), and finally the use of GPIb-IX mouse models to examine the impact of platelet biology beyond clotting. The humanization of the receptor and ligand axis is likely to be a major advancement in the characterization of therapeutics in the complex pathogenesis that drives thrombosis. When appropriate, we highlight some limitations of each mouse model, but this is not to minimize the contributions these models to the field. Rather, the limitations are meant to provide context for any direct application to the important mechanisms supporting human primary hemostasis and thrombosis.

Perforated hemorrhagic cholecystitis in a patient with Bernard-Soulier syndrome

Bernard-Soulier syndrome is an inherited coagulopathy, with an incidence of one per million. Hemorrhagic cholecystitis is a rare and life-threatening complication of acute cholecystitis. Less than 50 patients have been reported in the previous literature. Bleeding diathesis and anticoagulant treatment are well-known predisposing factors for hemorrhagic cholecystitis. We present a 57-year-old male patient who was referred to our department with a complaint of right upper quadrant abdominal pain. Contrast-enhanced computed tomography revealed a high-density mass associated with the gallbladder lumen, and blood clot in the gallbladder lumen and hemoperitoneum which were compatible for hemorrhagic cholecystitis and gallbladder perforation. The patient underwent urgent cholecystectomy. Hemorrhagic cholecystitis often manifests as typical acute cholecystitis presentation; but several clinical findings such as fever, lower gastrointestinal bleeding or severe intraabdominal bleeding-related hypovolemic shock may also occur. Most of the described cases in prior literature have been reported to use anticoagulant medications. This report describes the second hemorrhagic cholecystitis patient with inherited bleeding diathesis and the first case with Bernard-Soulier syndrome.

Genetic variants associated with platelet count are predictive of human disease and physiological markers

Platelets play an important role in hemostasis and other aspects of vascular biology. We conducted a meta-analysis of platelet count GWAS using data on 536,974 Europeans and identified 577 independent associations. To search for mechanisms through which these variants affect platelets, we applied cis-expression quantitative trait locus, DEPICT and IPA analyses and assessed genetic sharing between platelet count and various traits using polygenic risk scoring. We found genetic sharing between platelet count and counts of other blood cells (except red blood cells), in addition to several other quantitative traits, including markers of cardiovascular, liver and kidney functions, height, and weight. Platelet count polygenic risk score was predictive of myeloproliferative neoplasms, rheumatoid arthritis, ankylosing spondylitis, hypertension, and benign prostate hyperplasia. Taken together, these results advance understanding of diverse aspects of platelet biology and how they affect biological processes in health and disease.

Evgenia Mikaelsdottir et al. report a study of variants associated with platelet count among European individuals where they identify 577 associations. They also report a genetic overlap between platelet count and human diseases, including myeloproliferative neoplasms, rheumatoid arthritis, and hypertension, as well as a genetic overlap between platelet count and various physiological markers.

A homozygous loss-of-function mutation in GP1BB causing variable clinical phenotypes in a family with Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare autosomal recessive bleeding disorder and has a low incidence. Bernard-Soulier syndrome is caused by the deficiency of glycoprotein GPIb-V-IX complex, a receptor for von Willebrand factor and is characterized by thrombocytopenia, giant platelets and bleeding tendency. We are reporting three members of a same family with variable phenotypic clinical presentation. The index case is a 20-year-old boy who has a frequent presentation with epistaxis, and low platelet counts (25 × 109/l). He had been hospitalized multiple times and received platelet transfusions. His brother and cousin reported bleeding symptoms with less frequent medical intervention. Genetic analysis by next-generation sequencing identified a homozygous GP1BB variant (c.423C>A:p.Cys141Ter), which segregated amongst the family members. The results led us to an improved insight into the disease for this family with variable phenotypic expression, in addition to the identification of a variant for further structural and functional characterization.

Platelet transfusion for patients with platelet dysfunction: effectiveness, mechanisms, and unanswered questions

PURPOSE OF REVIEW

In this review, we discuss current clinical guidelines and potential underlying mechanisms regarding platelet transfusion therapy in patients at risk of bleeding, comparing management of patients with thrombocytopenia versus those with qualitative platelet disorders.

RECENT FINDINGS

Platelet transfusion therapy is highly effective in managing bleeding in patients with hypoproliferative thrombocytopenia. Clinical trials have demonstrated that platelet transfusion can be used at a lower trigger threshold and reduced platelet doses, and may be used therapeutically rather than prophylactically in some situations, although additional data are needed. In patients with inherited platelet disorders such as Glanzmann's Thrombasthenia or those with RASGRP2 mutations, platelet transfusion may be ineffective because of competition between transfused and endogenous platelets at the site of vascular injury. Successful management of these patients may require transfusion of additional platelet units, or mechanism-driven combination therapy with other pro-hemostatic agents. In patients on antiplatelet therapy, timing of transfusion and inhibitor mechanism-of-action are key in determining therapeutic success.

SUMMARY

Expanding our understanding of the mechanisms by which transfused platelets exert their pro-hemostatic function in various bleeding disorders will improve the appropriate use of platelet transfusion.

Activation of von Willebrand factor via mechanical unfolding of its discontinuous autoinhibitory module

Von Willebrand factor (VWF) activates in response to shear flow to initiate hemostasis, while aberrant activation could lead to thrombosis. Above a critical shear force, the A1 domain of VWF becomes activated and captures platelets via the GPIb-IX complex. Here we show that the shear-responsive element controlling VWF activation resides in the discontinuous autoinhibitory module (AIM) flanking A1. Application of tensile force in a single-molecule setting induces cooperative unfolding of the AIM to expose A1. The AIM-unfolding force is lowered by truncating either N- or C-terminal AIM region, type 2B VWD mutations, or binding of a ristocetin-mimicking monoclonal antibody, all of which could activate A1. Furthermore, the AIM is mechanically stabilized by the nanobody that comprises caplacizumab, the only FDA-approved anti-thrombotic drug to-date that targets VWF. Thus, the AIM is a mechano-regulator of VWF activity. Its conformational dynamics may define the extent of VWF autoinhibition and subsequent activation under force.

Von Willebrand factor (VWF) is a large glycoprotein in the blood secreted from endothelial cells lining the blood vessel and activation of VWF leads to formation of VWF-platelet complexes or thrombi. Here authors use single-molecule force measurement, X-ray crystallography and functional measurements to monitor the activation of VWF via mechanical unfolding of the autoinhibitory module (AIM).

A novel frameshift GP1BB mutation causes autosomal dominant macrothrombocytopenia with decreased vWF receptor expression but normal platelet aggregation

GP1bβ is a component of the von Willebrand factor (vWF) receptor complex that is necessary for platelet formation and activation. A novel frameshift variant in GP1BB has been identified in a family with macrothrombocytopenia. The variant leads to a protein that is 101 amino acids longer than wild type with loss of the transmembrane domain. As there is no defect in platelet aggregation, the family are classified as heterozygous carriers of a Bernard-Soulier syndrome-related mutation. The levels of the vWF receptor on platelets are reduced to 50% of the controls, with the presence of large platelets but normal platelet aggregation demonstrating that decreased vWF receptor expression impacts proplatelet formation but not platelet function.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

The Role of a Disintegrin and Metalloproteinase Proteolysis and Mechanical Damage in Nonphysiological Shear Stress-Induced Platelet Receptor Shedding

In order to explore the role of a disintegrin and metalloproteinase (ADAM) proteolysis and direct mechanical damage in non-physiologic shear stress (NPSS)-caused platelet receptor shedding, the healthy donor blood treated with/without ADAM inhibitor was exposed to NPSS (150 Pa). The expression of the platelet surface receptors glycoprotein (GP) Ibα and glycoprotein (GP) VI (GPVI) in NPSS-damaged blood was quantified with flow cytometry. The impact of ADAM inhibition on adhesion of NPSS-damaged platelets on von Willibrand factor (VWF) and collagen was explored with fluorescence microscopy. The impact of ADAM inhibition on ristocetin- and collagen-caused aggregation of NPSS-damaged platelets was examined by aggregometry. The results showed that ADAM inhibition could lessen the NPSS-induced loss of platelet surface receptor GPIbα (12%) and GPVI (9%), moderately preserve adhesion of platelets on VWF (7.4%) and collagen (8.4%), and partially restore the aggregation of NPSS-sheared platelets induced by ristocetin (18.6 AU*min) and collagen (48.2 AU*min). These results indicated that ADAM proteolysis played a role in NPSS-induced receptor shedding. However, the ADAM inhibition couldn't completely suppress the NPSS-caused loss of the platelet surface receptors (GPIbα and GPVI), only partially prevented the NPSS-induced reduction of platelet adhesion to VWF and collagen, and the agonist (ristocetin and collagen)-caused platelet aggregation. These results suggested that the direct mechanical damage is partially responsible for NPSS-induced receptor shedding in addition to the ADAM proteolysis. In conclusion, NPSS relevant to blood contacting medical devices can induce ADAM proteolysis and direct mechanical damage on the platelet receptor GPIbα and GPVI, leading to comprised hemostasis.

Bleeding Disorders Associated with Abnormal Platelets: Glanzmann Thrombasthenia and Bernard-Soulier Syndrome

Platelets, the smallest cells in the blood, are associated with hemostasis, bowel formation, tissue remodeling, and wound healing. Although the prevalence of inherited platelet disorders is not fully known, it is a rare disease group and is encountered in approximately between 10000 and 1000000. Glanzmann thrombasthenia (GT) and Bernard-Soulier syndrome (BSS) are more frequently observed in inherited platelet disorders. In GT, the platelet aggregation stage due to deficiency or dysfunction of the platelet GPIIb/IIIa complex cannot take place. BSS is a platelet adhesion disorder due to the absence or abnormality of GPIb/IX complex on the platelet surface. If there is bleeding after easy bruising, mucous and oral cavities, menorrhagia, tooth extraction, tonsillectomy, or other surgical interventions, inherited platelet dysfunction should be considered if the platelet count is normal while the bleeding time is long. Firstly, other causes should be investigated by making differential diagnosis of GT and BSS. In this chapter, the definition, etiology, historical process, epidemiology, genetic basis, pathophysiology, clinical findings, diagnosis, differential diagnosis, and the follow-up and treatment approach of GT and BSS will be reviewed according to the current medical literature.

A Comprehensive Review of Congenital Platelet Disorders, Thrombocytopenias and Thrombocytopathies

Platelets play an important role in hemostasis through platelet plug formation by a phenomenon of adhesion; activation; secretion and aggregation. Defects in platelet hemostatic mechanisms can be congenital or acquired. Congenital platelet disorders are rare and manifestations range from asymptomatic to sometimes severe bleeding. The disorders arise due to diverse mechanisms. Congenital platelet disorders include thrombocytopathies and thrombocytopenia (platelet count <150 x 10⁹/L) or thrombocytosis (platelet count > 450 x 10⁹/L). Congenital thrombocytopathies include disorders of adhesion like von Willebrand's disease or Bernard-Soulier syndrome. The disorders of aggregation include congenital afibrinogenemia and Glanzmann thrombasthenia. Disorders of storage granules are gray platelet syndrome and Quebec platelet disorder. Congenital thrombocythopathy and thrombocytopenia often occur in conjunction. In this article, we have a detailed literature review of these rare thrombocytopathies, their presentation and treatment.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Von Willebrand factor as a thrombotic and inflammatory mediator in critical illness

The endothelial exocytosis of high-molecular-weight multimeric von Willebrand factor (vWF) may occur in critical illness states, including trauma and sepsis, leading to the sustained elevation and altered composition of plasma vWF. These critical illnesses involve the common process of sympathoadrenal activation and loss of the endothelial glycocalyx. As a prothrombotic and proinflammatory molecule that interacts with the endothelium, the alterations exhibited by vWF in critical illness have been implicated in the development and damaging effects of downstream pathologies, such as disseminated intravascular coagulation and systemic inflammatory response syndrome. Given the role of vWF in these pathologies, there has been a recent push to further understand how the molecule may be involved in the pathophysiology of related diseases, such as trauma-induced coagulopathy and acute renal injury, which are also known to develop secondarily to critical illness states. Elucidation of the role of vWF across the broader spectrum of generalized pathologies may provide a basis for the development of novel preventative and restorative measures, while also bolstering the scaffold of more widely used treatments, such as the administration of plasma-containing blood products.

Co-Expression Network Analysis of Spleen Transcriptome in Rock Bream (Oplegnathus fasciatus) Naturally Infected with Rock Bream Iridovirus (RBIV)

Rock bream iridovirus (RBIV) is a notorious agent that causes high mortality in aquaculture of rock bream (Oplegnathus fasciatus). Despite severity of this virus, no transcriptomic studies on RBIV-infected rock bream that can provide fundamental information on protective mechanism against the virus have been reported so far. This study aimed to investigate physiological mechanisms between host and RBIV through transcriptomic changes in the spleen based on RNA-seq. Depending on infection intensity and sampling time point, fish were divided into five groups: uninfected healthy fish at week 0 as control (0C), heavy infected fish at week 0 (0H), heavy mixed RBIV and bacterial infected fish at week 0 (0MH), uninfected healthy fish at week 3 (3C), and light infected fish at week 3 (3L). We explored clusters from 35,861 genes with Fragments Per Kilo-base of exon per Million mapped fragments (FPKM) values of 0.01 or more through signed co-expression network analysis using WGCNA package. Nine of 22 modules were highly correlated with viral infection (|gene significance (GS) vs. module membership (MM) |> 0.5, p-value < 0.05). Expression patterns in selected modules were divided into two: heavy infected (0H and 0MH) and control and light-infected groups (0C, 3C, and 3L). In functional analysis, genes in two positive modules (5448 unigenes) were enriched in cell cycle, DNA replication, transcription, and translation, and increased glycolysis activity. Seven negative modules (3517 unigenes) built in this study showed significant decreases in the expression of genes in lymphocyte-mediated immune system, antigen presentation, and platelet activation, whereas there was significant increased expression of endogenous apoptosis-related genes. These changes lead to RBIV proliferation and failure of host defense, and suggests the importance of blood cells such as thrombocytes and B cells in rock bream in RBIV infection. Interestingly, a hub gene, pre-mRNA processing factor 19 (PRPF19) showing high connectivity (kME), and expression of this gene using qRT-PCR was increased in rock bream blood cells shortly after RBIV was added. It might be a potential biomarker for diagnosis and vaccine studies in rock bream against RBIV. This transcriptome approach and our findings provide new insight into the understanding of global rock bream-RBIV interactions including immune and pathogenesis mechanisms.

Genetic classification and confirmation of inherited platelet disorders: current status in Korea

Inherited platelet disorders (IPDs), which manifest as primary hemostasis defects, often underlie abnormal bleeding and a family history of thrombocytopenia, bone marrow failure, hematologic malignancies, undefined mucocutaneous bleeding disorder, or congenital bony defects. Wide heterogeneity in IPD types with regard to the presence or absence of thrombocytopenia, platelet dysfunction, bone marrow failure, and dysmegakaryopoiesis is observed in patients. The individual processes involved in platelet production and hemostasis are genetically controlled; to date, mutations of more than 50 genes involved in various platelet biogenesis steps have been implicated in IPDs. Representative IPDs resulting from defects in specific pathways, such as thrombopoietin/MPL signaling; transcriptional regulation; granule formation, trafficking, and secretion; proplatelet formation; cytoskeleton regulation; and transmembrane glycoprotein signaling are reviewed, and the underlying gene mutations are discussed based on the National Center for Biotechnology Information database and Online Mendelian Inheritance in Man accession number. Further, the status and prevalence of genetically confirmed IPDs in Korea are explored based on searches of the PubMed and KoreaMed databases. IPDs are congenital bleeding disorders that can be dangerous due to unexpected bleeding and require genetic counseling for family members and descendants. Therefore, the pediatrician should be suspicious and aware of IPDs and perform the appropriate tests if the patient has unexpected bleeding. However, all IPDs are extremely rare; thus, the domestic incidences of IPDs are unclear and their diagnosis is difficult. Diagnostic confirmation or differential diagnoses of IPDs are challenging, time-consuming, and expensive, and patients are frequently misdiagnosed. Comprehensive molecular characterization and classification of these disorders should enable accurate and precise diagnosis and facilitate improved patient management.

Filamin A: key actor in platelet biology

Filamins (FLNs) are large dimeric actin-binding proteins that regulate actin cytoskeleton remodeling. In addition, FLNs serve as scaffolds for signaling proteins, such as tyrosine kinases, GTPases, or phosphatases, as well as for adhesive receptors, such as integrins. Thus, they connect adhesive receptors to signaling pathways and to cytoskeleton. There are 3 isoforms of FLN (filamin a [FLNa], FLNb, FLNc) that originate from 3 homologous genes. FLNa has been the recent focus of attention because its mutations are responsible for a wide spectrum of defects called filaminopathies A, affecting brain (peri-ventricular nodular heterotopia), heart (valve defect), skeleton, gastrointestinal tract, and, more recently, the megakaryocytic lineage. This review will focus on the physiological and pathological roles of FLNa in platelets. Indeed, FLNa mutations alter platelet production from their bone marrow precursors, the megakaryocytes, yielding giant platelets in reduced numbers (macrothrombocytopenia). In platelets per se, FLNa mutations may lead to impaired αIIbβ3 integrin activation or in contrast, increased αIIbβ3 activation, potentially enhancing the risk of thrombosis. Experimental work delineating the interaction of FLNa with its platelet partners, including αIIbβ3, the von Willebrand factor receptor GPIb-IX-V, the tyrosine kinase Syk, and the signaling pathway of the collagen receptor GPVI, will also be reviewed.

Unaccompanied mechanosensory domain mediates low expression of glycoprotein Ibα: implications for Bernard-Soulier syndrome

BACKGROUND

Disruption of protein folding or inter-subunit interactions in the platelet glycoprotein (GP)Ib-IX complex leads to its abnormally low expression in the plasma membrane, the hallmark of Bernard-Soulier syndrome (BSS).

OBJECTIVE

To discover the molecular mechanism by which GPIbα in the absence of GPIbβ and GPIX subunits is targeted for rapid degradation.

METHOD

The expression of GPIbα mutants with deletion or replacement of various domains were measured in transiently transfected Chinese hamster ovary cells.

RESULTS

We report evidence to suggest that induction of the unfolded protein response by the unaccompanied mechanosensory domain (MSD) is a major factor for intracellular degradation and low expression of GPIbα. Removal of the MSD produced the first GPIbα variant that, even in the absence of GPIbβ and GPIX, expressed at a level comparable to that of wild-type GPIbα in the GPIb-IX complex, while retaining its native ligand-binding activity.

CONCLUSION

Our finding has important implications on the molecular pathogenesis of BSS and the function of the GPIb-IX complex.

Septins, a cytoskeletal protein family, with emerging role in striated muscle

Appropriate organization of cytoskeletal components are required for normal distribution and intracellular localization of different ion channels and proteins involved in calcium homeostasis, signal transduction, and contractile function of striated muscle. Proteins of the contractile system are in direct or indirect connection with the extrasarcomeric cytoskeleton. A number of other molecules which have essential role in regulating stretch-, voltage-, and chemical signal transduction from the surface into the cytoplasm or other intracellular compartments are already well characterized. Sarcomere, the basic contractile unit, is comprised of a precisely organized system of thin (actin), and thick (myosin) filaments. Intermediate filaments connect the sarcomeres and other organelles (mitochondria and nucleus), and are responsible for the cellular integrity. Interacting proteins have a very diverse function in coupling of the intracellular assembly components and regulating the normal physiological function. Despite the more and more intense investigations of a new cytoskeletal protein family, the septins, only limited information is available regarding their expression and role in striated, especially in skeletal muscles. In this review we collected basic and specified knowledge regarding this protein group and emphasize the importance of this emerging field in skeletal muscle biology.

Novel Compound Heterozygous Mutations in Two Families With Bernard-Soulier Syndrome

Background: Bernard-Soulier Syndrome (BSS) is a rare autosomal recessive bleeding disorder with large platelets and thrombocytopenia. It is caused by homozygous or compound heterozygous mutations in the GP1BA, GP1BB, or GP9 genes, which together encode the platelet surface receptor glycoprotein complex GPIb-IX-V. Objectives: We report two novel heterozygous mutations in the GP1BA and the GP9 genes, respectively. Patients/Methods: We analyzed the platelet glycoprotein expression by flow cytometry and screened the relevant genes for responsible mutations in two unrelated families. Results: Flow cytometric analyses revealed the absence of CD42a (GPIX) and CD42b (GPIb) on the platelets in the two affected siblings of family 1 and a significantly reduced expression of CD42b (GPIb) in the patient of family 2. In the two siblings, we identified a known frameshift (c.1601_1602delAT) and a novel nonsense mutation (c.1036C>T) in the GP1BA gene that abrogated the production of GP1bα. In the other patient, we found a novel missense mutation (c.112T>C) that was co-inherited with a common one (c.182A>G) in the GP9 gene, respectively. All analyzed heterozygous carriers were asymptomatic and had a normal GPIb-IX-V expression. Conclusions: The two novel GP1BA and GP9 mutations reported herein increment the number of causative genetic defects in BSS.

A large deletion in the GP9 gene in Cocker Spaniel dogs with Bernard-Soulier syndrome

Inherited bleeding disorders including abnormalities of platelet number and function rarely occur in a variety of dog breeds, but are probably underdiagnosed. Genetically characterized canine forms of platelet disorders provide valuable large animal models for understanding similar platelet disorders in people. Breed-specific disease associated genetic variants in only eight different genes are known to cause intrinsic platelet disorders in dogs. However, the causative genetic variant in many dog breeds has until now remained unknown. Four cases of a mild to severe bleeding disorder in Cocker Spaniel dogs are herein presented. The affected dogs showed a platelet adhesion defect characterized by macrothrombocytopenia with variable platelet counts resembling human Bernard-Soulier syndrome (BSS). Furthermore, the lack of functional GPIb-IX-V was demonstrated by immunocytochemistry. Whole genome sequencing of one affected dog and visual inspection of the candidate genes identified a deletion in the glycoprotein IX platelet (GP9) gene. The GP9 gene encodes a subunit of a platelet surface membrane glycoprotein complex; this functions as a receptor for von Willebrand factor, which initiates the maintenance of hemostasis after injury. Variants in human GP9 are associated with Bernard-Soulier syndrome, type C. The deletion spanned 2460 bp, and included a significant part of the single coding exon of the canine GP9 gene on dog chromosome 20. The variant results in a frameshift and premature stop codon which is predicted to truncate almost two-thirds of the encoded protein. PCR-based genotyping confirmed recessive inheritance. The homozygous variant genotype seen in affected dogs did not occur in 98 control Cocker Spaniels. Thus, it was concluded that the structural variant identified in the GP9 gene was most likely causative for the BSS-phenotype in the dogs examined. These findings provide the first large animal GP9 model for this group of inherited platelet disorders and greatly facilitate the diagnosis and identification of affected and/or normal carriers in Cocker Spaniels.

The use of prophylaxis in the treatment of rare bleeding disorders

Rare bleeding disorders (RBDs) are a heterogeneous group of coagulation factor deficiencies that include fibrinogen, prothrombin, α₂-antiplasmin, plasminogen activator inhibitor-1, and factors II, V, V/VIII, VII, X, XI and XIII. The incidence varies based upon the disorder and typically ranges from 1 in 500,000 to 1 per million population. Symptoms vary with the disorder and residual level of the clotting factor, and can range from relatively minor such as epistaxis, to life threatening, such as intracranial hemorrhage. Rapid treatment of bleeding episodes in individuals with severe bleeding phenotypes is essential to preserve life or limb and to prevent long-term sequelae; therapeutic options depend on the deficiency and range from plasma-derived (eg, fresh frozen plasma, prothrombin complex concentrates, factor X concentrate) to highly purified and recombinant single factor concentrates. The rarity of these disorders limits the feasibility of conventional prospective clinical trials; instead, clinicians rely upon registries, published case reports/series and experience to guide treatment. In some disorders, long-term prophylactic therapy is administered in response to the bleeding phenotype in an individual patient or based on the known natural history and severity of the deficiency. Intermittent prophylaxis, surrounding surgery, pregnancy, labor, and menstruation may be required to prevent or control excessive bleeding. This review summarizes therapeutic options, guidelines, recommendations and observations from the published literature for long-term, surgical, gynecological, and obstetric prophylaxis in deficiencies of fibrinogen; prothrombin; factors II, V, V/VIII, VII, X, XI and XIII; combined vitamin-K dependent factors; α2-antiplasmin; and plasminogen activator inhibitor 1. Platelet disorders including Glanzmann's thrombasthenia and Bernard-Soulier syndrome are also addressed.

The impact of GPIbα on platelet-targeted FVIII gene therapy in hemophilia A mice with pre-existing anti-FVIII immunity

Essentials Platelet-specific FVIII gene therapy is effective in hemophilia A mice even with inhibitors. The impact of platelet adherence via VWF/GPIbα binding on platelet gene therapy was investigated. GPIbα does not significantly affect platelet gene therapy of hemophilia A with inhibitors. Platelet gene therapy induces immune tolerance in hemophilia A mice with pre-existing immunity. SUMMARY: Background We have previously demonstrated that von Willebrand factor (VWF) is essential in platelet-specific FVIII (2bF8) gene therapy of hemophilia A (HA) with inhibitory antibodies (inhibitors). At the site of injury, platelet adherence is initiated by VWF binding to the platelet GPIb complex. Objective To investigate the impact of GPIbα on platelet gene therapy of HA with inhibitors. Methods Platelet-FVIII expression was introduced by 2bF8 lentivirus (2bF8LV) transduction of hematopoietic stem cells (HSCs) from GPIbαnull (Ibnull ) mice or rhF8-primed FVIIInull (F8null ) mice followed by transplantation into lethally irradiated rhF8-primed F8null recipients. Animals were analyzed by flow cytometry, FVIII assays and the tail bleeding test. Results After transplantation, 99% of platelets were derived from donors. The macrothrombocytopenia phenotype was maintained in F8null mice that received 2bF8LV-transduced Ibnull HSCs (2bF8-Ibnull /F8null ). The platelet-FVIII expression level in 2bF8-Ibnull /F8null recipients was similar to that obtained from F8null mice that received 2bF8LV-transduced F8null HSCs (2bF8-F8null /F8null ). The tail bleeding test showed that the remaining hemoglobin level in the 2bF8-Ibnull /F8null group was significantly higher than in the F8null control group, but there was no significant difference between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The half-life of inhibitor disappearance time was comparable between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The rhF8 re-challenge did not elicit a memory immune response once inhibitor titers dropped to undetectable levels after 2bF8 gene therapy. Conclusion GPIbα does not significantly impact platelet gene therapy of HA with inhibitors. 2bF8 gene therapy restores hemostasis and promotes immune tolerance in HA mice with pre-existing immunity.

Hemizygosity for the gene encoding glycoprotein Ibβ is not responsible for macrothrombocytopenia and bleeding in patients with 22q11 deletion syndrome

Essentials How thrombocytopenia relates to bleeding in 22q11 deletion syndrome (22q11DS) is not clear. Bleeding severity, platelet count and volume, and GPIBB were examined in patients with 22q11DS. Macrothrombocytopenia and bleeding typified imperfectly overlapping subsets of 22q11DS patients. GPIBB hemizygosity does not cause macrothrombocytopenia or bleeding in patients with 22q11DS. SUMMARY: Background and objectives Macrothrombocytopenia and bleeding are frequently associated with 22q11 deletion syndrome (22q11DS). GPIBB, which encodes the glycoprotein (GP) Ibβ subunit of GPIb-IX-V, is commonly deleted in patients with 22q11DS. Absence of functional GPIb-IX-V causes Bernard-Soulier syndrome, which is a severe bleeding disorder characterized by macrothrombocytopenia. Patients with 22q11DS are often obligate hemizygotes for GPIBB, and those with only a pathogenically disrupted copy of GPIBB present with Bernard-Soulier syndrome. The objective of this study was to determine how GPIBB hemizygosity and sequence variation relate to macrothrombocytopenia and bleeding in patients with 22q11DS who do not have Bernard-Soulier syndrome. Patients/methods We thoroughly characterized bleeding severity, mean platelet volume, platelet count and GPIBB copy number and sequence in patients with 22q11DS. Results and conclusions Macrothrombocytopenia and mild bleeding were observed in incompletely overlapping subsets of patients, and GPIBB copy number and sequence variation did not correlate with either macrothrombocytopenia or bleeding in patients with 22q11DS. These findings indicate that GPIBB hemizygosity does not result in either macrothrombocytopenia or bleeding in these patients. Alternative genetic causes of macrothrombocytopenia, potential causes of acquired thrombocytopenia and bleeding and ways in which platelet size, platelet count and GPIBB sequence information can be used to aid in the diagnosis and management of patients with 22q11DS are discussed.

Pathophysiology 2: The Role of Platelets in Cancer Biology

For over 100 years, a link has been recognized between thrombosis and cancer. However, whether this was a causal or correlational relationship was debated. It is now well established that cancer and thrombosis are mechanistically related in intricate ways and can directly fuel each other. Here, we present an historical perspective of platelets and how their physiological function in hemostasis can contribute to tumor development and metastasis. This emerging field has garnered great interest as aspirin therapy has been proposed as a prevention strategy for some malignancies. We highlight the advances that have been made, presenting platelets as a key component that supports many of the hallmarks of cancer that have been described and conclude with future directions and studies that are needed to clarify the role of platelets in cancer and solidify platelet modulating therapies within oncology.