Targeted long-read sequencing identifies and characterizes structural variants in cases of inherited platelet disorders

BACKGROUND

Genetic diagnosis of inherited platelet disorders (IPDs) is mainly performed by high-throughput sequencing (HTS). These short-read-based sequencing methods sometimes fail to characterize the genetics of the disease.

OBJECTIVES

To evaluate nanopore long-read DNA sequencing for characterization of structural variants (SVs) in patients with IPDs.

METHODS

Four patients with a clinical and laboratory diagnosis of Glanzmann thrombasthenia (GT) (P1 and P2) and Hermansky-Pudlak syndrome (HPS) (P3 and P4) in whom HTS missed the underlying molecular cause were included. DNA was analyzed by both standard HTS and nanopore sequencing on a MinION device (Oxford Nanopore Technologies) after enrichment of DNA spanning regions covering GT and HPS genes.

RESULTS

In patients with GT, HTS identified only 1 heterozygous ITGB3 splice variant c.2301+1G>C in P2. In patients with HPS, a homozygous deletion in HPS5 was suspected in P3, and 2 heterozygous HPS3 variants, c.2464C>T (p.Arg822∗) and a deletion affecting 2 exons, were reported in P4. Nanopore sequencing revealed a complex SV affecting exons 2 to 6 in ITGB3 (deletion-inversion-duplication) in homozygosity in P1 and compound heterozygosity with the splice variant in P2. In the 2 patients with HPS, nanopore defined the length of the SVs, which were characterized at nucleotide resolution. This allowed the identification of repetitive Alu elements at the breakpoints and the design of specific polymerase chain reactions for family screening.

CONCLUSION

The nanopore technology overcomes the limitations of standard short-read sequencing techniques in SV characterization. Using nanopore, we characterized novel defects in ITGB3, HPS5, and HPS3, highlighting the utility of long-read sequencing as an additional diagnostic tool in IPDs.

Effect of c.1431C > T mutation, a causative mutation of Glanzmann's thrombasthenia, on ITGB3 splicing, gene and protein expression

BACKGROUND/AIM

Recently, it was reported that the non-synonymous c.1431C > T (p. G477=) mutation of the integrin subunit β3 (ITGB3) gene is the cause of Glanzmann's thrombasthenia (GT). However, the functional consequences of this mutation on the ITGB3 gene and protein expression remain to be elucidated. Therefore, this study was conducted to cover this scientific shortage.

METHODS

Peripheral blood samples were collected from Chinese family members (parents and proband and his sister), and DNA was extracted and sequenced using whole-exome and Sanger sequencing. The effect of c.1431C > T mutation on the splicing of mRNA was verified by the in vitro minigene assay and the three variants that resulted from the mutation were cloned into a phage vector and pEGFP-C1 vector, and ITGB3 gene and protein expression was detected in the transfected 293 T cells using qPCR and Western blotting.

RESULTS

Minigene splicing assay showed that c.1431C > T mutation causes three kinds of alternative splicing; (1) a 95 bp deletion in the middle of exon10, (2) a 155 bp deletion (95 bp deletion in the middle of exon10 plus a 60 bp deletion in the right side of exon10), and (3) a 261 bp deletion in the right side of exon10. The in vitro expression assay showed that the c.1431C > T variant did not affect the ITGB3 mRNA levels, but directly led to protein truncation and declined expression.

CONCLUSION

Due to its significant impact on protein expression, c.1431C > T mutation in ITGB3 could be considered a pathogenic variant of GT. This could enrich the ITGB3 mutation spectrum and provide a base for the genetic diagnosis of GT.

SINE-VNTR-Alu retrotransposon insertion as a novel mutational event underlying Glanzmann thrombasthenia

BACKGROUND

Glanzmann thrombasthenia (GT) is an autosomal recessive platelet aggregation disorder caused by mutations in ITGA2B or ITGB3.

OBJECTIVES

We aimed to assess the phenotype and investigate the genetic etiology of a GT pedigree.

METHODS

A patient with bleeding manifestations and mild mental retardation was enrolled. Complete blood count, coagulation, and platelet aggregation tests were performed. Causal mutations were identified via whole exome and genome sequencing and subsequently confirmed through polymerase chain reaction and Sanger sequencing. The transcription of ITGB3 was characterized using RNA sequencing and reverse transcription polymerase chain reaction. The αⅡb and β3 biosynthesis was investigated via whole blood flow cytometry and in vitro studies.

RESULTS

GT was diagnosed in a patient with defective platelet aggregation. Novel compound heterozygous ITGB3 variants were identified, with a maternal nonsense mutation (c.2222G>A, p.Trp741∗) and a paternal SINE-VNTR-Alu (SVA) retrotransposon insertion. The 5' truncated SVA element was inserted in a sense orientation in intron 11 of ITGB3, resulting in aberrant splicing of ITGB3 and significantly reducing β3 protein content. Meanwhile, both the expression and transportation of β3 were damaged by the ITGB3 c.2222G>A. Almost no αⅡb and β3 expressions were detected on the patient's platelets surface.

CONCLUSION

Novel compound heterozygous ITGB3 mutations were identified in the GT pedigree, resulting in defects of αⅡbβ3 biosynthesis. This is the first report of SVA retrotransposon insertion in the genetic pathogenesis of GT. Our study highlights the importance of combining multiple high-throughput sequencing technologies for the molecular diagnosis of genetic disorders.

A single F153Sβ3 mutation causes constitutive integrin αIIbβ3 activation in a variant form of Glanzmann thrombasthenia

This report identifies a novel variant form of the inherited bleeding disorder Glanzmann thrombasthenia, exhibiting only mild bleeding in a physically active individual. The platelets cannot aggregate ex vivo with physiologic agonists of activation, although microfluidic analysis with whole blood displays moderate ex vivo platelet adhesion and aggregation consistent with mild bleeding. Immunocytometry shows reduced expression of αIIbβ3 on quiescent platelets that spontaneously bind/store fibrinogen, and activation-dependent antibodies (ligand-induced binding site-319.4 and PAC-1) report β3 extension suggesting an intrinsic activation phenotype. Genetic analysis reveals a single F153Sβ3 substitution within the βI-domain from a heterozygous T556C nucleotide substitution of ITGB3 exon 4 in conjunction with a previously reported IVS5(+1)G>A splice site mutation with undetectable platelet messenger RNA accounting for hemizygous expression of S153β3. F153 is completely conserved among β3 of several species and all human β-integrin subunits suggesting that it may play a vital role in integrin structure/function. Mutagenesis of αIIb-F153Sβ3 also displays reduced levels of a constitutively activated αIIb-S153β3 on HEK293T cells. The overall structural analysis suggests that a bulky aromatic, nonpolar amino acid (F,W)153β3 is critical for maintaining the resting conformation of α2- and α1-helices of the βI-domain because small amino acid substitutions (S,A) facilitate an unhindered inward movement of the α2- and α1-helices of the βI-domain toward the constitutively active αIIbβ3 conformation, while a bulky aromatic, polar amino acid (Y) hinders such movements and restrains αIIbβ3 activation. The data collectively demonstrate that disruption of F153β3 can significantly alter normal integrin/platelet function, although reduced expression of αIIb-S153β3 may be compensated by a hyperactive conformation that promotes viable hemostasis.

[Effects of the ITGA2B Nonsense Mutation (c.2659C > T, p.Q887X) on Platelet Function in a Mouse Model of Glanzmann's Thrombasthenia Generated with CRISPR/Cas9 Technology]

OBJECTIVE

To construct a mouse model of Glanzmann's thrombasthenia (GT) with ITGA2B c.2659 C>T (p.Q887X) nonsense mutation by CRISPR/Cas9 technology, and then further explore the expression and function of glycoprotein αIIbβ3 on the surface of platelet membrane.

METHODS

The donor oligonucleotide and gRNA vector were designed and synthesized according to the ITGA2B gene sequence. The gRNA and Cas9 mRNA were injected into fertilized eggs with donor oligonucleotide and then sent back to the oviduct of surrogate mouse. Positive F0 mice were confirmed by PCR genotyping and sequence analysis after birth. The F1 generation of heterozygous GT mice were obtained by PCR and sequencing from F0 bred with WT mice, and then homozygous GT mice and WT mice were obtained by mating with each other. The phenotype of the model was then further verified by detecting tail hemorrhage time, saphenous vein bleeding time, platelet aggregation, expression and function of αIIbβ3 on the surface of platelet.

RESULTS

The bleeding time of GT mice was significantly longer than that of WT mice (P<0.01). Induced by collagen, thrombin, and adenosine diphosphate (ADP), platelet aggregation in GT mice was significantly inhibited (P<0.01, P<0.01, P<0.05). Flow cytometry analysis showed that the expression of αIIbβ3 on the platelet surface of GT mice decreased significantly compared with WT mice (P<0.01), and binding amounts of activated platelets to fibrinogen were significantly reduced after thrombin stimulation (P<0.01). The spreading area of platelet on fibrinogen in GT mice was significantly smaller than that in WT mice (P<0.05).

CONCLUSION

A GT mouse model with ITGA2B c.2659 C>T (p.Q887X) nonsense mutation has been established successfully by CRISPR/Cas9 technology. The aggregation function of platelet in this model is defective, which is consistent with GT performance.

Flow cytometric analysis of platelet surface glycoproteins in the diagnosis of thirty-two Turkish patients with Glanzmann thrombasthenia: a multicenter experience

Background/aim

Glanzmann thrombasthenia (GT) is a rare autosomal recessively inherited bleeding disorder characterized by the quantitative (type 1 and type 2) or qualitative (type 3) deficiency in platelet membrane glycoprotein (GP) IIb/IIIa (CD41a/CD61) fibrinogen receptors. In type 1, 2, and 3, CD41a/CD61 expression is 5%, 5%–20% and above 20%, respectively. In this study, diagnosis of GT was confirmed and subgroups were identified in 32 Turkish patients by flow cytometry analysis.

Materials and methods

CD41a/CD61 expression levels in platelet-rich plasma (PRP) obtained from peripheral venous EDTA blood samples were analyzed with a BD FACSCanto II flow cytometer (Becton Dickinson, Franklin Lakes, NJ, USA). GT subgroup analysis was performed by counting 50,000 events in the BD FACSDiva Software v6.1.3 program of the instrument.

Results

In the present study, in blood samples of 32 patients from 23 families with GT and 22 healthy controls, co-expression levels of CD41a and CD61 in PRP was analyzed. 12 out of 23 families were consistent with type 1 GT (52.2%), 4 were consistent with type 2 GT (17.4%), and 7 were consistent with type 3 GT (30.4%).

Conclusion

Especially due to consanguineous marriages, GT with various glycoprotein levels may be detected. As a result of the flow cytometry analysis of the present study with the highest GT patient population in Turkey, type 1 GT patients were the most common subgroup. In the determination of the GT subgroups; especially in the detection of type 3 GT, flow cytometry is the most sensitive glycoprotein analysis method. In addition to light transmission aggregometry, CD41a/CD61 study by flow cytometer confirms diagnosis when mutation analysis cannot be performed.

A Novel Homozygous Frameshift Mutation in ITGB3 Causes Glanzmann's Thrombasthenia

The objective of this study was to elucidate the molecular characteristics of a Chinese family with Glanzmann's thrombasthenia (GT). The proband was diagnosed with GT based on clinical manifestations, platelet aggregation, and the expression of CD41 and CD61 in platelets. Whole-exome and Sanger sequencing were used to detect genetic defects related to GT in the proband and the family of the pedigree. Whole-exome sequencing showed a c.1784-1802delinsGTCACA, p. S595Cfs*70 homozygous mutation in exon 11 of the ITGB3 gene in the proband. Heterozygous mutations were found in the proband's parents, grandmother, uncle, aunt, and younger brother. This novel p. S595Cfs*70 ITGB3 gene mutation is not present in the 1000 Genomes and ExAC databases.

Nonsense-mediated mRNA decay efficiency influences bleeding severity in ITGA2B c.2659C > T (p.Q887X) knock-in mice

Glanzmann's thrombasthenia (GT) is a severe hemorrhagic disease. It is caused by mutations in ITGA2B or ITGB3, which are the respective genes encoding integrin αIIb and β3. Despite widespread mutational analysis, the mechanisms underlying the extensive variability in bleeding severity observed among affected individuals remains poorly understood. In order to explore the mechanisms conferring for bleeding heterogeneity, three GT patients with ITGA2B c.2671C > T (p.Q891X) who possessed different bleeding scores were studied. Analysis showed that there was significant difference in nonsense-mediated mRNA decay (NMD) efficiency among the three patients. These differences positively correlated with their bleeding score. Next, a knock-in mouse model (KI mice) with the ITGA2B c.2659C > T (p.Q887X) was generated using CRISPR/Cas9. Importantly, this mutation is homologous to ITGA2B c.2671C > T (p.Q891X) in humans. The bleeding time of KI mice was significantly in comparison to the wide-type mice. Interestingly, bleeding was stopped after treatment with caffeine, which is a known NMD inhibitor. This suggests that NMD efficiency potentially influences bleeding severity in ITGA2B c.2659C > T (p.Q887X) KI mice.

Genetic Confirmation and Identification of Novel Variants for Glanzmann Thrombasthenia and Other Inherited Platelet Function Disorders: A Study by the Korean Pediatric Hematology Oncology Group (KPHOG)

The diagnosis of inherited platelet function disorders (IPFDs) is challenging owing to the unavailability of essential testing methods, including light transmission aggregometry and flow cytometry, in several medical centers in Korea. This study, conducted by the Korean Pediatric Hematology Oncology Group from March 2017 to December 2020, aimed to identify the causative genetic variants of IPFDs in Korean patients using next-generation sequencing (NGS). Targeted exome sequencing, followed by whole-genome sequencing, was performed for diagnosing IPFDs. Of the 11 unrelated patients with suspected IPFDs enrolled in this study, 10 patients and 2 of their family members were diagnosed with Glanzmann thrombasthenia (GT). The variant c.1913+5G>T of ITGB3 was the most common, followed by c.2333A>C (p.Gln778Pro) of ITGB2B. Known variants of GT, including c.917A>C (p.His306Pro) of ITGB3 and c.2975del (p.Glu992Glyfs*), c.257T>C (p.Leu86Pro), and c.1750C>T (p.Arg584*) of ITGA2B, were identified. Four novel variants of GT, c.1451G>T (p.Gly484Val) and c.1595G>T (p.Cys532Phe) of ITGB3 and c.1184G>T (p.Gly395Val) and c.2390del (p.Gly797Valfs*29) of ITGA2B, were revealed. The remaining patient was diagnosed with platelet type bleeding disorder 18 and harbored two novel RASGRP2 variants, c.1479dup (p.Arg494Alafs*54) and c.813+1G>A. We demonstrated the successful application of NGS for the accurate and differential diagnosis of heterogeneous IPFDs.

Ferric carboxymaltose for sub-acute and chronic iron deficiency anemia in inherited platelet function defects

Inherited platelet function defects are characterized by sub-acute and chronic mucocutaneous bleedings leading to iron deficiency anemia (IDA). Oral supplementation is the mainstay of treatment of IDA; however, it can be insufficient to compensate the losses and is often associated with gastrointestinal (GI) side effects. Intravenous (IV) iron is indicated for severe anemia or to overcome GI intolerance. Previous IV iron formulations were limited by the risk of free iron toxicity and immunogenicity, while currently available compounds (ferumoxytol, iron isomaltoside and ferric carboxymaltose (FCM)) allow the administration of high doses with low immunogenicity. There are neither any randomized studies nor case reports evaluating the efficacy of FCM in patients with inherited platelet disorders. We herein present three cases of patients with IDA related to Glanzmann thrombasthenia and Bernard-Soulier syndrome, who have been successfully treated with FCM with increase in hemoglobin levels, reduced hospital visits and improvement in quality of life.

Immunization against αIIb β3 and αv β3 in Glanzmann thrombasthenia patients carrying the French Gypsy mutation

Essentials The c.1544+1G>A mutation was identified in Gypsy Glanzmann thrombasthenia (GT) patients. Gypsy GT patients express normal αv β3 carrying HPA-1b epitopes. To demonstrate HPA-1a alloimmunization by modified antigen capture assays. Gypsy GT patients could develop anti-HPA-1a alloantibodies against β3 and αv β3 . ABSTRACT: Background Glanzmann thrombasthenia (GT) is a rare bleeding disorder caused by the absence or the dysfunction of the platelet αIIb β3 integrin. A founder mutation in the ITGA2B gene was previously identified in French Gypsy patients. Interestingly, this mutation was strongly linked to the human platelet antigen-1b (HPA-1b). The HPA-1bb Gypsy patients are at risk of isoimmunization against αIIb β3 , as this complex is not expressed at their platelet surface. Tentatively, they would, however, not have an increased risk of developing anti-HPA-1a alloantibodies by exposure of αIIb β3 on platelets from random platelet transfusions. However, the β3 chain can also associate with the αv subunit expressed at the platelet surface. Because Gypsy GT patients express normal αv β3 carrying HPA-1b epitopes, these patients might develop anti-HPA-1a alloantibodies reacting with αv β3 and/or β3 . Objectives/Patients/Methods To demonstrate this hypothesis, sera from HPA-1bb (n = 5) and HPA-1ab (n = 1) Gypsy GT patients were investigated by modified antigen capture assay using platelets or stable transfected cells. Furthermore, stable transfected cells expressing either αIIb β3 or αv β3 together with soluble monomeric chimeric β3 (as absorbent) were used to differentiate anti-β3 and anti-αv β3 reactivity. Results Only HPA-1bb patients developed alloantibodies reacting with HPA-1a cells. Further analysis showed that HPA-1bb patients developed anti-HPA-1a alloantibodies reacting with β3 and/or αv β3 . Conclusion In this study, we found that HPA-1bb patients who failed to express αIIb β3 on the platelet surface can develop alloantibodies against HPA-1a reacting with β3 as well as αv β3 . This is of particular importance as anti-HPA-1a alloantibodies might cause fetal neonatal alloimmune thrombocytopenia and/or platelet transfusion refractoriness.

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.

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.

A Glanzmann thrombasthenia family associated with a TUBB1-related macrothrombocytopenia

BACKGROUND

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

OBJECTIVES

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Glanzmann Thrombasthenia in a Newborn with Heterozygous Factor V Leiden and Heterozygous MTHFR C677T Gene Mutations

INTRODUCTION

Glanzmann thrombasthenia is a rare congenital platelet dysfunction.

CASE CHARACTERISTICS

A 2-day-old male neonate delivered at 35 weeks' gestation was referred with extensive bruising and jaundice. His elder sibling had Glanzmann thrombasthenia, and his mother had thrombophilic risk factors. Flow cytometric analysis revealed absent CD41/CD61. A molecular thrombophilia panel revealed the presence of heterozygous factor V Leiden G1691A and methylenetetrahydrofolate reductase C677T gene mutations.

OUTCOME

General precautions to avoid injuries and spontaneous bleeding were advised.

MESSAGE

Life-threatening bleeding may not be the first finding in cases of thrombasthenia accompanied by thrombophilic risk factors.

In silico analysis of structural modifications in and around the integrin αIIb genu caused by ITGA2B variants in human platelets with emphasis on Glanzmann thrombasthenia

BACKGROUND

Studies on the inherited bleeding disorder, Glanzmann thrombasthenia (GT), have helped define the role of the αIIbβ3 integrin in platelet aggregation. Stable bent αIIbβ3 undergoes conformation changes on activation allowing fibrinogen binding and its taking an extended form. The αIIb genu assures the fulcrum of the bent state. Our goal was to determine how structural changes induced by missense mutations in the αIIb genu define GT phenotype.

METHODS

Sanger sequencing of ITGA2B and ITGB3 in the index case followed by in silico modeling of all known GT-causing missense mutations extending from the lower part of the β-propeller, and through the thigh and upper calf-1 domains.

RESULTS

A homozygous c.1772A>C transversion in exon 18 of ITGA2B coding for a p.Asp591Ala substitution in an interconnecting loop of the lower thigh domain of αIIb in a patient with platelets lacking αIIbβ3 led us to extend our in silico modeling to all 16 published disease-causing missense variants potentially affecting the αIIb genu. Modifications of structuring H-bonding were the major cause in the thigh domain although one mutation gave mRNA decay. In contrast, short-range changes induced in calf-1 appeared minor suggesting long-range effects. All result in severe to total loss of αIIbβ3 in platelets. The absence of mutations within a key Ca2+-binding loop in the genu led us to scan public databases; three potential single allele variants giving major structural changes were identiffied suggesting that this key region is not protected from genetic variation.

CONCLUSIONS

It appears that the αIIb genu is the object of stringent quality control to prevent platelets from circulating with activated and extended integrin.

Platelet collagen receptor Glycoprotein VI-dimer recognizes fibrinogen and fibrin through their D-domains, contributing to platelet adhesion and activation during thrombus formation

Essentials Glycoprotein VI (GPVI) binds collagen, starting thrombogenesis, and fibrin, stabilizing thrombi. GPVI-dimers, not monomers, recognize immobilized fibrinogen and fibrin through their D-domains. Collagen, D-fragment and D-dimer may share a common or proximate binding site(s) on GPVI-dimer. GPVI-dimer-fibrin interaction supports spreading, activation and adhesion involving αIIbβ3.

SUMMARY

Background Platelet collagen receptor Glycoprotein VI (GPVI) binds collagen, initiating thrombogenesis, and stabilizes thrombi by binding fibrin. Objectives To determine if GPVI-dimer, GPVI-monomer, or both bind to fibrinogen substrates, and which region common to these substrates contains the interaction site. Methods Recombinant GPVI monomeric extracellular domain (GPVIex ) or dimeric Fc-fusion protein (GPVI-Fc2 ) binding to immobilized fibrinogen derivatives was measured by ELISA, including competition assays involving collagenous substrates and fibrinogen derivatives. Flow adhesion was performed with normal or Glanzmann thrombasthenic (GT) platelets over immobilized fibrinogen, with or without anti-GPVI-dimer or anti-αIIbβ3. Results Under static conditions, GPVIex did not bind to any fibrinogen substrate. GPVI-Fc2 exhibited specific, saturable binding to both D-fragment and D-dimer, which was inhibited by mFab-F (anti-GPVI-dimer), but showed low binding to fibrinogen and fibrin under our conditions. GPVI-Fc2 binding to D-fragment or D-dimer was abrogated by collagen type III, Horm collagen or CRP-XL (crosslinked collagen-related peptide), suggesting proximity between the D-domain and collagen binding sites on GPVI-dimer. Under low shear, adhesion of normal platelets to D-fragment, D-dimer, fibrinogen and fibrin was inhibited by mFab-F (inhibitor of GPVI-dimer) and abolished by Eptifibatide (inhibitor of αIIbβ3), suggesting that both receptors contribute to thrombus formation on these substrates, but αIIbβ3 makes a greater contribution. Notably, thrombasthenic platelets showed limited adhesion to fibrinogen substrates under flow, which was further reduced by mFab-F, supporting some independent GPVI-dimer involvement in this interaction. Conclusion Only dimeric GPVI interacts with fibrinogen D-domain, at a site proximate to its collagen binding site, to support platelet adhesion/activation/aggregate formation on immobilized fibrinogen and polymerized fibrin.

A novel homozygous mutation (1619delC) in GPIIb gene associated with Glanzmann thrombasthenia, the decay of GPIIb-mRNA and the synthesis of a truncated GPIIb unable to form complex with GPIIIa

The absence of agonist-induced platelet aggregation and the lack of fibrinogen receptor (GPIIb/IIIa) on the platelet surface demonstrated that the severe hemorrhagic complications of a child of Romany descent were caused by Glanzmann thrombasthenia. DNA sequencing revealed a novel homozygous deletion of a cytosine (1619delC) in the GPIIb gene causing a frameshift and predicting a novel stop codon at position 533 following 24 altered amino acids. Both parents possessed the same deletion in heterozygous form. The amount of GPIIb mRNA in the patient’s platelets was 0.06% of the amount measured in control platelets. Neither GPIIb nor its truncated form could be detected in the platelets of the patient by Western blotting, while a small amount of GPIIIa was demonstrated. Quantitative flow cytometric analysis showed an elevated number of vitronectin receptors, a component of which is GPIIIa, on the patient’s platelets. The surface expression of vitronectin receptor on thrombasthenic, but not on normal platelets was further increased by activation with thrombin receptor agonist peptide. BHK cells transfected with wild type GPIIIa and mutated GPIIb failed to express any mature GPIIb or pro-GPIIb. Immunoprecipitation with a polyclonal antibody recognizing both GPIIb and GPIIIa recovered a 60 kDa truncated form of GPIIb. This band was absent when immunoprecipitation was carried out with an antibody recognizing GPIIIa, suggesting that the truncated protein, lacking calf-1, calf-2 domains and major part of the thigh domain, is unable to form complex with GPIIIa.

Identification of Clinicopathological Spectrum, Platelet Glycoprotein IIb/IIIa complex and Platelet Antibodies in Egyptian Children with Glanzmann's Thrombasthenia

Glanzmann's thrombasthenia (GT) is a rare genetic bleeding disorder. The aim of our study was to evaluate the clinicopathological spectrum of this syndrome and to study the platelet glycoprotein IIb/IIIa complex and platelet antibodies by flow cytometry in a cohort of children with GT in a tertiary care center in Upper Egypt. Forty children with GT were assessed for the expression of GPIIb-IIIa on the platelet surface and platelet antibodies by using flow cytometry, to determine the most common GT subtypes among Egyptian children. By analysis of platelet GP IIb-IIIa by flow cytometry the classification of patients with GT in our study was type I GT (47.5%), type II GT (32.5%) and type III GT (20%). In this study, we have delineated that type I is the most common type of GT in Upper Egypt. Our data suggested that there is a good correlation between quantitative changes in the surface expression of platelet membrane glycoproteins detected by flow cytometry and the clinical severity of bleeding. Therefore, classifying of severity of bleeding in patients with GT could possibly aid the pediatricians and hematologists in the implementation of ideal prophylactic measures.

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

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

SUMMARY

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

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Endometriosis and Glanzmann’s thrombasthenia

Glanzmann’s thrombasthenia (GT) is a rare bleeding syndrome characterized by deficiency or defect of platelet aggregation complex. The pathogenesis of endometriosis is controversial but the strongest evidence leans towards retrograde menstruation. GT probably predisposes to endometriosis. The management of women affected by this disease can be difficult due to the risk of bleeding complications, especially during surgical treatment. We describe the cases of three sisters affected by endometriosis and GT, referred to our Department, who received different therapeutic management.

αIIbβ3 variants defined by next-generation sequencing: predicting variants likely to cause Glanzmann thrombasthenia

Next-generation sequencing is transforming our understanding of human genetic variation but assessing the functional impact of novel variants presents challenges. We analyzed missense variants in the integrin αIIbβ3 receptor subunit genes ITGA2B and ITGB3 identified by whole-exome or -genome sequencing in the ThromboGenomics project, comprising ∼32,000 alleles from 16,108 individuals. We analyzed the results in comparison with 111 missense variants in these genes previously reported as being associated with Glanzmann thrombasthenia (GT), 20 associated with alloimmune thrombocytopenia, and 5 associated with aniso/macrothrombocytopenia. We identified 114 novel missense variants in ITGA2B (affecting ∼11% of the amino acids) and 68 novel missense variants in ITGB3 (affecting ∼9% of the amino acids). Of the variants, 96% had minor allele frequencies (MAF) < 0.1%, indicating their rarity. Based on sequence conservation, MAF, and location on a complete model of αIIbβ3, we selected three novel variants that affect amino acids previously associated with GT for expression in HEK293 cells. αIIb P176H and β3 C547G severely reduced αIIbβ3 expression, whereas αIIb P943A partially reduced αIIbβ3 expression and had no effect on fibrinogen binding. We used receiver operating characteristic curves of combined annotation-dependent depletion, Polyphen 2-HDIV, and sorting intolerant from tolerant to estimate the percentage of novel variants likely to be deleterious. At optimal cut-off values, which had 69-98% sensitivity in detecting GT mutations, between 27% and 71% of the novel αIIb or β3 missense variants were predicted to be deleterious. Our data have implications for understanding the evolutionary pressure on αIIbβ3 and highlight the challenges in predicting the clinical significance of novel missense variants.

C560Rβ3 caused platelet integrin αII b β3 to bind fibrinogen continuously, but resulted in a severe bleeding syndrome and increased murine mortality

BACKGROUND AND OBJECTIVES

β(3)-Deficient megakaryocytes were modified by human β(3)-lentivirus transduction and transplantation to express sufficient levels of a C560Rβ(3) amino acid substitution, for investigation of how an activated αII b β(3) conformation affects platelets in vivo in mice.

PATIENT/METHODS

As in our previous report of an R560β(3) mutation in a patient with Glanzmann thrombasthenia, R560β(3) murine platelets spontaneously bound antibody that only recognizes activated αII b β3 bound to its ligand, fibrinogen.

RESULTS

With this murine model, we showed that αII b -R560β3 mutation-mediated continuous binding of fibrinogen occurred in the absence of P-selectin surface expression, indicating that the integrin was in an active conformation, although the platelets circulated in a quiescent manner. Remarkably, only 35% of R560β(3) 'mutant' mice survived for 6 months after transplantation, whereas 87% of C560β(3) 'wild-type' mice remained alive. Pathologic examination revealed that R560β(3) mice had enlarged spleens with extramedullary hematopoiesis and increased hemosiderin, indicating hemorrhage. R560β(3) megakaryocytes and platelets showed abnormal morphology and irregular granule distribution. Interestingly, R560β(3) washed platelets could aggregate upon simultaneous addition of fibrinogen and physiologic agonists, but aggregation failed when platelets were exposed to fibrinogen before activation in vitro and in vivo.

CONCLUSIONS

The results demonstrate that continuous occupancy of αIIb β3 with fibrinogen disrupts platelet structure and function, leading to hemorrhagic death consistent with Glanzmann thrombasthenia rather than a thrombotic state.

Abnormal whole blood thrombi in humans with inherited platelet receptor defects

To delineate the critical features of platelets required for formation and stability of thrombi, thromboelastography and platelet aggregation measurements were employed on whole blood of normal patients and of those with Bernard-Soulier Syndrome (BSS) and Glanzmann's Thrombasthenia (GT). We found that separation of platelet activation, as assessed by platelet aggregation, from that needed to form viscoelastic stable whole blood thrombi, occurred. In normal human blood, ristocetin and collagen aggregated platelets, but did not induce strong viscoelastic thrombi. However, ADP, arachidonic acid, thrombin, and protease-activated-receptor-1 and -4 agonists, stimulated both processes. During this study, we identified the genetic basis of a very rare double heterozygous GP1b deficiency in a BSS patient, along with a new homozygous GP1b inactivating mutation in another BSS patient. In BSS whole blood, ADP responsiveness, as measured by thrombus strength, was diminished, while ADP-induced platelet aggregation was normal. Further, the platelets of 3 additional GT patients showed very weak whole blood platelet aggregation toward the above agonists and provided whole blood thrombi of very low viscoelastic strength. These results indicate that measurements of platelet counts and platelet aggregability do not necessarily correlate with generation of stable thrombi, a potentially significant feature in patient clinical outcomes.

A mutation in the β3 cytoplasmic tail causes variant Glanzmann thrombasthenia by abrogating transition of αIIb β3 to an active state

BACKGROUND

The cytoplasmic tails of α(IIb) and β(3) regulate essential α(IIb) β(3) functions. We previously described a variant Glanzmann thrombasthenia mutation in the β(3) cytoplasmic tail, IVS14: -3C>G, which causes a frameshift with an extension of β(3) by 40 residues.

OBJECTIVES

The aim of this study was to characterize the mechanism by which the mutation abrogates transition of α(IIb) β(3) from a resting state to an active state.

METHODS

 We expressed the natural mutation, termed 742ins, and three artificial mutations in baby hamster kidney (BHK) cells along with wild-type (WT) α(IIb) as follows: β(3) -742stop, a truncated mutant to evaluate the effect of deleted residues; β(3) -749stop, a truncated mutant that preserves the NPLY conserved sequence; and β(3) -749ins, in which the aberrant tail begins after the conserved sequence. Flow cytometry was used to determine ligand binding to BHK cells.

RESULTS AND CONCLUSIONS

Surface expression of α(IIb) β(3) of all four mutants was at least 60% of WT expression, but there was almost no binding of soluble fibrinogen following activation with activating antibodies (anti-ligand-induced-binding-site 6 [antiLIBS6] or PT25-2). Activation of the α(IIb) β(3) mutants was only achieved when both PT25-2 and antiLIBS6 were used together or following treatment with dithiothreitol. These data suggest that the ectodomain of the four mutants is tightly locked in a resting conformation but can be forced to become active by strong stimuli. These data and those of others indicate that the middle part of the β(3) tail is important for maintaining α(IIb) β(3) in a resting conformation.

Characterization of the cDNA and genomic DNA sequence encoding for the platelet integrin alpha IIB and beta III in a horse with Glanzmann thrombasthenia

Glanzmann thrombasthenia (GT) is characterized by a defect of platelet aggregation. This autosomal recessive genetic disorder is caused by an abnormality of the platelet glycoprotein receptors alpha IIb or beta III. Recently, we identified a horse with clinical and pathological features of GT. The aim of this study was to describe this case of GT at the molecular level. A point mutation from G to C in exon 2 of ITGA2B causing a substitution of the expected amino acid arginine 72 (Arg(72)) by a proline (Pro(72)) was encountered. This amino acid change may result in abnormal structural conformations that yield an inactive alpha IIb subunit. The genomic DNA analysis showed that this horse was homozygous for the missense mutation.

An αIIb mutation in patients with Glanzmann thrombasthenia located in the N-terminus of blade 1 of the β-propeller (Asn2Asp) disrupts a calcium binding site in blade 6

BACKGROUND

Studies of Glanzmann thrombasthenia (GT)-causing mutations has generated invaluable information on the formation and function of integrin αIIbβ(3).

OBJECTIVE

To characterize the mutation in four siblings of an Israeli Arab family affected by GT, and to analyze the relationships between the mutant protein structure and its function using artificial mutations.

METHODS AND RESULTS

Sequencing disclosed a new A97G transversion in the αIIb gene predicting Asn2Asp substitution at blade 1 of the β-propeller. Alignment with other integrin α subunits revealed that Asn2 is highly conserved. No surface expression of αIIbβ(3) was found in patients' platelets and baby hamster kidney (BHK) cells transfected with mutated αIIb and WT β(3). Although the αIIbβ(3) was formed, the mutation impaired its intracellular trafficking. Molecular dynamics simulations and modeling of the αIIbβ(3) crystal indicated that the Asn2Asp mutation disrupts a hydrogen bond between Asn2 and Leu366 of a calcium binding domain in blade 6, thereby impairing calcium binding that is essential for intracellular trafficking of αIIbβ(3). Substitution of Asn2 to uncharged Ala or Gln partially decreased αIIbβ(3) surface expression, while substitution by negatively or positively charged residues completely abolished surface expression. Unlike αIIbβ(3), αVβ(3) harboring the Asn2Asp mutation was surface expressed by transfected BHK cells, which is consistent with the known lower sensitivity of αVβ(3) to calcium chelation compared with αIIbβ(3).

CONCLUSION

The new GT causing mutation highlights the importance of calcium binding domains in the β-propeller for intracellular trafficking of αIIbβ(3). The mechanism by which the mutation exerts its deleterious effect was elucidated by molecular dynamics.

Atomic force microscopy-based molecular recognition of a fibrinogen receptor on human erythrocytes

The established hypothesis for the increase on erythrocyte aggregation associated with a higher incidence of cardiovascular pathologies is based on an increase on plasma adhesion proteins concentration, particularly fibrinogen. Fibrinogen-induced erythrocyte aggregation has been considered to be caused by its nonspecific binding to erythrocyte membranes. In contrast, platelets are known to have a fibrinogen integrin receptor expressed on the membrane surface (the membrane glycoprotein complex alpha(IIb)beta(3)). We demonstrate, by force spectroscopy measurements using an atomic force microscope (AFM), the existence of a single molecule interaction between fibrinogen and an unknown receptor on the erythrocyte membrane, with a lower but comparable affinity relative to platelet binding (average fibrinogen--erythrocyte and --platelet average (un)binding forces were 79 and 97 pN, respectively). This receptor is not as strongly influenced by calcium and eptifibatide (an alpha(IIb)beta(3) specific inhibitor) as the platelet receptor. However, its inhibition by eptifibatide indicates that it is an alpha(IIb)beta(3)-related integrin. Results obtained for a Glanzmann thrombastenia (a rare hereditary bleeding disease caused by alpha(IIb)beta(3) deficiency) patient show (for the first time) an impaired fibrinogen--erythrocyte binding. Correlation with genetic sequencing data demonstrates that one of the units of the fibrinogen receptor on erythrocytes is a product of the expression of the beta(3) gene, found to be mutated in this patient. This work demonstrates and validates the applicability of AFM-based force spectroscopy as a highly sensitive, rapid and low operation cost nanotool for the diagnostic of genetic mutations resulting in hematological diseases, with an unbiased functional evaluation of their severity.

Identification of compound heterozygous mutations in the ITGA2B gene in a Chinese patient with Glanzmann thrombasthenia

BACKGROUND

Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder characterized by the tendency to hemorrhage and the inability of platelets to aggregate in response to agonists. GT is caused by a defect of the platelet glycoprotein IIb/IIIa complex. The objective of this study was to describe the clinical features and the genetic cause of GT in a 6-year-old girl from south China.

METHODS

A three-generation family was studied. The proband patient aged 6 years and her parents undertook examinations of platelet counts, blood film, bleeding time, platelet aggregation, and flow cytometry. All coding exons of the ITGA2B and ITGB3 genes were amplified by polymerase chain reaction (PCR), and direct sequencing was performed for mutational screening on the patient and normal controls consisted of 52 healthy blood donors. Reverse transcription PCR was conducted to test for exon skipping.

RESULTS

The proposita patient showed dispersing platelets, prolonged bleeding time, and severely reduced platelet aggregation in response to the physiological agonists adenosine diphosphate (ADP), epinephrine, collagen, and ristocetin. Flow cytometric measurements showed that the contents of alphaIIb and beta3 were significantly decreased. Sequencing results demonstrated two different types of heterozygous mutations existed in the alphaIIb gene (c.2930delG and IVS15-1delG). The compound mutations were also confirmed in the patient's mother and father separately.

CONCLUSIONS

The alphaIIbbeta3 deficiency of the proband was caused by two compound ITGA2B mutations, which were first reported in Chinese GT patients. The IVS15-1delG was first confirmed to cause an exon skipping.

[Inherited bleeding disorders common in Jews]

Four inherited disorders of hemostasis have been identified in Jews with a relatively high frequency: Factor XI deficiency, factor VII deficiency, combined factor V and VIII deficiency and GLanzmann thrombasthenia. During the past decades, the bleeding manifestations of these disorders, the diagnosis, the molecular-genetic basis and therapy have been discerned. Furthermore, the prevalence of the respective mutant genes have been delineated in various Jewish Communities. Each one of the disorders can serve as a model enabling better understanding of the pathophysioLogy of the coagulation systems. On the basis of data obtained from the research of Glanzmann thrombasthenia, several widely used drugs have been developed as effective antithrombotic agents.

The study of the Golgi apparatus in blood--basic science and clinical applications

The Golgi apparatus (GA) is a cytoplasmic organelle that is of great interest to all scientists for its key role in the biosynthesis, transporting and sorting of both lipids and proteins located at the intersection of the secretory and endocytic pathways. Recently, more and more evidence shows that changes in the Golgi apparatus play an important role in the clinical progression and pathological development of many diseases. In this review, we will summarize the alteration of the Golgi apparatus in blood cells and anti-Golgi complex antibodies in blood serum under different conditions and further clarify the contribution of the Golgi apparatus dysfunction to the course of these diseases and its pathophysiological basis, which will significantly improve our understanding and impact our ability to develop more effective therapies for these diseases.

Molecular defects in ITGA2B and ITGB3 genes in patients with Glanzmann thrombasthenia

BACKGROUND

Glanzmann thrombasthenia (GT) is an autosomal recessive inherited platelet function defect that is characterized by reduction in, or absence of, platelet aggregation in response to multiple physiologic agonists. The defect is caused by mutations in the genes encoding ITGA2B or ITGB3. This results in qualitative or quantitative abnormalities of the platelet receptor, alpha IIb-beta 3.

OBJECTIVES

The aim of this study was to identify the mutations in GT patients and to correlate these with patient phenotype.

SUBJECTS AND METHODS

A total of 45 unrelated patients with GT were enrolled in the present study to identify the causative molecular defects, and also to correlate their phenotype with their genotype. Platelet aggregation, flow cytometry, Western blotting, and mutation screening by conformation sensitive gel electrophoresis (CSGE) followed by sequencing were performed in all patients. Novel mutations were analyzed for penetrance in individual families.

RESULTS

A total of 22 novel mutations were identified in 45 unrelated GT patients. Mutations were identified in 36 of the 45 (80%) patients. Missense mutations were seen in most of the GT patients (59%). The remaining mutations were heterogeneous and were distributed throughout the length of the gene. Analysis of family members showed heterozygous mutations in all families.

CONCLUSIONS

The severe type I GT was the most common subtype found in this study. Missense mutations were identified as the defects responsible for most GT patients. Carrier detection and genetic counseling in these families is a potentially effective alternative for decreasing the burden of severe type of GT.

Mutations in the β3 gene giving rise to type I Glanzmann thrombasthenia in two families in Portugal

Glazzmann thrombasthenia is an inherited bleeding syndrome in which an absence of platelet aggregation is associated with quantitative or qualitative deficiencies of the alphaIIbbeta3 integrin. We now describe biochemical and molecular studies on two Portuguese families where platelets lack both surface and intracellular pools of alphaIIbbeta3. DNA extraction was followed by PCR-SSCP analysis of all exons and intronic boundaries in the alphaIIb and beta3 genes. Migration abnormalities were found for PCR fragments encompassing exon 12 (family 1) and exon 10 (family 2). For patient 1, there was a homozygous G to T transition at position 1846 which resulted in a stop codon at codon 616 in the beta3 gene. For patient 2, direct sequencing revealed a homozygous 1347C insert which led to a stop codon at codon 444 in the beta3 gene. For both patients a single mutated allele was inherited from each parent. Evidence is accumulating that nonsense mutations leading to a truncated beta3 may be a frequent cause of type I Glanzmann thrombasthenia in the Iberian peninsula.

Glanzmann's thrombasthenia: an overview

Glanzmann's thrombasthenia (GT) is an autosomal recessive inherited bleeding disorder due to a defect in platelet function. The hallmark of this disease is severely reduced/absent platelet aggregation in response to multiple physiological agonists. Bleeding signs in GT include epistaxis, bruising, gingival hemorrhage, gastrointestinal hemorrhage, hematuria, menorrhagia, and hemarthrosis. Homozygous or compound heterozygous mutations in the genes of GPIIb and GPIIIa lead to GT. A patient with GT, with no possible causative mutations in GPIIb and GPIIIa genes, may harbor defects in a regulatory element affecting the transcription of these 2 genes. GT occurs in high frequency in certain ethnic populations with an increased incidence of consanguinity such as in Indians, Iranians, Iraqi Jews, Palestinian and Jordanian Arabs, and French Gypsies. Carrier detection in GT is important to control the disorder in family members. Carrier detection can be done both by protein analysis and direct gene analysis.

The GPIIb/IIIa (integrin alphaIIbbeta3) odyssey: a technology-driven saga of a receptor with twists, turns, and even a bend

Starting 90 years ago with a clinical description by Glanzmann of a bleeding disorder associated with a defect in platelet function, technologic advances helped investigators identify the defect as a mutation(s) in the integrin family receptor, alphaIIbbeta3, which has the capacity to bind fibrinogen (and other ligands) and support platelet-platelet interactions (aggregation). The receptor's activation state was found to be under exquisite control, with activators, inhibitors, and elaborate inside-out signaling mechanisms controlling its conformation. Structural biology has produced high-resolution images defining the ligand binding site at the atomic level. Research on alphaIIbbeta3 has been bidirectional, with basic insights resulting in improved Glanzmann thrombasthenia carrier detection and prenatal diagnosis, assays to identify single nucleotide polymorphisms responsible for alloimmune neonatal thrombocytopenia, and the development of alphaIIbbeta3 antagonists, the first rationally designed antiplatelet agents, to prevent and treat thrombotic cardiovascular disease. The future looks equally bright, with the potential for improved drugs and the application of gene therapy and stem cell biology to address the genetic abnormalities. The alphaIIbbeta3 saga serves as a paradigm of rigorous science growing out of careful clinical observations of a rare disorder yielding both important new scientific information and improved diagnosis, therapy, and prevention of other disorders.

[Molecular mechanisms of Glanzmann thrombasthenia caused by alpha II b L721R and Q860X compound heterozygous mutation]

OBJECTIVE

To explore the molecular mechanisms of Glanzmann thrombasthenia caused by alpha II b L721R and Q860X compound heterozygous mutation.

METHODS

All exons and exon-intron boundaries of alpha II b and beta3 gene were amplified by PCR and analyzed by direct DNA sequencing. Gene polymorphisms were excluded by direct DNA sequencing. Alpha II b L721R and Q860X mutants expressing vectors were constructed by in vitro site-directed mutagenesis. The expression of alpha II b L721R and Q860X mutants on transfected cell membrane were analyzed by flow cytometry and the whole expression level was confirmed by Western blot. The subcellular localizations of alpha II b L721R and Q860X mutants were determined by immunofluorescent confocal scanning microscopy.

RESULTS

The alpha II b compound heterozygous mutations, T2255G (L721R) and C2671T (Q860X), were identified in the proband, the former being inherited from the maternal side and the latter the paternal side. The 293T cells cotransfected with mutated alpha II b L721R and wild-type beta3 expression plasmids expressed 2.1% of normal amount of alpha II b on the cell surface as shown by FACS, in contrast to 31.9% of normal amount of alpha II b on the cells cotransfected with cDNAs of mutated alpha II b Q860X and wildtype beta3 expression plasmids. Western blot of the cell lysates showed no detectable mature alpha II b in cells lysates with L721R mutant. While, truncated alpha II b protein was detected in cell lystes with Q860X mutant. Immunofluorescence studies demonstrated that both L721R and Q860X mutant pro-alpha II bbeta33 complex colocalized in endoplasmic reticulum, but a little in Golgi.

CONCLUSIONS

The L721R and Q860X mutations of alpha II b prevent transport of the pro-alpha II bbeta3 complex from the endoplasmic reticulum to the Golgi, hindering its maturation and surface expression. The impaired alpha II bbeta3 transport is responsible for the thrombasthenia.

[Glanzmann's thrombasthenia: first case descriptions in sub-Saharan Africa]

BACKGROUND

Constitutive hemorrhagic diseases that affect primary haemostasis are reportedly rare in sub-Saharan Africa.

AIM

This study arrived to report within a Congolese family five cases of Glanzmann's thromboasthenia.

PATIENT

5 cases of the congenital form of Glanzmann's thromboasthenia were depicted in a Congolese family. The disease was first discovered with a young student who was transferred in France, who had shown a tendency to develop hemorrhages since childhood. This tendency was enhanced following abdominal surgery to treat peritonitis. Like the other 3 cases, she had a prolonged bleeding time, albeit with normal von Willebrand factor plasma values. A 7 year old girl died following appendectomy from post-surgery hemorrhages. In this young patient, platelet aggregation could be induced only by ristocetine, all other conventional agonists failed. Flow cytometric analysis showed the total absence of GPIIbIIIa. The hemorrhages in the girls could be managed by cyclic administration of oestrogens and iron supplementation. Serologic analysis showed this patient to be positive for hepatitis C virus antibodies.

CONCLUSION

This first description of Glanzmann's thrombo-asthenia in Blacks in sub-Saharan Africa shows the necessity of establish inter-hospital cooperation for the improvement of the management of constitutive hemorrhagic diseases in the Hematology wards.

[Molecular mechanisms of Glanzmann thrombasthenia caused by alphaII b P126H mutation: an in vitro experiment]

OBJECTIVE

To explore the molecular mechanisms of Glanzmann thrombasthenia caused by alpha IIb P126H mutation.

METHODS

Eukaryotic vector of alpha IIb P126H was constructed by PCR site-directed mutagenesis and then co-transfected with eukaryotic vector PCDM8 II a expressing the subunit beta3 into human renal epithelial cells of the line 293& and Chinese hamster ovarian cancer cells of the line CHO after sequencing. The membrane expression of alpha IIb P126H mutant was analyzed by flow cytometry and the whole expression was confirmed by Western blotting. The alpha IIb P126H mutant subcellular localization was determined by laser confocal scanning microscopy.

RESULTS

The 293T cells cotransfected with cDNAs of mutated alpha IIb and wild-type beta3 failed to express alpha II bbeta3 on the cell surface as shown by FACS. Western blotting of the cell lysate showed no detectable mature alpha IIb. Immunofluorescence studies demonstrated proa II bbeta3 complex colocalized with an endoplasmic reticulum (ER) marker, but showed minimal colocalization with an Golgi marker.

CONCLUSION

The P126H mutation in alpha IIb prevents transport of the pro-alpha II bbeta3 complex from ER to the Golgi body, thus hindering its maturation and surface expression. The impaired alpha II bp33 transport is responsible for the thrombasthenia.

[Analysis of clinical features and genotype in three Chinese pedigrees with Glanzmann thrombasthenia]

OBJECTIVE

To study the clinical feature and alpha II b beta 3 gene mutations of three Glanzmann thrombasthenia (GT) pedigrees.

METHODS

Platelet counts (BPC), blood film, bleeding time, platelet aggregation and flow cytometry were used for phenotype diagnosis of all the patients. All the exons of alpha II b and beta 3 genes were amplified by polymerase chain reaction (PCR) and direct sequencing was performed for mutational screening. One hundred and three healthy blood donors were as normal controls.

RESULTS

Three probands showed normal BPC, defective platelets aggregation, prolonged bleeding time and significantly reduced platelet aggregation to ADP, epinephrine, and collagen, while relatively normal aggregation to ristocetin. Flow cytometry showed platelet surface expressed alpha II b beta 3 was strongly reduced in proband 1 and proband 3 and mildly reduced in the amount of surface expressed alpha II b beta 3 (63%) in proband 2. Sequencing results showed that proband 1 had a G10A homozygous mutation in alpha II b, and a G1412T homozygous mutation in beta3. Compound heterozygous mutations in beta3, G1199A and 1525delC were identified in proband 2. No mutations in alpha II b beta 3 gene were identified in proband 3.

CONCLUSIONS

Compound homozygous mutations, GI0A in alpha II b and G1412T in beta3, lead to GT in proband 1. Compound heterozygous mutations in beta3, G1199A and 1525delC, lead to GT in proband 2. The mutations of G10A, G1412T and 1525delC were reported for the first time in GT patients.

[Novel frame-shift mutation of 540 A deletion in GP IIb gene from a patient with Glanzmann thrombasthenia]

OBJECTIVE

To explore the molecular mechanism of Glanzmann thrombasthenia (GT).

METHODS

All 45 exons of alphaIIb and beta3 subunit genes as well as their splicing sites were amplified by polymerase chain reaction(PCR) with 40 primer pairs, and then the PCR products were used to screen the gene mutation by single strand conformation polymorphism-polyacrylamide gel electrophoresis (SSCP-PAGE). The mutation was further confirmed by direct DNA sequencing.

RESULTS

A DNA band alterated migration was detected after SSCP-PAGE. DNA sequencing showed that a base deletion within the band at the site of 540 in GPIIb gene(540A) was found.

CONCLUSION

The frame-shift mutation caused by the deletion of 540A in GPIIb gene is a novel mutation which is a genetic defect in patients with GT.