αIIbβ3 integrin: new allelic variants in Glanzmann thrombasthenia, effects onITGA2BandITGB3mRNA splicing, expression, and structure-function

Analysis of Integrin αIIb Subunit Dynamics Reveals Long-Range Effects of Missense Mutations on Calf Domains

Integrin α_IIbβ₃, a glycoprotein complex expressed at the platelet surface, is involved in platelet aggregation and contributes to primary haemostasis. Several integrin α_IIbβ₃ polymorphisms prevent the aggregation that causes haemorrhagic syndromes, such as Glanzmann thrombasthenia (GT). Access to 3D structure allows understanding the structural effects of polymorphisms related to GT. In a previous analysis using Molecular Dynamics (MD) simulations of α_IIbCalf-1 domain structure, it was observed that GT associated with single amino acid variation affects distant loops, but not the mutated position. In this study, experiments are extended to Calf-1, Thigh, and Calf-2 domains. Two loops in Calf-2 are unstructured and therefore are modelled expertly using biophysical restraints. Surprisingly, MD revealed the presence of rigid zones in these loops. Detailed analysis with structural alphabet, the Proteins Blocks (PBs), allowed observing local changes in highly flexible regions. The variant P741R located at C-terminal of Calf-1 revealed that the Calf-2 presence did not affect the results obtained with isolated Calf-1 domain. Simulations for Calf-1 + Calf-2, and Thigh + Calf-1 variant systems are designed to comprehend the impact of five single amino acid variations in these domains. Distant conformational changes are observed, thus highlighting the potential role of allostery in the structural basis of GT.

New αIIbβ3 variants in 28 Turkish Glanzmann patients; Structural hypothesis for complex activation by residues variations in I-EGF domains

Glanzmann thrombasthenia (GT) is a rare autosomal recessive bleeding disorder characterized by impaired platelet aggregation due to defects in integrin αIIbβ3, a fibrinogen receptor. Platelet phenotypes and allelic variations in 28 Turkish GT patients are reported. Platelets αIIbβ3 expression was evaluated by flow cytometry. Sequence analyzes of ITGA2B and ITGB3 genes allowed identifying nine variants. Non-sense variation effect on αIIbβ3 expression was studied by using transfected cell lines. 3D molecular dynamics (MDs) simulations allowed characterizing structural alterations. Five new alleles were described. αIIb:p.Gly423Asp, p.Asp560Ala and p.Tyr784Cys substitutions impaired αIIbβ3 expression. The αIIb:p.Gly128Val substitution allowed normal expression; however, the corresponding NM_000419.3:c.476G>T variation would create a cryptic donor splicing site altering mRNA processing. The β3:p.Gly540Asp substitution allowed αIIbβ3 expression in HEK-293 cells but induced its constitutive activation likely by impairing αIIb and β3 legs interaction. The substitution alters the β3 I-EGF-3 domain flexibility as shown by MDs simulations. GT variations are mostly unique although the NM_000419.3:c.1752 + 2 T > C and NM_000212.2:c.1697 G > A variations identified in 4 and 8 families, respectively, might be a current cause of GT in Turkey. MD simulations suggested how some subtle structural variations in the β3 I-EGF domains might induce constitutive activation of αIIbβ3 without altering the global domain structure.

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

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

Two homozygous missense mutations in ITGB3 gene as a cause of Glanzmann Thrombasthenia in four consanguineous Pakistani pedigrees

INTRODUCTION

Glanzmann thrombasthenia (GT) is most common of inherited platelet disorders, resulting from quantitative/qualitative defects in platelet surface integrin αIIbβ3, encoded by ITGA2B and ITGB3 genes. Little is known about clinical and molecular characteristics of GT patients from highly consanguineous Pakistani population.

METHODS

This study analyzed the clinical and molecular spectrum of six GT patients from four unrelated but consanguineous families. Platelet surface expression of αIIbβ3 integrin was determined using flow cytometry analysis. ITGA2B and ITGB3 genes were screened for causative mutations by DNA sequencing. Detected mutations were characterized for their pathogenicity using a variety of in silico tools.

RESULTS

Glanzmann thrombasthenia patients in this study generally presented early in life, had a severe course of clinical disease with transfusion dependency for management of bleeding episodes. Molecular analysis revealed 2 homozygous missense mutations in ITGB3 gene, c.422 A˃G (p.Y141C) in three GT patients from a single pedigree with familial segregation and c.1641 C>G (p.C547W) in three unrelated GT patients from three families manifesting type I GT with severe reduction in platelet αIIbβ3 levels. In silico pathogenicity predictions, multiple sequence alignment and 3D protein modeling unanimously suggested deleterious nature of the detected mutations, possibly due to aberrant disulfide bonding. Of note, clinical diversity was observed even among GT patients with same mutation in GT1 family.

CONCLUSION

This study provides an initial yet important account of clinical and genetic characterization of GT in local patients which may spark further studies to help molecular diagnosis, optimal disease management, and genetic counseling based prevention efforts.

Application of High-Throughput Sequencing in the Diagnosis of Inherited Thrombocytopenia

Inherited thrombocytopenia is a group of hereditary diseases with a reduction in platelet count as the main clinical manifestation. Clinically, there is an urgent need for a convenient and rapid diagnosis method. We introduced a high-throughput, next-generation sequencing (NGS) platform into the routine diagnosis of patients with unexplained thrombocytopenia and analyzed the gene sequencing results to evaluate the value of NGS technology in the screening and diagnosis of inherited thrombocytopenia. From a cohort of 112 patients with thrombocytopenia, we screened 43 patients with hereditary features. For the blood samples of these 43 patients, a gene sequencing platform for hemorrhagic and thrombotic diseases comprising 89 genes was used to perform gene detection using NGS technology. When we combined the screening results with clinical features and other findings, 15 (34.9%) of 43patients were diagnosed with inherited thrombocytopenia. In addition, 19 pathogenic variants, including 8 previously unreported variants, were identified in these patients. Through the use of this detection platform, we expect to establish a more effective diagnostic approach to such disorders.

Clinical and molecular insights into Glanzmann's thrombasthenia in China

Glanzmann's thrombasthenia (GT) is a rare bleeding disorder characterized by spontaneous mucocutaneous bleeding. The disorder is caused by quantitative or qualitative defects in integrin αIIbβ3 (encoded by ITGA2B and ITGB3) on the platelet and is more common in consanguineous populations. However, the prevalence rate and clinical characteristics of GT in non-consanguineous populations have been unclear. We analyzed 97 patients from 93 families with GT in the Han population in China. This analysis showed lower consanguinity (18.3%) in Han patients than other ethnic populations in GT-prone countries. Compared with other ethnic populations, there was no significant difference in the distribution of GT types. Han females suffered more severe bleeding and had a poorer prognosis. We identified a total of 43 different ITGA2B and ITGB3 variants, including 25 previously unidentified, in 45 patients. These variants included 14 missense, 4 nonsense, 4 frameshift, and 3 splicing site variants. Patients with the same genotype generally manifested the same GT type but presented with different bleeding severities. This suggests that GT clinical phenotype does not solely depend on genotype. Our study provides an initial, yet important, clinical and molecular characterization of GT heterogeneity in China.

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.

Perioperative management of a patient with Glanzmann thrombasthenia undergoing a coronary artery bypass graft surgery: a case report

: We report herein the successful perioperative management of a 57-year-old man with a type I Glanzmann thrombasthenia undergoing coronary artery bypass graft surgery and right carotid endarterectomy. The patient suffered from several lesions in the three major coronary arteries and in the right carotid necessitating surgery. Prophylactic human leukocyte antigen (HLA)-matched platelets transfusions were continuous administrated before, and through the immediate perioperative period. Posttransfusion platelet recovery was monitored using flow cytometry to determine the percentage of circulating platelet expressing CD61 (β3). No bleeding complications occurred during and following the procedure. The patient did not develop HLA antibodies or αIIbβ3 antibodies. Thrombophilia screening revealed a heterozygous G20210A prothrombin gene mutation. The patient also suffered from an atrial fibrillation, necessitating anticoagulation therapy. During the hospital stay, a treatment with vitamin K antagonists for stroke prevention was initiated. The patient was discharged 8 days following surgery, and no further complications occurred during the 6 months follow-up.

Angeborene Thrombozytenfunktionsstörungen

Angeborene Thrombozytopathien können zu Blutungssymptomen unterschiedlichen Schweregrades führen, da die Thrombozyten nicht mehr ihre Funktion nach einer Gefäß-verletzung erfüllen können. In manchen Fällen sind Thrombozytopathien schwierig zu diagnostizieren und können Probleme in der Therapie und im Management verursachen. Dieser Review beschreibt den klinischen und molekulargenetischen Phänotyp der verschiedenen angeborenen Thrombozytopathien. Die angeborenen Thrombozytopathien werden entsprechend des Thrombozytendefekts eingeteilt: Rezeptordefekte (Adhäsion oder Aggregation), Sekretionsdefekte und Zytoskelettdefekte.

Die am besten charakterisierten thrombozytären Rezeptordefekte sind die Glanzmann Thrombasthenie (Defekt des Integrins [uni03B1]IIb[uni03B2]3) und das Bernard-Soulier Syndrom (Defekt des GPIb/IX/V Rezeptors). Umfassende Fall-berichte über die Blutungsdiathese sowie die Untersuchung der Thrombozytenaggregation bzw. -agglutination und Rezeptorexpression von Patienten, die an der Glanzmann Thrombasthenie (GT) oder am Bernard-Soulier Syndrom (BSS) leiden, sollen diesen Review ergänzen. Darüber hinaus wird das HermanskyPudlak Syndrom (HPS) als eine bedeutende Störung der [uni03B4]-Granula Sekretion zusammen mit einer Fallbeschreibung eines Patienten, der an HPS Typ 1 leidet, beschrieben.

Molecular characterization of Glanzmann's thrombasthenia in Iran: identification of three novel mutations

: Quantitative and/or qualitative defects of the platelet membrane glycoprotein IIb/IIIa complex lead to the clinical entity of Glanzmann's thrombasthenia. A large variety of mutations and polymorphisms are responsible for the aberrant expression and defective activity of this heterodimeric complex. The current study aimed to determine the pattern of mutations among Iranian population with Glanzmann's thrombasthenia. A total of 20 patients with Glanzmann's thrombasthenia have been evaluated. All exons and splice sites of ITGA2B and ITGB3 genes were amplified using touchdown PCR. Mutation screening was analyzed using conformation sensitive gel electrophoresis heteroduplex PCR, and DNA sequencing. In addition to finding one previously identified mutation and polymorphism, the experimenters explored 3 and 2 novel mutations and polymorphisms, respectively. One substitution mutation, two deletions of a single nucleotide, one insertion of a single nucleotide, two synonymous polymorphisms, and one missense polymorphism were found using Sanger sequencing method. All detected mutations were homozygous which will most likely contribute to the pathogenesis of Glanzmann's thrombasthenia. Furthermore, it suggested ITGB3 as the mainly affected gene impaired in the patients with Glanzmann's thrombasthenia. As expected, the molecular results were consistent with the phenotypic findings so that GPIIb/IIIa complex was disrupted due to mutations in all type-I Glanzmann's thrombasthenia patients. It is concluded that intronic alterations or epigenetic regulations could be responsible for aberrant expression and/or defective activity of GPIIb/IIIa complex among other patients.

Introducing high-throughput sequencing into mainstream genetic diagnosis practice in inherited platelet disorders

Inherited platelet disorders are a heterogeneous group of rare diseases, caused by inherited defects in platelet production and/or function. Their genetic diagnosis would benefit clinical care, prognosis and preventative treatments. Until recently, this diagnosis has usually been performed via Sanger sequencing of a limited number of candidate genes. High-throughput sequencing is revolutionizing the genetic diagnosis of diseases, including bleeding disorders. We have designed a novel high-throughput sequencing platform to investigate the unknown molecular pathology in a cohort of 82 patients with inherited platelet disorders. Thirty-four (41.5%) patients presented with a phenotype strongly indicative of a particular type of platelet disorder. The other patients had clinical bleeding indicative of platelet dysfunction, but with no identifiable features. The high-throughput sequencing test enabled a molecular diagnosis in 70% of these patients. This sensitivity increased to 90% among patients suspected of having a defined platelet disorder. We found 57 different candidate variants in 28 genes, of which 70% had not previously been described. Following consensus guidelines, we qualified 68.4% and 26.3% of the candidate variants as being pathogenic and likely pathogenic, respectively. In addition to establishing definitive diagnoses of well-known inherited platelet disorders, high-throughput sequencing also identified rarer disorders such as sitosterolemia, filamin and actinin deficiencies, and G protein-coupled receptor defects. This included disease-causing variants in DIAPH1 (n=2) and RASGRP2 (n=3). Our study reinforces the feasibility of introducing high-throughput sequencing technology into the mainstream laboratory for the genetic diagnostic practice in inherited platelet disorders.

Novel mutations in Thai patients with glanzmann thrombasthenia

OBJECTIVES

Glanzmann thrombasthenia (GT) is an autosomal recessive platelet disorder, caused by defects of the platelet integrin αIIbβ3 (GPIIb/IIIa) resulting from pathogenic mutations in either ITGA2B or ITGB3. It is characterized by spontaneous mucocutaneous bleeding. The molecular features of GT in Thailand have not been identified. This study aimed to determine the clinical and molecular features of unrelated Thai patients with GT.

METHODS

Four patients with clinically suspected GT were recruited at the Division of Pediatric Hematology/Oncology, King Chulalongkorn Memorial Hospital. The diagnosis was based on clinical and hematological parameters as well as genetic analysis. Whole exome sequencing (WES) was performed in all cases.

RESULTS

Of the four patients studied, the median age at first suspicion of GT was 2.5 years. All presented with severe bleeding symptoms (WHO bleeding scale 3). Flow cytometry to assess the surface GPIIb/IIIa complex showed reduced expression. By WES, we successfully identified seven mutant alleles in ITGA2B. One alteration, the c.2915dup (p.Leu973Alafs*63), was detected in two unrelated families. One patient was homozygous for the c.617T>A (p.Val206Asp). Of the five different mutations, three have never been previously described. These include a missense, c.617T>A (p.Val206Asp), a deletion, c.1524_1533del (p.Gln508Hisfs*3), and a nonsense, c.2344C>T (p.Arg782Ter).

CONCLUSION

This study reported three novel mutations expanding the genotypic spectrum of ITGA2B causing GT.

In silico analysis of Glanzmann variants of Calf-1 domain of αIIbβ3 integrin revealed dynamic allosteric effect

Integrin α_IIbβ₃ mediates platelet aggregation and thrombus formation. In a rare hereditary bleeding disorder, Glanzmann thrombasthenia (GT), α_IIbβ₃ expression / function are impaired. The impact of deleterious missense mutations on the complex structure remains unclear. Long independent molecular dynamics (MD) simulations were performed for 7 GT variants and reference structure of the Calf-1 domain of α_IIb. Simulations were analysed using a structural alphabet to describe local protein conformations. Common and flexible regions as well as deformable zones were observed in all the structures. The most flexible region of Calf-1 (with highest B-factor) is rather a rigid region encompassed into two deformable zones. Each mutated structure barely showed any modifications at the mutation sites while distant conformational changes were observed. These unexpected results question the relationship between molecular dynamics and allostery; and the role of these long-range effects in the impaired α_IIbβ₃ expression. This method is aimed at studying all α_IIbβ₃ sub-domains and impact of missense mutations at local and global structural level.

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

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

Identification of two rare and novel large deletions in ITGB4 gene causing epidermolysis bullosa with pyloric atresia

Epidermolysis bullosa with pyloric atresia (EB-PA) is a rare autosomal recessive hereditary disease with a variable prognosis from lethal to very mild. EB-PA is classified into Simplex form (EBS-PA: OMIM #612138) and Junctional form (JEB-PA: OMIM #226730), and it is caused by mutations in ITGA6, ITGB4 and PLEC genes. We report the analysis of six patients with EB-PA, including two dizygotic twins. Skin immunofluorescence epitope mapping was performed followed by PCR and direct sequencing of the ITGB4 gene. Two of the patients presented with non-lethal EB-PA associated with missense ITGB4 gene mutations. For the other four, early postnatal demise was associated with complete lack of β4 integrin due to a variety of ITGB4 novel mutations (2 large deletions, 1 splice-site mutation and 3 missense mutations). One of the deletions spanned 278 bp, being one of the largest reported to date for this gene. Remarkably, we also found for the first time a founder effect for one novel mutation in the ITGB4 gene. We have identified 6 novel mutations in the ITGB4 gene to be added to the mutation database. Our results reveal genotype-phenotype correlations that contribute to the molecular understanding of this heterogeneous disease, a pivotal issue for prognosis and for the development of novel evidence-based therapeutic options for EB management.

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Glanzmann thrombasthenia in Pakistan: molecular analysis and identification of novel mutations

Glanzmann thrombasthenia (GT) is an inherited genetic disorder affecting platelets, which is characterized by spontaneous mucocutaneous bleeding and abnormally prolonged bleeding in response to injury or trauma. The underlying defect is failure of platelet aggregation due to qualitative and/or quantitative deficiency of platelet integrin αIIbβ3 resulting from molecular genetic defects in either ITGA2B or ITGB3. Here, we examine a Pakistani cohort of 15 patients with clinical symptoms of GT who underwent laboratory and molecular genetic analysis. In patients with a broad range of disease severity and age of presentation, we identified pathogenic mutations in ITGA2B in 11 patients from 8 different families, including 2 novel homozygous mutations and 1 novel heterozygous mutation. Mutations in ITGB3 were identified in 4 patients from 3 families, two of which were novel homozygous truncating mutations. A molecular genetic diagnosis was established in 11 families with GT, including 5 novel mutations extending the spectrum of mutations in this disease within a region of the world where little is known about the incidence of GT. Mutational analysis is a key component of a complete diagnosis of GT and allows appropriate management and screening of other family members to be performed.

Protein flexibility in the light of structural alphabets

Protein structures are valuable tools to understand protein function. Nonetheless, proteins are often considered as rigid macromolecules while their structures exhibit specific flexibility, which is essential to complete their functions. Analyses of protein structures and dynamics are often performed with a simplified three-state description, i.e., the classical secondary structures. More precise and complete description of protein backbone conformation can be obtained using libraries of small protein fragments that are able to approximate every part of protein structures. These libraries, called structural alphabets (SAs), have been widely used in structure analysis field, from definition of ligand binding sites to superimposition of protein structures. SAs are also well suited to analyze the dynamics of protein structures. Here, we review innovative approaches that investigate protein flexibility based on SAs description. Coupled to various sources of experimental data (e.g., B-factor) and computational methodology (e.g., Molecular Dynamic simulation), SAs turn out to be powerful tools to analyze protein dynamics, e.g., to examine allosteric mechanisms in large set of structures in complexes, to identify order/disorder transition. SAs were also shown to be quite efficient to predict protein flexibility from amino-acid sequence. Finally, in this review, we exemplify the interest of SAs for studying flexibility with different cases of proteins implicated in pathologies and diseases.

Functional and molecular characterization of inherited platelet disorders in the Iberian Peninsula: results from a collaborative study

Background

The diagnostic evaluation of inherited platelet disorders (IPDs) is complicated and time-consuming, resulting in a relevant number of undiagnosed and incorrectly classified patients. In order to evaluate the spectrum of IPDs in individuals with clinical suspicion of these disorders, and to provide a diagnostic tool to centers not having access to specific platelets studies, we established the project “Functional and Molecular Characterization of Patients with Inherited Platelet Disorders” under the scientific sponsorship of the Spanish Society of Thrombosis and Haemostasis.

Patients/methods

Subjects were patients from a prospective cohort of individuals referred for clinical suspicion of IPDs as well as healthy controls. Functional studies included light transmission aggregation, flow cytometry, and when indicated, Western-blot analysis of platelet glycoproteins, and clot retraction analysis. Genetic analysis was mainly performed by sequencing of coding regions and proximal regulatory regions of the genes of interest.

Results

Of the 70 cases referred for study, we functionally and molecularly characterized 12 patients with Glanzmann Thrombasthenia, 8 patients with Bernard Soulier syndrome, and 8 with other forms of IPDs. Twelve novel mutations were identified among these patients. The systematic study of patients revealed that almost one-third of patients had been previously misdiagnosed.

Conclusions

Our study provides a global picture of the current limitations and access to the diagnosis of IPDs, identifies and confirms new genetic variants that cause these disorders, and emphasizes the need of creating reference centers that can help health care providers in the recognition of these defects.

Characterisation of patients with Glanzmann thrombasthenia and identification of 17 novel mutations

Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder characterised by quantitative and/or qualitative defects of the platelet glycoprotein (GP) IIb/IIIa complex, also called integrin αIIbβ3. αIIbβ3 is well known as a platelet fibrinogen receptor and mediates platelet aggregation, firm adhesion, and spreading. This study describes the molecular genetic analyses of 19 patients with GT who were diagnosed on the basis of clinical parameters and platelet analyses. The patients' bleeding signs include epistaxis, mucocutaneous bleeding, haematomas, petechiae, gastrointestinal bleeding, and menorrhagia. Homozygous or compound heterozygous mutations in ITGA2B or ITGB3 were identified as causing GT by sequencing of genomic DNA. All exons including exon/intron boundaries of both genes were analysed. In a patient with an intronic mutation, splicing of mRNA was analysed using reverse transcriptase (RT)-PCR of platelet-derived RNA. In short, 16 of 19 patients revealed 27 different mutations (ITGA2B: n=17, ITGB3: n=10). Seventeen of these mutations have not been published to date. Mutations in ITGA2B or ITGB3 were identified as causing GT in 16 patients. We detected a total of 27 mutations in ITGA2B and ITGB3 including 17 novel missense, nonsense, frameshift and splice site mutations. In addition, three patients revealed no molecular genetic anomalies in ITGA2B or ITGB3 that could explain the suspected diagnosis of GT. We assume that these patients may harbour defects in a regulatory element affecting the transcription of these genes, or other proteins may exist that are important for activating the αIIbβ3 complex that may be affected.

Glanzmann thrombasthenia: state of the art and future directions

Glanzmann thrombasthenia (GT) is the principal inherited disease of platelets and the most commonly encountered disorder of an integrin. GT is characterized by spontaneous mucocutaneous bleeding and an exaggerated response to trauma caused by platelets that fail to aggregate when stimulated by physiologic agonists. GT is caused by quantitative or qualitative deficiencies of αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes and which by binding fibrinogen and other adhesive proteins joins platelets together in the aggregate. Widespread genotyping has revealed that mutations spread across both genes, yet the reason for the extensive variation in both the severity and intensity of bleeding between affected individuals remains poorly understood. Furthermore, although genetic defects of ITGB3 affect other tissues with β3 present as αvβ3 (the vitronectin receptor), the bleeding phenotype continues to dominate. Here, we look in detail at mutations that affect (i) the β-propeller region of the αIIb head domain and (ii) the membrane proximal disulfide-rich epidermal growth factor (EGF) domains of β3 and which often result in spontaneous integrin activation. We also examine deep vein thrombosis as an unexpected complication of GT and look at curative procedures for the diseases, including allogeneic stem cell transfer and the potential for gene therapy.

Cis-trans isomerization of omega dihedrals in proteins

Peptide bonds in protein structures are mainly found in trans conformation with a torsion angle ω close to 180°. Only a very low proportion is observed in cis conformation with ω angle around 0°. Cis-trans isomerization leads to local conformation changes which play an important role in many biological processes. In this paper, we reviewed the recent discoveries and research achievements in this field. First, we presented some interesting cases of biological processes in which cis-trans isomerization is directly implicated. It is involved in protein folding and various aspect of protein function like dimerization interfaces, autoinhibition control, channel gating, membrane binding. Then we reviewed conservation studies of cis peptide bonds which emphasized evolution constraints in term of sequence and local conformation. Finally we made an overview of the numerous molecular dynamics studies and prediction methodologies already developed to take into account this structural feature in the research area of protein modeling. Many cis peptide bonds have not been recognized as such due to the limited resolution of the data and to the refinement protocol used. Cis-trans proline isomerization reactions represents a vast and promising research area that still needs to be further explored for a better understanding of isomerization mechanism and improvement of cis peptide bond predictions.

Modeling and molecular dynamics of HPA-1a and -1b polymorphisms: effects on the structure of the β3 subunit of the αIIbβ3 integrin

Background

The HPA-1 alloimmune system carried by the platelet integrin αIIbβ3 is the primary cause of alloimmune thrombocytopenia in Caucasians and the HPA-1b allele might be a risk factor for thrombosis. HPA-1a and -1b alleles are defined by a leucine and a proline, respectively, at position 33 in the β3 subunit. Although the structure of αIIbβ3 is available, little is known about structural effects of the L33P substitution and its consequences on immune response and integrin functions.

Methodology/Principal Findings

A complete 3D model of the L33-β3 extracellular domain was built and a P33 model was obtained by in silico mutagenesis. We then performed molecular dynamics simulations. Analyses focused on the PSI, I-EGF-1, and I-EGF-2 domains and confirmed higher exposure of residue 33 in the L33 β3 form. These analyses also showed major structural flexibility of all three domains in both forms, but increased flexibility in the P33 β3 form. The L33P substitution does not alter the local structure (residues 33 to 35) of the PSI domain, but modifies the structural equilibrium of the three domains.

Conclusions

These results provide a better understanding of HPA-1 epitopes complexity and alloimmunization prevalence of HPA-1a. P33 gain of structure flexibility in the β3 knee may explain the increased adhesion capacity of HPA-1b platelets and the associated thrombotic risk. Our study provides important new insights into the relationship between HPA-1 variants and β3 structure that suggest possible effects on the alloimmune response and platelet function.

The αIIb p.Leu841Met (Cab3(a+) ) polymorphism results in a new human platelet alloantigen involved in neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal-neonatal alloimmune thrombocytopenia (FNAIT) diagnosis relies on maternofetal incompatibility and alloantibody identification. Genotyping for rare platelet (PLT) polymorphisms allowed the identification of three families with suspected or confirmed maternofetal incompatibility for the αIIb-c.2614C>A mutation (Halle et al., Transfusion 2008;48:14-15).

STUDY DESIGN AND METHODS

A polymerase chain reaction-sequence-specific primers amplification assay was designed to genotype the αIIb-c.2614C>A mutation. HEK293 cells expressing αIIb-Leu841 or αIIb-Met841 αIIbβ3 forms were used to probe the reactivity of maternal sera from these families and to study the effects of the substitution on αIIbβ3 expression and functions.

RESULTS

Tested by flow cytometry (FCM), one serum sample specifically reacted with αIIb-Met841 but not with αIIb-Leu841 αIIbβ3. This specificity revealed the αIIb-Leu841 polymorphism as a new alloantigen named Cab3(a+) . Cross-match testing using FCM also showed the Cab3(a+) antigen to be expressed at the PLT surface. As for anti-human PLT alloantigen (HPA)-3a (or -3b) and anti-HPA-9bw, detection of anti-Cab3(a+) alloantibodies appeared difficult and required whole PLT assays when classical monoclonal antibody-specific immobilization of PLT antigen test failed. In our FNAIT set, the immune response to Cab3(a+) maternofetal incompatibility could induce severe thrombocytopenias and life-threatening hemorrhages. The p.Leu841Met substitution has limited effects, if any, on local αIIb structure, preserving both αIIbβ3 expression and functions.

CONCLUSION

The Cab3(a+) polymorphism is a new rare alloantigen (allelic frequency <1%) carried by αIIb that might result in severe life-threatening thrombocytopenias. In Sub-Saharan African populations, higher Cab3(a+) gene frequencies (up to 8.2%; Halle et al., Transfusion 2008;48:14-15) and homozygous people are observed.

Point mutations regarded as missense mutations cause splicing defects in the factor XI gene

BACKGROUND

Point mutations within exons are frequently defined as missense mutations. In the factor (F)XI gene, three point mutations, c.616C>T in exon 7, c.1060G>A in exon 10 and c.1693G>A in exon 14 were reported as missense mutations P188S, G336R and E547K, respectively, according to their exonic positions. Surprisingly, expression of the three mutations in cells yielded substantially higher FXI antigen levels than was expected from the plasma of patients bearing these mutations.

OBJECTIVES

To test the possibility that the three mutations, albeit their positions within exons, cause splicing defects.

METHODS AND RESULTS

Platelet mRNA analysis of a heterozygous patient revealed that the c.1693A mutation caused aberrant splicing. Platelet mRNA of a second compound heterozygote for c.616T and c.1060A mutations was undetectable suggesting its degradation. Cells transfected with a c.616T minigene favored production of an aberrantly spliced mRNA that skips exon 7. Cells transfected with a mutated minigene spanning exons 8-10 exhibited a significant decrease in the amount of normally spliced mRNA. In silico analysis revealed that the three mutations are located within sequences of exonic splicing enhancers (ESEs) that bind special proteins and are potentially important for correct splicing. Compensatory mutations created near the natural mutations corrected the putative function of ESEs thereby restoring normal splicing of exons 7 and 10.

CONCLUSIONS

The present findings define a new mechanism of mutations in F11 and underscore the need to perform expression studies and mRNA analysis of point mutations before stating that they are missense mutations.

New K103 β3 allele identified in a context of severe neonatal thrombocytopenia

BACKGROUND

A new β3 allele was identified in a severe case of neonatal alloimmune thrombocytopenia (<7 × 10(9) /L).

STUDY DESIGN AND METHODS

Diagnosis was done by use of monoclonal antibody-specific immobilization of platelet (PLT) antigen for serologic analyses and polymerase chain reaction (PCR)-sequence-specific primers (SSP) and PCR-restriction fragment length polymorphism (RFLP) for genotyping. Direct sequencing of PCR product was done and mutant αIIbβ3 expressed in HEK-293 cells.

RESULTS

Serologic analysis revealed in the maternal serum an anti-human PLT alloantigen (HPA)-1a alloantibody associated to an anti-α2β1. Anti-HPA-1a alloimmunization diagnosis was confirmed by genotyping showing maternofetal incompatibility. However, investigation of rare HPA polymorphisms revealed discrepant HPA-16b assignation between PCR-RFLP and PCR-SSP. Sequencing revealed a new c.385C>A mutation in the β3 coding sequence resulting in a false assignation of the HPA-16b allele by PCR-RFLP. This mutation leads to a Q103K substitution in mature β3. The K103-β3 form of the complex was expressed in HEK-293 cells but did not react with the maternal serum.

CONCLUSION

We have characterized a new rare allele (frequency < 1%) of β3 that yields false HPA-16b genotyping in PCR-RFLP. This new case of false typing assignation emphasizes the necessity to use two genotyping techniques in diagnosis. This particularly applies for rare HPA polymorphisms when PLT phenotyping cannot be used.