The low-frequency allele of the platelet collagen signaling receptor glycoprotein VI is associated with reduced functional responses and expression

Genetics of platelet reactivity in normal, healthy individuals

BACKGROUND

The Platelet Function Analyzer-100 (PFA-100) is widely used to measure platelet reactivity in whole blood under high shear.

OBJECTIVE

To characterize the genetic component of platelet reactivity among normal individuals, using the PFA-100.

METHODS

We compared baseline platelet reactivity with sex, age, platelet count, hematocrit, plasma von Willebrand factor antigen (VWF:Ag), and alleles of seven candidate genes: integrin subunits alpha2 (ITGA2) and beta3 (ITGB3), platelet glycoproteins GPIbalpha (GP1BA) and GPVI (GP6), purinogenic receptors (P2RY1 and P2RY12) and cyclooxygenase-1 (COX1).

RESULTS

Based on linear and logistic regression models, we report an inverse correlation between baseline closure time (CT) initiated by collagen plus epinephrine (CEPI) and plasma VWF:Ag level, ITGA2 807T and P2RY1 893C, and an inverse correlation between baseline CT initiated by collagen plus adenosine diphosphate (CADP) and P2RY1 893C or GP1BA -5C.

CONCLUSIONS

These results indicate that genetic polymorphisms in ITGA2 and P2RY1 combine with plasma VWF:Ag levels to modulate baseline platelet reactivity in response to collagen plus EPI, while genetic differences in P2RY1 and GP1BA significantly effect platelet responses to collagen plus ADP. Our results demonstrate that the PFA-100 can be used to evaluate the effects of genetic predictors of platelet function.

GPVI: the inside story

The search is on for natural regulators of platelet reactivity. In this issue of Blood, Trifiro and colleagues confirm that the low frequency isoform of platelet GPVI (VIb) attenuates ligand-mediated signal transduction and dampens down platelet reactivity with collagen while not affecting GPVI receptor copy number.

An acquired inhibitor to the GPVI platelet collagen receptor in a patient with lupus nephritis

BACKGROUND

GPVI is a major platelet collagen signaling receptor. In rare cases of immune thrombocytopenic purpura (ITP), autoantibodies to GPVI result in receptor shedding.

OBJECTIVES

To investigate a possible pathogenic role of plasma anti-GPVI antibody located in a woman with lupus nephritis.

METHODS

Measured were (i) platelet aggregation to collagen and convulxin, (ii) platelet GPVI expression (flow cytometry and western blotting), (iii) plasma soluble GPVI (sGPVI, dual antibody ELISA), and (iv) plasma anti-GPVI antibody (ELISA using recombinant sGPVI).

RESULTS

In 2006 and early 2007, the patient had a normal platelet count but a virtual absence of platelet aggregation to collagen and convulxin. Her platelets responded normally to other agonists including cross-linking ITAM-dependent FcgammaRIIA by monoclonal antibody, IV.3. Flow cytometry and western blotting showed a platelet deficiency of GPVI. Plasma sGPVI levels were undetectable whereas ELISA confirmed the presence of anti-GPVI antibody. Sequencing revealed a normal GPVI cDNA structure. The patient's plasma and the isolated IgG3 fraction activated and induced GPVI shedding from normal platelets. A deteriorating clinical condition led to increasingly strict immunosuppressive therapy. This was globally associated with a fall in plasma anti-GPVI titres, the restoration of platelet GPVI and the convulxin response, and the loss of her nephrotic syndrome.

CONCLUSIONS

Our results show that this patient acquired a potent anti-GPVI IgG3 antibody with loss of GPVI and collagen-related platelet function. Further studies are required to determine whether anti-GPVI antibodies occur in other lupus patients with nephritis.

A compound heterozygous mutation in glycoprotein VI in a patient with a bleeding disorder

BACKGROUND

The physiological relevance of the collagen glycoprotein VI (GPVI) receptor was known prior to its recognition as a platelet membrane receptor as several patients lacking GPVI as a consequence of autoantibody inhibition presented with a mild bleeding diathesis. Remarkably, patients with a proven GPVI gene mutation have not yet been identified.

RESULTS

In the present study, we describe a patient with a lifelong history of bleeding problems, structurally normal platelets but a functional platelet defect. Platelet aggregations are normal except for an absent response to Horm collagen, convulxin and the collagen-related peptide (CRP). ATP dense granule secretion is normal with ADP but absent with Horm collagen. Thrombus formation on a collagen surface in flowing blood is reduced but more single platelets are attached. Remarkably, the platelet function analyzer-100 shows a shortened collagen/ADP closure time. Flow cytometry demonstrates an absent expression of GPVI whereas immunoblot analysis shows strongly reduced levels of GPVI. The patient is compound heterozygous for an out-of-frame 16-bp deletion and a missense mutation S175N in a highly conserved residue of the 2nd Ig-like GPVI domain. The parents without clinical bleeding problems are heterozygous carriers. The mother carries the S175N mutation and presents with a mild functional platelet defect. In vitro studies show a reduced membrane expression and convulxin binding with the mutated S175N compared with the wild-type (WT) GPVI receptor.

CONCLUSIONS

This study presents the first patient with a proven genetic GPVI defect.

The low-frequency isoform of platelet glycoprotein VIb attenuates ligand-mediated signal transduction but not receptor expression or ligand binding

The 2 most common haplotypes of human GP6, GP6a and GP6b, generate the allelic isoforms glycoprotein VI (GPVI)a and GPVIb that differ by 5 amino acids: S219P, K237E, and T249A in the ectodomains, and Q317L and H322N in the cytoplasmic domain. By quantitative Western blot, we found no association between GP6 genotype and total platelet GPVI content among 132 normal subjects. When expressed as soluble products or as membrane-associated receptors, GPVIa and GPVIb have identical affinities for type I collagen, collagen-related peptide, or convulxin. However, the cytoplasmic domain substitutions in GPVIb have a significant effect on GPVI-dependent subcellular associations and ligand-induced signal transduction. L317 increases binding to calmodulin, whereas N322 attenuates binding to Fyn/Lyn. Consistent with the latter finding, convulxin-induced Syk phosphorylation is significantly attenuated in Dami cells stably transfected with GPVIb, relative to GPVIa. This represents direct evidence that haplotype-related GPVI functional differences are inherent in the cytoplasmic domain substitutions, whereby GPVIb binds less strongly to Fyn/Lyn and attenuates the rate and extent of Syk phosphorylation. These allelic differences in GP6a and GP6b explain functional differences in the respective isoforms, but the molecular basis for the several-fold range in GPVI levels of human platelets remains to be determined.

A functional genomics approach reveals novel quantitative trait loci associated with platelet signaling pathways

Platelet response to activation varies widely between individuals but shows interindividual consistency and strong heritability. The genetic basis of this variation has not been properly explored. We therefore systematically measured the effect on function of sequence variation in 97 candidate genes in the collagen and adenosine-diphosphate (ADP) signaling pathways. Resequencing of the genes in 48 European DNA samples nearly doubled the number of known single nucleotide polymorphisms (SNPs) and informed the selection of 1327 SNPs for genotyping in 500 healthy Northern European subjects with known platelet responses to collagen-related peptide (CRP-XL) and ADP. This identified 17 novel associations with platelet function (P < .005) accounting for approximately 46% of the variation in response. Further investigations with platelets of known genotype explored the mechanisms behind some of the associations. SNPs in PEAR1 associated with increased platelet response to CRP-XL and increased PEAR1 protein expression after platelet degranulation. The minor allele of a 3' untranslated region (UTR) SNP (rs2769668) in VAV3 was associated with higher protein expression (P = .03) and increased P-selectin exposure after ADP activation (P = .004). Furthermore the minor allele of the intronic SNP rs17786144 in ITPR1 modified Ca(2+) levels after activation with ADP (P < .004). These data provide novel insights into key hubs within platelet signaling networks.

Genetic polymorphisms in platelet-related proteins and coronary artery disease: investigation of candidate genes, including N-acetylgalactosaminyltransferase 4 (GALNT4) and sulphotransferase 1A1/2 (SULT1A1/2)

BACKGROUND

Both platelet function and heart disease show strong genetic components, many of which remain to be elucidated.

MATERIALS AND METHODS

The roles of candidate polymorphisms in ten platelet-associated genes were compared between 1,237 Acute Coronary Syndrome (ACS) cases (with myocardial infarction and unstable angina) and 386 controls, from an Irish Caucasian population. Additionally, 361 stable angina patients were investigated. Two genes of interest were followed up in a separate Irish study of 1,484 individuals (577 with IHD and 907 unaffected).

RESULTS

The GALNT4 (N-acetyl galactosaminyl transferase 4) 506I allele was significantly underrepresented in ACS (OR = 0.66, CI = 0.52-0.84; P = 0.001; P = 0.01 after correction for multiple testing), while the SULT1A1 (Sulphotransferase 1A1) 213H allele was associated with risk of ACS (OR = 1.37, CI = 1.08-1.74; P = 0.01; P = 0.1 after correction for multiple testing). Subsequent genotyping of further SNPs in GALNT4 in the family-based (IHD) group revealed that the 506I allele showed the same trend towards protecting against ACS but the haplotypic test over the four commonest haplotypes was not significant (P = 0.55). In contrast, the SULT1A1/SULT1A2 gene complex showed suggestive haplotypic association in the family-based study (P = 0.07), with the greatest increase in risk conferred by the SULT1A2 235T allele (P = 0.025).

CONCLUSION

We have identified two risk genes for cardiovascular disease, one of whose (GALNT4) effects may be on either platelet or endothelial function through modifications of PSGL1 or other important glycosylated proteins. The role of sulphotransferases (SULT1A1/2) in cardiovascular disease requires further exploration. Further validation of cardiovascular risks conferred by both genes in other populations (including gene copy number variation) is warranted.

Identification of variation in the platelet transcriptome associated with glycoprotein 6 haplotype

Platelet Glycoprotein VI (GPVI) is the activatory collagen signalling receptor that transmits an outside-in signal via the FcR gamma-chain. In Caucasians two GP6 haplotypes have been identified which encode GPVI isoforms that differ by five amino-acids. The minor haplotype is associated with a modest but statistically significant reduction in GPVI abundance and reduced downstream signalling events. As GPVI is also expressed on megakaryocytes, different GPVI isoforms may imprint on the platelet transcriptome. We investigated the association of GP6 haplotype with transcription by comparing the transcriptomes of platelets from individuals homozygous for the major ('a') and minor ('b') haplotypes to identify differentially expressed (DE) transcripts. Platelet RNA was isolated from apheresis concentrates from 16 'aa' donors and eight 'bb' donors. mRNA was amplified using a template-switching PCR based protocol and fluorescently labelled. Samples were randomly paired both within and between haplotypes and compared on a cDNA microarray. No consistently DE transcripts were identified within the 'aa' haplotype but 52 significantly DE transcripts were observed between haplotypes. Generally the fold differences were low (two to four-fold) but were confirmed by qRT-PCR for selected transcripts (TUBB1, P = 0.0004; VWF, P = 0.0126). The results of this study indicate that there are subtle differences between the platelet transcriptomes of individuals who differ by GP6 haplotype. The identification of DE genes may identify critical pathways and nodes not previously known to be involved in platelet development and function.

Platelet glycoprotein VI-related clinical defects

Human patients with defects associated with the platelet collagen receptor, glycoprotein (GP)VI, are rare and usually described as having a mild bleeding disorder. However, here we review clinical profiles of patients with familial or acquired GPVI defects, revealing the bleeding defect is often severe and associated with immune dysfunction. GPVI is a member of the immunoreceptor family, and co-expressed on platelets with Fc receptor gamma-chain (FcRgamma). Ligand binding to GPVI leads to activation of platelet integrins, in particular alpha(IIb)beta(3) that mediates platelet aggregation; and activation of endogenous platelet metalloproteinases resulting in ectodomain shedding and release of a soluble GPVI fragment. Increasing evidence supports the functional importance of GPVI/FcRgamma in thrombus formation at arterial shear rates, and expression levels of platelet GPVI may be a marker of thrombotic risk. Over the past 20 years, patients have been reported with GPVI-related defects involving: (i) an acquired deficiency, resulting from (a) anti-GPVI autoantibodies or (b) other causes; or (ii) a congenital deficiency, where (c) GPVI is not expressed or (d) is expressed in a dysfunctional form with defective signalling to alpha(IIb)beta(3). Clinical consequences of GPVI-related defects may be uniquely informative about the role of platelet GPVI in health and disease.

Mapping the platelet profile for functional genomic studies and demonstration of the effect size of the GP6 locus

BACKGROUND

Evidence suggests the wide variation in platelet response within the population is genetically controlled. Unraveling the complex relationship between sequence variation and platelet phenotype requires accurate and reproducible measurement of platelet response.

OBJECTIVE

To develop a methodology suitable for measuring signaling pathway-specific platelet phenotype, to use this to measure platelet response in a large cohort, and to demonstrate the effect size of sequence variation in a relevant model gene.

METHODS

Three established platelet assays were evaluated: mobilization of [Ca(2+)](i), aggregometry and flow cytometry, each in response to adenosine 5'-diphosphate (ADP) or the glycoprotein (GP) VI-specific crosslinked collagen-related peptide (CRP). Flow cytometric measurement of fibrinogen binding and P-selectin expression in response to a single, intermediate dose of each agonist gave the best combination of reproducibility and inter-individual variability and was used to measure the platelet response in 506 healthy volunteers. Pathway specificity was ensured by blocking the main subsidiary signaling pathways.

RESULTS

Individuals were identified who were hypo- or hyper-responders for both pathways, or who had differential responses to the two agonists, or between outcomes. 89 individuals, retested three months later using the same methodology, showed high concordance between the two visits in all four assays (r(2) = 0.872, 0.868, 0.766 and 0.549); all subjects retaining their phenotype at recall. The effect of sequence variation at the GP6 locus accounted for approximately 35% of the variation in the CRP-XL response.

CONCLUSION

Genotyping-phenotype association studies in a well-characterized, large cohort provides a powerful strategy to measure the effect of sequence variation in genes regulating the platelet response.

Platelet genomics and the risk of atherothrombosis

Platelets play a pivotal role in atherothrombosis after coronary artery plaque rupture. The extent of response of platelets to such an event varies between individuals. This variation is for a large extent genetically controlled. A comprehensive study of sequence variation that modifies the platelet response to agonists is, however, lacking. We set out to discover the regulatory nodes of platelet function by an integrated systems biology approach. The high density genotyping of 110 genes in a cohort of more than 500 individuals, in whom the platelet response to ADP and collagen-related peptide was determined, allowed the robust definition of the first set of regulatory nodes. Microarray and proteomics studies on platelets from individuals with a so-called 'extreme end' response phenotype provided further insight into key regulators of platelet function. In addition, the completion of the HapMap project allows the comprehensive surveying of the genome for sequence variation by the testing of a limited number of single nucleotide polymorphisms (SNPs). With the advent of high density (i.e. 500,000 SNPs) genotyping arrays large number of case and control samples can be tested at an affordable cost. The recently completed Wellcome Trust Case Control Consortium (WTCCC) study has allowed us to address the question of whether common sequence variation confers risk for seven common diseases, one being myocardial infarction. The results of the WTCCC genome-wide association study and issues of case-control study design, particularly the selection of suitable controls, will be reviewed. In conclusion the integration of the results from the platelet systems biology study with those of the WTCCC project enhances our understanding of the mechanisms underlying common conditions such as atherothrombosis and provides pointers to novel cellular mechanisms and pathways.

Improving the power to detect differentially expressed genes in comparative microarray experiments by including information from self-self hybridizations

Our ability to detect differentially expressed genes in a microarray experiment can be hampered when the number of biological samples of interest is limited. In this situation, we propose the use of information from self-self hybridizations to acuminate our inference of differential expression. A unified modelling strategy is developed to allow better estimation of the error variance. This principle is similar to the use of a pooled variance estimate in the two-sample t-test. The results from real dataset examples suggest that we can detect more genes that are differentially expressed in the combined models. Our simulation study provides evidence that this method increases sensitivity compared to using the information from comparative hybridizations alone, given the same control for false discovery rate. The largest increase in sensitivity occurs when the amount of information in the comparative hybridization is limited.

The novel inhibitory receptor G6B is expressed on the surface of platelets and attenuates platelet function in vitro

The G6B cell-surface receptor, which contains a single Ig-like domain, has been shown to bind to SHP-1 and SHP-2 after phosphorylation of 2 immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in its cytoplasmic tail, classifying this protein as a new member of the family of inhibitory receptors. In this study, we demonstrate by real-time polymerase chain reaction (PCR) and Western-blot analysis that G6B is expressed on platelets. Cross-linking of G6B with polyclonal antisera has a significant inhibitory effect on platelet aggregation and activation by agonists such as ADP and collagen-related peptide (CRP). This inhibitory function of G6B appears to operate in a calcium-independent manner. Our results suggest that G6B represents a novel inhibitory receptor found on the surface of platelets and that it could be an antithrombotic drug target.

Establishment of a flow cytometric assay for determination of human platelet glycoprotein VI based on a mouse polyclonal antibody

Glycoprotein VI (GPVI) is the major signaling receptor for collagen on platelets. Recent studies suggest that the surface density of GPVI is related to the activation of platelets by collagen. To measure the level of GPVI on platelets, a mouse polyclonal antibody BJ010 was prepared using an amplified fragment of extracellular domain in GPVI. The specific reactivity of BJ010 was identified by anti-GPVI specific monoclonal antibody 11a12 using immunoprecipitation, sandwich enzyme-linked immunosorbent assay (ELISA) and flow cytometry. The antibody BJ010 recognized both native and denatured human GPVI so that it was used to set up a flow cytometric assay to detect the level of GPVI in normal subjects. The relative level of GPVI on platelets was detected in 101 healthydonors. The median geometric mean fluorescence intensity (GMFI) of platelet GPVI level was 57.7. The variation between minimum and maximum values of platelet GPVI and integrin alpha2beta1 were found to be 3.5- and 4.1-fold, respectively, in the normal subjects. There was a week correlation between the amount of GPVI and integrin alpha2beta1 on platelet surfaces. For the method, the intraassay and interassay coefficient of variation was 6.3% and 8.8%, respectively. The flow cytometric assay described here provides a simple, reliable, reproducible, and readily available means of quantitation of collagen receptor GPVI density on the platelet surface in a larger number of blood samples.

Blood flow devices in medical research and clinical testing in humans: are we approaching personalized medicine?

This review focuses on studies of blood flow devices employed in man to unravel the mechanisms of bleeding and thrombotic disorders, and on the characterization of novel experimental antithrombotic entities and drug candidates in biopharmaceutical research and development. Clinical studies with drug candidates and new therapeutic strategies have also been performed, and the predictability of these experimental approaches to clinical situations is excellent. Based on the solid validation of these flow devices, miniature flow devices employing nonanticoagulated blood drawn directly from an antecubital vein should be developed for diagnostic purposes. It is anticipated that such a diagnostic flow device could develop into a personalized medicine approach.

Structural Basis for the Platelet-Collagen Interaction

Collagen-related peptide is a selective agonist for the platelet collagen receptor Glycoprotein VI. The triple helical peptide contains ten GPO triplets/strand (single letter amino acid nomenclature, where O is hydroxyproline) and so over-represents GPO compared with native collagen sequence. To investigate the ability of Glycoprotein VI to recognize GPO triplets in a setting more representative of the collagens, we synthesized a set of triple helical peptides containing fewer GPO triplets, varying their number and spacing within an inert (GPP)n backbone. The adhesion of recombinant human Glycoprotein VI ectodo-main, like that of human platelets, to these peptides increased with their GPO content, and platelet adhesion was abolished by the specific anti-Glycoprotein VI-blocking antibody, 10B12. Platelet aggregation and protein tyrosine phosphorylation were induced only by cross-linked peptides and only those that contained two or more GPO triplets. Such peptides were less potent than cross-linked collagen-related peptide. Our data suggest that both the sequences GPOGPO and GPO.........GPO represent functional Glycoprotein VI recognition motifs within collagen. Furthermore, we propose that the (GPO)4 motif can support simultaneous binding of two glycoprotein VI molecules, in either a parallel or anti-parallel stacking arrangement, which could play an important role in activation of signaling.

Novel Platelet and Vascular Roles for Immunoreceptor Signaling

The immunoreceptor signaling pathway has classically been defined by its role in mediating intracellular signals downstream of immune receptors on circulating cells, but recent studies have revealed new and unexpected roles for this pathway in vascular biology. In platelets the immunoreceptor signaling pathway is coupled to 2 structurally distinct platelet collagen receptors, glycoprotein VI and integrin α2β1, and is required for the activation of platelets after exposure to vessel wall collagen during plaque rupture. During vascular development immunoreceptor signaling is required for proper formation of the lymphatic system, a role that has revealed the contribution of hematopoietic endothelial progenitors to that process. In conjunction with the identification of new biological roles in vascular cell types, new molecular mechanisms of activating this signaling pathway have been discovered, including activation by integrins and immunoreceptor tyrosine activation motifs (ITAMs) on receptors that do not function as part of the immune response. Here we discuss some of these recent findings and their implications for vascular biology and the treatment of human vascular diseases.

Effect of genetic variations in platelet glycoproteins Ibalpha and VI on the risk for coronary heart disease events in postmenopausal women taking hormone therapy

Millions of women still use postmenopausal hormone therapy (HT). We genotyped 2090 women in Heart and Estrogen/progestin Replacement Study for functional polymorphisms in GP1BA and GP6 and assessed the coronary heart disease (CHD) event rate over 5.8 years of follow-up. In patients receiving placebo, there was an increased CHD death/myocardial infarction (MI)/unstable angina (UA) event rate in carriers of the GP1BA -5C allele (adjusted [adj] P = .006). HT increased the hazard ratio (HR) of CHD events in patients with the GP1BA -5TT genotype by 16% and reduced the HR in patients with the TC+CC genotypes by 46% (adj interaction P < .001). HT reduced the HR in patients with the GP6 13254TT genotype by 17% but increased the HR in patients with the TC+CC genotypes by 35% (adj interaction P < .001). Furthermore, HT increased the HR of CHD events in patients with the GP1BA -5TT plus GP6 13254TC+CC genotypes by 57% and reduced the HR in patients with the GP1BA -5TC+CC plus GP6 13254TT genotypes by 55% (adj interaction P < .001). In postmenopausal women with established CHD, these polymorphisms of platelet genes were predictors of CHD events and significantly modified the effects of HT on CHD risk. It will be important to replicate these findings in other studies.

Selective Blockade of Glycoprotein VI Clustering on Collagen Helices

Platelet activation by collagen relies on the interaction of the receptor glycoprotein VI (GPVI) with collagen helices. We have previously generated two recombinant single chain human antibodies (scFvs) to human GPVI. The first, 10B12, binds to the collagen-binding site on the apical surface between the two immunoglobulin-like domains (D1D2) of the receptor and so directly inhibits GPVI function. The second, 1C3, binds D1D2 independently of 10B12 and has been shown to have a more subtle effect on platelet responses to collagen. Here we have shown that 1C3 potentiates the effect of 10B12 on platelet aggregation induced by collagen and cross-linked collagen-related peptide (CRP-XL). We investigated this by measuring the effect of both scFvs on the binding of D1D2 to immobilized collagen and CRP. As expected, 10B12 completely inhibited binding of GPVI to each ligand in a dose-dependent manner. However, 1C3 inhibited only a proportion of GPVI binding to its ligands, implying that it interferes with another aspect of ligand recognition by GPVI. To further understand the mode of inhibition, we used a unique set of CRPs in which the content of critical glycine-proline-hydroxyproline (GPO) triplets was varied in relation to an "inert" scaffold sequence of GPP motifs. We observed that a stepwise increase in D1D2 binding with (GPO)(2) content was blocked by 1C3. Together these results indicate that 1C3 inhibits clustering of the immunoglobulin-like domains of GPVI on collagen/CRPs, a conclusion that is supported by mapping the 1C3 epitope to the region including isoleucine 148 in D2.

Production of calmodulin-tagged proteins in Drosophila Schneider S2 cells: A novel system for antigen production and phage antibody isolation

We report the development of an expression system for the production of soluble, calmodulin (CaM)-tagged proteins in Drosophila Schneider S2 cells and the subsequent use of these proteins for the selection of phage displayed antibodies. The CaM-tag permitted the purification of recombinant protein to >90% purity in a single step at yields of >20 mg/l. Using platelet glycoprotein VI (GP6) as a model, we demonstrated that the recombinant CaM-tagged protein was post-translationally N-glycosylated and had identical ligand specificity to native protein. A novel selection strategy, exploiting the CaM tag, was then used to isolate four single chain Fv fragments (scFvs) specific for GP6 from a non-immune phage display library. In contrast to other selection methods, which can result in antibodies that do not recognise native protein, all of the scFvs we selected bound cell surface expressed GP6. In conclusion, the production of CaM-tagged proteins in Drosophila Schneider S2 cells and the selection strategy reported here offer advantages over previously published methods, including simple culture conditions, rapid protein purification, specific elution of phage antibodies and preferential selection of phage antibodies that recognise native, cell surface expressed protein.

Definition of novel GP6 polymorphisms and major difference in haplotype frequencies between populations by a combination of in-depth exon resequencing and genotyping with tag single nucleotide polymorphisms

BACKGROUND

Common genetic variants of cell surface receptors contribute to differences in functional responses and disease susceptibility. We have previously shown that single nucleotide polymorphisms (SNPs) in platelet glycoprotein VI (GP6) determine the extent of response to agonist. In addition, SNPs in the GP6 gene have been proposed as risk factors for coronary artery disease.

METHODS

To completely characterize genetic variation in the GP6 gene we generated a high-resolution SNP map by sequencing the promoter, exons and consensus splice sequences in 94 non-related Caucasoids. In addition, we sequenced DNA encoding the ligand-binding domains of GP6 from non-human primates to determine the level of evolutionary conservation.

RESULTS

Eighteen SNPs were identified, six of which encoded amino acid substitutions in the mature form of the protein. The single non-synonymous SNP identified in the exons encoding the ligand-binding domains, encoding for a 103Leu > Val substitution, resulted in reduced ligand binding. Two common protein isoforms were confirmed in Caucasoid with frequencies of 0.82 and 0.15. Variation at the GP6 locus was characterized further by determining SNP frequency in over 2000 individuals from different ethnic backgrounds.

CONCLUSIONS

The SNPs were polymorphic in all populations studied although significant differences in allele frequencies were observed. Twelve additional GP6 protein isoforms were identified from the genotyping results and, despite extensive variation in GP6, the sequence of the ligand-binding domains is conserved. Sequences from non-human primates confirmed this observation. These data provide valuable information for the optimal selection of genetic variants for use in future association studies.

Inherited disorders of platelets: an update

PURPOSE OF REVIEW

To overview inherited syndromes that affect platelets and to discuss current data on the molecular origin and management of these rare diseases.

RECENT FINDINGS

An increasing number of genes responsible for inherited thrombocytopenias have been identified and these now extend to glycosylation defects. Although Glanzmann thrombasthenia remains the predominant disorder of platelet function, knowledge is increasing of pathologies concerning primary receptors for adhesion and signalling, the activation and secretory pathways, and even the development of procoagulant activity.

SUMMARY

These syndromes affect cell adhesion, cell activation, and cell-to-cell contact interactions fundamental in cell biology. Studies on the pathophysiology of alphaIIbbeta3 in platelets have helped unravel the molecular mechanisms of integrin function, and the information gained has resulted in improved antithrombotic therapy. The establishment of national registries and the use of state-of-the-art genomic and proteomic technologies will accelerate progress and help to define how mutations affecting a much larger range of proteins contribute alone or in combination to defining specific platelet phenotypes.

Platelet genomics and proteomics in human health and disease

Proteomic and genomic technologies provide powerful tools for characterizing the multitude of events that occur in the anucleate platelet. These technologies are beginning to define the complete platelet transcriptome and proteome as well as the protein-protein interactions critical for platelet function. The integration of these results provides the opportunity to identify those proteins involved in discrete facets of platelet function. Here we summarize the findings of platelet proteome and transcriptome studies and their application to diseases of platelet function.

Platelet function analyzer (PFA)-100 closure time in the evaluation of platelet disorders and platelet function

BACKGROUND

Closure time (CT), measured by platelet function analyzer (PFA-100) device, is now available to the clinical laboratory as a possible alternative or supplement to the bleeding time test.

AIM

On behalf of the Platelet Physiology Subcommittee of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH-SSC), a working Group was formed to review and make recommendations on the use of the PFA-100 CT in the evaluation of platelet function within the clinical laboratory.

METHODS

The Medline database was searched to review the published information on the PFA-100 CT in the evaluation of platelet disorders and platelet function. This information, and expert opinion, was used to prepare a report and generate consensus recommendations.

RESULTS

Although the PFA-100 CT is abnormal in some forms of platelet disorders, the test does not have sufficient sensitivity or specificity to be used as a screening tool for platelet disorders. A role of the PFA-100 CT in therapeutic monitoring of platelet function remains to be established.

CONCLUSIONS

The PFA-100 closure time should be considered optional in the evaluation of platelet disorders and function, and its use in therapeutic monitoring of platelet function is currently best restricted to research studies and prospective clinical trials.

Qualitative disorders of platelets and megakaryocytes

Qualitative disorders of platelet function and production form a large group of rare diseases which cover a multitude of genetic defects that by and large have as a common symptom, excessive mucocutaneous bleeding. Glanzmann thrombasthenia, is enabling us to learn much about the pathophysiology of integrins and of how alphaIIb beta3 functions. Bernard-Soulier syndrome, an example of macrothrombocytopenia, combines the production of large platelets with a deficit or non-functioning of the major adhesion receptor of platelets, the GPIb-IX-V complex. Amino acid substitutions in GPIb alpha, may lead to up-regulation and spontaneous binding of von Willebrand factor as in Platelet-type von Willebrand disease. In disorders with defects in the MYH9 gene, macrothrombocytopenias are linked to modifications in kidney, eye or ear, whereas other inherited thrombocytopenias variously link a low platelet count with a propensity to leukemia, skeletal defects, learning impairment, and abnormal red cells. Defects of secretion from platelets include an abnormal alpha-granule formation as in the gray platelet syndrome (with marrow myelofibrosis), and of organelle biogenesis in the Hermansky-Pudlak and Chediak-Higashi syndromes where platelet dense body defects are linked to abnormalities of other lysosomal-like organelles including melanosomes. Finally, defects involving surface receptors (P2Y(12), TPalpha) for activating stimuli, of proteins essential for signaling pathways (including Wiskott-Aldrich syndrome), and of platelet-derived procoagulant activity (Scott syndrome) show how studies on platelet disorders are helping unravel the pathways of primary hemostasis.

A novel human CD32 mAb blocks experimental immune haemolytic anaemia in FcgammaRIIA transgenic mice

A fully human IgG1 kappa antibody (MDE-8) was generated, which recognised Fc-gamma receptor IIa (FcgammaRIIa) molecules on CD32 transfectants, peripheral blood monocytes, polymorphonuclear cells and platelets. This antibody blocked FcgammaRIIa ligand-binding via its F(ab')(2) fragment. Overnight incubation of monocytes with F(ab')(2) fragments of MDE-8 leads to a c. 60% decrease in cell surface expression of FcgammaRIIa. MDE-8 whole antibody induced a concomitant c. 30% decrease of FcgammaRI on THP-1 cells and monocytes. In humans FcgammaRIIa plays an important role in the clearance of antibody-coated red blood cells in vivo. As an equivalent of FcgammaRIIa does not exist in mice, the in vivo effect of MDE-8 was studied in an FcgammaRIIa transgenic mouse model. In these mice, antibody-induced anaemia could readily be blocked by MDE-8. These data document a new human antibody that effectively blocks FcgammaRIIa, induces modulation of both FcgammaRIIa and FcgammaRI from phagocytic cells, and ameliorates antibody-induced anaemia in vivo.

Genetic analysis of patients with defects in early B-cell development

Approximately 85% of patients with defects in early B-cell development have X-linked agammaglobulinemia (XLA), a disorder caused by mutations in the cytoplasmic Bruton's tyrosine kinase (Btk). Although Btk is activated by cross-linking of a variety of cell-surface receptors, the most critical signal transduction pathway is the one initiated by the pre-B cell and B-cell antigen receptor complex. Mutations in Btk are highly diverse, and no single mutation accounts for more than 3% of patients. Although there is no strong genotype/phenotype correlation in XLA, the specific mutation in Btk is one of the factors that influences the severity of disease. Mutations in the components of the pre-B cell and B-cell antigen receptor complex account for an additional 5-7% of patients with defects in early B-cell development. Patients with defects in these proteins are clinically indistinguishable from those with XLA. However, they tend to be younger at the time of diagnosis, and whereas most patients with XLA have a small number of B cells in the peripheral circulation, these cells are not found in patients with defects in micro heavy chain or Igalpha. Polymorphic variants in the components of the pre-B cell and B-cell receptor complex, particularly micro heavy chain and lambda5, may contribute to the severity of XLA.

Platelet glycoprotein VI: its structure and function

Glycoprotein (GP) VI is a platelet membrane protein with a molecular weight of 62 kDa that was identified as a physiological collagen receptor from studies of patients deficient in this protein. GPVI-deficient platelets lacked specifically collagen-induced aggregation and the ability to form thrombi on a collagen surface under flow conditions, suggesting that GPVI makes an indispensable contribution to collagen-induced platelet activation. On the platelet surface, GPVI is present as a complex with the Fc receptor (FcR) gamma-chain, probably composed of two GPVI molecules and one FcR gamma-chain dimer. GPVI must form such a dimeric complex to exhibit high affinity binding to collagen. The GPVI-induced activation mechanism is initiated by tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of the FcR gamma-chain, and then this signal is transduced to many related proteins, mainly by tyrosine phosphorylation. GPVI is widely recognized as a requisite factor for the formation of platelet aggregates on a collagen surface under blood flow. However, individuals with GPVI-deficient or null platelets do not exhibit any strong bleeding tendency. Analyzing this apparent dichotomy should provide us with a more precise understanding of the mechanism of thrombus formation.

Multivariate QTL linkage analysis suggests a QTL for platelet count on chromosome 19q

Platelet count is a highly heritable trait with genetic factors responsible for around 80% of the phenotypic variance. We measured platelet count longitudinally in 327 monozygotic and 418 dizygotic twin pairs at 12, 14 and 16 years of age. We also performed a genome-wide linkage scan of these twins and their families in an attempt to localize QTLs that influenced variation in platelet concentrations. Suggestive linkage was observed on chromosome 19q13.13-19q13.31 at 12 (LOD = 2.12, P = 0.0009), 14 (LOD = 2.23, P = 0.0007) and 16 (LOD = 1.01, P = 0.016) years of age and multivariate analysis of counts at all three ages increased the LOD to 2.59 (P = 0.0003). A possible candidate in this region is the gene for glycoprotein VI, a receptor involved in platelet aggregation. Smaller linkage peaks were also seen at 2p, 5p, 5q, 10p and 15q. There was little evidence for linkage to the chromosomal regions containing the genes for thrombopoietin (3q27) and the thrombopoietin receptor (1q34), suggesting that polymorphisms in these genes do not contribute substantially to variation in platelet count between healthy individuals.

Aspirin and clopidogrel: efficacy, safety, and the issue of drug resistance

Aspirin and the thienopyridines ticlopidine and clopidogrel are antiplatelet agents that display good antithrombotic activity. In the past few years, the concept of aspirin resistance has been largely emphasized in the medical literature, although its definition is still uncertain. I suggest that "aspirin-resistant" should be considered as a description for those individuals in whom aspirin fails to inhibit thromboxane A2 production, irrespective of the results of unspecific tests of platelet function, such as the bleeding time, platelet aggregation, or the PFA-100 system. Less well known than aspirin resistance, but certainly better characterized, is the issue of "clopidogrel resistance," which is probably mostly caused by inefficient metabolism of the prodrug clopidogrel to its active metabolite. At present, aspirin and clopidogrel resistance should not be looked for in the clinical setting, because there is no definite demonstration of an association with clinical events conditioning cost-effective changes in patient management.

Platelet membrane collagen receptor glycoprotein VI polymorphism is associated with coronary thrombosis and fatal myocardial infarction in middle-aged men

Glycoprotein VI is a platelet collagen receptor binding to subendothelial collagen after a rupture of an atherosclerotic plaque. The GPVI gene is polymorphic with several SNPs and the T13254C polymorphism predicting amino acid substitution (serine to proline) has been associated with the risk of MI in a preliminary study. We studied the association of the GPVI T13254C with fatal myocardial infarction (MI) and coronary artery disease among the 300 men of the Helsinki Sudden Death Study (HSDS). Genotype frequencies were 77.9% for TT, 20.7% for CT and 1.4% for CC. We found a significant association (P = 0.02) between the C-allele carriers (CT or CC) and coronary thrombosis (OR 2.5, 95% CI: 1.05-6.2). There was also a tendency (P = 0.07) for an association between the C-allele and acute myocardial infarction (AMI) (OR 2.2). The average area of complicated coronary lesions was also significantly (P = 0.01) larger in carriers compared to non-carriers of the C-allele. Our findings support previous results on the role of this GPVI polymorphism, or another linked polymorphism, as a possible predictor of the risk of coronary thrombosis.

Facilitating roles of murine platelet glycoprotein Ib and alphaIIbbeta3 in phosphatidylserine exposure during vWF-collagen-induced thrombus formation

Vessel wall damage exposes collagen fibres, to which platelets adhere directly via the collagen receptors glycoprotein (GP) VI and integrin alpha(2)beta(1) and indirectly by collagen-bound von Willebrand factor (vWF) via the GPIb-V-IX and integrin alphaIIbbeta3 receptor complexes. Platelet-collagen interaction under shear stimulates thrombus formation in two ways, by integrin-dependent formation of platelet aggregates and by surface exposure of procoagulant phosphatidylserine (PS). GPVI is involved in both processes, complemented by alpha2beta1. In mouse blood flowing over collagen, we investigated the additional role of platelet-vWF binding via GPIb and alphaIIbbeta3. Inhibition of GPIb as well as blocking of vWF binding to collagen reduced stable platelet adhesion at high shear rate. This was accompanied by delayed platelet Ca(2+) responses and reduced PS exposure, while microaggregates were still formed. Inhibition of integrin alphaIIbbeta3 with JON/A antibody, which blocks alphaIIbbeta3 binding to both vWF and fibrinogen, reduced PS exposure and aggregate formation. The JON/A effects were not enhanced by combined blocking of GPIb-vWF binding, suggesting a function for alphaIIbbeta3 downstream of GPIb. Typically, with blood from FcR gamma-chain +/- mutant mice, expressing 50% of normal platelet GPVI levels, GPIb blockage almost completely abolished platelet adhesion and PS exposure. Together, these data indicate that, under physiological conditions of flow, both adhesive receptors GPIb and alphaIIbbeta3 facilitate GPVI-mediated PS exposure by stabilizing platelet binding to collagen. Hence, these glycoproteins have an assistant procoagulant role in collagen-dependent thrombus formation, which is most prominent at reduced GPVI activity and is independent of the presence of thrombin.

An association of candidate gene haplotypes and bleeding severity in von Willebrand disease (VWD) type 1 pedigrees

von Willebrand disease (VWD) type 1 is difficult to diagnose because of bleeding variability and low heritability of von Willebrand factor (VWF) levels. We compared a bleeding severity score and bleeding times to candidate gene haplotypes within pedigrees of 14 index cases, using a covariance components model for multivariate traits (Mendel: QTL Association). These pedigrees included 13 affected and 40 unaffected relatives, as defined by plasma ristocetin cofactor (VWF:RCo) levels. The bleeding severity score was derived from a detailed history. Donors were genotyped using a primer extension method, and 9 candidate genes were selected for analysis. VWF:RCo levels had the strongest influence on bleeding severity score and bleeding time. ITGA2 haplotype 2 (807C) and ITGA2B haplotype 1 (Ile(843)) were each associated with increased bleeding severity scores (P < .01 and P < .01, respectively). GP6 haplotype b (Pro(219)) was also associated with increased scores (P = .03) after adjustment for donor age. No association was observed with 6 other candidate genes, GP1BA, ITGB3, VWF, FGB, IL6, or TXA2R. Increased plasma VWF:Ag levels were associated with VWF haplotype 1 (-1793G; P = .02). These results establish that genetic differences in the adhesion receptor subunits alpha(2), alpha(IIb,) and GPVI can influence the phenotype of VWD type 1.

Severe deficiency of glycoprotein VI in a patient with gray platelet syndrome

We report a novel case of gray platelet syndrome (GPS) where a severe deficiency of the platelet collagen receptor, glycoprotein (GP) VI, accompanies classical symptoms of a low platelet count and platelets lacking alpha-granules. Dense granules were normally present. Platelet aggregation with collagen was severely decreased, as was the response to convulxin (Cvx), a GPVI agonist. Quantitative analysis of GPVI using fluorescein isothiocyanate (FITC)-Cvx in flow cytometry showed its virtual absence on the patient's platelets. The GPVI deficiency was confirmed using monoclonal antibodies in Western blotting and in immunogold labeling on frozen thin sections where internal pools of GPVI were confirmed for normal platelets. The Fc receptor gamma-chain, constitutively associated with GPVI in normal platelets, was present in subnormal amounts, and the phospholipase C gamma 2-dependent activation pathway appeared to function normally. No autoantibodies to GPVI were found in the patient's serum using monoclonal antibody immobilization of platelet antigen (MAIPA). Sequencing of coding regions of the GPVI gene failed to show abnormalities, and mRNA for GPVI was present in the patient's platelets, pointing to a probable acquired defect in GPVI expression. Our results may provide a molecular explanation for the subgroup of patients with severely deficient collagen-induced platelet aggregation as previously described for GPS in the literature.

Platelet receptor interplay regulates collagen-induced thrombus formation in flowing human blood

The platelet glycoproteins (GPs) Ib, integrin α2β1, and GPVI are considered central to thrombus formation. Recently, their relative importance has been re-evaluated based on data from murine knockout models. To examine their relationship during human thrombus formation on collagen type I fibers at high shear (1000 s–1), we tested a novel antibody against GPVI, an immunoglobulin single-chain variable fragment, 10B12, together with specific antagonists for GPIbα (12G1 Fab2) and α2β1 (6F1 mAb or GFOGER-GPP peptide). GPVI was found to be crucial for aggregate formation, Ca2+ signaling, and phosphatidylserine (PS) exposure, but not for primary adhesion, even with more than 97% receptor blockade. Inhibiting α2β1 revealed its involvement in regulating Ca2+ signaling, PS exposure, and aggregate size. Both GPIbα and α2β1 contributed to primary adhesion, showing overlapping function. The coinhibition of receptors revealed synergism in thrombus formation: the coinhibition of adenosine diphosphate (ADP) receptors with collagen receptors further decreased adhesion and aggregation, and, crucially, the complete eradication of thrombus formation required the coinhibition of GPVI with either GPIbα or α2β1. In summary, human platelet deposition on collagen depends on the concerted interplay of several receptors: GPIb in synergy with α2β1 mediating primary adhesion, reinforced by activation through GPVI, which further regulates the thrombus formation.

Platelet function and immune response

Altered platelet function may cause abnormal bleeding tendency or thrombosis. The goal of this article is to provide insights for understanding how platelet functions are related to immune response. Autoantibodies and drug-induced platelet antibodies have been demonstrated to downregulate or enhance platelet function. Drug-induced immune thrombocytopenia is an important adverse effect of glycoprotein IIb/IIIa antagonists. Platelets respond to binding of glycoprotein IIb/IIIa by partial platelet activation. This includes conformational changes of glycoprotein IIb/IIIa. Membrane changes may expose immunogenic neoantigens capable of abnormally altering immune responses. The presence of drug-dependent antibodies in an unexpectedly high frequency compared with the frequency of overt thrombocytopenia has opened a model for further studies. These may include monitoring of antiplatelet immune responses when new platelet antagonists are developed and comparisons of specific immune responses in other acute thrombocytopenias, such as those induced by quinidine or heparin and that associated with gold therapy or in acute profound thrombocytopenia, which may follow vaccination with live attenuated viruses.

Identification of the primary collagen-binding surface on human glycoprotein VI by site-directed mutagenesis and by a blocking phage antibody

Glycoprotein (GP) VI is the major receptor responsible for platelet activation by collagen, but the collagen-binding surface of GPVI is unknown. To address this issue we expressed, from insect cells, the immunoglobulin (Ig)-like ectodomains (residues 1-185) of human and murine GPVI, called hD1D2 and mD1D2, respectively. Both proteins bound specifically to collagen-related peptide (CRP), a GPVI-specific ligand, but hD1D2 bound CRP more strongly than did mD1D2. Molecular modeling and sequence comparison identified key differences between hD1D2 and mD1D2. Ten mutant hD1D2s were expressed, of which 4 had human residues replaced by their murine counterpart, and 6 had replacements by alanine. CRP binding studies with these mutants demonstrated that the exchange of lysine at position 59 for the corresponding murine glutamate substantially reduced binding to CRP. The position of lysine59 on the apical surface of GPVI suggests a mode of CRP binding analogous to that used by the related killer cell Ig-like receptors to bind HLA. This surface was confirmed as critical for collagen binding by epitope mapping of an inhibitory phage antibody against GPVI. This anti-GPVI, clone 10B12, gave dose-dependent inhibition of the hD1D2-collagen interaction. Clone 10B12 inhibited activation of platelets by CRP and collagen in aggregometry and thrombus formation by the latter in whole blood perfusion. Antibody 10B12 showed significantly reduced binding to the hD1D2-E59, and, on that basis, the GPVI:10B12 interface was modeled.