The effect of human platelet alloantigen polymorphisms on the in vitro responsiveness to adrenaline and collagen

Platelet genomics: the role of platelet size and number in health and disease

Taken together, there is ample evidence of the association of cardiovascular disease, cerebrovascular, and inflammatory disease with single nucleotide variants (SNV) due to their impact on platelet size, number, and function. With the use of electronic medical record (EMR) or other phenotypic-linked bioinformatics sources, the more important "functional" variants are emerging and provide valuable information on their specific role in promoting early onset of disease or poor response to therapeutic measures. This review will focus upon the recognized common polymorphisms or gene variants with small, but functional effects, as it is becoming clear that these contribute to hyper- or hypo-responsive platelet phenotypes. The impact of these gene variants is distinguishable among normal individuals, and they are suspected contributors to increased risk of adverse outcomes in patients with underlying disease. There are thousands of gene variants and environmental factors that may mitigate risk or amplify the potential for disease within each of us. When combined with the environment and epigenetic influences, it is clear that whole-genome sequencing and bioinformatics alone will not be enough to truly predict "risk" or probability, but awareness of their potential influence may be a starting point in selective screening and generating prevention strategies to promote a healthy lifestyle or fine-tune therapeutic choices in the future.

Genetic variants that affect platelet function

PURPOSE OF REVIEW

This review summarizes our current knowledge of common gene variants (polymorphisms) that have small individual effects on platelet function in humans, but can cumulatively lead to hyperreactive platelets and increase risk for negative outcomes in thrombotic disorders.

RECENT FINDINGS

Candidate gene association and genome-wide association studies (GWAS) have identified loci that include single nucleotide polymorphisms, which exert a cumulative effect on platelet function by modifying basic platelet parameters, such as mean platelet volume (MPV) or platelet count, by altering the expression or activity of key platelet receptors, or by influencing downstream effector pathways utilized by these receptors.

SUMMARY

Variation in MPV between normal individuals is responsible for roughly a two-fold range in platelet protein content, including key surface receptors and reactive granule constituents, the association of ADRA2, GP1BA, GP6, ITGA2 and P2Y12 variants with platelet reactivity, initially identified by candidate gene analyses, has now been validated by genome-wide approaches in much larger individual cohorts, and GWAS have identified novel gene variants, most notably PEAR1, that participate in variation in platelet reactivity among normal individuals, all of which contribute to a genetic basis for differences in platelet reactivty among normal individuals.

The Similarity Between Aspirin Treatment and the Lack of Response to Epinephrine and the Frequency of Aspirin Resistance in Healthy Males

Objectives: The lack of response of platelets against epinephrine has been discovered with a frequency of 14% to 40% in previous studies. There are studies that have demonstrated the effect of aspirin on platelets may resemble the lack of response to epinephrine. In this study, the extent of the effects of aspirin treatment on aggregation and secretion in healthy males with a lack of response to epinephrine and the frequency of aspirin resistance were investigated. Methods: Blood samples were collected at the beginning and at the end of a 10-day aspirin usage in 52 healthy males. Epinephrine, adenosine diphosphate (ADP), collagen, arachidonic acid (AA) and thrombin aggregations, and adenosine triphosphate (ATP) secretion were studied. Participants were assigned to nonresponder (<20%), semiresponder (20%-60%), and responder (>60%) groups, depending on their maximum aggregation responses to epinephrine. Participants who displayed an aggregation to AA at the end of the aspirin treatment were accepted to be aspirin resistant. Results: Of the 52 participants, 4 were found to be nonresponders and 3 of 52 of the participants were found to be semiresponders. Although the lack of response to epinephrine and aspirin treatment displayed similarities in aggregations using epinephrine, ADP, collagen, and thrombin, they differed in aggregations using AA and for ATP secretion. The ratio of aspirin resistance was determined to be 4:52. Conclusions: The observation of AA aggregation in the participants with a lack of response to epinephrine demonstrates that epinephrine nonresponse cannot substitute aspirin treatment. The fact that aspirin resistance is observed in healthy males supports the view that aspirin resistance exists even before the first usage.

The genetics of normal platelet reactivity

Genetic and environmental factors contribute to a substantial variation in platelet function seen among normal persons. Candidate gene association studies represent a valiant effort to define the genetic component in an era where genetic tools were limited, but the single nucleotide polymorphisms identified in those studies need to be validated by more objective, comprehensive approaches, such as genome-wide association studies (GWASs) of quantitative functional traits in much larger cohorts of more carefully selected normal subjects. During the past year, platelet count and mean platelet volume, which indirectly affect platelet function, were the subjects of GWAS. The majority of the GWAS signals were located to noncoding regions, a consistent outcome of all GWAS to date, suggesting a major role for mechanisms that alter phenotype at the level of transcription or posttranscriptional modifications. Of 15 quantitative trait loci associated with mean platelet volume and platelet count, one located at 12q24 is also a risk locus for coronary artery disease. In most cases, the effect sizes of individual quantitative trait loci are admittedly small, but the results of these studies have led to new insight into regulators of hematopoiesis and megakaryopoiesis that would otherwise be unapparent and difficult to define.

Impact of genetic polymorphisms on platelet function and aspirin resistance

Genetic polymorphisms may affect platelets' responses to the antiplatelet therapy. Our aim was to determine the role of genetic polymorphisms on aspirin resistance in patients with coronary heart disease (CHD). A total of 126 consecutive patients (35-85 years old, 32% women) with chronic stable CHD was enrolled in the study. Platelet function assays were realized by the platelet function analyzer (PFA)-100 with collagen and epinephrine (Col/Epi) and collagen and adenosine diphosphate (Col/ADP) cartridges. Aspirin resistance was defined as having a closure time of less than 186 s with Col/Epi cartridges despite regular aspirin therapy. Factor V, prothrombin, factor XIII, beta-fibrinogen, plasminogen activator inhibitor I (PAI-1), glycoprotein IIIa, methylene tetrahydrofolate reductase, ACE and ApoB gene polymorphisms were determined by three consecutive steps: isolation and amplification of DNA and reverse hybridization. We determined that 30 patients (23.8%) had aspirin resistance by the PFA-100. Mean closure time measured with the Col/ADP cartridges was 74 +/- 12 s (51-104 s). Ten of the 30 patients with aspirin resistance were women (33.3%). Genetic polymorphisms were determined in 30 aspirin-resistant and 17 aspirin-sensitive patients. No statistically significant relationship was determined between aspirin resistance and the genetic panel. In our study we did not determine a significant relationship between the aspirin resistance and factor V, prothrombin, factor XIII, beta-fibrinogen, PAI-1, glycoprotein IIIa, methylene tetrahydrofolate reductase, ACE and ApoB gene polymorphisms.

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.

Genotype-phenotype relationship for six common polymorphisms in genes affecting platelet function from 286 healthy subjects and 160 patients with mucocutaneous bleeding of unknown cause

Polymorphisms affecting platelet receptors and intracellular proteins have been extensively studied in relation to their potential influence in thrombosis and haemorrhages. However, few reports have addressed their impact on platelet function, with contradictory results. Limitations of these studies include, among others, small number of patients, the platelet functional parameters analyzed and their known variability in the healthy population. We studied the effect of six polymorphisms [ITGB3 1565T > C (HPA-1), GPIBA variable number tandem repeat and 524C > T (HPA-2), ITGA2 807C > T, ADRA2A 1780A > G, and TUBB1 Q43P] on platelet function in 286 healthy subjects and their potential pathogenetic role in 160 patients with hereditary mucocutaneous bleeding of unknown cause. We found no effect of any of these polymorphisms on platelet aggregation, secretion, PFA-100, and thrombin generation in platelet rich plasma. Furthermore, patients and controls showed no significant differences in the frequency of any of these polymorphisms. Thus, our study demonstrated that polymorphisms in genes affecting platelet function do not influence significantly major platelet functions and appear irrelevant in the pathogenesis of bleeding disorders.

No evidence for involvement of prothrombotic platelet receptor polymorphisms in acute coronary stent thrombosis

Polymorphisms of receptors involved in platelet adhesion and aggregation modulate platelet thrombogenicity and were found to predispose to premature arterial thromboses in individuals at risk. In our current study, we assessed the potential relevance of prothrombotic platelet receptor polymorphisms for the pathogenesis of acute coronary stent thrombosis. Comparing the genotype prevalences of respective polymorphisms in patients with acute coronary stent thrombosis and healthy control subjects, our data do not indicate an increased risk of carriers of prothrombotic variants of platelet receptors for this complication. Other factors such as the remodelling process and antiplatelet medication appear to be more relevant in this clinical setting. Along with our findings, screening for respective polymorphisms for risk assessment prior to coronary stenting is not indicated.

Evaluation of platelet function and lack of response to epinephrine in pregnant women

Previous studies in healthy subjects have demonstrated a lack of response of platelets to epinephrine at a rate of 16-40% on an aggregometer. An association between the increased procoagulant factors during pregnancy and venous thromboembolism is known, and it has also been shown that prolactin levels increase platelet aggregation. We evaluated whether platelet functions in pregnant women and also assessed the lack of response to epinephrine during this period. We compared 27 healthy and volunteering pregnant women with 26 similar control subjects. Platelet functions were assessed with an aggregometer and a Platelet Function Analyzer (PFA-100). Less than 40% response to epinephrine on the aggregometer was defined as an impaired epinephrine response. The aggregation response of epinephrine was normal in 25 of the 27 pregnant women, while two of them showed a late-rising response. Eight of the 26 subject control group (30.8%) showed an impaired response to epinephrine. When we compared the 25 pregnant and 18 control subjects with normal aggregation responses, the maximum aggregation responses to ADP and epinephrine, and the Col/Epi and Col/ADP cartridge closure time values were significantly lower in pregnant women. There were no difference between second and third trimesters as regards platelet function parameters. The fact that no impaired response to epinephrine was detected in pregnant women while a 30% rate was observed in non-pregnant women indicates that the platelet malfunction caused by a disorder in the Gi protein and intracellular mechanisms is bypassed during pregnancy thanks to some physiological changes.

Platelet functional implications of glycoprotein Ibalpha polymorphisms in African Americans

The platelet glycoprotein Ibalpha is crucial in the binding of platelets to Von Willebrand Factor within areas of high stress. A single nucleotide polymorphism of GP Ibalpha gives rise to the Ko(a) (HPA-2b) and the -5C Kozak polymorphism. The presence of these polymorphisms has been associated with an increased risk for atherothrombotic disease. The Ko(a) polymorphism has been shown to have a higher prevalence in African Americans compared to American Caucasians. However, very little is known regarding any functional consequences of these platelet polymorphisms in African Americans. We assessed the prevalence of the Ko and -5C Kozak polymorphisms in a population of both African American and American Caucasian patients with and without CAD and determined whether there were platelet functional consequences in both groups. We studied 99 patients of which 22 were African American and 77 were American Caucasian. Aggregations were performed and shear induced platelet plug formation was tested using a platelet function analyzer. The HPA-2b allele was significantly higher in African Americans when compared to Caucasians (P = 0.001). Genotype frequencies of the -5C Kozak polymorphism were not significantly different between the two groups. We found no differences in platelet aggregation in African Americans who were either heterozygous or homozygous for the HPA-2b allele or the -5C Kozak allele when compared to American Caucasians of the same category. We found no significant differences in PFA-100 testing. We conclude from our study that these polymorphisms do not lead to altered platelet function in African Americans.

Investigation of platelet glycoprotein IIIa polymorphism using flow cytometry in patients with rheumatoid arthritis

OBJECTIVES

Previous work has shown that the human platelet antigen (HPA) 1b polymorphism of platelet glycoprotein IIIa (GPIIIa) is implicated in the development of ischaemic vascular disease. HPA1b positive platelets have a lower threshold for activation and may exert a greater thrombotic tendency than those without the 1b allele. However, platelets heterozygous for the polymorphism are also more sensitive to aspirin than those homozygous for the 1b allele, which have a similar sensitivity to those without the 1b allele. A flow cytometric method has become available to identify this polymorphism. The aim of our study was to evaluate the use of this assay in patients with rheumatoid arthritis (RA) and to determine the incidence of the 1b allele in these patients. We also compared platelet aggregation and platelet/white blood cell interaction in patients with or without this polymorphism.

METHODS

We enrolled 99 patients and measured platelet aggregation in whole blood and platelet-rich plasma (prp), platelet/white blood cell interaction and C-reactive protein (CRP).

RESULTS

Thirty-four of the 99 patients were unsuitable for analysis because their baseline expression of GPIIIa was outwith the normal range, making the results outwith the limits of the flow cytometric method. The incidence of the 1b allele in the patients was 29%, with incidence being higher in females, although this failed to reach statistical significance. The number of circulating platelet aggregates and adenosine diphosphate (ADP)-induced aggregation in prp was significantly higher in those patients with the 1b allele.

CONCLUSIONS

This method may be of use as an initial screening test.

Genetics of platelet glycoprotein receptors: risk of thrombotic events and pharmacogenetic implications

Platelet aggregation and coronary thrombosis have a central role in the development of acute coronary syndromes and myocardial infarction (MI). Therapies aimed at inhibiting platelet aggregation have shown great benefit in individuals with coronary disease or with multiple risk factors for coronary disease. Genetic variation in platelet surface receptors mediating thrombus formation has been suggested to be associated with platelet hyperreactivity, with increased risk of MI and possibly with the benefit received from various antithrombotic drug treatments. This review focuses on discrepancies and their likely explanations in studies on platelet glycoprotein genetics. Current knowledge on important issues concerning coronary event phenotypes and pharmacogenetics is analyzed. Possible future applicability of these data to patient treatment is also discussed.

Role of platelet glycoprotein polymorphisms in cardiovascular diseases

Atherothrombosis is the leading cause of death in western countries. Major complications of atherothrombotic disease, which are responsible for a large burden of morbidity and mortality, are acute coronary syndromes, ischemic stroke, and peripheral occlusive disease. Plaque rupture, platelet adhesion, aggregation, and thrombosis may lead to unstable angina and may progress to myocardial infarction as well as to ischemic stroke. Platelet membrane glycoprotein receptors mediate crucial reactions in acute thrombosis and chronic processes of atherogenesis. The platelet glycoprotein GP IIb/IIIa, which is the most abundant platelet receptor, also represents the drug target of a novel class of anti-platelet drugs, which includes abciximab, tirofiban, and eptifibatide. The genes encoding the three major platelet glycoprotein receptors (GP Ib/IX/V, GP Ia/IIa, and GP IIb/IIIa) are subject to considerable genetic variability. This paper reviews how polymorphisms in the platelet glycoprotein receptors affect platelet function, susceptibility to atherothrombosis and its major complications including myocardial infarction, stroke, and complications following percutaneous coronary interventions, and individual variability of drug response. Recent data on platelet glycoprotein receptor polymorphisms as modifiers of drug action and as predictors of drug response offer the perspective of individualized drug treatment. Prospective studies will show whether this approach is useful or not. As the data reviewed here show clearly, future clinical trials should routinely take into account genetic susceptibility factors and modifiers, both for study design and for predefined patient stratification.

Shear stress augments the enhanced adhesive phenotype of cells expressing the Pro33 isoform of integrin beta3

Adhesion of platelets to the exposed extracellular matrix proteins at sites of vascular injury is partly regulated by the local fluid shear stress. Because the Leu33Pro (Pl(A)) polymorphism of integrin beta(3) confers only a modest increase in adhesion under static conditions, we used CHO and 293 cells expressing the Leu33 or Pro33 isoform of beta(3) in flow chamber experiments to test whether shear forces would alter the Pl(A) adhesive phenotype. We found that shear force augmented the Pro33-mediated enhanced adhesion to fibrinogen. This Pro33-dependent enhancement was aspirin-sensitive and was also observed on immobilized von Willebrand factor and cryoprecipitate, but not fibronectin. Thus, shear stress enhances the adhesive phenotype of the Pro33 cells to multiple physiologic substrates.

PlA polymorphism of integrin beta 3 differentially modulates cellular migration on extracellular matrix proteins

OBJECTIVE

Cell migration is central to multiple physiological and pathologic processes and involves interactions between integrins on the cell surface and the extracellular matrix. The Leu33Pro (PlA) polymorphism of integrin beta3 has been reported to be associated with a greater rate of restenosis after angioplasty, a process involving endothelial and smooth muscle cell migration. We have addressed the possibility that the Leu33Pro polymorphism could modify the migratory behavior of Chinese hamster ovary (CHO) cells expressing the beta3-containing integrin complexes.

METHODS AND RESULTS

Haptotactic migratory responses of CHO alpha(IIb)beta3 cells to fibronectin and vitronectin were not statistically different between the Leu33 and Pro33 cells. However, CHO cells with the Pro33 (PlA2) polymorphism had an enhanced haptotactic migratory response to fibrinogen and von Willebrand Factor. This enhanced migration (1) could be blocked by the alpha(IIb)beta3-complex-specific neutralizing mAb 10E5, by 7E3, a neutralizing mAb specific for the beta3 integrin, and by the alpha(IIb)beta3-blocking peptide Integrilin; (2) was not observed with a CHO cell line expressing an activating beta3 Cys435 to Ala mutation; and (3) was attributable to increased activity of mitogen-activated protein kinase and cyclooxygenase. CHO cell lines expressing the Pro33 isoform of alpha(v)beta3 had an enhanced haptotactic migratory response to vitronectin and osteopontin but not fibrinogen.

CONCLUSIONS

The Leu33Pro polymorphism alters the migratory behavior of cells on extracellular matrix substrates, and the alpha subunit influences the substrate specificity of this genetic effect.