Platelet GP IIIa Pl(A) polymorphisms display different sensitivities to agonists

Pharmacogenomics and drug response in cardiovascular disorders

There are a total of 17 families of drugs that are used for treating the heterogeneous group of cardiovascular diseases. We propose a comprehensive pharmacogenomic approach in the field of cardiovascular therapy that considers the five following sources of variability: the genetics of pharmacokinetics, the genetics of pharmacodynamics (drug targets), genetics linked to a defined pathology and its corresponding drug therapies, the genetics of physiologic regulation, and environmental–genetic interactions. Examples of the genetics of pharmacokinetics are presented for phase I (cytochromes P450) and phase II (conjugating enzymes) drug-metabolizing enzymes and for phase III drug transporters. The example used to explain the genetics of pharmacodynamics is glycoprotein IIIa and the response to antiplatelet effects of aspirin. Genetics linked to a defined pathology and its corresponding drug therapies is exemplified by ADRB1, ACE, CETP and APOE and drug response in metabolic syndrome. The examples of cytochrome P450s, APOE and ADRB2 in relation to ethnicity, age and gender are presented to describe genetics of physiologic regulation. Finally, environmental–genetic interactions are exemplified by CYP7A1 and the effects of diet on plasma lipid levels, and by APOE and the effects of smoking in cardiovascular disease. We illustrate this five-tiered approach using examples of cardiovascular drugs in relation to genetic polymorphism.

Relationship of the platelet glycoprotein PlA and fibrinogen T/G+1689 polymorphisms with peripheral arterial disease and ischaemic heart disease

INTRODUCTION

Genetic variation in plasma fibrinogen and the platelet receptor GP IIIa locus has been independently associated with increased risks of ischaemic heart disease, but there have been few reports on the relationship with peripheral arterial disease. This study determined the risk of peripheral arterial disease and ischaemic heart disease associated with polymorphisms of fibrinogen T/G(+1689) and platelet glycoprotein Pl(A) genes and the effects of cigarette smoking and fibrinogen.

MATERIALS AND METHODS

In the 5-year follow-up phase of the Edinburgh Artery Study, 939 subjects (60-79 years) had DNA extracted from a venous blood sample. One hundred sixteen subjects were identified as having angina, 87 a myocardial infarction, 104 had intermittent claudication and 663 subjects comprised a healthy group.

RESULTS

Distribution of the fibrinogen genotype was similar across the disease and healthy groups. Logistic regression analyses found no significant association between fibrinogen genotype and ischaemic heart disease and peripheral arterial disease. A lower percentage of claudicants had the Pl(A2) allele (8.3% vs. 15.2%, p=0.025). After adjustment for age and sex, the risk of IC associated with the Pl(A2) was half that of the homozygous Pl(A1) genotype (OR 0.49, 95% CI 0.25, 0.88; p<==0.05). Adjustment for lifetime smoking and fibrinogen levels increased the odds slightly to nonsignificance.

CONCLUSIONS

The Pl(A2) genotype was associated with a decreased risk of developing IC. There was no significant relationship between fibrinogen T/G(+1689) genotype and ischaemic and peripheral heart disease in this older population.

Protein phosphatase 1 associates with the integrin alphaIIb subunit and regulates signaling

Regulation of integrin activation occurs by specific interactions among cytoplasmic proteins and integrin alpha and beta cytoplasmic tails. We report that the catalytic subunit of protein phosphatase 1 (PP1c) constitutively associates with the prototypic integrin alphaIIbbeta3 in platelets and in cell lines overexpressing the integrin. PP1c binds directly to the cytoplasmic domain of integrin alphaIIb subunit containing a conserved PP1c binding motif 989KVGF992. Anchored PP1c is inactive, while thrombin-induced platelet aggregation or fibrinogen-alphaIIbbeta3 engagement caused PP1c dissociation and concomitant activation as revealed by dephosphorylation of PP1c substrate, myosin light chain. Inhibition of ligand binding to activated alphaIIbbeta3 blocks PP1c dissociation and represses PP1c activation. These studies reveal a previously unrecognized role for integrins whereby the alpha subunit cytoplasmic tail localizes the machinery for initiating and temporally maintaining the regulatory signaling activity of a phosphatase.

The platelet polymorphism PlA2 is a genetic risk factor for myocardial infarction

OBJECTIVES

Platelet glycoprotein (GP) receptor IIb/IIIa plays a key role in the development of myocardial infarction (MI), and Pl(A2) is a polymorphism in the gene encoding this receptor. The prevalence of Pl(A2) shows pronounced geographical variation and has to our knowledge not been presented for a Scandinavian population before. Platelets from Pl(A2)-positive individuals show increased aggregability compared with platelets from Pl(A2)-negative individuals, and Pl(A2) genotypes might be associated with MI. The purpose of this study was to investigate the relation between the Pl(A2) polymorphism and MI in a large Scandinavian population.

DESIGN

Case-control study. We included patients with angiographically verified CAD with and without previous MI and a group of healthy individuals matched for age, race, and sex.

RESULTS

We studied the frequency of Pl(A2) in 1191 healthy individuals and 1019 patients with coronary artery disease (CAD). Amongst these patients, 529 subjects had suffered an MI previously. Pl(A2) was present in 28% of healthy individuals, 28% of patients with CAD but no MI, and in 35% of patients with CAD and MI. The difference between healthy individuals and MI patients was significant (P = 0.002). Furthermore, a graded relationship between the number of Pl(A2) alleles and the risk of MI was seen (P = 0.011). Associations between Pl(A2) and traditional cardiovascular risk factors as well as mean platelet volume were investigated. We found a significant interaction between Pl(A2) and serum cholesterol.

CONCLUSION

In our Scandinavian study population the common platelet polymorphism Pl(A2) is significantly associated with an increased risk of MI, but not of CAD. Clinically, typing for Pl(A2) might have implications for antiplatelet therapy of patients with MI.

Clinical significance of PlA polymorphism of platelet GP IIb/IIIa receptors during long-term VAD support

BACKGROUND

Although bleeding and thromboembolism remain major complications after implantation of ventricular assist devices (VADs), no standard anticoagulation protocols are available. Genetic polymorphism of platelet glycoprotein IIb/IIIa may contribute to the development of complications. The present study demonstrates a relationship between the PlA genotype and postoperative complications in patients implanted with pulsatile and axial flow VADs.

METHODS

The PlA genotype was determined in 41 consecutive patients treated with a VAD who received anticoagulation with phenprocoumon and aspirin. Pulsatile Novacor (Novacor Corp, Oakland, CA) and Berlin Heart VADs (Berlin Heart, Berlin, Germany) were implanted in 28 patients and the axial flow MicroMed DeBakey VAD (MicroMed Technology, Inc, Houston, TX) in 13. The relationship between the PlA genotype, the anticoagulation regime, and bleeding and thromboembolic events was analyzed.

RESULTS

There were no differences between patients with the A1A1 and A1A2 genotype regarding demographic characteristics, weight, or infection episodes. The international normalized ratio (INR), platelet activation tests, and doses of aspirin and dipyridamol before events were similar in both groups. Patients with the A1A1 genotype developed more bleeding complications (39% vs 0%, p = 0.021), while patients with the A1A2 genotype showed a tendency toward more thromboembolic events (13% vs 30%, p = 0.33). With regard to different types of VAD, patients with the axial flow DeBakey VAD and the A1A1 genotype developed significantly more bleeding complications (70% vs 0%, p = 0.033).

CONCLUSIONS

In patients with a long-term VAD determination of PlA polymorphism and subsequent adjustment of the anticoagulation regime may lead to a reduction of bleeding and thromboembolic complications.

Genetic influences on blood pressure within the Stanislas Cohort

OBJECTIVE

To investigate the association of 21 polymorphisms within 13 genes, APOE, APOB, APOC3, CETP, LPL, PON1, MTHFR, FGB, F5, GPIIIa, SELE, ACE and AGT, with inter-individual blood pressure (BP) variation.

PARTICIPANTS

Seven hundred and seventy-six men and 836 women, free of antihypertensive and lipid-lowering medications, were selected from the Stanislas Cohort.

RESULTS

ANOVA on blood pressure values after adjustment for covariates [age, body mass index (BMI), contraceptive pill, tobacco and alcohol] showed that lipoprotein lipase (LPL) Ser447Ter and glycoprotein IIIA (GpIIIa) Pl polymorphisms were significantly associated with BP in women (0.01 < or = P < or = 0.05), whereas BP levels in men were significantly different according to apolipoprotein CIII (APOC3) 3206T/G and -482C/T polymorphisms (P < or = 0.05). In women, compared to the most common allele, the GpIIIa Pl allele was associated with increased mean arterial pressure (MAP) (P < 0.05) and pulse pressure (PP) (P < 0.001), and the LPL 447Ter allele was associated with decreased systolic blood pressure (SBP) and PP levels (0.001 < or = P < or = 0.05). These two polymorphisms appeared to act independently. In men, the APOC3 3206GG genotype was related to decreased diastolic blood pressure (DBP) and MAP levels (P < or = 0.01), and the APOC3 -482T allele with decreased PP levels (P < or = 0.05). The presence of both the -482C allele and the 3206GG genotype was related to decreased DBP, suggesting that specific haplotypes might be involved.

CONCLUSION

The APOC3, LPL and GpIIIa genes were found to be associated with BP levels. The contributions of these genes, although modest, are consistent with the polygenic nature of BP levels.

Interaction between a genetic variant of the platelet fibrinogen receptor and fibrinogen levels in determining the risk of cardiovascular events

BACKGROUND

The PlA1A2 polymorphism of glycoprotein IIIa (GPIIIa), which affects postoccupancy signaling by the platelet fibrinogen receptor IIbIIIa, has been investigated as a potential genetic risk factor for cardiovascular events in numerous studies, without consistent results. We investigated whether the effect of this genetic variant of the platelet fibrinogen receptor on the risk of cardiovascular events is affected by fibrinogen plasma levels.

METHODS

The GPIIIa PlA1A2 polymorphism and fibrinogen levels were determined in 455 men with angiographically documented coronary atherosclerosis.

RESULTS

Neither carriership of the rare PlA2 allele nor fibrinogen plasma levels affected the time to cardiovascular event, as assessed in a proportional hazards model. However, there was a significant interaction between PlA2 carriership and fibrinogen plasma levels (P =.002). Carriership of the variant PlA2 allele significantly affected event-free survival only in individuals within the highest fibrinogen quartile (hazard ratio, 2.7; 95% CI, 1.1 to 7.1; P =.03).

CONCLUSIONS

We observed a statistically significant interaction between a genetic variant of the platelet fibrinogen receptor and fibrinogen levels in determining the risk of cardiovascular events. This interaction may account for the inconsistent results of genetic association studies investigating this genotype as a genetic risk factor in thrombotic cardiovascular events.

Failure of aspirin to prevent atherothrombosis: potential mechanisms and implications for clinical practice

Aspirin (acetylsalicylic acid) reduces the odds of serious atherothrombotic vascular events and death in a broad category of high risk patients by about one-quarter. The mechanism is believed to be inhibition of thromboxane biosynthesis by inactivation of platelet cyclo-oxygenase-1 enzyme. However, aspirin is not that effective; it still fails to prevent the majority of serious vascular events. Mechanisms that may account for the failure of aspirin to prevent vascular events include non-atherothrombotic causes of vascular disease, non-adherence to aspirin therapy, an inadequate dosage, alternative "upstream" pathways of platelet activation (e.g. via stimulation of the ADP, collagen or thrombin receptors on platelets), aspirin-insensitive thromboxane biosynthesis (e.g. via monocyte cyclo-oxygenase-2), or drugs that interfere with the antiplatelet effects of aspirin. Genetic or acquired factors may further modify the inhibitory effects of aspirin on platelets (e.g. polymorphisms involving platelet-associated proteins, increased platelet turnover states). Identification and treatment of the potential causes of aspirin failure could prevent at least another 20% of serious vascular events (i.e. over and above those that are currently prevented by aspirin). There is currently no role for routine laboratory testing to measure the antiplatelet effects of aspirin. Clinicians should ensure that patients at high risk of atherothrombosis (>3% risk over 5 years) are compliant with aspirin therapy and are taking the correct dosage (75-150 mg/day). Patients who cannot tolerate aspirin, are allergic to aspirin, or have experienced recurrent serious atherothrombotic events whilst taking aspirin, should be treated with clopidogrel, and patients with acute coronary syndromes benefit from the combination of clopidogrel plus aspirin. Future research is required to standardize and validate laboratory testing of the antiplatelet effects of aspirin and to identify treatments that can both improve these laboratory measures and reduce the risk of future atherothrombotic events.

Genetically Determined Procoagulant States and Heparin Use

The propensity for both arterial and venous thrombotic disorders involves a genetic predetermination that operates In concert with environmental factors or triggers. Appropriate clinical assessment and therapeutic recommendations for patients with thrombosis requires a thorough knowledge of genetic variables that influence this propensity. This review focuses on the pathophysiology, natural history, and molecular biology of defined thrombophilic risk factors relevant to the care of patients with thrombotic disorders.

Simultaneous modeling of abciximab plasma concentrations and ex vivo pharmacodynamics in patients undergoing coronary angioplasty

An integrated structural pharmacokinetic/pharmacodynamic (PK/PD) model was developed for the glycoprotein IIb/IIIa antagonist abciximab administered to patients undergoing percutaneous transluminal coronary angioplasty. PK/PD data, in the form of plasma abciximab concentrations and ex vivo platelet aggregation in the presence of 20 microM adenosine diphosphate, were obtained from two previously conducted clinical studies. Study 1 consisted of patients who were given abciximab as a single intravenous injection of 0.25 mg/kg (n = 32). Patients in study 2 received an identical bolus dose, followed by a 36-h infusion at 0.125 microg/kg/min (n = 15). The PK component of the final model included drug-receptor binding, nonspecific distribution, and linear systemic clearance, whereas the PD module assumed that ex vivo dynamics were controlled by free plasma drug concentration. Mean PK/PD data from both studies were fitted simultaneously using nonlinear regression. PK profiles from both studies show rapidly decreasing plasma abciximab concentrations at early time points, but with extended terminal disposition phases. The maximum effect (Emax = 83.6%) was achieved rapidly and gradually returned to baseline values, although inhibition could be measured long after abciximab concentrations dropped below the detection limit. The final model well described the resulting PK/PD profiles and allowed for parameter estimation with relatively small coefficients of variation. Simulations were conducted to assess predicted receptor-occupancy and effects of selected parameters on PK/PD profiles. Models such as the one developed in this study demonstrate how drug binding to pharmacological targets may influence the PK of certain drugs and also provide a suitable paradigm for defining the PK/PD relationships of similar compounds.

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.

Resistance in vitro to low-dose aspirin is associated with platelet PlA1(GP IIIa) polymorphism but not with C807T(GP Ia/IIa) and C-5T kozak (GP Ibα) polymorphisms

OBJECTIVES

We investigated whether three platelet gene polymorphisms, Pl(A1/A2), C807T, and C-5T Kozak (encoding, respectively, for platelet membrane glycoproteins (GP) IIIa, GP Ia/IIa, GP Ibalpha), could contribute to the resistance to a low dose of aspirin (160 mg/day).

BACKGROUND

Aspirin antiplatelet effect is not uniform in all patients, and the mechanism by which some patients are in vitro resistant to aspirin remains to be determined. However, it has been suggested that polymorphisms of platelet membrane glycoproteins might contribute to aspirin resistance.

METHODS

Ninety-eight patients on aspirin (160 mg/day) for at least one month were enrolled. Aspirin resistance was measured by the platelet function analyzer (PFA)-100 analyzer; genotyping of the three polymorphisms was performed using a polymerase chain reaction-based restriction fragment-length polymorphism analysis.

RESULTS

Using a collagen/epinephrine-coated cartridge on the PFA-100, the prevalence of aspirin resistance was 29.6% (n = 29). Aspirin-resistant patients were significantly more often Pl(A1/A1) (86.2%; n = 25) than sensitive patients (59.4%; n = 41; p = 0.01). Of the 29 patients, 25 were reevaluated after having taken 300 mg/day aspirin for at least one month. Only 11 patients still have nonprolonged collagen epinephrine closure time, and these were all Pl(A1/A1). No relation was found between resistance status and C-5T Kozak or C807T genotypes.

CONCLUSIONS

Platelets homozygous for the Pl(A1) allele appear to be less sensitive to inhibitory action of low-dose aspirin. This differential sensitivity to aspirin may have potential clinical implications whereby specific antiplatelet therapy may be best tailored according to the patient's Pl(A) genotype.

Platelet glycoprotein IIb/IIIa Pl(A2)/Pl(A2) homozygosity associated with risk of ischemic cardiovascular disease and myocardial infarction in young men: the Copenhagen City Heart Study

OBJECTIVES

We tested the hypothesis that platelet glycoprotein (GP) IIb/IIIa Pl(A2)/Pl(A2) homozygotes or Pl(A1)/Pl(A2) heterozygotes versus Pl(A1)/Pl(A1) noncarriers have increased risk of ischemic cardiovascular disease and myocardial infarction (MI), stratified for age and gender.

BACKGROUND

The GP IIb/IIIa Pl(A1)/Pl(A2) polymorphism influences aggregation of platelets; however, an association between ischemic cardiovascular disease and heterozygosity remains controversial, and association with homozygosity is largely unexplored.

METHODS

We genotyped the participants of the Copenhagen City Heart Study, a prospective cardiovascular investigation of the Danish general population (n = 9,149, 22-year follow-up) and assessed the risk of ischemic cardiovascular disease in heterozygotes or homozygotes versus noncarriers.

RESULTS

Of the participants, 70.0%, 27.3%, and 2.7% were noncarriers, heterozygotes, or homozygotes, respectively. Incidence of ischemic cardiovascular disease was 167 and 103 per 10,000 person-years in homozygous and noncarrier men (log-rank: p = 0.006), whereas this difference was not observed in women (p = 0.33) (genotype.gender interaction: p = 0.03). In homozygous versus noncarrier men <40 years of age, 40 to 50 years, and >50 years at entry, age-adjusted relative risks (RRs) of ischemic cardiovascular disease were 3.6 (1.4 to 9.0), 2.4 (1.3 to 4.6), and 1.0 (0.6 to 1.8), respectively (age.genotype interaction in men: p = 0.04); equivalent multifactorially adjusted RRs were 3.0 (1.1 to 8.0), 2.0 (1.0 to 3.9), and 1.0 (0.6 to 1.8), respectively. The corresponding age-adjusted RR values of MI in men were 5.2 (1.5 to 18), 3.5 (1.6 to 7.5), and 0.5 (0.1 to 1.5), respectively (age.genotype interaction in men: p = 0.002); equivalent multifactorially adjusted RRs were 3.8 (1.0 to 15), 3.1 (1.4 to 6.9), and 0.5 (0.2 to 1.5), respectively.

CONCLUSIONS

Pl(A2)/Pl(A2) homozygosity is associated with a three-fold and four-fold risk of ischemic cardiovascular disease and MI in young men.

Integrin beta3 Leu33Pro homozygosity and risk of cancer

BACKGROUND

Increased tumor cell expression of integrins containing the beta3 subunit is associated with increased progression to invasive tumors, whereas inhibition of beta3 integrin expression and/or function may reduce tumor growth and metastasis. The Leu33Pro polymorphism of the beta3 subunit modulates the function of alpha(IIb)beta3 integrin. We examined whether this polymorphism influences cancer risk.

METHODS

Using participants (n = 9242) from the Copenhagen City Heart Study with 24 years of follow-up and endpoints from the Danish Cancer Registry, we assessed the risk of all cancers and of 27 cancer types in individuals who carry the Leu33Pro polymorphism (heterozygotes and homozygotes) relative to those without the polymorphism (non-carriers). Relative risks (RRs) of cancer and 95% confidence intervals (CIs) were calculated by Cox proportional hazards regression analysis. Differences in cumulative cancer incidence (per 10 000 person-years) were tested using log-rank statistics. Statistical tests were two-sided.

RESULTS

Among the participants, 70.0% were non-carriers, 27.3% were heterozygotes, and 2.7% were homozygotes. We detected 1296 participants with a first cancer. Cumulative incidences in non-carriers, heterozygotes, and homozygotes were 81, 83, and 112, respectively (homozygotes versus non-carriers, P =.02). The age-adjusted RR of all cancers in homozygotes relative to non-carriers was 1.4 (95% CI = 1.1 to 1.9). Incidences in non-carriers, heterozygotes, and homozygotes were 3, 4, and 16 for ovarian cancer; 19, 24, and 36 for breast cancer; and 2, 3, and 7 for melanoma (homozygotes versus non-carriers; P =.002, P =.06, and P =.03, respectively). The age-adjusted RR in homozygotes relative to non-carriers was 4.7 (95% CI = 1.6 to 14) for ovarian cancer, 1.9 (95% CI = 1.0 to 3.7) for breast cancer, and 3.5 (95% CI = 1.1 to 12) for melanoma. Adjustment for other cancer risk factors did not alter these results. Heterozygotes did not differ from non-carriers with respect to cancer risk.

CONCLUSION

Individuals homozygous for the Leu33Pro polymorphism of the beta3 integrin subunit have an increased cancer risk.

Variation of the platelet glycoprotein IIIa PI(A1/A2) allele frequencies in the three ethnic groups of Singapore

BACKGROUND

Polymorphisms of the glycoprotein IIIa receptor have been shown to be associated with differences in platelet aggregability. The PI(A2) variant of the polymorphism has been reported to be an inherited risk factor for acute coronary events. Although the allele frequency of this polymorphism is well documented in Caucasian populations, studies involving Asian Indians, Malays and Chinese are lacking. We studied 706 random male individuals to determine the genotypic distribution of this polymorphism in Singapore.

METHODS

Male subjects included in this study were drawn from those undergoing routine annual medical examinations offered by their employers. Venous blood was obtained from these patients after an overnight fast and from which genomic DNA was extracted. Genotyping was carried out by polymerase chain reaction (PCR) followed by digestion with restriction enzyme NciI. Personal and family medical history of the subjects were also taken.

RESULTS

The genotype distribution of the individuals studied was in accordance to a population at Hardy Weinberg equilibrium. The frequency of the PI(A2) allele was 0.1, 0.01 and 0.01 in the Indians, Malays and Chinese, respectively. The differences in frequencies of the PI(A2) variant are significant among different ethnic groups (P<0.001 for Indians vs. Chinese and Indians vs. Malays).

CONCLUSIONS

We observed a significantly higher frequency of the PI(A2) allele among Indians relative to the Chinese and Malays in Singapore. The effect of this genotype may partially explain the higher rate of ischaemic heart disease seen among Indians compared to the Chinese and Malay ethnic groups.

Genetic polymorphisms of platelet adhesive molecules: association with breast cancer risk and clinical presentation

The main platelet adhesive receptors integrin alpha2beta1, integrin alphaIIbbeta3 and glycoprotein (GP) Ibalpha are also expressed in breast carcinoma cells. They play a key role in tumor cell-induced platelet aggregation and in adhesive interactions necessary for tumoral invasion and metastasis. Several polymorphisms affecting these molecules, two in integrin alpha2 (C807T and G1648A). one in integrin beta3 (T1565C) and one in GP Ibalpha (VNTR), influencing their levels, structure, and possibly their function, have been previously described and associated with cardiovascular diseases. In this study, we investigated the association of these polymorphisms with breast cancer risk or clinical presentation. We studied 101 patients with invasive breast cancer. The main prognostic variables were recorded, and genomic PCR analysis of these polymorphisms was performed. A group of 101 control subjects matched on age and sex was studied and compared with patients. No association was found between VNTR (GP Ibalpha) polymorphism and breast cancer risk or presentation. Genotype and allele frequencies of C807T and G1648A polymorphisms of integrin alpha2 were not statistically different in breast cancer patients and controls, although we found an association between the 1648G/G genotype and higher disease stages (III and IV) (p = 0.02). Breast cancer risk was higher in carriers of beta3 integrin T/T genotype (OR = 2.08, 95% CI = 1.04-4.16, p = 0.04). Furthermore, genotype 1565T/T was also associated with axillary nodal metastasis (p = 0.017) and with tumoral diameter greater than 2cm (p = 0.02). Although confirmatory studies are needed, our results suggest that polymorphic genetic variation of integrins expressed in platelets and epithelial breast cells could modify the risk and the biological aggressiveness of breast carcinomas.

Genetic testing for coronary heart disease: the approaching frontier

The prospect for genetic testing to better delineate the risk for coronary heart disease will become a reality in the next decade. Advances in this area will follow the accelerated trajectory in our ability to dissect the genetics of complex diseases including coronary heart disease. A brief overview of the present state of knowledge will follow the discussion of present approaches in discovering these genetic predispositions. The key issues that will likely shape the field in the near future will also be presented.

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.

Association between the Pl(A) platelet glycoprotein GPIIIa polymorphism and extent of coronary artery disease

BACKGROUND

The Pl(A2) allele of the gene encoding for GPIIIa subunit of the platelet membrane receptor glycoprotein (GP) IIb/IIIa has been suggested as a significant risk factor for thrombotic complications of coronary artery disease (CAD). The aim of the current investigation was to investigate the association between Pl(A) GPIIIa polymorphism and the extent of angiographically confirmed CAD in patients from the north region of Poland.

METHODS

The study was performed in 397 male Caucasian patients. All subjects had significant coronary artery stenosis confirmed by elective coronary angiography. Screening for the Pl(A) GPIIIa genotypes was performed by polymerase chain reaction of genomic DNA, followed by NciI digestion and agarose gel electrophoresis.

RESULTS

The genotype distribution of the Pl(A) GPIIIa polymorphism in our study group was Pl(A1/A1)-75%, Pl(A1/A2)-24% and Pl(A2/A2)-1% with Pl(A1) and Pl(A2) allele frequencies of 0.87 and 0.13, respectively. The prevalence of the homozygous Pl(A1/A1) genotype among subjects with multiple-vessel CAD (two or three vessels with at least 50% stenosis) was significantly higher than in patients with single-vessel disease; the odds ratio of Pl(A2/A2) or Pl(A1/A2) patients for having multiple-vessel CAD was 0.46 (95% CI 0.27-0.77, P<0.01). The mean CAD score for Pl(A1/A1) patients was significantly higher in comparison to Pl(A2/A2) and Pl(A1/A2) patients (7.58+/-2.20 and 6.98+/-2.37, respectively, P<0.05).

CONCLUSIONS

Our results suggest, that the Pl(A1/A1) genotype of Pl(A) GPIIIa polymorphism is associated with more severe CAD in male Caucasian patients from the north region of Poland.

Implications of pharmacogenetics for individualizing drug treatment and for study design

Adverse drug reactions and ineffective drug treatment are responsible for a large health care burden. Considerable variability in drug response makes the prediction of the individual reaction difficult. Pharmacogenetics can help to individualize drug treatment in accordance with the genetic make-up of the patient. Drug response is best understood as a complex interplay between pharmacokinetics, pharmacodynamics, and other disease-associated factors. There are a large number of genetic variants in the enzymes of phase I and phase II drug metabolism, in drug transporters, and drug targets, all of which account for differences in drug response. The polymorphisms in the cytochrome P450 enzyme system have been investigated most extensively. Genotype-based dose adjustment which should ensure "bioequivalent" drug concentrations in all patients has been derived from pharmacokinetic parameters, but this approach will have to be verified in prospective studies. Drug transport has recently been recognized as a further crucial determinant in pharmacokinetics. The effect of genetics on disease susceptibility and drug treatment has been studied quite extensively; however, hardly any of this progress is at present reflected in routine health care. The integration of pharmacogenetic factors in clinical trials requires novel considerations for study design and data interpretation. It is to be hoped that the new science bioinformatics will (a) help us identify the contribution of genetics to disease and treatment response and will (b) create data-processing devices which help the physician in the face of the enormously expanding scientific knowledge in selecting the best individually adapted treatment for the patient.

Cardiovascular pharmacogenomics: current status, future prospects

Pharmacogenomics is a field dedicated to exploring the contribution of genetics to interindividual variability in drug response. A goal of cardiovascular pharmacogenomics is to guide cardiovascular drug development and selection so as to optimize therapeutic benefit and minimize the potential for toxicity. Genetic-based differences in drug metabolism have long been recognized but just now are on the verge of wider clinical application. Differences in efficacy of cardiovascular drugs (independent of drug concentration) based on common genetic variations (polymorphisms) only recently have begun to be explored, but the potential for clinical application appears promising. Examples are presented of important pharmacodynamic effects of genetic variants on several drugs, including those in antiarrhythmic, reninangiotensin, beta-blocker, lipid-lowering, and antithrombotic classes. Principles of pharmacogenomics applied to drug metabolism are discussed that are relevant to drug development and clinical use, and examples are given for CYP450 phase I enzymes, phase II enzymes, and drug transporters. Challenges in establishing true pharmacogenetic associations are discussed, and current and future clinical potential is summarized. Rapid research progress and initial clinical applications with pharmacogenomics are foreseen in the near future.

Enhanced activation of mitogen-activated protein kinase and myosin light chain kinase by the Pro33 polymorphism of integrin beta 3

Integrin beta(3) is polymorphic at residue 33 (Leu(33) or Pro(33)), and the Pro(33) variant exhibits increased outside-in signaling to focal adhesion kinase and greater actin reorganization. Because focal adhesion kinase activation and an intact cytoskeleton are critical links for integrin-mediated signaling to MAPK, we explored the role of integrin alpha(IIb)beta(3) in this signaling using Chinese hamster ovary and human kidney 293 cell lines expressing either the Leu(33) or Pro(33) isoform of beta(3). Compared with Leu(33) cells, Pro(33) cells demonstrated substantially greater activation of ERK2 (but not MAPK family members JNK and p38) upon adhesion to immobilized fibrinogen (but not fibronectin) and upon integrin cross-linking. ERK2 activation was mediated through MAPK kinase and required phosphoinositide 3-kinase signaling and an intact actin cytoskeleton. Human platelets and Chinese hamster ovary cells expressing the Pro(33) isoform showed enhanced activation of the ERK2 substrate myosin light chain kinase (MLCK) upon adhering to fibrinogen. Furthermore, compared with platelets and cells expressing the Leu(33) isoform, the Pro(33) variant showed greater alpha-granule release, clot retraction, and adhesion to fibrinogen under shear stress, and these functional differences were abolished by MLCK and MAPK kinase inhibition. Post-integrin occupancy signaling through MAPK and MLCK after alpha(IIb)beta(3) cross-linking may explain in part the increased adhesive properties of the Pro(33) variant of integrin beta(3).

Platelet FcgammaRIIA His131Arg polymorphism and platelet function: antibodies to platelet-bound fibrinogen induce platelet activation

The His131Arg polymorphism of platelet FcgammaRIIA affects the binding affinity of certain IgG subclasses. The Arg131 allele has been associated with (auto)immune thrombocytopenia and heparin-induced thrombocytopenia in some studies. Because FcgammaRIIA can transmit platelet activation signals, we studied platelet responsiveness from 73 healthy donors to determine if this polymorphism modulated platelet function. Platelet function was studied by agonist and shear-induced activation, and standard aggregation. FcgammaRIIA was genotyped by allele-specific PCR. Compared with His131, the Arg131 allele was associated with significantly greater binding of activation-dependent antibodies. This effect was most prominent for the receptor-induced binding site (RIBS) antibodies F26 (P < 0.0001) and RIBS1 (P = 0.0057), and the ligand-induced binding site antibody LIBS1 (P = 0.0367). Unexpectedly, Arg131-positive platelets did not show greater fibrinogen binding, platelet aggregation or shear-induced platelet activation. We considered whether enhanced Fc binding and FcgammaRIIA cross-linking were responsible for those discrepancies. The increased binding of the two RIBS antibodies to the Arg131 isoform was abolished by blocking FcgammaRIIA, and the FcgammaRIIA genotype effect on F26 IgG binding was lost when F26 F(ab')2 fragments were used. Furthermore, intact F26 and RIBS1 IgG directly and specifically induced P-selectin expression, and this effect was greatest in Arg131-positive platelets. We concluded that (a) the His131Arg polymorphism of FcgammaRIIA does not affect intrinsic platelet reactivity; (b) RIBS antibodies are able to cross-link FcgammaRIIA and activate platelets, and this activation has a modest effect on Arg131 platelets; and (c) flow cytometric based platelet assays may need to compensate for this FcgammaRIIA His131Arg effect on platelet activation.

Association of C807T, PlA, and −5 C/T Kozak genotypes with density of glycoprotein receptors on platelet surface

This study investigates whether three platelet glycoprotein (GP) polymorphisms, C807T in GP Ia, Pl(A1/A2) in GP IIIa, and -5 T/C Kozak in GP Ibalpha gene, influence the density of the three important adhesion and activation receptors on the platelet surface. Fifty-four healthy donors were genotyped according to the three polymorphisms, and densities of the corresponding GPs were measured by flow cytometry. Our study confirmed the association between C807T polymorphism and platelet surface expression of GP Ia-IIa and GP Ia and demonstrated that the density of GP Ibalpha or GP IX is not associated with the Kozak polymorphism. Although the Pl(A1/A2) polymorphism did not affect the expression of GP IIb-IIIa and GP IIIa on the platelet surface, flow-cytometric analysis employing murine monoclonal antibody SZ21 against GP IIIa can be applied to distinguish Pl(A1/A1) and Pl(A1/A2) polymorphism.

Scientific and therapeutic advances in antiplatelet therapy

Over the past decade, the platelet has emerged as a pivotal entity in cardiovascular diseases. Indeed, the 'preeminence of the platelet' has spawned a variety of drugs that have been shown in large-scale randomized trials to improve patient outcomes in acute coronary syndromes and percutaneous revascularization procedures. Although the platelet was initially viewed only as a bystander in haemostasis, it is now evident that the platelet is in fact a key mediator of thrombosis as well as of inflammation. New insights at the cellular and genomic levels will probably generate novel drugs to inhibit platelet function more effectively and safely than previously possible.

Antagonists of platelet fibrinogen receptor are less effective in carriers of Pl(A2) polymorphism of beta(3) integrin

The polymorphism Leu(33)-->Pro (platelet-specific antigen; Pl(A1/A2)) of platelet GPIIIa is a potential risk factor for arterial thrombosis. However, its influence on platelet function remains controversial and little is known about its impact on platelet sensitivity to GPIIb-IIIa antagonists. Our objective was to compare the effectiveness of various GPIIb-IIIa antagonists in Pl(A2)(+) and Pl(A2)(-) carriers. Platelet aggregation was monitored in healthy donors including Pl(A2)(-) (N=31) and Pl(A2)(+) subjects (N=27; 23 Pl(A1/A2), 4 Pl(A2)/(A2)) using the impedance and turbidimetric aggregation techniques. We evaluated the inhibition of ADP- and collagen-induced aggregation by disintegrins, kistrin and echistatin, and the low-molecular-weight blockers, GR144053F ((4-[4-[4-(aminoiminomethyl]-1-piperazinyl]-1-piperidineacetic acid, hydrochloride trihydrate) and eptifibatide (N(6)-(aminoiminomethyl)-N(2)-(3-mercapto-1-oxopropyl-L-lysylglycyl-L-alpha-aspartyl-L-tryptophyl-L-propyl-L-cysteinamide, cyclic (1-->6)-disulfide). Kistrin (10-30 nmol/l) inhibited ADP- and collagen-induced aggregation stronger in Pl(A2)(-) donors than in Pl(A2)(+) donors; there was a significant difference between 50% inhibitory concentrations (IC(50)). The same tendency occurred with moderate concentrations of eptifibatide (40-100 nmol/l) and also at low concentrations of GR144053F (5-10 nmol/l) and high concentrations of echistatin (80-150 nmol/l), although in the case of the two latter inhibitors, the estimated IC(50) values were not significantly different. In conclusion, GPIIb-IIIa blockers representing various classes are less effective inhibitors of platelet aggregation in Pl(A2)(+) carriers; however, the effect of the genotype is both agonist- and antagonist-dependent.

No evidence for an influence of the human platelet antigen-1 polymorphism on the antiplatelet effects of glycoprotein IIb/IIIa inhibitors

This study investigated the hypothesis that the human platelet antigen-1 (HPA-1) polymorphism may influence the antiplatelet effects of glycoprotein (GP)IIb/IIIa inhibitors. Adenosine diphosphate (30 micro mol)-induced fibrinogen binding was measured by flow cytometry. Abciximab (0.03-3 micro g/ml), tirofiban (0.3-30 nmol/l) or eptifibatide (0.01-1 micro g/ml) were incubated for 15 min with the samples prior to stimulation. IC(50) values for the inhibition of fibrinogen binding were determined from each experiment. All subjects were genotyped by GALIOS and automated fluorescence correlation spectroscopy. Although a marked variability in the inhibitory effects of all three GPIIb/IIIa inhibitors was confirmed, there were no significant differences between the genotypes with respect to the inhibition of fibrinogen binding. Thus, the present study does not provide evidence for an effect of HPA-1 polymorphism on the inter-individual variability in the platelet inhibitory effects of the three GPIIb/IIIa inhibitors approved for clinical use.

Enhanced platelet aggregation with TRAP-6 and collagen in platelet aggregometry in patients with venous thromboembolism

The role of platelet hyperaggregability as a possible risk factor for venous thromboembolism is not well defined. Some authors described enhanced maximal platelet aggregation in platelet aggregometry as a contributing factor for arterial and venous thrombosis. This syndrome has been termed "sticky-platelet syndrome" (SPS). The diagnosis of SPS is based on the demonstration of platelet hyperaggregability in aggregometry after stimulation with epinephrine (EPI) and/or adenosine diphosphate (ADP). We investigated platelet hyperaggregability in platelet-rich plasma (PRP) of patients (n = 34) with unexplained venous thromboembolism in comparison to healthy individuals (n = 53). For analysis, platelet aggregometry was performed and the influence of epinephrine, adenosine diphosphate, collagen (Coll) and thrombin receptor-activated peptide (TRAP-6) as agonist were determined. Compared to the control group, patients with venous thromboembolism showed an enhanced maximal platelet aggregation with low concentrations of TRAP-6 (2 microM) and collagen (0.05 microM). In contrast, we could not detect an increased platelet aggregation with EPI or ADP. Our results indicate that platelet hyperaggregability may represent an independent risk factor in patients with otherwise unexplained venous thromboembolism. In our study, low concentrations of TRAP-6 and collagen are superior to EPI and ADP to define platelet hyperreactivity in platelet aggregometry.

Aspirin resistance and genetic polymorphisms

Differences in genetic makeup or polymorphisms can affect individual drug response. Detecting genetic variation may help predict how a patient will respond to a drug and could be used as a tool to select optimal therapy, tailor dosage regimens, and improve clinical outcomes. The data are replete relative to the therapeutic efficacy of aspirin (ASA) for the prevention of ischemic events. However, there is a paucity of published data on the relationship between polymorphisms and the clinical effects on ASA. Prothrombotic genetic variations that may contribute to ASA resistance, and increased risk of cardiovascular events may involve: (1) a polymorphism on the cyclooxygenase-1 (COX-1) gene affecting Ser529; (2) overexpression of COX-2 mRNA on platelets and endothelial cells; (3) polymorphism PLA1/A2 of the gene encoding glycoprotein IIIa (GPIIIa); and (4) the homozygous 807T (873A) polymorphism allied with increased density of platelet GP Ia/IIa collagen-receptor gene. Because of the possible increased risk of ischemic vascular events, carriers of these genetic polymorphisms may be resistant to the antithrombotic effects of ASA and should be considered for additional or alternative treatment.

The influence of platelet glycoprotein polymorphisms on receptor function and risk for thrombosis

With regard to hemostasis and thrombosis, collagens are among the most important physiologic components of the extracellular matrix in their role as platelet activators. Differences in the rate of platelet activation markedly influence normal hemostasis and the pathological outcome of thrombosis. Thus, collagen receptors, such as the integrin alpha2beta1 and indirectly, GPIb alpha, represent a relatively unexploited target of pharmacological control and are only recently becoming appreciated as potential factors in the generic risk for thrombosis. In general, the importance platelet glycoprotein polymorphisms as genetic risk factors for arterial thrombosis is a new area of human genomics that needs to be carefully addressed. Discrepancies in the degree to which they are reported to contribute to risk for clinical thrombosis will only be resolved once there is a universal standard for clinical study design. Most of the clinical studies differ by patient population size, ethnicity, bias in the selection of patients and controls, plurality in clinical endpoints and variation of environmental factors. Despite these differences, there is substantial evidence that the integrin beta3 PlA2 haplotype, the GPIb alpha Met145 haplotype, the GPIb alpha -5C haplotype and the integrin alpha2 haplotype 1 (807T) each contribute to the risk for and morbidity of thrombotic disease. There may remain dispute as to the extent of their contribution. However, well-designed, large, prospective, genetic and epidemiologic studies are needed to clarify the role of these and other platelet receptor polymorphisms. Most importantly, the cumulative effects of multiple platelet and plasma glycoproteins SNPs to thrombotic risk must be evaluated concurrently. Additional in vitro studies of the functional relevance underlying these polymorphisms are needed to provide a sound biological explanation for the results of clinical correlations. The opportunity now exists to make significant inroads into the development of strategies for the prevention of thrombotic disease.

Drug target pharmacogenomics: an overview

Pharmacogenomics is a field aimed at understanding the genetic contribution to variability in drug efficacy and toxicity. The goal is to be able to select the drugs with the greatest likelihood of benefit and the least likelihood of harm in individual patients, based on their genetic make-up. Pharmacogenetics has historically been a field focused primarily on genetic polymorphisms in drug metabolizing enzymes and their impact on drug efficacy and toxicity. More recently, investigators have begun to study the relationship between drug target polymorphisms and drug efficacy and toxicity. There are now numerous examples in the literature of associations between drug target polymorphisms and drug effect. Drug targets can be broken into three main categories: the direct protein target of the drug, signal transduction cascades or downstream proteins, and disease pathogenesis proteins. While the drug target pharmacogenetics literature provides 'proof of concept' that genetic variability contributes to the variability in drug response, the data are not to the point of being clinically useful in most cases. Specific problems to date include the inability of a single polymorphism to be highly predictive of response, and inconsistencies across studies of the same polymorphism. It seems likely that an important factor in the above limitations is the approach of focusing on a single polymorphism in a single gene. Given that most drug responses involve a large number of proteins, all of whose genes could have several polymorphisms, it seems unlikely that a single polymorphism in a single gene would explain a high degree of drug response variability in a consistent fashion. Thus, it seems that a polygenic, or genomic approach will be more appropriate. Candidate gene and genome scanning approaches to pharmacogenomics have shown promise in relating drug target polymorphisms to response or toxicity, and pharmacogenomic strategies for drug discovery and drug development are now being implemented by most major pharmaceutical companies. Pharmacogenomics has the potential to significantly enhance the ability of clinicians to use medications in a safe and effective manner and, as such, represents an exciting field with tremendous clinical potential.

Low incidence of paradoxical platelet activation by glycoprotein IIb/IIIa inhibitors

The human platelet antigen-1 (HPA-1, Pl(A)) polymorphism has been proposed to influence the inhibitory actions of abciximab. Thus, we hypothesized that this polymorphism might also be the cause for paradoxical activation of platelets by GPIIb/IIIa inhibitors. The effects of abciximab (1-10 microg/ml), tirofiban (3-30 nM), or eptifibatide (0.3-3 microg/ml) on basal and ADP (3 microM)-induced CD62P externalization were measured in n=62 healthy blood donors and n=177 patients with stable coronary artery disease. All subjects were genotyped for the human platelet antigen-1 (HPA-1, Pl(A)) polymorphism by GALIOS(R) and fluorescence correlation spectroscopy. Although a significant platelet hyperreactivity was observed in the patients, the HPA-1 genotype did not influence basal or ADP-induced CD62P expression. A moderate (twofold) stimulation of CD62P expression by abciximab but not by tirofiban or eptifibatide was observed in one patient. Interestingly, this patient carried the HPA-1 b/b genotype. In no other subject any activation of platelets by GP IIb/IIIa inhibitors was observed and there were no statistically significant differences between HPA-1 genotypes with respect to the effects of GP IIb/IIIa inhibitors on basal or ADP-stimulated CD62P expression. It is concluded that paradoxical platelet activation by abciximab is a rare (<2%) phenomenon. HPA-1 b/b genotype might be a contributing factor but clearly does not predict platelet activation by GP IIb/IIIa inhibitors.

Genotyping of human platelet antigen-1 by gene amplification and labelling in one system and automated fluorescence correlation spectroscopy

Genotyping of human platelet antigen-1 (HPA-1) is required for the diagnosis and appropriate therapy of alloimmunization. Recently, the HPA-1 polymorphism has been identified as an inherited risk factor for thrombosis. Most currently used methods for HPA-1 genotyping have the disadvantage of time-consuming post-polymerase chain reaction (PCR) processes such as ligation (oligonucleotide ligation assay), restriction enzyme digestion (allele-specific restriction enzyme analysis) and electrophoresis (single-strand conformation polymorphism). We present a novel method for HPA-1 genotyping based on a homogeneous PCR strategy (GALIOS, gene amplification and labelling in one system) combined with automated fluorescence correlation spectroscopy (FCS). The PCR uses one pair of gene-specific amplification primers and two allele-specific, semi-nested labelling primers. The allele-specific labelling primers differ in a single nucleotide (T for HPA-1a/1a, C for HPA-1b/1b) and are coupled to different fluorescent dyes. The quantities of generated fluorescent PCR products are analysed by FCS at 543 nm and 633 nm excitation wavelength respectively. The genotypes determined using this method were in 100% concordance with the results obtained by allele-specific restriction analysis (n = 380 samples). The assay was validated for specificity, reliability and the dynamic range. This innovative method of rapid HPA-1 genotyping offers a specific and robust system, which is applicable for routine HPA-1 genotyping.

Pathophysiology of plaque instability: insights at the genomic level

Atherosclerosis and plaque rupture represent complex "traits" of unknown cause that involve multiple genes and their variants. Novel genomic technologies provide us with the tools that will allow for the identification of groupings of genes that determine either susceptibility or resistance relative to the development of atherosclerosis and its thromboembolic complications. This information may, in turn, lead to a clearer understanding of the cause and risk for atherosclerosis. Diagnostic tools, as well as preventive and therapeutic strategies, will be derived from such heightened understanding of the disease process. With this chapter, we have presented the current state of knowledge of atherosclerosis genomics.

Interaction of hemostatic genetics with hormone therapy: new insights to explain arterial thrombosis in postmenopausal women

Genetic variants of key hemostatic mediators increasingly have been proposed as risk factors for atherothrombosis. The Hormone and Estrogen/Progestin Replacement Study group recently reported that the initiation of estrogen replacement in postmenopausal women with known coronary heart disease is associated with an early increase in cardiovascular events. A putative genetic susceptibility factor has been proposed a potential mediator of this increased event risk. This review outlines the recent literature to support the premise for this important proposal. Genetic profiling has great potential to improve the safety and efficacy of individualized pharmacotherapy in postmenopausal women and other at-risk populations for the prevention of cardiovascular disease.

Pharmacogenomics: the inherited basis for interindividual differences in drug response

It is well recognized that most medications exhibit wide interpatient variability in their efficacy and toxicity. For many medications, these interindividual differences are due in part to polymorphisms in genes encoding drug metabolizing enzymes, drug transporters, and/or drug targets (e.g., receptors, enzymes). Pharmacogenomics is a burgeoning field aimed at elucidating the genetic basis for differences in drug efficacy and toxicity, and it uses genome-wide approaches to identify the network of genes that govern an individual's response to drug therapy. For some genetic polymorphisms (e.g., thiopurine S-methyltransferase), monogenic traits have a marked effect on pharmacokinetics (e.g., drug metabolism), such that individuals who inherit an enzyme deficiency must be treated with markedly different doses of the affected medications (e.g., 5%-10% of the standard thiopurine dose). Likewise, polymorphisms in drug targets (e.g., beta adrenergic receptor) can alter the sensitivity of patients to treatment (e.g., beta-agonists), changing the pharmacodynamics of drug response. Recognizing that most drug effects are determined by the interplay of several gene products that govern the pharmacokinetics and pharmacodynamics of medications, pharmacogenomics research aims to elucidate these polygenic determinants of drug effects. The ultimate goal is to provide new strategies for optimizing drug therapy based on each patient's genetic determinants of drug efficacy and toxicity. This chapter provides an overview of the current pharmacogenomics literature and offers insights for the potential impact of this field on the safe and effective use of medications.

Hypothermia-induced platelet aggregation in heparinized flowing human blood: identification of a high responder subpopulation

Cold-induced platelet aggregation (CIPA) in PRP has previously been documented in connection with platelet preservation (4-15 degrees C). This report describes hypothermia-induced platelet aggregation (HIPA) in whole blood and at temperatures used in open-heart surgery (24-32 degrees C). HIPA (specifically, the formation of occlusive aggregates) was studied in human whole blood. Fresh heparinized (1.5 U/ml) human blood was cooled and maintained at target temperatures (15, 20, 24, 28, 32, or 37 degrees C) as it flowed (1 ml/min) through 75-cm long 1/32 inches internal diameter polymer conduit. The formation of aggregates in the tubing was verified using optical video microscopy and was quantified by a light-scattering method and a constant-pressure filtration method. Donors were tested at least twice at each target temperature and were classified into three separate response regimes (Low, Medium, and High) on the basis of the number of aggregates and the duration of their appearance. The screening of 121 donors (average age 22.3 +/- 4.3 years) for HIPA at 24 degrees C (the temperature of maximum response) indicated 14% High Responders, 18% Medium Responders, and 68% Low Responders. HIPA was inhibited by EDTA, citrate, PGE1, and Tirofiban, but not by aspirin, and it was enhanced by elevated heparin levels. HIPA was consistently noted in the blood of a subpopulation of donors, and the associated platelet aggregates in the blood of High Responders were rigid and occlusive. It is postulated that such aggregates may contribute to cognitive dysfunction noted in patients undergoing hypothermic open-heart surgery, and that postulus is being investigated.

Reduced inhibition by abciximab in platelets with the PlA2 polymorphism

BACKGROUND

The PlA2 polymorphism of the glycoprotein IIb/IIIa (fibrinogen) receptor has been associated with increased restenosis and stent thrombosis. We postulated that this allele could alter the antiplatelet effect of abciximab in patients undergoing percutaneous coronary intervention.

METHODS

Optical platelet aggregation assays, Ultegra (Accumetrics, San Diego) rapid platelet function assays, and radiometric abciximab binding assays were performed in 66 Pl(A1/A1) and 21 PlA1/A2 patients undergoing percutaneous coronary interventions. The affinity of abciximab for the PlA1 and PlA2 receptors was determined with use of transfected cells.

RESULTS

Compared with PlA1/A1 homozygotes, PlA1/A2 platelets were less completely inhibited after abciximab bolus (P =.002) and at 24 hours (P =.02) as assessed by the rapid platelet function assays. Optical aggregation assays confirmed that Pl(A1/A2) platelets were less completely inhibited after abciximab bolus (P =.05). The radiometric abciximab binding assay demonstrated that the PlA1/A2 platelets had fewer baseline fibrinogen receptors than did the PlA1/A1 platelets (P =.04) and more free fibrinogen receptors at 24 hours (P =.008). Cells transfected to express homozygous PlA1 or PlA2 demonstrated a nonsignificant trend (P =.12) for reduced abciximab affinity for PlA2.

CONCLUSIONS

PlA1/A2 platelets are less completely inhibited with abciximab, contributing to the observed interindividual variability in platelet function inhibition. Because the extent of platelet inhibition is an independent predictor for the risk of major adverse coronary events after percutaneous coronary intervention, the relative resistance of PlA2-positive platelets may contribute to a less favorable outcome in these patients.

Genetic variation in glycoprotein IIb/IIIa (GPIIb/IIIa) as a determinant of the responses to an oral GPIIb/IIIa antagonist in patients with unstable coronary syndromes

This study examined the influence of the Pl(A) polymorphism of glycoprotein IIIa (GPIIIa) in determining the response to an oral GPIIb/IIIa antagonist, orbofiban, in patients with unstable coronary syndromes. Genotyping for the Pl(A) polymorphism was performed in 1014 patients recruited into the OPUS-TIMI-16 (orbofiban in patients with unstable coronary syndromes-thrombolysis in myocardial infarction 16) trial, in which patients were randomized to low- or high-dose orbofiban or placebo for 1 year. The primary end point (n = 165) was a composite of death, myocardial infarction (MI), recurrent ischemia requiring rehospitalization, urgent revascularization, and stroke. Overall, orbofiban failed to reduce ischemic events when compared with placebo, but increased the rate of bleeding. In the whole population, Pl(A2) carriers had a significant increase in MI (n = 33) during follow up, with a relative risk (RR) of 2.71 (95% CI, 1.37 to 5.38; P =.004). There was a significant interaction between treatment (placebo and orbofiban) and the Pl(A) polymorphism for bleeding (n = 187; P =.05). Thus, while orbofiban increased bleeding in noncarriers (RR = 1.87, 1.29 to 2.71; P <.001) in a dose-dependent fashion, it did not increase bleeding events in Pl(A2) carriers (RR = 0.87, 0.46 to 1.64). There was no interaction between treatment (placebo and orbofiban) and the Pl(A) polymorphism for the primary end point (P =.10). However, in the patients receiving orbifiban there was a higher risk of a primary event (RR = 1.55, 1.03 to 2.34; P =.04) and MI (RR 4.27, 1.82 to 10.03; P <.001) in Pl(A2) carriers compared with noncarriers. In contrast, there was no evidence that Pl(A2) influenced the rate of recurrent events in placebo-treated patients. In patients presenting with an acute coronary syndrome, the Pl(A) polymorphism of GPIIb/IIIa may explain some of the variance in the response to an oral GPIIb/IIIa antagonist.

Common variations in platelet glycoproteins: pharmacogenomic implications

Atherosclerosis and its complications are the result of complex interactions between the environment and genetic factors. Platelets play an important role in this disease process and antiplatelet agents are an essential part of its treatment. However, individual response to antiplatelet therapy is variable and agents that are safe and effective in one individual may be ineffective or harmful in another. It is likely that genetic factors are involved in this variance as platelet, and platelet-associated proteins are highly polymorphic. Up to 30% of natural variation in platelet reactivity is related to genetic inheritance. Rare inherited defects of platelet function due to the absence or reduced surface expression of platelet adhesion receptors have long been recognised. These cause minor bleeding defects and are usually clinically apparent. Antiplatelet agents should be avoided in these situations. The importance of the more common genetic variations or polymorphisms, which result in minor changes in the expressed protein and are often clinically silent, is unknown. Investigations are ongoing into the role of this variation in platelet physiology. A number of polymorphisms in platelet surface glycoproteins have received particular attention; the (A1/2) polymorphism resulting in conformational change at the amino terminus of the beta-3 chain of the platelet fibrinogen receptor glycoprotein (GP) IIb/IIIa and polymorphisms in the platelet collagen (GPIa/IIa and GPVI) and von Willebrand receptors (GPIb-IX). The (A2) allele has been associated with resistance to the antiplatelet agent aspirin and increased platelet responsiveness. The GPIa polymorphism has been associated with increased surface expression of GPIa and increased platelet adhesion to collagen. Recently, conflicting reports of the association of these polymorphisms with coronary artery disease (CAD) and its complications have been described. Mutations have also been identified in other platelet surface receptors including the recently identified G(i)-linked platelet adenosine diphosphate (ADP) receptor (P2Y(12)), targeted by the antiplatelet agents ticlopidine and clopidogrel. These discoveries have stimulated interest in the role of genetic factors in platelet physiology. In this article, the current knowledge of the influence of genetic make-up on antiplatelet therapy is discussed.

The looming polypharmacy crisis in the management of patients with heart failure. Potential solutions

Physicians may be on the road to a polypharmacy crisis in the development of new drugs for heart failure. They must somehow learn how to tailor the therapy to individual patients, rather than treating all patients with every potentially beneficial drug. This will not be an easy task, but the current model of rational drug development followed by a large megatrial is costly and not likely to be sustained indefinitely.

Age-dependent Prevalence of Vascular Disease-associated Polymorphisms among 2689 Volunteer Blood Donors

Background: The development of vascular disease involves the interaction of genetic and environmental factors. Because vascular disease is a major contributor to mortality in Western societies, we hypothesized that deleterious polymorphisms associated with hemostasis decrease in frequency among a healthy population as a function of age.

Methods: The frequencies of factor V G1691A Leiden (FVL), factor II (FII) G20210A, methylenetetrahydrofolate reductase (MTHFR) C677T, glycoprotein Ia (GPIa) C807T, glycoprotein IIIa (PlA1/PlA2) T1565C, and angiotensin-converting enzyme (ACE) intron 16 insertion/deletion (I/D) alleles were determined among 2689 healthy Caucasian whole-blood donors. For analysis, participants were divided into three age groups: 17–39 years (n = 979; 505 males and 474 females), 40–59 years (n = 900; 526 males and 374 females), and 60–85 years (n = 810; 530 males and 280 females).

Results: The PlA2 allele frequency decreased from 17.5% to 15.7% and 14.1% in the 17–39 years, 40–59 years, and 60–85 years age groups, respectively (n = 5094 alleles; P = 0.025). Among ACE DD males, the PlA2 allele frequency decreased from 20.8% to 16.1% and 9.1% in the same groups, respectively (n = 810 alleles; P = 0.001). No statistically significant decrease in genotype or allele frequency was observed among carriers of FVL, FII 20210A, MTHFR 677T, GPIa 807T, or ACE D.

Conclusions: These data suggest that PlA2 carriers, especially those who are ACE DD, are statistically less prevalent among older healthy blood donors compared with their younger counterparts. These observations suggest an important, deleterious, time-dependent impact of the PlA2 allele, as well as the ACE DD/PlA2 allelic combination, on overall health and longevity.

Platelet glycoprotein gene polymorphisms and risk of thrombosis: facts and fancies

Over the past several years, platelet glycoprotein gene polymorphisms have received increasing attention as possible inherited determinants of prothrombotic tendency. However, their role in genetic susceptibility to thrombotic disease remains controversial. The glycoprotein IIIa Leu33Pro amino acid substitution appears to be associated with a subtle effect on platelet thrombogenicity in vitro, but is not a major risk factor for arterial thrombotic disease among the general population. Evidence suggests that the glycoprotein IIIa Pro33 allele may be associated with increased risk of thrombotic events following coronary re-vascularization and possibly among younger subjects with atherosclerosis. The nucleotide 807T variant of glycoprotein Ia is associated with increased platelet glycoprotein Ia/IIa receptor density, collagen-induced platelet adhesion and an increased risk of early onset myocardial infarction and stroke. Evaluation of the roles of the glycoprotein Ibalpha Thr145Met and variable number of tandem repeat polymorphisms has been complicated by their lack of well-defined effects on platelet adhesive function and the strong linkage disequilibrium between the two sites. Future epidemiologic studies of platelet glycoprotein gene polymorphisms will require larger sample sizes and family based approaches to further elucidate clinically important associations with thrombotic disease, including gene-environment and gene-gene interactions. Other polymorphisms of potential functional significance within genes encoding platelet membrane proteins will undoubtedly be discovered. The challenge will be to integrate advances in platelet biology with molecular and genetic epidemiology to enhance our understanding of the genetic determinants of common, but etiologically complex thrombotic diseases.

Evaluation of an antibody-based genotype classification of the platelet fibrinogen receptor (GPIIb/IIIa)

Platelet membrane glycoprotein (GP) IIb/IIIa is the central molecule in platelet adhesion and aggregation by high-affinity binding of fibrinogen. Polymorphism of the beta chain of the receptor, especially the GPIIIa-proline33 allele [HPA-1b, Zwb, PI(A2)], has been suggested to be associated with a variety of vascular diseases, such as coronary stenosis, myocardial infarction, cerebral ischemia, or venous thrombosis. Using clinical chemistry standards, we evaluate a flow cytometric whole-blood, antibody-based method to determine the genotype [PI(A1A1), PI(A1A2), PI(A2A)] versus polymerase chain reaction (PCR)-based DNA restriction fragment length analysis in 220 individuals. Both homozygous and heterozygous genotypes differ in the expression of binding sites for the monoclonal antibody, SZ21. Agreement between the two methods was achieved in 187 cases, which reflects a test validity of 85%, a sensitivity of 83.6%, and a specificity of 85.4%. We conclude that flow cytometry is reliable for classifying the PI(AX) genotype. The performance characteristics are easy, fast, and cheap (genomics by proteomics). These features make it suitable for screening patients and broad populations for the future risk of cardiovascular ischemic events.

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

A number of clinical studies have suggested that carriage of the low frequency allele (b) of the human platelet antigen 1 (HPA-1) system is a risk factor for coronary thrombosis. We have examined the effect of a series of HPA biallelic polymorphisms (systems -1, -2, -3 and -5) on the in vitro platelet aggregation in response to adrenaline and collagen in 30 healthy volunteers. There was a significantly higher prevalence (10 out of 18) of carriers of the HPA-1b polymorphism among subjects showing a > 50% aggregation response to adrenaline ('responders') than the prevalence (1/12) in 'non-responders' (P < 0.05). Platelets heterozygous for the HPA-1b polymorphism showed a significantly higher rate (slope) and greater extent (%) of adrenaline-induced aggregation than platelets not carrying the HPA-1b allele (P < 0.05). A greater extent of collagen-induced aggregation was also demonstrated in HPA-1ab platelets (P < 0.05). Inhibition of adrenaline-induced aggregation following incubation with aspirin was greater (P < 0.01) in HPA-1ab than in HPA-1aa platelets. Collagen-induced aggregation was slower in carriers of the HPA-5b allele than in HPA-5aa subjects (P < 0.05). Polymorphisms of the HPA-2 and HPA-3 systems were not associated with different aggregation responses to either adrenaline or collagen. These results support the clinical observation that polymorphism HPA-1b may predispose to increased platelet thrombogenicity and suggest that the presence of polymorphism HPA-5b might render the platelet less reactive to collagen.

Profile and prevalence of aspirin resistance in patients with cardiovascular disease

We determined the prevalence and clinical predictors of aspirin resistance by prospectively studying 325 patients with stable cardiovascular disease who were receiving aspirin (325 mg/day for > or =7 days) but no other antiplatelet agents. We also compared the detection of aspirin resistance with optical platelet aggregation, a widely accepted method, with a newer, more rapid method, the platelet function analyzer (PFA)-100, a whole blood test that measures platelet adhesion and aggregation ex vivo. Blood samples were analyzed in a blinded fashion for aspirin resistance by optical aggregation using adenosine diphosphate (ADP) and arachidonic acid, and by PFA-100 using collagen and/or epinephrine and collagen and/or ADP cartridges to measure aperture closure time. Aspirin resistance was defined as a mean aggregation of > or =70% with 10 microM ADP and a mean aggregation of > or =20% with 0.5 mg/ml arachidonic acid. Aspirin semiresponders were defined as meeting one, but not both of the above criteria. Aspirin resistance by PFA-100 was defined as having a normal collagen and/or epinephrine closure time (< or =193 seconds). By optical aggregation, 5.5% of the patients were aspirin resistant and 23.8% were aspirin semiresponders. By PFA-100, 9.5% of patients were aspirin resistant. Of the 18 patients who were aspirin resistant by aggregation, 4 were also aspirin resistant by PFA-100. Patients who were either aspirin resistant or aspirin semiresponders were more likely to be women (34.4% vs 17.3%, p = 0.001) and less likely to be smokers (0% vs 8.3%, p = 0.004) compared with aspirin-sensitive patients. There was a trend toward increased age in patients with aspirin resistance or aspirin semiresponders (65.7 vs 61.3 years, p = 0.06). There were no differences in aspirin sensitivity by race, diabetes, platelet count, renal disease, or liver disease.