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(Circulation. 1997;96:1424-1431.)
© 1997 American Heart Association, Inc.

Articles

Association of the Platelet Pl^A Polymorphism of Glycoprotein IIb/IIIa and the Fibrinogen Bß 448 Polymorphism With Myocardial Infarction and Extent of Coronary Artery Disease

Angela M. Carter, BSc; Nicholas Ossei-Gerning, MRCP; Ian J. Wilson, MD; ; Peter J. Grant, MD

From the Unit of Molecular Vascular Medicine, Research School of Medicine, University of Leeds, Leeds General Infirmary, Leeds, and the Department of Cardiology (I.J.W.), Pinderfields Hospital, Wakefield, UK.

Correspondence to Angela M. Carter, Unit of Molecular Vascular Medicine, Research School of Medicine, G Floor, Martin Wing, General Infirmary at Leeds, Leeds LS1 3EX UK. E-mail Medamc{at}Medphysics.Leeds.ac.UK

	Abstract

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Background Platelets and fibrinogen play an integral role in the development of thrombosis and are implicated in the process of atherosclerosis. The fibrinogen Bß 448 polymorphism and the Pl^A polymorphism of platelet glycoprotein IIIa are reported to be independently associated with coronary artery disease. The aim of this study was to determine the association of the fibrinogen Bß 448 and the platelet glycoprotein IIIa Pl^A polymorphisms in relation to extent of coronary atheroma as characterized by angiography and a past history of myocardial infarction (MI) as assessed by World Health Organization criteria.

Methods and Results Caucasian patients (n=405) admitted for routine angiography for investigation of chest pain or suspected coronary artery disease were recruited. Caucasian control subjects (n=216) were recruited from local Family Health Services Authority general practice registers. Fibrinogen levels were higher (P=.04) in male patients (3.24 g/L; CI, 3.14 to 3.35) than male control subjects (3.06 g/L; CI, 2.91 to 3.21). There was a trend toward a difference (P=.06) in fibrinogen genotype distributions between female patients (1/1=93, 1/2=31, and 2/2=1) and female control subjects (1/1=67, 1/2=34, and 2/2=5). In logistic regression models the Pl^A2 genotype was associated with MI (odds ratio, 1.66; CI, 1.15 to 2.39; P=.007) and stenosis of more than one vessel (odds ratio, 1.5; CI, 1.01 to 2.26; P=.04). In men suffering an MI before the age of 47 years there was a 50% incidence of the Pl^A2 allele (P=.05), and in these subjects there was evidence of an interaction with cholesterol (P=.04).

Conclusions We found evidence of an association of the Pl^A2 polymorphism in MI and multiple-vessel stenosis. The association with MI was strongest in young men, in whom there was also evidence of an interaction with cholesterol.

Key Words: myocardial infarction • molecular biology • platelets • stenosis

	Introduction

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Platelets bind to subendothelium at sites of vascular injury, leading to activation and expression of functional GPIIb/IIIa receptors (integrin _IIbß₃) on the cell surface.¹ Fibrinogen causes platelet aggregation via binding to GPIIb/IIIa receptors on adjacent activated platelets.² Activated platelets also provide the negatively charged phospholipid surface necessary for the activation of the coagulation system, with resultant generation of thrombin, leading to cleavage of fibrinogen to form fibrin and further activation of circulating platelets.³ In addition, activated platelets release a number of factors, including ADP and thromboxane A₂, which cause further platelet activation, and factor XIII, which is essential for cross-linking of fibrin strands, leading to stabilization of the fibrin clot.⁴ Thus, fibrinogen and platelets form an integral part of the thrombotic process; this is supported by studies relating increased fibrinogen levels and increased platelet activity to risk of ischemic heart disease.^{5 6 7 8 9}

Both fibrinogen and platelets have also been implicated in the atherosclerotic process. Fibrinogen levels have been found to be associated with both the severity and extent of coronary and carotid atherosclerosis,^{10 11 12 13 14} and fibrinogen, fibrin, and fibrin(ogen) degradation products have been identified as components of atherosclerotic plaques.¹⁵ Increased platelet activity has also been associated with the number and severity of discrete coronary lesions.^{16 17} Activated platelets release a wide variety of substances with mitogenic and chemotactic properties, such as platelet-derived growth factor and transforming growth factor-ß, which may play a role in the smooth muscle cell proliferation and infiltration of monocytes that lead to the progression of atherosclerosis.¹⁸

Several polymorphisms in the genes encoding fibrinogen and the platelet GPIIb/IIIa have been identified. The fibrinogen Bß 448 polymorphism, which codes for an argininelysine substitution 13 amino acids from the carboxy terminal of the ß fibrinogen gene,¹⁹ has been associated with macrovascular disease.^{20 21} In addition, the Pl^A polymorphism of platelet GPIIIa, which codes for a leucineproline substitution at position 33,²² is reported to be independently associated with coronary thrombosis.²³

The aim of this study was to determine the association of the Pl^A and Bß 448 polymorphisms and fibrinogen levels and their interactions with other environmental factors with the extent of coronary stenosis and a past history of MI in 405 patients with CAD characterized for extent of disease by coronary angiography compared with 216 healthy control subjects.

	Methods

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Subjects
Caucasian patients (n=405) admitted for routine angiography for investigation of chest pain or suspected CAD were recruited from Leeds General Infirmary and Pinderfields Hospital in Wakefield. Healthy Caucasian control subjects (n=216) were recruited from local Family Health Services Authority general practice registers. All subjects gave informed consent according to a protocol approved by the United Leeds Teaching Hospitals National Health Service Trust and the Pinderfields Health Trust ethics committees. Subjects were classified as smokers if they currently smoked of if they had smoked within the last 10 years; all others were classified as nonsmokers. BMI was calculated as weight in kilograms divided by the height in meters squared.

Identification of CAD
Angiography results were reported by cardiologists blind to patient status. Presence of CAD was defined as stenosis 50% in a major coronary artery or a major coronary artery branch. The extent of disease was classified as the number of arteries with stenosis 50% as either no stenosis or one-, two-, or three-vessel disease. MI was ascertained by reference to patients' hospital case notes by using World Health Organization criteria (at least two of the following: ST elevation of 1 mm in two or more successive leads, typical chest pain longer than 20 minutes in duration, or a rise in creatinine kinase of more than twice the baseline level).²⁴ Subjects with equivocal evidence of MI from case notes (n=5) and those with equivocal angiography results (n=9) were excluded from the relevant analyses.

Analysis of Circulating Factors
Venous blood samples were obtained after an overnight fast of at least 8 hours from an antecubital vein by using a 19G butterfly needle. Blood was collected in tubes containing EDTA for the determination of platelet count and extraction of DNA and in tubes containing lithium/heparin for the determination of plasma lipids as described.²⁵ Blood samples for the determination of fibrinogen, which were available only from subjects recruited from the Leeds General Infirmary (n=259) and control subjects, were taken into 0.1 mol/L trisodium citrate and centrifuged at 2500g at room temperature; plasma was snap-frozen in liquid nitrogen and stored at -40°C until analysis.

Circulating fibrinogen levels were determined by using the method of Clauss²⁶ ; reference plasma was supplied by Organon Teknika. Intra-assay and interassay coefficients of variation were 2% and 3.5%, respectively. Platelet counts were determined by using a Coulter STRK Analyser (Coulter Electronics Limited).

DNA Analysis
Fibrinogen Bß 448 and platelet GPIIIa Pl^A genotypes were determined by polymerase chain reaction amplification of fragments of DNA by using specific oligonucleotide primers^{19 27} in a PTC 100 thermal cycler (Cetus). Standard polymerase chain reaction conditions of 50 pmol of each primer, 100 ng DNA, 200 µmol/L of each dNTP, 10 mmol/L Tris HCl (pH 8.8), 1.5 mmol/L MgCl₂, 50 mmol/L KCl, 0.1% Triton X-100, and 1.0 U Dynazyme II DNA polymerase (Flowgen) were used, involving 30 cycles at 93°C for 1 minute denaturing, 1 minute annealing (at 54°C for Bß 448 and 72°C for Pl^A), and 1 minute at 72°C for extension followed by a final 5-minute extension at 72°C. Polymerase chain reaction products were subjected to overnight digestion by using either 5 U Mnl I (Bß 448) or 5 U ScrFI (Pl^A) restriction enzymes at 37°C following the recommendations of the manufacturer (New England BioLabs). Restricted DNA products were then separated by using 2% agarose gel electrophoresis containing ethidium bromide and visualized by UV light. The Bß 448 genotype was classified as 1/1 (arg, arg), 1/2 (arg, lys), and 2/2 (lys, lys); the Pl^A genotype was classified as A1/A1(leu, leu), A1/A2 (leu, pro), A2/A2 (pro, pro).

Statistics
The distributions of fibrinogen, BMI, and TGs were positively skewed, and values were log transformed to normalize the distribution and to allow analysis by parametric tests. Log-transformed results are expressed as geometric mean and anti-logged 95% CI. All other values are expressed as mean (95% CI). Differences in levels between groups were compared by using an unpaired Student's t test. ANOVA was used to investigate the relationship between genotype, extent of disease, and levels. Multiple regression analysis was used to identify the determinants of fibrinogen levels in each group. Allele frequencies between groups were compared by gene counting and ² analysis. Logistic regression models that included age, gender, cholesterol level, BMI, smoking, hypertension, platelet count, fibrinogen level, and Bß 448 and Pl^A genotypes as covariates were used to identify determinants of stenosis and MI, and ORs are presented with 95% CIs. All statistical analyses were performed by using the SPSS statistical package (SPSS Inc).

	Results

Top Abstract Introduction Methods Results Discussion References

 
Subject Characteristics
The characteristics of the patient and control groups are presented in Table 1. The characteristics of the patient group have been described.²⁵ The women in both patient and control groups were significantly older than the men. The ratio of men to women was equal in the control group, but in the patient group there were significantly more men than women. Fibrinogen levels in the male patients were significantly higher than in the male control subjects (P=.04). There was no significant difference in fibrinogen levels in female patients and control subjects (P=.2). Of the patients with a history of MI, 15 were studied <3 months following MI, but there was no difference in fibrinogen levels in these patients compared with the patients with MI studied after 3 months (data not shown). Platelet count was significantly higher in female patients than male patients (P=.01) and female control subjects (P=.04).

View this table: [in this window] [in a new window]   Table 1. Characteristics of Patient and Control Groups by Gender

The genotype distributions of the Bß 448 and Pl^A polymorphisms in both patients and control subjects were in Hardy-Weinberg equilibrium. There were no significant differences in Pl^A and Bß 448 genotype distributions in patients and control subjects, although there was a trend toward a lower than expected number of 1/2+2/2 individuals among the female patients and more 1/2+2/2 individuals than expected among the female control subjects (P=.06) (Table 1).

Relationship of Bß 448 Polymorphism to Fibrinogen Levels
In control subjects, fibrinogen levels were significantly associated with the Bß 448 genotype (1/1=3.07 [2.95 to 3.20] g/L; 1/2=3.41 [3.18 to 3.67] g/L; 2/2=3.51 [2.93 to 4.20] g/L; P=.01). In the patients, there was no association of genotype with fibrinogen levels (1/1=3.27 [3.17 to 3.38] g/L; 1/2+2/2=3.41 [3.26 to 3.57] g/L; P=.16).

In control subjects, fibrinogen levels were significantly associated with age (r=.27, P=.01), platelet count (r=.51, P=.0001), smoking (nonsmokers=3.12 [3.01 to 3.23] g/L; smokers=3.48 [3.20 to 3.80] g/L; P=.01) gender (men=3.06 [2.91 to 3.21] g/L; women=3.33 [3.16 to 3.50] g/L; P=.02), and genotype. In a stepwise linear regression model, only age, genotype, and platelet count were significantly associated with fibrinogen levels, accounting for 31%, 7%, and 2% of the variation in levels, respectively. After adjustment for these covariates, mean fibrinogen levels in the genotype groups were 3.29 g/L for 1/1 and 3.79 g/L for 1/2+2/2 (P=.003).

In the patients, fibrinogen levels were significantly associated with age (r=.30, P<.0001), platelet count (r=.30, P<.0001), and gender (men=3.24 [3.14 to 3.35] g/L; women=3.46 [3.31 to 3.62] g/L; P=.02). In a stepwise linear regression model, age and platelet count were the only factors significantly associated with fibrinogen levels, accounting for 8.8% and 8.3% of the variation, respectively. After adjustment for covariates there remained no association between fibrinogen levels and genotype (1/1=3.32 g/L; 1/2+2/2=3.36 g/L; P=.7). In both patients and control subjects, we found no difference in the association of fibrinogen levels and genotype in smokers and nonsmokers; higher levels were observed in the 1/2+2/2 genotype groups in both smokers and nonsmokers (data not shown).

Because we have found the association of the Bß 448 polymorphism with fibrinogen levels to be restricted to men,²⁰ we examined the association of genotype with levels by gender. In both patients and control subjects, genotype was significantly associated with fibrinogen levels in the male subjects (Table 2). There was no association of genotype with fibrinogen levels in the female patients, but in the female control subjects there was a trend toward higher levels of fibrinogen in 1/2 and 2/2 subjects.

View this table: [in this window] [in a new window]   Table 2. Association of Fibrinogen Levels and Bß 448 Genotype by Gender in Patients and Control Subjects

Relationship of the Pl^A Polymorphism to Platelet Count and CardiovascularRisk Factors
Pl^A genotype was not significantly associated with platelet count in patients (A1/A1=250 [241 to 259]; A1/A2+A2/A2=252 [239 to 264]; P=.8), or control subjects (A1/A1=229 [204 to 254]; A1/A2+A2/A2=224 [180 to 270]; P=.8), or any of the other cardiovascular risk factors presented in Table 1 (data not shown). There was no evidence for gender-specific associations.

Fibrinogen Levels and Bß 448 and Pl^A Genotype in Relation to Stenosis
Characteristics of patients by extent of stenosis are presented in Table 3. Subjects with no stenosis were younger than those with one or more stenosed vessels. The sex distributions in the groups were significantly different, with an approximately even distribution of men and women in the group with no stenosis and an average of 76% men in those with stenosis. There were significantly more smokers and hypertensive subjects among those with one or more stenosed vessels; these subjects also had higher cholesterol and TG levels compared with those with no stenosis. There was no significant difference in fibrinogen levels by extent of disease, although there was a trend toward higher levels in those with two or three diseased vessels compared with those with either no evidence of significant stenosis, those with only one stenosed vessel, or control subjects. In logistic regression models comparing those with stenosis to those without stenosis and to control subjects, there was no evidence for an association of fibrinogen levels or the two polymorphisms by ² testing (data not shown). In a logistic regression model comparing patients with one stenosed vessel to those with two or three stenosed vessels, the GPIIIa A2 allele was the only factor significantly associated with multiple-vessel stenosis (OR, 1.5; CI, 1.01 to 2.26; P=.04).

View this table: [in this window] [in a new window]   Table 3. Characteristics of Patients by Extent of Stenosis

Fibrinogen Levels and Bß 448 and Pl^A Genotypes in Relation to MI
The characteristics of patients with and without MI compared with control subjects are presented in Table 4. There were significantly more men among patients with MI compared with patients without, but there were no other significant differences in risk factors between these two groups (Table 4). The groups with and without MI had significantly higher cholesterol and TG levels, higher BMI, and a greater proportion of men, smokers, and hypertension than the control group.

View this table: [in this window] [in a new window]   Table 4. Characteristics of Patients With and Without MI and Control Subjects

There was no significant difference in fibrinogen levels or genotype distributions of the Bß 448 and Pl^A polymorphisms between patients with and without MI (Table 4). In a logistic regression model, Pl^A genotype, hypertension, and male sex were significantly associated with MI: OR for subjects possessing the A2 allele compared with those homozygous for the A1 allele was 1.57 (1.08 to 2.27), P=.02; OR for hypertensive compared with nonhypertensive subjects was 1.54 (1.03 to 2.31), P=.04; and OR for men compared with women was 1.49 (1.02 to 2.16), P=.04. Subanalysis of these data indicated that A2 was associated with MI only in those subjects with three stenosed vessels (OR, 2.27 [1.11 to 4.64], P=.03). In addition, in the patients with MI, Pl^A genotype was the only factor significantly associated with multiple-vessel compared with single-vessel stenosis in a logistic regression model (OR, 5.67 [1.20 to 26.9], P=.03).

There was a trend toward higher levels of fibrinogen in the patients with MI than in the control subjects (Table 4). There was no significant difference in the genotype distributions of the two polymorphisms by ² testing. In a logistic regression model, the Pl^A genotype was significantly associated with MI, as were age, gender, cholesterol level, BMI, and smoking. ORs for these factors are presented in Table 5. We have found a particularly strong association of the A2 allele in men with a first MI before the age of 47 years,²⁸ 50% of whom carried the A2 allele, compared with 27% of age- and sex-matched control subjects (P=.05 by ² testing) (Table 6). In a logistic regression model, A2, smoking, and BMI were significantly associated with MI (see Table 6 for ORs). By creating interaction terms, there was evidence for an interaction of the Pl^A polymorphism and cholesterol in these subjects (P=.04).

View this table: [in this window] [in a new window]   Table 5. ORs for Factors Significantly Associated With Patients With a History of MI (n=156) Compared With Healthy Control Subjects (n=216)

View this table: [in this window] [in a new window]   Table 6. Characteristics of Male Patients <47 Years Old and Control Subjects

There was no evidence for a gender-specific association of the Pl^A and Bß 448 polymorphisms or fibrinogen levels with stenosis or MI (data not shown). An interaction term for the Bß 448 and Pl^A polymorphisms was created in the logistic regression models to determine if there was any association between these polymorphisms in the development of MI or stenosis. There was no evidence to suggest an interaction between these polymorphisms (data not shown).

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Epidemiological and case-control studies have demonstrated elevated fibrinogen levels and increased platelet activity to be independent risk factors for ischemic heart disease.^{5 6 7 8 9} Increased platelet activity has been associated with the number and severity of discrete coronary lesions,^{16 17} and fibrinogen has been associated with the extent and severity of coronary atheroma.^{10 11 12 13 14} Polymorphisms of the fibrinogen genes have been associated with the severity of CAD, cerebrovascular disease, and peripheral atherosclerosis,^{20 21 29} and the platelet Pl^A polymorphism has been associated with coronary thrombosis.²³ We have found that the A2 allele is associated with MI and the extent of stenosis in subjects with CAD characterized for extent of disease by coronary angiography and a history of MI as assesessed by World Health Organization criteria. In subjects with MI the association with extent of stenosis was greater than for the group as a whole, suggesting that this polymorphism is associated with an increased risk of MI in the presence of more extensive atheroma. We found no evidence for an association of the fibrinogen Bß 448 polymorphism or for any interaction between these two polymorphisms with MI or extent of stenosis. It is possible that the Pl^A polymorphism results in a functional change within the GPIIb/IIIa receptor protein, resulting in increased platelet aggregation and activation with a concomitant increase in platelet recruitment at the site of vascular damage, resulting in a greater thrombotic response and increase in release of factors involved in the progression of atherosclerosis.

Fibrinogen Levels and Genotype Distributions in Patients and Control Subjects
Fibrinogen levels in male patients were higher than in male control subjects. There was no significant difference in levels between female patients and female control subjects, although this may merely reflect the relatively small number of female patients in this study. We found no difference in Pl^A and Bß 448 genotype distributions between patients and control subjects. When considered by gender, there was a trend toward a difference in Bß 448 genotype distributions between female patients and control subjects. We have found a similar difference in Bß 448 genotype distributions in female patients with acute cerebrovascular disease and female control subjects²⁰ (ie, fewer than expected heterozygotes among the female patients and more than expected heterozygotes among the control subjects). The observation of a similar trend in the present study suggests that variation at this polymorphic site may be associated with a differing risk of CAD in women. These results and those from our previous study²⁰ suggest that possession of the 2 allele may be protective for the development of vascular disease in women, possibly due to an altered structure of the fibrinogen protein, which may be more susceptible to lysis by plasmin or may be less stable. Further investigation of a larger number of female subjects is required to confirm these results.

Association of Bß 448 Genotype and Fibrinogen Levels
We found the Bß 448 fibrinogen polymorphism to be associated with fibrinogen levels in both male patients and male control subjects. This is in keeping with our previous finding²⁰ of an association of this polymorphism with fibrinogen levels in male patients with acute cerebrovascular disease as well as with a number of studies that have investigated the association of the -455 G/A polymorphism in the 5' flanking region of the ß fibrinogen gene and levels.^{30 31} There was no association of fibrinogen levels and genotype in female patients, but in female control subjects there was a trend toward higher levels in those possessing the 2 allele, which may indicate that in women the effect of genotype on levels is masked by other environmental or genetic factors. The Bß 448 polymorphism is in strong linkage disequilibrium with the -455 G/A polymorphism,³² and it is likely that the Bß 448 polymorphism acts as a marker for this polymorphism in relation to the observed association with fibrinogen levels in the present study. Others have reported^{30 33} a genotype-smoking interaction with the association of the -455 G/A polymorphism and fibrinogen levels; an association between levels and genotype was observed only in smokers. We found no evidence to suggest such an interaction in this study; the association of genotype and levels was observed in both smokers and nonsmokers. This is consistent with our previous finding²⁰ concerning the association of this polymorphism with fibrinogen levels.

Association of Fibrinogen Levels and Bß 448 Polymorphism With MI and Extent of Stenosis
There was a trend toward higher fibrinogen levels in patients with a history of MI and control subjects, but no difference was observed between patients with and without MI. As this is a retrospective study this may merely represent lifestyle modification following MI and onset of symptoms of CAD. Fibrinogen levels were not significantly associated with the extent of disease, in contrast to the findings of studies^{10 11 12 13 14} that have demonstrated fibrinogen levels to be associated with the extent and severity of stenosis in both symptomatic and asymptomatic subjects. In keeping with our results, the ECAT (European Concerted Action on Thrombosis and Disabilities) study found fibrinogen levels were associated only with the number of occluded vessels, and no significant association with extent of disease was found as assessed by the number of vessels with >50% stenosis.³⁴

We found no association between the Bß 448 polymorphism and extent of stenosis or MI in patients compared with control subjects or in subgroup analyses. In contrast, Behague et al²¹ found that in the patients from the ECTIM (Etude Cas-Temoins sur l'Infarctus du Myocarde) study who had undergone angiography there was an association of a number of fibrinogen gene polymorphisms, including the Bß 448 polymorphism and extent of disease: there was a greater incidence of the rarer allele in patients with three coronary arteries with >50% stenosis compared with those with one or two stenosed vessels. They suggested²¹ that the most likely interpretation of these results was that the rare allele was associated with an increased risk of MI in individuals with severe atherosclerosis rather than that the polymorphism was associated with the extent of stenosis. We did not find a greater number of subjects with the rarer allele of this polymorphism in those with three diseased vessels compared with those with fewer, but one must consider that in the ECTIM study the association with stenosis was demonstrated in subjects who had all suffered an MI. However, when we restricted our analyses to those subjects with a confirmed history of MI we still did not find a greater number of subjects with the rare allele in the three-vessel disease group.

Association of the Pl^A Genotype With MI and Extent of Stenosis
Weiss et al²³ report that the Pl^A polymorphism of platelet GPIIIa is strongly associated with coronary thrombosis as assessed by the presence of acute MI or unstable angina. We found that this polymorphism was associated with patients with MI compared with those without MI and control subjects free of symptoms of CAD, but only when considered in a logistic regression model after accounting for other risk factors for MI. This association was observed only in those subjects with three stenosed vessels, which may merely reflect the greater number of subjects with MI in this group compared with those with fewer stenosed vessels, or it may indicate an interaction of this polymorphism with other factors associated with the development of atherosclerosis and thrombosis. As reported by Weiss et al,²³ we found the increased risk was associated with possession of the rare A2 allele. There are a number of differences in the populations in the two studies. Weiss et al²³ studied subjects with acute symptoms of MI and unstable angina and a group of inpatient control subjects who appeared to have a high incidence of coronary risk factors. Our study was a retrospective investigation of this polymorphism in patients with CAD and a population of healthy, community-based control subjects with no history of CAD. It is possible that this polymorphism is more strongly associated with unstable angina, or it may be associated with fatal thrombosis, which may explain the weaker association with MI observed in the present study. The association of Pl^A2 with MI was observed only in multivariate analyses, which suggests that either there is an interaction of Pl^A2 with other risk factors for vascular disease or that this observation represents a type I statistical error. In support of the former hypothesis, we found a much stronger association of the A2 allele with MI in young men (<47 years),²⁹ in whom we found a 50% incidence of the A2 allele, which in turn supports the suggestion³⁵ that this polymorphism is more strongly associated with MI in the young. In these young subjects we also found evidence for an interaction of Pl^A with cholesterol. We have also found a significantly higher prevalence of the A2 allele in young subjects (n=37) with atherothrombotic stroke, in whom there was a 49% incidence of the A2 allele compared with 27% in healthy age- and sex-matched control subjects (n=74) (A.M. Carter, A.J. Catto, P.J. Grant, unpublished data, 1997). Platelets from subjects with hypercholesterolemia have been shown to be hypersensitive to agonists such as ADP and thrombin, to produce significantly more thromboxane B₂, and to bind significantly more fibrinogen than platelets from healthy control subjects.³⁶ The importance of the GPIIb/IIIa receptor in CAD is supported by several studies that have demonstrated that antagonists to this receptor reduce the incidence of restenosis following coronary angioplasty both acutely and at 6 months.^{37 38} It is possible that the amino acid substitution associated with this polymorphism leads to increased fibrinogen binding to GPIIb/IIIa, leading to increased platelet aggregation and thrombus formation. Thus, if the A2 allele enhances fibrinogen binding to GPIIb/IIIa, possession of A2 in the presence of elevated cholesterol may further enhance fibrinogen binding and aggregation of platelets, thereby increasing the risk of a thrombotic event even further.

In conclusion, we found the A2 allele of the platelet GPIIIa Pl^A polymorphism to be associated with MI. In subgroup analyses of the patients by extent of stenosis, this association was observed only in patients with more extensive stenosis, suggesting that this polymorphism may be interacting with other classic risk factors for MI. We found the association of this polymorphism to be particularly strong in young men <47 years old, in whom there was a 50% incidence of the A2 allele, compared with 27% in age- and sex-matched control subjects. In these young subjects we also found evidence for an interaction of the Pl^A polymorphism and cholesterol. A large-scale study in young people is required to confirm these results.

	Selected Abbreviations and Acronyms

 

BMI = body mass index CAD = coronary artery disease GP = glycoprotein MI = myocardial infarction OR = odds ratio TG = triglyceride

	Acknowledgments

 
This study was supported by the British Heart Foundation and the United Leeds Teaching Hospitals Special Trustees.

Received October 16, 1996; revision received March 10, 1997; accepted April 2, 1997.

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