Glycoprotein (GP) Ib-IX-V complex plays an important role in formation of platelet-fibrin clot at the area of damaged vessel wall. One polymorphism of GP Ibα, the main component of GP Ib-IX-V complex, is due to variable numbers of tandem repeats (VNTRs) in the macroglycopeptide region of this molecule. We studied the association between the presence of different VNTR alleles of GP Ibα and the frequency of coronary heart disease (CHD) among individuals recruited to a large community-based case-cohort study (Atherosclerosis Risk in Communities [ARIC] study). We found that the distribution of VNTR alleles of GP Ibα is different among whites and African Americans. The B allele (with 3 repeats) of GP Ibα is relatively more common among African Americans compared with whites. In African Americans, the CC genotype (homozygous with 2 repeats) is associated with a lower risk of CHD events than all other genotypes. (Blood. 2004;103:963-965)

The glycoprotein (GP) Ib-IX-V complex contains 4 polypeptides, GP Ibα, GP Ibβ, GP IX, and GP V.1  The largest is GP Ibα, which contains the binding site for von Willebrand factor (VWF) and for several other proteins important in the genesis of vascular disease: thrombin, factor XI, P-selectin, and leukocyte Mac-1.2-5 

GP Ibα is highly polymorphic, one polymorphism being due to variable numbers of tandem repeats (VNTRs) in a region encoding the macroglycopeptide, a mucin-like stalk separating the ligand-binding region from the plasma membrane. This region may have from one to four tandem 39-base repeats, each encoding a 13-amino acid sequence. The 4 variants are named A, B, C, and D, with 4, 3, 2, and 1 repeat, respectively. It has been hypothesized that because added repeats increase the length of the macroglycopeptide region of GP Ibα, they might also increase the sensitivity of the longer molecule to shear stress, a known modulator of the GP Ibα-VWF interaction.1 

Here, we analyzed the association of the GP Ibα VNTR alleles with risk of incident coronary heart disease (CHD) in the Atherosclerosis Risk in Communities (ARIC) case-cohort study.

Study population

We analyzed the ARIC study population, consisting of a population-based cohort totaling 15 792 men and women between 45 and 64 years of age from 4 US communities.6  We defined CHD incidence as (1) a definite or probable myocardial infarction (MI), (2) a silent MI detected by new electrocardiographic (ECG) changes between examinations, (3) a definite CHD death, or (4) a coronary revascularization procedure.

This was a case-cohort study, with frequencies of GP Ibα alleles determined for CHD cases and a stratified random sample of the ARIC cohort. We identified 349 individuals as cases with CHD. For the reference cohort, we oversampled participants with low average carotid intima-media thickness measurements at baseline (< 30th percentile) and also stratified the sampling by age and sex. We included 383 individuals as a reference cohort (of whom 22 were also CHD cases). Before selecting the case-cohort sample, we excluded participants identified as having prevalent CHD, stroke, or transient ischemic neurologic attack (TIA). The study included participants from 2 ethnic groups: whites and African Americans.

Genotyping

We genotyped each individual for the GP Ibα VNTR polymorphism by polymerase chain reaction as described previously.7 

Data analysis

We used analysis of covariance to compute age-, race-, and sex-adjusted frequencies of different genotypes, and means or percentage values of study variables (including lipid profile, plasma VWF and fibrinogen concentrations, history of hypertension, diabetes mellitus, tobacco or alcohol abuse) for CHD cases versus noncases after appropriate weighting for the stratified case-cohort sampling design. We computed the risk ratios and 95% confidence intervals for the time to development of CHD using a weighted proportional hazard regression, accounting for the stratified random sampling and the case-cohort design by the Barlow method.8  Because the prevalence of different GP Ibα genotypes was significantly different between whites and African Americans, we performed separate weighted proportional hazard regression analyses for whites and African Americans. In the weighted proportional hazard regression model, we adjusted for sex, age, and the other factors related to CHD. The number of subjects with BB (5 cases and 5 noncases) and DD (1 case and 3 noncases) genotypes were very small and were combined with BC and CD, respectively, in analysis. Only 4 individuals with the A allele were identified, and these were omitted from the analysis.

Ethnicity and GP Ibα genotype

The association of 3 genotype groups (BC+BB, CC, and CD+DD) with age, sex, ethnicity, and conventional risk factors is shown in Table 1. The CC and CD+DD groups were smaller by percentage in African Americans than whites, with a corresponding increase in the BC+BB group. Aramaki and Reiner have previously shown the higher frequency of the B allele in African Americans compared with whites.9 

Table 1.

Age-, race-, and sex-adjusted means and proportions of CHD risk factors in the cohort random sample in relation to the GP lbα genotypes



Genotypes

Variable
BC+BB
CC
CD+DD
P
Hypertension, %   24   27   19   .655  
Diabetes mellitus, %   3   6   9   .675  
Systolic blood pressure, mm Hg   117   119   118   .835  
Total cholesterol level, mM/L   5.28   5.62   5.41   .204  
Triglyceride level, mmol/L   1.39   1.39   1.40   .987  
HDL-C level, mmol/L   1.45   1.40   1.32   .453  
LDL-C level, mmol/L   3.21   3.60   3.44   .498  
Fibrinogen level, μmol/L   8.47   8.94   8.64   .475  
VWF, %   133   118   102   .170  
Cigarette-years   272   234   210   .693  
Ethanol intake, g/wk   32   30   31   .987  
Waist-to-hip ratio   0.89   0.92   0.91   .260  
Age, y   55   54   54   .482  
Males, %   40   43   47   .906  
African Americans, %
 
39
 
22
 
5
 
.003*
 


Genotypes

Variable
BC+BB
CC
CD+DD
P
Hypertension, %   24   27   19   .655  
Diabetes mellitus, %   3   6   9   .675  
Systolic blood pressure, mm Hg   117   119   118   .835  
Total cholesterol level, mM/L   5.28   5.62   5.41   .204  
Triglyceride level, mmol/L   1.39   1.39   1.40   .987  
HDL-C level, mmol/L   1.45   1.40   1.32   .453  
LDL-C level, mmol/L   3.21   3.60   3.44   .498  
Fibrinogen level, μmol/L   8.47   8.94   8.64   .475  
VWF, %   133   118   102   .170  
Cigarette-years   272   234   210   .693  
Ethanol intake, g/wk   32   30   31   .987  
Waist-to-hip ratio   0.89   0.92   0.91   .260  
Age, y   55   54   54   .482  
Males, %   40   43   47   .906  
African Americans, %
 
39
 
22
 
5
 
.003*
 

HDL-C indicates high-density lipoprotein cholesterol; and LDL-C, low-density lipoprotein cholesterol.

*

P values for BC+BB versus CC, BC+BB versus CD+DD, and CC versus CD+DD are .074, .0001, and .009, respectively.

CHD incidence and GP Ibα genotype

In the entire population, we found no significant differences in genotypes between incident CHD cases and noncases (Table 2). However, when we examined the 2 ethnic groups separately, we found an association in African Americans between the non-CC genotypes and incident CHD (Table 2). Put another way, African Americans with the CC genotype had a significantly lower incidence of CHD than those with either the BC+BB or CD+DD genotypes. The weighted proportional hazard regression analysis showed that GP Ibα genotypes correlate with the development of CHD in African Americans (Table 2) In contrast, whites in either the BC+BB or CD+DD groups did not have a significantly higher risk of CHD (risk ratios, 1.1 and 1.4, respectively).

Table 2.

GP lbα genotypes and CHD



All*

Whites

African Americans
Genotype
CHD, no.
Non-CHD, no.
RH (95% CI)
CHD, no.
Non-CHD, no.
RH (95% CI)
CHD, no.
Non-CHD, no.
RH (95% CI)
BC+BB   61   50   1.6 (0.8-3.2)   26   31   1.1 (0.4-2.6)   35   19   5.6 (1.2-26.6)  
CC   239   279   &   206   219   &   33   60   &  
CD+DD
 
45
 
30
 
1.8 (0.87-3.6)
 
33
 
25
 
1.4 (0.8-3.1)
 
12
 
5
 
21.7 (4.3-108.8)
 


All*

Whites

African Americans
Genotype
CHD, no.
Non-CHD, no.
RH (95% CI)
CHD, no.
Non-CHD, no.
RH (95% CI)
CHD, no.
Non-CHD, no.
RH (95% CI)
BC+BB   61   50   1.6 (0.8-3.2)   26   31   1.1 (0.4-2.6)   35   19   5.6 (1.2-26.6)  
CC   239   279   &   206   219   &   33   60   &  
CD+DD
 
45
 
30
 
1.8 (0.87-3.6)
 
33
 
25
 
1.4 (0.8-3.1)
 
12
 
5
 
21.7 (4.3-108.8)
 

RH (95% CI) indicates relative hazard (95% confidence interval), adjusted for age, sex, race, hypertension, diabetes mellitus, total cholesterol, HDL-C, smoking, and alcohol intake; &, reference value in calculation of adjusted relative hazard.

P values calculated for the comparison of GP lbα genotypes against incident CHD status. P is computed by adjusting the effects of age, sex, and race for all subjects, and by adjusting the effects of age and sex for whites and African Americans. In African Americans, P = .014 for CC versus BC+BB and P = .0001 for CC versus CD+DD.

*

P = .101.

P = .871.

P < .0001.

We also analyzed the association according to the presence of the C allele (on the Blood website; see the Supplemental Table link at the top of the online article). The C allele was associated with lower incident CHD in African Americans (P = .003), whereas no such association was noted in the whole group or in whites (P = .091 and .521, respectively). African Americans homozygous for the C allele had significantly lower incident CHD compared to those with one C allele.

We have found a protective effect of the CC genotype in African Americans participating in the ARIC study, but not in the group overall or in whites. This may indicate a interaction between GP Ibα genotype and another environmental or genetic variable that is more prevalent in African Americans than whites. Alternatively, it is possible that the increased prevalence of the B allele in African Americans unmasks an increased risk for cardiovascular disease associated with this variant.

Of interest, 2 GP Ibα haplotypes for GP Ibα have been reported with much higher frequency in African Americans than in whites, the C/Met145 variant and the B/Thr145 variant, present in this population with frequencies of 2.2% and 6.5%, respectively.9  Because Met145 has been associated with an increased prevalence and severity of coronary artery disease,10  the protective effect of the CC genotype in the African-American population is even more striking.

The hypothesis that the length of GP Ibα affects its function, and as a result the propensity to develop CHD, is not a new one.11  Other groups have also studied the role of the GP Ibα VNTR polymorphism in case-control studies, with contradictory results.10,12-14  All of these studies were cross-sectional case-control studies. The design of these studies makes it less likely that an ideal control group will be chosen, with the case and control subjects being fully matched for all other risk factors. In the current study, 15 792 individuals were followed prospectively over a period of 10 years.

Another mechanism by which the VNTR polymorphism may influence CHD susceptibility is suggested by the results of our study. Most of the individuals in the non-CC group were heterozygous for 2 length variants, either carrying the BC or the CD genotypes. Thus, it is possible that it is the disparity in length between the 2 allele products in a heterozygous individual that influences the interaction of GP Ibα with the receptor (it has been postulated that GP Ibα may be able to associate into dimers or tetramers to form the functional VWF-binding unit1 ). Nevertheless, because of the relative shortage of BB homozygotes, a larger study is needed to determine whether it is the length of the macroglycopeptide stalk or a length disparity in heterozygotes that determines the increased risk in the non-CC African-American individuals.

Prepublished online as Blood First Edition Paper, October 30, 2003; DOI 10.1182/blood-2003-05-1502.

Supported by grant R01 HL65205 from the National Institutes of Health and National Heart, Lung, and Blood Institute contracts N01 HC-55015, -55016, -55018, -55019, -55020, -55021, and -55022. V.A.-K. is a recipient of the LLS and MPD Foundation's Translational Research Grant and American Heart Association, Texas Affiliate BGIA.

The online version of this article contains a data supplement.

The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 U.S.C. section 1734.

The authors thank the staff and participants in the ARIC study for their important contributions.

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