(Circulation. 2001;103:1386.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Vascular Biology Research Center and Division of Hematology, Department of Internal Medicine (K.K.W., N.A., H.J.), Division of General Medicine, Department of Internal Medicine (C.A.), and Department of Human Genetics (E.B.), The University of Texas-Houston Health Science Center, Houston, Tex; and Division of Epidemiology, The University of Minnesota, School of Public Health, Minneapolis, Minn (A.R.F.).
| Abstract |
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Methods and ResultsWithin a large cohort study, we identified 467 incident CHD cases during an average of 5 years of follow-up. We determined TM-455 genotypes on 376 CHD cases (23% black, 77% white) and a reference sample of 461. The AA genotype was significantly more prevalent in noncases than in cases (P=0.016). The prevalences of the AA genotype in noncase blacks and whites were 93% and 67%, respectively. The AA genotype frequency was significantly reduced in black cases versus noncases (P=0.018). It was also lower in white cases than in noncases, but the difference was not statistically significant (P=0.066). Weighted proportional hazards regression analysis after adjustment for age, sex, and other CHD risk factors showed that having the V allele increased risk of CHD by 6.1-fold (risk ratio 6.1, 95% CI 1.7 to 22.9) in blacks but did not significantly increase the risk in whites.
ConclusionsThe TM A455V polymorphism predicts risk of developing CHD in blacks.
Key Words: thrombomodulin genetics coronary disease
| Introduction |
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| Methods |
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Incident CHD Cases and Cohort Random
Sample
A case-cohort design was used for this nested genetic
study.14 For this analysis,
we included CHD events that occurred between the initial visit (1987 to
1989) and December 31, 1993. The mean follow-up period was 5 years. We
defined CHD incidence as (1) definite or probable MI, (2) silent MI,
(3) definite CHD death, or (4) coronary revascularization. The CHD
events were ascertained by strategies described
previously.15 All identified
potential clinical events were reviewed and adjudicated by an ARIC
morbidity and mortality classification committee using published
criteria.15 Coronary
revascularization was defined as having hospital procedure codes for
coronary bypass graft, coronary angioplasty, or coronary
atherectomy.
A stratified random sample of the entire ARIC cohort was selected as a reference group. For this reference group (designated as noncases), we oversampled participants with an average carotid intima-media thickness measurement at baseline less than the 30th percentile and stratified the sampling by age and sex. As a result of these varying sampling fractions, we adjusted for the sampling design during analysis.
Participants were excluded if they were neither white nor black, had prevalent CHD at baseline, or had a history of stroke or transient ischemic attacks. After exclusions, the sample included 376 incident CHD cases and 461 noncases.
Baseline Measurements
Trained technicians measured blood pressure 3 times
using a random-zero sphygmomanometer. The mean of the last 2
measurements was used for analysis. Hypertension was defined as
systolic blood pressure
140 mm Hg, diastolic blood pressure
90
mm Hg, or current use of antihypertensive medication. Cigarette
smoking and ethanol intake were based on questionnaire described
previously.16
Venous blood was collected and processed at the baseline
examination according to procedures described
previously.17 Specimens were
sent by overnight courier to the Central Hemostasis Laboratory in
Houston, the Lipid and DNA Laboratory in Houston, and the Chemistry
Laboratory in Minneapolis, where they were stored at -70°C to
-80°C until assay. Plasma total cholesterol and triglycerides were
measured by enzymatic methods as described
previously.18 HDL
cholesterol was measured after dextran-magnesium precipitation to
remove non-HDL
lipoproteins.19 Fibrinogen
levels were measured by the thrombin time titration method, and von
Willebrand factor antigen levels were measured by enzyme immunoassay as
described previously.17
Soluble TM levels in plasma were measured by enzyme immunoassay as
described previously.20 The
laboratory coefficients of variation for soluble TM between and within
assays were 8.2% and 6.0%, respectively. Serum glucose was measured
by the hexokinase method. Diabetes mellitus was defined as a fasting
glucose level
126 mg/dL or a nonfasting glucose level
200 mg/dL or
a history of, or treatment for, diabetes. White blood cells were
counted by local laboratories.
Determination of TM-455 Genotypes
Genomic DNA was isolated from buffy-coat white blood
cells by phenol-chloroform extraction and ethanol precipitation.
Genotypes of the TM-455 dimorphism were determined by single strand
conformation polymorphism (SSCP) according to a procedure described
previously.6 10
Amplification of the TM gene by polymerase chain reaction was performed
with forward primer, 5'-CGGTACCTTCGAGTGCATCT-3', and reverse primer,
5'-ACGGCCGGAGGAGTCAAGGT-3'. This resulted in the generation of a 153-bp
fragment. SSCP for detecting the TM-455 dimorphism was done with the
amplified polymerase chain reaction product as described
previously.10
Data Analysis
To determine the relation of TM dimorphism with other
variables, some of which may be confounders in this analysis, we used
ANCOVA to compute age-, race-, and sex-adjusted mean levels or
percentages of the variables of interest for the AV/VV and AA
genotypes. We also used ANCOVA to compute age-, race-, and sex-adjusted
mean or percentage values of study variables for CHD cases versus
noncases after appropriate weighting for the stratified case-cohort
sampling design. We computed the risk ratios (RRs) and 95% CIs for the
time to development of CHD using a weighted proportional hazards
regression, accounting for the stratified random sampling and the
case-cohort design by Barlows
method.21 Because there was
a significant difference in the prevalence of AA and AV/VV genotypes
between whites and blacks, we performed separate weighted proportional
hazards regression analyses for whites and blacks. In the weighted
proportional hazards regression models, we adjusted for sex, age, and
the following factors related to CHD in this sample: white blood cell
count, hypertension, diabetes, total cholesterol, HDL cholesterol, and
cigarette smoking.
| Results |
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The RR of AV/VV versus AA for developing CHD was analyzed by
weighted proportional hazards regression analysis. In the entire
sample, regardless of race, the combined AV/VV genotype group was
associated with significantly higher CHD risk by 60% over the AA
genotype (RR 1.6, 95% CI 1.1 to 2.3) after adjustment for age, race,
and sex. However, the RR was reduced to 1.2, which was statistically
insignificant, after adjustment for additional CHD risk factors. When
the RR was determined according to race, the AV/VV genotype group
increased the risk of CHD in blacks by 4.4-fold after adjustment for
age and sex and by 6.1-fold when adjusted for other CHD risk factors.
In contrast, genotypes were not significantly associated with risk of
CHD in whites
(Table 3
).
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Soluble TM in plasma is a degradation product of endothelial TM. Results from a recent ARIC report20 suggest that the plasma concentration of soluble TM may reflect the level of endothelial TM expression. We therefore compared mean plasma soluble TM concentrations between the AA and AV/VV genotypes in the cohort random sample. The mean soluble TM level in the AA genotype (42.7 ng/mL) was not significantly different from that in the AV/VV genotype (39.2 ng/mL, P=0.31). This was also true for both whites (P=0.20) and blacks (P=0.22).
| Discussion |
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TM residue 455 is located at the epidermal growth factor (EGF)like domain of the extracellular region of endothelial membrane TM.1 There are 6 EGF-like repeats, and the last 3 repeats are functionally important for protein C activation and thrombin binding. TM-455 is located in the last EGF-like repeat. A valine substitution for alanine was therefore considered to have the potential of altering the TM activity. However, a recent report22 showed that TM-455A and TM-455V expressed in COS-1 cells by transient transfection possessed similar kinetic properties for activation of protein C and thrombin binding. We20 recently observed that the plasma soluble TM level was inversely associated with the risk of CHD. These results led us to postulate that the plasma soluble TM concentration reflects the expression level of TM on endothelial surface. Because the endothelial TM level may be suppressed by proinflammatory cytokines, which have been shown to play an important role in CHD,23 we chose to analyze the correlation of TM-455 genotypes with soluble TM concentrations only in the random cohort sample. Our results did not show a statistically significant difference in plasma soluble TM between AA and AV/VV genotypes in the entire random cohort sample nor in blacks or whites. A previous report on whites also did not show a difference in soluble TM between these 2 genotypes.
The functional basis for the marked difference in the influence of the TM-455 polymorphism on the risk of CHD between blacks and whites is unclear. One possible explanation for these race-specific effects is that the TM-455 mutation causes a subtle reduction in TM function in vivo and that endothelial TM levels in blacks are more sensitive to inhibition by proinflammatory mediators than those in whites, which would further compromise the TM activity in blacks carrying the VV+AV genotypes. Another possible explanation is that risk factors for CHD differ between blacks and whites and this TM mutation occupies a more important position in determining CHD relative risk in blacks than in whites. Finally, the TM-455 polymorphism could be a marker for a neighboring polymorphism that confers a true biological difference to account for CHD risk.
There are 2 previous reports on the association of the TM-455 polymorphism with thrombotic disorders. Both reports were based on cross-sectional studies. One study from the Netherlands6 examined TM-455 genotype frequency in 25 patients with venous thromboembolic diseases and 26 controls and did not find a significant difference between cases and controls. Another cross-sectional study from Sweden10 analyzed genotype frequency in 91 patients who had at least 1 episode of acute MI before the age of 50 years and 159 healthy volunteers and reported a significantly higher A allele frequency in patients. The control group and patient groups were not matched, and confounding risk factors were not adjusted for in this study. Our report represents the first prospective study on the association of the TM-455 polymorphism with CHD. Contrary to the results from the cross-sectional study, our results show a lower A allele frequency in white CHD cases, with an age- and sex-adjusted P value of 0.066. However, neither univariate nor multivariate analysis shows a significant association of this polymorphism with incident CHD in our white group, despite the inclusion of a relatively large number of white CHD cases (n=289) from 3 US communities (Washington County, Maryland; northwest Minneapolis suburb, Minn; and Forsyth County, North Carolina). It is thus unlikely that the TM-455 polymorphism is an independent risk factor for incident CHD in American whites.
The genotype frequency in our white group differs from that reported by the Swedish study. The AA genotype frequency in the Swedish healthy subjects was 56% compared with 67% in our white noncases. It is unclear whether the difference in genotype frequency between the Swedish whites and our American whites influences the association of TM-455 polymorphism with CHD. This important issue should be further investigated by prospective studies. The TM-455 genotype frequency in other ethnic groups was not reported previously. In the present study, we showed that the AA genotype was much more prevalent in blacks than in American whites (93% versus 67%). The black cohort in the ARIC study was recruited from Jackson, Miss, and Forsyth County, North Carolina. There is no significant difference in the genotype frequency between these 2 communities. Whether the genotype frequency is identical among all blacks in the United States will need to be documented. Likewise, it will be important to determine this genotype frequency and the association of this genetic polymorphism in blacks and other ethnic groups.
Our results should be interpreted with caution because the number of black cases was relatively small compared with that of whites. They should be confirmed with a larger sample size of black cases. It will be possible to do this in the next several years when additional black CHD cases are identified during continuing follow-up of the ARIC cohort.
| Acknowledgments |
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| Footnotes |
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Guest Editor for this article was Joseph Loscalzo, MD, PhD, Boston University School of Medicine, Boston, Mass.
Received September 25, 2000; revision received November 21, 2000; accepted November 28, 2000.
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