Participants were selected from the ARIC study, a prospective investigation of atherosclerosis and its clinical sequelae involving 15,792 individuals aged 45 to 64 years at recruitment (1987-1989). Institutional review boards approved the ARIC study and all participants provided written informed consent. A detailed description of the ARIC study design/methods and details on ascertainment/classification of CHD and stroke events have been published elsewhere.6-8
Participants were excluded from analyses (n=2,301) if they had a positive or unknown history of prevalent stroke or CHD or a history of TIA/stroke symptoms at baseline, prohibited use of their DNA, had an ethnic background other than white or African American, or had missing information for both ICAM-1 genotypes or for the endpoints/covariates included in the analyses. Incident CHD cases (N=1,629) were defined as a definite or probable myocardial infarction (MI), a silent MI detected by ECG, a definite CHD death, or a coronary revascularization. Incident ischemic stroke cases (N=517) were defined as validated definite/probable hospitalized embolic or thrombotic brain infarctions.
Seated blood pressure was measured three times with a random-zero sphygmomanometer and the last two measurements were averaged. Hypertension was defined as SBP≥140 mmHg or DBP≥90 mmHg or current use of antihypertensive medications (assessed by questionnaires and in-person interviews). Diabetes was defined by a fasting glucose level≥126 mg/dL, a non-fasting glucose level≥200 mg/dL, and/or history of or treatment for diabetes. Cigarette-smoking status compared current smokers to former/never smokers. Body mass index (BMI, kg/m2
) was calculated from height and weight measurements. Plasma total cholesterol was measured by an enzymatic method.9
High-density lipoprotein (HDL) cholesterol was measured after dextran-magnesium precipitation of non-HDL lipoproteins.10
Plasma ICAM-1 levels were determined by ELISA (R&D Systems, Abingdon, UK).
Genotyping of the ICAM-1 G241R (rs1799969) and K469E (rs5498) polymorphisms was performed using the TaqMan assay (Applied Biosystems, Foster City, CA). Primers and probes are available upon request. The percentage of agreement for blind duplicate genotyping data was 98.7% for G241R and 98.2% for K469E.
All statistical analyses were conducted utilizing STATA version 9.2 (College Station, TX). Hardy-Weinberg equilibrium expectations were tested using a χ2
goodness-of-fit test. All analyses were done separately by race due to ICAM-1 allele frequency differences between whites and African Americans. Cox proportional hazards models were used to estimate the hazard rate ratios (HRRs) of incident CHD and ischemic stroke. For incident cases, follow-up time was defined as the time between the initial clinical visit and the date of the first event. For non-cases, follow-up continued until December 31, 2003, the date of death, or the date of last contact if lost to follow-up, whichever came first. For CHD analyses, covariates included age, gender, field center and traditional CHD risk factors (BMI, HDL, total cholesterol, smoking, diabetes, hypertension). For stroke analyses, covariates included age, gender, field center, smoking, diabetes and hypertension (identified by the National Institute of Neurological Disorders and Stroke, www.ninds.nih.gov
). Covariates were assessed for statistical significance by the Wald χ2