Design of the Genetics of Early Onset Cardiovascular Disease
Background: Coronary artery disease (CAD) is the leading cause of death and a leading cause of disability in the developed world. Early onset (premature) coronary artery disease (EOCAD) is known to have a particularly strong genetic component. However, the actual genes leading to this increased risk of CAD remain obscure.
Methods: The primary goal of the Genetics of Early Onset Cardiovascular Disease (GENECARD) study is to perform a genetic linkage study in 920 families with at least 1 sibling pair having EOCAD. The study sites include a US network of 15 cardiology practices and 5 additional sites located in Europe and the United States. We propose to identify chromosomal regions associated with increased susceptibility to EOCAD in this large sample of affected sibling pairs and nuclear families, where EOCAD is defined on the basis of having acute coronary syndrome (unstable angina or myocardial infarction), a revascularization procedure, or a positive functional imaging study at or before the age of 50 years in men or 55 years in women. To identify which genomic regions and genes are associated with increased susceptibility to EOCAD, we will use a comprehensive strategy comprising genomic screening, fine mapping, candidate gene analysis, and family-based association studies.
Results: Herein we describe the clinical characteristics, family history, and risk factor profiles of the 1168 members from 438 nuclear families included in the first, exploratory analysis. Analysis of the study population revealed a strong concordance of known cardiac risk factors among affected sibling pairs. There was significant concordance (P <. 01) among siblings with EOCAD for presence of diabetes (78% concordance), dyslipoproteinemia (67%), obesity (63%), and hypertension (56%). This level of concordance of risk factors among siblings might be expected, given the significant genetic components demonstrated for these metabolic susceptibility traits. However, there was also substantial sibling pair concordance (P < .01) for smoking history (74%), regular alcohol consumption (81%), and sedentary lifestyle (63%), environmental traits without known inherited predisposition.
Conclusions: Analyses such as these will have implications for stratifying populations for the statistical analysis of the genome scan and on the choice of covariates for the follow-up studies of the initial genome screen analysis.

Coronary artery dise (CAD) is a complex disease with clear genetic predisposition and environmental risk factors. In like manner, metabolic risk factors for CAD, such as diabetes, dyslipidemia, hypertension, and obesity, may have both genetic and environmental contributors. Early onset (premature) coronary artery disease (EOCAD) is known to have a particularly strong genetic component. However, the genes leading to increased risk and to the pathogenesis of CAD remain obscure. Although several large case-control studies of polymorphisms in candidate genes and the presentation of CAD have been performed in diverse populations, few studies have used genome-wide linkage analysis for the identification of CAD susceptibility genes in genetically and geographically diverse populations.

Coronary artery disease is the leading cause of death in the United States and the majority of the industrialized countries. More than 14 million Americans are afflicted with clinically significant CAD. To illustrate the impact of CAD in developed countries, the medical and societal costs of this disease in the United States alone is in excess of $90 billion annually. More than 600,000 Americans each year have new cardiac events, the majority of which present without warning as sudden cardiac death. More than 10% of CAD occurs in Americans <50 years of age. Although a minority of the patient base, this 10% of early events represents a potentially fertile area for investigation of the genetics underlying the inheritance of cardiac risk, because a major risk factor for development of the disease in this population is family history. Identifying genetic predisposition to early onset disease could guide targeted prevention efforts, help in understanding basic mechanisms of disease, and identify new therapeutic targets. Also, the identification of genetic markers for the predisposition to develop CAD will promote interventions that may significantly impact on the natural history of the disease as well as reduce the costs to the American and numerous international economies.

A family history of CAD is a robust risk factor for CAD, even after adjustment for environmental risks that may be shared within families. Several studies have estimated relative risks of developing EOCAD in siblings between 3.8 and 12.1, depending on the age of onset in the index case (proband) in the family. Rissanen examined the risk of coronary heart disease in the first-degree relatives of 309 men with myocardial infarction and 106 healthy men <56 years of age. The relative risk of CAD to brothers of probands by age 55 years was 11.4, 8.3, and 1.3 in southern Finland and 6.7, 3.6, and 1.8 in eastern Finland, depending on whether the diagnosis of myocardial infarction in these siblings was made before 46 years of age, at 46 to 50 years of age, or 51 to 55 years of age, respectively. Marenberg et al investigated the risk of premature coronary heart disease in 3298 monozygotic and 5964 dizygotic male twins and 4012 monozygotic and 7730 dizygotic female twins. For men, the relative risk of death from CAD before the age of 55 years was 8.1 (95% CI 2.7-24.5) for monozygotic twins and 3.8 (1.4-10.5) for dizygotic twins when the index case died of CAD before the age of 55 years, compared with those whose twin did not die of CAD before age 55 years. For women, the corresponding relative risks for death from CAD before age 65 years were 15.0 (7.1-31.9) for monozygotic twins and 2.6 (1.0-7.1) for dizygotic twins when their twin died of CAD before age 65 years. Thus there is ample evidence that inherited factors provide a significant risk for the development of CAD and a rationale for the initiation of studies specifically designed to identify susceptibility genes for EOCAD. Recently, several groups have identified candidate genes and genomic regions associated with excessive risk in such populations by use of association studies and linkage analysis, respectively. In each of these studies, different mechanisms to enrich the sample for CAD from genetic causes were used, either by restricting the sample to a well-defined ethnic group with perhaps a small set of founders or restricting the CAD phenotype. The goal of the GENECARD study was to conduct a search for CAD susceptibility genes in families across the spectrum of the CAD phenotype with a sample size large enough to allow examination of linkage evidence in phenotypic subgroups. Our strategy to enhance the genetic component in our study was to set qualifying age criteria that would enhance selection for individuals with a strong genetic predisposition for disease.

Widely accepted environmental risk factors for cardiovascular disease include both major (conferring independent risk) and minor (no definitive evidence for independent risk) risk factors. Traditionally, major risk factors have included smoking history, dyslipoproteinemia, and hypertension, and minor risk factors have included physical inactivity, obesity, diabetes mellitus, psychological stress, and insulin resistance. Recently, the American Heart Association has raised physical inactivity, obesity, and diabetes to major independent risk factors, based on accumulated epidemiologic evidence. In fact, the relations of all of these risk factors to the development of CAD have been widely studied, and reviews of this topic are included in most major textbooks of cardiovascular medicine. More recently, it has become apparent that certain risk factors, among them smoking history, are of greater importance in younger than older individuals. Because of the importance of these factors in determining risk and in understanding the underlying cause of CAD, their contributions must be incorporated into studies aiming to identify genes for EOCAD. More important for the GENECARD study, identification of significant confounding factors and covariates within and between families with EOCAD will permit stratification of families during the statistical analysis of the genome scan. Stratification has 2 major purposes: 1) increasing the homogeneity of the genetic effect and 2) clarifying the relations between risk factors and genes in their contribution to the development of EOCAD. An initial indication of the importance of a risk factor in this setting may be the concordance of its presentation in the proband and siblings within a nuclear family. Determining concordance and stratifying families on that basis may also assist in differentiating genes that contribute to the development of intermediate metabolic risk factors—such as hypertension, dyslipidemia, diabetes and obesity—from those that present a newly identified mechanism of risk. Herein we describe the study design and baseline characteristics of the 438 families that comprise the first cohort of families for the Genetics of Early Onset Cardiovascular Disease (GENECARD) study.