Genetic and Environmental Contributions to Atherosclerosis Phenotypes in Men and Women

Abstract

Background and Purpose— Carotid intima-media thickness (IMT) is a quantitative measure of subclinical atherosclerosis that is predictive of subsequent myocardial infarction and stroke. There is controversy regarding the proportion of variability in IMT explained by genetic factors. Thus, it is uncertain whether carotid IMT is a heritable trait that can be used in genetic studies. Methods— From 1996 to 1998, we measured carotid IMT in 906 men (mean age, 56.7 years) and 980 women (mean age, 57.4 years) from 586 extended families (1630 sib pairs) in the Framingham Offspring cohort. B-mode carotid ultrasonography was used to define mean and maximum IMT of the common carotid artery (CCA) and internal carotid artery (ICA). Correlation coefficients were calculated in pairs of siblings. Variance component methods were used to estimate heritability with crude, age- and sex-adjusted, and multivariable-adjusted normalized deviates. Results— Multivariable-adjusted correlation coefficients for mean CCA and ICA IMT were 0.16 and 0.16, respectively. Crude, age- and sex-adjusted, and multivariable-adjusted heritabilities were 0.67, 0.44, and 0.38 for the mean CCA IMT (all P <0.001) and 0.43, 0.37, and 0.35 for the mean ICA IMT (all P <0.001). For CCA IMT, 27% of the overall variance was due to measured covariates; 38% was due to heritable factors. Conclusions— These data suggest that a substantial proportion of the variability in carotid IMT is explained by genetic factors. Further studies of genetic linkage and candidate gene association are warranted to identify specific genetic variants predisposing to subclinical atherosclerosis and stroke.