Heritability of subclinical atherosclerosis in Latino families ascertained through a hypertensive parent

A Genome-Wide Scan for Carotid Artery Intima-Media Thickness

BACKGROUND AND PURPOSE

Carotid artery intima-media thickness (CIMT), a subclinical measure of atherosclerosis, is associated with coronary artery disease (CAD), and stroke. CIMT is also an important predictor of clinical cardiovascular events. To systematically identify the genetic determinants of CIMT, we performed a genome-wide scan using data from 91 2-generation Mexican American families ascertained via a parent with CAD diagnosed.

METHODS

CIMT was measured in 274 adult offspring (mean age, 34.6 years) using high-resolution B-mode ultrasound; 413 subjects, including adult offspring and their parents, were genotyped using Marshfield screen set 12 (380 microsatellite markers at approximately 10-cM interval). Heritability was estimated using the variance component approach implemented in SOLAR. Linkage analyses were performed using both the sib-pair regression approach and the variance component approach.

RESULTS

The estimated heritability was 0.68, 0.45, and 0.40 for unadjusted, gender- and age-adjusted, and multivariate-adjusted CIMT, respectively. The strongest evidence of linkage was found on chromosome 2 at D2S2944 (logarithm of the odds [LOD]=3.08). Other suggestive linkages were also found on chromosome 6 at D6S1022 to D6S2410 (LOD=2.21) and chromosome 13 at D13S796 to D13S895 (LOD=1.34).

CONCLUSIONS

These results show that there is a strong genetic effect on CIMT in these Mexican American CAD families. The linkage peak on chromosome 2 suggests that there is a gene (or genes) at this chromosome location influencing CIMT.

Heritability of Carotid Artery Atherosclerotic Lesions

Background and Purpose— Ultrasound examination of the carotid arteries yields several quantitative measures that may serve as intermediate phenotypes in genetic studies. This study was undertaken to compare the heritabilities of 3 ultrasound measures: intima-media thickness (IMT), plaque score, and maximal stenosis.

Methods— We studied 565 individuals from 154 families ascertained by an affected parent with carotid artery atherosclerosis. IMT, plaque score, and maximal stenosis of the carotid arteries were examined by B-mode ultrasound and analyzed quantitatively. Heritability estimates were obtained by variance component analysis as implemented in the program SOLAR (sequential oligogenic linkage analysis routines). Covariates were age, sex, weight, height, body mass index (BMI), arterial hypertension, diabetes mellitus, amount of nicotine consumed, and plasma levels of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol, LDL/HDL ratio, lipoprotein(a) [Lp(a)], triglycerides, factor VIII, factor XIII, fibrinogen, and von Willebrand factor (vWF).

Results— After accounting for the covariables age, sex, hypertension, diabetes mellitus, and Lp(a), heritability of IMT was estimated as h 2 =0.61±0.17 ( P =0.001). Variation of plaque score was influenced by age, sex, hypertension, diabetes mellitus, hypercholesterolemia, amount of nicotine consumed, factor VIII, and vWF. When these were considered, no significant heritability could be detected. Heritability of stenosis was estimated as h 2 =0.47±0.07 ( P =0.006), with age, sex, BMI, hypertension, diabetes mellitus, amount of nicotine consumed, and LDL/HDL ratio as covariates.

Conclusions— Among the 3 ultrasound measures studied, IMT had the highest heritability. IMT was strongly influenced by genetic determinants other than those influencing known risk factors. This makes IMT a promising candidate for use as an intermediate phenotype in genetic studies aiming to identify novel genes for atherosclerosis.

Lipoprotein lipase locus and progression of atherosclerosis in coronary-artery bypass grafts

PURPOSE

Our aim was to test whether polymorphisms in the lipoprotein lipase (LPL) gene were associated with the progression of atherosclerosis in grafts examined in the Post-Coronary Artery Bypass Graft Trial (Post-CABG Trial).

METHODS

843 subjects in the post-CABG trial were genotyped for the LPL-D9N, N291S, PvuII, (TTTA)n, and HindIII polymorphisms. Associations between genotype and angiographically measured progression of atherosclerosis in grafts, medical history, and family history were examined.

RESULTS

Greater progression of atherosclerosis was observed in subjects with LPL-HindIII 2/2 (56% versus 42% of those with other LPL HindIII genotypes, P = 0.025) and with LPL (TTTA)n 4/4 (63% versus 43% of those with other (TTTA)n genotypes, P = 0.020). Mantel-Haenszel analysis yielded an odds ratio of 1.84 for the effect of LPL HindIII 2/2 genotype on the progression of atherosclerosis in grafts (P = 0.015) and demonstrated that the effect of genotype on progression was of the same magnitude as, but independent of, the effect of drug treatment.

CONCLUSION

The LPL-HindIII 2/2 genotype is a marker for genetic variation in the 3'-end of LPL that acts as an independent risk factor for the progression of atherosclerosis in grafts examined in the Post-CABG Trial.

Genetics of Ultrasonographic Carotid Atherosclerosis

The search for genes related to the cause of common complex disorders such as cardiovascular disease has been frustrating, partly because of the many factors known to contribute to cardiovascular disease and the potential "distance" of cardiovascular disease as a phenotype from genes and gene products. Linkage and association studies for phenotypes more proximal in the pathway from DNA sequence variation to overt clinical disease, such as ultrasound-defined carotid atherosclerosis, may potentially be more enlightening. Only one genetic variant previously reported to be associated with atherosclerosis or clinically evident cardiovascular disease, matrix metalloproteinase (MMP) 3, has shown consistently positive associations with carotid disease, although it has not been studied widely. Another, PON1 L55M, is weakly associated in subgroups only, and 2, ApoE and MTHFR, are equivocal. Genetic variants reported to be associated with clinical cardiovascular disease show weak or no relationship to carotid atherosclerosis. This may reflect the known inconsistency in associations of genetic variants with clinical cardiovascular disease itself. Alternatively, genetic determinants of ultrasound-defined carotid atherosclerosis may differ from those of clinically manifest cardiovascular disease and may require pursuit through large-scale genomic studies of carotid atherosclerosis as a distinct phenotype. Only 1 genetic variant, MMP 3, has shown consistently positive associations with ultrasonographic carotid disease, although it has not been studied widely. Another, PON1 L55 mol/L, is weakly associated in subgroups only. Genetic variants reported to be associated with clinical cardiovascular disease show weak or no relationship to carotid atherosclerosis.

Genetic and environmental contributions to carotid intima-media thickness and obesity phenotypes in the Northern Manhattan Family Study

BACKGROUND AND PURPOSE

Both carotid intima-media thickness (IMT) and obesity are independent determinants of stroke and cardiovascular disease. The prevalence of obesity is higher in Hispanics. The genetic basis of IMT and obesity has not been well-characterized in Caribbean Hispanics. The purpose of this study was to examine the genetic and environmental contributions to IMT and obesity in this population.

METHODS

The data included 440 subjects from 77 Caribbean Hispanic families. Mean IMT and maximum IMT were measured in the internal carotid artery, common carotid artery, and carotid bifurcation. The total IMT was calculated as the mean value of IMT at all segments. Obesity phenotypes included body mass index (BMI), waist circumference, waist-to-hip ratio (WHR), and skin-fold thickness. Variance component methods were used to estimate age-adjusted and sex-adjusted heritability. Bivariate analyses were conducted to test for genetic and environmental correlations between IMT and obesity.

RESULTS

Heritabilities for IMT ranged from 9% to 40%, with the highest for total maximum IMT and lowest for internal carotid artery maximum IMT. Heritabilities for BMI, waist circumference, WHR, and skin-fold thickness were 44%, 47%, 5%, and 36%, respectively. There were significant genetic, but not environmental, correlations between IMT and BMI, waist circumference, and skin-fold thickness. There were no genetic or environmental correlations between IMT and WHR.

CONCLUSIONS

We found a substantial genetic contribution to IMT, BMI, waist circumference, and skin-fold thickness. Obesity and IMT may share common genetic factors. Future gene mapping studies are warranted to identify genes predisposing to IMT and obesity in this population.

The signal sequence polymorphism of the MnSOD gene is associated with the degree of carotid atherosclerosis

Redox-state of the cells of vascular walls is an important determinant of atherosclerosis. Manganese superoxide dismutase (MnSOD) is an essential anti-oxidant enzyme working in mitochondria of mammalian cells. A potentially functional amino acid polymorphism (Ala16Val) has been described in the signal sequence of the enzyme. The aim of the current study was to test whether the signal sequence polymorphism of the MnSOD would be associated with the degree of carotid atherosclerosis. The polymorphism was genotyped in a sample of 989 middle-aged hypertensive and control subjects. Carotid atherosclerosis was quantified as intima-media thickness (IMT) by ultrasound. The signal sequence polymorphism was found to be a minor determinant of carotid IMT explaining 1.3% of the overall variation, the Val allele associated with the higher IMT. In women, a significant interaction with plasma levels of low-density lipoprotein (LDL) cholesterol was detected, since LDL cholesterol levels were positively correlated with carotid IMT only in the carriers of the Val allele and the Val allele was associated with higher IMT only in the subjects with highest plasma levels of LDL cholesterol. In conclusion, the signal sequence polymorphism of the MnSOD gene is a minor determinant of carotid IMT pointing out the importance of redox-balance in the atherogenesis.

Genetics of intima-media thickness

PURPOSE OF REVIEW

Increased carotid artery intima-media thickness is associated with an increased risk of coronary heart disease or cerebrovascular disease and is a powerful predictor of cardiovascular and cerebrovascular outcomes. Consequently, the measurement is now used in a number of case control, cohort and familial and linkage studies as an intermediate phenotype for the investigation of the genetic and environmental determinants of atherosclerosis. The aim of this paper is to review the most recent available data on the genetic determinants of carotid intima-media thickness.

RECENT FINDINGS

Genes could account for a significant amount of variation in carotid intima-media thickness: up to 30-66%. Carotid intima-media thickness progressed more rapidly with age in familial hypercholesterolemia patients than in patients without his condition. Familial hypercholesterolemia patients with a null LDL receptor allele tended to have higher carotid intima-media thickness than patients carrying the LDL receptor defective allele. Small association studies showing positive or negative results with the angiotensinogen gene variants as well as with the angiotensin converting enzyme I/D polymorphism add to the confusion in this largely controversial area. Differing results may depend on the vascular territory and genetic background. New associations have been described in small studies.

SUMMARY

Studies on the association of polymorphisms and intima-media thickness are frequently disappointing and lead occasionally to conflicting results. Among study limitations is the fact that mostly single gene effects are considered; longitudinal cohort studies may be more appropriate than case control studies. Ongoing large prospective population studies and clinical trials have integrated the measurements of intima-media thickness and genotype determination with a genomic approach. As a result, in the near future we may see more important and robust results with significant consequences on our understanding of genetic determinants of atherosclerotic cardiovascular disease.

Heritability of carotid artery structure and function: the Strong Heart Family Study

OBJECTIVE

Alterations in carotid artery structure and function may represent phenotypic measures of vascular disease that contain information beyond that which can be inferred from conventional cardiovascular disease risk assessment. However, apart from their associations with cardiovascular disease risk factors and outcome, the genetic basis of variations in carotid artery structure and function is largely unknown. The purpose of this study was to examine the genetic and environmental contributions to carotid artery structure and function in 3 large groups of American Indians.

METHODS AND RESULTS

Approximately 950 men and women, aged > or =18 years, in 32 extended families were examined between 1997 and 1999. By use of a variance component approach and the program Sequential Oligogenic Linkage Analysis Routines, heritabilities for carotid artery structure and function phenotypes were estimated. After accounting for the effects of covariates (sex, age, diabetes, impaired glucose tolerance, smoking, cholesterol, body surface area, and hypertension), we detected significant heritabilities (given as h2 values) for common carotid artery diastolic diameter (h2<0.44), intimal-medial wall thickness (h2<0.21), vascular mass (h2<0.27), arterial stiffness (h2<0.23), and the augmentation index (h2<0.18).

CONCLUSIONS

These results suggest that the additive effects of genes explain a moderate proportion of the variability of carotid artery structure and function.