Genetic basis of atherosclerosis: insights from mice and humans

Nutrigenetics and nutrigenomics of atherosclerosis

The latest genome-wide association studies (GWAS) have re-energized our effort to understand the genetic basis of atherosclerotic cardiovascular disease. Although the knowledge generated by GWAS has confirmed that mediators of inflammation and perturbed lipid metabolism are major players in cardiovascular disease (CVD) development, much of individual disease heritability remains unexplained by the variants identified through GWAS. Moreover, results from interventions that aim at the pharmaceutical modification of lipid parameters fall short of expectation. These elusive treatment goals based on heritability studies highlight a key supportive, and perhaps even primary, role of nutritional therapy to achieve better health outcomes. Nonetheless, effective and specific interventions for CVD prevention using principles of "personalized" nutrition require a better knowledge of gene-diet interactions, an area that remains poorly explored. Dietary fatty acids such as omega-3 polyunsaturated fatty acids (PUFAs) are an excellent example of a widely studied "environment" that interacts with the genetic makeup in relation to CVD. A thorough exploration of the nutrigenomics and nutrigenetics of omega-3 PUFAs is key to understanding the etiology, and developing effective preventive measures. In this review, we will summarize the current state of knowledge of genetic interactions with omega-3 PUFAs in modulating lipid metabolism and inflammation, and defining health outcomes. Nutrigenetics and nutrigenomics are still in their infancy with respect to CVD prediction and therapy. Integration of the progress in the omics, including metabolomics, lipidomics, transcriptomics, and proteomics, coupled with advances in nutrigenomic and nutrigenetic research will move us towards personalized medicine as the ultimate paradigm of responsible clinical practice.

Gene expression analyses of mouse aortic endothelium in response to atherogenic stimuli

OBJECTIVE

Endothelial cells are central to the initiation of atherosclerosis, yet there has been limited success in studying their gene expression in the mouse aorta. To address this, we developed a method for determining the global transcriptional changes that occur in the mouse endothelium in response to atherogenic conditions and applied it to investigate inflammatory stimuli.

APPROACH AND RESULTS

We characterized a method for the isolation of endothelial cell RNA with high purity directly from mouse aortas and adapted this method to allow for the treatment of aortas ex vivo before RNA collection. Expression array analysis was performed on endothelial cell RNA isolated from control and hyperlipidemic prelesion mouse aortas, and 797 differentially expressed genes were identified. We also examined the effect of additional atherogenic conditions on endothelial gene expression, including ex vivo treatment with inflammatory stimuli, acute hyperlipidemia, and age. Of the 14 most highly differentially expressed genes in endothelium from prelesion aortas, 8 were also perturbed significantly by ≥ 1 atherogenic conditions: 2610019E17Rik, Abca1, H2-Ab1, H2-D1, Pf4, Ppbp, Pvrl2, and Tnnt2.

CONCLUSIONS

We demonstrated that RNA can be isolated from mouse aortic endothelial cells after in vivo and ex vivo treatments of the murine vessel wall. We applied these methods to identify a group of genes, many of which have not been described previously as having a direct role in atherosclerosis, that were highly regulated by atherogenic stimuli and may play a role in early atherogenesis.

Atherosclerosis susceptibility Loci identified in an extremely atherosclerosis-resistant mouse strain

Background

C3H/HeJ (C3H) mice are extremely resistant to atherosclerosis, especially males. To understand the underlying genetic basis, we performed quantitative trait locus (QTL) analysis on a male F₂ (the second generation from an intercross between 2 inbred strains) cohort derived from an intercross between C3H and C57BL/6 (B6) apolipoprotein E–deficient (Apoe^−/−) mice.

Methods and Results

Two hundred forty‐six male F₂ mice were started on a Western diet at 8 weeks of age and kept on the diet for 5 weeks. Atherosclerotic lesions in the aortic root and fasting plasma lipid levels were measured. One hundred thirty‐four microsatellite markers across the entire genome were genotyped. Four significant QTLs on chromosomes (Chr) 2, 4, 9, and 15 and 4 suggestive loci on Chr1, Chr4, and Chr7 were identified for atherosclerotic lesions. Unexpectedly, the C3H allele was associated with increased lesion formation for 2 of the 4 significant QTLs. Six loci for high‐density lipoprotein (HDL), 6 for non‐HDL cholesterol, and 3 for triglycerides were also identified. The QTL for atherosclerosis on Chr9 replicated Ath29, originally mapped in a female F₂ cohort derived from B6 and C3H Apoe^−/− mice. This locus coincided with a QTL for HDL, and there was a moderate, but statistically significant, correlation between atherosclerotic lesion sizes and plasma HDL cholesterol levels in F₂ mice.

Conclusions

These data indicate that most atherosclerosis susceptibility loci are distinct from those for plasma lipids except for the Chr9 locus, which exerts effect through interactions with HDL.

Gene expression differences during the heterogeneous progression of peripheral atherosclerosis in familial hypercholesterolemic swine

BACKGROUND

The heterogeneous progression of atherosclerotic disease in the peripheral arteries is currently not well understood. In humans, artery specific disease progression is partly attributed to the local hemodynamic environments. However, despite similar hemodynamic environments, porcine brachial arteries are protected while femoral arteries are highly susceptible to advanced lesion formation. The aim of this investigation was to determine whether artery specific gene expression patterns contribute to the uneven distribution of peripheral arterial disease (PAD) in Rapacz Familial-Hypercholesterolemic (FHC) swine.

RESULTS

Histological results confirmed rapid atherosclerotic disease progression in femoral but not brachial arteries. A total of 18,922 probe sets had sufficient signal abundance. A main effect for age and artery was observed for 1784 and 1256 probe sets, respectively. A significant age x artery interaction was found for 184 probe sets. Furthermore, comparison between arteries found a decrease from 714 to 370 differentially expressed transcripts from nine months to two years of age. Gene ontology analysis of the 56 genes with a main effect for artery and an age x artery interaction identified vascular smooth muscle contraction as enhanced biological signaling pathway.

CONCLUSION

This is the first investigation to report that the total number of differential genes decreases with diverging atherosclerotic disease pattern between porcine brachial and femoral arteries.

Identification of CAD candidate genes in GWAS loci and their expression in vascular cells

Recent genome-wide association studies (GWAS) have identified 35 loci that significantly associate with coronary artery disease (CAD) susceptibility. The majority of the genes represented in these loci have not previously been studied in the context of atherosclerosis. To characterize the roles of these candidate genes in the vessel wall, we determined their expression levels in endothelial, smooth muscle, and macrophage cells isolated from healthy, prelesioned, and lesioned mouse aortas. We also performed expression quantitative locus (eQTL) mapping of these genes in human endothelial cells under control and proatherogenic conditions. Of the 57 genes studied, 31 were differentially expressed in one or more cell types in disease state in mice, and the expression levels of 8 were significantly associated with the CAD SNPs in human cells, 7 of which were also differentially expressed in mice. By integrating human and mouse results, we predict that PPAP2B, GALNT4, MAPKAPK5, TCTN1, SRR, SNF8, and ICAM1 play a causal role in the susceptibility to atherosclerosis through a role in the vasculature. Additionally, we highlight the genetic complexity of a subset of CAD loci through the differential expression of multiple candidate genes per locus and the involvement of genes that lie outside linkage disequilibrium blocks.

A coding variant in SR-BI (I179N) significantly increases atherosclerosis in mice

Human coding variants in scavenger receptor class B member 1 (SR-BI; gene name SCARB1) have recently been identified as being associated with plasma levels of HDL cholesterol. However, a link between coding variants and atherosclerosis has not yet been established. In this study we set out to examine the impact of a SR-BI coding variant in vivo. A mouse model with a coding variant in SR-BI (I179N), identified through a mutagenesis screen, was crossed with Ldlr (-/-) mice, and these mice were maintained on a Western-type diet to promote atherosclerosis. Mice showed 56 and 125 % increased atherosclerosis in female and male Ldlr (-/-) Scarb1 (I179N) mice, respectively, when compared to gender-matched Ldlr (-/-) control mice. As expected, HDL cholesteryl ester uptake was impaired in Ldlr (-/-) Scarb1 (I179N) mice compared to Ldlr (-/-) control mice, with a net effect of increased small and very small LDL cholesterol in Ldlr (-/-) Scarb1 (I179N) mice being the most probable cause of the observed increased atherosclerosis. Our data show that non-null coding variants in SR-BI can have a large significant impact on atherosclerosis, even if plasma lipid levels are not dramatically affected, and that human mutations in other candidate lipid genes could significantly impact atherosclerosis.

Myeloperoxidase G-463A polymorphism and susceptibility to coronary artery disease: a meta-analysis

Published data on the association between the myeloperoxidase (MPO) G-463A polymorphism and coronary artery disease (CAD) are inconclusive. To derive a more precise estimation of the relationship, a meta-analysis on this topic was performed. PubMed, EMBASE and Chinese national knowledge infrastructure were searched for studies regarding the association between the MPO G-463A polymorphism and CAD. A logistic regression analysis was used to estimate the genetic effect and the possible genetic model of action. Summary odds ratios (ORs) with their corresponding 95% confidence intervals (CIs) were calculated. There was strong evidence for an association between the MPO G-463A polymorphism and CAD. The genetic model of action was most likely to be co-dominant. Overall, the data showed that AA and GA genotypes were significantly associated with reduced risk of CAD (AA vs. GG: OR=0.37, 95% CI=0.17-0.78; GA vs. GG: OR=0.73, 95% CI=0.57-0.92). In subgroup analyses by study population and sources of controls, statistically significant results were observed in the Chinese population (AA vs. GG: OR=0.21, 95% CI=0.10-0.43; GA vs. GG: OR=0.57, 95% CI=0.44-0.74) and in hospital-based control studies (AA vs. GG: OR=0.20, 95% CI=0.10-0.39; GA vs. GG: OR=0.61, 95% CI=0.48-0.77). This meta-analysis suggests that the MPO G-463A variant genotypes may be associated with decreased risk of CAD. However, given the limited number of studies and the potential biases, the influence of this polymorphism on CAD risk needs further investigation.

Functional genomics of the human high-density lipoprotein receptor scavenger receptor BI: an old dog with new tricks

PURPOSE OF REVIEW

The athero-protective role of scavenger receptor BI (SR-BI) is primarily attributed to its ability to selectively transfer cholesteryl esters from high-density lipoproteins (HDLs) to the liver during reverse cholesterol transport (RCT). In this review, we highlight recent findings that reveal the impact of SR-BI on lipid levels and cardiovascular disease in humans. Moreover, additional responsibilities of SR-BI in modulating adrenal and platelet function, as well as female fertility in humans, are discussed.

RECENT FINDINGS

Heterozygote carriers of P297S, S112F and T175A-mutant SR-BI receptors were identified in patients with high HDL-cholesterol levels. HDL from P297S-SR-BI carriers was unable to mediate macrophage cholesterol efflux, whereas hepatocytes expressing P297S-SR-BI were unable to mediate the selective uptake of HDL-cholesteryl esters. S112F and T175A-mutant receptors exhibited similar impaired cholesterol transport functions in vitro. Reduced SR-BI function in P297S carriers was also associated with decreased steroidogenesis and altered platelet function. Further, human population studies identified SCARB1 variants associated with female infertility.

SUMMARY

Identification of SR-BI variants confirms the key role of this receptor in influencing lipid levels and RCT in humans. A deeper understanding of the contributions of SR-BI to steroidogenesis, platelet function and fertility is required in light of exploration of HDL-raising therapies aimed at reducing cardiovascular risk.

FOXP3 genetic variant and risk of acute coronary syndrome in Chinese Han population

Coronary artery disease is the most common type of heart disease and a leading cause of morbidity and mortality all over the world. Acute coronary syndrome (ACS) is the most serious form of coronary artery disease. Recently, many studies indicated that genetic susceptibility may play a vital role in the pathogenesis of coronary heart disease including ACS. Forkhead/winged helix transcription factor (FOXP3) gene polymorphisms have been previously found to be associated with inflammatory diseases. To determine whether FOXP3 polymorphisms are associated with ACS, we examined the single nucleotide polymorphism rs3761548 of FOXP3 gene by polymerase chain reaction-polyacrylamide gel electrophoresis in 226 ACS patients and 259 unrelated healthy subjects. Our results showed that single nucleotide polymorphism rs3761548 had association with ACS in Chinese Han population. These data indicate that, for the first time, FOXP3 gene polymorphism may appear to play an important role in the susceptibility of ACS in Chinese Han population.

Atherosclerosis, inflammation, genetics, and stem cells: 2012 update

Atherosclerosis is a peculiar form of inflammation triggered by cholesterol-rich lipoproteins and other noxious factors such as cigarette smoke, diabetes mellitus, and hypertension. Genetics also play an important role in the disease, accounting for about 40% of the risk. Of surprise in recent years of post-human genome sequencing, atherosclerosis-relevant genes discovered by non-biased techniques (ie, genome-wide association studies), did not rehash previously suspected pathways of lipid metabolism, diabetes, or hypertension. Instead these studies highlighted genes relevant to mechanisms of inflammation and stem cell biology. Only a minority of implicated genes were linked to lipid and other cardiac risk factor genes. Although such findings do not contradict the fact that atherosclerosis is triggered and exacerbated by elevated lipids, atherosclerosis "new genes" suggest that the mechanism responsible for the development of arterial lesions is more complex than a simple response to injury, where injury is necessary, but perhaps not sufficient, for disease progression.

Functional characterization of newly-discovered mutations in human SR-BI

In rodents, SR-BI has been firmly established as a physiologically relevant HDL receptor that mediates removal of HDL-cholesteryl esters (CE). However, its role in human lipoprotein metabolism is less defined. Recently, two unique point mutations in human SR-BI - S112F or T175A - were identified in subjects with high HDL-cholesterol (HDL-C) levels. We hypothesized that mutation of these conserved residues would compromise the cholesterol-transport functions of SR-BI. To test this hypothesis, S112F- and T175A-SR-BI were generated by site-directed mutagenesis. Cell surface expression was confirmed for both mutant receptors in COS-7 cells upon transient transfection, albeit at lower levels for T175A-SR-BI. Both mutant receptors displayed defective HDL binding, selective uptake of HDL-CE and release of free cholesterol (FC) from cells to HDL. Mutant receptors were also unable to re-organize plasma membrane pools of FC. While these impaired functions were independent of receptor oligomerization, inability of T175A-SR-BI to mediate cholesterol-transport functions could be related to altered N-linked glycosylation status. In conclusion, high HDL-C levels observed in carriers of S112F- or T175A-SR-BI mutant receptors are consistent with the inability of these SR-BI receptors to mediate efficient selective uptake of HDL-CE, and suggest that increased plasma HDL concentrations in these settings may not be associated with lower risk of cardiovascular disease.

Nicotinic acetylcholine receptor signaling in atherogenesis

Smoking is a major risk factor for the development of atherosclerosis, stroke and myocardial infarction. Cigarette smoke consists of a complex mixture of about 4000 compounds. Out of these, polycyclic hydrocarbons, tobacco-specific nitrosamines, oxidizing agents and carbon monoxide have been implicated in the development of atherosclerosis. Recent studies have shown that nicotine (the addictive component of cigarettes) binds to high affinity cell-surface receptors and accelerates the atherogenic process. These receptors are called nicotinic acetylcholine receptors (nAChRs) and are expressed ubiquitously in almost all cells existing in the blood vessels. The present review summarizes the pro-atherogenic effects of nAChR ligands such as nicotine and tobacco nitrosamines. The contribution of different nAChR subunits in plaque growth, progression and neovascularization are discussed in detail. The signaling pathways underlying the actions of the nAChRs ligands in blood vessels are also described. Finally, the feasibility of nAChR ligands as therapeutic targets for atherosclerosis is summarized. We believe that the information presented in this review is relevant for atherosclerosis patients who are active smokers, exposed to environmental tobacco smoke or use nicotine patches or gums for smoking cessation.

Differences in health status affect susceptibility and mapping of genetic loci for atherosclerosis (fatty streak) in inbred mice

OBJECTIVE

We observed differences in atherosclerosis susceptibility in mouse inbred strains over the years as the health status of our animal rooms increased. Therefore, we investigated the effect of animal room health status on atherosclerosis susceptibility in different strains. As these data can also be used for genome-wide association mapping, we performed a mapping study and compared our results with previously found quantitative trait loci for atherosclerosis in mouse and humans.

METHODS AND RESULTS

Males and females from 48 inbred strains were housed in 2 animal rooms with different health status and given an atherogenic diet. We compared atherosclerosis susceptibility between animal rooms and between sexes and found that susceptibility is dependent on both health status and sex. Subsequently, the data were used for associations with loci on the mouse genome using 63 222 single nucleotide polymorphism. Three loci in males and 4 loci in females were identified using the data from the low-health status room. No significant associations were identified using the data from the high-health status room.

CONCLUSIONS

Health status influences susceptibility to atherosclerosis and suggests that microbiological pressure plays an important role in the development of atherosclerosis in many strains. As we were only able to map susceptibility loci using the data from the lower health status room, we argue that susceptibility under these conditions is determined by a few key loci, whereas in the higher health status room different mechanisms might play a role in the differences in atherosclerosis susceptibility between strains and we did not have enough power to map the loci that are involved.

Associations of polymorphisms in the apolipoprotein APOA1-C3-A5 gene cluster with acute coronary syndrome

Background. Acute coronary syndromes (ACSs) are clinically cardiovascular events associated with dyslipidemia in common. Single nucleotide polymorphisms (SNPs) and haplotypes in the APOA1/C3/A5 gene cluster are associated with diabetes and familial combined hyperlipidaemia (FCH). Little is known about whether the polymorphisms in these genes affect lipid homeostasis in patients with ACSs. The present paper aimed to examine these associations with 4 SNPs in the APOA1 −75G > A, the APOC3 −455T > C, and APOA5 −1131T > C, c.553G > T variant to ACSs in Chinese Han. Methods. Chinese Han of 229 patients with ACSs and 254 unrelated controls were analyzed. Four SNPs in APOA1/C3/A5 cluster were genotyped and lipid was determined. Results. Our data show that minor allelic frequencies of APOC3 −455T > C, APOA5 −1131T > C, and c.553G > T polymorphisms in patients with ACSs were significantly higher than control group (P < 0.05). Furthermore, the 3 polymorphic sites were strongly of linkage disequilibrium, and minor alleles of 3 SNP sites had higher TG level than wild alleles (P < 0.05), APOC3 −455C and APOA5 c.553T allele carriers also had lower level of HDL-C. Conclusions. The minor alleles of APOC3 −455T > C, APOA5 −1131T > C, and c.553G > T polymorphisms are closely associated with ACSs.