Genetic analysis of atherosclerosis: a research paradigm for the common chronic diseases

Endothelial nitric oxide synthase Glu298Asp (G894T) gene polymorphism in coronary artery disease patients with type 2 diabetes mellitus

AIMS

Endothelial dysfunction is thought to be a significant risk factor for cardiovascular disease. This study determined the role of endothelial nitric oxide synthase (eNOS) Glu298Asp polymorphism and intergenotypic variation of plasma nitric oxide (NO) levels in coronary artery disease (CAD) patients with type 2 diabetes mellitus (DM).

METHODS

This case-control study included 28 documented CAD patients with type 2 DM and 32 non-diabetic patients with CAD. Fifty healthy volunteers without any major cardiovascular risk factors served as controls. NO was estimated by modified Griess method. The eNOS gene polymorphism was studied by amplifying DNA by PCR and digesting with BanII restriction enzyme. Restriction fragment length polymorphism was studied by using a gel documentation system.

RESULTS

The genotype frequencies for Glu298Asp (GT) genotype were 10.71% in diabetic CAD patients, 28.1% in non-diabetic CAD patients and 12% in controls. The T allele frequency was higher in the non-diabetic CAD group (14%) as compared with the diabetic CAD (5.4%) and control group (6%). NO level was significantly lower in non-diabetic CAD patients (10.25 mmol/L) but not in diabetic CAD patients (13.89 mmol/L) as compared to controls (16.78 mmol/L).

CONCLUSION

Glu298Asp polymorphism is not the mediator of increased incidence of CAD in diabetic patients.

Apolipoprotein A1 gene polymorphism (G-75A and C+83T) in patients with myocardial infarction: a pilot study in a north Indian population

The apolipoprotein A1 gene polymorphism (G-75A and C+83T) was studied in 100 subjects (50 patients diagnosed with myocardial infarction and 50 healthy subjects). Serum apolipoprotein (apo) A1 and apo B levels were estimated immunoturbidometrically. Extracted DNA from blood was amplified by polymerase chain reaction, digested with MspI restriction enzyme, run on 8% polyacrylamide gel, and restriction fragment length polymorphism was studied by using a gel documentation system. Serum (mean +/- SD) apo A1 levels were significantly higher in control subjects than the study group (100.80 +/- 7.06 mg/dL [1.0 +/- 0.07 g/L] and 72.56 +/- 9.86 mg/dL [0.73 +/- 0.1 g/L], respectively; P < .0001), whereas apo B levels were significantly lower (72.12 +/- 11.32 mg/dL [0.7 +/- 0.1 g/L] and 97.45 +/- 9.04 mg/dL [1.0 +/- 0.09 g/L], respectively; P < .0001). The G allele frequency at the -75-base-pair (bp) site was higher in the study group (79%) compared with the control group (58%). The T allele frequency at the +83-bp site was higher in the study group (56%) than in the control group (32%). G at -75 bp upstream from the start of transcription and T at +83 bp in the first intron may be susceptibility alleles for myocardial infarction.

Methylentetrahydrofolate reductase and nitric oxide synthase polymorphism in patients with atherosclerosis and diabetes

The development of the atherosclerosis is based on multifactorial causes. In addition to the traditional risk factors, gene polymorphisms can play a role in the disease. Therefore in this study we investigated whether the eNOS and MTHFR gene polymorphisms is associated with myocardial infarction and stroke in patients with or without diabetes. We have identified polymorphisms in the NOS 3 gene and one of these polymorphisms, Glu(298-->)Asp, was found to be a major risk factor for carotid artery disease and myocardial infarction. Our results indicate that the MTHFR G677T allele is significantly associated with MI. MTHFR 677 G/T genotyping may be of clinical importance as a prognostic and therapeutic marker, although further studies are needed to substantiate this hypothesis.

Genetic susceptibility to peripheral arterial disease: a dark corner in vascular biology

Peripheral arterial disease (PAD) is characterized by reduced blood flow to the limbs, usually as a consequence of atherosclerosis, and affects approximately 12 million Americans. It is a common cause of cardiovascular morbidity and an independent predictor of cardiovascular mortality. Similar to other atherosclerotic diseases, such as coronary artery disease, PAD is the result of the complex interplay between injurious environmental stimuli and genetic predisposing factors of the host. Genetic susceptibility to PAD is likely contributed by sequence variants in multiple genes, each with modest effects. Although many of these variants probably alter susceptibility both to PAD and to coronary artery disease, it is likely that there exists a set of variants specifically to alter susceptibility to PAD. Despite the prevalence of PAD and its high societal burden, relatively little is known about such genetic variants. This review summarizes our limited present knowledge and gives an overview of recent, more powerful approaches to elucidating the genetic basis of PAD. We discuss the advantages and limitations of genetic studies and highlight the need for collaborative networks of PAD investigators for shedding light on this dark corner of vascular biology.

Atherosclerosis: the path from genomics to therapeutics

Recent rapid advances in genomic tools and techniques hold great promise for transforming the practice of cardiovascular medicine. Resources including the Human Genome Project and the International HapMap project, major technological advances in high-throughput genotyping and methods of statistical analysis, and methods for high-throughput gene expression and small molecule profiling allow researchers to confront issues that will fundamentally change the practice of cardiovascular medicine during the 21st century. Genomic, proteomic, and metabolomic studies of complex cardiovascular diseases such as atherosclerosis will bridge epidemiology and basic biology, and promise increased understanding of cardiovascular disease processes. Genetic approaches applied to atherosclerosis will continue to identify genes and pathways involved in the predisposition to and pathophysiology of atherosclerosis. Gene expression profiling refines our understanding of the dynamic nature of the atherosclerotic vascular wall and promises discovery and validation of targets for therapeutic intervention. Opportunities to translate genetic, genomic, proteomic, and metabolomic information into cardiovascular clinical practice have never been greater, but their fruition requires validation in large independent cohorts, achieved only through collaborative effort. Their continued success will depend on ongoing cooperation within the cardiovascular research community.

Hyperhomocysteinemia, Endothelial Nitric Oxide Synthase Polymorphism, and Risk of Coronary Artery Disease

Background: Hyperhomocysteinemia is an independent, graded risk factor for coronary artery disease (CAD). The G894T variant of endothelial nitric oxide synthase (eNOS) was postulated to be associated with hyperhomocysteinemia and could influence individual susceptibility to CAD. The aims of this study were to investigate (a) the relationship of the eNOS G894T polymorphism with the presence and the severity of CAD and (b) the possible relationship between hyperhomocysteinemia and the eNOS G894T variant for the risk of CAD severity in a Tunisian population.

Methods: We used PCR with restriction fragment length polymorphism analysis to detect the G894T variant of the eNOS gene in 100 patients with CAD and 120 healthy controls. The severity of CAD was expressed by the number of affected vessels. Total plasma homocysteine concentrations were determined by direct chemiluminescence assay.

Results: The frequencies of the eNOS GG, GT, and TT genotypes in the CAD group were significantly different from those in the control group (45%, 44%, and 11% vs 60%, 35.8% and 4.2%, respectively; P = 0.035). There was no association between the eNOS G894T genotype frequencies and the number of stenosed vessels (P = 0.149). In the CAD group, the coexistence of the 894 GT or TT genotypes and hyperhomocysteinemia led to an increased risk of CAD severity.

Conclusion: The G894T polymorphism of the eNOS gene is associated with the presence of CAD, and in conjunction with hyperhomocysteinemia, increased the risk of CAD severity in a Tunisian population.

Association of CommonCRPGene Variants with CRP Levels and Cardiovascular Events

C-reactive protein (CRP) is a well-documented marker of atherosclerotic cardiovascular disease risk. We resequenced CRP to identify a comprehensive set of common SNP variants, then studied and replicated their association with baseline CRP level among apparently healthy subjects in the Women's Health Study (WHS; n = 717), Pravastatin Inflammation/CRP Evaluation trial (PRINCE; n = 1,110) and Physicians' Health Study (PHS; n = 509) cohorts. The minor alleles of four SNPs were consistently associated in all three cohorts with higher CRP, while the minor alleles of two SNPs were associated with lower CRP (p < 0.05 for each). Single marker and haplotype analysis in all three cohorts were consistent with functional roles for the 5'-flanking triallelic SNP -286C>T>A and the 3'-UTR SNP 1846G>A. None of the SNPs associated with higher CRP were associated with risk of incident myocardial infarction (MI) or ischemic stroke in a prospective, nested case-control study design from the PHS cohort (610 case-control pairs). One SNP, -717A>G, was unrelated to CRP levels but associated with decreased risk of MI (p = 0.001). Taken together, these data imply significant interactions between both genetic and environmental contributions to the increased CRP levels that predict a greater risk of future atherothrombotic events in epidemiological studies.

New Explicit Expressions for Relative Frequencies of Single-Nucleotide Polymorphisms With Application to Statistical Inference on Population Growth

We present new methodology for calculating sampling distributions of single-nucleotide polymorphism (SNP) frequencies in populations with time-varying size. Our approach is based on deriving analytical expressions for frequencies of SNPs. Analytical expressions allow for computations that are faster and more accurate than Monte Carlo simulations. In contrast to other articles showing analytical formulas for frequencies of SNPs, we derive expressions that contain coefficients that do not explode when the genealogy size increases. We also provide analytical formulas to describe the way in which the ascertainment procedure modifies SNP distributions. Using our methods, we study the power to test the hypothesis of exponential population expansion vs. the hypothesis of evolution with constant population size. We also analyze some of the available SNP data and we compare our results of demographic parameters estimation to those obtained in previous studies in population genetics. The analyzed data seem consistent with the hypothesis of past population growth of modern humans. The analysis of the data also shows a very strong sensitivity of estimated demographic parameters to changes of the model of the ascertainment procedure.

Evidence for association of a common variant of the endothelial nitric oxide synthase gene (Glu298-->Asp polymorphism) to the presence, extent, and severity of coronary artery disease

BACKGROUND

Genetic variants of endothelial nitric oxide synthase (eNOS) could influence individual susceptibility to coronary artery disease.

OBJECTIVE

To assess whether Glu298-->Asp polymorphism of the eNOS gene is associated with the occurrence and severity of angiographically defined coronary artery disease in the Italian population.

METHODS

Polymerase chain reaction/restriction fragment length polymorphism analysis was done to detect the Glu298-->Asp variant of the eNOS gene in 201 patients with coronary artery disease and 114 controls. The severity of coronary artery disease was expressed by the number of affected vessels and by the Duke scoring system.

RESULTS

The frequencies of the eNOS Glu/Glu, Glu/Asp, and Asp/Asp genotypes in the coronary artery disease group were significantly different from those of controls (45.3%, 38.8%, and 15.9% v 42.1%, 51.8%, and 6.1%, respectively; chi2 = 8.589, p = 0.0136). In comparison with subjects who had a Glu298 allele in the eNOS gene, the risk of coronary artery disease was increased among Asp/Asp carriers (odds ratio 2.9, 95% confidence interval 1.2 to 6.8, p = 0.01) and was independent of the other common risk factors (p = 0.04). There was a significant association between the eNOS Glu298-->Asp variant and both the number of stenosed vessels (mean (SEM), 2.3 (0.1) for Asp/Asp v 1.9 (0.1) and 1.8 (0.1) for Glu/Glu and Glu/Asp, respectively; p = 0.01) and the Duke score (56.1 (3.1) for Asp/Asp v 46.7 (2.0) and 46.1 (1.9) for Glu/Glu and Glu/Asp, respectively; p = 0.02).

CONCLUSIONS

Glu298-->Asp polymorphism of the eNOS gene appears to be associated with the presence, extent, and severity of angiographically assessed coronary artery disease.

Pathophysiology of plaque instability: insights at the genomic level

Atherosclerosis and plaque rupture represent complex "traits" of unknown cause that involve multiple genes and their variants. Novel genomic technologies provide us with the tools that will allow for the identification of groupings of genes that determine either susceptibility or resistance relative to the development of atherosclerosis and its thromboembolic complications. This information may, in turn, lead to a clearer understanding of the cause and risk for atherosclerosis. Diagnostic tools, as well as preventive and therapeutic strategies, will be derived from such heightened understanding of the disease process. With this chapter, we have presented the current state of knowledge of atherosclerosis genomics.

Haplotypes at ATM identify coding-sequence variation and indicate a region of extensive linkage disequilibrium

Genetic variation in the human population may lead to functional variants of genes that contribute to risk for common chronic diseases such as cancer. In an effort to detect such possible predisposing variants, we constructed haplotypes for a candidate gene and tested their efficacy in association studies. We developed haplotypes consisting of 14 biallelic neutral-sequence variants that span 142 kb of the ATM locus. ATM is the gene responsible for the autosomal recessive disease ataxia-telangiectasia (AT). These ATM noncoding single-nucleotide polymorphisms (SNPs) were genotyped in nine CEPH families (89 individuals) and in 260 DNA samples from four different ethnic origins. Analysis of these data with an expectation-maximization algorithm revealed 22 haplotypes at this locus, with three major haplotypes having frequencies > or = .10. Tests for recombination and linkage disequilibrium (LD) show reduced recombination and extensive LD at the ATM locus, in all four ethnic groups studied. The most striking example was found in the study population of European ancestry, in which no evidence for recombination could be discerned. The potential of ATM haplotypes for detection of genetic variants through association studies was tested by analysis of 84 individuals carrying one of three ATM coding SNPs. Each coding SNP was detected by association with an ATM haplotype. We demonstrate that association studies with haplotypes for candidate genes have significant potential for the detection of genetic backgrounds that contribute to disease.

Cladistic structure within the human Lipoprotein lipase gene and its implications for phenotypic association studies

Haplotype variation in 9.7 kb of genomic DNA sequence from the human lipoprotein lipase (LPL) gene was scored in three populations: African-Americans from Jackson, Mississippi (24 individuals), Finns from North Karelia, Finland (24), and non-Hispanic whites from Rochester, Minnesota (23). Earlier analyses had indicated that recombination was common but concentrated into a hotspot and that recurrent mutations at multiple sites may have occurred. We show that much evolutionary structure exists in the haplotype variation on either side of the recombinational hotspot. By peeling off significant recombination events from a tree estimated under the null hypothesis of no recombination, we also reveal some cladistic structure not disrupted by recombination during the time to coalescence of this variation. Additional cladistic structure is estimated to have emerged after recombination. Many apparent multiple mutational events at sites still remain after removing the effects of the detected recombination/gene conversion events. These apparent multiple events are found primarily at sites identified as highly mutable by previous studies, strengthening the conclusion that they are true multiple events. This analysis portrays the complexity of the interplay among many recombinational and mutational events that would be needed to explain the patterns of haplotype diversity in this gene. The cladistic structure in this region is used to identify four to six single-nucleotide polymorphisms (SNPs) that would provide disequilibrium coverage over much of this region. These sites may be useful in identifying phenotypic associations with variable sites in this gene. Evolutionary considerations also imply that the SNPs in the 3' region should have general utility in most human populations, but the 5' SNPs may be more population specific. Choosing SNPs at random would generally not provide adequate disequilibrium coverage of the sequenced region.

Abnormal blood lipids: is it environment or is it genes?

Genetic and environmental factors contribute to the development and progression of atherosclerotic coronary heart disease processes. Major independent risk factors for coronary heart disease, including adverse levels of plasma lipids and lipoproteins, are also influenced by genetic and potentially modifiable environmental factors. Recent advances in molecular biology have resulted in the identification of specific genes associated with lipid disorders. The complex gene-environment interplay that contributes to interindividual variation in components of the lipid profile is also partially clarified. This article provides an overview of available data on the genetic and environmental influences on lipids and lipoproteins with emphasis on implications for clinical practice and future research.

Uses of Evolutionary Theory in the Human Genome Project

▪ Abstract  The Human Genome Project (HGP) originally sought to sequence the human genome but excluded studies on genetic diversity. Now genetic diversity is a major focus, and evolutionary theory provides needed analytical tools. One type of diversity research focuses on complex traits. This is often done by screening genetic variation at candidate loci functionally related to a trait followed by gene/phenotype association tests. Linkage disequilibrium creates difficulties for association tests, but evolutionary analyses using haplotype trees can circumvent these problems and result in greater statistical power, better disease risk prediction, the elimination of some polymorphisms as causative, and physical localization of causative variation when combined with an analysis of recombination. The HGP also now proposes to map over 100,000 single nucleotide polymorphisms to test for gene/phenotype associations through linkage disequilibrium in isolated human populations affected by past founder or bottleneck events. This strategy requires prior knowledge of recent human evolutionary history and current population structure, but other evolutionary considerations dealing with disequilibrium and nonrandom mutation pose difficulties for this approach. Studies on population structure also focus upon traits of medical relevance, and an understanding of the evolutionary ultimate cause for the predisposition of some populations to certain diseases is a useful predictor for shaping public health policies. Studies on the genetic architecture of common traits reveal much epistasis and variation in norms of reaction, including drug response. Because of these interactions, context dependency and sampling bias exist in disease association studies that require population information for effective use. Overall, the population thinking of evolutionary biology is an important counterweight to naive genetic determinism in applying the results of the HGP to issues of human health and well-being.

A common variant of the endothelial nitric oxide synthase (Glu298-->Asp) is a major risk factor for coronary artery disease in the UK

BACKGROUND

Endothelium-derived nitric oxide (NO) is synthesised from L-arginine by endothelial nitric oxide synthase (eNOS) encoded by the NOS 3 gene on chromosome 7. Because reduced NO synthesis has been implicated in the development of coronary atherosclerosis, which has a heritable component, we hypothesised that polymorphisms of NOS 3 might be associated with increased susceptibility to this disorder.

METHODS AND RESULTS

Single-strand conformation polymorphism analysis of NOS 3 identified a G-->T polymorphism in exon 7 of the gene which encodes a Glu-->Asp amino acid substitution at residue 298 of eNOS. We investigated the relationship between this Glu(298)-->Asp variant and atherosclerotic coronary artery disease (CAD) using 2 independent case-controlled studies. In the first study (CHAOS), cases consisted of 298 unrelated patients with positive coronary angiograms and controls were 138 unrelated healthy individuals ascertained through a population health screen. In the second study (CHAOS II), the cases were 249 patients with recent myocardial infarction (MI), and a further 183 unrelated controls. There was an excess of homozygotes for the Asp298 variant among patients with angiographic CAD, and among patients with recent MI when compared with their respective controls (35.9% versus 10.2%, P<0.0001 in CHAOS, and 18.1% versus 8.7%, P<0.02 in CHAOS II). In comparison to Glu(298) homozygotes, homozygosity for Asp(298) was associated with an odds ratio of 4.2 (95% CI, 2.3 to 7.9) for angiographic CAD and 2.5 (95% CI, 1.3 to 4.2) for MI.

CONCLUSIONS

Homozygosity for a common NOS 3 polymorphism (894 G-->T) which encodes a Glu298-->Asp amino acid substitution in eNOS is a risk factor for angiographic CAD and recent MI in this population.

Ionizing radiation and genetic risks. VI. Chronic multifactorial diseases: a review of epidemiological and genetical aspects of coronary heart disease, essential hypertension and diabetes mellitus

This paper provides a broad overview of the epidemiological and genetical aspects of common multifactorial diseases in man with focus on three well-studied ones, namely, coronary heart disease (CHD), essential hypertension (EHYT) and diabetes mellitus (DM). In contrast to mendelian diseases, for which a mutant gene either in the heterozygous or homozygous condition is generally sufficient to cause disease, for most multifactorial diseases, the concepts of genetic susceptibility' and risk factors' are more appropriate. For these diseases, genetic susceptibility is heterogeneous. The well-studied diseases such as CHD permit one to conceptualize the complex relationships between genotype and phenotype for chronic multifactorial diseases in general, namely that allelic variations in genes, through their products interacting with environmental factors, contribute to the quantitative variability of biological risk factor traits and thus ultimately to disease outcome. Two types of such allelic variations can be distinguished, namely those in genes whose mutant alleles have (i) small to moderate effects on the risk factor trait, are common in the population (polymorphic alleles) and therefore contribute substantially to the variability of biological risk factor traits and (ii) profound effects, are rare in the population and therefore contribute far less to the variability of biological risk factor traits. For all the three diseases considered in this review, a positive family history is a strong risk factor. CHD is one of the major contributors to mortality in most industrialized countries. Evidence from epidemiological studies, clinical correlations, genetic hyperlipidaemias etc., indicate that lipids play a key role in the pathogenesis of CHD. The known lipid-related risk factors include: high levels of low density lipoprotein cholesterol, low levels of high density lipoprotein cholesterol, high apoB levels (the major protein fraction of the low density lipoprotein particles) and elevated levels of Lp(a) lipoprotein. Among the risk factors which are not related to lipids are: high levels of homocysteine, low activity of paraoxonase and possibly also elevated plasma fibrinogen levels. In addition to the above, hypertension, diabetes and obesity (which themselves have genetic determinants) are important risk factors for CHD. Among the environmental risk factors are: high dietary fat intake, smoking, stress, lack of exercise etc. About 60% of the variability of the plasma cholesterol is genetic in origin. While a few genes have been identified whose mutant alleles have large effects on this trait (e.g., LDLR, familial defective apoB-100), variability in cholesterol levels among individuals in most families is influenced by allelic variation in many genes (polymorphisms) as well as environmental exposures. A proportion of this variation can be accounted for by two alleles of the apoE locus that increase (&epsi;4) and decrease (&epsi;2) cholesterol levels, respectively. A polymorphism at the apoB gene (XbaI) also has similar effects, but is probably not mediated through lipids. High density lipoprotein cholesterol levels are genetically influenced and are related to apoA1 and hepatic lipase (LIPC) gene functions. Mutations in the apoA1 gene are rare and there are data which suggest a role of allelic variation at or linked LIPC gene in high density lipoprotein cholesterol levels. Polymorphism at the apoA1--C3 loci is often associated with hypertriglyceridemia. The apo(a) gene which codes for Lp(a) is highly polymorphic, each allele determining a specific number of multiple tandem repeats of a unique coding sequence known as Kringle 4. The size of the gene correlates with the size of the Lp(a) protein. The smaller the size of the Lp(a) protein, the higher are the Lp(a) levels. (ABSTRACT TRUNCATED)

From "magic bullet" to "specially engineered shotgun loads": the new genetics and the need for individualized pharmacotherapy

Steady progress in the identification of human pharmacogenetic variants and new discoveries of disease susceptibility genes makes the old notion of one disease/one drug untenable. Advances in the ability to rapidly identify these variants, when coupled with appropriate drug delivery systems, should revolutionize pharmacotherapy.