Atherosclerosis.

Atherosclerosis, disease of large and medium-size arteries, common in socially advanced countries. Atherosclerosis is the most common underlying cause of heart attacks and strokes.

The disease probably starts in childhood and develops slowly over main cause of heart attacks and strokes. Many years, giving no indication of its presence. It is rare for its effects to become apparent before the age of 40 or 50; thereafter, it becomes progressively more common with advancing age. When it does show itself, it usually does so as a heart attack, most commonly in middle-aged men. Strokes—brain damage from loss of an adequate blood supply—occur later and affect the sexes about equally, mainly because women are believed to have a remarkable degree of protection from the disease so long as they are secreting the female hormone oestrogen. After the menopause, this protection is lost and the condition tends to progress rapidly.

Nature of Atherosclerosis.

The disease affects the innermost of the three layers of the artery. This layer is lined with a dense, single-celled surface that in a healthy person prevents any abnormal substances in the blood from entering the thickness of the wall. It also prevents clotting of the blood. It has repeatedly been shown experimentally that diets high in saturated fats lead to the development of fatty streaks in this inner layer. If such diets are continued, the streaks thicken and become invaded by new muscle cells and cells from the immune system. Eventually new blood vessels invade these areas, which develop a core of cholesterol, other fats, and degenerate muscle cells and other debris, and are known as atheromatous plaques.

Effects of atherosclerosis.

The main effect of arterial plaques is that by growing out towards the centre of the vessel, they impede the flow of blood to an organ. Also, because plaques no longer have the properties of healthy vessel lining, blood will readily clot on their surface. Finally these conditions will lead to heart attacks, strokes etc.

Risk Factors.

There are two outstanding and avoidable risk factors. They are the unsuitable diet and cigarette smoking. The evidence is not purely epidemiological. Much scientific research has now elucidated the various pathological stages by which a diet high in saturated fats or the toxic substances in tobacco smoke can lead to damage to the artery lining and the progression of atherosclerosis. Regular exercise is valuable largely because it helps to improve circulation, control appetite, and control weight.
Blood cholesterol tests are of little value in the diagnosis of atherosclerosis (because such tests do not reveal the state of the arteries) except in cases in which there is a family history of early atherosclerosis-induced disease.
Recent studies shows that Atherosclerosis is most often associated with hypercholesterolemia & hyperlipidaemia.

Recent findings Show that atherosclerosis has a distinct relationship with hereditary.

Genes contribute to both the cause and the pathogenesis of virtually all abnormalities of human physiology and behaviour, including, of course, atherosclerosis. A variety of genetic factors, in addition to well studied errors of lipid metabolism, clearly predispose to atherosclerosis. Few genes apart other than those involved in lipid metabolism have such an overwhelming impact that they may be identified on the basis of family history. However, genes that predispose to hypertension and diabetes mellitus; control arterial diameter, reactivity and branching angles; affect platelet adhesiveness, thrombosis and fibrinolysis; and regulate endothelial and smooth muscle function can all be considered candidate genes for study in families predisposed to atherosclerosis.

Genetics of non-Mendelian forms of atherosclerosis

The Human Genome Project has ushered in new opportunities for studying the genetic non-Mendelian disorders through construction of SNP (Single nucleotide polymorphisms) maps and genome-wide SNPs association studies While there is a considerable debate regarding the best approach for genome -wide SNP association studies, candidate gene approach has emerged as the practical approach that could be pursued at multiple levels. Because SNPs do not exist in isolation, comprehensive analysis of the selected candidate genes is necessary. Moreover, because a positive association does not establish causality and often indicates linkage disequilibrium with the actual mutation, the results are considered provisional pending confirmation through in vitro and in vivo experimentations .

Rare and Common Genetic Differences Contributing to Susceptibility to Atherosclerosis.

Single-gene (mendelian) disorders with large effects are the most dramatic examples of the genetic contributions to atherosclerosis.1,2 However, most forms of the disease are the product of many genes with small effects that are modified by the environment and the effects of other genes, rather than of a single highly penetrant gene. Studies of identical twins are particularly informative because these twins share all of their genes, and such studies consistently indicate that genetic effects powerfully influence athrosclerosis as well as most of its risk factors.2,3,4

Familial Combined hyperlipidemia (FCH):

FCH is a relatively common disease that affects approximately 1 – 2:100 in the population. It is not yet clear whether FCH is an autosomal recessive or a polygenic disease, and the causal genes and mutations are still unknown. The primary defect is impaired transfer of cholesteryl ester from HDL to LDL and VLDL cholesterol, which results in elevation of LDL and VLDL cholesterol and premature atherosclerosis.

The classic genetic trait in heart disease & atherosclerosis are familial hypercholesterolemia (FH). Carl Miller, a physician at Oslo County Hospital, Norway, first described the disorder _70 years ago.1,2 He noted that the triad of elevated cholesterol, tendon xanthomas, and early heart disease segregated together in families, providing strong evidence for an association between blood lipids and atherosclerosis.

Joseph Goldstein and Michael Brown further examined the disease in the early 1970s, and over the last 3 decades, their landmark studies have fostered many insights in the control of cholesterol levels and the aetiology and treatment of atherosclerosis.2,3
Along the way, they discovered receptor-mediated endocytosis and mechanisms for transcriptional regulation by lipids. Their studies revealed that FH is the result of mutations that destroy the ability of the LDL receptor to mediate the binding, internalization, or degradation of LDL.
Individuals carrying 2 mutant copies have extremely high levels of cholesterol (600 mg/dL), whereas heterozygotes with 1 mutant copy have levels of 400 mg/dL. The penetrance of atherosclerosis in FH varies widely, dependent on modifier genes and the same lifestyle environmental risk factors that determine risk in noncarriers, including diet, smoking, and activity level.4

The frequency of heterozygotes in most populations is surprisingly high for a lethal disease, 1 in 500.5 Because a significant fraction of heterozygous individuals are unaware that they carry a lethal dominant gene that is shared by half of their first-degree relatives, it would seem that DNA screening of the disease would be worthwhile, particularly because effective treatments for the heterozygous disease are available.6 Unfortunately, FH is heterogeneous in most populations, the exception being founder populations such as French Canadians and Afrikaners, involving hundreds of different mutations of the LDL receptor gene. Nevertheless, rapid mutation screening methods for FH have been developed.7

Single nucleotide polymorphisms(SNP) in atherosclerosis.

It has become clear that the genome, carried by each of us and inherited from our parents, most often differs between individuals in terms of single base changes. In the 20th century only a few thousand of these so called single nucleotide polymorphisms (SNPs; Fig. 1) identified. But after a short period, In the 21st century number increased 1000- fold. The main use of the human SNP map will be to identify the contributions of individual genes to diseases that have a complex, multi gene basis, such as atherosclerosis. Knowledge of genetic variation already affects patient care to some degree. For example, gene variants lead to tissue and organ incompatibility, affecting the success of transplants. Studies on SNPs and atherosclerosis will become more efficient when a few more problems are solved. First, although 82% of SNP variants are found at a frequency above 10% in the global human population, the ‘micro distribution’ of SNPs in individual populations is not known.11

Second, not all SNPs are equal, and it will be essential to know as much as possible about their effects from computational analysis before studying their involvement in disease. For example, each SNP can be classified by whether it codes or not. Coding SNPs can be classified by whether they alter the sequence of protein encoded by the altered gene; changes that alter protein sequences can be classified by their effects on protein sequences. Non-coding SNPs can be classified according to whether they are found in gene-regulating segments of the genome; atherosclerosis may arise from quantitative rather than qualitative differences in gene products.11 Third, the technology for the assay of thousands of SNPs, in thousands of patients and control individuals, is not yet fully developed.11

Figure 1 The most common sources of variation
between humans are single nucleotide polymorphisms
(SNPs) – single base differences between genome
sequences. Fragments of two sequences with eight SNPs
are shown (indicated by lighter text).

Relationship with apolipoprotein B deficiency.

Familial defective apolipoprotein B (apoB), another relatively common hypercholesterolemia (1 in 800), is the result of the mutations of apoB, the major protein of LDL, which prevent its binding to the LDL receptor. In contrast to FH, this disorder is homogeneous, with most cases resulting from a single nucleotide substitution at codon 3500.

Although the cholesterol levels of these patients are somewhat lower than are those with FH, they are still highly elevated. Like FH, the disorder exhibits dominant inheritance; therefore, half of the first-degree relatives of an individual with familial defective apoB will be affected. Most other single-gene traits associated with CHD are rare and of lesser clinical significance.2As was the case with FH, studies of rare disorders have led to many novel insights into the pathways involved in atherogenesis..

Similarly, Lifton’s studies7of rare mendelian forms of hypertension have identified key processes in the control of blood pressure.During the past 20 years, the study of human genetics has been revolutionized by positional cloning, a combination of genetic and physical mapping that allows genetic differences to be identified solely on the basis of their locations in the genome.8,9To day different mendelian disease genes have been identified by positional cloning. Unfortunately, the complex etiology of common diseases, involving many different genetic factors as well as important environmental influences, has made it difficult to apply positional cloning. So there is a great success achieved in identifying single-gene disorders for atherosclerosis, but understanding of the common, complex forms remains limited.

ApoE plays a major role in lipid metabolism and has a close link with atherosclerosis.
Most of success in understanding the genetic basis of common forms of atherosclerosis has come from studies of “candidate genes,” genes identified by biochemists and subsequently examined for genetic differences and associations in populations. A good example of a candidate gene is apolipoprotein E (apoE). ApoE has been studied intensively as a central player in plasma lipid metabolism, in which it mediates the uptake of chylomicron and VLDL remnants by the LDL receptor and its cousin, LDL receptor-related protein (LRP).

Common genetic differences (polymor- phisms) of apoE were identified by Utermann and colleagues10 in the 1970s, and subsequent studies by many laboratories revealed clear associations with plasma cholesterol levels and type III hyperlipidemia (and later, Alzheimer disease).Hyperlipidemia is associated with several alleles,namely E1,E2,E3..etc.Almost all individuals with this uncommon (1 in several thousand) hyperlipidemia are homozygous for the E2 allele, but most individuals who are homozygous for the E2 allele do not have the disorder.10 Thus, other genetic or environmental interactions are required to produce hyperlipidemia in addition to homozygosity for the E2 allele.

Our knowledge of genetic differences contributing to CHD & atherosclerosis risk is greatest in lipid metabolism. In addition toapoE, convincing evidence suggests that common variations of hepatic lipase influence levels of HDL,11 that variations of the apoAI-CIII-AIV-AV locus contribute to plasma triglyceride levels,12 and that variations of the apo(a) gene determine _90% of the population variance of Lp(a) levels.13 Both hepatic lipase and the apoAI-CIII-AIV-AV cluster appear to influence LDL particle size, a predictor of CHD & atherosclerosis risk.14 However, these genetic differences together explain only a small fraction of the total variance of plasma lipid phenotypes

More than 2 dozen studies have revealed significant associations between polymorphisms of paraoxonase-1 (PON1) and measures of atherosclerosis.15,16 PON1 is a serum esterase bound to HDL, first recognized for its ability to hydrolyze a metabolite of the insecticide parathion. PON1 was identified as a candidate gene for atherosclerosis because of its ability to inhibit the oxidation of LDL and transgenic studies in mice have confirmed that PON1 is antiatherogenic in vivo.17 The studies with PON1 are especially significant in providing support for the oxidation hypothesis of atherosclerosis18 Many other candidate genes have been examined in population-association studies. Common variations in some of these genes have been convincingly associated with CHD or atherosclerosis and its risk factors.

Most associations, however, require confirmation, and many are likely to prove to be false positives.19 A clustering of atherosclerotic risk factors termed the “metabolic syndrome”. MetSyn is characterized by visceral adiposity, insulin resistance, low HDL cholesterol, hypertriglyceridemia, a systemic proinflammatory state, and small dense LDL, and it is a strong predictor of atherosclerosis,CHD and type 2 diabetes mellitus.20

The assessment and treatment of MetSyn have yet to be effectively integrated into clinical practice.20 MetSyn, like atherosclerosis, clearly is complex, with a host of genetic and environmental contributions. Genetic differences in the nuclear receptor peroxisome proliferator-activatorreceptor-(PPAR_) appear to play a key role in MetSyn and type 2 diabetes mellitus.21 Interestingly, PPAR agonists are effective in correcting a number of features of MetSyn. Recent studies also have implicated common variations of the lipoprotein lipase (LPL) gene in MetSyn.22LPL is an enzyme in capillary surfaces that hydrolyzes the triglyceride in plasma chylomicrons and VLDL, releasing free fatty acids for uptake by peripheral tissues. Human studies that suggest a role for PPAR and LPL in MetSyn are strongly supported by studies with transgenic mice.22

Therapies

Probably the most significant impact of genetic studies on medicine pertains to the development of therapies. The outstanding example is the impetus that genetic studies of familial hypercholesterolemia (FH) provided in the development of hepatic hydroxymethyl glutaryl coenzyme A reductase inhibitors (statins)23.Several therapeutic approaches based on genetic studies are under development.

The identification of a naturally occurring variant of apolipoprotein AI, known as apoAI Milano, in a village in northern Italy in 198024 led to investigations of the therapeutic potential of HDL and this protein variant as antiatherogenic therapy.

Carriers of the apoAI Milano gene are characterized by extremely low levels of HDL cholesterol (10 to 30 mg/dL) and a decreased risk of atherosclerosis relative to the low level of HDL. The apoAI Milano protein differs from native apoAI in that a cysteine is substituted for arginine at position 173. This cysteine confers different properties to this protein as compared with normal apoAI, including the ability to form disulfide-bonded dimers with other apoAI Milano molecules and other HDL proteins such as apoAII.

Recombinant apoAI Milano has been formulated in a complex with phospholipids to mimic the properties of nascent HDL (ETC-216, Pfizer)25. Studies in mice and rabbits with experimental atherosclerosis have demonstrated that recombinant apoAI Milano/phospholipids complexes rapidly reduce the lipid and macrophage content of atherosclerotic plaques after a single infusion.26

The effect of short-term intravenous recombinant apoAI Milano/ phospholipid complexes on atheroma burden in patients with acute coronary syndromes was studied in a recent clinical trial27.

The intravenous administration of ApoAI Milano for doses at weekly intervals produced statistically significant regression in coronary atheroma volume in the target segment as compared with baseline measurements by intravascular ultrasound. No change was seen with saline placebo control.26

It remains to be determined whether apoAI Milano has unique properties that result in greater antiatherogenic potential than normal apoAI and whether the exciting results of this first study in humans can be confirmed in large-scale randomized clinical outcome trials.

HDL is involved in reverse cholesterol transport, the process by which cholesterol is transported from peripheral tissues to the liver, where it can be excreted in bile. Studies since the mid-1990s have provided strong evidence that HDL also may protect against atherosclerosis by virtue of its antiinflammatory properties. These studies, based in part on genetic variations that influence the antioxidant properties of HDL, indicate that HDL can selectively remove and destroy proinflammatory oxidized lipids from the vessel wall, thereby inhibiting the vicious cycle of LDL trapping, LDL oxidation, and inflammation.

Navab et al28 have demonstrated that short, synthetic amphipathic helices, similar to those in apoAI, exert powerful protective effects against atherosclerosis when administered orally to mice or monkeys..

Genetic studies implicated 5-lipoxygenase (5-LO) in atherosclerosis susceptibility in mice and then humans,29 and recent studies suggest that polymorphisms of other enzymes in leukotriene metabolism also are associated with atherosclerosis 30.5-LO polymorphisms were implicated originally in asthma, and a variety of leukotriene synthesis inhibitors are widely used to treat asthma.31

Risk Stratification, Prevention, and Treatment

National guidelines from the American Heart Association and the National Heart, Lung, and Blood Institute’s National Cholesterol Education Program recommend an approach to initial global risk assessment of the asymptomatic patient to obtain an estimate of absolute cardiovascular risk.32,33 On the basis of standard risk factors and related risk correlates, the concept was set forth that asymptomatic patients can be placed into 1 of 3 risk categories: low, intermediate, and high. The techniques used in office assessments include history, physical examination, laboratory testing, and ECG.The focus of the examination is on the detection of risk factors that either can be directly modified or will modify the overall intensity of risk-reduction therapies.

The major causal risk factors identified for routine assessment include age, smoking, elevated blood pressure, elevated serum LDL cholesterol, low HDL cholesterol, and diabetes. The approach to therapy is guided by the principle that the intensity of risk-factor management should be adjusted by the severity of risk.

Low-risk patients should be encouraged to adhere to healthy lifestyle habits. High-risk patients are advised to directly begin a regimen of aggressive risk reduction through a combination of nondrug and drug regimens. Patients at intermediate risk become candidates for further risk stratification through the measurement of the inflammatory marker high-sensitivity C-reactive protein or non-invasive procedures that test for the presence of myocardial ischemia or coronary atherosclerotic burden, or both techniques.

Although several of the conventional causal risk factors used in global risk assessment clearly have a genetic basis, specific genetic testing has not been recommended for routine clinical practice. Because conventional risk factors account for only 50% of the variability in risk, the identification of genetic differences influencing the pathways of measurable atherosclerotic risk factors or novel pathways may allow for the determination of risk that is additive to the measurement of conventional risk factors. Moreover, some genetic differences, such as those in the genes for apoE, lipoprotein lipase (LPL), and interleukin-6, show evidence of specific environmental interactions (e.g., with smoking), in which the overall risk is more than additive.34,35

Thus, the predictive models used in the Framingham risk score may be greatly enhanced by theaddition of pivotal single-nucleotide polymorphisms and haplotypes that have been shown to affect CHD. It can be envisioned that genotypic information could transform the value of the risk score and be integrated in routine clinical practice to guide preventive therapy. It also seems likely that genetic differences would be useful in predicting specific complications of atherosclerosis.

Figure 2
Expressiveness of the genes that related to atherosclerosis is influenced by the environmental factors and other modifier genes. By all these means one can clearly say that atherosclerosis is a polygenic disease which has a strong genetic relationship.

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