Research Express@NCKU - Articles Digest (Volume 3 Issue 8)

Coronary artery disease (CAD) is an important and rapid growing disease among developed countries.

This is a warming sign supported by our rapidly developing westernized-food style on diet intake in Taiwan. Several predictors and disorders have been demonstrated as independent risk factor for CAD analyzed from several important and long-term prospective cohort data in human. They are: hypertension, diabetes mellitus, smoking, hyperlipidemia, male in sex and positive family history for premature CAD.

All the risk factors contribute to the progression of lumen narrowing in coronary arteries. As the lumen becomes narrow or stenotic, the blood flow and oxygen supply for the cardiac muscle region downstream the lesion site will become insufficient and hypoxia happens subsequently. With increasing exposure of these risk factors, some “plaques” which are composed of lipid accumulation beneath the unhealthy endothelial layer within the coronary arteries will ultimately become either very narrow or unstable. Several mechanisms can cause plaque instability or even ruptures. They include platelet aggregation, thrombus formation and coronary vasospasm or atherosclerosis.

The final and most serious event will be completely obstruction of the coronary flow and result in myocardial infarction (MI). Patients suffering from MI will be prone to have higher sudden death rate because of sudden and fatal irregular heart rate. They also tend to have higher possibility of poor heart function subsequently because most of the myocardium has been damaged. For cardiologist doctors, it is important not only to treat patients suffering from active cardiac attack, and also to understand which patients are at high-risk.

From the point view of pathophysiology in coronary artery plaque formation, the process of lipid oxidation plays as a common and important role in the progression of CAD. Recently, some scientists hypothesize that these unstable plaques are “over-flamed”, and inflammatory process plays some role in the triggering of instability. We are interested in what kind of inflammatory factors are involved when the coronary plaques become flamed and finally result in plaque rupture and causing MI in these patients.

During the process of plaque formation, oxidation of low density lipoproteins is an initial step of atherogenesis that generates pro-inflammatory phospholipids, including platelet-activating factor (PAF). PAF is degraded by PAF-acetylhydrolase (PAF-AH), which has been postulated to be a risk factor for MI. Platelet-activating factor acetylhydrolase (PAF-AH), also called PLA2G7, may play important roles in the pathophysiology of thrombosis and atherosclerosis related to its catalytic action in the degradation of PAF and oxidized phospholipids. The role of PAF-AH in atherosclerosis remains controversial. Experimental data support both the pro- and the anti-atherogenic roles of this enzyme. PAF-AH has been detected in human and rabbit atherosclerotic lesions, hence potentially contributing to the release of lyso-PC and free oxidized fatty acids, although it might also prevent biological activities of PAF-like substances in situ. Supporting the pro-atherogenic role of PAF-AH is the fact that inhibiting PAF-AH reduces atherosclerotic lesion formation in hypercholesterolemic rabbits. In favor of the anti-atherogenic role is the fact that recombinant PAF-AH shows anti-inflammatory properties in some animal models. In addition, the adenoviral overexpression of PAF-AH in atherosclerosis prone apoE deficiency mice has been shown to diminish substantially the macrophage homing to aortic roots and decrease the neointima formation, thus to inhibit foam cell formation.

PAF has been considered as a product from the signal transduction pathway via the activation of phospholipase A2 (PLA2). Secretory type II PLA2 (sPLA2) significantly contributes to the pathogenesis of various inflammatory diseases. The plasma levels of sPLA2 were also higher in patients with coronary artery disease (CAD), and play as a prognostic indicator in these patients. In our previous study, we had already demonstrated that CAD patients having higher levels of sPLA2might have poor outcome after coronary intervention.

Genetic approach is one of the best ways to elucidate the association of proteins or biomarkers with some important diseases. Usually, genetic background heritages since newborn from human chromosomes originated from parents. Usually, some genetic variations exist among human individuals. Some of them are functional mutations and but usually are non-functional. If the genetic variation rate is greater than 1% within a larger population sample, we called it as polymorphism academically. A gene polymorphism can exist at the promoter region, exon regions but sometimes in the intron regions. If the polymorphism is functional both in vivo and in vitro, we can clearly elucidate the relationship between the disease and this protein or gene.

For example, we had already demonstrated a very strong association between young premature MI in Taiwanese and a gene polymorphism at the promoter region of an enzyme, matrix metalloproteinase-3, that is associated with collagen digestion and thus influence the plaque stability. The polymorphism itself can affect the expression of putative enzyme in some cells carrying this gene mutation. Once patients carrying this polymorphism, they will have different tissue protein levels compared with those without this genetic change. Indeed, the association between cause and result will be much understandable.

Several PAF-AH gene polymorphisms, including mutation of V279F and A379V polymorphism, were reported to cause a loss of catalytic activity and thus increase the levels of PAF or sPLA2. Their associations with CAD had been reported in either Western or Japanese ethnic groups. However, the roles of these polymorphisms for the onset of premature MI (onset age <45 years of age) in Taiwanese population, their functional implications in PAF-AH activity or the severity of atherosclerosis all have not yet been well documented. We thus hypothesized that those patients who carried this genetic defect would have decreased activity of PAF-AH, which might result in an increased sPLA2activity, and thus be more prone to having advanced atherosclerosis or premature MI.

Figure 2. Factors associated with the severity or complex of coronary atherosclerosis. (A): Correlates with the V allele mutation numbers of the platelet-activating factor acetylhydrolase A379V gene. (B): Higher severity score (>5) indicates lower activity of platelet-activating factor acetylhydrolase.

Figure 1. Gene-phenotype associations between the platelet-activating factor acetylhydrolase polymorphism and plasma activities of (A): platelet-activating factor acetylhydrolase activity and (B): secretory phospholipase A2.

Polymorphisms located in putatively functional regions were investigated in a cohort of patients having premature MI onset prior to 46 years of age (n = 200) and a sex-age-matched control group (n = 200). The activity of PAFAH and coronary angiograms were evaluated for the severity of coronary atherosclerosis. The V allele of A379V (exon 11) polymorphism on PAF-AH gene was more frequent in patients with premature MI (P< 0.001). This V allele polymorphism was also associated with a lower activity of plasma PAF-AH and a more complex coronary atherosclerosis (p Trends <0.05). Multiple logistic regression analysis showed that this polymorphism was an independent risk factor (Odds Ratio [OR] 1.66, 95% CI 1.14.1 to 5.80, P = 0.008) as well as smoking (OR 3.72, 95% CI 1.77 to 9.28, P 0.001), diabetes mellitus (OR 2.25, 95% CI 1.40 to 5.32, P = 0.007) and hypertension (OR1.88, 95% CI 1.25 to 5.36, P = 0.003) for the onset of premature MI.

In this gene-disease association study, we conclude that a functional and significant association between the A379V polymorphism on exon 11 of PAF-AH gene and premature MI exists in Taiwanese population. This polymorphism is significantly associated with the PAF-AH activity and the severity of coronary atherosclerosis.