Elsevier

Atherosclerosis

Volume 149, Issue 1, March 2000, Pages 91-97
Atherosclerosis

Common paraoxonase gene variants, mortality risk and fatal cardiovascular events in elderly subjects

https://doi.org/10.1016/S0021-9150(99)00311-1Get rights and content

Abstract

Recent studies indicate that the enzyme paraoxonase may be an important modulator of cardiovascular disease risk because of its ability to protect LDL from oxidation. We tested for association between two functional variants of the paraoxonase gene (Met-55/Leu and Gln-192/Arg) and both all-cause mortality and fatal cardiovascular disease. This was done within a population-based study among subjects aged 85 years and over in a cross-sectional and a prospective design. In the cross-sectional analysis, the distribution of both paraoxonase genotypes was found to be similar in the subset of 364 elderly subjects who were born in Leiden, The Netherlands, as compared with 250 young subjects whose families originated from the same geographical region. The polymorphisms were in strong linkage disequilibrium (P<0.00001) and the frequency of the haplotype carrying both risk alleles was not lower in the elderly than in the young (0.313 vs. 0.284). The complete cohort of 666 elderly subjects was followed over 10 years. The risk of all-cause and cardiovascular mortality was not increased in elderly subjects with the paraoxonase Leu/Leu (RR, 1.1 [95% CI, 0.9–1.5] and 1.3 [95% CI, 0.8–2.0], respectively) or the Arg/Arg genotype (RR, 0.9 [95% CI, 0.7–1.2] and 0.7 [95% CI, 0.4–1.3], respectively). In a subset of patients with diabetes, the all-cause mortality risk was elevated in Arg/Arg carriers (RR, 2.1 [95% CI, 0.8–5.8]) but this did not reach statistical significance. Analysis of genotype combinations did not yield significant associations with mortality. The paraoxonase gene variants, previously associated with coronary artery disease, are thus not likely to have a major effect on the risk of fatal cardiovascular disease in the population at large. Adverse effects of the gene variants might be observed in subjects exposed to factors that enhance oxidative stress such as diabetes.

Introduction

Oxidised low-density lipoprotein (oxLDL) is thought to play a central role in atherogenesis [1], [2]. Evidence is accumulating that the enzyme paraoxonase protects LDL from oxidation. Paraoxonase, which is physically associated with apolipoprotein A-I in HDL, inhibits Cu2+-induced oxidation of LDL in vitro [3] by destroying proinflammatory lipid peroxides [4]. Subsequent studies using a cell co-culture model showed that HDL from wild-type mice but not from paraoxonase deficient mice inhibits the monocyte chemotactic activity of LDL, which becomes oxidised in the subendothelial matrix if HDL is absent [5]. Moreover, paraoxonase deficient mice are more susceptible to atherosclerosis than wild-type mice when fed a high-fat/high-cholesterol diet [5].

The contribution of oxLDL to cardiovascular disease in humans may thus be evaluated by studying functional variants of the paraoxonase gene (PON1). Two variants in the coding region have been identified leading to a methionine-55 to leucine and a glutamine-192 to arginine amino acid substitution [6]. The inter-individual variation in the ability of paraoxonase to hydrolyse organophosphorous compounds is determined by the Gln-192/Arg polymorphism. The effect, however, is substrate-dependent [7], [8]. In vitro, the Arg-isoform was found to be less effective in preventing LDL from oxidation by Cu2+ than the Gln-isoform [9]. Hence, the Arg-allele may be a risk factor for cardiovascular disease. Evidence for this hypothesis was obtained in three studies in Caucasian subjects, which found the Arg-allele to be more common in type 2 diabetic patients with coronary heart disease [10], [11] and in patients with more than 75% stenosis in a coronary artery [12] than in controls. Two small studies suggested that the Arg-allele was associated with coronary heart disease in the Japanese [13], [14]. In contrast, no increased frequency of the Arg-allele was observed in myocardial infarct patients in two studies [11], [15], in Finnish patients who underwent coronary bypass surgery [16] and in Italian patients with more than 50% stenosis [17].

The Met-55/Leu polymorphism has been associated with the level of paraoxonase in serum [8] and mRNA in liver biopsies [18]. Surprisingly, it was the high-level associated Leu-allele that was found to represent an increased risk of coronary heart disease in Caucasian patients with type 2 diabetes [8]. This might suggest that the Leu-allele is not only associated with paraoxonase serum level, but also has a detrimental effect on enzyme function. In a study among Asian Indians and Chinese, the Met-55/Leu polymorphism was not associated with coronary heart disease [19].

Until now, none of the studies that assessed the possible disease association of the paraoxonase polymorphisms were prospective, nor did they include fatal cases. Therefore, we explored whether both polymorphisms, either separately or in combination, are associated with all-cause and cardiovascular mortality in the general population. This was done within a population-based study among subjects aged 85 years and over (Leiden 85-plus Study [20]) using two designs [21]. The impact of the gene variants on mortality before the age of 85 years was studied cross-sectionally, by comparing the paraoxonase genotype distribution between subjects aged 85 years and over and young subjects with families from the same geographical region. In a prospective study with a 10-year follow-up period, the relation of the gene variants to all-cause and cardiovascular mortality above the age of 85 years was investigated. During follow-up, the all-cause mortality was 89% and the cardiovascular mortality 38%.

Section snippets

Methods

The Leiden 85-plus Study is a population-based study in which all inhabitants of Leiden, The Netherlands, aged 85 years and over were invited to take part [20]. Out of a total of 1258 eligible subjects, 221 died before enrolment. Of the 1037 remaining subjects, 977 (94%) participated and were medically interviewed at home. Diabetes was diagnosed on the basis of the medical interview, use of medication for the treatment of diabetes and/or a serum glucose level over 11.0 mmol/l in a non-fasting

Cross-sectional analysis

The genotype distributions of the paraoxonase polymorphisms were determined in a cohort of 666 subjects aged 85 years and over. The frequencies for the Met-55/Leu polymorphism were 10.8% (Met/Met), 48.3% (Met/Leu) and 40.8% (Leu/Leu); for the Gln-192/Arg polymorphism the frequencies were 47.0% (Gln/Gln), 44.9% (Gln/Arg) and 8.1% (Arg/Arg). Excess mortality before the age of 85 years among carriers of the putative risk-genotypes would be reflected in a reduced frequency of these genotypes in the

Discussion

Polymorphisms in the paraoxonase gene are associated with paraoxonase levels in serum (Met-55/Leu) [8] and differential susceptibility of LDL to oxidation in vitro (Gln-192/Arg) [9]. In this study we investigated the contribution of these paraoxonase polymorphisms to mortality in a cohort born between 1887 and 1901. We found that paraoxonase genotypes previously associated with an increased risk of cardiovascular disease (i.e. containing a Leu [8] or an Arg-allele [10], [12]) were not

Acknowledgements

We wish to thank Dr Hans Princen for critical review of the manuscript, the Red Cross Blood Bank Leidsenhage, and in particular Marjo Dirven, for collecting the control population, and the Central Bureau of Statistics for generously making available the mortality statistics and database linking. This study was supported by grant 94.047 from the Netherlands Heart Foundation.

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