Common polymorphisms of ATP binding cassette transporter A1, including a functional promoter polymorphism, associated with plasma high density lipoprotein cholesterol levels in Turks
Introduction
Atherosclerotic cardiovascular disease is a leading cause of death worldwide [1], and a low level of high density lipoprotein cholesterol (HDL-C) is a major independent risk factor for atherosclerosis [2], [3]. The protective role of HDL-C is generally attributed to its participation in reverse cholesterol transport, a process in which excess cholesterol is transported from peripheral cells to HDL particles for delivery to the liver and excretion. The ATP binding cassette transporter A1 (ABCA1) participates in apolipoprotein-mediated efflux of cholesterol and phospholipid from peripheral cells, especially macrophages, that is crucial for the initial step of reverse cholesterol transport. The identification of mutations in the ABCA1 gene in patients with Tangier disease, who have very low HDL-C, elevated triglyceride levels, and increased risk of premature coronary atherosclerosis, suggested a major role for ABCA1 in regulating plasma HDL-C levels [4], [5], [6], [7], [8]. Some common polymorphisms of ABCA1, including R219K [9] and I883M [9], [10], [11], are associated with elevated HDL-C levels, although not in all studies [10], [12], [13], [14], [15], [16], [17]. Interestingly, common polymorphisms of ABCA1 may significantly alter the severity of atherosclerosis, apparently without influencing plasma lipid levels [12], [13], [15], [18], [19]. Recently, it was shown that some polymorphisms of ABCA1 were associated with increases (V771M and V825I) or decreases (R1587K) in HDL-C in women and with some consistent trends in men in a large random Danish population [17].
Haplotype analysis of the ABCA1 gene with respect to plasma HDL-C levels [20], [21] and plasma apolipoprotein (apo) AI levels and myocardial infarction [19] have been reported. Haplotype analysis revealed that ABCA1 accounted for about 10% of HDL-C variation [21], but no haplotype effect on apoAI variability or on the risk of myocardial infarction was detected [21]. Certain haplotypes were more frequent among coronary artery disease (CAD) patients than controls in the Malay population, but not in the Chinese and Indian populations [20].
Although cardiovascular risk factor profiles and the frequency of coronary events differ by gender, the mechanisms for the differences remain to be resolved. Gender differences were observed in other lipid-related association studies [22], [23], [24], [25], [26], [27], suggesting that gender-related mechanisms or factors might interact differently with the variants of a particular gene. This phenomenon was also observed with ABCA1. In one study, where males and females were analyzed separately, R219K and I883M were associated with elevated HDL-C levels in females only [9].
We hypothesized that polymorphisms in ABCA1 are important for determining plasma lipid levels and that gender may modulate the role of these polymorphisms. To test this hypothesis, we studied male and female subjects from the Turkish Heart Study [28], a large, random epidemiological survey of the Turkish population. The main characteristic of this population is a very low level of plasma HDL-C, making this an ideal population in which to study genes that influence HDL-C levels [28], [29], [30], [31]. We screened the promoter region and the exons and exon/intron splice junctions of ABCA1 with denaturing high-performance liquid chromatography (dHPLC) to detect polymorphisms, and subsequently analyzed their associations with plasma lipid levels.
Section snippets
Study population
The study population (n = 2700) was randomly selected from the Turkish Heart Study database of more than 9000 volunteers from six regions of Turkey [28]. Detailed biodata were obtained from each participant. The protocol was approved by the Committee on Human Research of the University of California, San Francisco, and was in accordance with the Helsinki Declaration. Subjects who were taking any lipid-lowering medication or had a history of diabetes mellitus were excluded.
Biochemical analyses
Blood samples were
Population characteristics
The demographic and biochemical characteristics and the apoE genotypes of 2700 randomly selected Turkish Heart Study participants are presented in Table 1. Both men and women had very low plasma HDL-C levels and high total cholesterol/HDL-C ratios, as reported [28]. Even though plasma total cholesterol and LDL-C levels were not excessively elevated, low HDL-C with or without mildly elevated triglyceride levels could represent a significant risk factor for heart disease in the Turkish population
Discussion
Turks represent an ideal population for studying genes that influence HDL-C levels. Turks have extremely low levels of plasma HDL-C that appear to be, in part, of genetic origin [28], [31], [33]. More than 70% of Turkish men and 50% of Turkish women have HDL-C <40 mg/dl.
The cell-surface protein ABCA1, which controls the delivery of cholesterol and phospholipids from cells to form plasma HDL, has attracted significant attention because mutations in the ABCA1 gene have been linked to low or absent
Acknowledgments
We are indebted to our associates at the American Hospital, Istanbul, especially Guy Pépin, Sibel Tanir, Judy Dawson-Pépin, and Linda L. Mahley in the Gladstone Institute (Istanbul), and Dr. K. Erhan Palaoğlu at the American Hospital (Istanbul). The authors gratefully acknowledge the technical assistance of and helpful discussions with Dr. Erwin Ludwig. We thank Sylvia Richmond and Jennifer Polizzotto for manuscript preparation and Stephen Ordway and Gary Howard for editorial assistance. We
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