Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 56, Num. 3, 2010, pp. 173-175

Journal of Postgraduate Medicine, Vol. 56, No. 3, July-September, 2010, pp. 173-175

Original Article

Apolipoprotein E gene polymorphism and total serum cholesterol level in Iranian population

Bazzaz JT^1,2 , Nazari M³ , Nazem H³ , Amiri P² , Fakhrzadeh H² , Heshmat R² , Abbaszadeh S² , Amoli MM²

¹ Department of Medical Genetics, School of Medicine,
² Endocrinology and Metabolism Research Centre, Tehran University of Medical Sciences,
³ Department of Science, Payamnoor University, Tehran, Iran

Correspondence Address: Dr. Mahsa M Amoli, Endocrinology and Metabolism Research Centre, Tehran University of Medical Sciences, Tehran, Iran, mahsa_amoli@hotmail.com

Date of Submission: 04-Oct-2009
Date of Decision: 01-Mar-2010
Date of Acceptance: 28-Mar-2010

Code Number: jp10048

PMID: 20739760

DOI: 10.4103/0022-3859.68629

Abstract

Keywords: Apolipoprotein, cholesterol, diabetes

Introduction

Apolipoprotein E (APOE) is known as one of the elements of major lipoproteins including chylomicron (CM), very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) and also a part of high density lipoprotein (HDL). ^[1] The uptake of these particles by their receptors in the liver and their removal is mediated by APOE. ^[2] This explains the role of APOE in human cholesterol and triglyceride (TG) homeostasis. APOE has been suggested as a major regulator of blood lipid levels in humans. ^[3],[4]

A number of APOE gene allelic variants including E2, E3 and E4 and six different genotypes (E2/2, E3/2, E4/2, E3/3, E4/3, and E4/4) have been reported. The properties of the two APOE binding domains differ according to these three isoforms, leading to functional differences. ^[5] It has been established in many reports that the presence of allele E4 (APOE c471TNC, rs 429358) is associated with increased levels of total and LDL serum cholesterol, while allele E2 (APOE c609CNT, rs7412) seems to be related to low levels of LDL and increased concentrations of plasma TG and lipoprotein remnants. ^[5]

Recent studies have suggested a role for APOE in obesity. ^[6],[7] Zhang et al.,[8] in an association study have shown that the E4 allele of APOE was significantly associated with increased body mass index (BMI) and plasma lipid levels in obese children.

The aim of this study was to examine the association between APOE genetic variants and lipid profile in an Iranian population.

Materials and Methods

Subjects n=320 (age between 25 and 64 years) were recruited from Zone 17 of Tehran who participated in a study based on the WHO-designed MONICA (multinational monitoring of trends and determinants in cardiovascular disease) study for coronary artery disease risk assessment. The study was approved by the institutional ethical committee written, informed consent was obtained from all subjects. There were 105 males, and 215 females.

The mean age was 45.7±13 years and mean BMI was 29.6±5.5.

Level of TG and cholesterol were tested for all our subjects using standard enzymatic methods (Pars Azmun, Iran). Body weight and height were measured twice with standard scale and at first their means and BMI calculated as body weight /square height (kg/m2).

DNA was purified from leukocytes by the salting-out method. Genotyping for APOE was carried out as described previously. ^[9] In brief, genomic DNA was amplified by PCR with the primers F5'-TCCAA GGAGC TGCAG GCGGC GCA and R5'-GCCCC GGCCT GGTAC ACTGC CA to yield a 218-bp DNA fragment that spans both APOE polymorphic sites. Simultaneous digestion of the 218-bp amplified product by AflIII and HaeII enzymes yielded 145-bp, 168-bp, and 195-bp fragments that were specific for Apo E3, E2, and E4, respectively.

Statistical analysis

Strength of association between different groups and alleles or genotypes of APOE polymorphism was estimated using odds ratios (OR) and 95% confidence intervals (CI). Levels of significance were determined using contingency tables by either Chi-square or Fisher exact analysis using the STATA (v8) software. The analysis of regression and significance of differences for level of cholesterol and TG was established by one-way analysis of variance followed by Dunnett post hoc multiple comparison tests using SPSS software Version 11.5. The significance level was set at 5 percent.

Results

We examined the APOE allele frequencies in patients with normal and abnormal total serum cholesterol levels, and observed that the frequency of allele E2 was significantly higher in patients with total serum cholesterol level <200 mg/dl (P 0.01 OR 2.1 95% CI 1.1-4.2). While the frequency of allele E3 was higher in patients with total serum cholesterol level >200 mg/dl it did not reach levels of significance (P 0.05 OR 1.5 95% CI 0.9-2.5). We also observed that the frequency of genotype E2/E3 and E2/E4 was significantly higher in patients with normal total serum cholesterol level compared to patients with abnormal cholesterol (P=0.003 OR 2.4 95% CI; 1.3-4.6). It was still significant after adjustment for sex (OR 2.3 95% CI; 1.3-4.7) and age (OR 1.04 95% CI; 1.02-1.06) [Table - 1].

Furthermore, we examined if there was a significant difference for association between APOE gene polymorphisms and normal and abnormal serum cholesterol levels in two groups with BMI<30 or BMI≥30. No significant differences were found (P 0.5) [Table - 2].

Also no significant association between APOE gene polymorphism and mean serum TG levels was found in our population. There was no significant difference for association between APOE gene polymorphism and mean serum TG levels when we compared between different groups with BMI<30 or BMI≥30.

Discussion

Genetic variation of APOE is a major determinant of inter-individual variation in susceptibility to dyslipidemia or coronary heart disease (CHD). ^{[10],[11],[12],[13]} Kao et al.,[1] first reported APOE polymorphism and its effect on lipid parameters in 546 unrelated Chinese in Taiwan by isoelectric focusing (IEF) and immunoblotting method for APOE phenotyping. It has been suggested that E2/3/4 protein isoforms of APOE are the major causes of lipid and apolipoprotein variability in many populations including Dutch, Nigerian, Japanese and Mexican-American samples. ^{[14],[15],[16],[17]}

We observed that the frequency of allele E2 was significantly higher in patients with normal total cholesterol level. Also, the number of individuals carrying allele E2 (genotype E2/E3 and E2/E4) was significantly higher in those with normal total serum cholesterol level compared to patients with abnormal cholesterol. The association between allele E2 and lower total serum cholesterol level was not significantly different when we classified the groups based on BMI. No significant associations between APOE gene polymorphism and serum TG levels were found.

In this study we have found that APO E3 allele in the Iranian population was the most common allele which is in keeping with other reports. ^[5] However, the frequencies of allele E2 were slightly increased and allele E4 was somewhat decreased in our population compared to the other groups which reported between 0.10 and 0.20 for allele E4 and 0.05 and 0.10 for allele E2 in most Caucasian populations. ^[5] The association between allele E2 and lower total cholesterol level has been reported in previous studies ^[4] which has been confirmed in our study. In most reports an association between APOE E4 allele and increased levels of total and LDL serum cholesterol also has been reported ^[18] which was not found in our population whereas in our population we found an increase in the frequency of allele E3 in individuals with higher total serum cholesterol level, however, it did not reach levels of significance. This might be due to a discrepancy in genetic risk contributing to the serum lipid level in different populations.

APOE polymorphism frequencies vary in different ethnic groups. The E2 allele was found to be completely absent in the Bari and Yucpa Amerindians ^[18] and Saudis ^[19] while significantly increased among the Mestizo and Negroid populations in Venezuela. ^[18] In China, different communities showed different frequencies of APOE alleles. ^[13],[20] In our study we have found that APOE allele E2 is protective against abnormal total serum cholesterol level in an Iranian population. Data presented in this study needs to be confirmed in a larger population with more detailed information for serum lipid profile including HDL, LDL and their subgroups.

Acknowledgment

This study was supported by a grant from the Endocrinology and Metabolism Research Centre (EMRC), Tehran University of Medical Sciences.

References

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