Apolipoproteins (lipid-free) are lipid-binding proteins which are responsible for the clearance of lipoproteins. Apolipoprotein E (ApoE) is one of the several classes of this protein family. Its main function is the normal catabolism of triglyceride-rich lipoproteins (VLDL, chylomicrons) and their remnants via acting as a ligand for the low-density lipid (LDL) receptors. Maintaining normal levels of lipoproteins is essential for the prevention of disorders that affect the heart and blood vessels (cardiovascular diseases), including heart attack and stroke. Beside its main function in lipid homeostasis, it plays important roles in anti-inflammation and host defense.
The APOE gene is localized on 19q13.2 in an approximately 45 kb gene cluster containing the genes for APO CI, CII, CIV, and CI pseudogene, along with elements controlling their tissue-specific transcription. It consists of four exons and three introns. APOE is most highly expressed by hepatocytes and other peripheral tissues, especially macrophages. ApoE protein is a 317 amino acid long and weights 36,154 Da.
The APOE gene is polymorphic. With three frequently occurring alleles: APOE*2, APOE*3, and APOE*4. The corresponding three major isoforms are E2, E3, and E4. The most common allele is E3. These variants differ in one amino acid from ApoE3 at either position 112 from cysteine to arginine (APOE*4) or at residue 158 from arginine to cysteine (APOE*2) in exon 4 of the APOE gene locus. The APOE*4 allele is found associated with an increasing risk for development type 2 Alzheimer disease and cardiovascular disorders such as coronary atherosclerosis. Also, people who carry two copies of the APOE*2 are at risk for a condition known as hyperlipoproteinemia type III.
Al-Shammari et al. (2004) conducted a study on two groups of Gulf Arab nationals to assess the frequencies of APOE genotypes and their associations with serum levels of lipids, lipoproteins and apolipoproteins. The first group included 106 healthy blood donors (100 males, 6 females) with a mean age of 40.5 +/- 4.7 years, while the second group comprised of 41 combined hyperlipidemia (CH) patients (22 males, 19 females) with a mean age of 42.3 +/- 3.5 years. The healthy blood donor group and CH patients group demonstrated allele frequencies of 5.7% for E2, 85.4% for E3, and 9.0% for E4. Al-Shammari et al. (2004) concluded that APOE allelic arrangements in both groups did not reasonably anticipate circulating blood levels of lipids and lipoproteins.
Al-Bustan et al. (2005) carried out a molecular study to estimate allele and genotype frequencies of the APOE gene locus in the Kuwaiti population. This study was performed on 292 healthy Kuwaiti volunteers (210 males, 66 females, 16 unknown), ranging in the age from 18 years to 62 years. The samples were then divided into four groups on the basis of the ethnic background: Arab origin (Arabian Peninsula), Arab Bedouin tribes, Iranian origin, and the heterogeneous population, including Arab and non-Arab descent, as well as samples obtained from the blood bank, the ethnic backgrounds of which were difficult to determine. These blood-bank samples may include Kuwaitis whose origin is Arab, non-Arab, or both, hence representing the whole of the Kuwaiti population. DNA samples were subjected to polymerase chain reaction (PCR) followed by restriction fragment length polymorphism (RFLP) analysis. Statistical analysis showed no difference in the allele frequencies between the four groups. The frequency of APOE*3 in the Kuwaiti population was highest (88.4%) followed by the frequency of APOE*4 (6.5%) and APOE*2 (5.1%). The genotype and allele frequencies obtained for the Kuwaiti population fell within the reported worldwide distribution for the APOE gene locus. The results obtained in this study showed no statistical difference (p > 0.05) between the APOE allele and genotype frequencies between the subgroups for all six genotypes and three alleles, supporting the assumption of admixture in the Kuwaiti population that has occurred over the last 50 years in Kuwait. Also, Al-Bustan et al. (2005) found that the distribution of the APOE alleles in Kuwait differs somewhat from those reported in other Arab populations, suggesting that the Arabs originating from the Arabian Peninsula are different from those of Lebanon, Morocco, and Sudan.
Al-Shammri et al. (2005) performed a study to compare the frequencies of specific APOE genotypes and the alleles of patients with clinical features of multiple sclerosis (MS) with frequencies that were examined in a healthy Kuwaiti Arab cohort. The study included two groups, the first group consisted of 39 MS patients (17 males and 22 females) and the second group consisted of 106 healthy Kuwaitis (controls). Al-Shammri et al. (2005) found no significant variation in allele frequencies among patients and controls; however, an insignificant tendency via lower APOE2 allele frequency was noticed in patients. Also, APOE4 allele was found to be significantly associated with female patients and with severe illness.
Akanji et al. (2007) investigated the associations of specific APOE allelic patterns with LDL size and subfraction profiles in patients with coronary heart disease (CHD) and healthy control subjects. Akanji et al. (2007) recruited two groups of male subjects: (A) 65 apparently healthy control subjects, median age, 39.0 years (range, 25.0-60.0 years); (B) 50 patients with CHD, median age, 54.0 years (range, 40.0-76.0 years). As expected, patients with CHD were found to have a worse atherogenic lipoprotein profile (waist-hip ratio, LDL, uric acid, and apolipoprotein B) than the controls. APOE genotype and allele frequencies were found to be similar for both groups. In either group, median percent large buoyant LDL (pattern A) was found to be greater in controls (51.0% vs 46.5%, P<.001) and percent small dense LDL (pattern B) was found to be greater with CHD (9.0% vs 3.0%, P<.001). The latter also was found to have smaller median particle size (26.5 vs 26.9 nm, P<.001). In controls, percent LDL pattern B was found to be significantly lower with APOE2 than with APO non-E2 (4.0% vs 0.0%, P<.05); in patients with CHD, E2 patients were found to have smaller particle size, and pattern B was found to be significantly lower with non-E2 than with E2 (15.0 vs 8.0, P<.05). With respect to E4, control non-E4 was found to have a smaller median percent LDL pattern B than E4; otherwise, no significant findings were found in relation to APOE type and LDL size and subfractions in both subject groups. Akanji et al. (2007) concluded that these results confirm observations in other populations of increased levels of small dense LDL in patients with CHD. They also concluded that although the APOE allelic pattern, especially APOE2, could be related to LDL subfraction profiles in control subjects, such associations could not be demonstrated in those with CHD.
Al-Bustan et al. (2009) investigated the possible association of clinical variables and apolipoprotein (APOE, APOCI and APOB) polymorphisms with the development of myocardial infarction (MI) and coronary heart disease (CHD) in Kuwaitis. APOE genotype was determined by polymerase chain reaction followed by restriction fragment length polymorphism in 143 Kuwaiti CHD patients with (n = 88) and without (n = 55) MI and in 122 controls matched for gender and age. APOCI and APOB genotypes were also determined in the same population sample within this study. In this study, Al-Bustan et al. (2009) determined APOE genotype by studying APOE3 allele (wild-type), APOE4 allele (Cys112Arg), and APOE2 allele (Arg158Cys). A statistically significant association was found between CHD and medical history of diabetes mellitus (p < 0.001), hypertension (p < 0.01), high cholesterol (p < 0.05) and family history of CHD (p < 0.001). A highly significant association (p < 0.001) was found, with an adjusted odds ratio of 9.32, for family history and the development of MI. No significant differences were found for allele or genotype frequencies between CHD patients and controls. Al-Bustan et al. (2009) concluded that the strong effect of family history suggests a major genetic component for the development of CHD in Kuwaitis, but this association does not appear to be related to the APO genes investigated in this study. They also suggested that the results in this study encourage future research into these and other polymorphisms and their potential association with MI and CHD in the Kuwaiti population.
Hilkevich et al. (1999) examined the frequency of the APOE*4 allele in Jews originating from Libya, Buchara and Ethiopia and in Jews of Sephardic and Ashkenazi origins. Its frequency among Ethiopian immigrants was 0.27, significantly higher than in the other groups, in which the frequency was between 0.067 and 0.10.
A study by Al-Yahyaee et al. (2005) obtained the genotypes of apo E among 162 healthy Omanis of Arab Bedouin origin in order to determine the allele distribution of this gene in the Omani population. The resulting allele frequencies were 0.052, 0.886 and 0.062 for E2, E3 and E4, respectively. The Al-Yahyaee et al. (2005) found that these results make Omanis have the lowest E4 frequency and the highest E3 frequency in the world. Nevertheless, the abovementioned frequencies were very similar to these observed in other Arabs, Mediterranean Europeans, Indians, and Japanese.
The relationship between apo E polymorphism and lipid profiles and risk of coronary artery disease (CAD) was studied in Omani dyslipidemic patients by Al-Yahyaee et al. (2007). This retrospective study involved 244 dyslipidemic patients of whom 67 had CAD. The frequency of the alleles E2, E3 and E4 were 0.030, 0.894 and 0.076, respectively, among dyslipidemic patients, and no statistically significant difference was found between these frequencies among patients with or without CAD. However, the study found that among dyslipidemic patients, carriers of the allele E4 had significantly higher levels of LDL cholesterol and apoB when compared to patients with the genotype E3/E3.
Bowirrat et al. (2000) presented data they interpreted as suggesting an autosomal recessive form of AD. They screened all 821 elderly residents of an Arab community located in Wadi Ara. An unusually high prevalence of AD was observed (20% of those 65 years old or older; 60.5% of those 85 years old or older). Data on the APOE4 allele suggested that it could not explain the AD prevalence in this population. The APOE4 allele was relatively uncommon in Arabs in Wadi Ara; in fact, Bowirrat et al. (2000) stated that it was the lowest frequency of the allele ever recorded. Bowirrat et al. (2002) reaffirmed the observation that the ApoE epsilon4 allele is relatively uncommon in this population and that it cannot explain the high DAT prevalence.
A study was conducted by Dzimiri et al. (1999) to determine the distribution of apo E genotypes in the Saudi male population and assess its contribution to coronary heart disease (CHD). Healthy Saudi blood donors (317 men and 3 women) were recruited to uncover the apo E genotype frequency in the population. The clinical relevance of apo E genotypes in the context of CHD was analyzed by comparing a group of 96 patients with angiographically documented coronary narrowing exceeding 75% with a control group of 40 individuals. The patient group consisted of 83 men and 13 women while the control group consisted of 19 men and 21 women. As for the blood donors group, the frequency of the allele E3 reached 84%, while the frequency of E2 was 4% and that of E4 was 12%. Upon comparing the patients group with controls, the former displayed a higher frequency of the allele E4 (10%) as opposed to 6% in the control group, but the difference is not statistically significant. The results suggest that neither of the two alleles (E2 and E4) predispose Saudi individuals to CHD. The relationship between classic risk factors for CHD and apo E polymorphism was analyzed and the results suggest a strong link between E4 and hypertension, 3-vessel disease and restenosis resulting from carrying the genotype E3/E4. On the other hand, the allele E2 seems to be associated with predictors of atherosclerosis such as elevated triglycerides and cholesterol as well as hypertension and diabetes.
The allele 4 of the apolipoprotein E (apo E) gene is associated with higher mean plasma cholesterol and triglyceride concentrations, and consequently leads to a higher risk of coronary disease. This may account for the high prevalence of cardiovascular death in patients with progressive renal disease. This triggered Al-Muhanna et al. (2008) to determine the prevalence of this polymorphism in Saudi patients with end-stage renal disease (ESRD) on hemodialysis. The control group only included people with no past, present or family history of renal disease. The apo epselion 4 allele frequency and homozygous genotype distribution in patients were 7% and 2.4%, respectively compared to 13% and 2% in the control group, and the difference was not statistically significant.
Nakai et al. (2004) applied a chip-based MALDI-TOF mass spectrometry to screen for 15 single nucleotide polymorphisms (SNPs) from 14 candidate genes involved in the renin-angiotensin system, lipid metabolism, cytokines and adhesion molecules, and growth factors, and the coagulation-fibrinolysis system. The study included the analysis of 47 healthy Yemenite Jews, a population known for its low incidence of coronary artery disease (CAD). The incidence of the C/T (p.R158C) allele in the Apo E gene was 0.085 vs 0.011.