Apolipoprotein E (APOE, MIM: 107741) is involved in cholesterol transport, lipid metabolism, and protein synthesis through mediation of binding of the low-density lipoprotein receptor and the APOE receptor of lipid particles to specific lipoprotein receptors. It is also involved in numerous other functions, including tissue repair, immune response and regulation, and cell growth and differentiation. The APOE gene is located on human chromosome 19q13.2; three functionally distinct isoforms of the protein (E2, E3, and E4), encoded by corresponding alleles ε2, ε3, and ε4, have been described. These isoforms, originally identified by isoelectric focusing, are defined by amino acid changes at positions 112 (rs. 429358) and 158 (rs. 7412): alleles ε2, ε3, and ε4 are defined, respectively, by cysteine/cysteine, cysteine/arginine, and arginine/arginine at these two sites. The most common isoform is ε3, with a frequency of approximately 70-80% [1,2].

Individual case-control association studies are usually small and underpowered and, thus, are unable to provide a definitive answer, even in cases involving existence of a true association. Thus, meta-analysis can effectively combine data from several studies, resulting in increased statistical power (lower type II error rate). Findings from previous studies investigating the association between APOE polymorphisms and breast cancer risk have been inconsistent [3-10]. Some studies have reported an association of E4, compared with the E3E3 genotype, with increased risk of breast cancer [7,9], while other reported findings have indicated no association between APOE polymorphism and risk of breast cancer. To the best of our knowledge this is the first meta-analysis to investigate the association of APOE polymorphisms with risk of breast cancer.

We identified 10 eligible studies, including 3,835 subjects (2008 patients, and 1,827 healthy controls), that reported on polymorphisms of APOE and risk of breast cancer, which are summarized in Table 1 [3-10]. The forest plot of the meta-analysis of the APOE alleles is shown in Fig. 1. Comparison of prevalence of the ε4 vs. ε3 alleles among cases and controls showed statistically significant heterogeneity between studies (Q=36.2, df=9, p<0.001). In order to find the source of heterogeneity, we stratified the studies based on the ethnicity of participants. As a result, heterogeneity showed a dramatic decrease among studies reported from Asia (Q=2.31, df=2, p=0.315), and a significant association was revealed. Presence of the ε4 allele indicated increased risk of breast cancer, in comparison with the ε3 allele (OR, 1.56; 95% CI, 1.19 to 2.04; p=0.001). It should be noted that no association of the ε2 allele with breast cancer risk was observed (Table 2).

Among other studies, heterogeneity was still observed. Excluding studies reported by Yaylim et al. [5] (due to a very small sample size) and Menzel et al. [6] (because there was not an equal number of the polymorphism at positions 112 and 158), heterogeneity showed a decrease (p>0.381) (Table 2). When compared with the ε3 allele, no association of either the ε4 (OR, 0.99; 95% CI, 0.83 to 1.17; p=0.917) or the ε2 (OR, 0.92; 95% CI, 0.72 to 1.17; p=0.514) allele with susceptibility to breast cancer was observed.

From 10 studies [3-10], only five studies [3,5,7,9,10] reported frequencies of APOE genotypes; these are summarized in Table 3. The forest plot of the meta-analysis of the APOE genotypes is shown in Fig. 2. As shown in Table 4, carriers of the ε4 allele (E4E4, E4E3, and E4E2 genotypes), in comparison with the E3E3 genotype, showed an association with elevated risk of breast cancer only among Asians (OR, 1.75; 95% CI, 1.23 to 2.47; p=0.002).

In order to assess publication bias for reported comparisons of the alleles and genotypes of APOE polymorphism and breast cancer risk, Begg's funnel plots were prepared and Egger's test was performed on the set of 10 studies [3-10]. The funnel plots (Fig. 3) appeared to be symmetrical in shape, and results of statistical analysis showed no significant publication bias.

The main finding of the present study was that carriers of the ε4 allele (E4E4, E4E3, and E4E2 genotypes), in comparison with the E3E3 genotype, showed an association with elevated risk of breast cancer only among Asians. In conclusion, findings of this meta-analysis suggest that the APOE ε4 allele is a low-penetrant risk factor for development of breast cancer. The APOE ε4 allele has been associated with increased risk of several multifactorial diseases, including hypertension [14] and coronary heart disease [15].

Based on findings from animal studies, ecologic studies, and studies of migrants from areas with low fat intake to those with high fat intake, an association of dietary fat intake with breast cancer risk has been hypothesized [16]. Previous studies have reported increased risk of breast cancer in association with elevated triglyceride (TG) levels [17]. Association of increased TG concentrations with decreased levels of sex hormone binding globulin, resulting in elevated levels of free estardiol and subsequently increased risk of breast cancer, has been reported [18]. Association of elevated TG levels with breast cancer risk among women with the APOE E4 genotype has been reported. The APOE E4 genotype has been reported to reduce TG clearance from plasma, resulting in persistently elevated TG concentrations, which could result in decreased levels of sex hormone binding globulin and elevated levels of free estradiol [3,19].

Possible association of some genetic polymorphisms with altered risk of some types of cancers only in some ethnic groups has been previously reported [20-23]. For example, increased risk of gastric cancer in association with the GSTT1 polymorphism has been reported only among Caucasians [21]. In relation to the XRCC1 polymorphism, association of Arg399Gln with lung and breast cancers among Asians, but not among Western countries, has been reported [20,23]. Similarly, findings from the present study indicated the APOE ε4 allele as a risk factor for development of breast cancer in Asia.

The gene encoding APOE was mapped on human chromosome 19q13.2. Of particular interest, based on published meta-analysis, single nucleotide polymorphisms of several genes, including XPD, ERCC2, XRCC1, and TGFB1, located on human chromosome 19q13, have been associated with breast cancer risk [23-25]. Therefore, it is quite probable that our present finding indicates a true association. We know that the major limitation of this study is the small number of articles available for meta-analysis, as well as its geographical distribution; for example, there are no reports from China, Western Europe, etc. Future well-designed large epidemiological studies are warranted in order to provide validation for the present findings.

The present meta-analysis suggest that the APOE ε4 allele is a low-penetrant risk factor for development of breast cancer.