In this explorative study of 373 SNPs, mainly located in pathways related to lipid and glucose metabolism, we found a significant association between the Ile405Val genotype in the CETP gene and the Cys112Arg genotype in the APOE gene, with multiple features of the metabolic syndrome, i.e. the prevalence of abdominal obesity and prevalence of low HDL-cholesterol. For both SNPs, the association with abdominal obesity was partly independent of the association with HDL-cholesterol, and vice versa. No, association was found between SNPs in genes involved in glucose metabolism or blood pressure regulation and multiple MetS features.
In humans, CETP and ApoE are expressed in the liver and in peripheral tissues, such as adipose tissue [13, 14]. Both genes are involved in plasma lipid homeostasis. CETP stimulates the clearance of HDL-cholesterol from plasma . Furthermore, CETP increases the formation of small dense LDL particles and triglycerides . ApoE removes atherogenic lipoproteins, such as VLDL, from the circulation . This results in lower cholesterol and triglyceride levels. Besides having a role in lipid homeostasis, a few studies indicate that CETP and ApoE may be involved in other metabolic processes such as weight regulation. For example, APOE plays a role in the deposition of dietary fat in adipose tissue . As CETP is synthesized in the adipose tissue, CETP may affect adipose tissue characteristics .
The Ile405Val polymorphism in the CETP gene induces a change in amino acid sequence. Therefore it is likely to be a functional SNP. In our study, the Val allele of the Ile405Val genotype was associated with a lower prevalence of abdominal obesity and a lower prevalence of low HDL-cholesterol levels. The stratified and adjusted analyses in our study suggested that the association with prevalence of abdominal obesity and prevalence of low HDL-cholesterol levels was partly independent of each other. This suggests that CETP regulates weight and HDL-cholesterol via independent pathways.
In line with our results, a meta-analysis of 29 studies, showed that Val allele carriers had higher HDL levels . Furthermore, a Chinese case-control study in 934 obesity cases and 924 controls showed a decreased obesity risk for Val/Val homozygotes, which persisted after adjustment for HDL-cholesterol levels . In previous studies, the 405Val allele has been associated with lower CETP mass and lower CETP activity . Lower CETP plasma levels are correlated with a lower obesity risk . The 405Val allele has also been associated with other positive health outcomes such as, increased HDL and LDL particle size , decreased coronary heart disease risk , and increased longevity , all of which are related to the MetS. In summary, cumulative evidence indicates that Ile405Val is involved in several metabolic processes, including lipid level control and weight regulation.
The Cys112Arg genotype of the APOE gene is a non-synonymous genotype. Together with Arg158Cys (rs7412), the Cys112Arg forms the ε2ε3ε4 haplotype. The ε2, ε3 and ε4 ApoE isoforms differ markedly on the structural and functional level . In our study the Arg allele of the Cys112Arg genotype was associated with an increased prevalence of low HDL-cholesterol levels and an increased prevalence of abdominal obesity. Again the stratified and adjusted analyses suggested that the associations with the prevalence of abdominal obesity and prevalence of low HDL-cholesterol levels were partly independent of each other. The ε4 isoform showed a similar, though less pronounced, pattern of associations. No associations were observed with Arg158Cys or ε2 isoform, of the ε2ε3ε4 haplotype.
Previous studies generally focused on the ε2, ε3 and ε4 haplotype and did not take associations with the individual Arg158Cys and Cys112Arg into account. In line with our study, the ε4 isoform was associated with a more detrimental metabolic profile in most studies. A meta-analysis of 19 studies in 9751 subjects, showed that ε3/ε4 carriers had lower HDL-cholesterol levels than ε3/ε3 carriers . Most studies showed either a positive [24–27] or no [28–31] association between the ε4 isoform and body weight. However, some showed a negative association [30, 31]. Arbones Mainar et al.  showed that compared to ApoE3 mice, ApoE4 mice fed a western diet were more prone to the development of several MetS features, such as increased insulin resistance, decreased fat tolerance and increased fat cell size. However, they gained less body weight. This suggests that the positive association between the ε4 isoform and abdominal obesity may be driven by the development of other MetS features, such as insulin resistance . Furthermore, these results suggest that the ε4 isoform may be associated with MetS. This has indeed been shown in other epidemiological studies [25, 26, 29].
Strength of our pathway driven candidate gene study was the relatively large sample size. Contrast and precision were increased by exclusively including people with consistent MetS phenotype, i.e. classified as healthy or not healthy for a particular metabolic phenotype over two measurement rounds. Furthermore we tried to keep the probability of chance findings low by including only those SNPS that were related to two or more MetS features with P < 0.01 into the second round of data-analysis. We found 2 SNPs, which differed significantly from the expected 0.12 SNPs (p < 0.005 chi-square with Yates correction). However, the 0.12 expected SNPs were obtained assuming independent random outcomes. As HDL-cholesterol and abdominal obesity are not completely independent, this assumption is partly violated. However, the associations with abdominal obesity and HDL-cholesterol remained significant in our stratified and adjusted analyses. A weakness of our study may be that blood samples were taken from non-fasting subjects. This may have randomly affected the glucose measurements. Another weakness is that triglycerides levels were not measured in our study. Therefore, we may have missed SNPs which were related to hypertriglyceridemia and one or two other MetS feature. For example, the CETP Ile405Val mutation has been associated with triglycerides in previous studies . We therefore expect that in our study population this SNP will not only be associated with HDL-cholesterol and abdominal obesity, but also with triglyceride levels.
In this explorative study of 373 SNPs among 3575 subjects, we emphasized on the intricate links between several MetS features. We have showed that two SNPs, mainly known for their role in lipid metabolism, influenced both abdominal obesity and low HDL-cholesterol levels, partly independent of each phenotype. If the pleiotropic effects of these genes are further confirmed by others it might be possible to develop medication which increases HDL-cholesterol leves and reduces waist circumference, and so affects the development of MetS