This is the first study to investigate the APOA1/C3/A4/A5 gene cluster in relation to lipid response to fenofibrate treatment. We found significant associations between the SNPs of the gene cluster and lipid response.
The most pronounced association was for APOA5_S19W, which was associated with both TG and HDL-C response to fenofibrate. APOA5_S19W alters codon 19 of the predicted amino-terminal signal sequence of APOA5, which substitutes a serine residue with tryptophan. As serine 19 can be placed at the −5 position for the preapolipoprotein [27
], a tryptophan residue so close to the cleavage site of the APOA5 signal sequence could considerably reduce the processing of this preprotein. Although no functional studies and induced mutations in mice have been carried out to prove its actual role in lipid metabolism, studies using protein structure prediction algorithm [28
] suggested that APOA5_S19W may cause a change in the secondary structure of APOA5, with a concomitant change in tertiary structure [29
]. We hypothesize that this APOA5 structure alteration may lead to altered gene function and thus lipid response to fenofibrate treatment.
APOC3_M482 is located at the insulin response element in the promoter region of APOC3 and confers decreased responsiveness to insulin in in-vitro assays [30
]. APOC3_M482 is in strong LD with APOC3_3U386, which is significantly associated with TG response in this study. APOC3_3U386 was associated with combined hyperlipidemia and hypertriglyceridemia [31
]. APO-A4_N147S is a coding variant that results in a nonsynonymous change at position 147 of APOA4 from asparagine to serine. Studies showed that APOA4 may control intravascular transport of dietary lipids and was associated with postprandial clearance of TG-rich lipoproteins [32
]. We postulate that APOA4_N147S may cause altered conformation and function of APOA4 and thus differential response to fenofibrate treatment.
Pharmacogenetic studies of lipid-lowering therapy, in particular with fibrate, are relatively rare. In the Lopid Coronary Angiography Trial study, no association was observed between APOA5 variants and lipid response and disease progress to gemfibrozil (another fibrate) treatment [33
]. The Lopid Coronary Angiography Trial study was conducted in men who had undergone coronary artery bypass grafting with HDL-C ≤ 42.57 mg/dl, whereas our study samples were randomly selected and may represent the general population. Moreover, our sample size is larger and thus has higher statistical power.
A few pharmacogenetic studies of lipid-lowering therapy with statin were reported [34
]. An APOA1 promoter polymorphism was found to influence basal HDL-C and its response to pravastatin therapy [35
]. As statins are 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors and have different metabolic mechanisms from that of fibrates, it is difficult to compare the results between statin-based studies and ours here with fibrates. Currently, statins are regarded as first-line therapy for lipid disorder and seem to have eclipsed the clinical use of fibrates. Emerging evidence, however, emphasizes the necessity of refocusing on fibrates in the poststatin era [36
]. First, statins are primarily designed for correcting hypercholesterolemia as the sole abnormality, but up to half of coronary patients are normocholesterolemic. Second, clinical endpoint trials suggest that the relative risk reduction with statins over a period of 5 years is at best 35% [36
], suggesting that a significant proportion of patients sustain cardiovascular events independent of LDL-C-lowering or a direct effect of statins on atherogenesis. Finally, up to a third of coronary patients have elevated plasma TG levels with low HDL-C and normal or near-normal LDL-C [36
]. Given these, our findings here may have important clinical implications.
We were not able to find significant association in analyses of subsample with hypertriglyceridemia (a TG level > 200 mg/dl). A possible reason is that the SNP effects on lipid response to fenofibrate may be different between those who have higher TG levels and those who have lower TG levels. Another possible reason is that the largely decrease sample size (192 patients) may not be sufficiently powerful to detect the modest effects that truly exist.
The strengths of this study lie in: a rigorous fenofibrate trial, a large family-based sample, and comprehensive survey of the entire gene cluster. Our study has, however, some potential limitations. First, although multiple SNPs were genotyped, they were not sufficient to exhaustively cover the entire gene cluster. The LD pattern and haplotype block structure of the gene cluster in this study sample are, however, comparable with some earlier studies in which denser SNPs were genotyped [12
]. Second, validation/replication in an independent sample are generally required to confirm association findings. Currently, large-scale fibrate intervention studies are, however, extremely rare in the field (largely because of the high costs involved and low visit adherence). Therefore, we lack available data set for validation/replication at this stage. Third, a number of tests were performed because of multiple SNPs involved. After correction for multiple testing, some significant associations only reached nominal significance or even disappeared. As the tested SNPs are in LD, Bonferroni’s correction may, however, be too conservative. Finally, the GOLDN project is a genetic epidemiology study, which alone is not sufficient to define a functional variant unambiguously.
In summary, we found that the APOA1/C3/A4/A5 gene cluster is associated with lipid response to fenofibrate treatment, suggesting that variation in this gene cluster could be useful to tailor lipid-lowering therapies. Further studies in other populations and/or molecular functional studies are necessary to validate/replicate our findings.