In the present study, we conducted an updated meta-analysis to evaluate the association between APOC3 polymorphisms (-455T/C or -482C/T) and risk of NAFLD. The pooled results indicated that the APOC3 -455T/C polymorphism confers an increased risk for NAFLD under the allelic and recessive models of inheritance. In addition, a subgroup analysis showed statistically significant associations between the APOC3 -455T/C polymorphism and risk of NAFLD in the Caucasian and PCR-RFLP groups, respectively.
The APOC3 -455T/C and − 482 C/T polymorphisms were thought to be related to increased risk of hypertriglyceridemia, metabolic syndrome, and coronary heart disease.24,25 Recently, these genetic variants were shown to be associated with the susceptibility to NAFLD.4 An in vitro promoter assay study demonstrated that these polymorphic sites at -455 and − 482, which fall within a previously identified insulin-response element, prevent insulin binding and thus increase APOC3 mRNA and protein levels.13 Insulin resistance is recognized as an essential pathophysiological factor of NAFLD, which results in hepatic de novo lipogenesis and a subsequent reduction in adipose tissue lipolysis, with a consequent increase in fatty acids in the liver.26 Taken together, it was proposed that that the variant alleles led to increased amounts of APOC3 and inhibition of lipoprotein lipase activity and triglyceride clearance, resulting in hypertriglyceridemia due to increased hepatic uptake of circulating chylomicron-remnant particles, and this results in NAFLD.4,27
A meta-analysis conducted by Zhang et al. 15 reported no association between APOC3 promoter polymorphisms and risk of NAFLD in different populations. These two SNPs are shown in strong LD with each other. It is noteworthy that Zhang et al. 15 investigated a combined effect of both APOC3 polymorphisms by comparison with wild-type homozygotes (-455C/C and − 482T/T) to carriers of one or more at-risk alleles (-455T and − 482C), as Petersen et al. 4 had previously done. Several studies evaluated the combined effect of APOC3 polymorphisms on NAFLD. However, the function of these two SNPs may be independent 4. Furthermore, the effect of an individual SNP was not thoroughly demonstrated. In our updated meta-analysis, the pooled OR for these two SNPs related to the risk of NAFLD was estimated, in comparison to Zhang et al.15 Here, we found that one of the gene polymorphisms − 455T/C was weakly correlated with NAFLD (p = 0.041). On the other hand, the − 482C/T polymorphism showed no detectable effect on NAFLD. Also of note is that Zhang et al. 15 did not include very recent studies 14,20−23 in their analysis. Thus, the present study provided a more comprehensive estimation of the association between APOC3 -455T/C polymorphism and risk of NAFLD. The − 455 site falls within a previously identified promoter insulin-response element. Lee et al. 13 have demonstrated that the variant sequence at the − 455 site reduces affinity for transcription factors that mediate the insulin response and provides a potential explanation for the inability of the variant promoter allele to respond to insulin. Insulin resistance results in increased delivery of free fatty acids to the liver. Also, insulin resistance is often accompanied by a state of chronic low-grade inflammation. Therefore, the ectopic lipid accumulation and activated inflammation cascades increases susceptibility to hepatic injury, and finally resulting in NAFLD.26
PCR-RFLP is a conventional SNP detection method. Among seven included studies in the current meta-analysis, three studies 14,22,23 used this method for SNP genotyping. As shown in Table 2, in two of these three studies,22,23 genotype distributions in the control population violated HWE. Furthermore, a sensitivity analysis showed that statistical significance of -455T/C was lost if one of these three studies14,22,23 was excluded. Heterogeneity differences were significant among the studies which assessed − 455T/C polymorphism in this meta-analysis. This suggests that various genotyping methods may lie behind the association between the APOC3 -455T/C polymorphism and risk of NAFLD. Then we performed subgroup analyses and found significant results in the PCR-RFLP group, but not in the other real-time/sequencing group (Table 3). Meanwhile, heterogeneity differences decreased and were not significant in either subgroup (see Supplementary Table S1). These findings indicate that the conventional PCR-RFLP SNP detection method introduced some heterogeneity differences.
NAFLD is a major public health hazard globally. Prevalence of NAFLD varied greatly by ethnicity, with the highest prevalence in Hispanics (45%-58%), followed by Caucasians (33%~44%), and lowest prevalence in African Americans (24%~35%).28,29 Therefore, we performed subgroup analyses based on the ethnicity of study subjects and found the − 455C allele had a significant effect on the risk of NAFLD in Caucasian subjects, but not in Asian subjects. Different genetic backgrounds and environments may contribute to the ethnic disparities in NAFLD.
Several limitations should be noted when interpreting results of our study. First, this meta-analysis included a limited number of studies. Meanwhile, most of the subjects were from Asian populations. Therefore, the generalizability of our findings is not certain, especially for Caucasian populations. Second, there was heterogeneity in the methods of NAFLD diagnosis; two studies 19,21 used liver histology and the others used ultrasound. Furthermore, other factors such as age, gender, and ethnicity may also introduce significant between-study heterogeneity. Third, we were unable to adjust for potential confounding factors, which might have affected the accuracy in evaluating effects of APOC3 polymorphisms on the susceptibility to NAFLD.