Association of apoE gene polymorphisms with lipid metabolism in renal diseases

Background and Objectives Apolipoprotein E (apoE) plays a central role in the metabolism and homeostasis of lipids. ApoE gene encodes three major isoforms: ε2, ε3 a nd ε4 forming six phenotypes: E2E2, E2E3, E2E4, E3E3, E3E3 and E4E4. Disorders of the lipid metabolism and the homeostasis are frequently coexist in renal diseases. The association between gene polymorphisms of apoE and lipid metabolism were not consistent. This meta-analysis was performed to assess the association between gene polymorphisms of apoE and lipid metabolism in renal diseases. Methods A pre-defined literatures search and selection of eligible relevant investigations were performed to extract and collect data from electronic databases. Results Sixteen articles were enrolled for the analysis of association between apoE gene polymorphisms and lipid metabolism. Subjects with E3E4 had a higher total cholesterol (TC) than those with E3E3, and subjects with E2E3 had a lower TC than those with E3E3. Subjects with ε2, had a lower TC than those with ε3 or ε4, and subjects with ε4 had a higher TC than those with, ε3. Subjects with E2E2, E2E3 or E4E4 had a higher triglyceride (TG) than those with E3E3. Subjects with ε4 had a higher TG than those with ε3. Subjects with ε2, had a higher level of TG than those with non-ε2. Subjects with E3E4 had a slightly lower high-density lipoprotein (HDL) than those with E3E3. E3E4 appeared to be associated with lower levels of HDL. Subjects with E2E2, E2E3 had a notably lower low-density lipoprotein (LDL) than those with E3E3. Subjects with ε2, had a lower LDL than those with ε3 or ε4 ApoE gene polymorphisms were not associated with very low-density lipoprotein, and lipoprotein (a) [Lp(a)]. Subjects with E2E3 or E2E4 had higher apoE levels than those with E3E3, and subjects with E4E4 had lower apoE levels than those with E3E3. Conclusion ApoE gene polymorphisms are associated with the expression of TC, TG HDL, LDL, Lp(a) or apoE.

are frequently coexist in renal diseases 14 , such as nephrotic syndrome (NS), most kidney diseases have been associated with the high serum/plasma level of VLDL and impaired clearance of atherosclerotic residues. Nephrotic dyslipidemia is a risk factor to develop into systemic atherosclerosis, which may aggravate glomerular sclerosis and accelerate the progression of glomerular disease 15 .
Currently, a number of reports have been carried out to show the relationship between the gene polymorphisms of apoE and expression of TC, TG, HDL, LDL, VLDL, and Lp(a). However, the results were not consistent. Due to the sparseness of data, and disagreements among the reported studies, the available evidence is weak, Evidence from meta-analysis can provide more convincing evidence when compared with individual investigations 16 . There is no meta-analysis to detect the association between apoE gene polymorphisms and lipid metabolism in kidney disease. Therefore, we performed a meta-analysis to further explore the relationship between apoE gene polymorphisms with lipid metabolism in renal diseases.

Methods
Search strategy: The search term was"(ApoE OR apolipoprotein E) AND (renal OR kidney)". Studies published on Oct 1, 2017 were screened from PubMed, Embase, and Cochrane Library without language limitation. "Related articles" and the bibliographies were also screened to extend search spectrum. Only the most complete paper recruited for repetitive data.

Inclusion and Exclusion Criteria
Inclusion criteria: (1) Renal diseases were demanded.
Exclusion criteria: (1) article types such as review, editorial and meta-analysis, etc; (2) multiple duplicated data in different publications; (3) apoE was not the target gene; (4) not renal diseases.

Data extraction and synthesis
Basic information (author, year, patient, location, disease type) was extracted independtly from every study by different investigators. Outcomes included the level of TC, TG, HDL, LDL, VLDL, Lp(a) or apoE.

Statistical analysis
The relationship was analyzed between apoE gene polymorphisms and outcomes. The available values of every outcomes were entered into Cochrane Review Manager (RevMan, Version 5). Fixed effects model was tacit, unless p-value of the heterogeneity test was less than 0.1, random effect model was conducted. Results were measured by Weighted mean differences(WMD) and 95% confidence intervals(CI). P < 0.05 was deemed statistically significant for the overall OR. I2 was used to test the heterogeneity of included studies. The Begg adjusted rank correlation test 17 and the Egger regression asymmetry test 18 were used to detect the publication bias (P<0.1 was considered significant) for included studies exceeding fifteen.

Study characteristics
27 studies retrieved from PubMed, Embase, and Cochrane Library, enable to further analyze ( Figure 1).

Study characteristics for the association between gene polymorphisms of apoE with VLDL levels
Four studies were recruited into the meta-analysis for the association between gene polymorphisms of apoE with VLDL levels. One study 19 was for the compassion of E2E2 vs. E3E3. 3 reports 19,24,30 were recruited into this meta-analysis of E2E3 vs. E3E3. One study 19 was recruited into this study of E2E4 vs. E3E3. 3 reports 19,24,30 were entered into the meta-analysis of E3E4 vs. E3E3. One study 19 was recruited into the study of E4E4 vs. E3E3. Three studies 24,30,38 were included into the study of ε 2 vs. ε 3 (including 4 comparisons). Three studies (24,30,38) were included into the investigation of ε 4 vs. ε 3 (including 4 comparisons). Three studies 24,30,38 were included into the study of ε 2 vs. ε 4 (including 4 comparisons).

Study characteristics for the association between gene polymorphisms of apoE with Lp(a) levels
Three studies were included into the meta-analysis for the association between gene polymorphisms of apoE with Lp(a) levels. One study 20 was for the compassion of E2E2 vs. E3E3. Two reports 20, 26 were recruited into the meta-analysis of E2E3 vs. E3E3. One study 20 was recruited into the study of E2E4 vs. E3E3. Two reports 20, 26 were recruited into the investigation of E3E4 vs. E3E3. One study 20 was recruited into the study of E4E4 vs. E3E3. Three reports 20,26,43 were included into the study of ε 2 vs. ε 3. Three reports 20,26,43 were included into the study of ε 4 vs. ε 3. Three reports 20,26,43 were included into the investigation of ε 2 vs. ε 4.

Study characteristics for the association between gene polymorphisms of apoE with ApoE expression
Six studies were included into the meta-analysis for the association between gene polymorphisms of apoE with TC expression. Two studies 19,21 was for the compassion of E2E2 vs. E3E3. 3 reports 19, 21, 25 were recruited into this meta-analysis of E2E3 vs. E3E3. Two studies 19,21 were recruited into this investigation of E2E4 vs. E3E3. 3 reports 19, 21 , were recruited into our meta-analysis of E3E4 vs. E3E3. Two studies 19,21 were recruited into the study of E4E4 vs. E3E3. Three studies 25,39,41,43 were recruited into the pooled study of ε 2 vs. ε 3 (including 4 comparisons). Three studies 25,39,41,43 were included into this investigation of ε 4 vs. ε 3 (including 4 comparisons). Three studies 25,39,41,43 were included into the investigation of ε 2 vs. ε 4 (including 4 comparisons).

The relationship between gene polymorphisms of apoE and lipid metabolism Relationship between gene polymorphisms of apoE and TC levels
In the current meta-analysis, we presented results separately for comparisons where the number of recruited articles was larger than ten, and where the number of recruited investigations for comparisons was no fewer than 10, since results from < 10 studies might be less robust.
When compared with those patients with E3E3, patients suffering from E3E4 had an increased level of TC ( Figure 2), and persons suffering from E2E3 had a reduced TC level (Figure 3). Subjects suffering from ε 2 had a reduced TC level when compared with those persons with ε 3 or ε 4, and persons suffering from ε 4 had increased levels of TC than those persons suffering from ε 3 ( Table 1). These results suggest that E3E4 and ε 4 are related to up-regulated levels of TC, and there are an association between E2/E3 or ε 2 and the reduced levels of TC. Persons suffering from ε 2 can get lower TC levels when compared peoples with non-ε 2. For these results, the sample size of number of incorporated investigations for some comparisons was larger than 10. Although we can't find any statistical difference between groups, subjects suffering from E2E4 tended to have lower TC levels when compared with those persons with E3E3, subjects suffering from E2E2 tended to have a slightly lower TC level when compared with those subjects with E3E3, and patients suffering from E4E4 tended to have a slightly increased TC when compared with those with E3E3. Subjects with ε 4 also tended to display a slightly increased level of TC compared to those subjects suffering from nonε 4, although the statistical difference was no notable (Table  1), there appeared a tendency for E3E4 to be related to increased levels of TC, and E2E4 related to lower levels of TC.

Relationship between gene polymorphisms of apoE and TG levels
Subjects suffering from E2E2, E2E3 or E4E4 were with increased TG levels when comared with those persons with E3E3 genotype. Persons suffering from ε 4 suffered from higher TG levels when compared with those subjects with ε 3. Patients with ε 2 got an up-regulated TG level than subjects with non-ε 2 ( Table 2). These results suggest that E3E4 or E2E3 is associated with increased levels of TG, and ε 2 and ε 4 are related to the increased TG levels.
Similarly, although the tendencies were similar, no statistical difference for TG was found between subjects with E2E4 vs. E3E3, and ε subjects with E3E4 vs. E3E3. Subjects with ε 4 vs. non-ε 4, and subjects with ε 2 vs. ε 3 (Table 2). Furthermore, no statistical difference was found between subjects with ε 2 vs. ε 4 (Table 2). Thus, there were trends suggesting E2E2, E2E3, E4E4, ε 2 and ε 4 are associated with increased TG levels, whereas E2E4 is associated with reduced levels of TG. Relationship between gene polymorphisms of apoE and HDL levels Subjects suffering from E3E4 got a slightly lower HDL when compared with those persons with E3E3 genotype (Table 3). It indicated that E3E4 genotype was associated with the reduced levels of HDL. No significant differences in HDL were found for subjects with E2E2 vs. those subjects suffering from E3E3, Patients with E2E4, E2E3 or E4E4 vs. those with E3E3, or subjects with ε 2 and ε 4vs. nonε 2 and nonε 4, respectively. Interestingly, subjects with ε 4 tended to get a slightly reduced level of HDL when compared with those persons with ε 2, and ε 3, and subjects with ε 2 tended to have a slightly lower level of HDL than those with ε 3, although the statistical difference was no notable (Table 3). In these studies, ε 4 tended to be associated with lower level of HDL.

Relationship between gene polymorphisms of apoE and LDL levels
Persons suffering from E2E3 or E2E2 had a notably reduced LDL when compared with those patients with E3E3 genotype. Patients suffering from ε 2 had a decreased LDL compared to those patients suffering from ε 3 or ε 4 ( Table 4), suggesting that E2E2, E2E3 and ε 2 are associated with lower levels of LDL. No statis-tical difference was found between subjects with E2E4, E4E4 and E3E3, nor between subjects with E3E4 and E3E3. Subjects with ε 2 tended to have a reduced LDL level than those with non-ε 2, and subjects with ε 4 also tended to have a down-regulated level of LDL than the patients with non-ε 4, (Table 4), and patients with ε 4 have tended toward increased LDL levels than those with ε 3, but again tere were no statistical differences.

Relationship between gene polymorphisms of apoE and VLDL levels
For VLDL, no statistical difference was found between subjects with E2E2, E2E3, E2E4, E3E4 or E4E4 when compared with those patients with E3E3. Subjects suffering from E2E2 genotype got a lower VLDL when compared with those patients with the genotype of E3E3, although no statistical difference was observed.
Patients with ε 2 had a lower VLDL level when compared with those patients with the genotype of ε 3 or ε 4, although no statistical difference was found. Moreover, patients with ε 4 got a increased level of VLDL than the patients with ε 3, although no statistical difference was detected (Table 5). It indicated that ε 4 was related to a increased level of VLDL.

Relationship between gene polymorphisms of apoE and Lp(a) levels
Patients suffering from E2E2, E2E4 or E3E4 genotype had a lower Lp(a) when compared with those patients with E3E3, and persons suffering from E2E3, E4E4 got a increased level of Lp(a) compared to the patients with E3E3 genotype, although no statistical difference was found. Patients with ε 2 got a higher Lp(a) levewhen compared with those with ε 4, ε 3, and subjects with ε 4 had a lower level of Lp(a) than those with ε 3, although there was no statistical difference (Table 6). It indicated that ε 2 was related to a higher Lp(a) level.

Relationship between gene polymorphisms of apoE and apoE levels
Patients suffering from E2E3, or E2E4 got an increased apoE levels when compared with those patients with E3E3 genotype, and patients with E4E4 had a reduced apoE level than those subjects suffering from E3E3. Furthermore, patients suffering from E2E2 got a high-er apoE levels when compared with those patients with E3E3 and subjects suffering from E3E4 got lower apoE levels compared to the patients with E3E3, although no statistical difference was found. Patients suffering from ε 2 had a higher apoE level when compared with those with ε 3 or ε 4. Moreover, patients with ε 4 got a lower apoE level than those subjects with ε 3, although there was no statistical difference ( Table 7). Series of renal diseases, such as NS 53 , glomerulonephritis 54 and chronic kidney disease with dialysis, had high serum TG levels 55 . Increasing levels of TG might be a core sign for the onset susceptibility of renal diseases. This meta-analysis found that subjects with E2E2, E2E3 or E4E4 manifested significantly higher TG levels than those with E3E3. Subjects with ε 4 had a high- er TG than those with ε 3. Subjects with ε 2 showed a highr level of TG than those with non-ε 2. This suggests that E2E3 or E3E4, and ε 2 and ε 4 are related to higher levels of TG. These outcomes were credible to some extent. Stiefel 71 . Whether it play a role in the pathogenesis of renal diseases is not well elucidated. Our study did not showed association between apoE gene polymorphisms and VLDL expression in patients with renal diseases. However, the results might be less robust because less than ten studies was included. Increased serum Lp(a) level might be an core symbol for lots of renal diseases, including NS 72 , chronic kidney disease with dialysis 73 . Whether it plays a role in the pathogenesis of renal diseases, it is not well elucidated. In our study, both subjects with E2E2 and E2E4 had a lower Lp(a) than those with E3E3. Moreover, Lp(a) levels of subjects with E4E4 were higher than those with E3E3. However, the results might be less evident because less than ten studies was included.

Evaluation of publication bias apoE gene polymorphisms with TC levels
Whether apoE gene polymorphisms are associated with apoE expression, and whether low/high levels of apoE are associated with renal diseases are not well elucidated at present. Among subjects with E2E3, E2E4, E3E3, and E4E4, E2E3 and E2E4 had higher apoE levels than others, while E4E4 had a lowest apoE levels. Subjects with ε 2 had a higher level of apoE when compared with those with ε 3 or ε 4. E2E2, or E2E3 might be a protective factor in fighting against renal diseases, while E3/E4 could be a risk factor. Our data also suggest that increased apoE has a protective role in renal diseases, and lower apoE is a risk factor for renal diseases.
It is complicated to assess the roles of apoE in diseases. Lots of studies indicated that apoE is a positive factor when against diseases. apoE has antioxidant activity 74 , and is widely accepted in assisting with providing protection against mesangial cell injury 75 . ApoE also took part in the repairment of tissue injury; for incidence, increased amounts of apoE levels are discovered in sites of peripheral nerve injury and regeneration 76 . ApoE deficency in mice induces the progress of atherosclerosis and re-expression of apoE reduces the extent of the disease 77 . Thus, increased apoE appears to be a protective factor against disease.

Conclusion
ApoE gene polymorphisms are associated with the expression of TC, TG HDL, LDL, Lp(a) or apoE.