Relationship between apolipoprotein E gene polymorphism and total cholesterol level in patients with kidney diseases

Abstract

AIM: To evaluate the association between apolipoprotein E (apoE) gene polymorphism and total cholesterol (TC) level in patients with kidney diseases.

METHODS: A predefined literature search was performed to collect data from the electronic databases of PubMed, Embase and the Cochrane Library and eligible relevant studies reporting the association of apoE gene polymorphism with TC level in patients with kidney diseases were recruited for meta-analysis.

RESULTS: Twenty-one studies were identified for the analysis of association between apoE gene polymorphism and TC level in patients with kidney disease. Subjects with E3E4 had a higher TC than those with E3E3 [weighted mean differences (WMD) = 2.14, P = 0.01] and subjects with E2E3 had a lower TC than those with E3E3 (WMD = -1.93, P = 0.01). Subjects with ε2 had a lower TC than those with ε3 (ε2 vs ε3: WMD = -1.23, P = 0.002; ε2 vs ε4: WMD = -2.77, P ﹤0.0001) and subjects with 3 had a lower TC than those with 4 (WMD = -0.79, P = 0.03).

CONCLUSION: Subjects with apoE E3E4 and ε4 had a higher TC level and subjects with apoE E2E3 and ε2 had a higher TC level in patients with kidney disease. However, more well-designed studies should be performed in the future to confirm these findings.

Key Words: Apolipoprotein E, Gene polymorphism, Total cholesterol, Meta-analysis

Core tip: The available evidence for an association between apolipoprotein E (apoE) and total cholesterol (TC) in kidney disease is weak and has conflicting results. This meta-analysis based on 21 included studies indicated that subjects with apoE E3E4 and 4 had a higher TC level and subjects with apoE E2E3 and ε2 had a higher TC level in patients with kidney disease.

INTRODUCTION

Apolipoprotein E (apoE) is an important protein of the lipoprotein transport system and plays an important role in lipoprotein metabolism and lipid homeostasis[1-3]. It is associated with the metabolism of total cholesterol (TC) and triglyceride[4,5]. ApoE, a 229-amino-acid polypeptide, is classified into three major isoforms (ε2, ε3 and ε4) according to the differences in amino acids at positions 112 and 158, forming six genotypes: E2E2, E2E3, E2E4, E3E3, E3E3 and E4E4[6,7]. ε3 and E3E3 are the wild type of apoE and ε2, ε4, E2E2, E2E3, E2E4, E3E3 and E4E4 are the mutation type of apoE.

The represent one of the major systems that maintain the body homeostasis[8]. Abnormalities of lipid metabolism and homeostasis are commonly present in patients with kidney disease[9], such as nephrotic syndrome (NS). Most kidney diseases are associated with elevated serum/plasma concentration and the impaired clearance of very low density lipoprotein and their atherogenic remnants. Nephrotic dyslipidemia is a risk factor for the development of systemic atherosclerosis, it may aggravate the glomerulosclerosis lesion and enhance the progression of glomerular disease[10]. apoE lipoprotein plays an important role in lipid homeostasis and kidney diseases.

In renal tissue, apoE is mainly synthesized by mesangial cells under normal physiological conditions[11,12]. Investigators reported that apoE had a protective role for the kidney[13,14]. Some other investigations found that the apoE expression in renal glomerulus was elevated when compared with that of the normal group[15-19]. There were some investigations reporting that apoE polymorphism was associated with renal diseases. In our previous study[20], we reported that the apoE gene expression was associated with the NS susceptibility in experimental studies and the apoE ε3ε3, ε3ε4, ε3 and ε4 were associated with the onset of NS in human studies. Hayakawa et al[21] carried out an investigation on a patient with apoE ε3ε3 variant without lipoprotein glomerulopathy and illustrated that not all apoE variants resulted in lipoprotein glomerulopathy and that the location of mutations in the apoE protein was one of the important determinants for the development of lipoprotein glomerulopathy. Luo et al[22] reported that apoE mutation was associated with the lipoprotein glomerulopathy risk and found that the apoE mutation could cause a marked molecular conformational change of apoE and thus impaired its binding ability to lipids.

Some reports show that apoE gene polymorphism is associated with the expression of TC. However, some studies found that apoE gene polymorphism was not associated with the expression of TC. The available evidence for an association between apoE and TC in kidney disease is weak due to paucity of data or disagreements among the reported studies. Thus, evidence from a meta-analysis may be more powerful compared to single studies[23]. To date, no meta-analysis exists that determines the association between apoE gene polymorphism and TC level in patients with kidney diseases. This meta-analysis was performed to investigate the association of the apoE gene polymorphism with the TC level in patients with kidney diseases.

MATERIALS AND METHODS

Search strategy

The relevant studies were screened from the search engines of PubMed, Embase and the Cochrane Library on March 1, 2012. “(apoE AND apolipoprotein E) AND (renal OR kidney)” was used in PubMed, Embase and the Cochrane Library. We also extended our search spectrum to the “related articles” and the bibliographies of all retrieved studies. If multiple publications from the same study group using the same data were reported, we only recruited the most complete study for our analysis.

Inclusion and exclusion criteria

Inclusion criteria: (1) the study had to be about kidney diseases; (2) provide detailed gene distribution of apoE; and (3) report on the level of TC.

Exclusion criteria: (1) Editorials, reviews, case reports; (2) data from multiple publications; (3) investigating the association of other genes in kidney diseases; and (4) investigating the role of apoE in diseases other than kidney diseases.

Data extraction and synthesis

The following information was extracted from each study independently by the investigators: first author’s surname, year of publication, location of study, patient type, level of TC and the number of subjects. Study type, comparisons of apoE gene polymorphism and methodological quality assessment outcomes were also extracted. The results were compared and disagreements were resolved by discussion.

Statistical analysis

Available data was entered into Cochrane Review Manager (RevMan, Version 5) and analyzed. The pooled statistic was counted using the fixed effects model but a random effects model was conducted when the P value of heterogeneity test was less than 0.1. Results were expressed with weighted mean differences (WMD) for continuous data and 95%CI were also counted. P < 0.05 was required for the overall WMD to be deemed statistically significant. I² was used to test the heterogeneity between the included studies. The Begg adjusted rank correlation test[24] and the Egger regression asymmetry test[25] were used for exploring publication bias (P < 0.1 was considered significant) when the number of the included studies was more than fifteen.

RESULTS

Study characteristics for the relationship between apoE gene polymorphism with TC expression

Twenty-one studies were included in the meta-analysis for the relationship between apoE gene polymorphism with TC expression (Figure 1 and Table 1). One study[26] was for the comparison of E2E2 vs E3E3. Eleven reports[26-36] were recruited into the study of E2E3 vs E3E3 (including 13 comparisons). Three reports[26,27,29] were recruited into the study of E2E4 vs E3E3 (including 3 comparisons). 11 reports[26-36] were recruited into the study of E3E4 vs E3E3 (including 13 comparisons). Three reports[26,27,29] were recruited into the study of E4E4 vs E3E3 (including 3 comparisons). Two reports[37] were included for the meta-analysis of ε2 vs non-ε2 (including 2 comparisons). Three studies[38-40] were recruited into our meta-analysis for the comparison of ε4 with non-ε4 (including 4 comparisons). 13 studies[27,28,30-36,41-45] were included into the study of ε2 vs ε3 (including 20 comparisons). Fourteen studies[27,28,30-36,41,42,44-46] were included into the study of ε4 vs ε4 (including 20 comparisons). Thirteen studies[27,28,30-36,41,42,44,45] were included into the study of ε2 vs ε4 (including 19 comparisons).

Table 1 The detailed characteristics of included studies.

Author	Study type	Location of study	Ethnicity	Patient type	Number of subjects	Comparisons of apoE gene polymorphism	Genotyping method reported	Blinding of genotyping
Feussner et al[26]	Prospective	Germany	Caucasian	ESRD	141 males and 104 females	E2E2, E2E3, E2E4, E3E3, E3E4, E4E4	No	Not mention
Eggertsen et al[29]	Prospective	Sweden	Caucasian	ESRD: 19 cases glomerulonephritis, 11 cases diabetic nephropathy, nine cases interstitial nephritis	25 males and 26 females	E2E3, E2E4, E3E3, E3E4, E4E4	Yes	Not mention
Oda et al[28]	Prospective	Japan	Asian	107 with GN and 399 with ESRD	Patients with GN consisted of 42 men and 65 women	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Lim et al[38]	Prospective	China	Asian	ESRD: 85 patients with chronic GN, 30 patients with DN, 18 patients with chronic pyelonephritis, 3 patients with PKD	96 males and 60 females	ε4, non-ε4	Yes	Not mention
Kimura et al[39]	Prospective	Japan	Asian	DN	88 men and 90 women	ε4, non-ε4	Yes	Not mention
Werle et al[30]	Prospective	Germany	Caucasian	DN	159 men and 129 women	E2E3, E3E3, ε2, ε3, ε4	Yes	Not mention
Imura et al[31]	Prospective	Japan	Asian	ESRD	287 men and 206 women	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Xiang et al[43]	Prospective	China	Asian	DN	26 men and 20 women	ε2, ε3	Yes	Not mention
Güz et al[32]	Prospective	Turkey	Caucasian	ESRD: GN (107 cases), hypertension nephropathy (37 cases), unknown (36 cases), pyelonephritis (29 cases), amyloidosis (20 cases), DN (15 cases), obstructive uropathy (9 cases), PKD (7 cases), toxic nephropathy (6 cases), and Alport’s syndrome (3 cases)	149 men and 112 women	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Zahálková et al[34]	Prospective	Czech	Caucasian	ESRD	53 males and 34 females	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Lehtinen et al[40]	Prospective	Finland	Caucasian	DN	Not mention	ε4, non-ε4	Yes	Not mention
Kahraman et al[42]	Prospective	Turkey	Caucasian	Renal transplant recipients	80 males and 38 females	ε2, ε3, ε4	Yes	Not mention
Joss et al[36]	Prospective	United Kingdom	Caucasian	DN	Not mention	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Maluf et al[45]	Prospective	United States	Mix	Renal transplant recipients	21 males and 18 females	ε2, ε3, ε4	Yes	Not mention
Arikan et al[35]	Prospective	Turkey	Caucasian	ESRD	84 males and 60 females	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Kwon et al[41]	Retrospective	Korea	Asian	DN	32 males and 62 females	ε2, ε3, ε4	No	Not mention
Leiva et al[46]	Retrospective	Chile	South America	DN	53 males and 32 females	ε3, ε4	Yes	Not mention
Ma et al[37]	Prospective	China	Asian	DN	146 males and 259 females	ε2, non-ε2	Yes	Not mention
Erdogan et al[33]	Prospective	Turkey	Caucasian	DN	19 males and 27 females	E2E3, E3E3, E3E4, ε2, ε3, ε4	Yes	Not mention
Hu et al[27]	Prospective	China	Asian	MCNS	176 males and 74 females	E2E2, E2E3, E2E4, E3E3, E3E4, E4E4, ε2, ε3, ε4	Yes	Not mention
Li et al[44]	Prospective	China	Asian	Renal transplant recipients	59 males and 46 females	ε2, ε3, ε4	Yes	Not mention

ESRD: End-stage renal disease; GN: Glomerulonephritis; DN: Diabetic nephropathy; PKD: Polycystic kidney disease; MCNS: Minimal change nephrotic syndrome.

Figure 1 Flow diagram for this meta-analysis.

Association of apoE gene polymorphism with TC expression

In this meta-analysis, the number of included studies for some comparisons was more than ten (E2E3 vs E3E3, E3E4 vs E3E3, ε2 vs ε3, ε4 vs ε3, ε2 vs ε4) and the number of included studies for some comparisons was less ten (E2E2 vs E3E3, E2E4 vs E3E3, E4E4 vs E3E3, ε2 vs non-ε2, ε4 vs non-ε4). Those results from less than ten might be less robust. We presented those results independently.

Subjects with E3E4 had a higher TC than those with E3E3 (Figure 2A and Table 2) and subjects with E2E3 had a lower TC than those with E3E3 (Figure 2B and Table 2). Subjects with ε2 had a lower TC than those with ε3 or ε4 (Figure 2C for ε3 and Figure 2D for ε4; Table 2) and subjects with ε4 had a higher TC than those with ε3 (Figure 2E and Table 2). It seemed that E3E4 and ε4 were associated with higher level of TC and E2/E3 and ε2 were associated with lower level of TC. The number of included studies for some comparisons was more than ten and those results might be robust to some extent.

Table 2 Meta-analysis of the association of apoE gene polymorphism with total cholecterol level.

Genetic comparisons	Q test	P contrasts	Model seclected	Weighted mean differences (95%CI)	P value
E2E2 vs E3E3	1	-	Fixed	109.00 (-32.07-250.07)	0.13
E2E3 vs E3E3	13	< 0.00001	Random	-1.93 (-3.39--0.46)	0.01
E2E4 vs E3E3	3	0.48	Random	-2.48 (-4.23--0.72)	0.006
E3E4 vs E3E3	13	< 0.00001	Random	2.14 (0.46-3.83)	0.01
E4E4 vs E3E3	3	0.02	Random	0.30 (-4.25-4.84)	0.90
ε2 vs non-ε2	2	0.57	Fixed	-0.25 (-0.43--0.08)	0.005
ε4 vs non-ε4	4	0.0001	Random	0.20 (-1.42-1.83)	0.81
ε2 vs ε3	20	< 0.00001	Random	-1.23 (-1.99--0.46)	0.002
ε3 vs ε4	20	< 0.00001	Random	-0.79 (-1.50--0.08)	0.03
ε2 vs ε4	19	< 0.00001	Random	-2.77 (-4.05--1.49)	< 0.0001

Figure 2 Association of apoE gene polymorphism with total cholesterol level. A: Using the comparison of E3E4 vs E3E3; B Using the comparison of E2E3 vs E3E3; C: Using the comparison of ε2 vs ε3; D: Using the comparison of ε2 vs ε4; E: Using the comparison of ε3 vs ε4.

Subjects with E2E4 had a much lower TC level when compared with those with E3E3. Subjects with ε2 had a lower level of TC than those with non-ε2. Subjects with E2E2 or E4E4 had a slightly higher TC than those with E3E3, although there was no statistical difference. Subjects with 4 had a similar level of TC than those with non-ε4 (Table 2). It seemed that E2E4 and ε2 was associated with lower level of TC. The number of included studies for some comparisons was less than ten and more studies should be performed in the future.

Publication bias test for the association of apoE gene polymorphism with TC expression

There was a publication bias test for the comparisons of E2E3 vs E3E3, E3E4 vs E3E3, ε2 vs ε3, ε4 vs ε3, ε2 vs ε4. There was no publication bias for the comparisons of E2E3 vs E3E3, E3E4 vs E3E3, ε2 vs ε3, ε4 vs ε3, ε2 vs ε4 (E2E3 vs E3E3: Begg P = 0.951, Egger P = 0.936; E3E4 vs E3E3: Begg P = 0.951, Egger P = 0.710; ε2 vs ε3: Begg P = 0.871, Egger P = 0.970; ε4 vs ε3: Begg P=0.315, Egger P = 0.251; ε2 vs ε4: Begg P=0.294, Egger P = 0.495).

DISCUSSION

High serum TC level is an important characteristic for many kidney diseases, such as NS[47], glomerulonephritis[48], end-stage kidney disease requiring dialysis[49], etc. Raised TC is a consequence of kidney disease but may accelerate progression to end-stage kidney disease. In our meta-analysis, we found that subjects with E3E4 had a higher 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. There was no publication bias test for the comparisons of E2E3 vs E3E3, E3E4 vs E3E3, ε2 vs ε3, ε4 vs ε3, ε2 vs ε4. E3E4 and ε4 were associated with higher level of TC and E2/E3 and ε2 were associated with lower level of TC. The conclusions for those comparisons were robust to some extent. Toms et al[50] reported that the ε2 allele was associated with the lowest and ε4 allele with the highest level of TC in rheumatoid arthritis patients. Alvim et al[51] reported that the ε4 allele was associated with higher TC value in healthy urban Brazilian individuals. Rahimi et al[52] found that ε4 allele resulted in a significant increase in the level of TC in sickle cell disease. Smart et al[53] reported that significantly higher TC was observed in apoE ε4 carriers compared to E3E3 homozygotes and ε2 carriers in 882 Greek children. The results from those studies mentioned above were similar to our results in this meta-analysis.

The roles of apoE in diseases were also complicated. Many studies reported that apoE could play a protective role against diseases. apoE can play an antioxidant role[54], has been demonstrated to play an important role in providing protection against mesangial cell injury[55] and also appears to be involved in the repair response to tissue injury; for example, markedly increased amounts of apoE are found at sites of peripheral nerve injury and regeneration[56]. apoE deficiency in mice leads to the development of atherosclerosis and re-expression of the protein reduces the extent of the disease[57]. It appears that the increased apoE is a protective factor against disease progression. However, more studies need to be performed to explore the role of apoE in diseases.

There were some meta-analyses to evaluate the relationship between apoE gene polymorphism and TC level. Anthopoulos et al[58] performed a meta-analysis to detect the association of apoE gene polymorphism with TC level in patients with type 2 diabetes and the meta-regression analysis provided some weak evidence that the risk conferred by ε2 allele was mediated through altering serum TC level. Our study reported the association of apoE gene polymorphism with increased TC levels in patients with kidney diseases and showed that apoE gene polymorphism was associated with the increased levels of TC in patients with kidney disease.

In this meta-analysis, it was difficult to conduct the methodological quality assessment of included studies for the reason that the information in the included studies was few. More well-designed studies should be performed in the future.

In conclusion, our meta-analysis shows that apoE gene polymorphism is associated with increased levels of TC. However, larger studies should be performed in the future to confirm this association.

COMMENTS

Background

Apolipoprotein E (apoE), one of the major plasma lipoproteins, plays a major role in the transport and metabolism of lipids by acting as a ligand. apoE gene contains three potential alleles: ε2, ε3 and ε4, forming six genotypes: E2E2, E2E3, E2E4, E3E3, E3E3 and E4E4. An association between apoE gene polymorphism and total cholesterol (TC) level is still controversial.

Innovations and breakthroughs

To date, no meta-analysis exists that determines the association between the apoE gene polymorphism and TC level in patients with kidney diseases. This meta-analysis was performed to investigate the association of apoE gene polymorphism with the TC level in patients with kidney diseases.

Peer review

This paper performed a meta-analysis to investigate the relationship between apoE gene polymorphism and TC level in patients with renal diseases. The paper presents an interesting topic.