diabetic nephropathy ( dn ) is the leading cause of chronic renal disease and a major cause of cardiovascular mortality . several factors are involved in the pathophysiology of dn , including metabolic and hemodynamic alterations , oxidative stress , activation of the renin - angiotensin system , immunoregulatory cytokines   and genetic factors . the 2 main risk factors for diabetic nephropathy are hyperglycemia and arterial hypertension , but the genetic susceptibility in type 1 and type 2 diabetes is of great importance . previous studies have shown that type 2 diabetes ( t2d ) is a metabolic disorder characterized by hyperglycemia , developing insulin resistance , -cell dysfunction , and impaired insulin secretion . as the incidence of type 2 diabetes continues to rise world - wide , the personal and social burdens associated with this complication are becoming increasingly serious . a familial study has provided compelling evidence that genetic factors contribute to dn susceptibility in t2d   as have studies aimed at identifying the causal genes responsible for its development . the apolipoprotein e ( apo e ) gene , located on chromosome 19q13.2 , has 3 common alleles  2 , 3 , and 4  coding for the 3 main isoforms of the apo e protein : 2 ( argcys ) , 3 ( parent isoform ) , and 4 ( argcys ) . there are 6 common apo e polymorphisms : apo 3/3 , apo 4/4 , apo 2/2 , apo 3/2 , apo 4/2 , and apo 4/3 . many studies have investigated gene apo e polymorphism effects on susceptibility to type 2 diabetic nephropathy , and we have summarized the findings of those individual studies in the appendix 1 . one of its advantages is to increase the sample size , which may reduce the probability that random error will result in a false - positive or false - negative association . therefore , we performed a meta - analysis to quantitatively assess the association of apo e gene polymorphisms with dn . the medline , pubmed , embase , and web of science were searched ( the last search was updated on june , 10 , 2014 using the search terms :  diabetic nephropathy  or all searched studies were retrieved and their bibliographies were checked for other relevant publications . review articles and bibliographies of other relevant identified studies were hand - searched in addition to eligible studies . only published studies with full - text articles were included . when more than one of the same patient populations was included in several publications , only the one with the sample size largest or the most complete study was used in this meta - analysis . a flow diagram of the study selection process is shown in figure 1 . the inclusion and exclusion criteria were determined by discussion . the inclusion criteria were : ( 1 ) the study aimed to examine the association between apo e polymorphisms and susceptibility to dn ; ( 2 ) the design type of the study was a case - control study ; ( 3 ) the study used diabetic patients without nephropathy or healthy subjects as controls ; ( 4 ) the study provided the number of dn cases or controls and the frequency of apo e genotypes . the exclusion criteria were : ( 1 ) the study did not fit the diagnosis criteria ; ( 2 ) the study was conducted on animals ; ( 3 ) the study was not a case - control study ; ( 4 ) the study reported useless data ; ( 5 ) the study focused on type 1 diabetic subjects . all of the data were extracted independently by 2 reviewers ( yijin lin and jinlin pan ) according to the pre - specified selection criteria . the following data were extracted : control type , diabetic duration , study design , first authors name , publication year , and number of cases with normoalbuminuria , microalbuminuria , and macroalbuminuria , and number of healthy controls . allele frequencies at the apo e single - nucleotide polymorphisms ( snps ) from the studies were determined by the allele counting method . statistical analysis was conducted using stata 11.0 ( statacorp , college station , tx ) and a p - value 0.05 was considered to be statistically significant . dichotomous data are presented as the odds ratio ( or ) with a 95% confidence interval ( ci ) . statistical heterogeneity was measured using the q - statistic ( p0.10 was considered to be representative of statistically significant heterogeneity ) . we also quantified the effect of heterogeneity using the i statistic , which measures the degree of inconsistency in the studies by calculating what percentage of the total variation across studies is due to heterogeneity rather than by chance . a fixed - effects model was used when there was no heterogeneity of the results of the trials ; otherwise , the random effects model was used . for dichotomous outcomes , patients with incomplete or missing data and small - sample studies to establish the effect of clinical heterogeneity between studies on the conclusions of meta - analyses , subgroup analysis was conducted on the basis of race . begg s rank correlation method and egger s weighted regression method were also used to statistically assess the publication bias ( p0.05 was considered to be representative of statistically significant publication bias ) . the medline , pubmed , embase , and web of science were searched ( the last search was updated on june , 10 , 2014 using the search terms :  diabetic nephropathy  or all searched studies were retrieved and their bibliographies were checked for other relevant publications . review articles and bibliographies of other relevant identified studies were hand - searched in addition to eligible studies . only published studies with full - text articles were included . when more than one of the same patient populations was included in several publications , only the one with the sample size largest or the most complete study was used in this meta - analysis . the inclusion criteria were : ( 1 ) the study aimed to examine the association between apo e polymorphisms and susceptibility to dn ; ( 2 ) the design type of the study was a case - control study ; ( 3 ) the study used diabetic patients without nephropathy or healthy subjects as controls ; ( 4 ) the study provided the number of dn cases or controls and the frequency of apo e genotypes . the exclusion criteria were : ( 1 ) the study did not fit the diagnosis criteria ; ( 2 ) the study was conducted on animals ; ( 3 ) the study was not a case - control study ; ( 4 ) the study reported useless data ; ( 5 ) the study focused on type 1 diabetic subjects . all of the data were extracted independently by 2 reviewers ( yijin lin and jinlin pan ) according to the pre - specified selection criteria . the following data were extracted : control type , diabetic duration , study design , first authors name , publication year , and number of cases with normoalbuminuria , microalbuminuria , and macroalbuminuria , and number of healthy controls . allele frequencies at the apo e single - nucleotide polymorphisms ( snps ) from the studies were determined by the allele counting method . statistical analysis was conducted using stata 11.0 ( statacorp , college station , tx ) and a p - value 0.05 was considered to be statistically significant . dichotomous data are presented as the odds ratio ( or ) with a 95% confidence interval ( ci ) . statistical heterogeneity was measured using the q - statistic ( p0.10 was considered to be representative of statistically significant heterogeneity ) . we also quantified the effect of heterogeneity using the i statistic , which measures the degree of inconsistency in the studies by calculating what percentage of the total variation across studies is due to heterogeneity rather than by chance . a fixed - effects model was used when there was no heterogeneity of the results of the trials ; otherwise , the random effects model was used . for dichotomous outcomes , patients with incomplete or missing data and small - sample studies to establish the effect of clinical heterogeneity between studies on the conclusions of meta - analyses , subgroup analysis was conducted on the basis of race . begg s rank correlation method and egger s weighted regression method were also used to statistically assess the publication bias ( p0.05 was considered to be representative of statistically significant publication bias ) . this meta - analysis included 16 relevant studies of apo e snps , with 1754 cases and 3912 controls . the aim of this study was to use the meta - analysis method to quantitatively summarize the results from the selected individual studies . in comparing dn cases versus diabetic patients without nephropathy , our was aim to evaluate the relationship between apo e polymorphisms on the progress of diabetic patients . the carriers of the apo 2 allele were more likely to have dn than the over - all group , the east asia group , and the japan group , but not in the 3 other subgroups ( 2 allele vs. 3 allele : over - all : pooled or=1.669 , 95% ci=1.1942.332 , p=0.003 ; east asia group : pooled or=1.667 , 95% ci=1.1502.417 , p=0.007 ; japan group : pooled or=2.352 , 95% ci=1.2284.502 , p=0.010 . 2 group vs. 3 group : east asia group : pooled or=1.829 , 95% ci=1.2352.711 , p=0.003 ; japan group : pooled or=3.085 , 95% ci=1.8525.140 , p<0.001 ) ( table 1 and figure 2 ) to understand the influence of diabetes duration on the development of diabetes , we divided the included studies into 2 parts by duration of diabetes , comparing the group with > 10 years duration versus the group with duration < 10 years . as table 1 and figure 3 show , the carriers of the apo 2 allele were associated with progression of dn in the duration > 10 years group , but not in the duration < 10 years group ( 2 allele vs. 3 allele : pooled or=1.920 , 95% ci=1.3382.754 , p<0.001 ; 2 group vs. 3 group : pooled or=1.667 , 95% ci=0.9462.936 , p=0.077 ) . the aim of comparing dn cases and healthy people was to estimate the association of the apo e polymorphisms and susceptibility to dn . the apo 2 polymorphism increased the susceptibility to dn in the asian population ( 2 allele vs. 3 allele : pooled or=1.629 , 95% ci=1.0102.628 , p=0.045 ; 2 group vs. 3 group : pooled or=1.531 , 95% ci=0.9642.432 , p=0.071 ) ( figure 4 ) . to further verify the association of development of dn and apo e polymorphisms , we quantitatively summarized the results of microalbuminuria versus normoalbuminuria and macroalbuminuria versus normoalbuminuria . apo 2 allele polymorphism was associated with the progression of dn ( table 2 ) . there were 3 prospective studies among the papers included in this meta - analysis , and the pooled results verified the conclusion of the case - control studies  the apo 2 allele polymorphism was a risk factor in the development of dn ( progression vs. non - progression : 2 allele vs. 3 allele : pooled rr=1.636 , 95% ci=1.0932.449 , p=0.017 ; 2 group vs. 3 group pooled rr=1.711 , 95% ci=1.1242.606 , p=0.012 ) . 2 allele vs. 3 allele group among  progression vs. non - progression , but there were no other result supporting this conclusion ( table 2 ) . the heterogeneity was calculated among all studies using the q - statistic ( q>0.05 ) and the i statistic ( i=0.0% ) . heterogeneity was found in some groups , and the random - effects model was used . a single study was deleted each time to investigate the influence of the individual dataset on the pooled ors . the corresponding pooled ors were not materially altered ( data not shown ) , indicating that our results are statistically robust . begg s funnel plot and egger s test were performed to assess the publication bias of the literature . we found no asymmetry of the funnel plot , suggesting that there was no publication bias in our meta - analysis . this meta - analysis included 16 relevant studies of apo e snps , with 1754 cases and 3912 controls . the aim of this study was to use the meta - analysis method to quantitatively summarize the results from the selected individual studies . in comparing dn cases versus diabetic patients without nephropathy , our was aim to evaluate the relationship between apo e polymorphisms on the progress of diabetic patients . the carriers of the apo 2 allele were more likely to have dn than the over - all group , the east asia group , and the japan group , but not in the 3 other subgroups ( 2 allele vs. 3 allele : over - all : pooled or=1.669 , 95% ci=1.1942.332 , p=0.003 ; east asia group : pooled or=1.667 , 95% ci=1.1502.417 , p=0.007 ; japan group : pooled or=2.352 , 95% ci=1.2284.502 , p=0.010 . 2 group vs. 3 group : east asia group : pooled or=1.829 , 95% ci=1.2352.711 , p=0.003 ; japan group : pooled or=3.085 , 95% ci=1.8525.140 , p<0.001 ) ( table 1 and figure 2 ) to understand the influence of diabetes duration on the development of diabetes , we divided the included studies into 2 parts by duration of diabetes , comparing the group with > 10 years duration versus the group with duration < 10 years . as table 1 and figure 3 show , the carriers of the apo 2 allele were associated with progression of dn in the duration > 10 years group , but not in the duration < 10 years group ( 2 allele vs. 3 allele : pooled or=1.920 , 95% ci=1.3382.754 , p<0.001 ; 2 group vs. 3 group : pooled or=1.667 , 95% ci=0.9462.936 , p=0.077 ) . the aim of comparing dn cases and healthy people was to estimate the association of the apo e polymorphisms and susceptibility to dn . the apo 2 polymorphism increased the susceptibility to dn in the asian population ( 2 allele vs. 3 allele : pooled or=1.629 , 95% ci=1.0102.628 , p=0.045 ; 2 group vs. 3 group : pooled or=1.531 , 95% ci=0.9642.432 , p=0.071 ) ( figure 4 ) . to further verify the association of development of dn and apo e polymorphisms apo 2 allele polymorphism was associated with the progression of dn ( table 2 ) . there were 3 prospective studies among the papers included in this meta - analysis , and the pooled results verified the conclusion of the case - control studies  the apo 2 allele polymorphism was a risk factor in the development of dn ( progression vs. non - progression : 2 allele vs. 3 allele : pooled rr=1.636 , 95% ci=1.0932.449 , p=0.017 ; 2 group vs. 3 group pooled rr=1.711 , 95% ci=1.1242.606 , p=0.012 ) . 2 allele vs. 3 allele group among  progression vs. non - progression , but there were no other result supporting this conclusion ( table 2 ) . the heterogeneity was calculated among all studies using the q - statistic ( q>0.05 ) and the i statistic ( i=0.0% ) . heterogeneity was found in some groups , and the random - effects model was used . a single study was deleted each time to investigate the influence of the individual dataset on the pooled ors . the corresponding pooled ors were not materially altered ( data not shown ) , indicating that our results are statistically robust . begg s funnel plot and egger s test were performed to assess the publication bias of the literature . we found no asymmetry of the funnel plot , suggesting that there was no publication bias in our meta - analysis . diabetic nephropathy ( dn ) is a major contributor to the high mortality of patients with dm . several acquired risk factors , such as abnormal lipoprotein metabolism , hypertension , and hyperglycemia , have been identified for the development of dn . studies of patients with type 2 dm have shown either that the 2 allele is a risk factor for dn or no association between apo e polymorphism and dn exists in asian populations . a study conducted in korean patients with type 2 dm found that the apo 2 allele was significantly more frequent in the macroalbuminuria group compared with the normoalbuminuria group . a japanese study involving 158 patients with long - term type 2 dm obtained similar results , showing that the 2 allele could increase the risk of dn , and 4 was a protective factor . conversely , there are conflicting results regarding the impact of allele 2 and 4 on the development of dn . the apo 2 allele did not appear to be associated with increased risk of renal impairment in chinese type 2 diabetic patients   and a study indicated that the 4 allele of the apo e polymorphism is one of the prognostic risk factors involved in the development of dn with type 2 diabetes mellitus . the results of this study suggest that apo 2 allele is more likely to increase the risk of dn , while apo 4 allele is not associated with the dn development and susceptibility in an east asia population . specifically , the or value of most included studies ( 3/15 ) were larger than 1 when the 2 allele and apo 3 was compared ( figure 2 ) . this finding indicates that the negative results of those studies might be due to inadequate sample size . in addition to the sample size , another reason for this inconsistency is the duration of diabetes in the dn and non - dn groups . this meta - analysis shows that , in most individual studies in patients with diabetes duration > 10 years group , there is a significant correlation between dn and apo 2 , but none of the studies had positive results in subgroups of patients with diabetes duration < 10 years ( figure 3 ) . the defective ability of the apo 2 isoform to bind to apo e receptors may increase the risk of dn . firstly , because only published studies were included in the meta - analysis , publication bias may have occurred , even though it was not found by statistical tests . secondly , a meta - analysis essentially retains the methodological deficiencies of the included studies . finally , this meta - analysis is based on unadjusted estimates , while a more precise analysis could be performed if individual data were available . in conclusion , in spite of several limitations mentioned above , this meta - analysis suggests that apo 2 mutation increased the development of dn , especially in east asian populations .
backgroundmany studies have determined the correlation between the apolipoprotein e ( apo e ) gene polymorphisms and diabetic nephropathy , but their results are inconclusive.material/methodswith the aim to confirm this correlation , we performed a meta - analysis of 16 studies . 
 the dichotomous data are presented as the odds ratio ( or ) with a 95% confidence interval ( ci).resultsthe results of our study indicate that apo 2 allele among the pooled asian populations were more likely to show high risk of dn development ( 2 allele vs. 3 allele : pooled or = 1.629 , 95% ci=1.0102.628 , p=0.045 ) . for further analysis , the apo 2 allele was associated with progress of dn in the group with duration > 10 years , but not in the group with duration < 10 years ( 2 allele vs. 3 allele : pooled or=1.920 , 95% ci=1.3382.754 , p<0.001 ) . 
 the apo 2 polymorphism increased the susceptibility to dn in asian population compared with healthy people ( 2 allele vs. 3 allele : pooled or=1.629 , 95% ci=1.0102.628 , p=0.045).conclusionsdevelopment of dn is associated with apo e polymorphisms in asian populations , especially in east asians .