Following on previous work, we replicated the inverse association between APOE
e4 allele and low-GFR cases in non-Hispanic whites. The APOE
e4 allele was significantly associated with decreased prevalence of low-GFR cases in non-Hispanic whites, and higher levels of continuous GFR. Individuals with a copy of the e4 allele had 24% lower odds of a GFR less than 75 ml/min/1.73 m2
, and, on average, a 2.57 ml/min/1.73 m2
higher GFR. The e2 allele did not associate with low-GFR cases (or lower levels of continuous GFR). We observed a weak, but significant, association between increasing APOE
summary score points (greater number of e2 alleles) and higher prevalence of low-GFR cases. Additionally, this observed association between APOE
summary score and low-GFR cases was independent of a kidney damage mechanism, as seen in the model adjusting for albuminuria. We had previously implemented the APOE
summary score to detect an association of APOE
variation with kidney function [28
], and obtained similar effects after full adjustment for confounders. In light of previously published studies demonstrating positive relationship between APOE
variants and CKD, the suggestive evidence from this study, and considering the wide age range and the non-clinical nature of NHANES III, we believe our results support and replicate the association between genetic variations of APOE
and low-GFR cases in non-Hispanic whites.
While we did not observe a significant association of APOE
polymorphisms with prevalence of low-GFR in non-Hispanic blacks due to low power (see Univariate Association
) we did observe an association between the e2 polymorphism and lower levels of continuous GFR. Additionally, the odds ratios for blacks for e2, e4 and the APOE
score method from our study are consistent with the hazard ratios of APOE
and progression of CKD estimated from African Americans in the Atherosclerosis Risk in Communities (ARIC) study [28
]. No association was detected between APOE
polymorphism and prevalence of low-GFR or continuously measured GFR in Mexican Americans. The APOE
e2 and e4 allele frequencies were very low in Mexican Americans and our analysis did not have enough power to detect an association (see Univariate Association
), even if one existed.
This is the first study of APOE
and estimated kidney function in Mexican Americans and establishes population-based allele frequencies in a nationally representative population of Mexican Americans. APOE
variation is uncommon in Mexican Americans. The frequency of the e2 allele is 3.5% and the e4 allele is 11% in Mexican Americans from NHANES III. These estimates are consistent with results from smaller regional cohorts, e2: 2.4 - 4.8% and e4: 6.9 - 9.9% [50
]. The combination of low allele frequency and low prevalence of estimated GFR <75 ml/min/1.73 m2
(6.4%) in this population resulted in an underpowered analysis to detect the significant association observed in non-Hispanic whites. Moreover, the lack of observed association in this population could be due to the need for a validated equation to estimate kidney function in Mexican Americans, but could also indicate APOE
variants are not associated with or have minimal effect on kidney function in this population.
The odds ratios obtained from analysis of APOE
summary score and low-GFR cases were similar to the estimates previously observed in ARIC - an increase in the number of e2 alleles was observed with an increased prevalence of low-GFR cases, or alternately, the decrease in the number of e2 alleles was associated with lower prevalence of low-GFR cases [28
]. Our adjusted estimates were not significant, possibly due to the smaller sample size of NHANES III genetic study compared to ARIC. Nevertheless, the direction of association and magnitude of the association are consistent between the two studies.
The inverse association between the e4 allele and prevalence of low-GFR cases (GFR<75 ml/min/1.73 m2
) is an important finding in determining whether APOE
variation is a risk factor for kidney disease. Studies of end-stage renal disease (ESRD) patients show lower prevalence of the e4 allele in ESRD than the general population [53
]. The e4 allele and e3/e4 - e4/e4 genotypes are found in lower frequency in patients with glomerular nephropathy [21
]. The e4 allele was found to be protective for onset and progression of diabetic nephropathy in type 2 diabetes [20
Greater prevalence of the e2 allele has been observed in ESRD patients compared to the general population [53
]. The e2 allele was found to be a risk factor for onset and progression of diabetic nephropathy in type 2 diabetes [13
]. The e2 allele was also found to be associated with development of diabetic nephropathy in type 1 diabetics [14
]. Several studies have produced null results from investigations of ApoE and renal disease, but were generally underpowered [20
]. A substantial body of research has consistently demonstrated the e2 allele is associated with increased risk of renal dysfunction and the e4 allele is inversely associated with risk of renal disease.
In order to assess whether this relationship is causal, additional investigations into the molecular pathways of different forms of ApoE on the pathophysiology of CKD and kidney function need to be carried out. The pathway by which ApoE acts on the kidney has yet to be precisely delineated. However, dysfunctional kidney repair functions (ineffective remodeling and mesangial cell proliferation) have been implicated in previous studies [24
]. Change in membrane permeability also may lead to derangement of kidney function.
Differences in APOE
genotype frequencies may account for some of the differences in low-GFR prevalence across ethnic subpopulations. In non-Hispanic blacks, we expected higher e2 allele frequencies due to an increased prevalence of CKD, and we expected lower frequencies of the e2 allele in Mexican-Americans due to lower prevalence of CKD in this population. We have identified a very low prevalence of the e2 allele and thus low frequencies of the e2/e2, e2/e3 and e2/e4 genotypes in a population based sample of Mexican-Americans, consistent with previously published results [50
]. We cannot make further inferences of the effect of e2 on estimated kidney function beyond the consistent, yet non-significant association with higher prevalence of low-GFR.
We chose to use GFR<75 ml/min/1.73 m2
to define cases and not the National Kidney Foundation's (NKF) standard definition for stage 3 CKD (GFR<60 ml/min/1.73 m2
) since using the NKF definition resulted in severely underpowered analyses (see Univariate association
). Comparison of results from univariate analyses using a case definition of GFR<60 ml/min/1.73 m2
showed similar association of case status with APOE2
(OR: 1.07, 95%CI: 0.61, 1.87), APOE4
(OR: 0.82, 95%CI: 0.58-1.17) and APOE
score (OR: 1.15, 95%CI: 0.84-1.56) as our current results with case definition of GFR<75 ml/min/1.73 m2
, but were not statistically significant. Additionally, similar risk factors have been associated with both cases defined by GFR<75 ml/min/1.73 m2
and cases defined by GFR<60 ml/min/1.73 m2
This study is large and representative of the general U.S. population, and findings in this study are consistent with previous results. However there are several limitations. First, CKD is clinically defined by altered kidney function or structure for a period of three months or more. The single serum creatinine measurement used in our study to estimate GFR may have lead to misclassification of disease status and decreased power. Second, CKD was defined based on estimated GFR from the MDRD equation and the CKD-EPI formula rather than the gold standard methods of direct measurement of glomerular function using inulin clearance, iothalamate clearance and creatinine clearance. Third, serum creatinine measures are higher in individuals with large amounts of muscle mass [56
], in addition to individuals with impaired kidney function. Therefore, the low GFR estimates generated for those with high muscle mass could indicate worse kidney function than in reality. This misclassification could lead to an attenuation of the association between APOE
and CKD. Fourth, neither the MDRD equation nor the CKD-EPI formula has been validated in Mexican Americans. Lastly, there is the possibility the relationship between APOE
variation and CKD is due to survivor bias; we observed a slight decline in e4 allele frequency with increasing age in all three ethnic groups. However, analyses stratified by age groups revealed similar associations to unstratified analysis (data not shown).
One of the main strengths of the present study was the large, nationally representative, population-based sample of non-Hispanic whites, non-Hispanic blacks and Mexican Americans. Previous epidemiologic studies have shown risk of CKD associated with APOE
variants mostly in clinic-based and international samples [13
]. Our generalizable results provide evidence to support the effects of APOE
variants on CKD. In addition, reverse causality - often a problem with cross-sectional studies - is not an issue here since APOE
gene variants should precede all manifestations of CKD, and therefore cannot be affected by the presence of CKD.