Apolipoprotein E epsilon 4 allele and nephrotic glomerular diseases in children

Function and Dysfunction of Complement Factor H During Formation of Lipid-Rich Deposits

Complement-mediated inflammation or dysregulation in lipid metabolism are associated with the pathogenesis of several diseases. These include age-related macular degeneration (AMD), C3 glomerulonephritis (C3GN), dense deposit disease (DDD), atherosclerosis, and Alzheimer’s disease (AD). In all these diseases, formation of characteristic lipid-rich deposits is evident. Here, we will discuss molecular mechanisms whereby dysfunction of complement, and especially of its key regulator factor H, could be involved in lipid accumulation and related inflammation. The genetic associations to factor H polymorphisms, the role of factor H in the resolution of inflammation in lipid-rich deposits, modification of macrophage functions, and complement-mediated clearance of apoptotic and damaged cells indicate that the function of factor H is crucial in limiting inflammation in these diseases.

A misprocessed form of Apolipoprotein A-I is specifically associated with recurrent Focal Segmental Glomerulosclerosis

Apolipoprotein A-Ib (ApoA-Ib) is a high molecular weight form of Apolipoprotein A-I (ApoA-I) found specifically in the urine of kidney-transplanted patients with recurrent idiopathic focal segmental glomerulosclerosis (FSGS). To determine the nature of the modification present in ApoA-Ib, we sequenced the whole APOA1 gene in ApoA-Ib positive and negative patients, and we also studied the protein primary structure using mass spectrometry. No genetic variations in the APOA1 gene were found in the ApoA-Ib positive patients that could explain the increase in its molecular mass. The mass spectrometry analysis revealed three extra amino acids at the N-Terminal end of ApoA-Ib that were not present in the standard plasmatic form of ApoA-I. These amino acids corresponded to half of the propeptide sequence of the immature form of ApoA-I (proApoA-I) indicating that ApoA-Ib is a misprocessed form of proApoA-I. The description of ApoA-Ib could be relevant not only because it can allow the automated analysis of this biomarker in the clinical practice but also because it has the potential to shed light into the molecular mechanisms that cause idiopathic FSGS, which is currently unknown.

Lipid biology of the podocyte--new perspectives offer new opportunities

In the past 15 years, major advances have been made in understanding the role of lipids in podocyte biology. First, susceptibility to focal segmental glomerulosclerosis (FSGS) and glomerular disease is associated with an APOL1 sequence variant, is expressed in podocytes and encodes apolipoprotein L1, an important component of HDL. Second, acid sphingomyelinase-like phosphodiesterase 3b encoded by SMPDL3b has a role in the conversion of sphingomyelin to ceramide and its levels are reduced in renal biopsy samples from patients with recurrent FSGS. Furthermore, decreased SMPDL3b expression is associated with increased susceptibility of podocytes to injury after exposure to sera from these patients. Third, in many individuals with membranous nephropathy, autoantibodies against the phospholipase A2 (PLA2) receptor, which is expressed in podocytes, have been identified. Whether these autoantibodies affect the activity of PLA2, which liberates arachidonic acid from glycerophospholipids and modulates podocyte function, is unknown. Fourth, clinical and experimental evidence support a role for ATP-binding cassette sub-family A member 1-dependent cholesterol efflux, free fatty acids and glycerophospolipids in the pathogenesis of diabetic kidney disease. An improved understanding of lipid biology in podocytes might provide insights to develop therapeutic targets for primary and secondary glomerulopathies.

A form of apolipoprotein a-I is found specifically in relapses of focal segmental glomerulosclerosis following transplantation

Recurrence of idiopathic focal segmental glomerulosclerosis (FSGS) following kidney transplantation occurs in a large percentage of patients. Accurate prediction of recurrence and elucidation of its pathogenesis are major therapeutic goals. To detect differential proteins related to FSGS recurrence, proteomic analysis was performed on plasma and urine samples from 35 transplanted idiopathic FSGS patients, divided into relapsing and nonrelapsing. Several proteins were detected increased in urine of relapsing FSGS patients, including a high molecular weight form of apolipoprotein A-I, named ApoA-Ib, found exclusively in relapsing patients. This finding was verified by Western blot individually in the 35 patients and validated in an independent group of 40 patients with relapsing or nonrelapsing FSGS, plus two additional groups: FSGS-unrelated patients showing different proteinuria levels (n = 30), and familial FSGS transplanted patients (n = 14). In the total of 119 patients studied, the ApoA-Ib form was detected in 13 of the 14 relapsing FSGS patients, and in one of the 61 nonrelapsing patients. Only one of the 30 patients with FSGS-unrelated proteinuria tested positive for ApoA-Ib, and was not detected in familial patients. Urinary ApoA-Ib is associated with relapses in idiopathic FSGS and warrants additional investigation to determine its usefulness as biomarker of relapse following transplantation.

Association of apoE gene expression and its gene polymorphism with nephrotic syndrome susceptibility: a meta-analysis of experimental and human studies

Apolipoprotein E (apoE) is a major protein in the lipoprotein transport system that plays a well established role in lipids metabolism. apoE gene contains three potential alleles: ε2, ε3 and ε4, forming six genotypes: ε2ε2, ε2ε3, ε2ε4, ε3ε3, ε3ε3 and ε4ε4. The disorder of lipids metabolism is an important feature for nephrotic syndrome (NS). There were some investigations reporting that apoE and its gene polymorphism was associated with NS susceptibility. However, the mechanism was unclear and the association was still controversial. This meta-analysis was performed to evaluate the association between apoE and NS risk in experimental and human studies. A predefined literature search and selection of eligible relevant studies were performed to collect the data from electronic databases, and eligible investigations were synthesized using meta-analysis method. In experimental models, twelve comparisons were included and a definitely positive association was observed between apoE protein expression and NS susceptibility (WMD = 1.88, P < 0.00001). However, in human, there was only two studies included for meta-analysis and a positive association between apoE protein expression and NS susceptibility wasn't found (OR = 108.10, P = 0.32). Interestingly, ε3ε3, ε3ε4, ε3 and ε4 were associated with NS susceptibility (ε3ε3: OR = 0.56, P = 0.002; ε3ε4: OR = 1.91, P = 0.02; ε3: OR = 0.61, P = 0.001; ε4: OR = 1.85, P = 0.009). In conclusion, 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. This study supported that the disorder of apoE was one of the possible causes for NS risk. However, more studies should be performed to investigate this relationship in the future.

Circulating ApoE level is independently associated with urinary albumin excretion in type 2 diabetic patients

OBJECTIVE

Evidence from many studies suggests that dyslipidemia has a pathogenetic role in the development and progression of diabetic nephropathy. The objective of this study was to evaluate the relationships between serum lipid parameters and urinary albumin excretion in type 2 diabetic patients from Shanghai, China.

METHODS

A total of 1094 type 2 diabetic patients were recruited. The urinary albumin to creatinine ratio (ACR), serum total cholesterol, triglyceride, high density lipoprotein cholesterol (HDL-C), apolipoprotein A I (ApoA I), apolipoprotein B (ApoB), apolipoprotein E (ApoE) and lipoprotein (a) [Lp(a)] were measured and low density lipoprotein cholesterol (LDL-C) was calculated according to the Friedewald formula. An ACR of ≥30 µg/mg was defined as albuminuria, and <30 µg/mg as normoalbuminuria.

RESULTS

The levels of ApoE and triglyceride were significantly higher in albuminuric group compared with normoalbuminuric group (p<0.001 and p=0.003, respectively). Differences of ACR among the groups based on the tertile of ApoE and the tertile of triglyceride were significant (both p<0.001). In a multiple linear regression model, ApoE was independently associated with ACR (β=0.003, 95%CI 0.002-0.005, p<0.001).

CONCLUSION

Serum ApoE level was independently associated with urinary albumin excretion in type 2 diabetic patients from Shanghai, China.

Association of polymorphisms at restriction enzyme recognition sites of apolipoprotein B and E gene with dyslipidemia in children undergoing primary nephrotic syndrome

BACKGROUND

Dyslipidemia, a common complication, is very prevalent in children with primary nephrotic syndrome (PNS). Recent studies have shown that genetic basis may be involved in the onset of HLP secondary to PNS. ApoB and E have been identified as the important candidate genes for lipid abnormalities.

OBJECTIVE

To investigate the association of apolipoprotein B (apoB) and E (apoE) genetic polymorphisms (Xba I, EcoR I, Msp I, and Hha I) with parameters describing the serum lipid profiles in children undergoing PNS.

METHODS

Genomic DNA was extracted from 250 children diagnosed with PNS and 200 healthy controls with neither allergic nor renal disease. ApoB (Xba I, EcoR I, and Msp I) and apoE (Hha I) genotypes were determined by PCR-restriction fragment length polymorphism (RFLP) analysis. The fasting serum lipoprotein (a) [Lp(a)], total cholesterol (TC), triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), apolipoprotein A1 (apoA1), apoB, and total protein from a 24-h urine sample were measured.

RESULTS

No significant differences in genotypes and alleles frequencies were observed for the apoB Xba I, EcoR I, Msp I and the apoE Hha I restriction sites in PNS patients as compared to controls (P > 0.05). Patients and controls with X + allele exhibited significantly higher serum levels of Lp(a), TC, nonHDL-C, LDL-C, LDL-C/HDL-C ratio, and apoB than that with X- allele (P < 0.05), whereas for apoA1/B ratio the opposite was found (P < 0.01). E-/E- carriers had significantly higher Lp(a), TC, HDL-C, and apoA1 concentrations than did E+/E- or E+/E+ carriers in control group (P < 0.05). Healthy children carrying the rare EcoR I allele had higher mean Lp(a), TC, and HDL-C levels than homozygotes for E+ (P < 0.05). Higher Lp(a) serum concentrations were observed in patients with E- allele (P < 0.05). No significant differences in lipid parameters were determined for the apoB Msp I and apoE Hha I the polymorphisms study (P > 0.05). When genetic variations were compared with urinary protein excretion, the Xba I X- allele was more frequent in patients with elevated proteinuria (P < 0.01).

CONCLUSION

Presence of Xba I X+ allele and/or EcoR I E- at the apoB gene may be risk factors for lipid abnormalities secondary to childhood PNS.

Dyslipidemia and nephrotic syndrome: Recent advances

Patients with nephrotic syndrome (NS) have one of the most pronounced secondary changes in lipoprotein metabolism known, and the magnitude of the changes correlates with the severity of the disease. These changes are of a quantitative as well as a qualitative nature. All apolipoprotein B (apo B)-containing lipoproteins, such as very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and lipoprotein(a) [Lp(a)], are elevated in nephrotic syndrome. High-density lipoproteins (HDL) are reported to be unchanged or reduced. In addition to these quantitative changes, the lipoprotein composition is markedly changed, with a higher ratio of cholesterol to triglycerides in the apo B-containing lipoproteins and an increase in the proportion of cholesterol, cholesterol ester, and phospholipids compared with proteins. Also apolipoproteins show major changes, with an increase in apolipoprotein A-I, A-IV, B, C, and E. Particularly the changes in apo C-II, which is an activator of the enzyme lipoprotein lipase (LPL), and apo C-III, an inhibitor of LPL, with an increase of the C-III to C-II ratio, might contribute to the impaired lipoprotein catabolism in NS. The mechanisms for these changes in lipoprotein metabolism are discussed in this review as far as they are known. Furthermore, the tremendous elevations of Lp(a) in nephrotic syndrome and its primary and secondary causes are reviewed. Primary causes became recently apparent by a significantly higher frequency of low-molecular-weight apo(a) phenotypes in patients compared with controls. The secondary causes were shown by an increase of Lp(a) in all apo(a) isoform groups. Because Lp(a) is an LDL-like particle that is usually included in the measured or calculated LDL cholesterol fraction, the influence of the extremely high Lp(a) levels in NS on the measurement of LDL cholesterol is discussed.

Apolipoprotein E and renal disease

Apolipoprotein E (ApoE) is a major constituent of plasma lipoproteins with many biological actions of great significance. Beyond the known influence of ApoE polymorphisms on serum lipid profile, the pathogenesis of atherosclerosis, and the development of neurodegenerative disorders, ApoE also has a major role in the pathogenesis and progression of a variety of renal diseases, as well as in the atherosclerotic complications associated with them. Briefly, the polymorphisms of ApoE are major determinants of plasma lipid levels in uremic patients. They may affect the risk for cardiovascular disease in this population, predispose to the development of diabetic nephropathy, influence the severity of certain glomerulopathies, and regulate mesangial and glomerular functions locally in the kidney microenvironment. Finally, certain mutations of the ApoE gene are associated with a recently described nephropathy, termed lipoprotein glomerulopathy.

Apolipoprotein E polymorphism and clinical course in childhood nephrotic syndrome

Numerous studies have reported on the role of apolipoprotein E (apoE) polymorphism in the progression of diseases associated with lipid abnormalities. However, few studies have been performed to date on the relationship between apoE polymorphism and childhood nephrotic syndrome (NS). In the present study, we evaluated the allelic and genotypic frequencies of the apoE gene and the possible association between the polymorphic forms of the apoE gene on the clinical course in 190 patients with childhood NS, who were further classified into frequent relapsers (FR, 92) and nonrelapsers or infrequent relapsers (IR, 98). Controls included 132 healthy Koreans. Allele-specific primers were used to detect polymorphism of the apoE gene. The allelic frequencies at the apoE locus were 5.9%, 82.6%, and 11.8% for alleles epsilon2, epsilon3, and epsilon4, respectively in the childhood NS group, while those in the control group were 6.8% for epsilon2, 88.3% for epsilon3, and 4.9% for epsilon4. The allelic frequency for epsilon4 in childhood NS was twice that of controls. Moreover, the allelic frequency of the epsilon4 allele in the FR group was 3.4 times that of the control group and 2.5 times that of the IR group. The high frequency of epsilon4 in patients with childhood NS suggests that epsilon4 may serve as a genetic marker for predisposition to childhood NS. We believe that the apoE allele type is of considerable significance in predicting the course of the disease.

Apolipoprotein E in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis

BACKGROUND

Hyperlipemia characterizes nephrotic syndrome (NS) and contributes to the progression of the underlying nephropathy. The data in the literature support an implication of apolipoprotein E (apoE) in both hyperlipemia and focal segmental glomerulosclerosis (FSGS), a malignant condition associated with NS.

METHODS

The apoE genotype was determined in 209 nephrotic patients, who were classified according to age and their response to steroids as resistant children (N = 96) and adults (43), and steroid dependent (33) and steroid responder (37) children. A total of 123 presented the histological features of FSGS. In a subgroup of 28 patients, serum and urinary levels of apoE and renal deposits were evaluated by immunofluorescence.

RESULTS

The allelic frequencies of the three major haplotypes epsilon2, epsilon3, and epsilon4 were the same in nephrotic patients versus controls, and homozygosity for epsilon3epsilon3 was comparably the most frequent genotype (70 vs. 71%) followed by epsilon3epsilon4, epsilon2epsilon3, epsilon2epsilon4, epsilon4epsilon4. Serum levels of apoE were fivefold higher in NS and in FSGS patients than in controls, with a direct correlation with hypercholesterolemia and proteinuria. ApoE genotypes did not influence serum levels. Urinary levels were 1/10,000 of serum with an increment in nephrotic urines. Finally, immunofluorescence demonstrated the absence of apoE in sclerotic glomeruli, while comparably nephrotic patients with membranous nephropathy had an increased glomerular expression of apoE.

CONCLUSIONS

ApoE is dysregulated in NS with a marked increment in serum, which is a part of the complex lipid metabolism. Down-regulation of glomerular apoE instead is a peculiarity of FSGS and may contribute to the pathogenesis of the disease. The normal distribution of apoE genotypes in nephrotic patients with FSGS excludes a pathogenetic role of genetic variants.

Serum glomerular permeability activity in patients with podocin mutations (NPHS2) and steroid-resistant nephrotic syndrome

A plasma factor displaying permeability activity in vitro and possibly determining proteinuria has been hypothesized in idiopathic focal segmental glomerulosclerosis (FSGS). In vitro permeability activity (P(alb)) was determined in sera of five patients with autosomal recessive steroid-resistant nephrotic syndrome (NPHS2), an inherited condition indistinguishable from idiopathic FSGS on clinical grounds, but in which proteinuria is determined by homozygous mutations of podocin, a key component of the glomerular podocyte. All patients had presented intractable proteinuria with nephrotic syndrome; four developed renal failure and received a renal allograft. For comparison, sera from 31 children with nephrotic syndrome were tested. Pretransplant P(alb) was high in all cases (mean 0.81 +/- 0.06), equivalent to levels observed in idiopathic FSGS. Overall, P(alb) did not correlate with proteinuria. The posttransplant outcome was complicated in two patients by recurrence of proteinuria after 10 and 300 d, respectively, that responded to plasmapheresis plus cyclophosphamide. P(alb) levels were high at the time of the recurrence episodes and steadily decreased after plasmapheresis, to reach normal levels in the absence of proteinuria after the seventh cycle. In an attempt to explain high P(alb) in these patients, putative inhibitors of the permeability activity were studied. Coincubation of serum with homologous nephrotic urine reduced P(alb) to 0, whereas normal urine did not determine any change, which suggests loss of inhibitory substances in nephrotic urine. The urinary levels of the serum P(alb) inhibitors apo J and apo E were negligible in all cases, thus suggesting that other urinary inhibitors were responsible for the neutralizing effect. These data indicate that P(alb) is high in NPHS2, probably resulting from loss of inhibitors in urine. Lack of correlation of P(alb) with proteinuria suggests a selective loss of inhibitors. As in idiopathic FSGS, proteinuria may also recur after renal transplantation in NPHS2 patients, and post-transplant proteinuria is associated with high P(alb). The relationship between elevated P(alb) and proteinuria in NPHS2 remains to be determined.