Phenotyping of human apolipoprotein E from whole blood plasma by immunoblotting

Validation of a novel and accurate ApoE4 assay for automated chemistry analyzers

The allele ε4 of the apolipoprotein E gene (APOE ε4) is the major genetic risk factor for non-dominantly inherited Alzheimer’s Disease (AD). Current techniques for APOE ε4 carriers identification show good accuracy but have several disadvantages that limit its implementation in a clinical laboratory. These include the need for sample preprocessing, poor automation, low throughput, requirement of additional equipment, and high cost. We followed ISO 13485 guidelines to validate the e4Risk test, a new latex-enhanced immunoturbidimetric blood assay for apolipoprotein E4 (ApoE4) determination in human plasma samples. The test showed high performance in terms of lot to lot variability, precision, interferences, reagents stability, prozone, and detectability. Furthermore, diagnostic accuracy is almost equal (99%) to the gold standard, APOE ε4 genotyping by polymerase chain reaction (PCR). Furthermore, we demonstrated that the e4Risk test can be adapted to any clinical chemistry analyzer, including the high throughput analyzers present in most hospitals and clinical laboratories. The e4Risk test versatility, low cost, and easiness provides an excellent solution for APOE ε4 carriers identification using the same blood sample drawn for biochemical diagnostic work-up of AD patients, which can have important advantages for patient stratification in clinical trials, preventative strategies for AD, and clinical assessment of risk for brain amyloidosis.

A fast and cost-effective method for apolipoprotein E isotyping as an alternative to APOE genotyping for patient screening and stratification

Apolipoprotein E (apoE) is a 34 kDa glycoprotein involved in lipid metabolism. The human APOE gene encodes for three different apoE protein isoforms: E2, E3 and E4. The interest in apoE isoforms is high for epidemiological research, patient stratification and identification of those at increased risk for clinical trials and prevention. The isoform apoE4 is associated with increased risk for coronary heart and Alzheimer's diseases. This paper describes a method for specifically detecting the apoE4 isoform from biological fluids by taking advantage of the capacity of apoE to bind "specifically" to polystyrene surfaces as capture and a specific anti-apoE4 monoclonal antibody as reporter. Our results indicate that the apoE-polystyrene binding interaction is highly stable, resistant to detergents and acid and basic washes. The methodology here described is accurate, easily implementable, fast and cost-effective. Although at present, our technique is unable to discriminate homozygous APOE ε4/ε4 from APOE ε3/ε4 and ε2/ε4 heterozygous, it opens new avenues for the development of inexpensive, yet effective, tests for the detection of apoE4 for patients' stratification. Preliminary results indicated that this methodology is also adaptable into turbidimetric platforms, which make it a good candidate for clinical implementation through its translation to the clinical analysis routine.

Simple and effective determination of apolipoprotein E genotypes by positive/negative polymerase chain reaction products

Several protein and DNA-based methods have been previously described for the identification of apolipoprotein E isoforms or genotypes. However, all of them generate frequently false-positive results. The purpose of this study was to set up a new, simple, and effective method for the analysis of the apoE polymorphism. A total of 1,253 subjects previously examined for the apolipoprotein E polymorphism by restriction fragment length polymorphism were reanalyzed by our new method based on Taq DNA polymerase's inability to correctly initiate the replication in the presence of a mismatch at the 3' end of the primer. We conceived a combination of 4 specific primers in 3 different pairs sharing the same stringent polymerase chain reaction conditions to directly detect the presence/absence of polymerase chain reaction products, and thus reveal the 6 apolipoprotein E genotypes. We confirm our previous results in 1,171 subjects, whereas in 82 subjects out of 1,253 (about 6%), the results have been reinterpreted. The final analysis revealed a total of 12 homozygotic subjects for the e2 allele (1.0%), 874 homozygotes for the e3 allele (69.8 %), and 8 homozygotes for the e4 allele (0.6 %). The frequence of heterozygotes was 8.7% for the e2/e3 genotype (n=109), 1.4% for the e2/e4 genotype (n=17), and 0.6% for the e3/e4 genotype (n=8). Relative allele frequencies were e2=0.060, e3=0.834, and e4=0.106. We describe a new, simple, unequivocal, and nonexpensive method for the identification of the 6 apoE genotypes.

Efficiency of intestinal cholesterol absorption in humans is not related to apoE phenotype

The present study investigated the role of apolipoprotein E (apoE) phenotype on intestinal cholesterol absorption and cholesterol synthesis. Studies were carried out in eight subjects homozygous for the apoE4 and 12 subjects homozygous for the E2 allele (six normocholesterolemic volunteers and six patients with type III hyperlipoproteinemia). Cholesterol absorption did not differ between the three groups of subjects and averaged 38 +/- 2% (mean +/- SEM) in normolipemic E2/2, 37 +/- 4% in type III hyperlipemic E2/2, and 41 +/- 3% in E4/4 subjects, respectively. Dietary intake of fat and cholesterol had no influence on cholesterol absorption efficiency. A positive correlation between efficiency of cholesterol absorption and the ratio of campesterol to cholesterol in plasma, an indirect marker for cholesterol absorption, was observed after combining the results of the three groups (r = 0.504; P < 0.02). Bile acid and total cholesterol synthesis were also not affected by the different apoE alleles, but the well-known relationship between body weight and cholesterol synthesis was noticed (r = 0.574; P < 0.01). Thus, the present study provides evidence that the efficiency of intestinal absorption and synthesis of cholesterol in humans are not related to the apoE phenotype.

Analysis of blood plasma proteins in patients with Alzheimer's disease by two-dimensional electrophoresis, sequence homology and immunodetection

Blood plasma proteins of patients with Alzheimer's disease (AD; senile dementia) and non-AD-type dementia were resolved by two-dimensional electrophoresis and identified by migration position in the electrophoresis pattern, sequence homology, and immunodetection by using antibodies. For the control experiments, blood plasma proteins of a healthy young individual and non-dementia patients were examined in a manner similar to that of the plasma samples of AD patients. In the plasma sample of the healthy young individual, more than 350 spots of silver-stained proteins were observed and among these spots, 73 spots were identified. Blood plasma proteins of the AD and non-AD-type dementia patients were compared with those of the control and non-dementia patients. In the blood plasma samples of five AD patients, three patients had apolipoprotein E4, and another patient showed apolipoprotein L and complement factor H. For the AD-related proteins apolipoprotein E, tau-1, and presenilin 2, proteins were examined by immunostaining with antibodies, in both AD and non-AD patients. Among the three samples of non-AD-type dementia patients, one was distinguishable by amyloid A proteins, and the other by haptoglobin isoforms.

Relative effects of high saturated fatty acid levels in meat, dairy products, and tropical oils on serum lipoprotein and low-density lipoprotein degradation by mononuclear cells in healthy males

To determine the effects of three saturated fatty acid combinations on lipoprotein metabolism, we fed 18 21- to 32-year-old men three diets in a crossover design for 28-day periods separated by washout periods of 4 to 6 weeks. The men self-selected a prescribed diet at home emphasizing saturated fat as the visible fat for 1 week. Then, they ate experimental diets providing 40%, 15%, 17%, and 7% of food energy, respectively, as total, saturated, monounsaturated, and polyunsaturated fatty acids, levels representing amounts available in the US diet. Different test fatty acid combinations, given at 4 to 6 energy% (en%) each, were incorporated into food products: 12:0 + 14:0, 14:0 + 16:0, and 16:0 + 18:0. Test fatty acids were equalized by giving free myristic acid (14:0) with palm kernel oil or butter and sheanut butter (high in 18:0) with lard. The diet highest in 12:0 + 14:0 also provided 4.2 en% 16:0, the most common saturated fatty acid in the US diet. Mean apparent absorption of all fatty acids was at least 90%. The three diets produced similar concentrations of serum total and low-density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B-100 regardless of the apo E phenotype of the subjects. Compared with baseline, the experimental diets affected serum high-density lipoprotein (HDL) concentrations (P < .06), with the highest values occurring on diet 12:0 + 14:0. When the change from baseline in receptor-mediated degradation of 125I-LDL in freshly isolated mononuclear cells (MNC) was stratified by apo E phenotype, diet 16:0 + 18:0 produced a 30% increase, compared with a 9% decrease on diet 12:0 + 14:0 and a 6% increase on diet 14:0 + 16:0 in subjects with the apo E3/3 phenotype. These results suggested that different saturated fatty acid combinations, consumed at levels typical of availability in the United States and with diets providing ample unsaturated fat, had similar cholesterolemic properties in healthy males despite some subtly different effects on lipoprotein metabolism.

Apolipoprotein E polymorphism in Italy investigated in native plasma by a simple polyacrylamide gel isoelectric focusing technique. Comparison with frequency data of other European populations

A new polyacrylamide gel isoelectric focusing (PAGIEF) technique has been developed that allows rapid and reliable identification of Apolipoprotein E (APOE) phenotypes directly from plasma or serum without any prior treatment. This method was used to determine the APOE phenotypes in samples from Central and Southern Italy, Sicily, and Sardinia. The frequencies observed for the APOE*2, APOE*3, and APOE*4 alleles in Central and Southern Italy (Sicily included) were similar (0.066, 0.851, 0.083 and 0.056, 0.858, 0.085 respectively) though lower APOE*4 frequencies were found in the more southern regions. The Sardinian population showed APOE gene frequencies (APOE*2 = 0.050, APOE*3 = 0.898, APOE*4 = 0.052) to be significantly different from those of the rest of Italy owing to the low APOE*4 frequency, the lowest among Caucasian populations. The frequencies were compared with those found in other European populations. A clear cut North-South decreasing cline was found for APOE*4 allele frequencies and an opposite trend was found for APOE*3 frequencies. The overall dispersion of European populations as determined by the three APOE allele frequencies was graphically represented using coordinate analysis. The tendency of the APOE*4 frequency to decline with latitude both at the Italian and at the European level was discussed with reference to similar trends observed for dietary habits (saturated fat intake).

A new approach for the detection of type III hyperlipoproteinemia by RLP-cholesterol assay

Type III is a remnant hyperlipoproteinemia identified by the presence of beta-VLDL (remnant lipoprotein) as well as a genetic variant of apo E (apo E2/2). The RLP isolated from the serum of Type III patients by a new method we have developed, the RLPcholesterol assay, was identified as chylomicron and VLDL remnant. In addition, the RLP-C levels of the Type III patients were significantly higher than other hyperlipidemic patients with similar serum TG levels, while the ratio of TC/TG in RLP-C of both groups was not significantly different. The RLP-cholesterol assay appears to be useful for the screening and monitoring of Type III hyperlipoproteinemia when used in conjunction with the assays of serum TG level and genetic apo E isoform analysis.

Serum lipoprotein(a) levels differ in different phenotypes of primary hyperlipoproteinemia

Previous studies have indicated that serum lipoprotein(a) [Lp(a)] levels are mostly under genetic control. We have attempted to determine whether serum Lp(a) levels differ in different phenotypes of primary hyperlipoproteinemia (HL). A total of 129 subjects with HL (three with type I, 43 with familial hypercholesterolemia [FH], 17 with type IIa [non-FH], 11 with type IIb, six with type III [E2/2], 44 with type IV, and five with type V) and 18 normolipidemic controls were included in the study. Thirty-two FH subjects were being treated with hypolipidemic agents, but none of the other subjects were receiving any medication. Fasting blood samples were collected for determination of both serum lipid and Lp(a) levels. Lp(a) level was measured by enzyme-linked immunosorbent assay. The 18 controls had serum Lp(a) concentrations of 18.0 +/- 14.5 mg/dL (mean +/- SD), and four of them had high serum Lp(a) levels (> or = 25 mg/dL). Serum Lp(a) concentrations in FH subjects tended to be higher than in the controls (30.5 +/- 25.0 mg/dL), and the incidence of high Lp(a) levels in FH subjects was significantly higher than in the controls (51% v 22%, P < .01). There was no difference between serum Lp(a) levels of FH subjects depending on whether they were receiving medication. In contrast, most of the subjects with selective hypertriglyceridemia had very low serum Lp(a) levels (1.5 +/- 0.7, 8.1 +/- 8.3, and 3.5 +/- 5.3 mg/dL in type I, IV, and V, respectively; P < .01 v controls).(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid and simple method for the identification of apolipoprotein E isomorphic phenotypes from whole serum

For the identification of apolipoprotein E isomorphic phenotypes, fresh or thawed serum was analyzed without prior delipidation or other pretreatment. Using 5% polyacrylamide gels with a 40 mm interelectrode distance, the isoforms were separated by isoelectric focusing in immobilized pH gradients ranging from pH 5 to 6.5, and transferred onto polyvinylidene difluoride membranes by contact blotting for 1 h. The apolipoprotein E isoforms were identified following immunostaining. The electrophoresis required less than 2 h and the entire procedure could be completed within 6 h.

Apolipoprotein E polymorphism in a Danish population compared to findings in 45 other study populations around the world

Apolipoprotein E (apoE) phenotypes were determined in a random sample of 466 Danish men born in 1948. The frequencies of the common alleles of the apoE gene were (with 95% confidence intervals) epsilon 2 = 0.085 (0.068-0.105), epsilon 3 = 0.741 (0.712-0.769), and epsilon 4 = 0.174 (0.150-0.200). These frequencies were compared to findings in 45 other study populations around the world (n greater than 100). The Danish population was found to cluster with populations from Iceland, Norway, Iceland, Scotland, the Netherlands, Germany, France (Paris), and Caucasian populations in Canada and the USA. The compiled data further show that dissimilarities in apoE allele frequencies among Caucasian populations are comparable to dissimilarities between some Caucasian and Asian populations. Notably, the frequency of epsilon 4 appears to be higher in northern regions of Europe (the Nordic countries, Scotland, Germany, and the Netherlands) than in southern regions (Switzerland, Tyrol, France [Nancy], Italy, and Spain).

Apolipoprotein E phenotyping: a word of caution

Using two different techniques, phenotyping and genotyping, we have studied allelic variation at amino acids 112 and 158 of the apolipoprotein E gene locus in 52 patients with insulin-dependent diabetes and in 58 non-diabetic controls. Phenotypes were determined by isoelectric focusing and immunoblotting of delipidated, neuraminidase-treated plasma. Genotypes were determined by using the polymerase chain reaction to amplify a 227 base pair fragment of the apolipoprotein E gene spanning both allelic sites. This was then digested with the restriction endonuclease CfoI and the alleles identified by polyacrylamide gel electrophoresis. Discrepancies between phenotype and genotype were observed in 16 (15%) of the individuals studied, 7 (13%) in the diabetics and 9 (17%) in the controls. From these results it is concluded that isoelectric focusing can lead to the erroneous assignment of apolipoprotein E phenotype even after pretreatment with neuraminidase. It is suggested that genotyping by DNA analysis is the method of choice in determining apolipoprotein E status.

Isoelectric focusing in immobilized pH gradients: applications in clinical chemistry and forensic analysis

The applications of isoelectric focusing in immobilized pH gradients in clinical chemistry and forensic analysis are reviewed. Strong emphasis is given to the separation of serum proteins, in particular alpha 1-acidic glycoprotein, acid phosphatase, alkaline phosphatase, alpha 1-antitrypsin, apolipoproteins, complement component, factor B, factor XIIIB, group-specific component, lecithin:cholesterol acyltransferase, phosphoglucomutase, prealbumin, protein C and transferrin. The analysis of human parotid salivary proteins is discussed and an assessment is given of the state of the art in thalassaemia screening.

Phenotyping of apolipoprotein E by immunoblotting in immobilized pH gradients

An immunoblotting method for the determination of apolipoprotein E (apoE) phenotypes has been developed. Delipidated plasma proteins are focused in an immobilized pH gradient, and transferred to polyvinylidene difluoride (PVDF) membranes. ApoE isomorphs are identified by immunoperoxidase staining. The method allows reproducible assignment of apoE phenotypes without isolation of triglyceride-rich lipoproteins. Only small amounts of serum are required. There are several important steps in the procedure: (i) delipidation is indispensable, (ii) carrier ampholytes have to be added to the gels and to the sample buffer, and, (iii) on immunostaining, polyvinylidene difluoride membranes provide an excellent signal-to-background ratio.

Low-dose oral contraceptives lower plasma levels of apolipoprotein E

Three different oral contraceptive preparations were studied before and after a 3 month treatment period with respect to their effects on plasma lipoprotein parameters. A total of 58 healthy women requesting oral contraception were randomly assigned to three groups. Each woman received either monophasic preparations containing ethinylestradiol and desogestrel (M-DG); ethinylestradiol and gestodene (M-GD); or a triphasic preparation of ethinylestradiol and levonorgestrel (T-LN). As has been reported in other studies, the concentrations of total plasma cholesterol and apolipoproteins B and A-IV did not change significantly in any group. HDL cholesterol, triglycerides, apolipoproteins A-I and A-II increased or tended to increase. Despite the effects of the three hormone preparations on these lipoprotein parameters, however, each led to a highly significant decrease in apolipoprotein E plasma levels. Considering the recently reported observations that oral contraceptives increase the hepatic uptake of cholesterol-rich remnants, this decrease in apo-E plasma levels may in women that take oral contraceptives be directly correlated with increased hepatic lipoprotein metabolism.

Detection of apolipoprotein E polymorphisms using PCR/ASO probes and Southern transfer: application for routine use

The detection of apolipoprotein E genotypes is of importance both for diagnostic and research purposes. We have previously used the polymerase chain reaction to amplify a specific region of the apolipoprotein E gene which, when used in conjunction with allele specific oligonucleotide probes, permits the detection of the six common apolipoprotein genotypes. In our present report we have modified the above procedure by immobilising the amplified DNA using Southern blotting. This enables the technique to be used in routine laboratories with minimal expenditure and little risk of cross-contamination between samples. Furthermore, it is inherently robust and may be rapidly performed.

Genetic studies of human apolipoproteins: XIV. A simple agarose isoelectric focusing gel method for apolipoprotein E phenotyping

A new and simplified procedure is described for apolipoprotein E (APO E) phenotyping from native plasma or serum samples. Diluted or dialyzed samples are separated on agarose isoelectric focusing gels followed by protein blotting on nitrocellulose membranes. APO E banding patterns are localized immunologically using polyclonal goat anti-APO E antiserum as the primary antibody and rabbit anti-goat IgG conjugated with alkaline phosphatase as the secondary antibody. The method was used in parallel with our previously described polyacrylamide gel system to screen 110 unrelated and healthy US whites. Both gel systems gave identical APO E phenotypes, and allele frequencies were comparable with reported US white values. This simplified method can be used on a large number of population and clinical samples with minimum cost and effort.

Polymorphism of apolipoprotein E influences levels of serum apolipoproteins E and B in the human neonate

To gain more insight into the genetic vs. environmental influence of the apoE phenotypes on plasma lipoprotein variation we studied human umbilical cord sera at birth. Apolipoprotein E genetic phenotypes were determined in 110 individuals by immunoblotting and shown to be identical to the adult human isoforms with six phenotypes present and occurring at a similar frequency as reported previously for the adult population in the same area. Total serum cholesterol and triglyceride levels were low in the neonates and did not differ significantly between apoE phenotypes. On the other hand as in the adult, levels of apoE and B differed significantly between the phenotypes. ApoE was highest in individuals with the epsilon 2 allele and lowest in individuals expressing apoE4, and vice versa for apoB. We conclude that apoE phenotypes in human umbilical cord blood serum are already associated with pronounced differences in apoE and B levels in the newborn. The study demonstrates that the association of apoE and apoB levels with the apoE polymorphism occurs independently of significant enteral nutrition in the relatively constant in utero environment.

Linkage between serum cholinesterase 2 (CHE2) and gamma-crystallin gene cluster (CRYG): assignment to chromosome 2

Serum cholinesterase 2 (CHE2) was examined in a Danish material of normal families that has been tested earlier for 70-78 classical marker systems and 25 RFLP systems. DNA for RFLP typing was provided by transforming 16-year-old frozen lymphocytes. The frequency of allele CHE2*C5+ in the Danish population was found to be 0.0430. The highest lod score was between CHE2 and the gamma-crystallin gene cluster (CRYG) (zeta = 4.21 at theta = 0.00 in females). The scores were from a single family with 15 children. CHE2 may, accordingly, be assigned to the location of CRYG: chromosome 2, bands q33-q35.

Isoelectric focusing of apolipoproteins in immobilized pH gradients: improved determination of apolipoprotein E phenotypes

A method for isoelectric focusing of apolipoprotein E in an immobilized pH gradient with added carrier ampholytes has been developed. This method is an improvement over conventional isoelectric focusing of apolipoprotein E with respect to resolution, reproducibility, and simplicity. Since monosialo isoforms are resolved from the normally cofocusing asialo isoforms, unique patterns are obtained for all 6 common apolipoprotein E phenotypes. The method can also be applied to the screening of apolipoprotein A and C isoforms. Delipidated very low density lipoproteins (VLDL) have been used as the source of apolipoprotein E and C. Apolipoprotein A isoforms were focused directly from detergent-treated serum. Immunodetection of apolipoprotein E using capillary transfer was found to be compatible with the described method.