Phenotyping is an accurate means of analysing the principal apolipoprotein E isoforms

Apolipoprotein E polymorphisms and mortality in Italian type 2 diabetic patients

AIMS

To determine if apolipoprotein E polymorphism is associated with cardiovascular or all-cause mortality in Italian Type 2 diabetic patients.

METHODS

A prospective study of mortality in Type 2 diabetic patients (n = 433) as a function of apolipoprotein E phenotype, which was assessed at entry into the study. During follow up (10 years), 110 (25.4%) patients died of which 66 (15.2%) were the result of cardiovascular causes. Cause of death was established from death certificates and clinical records. The clinical status of the survivors was determined at the end of the study.

RESULTS

Apolipoprotein E polymorphisms were not associated with excess cardiovascular or all-cause mortality in the Italian Type 2 diabetic patients either in univariate or multivariate analyses. Age, duration of diabetes and glycated haemoglobin levels at entry were the primary determinants of premature mortality in the diabetic population.

CONCLUSIONS

Apolipoprotein E polymorphisms are not markers for premature mortality in Italian Type 2 diabetic patients. The impact of apolipoprotein E mutations may be attenuated by environmental factors, notably a healthier diet, in Italian patients.

Discrepancies between apolipoprotein E phenotyping and genotyping in the elderly

We estimated the frequencies of phenotype (isoelectric focusing; IEF) vs. genotype (PCR/Hhal) discordance in a sample of an aged population (> 65 years). Both phenotype and genotype techniques have been used in the study of apolipoprotein E (apoE) polymorphism in 125 elderly subjects. The discordance between phenotype and genotype was unresolved in 11 (8.8%) of the 125 unrelated subjects studied. We observed a significant association between the presence of the E4 allele and both Alzheimer's disease (chi2 = 13, p < 0.001) and increased cholesterol concentration (Mann Whitney, p < 0.03). These relationships were not affected by the techniques used. Our results indicate that transcriptional modulation and post-transductional modifications in normal ageing and in aged-related diseases may explain in part discrepancies between gene analysis and protein characterisation.

Clinical chemistry of common apolipoprotein E isoforms

Apolipoprotein E plays a central role in clearance of lipoprotein remnants by serving as a ligand for low-density lipoprotein and apolipoprotein E receptors. Three common alleles (apolipoprotein E(2), E(3) and E(4)) give rise to six phenotypes. Apolipoprotein E(3) is the ancestral form. Common apolipoprotein E isoforms derive from nucleotide substitutions in codons 112 and 158. Resulting cysteine-arginine substitutions cause differences in: affinities for low-density lipoprotein and apolipoprotein E receptors, low-density lipoprotein receptor activities, distribution of apolipoprotein E among lipoproteins, low-density lipoprotein formation rate, and cholesterol absorption. Accompanying changes in triglycerides, cholesterol and low-density lipoprotein may promote atherosclerosis development. Over 90% of patients with familial dysbetalipoproteinaemia have apolipoprotein E(2)/E(2). Apolipoprotein E(4) may promote atherosclerosis by its low-density lipoprotein raising effect. Establishment of apolipoprotein E isoforms may be important for patients with diabetes mellitus and several non-atherosclerotic diseases. Apolipoprotein E phenotyping exploits differences in isoelectric points. Isoelectric focusing uses gels that contain pH 4-7 ampholytes and urea. Serum is directly applied, or prepurified by delipidation, lipoprotein precipitation or dialysation. Isoelectric focusing is followed by immunofixation/protein staining. Another approach is electro- or diffusion blotting, followed by protein staining or immunological detection with anti-apolipoprotein E antibodies and an enzyme-conjugated second antibody. Apolipoprotein E genotyping demonstrates underlying point mutations. Analyses of polymerase chain reaction products are done by allele-specific oligonucleotide probes, restriction fragment length polymorphism, single-stranded conformational polymorphism, the primer-guided nucleotide incorporation assay, or denaturating gradient gel electrophoresis. Detection with primers that either or not initiate amplification is performed with the amplification refractory mutation system. Disparities between phenotyping and genotyping may derive from isoelectric focusing methods that do not adequately separate apolipoprotein E posttranslational variants, storage artifacts or faint isoelectric focusing bands.

Phenotyping of apolipoprotein E using immobilized pH gradient gels for one-dimensional and two-dimensional separations

Apolipoprotein E (apo E) is a normal component of several classes of plasma lipoproteins. Apo E phenotypes are closely related to total cholesterol, low density lipoprotein (LDL)-cholesterol and apo B concentration. The apo E 2/2 phenotype is related to the type III hyperlipoproteinemia due to the defective binding of apo E-2 to the hepatic receptors. The apo E 4/4 phenotype has been reported to be present in most elderly people suffering from the Alzheimer disease, and is associated with increased risk of coronary heart disease and Creutzfeld-Jakob disease. Therefore, apo E phenotyping is essential. The method described here uses a precast immobilized pH gradient, avoids time-consuming separation of lipoproteins from plasma, needs no pretreatment with neuraminidase and involves highly sensitive enhanced chemiluminescence for visualization. Therefore it has many advantages over previously published methods.