Search

Different isoforms of apoE modulate the concentrations of plasma lipoproteins and the risk for atherosclerosis. A novel apoE isoform, apoE4Freiburg, was detected in plasma by isoelectric focusing because its isoelectric point is slightly more acidic than that of apoE4. ApoE4Freiburg results from a base exchange in the APOE4 gene that causes the replacement of a leucine by a proline at position 28. Analysis of the allelic frequencies in whites in southwestern Germany revealed that this isoform is frequent among control subjects (10:4264 alleles) and is even more frequent in patients with coronary artery disease (21:2874 alleles; P=0.004; adjusted odds ratio, 3.09; 95% confidence interval, 1.20 to 7.97). ApoE4Freiburg affects serum lipoproteins by lowering cholesterol, apoB, and apoA-I compared with apoE4 (P<0.05). Our 4 apoE4Freiburg homozygotes suffered from various phenotypes of hyperlipoproteinemia (types IIa, IIb, IV, and V). In vitro binding studies excluded a binding defect of apoE4Freiburg, and in vivo studies excluded an abnormal accumulation of chylomicron remnants. ApoE4Freiburg and apoE4 accumulated to a similar extent in triglyceride-rich lipoproteins. HDLs, however, contained about 40% less apoE4Freiburg than apoE4. In conclusion, our data indicate that apoE4Freiburg exerts its possible atherogenic properties by affecting the metabolism of triglyceride-rich lipoproteins and HDL.

Screening of 6,840 plasma samples by isoelectric focusing (IEF) led to the identification of a novel apolipoprotein C-III variant. The underlying molecular defect was established by sequencing of exons 3 and 4 of the apoC-III gene subsequent to their amplification by the polymerase chain reaction (PCR). A G-->A transition in the first nucleotide of codon 45 results in a replacement of aspartic acid by asparagine. ApoC-III(Asp45-->Asn) was detected in a Turkish patient who previously had undergone coronary bypass surgery. Family studies identified two of the three children of the index patient as heterozygous variant carriers. The family was too small to demonstrate a significant effect of the variant on lipid metabolism. However, as judged by two-dimensional immunoelectrophoresis as well as IEF and subsequent scanning densitometry, the concentrations of the variant allele products were increased twofold in very low density lipoproteins (VLDL) and slightly decreased both in low density lipoproteins (LDL) and in high density lipoproteins (HDL) relative to the concentrations of the normal allele products. The disproportional distribution of the variant apoC-III isoproteins may indicate differences in the metabolism of variant and normal apoC-III. We conclude that genetically determined structural variants of apoC-III with changes in complete net charges are very rare and, hence, do not significantly contribute to the formation of dyslipidemia in the German population. Although heterozygosity for apoC-III(Asp45-->Asn) is not associated with severe dyslipidemia, the disproportional distribution of the allele products among plasma lipoproteins indirectly indicates some impact on lipoprotein metabolism.