Apolipoprotein E 2 ( Arg 136 → Cys ) mutation in the receptor binding domain of apoE is not associated with dominant type III hyperlipoproteinemia 1

Using apoE phenotyping by immunoblotting and apoE genotyping we identified four heterozygous carriers of a rare apolipoprotein (apo) E2 variant, apoE2 (Arg 136 → Cys). ApoE2 (Arg 136 → Cys) was not distinct from apoE2 (Arg 158 → Cys) by phenotyping, but produced a unique pattern of bands on Cfo I restriction typing of a 244 bp apoE gene fragment. Two of the four apoE2 (Arg 136 → Cys)/3 heterozygotes had elevated triglycerides, two were normolipidemic. The composition of very low density lipoproteins (VLDL) was normal in each of the four apoE2 (Arg 136 → Cys) carriers, regardless of the triglyceride concentrations. None of the apoE2 (Arg 136 → Cys) carriers displayed a broad b -band and none revealed b -migrating particles in the VLDL. The two hypertriglyceridemic carriers of apoE2 (Arg 136 → Cys) were, therefore, classified as having type IV rather than type III hyperlipoproteinemia. LDL receptor binding activities were studied using recombinant apoE loaded to dimyristoylphosphatidylcholine (DMPC) vesicles and to VLDL and from an apoE-deficient individual. LDL receptor binding of apoE2 (Arg 136 → Cys) was 14% of apoE3 and was thus higher than that of apoE2 (Arg 158 → Cys). Both apoE2 (Arg 136 → Cys) and apoE2 (Arg 158 → Cys) displayed substantial heparin binding (61 and 53% of apoE3, respectively). As the dominant apoE variants known so far are characterized by more pronounced reductions of heparin binding, we suggest that apoE2 (Arg 136 → Cys) is not associated with dominant expression of type III hyperlipoproteinemia. These findings lend support to the concept that apoE variants predisposing to dominant type III hyperlipoproteinemia differ from recessive mutations by a more severe defect in heparin binding.— März, W., M. M. Hoffmann, H. Scharnagl, E. Fisher, M. Chen, M. Nauck, G. Feussner, and H. Wieland . Apolipoprotein E2 (Arg 136 → Cys) mutation in the receptor binding domain of apoE is not associated with dominant type III hyperlipoproteinemia. J. Lipid Res. 1998. 39: 658–669. Supplementary key words very low density lipoproteins • apolipoprotein B • low density lipoprotein receptor • hypercholesterolemia • hypertriglyceridemia • dysbetalipoproteinemia • heparan sulfate proteoglycans Apolipoprotein (apo) E is a glycoprotein of 34 kDa (reviewed in refs. 1, 2). In plasma, it is associated with triglyceride-rich lipoproteins and high density lipoproteins. ApoE is a ligand of members of the LDL receptor family of membrane proteins including the LDL receptor (1), the LDL receptor-related protein (3), the VLDL receptor (4), and others (5, 6). The best characterized function of apoE is to mediate the uptake of chylomicron and very low density lipoprotein (VLDL) remnants into the liver. ApoE is polymorphic. In humans, three common alleles exist that are designated e 2, e 3, e 4; their products differ from one another at positions 112 and 158 of the amino acid sequence (1, 2, 7). ApoE3, the most frequent isoform, has arginine at position 112 and cysteine at position 158. ApoE4 has arginine, and apoE2 has cysteine at both positions. ApoE4 is associated with elevated LDL concentrations. It increases both the risk of artherosclerosis and Alzheimer’s disease (2, 8, 9). ApoE2 is defective in its binding to lipoprotein receptors (10, 11). Homozygosity for apoE2 is necessary, but not sufficient by itself to precipitate type III hyperlipoproteinemia (HLP) (1, 2, 12), a disorder of lipoprotein metabolism characterized by the accumulation of cholesterol-rich remnant lipoproteins derived from the partial catabolism of chyAbbreviations: apo, apolipoprotein; DMPC, dimyristoylphosphatidylcholine; DOTAP, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate; VLDL, LDL, HDL, very low, low, and high density lipoproteins; VLDL-C, LDL-C, HDL-C, cholesterol of VLDL, LDL, and HDL; HLP, hyperlipoproteinemia. 1 This article is dedicated to Prof. Dr. Werner Gross on the occasion of his 60th birthday. 2 To whom correspondence should be addressed. by gest, on O cber 8, 2017 w w w .j.org D ow nladed fom