The functional characteristics of a human apolipoprotein E variant (cysteine at residue 142) may explain its association with dominant expression of type III hyperlipoproteinemia

Type III hyperlipoproteinemia typically is associated with homozygosity for apolipoprotein (apo) E2(Arg¹⁵⁸ → Cys). Dominant expression of type III hyperlipoproteinemia associated with apoE phenotype E3/3 is caused by heterozygosity for a human apoE variant, apoE3(Cys¹¹² → Arg, Arg¹⁴² → Cys). However, this apoE3 variant was not separable from the normal apoE3 in these patients' plasma because the two proteins have identical amino acid composition, charge, and molecular weight. Therefore, to determine the functional characteristics of this protein, we used recombinant DNA techniques to produce this apoE variant in bacteria. We also produced a non-naturally occurring variant, apoE(Arg¹⁴² → Cys), that had only the cysteine substituted at residue 142. These two apoE variants were purified from cell lysates of the transfected Escherichia coli by ultracentrifugal flotation in the presence of phospholipid, by gel filtration chromatography, and by heparin-Sepharose chromatography. Both Cys¹⁴² apoE variants bound to lipoprotein receptors on human fibroblasts with only about 20% of normal binding activity. Therefore, cysteine at residue 142, not arginine at residue 112, is responsible for the decreased receptor binding activity of the variants. Cysteamine treatment and removal of the carboxyl-terminal domain had little effect on the binding activity, whereas both modulate the receptor binding activity of apoE2(Arg¹⁵⁸ → Cys). The mutation at residue 142 decreased the binding activity of apoE to both heparin and the monoclonal antibody 1D7 (this antibody inhibits receptor binding of apoE), whereas apoE2(Arg¹⁵⁸ → Cys), which is associated with recessive expression of type III hyperlipoproteinemia, binds normally to both. The Arg¹¹², Cys¹⁴² variant predominates 3:1 over normal apoE3 in the very low density lipoproteins of plasma from an affected subject, as assessed by differential reactivity with the antibody 1D7. The unique combination of functional properties of the Arg¹¹², Cys¹⁴² variant provides a possible explanation for its association with dominant expression of type III hyperlipoproteinemia.