Low-density lipoprotein receptor point mutation results in expression of both active and inactive surface forms of the same mutant receptor

Abstract

LDL receptors, expressed in cultured fibroblasts from patients homozygous for the FH Afrikaner-1 (FH1) mutation (Asp206 to Glu), are transported from the endoplasmic reticulum (ER) to the Golgi apparatus more slowly than in normal cells. In the present study, binding characteristics of FH1 cells for lipoprotein ligands (LDL and beta VLDL) and for receptor-specific monoclonal antibodies pointed to the existence of two surface forms of the same mutant receptor. One of these forms bound lipoproteins with normal high affinity whereas another did not. Binding studies of transfected hamster cells expressing only the mutant human gene confirmed the single-gene origin of the different forms. The existence of functionally distinct forms of the receptor protein was supported by the observation that only lipoprotein-binding receptor molecules were trapped intracellularly and degraded following ammonium chloride treatment of cells in the presence of ligand. The lipoprotein-binding receptor population was indistinguishable from normal receptors with respect to its affinity for LDL and beta VLDL, uptake and degradation of lipoprotein, and receptor recycling. Ligand blotting versus immunoblotting of receptors revealed normal-sized mutant receptors that were not recognized by lipoprotein ligand. Despite these differences, both mutant forms of the receptor were degraded at rates similar to those of normal receptors. We propose that the single amino acid substitution in this receptor interferes with the folding and/or posttranslational processing of precursor molecules in such a way that receptors adopt alternative stable structures.

Comparison between medium-chain acyl-CoA dehydrogenase mutant proteins overexpressed in bacterial and mammalian cells

“…The processing of the mutant protein found in cystic fibrosis (AF508-CFTR), complexed to hsp70, can be modulated by the temperature (Denning et al, 1992). A slow processing mutant of the LDL-receptor, which may be modulated under different conditions and in different cell types, has also been described (Fourie et al, 1992).…”

Characterization of six patients who are double heterozygotes for familial hypercholesterolemia and familial defective apo B-100.

“…13 FH Afrikaner-2 (VaLwe^Met) is also slowly processed, and very low numbers of FH Afrikaner-2 receptors are present on the cell surface, as these mature receptors are abnormally rapidly degraded.…”

Phenotypic variation among familial hypercholesterolemics heterozygous for either one of two Afrikaner founder LDL receptor mutations.

“…The FH Afrikaner-1 (FHl) mutation (Asp^ to Glu; GAC to GAG) in exon 4 of the LDLR gene results in the formation of functionally distinct forms of the mutant receptor; one form exhibits normal receptor activity, whereas another is unable to bind lipoprotein ligands. 8 ' 9 As a result, fully upregulated cells homozygous for this mutation express about 20% of normal receptor activity. The FH Afrikaner-2 (FH2) mutation (VaLwe to Met; GTG to ATG) in exon 9 causes the receptor to be rapidly degraded and results in very low receptor activity (<2% of normal receptor activity).…”