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

Electrothermal atomic absorption spectrometric diagnosis of familial hypercholesterolemia

We have developed a new nonradioactive assay to identify human low-density lipoprotein receptor defects. It is based on the incubation of cultured cells with colloidal gold-LDL conjugates and quantitation of the gold associated with the cells by electrothermal atomic absorption spectrometry. After an oxidative treatment with nitric and hydrochloric acids, the biological matrix interferes neither with the gold recovery nor with the gold measurements, which are linear, at least from 0.15 to 3 ng of gold. When cells expressing a functional LDL receptor are incubated with increasing amounts of colloidal-gold LDL conjugates, the obtained saturation curve parallels that described when [125I]LDL is used as ligand. Moreover, this new assay allows us to clearly distinguish among fibroblasts from normal subjects or from heterozygous or homozygous patients of familial hypercholesterolemia, a very common autosomal disease. The assay is easy to perform, is sensitive, and avoids the use of radioactive compounds. Therefore, it could be successfully employed in the clinical diagnosis of this disease. Furthermore, since the methodology developed here can be applied to quantify the association of other gold-conjugated ligands to cells, it could have a widespread use in a variety of clinical and basic research studies.

A deletion in the first cysteine-rich repeat of the low-density-lipoprotein receptor leads to the formation of multiple misfolded isomers

The ligand-binding domain of the low-density-lipoprotein (LDL) receptor comprises seven cysteine-rich repeats, each approximately 40 amino acids long. The deletion of two amino acids (Asp26 and Gly27) from the first of these repeats (LB1), leads to a defective LDL receptor, and the clinical syndrome of familial hypercholesterolemia [Leitersdorf, E., Hobbs, H. H., Fourie, A. M., Jacobs, M., van der Westhuyzen, D.R. & Coetzee, G.A. (1988) Proc. Natl Acad. Sci. USA 85, 7912-7916]. Receptors which reach the cell surface fail to bind IgG-C7, a conformation-specific monoclonal antibody directed to LB1. To determine the effects of the two-amino-acid deletion on the folding of the LB1 of the LDL receptor, we have expressed LB1 and the mutant repeat, des-Asp26, Gly27-LB1, as recombinant (rLB1 and des-Asp26, Gly27-rLB1) peptides, and have determined their ability to fold in vitro. Unlike rLB1, which folded into a single isomer that was recognized by IgG-C7 and had three disulfide bonds, des-Asp26, Gly27-rLB1 folded into an equilibrium mixture of four isomers. Each of these isomers contained three disulfide bonds, but none were recognized by IgG-C7. We suggest that LDL receptors in the endoplasmic reticulum (ER) of the cell also fold into an equilibrium mixture of distinct receptor molecules, each with an abnormally folded isomer of des-Asp26, Gly27-LB1, and that the retarded transport of receptors to the cell surface arises because only a subset of the isomers reaches the cell surface.

FH Afrikaner-3 LDL receptor mutation results in defective LDL receptors and causes a mild form of familial hypercholesterolemia

Three founder-related gene mutations (FH Afrikaner-1, -2, and -3) that affect the LDL receptor are responsible for 90% of the familial hypercholesterolemia (FH) in South African Afrikaners. Patients heterozygous for the FH Afrikaner-1 (FH1) mutation, which results in receptors having approximately 20% of normal receptor activity, have significantly lower plasma cholesterol levels and milder clinical symptoms than heterozygotes with the FH Afrikaner-2 mutation, which completely abolishes LDL receptor activity. In this study we re-created the FH3 mutation (Asp154-->Asn) in exon 4 by site-directed mutagenesis and analyzed the expression of the mutant receptors in Chinese hamster ovary cells. The mutation resulted in the formation of LDL receptors that are markedly defective in their ability to bind LDL, whereas binding of apoE-containing beta-VLDL is less affected. The mutant receptors are poorly expressed on the cell surface as a result of significant degradation of receptor precursors. The plasma cholesterol levels of 31 FH3 heterozygotes were similar to FH1 heterozygotes but significantly lower than FH2 heterozygotes. The FH1 and FH3 heterozygotes also tended to be less severely affected clinically (by coronary heart disease and xanthomata) than FH2 patients. This study demonstrates that mutational heterogeneity in the LDL receptor gene influences the phenotypic expression of heterozygous FH and that severity of expression correlates with the activity of the LDL receptor measured in vitro. The results further indicate that knowledge of the specific mutation underlying FH in heterozygotes is valuable in determining the potential risk of premature atherosclerosis and should influence the clinical management of FH patients.

Abnormal structure and co-operative binding of low-density lipoprotein receptors containing the Glu-80-->Lys mutation

The properties of low-density lipoprotein (LDL) receptors containing a Glu to Lys substitution at position 80 have been studied in fibroblasts from a homozygous familial hypercholesterolaemic subject (MB) and in monkey COS cells transfected with the mutant cDNA. Receptors containing the Glu-80-->Lys mutation were processed more slowly than the normal protein and only approx. 50% reached the surface as the mature form. Both cell types exhibited a normal concentration binding curve for beta-very-low-density lipoproteins (beta-VLDL) but an atypical, sigmoidal curve for LDL. The mature mutant receptor protein migrated abnormally slowly on SDS-PAGE under non-reducing conditions but normally under reducing conditions or after treatment with neuraminidase. It also showed an unusual ability to form dimers that were stable in detergents. Transfected normal and mutant receptors were apparently cleaved on the surface of the cells to give a product lacking the NH2-terminal portion of the protein, which was resistant to further proteolytic digestion. The results suggest that the Glu-80-->Lys substitution produces a change in the conformation of the protein, stabilized by polysaccharide chains, which results in a strong self-association of receptor molecules that affects their ability to bind LDL.

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

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a potentially lethal inherited defect in the beta-oxidation of fatty acids. By comparing the behaviour of five missense MCAD mutant proteins expressed in COS cells and in Escherichia coli, we can define some of these as "pure folding mutants." Upon expression in E. coli, these mutant proteins produce activity levels in the range of the wild-type enzyme only if the chaperonins GroESL are co-overproduced. When overexpressed in COS cells, the pure folding mutants display enzyme activities comparable to the wild-type enzyme. The results suggest that the MCAD mutations can be modulated by chaperones, a phenomenon that may influence the manifestation of the MCAD disease.

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

Two common founder-related gene mutations that affect the low-density lipoprotein receptor (LDLR) are responsible for approximately 80% of familial hypercholesterolemia (FH) in South African Afrikaners. The FH Afrikaner-1 (FH1) mutation (Asp206-->Glu) in exon 4 results in defective receptors with approximately 20% of normal activity, whereas the FH Afrikaner-2 (FH2) mutation (Val408-->Met) in exon 9 completely abolishes LDLR activity (< 2% normal activity). We analyzed the contribution of these mutations and other factors on the variation of hypercholesterolemia and clinical features in Afrikaner FH heterozygotes. The type of FH mutation, plasma triglyceride levels, and age of patients each contributed significantly to the variation in hypercholesterolemia, whereas smoking status, high-density lipoprotein cholesterol levels, and gender had no influence. Although all FH heterozygotes had frank hypercholesterolemia, patients with the FH1 mutation had significantly lower cholesterol levels than those with the FH2 mutation. FH1 heterozygotes also tended to have milder clinical features. The differences between the two FH groups could not be explained by a difference in the common apolipoprotein E variants. This study demonstrates that mutational heterogeneity in the LDLR gene influences the phenotypic expression of heterozygous FH.

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

Familial defective apolipoprotein B-100 (FDB) and familial hypercholesterolemia (FH) are the common causes of monogenic primary hypercholesterolemia. An individual of mixed English and Afrikaner descent with both FDB and the FH Afrikaner-1 low-density lipoprotein receptor mutation was identified in our laboratory. Subsequent analysis of her extended family revealed the presence of heterozygotes for either FH Afrikaner-1, FH Afrikaner-2, or FDB as well as five additional double heterozygotes for FH Afrikaner-1 and FDB and one "complex" heterozygote with all three mutations. The hypercholesterolemic and clinical features of the pure FDB subjects were similar to those of the pure FH heterozygotes. The double heterozygotes with both FH and FDB have lipid levels and clinical features that are intermediate in severity between heterozygous and homozygous FH.