The Molecular Mechanism for the Genetic Disorder Familial Defective Apolipoprotein B100

Abstract

Familial defective apolipoprotein B100 (FDB) is a genetic disorder in which low density lipoproteins (LDL) bind defectively to the LDL receptor, resulting in hypercholesterolemia and premature atherosclerosis. FDB is caused by a mutation (R3500Q) that changes the conformation of apolipoprotein (apo) B100 near the receptorbinding site. We previously showed that arginine, not simply a positive charge, at residue 3500 is essential for normal receptor binding and that the carboxyl terminus of apoB100 is necessary for mutations affecting arginine 3500 to disrupt LDL receptor binding. Thus, normal receptor binding involves an interaction between arginine 3500 and tryptophan 4369 in the carboxyl tail of apoB100. W4369Y LDL and R3500Q LDL isolated from transgenic mice had identically defective LDL binding and a higher affinity for the monoclonal antibody MB47, which has an epitope flanking residue 3500. We conclude that arginine 3500 interacts with tryptophan 4369 and facilitates the conformation of apoB100 required for normal receptor binding of LDL. From our findings, we developed a model that explains how the carboxyl terminus of apoB100 interacts with the backbone of apoB100 that enwraps the LDL particle. Our model also explains how all known ligand-defective mutations in apoB100, including a newly discovered R3480W mutation in apoB100, cause defective receptor binding. The interaction between low density lipoprotein (LDL)1 and the LDL receptor is fundamental for the regulation of plasma cholesterol in humans (1). The sole protein component of LDL is apolipoprotein (apo) B100, which serves as the ligand for the LDL receptor (2). We recently identified the sequence in apoB100 that interacts with the LDL receptor (3). However, the ability of apoB100 to interact with the LDL receptor depends not only on sequence, but also on conformation, because apoB100 binds to the LDL receptor only after the hydrolysis of triglyceride-rich very low density lipoproteins (VLDL) to smaller cholesterol-rich LDL (1).Familial defective apoB100 (FDB) is a genetic disorder of LDL metabolism characterized by hypercholesterolemia and premature atherosclerosis (4, 5). Estimated to occur in 1/500 to 1/700 people in several Caucasian populations in North America and Europe, FDB is one of the most common single-gene defects known to cause an inherited abnormality (6, 7). Almost everyone with FDB is of European descent; in most cases, the CGG-to-CAG mutation in the codon for amino acid 3500 is on a chromosome with a rare haplotype at the apoB locus, suggesting that most probands descended from a common ancestor who lived in Europe about 6,750 years ago (8). The mutation substitutes glutamine for the normally occurring arginine at position 3500 (4). Except for a few cases in which tryptophan substitutes for arginine 3500 (9) or cysteine for arginine 3531 (10), leading to a minor decrease in LDL receptor binding, extensive searches have not revealed any other apoB100 mutations that cause defective receptor binding of LDL (10). Both of these mutation...

Molecular Characterization of Iranian Patients with Possible Familial Hypercholesterolemia

“…Therefore mutations in the APOB 100 will drastically alter its functional activity leading to a decrease in its binding to LDLR, thereby delaying the clearance of LDL particles. The latter situation usually results in a mild or severe form of hypercholesterolemia together with an increased risk for early onset atherosclerosis [10,11]. In contrast to LDLR, only a small number of functional mutations have been identified in APOB gene such as R3500Q [12], R3500 W [13], and R3531C [14].…”

Update of the Portuguese Familial Hypercholesterolaemia Study

“…At present, more than 1000 mutations have been described worldwide in the LDLR gene (http://www.ucl.ac.uk/fh) and the residual LDL receptor activity varies considerably between those [4]. Mutations in exons 26 and 29 of the APOB gene have been identified in FH patients, the most common of which is the nucleotide change c.10708G>A, predicted to lead to the amino acid substitution p.Arg3527Gln [5]. From the PCSK9 mutations reported worldwide [6] none was found in the Portuguese population except the mutation, p.Asp374His, located at the same codon as the p.Asp374Tyr described previously [6], but leading to a different amino acid substitution.…”

Mutation in Apolipoprotein B Associated with Hypobetalipoproteinemia Despite Decreased Binding to the Low Density Lipoprotein Receptor

“…In this study, we report the characteristics of a new liganddefective mutation in APOB, R3480P, located in a region of apoB-100 critical for modulation of binding of VLDL, IDL, and LDL to the LDL receptor (4,7,12). Our data indicate that APOB R3480P heterozygosity occurs at a frequency of 0.04% (ϳ1 in 2,000) in a general population of whites and is associated with hypobetalipoproteinemia, despite an impaired ability to interact with the LDL receptor.…”

“…Furthermore, an accidentally identified R3480W heterozygote had hypobetalipoproteinemia as well as both reduced production and catabolism of LDL, supporting our findings for R3480P heterozygotes. Finally, R3480W has been shown by others to have reduced affinity for the LDL receptor in vitro (7).…”

“…All three mutations result in the loss of arginines and in varying degrees of decreased receptor affinity in vitro, reflected in the varying effects on lipid and lipoprotein levels in vivo; R3500Q and R3500W are associated with moderate to severe hypercholesterolemia, whereas R3531C has no effect or a marginal effect on lipid levels in the general population in vivo (5)(6)(7)(8). Mutations resulting in truncations of apoB-100 are known to abolish LDL receptor binding when located amino-terminal to the putative binding site at amino acids 3359 -3369 (site B), indicating that this site is crucial for receptor interaction.…”