Familial defective apolipoprotein B-100: a mutation of apolipoprotein B that causes hypercholesterolemia.

We previously carried out genetic and metabolic studies in a partially inbred herd of pigs carrying cholesterol-elevating mutations. Quantitative pedigree analysis indicated that apolipoprotein (apo)B and a second major gene were responsible for the hypercholesterolemia in these animals. In this study, we assessed LDL receptor function by three different methods: ligand blots of liver membranes using ␤ -very low density lipoprotein (VLDL) as a ligand; low density lipoprotein (LDL)-dependent proliferation of T-lymphocytes; and direct binding of 125 I-labeled LDL to cultured skin fibroblasts. All three methods demonstrated that LDL receptor ligands bound with decreased affinity to the LDL receptor in these animals. In skin fibroblasts from the hypercholesterolemic pigs, the K d of binding was about 4-fold higher than in cells from normal pigs. The cDNA of the pig LDL receptor from normal and hypercholesterolemic pigs was isolated and sequenced. We identified a missense mutation that results in an Arg ¨ Cys substitution at the position corresponding to Arg 94 of the human LDL receptor. The mutation is in the third repeat of the ligand binding domain of the receptor. By single-stranded conformational polymorphism (SSCP) analysis, we studied the relationship between LDL receptor genotype and plasma cholesterol phenotype. In contrast to humans, the hypercholesterolemia associated with the LDL receptor mutation in pigs was expressed as a recessive trait. The LDL receptor mutation made a far more significant contribution to hypercholesterolemia than did the apoB mutation, consistent with observations made in human subjects with apoB mutations. Within each genotypic group (mutated apoB or mutated receptor), there was a wide range in plasma cholesterol. As the animals were on a well-controlled low-fat diet, this suggests that there are additional genetic factors that influence the penetrance of cholesterol-elevating mutations. -Grunwald, K. A

Section: Discussionsupporting

confidence: 85%

Identification of a novel Arg→Cys mutation in the LDL receptor that contributes to spontaneous hypercholesterolemia in pigs

Grunwald

Schueler

Uelmen

et al. 1999

“…In com-parison, the fractional catabolic rates of VLDL were similar in the three groups of subjects, indicating that neither the defective LDL receptor function nor the defective apoB-100 results in impaired VLDL removal. Compared to previously published data, the absolute production rates of VLDL apoB in control subjects of the present study were in a similar range (present study: 17.9 mg/kg/day; previous reports: 17.6 (9.9-36.7) mg/kg/day [mean, range] (5,7,14,(19)(20)(21)(22)(23)(24)(25)(26). The control subjects of the present study were on the average 18 yrs younger than the subjects with FH; Millar et al (27) reported that VLDL apoB APR increased slightly with age, however not to an extent explaining the present results in FH subjects.…”

Section: Discussionsupporting

confidence: 85%

“…Kinetic studies have not yet been performed comparing subjects with FH and with familial defective apoB-100 (FDB). FDB is an autosomal dominantly inherited disorder caused by a point mutation in the apoB-100 gene (7). Its prevalence in Switzerland is higher than in other countries (1:200) (8).…”

mentioning

confidence: 99%

VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100

Zulewski¹,

Ninnis²,

Miserez³

et al. 1998

Mutations in the apolipoprotein (apo) B, E (LDL) receptor gene and in the apolipoprotein B-100 gene are the cause of familial hypercholesterolemia (FH) and of familial defective apo B-100 (FDB), respectively. Whether these abnormalities lead to altered production or uptake of very low density lipoprotein (VLDL) or intermediate density lipoprotein (IDL) has not been established previously. Therefore VLDL and IDL apo B-100 kinetics were measured in seven subjects with FH, in six subjects with FDB, and in five normocholesterolemic controls using primed-constant infusions of [1-13 C]leucine. Absolute production rates (APR) of VLDL apoB were higher in FH than in controls (27.1 Ϯ 1.9 vs. 17.9 Ϯ 2.1 mg/kg/day P Ͻ 0.03). VLDL APR in FDB were between those of FH and controls (24.3 Ϯ 4.8 mg/kg/day), and demonstrated a relatively large inter-individual variability. The increase in VLDL APR in FH resulted in higher fasting serum triglyceride concentrations than in controls ( P Ͻ 0.05), whereas in FDB triglycerides were between those observed in FH and in controls. A significant correlation was observed between VLDL apoB APR and serum triglycerides in FH and in FDB; the correlation coefficient for all subjects was r ϭ 0.84 ( P Ͻ 0.0001), indicating that the major determinant of serum triglyceride concentrations was VLDL apoB APR. IDL apoB APR was lower in FH and in FDB compared to controls ( P Ͻ 0.03 P Ͻ 0.02, respectively): and its fractional catabolic rate (FCR) was slightly lower in FH and in FDB, resulting in similar plasma IDL apoB concentrations in all three groups of subjects. IDL apoB APR in FH were negatively correlated with LDL cholesterol concentrations ( r ϭ Ϫ 0.89; P Ͻ 0.001); LDL cholesterol concentrations correlated positively with the part of VLDL that did not appear in IDL ( r ϭ 0.82 P Ͻ 0.02), bypassing therefore the delipidation cascade.In conclusion the data demonstrate increased VLDL apoB production rates in FH. VLDL and IDL kinetics differ when LDL concentrations are elevated either due to a LDL receptor defect or due to defective apolipoprotein B-100.-Zulewski H., R. Ninnis, A. R. Miserez, M. W. Baumstark, and U. Keller. VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100.

“…that we found previously (4) and also to that reported by others (13,23). In a competitive binding assay ( Fig.…”

Section: Receptor-binding Affinity Of Ldlsupporting

confidence: 90%

“…subjects screened were pre-selected for lipid abnormalities. By comparison, the R3500Q mutation is present at 1 in 500 in populations of European descent (23). Other workers have recently reported the detection of additional individuals with the R3531C mutation.…”

Section: Discussionmentioning

confidence: 96%

The apolipoprotein B R3531C mutation: characteristics of 24 subjects from 9 kindreds

Pullinger

Gaffney

Gutiérrez

et al. 1999

Familial ligand-defective apolipoprotein B (apoB) is a group of disorders caused by mutations in the apoB gene. In this report the R3531C mutation is characterized further using a monoclonal antibody MB19/dynamic laser light scattering technique to measure ratios of Cys 3531 to normal low density lipoprotein (LDL) particles. All six subjects studied showed a preferential accumulation of particles carrying the defective apoB allotype. We determined binding properties of LDL from R3531C heterozygotes by measurement of high-affinity binding to LDL receptors on fibroblasts and its ability promote growth of U937 cells. LDL from R3531C heterozygotes, compared to normal LDL, had 49.3% of the binding affinity and was 74% as effective in a U937 cell proliferation assay. To identify new probands, we screened 2570 subjects for the R3531C mutation. Nine probands were found with 15 affected relatives. Of the seven haplotypes we uncovered, two were novel, while five were identical to one initially reported as associated with Cys 3531 . Three silent mutations were detected also: T3540T, N3542N and T3552T. Analysis of lipid profiles of R3531C families showed, as with the R3500Q mutation, variable expression of the phenotype, modulated by environmental and other genetic factors. Both mutations tend to produce lower plasma levels of LDL in affected subjects than do defects of the LDL receptor (familial hypercholesterolemia, FH). This study shows that the Cys 3531 LDL particles are not only defective at binding to the LDL receptor, as determined by two separate methods, but that in all cases they accumulate preferentially compared to the normal allotype.-