ApoB-75, a truncation of apolipoprotein B associated with familial hypobetalipoproteinemia: genetic and kinetic studies.
We have identified a mutation of apolipoprotein B (apoB) in a kindred with hypobetalipoproteinemia. Four affected members had plasma concentrations of total cholesterol of 115 +/- 14, low density lipoprotein (LDL)-C of 48 +/- 11, and apoB of 28 +/- 9 (mg/dl mean +/- SD). The values correspond to approximately 30% the values for unaffected relatives. Triglyceride and high density lipoprotein (HDL)-C concentrations were 92 +/- 50 and 49 +/- 4, respectively, neither significantly different from unaffected relatives. Western blots of plasma apoB of affected subjects showed two major bands: apoB-100 and an apoB-75 (mol wt of approximately 418,000). DNA sequencing of the appropriate polymerase chain reaction (PCR)-amplified genomic DNA segment revealed a deletion of the cytidine at nucleotide position 10366, resulting in a premature stop codon at amino acid residue 3387. In apoB-75/apoB-100 heterozygotes, two LDL populations containing either apoB-75 or apoB-100 could be distinguished from each other by gel permeation chromatography and by immunoblotting of nondenaturing gels using monoclonal antibodies B1B3 (epitope between apoB amino acid residues 3506-3635) and C1.4 (epitope between residues 97-526). ApoB-75 LDL were smaller and more dense than apoB-100 LDL. To determine whether the low concentration of apoB-75 was due to its enhanced LDL-receptor-mediated removal, apoB-75 LDL were isolated from the proband's d 1.063-1.090 g/ml fraction (which contained most of the apoB-75 in his plasma) by chromatography on anti-apoB and anti-apoA-I immunoaffinity columns. The resulting pure apoB-75 LDL fraction interacted with the cells 1.5-fold more effectively than apoB-100 LDL (d 1.019-1.063 g/ml). To determine the physiologic mechanism responsible for the hypobetalipoproteinemia, in vivo kinetic studies were performed in two affected subjects, using endogenous labeling of apoB-75 and apoB-100 with [13C]leucine followed by multicompartmental kinetic analyses. Fractional catabolic rates of apoB-75 VLDL and LDL were 2- and 1.3-fold those of apoB-100 very low density lipoprotein (VLDL) and LDL, respectively. Production rates of apoB-75 were approximately 30% of those for apoB-100. This differs from the behavior of apoB-89, a previously described variant, whose FCRs were also increased approximately 1.5-fold relative to apoB-100, but whose production rates were nearly identical to those of apoB-100. Thus, in contrast to the apoB-89 mutation, the apoB-75 mutation imparts two physiologic defects to apoB-75 lipoproteins that account for the hypobetalipoproteinemia, diminished production and increased catabolism.
Krul
E S
ES
Lipid Research Center, Washington University School of Medicine, St. Louis, MO 63110.
Parhofer
K G
KG
Barrett
P H
PH
Wagner
R D
RD
Schonfeld
G
G
D000328:Adult; D000368:Aged; D000595:Amino Acid Sequence; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000097:blood / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D015153:Blotting, Western; D002478:Cells, Cultured; D002648:Child; D004247:DNA; D005260:Female; D006801:Humans; D006995:Hypobetalipoproteinemias / Q000097:blood / Q000235:genetics*; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D009154:Mutation; D010375:Pedigree; D016133:Polymerase Chain Reaction; D011973:Receptors, LDL / Q000378:metabolism
D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D008077:Lipoproteins, LDL; D011973:Receptors, LDL; D004247:DNA