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.