Identification of a novel apoB variant in a family exhibiting hypocholesterolemia: mechanistic insights

BACKGROUND Familial hypobetalipoproteinemia is a rare autosomal codominant disorder, often caused by a defect in apolipoprotein B (apoB) production required for lipoprotein formation and secretion. OBJECTIVE Characterization of the lipid profiles of 3 family members exhibiting very low circulating cholesterol levels. METHODS Plasma samples from the control sibling and the affected patients were analyzed. Fast protein liquid chromatography and high-performance liquid chromatography were used to characterize the lipid profiles, size, and distribution of lipoprotein particles. Exome sequencing of family members revealed a single-nucleotide deletion in APOB in the 3 affected individuals. The effect of the single-nucleotide deletion on the secretion of apoB was analyzed in Immortalized Human Hepatocyte (IHH) cells. RESULTS Plasma lipid profiles revealed that the affected individuals have low levels of total cholesterol and low-density lipoprotein cholesterol, with no difference in lipoprotein particle size. DNA sequencing of APOB revealed a single heterozygote deletion of an adenosine in exon 3 at the nucleotide position 1268 in all affected members. This deletion introduces a reading frame shift at glutamine 380, resulting in a stop codon at position 397. The C-terminally truncated apoB, called apoB9, is a variant spanning ∼9% of the full-length protein. Upon expression of apoB9 in IHH cells, the protein did not exit the endoplasmic reticulum/cis-Golgi and, hence, was not secreted into the media. Molecular modeling revealed that apoB9 lacks the βA- and βB-sheets that are required for lipid particle formation, which can explain the absence of apoB9 secretion. CONCLUSION Our data suggested that the affected family members have ∼50% to 60% lower apoB levels and are likely protected against the development of atherosclerosis and cardiovascular diseases.