A Novel Nontruncating APOB Gene Mutation, R463W, Causes Familial Hypobetalipoproteinemia

• Published in: The Journal of biological chemistry Vol. 278; no. 15; pp. 13442 - 13452
• Main Authors: Burnett, John R, Shan, Jing, Miskie, Brooke A, Whitfield, Amanda J, Yuan, Jane, Tran, Khai, McKnight, C James, Hegele, Robert A, Yao, Zemin
• Format: Journal Article
• Language: English
• Published: United States American Society for Biochemistry and Molecular Biology 11-04-2003
• Subjects: Amino Acid Substitution; Apolipoprotein B-100; Apolipoprotein B-48; Apolipoproteins B - chemistry; Apolipoproteins B - genetics; Arginine; Base Sequence; Female; Genetic Carrier Screening; Humans; Hypobetalipoproteinemias - genetics; Male; Models, Molecular; Mutation, Missense; Oligodeoxyribonucleotides - chemistry; Pedigree; Point Mutation; Protein Conformation
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M300235200

Familial hypobetalipoproteinemia (FHBL), an autosomal co-dominant disorder, is associated with reduced plasma concentrations
(<5th percentile for age and sex) of apolipoprotein (apo) B and β-migrating lipoproteins. To date, only mutations in APOB encoding prematurely truncated apoB have been found in FHBL. We discovered a novel APOB gene mutation, namely R463W, in an extended Christian Lebanese FHBL kindred. Heterozygotes for R463W had the typical FHBL
phenotype, whereas homozygotes had barely detectable apoB-100. The effect of the R463W mutation on apoB secretion was examined
using transfected McA-RH7777 cells that expressed one of two recombinant human apoBs, namely B48 and B17. In both cases, the
mutant proteins (B48RW and B17RW) were retained within the endoplasmic reticulum and were secreted poorly compared with their
wild-type counterparts. Pulse-chase analysis showed that secretion efficiencies of B48RW and B17RW were, respectively, 45
and 40% lower than those of the wild-types. Substitution of Arg 463 with Ala in apoB-17 (B17RA) decreased secretion efficiency by ∼50%, but substitution with Lys (B17RK) had no effect on secretion,
indicating that the positive charge was important. Molecular modeling of apoB predicted that Arg 463 was in close proximity to Glu 756 and Asp 456 . Substitution of Glu 756 with Gln (B17EQ) had no effect on secretion, but substitution of Asp 456 with Asn (B17DN) decreased secretion to the same extent as B17RW. In co-transfection experiments, the mutant B17RW showed
increased binding to microsomal triglyceride transfer protein as compared with wild-type B17. Thus, the naturally occurring
R463W mutant reveals a key local domain governing assembly and secretion of apoB-containing lipoproteins.