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Background We previously showed that patients without pathogenic variants in the LDLR and PCSK9 genes comprised approximately 40% of familial hypercholesterolemia (FH) cases. Objective Our aim was to identify novel causative variants in Japanese patients with FH. Methods Whole-exome sequencing was performed in 216 family members from 123 families without pathogenic variants in the LDLR and PCSK9 genes. Clinical and biochemical data were gathered from the family members. Results The known p.(Arg3527Gln) variant in the APOB gene was identified in one Japanese family. The other pathogenic variants in the APOB gene were not identified. The p.(Arg3527Gln) variant was not identified in the other 113 index cases without pathogenic variants in the LDLR and PCSK9 genes. The allele frequency of the p.(Arg3527Gln) variant was 0.0001 in the general Japanese population. Conclusion This is the first report of Japanese cases of FH caused by a known pathogenic APOB variant, p.(Arg3527Gln).

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 beta-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 approximately 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.

Familial hypobetalipoproteinemia (FHBL) exists in three forms: a) FHBL genetically linked to truncated forms of apolipoprotein B (apoB); b) FHBL linked to the apoB gene but with no apoB truncations; and c) FHBL not linked to the apoB gene. Mean production rate (PR) of apoB-100 in FHBL subjects heterozygous for apoB truncations is approximately 30% of normal. In a 49-member D-kindred (FHBL phenotype defined as apoB < 40 mg/dl), no apoB truncations were detectable either by immunoblotting of plasma or by sequencing of relevant stretches of the apoB gene. Herein we report on the kinetic parameters of apoB-100-containing lipoproteins in four affected members of the D-kindred, and compare their kinetic values to 14 normal subjects, and 8 previously reported FHBL subjects heterozygous for various truncated forms of apoB. After an 8-h primed intravenous infusion of [13C]-leucine, enrichments of apoB-100 were assessed by gas chromatography-mass spectrometry and kinetic parameters were calculated by multicompartmental modeling. The affected members of the D-kindred had similar very low, intermediate, and low density lipoprotein (VLDL, IDL, and LDL) PRs as normal controls, but their fractional catabolic rates (FCR) for VLDL and LDL were approximately 2 and 3 times higher, respectively, than those of normals. By contrast in apoB truncation subjects, apoB-100 PRs were uniformly reduced, while apoB-100 FCRs were similar to normals. Thus, diverse physiologic mechanisms are responsible for the low apoB levels in these two different, genetically determined forms of FHBL.

Sudden death constitutes one of the most frequent causes of death in adults. In order to look for genetic markers that could play a role in the sudden death in adults (SDA), the common deletions of 4977 bp in the mitochondrial DNA and variations in the apoB gene were studied. The correlation of mtDNA common deletion with SDA victims might offer a rapid and simple tool to provide additional information in complex SDA investigations