Evidence for a quantitative trait locus affecting low levels of apolipoprotein B and low density lipoprotein on chromosome 10 in Caucasian families

Plasma non-cholesterol sterols in primary hypobetalipoproteinemia

Primary hypobetalipoproteinemia (pHBL) is characterized by plasma cholesterol levels <5th percentile of a population distribution. Plasma non-cholesterol sterols (NCS) are markers of cholesterol liver synthesis and intestinal absorption. Plasma NCS were measured in 111 pHBL subjects, 108 low cholesterol (LC) and 253 normal cholesterol (NC) controls to gain information on cholesterol metabolism in pHBL, and to assess whether NCS measurements may aid in distinguishing pHBL from LC controls. pHBL subjects compared with LC controls were characterized by increased cholesterol absorption (campesterol/TC) while the synthesis (lathosterol/TC) was not increased. The analysis of pHBL subjects divided by gene defect showed a high campesterol/TC ratio in familial HBL (FHBL) carriers of apolipoproteinB (ApoB) truncations longer than ApoB48 and in FHBL without known gene defect ("not linked"). One not linked kindred was characterized by an increase of the 7-dehydrocholesterol/latho ratio. In a discriminant analysis plasma NCS did not improve the power of TC levels to distinguish FHBL from LC controls. In conclusion, increased cholesterol absorption was found in FHBL subjects harbouring truncations of ApoB>ApoB48, and FHBL harbouring as yet unknown molecular defects. Not linked FHBL kindred are not homogeneous in terms of plasma NCS levels. NCS cannot replace genetic HBL analysis.

Exome sequencing, ANGPTL3 mutations, and familial combined hypolipidemia

We sequenced all protein-coding regions of the genome (the "exome") in two family members with combined hypolipidemia, marked by extremely low plasma levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides. These two participants were compound heterozygotes for two distinct nonsense mutations in ANGPTL3 (encoding the angiopoietin-like 3 protein). ANGPTL3 has been reported to inhibit lipoprotein lipase and endothelial lipase, thereby increasing plasma triglyceride and HDL cholesterol levels in rodents. Our finding of ANGPTL3 mutations highlights a role for the gene in LDL cholesterol metabolism in humans and shows the usefulness of exome sequencing for identification of novel genetic causes of inherited disorders. (Funded by the National Human Genome Research Institute and others.).

Linkage and association analyses identify a candidate region for apoB level on chromosome 4q32.3 in FCHL families

Familial combined hyperlipidemia (FCHL) is a complex trait leading to cardiovascular disease (CVD) risk. Elevated levels and size of apolipoprotein B (apoB) and low-density lipoprotein (LDL) are associated with FCHL, which is genetically heterogeneous and is likely caused by rare variants. We carried out a linkage-based genome scan of four large FCHL pedigrees for apoB level that is independent of LDL: apoB level that is adjusted for LDL level and size. Follow-up included SNP genotyping in the region with the strongest evidence of linkage. Several regions with the evidence of linkage in individual pedigrees support the rare variant model. Evidence of linkage was strongest on chromosome 4q, with multipoint analysis in one pedigree giving LOD = 3.1 with a parametric model, and a log Bayes Factor = 1.5 from a Bayesian oligogenic approach. Of the 293 SNPs spanning the implicated region on 4q, rs6829588 completely explained the evidence of linkage. This SNP accounted for 39% of the apoB phenotypic variance, with heterozygotes for this SNP having a trait value that was approximately 30% higher than that of the high-frequency homozygote, thus identifying and considerably refining a strong candidate region. These results illustrate the advantage of using large pedigrees in the search for rare variants: reduced genetic heterogeneity within single pedigrees coupled with the large number of individuals segregating otherwise-rare single variants leads to high power to implicate such variants.