Familial hypobetalipoproteinemia: a review

Genetic variants of ApoE account for variability of plasma low-density lipoprotein and apolipoprotein B levels in FHBL

We report two novel APOB mutations causing short apolipoprotein B (apoB) truncations undetectable in plasma and familial hypobetalipoproteinemia (FHBL). In Family 56, a 5 bp deletion in APOB exon 7 (870_874del5) causes a frame shift, converting tyrosine to a stop codon (Y220X) and producing an apoB-5 truncation. In Family 59, a point mutation (1941G>T) in APOB exon 13 converts glutamic acid to stop codon (E578X), specifying apoB-13. A recurrent mutation in exon 26 (4432delT) produces apoB-30.9 in Family 58. In some members of these families, we observed that plasma low-density lipoprotein (LDL) cholesterol and apoB levels were unusually low even for subjects heterozygous for FHBL. To ascertain whether genetic variations in apolipoprotein E (apoE) would explain some of the variations of apoB and LDL cholesterol levels, apoE genotypes were assessed in affected subjects from a total of eight FHBL families with short apoB truncations. Heterozygous FHBL with the epsilon3/epsilon4 genotype had 10-1 5mg/dL higher plasma LDL cholesterol and apoB levels compared to subjects with the epsilon2/epsilon3 and epsilon3/epsilon3 genotypes. The apoE genotype has been reported to account for approximately 10% of the variation of LDL cholesterol in the general population. It accounted for 15-60% of the variability of plasma LDL cholesterol or apoB levels in our FHBL subjects. The physiologic bases for the greater effects of apoE in FHBL remain to be determined.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

A genomewide search using an original pairwise sampling approach for large genealogies identifies a new locus for total and low-density lipoprotein cholesterol in two genetically differentiated isolates of Sardinia

A powerful approach to mapping the genes for complex traits is to study isolated founder populations, in which genetic heterogeneity and environmental noise are likely to be reduced and in which extended genealogical data are often available. Using graph theory, we applied an approach that involved sampling from the large number of pairwise relationships present in an extended genealogy to reconstruct sets of subpedigrees that maximize the useful information for linkage mapping while minimizing calculation burden. We investigated, through simulation, the properties of the different sets in terms of bias in identity-by-descent (IBD) estimation and power decrease under various genetic models. We applied this approach to a small isolated population from Sardinia, the village of Talana, consisting of a unique large and complex pedigree, and performed a genomewide search through variance-components linkage analysis for serum lipid levels. We identified a region of significant linkage on chromosome 2 for total serum cholesterol and low-density lipoprotein (LDL) cholesterol. Through higher-density mapping, we obtained an increased linkage for both traits on 2q21.2-q24.1, with a LOD score of 4.3 for total serum cholesterol and of 3.9 for LDL cholesterol. A replication study was performed in an independent and larger set from a genetically differentiated isolated population of the same region of Sardinia, the village of Perdasdefogu. We obtained consistent linkage to the region for total serum cholesterol (LOD score 1.4) and LDL cholesterol (LOD score 2.2), with a level of concordance uncommon for complex traits, and refined the location of the quantitative-trait locus. Interestingly, the 2q21.1-22 region has also been linked to premature coronary heart disease in Finns, and, in the adjacent 2q14 region, significant linkage with triglycerides has been reported in Hutterites.

Liver dysfunction and steatosis in familial hypobetalipoproteinemia

A 32-year-old man presented with increases in serum alanine aminotransferase activity, iron concentration, and transferrin saturation, suggestive of hepatic dysfunction and iron overload. In addition, he had unusually low plasma concentrations of LDL-cholesterol and apolipoprotein (apo) B. Hepatic ultrasonography was consistent with fatty liver. On liver biopsy, marked steatosis and moderate to marked iron deposition were observed. The patient was found to carry the HFE C282Y and H63D mutations, which are associated with hereditary hemochromatosis, and the alpha(1)-antitrypsin PiZ variant. An immunoblot of plasma for apoB showed the presence of a truncated apoB species, indicative of familial hypobetalipoproteinemia. DNA sequence analysis revealed that the patient was heterozygous for the apoB-80.5 (c.11040T>G) mutation. This unique case shows an unusual combination of underlying disorders that could all be contributing to liver dysfunction and fatty liver.

Lipid Disorders and Mutations in the APOB Gene

Background: Plasma lipoproteins are important determinants of atherosclerosis. Apolipoprotein (apo) B is a large, amphipathic glycoprotein that plays a central role in human lipoprotein metabolism. Two forms of apoB are produced from the APOB gene by a unique posttranscriptional editing process: apoB-48, which is required for chylomicron production in the small intestine, and apoB-100, required for VLDL production in the liver. In addition to being the essential structural component of VLDL, apoB-100 is the ligand for LDL-receptor-mediated endocytosis of LDL particles.

Content: The study of monogenic dyslipidemias has revealed important aspects of metabolic pathways. In this review, we discuss the regulation of apoB metabolism and examine how APOB gene defects can lead to both hypo- and hypercholesterolemia. The key clinical, metabolic, and genetic features of familial hypobetalipoproteinemia and familial ligand-defective apoB-100 are described.

Summary: Missense mutations in the LDL-receptor-binding domain of apoB cause familial ligand-defective apoB-100, characterized by hypercholesterolemia and premature coronary artery disease. Other mutations in APOB can cause familial hypobetalipoproteinemia, characterized by hypocholesterolemia and resistance to atherosclerosis. These naturally occurring mutations reveal key domains in apoB and demonstrate how monogenic dyslipidemias can provide insight into biologically important mechanisms.

In vivo MRS measurement of liver lipid levels in mice

A magnetic resonance spectroscopy (MRS) procedure for in vivo measurement of lipid levels in mouse liver is described and validated. The method uses respiratory-gated, localized spectroscopy to collect proton spectra from voxels within the mouse liver. Bayesian probability theory analysis of these spectra allows the relative intensities of the lipid and water resonances within the liver to be accurately measured. All spectral data were corrected for measured spin-spin relaxation. A total of 48 mice were used in this study, including wild-type mice and two different transgenic mouse strains. Different groups of these mice were fed high-fat or low-fat diets or liquid diets with and without the addition of alcohol. Proton spectra were collected at baseline and, subsequently, every 4 weeks for up to 16 weeks. Immediately after the last MRS measurement, mice were killed and their livers analyzed for triglyceride level by conventional wet-chemistry methods. The excellent correlation between in vivo MRS and ex vivo wet-chemistry determinations of liver lipids validates the MRS method. These results clearly demonstrate that in vivo MRS will be an extremely valuable technique for longitudinal studies aimed at providing important insights into the genetic, environmental, and dietary factors affecting fat deposition and accumulation within the mouse liver.

Fatty liver in familial hypobetalipoproteinemia: roles of the APOB defects, intra-abdominal adipose tissue, and insulin sensitivity

Fatty liver is frequent in the apolipoprotein B (apoB)-defective genetic form of familial hypobetalipoproteinemia (FHBL), but interindividual variability in liver fat is large. To explain this, we assessed the roles of metabolic factors in 32 affected family members with apoB-defective FHBL and 33 related and unrelated normolipidemic controls matched for age, sex, and indices of adiposity. Two hour, 75 g oral glucose tests, with measurements of plasma glucose and insulin levels, body mass index, and waist-hip ratios were obtained. Abdominal subcutaneous, intraperitoneal (IPAT), and retroperitoneal adipose tissue masses were quantified by MR imaging, and hepatic fat was quantified by MR spectroscopy. Mean +/- SD liver fat percentage values of FHBL and controls were 14.8 +/- 12.0 and 5.2 +/- 5.9, respectively (P = 0.001). Means for these measures of obesity and insulin action were similar in the two groups. Important determinants of liver fat percentage were FHBL-affected status, IPAT, and area under the curve (AUC) insulin in both groups, but the strongest predictors were IPAT in FHBL (partial R(2) = 0.55, P < 0.0002) and AUC insulin in controls (partial R(2) = 0.59, P = 0.0001). Regression of liver fat percentage on IPAT fat was significantly greater for FHBL than for controls (P < 0.001). In summary, because apoB-defective FHBL imparts heightened susceptibility to liver triglyceride accumulation, increasing IPAT and insulin resistance exert greater liver fat-increasing effects in FHBL.

The c.419-420insA in the MTP gene is associated with abetalipoproteinemia among French-Canadians

Abetalipoproteinemia (ABL) is a rare autosomal recessive disease characterised by the absence of apolipoprotein B (apoB) containing lipoproteins and, in consequence, very low triglyceride and cholesterol levels. Microsomal triglyceride transfer protein (MTP) has been associated with ABL. A search for sequence variants in the large subunit of MTP in a kindred of 10 individuals from Saguenay-Lac-St Jean area with a propositus exhibiting ABL as well as in four independent patients from the greater Quebec city area and exhibiting very low apoB and LDL-cholesterol levels identified 12 variations. Only one sequence variation, the c.419-420insA, was observed, in the homozygous form, in the abetalipoproteinemic patient. The -493G/-400A/-164T/282G/383T/419-420insA/453T/891C/969T/1151A/2884G haplotype carries the insertion and was found in all members of the family studied. In conclusion, the present study showed that the c.419-420insA alone, in the homozygous form, is a cause of classical recessive inherited ABL in the French-Canadian population.

Hypobetalipoproteinemia with an apparently recessive inheritance due to a “de novo” mutation of apolipoprotein B

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder either linked or not linked to apolipoprotein (apo) B gene. Abetalipoproteinemia (ABL) is a recessive disorder due to mutations of microsomal triglyceride transfer protein (MTP) gene. We investigated a patient with apparently recessive hypobetalipoproteinemia consistent with symptomatic heterozygous FHBL or a "mild" form of ABL. The proband had fatty liver associated with LDL-cholesterol (LDL-C) and apo B levels <5th percentile but no truncated apo B forms detectable in plasma. MTP gene sequence revealed that he was a carrier of the I128T polymorphism and an unreported amino acid substitution (V168I) unlikely to be the cause of hypobetalipoproteinemia. Apo B gene sequence showed that he was heterozygous for two single base substitutions in exon 9 and 22 resulting in a nonsense (Q294X) and a missense (R1101H) mutation, respectively. Neither of his parents carried the Q294X; his father and paternal grandmother carried the R1101H mutation. Analysis of polymorphic genetic markers excluded non-paternity. In conclusion, the proband has a "de novo" mutation of apo B gene resulting in a short truncated apo B form (apo B-6.46). Sporadic cases of FHBL with an apparently recessive transmission may be caused by "de novo" mutations of apo B gene.