Radioimmunoassay of human arginine-rich apolipoprotein, apoprotein E. Concentration in blood plasma and lipoproteins as affected by apoprotein E-3 deficiency

A radioimmunoassay for apolipoprotein E in human blood serum has been developed that measures equally the major polymorphic species of the protein (apolipoproteins E-1, E-2, E-3, and E-4) and the apo E in the dimer of apolipoproteins E and A-II. The assay is specific and yields values for apolipoprotein E in very low density lipoproteins that agree closely with those obtained by a quantitative electrophoretic method. Apolipoprotein E is also present in at least one species of high density lipoprotein, but the content of apolipoprotein E in the lipoprotein fractions of plasma is uncertain owing to dissociation during ultracentrifugation. The concentration of apolipoprotein E is higher in serum of normolipidemic, premenopausal women than in men of comparable age and is a direct function of the serum triglyceride level. Apolipoprotein E levels are increased out of proportion to triglyceride levels in hyperlipidemic patients with familial dysbetalipoproteinemia (homozygotes for lack of apolipoprotein E-3). Heterozygous relatives of homozygotes have significantly higher apolipoprotein E levels in serum than unaffected relatives. The concentration of partially degraded (remnant) triglyceride-rich lipoproteins also appears to be increased in heterozygotes, who comprise about 15% of the population.

PCSK9 dominant negative mutant results in increased LDL catabolic rate and familial hypobetalipoproteinemia

OBJECTIVE

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a central player in the regulation of cholesterol homeostasis, increasing the low-density lipoprotein (LDL) receptor degradation. Our study aimed at exploring the pathogenic consequences in vivo and in vitro of a PCSK9 prodomain mutation found in a family with hypobetalipoproteinemia (FHBL).

METHODS AND RESULTS

A white 49-year-old diabetic man had profound FBHL (LDLC: 16 mg/dL) whereas his daughter and sister displayed a milder phenotype (LDLC 44 mg/dL and 57 mg/dL, respectively), all otherwise healthy with a normal liver function. A monoallelic PCSK9 double-mutant R104C/V114A cosegregated with FBHL, with no mutation found at other FHBL-causing loci. A dose-effect was also found in FBHL relatives for plasma APOB and PCSK9 (very-low to undetectable in proband, approximately 50% decreased in sister and daughter) and LDL catabolic rate (256% and 88% increased in proband and daughter). Transient transfection in hepatocytes showed severely impaired processing and secretion of the double mutant which acted as a dominant negative over secretion of wild-type PCSK9.

CONCLUSIONS

These results show that heterozygous PCSK9 missense mutations may associate with profound hypobetalipoproteinemia and constitute the first direct evidence in human that decrease of plasma LDLC concentrations associated to PCSK9 LOF mutations are attributable to an increased clearance rate of LDL.

Fatty liver in familial hypobetalipoproteinemia: triglyceride assembly into VLDL particles is affected by the extent of hepatic steatosis

Familial hypobetalipoproteinemia (FHBL) subjects may develop fatty liver. Liver fat was assessed in 21 FHBL with six different apolipoprotein B (apoB) truncations (apoB-4 to apoB-89) and 14 controls by magnetic resonance spectroscopy (MRS). Liver fat percentages were 16.7 +/- 11.5 and 3.3 +/- 2.9 (mean +/- SD) (P = 0.001). Liver fat percentage was positively correlated with body mass index, waist circumference, and areas under the insulin curves of 2 h glucose tolerance tests, suggesting that obesity may affect the severity of liver fat accumulation in both groups. Despite 5-fold differences in liver fat percentage, mean values for obesity and insulin indexes were similar. Thus, for similar degrees of obesity, FHBL subjects have more hepatic fat. VLDL-triglyceride (TG)-fatty acids arise from plasma and nonplasma sources (liver and splanchnic tissues). To assess the relative contributions of each, [2H2]palmitate was infused over 12 h in 13 FHBL subjects and 11 controls. Isotopic enrichment of plasma free palmitate and VLDL-TG-palmitate was determined by mass spectrometry. Non-plasma sources contributed 51 +/- 15% in FHBL and 37 +/- 13% in controls (P = 0.02). Correlations of liver fat percentage and percent VLDL-TG-palmitate from liver were r = 0.89 (P = 0.0001) for FHBL subjects and r = 0.69 (P = 0.01) for controls. Thus, apoB truncation-producing mutations result in fatty liver and in altered assembly of VLDL-TG.

Familial hypobetalipoproteinemia: a review

We review the genetics and pathophysiology of familial hypobetalipoproteinemia (FHBL), a mildly symptomatic genetically heterogeneous autosomal trait. The minority of human FHBL is caused by truncation-specifying mutations of the APOB gene on chromosome 2. In seven families, linkage to chromosome 2 is absent, linkage is instead to chromosome 3 (3p21). In others, linkage is absent to both APOB and to 3p21. Apolipoprotein B-100 (apoB-100) levels are approximately 25% of normal, instead of the 50% expected based on the presence of one normal allele due to reduced rates of production. The presence of the truncating mutation seems to have a "dominant recessive" effect on apoB-100 secretion. Concentrations of apoB truncations in plasma differ by truncation but average at approximately 10% of normal levels. Lipoproteins bearing truncated forms of apoB are cleared more rapidly than apoB-100 particles. In contrast with apoB-100 particles cleared primarily in liver via the LDL receptor, most apoB truncation particles are cleared in renal proximal tubular cells via megalin. Since apoB defects cause a dysfunctional VLDL-triglyceride transport system, livers accumulate fat. Hepatic synthesis of fatty acids is reduced in compensation. Informational lacunae remain about genes affecting fat accumulation in liver, and the modulation of liver fat in the presence apoB truncation defects.

Characterization of an abnormal species of apolipoprotein B, apolipoprotein B-37, associated with familial hypobetalipoproteinemia

Steinberg and colleagues have previously described a unique kindred with normotriglyceridemic hypobetalipoproteinemia (1979. J. Clin. Invest. 64:292-301). In a reexamination of this kindred, we found an abnormal apolipoprotein (apo) B species, apo B-37 (203,000 mol wt), in the plasma lipoproteins of multiple members of the kindred. In affected individuals apo B-37 was found in very low density lipoproteins, along with the normal apo B species, apo B-100 and apo B-48. High density lipoproteins (HDL) also contained apo B-37, but no other apo B species. The first 13 amino-terminal amino acids of apo B-37 were identical to those of normal apo B-100. We utilized a panel of 18 different apo B-specific monoclonal antibodies and polyclonal antisera specific for apo B-37 and the thrombin cleavage products of apo B-100 to map apo B-37 in relation to apo B-100, apo B-48, and the thrombin cleavage products of apo B-100. The results of those immunochemical studies indicated that apo B-37 contains only amino-terminal domains of apo B-100. In affected individuals, the majority of apo B-37 in plasma was contained in the HDL density fraction. Within that fraction apo B-37 was found on discrete lipoprotein particles, termed Lp-B37, that had properties distinct from normal HDL particles containing apo A-I. This report documents for the first time the existence of an abnormal apo B species in humans. Further study of apo B-37 and lipoprotein particles containing apo B-37 should lead to an improved understanding of apo B structure and function.

Truncated variants of apolipoprotein B cause hypobetalipoproteinaemia

Familial hypobetalipoproteinaemia is a rare autosomal dominant disorder in which levels of apo-B-containing plasma lipoproteins are approximately half-normal in heterozygotes and virtually absent in homozygotes. Here we describe mutations of the apo-B gene that cause two different truncated variants of apo-B in unrelated individuals with hypobetalipoproteinaemia. One variant, apo-B(His1795----Met-Trp-Leu-Val-Thr-Term) is predicted to be 1799 amino acids long and arises from deletion of a single nucleotide (G) from leucine codon 1794. This protein was found at low levels in very low density and low density lipoprotein fractions in the blood. The second, shorter variant, apo-B(Arg1306----Term), is caused by mutation of a CpG dinucleotide in arginine codon 1306 converting it to a stop codon and predicting a protein of 1305 residues. The product of this allele could not be detected in the circulation. The differences in size and behaviour of these two variants compared to apo-B100 or apo-B48 point to domains that may be important for the assembly, secretion or stability of apo-B-containing lipoproteins.

Hepatitis C virus-associated hypobetalipoproteinemia is correlated with plasma viral load, steatosis, and liver fibrosis

OBJECTIVES

A relationship between chronic hepatitis C virus (HCV) infection and lipid metabolism has recently been suggested. The aim of this study was to determine the correlation between lipid profile and virology, histologic lesions, and response to alpha interferon therapy in noncirrhotic, nondiabetic patients with hepatitis C.

METHODS

A total of 109 consecutive untreated chronic hepatitis C patients were studied to assess the following: 1) the effects of HCV genotype, viral load, steatosis, hepatic fibrosis, and body mass index (BMI) on lipid profile; and 2) whether lipid parameters could predict response to antiviral therapy.

RESULTS

The control group showed a significantly higher apolipoprotein B (apoB) concentration compared with patients with chronic hepatitis C. Hypobetalipoproteinemia (apo B <0.7 g/L) was found in 27 (24.7%) chronic HCV patients and in five (5.3%) control subjects (p = 0.0002). Levels of apo B were negatively correlated with steatosis and HCV viral load (r = -0.22; p = 0.03). This last correlation was strong for non-1 genotype and genotype 3 (r = -0.48; p = 0.0005, and r = -0.47; p = 0.007, respectively) but was not found in genotype 1. In multivariate analysis, low apo B concentration was significantly associated with fibrosis grade 2 or 3 versus grade 0 or 1 (p < 0.001), steatosis >5% (p < 0.001), low body mass index (p < 0.001), and high HCV viral load (p < 0.014). No correlation was found in the 76 treated patients between apo B and response to interferon therapy.

CONCLUSIONS

In chronic HCV patients, hypobetalipoproteinemia occurs already in the early stages of HCV infection before the development of liver cirrhosis. The correlation between apo B levels and HCV viral load seems to confirm the interaction between hepatitis C infection and beta-lipoprotein metabolism.

Targeted modification of the apolipoprotein B gene results in hypobetalipoproteinemia and developmental abnormalities in mice

Familial hypobetalipoproteinemia is an autosomal codominant disorder resulting in a dramatic reduction in plasma concentrations of apolipoprotein (apo) B, cholesterol, and beta-migrating lipoproteins. A benefit of hypobetalipoproteinemia is that mildly affected individuals may be protected from coronary vascular disease. We have used gene targeting to generate mice with a modified Apob allele. Mice containing this allele display all of the hallmarks of human hypobetalipoproteinemia: they produce a truncated apoB protein, apoB70, and have markedly decreased plasma concentrations of apoB, beta-lipoproteins, and total cholesterol. In addition, the mice manifest several characteristics that are occasionally observed in human hypobetalipoproteinemia, including reduced plasma triglyceride concentrations, fasting chylomicronemia, and reduced high density lipoprotein cholesterol. An unexpected finding is that the modified Apob allele is strongly associated with exencephalus and hydrocephalus. These mice should help increase our understanding of hypobetalipoproteinemia, atherogenesis, and the etiology of exencephalus and hydrocephalus.

Hepatic and cardiovascular consequences of familial hypobetalipoproteinemia

OBJECTIVE

Individuals with familial hypobetalipoproteinemia (FHBL) have been reported to be prone to fatty liver disease (FLD). Conversely, the profound reduction of low-density lipoprotein (LDL) cholesterol in this disorder might decrease cardiovascular risk. In the present study, we assessed hepatic steatosis as well as noninvasive surrogate markers for cardiovascular disease (CVD) in subjects with FHBL and in matched controls.

METHODS AND RESULTS

Hepatic steatosis was assessed by abdominal ultrasonography. Carotid intima-media thickness (IMT) and distal common carotid arterial wall stiffness as surrogate markers for CVD risk were measured using high-resolution B-mode ultrasonography. Whereas transaminase levels were only modestly elevated, both prevalence (54% versus 29%; P=0.01) and severity of steatosis were significantly higher in FHBL individuals compared with controls. Despite similar IMT measurements, arterial stiffness was significantly lower in FHBL (P=0.04) compared with controls. Additionally, the increase in arterial stiffness as seen in the presence of traditional risk factors was attenuated, suggesting that very low levels of apoB-containing lipoproteins can negate the adverse effects of other risk factors on the vasculature.

CONCLUSIONS

FHBL is characterized by an increased prevalence and severity of fatty liver disease. The observed decreased level of arterial wall stiffness, most pronounced in the presence of nonlipid risk factors, is indicative of cardiovascular protection in these subjects.

Genetic analysis of a kindred with familial hypobetalipoproteinemia. Evidence for two separate gene defects: one associated with an abnormal apolipoprotein B species, apolipoprotein B-37; and a second associated with low plasma concentrations of apolipoprotein B-100

In 1979 Steinberg and colleagues recognized a unique kindred with normotriglyceridemic hypobetalipoproteinemia (1979. J. Clin. Invest. 64:292-301). We have undertaken an intensive reexamination of this kindred and have studied 41 family members in three generations. In this family we document the presence of two distinct apo B alleles associated with low plasma concentrations of apolipoprotein (apo) B and low density lipoprotein (LDL) cholesterol and we trace the inheritance of these two alleles over three generations. One of the alleles resulted in the production of an abnormal, truncated apo B species, apo B-37. The other apo B allele was associated with reduced plasma concentrations of the normal apo B species, apo B-100. H.J.B., the proband, and two of his siblings had both abnormal apo B alleles and were therefore compound heterozygotes for familial hypobetalipoproteinemia. Their average LDL-cholesterol level was 6 +/- 9 mg/dl. All of the offspring of the three compound heterozygotes had hypobetalipoproteinemia, and each had evidence of only one of the abnormal apo B alleles. In the entire kindred, we identified six heterozygotes for familial hypobetalipoproteinemia who had only the abnormal apo B-37 allele and their average LDL cholesterol was 31 +/- 12 mg/dl. We identified 10 heterozygotes who had only the allele for reduced plasma concentrations of apo B-100 and their LDL cholesterol level was 31 +/- 15 mg/dl. Unaffected family members (n = 22) had LDL cholesterol levels of 110 +/- 27 mg/dl. This report describes the first kindred in which two distinct abnormal apo B alleles have been identified, both of which are associated with familial hypobetalipoproteinemia.

The hypobetalipoproteinemias

The fifth- and ninety-fifth-percentile concentrations of low-density lipoprotein (LDL) cholesterol in most Western populations are approximately 90 and 200 mg/dl, respectively. Persons with LDL cholesterol levels equal to or less than the fifth percentile are defined as having hypobetalipoproteinemia. Epidemiologic studies show that such individuals have lower-than-average risk for atherosclerotic cardiovascular disease but higher risk for a variety of cancers, pulmonary, and gastrointestinal diseases than persons with higher levels of cholesterol. The reasons for this are not known, nor are the causes of most cases of hypobetalipoproteinemia. However, in some well-studied kindreds the hypobetalipoproteinemia phenotype is inherited as an autosomal dominant trait. Heterozygotes in such kindreds are usually healthy and have no difficulty absorbing dietary fat. In most kindreds, the molecular variants responsible for the hypobetalipoproteinemia are unknown, but a subset of kindreds have strong genetic linkages between the low-cholesterol phenotype and truncation-producing mutations of the apolipoprotein (apo) B-100 gene. The truncations of apoB are named according to a centile nomenclature. The full-length 4536-amino acid protein is called apoB-100, and the 25 truncations identified to date have been named apoB-2 to apoB-89. The mutations introduce premature termination codons resulting from frameshift-producing base additions or deletions. The mutations produce slowed rates of secretion of the truncated apoBs relative to the apoB-100s present in the heterozygotes. In addition, the apoB-100 molecules of the heterozygotes are also secreted at rates slower than those observed in closely matched normolipidemic controls. These physiologic results account for the hypobetalipoproteinemia of these subjects. The response of the plasma lipoproteins of heterozygotes to the manipulation of various dietary components remains to be determined. Additional low-cholesterol syndromes are autosomal recessive forms of hypobetalipoproteinemia, chylomicron retention disease, and abetalipoproteinemia. The molecular causes of the first two are unknown. Abetalipoproteinemia is an autosomal recessive condition resulting from mutations of the microsomal triglyceride transfer protein. All three conditions are characterized by vanishingly small concentrations of LDL, dietary fat malabsorption, and failure to thrive in infancy.

Lipoproteins containing the truncated apolipoprotein, Apo B-89, are cleared from human plasma more rapidly than Apo B-100-containing lipoproteins in vivo

We have reported previously on two truncations of apolipoprotein B (apo B-40 and apo B-89) in a kindred with hypobetalipoproteinemia. Premature stop codons were found to be responsible for both apo B-40 and apo B-89, but the physiologic mechanisms accounting for the reduced plasma concentrations of these proteins have not been determined in vivo. This study investigates the metabolism of apo B-89 in two subjects heterozygous for apo B-89/apo B-100 and in one apo B-40/apo B-89 compound heterozygote. In both heterozygotes total apo B concentration is approximately 30% of normal and apo B-89 is present in lower concentrations in plasma than apo B-100. After the administration of [1-13C]leucine as a primed constant infusion over 8 h, 13C enrichments of plasma leucine as well as enrichments of VLDL-, IDL-, and LDL-apo B-89 leucine and VLDL-, IDL-, and LDL-apo B-100 leucine were measured over 110 h. Enrichment values were subsequently converted to tracer/tracee ratios and a multicompartmental model was used to estimate metabolic parameters. In both apo B-89/apo B-100 heterozygotes apo B-89 and apo B-100 were produced at similar rates. Respective transport rates of apo B-89 and apo B-100 for subject 1 were 2.13 +/- 0.18 and 2.56 +/- 0.13 mg.kg-1.d-1, and for subject 2, 6.59 +/- 0.18 and 8.23 +/- 0.39 mg.kg-1.d-1. However, fractional catabolic rates of VLDL, IDL, and LDL particles containing apo B-89 were 1.4-3 times higher than the rates for corresponding apo B-100-containing particles. Metabolic parameters of apo B-89 in the apo B-40/apo B-89 compound heterozygote compared favorably with those established for apo B-89 in apo B-89/apo B-100 heterozygotes. Thus, the enhanced catabolism of VLDL, IDL, and LDL particles containing the truncated apolipoprotein is responsible for the relatively low levels of apo B-89 seen in these subjects.

Hypobetalipoproteinemia and abetalipoproteinemia: liver disease and cardiovascular disease

PURPOSE OF REVIEW

Several mutations in the apolipoprotein (apo) B, proprotein convertase subtilisin kexin 9 (PCSK9) and microsomal triglyceride transfer protein genes result in low or absent levels of apoB and LDL cholesterol (LDL-C) in plasma which cause familial hypobetalipoproteinemia (FHBL) and abetalipoproteinemia (ABL). Mutations in the angiopoietin-like protein 3 ANGPTL3 gene cause familial combined hypolipidemia (FHBL2). Clinical manifestations range from none-to-severe, debilitating and life-threatening disorders. This review summarizes recent genetic, metabolic and clinical findings and management strategies.

RECENT FINDINGS

Fatty liver, cirrhosis and hepatocellular carcinoma have been reported in FHBL and ABL probably due to decreased triglyceride export from the liver. Loss of function mutations in PCSK-9 and ANGPTL3 cause FHBL but not hepatic steatosis. In 12 case-control studies with 57 973 individuals, an apoB truncation was associated with a 72% reduction in coronary heart disease (odds ratio, 0.28; 95% confidence interval, 0.12-0.64; P = 0.002). PCSK9 inhibitors lowered risk of cardiovascular events in large, randomized trials without apparent adverse sequelae.

SUMMARY

Mutations causing low LDL-C and apoB have provided insight into lipid metabolism, disease associations and the basis for drug development to lower LDL-C in disorders causing high levels of cholesterol. Early diagnosis and treatment is necessary to prevent adverse sequelae from FHBL and ABL.

Linkage of a gene for familial hypobetalipoproteinemia to chromosome 3p21.1-22

Familial hypobetalipoproteinemia (FHBL) is an apparently autosomal dominant disorder of lipid metabolism characterized by less than fifth percentile age- and sex-specific levels of apolipoprotein beta (apobeta) and low-density lipoprotein-cholesterol. In a minority of cases, FHBL is due to truncation-producing mutations in the apobeta gene on chromosome 2p23-24. Previously, we reported on a four-generation FHBL kindred in which we had ruled out linkage of the trait to the apobeta gene. To locate other loci containing genes for low apobeta levels in the kindred, a genomewide search was conducted. Regions on 3p21.1-22 with two-point LOD scores >1.5 were identified. Additional markers were typed in the region of these signals. Two-point LOD scores in the region of D3S2407 increased to 3.35 at O = 0. GENEHUNTER confirmed this finding with an nonparametric multipoint LOD score of 7.5 (P=.0004). Additional model-free analyses were conducted with the square root of the apobeta level as the phenotype. Results from the Loki and SOLAR programs further confirmed linkage of FHBL to 3p21.1-22. Weaker linkage to a region near D19S916 was also indicated by Loki and SOLAR. Thus, a heretofore unidentified genetic susceptibility locus for FHBL may reside on chromosome 3.

High frequency of APOB gene mutations causing familial hypobetalipoproteinaemia in patients of Dutch and Spanish descent

BACKGROUND

Familial hypobetalipoproteinaemia (FHBL) is an autosomal co-dominant hereditary disorder of lipoprotein metabolism characterised by decreased low density lipoprotein (LDL) cholesterol and apolipoprotein B (APOB) plasma levels. High levels of plasma APOB and LDL cholesterol are strong predictors for risk of cardiovascular disease (CVD), while individuals with low APOB and LDL cholesterol levels are thought to have lower than average risk for CVD, and in fact, heterozygous FHBL patients appear to be asymptomatic.

METHODS

Rather than identifying truncated APOB proteins in plasma fractions separated by gel electrophoresis, which will miss any mutations in proteins smaller than 30 kb, we analysed the APOB gene directly, using PCR.

RESULTS

We identified nine different mutations, six of which are novel. Each mutation showed complete co-segregation with the FHBL phenotype in the families, and statistically significant differences between carriers and non-carriers were found for plasma total, LDL, and HDL cholesterol, triglycerides, and APOB levels, but not for APOA1 levels. All carriers of an APOB mutation were completely free from CVD.

CONCLUSIONS

Prolonged low levels of LDL cholesterol and elevated levels of HDL cholesterol may reduce the progression of atherosclerotic disease, but this has not been unequivocally shown that this is indeed the case in individuals with FHBL, and is the subject of a current study.

A study of fatty liver disease and plasma lipoproteins in a kindred with familial hypobetalipoproteinemia due to a novel truncated form of apolipoprotein B (APO B-54.5)

BACKGROUND/AIMS

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low-density lipoproteins. It can be caused by mutations in the gene encoding apolipoprotein B-100 (apo B), leading to the formation of truncated apo Bs which have a reduced capacity to export lipids from the hepatocytes as lipoprotein constituents. Case reports suggest the occurrence of liver disease in FHBL, but there are no studies of liver involvement in FHBL with defined apo B gene mutations. The presence of fatty liver disease was investigated in a large FHBL kindred.

METHODS

Plasma lipoprotein and apolipoprotein analysis, liver function tests, and apo B gene sequence were performed in 16 members of a FHBL kindred. The presence of fatty liver was assessed by ultrasound and computed tomography scanning.

RESULTS

The proband, a non-obese heavy drinker male with hypobetalipoproteinemia, had steatohepatitis with fibrosis. He was heterozygous for a novel non-sense mutation of apo B gene producing a truncated apo B of 2745 amino acids (designated apo B-54.5, having half the size of normal apo B-100). Seven other members of his kindred carried apo B-54.5. Although all of them were hypolipidemic, their lipid levels showed a large inter-individual variability not accounted for by polymorphisms of genes involved in apo B metabolism. Four carriers (two heavy drinkers and two teetotallers), irrespective of their plasma lipid levels, had ultrasonographic evidence of fatty liver. In the other four carriers no evidence of fatty liver was found.

CONCLUSIONS

In this kindred apo B-54.5 predisposes to fatty liver, which however may require some additional factors to become clinically relevant.

A truncated species of apolipoprotein B (B67) in a kindred with familial hypobetalipoproteinemia

We describe a kindred in which the proband and 6 of his 12 children have hypobetalipoproteinemia. The plasma lipoproteins of the affected subjects contained a unique species of apolipoprotein (apo) B, apo B67, in addition to the normal species, apo B100 and apo B48. The size of apo B67 and immunochemical studies with a panel of apo B-specific antibodies indicated that apo B67 was a truncated species of apo B that contained approximately the amino-terminal 3,000-3,100 amino acids of apo B100. Sequencing of genomic apo B clones revealed that affected family members were heterozygous for a mutant apo B allele containing a single nucleotide deletion in exon 26 (cDNA nucleotide 9327). This frameshift mutation is predicted to result in the synthesis of a truncated apo B containing 3,040 amino acids. Apo B67 is present in low levels in the plasma but is easily detectable within the very low density lipoprotein and low density lipoprotein fractions. Examination of the proband's immediate family revealed seven normolipidemic subjects and seven subjects with hypobetalipoproteinemia. In the affected subjects, the mean total and low density lipoprotein cholesterol levels were 120 and 42 mg/dl, respectively. A significantly higher mean high density lipoprotein cholesterol level was found in the affected subjects (75 vs. 55 mg/dl). We hypothesize that the elevated high density lipoprotein cholesterol levels in subjects heterozygous for the apo B67 mutation may be metabolically linked to the low levels of apo B-containing lipoproteins in their plasma.

Inference of a molecular defect of apolipoprotein B in hypobetalipoproteinemia by linkage analysis in a large kindred

Heterozygous hypobetalipoproteinemia is characterized by reduced plasma concentrations of LDL cholesterol, total triglycerides, and apo B to less than 50% of normal values. The molecular basis of this disorder remains unknown. The phenotype cosegregates with a DNA haplotype of the apo B gene in an Idaho pedigree, with a maximum decimal logarithm of the ratio (LOD) score of 7.56 at a recombination rate of zero. Individuals carrying this haplotype had total cholesterol levels of 96 mg/dl, LDL cholesterol levels of 37 mg/dl, triglycerides levels of 51 mg/dl, and apo B levels of 38 mg/dl. This study strongly suggests that apo B mutations underlie hypobetalipoproteinemia, and demonstrates the power of the candidate gene approach in linkage analysis for unraveling genetic determinants in metabolic disorders of undefined etiology.

Prospective 10-year evaluation of hypobetalipoproteinemia in a cohort of 772 firefighters and cross-sectional evaluation of hypocholesterolemia in 1,479 men in the National Health and Nutrition Examination Survey I

Our specific aim in a 10-year prospective study of 772 Cincinnati firemen (predominantly aged 26 to 46 years) was to determine the prevalence, attributes, and etiology of persistent hypobetalipoproteinemia, defined by entry low-density lipoprotein cholesterol (LDLC) less than 75 mg/dL. A second specific aim was to cross-sectionally assess hypocholesterolemia (defined by total serum cholesterol [TC] < 130 mg/dL) in 1,314 white and 165 black men aged 26 to 46 years in the National Health and Nutrition Examination Survey (NHANES I). The 141 black and 631 white firemen had 4,973 person-years of follow-up time (median, 7.1 yr/man). Of 772 men, 44 (5.7%) had entry LDL levels less than 75 mg/dL; they had a mean follow-up time of 7.3 yr/man. Of these 44 men, there were 12 (1.8% of the cohort) with entry LDLC less than 75 mg/dL, and at least 67% of their follow-up LDLC levels were less than 75. Their mean entry TC and LDLC levels were low (130 and 58 mg/dL), mean triglyceride (TG) was low (63 mg/dL), and mean high-density lipoprotein cholesterol (HDLC) was high (60 mg/dL), LDLC remained at less than 75 mg/dL in 81% of their follow-up samples. Their mean entry and follow-up cholesterol and LDLC did not differ (P > .1, 130 v 133 mg/dL and 58 v 63 mg/dL). Compared with 32 men with entry LDLC less than 75 mg/dL but with less than 87% of follow-up LDLC less than 75 mg/dL, the 12 men with persistently low LDLC had lower mean Quetelet indices and diastolic blood pressure at entry (2.36 v 2.58, P = .056; 73 v 80 mm Hg, P = .03) and on follow-up study (2.45 v 2.69, P = .04; 72 v 79 mm Hg, P = .05). Of 12 men with persistently low LDLC, two had truncated apolipoprotein (apo) B (familial hypobetalipoproteinemia, two had the apo E genotype 2/3, and two had acquired hypobetalipoproteinemia that antedated mortality from melanoma by 9 years and from alcoholism by 2 years. Comparable to white and black firemen aged 26 to 46 years, 2.9% and 3.6% of whom had entry serum TC less than 130 mg/dL, of 1,314 white and 165 black men in the NHANES I study (aged 26 to 46), 1.8% and 3.6% had hypocholesterolemia (entry TC < 130 mg/dL). Daily mean calorie, fat, and protein intake (grams per day) did not differ (P > .05) in men with entry TC less than 130 mg/dL compared with those with TC 130 to 230 or greater than 230 mg/dL. Hypocholesterolemia in white and black men in NHANES I could not be attributed to hypocaloric intake or to protein, fat, or carbohydrate undernutrition. There appear to be racial differences in the prevalence of hypocholesterolemia. Blacks comprised 18% of the firemen's cohort but 42% of those with persistent hypobetalipoproteinemia; among NHANES I subjects, 3.6% of blacks were hypocholesterolemic versus 1.8% of whites. Unless persistent hypobetalipoproteinemia reflects an underlying disease, alcoholism, etc., it is often heritable, and may be associated with a reduced likelihood of coronary heart disease (CHD) and with increased longevity.

Identification and molecular analysis of two apoB gene mutations causing low plasma cholesterol levels

BACKGROUND

Familial hypobetalipoproteinemia (FHB) is an autosomal codominant disorder characterized by abnormally low plasma levels of apoB and LDL cholesterol. Heterozygotes for FHB almost always have plasma LDL cholesterol levels < 70 mg/dL and are asymptomatic. Because the low cholesterol levels may protect FHB heterozygotes from coronary heart disease, the mechanisms for FHB are of considerable interest.

METHODS AND RESULTS

The plasma lipoproteins of 29 subjects with LDL cholesterol levels < 70 mg/dL were examined by SDS-PAGE. One subject who had virtually undetectable levels of LDL cholesterol had a truncated apoB, apoB-44.4, in his lipoproteins; a second subject with an LDL cholesterol level of 44 mg/dL had apoB-55 in his lipoproteins. The apoB-44.4 (2014 amino acids in length) resulted from a frameshift caused by an 11-bp insertion in exon 26 of the apoB gene; the apoB-55 (2494 amino acids) was caused by a nonsense mutation in exon 26 of the apoB gene. The apoB-55 mutation occurred at a CpG dinucleotide pair, a mutational hot spot, and was identical to a mutation described previously in a subject with hypobetalipoproteinemia. Our subject with apoB-55, however, had a different haplotype than the subject described previously, suggesting that the two apoB-55 mutations may have arisen independently. Of note, the apoB-55 proband's father, who had very low cholesterol levels and who probably carried the apoB-55 mutation, had significant coronary and aortic atherosclerosis at autopsy.

CONCLUSIONS

In a study of adults with low LDL cholesterol levels, we discovered two subjects with truncated apoB proteins and identified the responsible mutations. ApoB gene mutations causing truncated apoB are not particularly rare in subjects with low cholesterol levels. The role of these mutations in preventing atherosclerosis deserves further study.

A form of familial hypobetalipoproteinaemia not due to a mutation in the apolipoprotein B gene

Familial hypobetalipoproteinaemia (FHBL) is a dominant disorder of lipoprotein metabolism characterized by levels of apolipoprotein B-carrying lipoproteins (VLDL, IDL and LDL) which are 50% of the normal levels in the heterozygotes and almost absent in the homozygotes. Several reports have recently shown that the underlying defect in FHBL involves different mutations in the apo B gene which lead to reduced levels of apo B mRNA or to the production of truncated forms of apo B having either a lower synthetic rate or a higher catabolic rate than normal apo B. We here present a three-generation family with several FHBL members in which the linkage analysis shows absence of co-segregation between apo B gene alleles and the hypocholesterolaemic phenotype. We conclude that a dominantly transmitted mutation in a gene other than that for apo B is responsible for the low plasma cholesterol levels.

Postprandial lipoprotein metabolism in familial hypobetalipoproteinemia

OBJECTIVE

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominantly inherited disorder of lipoprotein metabolism characterized by decreased plasma concentrations of low-density lipoprotein-cholesterol and apolipoprotein (apo) B. We examined the effect of truncated apoB variants (<apoB-48) causing FHBL on postprandial triglyceride-rich lipoprotein (TRL) metabolism.

METHODS AND RESULTS

A standardized oral fat load was given after a 12-h fast to six heterozygous [apoB-6.9 (n=3), apoB-25.8 (n=1), apoB-40.3 (n=2)] FHBL subjects and 10 normolipidemic controls. Plasma was obtained every 2 h for 10 h. Large TRLs [containing chylomicrons (CM)] and small TRLs (containing CM remnants) were isolated by ultracentrifugation. Compared with controls, FHBL subjects had significantly decreased fasting plasma cholesterol (2.3+/-0.5 vs. 4.8+/-0.5 mmol/liter), triglyceride (0.4+/-0.3 vs. 1.5+/-0.5 mmol/liter), low-density lipoprotein-cholesterol (0.6+/-0.4 vs. 3.0+/-0.5 mmol/liter), and apoB (0.22+/-0.05 vs. 0.95+/-0.14 g/liter) concentrations (all P<0.001). The postprandial incremental area under the curve in FHBL subjects was decreased for large TRL-triglyceride (-61%; P<0.005), small TRL-cholesterol (-86%; P<0.001), and small TRL-triglyceride (-86%; P<0.001) relative to controls. Multicompartmental modeling analysis showed that the delay time of apoB-48 was shorter and that apoB-48 production was decreased in FHBL subjects compared with controls.

CONCLUSIONS

We have demonstrated that heterozygous FHBL subjects with apoB truncations shorter than apoB-48, and therefore only a single fully-functional apoB-48 allele, have decreased TRL production but normal postprandial TRL particle clearance.

Threshold Effects of Circulating Angiopoietin-Like 3 Levels on Plasma Lipoproteins

CONTEXT

Angiopoietin-like 3 (ANGPTL3) deficiency in plasma due to loss-of-function gene mutations results in familial combined hypobetalipoproteinemia type 2 (FHBL2) in homozygotes. However, the lipid phenotype in heterozygotes is much milder and does not appear to relate directly to ANGPTL3 levels. Furthermore, the low-density lipoprotein (LDL) phenotype in carriers of ANGPTL3 mutations is unexplained.

OBJECTIVE

To determine whether reduction below a critical threshold in plasma ANGPTL3 levels is a determinant of lipoprotein metabolism in FHBL2, and to determine whether proprotein convertase subtilisin kexin type 9 (PCSK9) is involved in determining low LDL levels in this condition.

DESIGN

We studied subjects from 19 families with ANGPTL3 mutations and subjects with familial combined hypobetalipoproteinemia type 1 (FHBL1) due to truncated apolipoprotein B (apoB) species.

RESULTS

First, total cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, and HDL and LDL particle concentration correlated with plasma ANGPTL3 levels but only when the latter was <25% of normal (<60 ng/dL). Second, the very low-density lipoprotein particle concentration correlated strongly with plasma ANGPTL3 when the latter was <58% of normal. Third, both FHBL1 and FHBL2 subjects showed low levels of mature and LDL-bound PCSK9 and higher levels of its furin-cleaved form. Finally, LDL-bound PCSK9 is protected from cleavage by furin and binds to the LDL receptor more strongly than apoB-free PCSK9.

CONCLUSIONS

Our results suggest that the hypolipidemic effects of ANGPTL3 mutations in FHBL2 are dependent on a threshold of plasma ANGPTL3 levels, with differential effects on various lipoprotein particles. The increased inactivation of PCSK9 by furin in FHBL1 and FHBL2 is likely to cause increased LDL clearance and suggests novel therapeutic avenues.

Oxidation of apolipoprotein B-100 in circulating LDL is related to LDL residence time. In vivo insights from stable-isotope studies

5-Hydroxy-2-aminovaleric acid (HAVA) has been suggested to be a specific marker of oxidation of apolipoprotein (apo) B-100 proline (Pro) and arginine (Arg) side-chain residues in low density lipoprotein (LDL) in vitro. Here we describe the application of sensitive mass spectrometric techniques to the characterization of Pro/Arg-modified apoB-100 in LDL(1) (S(f) 7 to 12) and LDL(2) (S(f) 0 to 7) in vivo. We studied 7 subjects with familial defective apoB-100 (FDB) and 8 normolipidemic controls. In FDB subjects, the presence of a mutant apoB-100 (FDB(3500Q)) in LDL markedly reduced its affinity for the LDL receptor, leading to increased residence times (RTs) of LDL(1) (65+/-21 versus 32+/-12 hours, P<0.005) and LDL(2) (230+/-40 versus 53+/-7 hours, P:<0.001) when compared with controls, as determined by stable-isotope turnover studies. LDL(1) HAVA content was not different between the groups (FDB, 0.004+/-0. 001 mol/mol apoB-100 versus controls, 0.003+/-0.001 mol/mol apoB-100, P=0.200). LDL(2) HAVA content was higher in FDB subjects (0. 374+/-0.088 versus 0.013+/-0.002 mol/mol apoB-100, P<0.001). In both groups, LDL(2) HAVA was positively associated with LDL(2) RT (FDB, r=0.893, P:=0.003; controls, r=0.976, P=0.000) and negatively correlated with LDL(2) alpha-tocopherol content (FDB, r=-0.929, P=0. 003; controls, r=-0.903, P=0.002). No significant correlations could be found between LDL(1) HAVA, LDL(1) RT, and alpha-tocopherol, respectively. The low LDL(1) HAVA content observed in both FDB and control groups was thought to be due to the relatively lower RT as well as the higher alpha-tocopherol content of these lipoproteins. In contrast, LDL(2) seemed to be strongly prone to direct oxidation of apoB-100 in vivo. The longer these particles linger in the circulation, the more apoB-100 Pro/Arg residues become modified.