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

Rare primary dyslipidaemias associated with low LDL and HDL cholesterol values in Portugal

Background: Dyslipidaemia represents a group of disorders of lipid metabolism, characterized by either an increase or decrease in lipid particles, usually associated with triglycerides, LDL cholesterol (LDL-C) and/or HDL cholesterol (HDL-C). Most hyperlipidaemias and HDL deficiencies confer an increased cardiovascular risk, while hypolipidaemia, such as abeta or hypobetalipoproteinemia, may present different manifestations ranging from poor weight progression to neurological manifestations. The aim of this study is to present 7 cases with rare dyslipidaemias associated with low LDL or low HDL cholesterol values, referred to our laboratory for the genetic identification of the cause of the dyslipidaemia. Methods: Lipid profile was determined for each individual in an automated equipment Integra Cobas (Roche). Molecular analysis was performed by NGS with a target panel of 57 genes involved in lipid metabolism (Sure select QXT, Agilent) and samples were run in a NextSEQ Sequencer (Illumina). Only genes associated to rare forms of low HDL-c or LDL-c were analysed for this work, namely: ABCA1, APOA1, LCAT, SCARB1, APOB, PCSK9, MTTP, SAR1B, and ANGPTL3. All rare variants (MAF<5%) found in these genes were confirmed by Sanger sequencing. Results and discussion: This study includes 7 index cases (IC), with the following clinical diagnoses: Fish Eye Disease (1), Hypoalphalipoproteinemia (1) and Abetalipoproteinemia (ABL) / Familial Hypobetalipoproteinemia (FHBL) (5). We have identified one IC with a compound heterozygosity in LCAT causing Fish Eye Disease and one IC with a variant in ABCA1 in homozygosity causing Tangier disease. We found variants causing homozygous FHBL in 2 IC, one of whom has an undescribed pathogenic variant in homozygosity in APOB (c.12087+1G>A) and the other is a possible compound heterozygous for APOB variants c.2604+1G>A and c.4651C>T/p.(Gln1551*). In two patients only a variant in heterozygosity (c.3365delG/p.(Gly1122Vfs*62) and c.11095A>T/p.(Arg3699*)). In the remaining patient, no variants were identified. NGS proved to be a fundamental key for genetic testing of rare lipid disorders, allowing us to find the genetic cause of disease in 6/7 patients with low HDL-c and LDL-c. Patients with these rare conditions should be identified as early as possible in order to minimize or prevent clinical manifestations. The unsolved case is still under investigation.

Rare dyslipidaemias, from phenotype to genotype to management: a European Atherosclerosis Society task force consensus statement

Genome sequencing and gene-based therapies appear poised to advance the management of rare lipoprotein disorders and associated dyslipidaemias. However, in practice, underdiagnosis and undertreatment of these disorders are common, in large part due to interindividual variability in the genetic causes and phenotypic presentation of these conditions. To address these challenges, the European Atherosclerosis Society formed a task force to provide practical clinical guidance focusing on patients with extreme concentrations (either low or high) of plasma low-density lipoprotein cholesterol, triglycerides, or high-density lipoprotein cholesterol. The task force also recognises the scarcity of quality information regarding the prevalence and outcomes of these conditions. Collaborative registries are needed to improve health policy for the care of patients with rare dyslipidaemias.

Nutrition from the Inside Out

Nearly 50 years ago, I set out to investigate the clinical problem of hypoglycemia in children with illnesses that limited their food intake. My goal was to gather accurate and precise measurable data. At the time, I wasn't interested in nutrition as a discipline defined in its more general or popular sense. To address the specific problem that interested me required development of entirely new methods based on stable, nonradioactive tracers that satisfied the conditions of accuracy and precision. At the time, I had no inclination of the various theoretical and practical problems that would have to be solved to achieve this goal. Some are briefly described here. Nor did I have the slightest idea that developing the field would result in a fundamental change in how human clinical investigation was conducted, with the eventual replacement of radiotracers with stable isotopically labeled ones, even for adult clinical investigation. Additionally, I had no inclination that the original questions would open avenues to much broader questions of practical nutritional relevance. Moreover, only much later as the editor of The American Journal of Clinical Nutrition did I appreciate the policy implications of how nutritional data are presented in the scientific literature. At least in part, less accurate and precise measurements and less than full transparency in reporting nutritional data have resulted in widespread debate about the public policy recommendations and guidelines that are the intended result of collecting the data in the first place. This article provides a personal recollection (with all the known faults of self-reporting and retrospective memory) of the journey that starts with measurement certainty and ends with policy uncertainty.

Lipoprotein Metabolism in APOB L343V Familial Hypobetalipoproteinemia

CONTEXT

Familial hypobetalipoproteinemia (FHBL) is a codominant disorder of lipoprotein metabolism characterized by decreased plasma concentrations of low-density lipoprotein (LDL)-cholesterol and apolipoprotein B (apoB).

OBJECTIVE

The objective was to examine the effect of heterozygous APOB L343V FHBL on postprandial triglyceride-rich lipoprotein (TRL) and fasting lipoprotein metabolism.

METHODS

Plasma incremental area under the curve apoB-48 and apoB-48 kinetics were determined after ingestion of a standardized oral fat load using compartmental modeling. Very low-density lipoprotein (VLDL)-, intermediate-density lipoprotein (IDL)-, and LDL-apoB kinetics were determined in the fasting state using stable isotope methods and compartmental modeling.

RESULTS

The postprandial incremental area under the curve (0-10 h) in FHBL subjects (n = 3) was lower for large TRL-triglyceride (-77%; P < .0001), small TRL-cholesterol (-83%; P < .001), small TRL-triglyceride (-88%; P < .001), and for plasma triglyceride (-70%; P < .01) and apoB (-63%; P < .0001) compared with controls. Compartmental analysis showed that apoB-48 production was lower (-91%; P < .05) compared with controls. VLDL-apoB concentrations in FHBL subjects (n = 2) were lower by more than 75% compared with healthy, normolipidemic control subjects (P < .01). The VLDL-apoB fractional catabolic rate (FCR) was more than 5-fold higher in the FHBL subjects (P = .07). ApoB production rates and IDL- and LDL-apoB FCRs were not different between FHBL subjects and controls.

CONCLUSIONS

We conclude that when compared to controls, APOB L343V FHBL heterozygotes show lower TRL production with normal postprandial TRL particle clearance. In contrast, VLDL-apoB production was normal, whereas the FCR was higher in heterozygotes compared with lean control subjects. These mechanisms account for the marked hypolipidemic state observed in these FHBL subjects.

Insights from human congenital disorders of intestinal lipid metabolism

The intestine must challenge the profuse daily flux of dietary fat that serves as a vital source of energy and as an essential component of cell membranes. The fat absorption process takes place in a series of orderly and interrelated steps, including the uptake and translocation of lipolytic products from the brush border membrane to the endoplasmic reticulum, lipid esterification, Apo synthesis, and ultimately the packaging of lipid and Apo components into chylomicrons (CMs). Deciphering inherited disorders of intracellular CM elaboration afforded new insight into the key functions of crucial intracellular proteins, such as Apo B, microsomal TG transfer protein, and Sar1b GTPase, the defects of which lead to hypobetalipoproteinemia, abetalipoproteinemia, and CM retention disease, respectively. These "experiments of nature" are characterized by fat malabsorption, steatorrhea, failure to thrive, low plasma levels of TGs and cholesterol, and deficiency of liposoluble vitamins and essential FAs. After summarizing and discussing the functions and regulation of these proteins for reader's comprehension, the current review focuses on their specific roles in malabsorptions and dyslipidemia-related intestinal fat hyperabsorption while dissecting the spectrum of clinical manifestations and managements. The influence of newly discovered proteins (proprotein convertase subtilisin/kexin type 9 and angiopoietin-like 3 protein) on fat absorption has also been provided. Finally, it is stressed how the overexpression or polymorphism status of the critical intracellular proteins promotes dyslipidemia and cardiometabolic disorders.

Familial hypobetalipoproteinemia-induced nonalcoholic steatohepatitis

Familial hypobetalipoproteinemia (FHBL) is a rare genetic disorder of lipid metabolism that is associated with abnormally low serum levels of low-density lipoprotein (LDL) cholesterol and apolipoprotein B. It is an autosomal co-dominant disorder, and depending on zygosity, the clinical manifestations may vary from none to neurological, endocrine, hematological or liver dysfunction. Nonalcoholic fatty liver disease is common in persons with FHBL, however progression to nonalcoholic steatohepatitis is unusual. We describe here a patient with a novel APOB mutation, V703I, which appears to contribute to the severity of the FHBL phenotype. He had liver enzyme abnormalities, increased echogenicity of the liver consistent with steatosis, very low LDL cholesterol at 0.24 mmol/l (normal 1.8–3.5 mmol/l) and an extremely low apolipoprotein B level of 0.16 g/l (normal 0.6–1.2 g/l). APOB gene sequencing revealed him to be a compound heterozygote with two mutations (R463W and V703I). APOB R463W has previously been reported to cause FHBL. Genetic sequencing of his first-degree relatives identified the APOB V703I mutation in his normolipidemic brother and father and the APOB R463W mutation in his mother and sister, both of whom have very low LDL cholesterol levels. These results suggest that the APOB V703I mutation alone does not cause the FHBL phenotype. However, it is possible that it has a contributory role to a more aggressive phenotype in the presence of APOB R463W.

Apolipoprotein B levels, APOB alleles, and risk of ischemic cardiovascular disease in the general population, a review

Apolipoprotein B is a key component in lipid metabolism. Subendothelial retention of apolipoprotein B containing lipoproteins is a necessary initiating event in atherogenesis, and high plasma levels of apolipoprotein B is a risk factor for atherosclerosis, whereas low levels may provide protection. The present review examines, with focus on general population studies, apolipoprotein B levels as a predictor of ischemic cardiovascular disease, as well as the association of mutations and polymorphisms in APOB with plasma apolipoprotein B levels, and risk of ischemic cardiovascular disease. The studies can be summarized as follows: (1) apolipoprotein B predicts ischemic cardiovascular events in both genders, and is better than LDL cholesterol in this respect; (2) linkage disequilibrium structure in APOB is more complex than expected from HapMap data, because a minimal set of tag single nucleotide polymorphisms capturing the entire variation in APOB cannot be identified, and thus most polymorphisms must be evaluated separately in association studies; (3) APOB mutations and polymorphisms are associated with a range of apolipoprotein B and LDL cholesterol levels, although the magnitude of effect sizes of common polymorphisms are modest; (4) both mutations and polymorphisms are associated with LDL metabolism in vivo; (5) association of APOB mutations and polymorphisms with lipid and disease phenotype cannot be predicted in silico using evolutionary conservation or existing prediction programs; and finally, (6) except for the E4154K polymorphism that possibly predicts a reduction in risk of ischemic cerebrovascular disease and ischemic stroke, common APOB polymorphisms with modest effect sizes on lipid levels do not predict risk of ischemic heart disease, myocardial infarction, ischemic cerebrovascular disease, or ischemic stroke in the general population.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Familial hypobetalipoproteinemia in a Turkish family with hereditary spastic paraplegia

A 24-year-old male presented with progressive gait disturbance and was diagnosed with hereditary spastic paraplegia. His brother and possibly one uncle also had the condition. Routine biochemical testing found that the patient had unusually low plasma concentrations of low density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B, the hallmark of familial hypobetalipoproteinemia. DNA sequencing showed that he, along with other family members (n=5; mean LDL cholesterol 0.8 mmol/L, apoB 0.31 g/L), were heterozygous for a single nucleotide deletion in exon 26 of the APOB gene. This mutation is predicted to form a truncated apoB species of 3545 amino acids, which we have designated apoB-78.2.

Common and rare gene variants affecting plasma LDL cholesterol

The plasma level of LDL cholesterol is clinically important and genetically complex. LDL cholesterol levels are in large part determined by the activity of LDL receptors (LDLR) in the liver. Autosomal dominant familial hypercholesterolaemia (FH) - with its high LDL cholesterol levels, xanthomas, and premature atherosclerosis - is caused by mutations in either the LDLR or in APOB - the protein in LDL recognised by the LDLR. A third, rare form - autosomal recessive hypercholesterolaemia - arises from mutations in the gene encoding an adaptor protein involved in the internalisation of the LDLR. A fourth variant of inherited hypercholesterolaemia was recently found to be associated with missense mutations in PCSK9, which encodes a serine protease that degrades LDLR. Whereas the gain-of-function mutations in PCSK9 are rare, a spectrum of more frequent loss-of-function mutations in PCSK9 associated with low LDL cholesterol levels has been identified in selected populations and could protect against coronary heart disease. Heterozygous familial hypobetalipoproteinaemia (FHBL) - with its low LDL cholesterol levels and resistance to atherosclerosis - is caused by mutations in APOB. In contrast to other inherited forms of severe hypocholesterolaemia such as abetalipoproteinaemia - caused by mutations in MTP - and homozygous FHBL, a deficiency of PCSK9 appears to be benign. Rare variants of NPC1L1, the gene encoding the putative intestinal cholesterol receptor, have shown more modest effects on plasma LDL cholesterol than PCSK9 variants, similar in magnitude to the effect of common APOE variants. Taken together, these findings indicate that heritable variation in plasma LDL cholesterol is conferred by sequence variation in various loci, with a small number of common and multiple rare gene variants contributing to the phenotype.

Polymorphisms in apolipoprotein B and risk of ischemic stroke

CONTEXT

Apolipoprotein B levels associate with risk of ischemic stroke. APOB polymorphisms may influence levels of apolipoprotein B and low-density lipoprotein (LDL), but whether they associate with risk of ischemic stroke is unknown.

OBJECTIVE

We tested the hypothesis that the APOB T71I, A591V, P2712L, R3611Q, E4154K, and N4311S polymorphisms associate with risk of ischemic stroke in the general population and performed in vivo human LDL turnover studies of E4154K heterozygotes vs. K4154K homozygotes.

DESIGN

This was a prospective study (the Copenhagen City Heart Study) with 23-yr, 100% complete follow-up.

SETTING

The study was conducted with a Danish general population.

PARTICIPANTS

A total of 9157 women and men aged 20-80+ yr participated in the study.

MAIN OUTCOME MEASURES

Risk of ischemic cerebrovascular disease and ischemic stroke, apolipoprotein B and LDL levels, and LDL fractional catabolic rate were measured. The hypothesis was formulated before genotyping.

RESULTS

APOB K4154K homozygotes had an age-adjusted hazard ratio of 0.4 (95% confidence interval 0.2-0.9) for ischemic cerebrovascular disease and 0.2 (0.1-0.7) for ischemic stroke relative to E4154E homozygotes. Corresponding multifactorially adjusted hazard ratios were 0.5 (0.2-1.0) and 0.2 (0.1-0.8). Furthermore, E4154K heterozygotes and K4154K homozygotes had lower levels of apolipoprotein B and LDL cholesterol, compared with E4154E homozygotes. Finally, E4154K heterozygotes had an increased fractional catabolic rate of LDL relative to E4154E homozygotes. None of the other polymorphisms studied influenced risk of ischemic stroke.

CONCLUSIONS

APOB K4154K homozygosity predicts a 3- to 5-fold reduction in risk of ischemic cerebrovascular disease and ischemic stroke. This may be explained by lower plasma levels of apolipoprotein B and LDL cholesterol caused by an increased catabolism of LDL particles, although another yet-unknown mechanism is also possible.

Missense mutations in APOB within the betaalpha1 domain of human APOB-100 result in impaired secretion of ApoB and ApoB-containing lipoproteins in familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is associated with mutations in the APOB gene. We reported the first missense APOB mutation, R463W, in an FHBL kindred (Burnett, J. R., Shan, J., Miskie, B. A., Whitfield, A. J., Yuan, J., Tran, K., Mc-Knight, C. J., Hegele, R. A., and Yao, Z. (2003) J. Biol. Chem. 278, 13442-13452). Here we identified a second nonsynonymous APOB mutation, L343V, in another FHBL kindred. Heterozygotes for L343V (n = 10) had a mean plasma apoB at 0.31 g/liter as compared with 0.80 g/liter in unaffected family members (n = 22). The L343V mutation impaired secretion of apoB-100 and very low density lipoproteins. The secretion efficiency was 20% for B100wt and 10% for B100LV and B100RW. Decreased secretion of mutant apoB-100 was associated with increased endoplasmic reticulum retention and increased binding to microsomal triglyceride transfer protein and BiP. Reduced secretion efficiency was also observed with B48LV and B17LV. Biochemical and biophysical analyses of apoB domain constructs showed that L343V and R463W altered folding of the alpha-helical domain within the N terminus of apoB. Thus, proper folding of the alpha-helical domain of apoB-100 is essential for efficient secretion.

Molecular diagnosis of hypobetalipoproteinemia: an ENID review

Primary hypobetalipoproteinemia (HBL) includes a group of genetic disorders: abetalipoproteinemia (ABL) and chylomicron retention disease (CRD), with a recessive transmission, and familial hypobetalipoproteinemia (FHBL) with a co-dominant transmission. ABL and CRD are rare disorders due to mutations in the MTP and SARA2 genes, respectively. Heterozygous FHBL is much more frequent. FHBL subjects often have fatty liver and, less frequently, intestinal fat malabsorption. FHBL may be linked or not to the APOB gene. Most mutations in APOB gene cause the formation of truncated forms of apoB which may or may be not secreted into the plasma. Truncated apoBs with a size below that of apoB-30 are not detectable in plasma; they are more frequent in patients with the most severe phenotype. Only a single amino acid substitution (R463W) has been reported as the cause of FHBL. Approximately 50% of FHBL subjects are carriers of pathogenic mutations in APOB gene; therefore, a large proportion of FHBL subjects have no apoB gene mutations or are carriers of rare amino acid substitutions in apoB with unknown effect. In some kindred FHBL is linked to a locus on chromosome 3 (3p21) but the candidate gene is unknown. Recently a FHBL plasma lipid phenotype was observed in carriers of mutations of the PCSK9 gene causing loss of function of the encoded protein, a proprotein convertase which regulates LDL-receptor number in the liver. Inactivation of this enzyme is associated with an increased LDL uptake and hypobetalipoproteinemia. HBL carriers of PCSK9 mutations do not develop fatty liver disease.

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.

Thematic review series: patient-oriented research. What we have learned about VLDL and LDL metabolism from human kinetics studies

Lipoprotein metabolism is the result of a complex network of many individual components. Abnormal lipoprotein concentrations can result from changes in the production, conversion, or catabolism of lipoprotein particles. Studies in hypolipoproteinemia and hyperlipoproteinemia have elucidated the processes that control VLDL secretion as well as VLDL and LDL catabolism. Here, we review the current knowledge regarding apolipoprotein B (apoB) metabolism, focusing on selected clinically relevant conditions. In hypobetalipoproteinemia attributable to truncations in apoB, the rate of secretion is closely linked to the length of apoB. On the other hand, in patients with the metabolic syndrome, it appears that substrate, in the form of free fatty acids, coupled to the state of insulin resistance can induce hypersecretion of VLDL-apoB. Studies in patients with familial hypercholesterolemia, familial defective apoB, and mutant forms of proprotein convertase subtilisin/kexin type 9 show that mutations in the LDL receptor, the ligand for the receptor, or an intracellular chaperone for the receptor are the most important determinants in regulating LDL catabolism. This review also demonstrates the variance of results within similar, or even the same, phenotypic conditions. This underscores the sensitivity of metabolic studies to methodological aspects and thus the importance of the inclusion of adequate controls in studies.

In-vivo metabolism of VLDL-apolipoprotein-B, -CIII and -E in normolipidemic subjects

BACKGROUND AND AIM

ApoE and apoC-III are important components of lipoprotein metabolism. While the function of both apoproteins is relatively well understood, little is known about the in vivo metabolism of these proteins, partly because of the lack of a standardized method to isolate these apoproteins in large sample numbers.

METHODS AND RESULTS

We developed a new reverse phase HPLC method (acetonitril/phosphate gradient; Aquapore RP-300, 7 microm, 220 x 4.6 mm) to isolate a number of different apoproteins, including apoC-III and apoE from VLDL. This method was then used in a study which aimed at determining VLDL-apoE-3 and VLDL-apoC-III metabolism. In addition VLDL-apoB and LDL-apoB metabolism was determined. Endogenous labeling with d(3)-leucine, mass spectrometry and multicompartmental modeling was used in 6 normolipidemic healthy male subjects. Tracer/tracee ratios of free plasma leucine, VLDL-apoE, -apoC-III, -apoB, and LDL-apoB leucine were determined over 60 h following a bolus of d(3)-leucine (5 mg kg(-1)). In all subjects sufficient apoC-III could be isolated by reverse phase HPLC to derive metabolic parameters, while apoE metabolic parameters could only be determined if apoE plasma concentration was 0.75 mg dl(-1) or higher. Compared to VLDL-apoB (FCR 10.4 +/- 3.3 d(-1), production 17.8 +/- 4.5 mg kg(-1) d(-1)), VLDL-apoE-3 (FCR 1.03 +/- 0.11 d(-1), production 0.50 +/- 0.29 mg kg(-1) d(-1)) and VLDL-apoC-III (FCR 1.67 +/- 1.22 d(-1), production 0.44 +/- 0.24 mg kg(-1) d(-1)) parameters were much lower. This indicates that apoE-3 and apoC-III recirculate in plasma and that only a small fraction of apoE and apoC-III on VLDL is newly synthesized.

CONCLUSIONS

We conclude that HPLC methodology can be used to isolate VLDL-apoC-III and apoE for metabolic studies and that the metabolic fate of apoC-III and apoE is different from that of apoB because both apoproteins recycle through the VLDL fraction.

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.

Mutation in apolipoprotein B associated with hypobetalipoproteinemia despite decreased binding to the low density lipoprotein receptor

Mutations in apolipoprotein B (APOB) may reduce binding of low density lipoprotein (LDL) to the LDL receptor and cause hypercholesterolemia. We showed that heterozygotes for a new mutation in APOB have hypobetalipoproteinemia, despite a reduced binding of LDL to the LDL receptor. APOB R3480P heterozygotes were identified among 9,255 individuals from the general population and had reduced levels of apoB-containing lipoproteins. Most surprisingly, R3480P LDL bound with lower affinity to the LDL receptor than non-carrier LDL in vitro, and these results were confirmed by turnover studies of LDL in vivo. In very low density lipoprotein (VLDL) turnover studies, the amount of VLDL converted to LDL in R3480P heterozygotes was substantially reduced, suggesting that this was the explanation for the hypobetalipoproteinemia observed in these individuals. Our findings emphasized the importance of combining in vitro studies with both human in vivo and population-based studies, as in vitro studies often have focused on very limited aspects of complex mechanisms taken out of their natural context.

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.

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.

Hepatic secretion of apoB-100 is impaired in hypobetalipoproteinemic mice with an apoB-38.9-specifying allele

Apolipoprotein B (apoB) truncation-specifying mutations cause familial hypobetalipoproteinemia (FHBL). Lipoprotein kinetics studies have shown that production rates of apoB-100 are reduced by 70-80% in heterozygous FHBL humans, instead of the expected 50%. To develop suitable mouse models to study the underlying mechanism, apoB-38.9-only (Apob(38.9/38.9)) mice were crossbred with Apobec-1 knockout (Apobec-1(-/-)) mice or apoB-100-only (Apob(100/100)) mice to produce two lines of apoB-38.9 heterozygous mice that produce only apoB-38.9 and apoB-100, namely Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) mice. In vivo rates of apoB-100 secretion were measured using [35S]Met/Cys to label proteins and Triton WR-1339 to block apoB-100 VLDL lipolysis/uptake. Rates of secretion were reduced by 80%, rather than the expected 50%, in both Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) mice compared with those of the respective Apobec-1(-/-)/Apob(+/+) and Apob(100/100) control mice. Continuous labeling and pulse-chase experiments in primary hepatocyte cultures revealed that rates of apoB-100 synthesis by Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) hepatocytes were reduced to the expected 50% of those of the respective controls, but the efficiency of secretion of apoB-100 was significantly lower in apoB-38.9 heterozygous hepatocytes. The greater-than-expected decreases in apoB-100 production rates of FHBL heterozygous humans appear to be attributable to a defect in secretion rather than in the synthesis of apoB-100 from the unaffected apoB allele.

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.

A novel nontruncating APOB gene mutation, R463W, causes familial hypobetalipoproteinemia

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.

Amino terminal 38.9% of apolipoprotein B-100 is sufficient to support cholesterol-rich lipoprotein production and atherosclerosis

OBJECTIVE

Carboxyl terminal truncation of apolipoprotein (apo)B-100 and apoB-48 impairs their capacity for triglyceride transport, but the ability of the resultant truncated apoB to transport cholesterol and to support atherosclerosis has not been adequately studied. The atherogenicity of apoB-38.9 was determined in this study by using our apoB-38.9-only (Apob38.9/38.9) mice.

METHODS AND RESULTS

ApoB-38.9-lipoproteins (Lp-B38.9) circulate at very low levels in Apob38.9/38.9 mice as small LDLs or HDLs. Disruption of apoE gene in these mice caused accumulation of large amounts of betaVLDL-like LpB-38.9 in plasma. These betaVLDL particles were more enriched with cholesteryl esters but poor in triglycerides compared with the apoB-48-betaVLDL of the apoB-wild-type/apoE-null (Apob+/+/Apoe-/-) mice. Likewise, apoB-38.9-VLDL secreted by cultured Apob38.9/38.9 mouse hepatocytes also had higher ratios of total cholesterol to triglycerides than apoB-48-VLDL secreted by the apoB-48-only hepatocytes. Thus, despite its impaired triglyceride-transporting capacity, apoB-38.9 has a relatively intact capacity for cholesterol transport. Spontaneous aortic atherosclerotic lesions were examined in apoB-38.9-only/apoE-null (Apob38.9/38.9/Apoe-/-) mice at ages 9 and 13 months. Extensive lesions were found in the Apob38.9/38.9/Apoe-/- mice as well as in their Apob+/38.9/Apoe-/- and Apob+/+/Apoe-/- littermates.

CONCLUSIONS

Deleting the C-terminal 20% from apoB-48 does not impair its ability to transport cholesterol and to support atherosclerosis, thus narrowing the "atherogenic region" of apoB.

ApoB-48 and apoB-100 differentially influence the expression of type-III hyperlipoproteinemia in APOE*2 mice

Apolipoprotein E (apoE) is essential for the clearance of plasma chylomicron and VLDL remnants. The human APOE locus is polymorphic and 5-10% of APOE*2 homozygotes exhibit type-III hyperlipoproteinemia (THL), while the remaining homozygotes have less than normal plasma cholesterol. In contrast, mice expressing APOE*2 in place of the mouse Apoe (Apoe(2/2) mice) are markedly hyperlipoproteinemic, suggesting a species difference in lipid metabolism (e.g., editing of apolipoprotein B) enhances THL development. Since apoB-100 has an LDLR binding site absent in apoB-48, we hypothesized that the Apoe(2/2) THL phenotype would improve if all Apoe(2/2) VLDL contained apoB-100. To test this, we crossed Apoe(2/2) mice with mice lacking the editing enzyme for apoB (Apobec(-/-)). Consistent with an increase in remnant clearance, Apoe(2/2). Apobec(-/-) mice have a significant reduction in IDL/LDL cholesterol (IDL/LDL-C) compared with Apoe(2/2) mice. However, Apoe(2/2).Apobec(-/-) mice have twice as much VLDL triglyceride as Apoe(2/2) mice. In vitro tests show the apoB-100-containing VLDL are poorer substrates for lipoprotein lipase than apoB-48-containing VLDL. Thus, despite a lowering in IDL/LDL-C, substituting apoB-48 lipoproteins with apoB-100 lipoproteins did not improve the THL phenotype in the Apoe(2/2).Apobec(-/-) mice, because apoB-48 and apoB-100 differentially influence the catabolism of lipoproteins.

Hypobetalipoproteinemic mice with a targeted apolipoprotein (Apo) B-27.6-specifying mutation: in vivo evidence for an important role of amino acids 1254-1744 of ApoB in lipid transport and metabolism of the apoB-containing lipoprotein

Carboxyl-terminal deletion of apoB-100 may impair its triglyceride (TG)-transporting capability and alter its catabolism. Here, we compare our newly generated apoB gene (Apob)-targeted apoB-27.6-bearing mice to our previously reported apoB-38.9 mice to understand further the relationship between the size of a truncated apoB variant and its function/metabolism in vivo. The apoB-27.6-specifying mutation produces a premature stop codon six amino acids (aa) downstream of the last codon of mouse Apob exon 24 (corresponding to aa 1254 of human apoB-100). ApoB-27.6 transcripts were 3- and 5-fold more abundant than apoB wild type and apoB-38.9 transcripts in the liver. Likewise, hepatic secretion rates of apoB-27.6 were 7-fold higher than those of apoB-48 and apoB-38.9. In contrast, apoB-27.6 heterozygotes (Apob(27.6/+)) had lower hepatic TG secretion rates and higher liver TG contents than both apoB-38.9 heterozygotes (Apob(38.9/+)) and apoB wild type mice (Apob(+/+)). ApoB-27.6 was secreted by Apob(27.6/+) hepatocytes as dense high density lipoprotein particles. Moreover, despite its high secretion rates, apoB-27.6 was barely detectable in plasma. Disruption of apoE gene in Apob(38.9/+) and Apob(27.6/+) dramatically increased plasma levels of apoB-38.9 as well as apoB-48 but caused no change in plasma apoB-27.6 concentrations. Finally, the birth rate of apoB-27.6 homozygotes (Apob(27.6/27.6)) from intercrosses of Apob(27.6/+) was 7-fold lower than that of Apob(38.9/38.9) from Apob(38.9/+) intercrosses (1.8% versus 12%). Crossbreeding of Apob(27.6/27.6) and Apob(38.9/38.9) produced viable Apob(27.6/38.9) offspring, but Apob(27.6/27.6) intercrosses produced no offspring. Together, these results demonstrate in vivo that the apoB-27.6-apoB-38.9 peptide segment (aa 1254-1744) plays a critical role, not only in supporting hepatic TG-secretion and in modulating catabolism of apoB-containing lipoproteins, but also in normal mouse embryonic development.

A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides

Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.

Well-defined regions of apolipoprotein B-100 undergo conformational change during its intravascular metabolism

Apolipoprotein B (apoB)-100-containing lipoproteins are secreted from the liver as large triglyceride-rich very low density lipoproteins (VLDLs) into the circulation, where they are transformed, through the action of lipases and plasma lipid transfer proteins, into smaller, less buoyant, cholesteryl ester-rich low density lipoproteins (LDLs). As a consequence of this intravascular metabolism, apoB-containing lipoproteins are heterogeneous in size, in hydrated density, in surface charge, and in lipid and apolipoprotein composition. To identify specific regions of apoB that may undergo conformational changes during the intravascular transformation of VLDLs into LDLs, we have used a panel of 29 well-characterized anti-apoB monoclonal antibodies to determine whether individual apoB epitopes are differentially expressed in VLDL, intermediate density lipoprotein (IDL), and LDL subfractions isolated from 6 normolipidemic subjects. When analyzed in a solid-phase radioimmunoassay, the expression of most epitopes was remarkably similar in VLDLs, IDLs, and LDLs. Two epitopes that are close to the apoB LDL receptor-binding site show an increased expression in large (1.019 to 1.028 g/mL), medium (1.028 to 1.041 g/mL), and small (1.041 to 1.063 g/mL) LDLs compared with VLDLs and IDLs, and 2 epitopes situated between apoB residues 4342 and 4536 are significantly more immunoreactive in small and medium-sized LDLs compared with VLDLs, IDLs, and large LDLs. Therefore, as VLDL is converted to LDL, conformational changes identified by monoclonal antibodies occur at precise points in the metabolic cascade and are limited to well-defined regions of apoB structure. These conformational changes may correspond to alterations in apoB functional activities.

In vivo metabolism of ApoB, ApoA-I, and VLDL triglycerides in a form of hypobetalipoproteinemia not linked to the ApoB gene

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant disorder that may result from different mutations in the apolipoprotein B (apoB) gene or chromosome 2. However, linkage of FHBL to the apoB gene was ruled out in 2 kindreds reported to date, and the genetic and metabolic bases for FHBL remain unknown. One of the reported kindreds is our 40-member F kindred, in which we found linkage of FHBL to a novel susceptibility region on chromosome 3p21. 1-2. In addition to having low apoB levels, some, but not all, of the affected subjects in the F kindred also had low levels of high density lipoprotein (HDL) cholesterol and apoA-I. Our aim was to define the metabolic bases of the disorder in the F kindred. Therefore, we studied the in vivo kinetics of apoB and apoA-I and very low density lipoprotein (VLDL) triglycerides in 4 affected subjects and 5 normolipidemic relatives. Deuterated leucine and deuterated glycerol were used to label the apolipoproteins and triglycerides, respectively. Compartmental modeling was used to obtain the kinetic parameters. Affected subjects had (1) normal fractional catabolic rates (FCRs) for VLDL apoB, (2) increased FCRs for low density lipoprotein (LDL) apoB (0.050+/-0.009 versus 0. 030+/-0.006 pools per hour for normal subjects, P=0.005), and (3) decreased production rates of VLDL apoB (11.4+/-1.7 versus 25.6+/-4. 9 mg. kg(-1). d(-1), P=0.003), LDL apoB (7.8+/-1.3 versus 12.7+/-3.7 mg. kg(-1). d(-1), P=0.04), and VLDL triglycerides (8.2+/-4.5 versus 19.6+/-10.8 58 micromol. kg(-1). h(-1), P=0.09). These data differ from those obtained in previously studied FHBL heterozygotes bearing apoB-2 and apoB-9, 2 very short truncations of apoB. Low HDL cholesterol and apoA-I levels were caused by higher apoA-I FCRs (0. 035+/-0.005 versus 0.018+/-0.005 pools per hour in controls, P<0.01) without significant decrease in apoA-I production rates (18.7+/-2.7 versus 22.8+/-5.6 mg. kg(-1). d(-1)). In conclusion, decreased secretion of apoB-containing lipoproteins and hypercatabolism of LDL account for low apoB and cholesterol levels in this novel form of FHBL.

Decreased production rates of VLDL triglycerides and ApoB-100 in subjects heterozygous for familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominant disorder characterized by low levels of apolipoprotein (apo) B and low-density lipoprotein (LDL) cholesterol. Decreased production rates of apoB have been demonstrated in vivo in FHBL heterozygotes. In the present study, we wished to investigate whether the transport of triglycerides was similarly affected in these subjects. Therefore, we studied the in vivo kinetics of very-low-density lipoprotein (VLDL) triglycerides and VLDL apoB-100 simultaneously in 7 FHBL heterozygotes from 2 well-characterized kindreds and 7 healthy normolipidemic subjects. In both kindreds, hypobetalipoproteinemia is caused by mutations in the 5' portion of the apoB gene specifying short truncations of apoB undetectable in plasma. A bolus injection of deuterated palmitate and a primed constant infusion of deuterated leucine were given simultaneously, and their incorporation into VLDL triglycerides and VLDL apoB, respectively, were determined by gas chromatography-mass spectrometry. Kinetic parameters were calculated by using compartmental modeling. VLDL apoB fractional catabolic rates (FCRs) in FHBL heterozygotes and controls were similar (11. 6+/-3.9 and 10.9+/-2.4 pools per day, respectively, P=0.72). On the other hand, FHBL heterozygotes had a 75% decrease in VLDL apoB production rates compared with normal subjects (5.8+/-1.8 versus 23.4+/-7.1 mg/kg per day, P<0.001). The decreased production rates of VLDL apoB accounts for the very low concentrations of plasma apoB found in heterozygotes from these kindreds (24% of normal). Mean VLDL triglyceride FCRs in FHBL subjects and controls were not significantly different (1.06+/-0.74 versus 0.89+/-0.50 pools per hour, respectively, P=0.61). There was a good correlation between VLDL apoB FCR and VLDL triglyceride FCR in the 2 groups (r=0.84, P<0. 001). VLDL triglyceride production rates were decreased by 60% in FHBL heterozygotes compared with controls (9.3+/-6.0 versus 23.0+/-9. 6 micromol/kg per hour, P=0.008). Thus, the hepatic secretion of VLDL triglycerides is reduced in FHBL heterozygotes but to a lesser extent than the decrease in apoB-100 secretion. This is probably achieved by the secretion of VLDL particles enriched with triglycerides.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Dual mechanisms for the low plasma levels of truncated apolipoprotein B proteins in familial hypobetalipoproteinemia. Analysis of a new mouse model with a nonsense mutation in the Apob gene

Familial hypobetalipoproteinemia (FHbeta), a syndrome characterized by low plasma cholesterol levels, is caused by mutations in the apo-B gene that interfere with the synthesis of apo-B100. FHbeta mutations frequently lead to the synthesis of a truncated form of apo-B, which typically is present in plasma at < 5% of the levels of apo-B100. Although many FHbeta mutations have been characterized, the basic mechanisms causing the low plasma levels of truncated apo-B variants have not been defined. We used gene targeting to create a mutant allele that exclusively yields a truncated apo-B, apo-B83. In mice heterozygous for the Apob83 allele, plasma levels and the size and density distribution of apo-B83-containing lipoproteins were strikingly similar to those observed in humans with FHbeta and an apo-B83 mutation. Analysis of mice carrying the Apob83 mutation revealed two mechanisms for the low plasma levels of apo-B83. First, Apob83 mRNA levels and apo-B83 secretion were reduced 76 and 72%, respectively. Second, apo-B83 was removed rapidly from the plasma, compared with apo-B100. This mouse model provides a new level of understanding of FHbeta and adds new insights into apo-B metabolism.

Hypobetalipoproteinemia associated with apo B-48.4, a truncated protein only 14 amino acids longer than apo B-48

Familial hypobetalipoproteinemia is an autosomal codominant trait that can be caused by mutations in the apo B gene. Here we report a novel apo B gene mutation causing hypobetalipoproteinemia, that is associated with the synthesis of a truncated apo B protein in a young healthy male subject and his mother. The mutation is an A deletion at position 6627 of the apo B cDNA leading to a truncated protein of 2166 amino acids (apo B-48.4). This truncated apo B was detected mainly in VLDL, LDL and in trace amounts in HDL, but not in the lipoprotein deficient plasma fraction. Affected family members present with elevated levels of HDL-cholesterol, mainly due to an increase in HDL2 particles. Postprandial triglycerides and retinyl esters in the d < 1.006 g/ml lipoprotein in the proband showed a normal response to an oral fat load compared to a group of eight matched healthy controls. In summary this novel mutation is associated with hypobetalipoproteinemia with a normal fat absorption as expected for a protein with a length similar to that of apo B-48.

Organ loci of catabolism of short truncations of apoB

Truncations of apolipoprotein (apo) B shorter than 3200 amino acids (3200/4536 = apoB-70) do not possess the LDL receptor-recognition domain and are not recognized by altered cells with normally functioning LDL receptors. To ascertain which organs remove such truncated apoB-containing particles, we isolated apoB-31-, apoB-38.9-, and apoB-43.7-containing particles from plasmas of familial hypobetalipoproteinemia heterozygous humans by a combination of sequential ultracentrifugation and preparative electrophoresis. Particles with labeled 125I- or 131I-dilactitol tyramine (I-DLT), were injected into New Zealand White rabbits, along with I-DLT-apoB-100-containing LDLs, and the decay of 125I- and 131I-TCA-precipitated counts was followed over 24 hours. At the end of 24 hours, rabbits were anesthetized and their bodies perfused. Organs were removed and homogenized, and TCA-precipitable counts determined. Fractional catabolic rates of apoB truncation particles were two to five times greater than those of apoB-100 LDLs. ApoB truncations accumulated in adrenals at one fifth the rates of apoB-100 LDL, compatible with the functional absences of LDL receptor-recognition domains in truncated apoBs. The major organ of uptake for apoB-100-LDLs was the liver, whereas truncation particles were readily removed by the kidney (kidney: liver uptake ratios were 0.10 to 0.30 for apoB-100 LDLs and 1.03 to 3.77 for truncations). Spleens accumulated little of either apoB-100 or truncation particles, suggesting particles were not "damaged" or aggregated. Thus, the absence of > 56% of the carboxyl end of apoB-100 increases the plasma clearance and redirects the organ uptake of the apoB truncation-containing lipoproteins from liver to kidney.

Decreased production and increased catabolism of apolipoprotein B-100 in apolipoprotein B-67/B-100 heterozygotes

Apolipoprotein (apo) B-67 is a truncated form of apoB-100 due to deletion of an adenine at cDNA 9327. Heterozygotes have one allele making apoB-100; therefore, plasma apoB levels would be predicted to be at least 50% of normal. However, apoB-67 heterozygotes have total plasma apoB levels that are 24% of normal. To determine the mechanisms responsible for the lower-than-expected levels of apoB, in vivo kinetics of apoB-100 were performed in three apoB-67/apoB-100 heterozygotes and compared with those of six control subjects by using a primed-constant infusion of [5,5,5-2H3]leucine in the fed state. Kinetic parameters were calculated by multicompartmental modeling of the data. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8+/-6.1 mg/dL, or 24% of that of control subjects (89.6+/-24.1 mg/dL, P=.002). ApoB-67 subjects had lower mean VLDL apoB-100 production rates (3.6+/-1.2 versus 13.9+/-3.5 mg x kg(-1) x d(-1), P=.002) and lower mean transport rates of apoB-100 into LDL (3.5+/-1.4 versus 12.6+/-4.1 mg x kg(-1) x d(-1), P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2+/-0.5 versus 6.2+/-4.0 mg x kg(-1) x d(-1), P=.07). The fractional catabolic rate of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1+/-8.6 versus 7.6+/-1.6 mg x kg(-1) x d(-1), P=.017). ApoB-100 IDL and LDL fractional catabolic rates were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8+/-7.7 versus 141.6+/-33.7 mg, P=.0004) due to a 74% lower production rate (26% of control values) and a 2.4-fold higher fractional catabolic rate. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3+/-192.4 versus 2968.3+/-765.2 mg, P=.002) due primarily to a lower production rate (27% of control values). Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.

Use of stable isotopically labeled tracers for studies of metabolic kinetics: an overview

Stable isotopically labeled tracers offer a reliable and safe alternative to the use of radioactive tracers for studies of metabolic kinetics. This overview examines some of the principles and technical issues regarding mass spectrometry instrumentation, and reviews some of the approaches used in the application of stable isotopically labeled tracers to studies of protein, lipid, and carbohydrate metabolic kinetics.

A new apolipoprotein B truncation (apo B-43.7) in familial hypobetalipoproteinemia: genetic and metabolic studies

We describe a new truncation of apolipoprotein (apo) B in a white kindred with familial hypobetalipoproteinemia (FHBL). Apo B-43.7, found in a daughter and her father, was due to a C --> T change in base position 6162 of the apo B gene converting the arginine (residue 1986) codon CGA to a stop codon TGA. Both subjects were heterozygotes, and both apo B-43.7- and apo B-100-containing particles were present in plasma. On density gradient ultracentrifugation (DGUC), approximately 30% to 40% of apo B-43.7 floated with very-low-density lipoprotein (VLDL)/intermediate-density lipoprotein (IDL)-density particles and 60% to 70% floated with high-density lipoprotein (HDL)-density particles. To assess the metabolism of apo B, 13C-leucine was infused and its rates of appearance in and disappearance from apo B-43.7- and apo B-100-containing particles were quantified by multicompartmental kinetic analysis. Apo B-100 entered plasma via VLDL with a production rate of 30 mg x kg-1 x d-1. Fractional catabolic rates (FCRs) for apo B-100 VLDL, IDL, and low-density lipoprotein (LDL) were 20.0, 16.0, and 0.46 pools x d-1, respectively. The production rate of apo B-43.7 was 9.6 mg x kg-1 x d-1, and FCRs for apo B-43.7 VLDL- and HDL-like particles were 12.0 and 1.8 pools x d-1, respectively. Approximately 30% of apo B-43.7 in HDL-density particles was derived from VLDL apo B-43.7, and about 70% appeared to enter the plasma as HDLs. The relatively low production rate of apo B-43.7 is compatible with previous reports that apo B truncations are produced at lower rates than their apo B-100 counterparts.

Homozygous familial hypobetalipoproteinemia. Increased LDL catabolism in hypobetalipoproteinemia due to a truncated apolipoprotein B species, apo B-87Padova

Mutations on the apolipoprotein (apo) B gene that interfere with the full-length translation of the apoB molecule are associated with familial hypobetalipoproteinemia (FHBL), a disease characterized by the reduction of plasma apoB and LDL cholesterol. In this report, we describe an FHBL kindred carrying a unique truncated apoB form, apoB-87Padova. Sequence analysis of amplified genomic DNA identified a single G deletion at nucleotide 12032, which shifts the translation reading frame and causes a termination at amino acid 3978. Two homozygous subjects and seven heterozygous relatives were studied. Although homozygous individuals had only trace amounts of LDL, they were virtually free from the symptoms typical of homozygous FHBL subjects. We investigated the in vivo turnover of radiolabeled normal apoB-100 LDL and apoB-87 LDL in one homozygous patient and two normal control subjects. ApoB-87 LDL showed a similar metabolism in all three subjects, with a fractional catabolic rate more than double that of normal LDL. The rate of entry of apoB-87 in the LDL compartment was also markedly decreased compared with normal apoB-100. The increased in vivo catabolism of apoB-87 LDL was paralleled in vitro by a 2.5-fold increased ability of these particles to inhibit the uptake and degradation of normal apoB-100 LDL by normal human cultured fibroblasts. These results indicate that apoB-87 LDL has an enhanced ability to interact with the LDL receptor, the increased apoB catabolism contributes to the hypobetalipoproteinemia and may explain the mild expression of the disease in the two homozygous individuals.

Familial hypobetalipoproteinemia is not associated with low levels of lipoprotein(a)

To assess whether very low concentrations of LDL affected lipoprotein(a) [Lp(a)] concentrations and apo(a) associations with lipoproteins, we studied Lp(a) levels and associations in heterozygous subjects with familial hypobeta-lipoproteinemia FHBL) associated with several truncated forms of apoB-100, ranging from apoB-31 to apoB-89. Distributions of apo(a) isotypes were assessed by a combined electrophoresis-immunoblotting procedure that detects 34 isoforms. Lp(a) concentrations were quantified by two ELISAs, one detecting total apo(a) and the other apoB-bound apo(a) in plasma. Associations of apo(a) with plasma lipoproteins were evaluated by gel permeation chromatography (FPLC) and DGUC followed by analyses of elution and gradient fractions by apo(a) ELISA. In addition, associations were examined by nondenaturing electrophoresis or immunoprecipitation of whole plasma and examination of contents by immunoblotting. Finally, interactions between r-apo(a) and LDLs were evaluated in reconstitution experiments. The distributions of apo(a) isotypes did not differ between FHBL-affected and unaffected members of the same kindreds, and concentrations of Lp(a) were similar even when subjects were matched for isotypes both within and across kindreds. In subjects heterozygous for apo(a) isoforms, the smaller isoforms were inversely related to Lp(a) concentrations, the larger isoforms were not. The regression lines between Lp(a) concentrations and the smaller apo(a) isoforms were significant and negative in slope for both FHBL-affected and unaffected subjects, but the slopes of the lines did not differ. In multiple regression analyses, only the sizes of the smaller apo(a) isoforms contributed to the prediction of Lp(a) concentrations. ApoB-size made no difference. In simple apoB-100/apoB-truncation heterozygotes, virtually all apo(a) was complexed with apoB-100-containing particles but not apoB-truncation particles, and r-apo(a) recombined with apoB-100-containing LDLs but not with apoB-89-containing LDLs. Thus, (1) low apoB levels do not affect the plasma concentrations of Lp(a), (2) apo(a) binds apoB-100 to form Lp(a) particles of usual sizes and densities, and (3) apoB truncations even as large as apoB-89 do not form covalent bands with apo(a), although noncovalent associations with apoB-89 may be present in plasma.

Increased catabolic rate of low density lipoproteins in humans with cholesteryl ester transfer protein deficiency

The cholesteryl ester transfer protein (CETP) transfers lipids among lipoprotein particles and plays a central role in lipoprotein metabolism. Humans with genetic deficiency of CETP have both elevated HDL cholesterol and apolipoprotein A-I concentrations as well as decreased LDL cholesterol and apolipoprotein B levels. The present study was undertaken to elucidate the metabolic basis for the decreased LDL cholesterol and apo B levels in CETP deficiency. We conducted a series of in vivo apo B kinetic studies in tow unrelated homozygotes with CETP deficiency and in control subjects. A primed constant infusion of stable isotopically labeled phenylalanine was administered to the two CETP deficient subjects and control subjects and apo B kinetic parameters in VLDL, intermediate density lipoproteins, and LDL were obtained by using a multicompartmental model. The fractional catabolic rates (FCR) of LDL apo B were significantly increased in the CETP-deficient subjects (0.56 and 0.75/d) compared with the controls (mean FCR of 0.39/d). Furthermore, the production rates of apo B in VLDL and intermediate density lipoprotein were decreased by 55% and 81%, respectively, in CETP deficiency compared with the controls. In conclusion, CETP-deficient subjects were demonstrated to have substantially increased catabolic rates of LDL apo B as the primary metabolic basis for the low plasma levels of LDL apo B. This result indicates that the LDL receptor pathway may be up-regulated in CETP deficiency.

Apoprotein B-100 production is decreased in subjects heterozygous for truncations of apoprotein B

Among individuals who are heterozygous for familial hypobetalipoproteinemia (FHBL) and who have various truncations of apoprotein (apo) B (ie, FHBL with apoB truncation/apoB-100 genotypes), the plasma concentrations of apoB-100 are typically approximately 30% rather than the expected approximately 50% of those in unaffected family members. The metabolic basis for the low apoB-100 levels is unknown. Therefore, we compared the metabolism of apoB-100 in 8 subjects with heterozygous FHBL (2 apoB-89/apoB-100, 2 apoB-75/apoB-100, 2 apoB-54.8/apoB-100, 1 apoB-52/apoB-100, and 1 apoB-31/apoB-100) with the metabolism of apoB-100 in 8 apoB-100/apoB-100 control subjects who were paired with the heterozygotes by gender, age, height, weight, and race. Endogenous labeling of apoB-100 with [13C]leucine and a multicompartmental kinetic model were used to obtain kinetic parameters. FHBL heterozygotes had significantly reduced VLDL apoB-100 production rates (7.7 +/- 3.7 versus 21.2 +/- 6.2 mg.kg-1.d-1, P = .002) and LDL apoB-100 production rates (4.5 +/- 3.12 versus 15.3 +/- 1 mg.kg-1.d-1, P = .05) compared with control subjects. Fractional conversion rates of VLDL to LDL were not significantly different (0.67 +/- 0.36 versus 0.77 +/- 0.17 pools/d), and the respective fractional catabolic rates of apoB-100 in VLDL, IDL, and LDL also were similar in both groups. Thus, FHBL heterozygotes produced apoB-100 at about 30% of the rates of control subjects. We believe these reduced production rates largely account for the lower than expected levels of apoB-100 and LDL cholesterol in the plasma of FHBL heterozygotes.