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

A Review of Progress on Targeting LDL Receptor-Dependent and -Independent Pathways for the Treatment of Hypercholesterolemia, a Major Risk Factor of ASCVD

Since the discovery of the LDL receptor in 1973 by Brown and Goldstein as a causative protein in hypercholesterolemia, tremendous amounts of effort have gone into finding ways to manage high LDL cholesterol in familial hypercholesterolemic (HoFH and HeFH) individuals with loss-of-function mutations in the LDL receptor (LDLR) gene. Statins proved to be the first blockbuster drug, helping both HoFH and HeFH individuals by inhibiting the cholesterol synthesis pathway rate-limiting enzyme HMG-CoA reductase and inducing the LDL receptor. However, statins could not achieve the therapeutic goal of LDL. Other therapies targeting LDLR include PCSK9, which lowers LDLR by promoting LDLR degradation. Inducible degrader of LDLR (IDOL) also controls the LDLR protein, but an IDOL-based therapy is yet to be developed. Among the LDLR-independent pathways, such as angiopoietin-like 3 (ANGPTL3), apolipoprotein (apo) B, apoC-III and CETP, only ANGPTL3 offers the advantage of treating both HoFH and HeFH patients and showing relatively better preclinical and clinical efficacy in animal models and hypercholesterolemic individuals, respectively. While loss-of-LDLR-function mutations have been known for decades, gain-of-LDLR-function mutations have recently been identified in some individuals. The new information on gain of LDLR function, together with CRISPR-Cas9 genome/base editing technology to target LDLR and ANGPTL3, offers promise to HoFH and HeFH individuals who are at a higher risk of developing atherosclerotic cardiovascular disease (ASCVD).

Automated untargeted stable isotope assisted lipidomics of liver cells on high glucose shows alteration of sphingolipid kinetics

Untargeted lipidomics is a powerful tool to discover new biomarkers and to understand the physiology and pathology of lipids. The use of stable isotopes as tracers to investigate the kinetics of lipids is another tool able to supply dynamic information on lipid synthesis and catabolism. Coupling the two methodology is then very appealing in the study of lipid metabolism. The main issue to face is to perform thousands of calculations in order to obtain kinetic parameters starting from the MS raw data. An automated computerized routine able to do accomplish such task is presented in this paper. We analyzed the lipid kinetics of palmitic acid (PA) in hepatoma liver cells cultured in vitro in which insulin resistance has been induced by high glucose supplementation. The deuterated palmitate tracer (d5PA) was administered as a bolus and the cells were harvested daily for 48 h. 5dPA was incorporated into 326 monoisotopic compounds and in 84 of their [M + 1] isotopologues detected by high resolution orbitrap MS. The differences between the kinetics curves showed that at least four long chain triglycerides (TG) species incorporated more PA in glucose treated cells, while phosphocholines, sphingomyelins, mono- and di-glycerides and ceramides (Cer) incorporated less tracer under glucose treatment. Nevertheless, Cer amount was increased by glucose treatment. In conclusion we developed an automated powerful algorithm able to model simultaneously hundreds of kinetic curves obtained in a cell culture spiked with a stable isotope tracer, and to analyze the difference between the two different cell models.

Molecular analysis of APOB, SAR1B, ANGPTL3, and MTTP in patients with primary hypocholesterolemia in a clinical laboratory setting: Evidence supporting polygenicity in mutation-negative patients

BACKGROUND AND AIMS

Primary hypobetalipoproteinemia is generally considered a heterogenic group of monogenic, inherited lipoprotein disorders characterized by low concentrations of LDL cholesterol and apolipoprotein B in plasma. Lipoprotein disorders include abetalipoproteinemia, familial hypobetalipoproteinemia, chylomicron retention disease, and familial combined hypolipidemia. Our aim was to review and analyze the results of the molecular analysis of hypolipidemic patients studied in our laboratory over the last 15 years.

METHODS

The study included 44 patients with clinical and biochemical data. Genomic studies were performed and genetic variants were characterized by bioinformatics analysis. A weighted LDL cholesterol gene score was calculated to evaluate common variants associated with impaired lipid concentrations and their distribution among patients.

RESULTS

Twenty-three patients were genetically confirmed as affected by primary hypobetalipoproteinemia. In this group of patients, the most prevalent mutated genes were APOB (in 17 patients, with eight novel mutations identified), SAR1B (in 3 patients, with one novel mutation identified), ANGPTL3 (in 2 patients), and MTTP (in 1 patient). The other 21 patients could not be genetically diagnosed with hypobetalipoproteinemia despite presenting suggestive clinical and biochemical features. In these patients, two APOB genetic variants associated with lower LDL cholesterol were more frequent than in controls. Moreover, the LDL cholesterol gene score, calculated with 11 SNPs, was significantly lower in mutation-negative patients.

CONCLUSIONS

Around half of the patients could be genetically diagnosed. The results suggest that, in at least some of the patients without an identified mutation, primary hypobetalipoproteinemia may have a polygenic origin.

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.

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.

Effect of hypothyroidism on kinetics of metabolism of very-low-density lipoprotein in mares

OBJECTIVE

To compare kinetics of the metabolism of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) before and after thyroidectomy in mares.

ANIMALS

5 healthy adult mares.

PROCEDURE

Thyroidectomy was performed in euthyroid mares. Kinetics of VLDL apoB metabolism were measured before and after thyroidectomy by use of a bolus IV injection of 5,5,5-2H3 (98%) leucine (5 mg/kg) and subsequent isolation of labeled amino acid from plasma and VLDL. Labeled leucine was quantified by use of gas chromatography-mass spectrometry. Production rate (PR), delay time, and fractional catabolic rate (FCR) were calculated for the 2 forms of equine VLDL, apoB-48 VLDL, and apoB-100 VLDL. Plasma lipid concentrations were measured, and VLDL composition was determined.

RESULTS

Physical appearance of horses was not altered by thyroidectomy. Significantly lower mean blood concentrations of thyroid hormones and non-esterified fatty acids were detected following thyroidectomy. Mean percentage of free cholesterol in VLDL was significantly higher after thyroidectomy. Mean plasma VLDL concentration or kinetics of apoB-48 or apoB-100 were not significantly altered by thyroidectomy. Mean +/- SEM PR was significantly lower (8.70 +/- 1.61 mg/kg/d) and mean delay time significantly longer (1.58 +/- 0.12 hours) for apoB-48 VLDL in euthyroid mares, compared with values for thyroidectomized mares (16.15 +/- 2.24 mg/kg/d and 0.93 +/- 0.10 hours, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

Hypothyroidism did not significantly alter plasma VLDL concentrations or kinetics of VLDL apoB metabolism. Metabolism of apoB-48 VLDL differed significantly from that of apoB-100 VLDL in euthyroid mares.

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.

Regulation of the apolipoprotein B in heterozygous hypobetalipoproteinemic knock-out mice expressing truncated apoB, B81. Low production and enhanced clearance of apoB cause low levels of apoB

Low levels of cholesterol are protective against development of coronary artery disease. Heterozygous hypobetalipoproteinemic individuals expressing truncated apolipoprotein (apo)B as a result of mutation in the apob gene have low levels of cholesterol and apoB in their plasma. To study the molecular mechanism of low levels of apoB in these individuals, we employed a previously reported knock out mouse model generated by targeted modification of the apob gene. The heterozygous, apoB-100/B-81, mice express full length and truncated apoB, B-81, and have 20 and 35% lower levels of total cholesterol and apoB, respectively, when compared to WT (apoB-100/B-100) mice. The majority of the truncated apoB, B-81, fractionated in the VLDL- density range. The mechanism of low levels of apoB in B-100/B-81 mice was examined. Total hepatic apoB mRNA levels decreased by 15%, primarily due to lower levels of apoB-81 mRNA. Since apoB mRNA transcription rates were similar in B-100/B-100 and B-100/B-81 mice, low levels of mutant apoB-81 mRNA occurred by enhanced degradation of apoB mRNA transcript containing premature translational stop codon. ApoB synthesis measured on isolated hepatocytes decreased in B-100/B-81 mice by 35%, while apoB-48, apoE, and apoAI syntheses remained unchanged. Metabolic studies using whole animal showed a 32% decrease in triglyceride secretion rates, consistent with the apoB secretion rates. Inhibition of receptor-mediated clearance of apoB-81-containing particles resulted in greater relative accumulation of apoB-81 in plasma than apoB-100, suggesting enhanced clearance of apoB-81-containing particles. These results demonstrate that low levels of apoB in heterozygous hypobetalipoproteinemic mice occurs by low rates of apoB secretion, and increased clearance of truncated apoB. Similar mechanisms appear to contribute to low levels of apoB in hypobetalipoproteinemic humans.

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.

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.