Homozygous familial defective apolipoprotein B-100. Enhanced removal of apolipoprotein E-containing VLDLs and decreased production of LDLs

Identification of a novel LDLR disease-causing variant using capture-based next-generation sequencing screening of familial hypercholesterolemia patients in Taiwan

BACKGROUND AND AIMS

Familial hypercholesterolemia (FH) is an autosomal dominant disorder with paramount health impacts. However, less than 1% FH patients in Taiwan were formally diagnosed, partly due to the lack of reliable cost-effective genetic testing. We aimed at using a next-generation sequencing (NGS) platform as the clinical genetic testing method for FH.

METHODS

We designed probes to capture the whole LDLR gene and all coding sequences of APOB and PCSK9, and then sequenced with Illumina MiSeq platform (2 × 300 bps). The entire pipeline was tested on 13 DNA samples with known causative variants (including 3 large duplications and 2 large deletions). Then we enrolled a new cohort of 28 unrelated FH patients with Dutch Lipid Clinic Network score ≥5. Relatives were included in the cascade screening.

RESULTS

From the 13 DNA samples, we correctly identify all the variants, including big duplications and deletions. From the new cohort, we identified the causative variants in 21 of the 28 unrelated probands; five of them carrying a novel splice site variant c.1186+2T>G in LDLR. Among the family members, the concentration of LDL cholesterol was 7.82 ± 2.13 mmol/l in LDLR c.1186+2T>G carrier group (n = 26), and was significantly higher than 3.18 ± 1.36 mmol/l in the non-carrier group (n = 25).

CONCLUSIONS

This is the first capture-based NGS testing for FH to cover the whole LDLR genomic region, and therefore making reliable structural variation detection. This panel can comprehensively detect disease-causing variants in LDLR, APOB, and PCSK9 for FH patients.

Molecular description of familial defective APOB-100 in Malaysia

Familial ligand-defective apolipoprotein B-100 is characterized by elevated plasma low-density lipoprotein levels and premature heart disease. This study aims to determine apolipoprotein B gene mutations among Malaysians with clinical diagnoses of familial hypercholesterolemia and to compare the phenotype of patients with apolipoprotein B gene mutations to those with a low-density lipoprotein receptor gene mutation. A group of 164 patients with a clinical diagnosis of familial hypercholesterolemia was analyzed. Amplicons in exon 26 and exon 29 of the apolipoprotein B gene were screened for genetic variants using denaturing gradient high-performance liquid chromatography; 10 variants were identified. Five novel mutations were detected (p.Gln2485Arg, p.Thr3526Ala, p.Glu3666Lys, p.Tyr4343CysfsX221, and p.Arg4297His). Those with familial defective apolipoprotein had a less severe phenotype than those with familial hypercholesterolemia. An apolipoprotein gene defect is present among Malaysian familial hypercholesterolemics. Those with both mutations show a more severe phenotype than those with one gene defect.

Low molecular weight chitosan nanoparticulate system at low N:P ratio for nontoxic polynucleotide delivery

Chitosan, a natural polymer, is a promising system for the therapeutic delivery of both plasmid DNA and synthetic small interfering RNA. Reports attempting to identify the optimal parameters of chitosan for synthetic small interfering RNA delivery were inconclusive with high molecular weight at high amine-to-phosphate (N:P) ratios apparently required for efficient transfection. Here we show, for the first time, that low molecular weight chitosan (LMW-CS) formulations at low N:P ratios are suitable for the in vitro delivery of small interfering RNA. LMW-CS nanoparticles at low N:P ratios were positively charged (ζ-potential ~20 mV) with an average size below 100 nm as demonstrated by dynamic light scattering and environmental scanning electron microscopy, respectively. Nanoparticles were spherical, a shape promoting decreased cytotoxicity and enhanced cellular uptake. Nanoparticle stability was effective for at least 20 hours at N:P ratios above two in a slightly acidic pH of 6.5. At a higher basic pH of 8, these nanoparticles were unravelled due to chitosan neutralization, exposing their polynucleotide cargo. Cellular uptake ranged from 50% to 95% in six different cell lines as measured by cytometry. Increasing chitosan molecular weight improved nanoparticle stability as well as the ability of nanoparticles to protect the oligonucleotide cargo from nucleases at supraphysiological concentrations. The highest knockdown efficiency was obtained with the specific formulation 92-10-5 that combines sufficient nuclease protection with effective intracellular release. This system attained >70% knockdown of the messenger RNA, similar to commercially available lipoplexes, without apparent cytotoxicity. Contrary to previous reports, our data demonstrate that LMW-CS at low N:P ratios are efficient and nontoxic polynucleotide delivery systems capable of transfecting a plethora of cell lines.

Genetics and kinetics of familial hypercholesterolemia, with the special focus on FH-(Marburg) p.W556R

OBJECTIVE

Familial hypercholesterolemia (FH) is an autosomal dominant inherited disorder, caused by mutations in the low density lipoprotein receptor (LDLR) gene. FH is characterized by elevated plasma LDL cholesterol, premature atherosclerosis and high risk of premature myocardial infarction. Extended work has been done to understand both, the primary genetic defect as well as the in vivo kinetic consequences of this disease. Both approaches, genetics and kinetics, are challenging but also fruitful approaches for a better understanding of this devastating disease. For this we reviewed the recent literature and used our in vitro and in vivo data on one of the most frequently occurring types of FH, the FH(Marburg) p.W556R.

METHODS

To identify the primary genetic defect of the FH(Marburg) we used denaturing gradient gel electrophoresis (DGGE) mutation analysis. In vivo kinetic studies were performed in a heterozygote FH(Marburg) subject and in 5 healthy control subjects utilizing a stable isotope tracer kinetic approach with 3D-leucine.

RESULTS

DGGE screening of the LDLR gene identified a tryptophan (W) to arginine (R) substitution at residue 556 (p.W556R) in the fifth conserved YWTD repeat of the LDLR-beta-propeller in FH(Marburg). In vivo kinetic studies in a heterozygote FH subject for FH(Marburg) and in 5 healthy control subjects demonstrated a severe decrease in LDL FCR and a mild increase of LDL PR in FH compared to healthy controls.

CONCLUSIONS

The LDLR mutation p.W556R is a frequent and severe defect for FH. This defect has a major influence on the in vivo lipoprotein kinetics and lipid levels. In a heterozygote FH patient we found a dual defect for the increase in LDL cholesterol, namely a decrease in the fractional catabolic rate (FCR) of LDL but also an increase in LDL production rate (PR). By this a well defined, single genetic defect may have a series of different in vivo metabolic consequences which could be used for potential therapeutic approaches to this disease.

The genetic basis of familial hypercholesterolemia: inheritance, linkage, and mutations

Familial hypercholesterolemia (FH) is a genetic disorder of lipoprotein metabolism characterized by high plasma concentrations of low-density lipoprotein cholesterol (LDLc), tendon xanthomas, and increased risk of premature coronary heart disease. FH is one of the most common inherited disorders; there are 10,000,000 people with FH worldwide, mainly heterozygotes. The most common FH cause is mutations along the entire gene that encode for LDL receptor (LDLR) protein, but it has been also described that mutations in apolipoprotein B (APOB) and proprotein convertase subtilisin/kexin type 9 genes produce this phenotype. About 17%-33% of patients with a clinical diagnosis of monogenic hypercholesterolemia do not harbor any genetic cause in the known loci. Because FH has been considered as a public health problem, it is very important for an early diagnosis and treatment. Recent studies have demonstrated the influence of the LDLR mutation type in the FH phenotype, associating a more severe clinical phenotype and worse advanced carotid artherosclerosis in patients with null than those with receptor-defective mutations. Since 2004, a molecular FH diagnosis based on a genetic diagnostic platform (Lipochip(®); Progenika-Biopharma, Derio, Spain) has been developed. This analysis completes the adequate clinical diagnosis made by physicians. Our group has recently proposed new FH guidelines with the intention to facilitate the FH diagnosis. The treatment for this disease is based on the benefit of lowering LDLc and a healthy lifestyle. Actually, drug therapy is focused on using statins and combined therapy with ezetimibe and statins. This review highlights the recent progress made in genetics, diagnosis, and treatment for FH.

Clinical observations, molecular genetic analysis, and treatment of sitosterolemia in infants and children

The clinical observation and treatment of young children with sitosterolemia has rarely been reported. We report clinical, biochemical, and molecular genetic observations and treatment outcomes for five Chinese children from four separate families presenting with sitosterolemia in whom we identified two new (Y329X, G269R) and three known (R446X, N437K, R389H) mutations in the ABCG5 gene. The R389H mutation was found in 50% of alleles. Three of these five patients received cholestyramine therapy with a very good response. However, all patients discontinued this therapy because of poor compliance. Finally, all patients were on ezetimibe therapy and had satisfactory total serum cholesterol levels, though their plant sterol levels were still higher than normal. Another noteworthy finding is that a female infant had a serum cholesterol level of 654 mg/dl at 7 months of age, despite being breast fed (with very tiny amounts of plant sterols) since birth and undergoing 4 months of ezetimibe administration. Although she failed to respond to ezetimibe during this period, she did show improvement when the therapy was started again at 2 years of age. It is possible that another 23-month-old female patient also responded more slowly to ezetimibe treatment than older patients.

Absence of hyperlipidemia in LDL receptor-deficient mice having apolipoprotein B100 without the putative receptor-binding sequences

OBJECTIVE

To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo.

METHODS AND RESULTS

Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different.

CONCLUSIONS

While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.

Evaluation of clinical diagnosis criteria of familial ligand defective apoB 100 and lipoprotein phenotype comparison between LDL receptor gene mutations affecting ligand-binding domain and the R3500Q mutation of the apoB gene in patients from a South European population

Familial hypercholesterolemia (FH) and familial defective apoB 100 (FDB) are characterized by increased plasma low-density lipoprotein cholesterol (LDLc) levels and risk of coronary heart disease (CHD). FDB is clinically indistinguishable from FH. The aims of this study were to evaluate clinical diagnosis criteria for FDB and to compare the lipoprotein phenotype between carriers of LDL receptor (LDLR) gene mutations that affect the ligand-binding domain and subjects with the R3500Q mutation in apoB gene. We studied 213 subjects (113 probands) with FH and 19 heterozygous FDB subjects. Genetic diagnosis was determined by following a protocol based on Southern blot and polymerase chain reaction-single strand conformation polymorphism (SSCP) analysis. Thirty FH carriers of LDLR gene missense mutations that affect ligand-binding domain were matched by age, gender, and body mass index to the 19 FDB subjects (R3500Q mutation). Lipoprotein phenotype comparison was conducted between the 2 groups. FH patients showed plasma total and LDL cholesterol levels significantly higher than those in FDB patients. Three FDB showed plasma total and LDLc values in the normal range. Using the 1999 clinical Med-Ped criteria for diagnosis of genetic hypercholesterolemia, no FDB subjects had a confirmed diagnosis; it was probable in 36% of the subjects, it was possible in 32% of the subjects, and it could be excluded in the remaining 32% of the subjects. We conclude that the FDB lipoprotein phenotype was significantly less severe than that observed in FH carriers of LDLR gene missense ligand-binding domain mutations. Clinical Med-Ped diagnosis criteria tend to under-diagnose FDB.

Studying apolipoprotein turnover with stable isotope tracers: correct analysis is by modeling enrichments

Lipoprotein kinetic parameters are determined from mass spectrometry data after administering mass isotopes of amino acids, which label proteins endogenously. The standard procedure is to model the isotopic content of the labeled precursor amino acid and of proteins of interest as tracer-to-tracee ratio (TTR). It is shown here that even though the administered tracer alters amino acid mass and turnover, apolipoprotein synthesis is unaltered and hence the apolipoprotein system is in a steady state, with the total (labeled plus unlabeled) masses and fluxes remaining constant. The correct model formulation for apolipoprotein kinetics is shown to be in terms of tracer enrichment, not of TTR. The needed mathematical equations are derived. A theoretical error analysis is carried out to calculate the magnitude of error in published results using TTR modeling. It is shown that TTR modeling leads to a consistent underestimation of the fractional synthetic rate. In constant-infusion studies, the bias error percent is shown to equal approximately the plateau enrichment, generally <10%. It is shown that, in bolus studies, the underestimation error can be larger. Thus, for mass isotope studies with endogenous tracers, apolipoproteins are in a steady state and the data should be fitted by modeling enrichments.

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.

Low Density Lipoprotein Apolipoprotein B Metabolism in Treatment-Naive HIV Patients and Patients on Antiretroviral Therapy

Background

Dyslipidaemia and lipodystrophy have been described in treated HIV patients and in a small percentage of untreated HIV patients. Lipodystrophy in these patients has been shown to be associated with a lower expression of low density lipoprotein (LDL) receptors.

Methods

We have investigated the effect of antiretroviral treatment with either a protease inhibitor (PI) or a non-nucleoside reverse transcriptase inhibitor (NNRTI) on body fat distribution and LDL apolipoprotein B (apoB) kinetics in 12 HIV-negative controls and 52 HIV-infected patients, including antiretroviral treatment-naive (TN) patients ( n=13) and patients taking two nucleoside analogues plus either a PI ( n=15) or an NNRTI ( n=24).

Results

LDL cholesterol was not different between groups. Compared with the controls, LDL apoB absolute synthetic rate (ASR) and fractional catabolic rate (FCR) were lower and residence time (RT) was higher in the PI and NNRTI groups ( P<0.05). In the TN patients, LDL ASR was lower ( P<0.05) and there was a trend for a lower FCR and higher RT compared with the controls ( P=0.07). LDL apoB pool size was greater in the PI group compared with the controls ( P<0.05). In the PI group, patients on ritonavir (RTV)-containing regimens had a lower LDL apoB ASR ( P=0.009) and a trend to a lower LDL apoB FCR and increased RT compared with non-RTV-containing PI regimens ( P=0.05). There was a positive correlation between LDL apoB FCR and limb fat/lean body mass ( P=0.004) in all subjects.

Conclusions

Decreased LDL FCR, despite unchanged LDL cholesterol, was demonstrated in both treated and untreated HIV patients. It was more marked with RTV-containing regimens and was associated with reduced limb fat. The increased LDL RT may lead to an increased risk of atherogenesis thus contributing to the risk for cardiovascular disease in these patients.

Estudio del defecto familiar de unión de la apolipoproteína B100 en una población mediterránea

BACKGROUND AND OBJECTIVE

To compare the lipoprotein phenotype between FDB and heterozygous familial hypercholesterolemia (FH); to study the prevalence and possible founder effect of familial ligand-defective apo B100 (FDB) in a Mediterranean population, and to analyze the clinical and biochemical characteristics of FDB patients.

SUBJECTS AND METHOD

We studied 19 heterozygous FDB subjects (8 males) from 12 related families, carriers of the R3500Q mutation on the apo B gene, and 57 heterozygous FH (24 males) genetically characterized, randomly selected from a total of 213 FH. The genetic diagnosis was established with Southern blot analysis, PCR-SSCP analysis and automatic sequencing. In all subjects, plasma lipids and apolipoprotein levels were determined with standard procedures.

RESULTS

We demonstrated a founder effect for the R3500Q mutation in a geographically isolated rural area from our community. The prevalence of FDB in this area is high: 4/350. Heterozygous FDB subjects showed a statistical significantly lower prevalence of xanthomas and coronary heart disease, plasma concentrations of total and LDL cholesterol, HDL cholesterol, apo B and apo A-I values than heterozygous FH subjects.

CONCLUSIONS

A founder effect for the R3500Q mutation was found in a rural population with a high prevalence of FDB. In our population, FDB patients showed a mild clinical expression and lipoprotein phenotype compared with FH patients.

A new but frequent mutation of apoB-100—apoB His3543Tyr

ApolipoproteinB 100 (apoB-100) is an important component of atherogenic lipoproteins such as LDL and serves as a ligand for the LDL-receptor. Familial defective apolipoproteinB 100 (FDB) is caused by a R3500Q mutation of the apoB gene and results in decreased binding of LDL to the LDL-receptor. So far FDB is the most frequent and best studied alteration of apoB-100. Apart from this, three other apoB mutations, R3500W, R3531C and R3480W, affecting binding to the LDL-receptor are known to date. We screened the apoB gene segment of codons 3448-3561 by denaturing gradient gel electrophoresis (DGGE) analysis in a total of 853 consecutively sampled German patients undergoing diagnostic coronary angiography for suspected CAD. By this, a new single base mutation was detected and confirmed by DNA sequencing. The mutation, CAC(3543)TAC results in a His3543Tyr substitution in apoB-100 (H3543Y). The prevalence of heterozygotes for H3543Y in the study population was 0.47% compared to 0.12% for the known Arg 3500 Gln (R3500Q) mutation. In conclusion, the new mutation is four times more frequent than "classical" FDB and thus appears to be the most common apoB mutation in Germany.

Influence of LDL receptor gene mutations and the R3500Q mutation of the apoB gene on lipoprotein phenotype of familial hypercholesterolemic patients from a South European population

Few data are available on genotype-phenotype interactions among familial hypercholesterolemia (FH) patients in South European populations and there are no data about the influence of R3500Q mutation on lipoprotein phenotype compared to low-density lipoprotein receptor (LDLR) mutations. The objective of the study is to analyze the influence of mutations in the LDLR and apolipoprotein B (apoB) genes on lipoprotein phenotype among subjects clinically diagnosed of FH living in East Spain. In all, 113 FH index patients and 100 affected relatives were studied. Genetic diagnosis was carried out following a protocol based on Southern blot and PCR-SSCP analysis. A total of 118 FH subjects could be classified into three groups according to the type of LDLR mutations (null mutations, missense mutations affecting the ligand binding 3-5 repeat, and missense mutations outside this domain). In addition, the lipoprotein phenotype of these FH groups was compared with 19 heterozygous subjects with familial ligand-defective apoB (FDB), due to R3500Q mutation. FH patients carrying missense mutations affecting the ligand binding repeat 3-5 showed total and LDL cholesterol levels significantly higher than FH patients with missense mutations in other LDLR domains or FDB patients. FH subjects carrying null mutations showed lower high-density lipoprotein cholesterol plasma values compared to FH carrying missense mutations. FDB subjects showed the lowest total and LDL cholesterol plasma values. In conclusion, the type of LDLR gene mutation and R3500Q mutation influences the lipoprotein phenotype of FH population from East Spain.

Comparison of apolipoprotein B metabolism in familial defective apolipoprotein B and heterogeneous familial hypercholesterolemia

Both defective LDL receptors (familial hypercholesterolaemia, FH) and mutations in apolipoprotein B (apoB) on LDL (familial defective apoB, FDB) give rise to a phenotype of elevated LDL cholesterol. We sought to compare the metabolic basis of the two conditions by examining apoB turnover in FDB and FH subjects. A group comprising three heterozygous and one homozygous FDB subjects were compared with five FH heterozygotes and 17 control subjects using a deuterated leucine tracer. Kinetic parameters were derived by multicompartmental modelling. FH heterozygotes had a reduced delipidation rate for VLDL, which led to a moderate increase in plasma triglyceride. Compared with controls and FH, the FDB subjects converted 44% less IDL to LDL. The LDL FCR was reduced to a similar extent in FDB and FH. In all subjects LDL plasma levels appeared to be regulated by the LDL FCR and the rate of production of small VLDL. We conclude that disturbances in IDL metabolism provide the basis for understanding why FDB is less severe than FH. Our findings suggest that an apoB-LDL receptor interaction is important in the IDL to LDL conversion.

Shifting the LDL-receptor paradigm in familial hypercholesterolemia: novel insights from recent kinetic studies of apolipoprotein B-100 metabolism

Familial hypercholesterolemia (FH) is a dominantly inherited disorder associated with elevated plasma cholesterol concentrations and premature cardiovascular disease. In addition to impaired low density lipoprotein (LDL) receptor-mediated clearance of low density lipoproteins in FH, evidence from in vitro and in vivo studies suggests that hepatic oversecretion of apoB may contribute to the hypercholesterolemia. The proposed association between apoB secretion and FH may, however, be a function of the class of LDL receptor defect. Hepatic cholesterol pools appear to regulate apoB secretion and LDL receptor activity. Therefore, therapeutic regulation of cholesterogenesis in FH may have the dual effect of reducing hepatic apoB secretion and upregulating the LDL receptor. These effects may also be genetically determined.

The significance of genetic polymorphisms in modulating the response to lipid-lowering drugs

The response to lipid-lowering drugs is modified by a number of factors like age, gender, concomitant disease and genetic determinants. Even within homogenous groups of patients, individual responses vary greatly. Until now, no clinical or biochemical parameter exists which predicts whether a subject will respond well to a particular lipid-lowering drug or, in the extreme case, will develop adverse, life-threatening effects (e.g., myositis or rhabdomyolysis). The recent advances in the human genome project promises to have a great impact on our understanding of lipid and lipoprotein metabolism and of the individual response to lipid-lowering drugs. Monogenetic disorders of the lipid metabolism produce severe clinical phenotypes, such as Tangier disease, but have a minor role in the evaluation of cardiovascular risk in the general population. On the other hand, several polymorphisms in genes involved in lipoprotein metabolism (e.g., apolipoprotein E) are associated with the plasma levels of lipoproteins, explaining a substantial fraction of the variance of LDL or HDL concentrations. In combination, the knowledge of these polymorphisms, further variants yet to be discovered and variants within the genes involved in the metabolism of lipid-lowering drugs will in the future allow these drugs to be selected according to the patients needs and thus increase both efficacy and cost-effectiveness of lipid-lowering regimes.

Human triglyceride-rich lipoprotein apo E kinetics and its relationship to LDL apo B-100 metabolism

Apolipoprotein (apo) E is a multifunctional protein that can act as a ligand for lipoprotein receptors. The receptor-mediated clearance of the triglyceride-rich lipoproteins (TRL) chylomicrons and VLDL from plasma is, in part, dependent on apo E. Enrichment of VLDL with apo E is thought to enhance receptor-mediated clearance of VLDL resulting in a low rate of conversion of VLDL to LDL. However, the kinetic mechanism controlling the concentration of apo E in VLDL is not known. We conducted kinetic studies on apo E in the TRL fraction (d < 1.006 g/ml) and apo B-100 in the TRL and LDL (d = 1.019-1.063 g/ml) fractions to assess the kinetic determinants of apo E concentration in TRL and to determine the effects that TRL apo E production and clearance rates have on the production rate of LDL apo B-100. Nineteen males between the ages of 24 and 73 underwent a primed-constant infusion with deuterated leucine tracer in the constantly-fed state. Apo B-100 from TRL and LDL, and apo E from TRL were isolated and their tracer incorporation measured by gas chromatography/mass spectrometry. The residence time and production rates of each protein were determined from the kinetic data using the SAAM II modeling program. The residence time and production rate of TRL apo E were about one-half that of TRL apo B-100 (1.8 +/- 1.0 vs. 2.9 +/- 2.1 h and 14.5 +/- 11.0 vs. 27.6 +/- 17.3 mg/kg per day, respectively). The production rate of TRL apo E was weakly correlated with the production rate of TRL apo B-100 (r = 0.424, P = 0.07). Multiple regression analysis showed that the residence time of TRL apo B-100 and the relative TRL apo E production rate (relative to the TRL apo B100 production rate) were negatively associated with LDL apo B-100 production rate, accounting for 68% of its variability. We conclude that (1) the concentration of apo E in TRL is highly correlated to its production rate, suggesting that production rate regulates the TRL apo E concentration, and (2) individuals with a relatively short TRL apo B-100 residence time and those producing TRL with a relatively low apo E content have the highest LDL apo B-100 production rates.

Frequency of the R3500Q mutation of the apolipoprotein B-100 gene in a sample screened clinically for familial hypercholesterolemia in Hungary

Familial hypercholesterolemia (FH) and familial defective apolipoprotein B-100 (FDB) cause early onset of coronary heart diseases (CHD). According to the recommendations of the international MEDPED program, we tried to find FH cases. We analyzed 73 FH probands and their 304 first-degree relatives. A total of 39 probands were found from the 21000 subjects screened (1:538) from family doctors' registers recording all citizens, while the remaining 34 were derived from screened patients from lipid clinics. In our FH probands, four cases of FDB (R3500Q mutation) were diagnosed with allele-specific PCR, and the mutation was also detectable in five cases out of seven living family members. In the remaining 69 FH families, 156 people were diagnosed clinically with FH, and 31.8% of the males (against 13% of the not clinically diagnosed FH males, P<0.01), and 32.4% of the females (against 13.5% of the not clinically diagnosed FH females, P<0.01) suffered from early onset CHD. The plasma total cholesterol level of the FDB patients, especially in the younger patients, was very close to normal values. Therefore, the FDB patients seem to be under-represented in this type of survey. Because FDB is one of the independent causes of early onset CHD, the R3500Q mutation should be considered in families with a high frequency of cardiovascular diseases.

HDL steady state levels are not affected, but HDL apoA-I turnover is enhanced by Lifibrol in patients with hypercholesterolemia and mixed hyperlipidemia

Lifibrol (4-(4'-tert-butylphenyl)-1-(4'carboxyphenoxy)-2-butanol) is a new hypocholesterolemic drug effectively reducing total cholesterol, LDL cholesterol, and apolipoprotein (apo) B in experimental animals and in humans. In contrast to fibrates and HMG-CoA reductase inhibitors the cholesterol and triglyceride lowering effect of Lifibrol is not accompanied by increases in HDL cholesterol and apoA-I levels. We examined the impact of Lifibrol on the metabolism of HDL apoA-I in patients with hyperlipoproteinemia, using endogenous labeling with stable isotopes. Kinetic studies were performed in five male hypercholesterolemic individuals (type IIa), before and on treatment with 450 mg of Lifibrol daily for 4 weeks and in five male individuals suffering from mixed hyperlipidemia (type IIb), before and on therapy, for 12 weeks. Lifibrol reduced total cholesterol by 14% (P=0.02) and LDL cholesterol by 16% (P=0. 014) in all patients, and decreased triglycerides by 34% in type IIb patients. During Lifibrol therapy, HDL cholesterol and ApoA-I concentrations did not change. Tracer kinetics revealed that the fractional catabolic rate (FCR) of HDL apoA-I increased by 22% (P=0. 013). This increase in the apoA-I FCR was accompanied by a 23% increase in HDL apoA-I production rate (P=0.006). We conclude that Lifibrol, although not changing HDL steady state concentrations, enhances the turnover of apoA-I containing HDL particles.

Kinetic study of apo B100 containing lipoprotein metabolism using amino acids labeled with stable isotopes: methodological aspects

Kinetic disturbances of lipoprotein metabolism are important to know for a better understanding of lipid diseases or effects of drugs. These kinetic aspects were previously studied with radioactive tracers. The ethical concerns related to these tracers can be now overcome at a reasonable cost with the new development of small bench top mass spectrometers and the increased production of stable isotope tracers. In this review, we will discuss some methodological aspects related to stable isotope tracers and the analysis of the data with non-compartmental or compartmental models.

Genetics of lipoprotein disorders

The study of lipoprotein metabolism has led to major breakthroughs in the fields of cellular physiology, molecular genetics, and protein chemistry. These advances in basic science are reflected in medicine in the form of improved diagnostic methods and better therapeutic tools. Perhaps the greatest benefit is the improved ability to identify at an early stage patients who are at high risk for atherosclerosis, providing clinicians the opportunity to proceed swiftly with intensive lipid-lowering therapy for the prevention of cardiovascular complications. Recent clinical trials have shown that such an approach is not only cost-effective but saves lives while improving the quality of life. They also emphasize the important role physicians can have in prevention. More than half of patients with premature CAD have a familial form of dyslipoproteinemia. This review of the genetics of atherogenic lipoprotein disorders underscores the importance of identifying major genetic defects. It also stresses the need to take into account multifactorial etiologies and clustering of risk factors, as well as gene-gene and gene-environment interactions in assessing the atherogenic potential of a lipid transport disorder. Table 2 summarizes the key points in the diagnosis, clinical implications, and treatment of the major inherited atherogenic dyslipidemias.

Heterogeneity of apolipoprotein B-100-containing lipoproteins: what we have learnt from kinetic studies

Apolipoprotein B-100-containing lipoprotein assembly is dependent on the successive addition of triglyceride by microsomal transfer protein. Assembly may end at this point and the lipoprotein secreted or it may continue with the bulk addition of triglyceride by an unknown transfer process. These steps are independently regulated and result in the secretion of a spectrum of apolipoprotein B-100-containing particles. The production of small, dense LDL is determined by the type of VLDL secreted by the liver. Large, triglyceride-rich VLDL1 results in the formation of small, dense LDL through triglyceride exchange and subsequent hydrolysis. Small, dense LDL are cleared from plasma relatively slowly and tend to accumulate in the circulation where they exert their atherogenic effects.