Hyperlipoproteinemia

On the possible existence of a liver LDL-ostat, and its malfunctioning in familial hypercholesterolemia

The investigation of familial hypercholesterolemia (FH) and its relationship to atherosclerosis has led to enormous scientific and medical progress, including the identification of genetic defects underlying FH, the elucidation of molecular mechanisms crucial for cellular cholesterol homeostasis and the development of current pharmaceutical tools for FH treatment (which are directed at increasing LDL uptake). These successes also led to the establishment of a model centered on cellular rather than whole organism processes, and a view of FH as resulting from a deficiency in LDL uptake. On the other hand, whole organism fluxes of cholesterol (like those of other nutrients) are centered on the liver, LDL (ultimately derived from the liver) is the main cholesterol transporter in plasma, and there is evidence of evolutionary pressure favoring mechanisms to maintain LDL plasma concentrations. Furthermore, the alterations in cellular metabolism observed in FH are consistent with a coordinated response by the liver to increase the levels of plasma LDL, suggesting that a signaling defect (rather than an uptake deficiency) is the fundamental problem underlying hypercholesterolemia - an hypothesis that explains the occurrence of hypercholesterolemia in CESD, despite normal LDL binding and uptake. I therefore propose that the liver contains a mechanism to assess and regulate plasma levels of LDL (an "LDL-ostat"), and that hypercholesterolemia is caused by defects in it. This model has implications for future research directions, and suggests alternative therapeutic approaches, particularly centered on efforts to restore LDL measurement/signaling (rather than its uptake), some of which are in stark contrast to those currently in use.

Lipid-based biomarkers for CVD, COPD, and aging - A translational perspective

For many diseases, there is an unmet need for new or better biomarkers for improved disease risk assessment and monitoring, as available markers lack sufficient specificity. Lipids are drawing major interest as potential candidates for filling these gaps. This has recently been demonstrated by the identification of selective ceramides for prediction of cardiovascular mortality, enabling improved risk assessment of cardiovascular disease compared with conventional clinical markers. In this review, we discuss current lipid biomarker findings and the possible connection between cardiovascular disease, chronic obstructive pulmonary disease, and aging. Moreover, we discuss how to overcome the current roadblocks facing lipid biomarker research. We stress the need for improved quantification, standardization of methodologies, and establishment of initial reference values to allow for an efficient transfer path of research findings into the clinical landscape, and, ultimately, to put newly identified biomarkers into practical use.

An Evaluation of the Arcus Corneae For Age Estimation

Although the arcus corneae (AC) has long been used as an age indicator for forensic purposes, its diagnostic value has not been evaluated. To evaluate the AC as a predictor of chronological age, the author has studied the correlation of AC with respect to age of the deceased. A cross-sectional study was conducted of 342 Thai corpses at the Maharat Nakhon Ratchasima Hospital, Thailand. AC was graded into three levels: no AC, incomplete ring, and complete ring. One-way analysis of variance, chi-square test, binomial logistic regression, sensitivity, specificity, predictive values, and likelihood ratios were used for analysis. The Cohen's kappa was used to determine the intraobserver and interobserver reliability. The prevalence of AC and the probability of complete AC were significant higher in corpses aged 60 years and above than in those under 60 years. Consequently, this study confirmed that the prevalence of AC was significantly correlated with the age of Thai individuals. If the complete AC is used as an indicator of age of 60 years and above, complete AC has high sensitivity (92.56%) but low specificity (72.85%), low positive predictive value (65.12%), but high negative predictive value (94.71%). For diagnostic value, the presence of AC can be used for age screening but not for absolute confirmation. The absence of AC indicates young age, incomplete AC indicates middle age, and complete AC indicates old age. The high intraobserver and interobserver reliability provides assurance of the value of AC as a means to estimate personal age.

Apolipoprotein E in lipoprotein metabolism, health and cardiovascular disease

Apolipoprotein E (apoE), a 34 kDa circulating glycoprotein of 299 amino acids, predominantly synthesised in the liver, associates with triglyceride-rich lipoproteins to mediate the clearance of their remnants after enzymatic lipolysis in the circulation. Its synthesis in macrophages initiates the formation of high density-like lipoproteins to effect reverse cholesterol transport to the liver. In the nervous system apoE forms similar lipoproteins which perform the function of distributing lipids amongst cells. ApoE accounts for much of the variation in plasma lipoproteins by three common variants (isoforms) that influence low-density lipoprotein concentration and the risk of atherosclerosis. ApoE2 generally is most favourable and apoE4 least favourable for cardiovascular and neurological health. The apoE variants relate to different amino acids at positions 112 and 158: cysteine in both for apoE2, arginine at both sites for apoE4, and respectively cysteine and arginine for apoE3 that is viewed as the wild type. Paradoxically, under metabolic stress, homozygosity for apoE2 may result in dysbetalipoproteinaemia in adults owing to impaired binding of remnant lipoproteins to the LDL receptor and related proteins as well as heparan sulphate proteoglycans. This highly atherogenic condition is also seen with other mutations in apoE, but with autosomal dominant inheritance. Mutations in apoE may also cause lipoprotein glomerulopathy. In the central nervous system apoE binds amyloid β-protein and tau protein and fragments may incur cellular damage. ApoE4 is a strong risk factor for the development of Alzheimer's disease. ApoE has several other physiological effects that may influence health and disease, including supply of docosahexaenoic acid for the brain and modulating immune and inflammatory responses. Genotyping of apoE may have application in disorders of lipoprotein metabolism as well as glomerulopathy and may be relevant to personalised medicine in understanding cardiovascular risk, and the outcome of nutritional and therapeutic interventions. Quantitation of apoE will probably not be clinically useful. ApoE is also of interest as it may generate peptides with biological function and could be employed in nanoparticles that may allow crossing of the blood-brain barrier. Therapeutic options may emerge from these newer insights.

A century of cholesterol and coronaries: from plaques to genes to statins

One-fourth of all deaths in industrialized countries result from coronary heart disease. A century of research has revealed the essential causative agent: cholesterol-carrying low-density lipoprotein (LDL). LDL is controlled by specific receptors (LDLRs) in liver that remove it from blood. Mutations that eliminate LDLRs raise LDL and cause heart attacks in childhood, whereas mutations that raise LDLRs reduce LDL and diminish heart attacks. If we are to eliminate coronary disease, lowering LDL should be the primary goal. Effective means to achieve this goal are currently available. The key questions are: who to treat, when to treat, and how long to treat.

Corneal arcus is a sign of cardiovascular disease, even in low-risk persons

PURPOSE

To examine the association of corneal arcus to cardiovascular disease (CVD) in an adult, ethnic Indian population.

DESIGN

Population-based cross-sectional study.

METHODS

Population-based study of ethnic South Asian Indians 40 to 80 years of age in Singapore from June 2007 through March 2009. We obtained a 75.5% response rate (3397/4497). All participants underwent standardized interview and systemic and ocular examinations, followed by nonfasting blood sampling. Corneal arcus was detected using a standardized slit-lamp examination. The main outcome measure was CVD, defined from a self-reported history of previous myocardial infarction, angina, or stroke.

RESULTS

Corneal arcus, found in 1701 (50.1%) of 3397 participants, was associated with older age (odds ratio [OR], 3.07; 95% CI, 2.78 to 3.40; P < .001), male gender (OR, 2.17; 95% CI, 1.81 to 2.62; P < .001), higher levels of total cholesterol (OR, 1.14; 95% CI, 1.05 to 1.24; P = .002), hypertension (OR, 1.14; 95% CI, 1.05 to 1.24; P = .013), and cigarette smoking (OR, 1.59; 95% CI, 1.25 to 2.03; P < .001). Corneal arcus was associated with CVD (OR, 1.31; 95% CI, 1.02 to 1.7; P = .0038) independent of the above-named cardiovascular risk factors. Participants with low-risk Framingham scores were more likely to be associated with CVD if they had corneal arcus (men: OR, 2.02; 95% CI, 1.20 to 3.40; P = .008; women: OR, 2.78; 95% CI, 1.36 to 3.01; P < .001). Corneal arcus was associated with CVD independent of the Framingham score (men: Akaike information criterion, 1524.39 for Framingham Score and corneal arcus vs 1527.38 for Framingham Score alone; women: 1000.14 vs 1003.54, respectively).

CONCLUSIONS

Corneal arcus is associated with CVD, independent of risk factors in ethnic Indian adults, even in those at low risk for vascular disease.

Corneal arcus as coronary artery disease risk factor

Corneal arcus is a lipid-rich and predominantly extracellular deposit that forms at the corneoscleral limbus. It represents the most common peripheral corneal opacity and is not associated with tissue breakdown but rather with the deposition of lipids. The deposition of cholesterol in the peripheral cornea and arterial wall are similar in that both are accelerated by elevated serum levels of atherogenic lipoproteins, such as low-density lipoproteins (LDL). Corneal arcus is more prevalent in men than in women and in Blacks than in Whites. Its prevalence increases with advancing age. It has been associated with hypercholesterolemia, xanthelasmas, alcohol, blood pressure, cigarette smoking, diabetes, age, and coronary heart disease. Nevertheless, it is not clear whether or not corneal arcus is an independent risk factor for coronary heart disease (CHD). The present systematic review examines the relationship of corneal arcus and CHD to determine if corneal arcus is an independent CHD risk factor. We conclude that there is no consensus that corneal arcus is an independent risk factor. The presence of corneal arcus in a young person should prompt a search for lipid abnormalities. Also, because corneal arcus represents physical evidence of early lipid deposition, its presence suggests the need for aggressive lipid therapy.

[Eruptive xanthomas]

A 41-year old male patient presented with yellowish papules on the elbows, upper arms, back and the buttocks, as well as yellow streaks in the palmar creases. Laboratory examination revealed Fredrickson type IIb hyperlipidemia. Histologic changes were consistent with eruptive xanthomas. Treatment was started with HMG-CoA reductase inhibitors and dietary measures were taken to lower serum levels of cholesterol and triglycerides. Hyperlipidemia is the cause of eruptive xanthomas and strongly increases the risk of cardiovascular diseases.

Cryoelectron microscopy of low density lipoprotein in vitreous ice

In this report, images of low density lipoprotein (LDL) in vitreous ice at approximately 30 A resolution are presented. These images show that LDL is a quasi-spherical particle, approximately 220-240 A in diameter, with a region of low density (lipid) surrounded by a ring (in projection) of high density believed to represent apolipoprotein B-100. This ring is seen to be composed of four or five (depending on view) large regions of high density material that may represent protein superdomains. Analysis of LDL images obtained at slightly higher magnification reveals that areas of somewhat lower density connect these regions, in some cases crossing the projectional interiors of the LDL particles. Preliminary image analysis of LDL covalently labeled at Cys3734 and Cys4190 with 1.4-nm Nanogold clusters demonstrates that this methodology will provide an important site-specific marker in studies designed to map the organization of apoB at the surface of LDL.

The role of lipid peroxidation and antioxidants in oxidative modification of LDL

The purpose of this study is to provide a comprehensive survey on the compositional properties of LDL (e.g., lipid classes, fatty acids, antioxidants) relevant for its susceptibility to oxidation, on the mechanism and kinetics of LDL oxidation, and on the chemical and physico-chemical properties of LDL oxidized by exposure to copper ions. Studies on the occurrence of oxidized LDL in plasma, arteries, and plaques of humans and experimental animals are discussed with particular focus on the use of poly- and monoclonal antibodies for immunochemical demonstration of apolipoprotein B modifications characteristic for lipid peroxidation. Apart from uptake of oxidized LDL by macrophages, studies describing biological effects of heavily or minimally oxidized LDL are only briefly addressed, since several reviews dealing with this subject were recently published. This article is concluded with a section on the role of natural and synthetic antioxidants in protecting LDL against oxidation, as well as some previously unpublished material from our laboratories.

Structure of apolipoprotein B-100 of human low density lipoproteins

We have analyzed low density lipoproteins (LDL) apolipoprotein (apop) B structure by direct sequence analysis of LDL apo B-100 tryptic peptides. Native LDL were digested with trypsin, and the products were fractionated on a Sephadex G-50 column. The partially digested apo B-100 still associated with lipids was recovered in the void volume (designated trypsin-nonreleasable, TN, peptides). The released peptides (designated trypsin-releasable, TR, peptides) in subsequent peaks were repurified on two successive high-performance liquid chromatography (HPLC) columns. The TN peak was delipidated and redigested with trypsin, and the resulting peptides were purified on two successive HPLC columns. Using this approach, we sequenced over 88% of LDL apo B-100, extending and refining our previous study (Nature 1986;323:738-742) which covered 52% of the protein. TN peptides made up 31%, and the TR peptides, 34% of the apo B-100 sequence; 23.7% were found under both TN and TR categories. Based on its differential trypsin releasability, apo B-100 can be divided into five domains: 1) residues 1----1000, largely TR; 2) residues 1001----1700, alternating TR and TN; 3) residues 1701----3070, largely TN; 4) residues 3071----4100, mainly TR and mixed; and 5) residues 4101----4536, almost exclusively TN. Domain 1 contained 14 of the 25 Cys residues in apo B. Domain 4 encompassed seven N-glycosylation sites, and contained the putative receptor binding domains. All 19 potential N-glycosylation sites were directly sequenced: 16 were found to be glycosylated and three were not. Three pairs of disulfide bridges were also mapped. Finally, a combination of cDNA sequencing, direct mRNA sequencing, and comparison of published apo B-100 sequences allowed us to identify specific amino acid residues within apo B-100 that seem to represent bona fide allelic variations. Our study provides information on LDL apo B-100 structure that will be important to our understanding of its conformation and metabolism.

The effect of etophylline clofibrate on HDL subfractions

The effect of etophylline clofibrate on lipids and apolipoproteins of the high density lipoprotein (HDL) subfractions HDL2 and HDL3 as well as on very low density (VLDL) and low density lipoproteins (LDL) and the post heparin lipolytic activities (PHLA) of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) has been studied in 14 patients with type II hyperlipoproteinemia (HLP). The study was preceded by a 4-week washout phase, followed by a 6-week placebo period. During the next 12 weeks, the patients received 750 mg etophylline clofibrate per day. Then the drug was again replaced by placebo for another 6 weeks. During the study the patients were on a low fat diet poor in cholesterol with a P/S ratio over 1.0. HDL cholesterol and apoproteins increased significantly during treatment. In the first verum phase this effect was related to the rise in HDL2 components with minor changes in HDL3 concentrations, whereas in the second verum period a distinct increase of the HDL3 components could be detected. This development was accompanied by a significant increase of the LPL activities during the first 6 weeks of treatment, followed by a decrease to initially measured values after 12 weeks. The drug lowered plasma- and LDL-cholesterol levels by 19% and 22%, and plasma and VLDL triglycerides by 22% and 25%, respectively. VLDL-C apoproteins (C-I, C-II, C-III) declined by 31% with a percentage increase of apo C-II compared with apo C-I and apo C-III.

Catabolism of human low density lipoproteins by human hepatoma cell line HepG2

The mechanism of hepatic catabolism of human low density lipoproteins (LDL) by human-derived hepatoma cell line HepG2 was studied. The binding of 125I-labeled LDL to HepG2 cells at 4 degrees C was time dependent and inhibited by excess unlabeled LDL. The specific binding was predominant at low concentrations of 125I-labeled LDL (less than 50 micrograms protein/ml), whereas the nonsaturable binding prevailed at higher concentrations of substrate. The cellular uptake and degradation of 125I-labeled LDL were curvilinear functions of substrate concentration. Preincubation of HepG2 cells with unlabeled LDL caused a 56% inhibition in the degradation of 125I-labeled LDL. Reductive methylation of unlabeled LDL abolished its ability to compete with 125I-labeled LDL for uptake and degradation. Chloroquine (50 microM) and colchicine (1 microM) inhibited the degradation of 125I-labeled LDL by 64% and 30%, respectively. The LDL catabolism by HepG2 cells suppressed de novo synthesis of cholesterol and enhanced cholesterol esterification; this stimulation was abolished by chloroquine. When tested at a similar content of apolipoprotein B, very low density lipoproteins (VLDL), LDL and high density lipoproteins (HDL) inhibited the catabolism of 125I-labeled LDL to the same degree, indicating that in HepG2 cells normal LDL are most probably recognized by the receptor via apolipoprotein B. The current study thus demonstrates that the catabolism of human LDL by HepG2 cells proceeds in part through a receptor-mediated mechanism.

Familial dysbetalipoproteinemia. New aspects of pathogenesis and diagnosis

New aspects in the pathogenesis and diagnosis of familial dysbetalipoproteinemia are discussed, including the clarification of the chemical basis of the polymorphism of apoprotein E, the allelic nature of the primary isoforms of the protein, the relationship of the abnormality of apoprotein E to the accumulation of remnant lipoproteins in dysbetalipoproteinemia and in persons carrying the trait for abnormal apoprotein E, and the pathogenesis of hyperlipidemia in this disorder. The related clinical features of dysbetalipoproteinemia are included in the discussion.

Structure of two subfractions of normal porcine (Sus domesticus) serum low-density lipoproteins. X-ray small-angle scattering studies

Two subfractions of low-density lipoproteins (LDL) were isolated from normal pig (Sus domesticus) serum by a combined method including precipitation, ultracentrifugation, and gel chromatography. The fractions recovered from the buoyant density ranges 1.020-1.050 and 1.050-1.090 g/mL, denoted as LDL1 and LDL2, respectively, were studied with regard to structure and thermotropic behavior by X-ray small-angle scattering and were compared to human serum low-density lipoprotein of density 1.020-1.063 g/mL. The average molecular weights determined from the scattering intensities on absolute scale were 2.6 X 10(6) and 2.0 X 10(6) for LDL1 and LDL2, respectively. The maximum particle diameters were found to be 24 and 21 nm, respectively. Both species were found to have quasi-spherical symmetry and to display the thermotropic transition of the apolar lipids within the particle core similar to human LDL. The width of the transition was approximately 9 degrees C in both cases, but the midpoint transition temperature was higher by 8 degrees C for LDL1 (33 degrees C) than for LDL2 (25 degrees C). Despite their different sizes and thermotropic behavior, the two porcine LDL subfractions appear to be built according to the same structural principle as human LDL in the molecular organization of the apolar lipids within the particle core.

Genetic transmission of isoapolipoprotein E phenotypes in a large kindred: relationship to dysbetalipoproteinemia and hyperlipidemia

The largest reported kindred of a proband with type III hyperlipoproteinemia was investigated by assessment of lipid and lipoprotein levels and very low density lipoprotein (VLDL) isoapolipoprotein E distributions in all accessible family members (56% of the 124 living blood relatives and 59% of the 37 spouses). The results confirm in this kindred a trimodal distribution of apoE3/E2 ratios, and segregation analysis of 16 informative matings classified according to E3/E2 ratio demonstrated classical Mendelian inheritance of the autosomal codominant type: the E3/E2 ratio is determined by two alleles, apoE3d and apoE3n, which produce three phenotypes apoE3-D, apoE3-ND, and apoE3-N, corresponding to the low, intermediate, and high modes, respectively. Vertical transmission of the apoE3-D phenotype occurred in two branches of the second generation. In both instances this represented pseudodominance; i.e., products of heterozygous (apoE3-ND) x homozygous (apoE3-D) matings. Hyperlipidemia (defined as a low density lipoprotein cholesterol and/or plasma triglyceride level exceeding the respective age-, sex-, and sex-steroid-specific 95th percentiles derived from Lipid Research Clinics population studies) was present in 15 blood relatives in multiple lipoprotein patterns, consistent with the presence of familial combined hyperlipidemia in this kindred. Eight of nine members with the apoE3-D phenotype had either type III hyperlipoproteinemia or, in the absence of hyperlipidemia, beta-VLDL and at least marginally cholesterol-rich VLDL (VLDL-cholesterol/plasma triglyceride greater than 0.25) (defined as dysbetalipoproteinemia). The ninth such member, the only child with this phenotype, was normal. beta-VLDL and marginally cholesterol-rich VLDL was seen in but one of six hyperlipidemic family members of phenotype apoE3-ND, in none of seven hyperlipidemic blood relatives of phenotype apoE3-N, in no normolipidemic family members of phenotype apoE3-ND or apoE3-N, and in no spouses (three of whom were hyperlipidemic and nine of phenotype apoE3-ND). Thus, among adult members of the O'D kindred the apo3-D phenotype was nearly specifically associated with dysbetalipoproteinemia or, when hyperlipidemia was present, type III hyperlipoproteinemia.

Type III hyperlipoproteinemia: a comparative study of current diagnostic techniques

Research into the prevalence, genetic transmission and pathophysiology of Type III hyperlipoproteinemia has suffered from the lack of a practical specific diagnostic procedure. In this study, very low density lipoprotein (VLDL) compositional criteria were established in a population lacking the beta-migrating VLDL characteristic of this disorder. Diagnosis by these criteria was compared to diagnosis using current criteria for the Type III lipoprotein pattern. In addition two techniques for detecting Type III without preliminary VLDL isolation by ultracentrifugation were evaluated. Plasma triglyceride (TG) concentration dependent cutlines for the compositional criteria reduced false positives at low TG levels and false negatives at high TG levels. Furthermore, an agarose electrophoresis heparin-manganese precipitation technique was effective for screening for a possible Type III pattern in plasma whereas the combination agarose-polyacrylamide gel electrophoresis system was not effective.

Inheritance of type-III hyperlipoproteinemia. Lipoprotein patterns in first-degree relatives

The lipoprotein (LP) patterns were studied in the families of 19 index cases with type-III hyperlipoproteinemia (HLP). Seventy adult first-degree relatives (93% ascertainment) to the 19 probands were analyzed. The diagnosis of HLP type III among the first-degree relatives was based on three criteria, all of which had to be fulfilled to make the diagnosis: (1) presence of a slow-moving band in very-low-density LP (VLDL) on agarose gel electrophoresis migrating in beta or close to beta position; (2) A cholesterol/triglyceride ratio (mg/100 ml: mmoles/liter) in VLDL greater than 29.0; and (3) A "III-index" [cholesterol/triglycerides in VLDL x 10 divided by cholesterol/triglycerides in low-density LP (LDL)] greater than 1.30. When defined according to these criteria there was a marked over-representation of HLP type III among the relatives (27%). There was also an increased frequency of hypertriglyceridemia (28% against expected 15%), mainly because of a high prevalence of HLP type IV (24%). On agarose gel electrophoresis a "late pre-beta" band, probably indicative of an increased amount of intermediary LP particles, was frequently present (47%) among relatives not classified as HLP type III. Type-III patients with hypertriglyceridemia were characterized by a significantly higher body weight than those with normotriglyceridemic type III. However, there was no qualitative difference in the composition of the lipoproteins in normotriglyceridemic and hypertriglyceridemic type-III patients. A genetic analysis of the LP patterns within the families showed several examples of vertical transmission of HLP type III. There was no sex linkage. Six of thirteen analyzed parents showed LP patterns classified as HLP type III. Another two parents were most probably carriers of the gene. Of the siblings to the probands, 23% showed a type-III pattern and another four (7%) showed LP patterns very similar to type III, fulfilling two of three criteria for HLP type III. The data support the concept that HLP type III is inherited as an autosomal dominant gene. It was indicated that HLP type IV with a late pre-beta band in VLDL may represent another expression of the gene for HLP type III. It is suggested that HLP type III may be a pathogenetically heterogenous group of lipid disorders. A separation of type III into two subgroups with low or normal and high LDL cholesterol concentration, respectively, may facilitate the understanding of the inheritance of type III as well as of the pathogenesis behind this LP abnormality.

Conversion of type III hyperlipoproteinaemia to type IV hyperlipoproteinaemia by a fat-free, carbohydrate rich diet

Hyperlipoproteinaemia type III has been considered a clear cut clinical entity characterized by the presence of cholesterol rich "floating" beta very low density lipoproteins (VLDL) after ultracentrifugation. Carbohydrate inducibility is pronounced in type III. This study was undertaken to elucidate the effect of a high carbohydrate diet in type III as an effort to clarify a suggested association between type III and IV hyperlipoproteinaemia. On an isocaloric fat free carbohydrate rich diet the lipoprotein pattern changed as the VLDL triglycerides increased 31 per cent on average and LDL and HDL cholesterol decreased by 40 and 22 per cent. The high ratio cholesterol/triglycerides in VLDL was normalized. The lipoprotein levels in serum after carbohydrate induction showed all characteristics of a type IV with high VLDL triglycerides, normal cholesterol/triglyceride ratio in VLDL, subnormal cholesterol levels in LDL and HDL with changed relation between LDL1 and LDL2. On electrophoresis the floating beta band disappeared. It is probable that depending on the nutritional situation, the metabolic defect in type III may be expressed either as a type III or a type IV. An explanation of the effects of the carbohydrate rich diet on the lipoprotein pattern is suggested.

Broad-beta disease versus endogenous hypertriglyceridemia: levels and lipid composition of chylomicrons and very low density lipoproteins during fat-free feeding and alimentary lipemia

To assess the roles of endogenous and exogenous lipid in the production of the abnormal lipoprotein patterns characteristic of broad-beta desease (with a type IVIII lipoprotein pattern) and endogenous hypertriglyceridemia (with a type IV pattern), oral fat loads (50 g/M-2) were administered to six subjects with broad-beta disease and to eight with endogenous hypertriglyceridemia following at leat 72 hr of 0% fat, 85% carbohydrate isocaloric formula feeding. Total plasma and Sf greater than 400, 100-400, 60-100, 30-60, and 20-30 lipoprotein cholesterol, triglyceride, and phospholipid levels were measured at 0 hr, 6 hr (at or before the peak of alimentary lipemia), and 24 hr following the fat load. Following fat-free feeding the levels and composition of the endogenous Sf greater than 400 lipoproteins were similar in both disorders; whereas total Sf20-400, and most notably, Sf 30-60 and 20-30 levels were increased and enriched in cholesterol in the subjects with broad-beta disease.

Hyperlipidemia in coronary heart disease. 3. Evaluation of lipoprotein phenotypes of 156 genetically defined survivors of myocardial infarction

Although analysis of lipoprotein phenotypes is widely used to diagnose and classify the familial hyperlipidemias, an evaluation of this system as a method for genetic classification has hitherto not been published. The present study of 156 genetically defined survivors of myocardial infarction was therefore designed to examine the relationship between lipoprotein phenotypes and genetic lipid disorders. The lipoprotein phenotypes of each survivor was determined primarily by measurement of his plasma triglyceride and low density lipoprotein (LDL)-cholesterol concentrations; his genetic disorder was identified by analysis of whole plasma cholesterol and triglyceride levels in relatives. The mean levels of LDL-cholesterol discriminated statistically among the three monogenic lipid disorders; it was highest in survivors with familial hypercholesterolemia (261+/-61 mg/100 ml [mean +/-SD]); intermediate in those with familial combined hyperlipidemia (197+/-50); and lowest in those with familial hypertriglyceridemia (155+/-36) (P < 0.005 among the three groups). However, on an individual basis no lipoprotein pattern proved to be specific for any particular genetic lipid disorder; conversely, no genetic disorder was specified by a single lipoprotein pattern. This lack of correlation occurred for the following reasons: (a) individual LDL-cholesterol levels frequently overlapped between disorders; (b) in many instances a small quantitative change in the level of either LDL-cholesterol or whole plasma triglyceride caused qualitative differences in lipoprotein phenotypes, especially in individuals with familial combined hyperlipidemia, who showed variable expression (types IIa, IIb, IV, or V); (c) lipoprotein phenotypes failed to distinguish among monogenic, polygenic, and sporadic forms of hyperlipidemia; (d) clofibrate treatment of some survivors with genetic forms of hyperlipidemia caused their levels of triglyceride and LDL-cholesterol to fall below the 95th percentile, thus resulting in a normal phenotype; and (e) beta-migrating very low density lipoproteins (beta-VLDL), previously considered a specific marker for the type III hyperlipidemic disorder, was identified in several survivors with different lipoprotein characteristics and familial lipid distributions. These studies indicate that lipoprotein phenotypes are not qualitative markers in the genetic sense but instead are quantitative parameters which may vary among different individuals with the same genetic lipid disorder. It would therefore seem likely that a genetic classification of the individual hyperlipidemic patient with coronary heart disease made from a quantitative analysis of lipid levels in his relatives may provide a more meaningful approach than determination of lipoprotein phenotypes.