Apolipoprotein A-I(Zavalla) (Leu159-->Pro): HDL cholesterol deficiency in a kindred associated with premature coronary artery disease

Very low HDL levels: clinical assessment and management

In individuals with very low high-density lipoprotein (HDL-C) cholesterol, such as Tangier disease, LCAT deficiency, and familial hypoalphalipoproteinemia, there is an increased risk of premature atherosclerosis. However, analyzes based on comparisons of populations with small variations in HDL-C mediated by polygenic alterations do not confirm these findings, suggesting that there is an indirect association or heterogeneity in the pathophysiological mechanisms related to the reduction of HDL-C. Trials that evaluated some of the HDL functions demonstrate a more robust degree of association between the HDL system and atherosclerotic risk, but as they were not designed to modify lipoprotein functionality, there is insufficient data to establish a causal relationship. We currently have randomized clinical trials of therapies that increase HDL-C concentration by various mechanisms, and this HDL-C elevation has not independently demonstrated a reduction in the risk of cardiovascular events. Therefore, this evidence shows that (a) measuring HDL-C as a way of estimating HDL-related atheroprotective system function is insufficient and (b) we still do not know how to increase cardiovascular protection with therapies aimed at modifying HDL metabolism. This leads us to a greater effort to understand the mechanisms of molecular action and cellular interaction of HDL, completely abandoning the traditional view focused on the plasma concentration of HDL-C. In this review, we will detail this new understanding and the new horizon for using the HDL system to mitigate residual atherosclerotic risk.

Leveraging knowledge of HDLs major protein ApoA1: Structure, function, mutations, and potential therapeutics

Apolipoprotein A1 (ApoA1) is a member of the Apolipoprotein family of proteins. It's a vital protein that helps in the production of high-density lipoprotein (HDL) particles, which are crucial for reverse cholesterol transport (RCT). It also has anti-inflammatory, anti-atherogenic, anti-apoptotic, and anti-thrombotic properties. These functions interact to give HDL particles their cardioprotective characteristics. ApoA1 has recently been investigated for its potential role in atherosclerosis, diabetes, neurological diseases, cancer, and certain infectious diseases. Since ApoA1's discovery, numerous mutations have been reported that affect its structural integrity and alter its function. Hence these insights have led to the development of clinically relevant peptides and synthetic reconstituted HDL (rHDL) that mimics the function of ApoA1. As a result, this review has aimed to provide an organized explanation of our understanding of the ApoA1 protein structure and its role in various essential pathways. Furthermore, we have comprehensively reviewed the important ApoA1 mutations (24 mutations) that are reported to be involved in various diseases. Finally, we've focused on the therapeutic potentials of some of the beneficial mutations, small peptides, and synthetic rHDL that are currently being researched or developed, since these will aid in the development of novel therapeutics in the future.

Dysfunctional HDL in diabetes mellitus and its role in the pathogenesis of cardiovascular disease

Coronary artery disease, the leading cause of death in the developed and developing countries, is prevalent in diabetes mellitus with 68% cardiovascular disease (CVD)-related mortality. Epidemiological studies suggested inverse correlation between HDL and CVD occurrence. Therefore, low HDL concentration observed in diabetic patients compared to non-diabetic individuals was thought to be one of the primary causes of increased risks of CVD. Efforts to raise HDL level via CETP inhibitors, Torcetrapib and Dalcetrapib, turned out to be disappointing in outcome studies despite substantial increases in HDL-C, suggesting that factors beyond HDL concentration may be responsible for the increased risks of CVD. Therefore, recent studies have focused more on HDL function than on HDL levels. The metabolic environment in diabetes mellitus condition such as hyperglycemia-induced advanced glycation end products, oxidative stress, and inflammation promote HDL dysfunction leading to greater risks of CVD. This review discusses dysfunctional HDL as one of the mechanisms of increased CVD risks in diabetes mellitus through adversely affecting components that support HDL function in cholesterol efflux and LDL oxidation. The dampening of reverse cholesterol transport, a key process that removes cholesterol from lipid-laden macrophages in the arterial wall, leads to increased risks of CVD in diabetic patients. Therapeutic approaches to keep diabetes under control may benefit patients from developing CVD.

Structural Insights into High Density Lipoprotein: Old Models and New Facts

The physiological link between circulating high density lipoprotein (HDL) levels and cardiovascular disease is well-documented, albeit its intricacies are not well-understood. An improved appreciation of HDL function and overall role in vascular health and disease requires at its foundation a better understanding of the lipoprotein's molecular structure, its formation, and its process of maturation through interactions with various plasma enzymes and cell receptors that intervene along the pathway of reverse cholesterol transport. This review focuses on summarizing recent developments in the field of lipid free apoA-I and HDL structure, with emphasis on new insights revealed by newly published nascent and spherical HDL models constructed by combining low resolution structures obtained from small angle neutron scattering (SANS) with contrast variation and geometrical constraints derived from hydrogen-deuterium exchange (HDX), crosslinking mass spectrometry, electron microscopy, Förster resonance energy transfer, and electron spin resonance. Recently published low resolution structures of nascent and spherical HDL obtained from SANS with contrast variation and isotopic labeling of apolipoprotein A-I (apoA-I) will be critically reviewed and discussed in terms of how they accommodate existing biophysical structural data from alternative approaches. The new low resolution structures revealed and also provided some answers to long standing questions concerning lipid organization and particle maturation of lipoproteins. The review will discuss the merits of newly proposed SANS based all atom models for nascent and spherical HDL, and compare them with accepted models. Finally, naturally occurring and bioengineered mutations in apoA-I, and their impact on HDL phenotype, are reviewed and discuss together with new therapeutics employed for restoring HDL function.

The Effect of Natural LCAT Mutations on the Biogenesis of HDL

We have investigated how the natural LCAT[T147I] and LCAT[P274S] mutations affect the pathway of biogenesis of HDL. Gene transfer of WT LCAT in LCAT(-/-) mice increased 11.8-fold the plasma cholesterol, whereas the LCAT[T147I] and LCAT[P274S] mutants caused a 5.2- and 2.9-fold increase, respectively. The LCAT[P274S] and the WT LCAT caused a monophasic distribution of cholesterol in the HDL region, whereas the LCAT[T147I] caused a biphasic distribution of cholesterol in the LDL and HDL region. Fractionation of plasma showed that the expression of WT LCAT increased plasma apoE and apoA-IV levels and shifted the distribution of apoA-I to lower densities. The LCAT[T147I] and LCAT[P274S] mutants restored partially apoA-I in the HDL3 fraction and LCAT[T147I] increased apoE in the VLD/IDL/LDL fractions. The in vivo functionality of LCAT was further assessed based on is its ability to correct the aberrant HDL phenotype that was caused by the apoA-I[L159R]FIN mutation. Co-infection of apoA-I(-/-) mice with this apoA-I mutant and either of the two mutant LCAT forms restored only partially the HDL biogenesis defect that was caused by the apoA-I[L159R]FIN and generated a distinct aberrant HDL phenotype.

Role of Conserved Proline Residues in Human Apolipoprotein A-IV Structure and Function

Apolipoprotein (apo)A-IV is a lipid emulsifying protein linked to a range of protective roles in obesity, diabetes, and cardiovascular disease. It exists in several states in plasma including lipid-bound in HDL and chylomicrons and as monomeric and dimeric lipid-free/poor forms. Our recent x-ray crystal structure of the central domain of apoA-IV shows that it adopts an elongated helical structure that dimerizes via two long reciprocating helices. A striking feature is the alignment of conserved proline residues across the dimer interface. We speculated that this plays important roles in the structure of the lipid-free protein and its ability to bind lipid. Here we show that the systematic conversion of these prolines to alanine increased the thermodynamic stability of apoA-IV and its propensity to oligomerize. Despite the structural stabilization, we noted an increase in the ability to bind and reorganize lipids and to promote cholesterol efflux from cells. The novel properties of these mutants allowed us to isolate the first trimeric form of an exchangeable apolipoprotein and characterize it by small-angle x-ray scattering and chemical cross-linking. The results suggest that the reciprocating helix interaction is a common feature of all apoA-IV oligomers. We propose a model of how self-association of apoA-IV can result in spherical lipoprotein particles, a model that may have broader applications to other exchangeable apolipoprotein family members.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Oxidizability assay of unfractionated plasma of patients' with different plasma profile: a methodological study

BACKGROUND

Present study describe the in vitro model of plasma oxidation of patients with different lipid profile, that can be correlated to their invivo plasma oxidizability in order to find the arterial diseases prone patient groups.

METHOD

The method applied here to measure the invitro plasma oxidizability, accounts a convenient way that can be well suited in any clinical laboratory settings. Un-fractionated plasma was exposed to CuSO4 (5.0 mmol/L), a pro-oxidant, and low frequency ultrasonic wave to induce oxidation, and finally oxidizability was calculated by TBARS and Conjugated Diene methods.

RESULT

In our study, plasma LDL greater than 150 mg/dL possess 1.75 times more risk to undergo oxidation (CI, 0.7774 to 3.94; p = 0.071) than the low LDL plasma, percent of oxidation increased from 38.3% to 67.1% for the LDL level upto 150 mg/dL and high. Lag phase, which is considered as the plasma antioxidative protection, was also influenced by the higher LDL concentration. The mean lag time was 65.27 ± 20.02 (p = 0.02 compared to healthy), where as for 94.71 ± 35.11 min for the normolipidemic subject. The plasma oxidizability was also changed drastically for total cholesterol level, oxidative susceptibility shown 35% and 55.02% for 200 mg/dL and high respectively, however it didn't appear as risk factor. Patient samples were also stratified according to their age, gender, and blood glucose level. Older persons (≥40 years) were 1.096 times (95% CL, 0.5607 to 2.141, p = 0.396) than younger (≤39 years age), males are 1.071 (95% CI, 0.5072- 2.264) times than the females, and diabetic patients are 1.091 (CI, 0.6153 to 1.934, p = 0.391) times in more risk than the non-diabetic counterpart.

CONCLUSION

This method addressing its easy applicability in biomedical research. And by this we were able to show that patients with high LDL (≥150 mg/dL) are in alarming condition besides diabetic and elderly (≥40 years age) males are considered to be susceptible and more prone to develop vascular diseases.

High prevalence of mutations in LCAT in patients with low HDL cholesterol levels in The Netherlands: identification and characterization of eight novel mutations

Lecithin:cholesterol acyltransferase (LCAT) is crucial to the maturation of high-density lipoprotein (HDL). Homozygosity for LCAT mutations underlies rare disorders characterized by HDL-cholesterol (HDL-c) deficiency while heterozygotes have half normal HDL-c levels. We studied the prevalence of LCAT mutations in referred patients with low HDL-c to better understand the molecular basis of low HDL-c in our patients. LCAT was sequenced in 98 patients referred for HDL-c <5th percentile and in four patients referred for low HDL-c and corneal opacities. LCAT mutations were highly prevalent: in 28 of the 98 participants (29%), heterozygosity for nonsynonymous mutations was identified while 18 patients carried the same mutation (p.T147I). The four patients with corneal opacity were compound heterozygotes. All previously identified mutations are documented to cause loss of catalytic activity. Nine novel mutations-c.402G>T (p.E134D), c.403T>A (p.Y135N), c.964C>T (p.R322C), c.296G>C (p.W99S), c.736G>T (p.V246F), c.802C>T (p.R268C), c.945G>A (p.W315X), c.1012C>T (p.L338F), and c.1039C>T (p.R347C)--were shown to be functional through in vitro characterization. The effect of several mutations on the core protein structure was studied by a three-dimensional (3D) model. Unlike previous reports, functional mutations in LCAT were found in 29% of patients with low HDL-c, thus constituting a common cause of low HDL-c in referred patients in The Netherlands.

The genetics of high-density lipoprotein metabolism: clinical relevance for therapeutic approaches

The risk for coronary artery disease (CAD) is inversely correlated with high-density lipoprotein (HDL) cholesterol plasma levels. These plasma HDL cholesterol levels are influenced by the activity of a number of enzymes and receptors, and therefore, variations in the genes encoding for these proteins may consequently result in an altered CAD risk. Identification of such pivotal players in HDL cholesterol metabolism that are also strongly associated with CAD risk is crucial for the materialization of novel therapeutic modalities. A large amount of knowledge has been obtained by studies involving families with extreme HDL phenotypes specific to molecular defects. In fact, thus far, monogenetic defects have been described in the genes coding for apolipoprotein A-I, adenosine triphosphate-binding cassette transporter A1, cholesterol ester transfer protein, the lack of endothelial lipase (LIPG), phospholipid transfer protein, and lecithin-cholesterol acyltransferase. Despite the fact that the total number of carriers of such mutations is rather small, much can be gained by extensively studying the metabolic and vascular consequences of these mutations. Surrogate markers for atherosclerosis have proved to be useful to overcome this sample size limitation and have been widely exploited to study families with decreased or increased HDL cholesterol levels in order to correlate HDL cholesterol phenotypes to atherosclerotic burden in cases and controls. Apart from such extreme phenotype approaches, novel population-based genome-wide association studies have been used to decipher the link between genetic loci and HDL cholesterol levels, and the identification of novel HDL cholesterol-related genes is eagerly awaited. These might be instrumental in the ongoing fight against atherosclerosis.

Relevance of the amino acid conversions L144R (Zaragoza) and L159P (Zavalla) in the apolipoprotein A-I binding site for haptoglobin

The high-density lipoprotein apolipoprotein A-I (ApoA-I) stimulates the enzyme lecithin-cholesterol acyltransferase (LCAT) in the reverse cholesterol transport pathway. Two ApoA-I variants, Zaragoza (L144R) and Zavalla (L159P), are associated with low levels of HDL-cholesterol but normal LCAT activity. Haptoglobin interacts with ApoA-I, impairing LCAT stimulation. Synthetic peptides matching the haptoglobin-binding site of native or variant ApoA-I (native, P2a; variants, Zav-pep and Zar-pep) bound haptoglobin with different activity: Zar-pep>P2a>Zav-pep. They also differently rescued LCATin vitroactivity in the presence of haptoglobin (P2a=Zar-pep>Zav-pep). Therefore, both amino acid conversions affect haptoglobin binding and LCAT regulation. We highlight the role of haptoglobin in LCAT regulation in subjects with ApoA-I variants.

Naturally occurring and bioengineered apoA-I mutations that inhibit the conversion of discoidal to spherical HDL: the abnormal HDL phenotypes can be corrected by treatment with LCAT

In the present study we have used adenovirus-mediated gene transfer of apoA-I (apolipoprotein A-I) mutants in apoA-I-/- mice to investigate how structural mutations in apoA-I affect the biogenesis and the plasma levels of HDL (high-density lipoprotein). The natural mutants apoA-I(R151C)Paris, apoA-I(R160L)Oslo and the bioengineered mutant apoA-I(R149A) were secreted efficiently from cells in culture. Their capacity to activate LCAT (lecithin:cholesterol acyltransferase) in vitro was greatly reduced, and their ability to promote ABCA1 (ATP-binding cassette transporter A1)-mediated cholesterol efflux was similar to that of WT (wild-type) apoA-I. Gene transfer of the three mutants in apoA-I-/- mice generated aberrant HDL phenotypes. The total plasma cholesterol of mice expressing the apoA-I(R160L)Oslo, apoA-I(R149A) and apoA-I(R151C)Paris mutants was reduced by 78, 59 and 61% and the apoA-I levels were reduced by 68, 64 and 55% respectively, as compared with mice expressing the WT apoA-I. The CE (cholesteryl ester)/TC (total cholesterol) ratio of HDL was decreased and the apoA-I was distributed in the HDL3 region. apoA-I(R160L)Oslo and apoA-I(R149A) promoted the formation of prebeta1 and alpha4-HDL subpopulations and gave a mixture of discoidal and spherical particles. apoA-I(R151C)Paris generated subpopulations of different sizes that migrate between prebeta and alpha-HDL and formed mostly spherical and a few discoidal particles. Simultaneous treatment of mice with adenovirus expressing any of the three mutants and human LCAT normalized plasma apoA-I, HDL cholesterol levels and the CE/TC ratio. It also led to the formation of spherical HDL particles consisting mostly of alpha-HDL subpopulations of larger size. The correction of the aberrant HDL phenotypes by treatment with LCAT suggests a potential therapeutic intervention for HDL abnormalities that result from specific mutations in apoA-I.

Do mutations causing low HDL-C promote increased carotid intima-media thickness?

BACKGROUND

Although observational data support an inverse relationship between high-density lipoprotein (HDL) cholesterol and coronary heart disease (CHD), genetic HDL deficiency states often do not correlate with premature CHD.

METHODS

Carotid intima-media thickness (cIMT) measurements were obtained in cases comprising 10 different mutations in LCAT, ABCA1 and APOA1 to further evaluate the relationship between low HDL resulting from genetic variation and early atherosclerosis.

RESULTS

In a 1:2 case-control study of sex and age-related (+/-5 y) subjects (n=114), cIMT was nearly identical between cases (0.66+/-0.17 cm) and controls (0.65+/-0.18 cm) despite significantly lower HDL cholesterol (0.67 vs. 1.58 mmol/l) and apolipoprotein A-I levels (96.7 vs. 151.4 mg/dl) (P<0.05)

CONCLUSIONS

Genetic variants identified in the present study may be insufficient to promote early carotid atherosclerosis.

Human genetics of variation in high-density lipoprotein cholesterol

Longitudinal population studies have confirmed plasma levels of high-density lipoprotein (HDL) cholesterol to be an important inverse coronary risk factor. Although environmental influences are known to regulate HDL cholesterol levels, genetic factors are also known to be important, and over 25 candidate genes have been proposed to be associated with variation in HDL cholesterol levels. A variety of monogenic conditions of extremely low or high HDL cholesterol has helped to delineate the physiology of HDL cholesterol metabolism in humans, which has led to the development of new therapeutic approaches to HDL cholesterol. However, most causes of genetic variation in HDL cholesterol in the general population are likely oligogenic or polygenic. We review the monogenic disorders associated with both high and low HDL cholesterol and the relevance of mutations and polymorphisms in these genes to variation in HDL cholesterol levels in the general population.

Differential diagnosis of familial high density lipoprotein deficiency syndromes

Monogenic high density lipoprotein (HDL) deficiency, because of defects in the genes of apolipoprotein A-I (apoA-I), adenosine triphosphate binding cassette transporter A1 (ABCA1) or lecithin:cholesterol acyltransferase (LCAT), can be assumed in patients with HDL cholesterol levels below the fifth percentile within a given population. As in a first step underlying diseases should be excluded. Patients with a virtual absence of HDL must undergo careful physical examination to unravel the clinical hallmarks of certain HDL deficiency syndromes. In addition, family studies should be initiated, to demonstrate the vertical transmission of the low HDL cholesterol phenotype. Definitive diagnosis requires specialized biochemical tests and the demonstration of a functionally-relevant mutation in one of the three discussed candidate genes. As yet no routinely used drug is able to increase HDL cholesterol levels in patients with familial low HDL cholesterol so that prevention of cardiovascular disease in these patients must be focused on the avoidance and treatment of additional risk factors.

Genetic determinants of low high-density lipoprotein cholesterol

PURPOSE OF REVIEW

High-density lipoprotein cholesterol (HDL-C) has been well established as an inverse predictor of coronary heart disease (CHD), and in recent years, investigations have focused on the genetic regulation of high-density lipoprotein. Although numerous candidate genes contribute to the low HDL-C phenotype, their impact on CHD is heterogeneous, reflecting diverse gene-gene interactions and gene-environmental relationships. This review summarizes recent data involving HDL regulatory genes and their role in atherothrombosis.

RECENT FINDINGS

The primary genetic determinants associated with relative HDL-C deficiency states are the ATP binding cassette protein, ABCA1; apolipoprotein (APO) A1; and lecithin cholesteryl acyl transferase. Other potentially important candidates invoked in low HDL-C syndromes in humans include APOC3, lipoprotein lipase, sphingomyelin phosphodiesterase 1, and glucocerebrosidase. Molecular variation in ABCAI and APOAI and, in selected cases, lecithin cholesteryl acyl transferase deficiency have been associated with increased CHD, whereas two notable variants, APOAIMilano and APOAIParis, are associated with reduced risk.

SUMMARY

Low HDL-C syndromes have generally been correlated with an increased risk of CHD. However, single-gene abnormalities responsible for HDL-C deficiency states may have variable effects on atherothrombotic risk.

A novel apoA-I mutation (L178P) leads to endothelial dysfunction, increased arterial wall thickness, and premature coronary artery disease

OBJECTIVES

We investigated the consequences of an apolipoprotein A-I (apoA-I) gene defect with regard to lipid metabolism, endothelial function, arterial wall thickness, and coronary artery disease (CAD) risk.

BACKGROUND

Due to limited numbers of carriers of the apoA-I defects, data on the consequences of such defects have remained inconclusive.

METHODS

Lipids and lipoproteins were measured in 54 apoA-I (L178P) carriers and 147 nonaffected siblings. Flow-mediated dilation (FMD) was assessed in 29 carriers and 45 noncarriers, and carotid intima-media thickness (IMT) could be determined in 33 heterozygotes and 40 controls. Moreover, CAD risk was evaluated for all apoA-I mutation carriers.

RESULTS

Heterozygotes exhibited lower plasma levels of apoA-I (-50%; p < 0.0001) and high-density lipoprotein cholesterol (-63%; p < 0.0001). In addition, carriers had impaired FMD (p = 0.012) and increased carotid IMT (p < 0.001), whereas multivariate analysis revealed that heterozygotes had a striking 24-fold increase in CAD risk (p = 0.003).

CONCLUSIONS

Heterozygosity for a novel apoA-I mutation underlies a detrimental lipoprotein profile that is associated with endothelial dysfunction, accelerated carotid arterial wall thickening, and severely enhanced CAD risk. Importantly, the extent of atherosclerosis in these subjects was similar to the burden of premature arterial wall abnormalities seen in patients with familial hypercholesterolemia. These data illustrate the pivotal role in humans of apoA-I in the protection against CAD.

Genetics of HDL regulation in humans

PURPOSE OF REVIEW

To review gene regulation of HDL-cholesterol and discuss molecular abnormalities in HDL candidate genes that may lead to human pathologic states.

RECENT FINDINGS

The inverse association between HDL-cholesterol and vascular disease, especially coronary heart disease, has long been recognized, but understanding gene regulation of HDL in humans gained considerable momentum following the identification of ABCA1 as playing a pivotal role in reverse cholesterol transport. Recent data suggest that potentially important targets for upregulating HDL in humans include upregulators of ABCA1 and APOA1 (e.g. peroxisome proliferator activated receptor and liver X receptor agonists) and downregulators of CETP (e.g. JTT-705). A host of other nuclear receptors under investigation in animal models may advance to human testing in the near future.

SUMMARY

Disorders affecting HDL metabolism are complex because monogenic disorders causing low HDL do not necessarily correlate with premature vascular disease. To date, pathologic phenotypes have only been deduced among several HDL candidate genes. Understanding the genetic underpinnings associated with variant HDL and reverse cholesterol transport provides an exceptional opportunity to identify novel agents that may optimize this process and reduce vascular event rates beyond currently available LDL lowering therapies.

Recurrent mutations of the apolipoprotein A-I gene in three kindreds with severe HDL deficiency

Two siblings with high density lipoprotein (HDL) deficiency and no plasma apolipoprotein A-I (Apo A-I) were found to be homozygous for a cytosine deletion in exon 3 of Apo A-I gene (c.85 del C, Q5FsX11). This mutation causes a frameshift leading to a premature stop codon and abolishes the synthesis of Apo A-I. Although both siblings had corneal opacifications and planar xanthomas, only one of them had premature coronary artery disease, probably as the result of mildly elevated LDL levels. In two other unrelated subjects HDL deficiency was due to heterozygosity for a nucleotide substitution in exon 4 of Apo A-I gene (c.494 T>G, L141R). Both Apo A-I mutations were reported previously in an Italian kindred which included compound heterozygotes and simple heterozygotes. We investigated all carriers of these mutations in the three kindreds and in the one previously reported. Plasma Apo A-I and HDL-C levels were lower in the mutation carriers than in non-carrier family members. These levels, however, were lower in L141R carriers than in carriers of c.85 del C. Haplotype analysis performed using several polymorphisms suggested that both the c.85 del C and L141R are likely to be recurrent mutations.

Lack of Association between Increased Carotid Intima-Media Thickening and Decreased HDL-Cholesterol in a Family with a Novel ABCA1 Variant, G2265T

Low HDL-cholesterol (HDL-C) concentrations are inversely correlated with cardiovascular disease, and previous studies have demonstrated that variants in the ATP-binding cassette transporter, ABCA1, are responsible for a proportion of HDL-C deficiency states. We identified a novel variant in ABCA1 in a kindred with decreased HDL-C. This variant was not identified in &gt;200 chromosomes of healthy individuals. The proband, a heterozygote for G2265T, developed premature coronary artery disease. In addition to low HDL-C, six biological family members heterozygous for the ABCA1 variant exhibited low HDL-C concentrations compared with unaffected family members (0.83 ± 0.32 vs 1.33 ± 0.36 mmol/L; P = 0.009). Despite the decreased HDL-C, carotid artery B-mode ultrasound studies failed to reveal increased intima-media thickening in affected individuals compared with age- and sex-matched controls. Although these data extend previous observations that a single defective ABCA1 allele may lead to decreased HDL-C, associated evidence of early atherosclerosis was not confirmed.

The effects of altered apolipoprotein A-I structure on plasma HDL concentration

Approximately 46 human apolipoprotein A-I (apoA-I) coding sequence mutations have been reported to date. Roughly half of these mutations are associated with lower than average plasma concentrations of high-density lipoprotein (HDL) apoA-I. Mutations associated with low HDL apoA-I concentrations fall into two main categories: those which poorly activate the enzyme lecithin:cholesterol acyltransferase (LCAT) and those associated with amyloidosis. These phenotypically distinct groups of mutations are uniquely localized in different regions of the apoprotein sequence. Mutations associated with abnormal LCAT activation are located within repeats 5, 6, and 7, corresponding to amino acids 121 to 186, while many of the mutations found in amyloid deposits are clustered at the amino terminus of the protein, namely residues 1 to 90. These observations strongly support the idea that the tertiary structure of apoA-I determines its intravascular fate and ultimately the steady state concentration of plasma HDL.

In vivo metabolism of HDL, apo A-I, and lp A-I, and function of HDL--a clinical perspective

Serum levels of high density lipoprotein cholesterol (HDL-C) are inversely correlated with coronary heart disease (CHD). Kinetic studies indicate that the mechanism for the variation in HDL levels associated with various pathophysiologic states includes changes in the fractional catabolic rate (FCR) and/or the synthesis rate of HDL and its major proteins apolipoprotein (apo) A-I and apo A-II. The antiatherogenic effects of HDL are thought to be mainly due to its role in reverse cholesterol transport. HDL is an assembly of heterogeneous particles. HDL enlarges when it takes up cellular cholesterol, and shrinks when HDL cholesterol ester (CE) is transfered to low density lipoprotein (LDL) and very low density lipoprotein (VLDL) particles. The functional ability of HDL (to remove cellular cholesterol) has drawn considerable attention. The fractional esterification rate of cholesterol in HDL (FER(HDL)) has been established as a functional assay of HDL, and reflects the size of HDL particles. We investigated the clinical significance of FER(HDL) and its relationship to the quantity of HDL. FER(HDL) values were inversely correlated with levels of HDL-C and large lipoprotein containing apo A-I (LpA-I). The association between FER(HDL) and CHD changed with serum HDL-C levels: increased FER(HDL) values significantly increased the risk of CHD when serum HDL-C levels were low, while there was no such relationship when HDL-C levels were high. We concluded that the combination of HDL-C levels and FER(HDL) is a stronger indicator of CHD than either the HDL-C level (quantitative measure of HDL) or FER(HDL) (functional measure of HDL) alone.

Proteolytic degradation and impaired secretion of an apolipoprotein A-I mutant associated with dominantly inherited hypoalphalipoproteinemia

We have devised a combined in vivo, ex vivo, and in vitro approach to elucidate the mechanism(s) responsible for the hypoalphalipoproteinemia in heterozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) variant (Leu(159) to Arg) known as apoA-I Finland (apoA-I(FIN)). Adenovirus-mediated expression of apoA-I(FIN) decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-deficient mice expressing native human apoA-I (hapoA-I). Interestingly, apoA-I(FIN) was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA-I(FIN) had impaired activation of lecithin:cholesterol acyltransferase in vitro compared with hapoA-I, but in a mixed lipoprotein preparation consisting of both hapoA-I and apoA-I(FIN) there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decreased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expressed with apoA-I(FIN) following infection with recombinant adenoviruses, a condition that mimics secretion in heterozygotes. Thus, this is the first demonstration of an apoA-I point mutation that decreases LCAT activation, impairs hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolysis leading to dominantly inherited hypoalphalipoproteinemia.

Enhanced fractional catabolic rate of apo A-I and apo A-II in heterozygous subjects for apo A-I(Zaragoza) (L144R)

We have recently reported a new apolipoprotein (apo) A-I variant (apo A-I(Zaragoza) L144R) in a Spanish family with HDL-C levels below the 5th percentile for age and sex and low apo A-I concentrations. All the apo A-I(Zaragoza) subjects were heterozygous and none of them showed evidence of coronary artery disease (CAD). Mean plasma HDL-C, apo A-I, and apo A-II levels were lower in apo A-I(Zaragoza) carriers as compared to control subjects (40, 60, and 50%, respectively). Lipid composition analysis revealed that apo A-I(Zaragoza) carriers had HDL particles with a higher percentage of HDL triglyceride and a lower percentage of HDL esterified cholesterol as compared to those of control subjects. Lecithin:cholesterol acyltransferase (LCAT) activity and cholesterol esterification rate of apo A-I(Zaragoza) carriers were normal. Apo A-I and apo A-II metabolic studies were performed on two heterozygous apo A-I(Zaragoza) carriers and on six control subjects. We used a primed constant infusion of [5,5,5-2H3]leucine and HDL apo A-I and apo A-II tracer/tracee ratios were determined by gas chromatography mass spectrometry and fitted to a monoexponential equation using SAAM II software. Both subjects carrying apo A-I(Zaragoza) variant showed mean apo A-I fractional catabolic rate (FCR) values more than two-fold higher than mean FCR values of their controls (0.470+/-0.0792 vs. 0.207+/-0.0635 x day(-1), respectively). Apo A-I secretion rate (SR) of apo A-I(Zaragoza) subjects was slightly increased compared with controls (17.32+/-0.226 vs. 12.76+/-3.918 mg x kg(-l) x day(-1), respectively). Apo A-II FCR was also markedly elevated in both subjects with apo A-I(Zaragoza) when compared with controls (0.366+/-0.1450 vs. 0.171+/-0.0333 x day(-1), respectively) and apo A-II SR was normal (2.31+/-0.517 vs. 2.1+/-0.684 mg x kg(-l) x day(-1), respectively). Our results show that the apo A-I(Zaragoza) variant results in heterozygosis in abnormal HDL particle composition and in enhanced catabolism of apo A-I and apo A-II without affecting significantly the secretion rates of these apolipoproteins and the LCAT activation.

Apolipoprotein A-I-derived amyloid in atherosclerosis. Its association with plasma levels of apolipoprotein A-I and cholesterol

Wild-type apolipoprotein A-I (apo A-I)-derived amyloid commonly occurs in atherosclerotic plaques. To clarify apo A-I amyloid formation, plasma levels of apo A-I and cholesterol were related to the presence of amyloid in atherosclerotic plaques in 15 patients with peripheral atherosclerosis, subjected to arterial reconstruction. Plasma levels of apo A-I and high-density lipoprotein (HDL) cholesterol were slightly higher in patients with apo A-I-derived amyloid than in those without, but the difference was not significant. Levels of low-density lipoprotein cholesterol and total cholesterol were significantly higher in the group with amyloid. High concentrations of apo A-I in the arterial intima are probably of greater importance to amyloid formation than high plasma levels of the protein. During atherosclerosis, the acute phase reactant serum amyloid A may displace apo A-I from HDL, leading to increased concentration of lipid-free apo A-I in the intima and conformational changes of apo A-I, which make it more fibrillogenic. Some forms of amyloid fibrils have been shown to be cytotoxic. Apo A-I-derived amyloid is possibly a pathogenically important factor in atherosclerosis.