Familial High-density Lipoprotein Deficiency Research Articles

Expression and Functional Analyses of Novel Mutations of ATP-Binding Cassette Transporter-1 in Japanese Patients with High-Density Lipoprotein Deficiency

ATP-binding cassette transporter-1 (ABCA1) gene is mutated in patients with familial high-density lipoprotein deficiency (FHD). In order to know the molecular basis for FHD, we characterized three different ABCA1 mutations associated with FHD (G1158A/A255T, C5946T/R1851X, and A5226G/N1611D) with respect to their expression in the passaged fibroblasts from the patients and in the cells transfected with the mutated cDNAs. Fibroblasts from the all patients showed markedly decreased cholesterol efflux to apolipoprotein (apo)-Al. In the fibroblasts homozygous for G1158A/A255T, the immunoreactive mass of ABCA1 could not be detected, even when stimulated by 9-cis-retinoic acid and 22-R-hydroxycholesterol. In the fibroblasts homozygous for C5946T/R1851X, ABCA1 mRNA was comparable. Because the mutant ABCA1 protein (R1851X) was predicted to lack the epitope for the antibody used, we transfected FLAG-tagged truncated mutant (R1851X/ABCA1-FLAG) cDNA into Cos-7 cells, showing that the mutant protein expression was markedly reduced. The expression of N1611D ABCA1 protein was comparable in both fibroblasts and overexpressing cells, although cholesterol efflux from the cells was markedly reduced. These data indicated that, in the three patients investigated, the abnormalities and dysfunction of ABCA1 occurred at the different levels, providing important information about the expression, regulation, and function of ABCA1.

High-density single-nucleotide polymorphism (SNP) map of the 150-kb region corresponding to the human ATP-binding cassette transporter A1 (ABCA1) gene.

Highly dense catalogs of human genetic variations, in combination with high-throughput genotyping technologies, are expected to clarify individual genetic differences in pharmacological responsiveness and predispositions to common diseases. Here we report single-nucleotide polymorphisms (SNPs) present among 48 Japanese individuals at the locus for the human ATP-binding cassette transporter A1 (ABCA1) gene. ABCA1 plays a key role in apolipoprotein-mediated cholesterol transport, and mutations in this gene are responsible for Tangier disease and familial high-density lipoprotein deficiency associated with reduced cholesterol efflux. We identified a total of 162 SNPs, 149 of which were novel, within the 150-kb region encompassing the entire ABCA1 gene. Eight of the SNPs lie within coding elements, two in 5' flanking regions, 147 in introns, and five in 3' untranslated regions, but none were found in 5' untranslated or 3' flanking regions. The ratio of transitions to transversions was approximately 2.37 to 1. Our dense SNP map of this region could serve as a powerful resource for studies of complex genetic diseases that may be associated with ABCA1 and of individual responses to drug therapy.

Novel mutations in ABCA1 gene in Japanese patients with Tangier disease and familial high density lipoprotein deficiency with coronary heart disease

Mutations in the ATP-binding cassette transporter 1 (ABCA1) gene have been recently identified as the molecular defect in Tangier disease (TD) and familial high density lipoprotein deficiency (FHA). We here report novel mutations in the ABCA1 gene in two sisters from a Japanese family with TD who have been described previously (S. Ohtaki, H. Nakagawa, N. Kida, H. Nakamura, K. Tsuda, S. Yokoyama, T. Yamamura, S. Tajima, A. Yamamoto, Atherosclerosis 49 (1983)) and a family with FHA. Both probands of TD and FHA developed coronary heart disease. Sequence analysis of the ABCA1 gene from the patients with TD revealed a homozygous G to A transition at nucleotide 3805 of the cDNA resulting in the substitution of Asp 1229 with Asn in exon 27, and a C to T at nucleotide 6181 resulting in the substitution of Arg 2021 with Trp in exon 47. Sequence analysis of the ABCA1 gene from the FHA patient revealed a homozygous 4 bp CGCC deletion from nucleotide 3787 to 3790 resulting in premature termination by frameshift at codon 1224. These mutations were confirmed by restriction digestion analysis, and were not found in 141 control subjects. Our findings indicate that mutations in the ABCA1 gene are associated with TD as well as FHA.

Homogeneous assay based on 52 primer sets to scan for mutations of the ABCA1 gene and its application in genetic analysis of a new patient with familial high-density lipoprotein deficiency syndrome

Familial high-density lipoprotein (HDL)-deficiency syndromes are caused by mutations of the ABCA1 gene, coding for the ATP-binding cassette transporter 1. We have developed a homogeneous assay based on 52 primer sets to amplify all 50 ABCA1 exons and approximately 1 kb of its promoter. The assay allows for convenient amplification of the gene from genomic DNA and easy mutational analysis through automatic sequencing. It obviates the need to use mRNA preparations, which were difficult to handle and posed a risk to miss splice junction or promoter mutations. The application of the test to the molecular analysis of a new patient with familial HDL-deficiency (Tangier disease) led to a discovery of two novel ABCA1 mutations: C2665del and C4457T.

Cholesterol efflux regulatory protein, Tangier disease and familial high-density lipoprotein deficiency.

Cellular cholesterol efflux, by which cholesterol is transported from peripheral cells to HDL acceptor molecules for transport to the liver, is the first step of reverse cholesterol transport. Two genetic disorders, Tangier disease and some cases of familial HDL deficiency, have defects of cellular cholesterol efflux. The recent discovery of mutations in the ABC1 gene, which encodes the cholesterol efflux regulatory protein, in both these disorders establishes cholesterol efflux regulatory protein as a rate-limiting factor in reverse cholesterol transport.

Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency.

Genes have a major role in the control of high-density lipoprotein (HDL) cholesterol (HDL-C) levels. Here we have identified two Tangier disease (TD) families, confirmed 9q31 linkage and refined the disease locus to a limited genomic region containing the gene encoding the ATP-binding cassette transporter (ABC1). Familial HDL deficiency (FHA) is a more frequent cause of low HDL levels. On the basis of independent linkage and meiotic recombinants, we localized the FHA locus to the same genomic region as the TD locus. Mutations in ABC1 were detected in both TD and FHA, indicating that TD and FHA are allelic. This indicates that the protein encoded by ABC1 is a key gatekeeper influencing intracellular cholesterol transport, hence we have named it cholesterol efflux regulatory protein (CERP).

Cellular cholesterol transport and efflux in fibroblasts are abnormal in subjects with familial HDL deficiency.

Familial high density lipoprotein (HDL) deficiency (FHD) is a genetic lipoprotein disorder characterized by a severe decrease in the plasma HDL cholesterol (-C) level (less than the fifth percentile). Unlike Tangier disease, FHD is transmitted as an autosomal dominant trait. FHD subjects have none of the clinical manifestations of Tangier disease (lymphoid tissue infiltration with cholesteryl esters and/or neurological manifestations). Plasmas from FHD subjects contain pre-beta-migrating HDLs but are deficient in alpha-migrating HDLs. We hypothesized that a reduced HDL-C level in FHD is due to abnormal transport of cellular cholesterol to the plasma membrane, resulting in reduced cholesterol efflux onto nascent HDL particles, leading to lipid-depleted HDL particles that are rapidly catabolized. Cellular cholesterol metabolism was investigated in skin fibroblasts from FHD and control subjects. HDL3- and apolipoprotein (apo) A-I-mediated cellular cholesterol and phosphatidylcholine efflux was examined by labeling cells with [3H]cholesterol and [3H]choline, respectively, during growth and cholesterol loading during growth arrest. FHD cells displayed an approximately 25% reduction in HDL3-mediated cellular cholesterol efflux and an approximately 50% to 80% reduction in apoA-I-mediated cholesterol and phosphatidylcholine efflux compared with normal cells. Cellular cholesterol ester levels were decreased when cholesterol-labeled cells were incubated with HDL3 in normal cells, but cholesterol ester mobilization was significantly reduced in FHD cells. HDL3 binding to fibroblasts and the possible role of the HDL binding protein/vigilin in FHD were also investigated. No differences were observed in 125I-HDL3 binding to LDL-loaded cells between FHD and control cells. HDL binding protein/vigilin mRNA levels and its protein expression were constitutively expressed in FHD cells and could be modulated ( approximately 2-fold increase) by elevated cellular cholesterol in normal cells. In conclusion, FHD is characterized by reduced HDL3- and apoA-I-mediated cellular cholesterol efflux. It is not associated with abnormal cellular HDL3 binding or a defect in a putative HDL binding protein.

Familial HDL deficiency characterized by hypercatabolism of mature apoA-I but not proapoA-I.

We have previously described patients with familial high density lipoprotein (HDL) deficiency (FHD) having a marked reduction in the plasma concentration of HDL cholesterol and apolipoprotein (apo) A-I but lacking clinical manifestations of Tangier disease or evidence of other known causes of HDL deficiency. To determine whether FHD in these individuals was associated with impaired HDL production or increased HDL catabolism, we investigated the kinetics of plasma apoA-I and apoA-II in two related FHD patients (plasma apoA-I, 17 and 37 mg/dL) and four control subjects (apoA-I, 126+/-18 mg/dL, mean+/-SD) by using a primed constant infusion of deuterated leucine. Kinetic analysis of plasma apolipoprotein enrichment curves demonstrated that mature plasma apoA-I production rates (PRs) were similar in patients and control subjects (7.9 and 9.1 versus 10.5+/-1.7 mg x kg[-1] x d[-1]). Residence times (RTs) of mature apoA-I were, however, significantly less in FHD patients (0.79 and 1.66 days) compared with controls (5.32+/-1.05 days). Essentially normal levels of plasma proapoA-I (the precursor protein of apoA-I) in FHD patients were associated with normal plasma proapoA-I PRs (7.8 and 10.4 versus 10.9+/-2.6 mg x kg[-1] x d[-1]) and proapoA-I RTs (0.18 and 0.15 versus 0.16+/-0.03 day). The RTs of apoA-II were, however, less in patients (3.17 and 2.92 days) than control subjects (7.24+/-0.71 days), whereas the PRs of apoA-II were similar (1.8 and 1.9 versus 1.7+/-0.2 mg x kg[-1] x d[-1]). Increased plasma catabolism of apoA-II in FHD patients was associated with the presence in plasma of abnormal apoA-II-HDL (without apoA-I). These results demonstrate that FHD in our patients is characterized, like Tangier disease, by hypercatabolism of mature apoA-I and apoA-II, but unlike Tangier disease, by essentially normal plasma catabolism and concentration of proapoA-I.

The pathology of cornea in Tangier disease (familial high density lipoprotein deficiency).

To clarify the underlying causes of corneal opacification in Tangier disease. Both corneas were removed at death from a 62 year old man with Tangier disease, and were examined by direct and transmission electron microscopy, histochemistry, biochemical analysis by thin-layer and gas-liquid chromatography after extraction, and by differential scanning calorimetry. Membranous inclusions in the stroma were seen on transmission electron microscopy. Direct analysis confirmed enrichment with phospholipids and cholesterol, with acyl patterns and proportions as ester broadly similar to those of normal cornea. Tangier cornea showed major thermotropic phase transitions in the range 28-37 degrees C, peak 30-33 degrees C, extending above profiles of normal clear cornea and without the complexity of those seen with cornea with heavy arcus involvement. Lipid accumulation underlies corneal opacification in Tangier disease. The excess material is mainly phospholipid and cholesterol esters. As at other sites which are below body core temperature, notably tonsil, accumulation may be enhanced by local impaired mobilisation of material as the phase transitions of the excess lipid present extend above ambient corneal temperatures.

Characterization of atherosclerosis in a patient with familial high-density lipoprotein deficiency

We describe the cardiovascular state of a 60-year-old homozygous patient with familial HDL deficiency (Tangier disease). The patient was examined by coronary angiography and intravascular ultrasound because of chest pain at rest and on exertion. We found a normal left ventricular function, moderately diffuse coronary sclerosis without stenosis and no critical stenosis of peripheral arteries. Intravascular ultrasound revealed the three layer appearance of arterial intima, media and adventitia with normal thickness. No calcified plaques or intimal hyperplasia could be detected apart from a single, discrete atherosclerotic lesion in one iliac artery segment. Concentric non-occlusive atherosclerotic lesions which are readily detectable with intravascular ultrasound were not found. The lack of severe atherosclerosis was remarkable insofar as massive foam cell formation and the virtually complete absence of circulating HDL is characteristic of Tangier disease and has been previously demonstrated in this patient. Our findings suggest that HDL deficiency and foam cell formation in Tangier disease are not necessarily associated with accelerated development of atherosclerosis.

Familial HDL deficiency due to marked hypercatabolism of normal apoA-I.

In this article, we describe a 46-year-old man with severe high-density lipoprotein (HDL) deficiency and his kindred. In the proband, HDL cholesterol and apolipoprotein (apo) A-I levels were 5 and 4.5 mg/dL, respectively. Xanthomata, xanthelasma, arcus corneae, and hepatosplenomegaly were not present. The proband had coronary artery disease, but it was impossible to state whether the HDL deficiency cosegregated with premature coronary artery disease in this kindred. Pedigree analysis was suggestive of a codominant familial disease. Polymerase chain reaction amplification of the apoA-I gene of the proband, followed by subcloning and sequencing, did not reveal any mutation in either the coding regions or intron-exon junctions. A kinetic study using deuterated leucine to endogenously label apoA-I was performed to elucidate the metabolic basis of the apoA-I deficiency. We demonstrated marked hypercatabolism of apoA-I in the proband, with a fractional catabolic rate more than 10 times faster than normal; the plasma residence time of apoA-I in the proband was only 0.38 day compared with 4.10 days in a control subject. The apoA-I production rate was also substantially decreased in the proband. The association of a normal apoA-I gene sequence with marked hypercatabolism of apoA-I is similar to that described in Tangier disease. However, except for the presence of mild, diffuse, corneal deposits, this patient had no evidence of the reticuloendothelial cholesterol deposition characteristic of Tangier disease. This study establishes that a form of severe hypoalphalipoproteinemia distinct from Tangier disease can be caused by marked hypercatabolism of a normal A-I apolipoprotein.

The cornea in familial high density lipoprotein deficiency, tangier disease

The cornea in familial high density lipoprotein deficiency, tangier disease

Cholesteryl ester transfer activity in plasma of patients with familial high-density lipoprotein deficiency.

We determined cholesteryl ester transfer activity in whole plasma and in lipoprotein-depleted plasma of normolipidemic subjects and of patients with severe high-density lipoprotein (HDL) deficiency: Tangier disease, lecithin:cholesterol acyltransferase (LCAT) deficiency, and "fish-eye" disease. Transfer rates in plasma were positively correlated (r = 0.950) with rates measured in the absence of the endogenous lipoproteins. This suggests that lipoprotein composition and content may not affect total cholesteryl ester transfer activity in normolipidemic and the HDL-deficient subjects. Cholesteryl ester transfer from solid-phase-bound HDL to plasma lipoproteins was decreased by 39% in fish-eye disease and 33% in LCAT deficiency but increased by 57% in Tangier disease, as compared with normal values. Changes were similar for lipoprotein-depleted plasma from the same individuals. Transfer to plasma HDL was significantly decreased in all HDL-deficient patients, whereas transfer to very-low- and low-density lipoproteins was increased only in Tangier disease. Differences in transfer rates between the patients studied appeared to reflect the LCAT activity and the need to transport cholesteryl ester rather than the HDL cholesterol concentration. Thus, the concentration of HDL in plasma does not directly affect total cholesteryl ester transfer activity in HDL deficiency.

Ocular complications of tangier disease

Tangier disease, or familial high-density lipoprotein deficiency, is an inherited disorder resulting in tissue deposition of excessive cholesterol esters. Although associated corneal clouding has been reported to produce little visual impairment, this patient with Tangier disease had corneal clouding, decreased corneal sensation, and cicatricial ectropion and experienced slowly progressive marked visual impairment. All ocular cases of Tangier disease are reviewed. Ectroplon and incomplete eyelid closure may precede corneal clouding and should be recognized as signs associated with Tangier disease. The combination of exposure keratopathy and corneal infiltration can cause significant visual impairment.

The degradation of platelet-activating factor in the plasma of a patient with familial high density lipoprotein deficiency (Tangier disease)

Platelet Activating Factor (PAF) (1-O-alkyl-2-acetyl sn-glycerol 3- phosphocholine) has been characterized by its ability to aggregate platelets at low concentrations and its profound hypotensive effects. There is evidence that the rate of catabolism of this compound in the plasma regulates its concentration. In humans, we and others have shown that a PAF acetylhydrolase is associated with low density lipoprotein (LDL). The LDL particle in the plasma of patients with Tangier disease is quite different from normal as its lipid core appears to be enriched with triacylglycerol. Thus, we have studied the potential of this abnormal lipoprotein to degrade PAF. The assay for PAF acetylhydrolase was based on the release of 3H from PAF that was labelled in the acetate moiety of the sn-2 position. Tangier disease plasma had approximately 3.3-fold higher PAF acetylhydrolase activity (208 +/- 9 nmol/min/mL) than controls (63 +/- 18 nmol/min/mL). This increase was brought about by an increase in the Vmax (400 +/- 40, Tangier disease; 54 +/- 5, controls) and Km for PAF (120 +/- 20 mumol/L, Tangier disease; 28 +/- 4 mumol/L, controls). The activity appears to be a specific acetylhydrolase rather than a phospholipase A2 as preincubation of the substrate with 0 to 100 mumol/L phosphatidylcholine did not affect the amount of [3H] acetate released. The role of PAF, and its degradation by LDL-bound PAF acetylhydrolase in the phenotypic expression of this patient with Tangier disease, is not known. However, this is the first patient so far described who has an increased ability to degrade PAF in the plasma.

Clinical, biochemical, and genetic features in familial disorders of high density lipoprotein deficiency.

This review assesses current knowledge of the clinical, genetic, and biochemical features of familial high density lipoprotein (HDL) deficiency syndromes. The focus is on HDL deficiency states occurring in the absence of severe hypertriglyceridemia or lecithin/cholesterol acyltransferase deficiency. Specific entities falling within this category include Tangier disease, familial HDL deficiency with planar xanthomas, familial apolipoprotein A-I and C-III deficiency (formerly known as apolipoprotein A-I absence), familial deficiency of apolipoprotein A-I and C-III, fish-eye disease, familial hypoalphalipoproteinemia, and apolipoprotein A-I variants (apo A-I Milano, apo A-I Marburg, apo A-I Giessen, and apo A-I Munster 1-3). Diffuse corneal opacification and premature coronary artery disease are common features in many of these kindreds. No striking clinical abnormalities have been noted in patients with currently known apolipoprotein A-I variants, possibly because these subjects are heterozygotes for their respective defects. The HDL deficiency in many of these disorders has been associated with abnormalities or deficiencies of apolipoprotein A-I. Further research will undoubtedly define the defects in all the disorders that have been described, uncover new mutations, as well as provide additional insights into the precise relationship between HDL deficiency and atherosclerosis.

A Japanese family with high density lipoprotein deficiency

Two siblings with marked reduction of plasma high density lipoprotein (HDL) were found in a Japanese family. Their plasma cholesterol levels were very low (30–60 mg/dl), especially in the HDL fraction (0–1 mg/dl). The concentration of apolipoprotein (Apo) A-I in their plasma was 2–3 mg/dl and that of Apo A-II was 1.5–2.0 mg/dl, determined by means of a single radial immunodiffusion technique. An ultracentrifugally separated HDL fraction contained two different populations of lipoprotein particles, as shown by electron microscopy; a small particle with a diameter of 50–70 Å and a relatively large particle at 200 Å. Plasma lecithin: cholesterol acyltransferase activity was substantially retained in both cases. Hepatosplenomegaly was present and liver biopsy revealed lipid deposition in reticuloendothelial cells, although the tonsils were apparently normal. No severe atherosclerotic lesions were noticed. The results from these two cases were consistent with the characteristic features of homozygotes of familial HDL deficiency (Tangier disease). HDL cholesterol levels were relatively low in the parents and two children from one patient, which is consistent with the heterozygote state. Two other cases in the kindred were also found to have relatively low HDL cholesterol levels, besides these 4 cases of obligate heterozygotes. Apo A-I and Apo A-II levels in the plasms of the obligate heterozygotes, however, were within the normal range. Plasma low density lipoprotein in the patients moved faster in polyacrylamide gel electrophoresis than those of normal subjects, as did those in the heterozygotes.

Metabolism of high density lipoprotein subfractions and constituents in Tangier disease following the infusion of high density lipoproteins.

The metabolism of apolipoproteins A-I and A-II, as well as other high density lipoprotein (HDL) constituents, was studied in patients with homozygous familial HDL deficiency (Tangier disease) prior to and after plasma exchange or HDL infusion. Mean plasma apoA-I, apoA-II, and HDL cholesterol values in homozygotes (n = 2) were 2.0 mg/dl, 2.7 mg/dl, and 1.5 mg/dl, respectively, and in a normal control subject were 125.1 mg/dl, 23.0 mg/dl, and 53.0 mg/dl, respectively. Based on radioiodinated apoA-I and apoA-II kinetic studies in the baseline state, synthesis rates for apoA-I and apoA-II in mg/kg/day were 3.81 and 1.61, respectively, in one homozygote (patient B) and 11.82 and 1.99, respectively, in the normal subject. ApoA-I and apoA-II plasma residence times in days were 0.22 and 0.81, respectively, in the homozygote, and 4.04 and 4.44, respectively, in the normal subject. These data indicate that this homozygote had both a moderate decrease in the synthetic rates of apoA-I and apoA-II, as well as a marked decrease in the plasma residence times of these two apolipoproteins. In one homozygote (patient A) following a complete plasma exchange during cardiopulmonary bypass, plasma HDL cholesterol, apoA-I, and apoA-II levels were very similar to pre-exchange values within 64 hr after exchange. A second homozygote (patient B) received HDL intravenously as well as 125I-labeled apoA-I and 131I-labeled apoA-II. Following infusion, the residence time in days for HDL subfractions, HDL2b, HDL2a, and HDL3 were 0.1, 0.8, and 2.7, respectively. HDL protein and phospholipid both had a monoexponential decay, with residence times of 0.7 days, while HDL triglyceride disappeared monoexponentially with a residence time of 0.5 days. HDL cholesterol had a biexponential decay, with the residence time of the slow component being 0.7 days. Plasma and HDL apoA-I decayed down to baseline values significantly faster than did plasma and HDL apoA-II. ApoA-II specific radioactivity decreased throughout the course of the infusion study in both plasma and HDL, while apoA-I specific radioactivity decreased slightly, then rose, and subsequently declined in both plasma and HDL. The data indicate that the rapid and altered catabolism of apoA-I and apoA-II in Tangier homozygotes persists despite major increases in the plasma pool size of these proteins. In addition, following HDL infusion, HDL2b and HDL2a disappeared at a faster rate than HDL3, HDL cholesterol and triglyceride were catabolized at a faster rate than HDL protein and phospholipid, and apoA-I disappeared more rapidly than apoA-II. These observations may have important implications with regard to the catabolism of HDL subfractions and constituents in normal man.

Coronary heart disease prevalence and other clinical features in familial high-density lipoprotein deficiency (Tangier disease).

High-density (HD) lipoprotein cholesterol levels have been inversely associated with the incidence of coronary heart disease. The clinical features were reviewed and the prevalence of clinically evident coronary artery or other vascular disease was ascertained in 27 homozygous and 22 obligate heterozygous patients with familial HD lipoprotein deficiency (Tangier disease). Mean HD lipoprotein cholesterol levels were 4 mg/dL in homozygotes, 26 mg/dL in heterozygotes, and 54 mg/dL in normal subjects. Mean low-density (LD) lipoprotein cholesterol levels were 47 mg/dL in homozygotes, 120 mg/dL in heterozygotes, and 124 mg/dL in normal subjects. In heterozygotes or homozygotes 35 years of age or less no evidence of vascular disease was noted. In subjects between 35 and 65 years of age, five of 19 heterozygotes (26%) and five of 11 homozygotes (45%) had some evidence of coronary artery or cerebrovascular disease, as compared to 4% or less in a normal population. The data are consistent with the concept that patients with familial HD lipoprotein deficiency may be at increased risk for premature vascular disease. However, the strikingly accelerated atherosclerosis seen in patients with familial hypercholesterolemia is not seen in patients with Tangier disease possibly due to their normal or reduced LD lipoprotein cholesterol levels.

High density lipoproteins, 1978—An overview

High density lipoproteins (HDL) have come of age. For years it has been fashionable to study HDL as an approach to understanding lipoprotein structure and lipid binding. Available in abundant amounts from normal human plasma, readily separable into its individual lipid and soluble apolipoprotein components, HDL has provided much information for lipoprotein model building. Suddenly it has been thrust center stage clinically by a host of convincing epidemiologic studies that clearly establishes an inverse relationship between HDL levels and coronary vascular events. Biochemists, clinicians, cardiologists and epidemiologists are simultaneously focusing attention on HDL. Familial High Density Lipoprotein Deficiency (Tangier Disease) has been well described but is poorly understood as a clinical syndrome complex. We have suddenly become aware of how little we understand about HDL's normal ultracentrifugal and apoprotein heterogeneity, about its functional role(s) or the determinant(s) of its concentration in plasma. The relative contributions of the two sites of HDL origin, the liver and intestine, are yet to be determined as are the site(s) of degradation. Awareness of a problem and its importance is the first step toward the solution(s) of the problem.