In vivo metabolism of apo A-I and apo A-II in subjects with apo A-I(Lys107-->0) associated with reduced HDL cholesterol and Lp(AI w AII) deficiency

Human apoA-I[Lys107del] mutation affects lipid surface behavior of apoA-I and its ability to form large nascent HDL

Population studies have found that a natural human apoA-I variant, apoA-I[K107del], is strongly associated with low HDL-C but normal plasma apoA-I levels. We aimed to reveal properties of this variant that contribute to its unusual phenotype associated with atherosclerosis. Our oil-drop tensiometry studies revealed that compared to WT, recombinant apoA-I[K107del] adsorbed to surfaces of POPC-coated triolein drops at faster rates, remodeled the surfaces to a greater extent, and was ejected from the surfaces at higher surface pressures on compression of the lipid drops. These properties may drive increased binding of apoA-I[K107del] to and its better retention on large triglyceride-rich lipoproteins, thereby increasing the variant's content on these lipoproteins. While K107del did not affect apoA-I capacity to promote ABCA1-mediated cholesterol efflux from J774 cells, it impaired the biogenesis of large nascent HDL particles resulting in the formation of predominantly smaller nascent HDL. Size-exclusion chromatography of spontaneously reconstituted 1,2-dimyristoylphosphatidylcholine-apoA-I complexes showed that apoA-I[K107del] had a hampered ability to form larger complexes but formed efficiently smaller-sized complexes. CD analysis revealed a reduced ability of apoA-I[K107del] to increase α-helical structure on binding to 1,2-dimyristoylphosphatidylcholine or in the presence of trifluoroethanol. This property may hinder the formation of large apoA-I[K107del]-containing discoidal and spherical HDL but not smaller HDL. Both factors, the increased content of apoA-I[K107del] on triglyceride-rich lipoproteins and the impaired ability of the variant to stabilize large HDL particles resulting in reduced lipid:protein ratios in HDL, may contribute to normal plasma apoA-I levels along with low HDL-C and increased risk for CVD.

Vascular alterations in apolipoprotein A-I amyloidosis (Leu75Pro). A case-control study

BACKGROUND

Among hereditary amyloidoses, apolipoprotein A-I (APO A-I) amyloidosis (Leu75Pro) is a rare, autosomal dominant condition in which renal, hepatic, and testicular involvement has been demonstrated.

OBJECTIVE

To investigate vascular structural as well as functional alterations.

METHODS

In 131 carriers of the amyloidogenic Leu75Pro APO A-I mutation (mean age 52 + 16 years, 56 women) and in 131 subjects matched for age, sex, body mass index and clinic blood pressure (BP), arterial stiffness (pulse wave velocity, PWV) and carotid intima-media thickness (IMT) were measured.

RESULTS

By definition no differences for age, sex, body mass index, and BP were observed. Meanmax IMT (Mmax-IMT) in the common (CC), bifurcation (BIF) and internal (ICA) carotid artery were comparable in the two groups. After adjustment for high-density lipoprotein cholesterol and renal function differences between the two groups, a lower meanmax-IMT was observed in APO A-I Leu75Pro mutation carriers than in controls (CC Mmax-IMT 0.87 ± 0.21 versus 0.93 ± 0.2 mm, p = 0.07; BIF Mmax-IMT 1.19 ± 0.48 versus 1.36 ± 0.46 mm, p = 0.025; ICA Mmax-IMT 0.9 ± 0.37 versus 1.02 ± 0.35 mm, p = 0.028). On the other hand, aortic stiffness was significantly greater in patients with APO A-I amyloidosis than controls (PWV 11.5 ± 2.9 and 10.7 ± 2.3 m/s, p < 0.05), even after adjusting for confounders.

CONCLUSIONS

In carriers of the amyloidogenic Leu75Pro APO A-I mutation, a significant increase in arterial stiffness is observed; on the contrary, carotid artery IMT is comparable to that of control subjects. These results may add significant information to the clinical features of this rare genetic disorder.

Population-based resequencing of APOA1 in 10,330 individuals: spectrum of genetic variation, phenotype, and comparison with extreme phenotype approach

Rare genetic variants, identified by in-detail resequencing of loci, may contribute to complex traits. We used the apolipoprotein A-I gene (APOA1), a major high-density lipoprotein (HDL) gene, and population-based resequencing to determine the spectrum of genetic variants, the phenotypic characteristics of these variants, and how these results compared with results based on resequencing only the extremes of the apolipoprotein A-I (apoA-I) distribution. First, we resequenced APOA1 in 10,330 population-based participants in the Copenhagen City Heart Study. The spectrum and distribution of genetic variants was determined as a function of the number of individuals resequenced. Second, apoA-I and HDL cholesterol phenotypes were determined for nonsynonymous (NS) and synonymous (S) variants and were validated in the Copenhagen General Population Study (n = 45,239). Third, observed phenotypes were compared with those predicted using an extreme phenotype approach based on the apoA-I distribution. Our results are as follows: First, population-based resequencing of APOA1 identified 40 variants of which only 7 (18%) had minor allele frequencies >1%, and most were exceedingly rare. Second, 0.27% of individuals in the general population were heterozygous for NS variants which were associated with substantial reductions in apoA-I (up to 39 mg/dL) and/or HDL cholesterol (up to 0.9 mmol/L) and, surprisingly, 0.41% were heterozygous for variants predisposing to amyloidosis. NS variants associated with a hazard ratio of 1.72 (1.09–2.70) for myocardial infarction (MI), largely driven by A164S, a variant not associated with apoA-I or HDL cholesterol levels. Third, using the extreme apoA-I phenotype approach, NS variants correctly predicted the apoA-I phenotype observed in the population-based resequencing. However, using the extreme approach, between 79% (screening 0–1^st percentile) and 21% (screening 0–20^th percentile) of all variants were not identified; among these were variants previously associated with amyloidosis. Population-based resequencing of APOA1 identified a majority of rare NS variants associated with reduced apoA-1 and HDL cholesterol levels and/or predisposing to amyloidosis. In addition, NS variants associated with increased risk of MI.

Rare genetic variants, identified by in-detail resequencing of loci, may contribute to complex traits. We used the apolipoprotein A-I gene (APOA1), a major high-density lipoprotein (HDL) gene, and population-based resequencing to determine the spectrum of genetic variants, the phenotypic characteristics of these variants, and how these results compared with results based on resequencing only the extremes of the apolipoprotein A-I (apoA-I) distribution. By resequencing APOA1 in >10,000 Danes and genotyping an additional >45,000, we show that population-based resequencing of APOA1 identifies a majority of rare genetic variants that together are relatively frequent: 0.27% of the population are heterozygous for nonsynonymous (NS) variants in APOA1 that associate with substantial reductions in apoA-I and HDL cholesterol, and 0.41% are heterozygous for variants predisposing to amyloidosis. NS variants associated with a hazard ratio of 1.72 (1.09–2.70) for myocardial infarction (MI), largely driven by A164S, a variant not associated with apoA-I or HDL cholesterol levels. Resequencing only the extremes of the apoA-I distribution, between 79% and 21% of all variants are not identified; among these are variants previously associated with amyloidosis. These results provide direct evidence that rare NS variants in APOA1 contribute to low apoA-I and HDL cholesterol levels, to susceptibility to amyloidosis, and to risk of MI in the general population.

Renal apolipoprotein A-I amyloidosis associated with a novel mutant Leu64Pro

Apolipoprotein A-I amyloidosis (Apo A-I) is an inherited systemic disease that results from the pathologic deposition in tissues throughout the body of fibrils composed of Apo A-I-related molecules. This disorder has been linked to mutations occurring within the coding region of the Apo A-I gene and, to date, 11 such substitutions have been documented. In 4 of these cases, the kidney was the target organ of the disease process. The authors report their studies on a patient with renal amyloidosis and a heretofore undescribed alteration in the amyloidogenic precursor protein. Analyses of genomic DNA evidenced a transition in the second base of codon 64 (T-->C) in one Apo A-I allele that resulted in the replacement of leucine by proline at position 64 (Leu64Pro). Additionally, fibrils extracted from the kidney and characterized chemically were found to be composed almost exclusively of an approximately 96-residue N-terminal Apo A-I fragment that contained the Leu64Pro substitution. These studies have provided further evidence for Apo A-I amyloidogenicity and the propensity of certain mutants to deposit in renal parenchyma.

Hepatic amyloidosis resulting from deposition of the apolipoprotein A-I variant Leu75Pro

Apolipoprotein A-I amyloidosis (AApo A-I) is an inherited systemic disease that results from pathologic deposition in tissues of fibrils composed of Apo A-I-related molecules. This disorder has been linked to mutations occurring within the coding region of the Apo A-I gene and heretofore, nine such variants had been described. Recently, a tenth alteration was found in an Italian population where the substitution of proline for leucine at position 75 (Leu75Pro) was associated with amyloid deposits in the liver. We now report our studies on a patient of different ethnicity who has hepatic amyloidosis and a similar mutation in the amyloidogenic precursor protein, as evidenced from analyses of genomic Apo A-I-encoding DNA. Additionally, fibrils extracted from the liver and characterized chemically were found to be composed almost exclusively of a approximately 96 residue N-terminal Apo A-I fragment that contained the Leu75Pro substitution. RFLP analyses revealed that the patient was heterozygous for this mutation; however, < 10% of the plasma Apo A-I consisted of the aberrant protein while the remainder had the normal (wild-type) sequence. Our findings provide further evidence that the Leu75Pro variant is associated with a predominant hepatic phenotype and can occur in individuals of diverse ethnic backgrounds.

Apolipoprotein A-II, HDL metabolism and atherosclerosis

Apolipoprotein (Apo) A-I and apo A-II are the major apolipoproteins of HDL. It is clearly demonstrated that there are inverse relationships between HDL-cholesterol and apo A-I plasma levels and the risk of coronary heart disease (CHD) in the general population. On the other hand, it is still not clearly demonstrated whether apo A-II plasma levels are associated with CHD risk. A recent prospective epidemiological (PRIME) study suggests that Lp A-I (HDL containing apo A-I but not apo A-II) and Lp A-I:A-II (HDL containing apo A-I and apo A-II) were both reduced in survivors of myocardial infarction, suggesting that both particles are risk markers of CHD. Apo A-II and Lp A-I:A-II plasma levels should be rather related to apo A-II production rate than to apo A-II catabolism. Mice transgenic for both human apo A-I and apo A-II are less protected against atherosclerosis development than mice transgenic for human apo A-I only, but the results of the effects of trangenesis of human apo A-II (in the absence of a co-transgenesis of human apo A-I) are controversial. It is highly suggested that HDL reduce CHD risk by promoting the transfer of peripherical free cholesterol to the liver through the so-called 'reverse cholesterol transfer'. Apo A-II modulates different steps of HDL metabolism and therefore probably alters reverse cholesterol transport. Nevertheless, some effects of apo A-II on intermediate HDL metabolism might improve reverse cholesterol transport and might reduce atherosclerosis development while some other effects might be deleterious. In different in vitro models of cell cultures, Lp A-I:A-II induce either a lower or a similar cellular cholesterol efflux (the first step of reverse cholesterol transport) than Lp A-I. Results depend on numerous factors such as cultured cell types and experimental conditions. Furthermore, the effects of apo A-II on HDL metabolism, beyond cellular cholesterol efflux, are also complex and controversial: apo A-II may inhibit lecithin-cholesterol acyltransferase (LCAT) (potential deleterious effect) and cholesteryl-ester-transfer protein (CETP) (potential beneficial effect) activities, but may increase the hepatic lipase (HL) activity (potential beneficial effect). Apo A-II may also inhibit the hepatic cholesteryl uptake from HDL (potential deleterious effect) probably through the SR-BI depending pathway. Therefore, in terms of atherogenesis, apo A-II alters the intermediate HDL metabolism in opposing ways by increasing (LCAT, SR-BI) or decreasing (HL, CETP) the atherogenicity of lipid metabolism. Effects of apo A-II on atherogenesis are controversial in humans and in transgenic animals and probably depend on the complex effects of apo A-II on these different intermediate metabolic steps which are in weak equilibrium with each other and which can be modified by both endogenous and environmental factors. It can be suggested that apo A-II is not a strong determinant of lipid metabolism, but is rather a modulator of reverse cholesterol transport.

Recombinant proapoA-I(Lys107del) shows impaired lipid binding associated with reduced binding to plasma high density lipoprotein

In the present study apoA-I (Lys 107del), a naturally occurring human apoA-I variant with a deletion of Lys 107, was expressed in E. coli to examine the effect of this mutation on lipid binding, cholesterol efflux and lecithin:cholesterol acyltranferase (LCAT) activation. Dimyristoyl phosphatidylcholine (DMPC) binding studies revealed slow interaction of proapoA-I(Lys107del) with DMPC relative to normal proapoA-I. After preincubation with human plasma lipoprotein (d<1.225 g/ml) for 1 h at 37 degrees C, 125I-labeled normal proapoA-I chromatographed as a single peak with the high density lipoprotein (HDL) fraction, whereas 125I-labeled proapoA-I(Lys107del) chromatographed with both HDL and free proapoA-I (26% of the radioactivity). Circular dichroism measurements showed that the alpha-helical content of lipid-bound proapoA-I (Lys107del) was reduced to 64 versus 73% of normal proapoA-I. Non-denaturing gradient gel electrophoresis of reconstituted HDL assembled with either proapoA-I(Lys107del) or normal proapoA-I showed that the mutation led to the formation of a second population of smaller rHDL particles. DMPC/proapoA-I(Lys107del) and normal DMPC/proapoA-I complexes exhibited a similar capacity to promote cholesterol efflux from fibroblasts. ProapoA-I (Lys107del) also activated LCAT similar to wild type proapoA-I and human plasma apoA-I. We conclude that deletion of Lys 107 substantially alters the lipid binding properties of the protein, which correlated with reduced binding to plasma HDL in vitro, but did not affect the capacity of the mutant/lipid complex to promote cholesterol efflux or activate LCAT.

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-1-derived amyloid in atherosclerotic plaques of the human aorta

Previous studies have shown that the amyloid localized to the aortic intima may be a biochemical entity different from other forms of localized amyloid. The amyloid fibril protein in one patient studied consisted of an N-terminal fragment of apolipoprotein A-1 (apo A-1). Since this patient was later shown to carry a missense mutation in the apo A-1 gene, leading to a deletion at position 107 of the mature protein, the question remained whether wild-type apo A-1 is amyloidogenic. In autopsy specimens from the thoracic aorta from 69 individuals, intimal atherosclerotic plaque-related amyloid was present in 11 cases (16%) and amyloid outside plaques in 37 cases (54%). The immunoreactivity of amyloid localized to the aortic intima was evaluated with the aid of antisera against N-terminal segments of apo A-1. The amyloid in association with atherosclerotic plaques was positively labelled by immunohistochemistry. The amyloid fibril protein from one patient, previously shown not to carry any mutation in the apo A-1 gene, was purified and shown by amino acid sequence analysis to be of apo A-1 nature. The result shows that wild-type apo A-1 is amyloidogenic and gives rise to a common localized form of amyloid associated with atherosclerosis.

In vivo kinetics of human apolipoprotein A-I variants in rabbits

BACKGROUND

Genetic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits.

MATERIALS AND METHODS

Apo A-I variants from heterozygous carriers of Lys-107-->0, Lys-107-->Met, Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys and the corresponding normal apo A-I were purified and then radioiodinated with 131I and 125I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve.

RESULTS

Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107-->0, and Lys-107-->Met) were catabolized faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P </= 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys) were catabolized more slowly (FCR 1.470 +/- 0.380 per day vs. 1.654 +/- 0.430 per day, P </= 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0. 90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits.

CONCLUSIONS

Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.

A novel homozygous missense mutation in the apo A-I gene with apo A-I deficiency

We analyzed the genetic defect in a 67-year-old Japanese male patient with apolipoprotein (apo) A-I and high density lipoprotein (HDL) deficiencies, corneal opacities, and coronary artery disease. The plasma concentrations of apoA-I and HDL cholesterol were 2.9 to 7.3 mg/dL and 0.08 to 0.19 mmol/L, respectively. The lecithin:cholesterol acyltransferase (LCAT) activity and cholesterol esterification rate were <40% of normal control values. LCAT mass was 550% of normal control. Sequence analysis of polymerase chain reaction-amplified DNA of the proband's apoA-I gene showed a homozygous T-to-A transition resulting in the substitution of Val 156 with Glu (apoA-I Oita). Direct sequencing of samples obtained from other family members showed that the brother was homozygous, whereas the son was a heterozygous carrier of apoA-I Oita. The heterozygote for apo A-I Oita showed nearly 60% of normal apoA-I and normal HDL cholesterol levels. In vivo turnover studies in rabbits demonstrated that the variant apoA-I was rapidly cleared from plasma compared with normal human apoA-I. Our data suggest that the Val156Glu substitution is associated with apoA-I and HDL deficiency, partial LCAT deficiency, and corneal opacities and that Val156 of apoA-I may play an important role in apoA-I function.