Structural analysis of human apolipoprotein A-I variants. Amino acid substitutions are nonrandomly distributed throughout the apolipoprotein A-I primary structure

Identification of two apolipoprotein variants, A-I Kaho (Asp 51-->Val) and A-I Lys 107 deletion

We have identified two apolipoprotein (apo) A-I variants using isoelectric focusing gel electrophoresis: apo A-I Kaho which has a relative charge of +1 compared to normal apo A-I4, and apo A-I Nanakuma2 which has a relative charge of -1. Sequence analysis of PCR-amplified DNA from the proband of apo A-I Kaho revealed a single substitution of aspartic acid (GAC) for valine (GTC) at residue 51. Sequence analysis of PCR-amplified DNA from the proband of apo A-I Nanakuma2 revealed a three-base (AAG or AGA) deletion between bases 186 and 193 from the 5' end of exon 4 that leads to deletion of Lys 106 or 107. This mutation may be the same as that of apo A-I Marburg or A-I Munster-2 reported by Rall et al. (Rall SC Jr, Weisgraber KH, Mahley RW, Ogawa Y, Fielding CJ, Utermann G, Haas J, Steinmetz A, Menzel HJ, and Assmann G, J Biol Chem, 259: 10063-10070, 1984). Because of its unique sequence between 185 and 193 from the 5' end of exon 4 of apo A-I gene, we could not define whether AAG or AGA is deleted by DNA sequencing.

HDL3 stimulates multiple signaling pathways in human skin fibroblasts

The influence of HDL3 on phospholipid breakdown was examined in human skin fibroblasts. HDL3 elicited phosphatidylcholine (PC) and phosphatidylinositol (PI) turnover and activated multiple phospholipases. In [14C]lyso-PC-labeled or [14C]choline (Cho)-labeled cells, a biphasic activation of PC-specific phospholipase D (PLD) with peak maxima 30 to 60 seconds and 5 to 7 minutes after stimulation with 20 micrograms/mL HDL3 was shown by (1) a 1.5- to 3-fold increase in Cho release, and (3) transphosphatidylation of PC to phosphatidylbutanol in the presence of 0.3% butanol. Activation of PC-specific PLD was paralleled by an activation of PC-specific phospholipase C (PLC). A significant increase in [14C]diacylglycerol (DG) was seen from 2 minutes after stimulation onward and remained for at least 2 hours. By means of butanol, the PA-phosphohydrolase (PPH) inhibitor propranolol, and the PC-PLC inhibitor D609, we demonstrated that the initial PC-derived DG formation occurred primarily by a coupled PLD/PPH pathway and that a major part of the sustained DG formation was derived directly from PC by PC-PLC. By down-regulating protein kinase C (PKC) we demonstrated that PKC activates PC-PLC and desensitizes PC-PLD at no longer incubation times. The sustained PC hydrolysis as well as HDL3-mediated PI turnover and PC resynthesis was observed on stimulation with 5 to 75 micrograms/mL HDL3, whereas the rapid activation of PC-PLD/PPH was detected only on stimulation with HDL3 at concentrations of between 10 and 75 micrograms/mL. Only the latter response could be mimicked by apolipoprotein A-I and apolipoprotein A-II proteoliposomes, and only this response was inducible by cholesterol loading. The HDL3-mediated second-messenger responses were inhibited by modification of HDL3 by tetranitromethane and could not be mimicked by protein-free liposomes. These data suggest that HDL3-induced cell signaling in human skin fibroblasts is mediated by specific protein-receptor interaction and that more than one agonist activity may be involved.

A new case of apoA-I deficiency showing codon 8 nonsense mutation of the apoA-I gene without evidence of coronary heart disease

We report a 39-year-old Japanese man with HDL and apoA-I deficiency as well as data from members of his family. Corneal opacity and a stomatocyte were found but not tonsillar hypertrophy, xanthomas, or splenomegaly. His serum HDL cholesterol, apoA-I, apoA-II, and LDL cholesterol levels were t mg/dL, < 3 mg/dL, 6 mg/dL, and 175 mg/dL, respectively. Plasma triglyceride, phospholipid, apoB, apoC-III, and apoE levels were all within normal limits. Lecithin:cholesterol acyltransferase activity was half of normal, while lipoprotein lipase and hepatic triglyceride lipase activities were within normal limits. ApoA-I deficiency was confirmed by combined isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by an immunoblotting method. We surveyed the apoA-I gene of the patient and five of his family members by direct sequencing after amplification by polymerase chain reaction and found a codon 8 nonsense mutation (TGG --> TAG, Trp --> stop) in exon 3 of the apoA-I gene. The results of a pedigree analysis by DNA sequencing and restricted fragment length polymorphism (Sty I) were consistent with an autosomal codominant trait. Coronary angiography was performed to evaluate coronary atherosclerosis, but no significant luminal narrowing was detected. An intracoronary ultrasound study showed mild intimal hyperplasia in segment 6. In summary, this is a case of apoA-I deficiency without evidence of coronary heart disease.

ApoA-IHelsinki (Lys107-->0) associated with reduced HDL cholesterol and LpA-I:A-II deficiency

A Finnish kindred with premature coronary heart disease and decreased HDL cholesterol levels was identified as having an apoA-I variant, apoA-I (Lys107-->0), caused by a 3-bp deletion of nucleotides 1396 through 1398 in exon 4 of the apoA-I gene. These subjects (n = 10) were heterozygous for this mutation. The mean serum HDL cholesterol concentration (26.7 +/- 9.7 mg/dL) of affected family members was 36%, lower than that of unaffected family members (P < .05). Mean serum apoA-I and apoA-II concentrations in heterozygotes were reduced by 18% and 22%, respectively, compared with normal family members (P < .05). In heterozygotes the mean concentration of lipoprotein containing both apoA-I and apoA-II (LpA-I:A-II) was 31% lower than in those with normal apoA-I (P < .001), while the mean level of lipoproteins containing apoA-I without apoA-II was similar in the two groups. HDL density-gradient ultracentrifugation showed a lack of HDL2 and small dense HDL3 in heterozygotes compared with unaffected family members. The HDL particle size distribution, as analyzed by nondenaturing gradient gel electrophoresis of heterozygotes, revealed one major peak at 8.0 to 9.7 nm, a minor peak at 7.8 to 8.5 nm, and an absence of HDL2b and HDL2a peaks. These latter peaks were observed in unaffected family members. Serum levels of LDL cholesterol, triglycerides, VLDL, IDL, and LDL subclasses were similar in the two groups. However, in heterozygotes the cholesterol-to-triglyceride ratios in VLDL2, LDL1, LDL3, HDL2b, HDL2a, and HDL3a were 8% to 54% lower than in unaffected family members (P < .05). Cholesteryl ester transfer protein activity in heterozygotes was reduced by 25% compared with unaffected family members (P < .05), while the plasma lecithin:cholesterol acyltransferase (LCAT) activity did not differ between heterozygotes and unaffected family members. The ability of isolated variant apoA-I to serve as a cofactor for LCAT in vitro did not differ from that of normal apoA-I. Our data are consistent with the concept that a low HDL cholesterol level in subjects heterozygous for the apoA-IHelsinki mutation (Lys107-->0) having normal LCAT activity is a consequence of decreased concentration of LpA-I:A-II particles and of a smaller size and reduced cholesterol content of HDL particles.

Screening for naturally occurring apolipoprotein A-I variants: apo A-I(delta K107) is associated with low HDL-cholesterol levels in men but not in women

Isoelectric focussing (IEF) in carrier ampholyte-generated pH gradients and hybrid isoelectric focussing (HIEF) in immobilized pH gradients under nondenaturing conditions were used in parallel to screen 5,500 plasma samples for naturally occurring variants of apolipoprotein A-I (apo A-I). The following defects were identified in four unrelated subjects heterozygous for apo A-I variants: apo A-I(delta K107)(2 x), apo A-I(K107M)(1 x), and apo A-I(E41R)(1 x). The later variant is a novel finding. Family studies did not reveal any association of apo A-I(K107M) and apo A-I(E41R) with dyslipidemia, but identified several heterozygotes for apo A-I(delta K107) who had low levels of high density lipoprotein (HDL)-cholesterol. Therefore, and since the apo A-I(delta K107) is the most frequent apo A-I variant in Germany (1: 5,000) we evaluated our data and that reported from 11 families with 32 heterozygous carriers and 30 unaffected controls. This analysis revealed that apo A-I(delta K107) is associated with lower HDL-cholesterol (-30%) and higher triglycerides (+48%) in men but not in women as compared with unaffected family members as well as with controls from the Prospective Cardiovascular Münster (PROCAM) study. Moreover, 11 of 15 male apo A-I(delta K107) heterozygotes but only 2 of 17 female apo A-I(delta K107) heterozygotes had HDL-cholesterol levels below the 20th percentile of sex-matched controls from the PROCAM study. We conclude that heterozygosity for apo A-I(delta K107) decreases HDL-cholesterol and increases triglycerides in men but not in women.

Severe familial HDL deficiency in French-Canadian kindreds. Clinical, biochemical, and molecular characterization

A decreased level of HDL cholesterol (HDL-C) is the most common lipoprotein abnormality seen in people with premature coronary artery disease (CAD). In many cases, HDL-C reduction in patients with CAD may be the result of increased apo B-containing lipoprotein production by the liver with secondary hypoalphalipoproteinemia. Primary hypoalphalipoproteinemia is seen in approximately 4% of people with CAD. We report findings in four subjects with severe familial HDL deficiency (HDL-C << 5th percentile for age and sex; 0.08 to 0.38 mmol/L) in three French-Canadian kindreds with autosomal codominant inheritance. By inclusion criteria, all four subjects had normal fasting triglycerides and none were diabetic. HDL particle size by gradient gel electrophoresis revealed small HDL particles (estimated Stokes' diameter, 8.14 to 8.30 nm). Apo AI analysis by polyacrylamide gel electrophoresis and use of isoelectrofocusing gels in affected subjects revealed normal molecular weight (28.3 kD) and normal isoelectrofocusing point but a relative increase in proapoliprotein AI, with near-normal levels of proapolipoprotein AI in plasma, suggesting normal secretion of apo AI. Quantitative Southern blot analysis of the apo AI-CIII-AIV gene cluster reveals no gene rearrangements or allele deletion. Haplotypes of the apo AI gene, determined by use of the restriction enzymes Pst I, Xmn I, and Sst I and of the apo AII gene by use of the enzyme Msp I, did not reveal segregation of the low HDL-C trait with either the apo AI or the AII gene. Sequence analysis of the promoter region of the apo AI gene reveals heterozygosity for guanine-to-adenine substitution at position 76 in two kindreds with no evidence of segregation with the low HDL trait. None of the patients had mutations of the lipoprotein lipase gene common in subjects of French-Canadian descent. Haplotype analysis of the lipoprotein lipase gene did not show segregation with the low HDL trait. Plasma lecithin: cholesterol acyltransferase (LCAT) activity was found to be within normal levels in affected subjects and in nonaffected first-degree relatives. None of the affected subjects had clinical manifestations of Tangier disease. Two of the four cases examined, both men, had severe CAD and had undergone revascularization procedures. The third is a younger brother of one of these probands and the fourth is a 30-year-old woman, and both were free of clinical CAD. However, in none of the families did the low HDL trait unequivocally cosegregate with CAD.(ABSTRACT TRUNCATED AT 400 WORDS)

Mannitol prevents methionine sulphoxidation mediated electrophoretic heterogeneity of apolipoprotein A-I

Hybrid isoelectric focusing of apolipoprotein A-I in polyacrylamide gels with immobilized pH-gradients under non-denaturing conditions resulted in the occurrence of additional bands which could prevent the specific and sensitive detection of genetic variants. Hybrid isoelectric focusing of two chromatographically distinguishable apolipoprotein A-I isoforms that differ by sulphoxidation of methionine residues, apo A-I(Met) and apo A-I(MetSO), revealed that the additional bands were caused by this post-translational modification. Several antioxidative additives and conditions were compared for their ability to prevent methionine sulphoxidation in apolipoprotein A-I. In the presence of 200 g/L mannitol in the gel, apolipoprotein A/I focused as a single band. Since methionine sulphoxidation in proteins is a general phenomenon either taking place in vivo or in vitro by isoelectric focusing, we conclude that isoelectric focusing in the presence of mannitol will improve the quality of resolution of many proteins in gels with immobilized pH-gradients.

Familial lipoprotein disorders and premature coronary artery disease

Significant risk factors for premature coronary heart disease include: (1) family history, (2) elevated low density lipoprotein (LDL) cholesterol level > or = 160 mg/dl, l, (3) decreased high density lipoprotein (HDL) cholesterol level < 35 mg/dl, l, (4) cigarette smoking, (5) high blood pressure and (6) diabetes mellitus. All of these risk factors are common in patients with premature heart disease. Common familial lipid disorders associated with premature heart disease include familial lipoprotein(a) excess, familial dyslipidemia (elevated triglycerides and decreased HDL cholesterol), familial combined hyperlipidemia (elevations of LDL cholesterol and triglycerides, and often decreased HDL cholesterol), familial hypoapobetalipoproteinemia (elevated apolipoprotein B levels), familial hypoalphalipoproteinemia (low HDL cholesterol levels), and familial hypercholesterolemia (elevated LDL cholesterol levels). All these disorders have been characterized using age and gender specific 90th and 10th percentile values from the normal population. The diagnosis and potential management of these disorders is reviewed.

Homozygous Tangier disease and cardiovascular disease

Decreased levels of plasma high density lipoprotein (HDL) cholesterol have been associated with premature cardiovascular disease (CVD). Tangier disease is an autosomal co-dominant disorder in which homozygotes have a marked deficiency of HDL cholesterol and apolipoprotein (apo) A-I levels (both < 10 mg/dl), decreased low density lipoprotein (LDL) cholesterol levels (about 40% of normal), and mild hypertriglyceridemia. Homozygotes develop cholesterol ester deposition in tonsils (orange tonsils), liver, spleen, gastrointestinal tract, lymph nodes, bone marrow, and Schwann cells. Our purpose was to assess the prevalence of CVD in Tangier disease. We reviewed published clinical information on 51 cases of homozygous Tangier disease, report 3 new cases and provide autopsy information on 3 cases. Mean (+/- S.D.) lipid values of all cases were as follows: total cholesterol 68 +/- 30 mg/dl (32% of normal), triglycerides 201 +/- 118 mg/dl (162% of normal), HDL cholesterol 3 +/- 3 mg/dl (6% of normal) and LDL cholesterol 50 +/- 38 mg/dl (37% of normal). The most common clinical finding in these subjects (n = 54) was peripheral neuropathy which was observed in 54% of cases versus < 1% of control subjects (n = 3130). CVD was observed in 20% of Tangier patients versus 5% of controls (P < 0.05), and in those that were between 35 and 65 years of age, 44% (11 of 25) had evidence of CVD (either angina, myocardial infarction or stroke) versus 6.5% in 1533 male controls and 3.2% in 1597 female controls in this age group (P < 0.01). In 9 patients who died, 2 died prior to age 20 of probable infectious diseases, 3 of documented coronary heart disease at ages 48, 64, and 72, 2 of stroke at ages 56 and 69, one of valvular heart disease, and 1 of cancer. In three autopsy cases, significant diffuse atherosclerosis was observed in one at age 64, moderate atherosclerosis and cerebral infarction in another at age 56, but no atherosclerosis was noted in the third case who died of lymphoma at age 62. In one patient with established coronary heart disease, none of the lipid lowering agents used (niacin, gemfibrozil, estrogen or lovastatin) raised HDL cholesterol levels above 5 mg/dl. However, these agents did have significant effects on lowering triglyceride and LDL cholesterol levels. Our data indicate that there may be heterogeneity in these patients with regard to CVD risk, that peripheral neuropathy is a major problem in many patients, and that CVD is a significant clinical problem in middle aged and elderly Tangier homozygotes.(ABSTRACT TRUNCATED AT 400 WORDS)

Familial lipoprotein disorders and premature coronary artery disease

Although there is consensus that lipid variables, especially lipoprotein(a), are heritable and that elevated LDL cholesterol levels should be treated, there are no clear definitions of the common familial lipid disorders associated with premature CHD (lipoprotein(a) excess, FCH, familial dyslipidemia, familial hypoalphalipoproteinemia, familial hypercholesterolemia), nor do we have clear guidelines for the treatment of most of these disorders. Implementation of therapy for elevated LDL cholesterol in familial lipid disorders often has not occurred even in the United States. Before recommendations can be made for subjects with lipoprotein(a) excess and HDL deficiency (who often have combined hyperlipidemia or hypertriglyceridemia), prospective studies documenting benefit of CHD risk reduction must be carried out in subjects with lipoprotein(a) excess and HDL deficiency. One such study is being carried out with gemfibrozil in CHD patients with HDL deficiency. Current data do justify treatment of CHD patients with lipoprotein(a) excess with niacin because niacin has been shown to lower lipoprotein(a) levels as well as lower CHD risk mortality in random CHD patients. With regard to CHD patients with or without HDL cholesterol levels less than 35 mg/dL (0.9 mmol/L), efforts should be made to optimize their lipid profile and reduce their LDL cholesterol levels to less than 100 mg/dL (2.6 mmol/L).

Familial hypoalphalipoproteinemia in premature coronary artery disease

Hypoalphalipoproteinemia (HA) is a common finding in patients with premature coronary artery disease. To characterize the common familial forms of HA, we studied 102 families of probands with premature coronary artery disease; 40 probands (39.2%) had HA. Of these, 25 had at least one first-degree relative affected with HA; 11 had familial hypertriglyceridemia with HA (FTgHA); 10 had familial combined hyperlipidemia (FCH); and 4 had familial HA (FHA) with no other lipoprotein abnormalities. In the remaining 15 families, no lipoprotein abnormalities were observed in first-degree relatives. We measured apolipoprotein (apo) A-I, B, C-III, and E levels as well as lipoprotein particle (Lp) levels of LpA-I (containing apoA-I only), LpA-I:A-II (containing both apoA-I and A-II), LpB:E, and LpB:C-III. Compared with a reference group of healthy men (n = 103) and women (n = 106), probands with familial forms of HA had lower high-density lipoprotein cholesterol levels by selection criteria. Triglyceride levels were higher in FTgHA and FCH probands than in the reference group or FHA subjects. Despite selection of FTgHA and FCH by low-density lipoprotein (LDL) cholesterol, the latter was not significantly different between the three groups and the reference group. ApoA-I levels were decreased in FCH, FHA, and FTgHA probands, and LpA-I and LpA-I:A-II were lower in FHA and FTgHA probands. ApoB levels were significantly higher in all familial HA groups compared with the reference group, being highest in FCH individuals, but not significantly higher between FCH, FTgHA, or FHA probands. LpB:E levels were higher in the FCH and FTgHA groups than in the reference group. There were no significant differences between groups for apoE, apoC-III, and LpB:C-III. LDL particle size was smaller in all three forms of FHA, which, in combination with higher apoB levels, reflects an increased number of smaller, denser LDL particles. Affected children had, on average, higher apoB and LpB:E levels than nonaffected siblings. Our data suggest that common forms of FHA in subjects with coronary artery disease represent a spectrum of overlapping disorders characterized by an increase in apoB-containing lipoproteins, especially LpB:E particles, and smaller, denser LDL particles. When using appropriate age- and gender-adjusted cutpoints, approximately half the offspring (in young adulthood) appeared to be affected.

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.

Structural and functional properties of natural and chemical variants of apolipoprotein A-I

Four isoforms of human apolipoprotein A-I (apo A-I): the normal allele product and the corresponding Lys-107 deletion mutant, and apo A-I with sulfoxidized Met-112 and Met-148 residues and the corresponding reduced form, were investigated in their lipid binding properties, structures, and abilities to activate lecithin-cholesterol acyltransferase. All apo A-I isoforms reacted completely with palmitoyloleoylphosphatidylcholine to give reconstituted high density lipoprotein (rHDL) particles with diameters of 96 A. These particles reacted with low density lipoprotein (LDL) and lecithin-cholesterol acyltransferase (LCAT) equally well, except that the Lys-107 deletion mutant was resistant to structural rearrangements in the presence of LDL. The spectral measurements revealed only minor structural differences among the free apo A-I forms or among their rHDL products, but showed a decreased stability of the Lys-107 deletion mutant and the isoform with reduced Met towards denaturation by guanidine hydrochloride. The results demonstrate that these specific alterations of the apo A-I sequence, which change the helix orientation and hydrophobic moment in one or two putative lipid binding regions, are not sufficient to disrupt the overall properties of the apo A-I complexes with lipid nor to impair significantly their ability to activate LCAT.

A mutation in apolipoprotein A-I in the Iowa type of familial amyloidotic polyneuropathy

Immunoblotting of isoelectric focusing gels of plasma and direct genomic DNA sequencing have been used to characterize a mutation in apolipoprotein A-I associated with the familial amyloidotic polyneuropathy originally described by Van Allen in an Iowa kindred. An arginine for glycine substitution in apolipoprotein A-I identified in the proband's amyloid fibrils was determined to be the result of a mutation of guanine to cytosine in the apolipoprotein A-I gene at the position corresponding to the first base of codon 26. Direct sequencing of genomic DNA of three affected individuals who died in the 1960s confirmed the inheritance of the disorder. Immunoblot analysis detected the variant apolipoprotein A-I in the proband's plasma and in several at-risk members of the kindred. In addition, allele-specific amplification by the polymerase chain reaction was used to detect carriers of the variant gene.