Apolipoprotein E2 (Arg-136→Cys), a variant of apolipoprotein E associated with late-onset dominance of type III hyperlipoproteinaemia

Type III hyperlipoproteinaemia (HLP) is usually associated with homozygosity for apolipoprotein (apo) E2 (Arg-158→Cys). We identified a 46-year-old white female with severe hyperlipidaemia and the heterozygous apo E3/2* phenotype. Typical clinical characteristics of type III HLP, i.e. palmar xanthomas (orange–yellowish discolorations of the palmar creases) and tuberoeruptive xanthomas, were present in the patient. Without therapy the patient's serum triglycerides (1.098 mg dL⁻¹), cholesterol (546 mg dL^–1), very low-density lipoprotein (VLDL) cholesterol (372 mg dL⁻¹) and the apo E concentration (25.0 mg dL⁻¹) were distinctly elevated as well as her VLDL cholesterol to serum triglyceride (TG) ratio at 0.34 (normal ratio about 0.2). Direct sequencing of polymerase chain reaction (PCR)-amplified segments of the apo ε gene identified a thymine for cytosine (C→T) exchange in the first base of codon 136 that is predictive for a Cys ( T GC) for Arg ( C GC) substitution in the encoded amino acid sequence. Two children, an 18-year-old female with the heterozygous apo E4/2* phenotype, a 25-year-old female with the heterozygous apo E3/2* phenotype and the 73-year-old father of the proband with the heterozygous apo E3/2* phenotype are also carriers of the rare mutant. The father has severe atherosclerosis and lipid values compatible with the diagnosis of type III HLP. The affected children have hyper/dyslipidaemia but as yet no clinical expression of the disease. We propose that in the analysed family this rare apo E2 (Arg-136→Cys) variant is associated with late-onset dominance of type III HLP.     

Severe type III hyperlipoproteinemia associated with unusual apolipoprotein E1 phenotype and & epsilon;1/lsquo;null genotype‡

A 60-year-old white male (KH) was diagnosed to suffer from severe type III hyperlipoproteinemia (HLP) and premature cardiovascular disease. Biochemical analysis revealed an unusual apolipoprotein (apo) E phenotype and genotype. All clinical characteristics of type III HLP were present in the patient. His very low density lipoprotein (VLDL) cholesterol to plasma triglyceride (TG) ratio was elevated at 0.97 without therapy which is unusually high (normal ratio about 0.18). By contrast his plasma apo E level was only moderately elevated (6.8 mg dl-1). The patient's apo E migrated in the apo E1 position on isoelectric focusing gels. Chemical modification with cysteamine and treatment with neuraminidase confirmed the presence of two cysteine residues in the patient's apo E and a normal sialylation pattern. Pedigree analysis suggested that the patient was a compound heterozygote with one apo epsilon 1 allele and another allele whose product did not appear in the plasma compartment ('null' allele). Direct sequencing of polymerase chain reaction (PCR) amplified segments of the apo E gene as well as restriction fragment length polymorphism (RFLP) analysis with the endonuclease Taq I identified an adenosine for guanosine (G-->A) exchange in the second base of codon 127 that is predictive for an Asp for Gly substitution in the encoded apo E amino acid sequence. This mutation is the structural basis for the apo E1 isoform identified upon isoelectric focusing. Five other family members are also carriers of the mutant apo epsilon 1 allele. Two of those were hyperlipidemic and exhibited biochemical characteristics of type III HLP. A second mutation, a deletion of a G in codon 31, is predictive for a reading frameshift that encodes for a premature stop in codon 60. Our inability to identify the product of a second apo E allele in the plasma of the patient and two other members of the KH family corresponds with the heterozygous presence of this mutation in the affected individuals. Both relatives (like the index case) had an increased VLDL cholesterol to plasma TG ratio, which indicates the presence of cholesterol-enriched VLDL particles. We propose that the single base deletion in the apo E gene which is the cause of a non-functional 'null' allele in addition to a probably dominant apo E1 (Gly127-->Asp, Arg158-->Cys) variant of late or incomplete penetrance are the primary genetic defects in this kindred leading to severe dysbetalipoproteinemia.     

No evidence for feedback inhibition of hepatic apolipoprotein B (apo B) production after extracorporeal low density lipoprotein precipitation as determined by [I-13C]leucine infusion in normal volunteers

Abstract. To determine the impact of an acute reduction of the circulating mass of apolipoprotein B (apo B) on apo B metabolism we studied six healthy male volunteers before (day 0), 1 day after (day 2), and 7 days after (day 8) an LDL apheresis treatment which reduced apo B mass by 59%. Appearance of newly synthesized apo B in plasma VLDL and LDL was studied using a primed-constant infusion of [I-¹³C]-leucine. VLDL apo B pool size and fractional VLDL apo B production rate calculated using a one-compartment model were similar on all 3 study days. Absolute VLDL apo B production was not statistically different throughout the study (19.7±12.3, 19.5 ± 7.5, 29.1 ± 17.7 mg kg^-1 day^-1). LDL apo B fractional production rate was increased on day 2 (0.38 ± 0.17, 0.68±0.08, 0.37±0.06 pools day^-1on days 0, 2, and 8; P <0.01). Absolute LDL apo B production, however, remained constant throughout the study (10.8 ± 3.3, 11.0±1.9, 10.8 ± 3.1 mg kg^-1 day^-1). We conclude that in healthy male volunteers acute reduction of the circulating apo B mass by LDL apheresis does not affect apo B metabolism significantly.     

Influence of apolipoprotein E polymorphism on apolipoprotein B-100 metabolism in normolipemic subjects.

This study examined apolipoprotein (apo) B metabolism in normolipemic subjects homozygous for the apo E2 (n = 4), apo E3 (n = 5), or apo E4 (n = 5) phenotype. Radioiodinated very low density lipoprotein (VLDL1) (ultracentrifuge flotation rate [Sf] 60-400) and VLDL2 (Sf 20-60) were injected into volunteers and the conversion of apo B was followed through intermediate density lipoprotein (IDL) to low density lipoprotein (LDL). Subjects homozygous for E3 converted approximately 50% of LVDL2 to LDL, the remainder being lost by direct catabolism. Those with the E2 phenotype produced less VLDL1, but converted more of it to VLDL2 (compared to E3 subjects). They displayed a characteristic dyslipidemia with the presence of slowly catabolized VLDL1 and VLDL2 remnants. LDL levels were low owing to increased direct catabolism of VLDL2 and IDL and a reduced efficiency of delipidation; only 25% of VLDL2 apo B was directed to LDL production. In contrast, E4 subjects converted more VLDL2 apo B to LDL than E3 subjects. About 70% of VLDL2 apo B was found in LDL; direct catabolism of VLDL and IDL was reduced as was the fractional catabolic rate of LDL (0.2 vs. 0.26 in E3 subjects). These changes in the VLDL----IDL----LDL metabolic cascade can in part be explained by alterations in hepatic LDL receptors with E2 subjects having higher and E4 subjects lower activities than those in E3 homozygotes.     

Effect of heparin-induced lipolysis on the distribution of apolipoprotein e among lipoprotein subclasses. Studies with patients deficient in hepatic triglyceride lipase and lipoprotein lipase

In normal subjects, apolipoprotein E (apo E) is present on very low density lipoproteins (VLDL) (fraction I) and on particles of a size intermediate between VLDL and low density lipoproteins (LDL) (fraction II). The major portion of apo E is, however, on particles smaller than LDL but larger than the average high density lipoproteins (HDL) (fraction III). To investigate the possible role of the vascular lipases in determining this distribution of apo E among the plasma lipoproteins, we studied subjects with primary deficiency of either hepatic lipase or of lipoprotein lipase and compared them with normal subjects. Subjects with familial hepatic triglyceride lipase deficiency (n = 2) differ markedly from normal in that fraction II is the dominant apo E-containing group of lipoproteins. When lipolysis of VLDL was enhanced in these subjects upon release of lipoprotein lipase by intravenous heparin, a shift of the apo E from VLDL into fractions II and III was observed. In contrast, apolipoproteins CII and CIII (apo CII and CIII, respectively) did not accumulate in intermediate-sized particles but were shifted markedly from triglyceride rich lipoproteins to HDL after treatment with heparin. In subjects with primary lipoprotein lipase deficiency (n = 4), apo E was confined to fractions I and III. Release of hepatic triglyceride lipase by heparin injection in these subjects produced a shift of apo E from fraction I to III with no significant increase in fraction II. This movement of apo E from large VLDL and chylomicron-sized particles occurred with little hydrolysis of triglyceride and no significant shift of apo CII or CIII into HDL from triglyceride rich lipoproteins. When both lipoprotein lipase and hepatic triglyceride lipase were released by intravenous heparin injection into normal subjects (n = 3), fraction I declined and the apo E content of fraction III increased by an equivalent amount. Either moderate or no change was noted in the intermediate sized particles (fraction II). These data strongly support the hypothesis that fraction II is the product of the action of lipoprotein lipase upon triglyceride rich lipoproteins and is highly dependent on hepatic triglyceride lipase for its further catabolism. In addition, the hydrolysis by hepatic triglyceride lipase of triglyceride rich lipoproteins in general results in a preferential loss of apo E and its transfer to a specific group of large HDL.     

A Phenocopy of Type III Dysbetalipoproteinemia Occurring in a Candidate Family for a Putative Apo E Receptor Defect

On theoretical grounds, an apo E receptor defect should be manifested by the accumulation of lipoprotein remnants that are normally cleared by this receptor and cannot be processed by the normal apo B, E receptor (LDL-receptor). Furthermore, the defect should not be selective for a specific apo E phenotype since none of the isoforms would be cleared preferentially. Our search for such an occurrence led us to the discovery, in five members of a family of ten, of a unique dyslipoproteinemia mimicking type III. As in type III, plasma levels of cholesterol, triglycerides, VLDL-cholesterol, VLDL-triglycerides and apo E, as well as the VLDL-C/TG ratio, were high. LDL-cholesterol and HDL-cholesterol tended to be low. The clearance of plasma triglycerides after a fat load was impaired. Tubero-eruptive xanthomas, arcus cor-neae and manifestations of atherosclerosis were present in some individuals.

In contrast to type III, the dyslipoproteinemia occurred in subjects bearing three different apo E phenotypes: E4/2, E4/3 and E3/2. VLDL-apo B levels were markedly increased, the VLDL-C/VLDL-B ratio was low and a double pre-β band was present on lipoprotein electrophoresis. In spite of high apo E and borderline high apo CHI plasma levels, levels of the lipoprotein particles LpCIII:B and LpE:B, which characterize type III, were not raised. Rapid weight loss or treatment with a fibrate was observed to normalize the lipoprotein profile. It is surmised that the apo E-rich lipoprotein particles accumulating in this type III phenocopy with “hyperapoprebe-talipoproteinemia” could be those that are normally cleared by an apo E receptor.     

Apolipoprotein E2-Christchurch (136 Arg----Ser). New variant of human apolipoprotein E in a patient with type III hyperlipoproteinemia.

The primary structure of apolipoprotein E (apo E) was investigated in seven type III hyperlipoproteinemic patients with the apo E-2/2 phenotype. Six of the patients had identical two-dimensional tryptic peptide maps. These differed from the normal apo E3 map by the altered mobility of a single peptide. Amino acid analysis and sequencing showed that apo E2 in these patients had a substitution of 158 Arg----Cys. The presence of this mutation in six of the seven type III patients confirms that this is the most common form of apo E2. The seventh type III patient had a unique map with a new peptide resulting from a substitution of 136 Arg----Ser. He was heterozygous for this and for the more common apo E2 (158 Arg----Cys) variant. His very low-density lipoprotein contained approximately five times more apo E2 (136 Arg----Ser) than apo E2 (158 Arg----Cys), as determined by cysteamine treatment and peptide mapping. This new apo E2 mutant thus appears to contribute significantly to the patient's hyperlipidemia.     

Type III hyperlipoproteinemic phenotype in transgenic mice expressing dysfunctional apolipoprotein E.

Transgenic mice were prepared that expressed a dysfunctional apo E variant, apo E (Arg-112, Cys-142), which is associated with dominant inheritance of type III hyperlipoproteinemia (type III HLP) in humans. Among eight founder mice, plasma apo E (Arg-112, Cys-142) levels varied 100-fold and directly correlated with plasma cholesterol and triglyceride levels. On a normal chow diet, mice expressing high levels (> 70 mg/dl) of the dysfunctional apo E had grossly elevated plasma lipids, with cholesterol levels of up to 410 mg/dl and triglyceride levels of up to 1,210 mg/dl. Upon agarose electrophoresis, plasma from these mice demonstrated beta-very low density lipoproteins (beta-VLDL). Mice expressing low (< 2.5 mg/dl) or intermediate (21 mg/dl) levels of the apo E variant had much less severe hyperlipidemia and did not have beta-VLDL. Although the transgenic mouse beta-VLDL were enriched in cholesteryl esters compared with normal mouse VLDL, they were not as cholesterol enriched as human beta-VLDL from type III HLP subjects. Transgenic mouse beta-VLDL injected into normal mice were cleared from plasma at a significantly slower rate than normal mouse VLDL, demonstrating the impaired catabolism of beta-VLDL. Thus, transgenic mice expressing high levels of the dysfunctional apo E (Arg-112, Cys-142) variant have many characteristics of the human type III HLP phenotype and appear to be a suitable animal model for this disorder.     

Bile acid metabolism in familial dysbetalipoproteinaemia: studies in subjects with the apolipoprotein E-2/2 phenotype

Bile acid kinetics and biliary lipid composition were determined in seven subjects with primary dysbetalipoproteinaemia. They were all homozygous for the apolipoprotein E isoform E-2 and six of them were hyperlipidaemic (type III hyperlipoproteinaemia). With or without hyperlipidaemia, the apo E-2/2 phenotype was associated with increased bile acid formation (mean increase compared with 32 normolipidaemic controls, 43%; P less than 0.025). The biliary lipid composition was not different from that seen in the controls. The results indicate that the uptake by the liver of apo E-containing remnant particles is of importance for the regulation of hepatic cholesterol metabolism in man. It is suggested that hepatic cholesterol synthesis is stimulated in dysbetalipoproteinaemia, and that this leads to a compensatory increase in bile acid synthesis.     

Lipoprotein lipase gene mutations D9N and N291S in four pedigrees with familial combined hyperlipidaemia

Abstract. The role of the lipoprotein lipase (LPL) gene in familial combined hyperlipidaemia (FCH) is unclear at present. We screened a group of 28 probands with familial combined hyperlipidaemia and a group of 91 population controls for two LPL gene mutations. D9N and N291S. LPL-D9N was found in two probands and one normolipidaemic population control. LPL-N291S was found in four probands and four population controls. Subsequently, two pedigrees from probands with the D9N mutation and two pedigrees from probands with the N291S mutation were studied, representing a total of 24 subjects. Both LPL gene mutations were associated with a significant effect on plasma lipids and apolipoproteins. Presence of the D9N mutation (n = 7) was associated with hypertriglyceridaemia [2.69± 1.43 (SD) mmol L^-1] and reduced plasma high-density lipoprotein cholesterol (HDL-C) concentrations (0.92± 0.21 mmol L^-1) compared with 11 non-carriers (triglyceride 1.75± 0.64 mmol L^-1; HDL-C 1.23± 0.30 mmol L^-1, P = 0.03 and P = 0.025 respectively). LPL-D9N carriers had higher diastolic blood pressures than non-carriers. LPL-N291S carriers ( n = 6) showed significantly higher (26%) apo B plasma concentrations (174± 26 mg dL^-1) than non-carriers (138± 26 mg dL^-1; P = 0.023), with normal post-heparin plasma LPL activities. Linkage analysis revealed no significant relationship between the D9N or N291S LPL gene mutations and the FCH phenotype (hypertriglyceridaemia, hypercholesterolaemia or increased apo B concentrations). It is concluded that the LPL gene did not represent the major single gene causing familial combined hyperlipidaemia in the four pedigrees studied, but that the LPL-D9N and LPL-N291S mutations had significant additional effects on lipid and apolipoprotein phenotype.     

Variable expression of familial dysbetalipoproteinemia in apolipoprotein E*2 (Lys146-->Gln) Allele carriers.

Genetic and biochemical studies were carried out in 96 relatives of six independently ascertained probands with familial dysbetalipoproteinemia (FD) carrying the APOE*2 (Lys146-->Gln) allele. Compared to noncarriers, the 40 heterozygous APOE*2 (Lys146-->Gln) allele carriers exhibited markedly increased mean levels of cholesterol and triglyceride in the very low density lipoproteins (VLDL) (1.89 +/- 0.37 vs 0.30 +/- 0.27 and 1.86 +/- 0.37 vs 0.68 +/- 0.27 mmol/liter, respectively) and plasma apolipoprotein (apo) E levels (28.1 +/- 1.6 vs 4.6 +/- 1.1 mg/dl), which is characteristic for FD. By means of a pedigree-based maximum likelihood method we calculated that carrier-status accounted for 57% and 71%, respectively, of the total variance of the ratio (VLDL + IDL)-cholesterol/plasma triglyceride and plasma apoE levels. APOE*2 (Lys146-->Gln) and APOE*3-Leiden allele carriers were found to differ significantly in: (a) plasma apoE levels, (b) in the amounts of triglycerides in the VLDL and VLDL + IDL fraction, and (c) in the amount of cholesterol in the VLDL and VLDL + IDL fraction relative to the amount of triglyceride in these fractions. In the APOE*2 (Lys146-->Gln) allele carriers the VLDL and VLDL + IDL fraction is relatively rich in triglycerides as compared with that in APOE*3-Leiden carriers. We hypothesize that these two rare mutations of apoE both lead to dominantly inherited forms of FD along different underlying metabolic defects.     

Evidence for a dominant gene that suppresses hypercholesterolemia in a family with defective low density lipoprotein receptors.

This paper describes an unusual kindred with familial hypercholesterolemia in which one-third of the relatives with a mutant LDL receptor gene have normal plasma cholesterol concentrations. The proband, a 9-yr-old boy with a plasma cholesterol value greater than 500 mg/dl, is homozygous for a point mutation that changes Ser156 to Leu in the LDL receptor. This substitution in the fourth repeat of the ligand binding domain slows the transport of the protein to the cell surface. The defective receptor cannot bind LDL, which contains apo B-100, but it does bind beta-migrating VLDL, which contains apo E in addition to apo B-100. Although the mother is heterozygous for this mutation, her LDL-cholesterol concentration is consistently in the 28th percentile for the population. Through direct examination of genomic DNA, we identified the mutant gene in heterozygous form in 17 of the mother's relatives, five of whom had normal LDL-cholesterol values. The pedigree was consistent with dominant transmission of a single gene that ameliorates or suppresses the hypercholesterolemic effect of the LDL receptor mutation. Through linkage analysis, we excluded the possibility that this suppressor gene was an allele at the LDL receptor locus. We also excluded the genes for the two ligands for the LDL receptor, apo B-100 and apo E. The existence of this putative suppressor gene may explain the occasional observation of normal LDL-cholesterol concentrations in heterozygotes for LDL receptor mutations.     

Isolation and characterization of apolipoprotein B-48 and B-100 very low density lipoproteins from type III hyperlipoproteinemic subjects.

Two major species of human apolipoprotein (apo) B have been identified, apo B-48 and apo B-100, which are the predominant forms in chylomicrons and very low density lipoproteins (VLDL), respectively. Due to defective hepatic clearance, apo B-48 containing lipoproteins accumulate in the plasma of subjects with type III hyperlipoproteinemia. In the present study, we have used immunoaffinity chromatography to separate type III VLDL into a nonretained (apo B-48 VLDL) and a retained (apo B-100 VLDL) fraction. To achieve complete separation, as determined by electrophoresis and radioimmunoassay, it was necessary to employ two different insolubilized anti-apo B-100 monoclonal antibodies because of immunochemical heterogeneity within the apo B-100 VLDL fraction. The ability to separate apo B-100 VLDL from apo B-48 VLDL shows that the two apo B species are found on different particles. The apo B-48 VLDL had an electrophoretic mobility similar to chylomicrons, whereas the apo B-100 VLDL migrated similarly to total type III VLDL. Both fractions showed a concentration of particles with diameters approximately 100 nm, with apo B-48 VLDL being somewhat more heterogeneous in particle size. The two fractions were qualitatively similar in apolipoprotein composition but apo B-48 VLDL was enriched in apo E, relative to apo B-100 VLDL. Apo B-48 VLDL was enriched in cholesterol esters and deficient in triglycerides and phospholipids when compared with apo B-100 VLDL. The existence of immunochemical heterogeneity in the apo B-100 VLDL may reflect different functional subpopulations of particles within this fraction.     

Relationships between the metabolism of high-density and very-low-density lipoproteins in man: studies of apolipoprotein kinetics and adipose tissue lipoprotein lipase activity

Abstract. In order to gain further insight into the relationship between high-density lipoprotein (HDL) metabolism and plasma triglyceride transport, measurements were made of HDL cholesterol concentration, apoprotein (apo) AI and AII metabolism, very-low-density lipoprotein (VLDL) apo B metabolism, and heparin-elutable adipose tissue lipoprotein lipase (LPL) activity in seventeen subjects with a wide range of plasma triglyceride concentrations (0.8–25 mmol/l).
The fractional catabolic rate (FCR) of VLDL apo B was directly related to LPL activity ( r =+ 0.80), providing evidence that the activity of the enzyme in adipose tissue is a determinant of the rate of lipolysis of VLDL in man. HDL cholesterol concentration was a positive function of both VLDL apo B FCR ( r =+ 0.74) and LPL activity, a finding consistent with previous evidence for the origin of a proportion of HDL cholesterol from 'surface remnants' liberated during VLDL catabolism. The FCRs of both apo AI and apo AII were inversely related to VLDL apo B FCR (AI, r = - 0.52; AII, r = - 0.69) and to LPL activity. The synthetic rate of apo AII, but not that of apo AI, was positively correlated with VLDL apo B synthesis ( r =+ 0.71). Thus, the metabolism of the major proteins of HDL in man appears to be closely associated with VLDL metabolism.     

Type III hyperlipoproteinemia associated with apolipoprotein E phenotype E3/3. Structure and genetics of an apolipoprotein E3 variant.

A family has been described in which type III hyperlipoproteinemia is associated with apo E phenotype E3/3 (Havel, R. J., L. Kotite, J. P. Kane, P. Tun, and T. Bersot. 1983. J. Clin. Invest. 72:379-387). In the current study, the structure of apo E from the propositus of this family was determined using both protein and DNA analyses. The propositus is heterozygous for two different apo E alleles, one coding for normal apo E3 and one for a previously undescribed variant apo E3 in which arginine replaces cysteine at residue 112 and cysteine replaces arginine at residue 142. Apo E gene analysis of nine other family members spanning four generations indicated that only those five members having type III hyperlipoproteinemia possess the variant apo E3. Like the propositus, all five are heterozygous for this variant, suggesting that the disorder in this family is transmitted in a dominant fashion. The variant apo E3 was defective in its ability to bind to lipoprotein receptors, and this functional defect probably contributes to the expression of type III hyperlipoproteinemia in this family.     

Impaired fatty acid metabolism in familial combined hyperlipidemia. A mechanism associating hepatic apolipoprotein B overproduction and insulin resistance.

To establish whether insulin resistance and/or postprandial fatty acid metabolism might contribute to familial combined hyperlipidemia (FCH) we have examined parameters of insulin resistance and lipid metabolism in six FCH kindreds. Probands and relatives (n = 56) were divided into three tertiles on the basis of fasting plasma triglycerides (TG). Individuals in the highest tertile (TG > 2.5 mM; n = 14) were older and had increased body mass index, systolic blood pressure, and fasting plasma insulin concentrations compared with individuals in the lowest tertile (n = 24). The former also presented with decreased HDL cholesterol and increased total plasma cholesterol, HDL-TG, and apoprotein B, E, and CIII concentrations. Insulin concentrations were positively correlated with plasma apo B, apo CIII, apo E, and TG, and inversely with HDL cholesterol. Fasting nonesterified fatty acids (NEFA) were elevated in FCH subjects compared to six unrelated controls and five subjects with familial hypertriglyceridemia. Prolonged and exaggerated postprandial plasma NEFA concentrations were found in five hypertriglyceridemic FCH probands. In FCH the X2 minor allele of the AI-CIII-AIV gene cluster was associated with increased fasting plasma TG, apo CIII, apo AI, and NEFA concentrations and decreased postheparin lipolytic activities. The clustering of risk factors associated with insulin resistance in FCH indicates a common metabolic basis for the FCH phenotype and the syndrome of insulin resistance probably mediated by an impaired fatty acid metabolism.     

Distribution of apolipoprotein(a) in the plasma from patients with lipoprotein lipase deficiency and with type III hyperlipoproteinemia. No evidence for a triglyceride-rich precursor of lipoprotein(a).

Lipoprotein(a) consists of a low-density lipoprotein containing apolipoprotein (apo) B-100 and of the genetically polymorphic apo(a). It is not known where and how lipoprotein(a) is assembled and whether there exists a precursor for lipoprotein(a). We have determined the phenotype, concentration, and distribution of apo(a) in plasma from patients with lipoprotein lipase (LPL) deficiency (type I hyperlipoproteinemia, n = 14), in apo E 2/2 homozygotes with type III hyperlipoproteinemia (n = 12) and in controls (n = 16). In the two genetic conditions, there is grossly impaired catabolic conversion of apo B-100-containing precursor lipoproteins to low-density lipoproteins. Considering apo(a) type, the plasma concentration of apo(a) was normal in type III patients but significantly reduced in LPL deficiency. Despite the defects in the catabolism of other apo B-containing lipoproteins, the distribution of apo(a) was only moderately affected in both metabolic disorders, with 66.7% (type I) and 74.7% (type III) being present as the characteristic lipoprotein(a) in the density range of 1.05-1.125 g/ml (controls 81.6%). The remainder was distributed between the triglyceride-rich lipoproteins (type I 12.4%, type III 8.5%, controls 4.7%) and the lipid-poor bottom fraction (type I 19.3%, type III 15.3%, controls 12.6%). In all conditions most apo(a) (57-88%) dissociated from the triglyceride-rich lipoproteins upon recentrifugation and was recovered as lipoprotein(a). These data suggest that lipoprotein(a) is not generated from a triglyceride-rich precursor. Lipoprotein(a) may be secreted directly into plasma or may be formed by preferential binding of secreted apo(a) to existing low-density lipoprotein.     

Familial apolipoprotein E deficiency.

A unique kindred with premature cardiovascular disease, tubo-eruptive xanthomas, and type III hyperlipoproteinemia (HLP) associated with familial apolipoprotein (apo) E deficiency was examined. Homozygotes (n = 4) had marked increases in cholesterol-rich very low density lipoproteins (VLDL) and intermediate density lipoproteins (IDL), which could be effectively lowered with diet and medication (niacin, clofibrate). Homozygotes had only trace amounts of plasma apoE, and accumulations of apoB-48 and apoA-IV in VLDL, IDL, and low density lipoproteins. Radioiodinated VLDL apoB and apoE kinetic studies revealed that the homozygous proband had markedly retarded fractional catabolism of VLDL apoB-100, apoB-48 and plasma apoE, as well as an extremely low apoE synthesis rate as compared to normals. Obligate heterozygotes (n = 10) generally had normal plasma lipids and mean plasma apoE concentrations that were 42% of normal. The data indicate that homozygous familial apoE deficiency is a cause of type III HLP, is associated with markedly decreased apoE production, and that apoE is essential for the normal catabolism of triglyceride-rich lipoprotein constituents.     

Inter-individual variability of plasma PAF-acetylhydrolase activity in ARDS patients and PAFAH genotype

Background:  Platelet activating factor (PAF), a pro-inflammatory phospholipid, stimulates cytokine secretion from polymorphonuclear leukocytes expressing the transmembrane G-protein coupled PAF receptor. Elevated PAF levels are associated with acute respiratory distress syndrome (ARDS) and sepsis severity. The pro-inflammatory effects of PAF are terminated by PAF acetylhydrolase (PAF-AH).
Objective:  We sought to determine whether allelic variants in the human PAFAH gene (Arg92His, Ile198Thr, and Ala379Val) contribute to variability in PAF-AH activity in patient plasma obtained within 72 h of ARDS diagnosis.
Results:  Plasma PAF-AH activity (mean ± SD) was higher in patients homozygous for the Arg92 allele compared to His92 allele carriers (2·21 ± 0·77 vs. 1·64 ± 0·68 U/min; P  < 0·01; n  = 31 and 21 respectively). Baseline plasma PAF-AH activity was higher among day 7 survivors vs. day 7 non-survivors (2·05 ± 0·75 vs. 1·27 ± 0·63, P  = 0·05).
Conclusion:  These data demonstrate an association between PAF-AH allelic variation, plasma activity, and outcome in ARDS.     

Erythrocyte uroporphyrinogen-I-synthase activity in β-thalassaemic patients

Abstract. The haem pathway enzyme uroporphyrino-gen-I-synthase (UPGS) was assayed in erythrocyte samples from twenty normal, twenty β-thalassaemia heterozygotic and twenty jS-thalassaemia homozygotic subjects, after partial separation of the erythrocytes according to their age. UPGS erythrocyte enzyme concentration activity was significantly higher in the young than in the old erythrocytes of normal (66·5 ± 11·8 v. 45 ± 9·5 nmol h^-1 I^-1, mean ± SD, P < 0·001) and β-thalassaemia heterozygotic subjects (70·1 ± 18·7 v. 49·8 ± 14·5 nmol h^-1 I^-1, P lt; 0·001), but not in patients with homozygous β-thalassaemia (46·0 ± 12·8 v. 44·1 ± 12·5 nmol h^-1 I^-1, P = 0·65). Furthermore, UPGS enzyme concentration of both young and old erythrocytes of homozygous β-thalassaemia was significantly lower than that of the young ( P < 0·001) but similar to that of the old ( P > 0·2) erythrocytes of either normal or β-thalassaemia heterozygotic subjects. Since severe chronic haemolysis due to haemoglobinopathies is associated with increased UPGS enzyme concentration, these results suggest that UPGS activity may be suppressed in homozygous β-thalassaemia.