Enigmatic role of lipoprotein(a) in cardiovascular disease

Lipoprotein (a), [Lp(a)] has many properties in common with low-density lipoprotein, (LDL) but contains a unique protein apolipoprotein(a), linked to apolipoprotein B-100 by a single disulfide bond. There is a substantial size heterogeneity of apo(a), and generally smaller apo(a) sizes tend to correspond to higher plasma Lp(a) levels, but this relation is far from linear, underscoring the importance to assess allele-specific apo(a) levels. The presence of apo(a), a highly charged, carbohydrate-rich, hydrophilic protein may obscure key features of the LDL moiety and offer opportunities for binding to vessel wall elements. Recently, interest in Lp(a) has increased because studies over the past decade have confirmed and more robustly demonstrated a risk factor role of Lp(a) for cardiovascular disease. In particular, levels of Lp(a) carried in particles with smaller size apo(a) isoforms are associated with coronary artery disease (CAD). Other studies suggest that proinflammatory conditions may modulate risk factor properties of Lp(a). Further, Lp(a) may act as a preferential acceptor for proinflammatory oxidized phospholipids transferred from tissues or from other lipoproteins. However, at present only a limited number of agents (e.g., nicotinic acid and estrogen) has proven efficacy in lowering Lp(a) levels. Although Lp(a) has not been definitely established as a cardiovascular risk factor and no guidelines presently recommend intervention, Lp(a)-lowering therapy might offer benefits in subgroups of patients with high Lp(a) levels.     

Lipoprotein(a): a unique risk factor for cardiovascular disease

Lipoprotein(a) (Lp(a)) is present in humans and primates. It has many properties in common with low-density lipoprotein, but contains a unique protein moiety designated apo(a), which is linked to apolipoprotein B-100 by a single disulfide bond. International standards for Lp(a) measurement and optimized Lp(a) assays insensitive to isoform size are not yet widely available. Lp(a) is a risk factor for coronary artery disease, and smaller size apo(a) is associated with coronary artery disease. The physiologic role of Lp(a) is unknown.     

Sequence polymorphisms in the apolipoprotein (a) gene. Evidence for dissociation between apolipoprotein(a) size and plasma lipoprotein(a) levels.

Apolipoprotein(a) [apo(a)], an apolipoprotein unique to lipoprotein(a) [Lp(a)], is highly polymorphic in size. Previous studies have indicated that the size of the apo(a) gene tends to be inversely correlated with the plasma level of Lp(a). However, several exceptions to this general trend have been identified. Individuals with apo(a) alleles of identical size do not always have similar plasma concentrations of Lp(a). To determine if these differences in plasma Lp(a) concentrations were due to sequence variations in the apo(a) gene, we examined the sequences of apo(a) alleles in 23 individuals homozygous for same-sized apo(a) alleles. We identified four single-strand DNA conformation polymorphisms (SSCPs) in the apo(a) gene. Of the 23 homozygotes, 21 (91%) were heterozygous for at least one of the SSCPs. Analysis of a family in which a parent was homozygous for the same-sized apo(a) allele revealed that each allele, though identical size, segregated with different plasma concentrations of Lp(a). These studies indicate that the apo(a) gene is even more polymorphic in sequence than was previously appreciated, and that sequence variations at the apo(a) locus, other than the number of kringle 4 repeats, contribute to the plasma concentration of Lp(a).     

Further characterization of the plasma lipoprotein(a) distribution

The plasma lipoprotein(a) [Lp(a)] distribution in caucasians is heavily skewed to the right, with evidence of bimodality. As there is a well-described inverse relationship between apolipoprotein(a) [apo(a)] size and Lp(a) concentration, it is likely that the presence of multiple apo(a) isoforms of differing frequency has a significant impact on the final distribution of Lp(a) concentrations. We have previously described an immunoblot method for examining the relationship between apolipoprotein(a) [apo(a)] size and lipoprotein(a) [Lp(a)] mass among samples heterozygous for apo(a) size, thus eliminating confounding by null or undetected apo(a) isoforms. In the present study, this method has been applied to examine the plasma Lp(a) distribution, independent of the effects of apo(a) isoform size and frequency. Seventy subjects heterozygous for apo(a) size were studied. To take into account the inverse relationship (P <0.001) between apo(a) isoform size and Lp(a) concentration, Lp(a) data associated with each apo(a) isoform were normalized as multiples of the median Lp(a) concentration for that isoform. These apo(a) isoform-independent Lp(a) data demonstrated a strikingly multimodal distribution, with five major peaks. The relative frequencies of Lp(a) peaks 1–5 were 17.1%, 15.0%, 35.7%, 23.6%, and 8.6%, and associated median Lp(a) concentrations were 1.0, 6.2,15.0, 21.8, and 39.6 mg/dL, respectively. Multivariate analysis demonstrated that apo(a) isoform size accounted for 23% and isoform-independent Lp(a) peaks for 59.5% of the variation in Lp(a) concentration. Further investigation of the characteristics of the apo(a) isoform-independent Lp(a) distribution is warranted.     

A monoclonal-antibody-based enzyme-linked immunosorbent assay of lipoprotein(a)

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein (LDL)-like lipoprotein particle recently described as a risk factor for premature coronary heart disease, stroke, and atherosclerosis. Structurally, Lp(a) is similar to LDL in that it has comparable lipid composition and contains apolipoprotein B-100 (apo B-100). In addition, Lp(a) contains the glycoprotein apolipoprotein(a) [apo(a)], which is disulfide-linked to apo B-100. The recent awareness of a striking correlation between atherosclerosis and concentrations of Lp(a) in plasma prompted our development of an accurate quantitative assay for plasma Lp(a), a monoclonal-antibody-based enzyme-linked immunosorbent assay for Lp(a) that is shown to be sensitive, precise, and highly specific. The response to several isoforms of Lp(a) is linear, and as many as 80 samples can be quantified on one plate. This easily performed assay is suitable for use in the clinical laboratory and for screening large populations.     

Elevated plasma concentrations of lipoprotein(a) in patients with end-stage renal disease are not related to the size polymorphism of apolipoprotein(a).

Patients with terminal renal insufficiency suffer from an increased incidence of atherosclerotic diseases. Elevated plasma concentrations of lipoprotein(a) [Lp(a)] have been established as a genetically controlled risk factor for these diseases. Variable alleles at the apo(a) gene locus determine to a large extent the Lp(a) concentration in the general population. In addition, other genetic and nongenetic factors also contribute to the plasma concentrations of Lp(a). We therefore investigated Apo(a) phenotypes and Lp(a) plasma concentrations in a large group of patients with end-stage renal disease (ESRD) and in a control group. Lp(a) concentrations were significantly elevated in ESRD patients (20.1 +/- 20.3 mg/dl) as compared with the controls (12.1 +/- 15.5 mg/dl, P < 0.001). However, no difference was found in apo(a) isoform frequency between the ESRD group and the controls. Interestingly, only patients with large size apo(a) isoforms exhibited two- to fourfold elevated levels of Lp(a), whereas the small-size isoforms had similar concentrations in ESRD patients and controls. Beside elevated Lp(a) concentrations, ESRD patients had lower levels of plasma cholesterol and apolipoprotein B. These results show that elevated Lp(a) plasma levels might significantly contribute to the risk for atherosclerotic diseases in ESRD. They further indicate that nongenetic factors related to renal insufficiency or other genes beside the apo(a) structural gene locus must be responsible for the high Lp(a) levels.     

Lp(a) glycoprotein phenotypes. Inheritance and relation to Lp(a)-lipoprotein concentrations in plasma.

The Lp(a) lipoprotein represents a quantitative genetic trait. It contains two different polypeptide chains, the Lp(a) glycoprotein and apo B-100. We have demonstrated the Lp(a) glycoprotein directly in human sera by sodium dodecyl sulfate-gel electrophoresis under reducing conditions after immunoblotting using anti-Lp(a) serum and have observed inter- and intraindividual size heterogeneity of the glycoprotein with apparent molecular weights ranging from approximately 400,000-700,000 D. According to their relative mobilities compared with apo B-100 Lp(a) patterns were categorized into phenotypes F (faster than apo B-100), B (similar to apo B-100), S1, S2, S3, and S4 (all slower than apo B-100), and into the respective double-band phenotypes. Results from neuraminidase treatment of isolated Lp(a) glycoprotein indicate that the phenotypic differences do not reside in the sialic acid moiety of the glycoprotein. Family studies are compatible with the concept that Lp(a) glycoprotein phenotypes are controlled by a series of autosomal alleles (Lp[a]F, Lp[a]B, Lp[a]S1, Lp[a]S2, Lp[a]S3, Lp[a]S4, and Lp[a]0) at a single locus. Comparison of Lp(a) plasma concentrations in different phenotypes revealed a highly significant association of phenotype with concentration. Phenotypes B, S1, and S2 are associated with high and phenotypes S3 and S4 with low Lp(a) concentrations. This suggests that the same gene locus is involved in determining Lp(a) glycoprotein phenotypes and Lp(a) lipoprotein concentrations in plasma and is the first indication for structural differences underlying the quantitative genetic Lp(a)-trait.     

The inverse association of plasma lipoprotein(a) concentrations with apolipoprotein(a) isoform size is not due to differences in Lp(a) catabolism but to differences in production rate.

Lipoprotein(a) (Lp[a]) is an atherogenic lipoprotein which is similar in structure to low density lipoproteins (LDL) but contains an additional protein called apolipoprotein(a) (apo[a]). Apo(a) is highly polymorphic in size, and there is a strong inverse association between the size of the apo(a) isoform and the plasma concentration of Lp(a). We directly compared the in vivo catabolism of Lp(a) particles containing different size apo(a) isoforms to establish whether there is an effect of apo(a) isoform size on the catabolic rate of Lp(a). In the first series of studies, four normal subjects were injected with radio-labeled S1-Lp(a) and S2-Lp(a) and another four subjects were injected with radiolabeled S2-Lp(a) and S4-Lp(a). No significant differences in fractional catabolic rate were found between Lp(a) particles containing different apo(a) isoforms. To confirm that apo(a) isoform size does not influence the rate of Lp(a) catabolism, three subjects heterozygous for apo(a) were selected for preparative isolation of both Lp(a) particles. The first was a B/S3-apo(a) subject, the second a S4/S6-apo(a) subject, and the third an F/S3-apo(a) subject. From each subject, both Lp(a) particles were preparatively isolated, radiolabeled, and injected into donor subjects and normal volunteers. In all cases, the catabolic rates of the two forms of Lp(a) were not significantly different. In contrast, the allele-specific apo(a) production rates were more than twice as great for the smaller apo(a) isoforms than for the larger apo(a) isoforms. In a total of 17 studies directly comparing Lp(a) particles of different apo(a) isoform size, the mean fractional catabolic rate of the Lp(a) with smaller size apo(a) was 0.329 +/- 0.090 day-1 and of the Lp(a) with the larger size apo(a) 0.306 +/- 0.079 day-1, not significantly different. In summary, the inverse association of plasma Lp(a) concentrations with apo(a) isoform size is not due to differences in the catabolic rates of Lp(a) but rather to differences in Lp(a) production rates.     

The unique lipoprotein(a): properties and immunochemical measurement

Lipoprotein (a) [Lp(a)] represents a class of lipoprotein particles defined by the presence of apolipoprotein(a), a unique glycoprotein linked by a disulfide bond to apolipoprotein B-100 to form a single macromolecule. Apolipoprotein(a) is formed by three different structural domains having high amino acid sequence homology with plasminogen. One of the domains, called kringle 4, is present in multiple copies, the number of which varies and is genetically determined. This accounts for the size heterogeneity of apolipoprotein(a) and thus of Lp(a). Because high concentrations of Lp(a) are associated with atherosclerotic cardiovascular and cerebrovascular disease and may inhibit fibrinolysis, interest in measuring Lp(a) has increased considerably, leading to a rapid development of commercially available immunoassays for the measurement of Lp(a) in human plasma. However, the immunochemical measurement of Lp(a) has several peculiar problems in addition to those encountered by the measurements of other apolipoproteins. The major problems that need to be carefully evaluated are (a) the structural complexity and heterogeneity of Lp(a), (b) the homology of apolipoprotein(a) with plasminogen, (c) the lack of standardization of the methods, and (d) the lack of a common means of expressing the Lp(a) values.     

Genetics and metabolism of lipoprotein(a) and their clinical implications (Part 1)

The human plasma lipoprotein Lp(a) has gained considerable clinical interest as a genetically determined risk factor for atherosclerotic vascular diseases. Numerous (including prospective) studies have described a correlation between elevated Lp(a) plasma levels and coronary heart disease, stroke and peripheral atherosclerosis. Lp(a) consists of a large LDL-like particle to which the specific glycoprotein apo(a) is covalently linked. The apo(a) gene is located on chromosome 6 and belongs to a gene family including the highly homologous plasminogen. Lp(a) plasma concentrations are controlled to a large extent by the extremely polymorphic apo(a) gene. More than 30 alleles at this locus determine a size polymorphism. The size of the apo(a) isoform is inversely correlated with Lp(a) plasma concentrations, which are non-normally distributed in most populations. To a minor extent, apo(a) gene-independent effects also influence Lp(a) concentrations. These include diet, hormonal status and diseases like renal disease and familial hypercholesterolemia. The standardisation of Lp(a) quantification is still an unresolved problem due to the enormous particle heterogeneity of Lp(a) and homologies of other members of the gene family. Stability problems of Lp(a) as well as statistical pitfalls in studies with small group sizes have created conflicting results. The apo(a)/Lp(a) secretion from hepatocytes is regulated at various levels including postranslationally by apo(a) isoform-dependent prolonged retention in the endoplasmic reticulum. This mechanism can partly explain the inverse correlation between apo(a) size and plasma concentrations. According to numerous investigations, Lp(a) is assembled extracellularly from separately secreted apo(a) and LDL. The sites and mechanisms of Lp(a) removal from plasma are only poorly understood. The human kidney seems to represent a major catabolic organ for Lp(a) uptake. The underlying mechanism is rather unclear; several candidate receptors from the LDL-receptor gene family do not or poorly bind Lp(a) in vitro. Lp(a) plasma levels are elevated over controls in patients with renal diseases like nephrotic syndrome and end-stage renal disease. Following renal transplantation, Lp(a) concentrations decrease to values observed in controls matched for apo(a) type. Controversial data on Lp(a) in diabetes mellitus mainly result from insufficient sample sizes in numerous studies. Large studies and those including apo(a) phenotype analysis have come to the conclusion that Lp(a) levels are not or only moderately elevated in insulin-dependent patients. In non-insulin-dependent diabetics Lp(a) is not elevated. Several rare disorders, such as LCAT and LPL deficiency, as well as liver diseases and abetalipoproteinemia are associated with low plasma levels or lack of Lp(a).     

Lipoprotein(a) as a risk factor for atherosclerosis and thrombosis: mechanistic insights from animal models

Evidence continues to accumulate from epidemiological studies that elevated plasma concentrations of lipoprotein(a) [Lp(a)] are a risk factor for a variety of atherosclerotic and thrombotic disorders. Lp(a) is a unique lipoprotein particle consisting of a moiety identical to low-density lipoprotein to which the glycoprotein apolipoprotein(a) [apo(a)] that is homologous to plasminogen is covalently attached. These features have suggested that Lp(a) may contribute to both proatherogenic and prothrombotic/antifibrinolytic processes and in vitro studies have identified many such candidate mechanisms. Despite intensive research, however, definition of the molecular mechanisms underlying the epidemiological data has proven elusive. Moreover, an effective and well-tolerated regimen to lower Lp(a) levels has yet to be developed. The use of animal models holds great promise for resolving these questions. Establishment of animal models for Lp(a) has been hampered by the absence of this lipoprotein from common small laboratory animals. Transgenic mice and rabbits expressing human apo(a) have been developed and these have been used to: (i) examine regulation of apo(a) gene expression; (ii) study the mechanism and molecular determinants of Lp(a) assembly from LDL and apo(a); (iii) demonstrate that apo(a)/Lp(a) are indeed proatherogenic and antifibrinolytic; and (iv) identify structural domains in apo(a) that mediate its pathogenic effects. The recent construction of transgenic apo(a) rabbits is a particularly promising development in view of the excellent utility of the rabbit as a model of advanced atherosclerosis.     

Apolipoprotein(a) gene accounts for greater than 90% of the variation in plasma lipoprotein(a) concentrations.

Plasma lipoprotein(a) [Lp(a)], a low density lipoprotein particle with an attached apolipoprotein(a) [apo(a)], varies widely in concentration between individuals. These concentration differences are heritable and inversely related to the number of kringle 4 repeats in the apo(a) gene. To define the genetic determinants of plasma Lp(a) levels, plasma Lp(a) concentrations and apo(a) genotypes were examined in 48 nuclear Caucasian families. Apo(a) genotypes were determined using a newly developed pulsed-field gel electrophoresis method which distinguished 19 different genotypes at the apo(a) locus. The apo(a) gene itself was found to account for virtually all the genetic variability in plasma Lp(a) levels. This conclusion was reached by analyzing plasma Lp(a) levels in siblings who shared zero, one, or two apo(a) genes that were identical by descent (ibd). Siblings with both apo(a) alleles ibd (n = 72) have strikingly similar plasma Lp(a) levels (r = 0.95), whereas those who shared no apo(a) alleles (n = 52), had dissimilar concentrations (r = -0.23). The apo(a) gene was estimated to be responsible for 91% of the variance of plasma Lp(a) concentration. The number of kringle 4 repeats in the apo(a) gene accounted for 69% of the variation, and yet to be defined cis-acting sequences at the apo(a) locus accounted for the remaining 22% of the inter-individual variation in plasma Lp(a) levels. During the course of these studies we observed the de novo generation of a new apo(a) allele, an event that occurred once in 376 meioses.     

Molecular basis of apolipoprotein (a) isoform size heterogeneity as revealed by pulsed-field gel electrophoresis.

Lipoprotein(a) [Lp(a)] is a cholesterol-rich lipoprotein that is distinguished by its content of a glycoprotein called apolipoprotein(a) [apo(a)]. Apo(a) varies in size among individuals owing to different numbers of cysteine-rich sequences that are homologous to kringle 4 of plasminogen. The genetic basis for this variation is not understood at the genomic level. In this study we used pulsed-field gel electrophoresis and genomic blotting to identify a highly polymorphic restriction fragment from the apo(a) gene. The fragment contains multiple tandem repeats of a kringle 4-encoding sequence and varies in length from 48 to 190 kb depending on the number of kringle 4-encoding sequences. A total of 19 different alleles were identified among 102 unrelated Caucasian Americans. 94% of individuals studied had two different alleles which could be distinguished by size on pulsed-field gel electrophoresis. The degree of size heterogeneity was much greater than had been previously appreciated based on the analysis of the apparent molecular mass of the protein. The size of the apo(a) gene correlated directly with the size of the apo(a) protein, and inversely with the concentration of Lp(a) in plasma. Segregation analysis of the apo(a) gene was performed in families; siblings with identical apo(a) genotypes had similar plasma levels of Lp(a). These results suggest that in the normal population, the level of plasma Lp(a) is largely determined by alleles at the apo(a) locus.     

Electrophoretic measurement of lipoprotein(a) cholesterol in plasma with and without ultracentrifugation: comparison with an immunoturbidimetric lipoprotein(a) method

OBJECTIVES: Elevated plasma lipoprotein(a) [Lp(a)] is a significant risk factor for vascular disease. Standardization of Lp(a) mass measurement is complicated by the heterogeneity of apolipoprotein(a) [apo(a)]. We investigated whether Lp(a) cholesterol measurement, which is not influenced by apo(a) size, is a viable alternative to measuring Lp(a) mass. DESIGN AND METHODS: Plasma Lp(a) cholesterol was measured electrophoretically, with and without ultracentrifugation, and results were compared to each other and to immunoturbidimetrically measured Lp(a) mass in 470 subjects. RESULTS: Ultracentrifuged and whole plasma Lp(a) cholesterol levels demonstrated high correlation (R = 0.964). All samples with detectable (>/=2.0 mg/dl) Lp(a) cholesterol had Lp(a) mass >30 mg/dl (the clinically relevant cutpoint), while 59 samples with Lp(a) mass >30 mg/dl did not have detectable Lp(a) cholesterol. CONCLUSIONS: Lp(a) cholesterol can be measured in whole plasma without interference from VLDL lipoproteins. The relative clinical merits of measuring Lp(a) cholesterol vs. Lp(a) mass or both in combination deserves investigation.     

The susceptibility of lipoprotein(a) to copper oxidation is correlated with the susceptibility of autologous low density lipoprotein to oxidation

OBJECTIVES: Lipoprotein(a) [Lp(a)] can be oxidized by copper in vitro in a way comparable to low-density lipoprotein (LDL). We sought to determine whether the susceptibility of Lp(a) to oxidation is correlated with the susceptibility of autologous heterogeneous LDL, with apolipoprotein(a) [apo(a)] molecular size, or with both factors. DESIGN AND METHODS: We examined shifts in electrophoretic mobility of Lp(a) and LDL caused by copper oxidation in plasma samples from 81 healthy men. The effect of copper oxidation on different-sized apo(a) was also evaluated. RESULTS: There was a close correlation between the relative electrophoretic mobilities of oxidized Lp(a) and oxidized LDL in subjects, especially with small-sized apo(a) (n = 25, r = 0.72, p < 0.0001). Oxidative processes in Lp(a) resulted in the degradation of large-, but not small-sized apo(a). CONCLUSIONS: The susceptibility of Lp(a) to oxidation is correlated with that of autologous LDL. Large-sized apo(a) may be involved in the Lp(a) oxidation.     

The antifibrinolytic effect of lipoprotein(a) in heterozygous subjects is modulated by the relative concentration of each of the apolipoprotein(a) isoforms and their affinity for fibrin

Individuals heterozygous for the apolipoprotein(a) [apo(a)] trait have phenotypes combining two different lipoprotein(a) [Lp(a)] particle suspecies that are present in plasma at a different concentration. Evaluation of the ability of each of these isoforms to bind to fibrin and affect plasminogen binding is essential to assess the pathogenic role of Lp(a) in these subjects; therefore, fractions containing different ratios of Lp(a) with distinct apo(a) isoforms (e.g. B/S3, S1/S4) were prepared by density gradient ultracentrifugation of plasma, and tested. Lp(a) fractions containing mainly small apo(a) isoforms (either B or S1) showed the highest affinity for fibrin ( K _d ∼ 150 nmol L⁻¹) and the best competitor activity for plasminogen, whereas fractions containing mainly the high molecular mass isoforms (either S3 or S4) showed the lowest affinities ( K _d ≥ 500 nmol L⁻¹). An increase in K _d was observed as a function of the relative content in isoforms of high molecular mass in these fractions. This inverse relationship between affinity for fibrin and apo(a) size indicates that Lp(a) subspecies in heterozygotes may have different pathogenic potential. Thus, the antifibrinolytic effect of Lp(a) in heterozygous subjects would depend on the relative concentration of the isoform with the highest affinity for fibrin.     

Apolipoprotein(a) inhibits the conversion of Glu‐plasminogen to Lys‐plasminogen: a novel mechanism for lipoprotein(a)‐mediated inhibition of plasminogen activation

Summary. Background: Elevated plasma concentrations of lipoprotein(a) [Lp(a)] are associated with an increased risk for thrombotic disorders. Lp(a) is a unique lipoprotein consisting of a low‐density lipoprotein‐like moiety covalently linked to apolipoprotein(a) [apo(a)], a homologue of the fibrinolytic proenzyme plasminogen. Several in vitro and in vivo studies have shown that Lp(a)/apo(a) can inhibit tissue‐type plasminogen activator‐mediated plasminogen activation on fibrin surfaces, although the mechanism of inhibition by apo(a) remains controversial. Essential to fibrin clot lysis are a number of plasmin‐dependent positive feedback reactions that enhance the efficiency of plasminogen activation, including the plasmin‐mediated conversion of Glu‐plasminogen to Lys‐plasminogen. Objective: Using acid–urea gel electrophoresis to resolve the two forms of radiolabeled plasminogen, we determined whether apo(a) is able to inhibit Glu‐plasminogen to Lys‐plasminogen conversion. Methods: The assays were performed in the absence or presence of different recombinant apo(a) species, including point mutants, deletion mutants and variants that represent greater than 90% of the known apo(a) isoform sizes. Results: Apo(a) substantially suppressed Glu‐plasminogen conversion. Critical roles were identified for the kringle IV types 5–9 and kringle V; contributory roles for sequences within the amino‐terminal half of the molecule were also observed. Additionally, with the exception of the smallest naturally‐occurring isoform of apo(a), isoform size was found not to contribute to the inhibitory capacity of apo(a). Conclusion: These findings underscore a novel contribution to the understanding of Lp(a)/apo(a)‐mediated inhibition of plasminogen activation: the ability of the apo(a) component of Lp(a) to inhibit the key positive feedback step of plasmin‐mediated Glu‐plasminogen to Lys‐plasminogen conversion.     

Normal lipoprotein(a) concentrations and apolipoprotein(a) isoforms in patients with insulin-dependent diabetes mellitus

Lipoprotein(a) [Lp(a)] is an LDL particle in which apoliporotein B-100 is attached to a large plasminogen-like protein called apolipoprotein(a) [apo(a)]. Apo(a) has several genetically determined phenotypes differing in molecular weight, to which Lp(a) concentrations in plasma are inversely correlated, and plasma Lp(a) concentrations above 20-30 mg dl-1 are an independant risk factor for ischaemic heart disease (IHD). To investigate whether Lp(a) could be important for the high cardiovascular mortality rate in patients with insulin dependent diabetes mellitus (IDDM), we determined Lp(a) concentrations and phenotypes in a group of 108 men (median age 32 years) with IDDM without nephropathy. A group of 40-year-old men (n = 466) served as controls. The median Lp(a) concentration was 7.4 mg dl-1 [95% CI 4.9 to 11.7] in the diabetic patients and 6.3 mg dl-1 [95% CI 5.2 to 7.0] in controls. The Lp(a) concentration exceeded 30 mg dl-1 in 22% of IDDM patients and in 20% of controls (P = 0.13). Moreover, the distribution of apo(a) phenotypes did not differ between patients and control. Lp(a) levels and apo(a) phenotypes are thus apparently the same in IDDM patients without nephropathy and controls. These findings do not exclude the possibility that Lp(a) may be increased in patients with nephropathy in whom coronary artery disease frequently co-exist or that Lp(a) in a given concentration is more atherogenic in IDDM patients than in persons without IDDM.     

Relationship between apolipoprotein(a) phenotype, lipoprotein(a) concentration in plasma, and low density lipoprotein receptor function in a large kindred with familial hypercholesterolemia due to the pro664----leu mutation in the LDL receptor gene.

In a large kindred of 66 individuals, 22 were identified as heterozygous and 3 as homozygous for a mutation (pro664----leu) in the LDL-receptor gene that gives rise to familial hypercholesterolaemia (FH). All the heterozygotes had a raised level of plasma total cholesterol and low density lipoprotein cholesterol, but were remarkably free from premature coronary disease. Determination of apolipoprotein(a) (apo(a)) phenotype and lipoprotein(a) (Lp(a)) concentration in plasma revealed that in many instances, involving individuals with various apo(a) phenotypes, there was no difference in plasma Lp(a) concentration between an FH heterozygote and an unaffected sibling with the same apo(a) phenotype. No significant difference in Lp(a) concentration was observed between groups of FH and non-FH of the same apo(a) phenotype, although in each case the mean value for the FH group was greater than that for the non-FH group. There was also evidence for an inherited trait that markedly increased Lp(a) concentration, which did not segregate with apo(a) phenotype or the defective LDL-receptor allele. The data provide no evidence for a strong multiplicative interaction between the gene loci for apo(a) and the LDL receptor.     

Influence of apolipoprotein(a) phenotype on lipoprotein(a) quantification: Evaluation of three methods

Three commercially available assays (an enzyme-linked immunosorbent assay ELISA, an immunoradiometric assay, IRMA, and a nephelometric assay) for the determination of lipoprotein(a) [Lp(a)] were compared with respect to the dependency of these assays on the various apolipoprotein(a) [apo(a)] isoforms. Although there was a strong correlation between the three methods, a significant difference between the absolute values (mg/L) was observed (p < 0.001). Using purified Lp(a) preparations, we showed that the ELISA assay quantifies the Lp(a) concentration on a molar basis, independently of the apo(a) isoform size. The IRMA and the nephelometric assay however are apo(a) isoform size dependent and overestimate the Lp(a) concentration of large apo(a) isoforms whereas the amount of small apo(a) isoforms is underestimated. In general, the isoform dependency of the Lp(a) quantification is of limited clinical relevance. In this study, inconsistent risk assignments are made in approximately 3% of the cases, when the Lp(a) concentrations obtained with the apo(a) isoform dependent assays are compared with the isoform independent ELISA.