Is Lipoprotein(a) Cholesterol a Significant Indicator of Cardiovascular Risk?

In the July 1999 issue of Clinical Chemistry, Seman et al. (1) present reference ranges and prevalence data for lipoprotein(a) cholesterol [Lp(a)-C], based on the examination of offspring and spouses of participants of the Framingham Heart study. These data will be potentially useful provided that the determination of Lp(a)-C is included in routine cardiovascular risk assessment in the future. Seman et al. (1) measured Lp(a)-C by nonspecifically trapping Lp(a) with a lectin and subsequently determining cholesterol enzymatically. They claim that their method, in contrast to immunoassays for Lp(a), was not sensitive to the genetic size polymorphism of apolipoprotein(a) [apo(a)]. This, however, has not been demonstrated conclusively. Thus, to date, there are no data available as to whether the apo(a) size polymorphism affects the binding of Lp(a) to the lectin matrix. In 1995, we published a method to measure Lp(a)-C using a two-step procedure: (a) we first removed lipoproteins with a density <1.006 kg/L by preparative ultracentrifugation; and (b) we then detected Lp(a) by electrophoresis of the infranate and subsequent cholesterol staining (2). In addition, we evaluated a commercially available agarose gel electrophoresis method that separated HDL, LDL, VLDL, and Lp(a) in one run (3)(4). Both of these methods are completely independent of the apo(a) size polymorphism, and they have the additional advantage that LDL-cholesterol (LDL-C) is already “corrected” for Lp(a)-C. Immunoassays for Lp(a) differ widely from each other regarding their sensitivity to the apo(a) genetic polymorphism. Probably the least sensitive ones are enzyme-linked immunosorbent assays that use capture antibodies against apo(a) and detection antibodies against apoB (5)(6). To increase the accuracy and comparability of Lp(a) mass assays, an IFCC standard will soon be available in which the Lp(a) concentration is expressed in mmol/L. This standard will not, however, eliminate the problem that Lp(a) mass determinations are affected by the apo(a) polymorphism because it will be used in connection with assays of varying architecture that are themselves influenced by the size of apo(a). In addition, the current cutoff values of the National Cholesterol Education Program (NCEP) guidelines already account for the bias of LDL-C by Lp(a)-C (7). The question, however, of whether quantification of Lp(a) in terms of cholesterol will be preferable over immunoassays will probably not be answered on the basis of methodical considerations only. Instead, this will depend on the ability of this marker to identify subjects at an increased risk for coronary artery disease (CAD) better than Lp(a) protein determination performed by methods independent of the apo(a) size polymorphism. From this point of view, we miss in the publication by Seman et al. (1) the discussion of our reports, in which we measured Lp(a)-C directly using electrophoretic techniques (2)(4). Using our combined ultracentrifugation and electrophoresis assay, we performed a case-control study that included 399 female and male subjects. Lp(a) mass was determined on a Behring nephelometer, using a polyclonal antibody (Incstar). This method was standardized with a commercial calibrator (Immuno) (2). In that study, both Lp(a)-C and Lp(a) mass concentrations were significantly higher in patients with CAD demonstrated by coronary angiography than in healthy controls (P <0.05). The odds ratios were 1.71 (95% confidence interval, 1.024–2.855) and 1.59 (95% confidence interval, 1.019–2.481) for Lp(a)-C and Lp(a) mass, respectively, suggesting that both methods have similar discrimination power. This was confirmed by virtually identical ROC curves. These data are very similar to the results of Seman et al. (1), who reported an adjusted odds ratio for Lp(a)-C of 2.293 in men. Interestingly, in our study, the ratio of Lp(a)-C to Lp(a) mass tended to be lower in CAD patients than in controls (P = 0.097). The nephelometric assay used in this study was sensitive to the apo(a) size polymorphism, with larger apo(a) isoforms producing higher signals than smaller ones. Cross-sectional studies have shown that CAD patients possess smaller apo(a) isoforms than healthy individuals. Because our nephelometric Lp(a) immunoassay is influenced by the size of the apo(a) antigen, this would produce an increased Lp(a)-C/Lp(a) mass ratio in CAD patients if the amount of cholesterol per particle was not different between CAD and controls. However, the opposite was observed (2). This raises the question of whether Lp(a) particles from patients with CAD contain less cholesterol molecules, similar to atherogenic “small, dense” LDL particles (8). Regarding the average percentage of cholesterol content in Lp(a) particles, the averages (29.1% and 26.7%) we found in our studies (2)(4) are in good agreement with the average (27.5%) presented by Seman et al. (1). Seman et al. now have the data to answer the interesting question of whether the composition of Lp(a) is of predictive significance because they measured Lp(a) mass in addition to Lp(a)-C in a subset of 1000 samples, using an immunoassay that is not biased by the apo(a) polymorphism. If they are able to reproduce our finding suggesting the existence of small, dense Lp(a) particles, then the simultaneous assessment of Lp(a)-C and Lp(a) mass might represent a valuable tool to assess the contribution of Lp(a) to an individual’s risk of CAD.