Evaluation of a new apolipoprotein(a) isoform-independent assay for serum Lipoprotein(a)

Supporting evidence for lipoprotein(a) measurements in clinical practice

High levels of lipoprotein(a) [Lp(a)] are causal for development of atherosclerotic cardiovascular disease and highly regulated by genetics. Levels are higher in Blacks compared to Whites, and in women compared to men. Lp(a)'s main protein components are apolipoprotein (apo) (a) and apoB100, the latter being the main component of Low-Density Lipoprotein (LDL) particles. Studies have identified Lp(a) to be associated with inflammatory, coagulation and wound healing pathways. Lack of validated and accepted assays to measure Lp(a), risk cutoff values, guidelines for diagnosis, and targeted therapies have added challenges to the field. Scientific efforts are ongoing to address these, including studies evaluating the cardiovascular benefits of decreasing Lp(a) levels with targeted apo(a) lowering treatments. This review will provide a synopsis of evidence-based effects of high Lp(a) on disease presentation, highlight available guidelines and discuss promising therapies in development. We will conclude with current clinical information and future research needs in the field.

Lp(a)-Associated Oxidized Phospholipids in Healthy Black and White Participants in Relation to apo(a) Size, Age, and Family Structure

Background

Lp(a) (lipoprotein(a)) is the major lipoprotein carrier of oxidized phospholipids (OxPL) and this function mediates Lp(a) atherogenicity. However, the relationship between OxPL, Lp(a), and genetic and biological characteristics remains poorly understood. We assessed the relationship between Lp(a)‐bound OxPL, apolipoprotein(a) (apo(a)) size, age, and family structure in 2 racial groups.

Methods and Results

Healthy Black and White families were recruited from the general population (age: 6–74 years, n=267). OxPL and Lp(a) levels were assayed enzymatically; apo(a) isoform, LPA allele sizes, and allele‐specific Lp(a) levels were determined. Lp(a)‐OxPL levels did not differ significantly by racial and age groups. Lp(a)‐OxPL levels were associated with total plasma Lp(a) in all participants and in race‐specific analyses. Further, OxPL levels were significantly associated with allele‐specific Lp(a) levels carried by the smaller apo(a) size in all participants (β=0.33, P=0.0003) as well as separately for Black (β=0.50, P=0.0032) and White (β=0.26, P=0.0181) participants. A significant association of OxPL with allele‐specific Lp(a) levels for larger apo(a) sizes was seen only in Black participants (β=0.53, P=0.0076). In this group, Lp(a)‐OxPL levels were also heritable (h²=0.29, P=0.0235), resulting in a significant interracial difference in heritability between Black and White people (P=0.0352).

Conclusions

Lp(a)‐OxPL levels were associated with allele‐specific Lp(a) level carried on smaller apo(a) sizes and among Black participants also for larger apo(a) sizes. The heritability estimates for Lp(a)‐bound OxPL differed by race.

The impact of race and ethnicity on lipoprotein(a) levels and cardiovascular risk

PURPOSE OF REVIEW

Lipoprotein(a) [Lp(a)] is a plasma circulating apoB100 (apoB) containing lipoprotein. It has a unique glycoprotein bound to the apoB100, apolipoprotein(a) [apo(a)]. The majority of the population expresses two apo(a) isoforms, when bound to apoB100 they create two circulating Lp(a) particles. Lp(a) levels are genetically determined and epidemiological studies have established elevated levels of Lp(a) to be a causal risk factor of cardiovascular disease (CVD). Lp(a) levels differ across racial groups and Blacks of Sub-Saharan decent have higher levels when compared to white. In comparison to white populations, studies in minorities are less represented in the published literature. Additionally, there is a lack of standardization in the commercial assays used to measured Lp(a) levels, and hence it is difficult to assess risk based on individual Lp(a) levels, but risk seems to occur in the upper percentiles of the population.

RECENT FINDINGS

A recent study using data from the UK biobank highlights the racial differences in Lp(a) levels and the increase risk in CVD amongst all races.

SUMMARY

This review will highlight Lp(a) biology and physiology with a focus on available data from racially diverse cohorts. There is a need to perform studies in diverse populations to understand if they are at higher risk than whites are.

PCSK9 in African Americans and Caucasians in Relation to Lp(a) Level, Apo(a) Size and Heritability

Context

Inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) reduces lipoprotein(a) [Lp(a)] levels, but the association of PCSK9 with Lp(a) level and its major determinant, apolipoprotein(a) [apo(a)] size, is not fully understood.

Objective

To assess the relationship between PCSK9, Lp(a) level, apo(a) size, age, and ethnicity/race.

Design

Cross-sectional

Setting

General population

Participants

Healthy African Americans and Caucasians (n = 267); age range: 6 to 74 years.

Interventions

None.

Main outcome measure(s)

PCSK9 levels, apo(a) isoform and LPA allele sizes, and isoform-specific Lp(a) levels.

Results

Plasma PCSK9 levels were significantly higher in African Americans vs Caucasians, in females vs males, and in adults vs children. PCSK9 levels were not associated with total plasma Lp(a) levels either in all participants or in ethnicity-specific analyses. However, PCSK9 levels were significantly positively associated with isoform-specific Lp(a) levels carried by the larger apo(a) size in all participants (r = 0.139, P = 0.0361). In ethnicity/race analyses, a significant association was seen for African Americans (r = 0.268, P = 0.0199), but not for Caucasians. In contrast, there were no significant associations of PCSK9 with isoform-specific Lp(a) levels for the smaller apo(a) sizes in all participants nor in ethnic-specific analyses. Furthermore, heritability (h²) analyses revealed a significant heritability for PCSK9 level in both ethnic groups, with a higher estimate in Caucasians than in African Americans (47% vs 22%, respectively).

Conclusions

Among African Americans, but not Caucasians, PCSK9 levels were associated with isoform-specific Lp(a) levels carried on larger, but not smaller, apo(a) sizes. The findings illustrate a diverging relationship of PCSK9 with isoform-specific Lp(a) levels across ethnicity.

Effect of antiretroviral therapy on allele-associated Lp(a) level in women with HIV in the Women's Interagency HIV Study

We previously demonstrated an association between lipoprotein (a) [Lp(a)] levels and atherosclerosis in human immunodeficiency virus (HIV)-seropositive women. The effects of antiretroviral therapy (ART) on Lp(a) levels in relation to apo(a) size polymorphism remain unclear. ART effects on allele-specific apo(a) level (ASL), an Lp(a) level associated with individual apo(a) alleles within each allele-pair, were determined in 126 HIV-seropositive women. ART effects were tested by a mixed-effects model across pre-ART and post-ART first and third visits. Data from 120 HIV-seronegative women were used. The mean age was 38 years; most were African-American (∼70%). Pre-ART ASLs associated with the larger (4.6 mg/dl vs. 8.0 mg/dl, P = 0.024) or smaller (13 mg/dl vs. 19 mg/dl, P = 0.041) apo(a) sizes were lower in the HIV-seropositive versus HIV-seronegative group, as was the prevalence of a high Lp(a) level (P = 0.013). Post-ART ASL and prevalence of high Lp(a) or apo(a) sizes and frequency of small size apo(a) (22 kringles) did not differ between the two groups. ART increased Lp(a) level (from 18 to 24 mg/dl, P < 0.0001) and both ASLs (P < 0.001). In conclusion, regardless of genetic control, Lp(a) can be modulated by HIV and its treatment. ART initiation abrogates HIV-induced suppression of Lp(a) levels and ASLs, contributing to promote CVD risk in HIV-seropositive individuals.

Lipoprotein(a) and apolipoprotein(a) in polycystic ovary syndrome

OBJECTIVE

Levels of lipoprotein(a), Lp(a), an independent risk factor for cardiovascular disease (CVD), are affected by sex hormones. Women with polycystic ovary syndrome (PCOS) have elevated androgen levels and are at increased CVD risk. We investigated the impact of PCOS-related hormonal imbalance on Lp(a) levels in relation to apo(a) gene size polymorphism, a major regulator of Lp(a) level.

DESIGN

Cross-sectional.

PATIENTS

Forty-one Caucasian women with PCOS based on the NIH criteria.

MEASUREMENTS

(1) Apo(a) gene size polymorphism measured as Kringle (K) 4 repeat number; (2) total plasma Lp(a) level; (3) allele-specific apo(a) level assessing the amount of Lp(a) carried by an individual apo(a) allele/isoform; and (4) sex hormone levels.

RESULTS

The mean age was 32 ± 6 years, and the mean BMI was 35 ± 8 with 66% of women classified as obese (BMI >30 kg/m² ). LDL cholesterol was borderline high (3·37 mmol/l), and HDL cholesterol was low (1·06 mmol/l). The distribution of Lp(a) level was skewed towards lower levels with a median level of 22·1 nmol/l (IQR: 6·2-66·5 nmol/l). Lp(a) levels were not correlated with age, body weight or BMI. The median allele-specific apo(a) level was 10·6 nmol/l (IQR: 3·1-31·2 nmol/l), and the median apo(a) size was 27 (IQR: 23-30) K4 repeats. Allele-specific apo(a) levels were significantly and inversely correlated with K4 repeats (r = -0·298, P = 0·007). Neither Lp(a) nor allele-specific apo(a) levels were significantly associated with testosterone or dehydroepiandrosterone sulphate levels.

CONCLUSIONS

The apo(a) genetic variability remains the major regulator of plasma Lp(a) levels in women with PCOS.

Rhesus monkey (Macaca mulatta) lipoprotein(a) and apolipoprotein(a): high frequency of small size apolipoprotein(a) isoforms

BACKGROUND

Levels of lipoprotein(a), Lp(a), a genetically regulated independent cardiovascular risk factor present in humans and Old World monkeys, are impacted by the apolipoprotein(a), apo(a), gene. Allele-specific apo(a) levels, taking both the apo(a) genotypic and phenotypic characteristics into account, are useful markers to determine atherosclerotic cardiovascular risk.

METHODS

We determined (i) the genetic variability of apo(a), (ii) Lp(a) levels, and (iii) allele-specific apo(a) levels in rhesus monkeys (n = 95).

RESULTS

Lp(a) levels differed substantially between animals (range: 4-247 nmol/l) with a skewed distribution toward lower levels. Lp(a) and allele-specific apo(a) levels were inversely related to the number of apo(a) Kringle 4 (K4) repeats. The median apo(a) size was 23 K4 repeats, and the prevalence of a small size apo(a) (≤22 K4) was 43%.

CONCLUSIONS

Distribution of Lp(a) and allele-specific apo(a) levels in rhesus monkeys reflected the corresponding human patterns, but with a high prevalence of smaller apo(a) sizes.

Clinically significant novel biomarkers for prediction of first ever myocardial infarction: the Tromsø Study

BACKGROUND

Identification of individuals with high risk for first-ever myocardial infarction (MI) can be improved. The objectives of the study were to survey multiple protein biomarkers for association with the 10-year risk of incident MI and identify a clinically significant risk model that adds information to current common risk models.

METHODS AND RESULTS

We used an immunoassay platform that uses a sensitive, sample-efficient molecular counting technology to measure 51 proteins in samples from the fourth survey (1994) in the Tromsø Study, a longitudinal study of men and women in Tromsø, Norway. A case control design was used with 419 first-ever MI cases (169 females/250 males) and 398 controls (244 females/154 males). Of the proteins measured, 17 were predictors of MI when considered individually after adjustment for traditional risk factors either in men, women, or both. The 6 biomarkers adjusted for traditional risk factors that were selected in a multivariable model (odds ratios [OR] per standard deviation) using a stepwise procedure were apolipoprotein B/apolipoprotein A1 ratio (1.40), kallikrein (0.73), lipoprotein a (1.29), matrix metalloproteinase 9 (1.30), the interaction term IP-10/CXCL10×women (0.69), and the interaction term thrombospondin 4×men (1.38). The composite risk of these biomarkers added significantly to the traditional risk factor model with a net reclassification improvement of 14% (P=0.0002), whereas the receiver operating characteristic area increased from 0.757 to 0.791, P=0.0004.

CONCLUSIONS

Novel protein biomarker models improve identification of 10-year MI risk above and beyond traditional risk factors with 14% better allocation to either high or low risk group.

Dramatic lowering of very high Lp(a) in response to niacin

We describe a patient with markedly elevated lipoprotein(a) (Lp(a)) without any other lipid abnormalities. After a myocardial infarction, she was treated with combination of extended-release niacin and statin. An 88% reduction in Lp(a) was observed during 5 years of treatment, which is much better response than usually reported.

HIV disease activity as a modulator of lipoprotein(a) and allele-specific apolipoprotein(a) levels

OBJECTIVE

Mechanisms underlying the cardiovascular risk of lipoprotein(a) are poorly understood. We investigated the relationship of apolipoprotein(a) (apo(a)) size, lipoprotein(a), and allele-specific apo(a) levels with HIV disease activity parameters in a biethnic population.

METHODS AND RESULTS

Lipoprotein(a) and allele-specific apo(a) levels were determined in 139 white and 168 black HIV-positive patients. Plasma HIV RNA viral load and CD4+ T-cell count were used as surrogates for disease activity. Lipoprotein(a) and allele-specific apo(a) levels were higher in blacks than whites (for both P<0.001). Apo(a) allele size distribution was similar between the 2 ethnic groups, with a median apo(a) size of 28 kringle 4 repeats. Allele-specific apo(a) levels were positively associated with CD4+ T-cell count (P=0.027) and negatively with plasma HIV RNA viral load (P<0.001). Further, allele-specific apo(a) levels associated with smaller (<28 kringle 4) atherogenic apo(a) sizes were higher in subjects with CD4+ T-cell counts of ≥350 (P=0.002).

CONCLUSIONS

Allele-specific apo(a) levels were higher in subjects with high CD4+ T-cell count or low plasma HIV RNA viral load. The findings suggest that HIV disease activity reduced allele-specific apo(a) levels. Higher allele-specific apo(a) levels associated with atherogenic small apo(a) sizes might contribute to increased cardiovascular risk in HIV-positive subjects with improved disease status.

High lipoprotein (a) levels are associated with an increased risk of retinal vein occlusion

INTRODUCTION

Retinal vein occlusion (RVO) is one of the most common retinal vascular disorders affecting ocular vessels. Few studies, with conflicting results and conducted in limited study populations, have hypothesised the role of high levels of lipoprotein (a) [Lp(a)] in the occurrence of RVO. The aim of this study was to investigate, in a large group of RVO patients, the role of such an emerging thrombophilic parameter on the pathogenesis of RVO.

MATERIALS AND METHODS

We compared 262 patients [median age: 66 years (15-88); 122 M, 140 F] with 262 age- and sex-comparable healthy subjects.

RESULTS

Circulating concentrations of Lp(a) were found to be significantly different in patients when compared to healthy subjects [189 (60-1898)mg/L vs. 119.5 (6-1216)mg/L; p<0.0001, respectively]. No significant differences were observed relating to the different types of occlusion (central or branch occlusion). In order to investigate the possible association between high Lp(a) levels and the disease we performed a logistic regression analysis. In the univariate analysis, Lp(a) levels>300mg/L were found to be associated with an increased risk of RVO (OR: 2.39, 95%CI 1.39-3.59; p<0.0001). Following this, three models of multivariate analysis were performed, firstly by adjusting for age, gender, and traditional cardiovascular risk factors, secondly for triglycerides and thirdly for homocysteine levels. In all the models, Lp(a) levels>300mg/L confirmed their role as a risk factor for RVO [first model, OR: 2.15 (95%CI 1.39-3.32), p=0.0001; second model, OR: 3.11 (95%CI 1.77-5.62), p<0.00001; third model, OR: 3.48 (95%CI 1.88-6.43), p<0.00001].

CONCLUSIONS

This study reports that, in a large population of RVO patients, high Lp(a) concentrations are significantly related to RVO, independent from other traditional and emerging risk factors, suggesting that they may play a role in its pathogenesis.

Lipoprotein(a) level as a predictor of cardiovascular disease and small apoliprotein(a) isoforms in dialysis patients: assay-related differences are important

BACKGROUND

Lipoprotein(a) assays sensitive to apolipoprotein(a) size may underestimate associations of lipoprotein(a) with cardiovascular disease (CVD) and low molecular weight (LMW) apolipoprotein(a) isoforms. This study among 629 dialysis patients compares the value of two lipoprotein(a) assays in predicting CVD events and small isoforms.

METHODS

Lipoprotein(a) level was measured by an apolipoprotein(a) size-insensitive ELISA and apolipoprotein(a) size-sensitive immunoturbidometric (IT) assay; and apolipoprotein(a) size by Western blot. Positive/negative predictive values (PPV/NPV) for small isoforms were calculated, and CVD events ascertained prospectively.

RESULTS

The ELISA assay predicted CVD more strongly [Relative Hazard, RH=1.8; p=0.045, at the 85th Lipoprotein(a) percentile] than the IT assay (RH=1.3; p=0.37). The PPV for LMW isoforms using the ELISA (Whites, 98%; Blacks, 90%) were much higher than the IT assay (Whites, 75%; Blacks, 68%). Relative to the ELISA assay values, a positive bias in the IT assay values was seen for participants with larger apolipoprotein(a) isoforms, which may explain these findings.

CONCLUSIONS

When measured by an apolipoprotein(a) size-insensitive ELISA assay, but not a size-sensitive IT assay, high lipoprotein(a) levels predict both incident CVD and LMW isoforms in dialysis patients. Clinicians ordering lipoprotein(a) levels and research studies of lipoprotein(a) should determine if an apolipoprotein(a)-size related bias is present in the assay they use.

Evaluation of a fully-automated particle-enhanced turbidimetric immunoassay for the measurement of plasma lipoprotein(a). population-based reference values in an area with low incidence of cardiovascular disease

OBJECTIVES

Lipoprotein(a) has been proposed as an independent risk factor for cardiovascular disease. This lipoprotein possesses a marked size polymorphism that makes difficult to measure accurately its concentration in plasma. The International Federation of Clinical Chemistry recently recommended to carefully evaluate new commercial methods for lipoprotein(a) measurement to discard the possible influence of lipoprotein(a) isoforms on immunoreactivity. They also recommended to perform population-based studies for different ethnic and geographic groups. Therefore, in the evaluation of a fully automated, particle-enhanced turbidimetric immunoassay for the measurement of lipoprotein(a) we have determined its reference interval in the Spanish population, an area with the lowest incidence of cardiovascular disease in Europe.

DESIGN AND METHOD

We evaluated a commercial kit of reagents calibrated against the Proposed Reference Material and determined the effect of lipoprotein(a) size polymorphism on the measurements. A population-based study was carried out in two different villages on the Mediterranean coast of Spain.

RESULTS

Imprecision at different lipoprotein(a) concentrations ranged between 3.0 and 15.4%. Recovery was 98.5 +/- 2.1. Detection limit was 4.8 nmol/L. There were no significant interferences from lipemia, jaundice, hemolysis, paraproteinemia, apolipoprotein B or plasminogen. We did not observe any effect of the lipoprotein(a) size polymorphism on the measurements. Mean (and SD) values for plasma lipoprotein(a) (n = 369) were 53.6 (65.3) nmol/L, the median was = 25.3 nmol/L and range varied between <4.8 and 356.0 nmol/L.

CONCLUSION

The present article presents an accurate and practical assay for measuring plasma lipoprotein(a) concentrations and describes its reference values in a population of Spanish Caucasians. Our results are similar to those obtained in other Caucasian populations (between 10 and 25% higher than in participants of the CARDIA study).