Supporting evidence for lipoprotein(a) measurements in clinical practice

Abstract

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.

What is lipoprotein (a)?

Lipoprotein (a) [Lp(a)] is a plasma lipid particle containing two main apolipoproteins: apolipoprotein B100 [apoB] and apolipoprotein(a) [apo(a)] [1]. It was first discovered by Kare Berg in 1963 [2]. Current evidence supports that the assembly of the Lp(a) particle occurs within the liver, including the binding of apo(a) to apoB via a disulfide bond [3]. Most individuals produce two isoforms of apo(a), as determined by the number of Kringle-IV type 2 (KIV-2) repeats, which are highly regulated by the LPA gene [4,5]. The location of the LPA gene is in close proximity to the plasminogen gene and the latter has a high degree of homology with apo(a) [6]. The proteome of isolated Lp(a) particles has been described [7] and pathway analysis of the expressed proteins link Lp(a) to wound healing, lipoprotein metabolism and complement activation. The specific role of these pathways in cardiovascular disease development and progression due to high Lp(a) levels is yet to be determined (Fig. 1).

Fig. 1.

Lipoprotein(a) current risk (A) and clinical considerations (B).

Metabolic regulation of circulating lipoprotein(a) levels: synthesis, production, and clearance

The level of circulating plasma lipoproteins is regulated by their production and clearance and, the specific pathways responsible for Lp(a) processing are still being investigated [8]. The production of Lp(a) is highly affected by the number of KIV-2 repeats and self-reported race/ethnicity [9,10]. It has been shown that levels of Lp(a) are higher in Blacks and Hispanics compared to Whites. Additionally, results from studies using low density lipoprotein cholesterol (LDL-C) and apoB lowering therapies, show that Lp(a) plasma levels decrease due combination of decreases in production and increases in the clearance of Lp(a). [11–13]. A study, where subjects received Anacetrapib, a Cholesteryl Ester Transfer Protein (CETP) inhibitor, showed that the drug lowered plasma Lp(a) levels by decreasing production [11]. Similar results were observed after apoB antisense treatment [12]. However, studies with proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor treatment, which works by upregulating the LDL receptor (LDL-R), lowered Lp(a) level in patients by increasing both fractional clearance rate and reducing production rate [13,14]. This might suggest that the LDL-R also participates in the catabolism of Lp (a) under certain conditions. There have been reports of Lp(a) being found in urine, suggesting that kidneys play a role in its excretion [15,16].

Lipoprotein(a): genetics and clinical perspective

Lp(a) levels are primarily genetically regulated [17]. The size of the apo(a) is determined by the KIV-2 copy number and is inversely related to the Lp(a) plasma levels. This copy number variance may account for the large variability (19–69%) of interindividual Lp(a) levels [18]. Genetic relationships with APOE ε2 allele have been linked to a small decreases in levels of Lp(a) [19]. Lastly, the work of various investigators [20–22], and most recent publications from Dr. Kronenberg’ s group [23], have described a large amount of single nucleotide polymorphisms (SNPs) in the LPA locus that are strongly associated with Lp(a) concentrations. The relationships of other gene associations are being investigated as reviewed in the recent Lp(a) consensus statement from the European Atherosclerosis Society (EAS) [24]. The clinical relevance of these associations is still to be confirmed. In the era of increase personalized medicine efforts, it is important to note the relationship of these common SNPs with specific patient populations. We highlight these in Table 1.

Table 1.

Genetic variations of the LPA gene by race/ethnicity.

SNP	Most prevalent populations
rs3798220	Hispanics (42.38%)
rs9457951	Blacks (32.92%)
rs10455872	Whites (14.27%)

There are associations between Lp(a) levels and ASCVD risk among different ethnicities. These racial differences affect the heritability of apo(a), with African-Americans having a lower heritability when compared to Caucasian populations [25]. In 2021, a study utilizing data from the UK Biobank highlighted higher levels of Lp(a) in Blacks compare to other racial groups [26]. Although Hispanics have not been included in these reports, Lp(a) levels have been examined in the following studies: Atherosclerosis Risk in Communities (ARIC) [27–29], Multi-Ethnic Study of Atherosclerosis (MESA) [30], the Dallas Heart Study (DHS) [10] and South Asians [31] living in the US. In all studies and racial/ethnic groups, high Lp(a) levels were linked to an increase in ASCVD. Currently, there are no guidelines for risk stratification based on race and/or sex.

A review of literature from the last decade shows that only about 25% of the studies were done in diverse populations compared to only White cohorts. Of the studies that do examine Lp(a) concentrations in underrepresented groups, the population sample sizes are much lower than sample sizes available in Whites. While larger sample sizes will not eliminate disparity, greater precision for serum levels per race/ethnicity may be achieved. Clinicians often use race as a valuable predictor for cardiovascular risk. More robust analyses of Lp(a) levels by race may yield a more quantifiable basis for cardiovascular risk stratification [32,33].

Lp(a) levels have been found to be affected by non-genetic influences such as diet, hormonal regulation, kidney disease, and chronic inflammation as mentioned in Table 2. These factors may be related to high Lp(a) levels and increased risk of disease and have been recently reviewed [24]. Briefly, Lp(a) levels have been found to be lower in individuals consuming high saturated diets as recently reviewed by Enkhmaa B et al. [34]. These diet effects have also been seen in past studies (the Delta study) [35] and in a more recent randomized diet study [36]. Mechanisms regulating levels of apoB containing particles upon consumption of a high saturated fat diet need to be further investigated given that Lp(a) contains this protein. Possibly, there could be a change in the available pool of liver apoB that binds to apo(a) during consumption of high saturated fat content, causing lower Lp(a) particles to be assembled and secreted into the circulation.

Table 2.

Non-genetic influences on Lp(a) concentration.

Influence	Increases Lp (a)	Decreases Lp (a)
Diet [34,36]	High carbohydrate and unsaturated fat diet	Low carbohydrate and high saturated fat diet
Hormonal changes [87–90]	Hypothyroidism Growth hormone Pregnancy	Hyperthyroidism Post-menopausal hormone replacement therapy
Renal disease [91]	Dialysis dependency Nephrotic syndrome
Chronic inflammation [92]	Rheumatoid arthritis Systemic lupus erythematosus (SLE) Antiphospholipid syndrome (APS)

Early reports by Mora et al. found an increase in the incidence of Type 2 Diabetes Mellitus in individuals with low Lp(a) levels [37]. These reports have been duplicated, and a meta-analysis within the recent EAS statement of all studies showed a 38% (95% CI 29–48%, P < 0.0001) higher risk for the bottom quintile (thresholds < 3–5 mg/dL) vs. top quintile of Lp(a) (thresholds > 27–55 mg/dL) with no significant heterogeneity across studies. Ongoing treatment development programs using targeted apo(a) lowering therapies to improve cardiovascular outcomes have not reported signals for increases in insulin resistance or glucose levels [38,39]. Importantly, Lp(a) assembly is linked to apoB availability in the liver, and apoB is highly regulated by insulin signaling. Further investigation into these mechanisms may provide some insight into the increase in insulin resistance with lower Lp(a) levels observed in some individuals.

Lp(a) and ASCVD and aortic valve stenosis

Lp(a) particles have been identified in human atheroma dissections [40,41], indicating it’s involvement in atherogenic mechanisms. In addition, genetic studies have provided convincing evidence that LPA is associated causally with coronary heart disease (CHD) and the development of aortic stenosis [22,42–45]. There is data supporting an association between high Lp(a) levels with myocardial infarction [46–48], aortic valve disease [49,50], stroke [51], peripheral arterial disease (PAD) and increased cardiovascular mortality [26,52,53].

Recent studies from the UK Biobank showed a linear rise in ASCVD risk with increasing Lp(a) concentrations, and specifically, these observations were seen across all racial groups [26]. More recent findings from MESA and DHS cohorts found that Lp(a) and coronary artery calcium (CAC) score were independently associated with ASCVD risk [30].

High levels of Lp(a) have also been associated with increased incidence, progression, and post-treatment recurrence of PAD [54,55]. This relationship may be driven by migration of small Lp(a) particles into the subendothelial space [56]. Lp(a) has also been shown to compete for binding of plasminogen and plasmin, generating a prothrombotic state [57]. A study published in COVID-19 patients, noted that thrombotic activity marked by elevated D-dimer was associated with high Lp(a) levels [58]. To this effort, a recent genomic analysis of the Aspirin in Reducing Events in the Elderly (ASPREE) trial revealed data which supports a link between high Lp(a) and a prothrombotic state [59]. This study used previously described Lp(a) genotypes [20,21] linked to high Lp(a) levels (rs3798220-C) and a polygenic Lp(a) risk, rather than a phenotype, as a predictor to examine the benefits of aspirin treatment. They provided data that support beneficial effects of low-dose aspirin in this genotyped high Lp(a) group [59]. However, the authors state that in clinical practice, the benefits of aspirin must be balanced with bleeding risks, which increase with age and other factors. Additional data using randomized and well-pheno-typed patient populations are needed to confirm these findings.

Lipoprotein(a) links to inflammatory pathways

Inflammatory pathways have been linked to the development and outcomes of ASCVD. Importantly, the Canakinumab Anti-inflammatory Thrombosis Outcomes Study (CANTOS) for the first time provided conclusive evidence that inflammation enhances cardiovascular disease (CVD) in humans [60]. Inter-leukins (IL-6 and IL-18), markers of the inflammatory state, are associated with CVD risk in humans [61–63], and use of Tocilizumab, an IL-6 antagonist, resulted in decreased serum Lp(a) concentrations [64]. Moreover, a post-hoc analysis of the Assessment of Clinical Effects of Cholesteryl Ester Transfer Protein Inhibition with Evacetrapib in Patients at a High Risk for Vascular Outcomes (ACCELERATE) trial, in patients with established vascular disease who were optimally treated, found that increased Lp(a) levels during treatment were significantly associated with cardiovascular death, myocardial infarction, and stroke only in individuals with high-sensitivity C-reactive protein (hsCRP) levels of 2 mg/L or higher during treatment, but not in those with levels less than 2 mg/L [65]. A more recent, large study using the MESA cohort assessed whether Lp(a)-associated ASCVD risk was modified by hsCRP in a primary prevention setting [66]. The investigators found a significant interaction between Lp(a) and hsCRP (p = 0.04). With hsCRP < 2 mg/L, no significant CVD risk was observed at any level of Lp(a) from < 50 mg/dL to > 100 mg/dL. However, with hsCRP ≥ 2 mg/L, a significant CVD risk was observed with Lp(a) of 50–99.9 mg/dL (HR: 1.36; 95% CI: 1.02–1.81) and Lp(a) ≥ 100 mg/dL (HR: 2.09; 95% CI: 1.40–3.13). Importantly, in this primary prevention cohort, isolated elevations of Lp(a) or hsCRP were not associated with increased CVD risk. However, the combination of elevated Lp(a) (≥50 mg/dL) and hsCRP (≥2 mg/L) is associated with significant CVD risk and all-cause mortality. The authors concluded that individuals with both high Lp(a) and CRP levels have greater ASCVD risk and all-cause mortality, and thus may merit closer surveillance and more aggressive ASCVD risk management. Reports from a retrospective study in COVID-19 patients, found that elevated inflammatory biomarkers, including IL-6, CRP, and procalcitonin were associated with lower Lp(a). However, elevated Lp(a) was not associated with rate of hospital death or discharge [58].

Lastly, there is strong evidence classifying oxidized phospholipids (OxPL) as a pro-atherogenic biomarkers, and Lp(a) is identified as the preferential carrier of OxPL [67,68]. These pathways of inflammation and Lp(a) have been recently reviewed [68]. Importantly, mice and humans have been used to describe the role of cholesterol and OxPL within the isolated Lp(a) particles, establishing that binding of OxPL and Lp(a) promotes atherosclerosis [69].

Measuring Lp(a) in patients: how and when?

Despite the lack of standardized assays, the data on Lp(a)’s causal link to ASCVD and multiple statements and guidelines have encouraged clinicians to measure Lp(a) levels at least once in a lifetime [53]. Although temporal variabilities in Lp(a) levels have been reported during a lifetime [70], these are not significant for individuals to be classified as having high levels in one measurement, and later in life finding these levels to be low with a second measurement. However, multiple measurements may help with risk assessment. Importantly, there are no established cut-offs that define low vs. high levels. The lack of standardization of the available assays prevents comparison of studies and a true measure of the population distribution and risk assessment based on Lp(a) levels [71–73].

Measurements of Lp(a) should be obtained in nmol/L via an assay that is isoform independent and is traceable to calibrators that are internationally accepted [53].

Various organizations have provided statements and guidelines to assist with raising awareness of Lp (a) as a causal risk for ASCVD, Table 3. These efforts have assisted clinicians with understanding the available evidence to use in clinical practice. In 2018, the NHLBI gathered a group of experts and published a statement to help the Lp(a) field move forward [74]. The experts outlined the top areas for research in the field: to define the mechanisms of Lp(a) synthesis, assembly, and clearance; to understand the mechanisms linking Lp(a) to oxidized phospholipids and inflammation that are driving ASCVD; to standardize Lp(a) measurements; to define population risk levels; to understand racial and ethnic differences in levels; and to understand the effects of lowering Lp(a) levels on cardiovascular disease outcomes. Soon after various organizations followed with their own statements [24,53,75] and guideline reports [76–78].

Table 3.

Highlights of global statements and guidelines.

Year	Publication Team	Highlight
2018	National Heart, Blood and Lung Institute (NHLBI) Workgroup on Lipoprotein(a) [93]	Outline of top areas for research and future directions to optimize Lp(a) risk assessment.
2019	National Lipid Association (NLA) Lipid Association Statement [94]	Use of Lipoprotein(a) in clinical practice.
2019	AHA/ACC/AACVPR/AAPA/ABC/ACPM/ADA/AGS/ APhA/ASPC/NLA/PCNA. 2018 Guideline on the Management of Blood Cholesterol: Executive Summary. A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines [75]	Guidance for and use of Lp(a) as a risk enhancer.
2019	European Society of Cardiology (ESC)/EAS Guidelines for the management of dyslipidemias: Lipid modification to reduce cardiovascular risk Atherosclerosis [95]	Guidelines suggesting once in a lifetime measurement of Lp(a).
2020	ESC Scientific Document Group. 2019 ESC/EAS Guidelines for the management of dyslipidemias: lipid modification to reduce cardiovascular risk [96]	Guidelines suggesting once in a lifetime measurement of Lp(a).
2020	Endocrine Society Clinical Practice Guidelines [97]	Guidelines suggesting once in a lifetime measurement of Lp(a) without repeating it later in life.
2021	Canadian Cardiovascular Society Guidelines for the Management of Dyslipidemia for the Prevention of Cardiovascular Disease in the Adult [76]	Guidelines suggesting measuring Lp(a) two times in a lifetime.
2022	AHA Lp(a) statement [53]	Rationale for targeted research efforts that can provide clinical direction for risk reduction. Encouragement for screening strategies and the need for further mechanistic and biological studies of Lp(a).
2022	EAS Scientific Statement on Lp(a) [24]	Updates on evidence for the role of Lp(a) in atherosclerotic cardiovascular disease (ASCVD) and Aortic Valve Stenosis. Clinical guidance for testing and treating elevated Lp(a) levels, and consideration of its inclusion in global risk estimation.

The latest guidelines from the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines suggest using Lp(a) levels as a risk enhancer [75]. They also indicate that relative indications for Lp(a) measurement are a family history of premature ASCVD (< 55 years of age in men; < 65 years of age in women), personal history of ASCVD not explained by major risk factors, primary severe hypercholesterolemia (LDL-C ≥190 mg/dL), or suspected familial hypercholesterolemia. It is important to recognize that patients who have a family history of elevated Lp(a) or premature ASCVD may not themselves have clinical signs of cardiovascular disease. Since 2018, various reports have aided in further interpretation of the findings and suggestions, with the most recent statement from the European Atherosclerosis Society [24], including new analysis of existing data (meta-analysis) and updates on the effects of race/ethnicity on Lp(a) levels. The authors agree with other efforts that measurement of Lp(a) should be done at least once in adults, and that a high Lp(a) concentration should be interpreted in the context of other risk factors, including absolute global cardiovascular risk, and addressed through intensified lifestyle and risk factor management. Table 4 provides an overview for using Lp(a) levels in clinical practice.

Table 4.

Lp(a) in Clinical Practice.

Relative AHA indications	ASCVD presentation	Clinical considerations
Lp(a) levels ≥ 50 mg/dL or ≥ 125nmol/L may be considered a risk enhancing factor A personal history of premature ASCVD Family history of premature ASCVD Primary severe hypercholesterolemia (LDL-C ≥ 190 mg/dl) or suspected Familial Hypercholesterolemia	Stroke Myocardial Infarction Aortic Valve Disease Peripheral Arterial Disease	Cut-off values for disease risk are not universally established Levels greater than 50 mg/dl have been shown to increase risk Levels are Higher in Women vs. Men Levels are Higher in Hispanics and Blacks vs. Whites

What to do when Lp(a) is high?

The current guidelines on management of Lp(a) levels suggest lowering all other risk factors [53,76]. In the published statement on Lp(a) from the American Heart Association (AHA), authors have provided user friendly tables to assess risk, these can aid clinicians to assess additional cardiovascular risk factors, including Lp(a) levels when/if available [53]. Some therapies that lower apoB and LDL-C also decrease Lp (a) modestly, such as niacin, CETP inhibitors, ApoB antisense. PSCK9 inhibitor showed a 25% reduction in Lp(a), and after one year of treatment reduced cardiovascular event rate. However, that could be due to the combined effect of lowering LDL-C with Lp(a). Although plasma LDL-C lowering agents, such as statins, are widely used, and have been proven to decrease ASCVD [79] in many populations, statins do not lower Lp(a) levels. In fact, they may increase Lp(a) levels [80]. Importantly, in patients with elevated LDL-C despite statin therapy, a substantial fraction of cholesterol may be carried by Lp(a) rather than LDL particles. However, the role of high or low cholesterol carried by Lp(a) particles and its link to cardiovascular disease and/or benefit has not been established.

The FDA approved lipoprotein apheresis for specific cases at significantly increased risk [81,82]. In some countries, apheresis is standard of care for individuals with high Lp(a) levels [81,83]. A list of current and new programs to lower Lp(a) is listed on Table 5. Various publications on the effects of apheresis in individuals show a benefit [84]. However, more rigorous randomized control studies are needed in this area. Diets rich in saturated fats have been linked to lower Lp(a) levels. However, the current data does not support the use of high saturated fats to lower Lp(a) plasma levels. Moreover, current data suggests that all individuals with high levels of Lp(a) should follow a heart healthy diet and lifestyle modification to lower all other cardiovascular risk factors [24,53,75].

Table 5.

Current management of high Lp(a).

A. Lowering all other risk factors with available therapies including lifestyle modifications

B. Lp(a) apheresis FDA approved for four categories:

1. Functional Hypercholesterolemic Homozygotes with LDL-C > 500 mg/dL

2. Functional Hypercholesterolemic Heterozygotes with LDL-C ≥ 300 mg/dL

3. Functional Hypercholesterolemic Heterozygotes with LDL-C ≥ 100 mg/dL and either documented coronary artery disease or documented peripheral artery disease

4. Lp(a) > 60 mg/dL and LDL-C > 100 mg/dL and with either documented coronary artery disease or documented peripheral artery disease.

C. Small interfering RNA (siRNA) and antisense technology are in Phase III of development

Drugs in development that lower Lp(a)

Over the last 5 years, there has been an increased interest in understanding Lp(a) disease presentation and its link to development of ASCVD. This has been driven by the investments being made in novel therapies that target apo(a) synthesis, using small interfering RNA (siRNA) and antisense technology in the liver, thereby lowering the production of Lp(a) [53]. Completed studies in these programs observed reductions in Lp(a) plasma levels between 71% and 97% with olpasiran, a N-acetylgalactosamine (GalNAc)-conjugated siRNA; and similar changes with the GalNAc-conjugated siRNA (SLN360, Silence Therapeutics) [85] (NCT05581303). Pelacarsen, an antisense oligonucleotide, reduced Lp(a) levels from 72% to 80% [86] (NCT04023552). The cardiovascular outcome trial with Pelacarsen is anticipated to conclude in 2025, providing long awaited results in patients with established cardiovascular disease (NCT04023552). The safety of these programs continues to be evaluated and additional therapies are also in development (NCT05565742).

Summary

This review described Lp(a), discussed the role of genetics and other factors that affect Lp(a) levels, and its role in cardiovascular disease with the emphasis on clinical implications. There is strong scientific support for measuring Lp(a) in every individual at least once and some suggest measuring it twice. Obtaining a thorough medical history in every patient will ensure correct follow-up and personalized advise from treating physicians. It is clear that in individuals with high Lp(a), current efforts should target lowering all other risk factors that increase ASCVD. Future studies and efforts should focus on understanding the racial/ethnic differences in Lp(a) levels and whether these higher levels in certain populations are linked to higher cardiovascular risk and/or additional mortality. Factors that determine disease presentation in individuals with high Lp(a) should be better defined.

Practice points.

• High Lp(a) levels are causal for ASCVD.

• Measurements of Lp(a) should be obtained in nmol/L via an assay that is isoform independent and is traceable to calibrators that are internationally accepted. However, physicians should measure Lp(a) with available assays (i.e.when levels are high all assays will report this).

• Various National and International Institutions suggest measuring Lp(a) once in a lifetime, Table 4.

• Current data suggests that all individuals with high levels of Lp(a) should follow a heart healthy diet and lifestyle modification to lower all other cardiovascular risk factors.

• Treatment for individuals with high Lp(a) levels should focus on decreasing all other risk. Current evidence supports the use of Lp(a) levels to assess ASCVD risk.

• Statins do not lower Lp(a), other apoB lowering therapies provide Lp(a) lowering.

Research agenda.

• Clinical studies are needed to help identify Lp(a) levels cut-offs that can help with risk assessments.

• Lp(a) levels are higher in Hispanics, South Asians and Blacks when compared to Whites, the mechanism underlying this is not clear. Whether these populations are at higher risk for development of ASCVD due to high Lp(a) levels is unclear.

• Outcome studies from ongoing development programs targeting apo(a), and hence lowering plasma Lp(a) levels, will provide data on cardiovascular benefits.