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Quantifying apolipoprotein(a) in the era of proteoforms and precision medicine
1Department of Clinical Chemistry and Laboratory Medicine, Leiden University Medical Center, Leiden, the Netherlands.
Insights
Lipoprotein(a) (Lp(a)) mass tests are flawed due to variable particle composition. Accurate measurement requires molar units, not mass, to reflect clinical risk for cardiovascular diseases.
Area of Science:
- Biochemistry
- Cardiovascular Medicine
- Metrology
Background:
- Lipoprotein(a) (Lp(a)) is a key risk factor for atherosclerotic cardiovascular diseases (ASCVD) and calcific aortic valve disease (CAVD).
- Lp(a) consists of an LDL-like particle with apolipoprotein (a) (apo(a)), which has variable kringle IV repeats.
- Current Lp(a) mass assays are unreliable due to inherent variability in particle composition.
Purpose of the Study:
- To investigate the determinants of variability in Lp(a) particle composition.
- To evaluate the metrological appropriateness of reporting Lp(a) in mass units.
- To advocate for accurate, molar-based quantification of Lp(a)/apo(a).
Main Methods:
- Analysis of Lp(a) composition, including apo(a) size polymorphism and post-translational modifications.
- Assessment of the impact of variable protein content on mass measurements.
- Exploration of mass spectrometry for molecular characterization and molar quantification.
Main Results:
- The number of kringle IV-2 repeats significantly affects the theoretical protein content (30-46% w/w) of Lp(a).
- This variability inherently confounds traditional mass-based Lp(a) measurements, leading to biased results.
- Mass spectrometry enables accurate, molar-based quantification of apo(a), independent of size polymorphism.
Conclusions:
- Reporting Lp(a) concentrations in mass units is metrologically inappropriate and should be discontinued.
- Accurate Lp(a)/apo(a) testing requires molar quantification for clinical utility.
- Implementing metrology principles in test development is crucial for reliable Lp(a) diagnostics.
Abstract:
Lipoprotein(a) (Lp(a)) is an independent risk factor in the development of atherosclerotic cardiovascular diseases (ASCVD) and calcific aortic valve disease (CAVD). Lp(a) is an LDL-like particle to which apolipoprotein (a) (apo(a)) is covalently bound. Apo(a) contains a variable number of kringle IV repeats, a kringle V and a protease domain. Serum/plasma Lp(a) concentrations are traditionally expressed as total particle mass in mg/L. Concern has arisen lately as flawed Lp(a) mass tests have masked its clinical utility. The determinants of variability in Lp(a) composition were investigated, including the apo(a) size polymorphism, post-translational modifications -N- and O-glycosylation- and the lipid:protein ratio. Depending on the number of kringle IV-2 repeats, the theoretical protein content of the Lp(a) particle varies between 30 and 46 (w/w) %, which inescapably confounds Lp(a) mass measurements. The authors advocate that reporting of Lp(a) particle concentrations in mass units is metrologically inappropriate and should be abandoned, as it results in systematically biased Lp(a) results. Enabling technology, such as mass spectrometry, allows unequivocal molecular characterization of the apo(a) measurand(s) and accurate quantitation of apo(a) in molar units, unaffected by apo(a) size polymorphism. To guarantee that Lp(a)/apo(a) tests are fit-for-clinical-purpose, basic metrology principles should be implemented upfront during test development.
