A Deep Dive in Production to Measurement of Lipoprotein(a)
The complexities of apolipoprotein(a) apo(a) and its association with lipoprotein(a) Lp(a), particularly focusing on the impact of genetic polymorphisms on the structure and molecular weight of apo(a) and their relation to Lp(a) concentrations and potential cardiovascular risk.Genetic Variation and Molecular Weight:
Differences in the number of kringle 4 subunits in apo(a) resulting from genetic polymorphisms can lead to variations in the molecular weight of apo(a).
Individuals can inherit different numbers of kringle 4 subtype 2 repeats from each parent, leading to the presence of both low and high molecular weight forms of apo(a) in the same individual.
The production rate of apo(a) is influenced by genetic factors determining whether the smaller, low molecular weight isoforms or the larger, high molecular weight isoforms are synthesized.
Ribosome Production and Particle Count:
In the endoplasmic reticulum, the ease of producing the smaller isoforms compared to the larger isoforms of apo(a) influences the overall concentration of Lp(a) in the plasma.
The ribosome finds it quicker and more efficient to produce the smaller, low molecular weight apo(a) isoforms than the larger, high molecular weight isoforms due to the number of repeats required.
The process of degradation is more likely for the larger apo(a) isoforms before they bind to LDL particles, resulting in lower production rates of Lp(a) particles comprising these larger isoforms.
Measurement Considerations:
Similar to the role of ApoB in cholesterol levels, the total number of Lp(a) particles is considered more critical in determining cardiovascular risk than the mass or size of individual apo(a) isoforms.
Measuring the particle count of Lp(a) is emphasized as a more meaningful metric for understanding the risk associated with high Lp(a) concentrations.
While different measurement units like milligrams per deciliter and nanomoles per liter are used in lab tests, focusing on the particle count offers valuable insights into Lp(a) concentrations and associated risk.
Lab Assays and Measurement Units:
Labs may report Lp(a) concentrations in mass (milligrams per deciliter) or molar concentration, with mass measurements focusing on the weight of the entire particle and molar concentration counting the number of atoms or particles.
Molar concentrations provide a more accurate assessment of the number of particles present, offering insights into the risk associated with high Lp(a) concentrations.
Mass Measurement vs. Molar Measurement:
Mass measurements in Lp(a) assays involve isolating the particles, including APO(a) and other associated molecules, and weighing the entire particle.
Molar measurements, on the other hand, count the number of particles by considering the molecular weight of individual components like APO(a), APO B, phospholipids, cholesterol esters, triglycerides, and other molecules on the particle.
Challenges with Conversion:
Converting mass concentrations of APO(a) to molar concentrations can be challenging due to the significant variability in APO(a) molecular weights, making it less accurate.
An average molecular weight conversion factor of 2 to 2.8 can be used as a rough estimate to convert mass concentrations to molar concentrations, but guidelines caution against this due to inaccuracies.
High Mass vs. High Particle Concentrations:
High mass concentrations in Lp(a) assays may indicate high molar concentrations, and vice versa. However, they do not always align perfectly, especially in cases of low mass measurements with high particle measurements.
Understanding the number of particles present is critical for guiding therapeutic interventions, akin to the shift from LDL cholesterol measurements to LDL particle counts using NMR technology for a more precise assessment.
Challenges with Standardization:
There is still no standardized assay that can consistently measure molar concentrations of Lp(a), leading to potential variations in results between different labs using diverse assays.
The World Health Organization has introduced an assay with five types of apolipoprotein(a) in the control, emphasizing the need for labs to match antibodies used for Lp(a) measurements to ensure consistency.
Calibrators and Assay Accuracy:
Some labs use five distinct calibrators to improve accuracy in Lp(a) testing, but variations can still occur due to the diverse isoform types present in Lp(a).
Achieving precise Lp(a) measurements can be challenging, as current immunoassays may not be super accurate in counting Lp(a) particle counts.
Test Result Variability:
Individuals may experience fluctuations in Lp(a) levels upon retesting, with reports of results being 20% lower or 10% higher.
While Lp(a) levels are primarily genetically determined, other factors like inflammatory states, acute phase reactions, hormonal changes (e.g., estrogen levels, menopause), and potentially diet can influence variations in Lp(a) levels.
Standardization Challenges:
The lack of standardized assays for Lp(a) measurements can lead to variability in results between labs, contributing to the reported fluctuations in Lp(a) levels upon retesting.
