Biochemistry, Apolipoprotein B

Fundamentals

ApoB-100-carrying particles such as LDL and lipoprotein (a) predispose to premature atherosclerotic cardiovascular disease.

Issues of Concern

Measurement

Currently available methods for apoB measurement include automated immunoassays. Reference materials have enabled the standardization of apoB measurements. Bias and imprecision were generally below 5% for 22immunonephelometric and immunoturbidimetric assays.[2] An additional advantage of measuring apoB rather than obtaining the standard lipid profile is that a fasting specimen is not required. However, despite the availability of accurate and precise methods for apoB measurement, the test is not as widely available or routinely used as traditional lipid profile tests, such as total cholesterol and low-density lipoprotein cholesterol (LDL-C) measurement or estimation. Limited use of apoB testing may reflect the cost of testing, the lack of immunoassay platforms in some clinical laboratories compared with chemistry platforms, the absence of consensus guidelines requiring apoB measurement, and the need for additional patient and clinician education regarding the utility of the test and commonly accepted target values.

Interest has recently increased in the role of remnant particles in atherosclerotic cardiovascular disease (ASCVD). Measurement of apoB-48 provides a reliable measure of chylomicron remnants. Apolipoprotein A (apoA) acts as a surrogate marker of the plasma concentration of high-density lipoprotein, and results from the INTERHEART study supported the utility of the apoB ratio for predicting cardiovascular disease. However, given the controversy regarding the role of high-density lipoprotein in preventing atherosclerosis, there is no strong indication for apoA measurement.

Non–high-density lipoprotein cholesterol (non-HDL-C), calculated as total cholesterol minus high-density lipoprotein cholesterol, is strongly supported as a predictor of cardiovascular disease and provides information similar to that obtained with apoB measurement. However, although the 2 measures are correlated, discordance has been noted, particularly in patients with dyslipoproteinemias; therefore, the measures should not be considered equivalent. Nonetheless, when LDL-C measurement is unreliable, such as in patients with hypertriglyceridemia, non-HDL-C provides a valuable adjunct for monitoring treatment.

Molecular Level

The APOB gene, located on the short arm of chromosome 2, consists of 29 exons and encodes both apoB-100, which contains 4536 amino acids and has a molecular mass of 550 kDa, and apoB-48, which has a molecular mass of 265 kDa and is approximately half the length of apoB-100. ApoB-48 is formed in the small intestine through a unique messenger RNA editing process involving highly specific posttranscriptional cytidine deamination by the apoB messenger RNA–editing enzyme apolipoprotein B messenger RNA editing enzyme catalytic subunit 1 (APOBEC1).[2]

Function

Apolipoprotein B-100 is the ligand on LDL that binds to the apolipoprotein B and apolipoprotein E receptor, also known as the LDL receptor. This binding promotes receptor-mediated catabolism of LDL. Each LDL particle contains a single molecule of apolipoprotein B-100.

Testing

Immunonephelometry is the most common method used to measure apolipoprotein B. Apolipoprotein B assays are standardized and calibrated using World Health Organization–certified reference materials. However, American Heart Association and American College of Cardiology guidelines do not recommend apolipoprotein B as a treatment target.

Pathophysiology

Familial defective apolipoprotein B is an autosomal dominant disorder caused by an APOB gene variant that impairs the binding of apolipoprotein B-100 to the low-density lipoprotein receptor. The disorder produces a clinical phenotype similar to classic familial hypercholesterolemia caused by low-density lipoprotein receptor variants, including elevated low-density lipoprotein cholesterol levels, xanthomas, and premature atherosclerotic cardiovascular disease. These manifestations underscore the importance of apolipoprotein B in atherogenesis. The most frequent variant is apolipoprotein B-3500, caused by a point mutation that substitutes glutamine for arginine at position 3500.[1] This ligand defect illustrates how impaired receptor binding can result in premature atherosclerotic cardiovascular disease.

Clinical Significance

The assessment of cardiovascular disease has, for the last several decades, been based on findings from the Framingham Heart Study cohorts, with an emphasis on total cholesterol and low-density lipoprotein cholesterol (LDL-C).[1][3] Apolipoprotein B (apoB) is a component of all atherogenic or potentially atherogenic particles, including small very low-density lipoprotein, intermediate-density lipoprotein, LDL, and lipoprotein(a), and each particle contains 1 apoB molecule. Therefore, apoB measurement directly reflects the number of atherogenic lipoprotein particles in circulation.

Apolipoprotein B–containing lipoproteins play a crucial role in atherogenesis, including the promotion of arterial plaque formation. Results from many recent studies demonstrated that elevated apoB levels predicted cardiovascular disease risk more accurately than the aforementioned traditional markers. Because each hepatically derived lipoprotein particle contains 1 apoB molecule, apoB measurement directly indicates the number of circulating atherogenic particles. Additionally, results from some studies showed that apoB measurement improved cardiovascular disease risk prediction in patients with diabetes or metabolic syndrome.[4] Beyond its role as a cardiovascular disease risk marker, apoB measurement may help monitor residual risk after the initiation of statin treatment.[2][3] However, standardized apoB treatment goals comparable to those established for LDL-C remain necessary.

Apolipoprotein B also serves as a ligand for low-density lipoprotein receptor–mediated clearance. Mutations affecting apoB, such as those causing familial defective apolipoprotein B, result in a phenotype resembling familial hypercholesterolemia. Conversely, familial hypobetalipoproteinemia is an inherited disorder associated with low low-density lipoprotein cholesterol levels and results from mutations in the APOB gene. Although the disorder may have no apparent clinical sequelae, vitamin E deficiency can develop; therefore, supplementation is generally recommended.

A promising new area in lipid-lowering treatment involves antisense oligonucleotides targeting the messenger RNA of proteins involved in cholesterol metabolism. Results from initial trials of apoB antisense oligonucleotides showed promise, with a 50% reduction in apoB levels, a 30% reduction in low-density lipoprotein cholesterol levels, and reduced cardiovascular risk. However, these agents can cause hepatic dysfunction. The 2018 cholesterol guideline identified an apoB level greater than 130 mg/dL as a risk-enhancing factor and recommended considering its measurement in selected patients undergoing primary prevention risk assessment, representing an important step toward incorporating apoB into cardiovascular risk evaluation.[5]