Biochemistry, Apolipoprotein B
Lipids such as cholesterol are insoluble in plasma, and for delivery to tissues such as the adrenal gland, gonads, etc., they have to be packaged into lipoproteins with cholesterol esters and triglycerides in the core and phospholipids, free cholesterol, and apolipoproteins on the surface. Such is the composition of the lipoproteins. Apolipoprotein B (ApoB) is the primary apolipoprotein and is the carrier for the following lipids: chylomicrons, low-density lipoprotein ( LDL), very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and lipoprotein (a). ApoB is not found in high-density lipoproteins (HDL). The latter are reconstituted into lipoproteins with Apo A. Hepatic ApoB has a molecular mass of 540000 Da. There are two circulating forms of Apo B, Apo B48 (from the small intestine) and Apo B100 (from the liver). Intestinal ApoB, which is present in chylomicrons, has a molecular mass of 48% of that of hepatic ApoB. Hence, hepatic ApoB is termed ApoB100, and intestinal Apo B as apoB 48. The same gene codes for both ApoB48 and ApoB 100. Apo B-100 contains 4536 amino acids and is necessary for the assembly of VLDL in the liver and also serves as the primary ligand for LDL receptor-mediated clearance of LDL particles from the blood. Apo B-48 has 2512 amino acids and is essential for the formation of chylomicrons, and serves in the absorption of dietary fats from the intestine.
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ApoB-100 carrying particles such as LDL and lipoprotein (a) predispose to premature atherosclerotic disease (ASCVD).
Measurement: Currently available methods for apo B measurement include automated immunoassays. Reference material is available that has allowed for the standardization of Apo B measurements. The bias and imprecision for 22 immunonephelometric and immunoturbidimetric assays ranged were usually below 5%. An additional advantage to measuring Apo B as opposed to the standard lipid profile is that a fasting specimen is not required. However, despite the availability of accurate and precise methods for Apo B measurement, the test is not as widely available or as routinely utilized as the traditional tests of the lipid profile, such as total cholesterol and LDL-C measurement/estimation. This could be due to the cost of testing, unavailability of immunoassay platforms in all clinical laboratories compared to chemistry platforms, lack of consensus guidelines that make Apo B measurements mandatory, and it may be that further patient and physician education is necessary with regards to the utility of the test as well as the availability of common target goals.
There is much recent interest in the role of remnant particles in ASCVD. Assaying Apo B48 serves as a reliable measure of chylomicron remnants.
Apolipoprotein A acts as a surrogate marker of HDL plasma concentration, and evidence of the utility of the ApoB: Apo A ratio as a predictor of cardiovascular disease (CVD) has been reported, especially in the INTERHEART study. However, given the controversy about the role of HDL in preventing atherosclerosis, there is no strong indication to measure apoA.
The estimated non-HDL-C, i.e., total cholesterol minus HDL cholesterol, has received strong backing as a predictor of CVD providing similar information as ApoB measurement. However, while there is a correlation between the two measures, discordance has been noted, particularly in patients with dyslipoproteinemias; thus, they should not be considered equivalent measures. Nonetheless, when measurement of LDL-C is unreliable, as with hypertriglyceridemia, non-HDL-cholesterol is a valuable adjunct to monitoring therapy.
The gene coding for apo B is located on the short arm of chromosome 2 and consists of 29 exons. This gene codes for both apo B-100, with 4536 amino acids (550 kDa), as well as for apo B-48 (265 kDa) - the latter is only about half the length of the native apo B-100 molecule. The formation of Apo B-48 occurs in the small intestine occurs through a unique mRNA editing process, a highly specific post-transcriptional cytidine deamination using an Apo B mRNA editing enzyme called apobec-1.
Apolipoprotein B-100 is the protein determinant on LDL that recognizes the apoB/E (LDL) receptor; this recognition of the receptor results in the receptor-mediated catabolism of LDL. There is a single molecule of Apo B per LDL particle.
Immunonephelometry is the most common means for measurement of apolipoprotein B. Tests for ApoB are now standardized and calibrated to the World Health Organization certified reference materials. However, the AHA/ACC guidelines do not recommend this as a target for treatment.
Familial defective apo B is an autosomal dominant disorder that arises due to a mutation in apo B that prevents binding of the defective apo B to the LDL receptor. This condition results in a clinical phenotype similar to classical familial hypercholesterolemia due to mutations in the LDL receptor with increased LDL-cholesterol, xanthomas, and premature ASCVD, underscoring the importance of apoB in atherogenesis. The most frequent variant is apoB-3500, caused by a point mutation resulting in a substitution of glutamine for arginine at position 3500. This mutation is an excellent illustration of a defect in an important ligand resulting in premature ASCVD.
The assessment of cardiovascular disease has, for the last several decades, had its basis in the findings of the Framingham Heart Study cohorts with emphasis on total cholesterol and low-density lipoprotein cholesterol (LDL-C). Apo B is a component of all atherogenic or potentially atherogenic particles, including small very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), LDL, and lipoprotein(a) [Lp(a)], and each particle contains 1 molecule of apo B. Therefore, apo B provides a direct measure of the number of atherogenic lipoprotein particles in circulation.
ApoB containing lipoproteins play a crucial role in atherogenesis, including promoting plaque formation within arteries. Many studies in the recent past have demonstrated that increased levels of apoB are a better predictor of risk for cardiovascular disease (CVD) than the aforementioned traditional markers. Measurement of ApoB serves as a direct indicator of the number of circulating atherogenic particles. Importantly, there is one Apo B molecule per hepatic derived lipoprotein particle. Additionally, in some studies, ApoB measurement improves CVD risk prediction in diabetes patients and those with metabolic syndrome. Apart from the role as a prediction marker for CVD risk, the measurement of Apo B levels may be used to monitor risk in patients following the initiation of statin therapy.[ However, common treatment goals as available for LDL-c need to be established for ApoB. Since each Apo B containing lipoprotein particle only contains one apoB molecule, the measurement of apoB also provides a measure of particle number
ApoB also acts as a ligand for LDL receptor-mediated clearance. Mutations affecting ApoB, such as in familial defective apoB, result in familial hypercholesterolemia. On the other hand, familial hypobetalipoproteinemia is an inherited disorder associated with low LDL levels and results from mutations in the Apo B gene. There are no apparent clinical sequelae with this disorder, but Vitamin E deficiency could develop, and hence supplementation is the usual recommendation.
A promising new area in lipid-lowering therapies is that of antisense oligonucleotides to target mRNA of proteins that are involved in cholesterol metabolism. Initial trials of apoB antisense oligos have shown promise with a 50% reduction in Apo B levels, a 30% reduction in LDL levels, and decreased cardiovascular risk. However, they cause liver dysfunction. A new goal of the 2018 NCEP guidelines is the inclusion of apoB, stating that an apoB greater than 130 mg/dL is a risk-enhancing factor and requires measurement in primary prevention treatment protocols, and this is an essential step towards the analysis of apoB for cardiovascular risk.
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