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APOB
apolipoprotein B
The APOB gene provides instructions for making apolipoprotein B, a key protein involved in the transport of fats and cholesterol throughout the body, playing a central role in lipid metabolism. APOB is essential for the formation and function of lipoproteins, which are particles that carry lipids in the bloodstream.
APOB is located on the short (p) arm of chromosome 2, at band 2p24.1. Arm ratio per GRCh38 - banding schematic.
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The APOB gene encodes apolipoprotein B, a structural component of lipoproteins. Lipoproteins are complex particles that transport fats, including cholesterol and triglycerides, within the bloodstream to various tissues. The APOB gene specifically produces two main forms of this protein: apolipoprotein B-48 and apolipoprotein B-100.
Apolipoprotein B-48 originates in the intestine and is critical for the assembly of chylomicrons, which carry dietary fats from the gut into circulation. Apolipoprotein B-100 is produced in the liver and is a fundamental component of very low-density lipoproteins (VLDLs), intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs). These lipoproteins are responsible for distributing fats and cholesterol from the liver to other cells throughout the body.
Pathogenic variants in the APOB gene are associated with inherited conditions that can lead to either excessively high or remarkably low levels of cholesterol and other lipids in the blood. These conditions include familial hypercholesterolaemia, characterised by elevated LDL cholesterol and increased cardiovascular risk, and familial hypobetalipoproteinaemia, which involves impaired fat absorption.
What the gene does
The APOB gene directs the synthesis of apolipoprotein B, a vital protein with two primary functional forms: apolipoprotein B-48 and apolipoprotein B-100. Both are integral to the transport of lipids, such as cholesterol and triglycerides, within the circulatory system.
Apolipoprotein B-48 is synthesised in the human intestine. Its primary role is to act as a crucial structural component of chylomicrons, which are large lipoprotein particles formed after a meal. Chylomicrons are responsible for absorbing dietary fats and fat-soluble vitamins (like vitamins A and E) from the intestine and transporting them into the bloodstream. This process is essential for distributing absorbed nutrients throughout the body.
Apolipoprotein B-100 is produced in the liver. It serves as a scaffolding protein for several classes of lipoproteins, including very low-density lipoproteins (VLDLs), intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs). VLDLs are secreted by the liver to transport endogenously synthesised triglycerides to peripheral tissues. As VLDLs lose triglycerides, they become IDLs and then LDLs. Apolipoprotein B-100 remains bound to these particles throughout their metabolism.
Crucially, apolipoprotein B-100 also acts as a ligand for the LDL receptor. This interaction allows LDL particles, which are the primary carriers of cholesterol in the blood, to bind to specific receptors on the surface of cells, particularly in the liver. Once bound, the LDL-receptor complex is internalised by the cell, enabling the release of cholesterol for cellular use, storage, or excretion. This receptor-mediated uptake mechanism is a critical pathway for the clearance of LDL cholesterol from the bloodstream, thereby regulating systemic cholesterol levels.
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Chromosome location
The APOB gene is situated on the short arm of chromosome 2, at position 2p24.1. This means it is found on the p-arm (short arm) of chromosome 2, within region 2, band 4, sub-band 1. The gene spans a significant length of DNA, encoding a large protein that plays a central role in lipid transport. The precise location on the chromosome helps in understanding its genetic neighbourhood and potential interactions with other genes.
Protein structure
The apolipoprotein B protein, encoded by the APOB gene, is a large and complex polypeptide, typically comprising 4563 amino acids. Its structural integrity and functional regions are critical for its role in lipid transport. The protein features several key domains and regions that facilitate its interactions with other molecules and lipoprotein components.
Multiple heparin-binding regions are present across the protein, indicating sites that can interact with heparin sulphate proteoglycans on cell surfaces, which can be involved in lipoprotein metabolism. These regions are located at amino acids 32-126, 232-306, 902-959, 2043-2178, 3161-3236, and 3383-3516.
A significant part of the protein is occupied by the Vitellogenin domain, spanning amino acids 46-672. This domain is structurally related to vitellogenin, a precursor of egg yolk proteins, suggesting an evolutionary link in lipid-binding mechanisms.
Two basic regions, potentially involved in receptor binding, are found at amino acids 3174-3184 and 3386-3394. These basic residues are often crucial for mediating interactions with negatively charged surfaces or receptors. A specific LDL receptor binding region is identified between amino acids 3373-3393, which is essential for the recognition and uptake of LDL particles by target cells.
Key variants
Variants within the APOB gene can influence its function, leading to a spectrum of associated clinical presentations. These genetic changes can range from single nucleotide substitutions, altering a single amino acid in the apolipoprotein B protein, to larger deletions or insertions that may result in truncated or non-functional proteins. The impact of a specific variant often depends on its location within the gene and how it affects critical functional domains of the protein, such as those involved in lipid binding or receptor interaction. Some variants may impair the protein's ability to bind to lipids or to the LDL receptor, while others might lead to its premature truncation. The relationship between genotype and phenotype can be complex, with some variants leading to severe disruptions in lipid metabolism and others having more subtle effects.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.10182G>T | p.Lys3394Asn | Pathogenic/Likely pathogenic | ★★☆☆ | Hypercholesterolemia, familial, 1 |
c.10238del | p.Thr3413fs | Pathogenic/Likely pathogenic | ★★☆☆ | Hypobetalipoproteinemia |
c.10579C>T | p.Arg3527Trp | Pathogenic/Likely pathogenic | ★★☆☆ | Hypercholesterolemia, autosomal dominant, type B |
c.10580G>A | p.Arg3527Gln | Pathogenic/Likely pathogenic | ★★☆☆ | Hypercholesterolemia, familial, 1 |
c.10781G>A | p.Trp3594Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Hypercholesterolemia, autosomal dominant, type B |
c.10848del | p.Gly3617fs | Pathogenic | ★★☆☆ | not provided |
c.11040T>G | p.Tyr3680Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Hypercholesterolemia, autosomal dominant, type B |
c.11124del | p.Phe3708fs | Pathogenic | ★★☆☆ | Hypercholesterolemia, autosomal dominant, type B |
c.11330C>A | p.Ser3777Ter | Pathogenic | ★★☆☆ | not provided |
c.1315C>T | p.Arg439Ter | Pathogenic | ★★☆☆ | not provided |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Variants in the APOB gene are primarily associated with conditions affecting lipid metabolism. Two significant inherited disorders linked to APOB are familial hypercholesterolaemia and familial hypobetalipoproteinaemia. Familial hypercholesterolaemia is characterised by substantially elevated levels of low-density lipoprotein (LDL) cholesterol in the blood, increasing the risk of early-onset cardiovascular disease. Conversely, familial hypobetalipoproteinaemia involves abnormally low levels of LDL cholesterol and can impair the body's ability to absorb and transport dietary fats, potentially leading to malabsorption of fat-soluble vitamins.
Inheritance pattern
Conditions caused by pathogenic APOB variants typically follow autosomal dominant inheritance.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
UK clinical status
The APOB gene is included in several panels within NHS England's Genomic Medicine Service (GMS) pathways, as listed on PanelApp. It is classified as 'green' for 'Additional findings health related' and 'Additional findings health related - children', indicating that variants in this gene are generally considered clinically actionable and relevant for both adults and children when identified as secondary findings. APOB is also a key gene on panels for 'Familial hypercholesterolaemia' and 'Familial hypercholesterolaemia (GMS) (R134)', reflecting its established role in this common inherited lipid disorder. Furthermore, it is listed on panels for 'Intestinal failure or congenital diarrhoea (R331)', 'Likely inborn error of metabolism (R98)', and 'Undiagnosed metabolic disorders', highlighting its broader involvement in metabolic health and specific gastrointestinal conditions.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Lifestyle factors can influence lipid metabolism and may interact with an individual's genetic predisposition, including variants in the APOB gene. While specific dietary or lifestyle interventions cannot reverse a genetic mutation, adopting a heart-healthy lifestyle is generally recommended, particularly for individuals with conditions like familial hypercholesterolaemia. Research suggests that a diet rich in fruits, vegetables, whole grains, and lean proteins, while being low in saturated and trans fats, can help manage cholesterol levels in the general population. Regular physical activity is also widely recognised for its benefits in maintaining cardiovascular health and influencing lipid profiles. Avoiding smoking and limiting alcohol intake are additional recommendations. These lifestyle adjustments are typically part of a comprehensive management strategy, working in conjunction with medical supervision, especially for individuals with genetic conditions affecting lipid metabolism.
Supplement considerations
The role of dietary supplements in managing conditions related to APOB gene variants, such as familial hypercholesterolaemia, is an area of ongoing research. While some supplements, such as plant sterols/stanols, soluble fibre, and omega-3 fatty acids, have been studied for their potential to help lower cholesterol in the general population, there is no conclusive evidence that they can prevent the progression or fully treat genetic lipid disorders. supplements are not regulated as medicines, and their efficacy and safety can vary. Individuals with APOB-related conditions should always discuss any potential use of supplements with a healthcare provider. A healthcare professional can offer personalised advice, considering an individual's specific genetic profile, existing health conditions, and current medications, to ensure a safe and effective management plan.
Frequently asked questions
What is the primary function of the APOB gene?
The APOB gene provides instructions for making apolipoprotein B proteins, which are essential components of lipoproteins. These lipoproteins are responsible for transporting fats and cholesterol throughout the bloodstream.
What are the two main forms of apolipoprotein B?
The APOB gene produces two main forms: apolipoprotein B-48, made in the intestine and part of chylomicrons for dietary fat transport, and apolipoprotein B-100, made in the liver and a component of VLDL, IDL, and LDL for internal fat transport.
What conditions are associated with variants in the APOB gene?
Variants in the APOB gene are primarily associated with familial hypercholesterolaemia, characterised by high LDL cholesterol, and familial hypobetalipoproteinaemia, which involves abnormally low LDL cholesterol and impaired fat absorption.
How does apolipoprotein B-100 affect cholesterol levels?
Apolipoprotein B-100 enables LDL particles to bind to LDL receptors on cells, primarily in the liver. This binding allows cells to take up and clear cholesterol from the bloodstream, thereby regulating overall cholesterol levels.
Is APOB included in NHS genetic testing panels?
Yes, the APOB gene is included in several NHS England Genomic Medicine Service (GMS) panels, particularly for familial hypercholesterolaemia and as an additional finding in general health-related screens for both children and adults.
Can lifestyle changes help manage APOB-related conditions?
While lifestyle changes cannot alter the gene itself, adopting a heart-healthy diet, engaging in regular physical activity, and avoiding smoking can contribute to overall cardiovascular health and help manage lipid levels, especially for individuals with genetic predispositions.
References
- Tarugi P, Averna M. Hypobetalipoproteinemia: genetics, biochemistry, and clinical spectrum. Advances in clinical chemistry. 2011. PMID: 21874758
- Tarugi P, Averna M, Di Leo E. Molecular diagnosis of hypobetalipoproteinemia: an ENID review. Atherosclerosis. 2007. PMID: 17570373
- Fouchier SW, Sankatsing RR, Peter J. High frequency of APOB gene mutations causing familial hypobetalipoproteinaemia in patients of Dutch and Spanish descent. Journal of medical genetics. 2005. PMID: 15805152
- Schonfeld G, Lin X, Yue P. Familial hypobetalipoproteinemia: genetics and metabolism. Cellular and molecular life sciences : CMLS. 2005. PMID: 15818469
- Benn M, Nordestgaard BG, Jensen JS. Polymorphism in APOB associated with increased low-density lipoprotein levels in both genders in the general population. The Journal of clinical endocrinology and metabolism. 2005. PMID: 16030169
- Olofsson SO, Borèn J. Apolipoprotein B: a clinically important apolipoprotein which assembles atherogenic lipoproteins and promotes the development of atherosclerosis. Journal of internal medicine. 2005. PMID: 16238675
- Hooper AJ, van Bockxmeer FM, Burnett JR. Monogenic hypocholesterolaemic lipid disorders and apolipoprotein B metabolism. Critical reviews in clinical laboratory sciences. 2005. PMID: 16390683
- Soufi M, Sattler AM, Maerz W. A new but frequent mutation of apoB-100-apoB His3543Tyr. Atherosclerosis. 2004. PMID: 15135245
- van Aalst-Cohen ES, Jansen AC, de Jongh S. Clinical, diagnostic, and therapeutic aspects of familial hypercholesterolemia. Seminars in vascular medicine. 2004. PMID: 15199431
- Vrablík M, Ceska R, Horínek A. Major apolipoprotein B-100 mutations in lipoprotein metabolism and atherosclerosis. Physiological research. 2001. PMID: 11551138
- Adam MP, Bick S, Mirzaa GM. APOB-Related Familial Hypobetalipoproteinemia. 1993. PMID: 33983694