Factor VIII

For the documentary, see Factor 8: The Arkansas Prison Blood Scandal

Coagulation factor VIII, procoagulant component
250px PDB rendering based on 2R7E.
Available structures PDB Ortholog search: PDBe, RCSB List of PDB id codes 1CFG, 1D7P, 1FAC, 1IQD, 2R7E, 3CDZ, 3HNB, 3HNY, 3HOB, 3J2Q, 3J2S, 4BDV, 4KI5, 4PT6
Identifiers
Symbols	F8 ; AHF; DXS1253E; F8B; F8C; FVIII; HEMA
External IDs	OMIM: 300841 MGI: 88383 HomoloGene: 49153 ChEMBL: 3143 GeneCards: F8 Gene
Gene ontology Molecular function • copper ion binding • protein binding • oxidoreductase activity Cellular component • extracellular region • extracellular space • plasma membrane • platelet alpha granule lumen Biological process • platelet degranulation • acute-phase response • blood coagulation • blood coagulation, intrinsic pathway • platelet activation • oxidation-reduction process Sources: Amigo / QuickGO
RNA expression pattern
File:PBB GE F8 205756 s at tn.png
More reference expression data
Orthologs
Species	Human	Mouse
Entrez	2157	14069
Ensembl	ENSG00000185010	ENSMUSG00000031196
UniProt	P00451	Q06194
RefSeq (mRNA)	NM_000132	NM_001161373
RefSeq (protein)	NP_000123	NP_001154845
Location (UCSC)	Chr X: 154.84 – 155.03 Mb	Chr X: 75.17 – 75.38 Mb
PubMed search	[1]	[2]

Factor VIII (FVIII) is an essential blood-clotting protein, also known as anti-hemophilic factor (AHF). In humans, factor VIII is encoded by the F8 gene. Defects in this gene results in hemophilia A, a recessive X-linked coagulation disorder. Factor VIII is produced in liver sinusoidal cells and endothelial cells outside of the liver throughout the body. This protein circulates in the bloodstream in an inactive form, bound to another molecule called von Willebrand factor, until an injury that damages blood vessels occurs. In response to injury, coagulation factor VIII is activated and separates from von Willebrand factor. The active protein (sometimes written as coagulation factor VIIIa) interacts with another coagulation factor called factor IX. This interaction sets off a chain of additional chemical reactions that form a blood clot.

Factor VIII participates in blood coagulation; it is a cofactor for factor IXa which, in the presence of Ca²⁺ and phospholipids forms a complex that converts factor X to the activated form Xa. The factor VIII gene produces two alternatively spliced transcripts. Transcript variant 1 encodes a large glycoprotein, isoform a, which circulates in plasma and associates with von Willebrand factor in a noncovalent complex. This protein undergoes multiple cleavage events. Transcript variant 2 encodes a putative small protein, isoform b, which consists primarily of the phospholipid binding domain of factor VIIIc. This binding domain is essential for coagulant activity.

People with high levels of factor VIII are at increased risk for deep vein thrombosis and pulmonary embolism. Copper is a required cofactor for factor VIII and copper deficiency is known to decrease levels of factor VIII.

It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.

Genetics

The gene for factor VIII is located on the X chromosome (Xq28). The gene for factor VIII presents an interesting primary structure, as another gene is embedded in one of its introns.

Physiology

FVIII is a glycoprotein procofactor. Although the primary site of release in humans is ambiguous, it is synthesized and released into the bloodstream by the vascular, glomerular, and tubular endothelium, and the sinusoidal cells of the liver. Hemophilia A has been corrected by liver transplantation. Transplanting hepatocytes was ineffective, but liver endothelial cells were effective.

In the blood, it mainly circulates in a stable noncovalent complex with von Willebrand factor. Upon activation by thrombin, (factor IIa), it dissociates from the complex to interact with factor IXa in the coagulation cascade. It is a cofactor to factor IXa in the activation of factor X, which, in turn, with its cofactor factor Va, activates more thrombin. Thrombin cleaves fibrinogen into fibrin which polymerizes and crosslinks (using factor XIII) into a blood clot.

No longer protected by vWF, activated FVIII is proteolytically inactivated in the process (most prominently by activated protein C and factor IXa) and quickly cleared from the blood stream.

Factor VIII is not affected by liver disease. In fact, levels usually are elevated in such instances.

Medical use

FVIII concentrated from donated blood plasma (Aafact or Alphanate, Monoclate-P®), or alternatively recombinant FVIII can be given to hemophiliacs to restore hemostasis.

The transfer of a plasma byproduct into the blood stream of a patient with hemophilia often led to the transmission of diseases such as hepatitis B and C and HIV before purification methods were improved.

Antibody formation to factor VIII can also be a major concern for patients receiving therapy against bleeding; the incidence of these inhibitors is dependent of various factors, including the factor VIII product itself.

Contamination scandal

Main article: Contaminated haemophilia blood products

In the 1980s, some pharmaceutical companies such as Bayer sparked controversy by continuing to sell contaminated factor VIII after new heat-treated versions were available. Under FDA pressure, unheated product was pulled from US markets, but was sold to Asian, Latin American, and some European countries. The product was tainted with HIV, a concern that had been discussed by Bayer and the U.S. Food and Drug Administration (FDA).

In the early 1990s, pharmaceutical companies began to produce recombinant synthesized factor products, which now prevent nearly all forms of disease transmission during replacement therapy.

See also

• Ralph Kekwick
• Ryan White
• Edward Tuddenham
• Edward Shanbrom

External links

• GeneReviews/NCBI/NIH/UW entry on Hemophilia A
• The Coagulation Factor VIII Protein
• Factor VIII at the US National Library of Medicine Medical Subject Headings (MeSH)