Historical Background
Paul Owren from Norway identified Factor V (FV) during World War II. His index patient had presented at age 3 years with prolonged epistaxis. She subsequently had symptoms of easy bruising, menorrhagia, and prolonged bleeding after trauma. Owren demonstrated that the patient lacked or was deficient in a previously unrecognized procoagulant, which he designated FV. He named the disorder parahemophilia.1 FV was later renamed proaccelerin and was shown to be the same as the labile factor that had been independently identified by Quick.2,3 Cloning of the cDNA led to the determination of the amino acid sequence of the protein.4 The entire genomic structure of the FV gene was characterized in 1992.5
FV Synthesis
FV is synthesized primarily by the liver, and levels can decrease when liver synthetic function is impaired. Plasma FV circulates as a 330-kDa single-chain polypeptide that is the inactive procoagulant. Although most FV is present in plasma, approximately 20% of the circulating FV is found within platelet alpha-granules. The source of platelet FV has not been definitively established, but evidence suggests that platelets or megakaryocytes can both endocytose and synthesize FV. Platelet FV is partially proteolysed and is stored bound to the protein multimerin in alpha-granules. [Reviewed in Kalafatis M. Curr Opin Hematol 2005, Asselta R. J Thromb Haemost 2006, and Gertz JM J Cell Biochem 2015]6-8
Relevance of Factor V in the Clotting Cascade
Activated FV (FVa) is the cofactor in the prothrombinase complex that cleaves and activates prothrombin to thrombin (reviewed in Kalafatis M. Curr Opin Hematol 2005 and Asselta R et al. J Thromb Haemost 20066,7; see references therein). This multicomponent enzyme complex consists of FVa, calcium, phospholipids, and activated factor X (FXa).
FVa increases the concentration of FXa at the membrane surface by acting as a receptor for FXa and allosterically alters the active site of FXa to optimize its ability to cleave prothrombin. By stabilizing the complex and increasing the rate at which FXa cleaves prothrombin, FVa enhances prothrombin activation by five orders of magnitude when compared with FXa in the absence of FVa.
FV is functionally and structurally similar to factor VIII (FVIII). Like FVIII, FV is composed of six domains: A1, A2, B, A3, C1, and C2. The A and C domains of FV and FVIII are approximately 40% homologous. A highly conserved pair of regions in the B domain, together called the pro-cofactor regulatory region, has an auto-inhibitory function and when cleaved off leads to exposure of a FXa binding site.9 As is the case with FVIII, FV activity is tightly regulated via site-specific proteolysis.
Thrombin, and to a lesser extent FXa, is primarily responsible for FV activation via proteolytic cleavages at arginine residues in positions 709, 1018, and 1545. These cleavages release the B domain and create a dimeric molecule composed of a 105-kDa heavy chain that contains the A1 and A2 domains and a 71- to 74-kDa light chain that contains the A3, C1, and C2 domains. These two chains are held together by calcium and hydrophobic interactions. The heavy chain provides the contacts for both FXa and prothrombin, whereas the two C domains in the light chain are needed for the interaction of FVa with the phospholipid surface. The A3 domain in the light chain is involved in both FXa and phospholipid interactions. Taken together, these two FVa chains link FXa to the phospholipid surface formed by the platelet plug at the site of injury and enable FXa to efficiently bind and cleave prothrombin to generate an effective thrombin burst.
Activated protein C (APC) mediates the inactivation of FVa. APC cleaves FVa at arginine residues in positions 506, 306, and 679 and at lysine 994. The cleavage at Arg 506 converts FVa to FV anticoagulant (FVac), which interacts with APC and protein S to inactivate FVIIIa. The cleavage at Arg 506 reduces both the cofactor procoagulant activity and its affinity for FXa, while the cleavage at Arg 306 completes the inactivation. APC thus not only turns off the FVa procoagulant activity but also converts it to an anticoagulant.
Pathophysiology
FV deficiency can be categorized as either congenital or acquired. Congenital deficiencies arise from mutations either in the FV gene itself or in genes that affect the processing or storage of FV. Examples of the latter are mutations in LMAN1 or MCFD2, which lead to combined FV and FVIII deficiency, and FV Quebec, a platelet alpha-granule defect. Mutations in the FV gene itself can result in either a quantitative (type I) or qualitative (type II) defect. Thus far, the only qualitative defect described is FV New Brunswick.6,10,11
Causative Genetic Mutations
Kingsley first described the autosomal recessive inheritance pattern of congenital FV deficiency in two South African families of Dutch ancestry.2 The cDNA was cloned 30 years later. The amino acid sequence of the protein was determined,4 and the entire genomic structure of the FV gene was characterized in 1992.5
The FV gene (GenBank accession no. NM_000130) is located on the long arm of chromosome 1 at 1q23. The entire gene spans approximately 80 kb, contains 25 exons, and is transcribed into a nearly 7-kb long mRNA encoding a 2224-amino acid protein that contains a 28-amino acid residue signal peptide. More than 150 mutations are associated with FV deficiency (defined as DNA changes that reduce FV activity or antigen levels by >50%) and more than 700 polymorphisms that do not have a clinical phenotype have now been identified.12,13
Relationship of Types of Genetic Mutations with Phenotypes
At present, no clear correlation between genotypes and the clinical phenotypes have been identified.6 Clinically important nonsense, frameshift, missense, and splice-site mutations in the FV gene have all been described. Recently a patient has been described with a FV level of 9% who has a complete deletion of one FV allele in association with a 1q deletion on one chromosome combined with a point mutation in the other FV allele.14 In light of the severe phenotype of the FV knockout mice, which die either in utero at embryonic day 9–10 or within a few hours of birth from massive hemorrhage,15 the lack of patients with complete gene deletions has led to the hypothesis that complete FV deficiency is incompatible with life.6
The FV activity level has limited correlation with the severity of bleeding. Overall, patients with lower levels are more likely to have bleeding episodes than those with higher levels. Patients with identical mutations or activity levels, including related patients with identical genotypes and equally low (<1%) FV activity, can vary greatly in their bleeding symptoms.6,16 These low levels of FV activity cannot be detected by available tests, but they do convey some protection from fatal hemorrhage in utero.
The severity of clinical manifestations in FV deficiency may be mediated in part by the amount of platelet-FV, TFPI, or procoagulant plasma components.17-20 Patients who come to medical attention are typically symptomatic homozygotes or compound heterozygotes with FV activity levels less than 5%, although one patient in the FV mutation database who is thought to be a compound heterozygote has 26% activity.10 In contrast, Kingsley found that the heterozygotes in the two families he studied had levels that ranged from 24% to 68%, and none had bleeding symptoms.2
Other causes of FV Deficiency
Aside from mutations in the FV gene, deficiencies of FV can also arise due to acquired specific inhibitors to FV and to defects that affect the storage and processing of FV. FV-specific inhibitors most often develop after exposure to preparations of bovine thrombin21 but have also been reported in patients who have underlying rheumatologic conditions or malignancies or who were being treated with antibiotics (for a review of acquired FV inhibitors, see Franchini M, J Thromb Thrombolysis 201122). Rarely, FV-deficient patients have developed inhibitors to FV after receiving fresh frozen plasma (FFP).6,23,24
In FV Quebec, the contents of platelet α-granules, including FV, are abnormally proteolyzed.25 Combined deficiency of both FV and FVIII can result from mutations in either LMAN1 or MCFD2, genes encoding proteins involved in the processing and transport of FV and FVIII26 (see Combined FV and FVIII Deficiency by Peyvandi et al on this website).
No precise epidemiologic data exist for congenital FV deficiency, but its prevalence has been estimated to be 1 in 1,000,000 persons with no apparent ethnic predisposition.6 In Iran, where a registry of rare bleeding disorders has been maintained since the early 1970s, 35 FV-deficient patients have been identified in a population of 65 million as of 1998.27 A similar Italian registry (overall population of 55 million) that has been enrolling patients since 1980 lists 35 FV-deficient patients.28 As ascertainment is almost certainly incomplete, the prevalence is likely higher than what is suggested by the number of patients in these registries. Both sexes are equally affected.
