Molecular characterization of 3 factor V mutations, R2174L, V1813M, and a 5-bp deletion, that cause factor V deficiency

Factor V variants in bleeding and thrombosis

A state-of-the-art lecture titled "Factor V variants in bleeding and thrombosis" was presented at the International Society on Thrombosis and Haemostasis (ISTH) congress in 2023. Blood coagulation is a finely regulated cascade of enzymatic reactions culminating in thrombin formation and fibrin deposition at the site of injury. Factor V (FV) plays a central role in this process, as its activated form is an essential procoagulant cofactor in prothrombin activation. However, other molecular forms of FV act as anticoagulant cofactors of activated protein C and tissue factor pathway inhibitor α, respectively, thereby contributing to the regulation of coagulation. This dual procoagulant and anticoagulant character makes FV a central regulator of the hemostatic balance, and quantitative and qualitative alterations of FV may be associated with an increased risk of bleeding or venous thrombosis. Here, we review the procoagulant and anticoagulant functions of FV and the manifold mechanisms by which F5 gene mutations may affect the balance between these opposite functions and thereby predispose individuals to bleeding or venous thrombosis. In particular, we discuss our current understanding of the 3 main pathological conditions related to FV, namely FV deficiency, activated protein C resistance, and the overexpression of FV-short, a minor splicing isoform of FV with tissue factor pathway inhibitor α-dependent anticoagulant properties and an emerging role as a key regulator of the initiation of coagulation. Finally, we summarize relevant new data on this topic presented during the 2023 ISTH Congress.

Prolonged α-thrombin-related activation and delayed active protein C-associated degradation confer mild phenotype in a patient with severe hemophilia A with F8 p.H118R

In hemophilia A, bleeding mostly correlates with factor VIII activity (FVIII:C), although some patients show discrepancy in bleeding severity and FVIII:C. We report a novel procoagulant mechanism associated with F8 p.H118R (c.353A > G) in a young Japanese man with few bleeding episodes despite low levels of FVIII:C (< 1 IU/dL). Plasma FVIII:C was < 1 IU/dL measured by one-stage clotting assay (OSA) and chromogenic substrate assay (CSA), whereas FVIII antigen (FVIII:Ag) was 9.7%. The global coagulation assay showed higher max speed in clot waveform analysis (CWA), shorter clotting time in rotation thromboelastometry (ROTEM) (1605 vs. > 5000 s), shorter lag time (4.87 vs. 12.47 min) and larger ETP (207.9 vs. 53.3 nM*min) in thrombin generation assay, compared with FVIII-deficient control. Expressed recombinant H118R mutant in culture media showed low FVIII:C (1-5 IU/dL) by OSA, with non-hemophilia level of FVIII:Ag. Western blot analysis using recombinant H118R showed longer persistence of heavy-chain of H118R after incubation with α-thrombin, compared with wild-type. Incubation of H118R with activated protein C (APC) also showed longer persistence of A1-A2 domain. In conclusion, H118R showed prolonged activation by α-thrombin and delayed APC-related FVIII degradation. These properties may confer the procoagulant activity and few bleeding episodes despite low FVIII:C.

Asp68His mutation in the A1 domain of human factor V causes impaired secretion and ineffective translocation

Congenital factor V (FV) deficiency is a rare inherited disorder. We determined the mechanism of a missense mutation, Asp68His, in the A1 domain of the FV protein, is associated with severe FV deficiency. We characterized the mutant FV-Asp68His protein using in vitro expression studies by using specific secretion and degradation pathway inhibitors and analysed the intracellular translocation of the mutant protein by immunofluorescence staining. The Asp68His mutation caused very low levels of FV protein in the conditioned media, with normal specific FV activity. Similar mRNA degradation rates between FV-wild-type (wt) and FV-Asp68His mRNA showed that the Asp68His mutation does not affect FV expression at the transcriptional level. A specific secretion pathway inhibitor, brefeldin A, was used to demonstrate that the lower efficiency of transport to the outside of the cell for FV-Asp68His mutant protein compared with that of the FV-wt protein. Furthermore, we showed that the Asp68His mutation resulted in increased intracellular degradation through a MG132-mediated proteasomal degradation pathway. In the transfected cell lysates, FV-wt protein had multiple posttranslational modified forms, but the FV-Asp68His protein was not completely glycosylated. We further observed that the FV-Asp68His protein was retrieved in the endoplasmic reticulum only and did not undergo transport to the Golgi apparatus, leading to impaired secretion. These results strongly suggest that the Asp68His mutation may result in intracellular defective trafficking and enhanced degradation, and impaired secretion of FV protein.

Novel FV mutation (W1920R, FVNara) associated with serious deep vein thrombosis and more potent APC resistance relative to FVLeiden

FVNara (W1920R), associated with serious deep vein thrombosis, is more resistant to APC relative to FVLeiden (R506Q). This mechanism results from significant decreases in FVa susceptibility to APC and FV cofactor activity for APC.

Factor V deficiency caused by a novel nonsense mutation (Gln2031stop) in a Chinese patient

Congenital factor V deficiency is a rare bleeding disorder characterized by low coagulant activity, associated with variable phenotypic expression. Among rare inherited coagulopathies, the molecular basis of factor V deficiency is rarely described because of its relatively low prevalence in the general population. Recently, we detected two genetic variations in factor V of a Chinese patient with hereditary factor V deficiency. One was a heterozygous nonsense mutation, C67868T in exon 22, which resulted in Gln2031stop substitution in the C1 domain of factor V. The other was a previously described polymorphism, G1618A in exon10, leading to Arg485Lys substitution. We deduced that the nonsense mutation is responsible for the factor V deficiency, whereas the Arg485Lys polymorphism is expected to compensate for the low plasma factor V levels. Of note, the nonsense mutation has been confirmed to be a novel mutation.

Residual platelet factor V ensures thrombin generation in patients with severe congenital factor V deficiency and mild bleeding symptoms

Coagulation factor V (FV), present in plasma and platelets, is indispensable to thrombin formation, yet patients with undetectable plasma FV seldom experience major bleeding. We used thrombin generation assays to explore the role of platelet FV in 4 patients with severe congenital FV deficiency (3 with plasma FV clotting activity [FV:C] < 1%). When triggered with tissue factor (TF) concentrations up to 50pM, platelet-poor plasma (PPP) from the patients with undetectable plasma FV showed no thrombin generation, whereas platelet-rich plasma (PRP) formed thrombin already at 1 to 5pM of TF. Thrombin generation in PRP from the FV-deficient patients was enhanced to near-normal levels by platelet activators (collagen or Ca(2+)-ionophore) and could be completely suppressed by specific FV inhibitors, suggesting FV dependence. Accordingly, platelet FV antigen and activity were measurable in all FV-deficient patients and platelet FVa could be visualized by Western blotting. Normalization of the tissue factor pathway inhibitor (TFPI) level, which is physiologically low in FV-deficient plasma, almost completely abolished thrombin generation in PRP from the FV-deficient patients. In conclusion, patients with undetectable plasma FV may contain functional FV in their platelets. In combination with low TFPI level, residual platelet FV allows sufficient thrombin generation to rescue these patients from fatal bleeding.

A novel in-frame deletion in the factor V C1 domain associated with severe coagulation factor V deficiency in a Korean family

Hereditary coagulation factor V deficiency is a rare bleeding disorder characterized by extremely low levels of functional and immunoreactive factor V in plasma associated with moderate-to-severe bleeding symptoms. The genetic bases of factor V deficiency have been characterized only in a limited number of cases and the majority of causative mutations are truncating mutations providing only limited information about the function of subdomains and of individual residues. Here, we present the first report on a Korean family with inherited factor V deficiency. The proband is a 25-year-old man who showed normal coagulation factor levels except those for factor V antigen and activity (3 and 4%, respectively), and was only suffering from bleeding after tooth extraction. Direct sequencing of the factor V gene disclosed the already known nonsense mutation (R1133X) and a novel in-frame 6-bp deletion (6116-6121delGAACAG, corresponding to the amino-acid deletion N1982-S1983) located in the factor V C1 domain; both mutations were found in the heterozygous state. The structural and functional importance of the in-frame deletion was examined by constructing a molecular model based on the crystal structure of bovine activated factor V that has been inactivated by activated protein C. N1982 and S1983 are located on a loop that is exposed on the surface of the C1 domain, and are in close contact with another loop belonging to the A3 domain. Even though the detailed mechanism of the association of the in-frame deletion of our patient and factor V deficiency needs further investigation, this model suggests the possibility that the N1982-S1983 deletion could destabilize the C1-A3 interaction by preventing the potential formation of hydrogen bonds between K1980 and N1986 of the C1 domain with D1604 of the A3 domain. Also, because N1982 is strongly expected to be N-glycosylated judging from its structural homology to factor VIII, loss of this residue can influence proper folding of factor V, resulting in unstable structure, which is vulnerable to intracellular degradation.

Identification of four novel mutations in F5 associated with congenital factor V deficiency

Coagulation factor V (FV) deficiency is a rare bleeding disorder characterized by low coagulant and antigen levels of FV with bleeding symptoms ranging from mild to severe. Only a limited number of mutations have been reported because of the large size of the factor V gene (F5) as well as the low prevalence. In this study, we have identified four novel mutations in F5 in five unrelated patients with congenital FV deficiency. All the patients, including two with undetectable FV activity, were asymptomatic and were found to have prolonged prothrombin time and activated partial thromboplastin time during preoperative screening or routine examinations. All four mutations found in this study are either missense or in-frame deletion. This is in contrast with previous reports of a high frequency of mutations introducing premature termination codons in inherited FV deficiency. Missense mutations of F5 might produce a mild phenotype and are not frequently diagnosed. Although FV deficiency is a very rare disorder with a predicted incidence of one in 1 million, this study suggests that the numbers of F5 mutations, especially missense mutations, are higher than estimated.