Sequences flanking Arg336 in factor VIIIa modulate factor Xa-catalyzed cleavage rates at this site and cofactor function

High Mutational Heterogeneity, and New Mutations in the Human Coagulation Factor V Gene. Future Perspectives for Factor V Deficiency Using Recombinant and Advanced Therapies

Factor V is an essential clotting factor that plays a key role in the blood coagulation cascade on account of its procoagulant and anticoagulant activity. Eighty percent of circulating factor V is produced in the liver and the remaining 20% originates in the α-granules of platelets. In humans, the factor V gene is about 80 kb in size; it is located on chromosome 1q24.2, and its cDNA is 6914 bp in length. Furthermore, nearly 190 mutations have been reported in the gene. Factor V deficiency is an autosomal recessive coagulation disorder associated with mutations in the factor V gene. This hereditary coagulation disorder is clinically characterized by a heterogeneous spectrum of hemorrhagic manifestations ranging from mucosal or soft-tissue bleeds to potentially fatal hemorrhages. Current treatment of this condition consists in the administration of fresh frozen plasma and platelet concentrates. This article describes the cases of two patients with severe factor V deficiency, and of their parents. A high level of mutational heterogeneity of factor V gene was identified, nonsense mutations, frameshift mutations, missense changes, synonymous sequence variants and intronic changes. These findings prompted the identification of a new mutation in the human factor V gene, designated as Jaén-1, which is capable of altering the procoagulant function of factor V. In addition, an update is provided on the prospects for the treatment of factor V deficiency on the basis of yet-to-be-developed recombinant products or advanced gene and cell therapies that could potentially correct this hereditary disorder.

Combining mutations that modulate inter-subunit interactions and proteolytic inactivation enhance the stability of factor VIIIa

FVIIIa is labile due to the dissociation of A2 subunit. Previously, we introduced hydrophobic mutations at select A1/A2/A3 subunit interfaces yielding more stable FVIII(a) variants. Separately we showed that altering the sequence flanking the primary FXa cleavage site in FVIIIa (Arg336) yielded reduced rates of proteolytic inactivation of FVIIIa. In this study we prepared the FXa-cleavage resistant mutant (336(P4-P3')562) combined with mutations of Ala108Ile, Asp519Val/Glu665Val or Ala108Ile/Asp519Val/Glu665Val and examined the effects of these combinations relative to FVIII thermal stability, rates of FVIIIa decay and proteolytic inactivation of FVIIIa by FXa. Thermal decay rates for 336(P4-P3')562/Ala108Ile, 336(P4-P3')562/Asp519Val/Glu665Val, and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ~2- to 5-fold as compared with wild-type (WT) primarily reflecting the effects of the A domain interface mutations. FVIIIa decay rates for 336(P4-P3')562/Asp519Val/Glu665Val and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ~25 fold, indicating greater stability than the control Asp519Val/Glu665Val variant (~14-fold). Interestingly, 336(P4-P3')562/Asp519Val/Glu665Val and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants showed reduced FXa-inactivation rates compared with the 336(P4-P3')562 control (~4-fold), suggesting A2 subunit destabilisation is a component of proteolytic inactivation. Thrombin generation assays using the combination variants were similar to the Asp519Val/Glu665Val control. These results indicate that combining multiple gain-of-function FVIII mutations yields FVIII variants with increased stability relative to a single type of mutation.

Residues flanking scissile bonds in Factor VIII modulate rates of cleavage and proteolytic activation catalyzed by Factor Xa

Factor Xa (FXa) proteolytically activates Factor VIII (FVIII) by cleaving P1 residues Arg(372), Arg(740), and Arg(1689). The Arg(372) site represents the rate-limiting step for procofactor activation, whereas cleavage at Arg(740) is a fast step. FXa also catalyzes inactivating cleavages that occur on a slower time scale than the activating ones. To assess the role of sequences flanking the Arg(372) and Arg(740) sites, recombinant FVIII variants in which P3-P3' sequences were swapped individually or in combination were prepared. Replacing the Arg(372) flanking sequence with that from the Arg(740) site increased the rate of cleavage at Arg(372), as judged by the ~5-fold increased rate in A1 subunit generation, and reduced the FVIIIa-dependent lag time for in situ FXa generation. The reciprocal swap yielded a nearly 2-fold increase in the rate of Arg(372) cleavage, while the combined double-swap variant showed a 10-fold rate increase at that site, consistent with the individual effects being additive. Although this cleavage represents the slow step for activation, the rate of this reaction appeared to be ~9-fold greater than the rate of the primary inactivating cleavage at Arg(336) in generating the A1(336) product. Interestingly, replacement of the Arg(372) flanking sequence with the Arg(740) sequence combined with an Arg(740)Gln mutation yielded both more rapid cleavage of the Arg(372) site and accelerated inactivating cleavages within the A1 subunit. These results indicate that flanking sequences in part modulate the reaction rates required for procofactor activation and influence the capacity of FXa as an initial activator of FVIII rather than an inactivator.

Replacing the factor VIII C1 domain with a second C2 domain reduces factor VIII stability and affinity for factor IXa

Factor VIII (FVIII) consists of a heavy chain (A1(a1)A2(a2)B domains) and light chain ((a3)A3C1C2 domains). To gain insights into a role of the FVIII C domains, we eliminated the C1 domain by replacing it with the homologous C2 domain. FVIII stability of the mutant (FVIIIC2C2) as measured by thermal decay at 55 °C of FVIII activity was markedly reduced (~11-fold), whereas the decay rate of FVIIIa due to A2 subunit dissociation was similar to WT FVIIIa. The binding affinity of FVIIIC2C2 for phospholipid membranes as measured by fluorescence resonance energy transfer was modestly lower (~2.8-fold) than that for WT FVIII. Among several anti-FVIII antibodies tested (anti-C1 (GMA8011), anti-C2 (ESH4 and ESH8), and anti-A3 (2D2) antibody), only ESH4 inhibited membrane binding of both WT FVIII and FVIIIC2C2. FVIIIa cofactor activity measured in the presence of each of the above antibodies was examined by FXa generation assays. The activity of WT FVIIIa was inhibited by both GMA8011 and ESH4, whereas the activity of FVIIIC2C2 was inhibited by both the anti-C2 antibodies, ESH4 and ESH8. Interestingly, factor IXa (FIXa) binding affinity for WT FVIIIa was significantly reduced in the presence of GMA8011 (~10-fold), whereas the anti-C2 antibodies reduced FIXa binding affinity of FVIIIC2C2 variant (~4-fold). Together, the reduced stability plus impaired FIXa interaction of FVIIIC2C2 suggest that the C1 domain resides in close proximity to FIXa in the FXase complex and contributes a critical role to FVIII structure and function.