Location of the platelet binding site in zymogen coagulation factor IX

Abnormal hemostasis in a knock-in mouse carrying a variant of factor IX with impaired binding to collagen type IV

BACKGROUND

Factor IX binds to collagen type IV, but this binding has no known consequence.

OBJECTIVES

To determine the effect of reduced binding of FIX to collagen IV.

METHODS

We constructed and characterized 'knock-in' mice containing the mutation lysine 5 to alanine (K5A) in the Gla domain of their FIX. The K5A mutation dramatically reduced the affinity of FIX for collagen type IV, but had no measurable effect on platelet binding, phospholipid binding, or in vitro clotting activity. However, K5AFIX mice had a mild bleeding tendency, despite their in vitro clotting activity being normal. Hemostatic protection from delayed rebleeding was intermediate between wild-type and hemophilia B mice (which had no detectable clotting activity); moreover, survival of K5A FIX mice after nascent clot removal was dramatically improved as compared with hemophilia B mice. Importantly, there was no detectable difference between K5AFIX and wild-type mice in either a laser-induced thrombosis model or the chromogenic FIX activity assay. In contrast, after ferric chloride injury, which exposes collagen IV as well as other basement membrane proteins, intravital microscopy revealed that vessel occlusion was significantly slower in K5AFIX mice than in wild-type mice.

CONCLUSIONS

Our results indicate that the FIX molecule with decreased affinity for collagen IV has altered hemostatic properties in vivo and that the binding of FIX to collagen IV probably plays a significant functional role in hemostasis.

Recent advances in clot biology and assessment of clotting

Since the first description of the haemostatic process by Morawitz in 1904, knowledge about the haemostasis mechanism has undergone substantial modifications. Increasing knowledge of enzymology, purification and characterisation of coagulation proteins led to the introduction of the waterfall or cascade model of coagulation. However, these models were based on in vitro studies in the presence of artificial phospholipids and the absence of cells. Two pathways to achieve the formation of a haemostatic fibrin plug were identified, the so-called `extrinsic system' involving both factors present in the circulation and from the extravascular space, and the `intrinsic system' using only factors present in the circulation. However, with increasing knowledge about the interaction between factors from the two systems, the relevance of this model was questioned. The availability of recombinant FVIIa has made further research of the role of FVII/FVIIa and TF feasible, resulting in the current concept of haemostasis according to which the process principally occurs on two cell surfaces, the TF-bearing cell and the thrombin-activated platelet. A limited amount of thrombin is generated by the FVIIa-TF complex on the TF-bearing cell resulting in activation of platelets, FIX, FVIII and FV. The further and full thrombin generation then takes place on the activated platelet surface. The most frequently used assays for evaluation of the global haemostatic capacity are the prothrombin time (PT) and the activated partial thromboplastin time (APTT). The PT measures the formation of a fibrin clot in the presence of an abundance of TF thereby principally reflecting the initial thrombin generation dependent especially on FVII, FV and FX, while the APTT mimics the processes on the activated platelet surface involving FVIII, FIX, FXl, FV, FX and prothrombin. For more specific analyses, assay systems measuring the level of various coagulation factors are available. Platelet function is measured by platelet count and bleeding time. The platelet aggregation response to different agonists can be measured in special aggregometers. The usefulness of these techniques in evaluating a potential bleeding risk is, however, doubtful.

Tissue factor and factor VIIa as therapeutic targets in disorders of hemostasis

For hemophilia patients with inhibitors against FVIII or FIX, the development of recombinant factor VIIa (rFVIIa) raises the possibility of a therapeutic alternative whose availability and convenience of treatment are comparable to those of FVIII or FIX. In support of this new concept for the treatment of bleeding episodes, pharmacological doses of FVIIa have been shown to induce hemostasis. Pharmacological doses of rFVIIa enhance thrombin generation on thrombin-activated platelets, thereby facilitating the formation of strong, well-structured fibrin plugs resistant to premature proteolysis. Modified rFVIIa molecules with a stronger hemostatic potential have been produced. Inhibition of the FVII-TF-dependent pathway (TFPI and rFVIIai) has been tried in attempts to prevent thrombosis, with promising results in animal models so far not confirmed in clinical trials.

Insight into molecular changes of the FIX protein in a series of Italian patients with haemophilia B

Deficiency or dysfunction of factor IX FIX leads to haemophilia B (HB), an X-linked, recessive, bleeding disorder. On a molecular basis, HB is due to a heterogeneous spectrum of mutations spread throughout the F9 gene. In several instances, a cause-effect relation has been elucidated, in others predicted possibilities have been offered by crystallography inspection and by software-constructed models of the protein. The aim of this study was to contribute to the understanding of HB molecular pathology. The F9 missense mutations we identified in 21 unrelated Italian HB patients by direct sequencing of the whole F9 coding regions were inspected for the causative effect they provoked on the ensuing transcript, and on the protein structure. Each alteration was studied in order to: (i) characterize the defect on the basis of the nature of the mutation; (ii) identify the predicted defect that is induced in the gene and (iii) speculate about the potential, detrimental effects which upset the protein functionality through an idealized FIX model. The resulting data may further contribute to the comprehension of the mechanisms underlying the disease.

Effects of genetic fusion of factor IX to albumin on in vivo clearance in mice and rabbits

Individuals with haemophilia B require replacement therapy with recombinant or plasma-derived coagulation factor IX (fIX). More benefit per injected dose might be obtained if fIX clearance could be slowed. The contribution of overall size to fIX clearance was explored, using genetic fusion to albumin. Recombinant murine fIX (MIX), and three proteins with C-terminal epitope tags were expressed in HEK 293 cells: tagged MIX (MIXT), tagged mouse serum albumin (MSAT) and MFUST, in which MIX and MSAT were fused in a single polypeptide chain. Proteins MFUST and MIXT were two- to threefold less active in clotting assays than MIX. In mice, the area under the clearance curve (AUC) was reduced for MFUST compared with MSAT or plasma-derived MSA (pd-MSA); the terminal catabolic half-life (t(0.5)) did not differ amongst the three proteins. Two minutes after injection, >40% of the injected MFUST was found in the liver, compared with <10% of either MSAT or pd-MSA. In rabbits, the AUC for MFUST was reduced compared to MIXT, MSAT, or pd-MSA, while the t(0.5) of the fusion protein fell between that of MIXT and MSAT or pd-MSA. Similar results were obtained with non-radioactive fused or non-fused recombinant human fIX in fIX knockout mice. The clearance behaviour of the fusion protein thus more closely resembled that of fIX than that of albumin despite a modest increase in terminal half-life, suggesting that fIX-specific interactions that are important in determining clearance were maintained in spite of the increased size of the fusion protein.