Wolfgang Miesbach: Three Cleavages, One Key to Factor VIII Activation
Wolfgang Miesbach, Head of Hemostasis/Hemophilia Center at Medical Clinic 2 at University Hospital Frankfurt, shared a post on LinkedIn about a recent article by Lacramioara Ivanciu et al., published in Blood, adding:
“Three cuts turn factor VIII on. Only one of them actually does it – the other two set the tempo.
FVIII circulates as an inert procofactor. Thrombin and FXa cleave it at three arginines:
- Arg372 — the decisive cut. It severs the A1–A2 linkage and is thought to expose the interface through which FVIIIa engages FIXa. Without it, no cofactor activity whatsoever.
- Arg740 — removes the residual B-domain from the heavy chain; no haemophilia-causing mutations are known at this site.
- Arg1689 — releases FVIII from von Willebrand factor and frees the light chain for membrane binding.
Ivanciu and colleagues built a FVIII in which Arg740 and Arg1689 are already processed during biosynthesis – a molecule one cut away from FVIIIa.
It is cleaved at Arg372 far faster, shows 7-10 higher apparent activity in the one-stage assay and shortens the thrombin generation lag time – yet is indistinguishable from conventional FVIII once activated.
The two accessory cuts therefore contribute no activity of their own. They govern how quickly Arg372 is reached.
In haemophilia A mice that was enough to normalise vessel occlusion at a tenth of the dose, and in the cremaster model it briefly out-clotted wild-type animals.
It still worked in mice lacking VWF. VWF guards FVIII in the circulation – without the a3 region the half-life drops 7- to 14-fold — but it is not required at the wound itself.
The variant is no drug: far too short-lived. The principle is what matters. Activation kinetics are a design axis we have barely used, and half-life extension need not run through VWF.”
Title: Enhanced factor VIII activation accelerates thrombin generation and hemostatic potency in mice
Authors: Lacramioara Ivanciu, Mettine H.A. Bos, Sudharsan Parthasarathy, Raffaella Toso, Rodney M. Camire
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