Summary
The clinical severity in some patients with haemophilia A appears to be unrelated
to the levels of factor (F)VIII activity (FVIII:C), but mechanisms are poorly understood.
We have investigated a patient with a FVIII gene mutation at Arg1781 to His (R1781H) presenting with a mild phenotype despite FVIII:C of 0.9 IU/dl. Rotational
thromboelastometry using the patient’s whole blood demonstrated that the clot time
and clot firmness were comparable to those usually observed at FVIII:C 5–10 IU/dl.
Thrombin and FXa assays using plasma samples also showed that the peak levels of thrombin
formation and the initial rate of FXa generation were comparable to those observed
at FVIII:C 5–10 IU/dl. The results suggested a significantly greater haemostatic potential
in this individual than in those with severe phenotype. The addition of incremental
amounts of FX to control plasma with FVIII:C 0.9 IU/dl in clot waveform analyses suggested
that the enhanced functional tenase assembly might have been related to changes in
association between FVIII and FX. To further investigate this mechanism, we prepared
a stably expressed, recombinant, B-domainless FVIII R1781H mutant. Thrombin generation
assays using mixtures of control plasma and FVIII revealed that the coagulation function
observed with the R1781H mutant (0.9 IU/dl) was comparable to that seen with wild-type
FVIII:C at ∼5 IU/dl. In addition, the R1781H mutant demonstrated an ∼1.9-fold decrease
in K
m for FX compared to wild type. These results indicated that relatively enhanced binding
affinity of FVIII R1781H for FX appeared to moderate the severity of the haemophilia
A phenotype.
Note: An account of this work was presented, in part, at the 23rd Congress of the International Society of Thrombosis and Haemostasis, July 27, 2011,
Kyoto, Japan.
Keywords
Haemophilia A - FVIII - clinical phenotype - FX - association