AN IN-VITRO ANALYSIS OF THE COMBINATION OF HEMOPHILIA-A AND FACTOR-V-LEIDEN

The classification of factor VIII deficiency, generally used based on plasma levels of factor VIII, consists of severe (<1% normal factor VI II activity), moderate (1% to 4% factor VIII activity), or mild (5% to 25% factor VIII activity). A recent communication described four indi viduals bearing identical factor VIII mutations. This resulted in a se vere bleeding disorder in two patients who carried a normal factor V g ene, whereas the two patients who did not display severe hemophilia we re heterozygous for the factor V-LEIDEN mutation, which leads to the s ubstitution of Arg506 --> Gln mutation in the factor V molecule. Based on the factor VIII level measured using factor VIII-deficient plasma, these two patients were classified as mild/moderate hemophiliacs. We studied the condition of moderate to severe hemophilia A combined with the factor V-LEIDEN mutation in vitro in a reconstituted model of the tissue factor pathway to thrombin. In the model, thrombin generation was initiated by relipidated tissue factor and factor VIIa in the pres ence of the coagulation factors X, IX, II, V, and VIII and the inhibit ors tissue factor pathway inhibitor, antithrombin-III, and protein C. At 5 pmol/L initiating factor VIIa.tissue factor, a 10-fold higher pea k level of thrombin formation (350 nmol/L), was observed in the system in the presence of plasma levels of factor VIII compared with reactio ns without factor VIII. Significant increase in thrombin formation was observed at factor VIII concentrations less than 42 pmol/L (similar t o 6% of the normal factor VIII plasma concentration). In reactions wit hout factor VIII, in which thrombin generation was downregulated by th e addition of protein C and thrombomodulin, an increase of thrombin fo rmation was observed with the factor V-LEIDEN mutation. The level of i ncrease in thrombin generation in the hemophilia A situation was found to be dependent on the factor V-LEIDEN concentration. When the factor V-LEIDEN concentration was varied from 50% to 150% of the normal plas ma concentration, the increase in thrombin generation ranged from thre efold to sevenfold. The data suggested that the analysis of the factor V genotype should be accompanied by a quantitative analysis of the pl asma factor V-LEIDEN level to understand the effect of factor V-LEIDEN in hemophilia A patients. The presented data support the hypothesis t hat the factor V-LEIDEN mutation can increase thrombin formation in se vere hemophilia A. (C) 1997 by The American Society of Hematology.