Further studies on factor IX antigen in hemophilia B.

Abstract

Hemophilia B is a sex-linked coagulation disorder characterized by low factor IX activity and by a variable factor IX antigen. Hemophilia A is also sex-linked, but factor VIII antigen is always normal. Nobody knows the reason for this discrepancy which is really striking if one takes into account the identical hereditary pattern and the common clinical features. The frequency of hemophilia B versus hemophilia A is about to 1 to 10. This disparity in frequency remains also unexplained. Hemophilia B shows a wide genetic heterogeneity [1-4,8,10,11]. We have recently reported that in a series of 23 hemophilia B cases, 69 % of patients had no detectable factor IX antigen in their plasma [5]. This was in agreement with the few studies available except for two which indicated either lower or higher percentages [7, 12]. The object of the present communication is to report further data on the factor IX antigen in a larger series of patients. The plasma of 34 patients with proven hemophilia B, including the 23 already presented, have been studied immunologically as previously reported [5]. The patients belonged to 28 different families and came from several Italian regions. In 26 of these patients no antigen was found. In 6 patients a normal factor IX antigen (60-160 %) was present. All these 6 patients had some factor IX activity (1-6 %) in their plasma but no correlation was found between factor IX activity and factor IX antigen. In 2 patients levels of factor IX antigen were reduced (20-35 %), and there was also some factor IX activity present (2.5-5 %). Patients with reduced or normal levels of factor IX antigen showed a less severe clinical picture as compared to those with no antigen detectable. This is in agreement with the presence of some factor IX activity in the former. The percentile figures obtained by us in this second study are almost identical to those previously reported on a smaller series of patients [5]. This indicates that the figure originally arrived at, namely 69 %, was very close to the real one. Among the additional 11 patients now reported, 10 patients had no detectable factor IX antigen (91%). This indicates that small series of patients may yield