Factor IX and prothrombin in amniotic fluid and fetal plasma: constraints
on prenatal diagnosis of hemophilia B and evidence of proteolysis
AR Thompson
Potential limitations of prenatal diagnosis of hemophilia B, as compared to
hemophilia A, include (1) occurrence of far more frequent defects with
abnormal circulating antigen, (2) lower levels of factor IX in fetal plasma
at 16 to 20 weeks gestation, and (3) the presence of factor IX antigen in
amniotic fluid. In addition, proteolysis could occur, especially with
amniotic fluid contamination of fetal plasma. A sensitive polyclonal
immunoradiometric assay for factor IX antigen was used to characterize the
range of levels in amniotic fluids and fetal plasma samples. To assess for
altered forms, factor IX species were compared to those of a homologous
clotting factor, prothrombin. Fourteen postmortem abortus blood samples
from fetuses of 14 to 23 weeks gestation had factor IX antigen levels that
averaged 5.1 U/dL and ranged from 1.7 to 15 U/dL. Amniotic fluid factor IX
antigen averaged 2.9 U/dL, with a range from 1.4 to 8.5 U/dL in 19 separate
amniocentesis samples. Thus, in a male fetus at risk of hemophilia B and
with a low circulating level of gene product, mixture of fetal plasma with
amniotic fluid could severely limit prenatal diagnosis, assuming that the
amniotic fluid factor IX is of maternal origin. Despite rapid processing of
amniotic fluid samples, the prothrombin was extensively cleaved, suggesting
that it had been activated in vivo. On gel electrophoresis of amniotic
fluid samples, however, factor IX was only minimally cleaved. In the
postmortem fetal blood specimens, prothrombin was partially cleaved. On
crossed-immunoelectrophoresis, fetal plasma prothrombin showed decreased
migration in calcium, compared to EDTA, indicative of mature gamma-glutamyl
carboxylation. The latter presumably resulted from fetal hepatic synthesis.
Volume 64,
Issue 4,
pp. 867-874,
10/01/1984
Copyright © 1984 by The American Society of Hematology
