Summary
Factor X (FX) is a vitamin K-dependent coagulation zymogen, which upon activation
to factor Xa assembles into the prothrombinase complex to activate prothrombin to
thrombin. FX can be activated by either factor VIIa-tissue factor or factor IXa-factor
VIIIa in extrinsic and intrinsic pathways, respectively. In this study, we identified
a bleeding patient with moderate FX deficiency who exhibits a clotting defect only
in the intrinsic pathway. Exome sequencing revealed that the patient carries a novel
homozygous missense mutation that results in substitution of Thr211 with Pro in the
activation peptide of FX. Thr211 is the site of an O-linked glycosylation in the activation
peptide of FX. We postulated that the lack of this post-translational modification
specifically impacts the activation of FX by intrinsic Xase, thereby impairing thrombin
generation in the subject. To test this hypothesis, we expressed both wild-type FX
and FX containing this mutation in mammalian cells and following the purification
of the zymogens to homogeneity characterized their properties in both purified and
plasma-based assay systems. Analysis of the results suggests that Thr211 to Pro substitution
renders the FX mutant a poor substrate for both physiological activators, however,
at physiological concentration of the substrate, the clotting defect manifest itself
only in the intrinsic pathway, thus explaining the bleeding phenotype for the patient
carrying this mutation.
Keywords
Coagulation factors - factor VIII - protein function / activity - thrombin - tissue
factor / factor VII