A novel congenital dysprothrombinemia leading to defective prothrombin maturation

- We describe a novel mutation in F2 gene, Val322Glu, causing dysprothrombinemia
- The mutation leads to reduced FII activity (0.82%) but normal FII antigen levels
- Activation of the mutant FII by ecarin results in accumulation of meizothrombin
- Accumulation of meizothrombin reveals defective prothrombin maturation into thrombin
- Residual activity of meizothrombin on fibrinogen may explain patient's mild bleeding

IntroductionProthrombin deficiency is a very rare disorder caused by mutations in the F2 gene that generate hypoprothrombinemia or dysprothrombinemia and is characterized by bleeding manifestations that can vary from clinically irrelevant to life-threatening.AimHere we characterize a patient with a novel missense mutation in F2, c.1090 T/A (p.Val322Glu), that causes severe dysprothrombinemia.MethodsCoagulation assays, prothrombin Western Blotting, FII activation by Ecarin, fibrinogen degradation products quantification and thrombin generation assay were carried out to assess prothrombin expression and function. PCR followed by direct sequencing was carried out to characterize the mutation. In silico analysis for missense variant and molecular modeling were applied to predict the mechanism that leads to dysprothrombinemia.Results and conclusionsThe homozygous patient had a markedly prolonged prothrombin time, strongly reduced FII activity (0.82%) but normal antigen levels. In the thrombin generation assay the lag time and the peak height were unmeasurable, suggesting that the Val322Glu mutation results in the inability of the mutant prothrombin to be fully activated to thrombin. In fact, prothrombin activation by ecarin was defective, with a massive accumulation of the meizothrombin intermediate. Molecular modeling and dynamic simulation studies showed that the Val322Glu mutation interferes with protein flexibility at Arg271 and Arg320. This impairs the switch of the protein from zymogen to proteinase, thus preventing the formation of thrombin. Accumulated meizothrombin, however, maintains some fibrinogen-degrading activity, as shown by the formation of FDPs, and this probably explains the patient's mild bleeding phenotype.