Molecular defects in the factor X gene caused by novel heterozygous mutations IVS5+1G>A and Asp409del

Characterization of a Missense Mutation in the Catalytic Domain and a Splicing Mutation of Coagulation Factor X Compound Heterozygous in a Chinese Pedigree

Background: Congenital coagulation factor X (FX) deficiency is a rare bleeding disorder with an incidence of one in one million caused by mutations in the FX-coding gene(F10), leading to abnormal coagulation activity and a tendency for severe hemorrhage. Therefore, identifying mutations in FX is important for diagnosing congenital FX deficiency. Results: Genetic analysis of the proband identified two single-base substitutions: c.794T > C: p.Ile265Thr and c.865 + 5G > A: IVS7 + 5G > A. His FX activity and antigen levels were < 1% and 49.7%, respectively; aPTT and PT were prolonged to 65.3 and 80.5 s, respectively. Bioinformatics analysis predicted the two novel variants to be pathogenic. In-vitro expression study of the missense mutation c.794T > C: p.Ile265Thr showed normal synthesis and secretion. Activation of FXs by RVV, FVII/TF, and FVIII/FIX all showed no obvious difference between the variant and the reference. However, clotting activity by PT and aPTT assays and activity of thrombin generation in a TGA assay all indicated reduced activity of the mutant FX-Ile265Thr compared to FX-WT. Minigene assay showed a normal splicing mode c.865 + 5G > A: IVS7 + 5G > A, which is inconsistent with clinical phenotype. Conclusions: The heterozygous variants c.794T > C: p.Ile265Thr or c.865 + 5G > A: IVS7 + 5G > A indicate mild FX deficiency, but the compound heterozygous mutation of the two causes severe congenital FX deficiency. Genetic analysis of these two mutations may help characterize the bleeding tendency and confirm congenital FX deficiency. In-vitro expression and functional study showed that the low activity of the mutant FX-Ile265Thr is caused by decrease in its enzyme activity rather than self-activation. The minigene assay help us explore possible mechanisms of the splicing mutation. However, more in-depth mechanism research is needed in the future.

A novel factor X mutation Cys81 by Arg and a reported factor VII polymorphism Arg353 replaced by Gln co-occured in a patient

: Coagulation factor X and factor VII (FVII) are both very important components in blood coagulation. To study the molecular pathogenic mechanism of the inherited factor X and FVII deficiency, the factor X activity (FX:C) and FVII activity were tested with one-stage clotting methods. The factor X antigen and factor FVII antigen were tested with ELISA. All the exons, intron-exon boundaries and 5',3'-flanking regions of F10 and F7 genes were amplified by PCR with direct sequencing. The ClustalX software was used to analyze the conservative property of the mutation sites. The PolyPhen-2 and Sorting Intolerant From Tolerant (SIFT) online bioinformatics softwares were taken to predict whether the point mutation could affect protein function. The software Swiss-pdb Viewer was brought to analyze the impact of mutations on the structure and function of the protein. The thrombin generation tests were applied to evaluate whether there were obstacles in producing thrombin about the mutant protein. The FX:C and FVII activity of the proband were reduced to 35 and 42%, and the factor X antigen and factor FVII antigen were decreased to 43 and 55%, simultaneously. Correspondingly, a FX:Cys81Arg (Cys81 by Arg) mutation and a FVII:Arg353 replaced by Gln polymorphism were detected in the proband. The Cys81 of factor X was conserved among homologous species, but the Arg353 of FVII was not. All softwares analysis results indicated protein features and structures might be affected by the mutation and the polymorphism. And the thrombin generation tests showed that the mutant protein had obstacles in thrombin generation. We identified a FX:Cys81Arg mutation and a FVII:Arg353 replaced by Gln polymorphism in the proband. And they accounted for the decrease of the activity and antigen of factor X and FVII. Of note, the Cys81Arg of factor X was first reported in the world.

Molecular basis of the clotting defect in a bleeding patient missing the Asp-185 codon in the factor X gene

Factor X (FX) is a vitamin K-dependent plasma zymogen, which following activation to factor Xa (FXa), converts prothrombin to thrombin in the blood clotting cascade. It was recently demonstrated that a natural variant of FX carrying the Asp-185 deletion (FX-D185del, chymotrypsinogen numbering) was associated with mild bleeding in a patient with severe FX deficiency. In this study, we expressed FX-D185del in mammalian cells and characterized its properties in appropriate kinetic assays in purified systems. We discovered that while the FX variant can be normally activated by physiological activators; both amidolytic and proteolytic activities of the mutant are dramatically impaired. Interestingly, factor Va (FVa) significantly improved the proteolytic defect when the mutant protease was assembled into the prothrombinase complex. Thus, in contrast to >50-fold catalytic defect in the absence of FVa, the variant activated prothrombin with only ~2.5-fold decreased catalytic efficiency in the presence of the cofactor. The FXa variant dramatically lost its susceptibility to inhibition by antithrombin and tissue factor pathway inhibitor, thus exhibiting ~2-3 orders of magnitude lower reactivity with the plasma inhibitors. Further studies revealed that Na(+) no longer activates the variant protease, suggesting that the functionally important allosteric linkage between the Na(+)-binding and the P1-binding sites of the protease has been eliminated. These results suggest that the lower catalytic efficiency of FXa-D185del in the bleeding patient may be partially compensated by the loss of its reactivity with plasma inhibitors, possibly explaining the basis for the paradoxical severe FX deficiency with only mild bleeding tendency for this mutation.