A recurrent Gly43Asp substitution in coagulation Factor X rigidifies its catalytic pocket and impairs catalytic activity and intracellular trafficking

Molecular mechanism of a novel Ser362Asn mutation causing inherited FX deficiency in a Chinese family

Factor X (FX) deficiency is an inherited autosomal recessive bleeding disorder. Here, we analyzed a proband with FX deficiency in a Chinese family. Genetic analysis revealed that the proband and his affected sister was homozygous for c.1085G>A mutation, corresponding to a Ser362Asn substitution. In vitro expression experiments showed that the FX Ser362Asn mutation led to a significant reduction in activity levels in the culture medium. This Ser to Asn substitution may change the shape of the active site. Moreover, simulations of molecular dynamics indicated that the binding energy of the FX Ser362Asn to the substrate is higher than that of wild type and the side-chain conformation of the catalytic residue His276 (His42) is changed. This impairs the conformational switch of the protein from zymogen to proteinase, thus causing the functional defect of FX protein. Our findings suggest that the Ser362Asn substitution is a pathogenic mutation that causes inherited FX deficiency.

Rare bleeding disorders and advances in gene therapy

: Rare bleeding disorders usually begin in childhood and manifest as varying degrees of bleeding, which can be life-threatening in severe cases. With the development of gene editing technology, it is expected that hereditary coagulation factor disorders will someday be fundamentally cured by gene therapy. On account of their rarity, comprehension of these diseases is essential for the application of new treatment strategies. We have compiled the features of some newly discovered mutations of prothrombin, factor VII, and factor X in recent years. In addition, this review introduces the advances and obstacles in gene therapy.

The carboxyl-terminal region of human coagulation factor X as a natural linker for fusion strategies

Fusion with human serum albumin (HSA), which represents a well-established technique to extend half-life of therapeutic proteins, commonly exploits intervening peptide linkers as key components. Here, we explored the human coagulation factor X (FX) carboxyl-terminal region, previously demonstrated by us to be dispensable for secretion and coagulant activity, as a natural linker for fusion purposes. To test our hypothesis, we compared direct FX-HSA fusion with the designed FX-HSA fusion proteins mimicking the recombinant activated factor VII (rFVIIa)-HSA or factor IX (FIX)-HSA chimeras, both strongly dependent from artificial linkers. Three constructs were produced by direct tandem fusion (FX-HSA) and through flexible (glycine/serine; FX-GS-HSA, mimicking rFVIIa-HSA) or cleavable (incorporating the FX activation site; FX-CL-HSA, mimicking FIX-HSA) linkers. The FX-HSA was efficiently secreted and displayed prolonged plasma persistence in mice. All chimeras possessed remarkable pro-coagulant activity, comparable to FX for FX-HSA (88.7 ± 6.0%) and FX-CL-HSA (98.0 ± 16.4%) or reduced for FX-GS-HSA (55.8 ± 5.4%). Upon incubation with activators, FX-HSA and FX-CL-HSA displayed a correct activation profile while the FX-GS-HSA activation was slightly defective. In fluorogenic-based assays, FX-HSA showed normal activity over time and a specific amidolytic activity (1.0 ± 0.12) comparable to that of FX. Overall, the FX-HSA features indicate that the FX carboxyl-terminal region represents an intrinsic sequence allowing direct tandem fusion. Our results provide the first experimental evidence for i) a coagulation factor fusion protein with biological properties independent from artificial linkers, ii) the suitability of FX carboxyl-terminal region as a natural linker for fusion purposes.

Frequency of the p.Gly262Asp mutation in congenital Factor X deficiency

INTRODUCTION

Congenital factor X (FX) deficiency is a rare bleeding disorder inherited as an autosomal recessive trait with an incidence of 1 : 500 000-1 000 000. A total or partial deficiency of FX causes an impairment of clot formation, leading to a haemorrhagic disease, which manifests with bleeding symptoms of different severity, also unprovoked.

AIM

We analysed the clinical manifestations, laboratory phenotype and genotype in 12 patients from Turkey affected with severe FX deficiency.

METHODS

Prothrombin time (PT), activated partial thromboplastin time (APTT), FX activity (FX:C) and FX antigen level (FX:Ag) were measured, and mutation analysis was performed for all patients.

RESULTS

The most frequent bleeding episodes in patients were epistaxis and easy bruising (11/12, 91%), followed by haemarthroses (10/12, 83%). FX:C was <1% in 11 patients, and 4% in one. FX:Ag was reduced in all patients, consistent with type II deficiency. Direct sequencing of the factor X gene (F10) identified two different mutations: the novel 33 bp in-frame deletion p.Thr176_Gln186, c.526_558del, which seems to be associated with milder bleeding symptoms and the c.785G>A, p.Gly262Asp missense mutation (previously reported as Gly222Asp), which is associated with severe bleeding symptoms.

CONCLUSION

The p.Gly262Asp missense mutation was identified in 11 of the 12 patients in this study. Previously published cases on the same p.Gly262Asp mutation were Iranian patients originating from the border between Turkey and Iran suggesting that this mutation may be candidate as a good tool for mutational screening analysis in this area.