Novel Y283C mutation of the A subunit for coagulation factor XIII: molecular modelling predicts its impaired protein folding and dimer formation

Genetic landscape in coagulation factor XIII associated defects – Advances in coagulation and beyond

Coagulation factor XIII (FXIII) acts as a fine fulcrum in blood plasma that maintains the balance between bleeding and thrombosis by covalently crosslinking the pre-formed fibrin clot into an insoluble one that is resistant to premature fibrinolysis. In plasma, FXIII circulates as a pro-transglutaminase complex composed of the dimeric catalytic FXIII-A encoded by the F13A1 gene and dimeric carrier/regulatory FXIII-B subunits encoded by the F13B gene. Growing evidence accumulated over decades of exhaustive research shows that not only does FXIII play major roles in both pathological extremes of hemostasis i.e. bleeding and thrombosis, but that it is, in fact, a pleiotropic protein with physiological roles beyond coagulation. However, the current FXIII genetic-epidemiological literature is overwhelmingly derived from the bleeding pathology associated with its deficiency. In this article we review the current clinical, functional, and molecular understanding of this fascinating multifaceted protein, especially putting into the same perspective its genetic landscape.

An update of the mutation profile of Factor 13 A and B genes

Mutational reports over the past two decades have accumulated an immense amount of literature for inherited Factor XIII deficiency. However, the genotype and phenotype correlations for inherited Factor XIII deficiency are complicated. While many studies clearly prove a cause and effect relationship for the reported mutations, others are lacking in this regard. The F13B gene remains an elusive component as far as inherited Factor XIII deficiencies are concerned. Also, an in-depth analysis into the heterozygous state of this deficiency is also lacking. In this review we have tried to analyze and present an exhaustive amount of mutational data from the past three decades. The source of our mutational data is our website dedicated to Factor XIII deficiencies (www.F13-database.de) as well as literature search done on the Pubmed (www.ncbi.nlm.nih.gov/pubmed).

Role of calcium in the conformational dynamics of factor XIII activation examined by hydrogen-deuterium exchange coupled with MALDI-TOF MS

Factor XIII catalyzes formation of γ-glutamyl-ε-lysyl crosslinks within fibrin clots. FXIII A(2) can be activated proteolytically with thrombin and low mM Ca(2+) or nonproteolytically with high monovalent/divalent cations along with low mM Ca(2+). Physiologically, FXIII A(2) is poised to respond to transient influxes of Ca(2+) in a Na(+) containing environment. A successful strategy to monitor FXIII conformational events is hydrogen-deuterium exchange (HDX) coupled with mass spectrometry. FXIII A(2) was examined in the presence of different cations (Ca(2+), Mg(2+), Ba(2+), Cu(2+), Na(+), TMAC(+), and EDA(2+)) ranging from 1 to 2mM, physiological Ca(2+) concentration, to 50-500mM for nonproteolytic activation. Increases in FXIII solvent exposure could already be observed at 1mM Ca(2+) for the dimer interface, the catalytic site, and glutamine substrate regions. By contrast, solvent protection was observed at the secondary cleavage site. These events occurred even though 1mM Ca(2+) is insufficient for FXIII activation. The metals 1mM Mg(2+), 1mM Ba(2+), and 1mM Cu(2+) each led to conformational changes, many in the same FXIII regions as Ca(2+). FXIII could also be activated nonproteolytically with 500mM tetramethylammonium chloride (TMAC(+)) and 500mM ethylenediamine (EDA(2+)), both with 2mM Ca(2+). These different HDX studies help reveal the first FXIII segments that respond to physiological Ca(2+) levels.

Factor XIII: novel structural and functional aspects

Factor (F)XIII is a protransglutaminase that, in addition to maintaining hemostasis, has multiple plasmatic and intracellular functions. Its plasmatic form (pFXIII) is a tetramer of two potentially active A (FXIII-A) and two inhibitory/carrier B (FXIII-B) subunits, whereas its cellular form (cFXIII) is a dimer of FXIII-A. FXIII-A belongs to the family of transglutaminases (TGs), which show modest similarity in the primary structure, but a high degree of conservatism in their domain and sub-domain secondary structure. FXIII-A consists of an activation peptide, a β-sandwich, a catalytic and two β-barrel domains. FXIII-B is a glycoprotein consisting of 10 repetitive sushi domains each held together by two internal disulfide bonds. The structural elements of FXIII-A involved in the interaction with FXIII-B have not been elucidated; in FXIII-B the first sushi domain seems important for complex formation. In the circulation pFXIII is bound to the fibrinogen γ'-chain through its B subunit. In the process of pFXIII activation first thrombin cleaves off the activation peptide from FXIII-A, then in the presence of Ca(2+) FXIII-B dissociates and FXIII-A becomes transformed into an active transglutaminase (FXIIIa). The activation is highly accelerated by the presence of fibrin(ogen). cFXIII does not require proteolysis for intracellular activation. The three-dimensional structure of FXIIIa has not been resolved. Based on analogies with transglutaminase-2, a three-dimensional structure of FXIIIa was developed by molecular modeling, which shows good agreement with the drastic structural changes demonstrated by biochemical studies. The structural requirements for enzyme-substrate interaction and for transglutaminase activity are also reviewed.

Factor XIII deficiency

Inherited factor XIII (FXIII) deficiency is a rare bleeding disorder that can present with umbilical bleeding during the neonatal period, delayed soft tissue bruising, mucosal bleeding and life-threatening intracranial haemorrhage. FXIII deficiency has also been associated with poor wound healing and recurrent miscarriages. FXIII plays an integral role in haemostasis by catalysing the cross-linking of fibrin, platelet membrane and matrix proteins throughout thrombus formation, thus stabilizing the blood clot. The molecular basis of FXIII deficiency is characterized by a high degree of heterogeneity, which contributes to the different clinical manifestations of the disease. There have been more than 60 FXIII mutations identified in the current literature. In addition, single nucleotide polymorphisms have been described, some of which have been shown to affect FXIII activity, contributing further to the heterogeneity in patient presentation and severity of clinical symptoms. Although there is a lifelong risk of bleeding, the prognosis is excellent when current prophylactic treatment is available using cryoprecipitate or plasma-derived FXIII concentrate.

Factor XIII A subunit-deficient mice developed severe uterine bleeding events and subsequent spontaneous miscarriages

To understand the molecular pathology of factor XIII (FXIII) deficiency in vivo, its A subunit (FXIIIA)-knockout (KO) mice were functionally analyzed. Although homozygous FXIIIA female KO mice were capable of becoming pregnant, most of them died due to excessive vaginal bleeding during gestation. Abdominal incisions revealed that the uteri of the dead mice were filled with blood and that some embryos were much smaller than others within a single uterus. A series of histologic examinations of the pregnant animals suggested that massive placental hemorrhage and subsequent necrosis developed in the uteri of the FXIIIA KO mice on day 10 of gestation. This was true regardless of the genotypes of fetuses. These results are reminiscent of spontaneous miscarriage in pregnant humans with FXIII deficiency and indicate that maternal FXIII plays a critical role in uterine hemostasis and maintenance of the placenta during gestation.

Identification of an inframe deletion and a missense mutation in the factor XIIIA gene in two Turkish patients

We report two novel mutations in factor XIIIA (FXIIIA) gene that caused congenital factor XIII deficiency in two unrelated patients. The first alteration, a missense mutation Leu235Arg in exon 6 of FXIIIA gene, is located in the putative calcium-binding part of the core domain of the enzyme. Replacement of non-polar hydrophobic leucine residue with positively charged arginine residue is likely to effect protein folding thus destabilizing the molecule. The second mutation is a 3-bp deletion in exon 14 of FXIIIA gene. This deletion is located in beta barrel 2 domain of the protein and results in translation of an aberrant FXIIIA molecule that lacks lysine residue either at positions 677 or 678. As this inframe deletion is located in a direct repetetive sequence of AAGAAG, that codes for two lysine residues, the exact location of deletion could not be detected.

Impaired protein folding, dimer formation, and heterotetramer assembly cause intra- and extracellular instability of a Y283C mutant of the A subunit for coagulation factor XIII

Factor XIII (XIII) is a heterotetramer consisting of two catalytic A subunits (XIIIA) and two noncatalytic B subunits (XIIIB). We examined the molecular mechanisms of a Y283C mutation which had previously been identified in a patient with XIIIA deficiency. The recombinant Y283C protein was labile when expressed in MEG-01 cells, which can endogenously synthesize XIIIA. We also included two other mutants, G562R and I464stop, previously characterized in a non-XIIIA-producing cell line. All these mutants exhibited decreased thermostability and resistance against proteolytic digestion when compared with the wild-type. Gel-filtration analysis revealed that the mutants were in monomer form, while the wild-type formed a dimer. These results were consistent with the prediction by molecular modeling that the mutant molecules would be misfolded. Although assembly of a heterotetramer with XIIIB was demonstrated for Y283C, its binding ability was 10% that of the wild-type. No complex formation was observed for the G562R or I464stop mutants. The wild-type was stabilized in plasma by complex formation with XIIIB, resulting in an increased resistance against proteolytic digestion. In contrast, the mutants were unstable in plasma even in the presence of XIIIB. Thus, impaired folding, dimer formation, and heterotetramer assembly of the mutant XIIIAs lead to both intra- and extracellular instability, which must be responsible for XIIIA deficiency in the patient.