Identification and Characterization of Two Novel Mutations (Q421K and R123P) in Congenital Factor XII Deficiency

Mechanism, Functions, and Diagnostic Relevance of FXII Activation by Foreign Surfaces

Factor XII (FXII) is a serine protease zymogen produced by hepatocytes and secreted into plasma. The highly glycosylated coagulation protein consists of six domains and a proline-rich region that regulate activation and function. Activation of FXII results from a conformational change induced by binding ("contact") with negatively charged surfaces. The activated serine protease FXIIa drives both the proinflammatory kallikrein-kinin pathway and the procoagulant intrinsic coagulation cascade, respectively. Deficiency in FXII is associated with a prolonged activated partial thromboplastin time (aPTT) but not with an increased bleeding tendency. However, genetic or pharmacological deficiency impairs both arterial and venous thrombosis in experimental models. This review summarizes current knowledge of FXII structure, mechanisms of FXII contact activation, and the importance of FXII for diagnostic coagulation testing and thrombosis.

Genetic analysis of a pedigree with hereditary coagulation factor XII deficiency

The aim of this study was to elucidate the molecular defects of a Chinese family with hereditary coagulation factor XII (FXII) deficiency. The FXII activity (FXII:C) and FXII antigen (FXII:Ag) levels were measured by clotting assay and ELISA, respectively. To identify mutations, the F12 gene sequencing was carried out. ClustalX-2.1-win and four online bioinformatics tools were applied to study the conservatism and harm of the mutation. The proband's FXII:C and FXII:Ag were 3 and 4%, respectively. Sequencing analysis revealed compound heterozygous mutations, including the deletion mutation (c.130delG) resulting in p.E26Sfs∗50 and the missense mutation (c.1561G>A) resulting in p.E502K. Bioinformatics indicated that mutations probably disrupt the function of the FXII protein. The c.130delG heterozygous deletion variation and the c.1561G>A heterozygous missense variation were responsible for the reduction of FXII:C in this family, of which c.130delG was first reported in the world.

Compound heterozygous mutations Glu502Lys and Met527Thr of the FXII gene in a patient with factor XII deficiency

OBJECTIVE

To study the gene mutation of human coagulation factor XII (FXII) in a Chinese family with FXII deficiency and it will help us to understand the pathogenesis of this type of disease.

CLINICAL PRESENTATION

The proband was a 50-year-old male who had a fracture of the right humerus. The routine presurgical coagulation test showed a significant prolonged activated partial thromboplastin time (APTT) at 59.1s (reference range, 29.0-43.0s).

TECHNIQUES

FXII activity (FXII:C) and FXII antigen (FXII:Ag) were detected by the one-stage clotting method and ELISA, respectively. To identify mutations, the FXII whole exon and flanking sequences were carried out. Suspected mutations were confirmed by reverse sequencing. The conservatism and possible impact of the amino acid substitution were analyzed by ClustalX-2.1-win and four online bioinformatics tools.

RESULTS

Phenotypic analysis revealed the FXII:C and FXII:Ag of the proband were 4% and 5%, respectively (normal range, 72-113%). Gene sequencing detected compound heterozygous mutations c.1561G > A (Glu502Lys) and c.1637T > C (Met527Thr) in exon 13. Bioinformatics and model analysis indicated that mutations probably had disrupted the function and structure of the FXII protein.

CONCLUSION

We detected two missense mutations Glu502Lys and Met527Thr in the catalytic domain of the proband, of which Met527Thr was first reported in the world. Our findings suggest that the double mutations in the FXII gene were the causing reasons for the decreased FXII:C and FXII:Ag. These results not only enriched the F12 mutation database in this condition, but also helped to identify the genetic defects of FXII in China.

Proteosomal degradation of naturally recurring R260C missense and exon-IV deletion mutants of factor XIII A-subunit expressed in mammalian cells

Congenital factor XIII (FXIII) deficiency is a severe bleeding disorder. We previously identified an Arg260Cys missense mutation and an exon-IV deletion in patients' A subunit genes, F13A. To characterize the molecular/cellular basis of this disease, we expressed a wild type and these mutant A subunits in baby hamster kidney (BHK) cells. The mutant proteins were expressed less efficiently than the wild type. These mutants gradually decreased inside BHK cells, whereas the wild type remained largely unchanged. The decline/decrease in these mutants was completely blocked/restored by a potent proteasome inhibitor, MG-132. This was consistent with the prediction by molecular modelling that the mutant molecules would lose the native structure of wild-type molecule, leading to their instability and degeneration and ultimately to degradation. These mutants might have significantly altered conformations, resulting in the rapid degradation by the proteasome inside the synthesizing cells, and ultimately leading to FXIII deficiency.

Compound heterozygous mutations (p.Leu13Pro and p.Tyr294*) associated with factor VII deficiency cause impaired secretion through ineffective translocation and extensive intracellular degradation of factor VII

INTRODUCTION

Congenital coagulation factor VII (FVII) deficiency is a rare coagulation disease. We investigated the molecular mechanisms of this FVII deficiency in a patient with compound heterozygous mutations.

METHODS

A 22-year-old Japanese female was diagnosed with asymptomatic FVII deficiency. The FVII activity and antigen were greatly reduced (activity, 13.0%; antigen, 10.8%). We analyzed the F7 gene of this patient and characterized mutant FVII proteins using in vitro expression studies.

RESULTS

Sequence analysis revealed that the patient was compound heterozygous with a point mutation (p.Leu13Pro) in the central hydrophobic core of the signal peptides and a novel non-sense mutation (p.Tyr294*) in the catalytic domain. Expression studies revealed that mutant FVII with p.Leu13Pro (FVII13P) showed less accumulation in the cells (17.5%) and less secretion into the medium (64.8%) than wild type showed. Truncated FVII resulting from p.Tyr294* (FVII294X) was also decreased in the cells (32.0%), but was not secreted into the medium. Pulse-chase experiments revealed that both mutants were extensively degraded intracellularly compared to wild type. The majority of FVII13P cannot translocate into endoplasmic reticulum (ER). However, a small amount of FVII13P was processed normally with post-translational modifications and was secreted into the medium. The fact that FVII294X was observed only in ER suggests that it is retained in ER. Proteasome apparently plays a central role in these degradations.

CONCLUSIONS

These findings demonstrate that both mutant FVIIs impaired secretion through ineffective translocation to and retention in ER with extensive intracellular degradation, resulting in an insufficient phenotype.

Genetic analysis of a pedigree with combined factor XII and factor XI deficiency

The objective of the present study was to identify the gene mutations of factor XI (FXI) and factor XII (FXII) in a Chinese pedigree with combined congenital FXI and FXII deficiencies. The proband was a 40-year-old woman with deficiency in both FXI (49%) and FXII (0%) activities. Blood samples from 10 other members of her family were collected and used for detection of FXI, FXII activities (FXI: C, FXII: C) and antigen levels. Genetic analysis to detect mutations in FXI, FXII genes was also performed. The proband's mother, three brothers, two sisters, her son and her daughter all have lowered FXII: C. Furthermore, her mother and one of her brothers also have lowered FXI: C. Gene sequencing for FXI in affected members revealed a heterozygous C23179T point mutation in exon 11 resulting in substitution of arginine 396 by cysteine. Gene sequencing for FXII revealed a C46T in the promoter region and a deletion mutation of two nucleotides CA at position 9160 and 9161 in exon 5. The deletion mutation can lead to frameshift mutation and premature termination of transcription in exon 6. We found a new heterozygous missense mutation in the FXI gene and a new nonsense mutation of two nucleotides deletion which caused frameshift mutation and premature termination of transcription in the FXII gene in a Chinese family with combined FXI and FXII deficiencies.

A case of atypical hemolytic uremic syndrome due to anti-factor H antibody in a patient presenting with a factor XII deficiency identified two novel mutations

A 9-year-old boy with pallor and macrohematuria showed hemolytic anemia, thrombocytopenia and renal failure. There was no history of diarrhea and the stool culture was negative. A diagnosis of atypical hemolytic uremic syndrome (HUS) was confirmed; however, the cause of the prolonged activated partial thromboplastin time (APTT) was unknown. Plasma exchange and hemodialysis were performed because of progressive hemolytic anemia and renal dysfunction. Fresh frozen plasma was administered frequently to correct the prolonged APTT after hemolysis was controlled and C3 levels had recovered. Factor H (FH) and factor I (IF) levels were normal and we did not detect mutations of FH, IF and membrane cofactor protein. Further investigation revealed the presence of anti-FH antibody in the patient's plasma and a deficiency of coagulation factor XII. Analysis of the patient's coagulation system displayed <3% functional activity of factor XII, whereas levels of other coagulation factors were within the normal range. Two novel mutations (W222G and R447S) were identified upon analysis of the factor XII gene in this patient. Moreover, further investigation revealed that compound heterozygous mutations were present in two of the patient's three siblings, while the third sibling only had a mutation at W222G. The patient was treated for atypical HUS; however, no treatment was required for factor XII deficiency as he did not display a hemorrhagic tendency. We report here a rare case of atypical HUS due to anti-FH antibody presenting with a coagulation factor XII deficiency.

Sequence variation and genetic evolution at the human F12 locus: mapping quantitative trait nucleotides that influence FXII plasma levels

The level of Factor XII (FXII) is an important phenotype that exhibits a high genetic component and is associated with thrombotic disease. In a genome-wide linkage scan, we demonstrated that the F12 gene represents a quantitative trait locus (QTL) that influences FXII levels. The current study investigated the genetic architecture of the F12 gene to locate polymorphism(s) responsible for the variation of FXII levels. Re-sequencing of the F12 gene in 40 unrelated individuals (selected from the tails of normal distribution of FXII levels) identified 26 polymorphisms which were genotyped in 398 individuals belonging to 21 families from the GAIT Project. By a measured genotype association analysis, eight of 26 SNPs showed significant P-values less than 10(-5) (after multiple test correction) with FXII levels. In addition, the Bayesian Quantitative Trait Nucleotide method, which infers those polymorphisms most likely to have a direct influence on the trait under study, provided evidence that only rs1801020 variation accounted for the variance attributed to this QTL. Moreover, we have analyzed the evolutionary processes that produced the variation in F12 gene and concluded that is evolutionarily neutral and that the T allele of the rs1801020 appeared approximately 100 000 years ago and spread to most human populations rising to high frequencies by genetic drift. Our study provides a template for future genetic studies of human quantitative traits, as we move beyond QTL localization to the polymorphisms responsible for the variation of important biomedical phenotypes.

Deep venous thrombosis and previous myocardial infarction in mild factor XII deficiency: a risk factor for both venous and arterial thrombosis

Factor XII deficiency is associated with increased risk for both arterial and venous thrombosis. We describe a case of DVT involving superficial femoral and popliteal vein occurred following total hip replacement and despite prophylaxis with low molecular weight heparin in a subject with previous acute myocardial infarction (AMI). Tests of haemostasis documented a slightly prolonged activated partial thromboplastin time (APTT) (45'') due to mild factor XII deficiency (clotting activity 32%). A therapeutic dose of enoxaparin was started, together with warfarin therapy. The patient was advised to continue oral anticoagulation indefinitely. Although cases of both venous and arterial thrombosis in carriers of severe factor XII deficiency have been already reported, to our knowledge this is the first case in the literature occurred in a carrier of partial factor XII deficiency. In conclusion, factor XII deficiency should be suspected if a patient presents with recurrent arterial and/or venous thrombosis and prolonged APTT. If this defect is diagnosed, in the presence of a history of thrombotic events, lifelong anticoagulation could be considered.

Phenotypic consequences of genetic variation at hemizygous alleles: Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency

PURPOSE

We tested the hypothesis that Sotos syndrome (SoS) due to the common deletion is a contiguous gene syndrome incorporating plasma coagulation factor twelve (FXII) deficiency. The relationship between FXII activity and the genotype at a functional polymorphism of the FXII gene was investigated.

METHODS

A total of 21 patients including those with the common deletion, smaller deletions, and point mutations, and four control individuals were analyzed. We examined FXII activity in patients and controls, and analyzed their FXII 46C/T genotype using direct DNA sequencing.

RESULTS

Among 10 common deletion patients, seven patients had lower FXII activity with the 46T allele of the FXII gene, whereas three patients had normal FXII activity with the 46C allele. Two patients with smaller deletions, whose FXII gene is not deleted had low FXII activity, but one patient with a smaller deletion had normal FXII. Four point mutation patients and controls all had FXII activities within the normal range.

CONCLUSION

FXII activity in SoS patients with the common deletion is predominantly determined by the functional polymorphism of the remaining hemizygous FXII allele. Thus, Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency.

Factor XII Shizuoka, a novel mutation (Ala392Thr) identified and characterized in a patient with congenital coagulation factor XII deficiency

We identified a novel mutation (Ala392Thr) in the factor XII (FXII) gene of a patient with congenital FXII deficiency, designated Factor XII Shizuoka. The proband was an asymptomatic 63-year-old Japanese male with an abnormal coagulation test, discovered by chance during preoperative testing. The FXII activity was under 3% and antigen level was under 10%. Sequence analysis of the proband's FXII gene revealed a homozygous nucleotide substitution G to A in exon 10, resulting in the amino acid substitution Ala392 to Thr in the catalytic domain. We constructed the mutant FXII cDNA in an expression plasmid vector and transfected it into Chinese hamster ovary (CHO) cells. The recombinant wild-type FXII antigen was detected in the culture medium by immunoprecipitation assay, but the mutant FXII (A392T) was not observed. Both the wild-type FXII and A392T cell lysates, however, contained equivalent levels of FXII antigen and FXII mRNA, as estimated by Western blotting and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), respectively. These findings suggest that the Ala392 to Thr substitution impairs intracellular protein processing and causes a cross-reacting material -negative FXII deficiency.

Genetic analyses and expression studies identified a novel mutation (W486C) as a molecular basis of congenital coagulation factor XII deficiency

We analyzed the factor XII (FXII) gene of a patient with congenital FXII deficiency and identified a novel amino acid substitution (W486C) in the catalytic domain. The proband was an asymptomatic 49-year-old Japanese female with abnormal coagulation test, discovered by chance. The FXII activity and antigen level were both under 10%, suggesting a cross-reacting material-negative FXII deficiency. Sequence analysis of the proband's FXII gene revealed a homozygous nucleotide substitution G --> C in exon 12, resulting in the amino acid substitution W486C in the catalytic domain. We constructed the mutant FXII cDNA in an expression plasmid vector and transfected it into Chinese hamster ovary cells. The recombinant wild-type FXII antigen was detected in the culture medium by immunoprecipitation assay, but the mutant FXII (W486C) was not observed. On the other hand, both the wild-type FXII and W486C cell lysates contained FXII antigen and FXII mRNA, as estimated by western blotting and quantitative reverse transcriptase-polymerase chain reaction. These findings suggest that the W486C substitution of FXII impairs intracellular processing of the protein and/or transport system.