Deficiency of coagulation factor XIII A subunit caused by the dinucleotide deletion at the 5' end of exon III

The History of Factor XIII Deficiency

Despite the early discovery of factor XIII (FXIII) in 1944, the diagnosis of FXIII deficiency was not made until 1960, after all the other coagulation factor deficiencies, most likely due to the normality of routine coagulation testing in FXIII deficiency. Although the first case was detected by the clot solubility test and this test has long since been used to detect FXIII deficiency, the test is no longer recommended by experts. Over the past 60 years, knowledge about FXIII deficiency has expanded considerably, between 1992, when the first variant was identified, and 2022, 197 mutations have been reported. Almost all missense mutations have a similar effect on FXIII, leading to instability and faster degradation of mutant FXIII protein. Therapeutic options have evolved from historical fresh frozen plasma (FFP), old plasma, whole blood, and cryoprecipitate, to plasma-derived and recombinant FXIII concentrates, respectively available since 1993 and 2012. These concentrate products were respectively approved by the Food and Drug Administration in 2011 and 2013. This historical review covers various aspects of FXIII related disorders, including the discovery of the FXIII, associated disorders, molecular basis, diagnosis, and treatment of FXIII deficiency.

Increased bleeding and thrombosis in myeloproliferative neoplasms mediated through altered expression of inherited platelet disorder genes

An altered thrombo-hemorrhagic profile has long been observed in patients with myeloproliferative neoplasms (MPNs). We hypothesized that this observed clinical phenotype may result from altered expression of genes known to harbor genetic variants in bleeding, thrombotic, or platelet disorders. Here, we identify 32 genes from a clinically validated gene panel that were also significantly differentially expressed in platelets from MPN patients as opposed to healthy donors. This work begins to unravel previously unclear mechanisms underlying an important clinical reality in MPNs. Knowledge of altered platelet gene expression in MPN thrombosis/bleeding diathesis opens opportunities to advance clinical care by: (1) enabling risk stratification, in particular, for patients undergoing invasive procedures, and (2) facilitating tailoring of treatment strategies for those at highest risk, for example, in the form of antifibrinolytics, desmopressin or platelet transfusions (not current routine practice). Marker genes identified in this work may also enable prioritization of candidates in future MPN mechanistic as well as outcome studies.

Molecular basis of rare congenital bleeding disorders

Rare bleeding disorders (RBDs), including factor (F) I, FII, FV, FVII, combined FV and FVIII (CF5F8), FXI, FXIII and vitamin-K dependent coagulation factors (VKCF) deficiencies, are a heterogeneous group of hemorrhagic disorder with a variable bleeding tendency. RBDs are due to mutation in underlying coagulation factors genes, except for CF5F8 and VKCF deficiencies. FVII deficiency is the most common RBD with >330 variants in the F7 gene, while only 63 variants have been identified in the F2 gene. Most detected variants in the affected genes are missense (>50% of all RBDs), while large deletions are the rarest, having been reported in FVII, FX, FXI and FXIII deficiencies. Most were located in the catalytic and activated domains of FXI, FX, FXIII and prothrombin deficiencies. Understanding the proper molecular basis of RBDs not only can help achieve a timely and cost-effective diagnosis, but also can help to phenotype properties of the disorders.

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.

Challenges imposed by minor reference alleles on the identification and reporting of clinical variants from exome data

Background

The conventional variant calling of pathogenic alleles in exome and genome sequencing requires the presence of the non-pathogenic alleles as genome references. This hinders the correct identification of variants with minor and/or pathogenic reference alleles warranting additional approaches for variant calling.

Results

More than 26,000 Exome Aggregation Consortium (ExAC) variants have a minor reference allele including variants with known ClinVar disease alleles. For instance, in a number of variants related to clotting disorders, the phenotype-associated allele is a human genome reference allele (rs6025, rs6003, rs1799983, and rs2227564 using the assembly hg19). We highlighted how the current variant calling standards miss homozygous reference disease variants in these sites and provided a bioinformatic panel that can be used to screen these variants using commonly available variant callers. We present exome sequencing results from an individual with venous thrombosis to emphasize how pathogenic alleles in clinically relevant variants escape variant calling while non-pathogenic alleles are detected.

Conclusions

This article highlights the importance of specialized variant calling strategies in clinical variants with minor reference alleles especially in the context of personal genomes and exomes. We provide here a simple strategy to screen potential disease-causing variants when present in homozygous reference state.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4433-3) contains supplementary material, which is available to authorized users.

Blood coagulation factor XIII and factor XIII deficiency

Factor XIII (FXIII) is a multifunctional pro-γ-transglutaminase that, in addition to its well-known role in hemostasis, has a crucial role in angiogenesis, maintenance of pregnancy, wound healing, bone metabolism, and even cardio protection. FXIII deficiency (FXIIID) is a rare bleeding disorder (RBD) with an estimated incidence of one per two million that is accompanied by life-threatening bleeding such as umbilical cord bleeding, recurrent spontaneous miscarriage, and intracranial hemorrhage (ICH). Today, the disease is successfully managed by FXIII concentrate and recombinant FXIII for prophylaxis, management of minor and major bleeding, treatment of ICH, and successful delivery in women with recurrent pregnancy loss. Molecular analysis of patients with FXIIID revealed a wide spectrum of mutations, most frequently missense mutations in the FXIII-A subunit, with a few recurrent mutations observed worldwide. In vitro expression studies revealed that most of the missense mutations cause intracellular instability of the FXIII protein and, subsequently, FXIIID.

Characterization of a novel large deletion caused by double-stranded breaks in 6-bp microhomologous sequences of intron 11 and 12 of the F13A1 gene

Coagulation Factor XIII is a heterotetrameric protransglutaminase which stabilizes preformed fibrin clots by covalent crosslinking them. Inherited homozygous or compound heterozygous deficiency of coagulation Factor XIII (FXIII) is a rare severe bleeding disorder affecting 1 in 2 million individuals. Most of the patients with inherited FXIII deficiency described in the literature carry F13A1 gene point mutations (missense, nonsense and splice site defects), whereas large deletions (>0.5 kb in size) are underrepresented. In this article we report for the first time the complete characterization of a novel homozygous F13A1 large deletion covering the entire exon 12 in a young patient with a severe FXIII-deficient phenotype from France. Using primer walking on genomic DNA we have identified the deletion breakpoints in the region between g.6.143,016–g.6.148,901 caused by small 6-bp microhomologies at the 5´ and 3´ breakpoints. Parents of the patient were heterozygous carriers. Identification of this large deletion offers the possibility of prenatal diagnosis for the mother in this family who is heterozygous for this deletion.

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).

The genetics of aspirin resistance

Aspirin is widely used for the prophylaxis of cardiovascular events in patients with cardiovascular risk factors or established atherosclerotic disease. However, despite aspirin treatment, a substantial number of patients experience recurrent events. Such 'aspirin resistance' is generally defined as failure of aspirin to produce an expected biological response, for example inhibition of platelet aggregation or of thromboxane A2 synthesis. Whilst its aetiology is multifactorial, genetic factors are also likely to play their part. Here we review the evidence for and against such a genetic contribution, as well as the data suggesting the involvement of specific genes.

Déficit congénital en facteur XIII de la coagulation dans le sud tunisien

Factor XIII deficiency is a rare autosomal recessive congenital disorder of haemostasis characterised by a plasmatic factor XIII level less than 1% in homozygote and bleeding as of the youth. We report a study about ten patients with congenital factor XIII deficiency from seven south-Tunisian families, there are seven females. Umbilical bleeding was common and only two patients had intracranial bleeding. The standard screening tests are normal. Factor XIII activity was less than 1% in all patients. A sub-unit A deficit was detected for the ten patients. Out hemorrhagic context, five patients receive regular prophylactic transfusion with fresh frozen plasma.

Congenital deficiency of factor XIII caused by two missense mutations in a Dutch family

We present the clinical, biochemical and genomic findings of a family with congenital factor XIII (FXIII) deficiency. Congenital FXIII deficiency is a very rare autosomal recessive bleeding disorder, characterized by umbilical cord bleeding at birth and spontaneous intracranial haemorrhage. Routine clotting tests are normal, which may delay the diagnosis, leading to an increased chance of severe sequelae. The propositus and her brother, known with haemorrhagic diathesis, were found to be compound heterozygous with a known missense mutation (1050 G --> T transversion in exon 7, Val316Phe substitution) and a novel mutation 889 G --> A in exon 6, which predicts a Gly262Glu substitution. As these mutations were known in the family, DNA obtained from cord blood of the youngest sister was analysed for mutations in exons 6 and 7 only. We postulate that the diagnosis was facilitated by determining the two different mutations in the genotype of this family. The analysis showed that she was heterozygous for the exon 7 mutation. Hence, she was not at risk of experiencing haemorrhagic diathesis. This diagnosis avoided the administration of FXIII concentrate to the newborn.

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.

A Novel Asn344 Deletion in the Core Domain of Coagulation Factor XIII A Subunit: Its Effects on Protein Structure and Function

In this study a previously undescribed 3 bp deletion, AAT1030-1032, in the factor XIII A subunit gene, has been detected in a Thai patient. The inframe deletion results in the translation of a factor XIII A subunit that lacks Asn344. This is the first inframe deletion to be identified in the factor XIII A subunit gene because six previously reported deletions have all caused frameshifts. The deletion has been introduced into a factor XIII A subunit cDNA and the deleted polypeptide expressed in yeast. The mRNA encoding the mutant enzyme appears to have normal stability but the translated protein is subject to premature degradation. In addition, the mutated enzyme exhibited very little transglutaminase activity compared with the wild-type enzyme. Structural modeling of the deleted enzyme suggests that the absence of Asn344 would have a potent impact on the catalytic activity by reorienting the residues associated with the catalytic center. Thus, the Asn344 deletion strongly confirms the significance of the residues surrounding the catalytic center of the factor XIII A subunit.

A Novel Asn344 Deletion in the Core Domain of Coagulation Factor XIII A Subunit: Its Effects on Protein Structure and Function

In this study a previously undescribed 3 bp deletion, AAT1030-1032, in the factor XIII A subunit gene, has been detected in a Thai patient. The inframe deletion results in the translation of a factor XIII A subunit that lacks Asn344. This is the first inframe deletion to be identified in the factor XIII A subunit gene because six previously reported deletions have all caused frameshifts. The deletion has been introduced into a factor XIII A subunit cDNA and the deleted polypeptide expressed in yeast. The mRNA encoding the mutant enzyme appears to have normal stability but the translated protein is subject to premature degradation. In addition, the mutated enzyme exhibited very little transglutaminase activity compared with the wild-type enzyme. Structural modeling of the deleted enzyme suggests that the absence of Asn344 would have a potent impact on the catalytic activity by reorienting the residues associated with the catalytic center. Thus, the Asn344 deletion strongly confirms the significance of the residues surrounding the catalytic center of the factor XIII A subunit.

Arg260-Cys mutation in severe factor XIII deficiency: conformational change of the A subunit is predicted by molecular modelling and mechanics

To explore the implications of the structure/ function relationships in factor XIII. a patient with severe A subunit deficiency was examined at the DNA and RNA levels. Nucleotide sequence analysis of the patient's DNA amplified by PCR revealed that the patient had a replacement of C by T in the codon for Arg260. RT-PCR analysis demonstrated that only one kind of mRNA coding for the Arg260-Cys mutation was expressed in the patient at a normal level. Another possible defective allele of the A subunit gene with a G-A polymorphism was not expressed (null allele). The substitution of Arg260 by Cys located on the interface of two A subunits would preclude the reciprocal ionic interaction (salt bridge) between Arg260 and Asp404. Molecular modelling and, for the first time, molecular mechanics calculated that Cys260 changed the local conformation of the A subunit and reduced the electrostatic interaction between two monomers, suggesting destabilization of the molecule's dimer.

Molecular Mechanisms of Type II Factor XIII Deficiency: Novel Gly562-Arg Mutation and C-Terminal Truncation of the A Subunit Cause Factor XIII Deficiency as Characterized in a Mammalian Expression System

To explore the biological and clinical implications of the structure/function relationships in factor XIII, mutations in two patients with type II deficiency were identified and characterized in a mammalian expression system. Nucleotide sequence analysis of the A subunit gene showed that case no. 1 had a deletion of 4 bp (AATT) in exon XI and that, in case no. 2, Gly562 (GGG) had been replaced by Arg(AGG). The deletion in case no. 1 leads to a premature termination at codon 464. Restriction digestion of amplified DNAs confirmed that both cases were homozygous for their respective mutations. Reverse transcription-polymerase chain reaction analysis demonstrated that the level of mRNA was greatly reduced in case no. 1, whereas the level of mutant mRNA expressed in case no. 2 was normal. Molecular modeling calculated that Arg562 changed the conformation of the A subunit, suggesting misfolding and/or destabilization of the molecule. To determine how these mutations impaired synthesis of the A subunit, recombinant A subunits bearing the mutations were expressed in mammalian cells. Pulse-chase experiments showed that the mutants were synthesized normally but disappeared rapidly, whereas the wild-type remained. These results indicate that both mutant proteins with an altered conformation become prone to rapid degradation, resulting in factor XIII deficiency in these patients.

Identification of a Large Deletion, Spanning Exons 4 to 11 of the Human Factor XIIIA Gene, in a Factor XIII-Deficient Family

Inherited deficiency of factor XIIIA subunit (FXIIIA) is an autosomal recessive disorder that is characterized by a life-long bleeding tendency and complications in wound healing. Molecular genetic studies have shown the deficiency can be due to small sequence changes within the FXIIIA gene, such as point mutations or microdeletions. On molecular analysis of the FXIIIA gene in an FXIII-deficient patient, of United Kingdom origin, we identified a putative homozygous missense mutation, Arg408Gln. However, the father of this patient is homozygous normal for arginine at codon 408. Having proved paternity in this pedigree by microsatellite analysis, we examined the FXIIIA RNA of the patient by reverse transcriptase-polymerase chain reaction and found the paternal allele to lack exons 4 through 11 inclusive. Hence, a huge deletion extending from intron 3 to intron 11 and the Arg408Gln mutation are jointly responsible for FXIIIA deficiency in this family. This is the first finding of such a large deletion in the FXIIIA gene.

Identification of a Large Deletion, Spanning Exons 4 to 11 of the Human Factor XIIIA Gene, in a Factor XIII-Deficient Family

Inherited deficiency of factor XIIIA subunit (FXIIIA) is an autosomal recessive disorder that is characterized by a life-long bleeding tendency and complications in wound healing. Molecular genetic studies have shown the deficiency can be due to small sequence changes within the FXIIIA gene, such as point mutations or microdeletions. On molecular analysis of the FXIIIA gene in an FXIII-deficient patient, of United Kingdom origin, we identified a putative homozygous missense mutation, Arg408Gln. However, the father of this patient is homozygous normal for arginine at codon 408. Having proved paternity in this pedigree by microsatellite analysis, we examined the FXIIIA RNA of the patient by reverse transcriptase-polymerase chain reaction and found the paternal allele to lack exons 4 through 11 inclusive. Hence, a huge deletion extending from intron 3 to intron 11 and the Arg408Gln mutation are jointly responsible for FXIIIA deficiency in this family. This is the first finding of such a large deletion in the FXIIIA gene.

Molecular Mechanism of a Mild Phenotype in Coagulation Factor XIII (FXIII) Deficiency: A Splicing Mutation Permitting Partial Correct Splicing of FXIII A-Subunit mRNA

Congenital factor XIII (FXIII) deficiency is potentially a severe bleeding disorder, but in some cases, the symptoms may be fairly mild. In this study, we have characterized the molecular mechanism of a mild phenotype of FXIII A-subunit deficiency in a Finnish family with two affected sisters, one of whom has even had two successful pregnancies without regular substitution therapy. In the screening tests for FXIII deficiency, no A-subunit could be detected, but by using more sensitive assays, a minute amount of functional A-subunit was seen. 3H-putrescine incorporation assay showed distinct FXIII activity at the level of 0.35% of controls, and also the fibrin cross-linking pattern in the patients clotted plasma showed partial γ-γ dimerization. In Western blot analysis, a faint band of full-length FXIII A-subunit was detected in the patients' platelets. The patients have previously been identified as heterozygotes for the Arg661 → Stop mutation. Here we report a T → C transition at position +6 of intron C in their other allele. The transition affected splicing of FXIII mRNA resulting in low steady state levels of several variant mRNA transcripts. One transcript contained sequences of intron C, whereas two transcripts resulted from skipping of one or two exons. Additionally, correctly spliced mRNA lacking the Arg661 → Stop mutation of the maternal allele could be detected. These results demonstrate that a mutation in splice donor site of intron C can result in several variant mRNA transcripts and even permit partial correct splicing of FXIII mRNA. Further, even the minute amount of correctly processed mRNA is sufficient for producing protein capable of γ-γ dimerization of fibrin. This is a rare example of an inherited functional human disorder in which a mutation affecting splicing still permits some correct splicing to occur and this has a beneficial effect to the phenotype of the patients.

Factor XIII(A) subunit deficiency due to a homozygous 13-base pair deletion in exon 3 of the A subunit gene

We investigated the molecular basis of factor XIII(A) subunit deficiency in a Greek family. Each of the 15 exons of the A subunit gene were individually amplified by polymerase chain reaction, using previously reported oligoprimers. The proband with severe deficiency was found to have a homozygous 13-base pair deletion in the 3' half of exon 3. The deleted sequence, extending from codons 82-86, results in a frameshift and generates a downstream termination codon in exon 4. Single strand conformation polymorphism (SSCP) analysis detected no additional mutations in the coding or consensus splice sequences of the A subunit gene. Both parents of the proband were heterozygous for the defect. Only one previous microdeletion (AG dinucleotide) has been reported in the A subunit gene, and was located at the intron B-exon 3 boundary. Further studies are necessary to determine whether this region of the gene is a "hot spot" for microdeletion mutations.

Molecular basis of inherited factor XIII deficiency: identification of multiple mutations provides insights into protein function

Factor XIII (FXIII) is a zymogen essential for normal haemostasis. In inherited FXIII deficiency the majority of cases show absence of the FXIIIa subunit. Molecular analysis of PCR-amplified FXIIIa subunit exonic regions, and of RT-PCR amplified cDNA from six patients with FXIIIa subunit deficiency, from five unrelated families, has revealed 10 sequence changes: three mutations resulting in abnormal splicing of pre-mRNA, one nonsense mutation, one deletion/insertion change, three point mutations producing Val34Leu, Asn60Lys and Arg408Gln changes, and two silent mutations. In three families the patients are homozygous for a specific deficiency causing mutation, and patients from the remaining two families are compound heterozygotes. Understanding the molecular pathology of the disorder provides insights into the structure-function relationships of the various domains within the FXIII protein. From a clinical point of view, it enables direct diagnosis at the DNA level and may aid the development of FXIII analogues to promote wound healing.

Factor XIIIA Calgary: a candidate missense mutation (Leu667Pro) in the beta barrel 2 domain of the factor XIIIA subunit

Molecular analysis performed on a Canadian family with congenital factor XIII deficiency revealed a homozygous missense mutation (Leu667Pro) in exon 14 of the A subunit gene in three affected siblings. The mutation results from a T-to-C transition at nucleotide position 2087 and generates a new Msp1 restriction site. Digestion of an amplified fragment containing exon 14 with this restriction enzyme enabled the heterozygous allele to be identified in both parents (who were third cousins) and three other family members. SSCP analysis detected no additional mutations in the coding or consensus splice sequences of the A subunit gene. The mutant nucleotide substitution was absent in 60 normal alleles and 10 unrelated patients with XIIIA deficiency. Leu667 is located in the carboxyl terminal beta barrel 2 domain of the A subunit molecule. Computer modelling based on 3D crystallographic data predicts that the mutant protein has aberrant folding and is likely to be rapidly degraded following translation.

Highly active soluble processed forms of the transglutaminase 1 enzyme in epidermal keratinocytes

The transglutaminase 1 (TGase 1) enzyme is required for the formation of a cornified cell envelope in epidermal keratinocytes. We show here that in addition to its membrane-anchored form, soluble forms of it are also important in keratinocytes. Proliferating cells contain soluble full-length enzyme of 106 kDa, but terminally differentiating cells contain a soluble 67-kDa form often complexed with a 33-kDa protein as well. The amino terminus of the 67 kDa form is residue 93 of the TGase 1 protein, corresponding to the site of proteolytic activation of the factor XIIIa TGase. The amino terminus of the 33-kDa protein is residue 573, corresponding to the site of a second proteolytic cleavage site of factor XIIIa, and of the site for proteolytic activation of the TGase 3 enzyme. The specific activity of the 67/33-kDa soluble complex is twice that of the soluble 67-kDa form and 10 times that of full-length TGase 1. The half-lives of the 67/33- and 106-kDa forms are about 7 or 20 h, respectively. Thus the TGase 1 enzyme is complex, since it exists in keratinocytes as multiple soluble forms, either intact or proteolytically processed at conserved sites, and which have varying specific activities and likely functions.

Molecular and cellular basis of deficiency of the b subunit for factor XIII secondary to a Cys430-Phe mutation in the seventh Sushi domain

We studied the defect responsible for deficiency of the b subunit for factor XIII in the first known case of this condition. The patient is a compound heterozygote of two genetic defects: deletion of A-4161 at the acceptor splice junction of intron A, resulting in a loss of the obligatory AG splicing sequence; and, replacement of G-11499 by T in exon VIII, resulting in an amino acid substitution of Cys430 by Phe. To determine how the latter mutation impaired b subunit synthesis, recombinant b subunit bearing the mutation was expressed in BHK cells. The mutant as well as wild-type b subunit was synthesized by the cells. However, the apparent molecular weight of the mutant was slightly higher than those of the wild-type and plasma b subunits under nonreducing conditions, probably because of destruction of a disulfide bond. The mutant b subunit was secreted from the cells much less effectively than the wild type and remained susceptible to endoglycosidase H, indicating that it was not transported from the endoplasmic reticulum to the Golgi apparatus where the processing of oligosaccharides occurs. Immunofluorescence study suggested that the mutant protein was retained in the endoplasmic reticulum. These studies demonstrate that a Cys430-Phe mutation does not prevent the de novo synthesis of the b subunit, but alters the conformation of the mutant protein sufficiently to impair its intracellular transport, resulting in its deficiency in this patient.

Factor XIII ABristol 1: detection of a nonsense mutation (Arg171-->stop codon) in factor XIII A subunit deficiency

Molecular analysis has been performed on a patient with coagulation factor XIII A subunit deficiency. A previously published genomic sequence indicates that exon 4 of the factor XIII A subunit gene contains two TaqI restriction sites within which arginine (CGA)-->stop (TGA) nonsense mutations are possible. Oligonucleotide primers were therefore used to amplify exon 4 by the polymerase chain reaction. TaqI digestion of the 326 base pair (bp) product derived from normal genomic DNA yielded expected fragments of 244, 73 and 9 bp in size. In the case of the patient, however, an additional fragment of 253 bp was present. Direct sequence analysis showed that the 5' TaqI site had been lost from one allele by a C-->T transition at nucleotide 598. Family studies demonstrated the mutation in the patient's father but no other first-degree relatives. This is the third independent mutation described in the factor XIII A subunit gene and the first to be identified in a patient compound heterozygous for the disorder.

Factor XIII: inherited and acquired deficiency

Factor XIII (XIII), an enzyme found in plasma (present as a pro-enzyme), platelets and monocytes, is essential for normal haemostasis. It may also have a role to play in the processes of wound healing and tissue repair. Inherited XIII deficiency results in a life-long, severe bleeding diathesis which, if untreated, carries a very high risk of death in early life from intracranial bleeding. XIII is a zymogen requiring thrombin and calcium for activation. In plasma, XIII has two subunits: the 'a' subunit, which is the active enzyme, and the 'b' subunit which is a carrier protein. Activated XIII modifies the structure of clot by covalently crosslinking fibrin through an epsilon (gamma-glutamyl)lysine link. It also crosslinks other proteins, including fibronectin and alpha-2-plasmin inhibitor (alpha-2PI), into the clot through the same link. Clot modified by XIII is physically stronger, relatively more resistant to fibrinolysis and may be a more suitable medium for the ingrowth of fibroblasts. Inheritance of factor XIII is autosomal recessive. The majority of patients with the inherited defect show no XIII activity and absence of 'a' subunit protein in plasma, platelets and monocytes. At the molecular level, the defect is not a major gene rearrangement or deletion, but most likely a single point mutation which may be different in each family. Because of the severity of the bleeding diathesis, prophylaxis is desirable and has been shown to be very effective as the in vivo half-life of plasma XIII is long, and low plasma levels are sufficient for haemostasis. Acquired inhibitors have been reported in only two cases with inherited XIII deficiency. Acquired XIII deficiency has been described in a variety of diseases and bleeding has been controlled by therapy with large doses of XIII in such conditions as Henoch-Schönlein purpura, various forms of colitis, erosive gastritis and some forms of leukaemia. Large dose XIII therapy has also been used in an endeavour to promote wound healing after surgery and bone union in non-healing fractures. The use of XIII in these conditions remains controversial. Very rarely a bleeding diathesis results from the development of a specific inhibitor to XIII arising de novo, often as a complication in the course of a disease or in association with long-term drug therapy. The bleeding diathesis in these patients is difficult to treat.