Effect of Val34Leu Polymorphism on the Activation of the Coagulation Factor XIII-A

Coagulation factor XIII (FXIII) is a protransglutaminase involved in the last step of the coagulation cascade by stabilising the fibrin clot. Recently, a common variation (FXIII Val34Leu) has been associated with a decreased risk of myocardial infarction and deep venous thrombosis. Val34Leu is critically located near the thrombin activation site of FXIII-A. In this study we investigated its effects on the activation of FXIII. Both recombinant and platelet-derived FXIII Val34Leu variants were shown to be more susceptible to thrombin cleavage than the wild type FXIII. The rate of enzymatic activation of FXIII Val34Leu was found increased, however, the specific activity of fully activated wild type FXIII and the Val34Leu mutant did not differ. During the course of thrombin-induced activation of FXIII fibrin γ-chain dimerisation and α-chain polymerisation developed more rapidly with the Val34Leu mutant. The increased rate of fibrin stabilisation brought about by the Val34Leu FXIII seems to be paradoxically associated with a protective effect against pathological thrombosis.

Abbreviations: AP, activation peptide of factor XIII; FXIII, blood coagulation factor XIII; FXIII-A, factor XIII subunit A; FXIII-A’, proteolytically activated subunit A; FXIII-B, factor XIII subunit B; SDS PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis

Longitudinal register study of attendance frequencies in public and private dental services in Finland

OBJECTIVE

The Public Dental Service (PDS) in Finland was recently opened to all adults. According to annual statistics, 75% of children and 51% of adults made dental attendances in 2008. This study aimed to survey the frequency of dental attendance across three years and compared attendance frequencies between age groups and treatment sectors.

METHODS

Data from municipal databases and the reimbursement register of the Social Insurance Institution were collected on all who had attended the PDS (733,000) or the private sector (473,000) in 2008 and they were retrospectively followed from 2008 to 2006.

RESULTS

Most children had attended the PDS in each year (57.4%) or in two of the three years (32.2%). Most working aged (57.3%) and elderly (69.1%) were annual attenders in the private sector. In addition, 27.1% of the former and 19.8% of the latter had attended in two of the three years. Attending in one year only was unusual. In the PDS, adult annual attendance was uncommon (31.9%), and adult attenders were fairly evenly distributed over the three categories, attending in one, two or all three years.

CONCLUSIONS

Annual or biannual attendances seemed to be the norm among children in the PDS and adults in the private sector. Adults in the PDS showed irregular attendance patterns probably partly due to scarcity of resources for recall patients in the PDS.

Industry structures in private dental markets in Finland

OBJECTIVE

To use industrial organisation and organisational ecology research methods to survey industry structures and performance in the markets for private dental services and the effect of competition.

DESIGN

Data on practice characteristics, performance, and perceived competition were collected from full-time private dentists (n = 1,121) using a questionnaire. The response rate was 59.6%. Cluster analysis was used to identify practice type based on service differentiation and process integration variables formulated from the questionnaire.

RESULTS

Four strategic groups were identified in the Finnish markets: Solo practices formed one distinct group and group practices were classified into three clusters Integrated practices, Small practices, and Loosely integrated practices. Statistically significant differences were found in performance and perceived competitiveness between the groups. Integrated practices with the highest level of process integration and service differentiation performed better than solo and small practices. Moreover, loosely integrated and small practices outperformed solo practises. Competitive intensity was highest among small practices which had a low level of service differentiation and was above average among solo practises.

CONCLUSIONS

Private dental care providers that had differentiated their services from public services and that had a high number of integrated service production processes enjoyed higher performance and less competitive pressures than those who had not.

Pricing and competition in the private dental market in Finland

OBJECTIVE

To investigate how the prices were set in private dental care, which factors determined prices and whether the recent National Dental Care Reform had increased competition in the dental care market in Finland.

DESIGN

A questionnaire to all full time private dentists (n = 1,121) in the ten largest cities. Characteristics of the practice, prices charged, price setting, perceived competition and expectations for the practices were requested. The response rate was 59.6%. Correlation analysis (Pearson's) was used to study relationships between the prices of different treatment items. Linear regression analysis was used to study determinants of the price of a one surface filling.

RESULTS

Most dentists' fee schedules were based on the price of a one surface filling and updated annually. Changes in practice costs calculated by the dentists' professional association and information on average prices charged on dental treatments in the country influenced pricing. High price levels were associated with specialisation, working in a group practice, working close to many other practices or in a town with a dental school. Less than half of the respondents had faced competition in dental services and price competition was insignificant.

CONCLUSIONS

Price setting followed traditional patterns and private markets in dental services were not found to be very competitive.

Probiotic intervention decreases serum gastrin-17 in Helicobacter pylori infection

BACKGROUND

Previously we showed that a probiotic combination with L. rhamnosus GG was beneficial as an adjuvant therapy during H. pylori eradication.

AIM

To evaluate whether probiotic combination with LGG adheres to the upper gastrointestinal mucosa and modifies H. pylori colonisation and H. pylori induced inflammation.

METHODS

Thirteen patients referred for gastroduodenoscopy received a drink consisting of equal doses (2.5x10(9)CFU) of LGG, L. rhamnosus LC705, Propionibacterium freudenreichii JS and Bifidobacterium lactis Bb12 daily. Recovery of probiotics in biopsies (antrum, corpus, duodenum) and faecal samples was evaluated by strain-specific quantitative polymerase chain reaction. H. pylori colonization and gastric inflammation was investigated by urease activity ((13)C-urea breath test), histology and serum pepsinogen I, II and gastrin-17 measurements.

RESULTS

Twelve patients were fully investigated; of these three of the patients had LGG adhering to the biopsies at end of the intervention. Other probiotic strains were not detected, even though the recovery of all individual probiotic strains from the faeces was significantly increased (p<0.01). After the treatment, the level of (13)C-urea breath test (p=0.063) and gastrin-17 (p=0.046) decreased.

CONCLUSIONS

The decreases in (13)C-urea breath test and gastrin-17 indicate that the probiotic combination exerts a beneficial effect on gastric mucosa in H. pylori infected patients. LGG showed marginal ability to adhere to the upper gastrointestinal tract mucosa.

Lentiviral-mediated gene transfer into haematopoietic stem cells

OBJECTIVES

Lentiviral vectors can transduce nondividing cells. As most haematopoietic stem cells (HSCs) are nondividing in vivo, lentiviral vectors are promising viral vectors to transfer genes into HSCs.

DESIGN AND SETTING

We have used HIV-1 based lentiviral vectors containing the green fluorescent protein (GFP) gene to transduce umbilical cord blood CD34+ and CD34+/CD38- cells prior to transplantation into NOD/SCID mice.

RESULTS

High level engraftment of human cells was obtained and transgene expression was seen in both myeloid and lymphoid lineages. Bone marrow from the primary transplant recipients mice was transplanted into secondary recipients. GFP expression was seen in both lymphoid and myeloid cells in the secondary recipients 6 weeks posttransplantation. Human haematopoietic progenitor colonies were grown from both primary and secondary recipients. Over 50% of the haematopoietic colonies in these recipients were positive for the GFP transgene by PCR. Following inverse PCR, amplified fragments were sequenced and integration of the vector into human genomic DNA was demonstrated. Several vectors containing different internal promoters were tested in NOD/SCID mice that had been transplanted with transduced CD34+ and CD34+/CD38- cells. The elongation factor-1alpha (EF-1alpha) promoter gave the highest level of expression, both in the myeloid and lymphoid progeny of the engrafting cells.

CONCLUSIONS

These data collectively indicate that candidate human HSCs can be efficiently transduced with lentiviral vectors and that the transgene is highly expressed in their progeny cells.

Lentivirus gene transfer in murine hematopoietic progenitor cells is compromised by a delay in proviral integration and results in transduction mosaicism and heterogeneous gene expression in progeny cells

Human immunodeficiency virus type 1-based lentivirus vectors containing the green fluorescent protein (GFP) gene were used to transduce murine Lin(-) c-kit(+) Sca1(+) primitive hematopoietic progenitor cells. Following transduction, the cells were plated into hematopoietic progenitor cell assays in methylcellulose and the colonies were scored for GFP positivity. After incubation for 20 h, lentivirus vectors transduced 27.3% +/- 6.7% of the colonies derived from unstimulated target cells, but transduction was more efficient when the cells were supported with stem cell factor (SCF) alone (42. 0% +/- 5.5%) or SCF, interleukin-3 (IL-3), and IL-6 (53.3 +/- 1.8%) during transduction. The, vesicular stomatitis virus glycoprotein-pseudotyped MGIN oncoretrovirus control vector required IL-3, IL-6, and SCF for significant transduction (39.3 +/- 9.4%). Interestingly, only a portion of the progeny cells within the lentivirus-transduced methylcellulose colonies expressed GFP, in contrast to the homogeneous expression in oncoretrovirus-transduced colonies. Secondary plating of the primary GFP(+) lentivirus vector-transduced colonies revealed vector PCR(+) GFP(+) (42%), vector PCR(-) GFP(-) (46%), and vector PCR(+) GFP(-) (13%) secondary colonies, indicating true genetic mosaicism with respect to the viral genome in the progeny cells. The degree of vector mosaicism in individual colonies could be reduced by extending the culture time after transduction and before plating into the clonal progenitor cell assay, indicating a delay in the lentiviral integration process. Furthermore, supplementation with exogenous deoxynucleoside triphosphates during transduction decreased mosaicism within the colonies. Although cytokine stimulation during transduction correlates with higher transduction efficiency, rapid cell division after transduction may result in loss of the viral genome in the progeny cells. Therefore, optimal transduction may require activation without promoting intense cell proliferation prior to vector integration.

Lentiviral gene transfer into primary and secondary NOD/SCID repopulating cells

The ability of lentiviral vectors to transfer genes into human hematopoietic stem cells was studied, using a human immunodeficiency virus 1 (HIV-1)-derived vector expressing the green fluorescence protein (GFP) downstream of the phosphoglycerate kinase (PGK) promoter and pseudotyped with the G protein of vesicular stomatitis virus (VSV). High-efficiency transduction of human cord blood CD34(+) cells was achieved after overnight incubation with vector particles. Sixteen to 28 percent of individual colony-forming units granulocyte-macrophage (CFU-GM) colonies derived from cord blood CD34(+) cells were positive by polymerase chain reaction (PCR) for the GFP gene. The transduction efficiency of SCID-repopulating cells (SRC) within the cord blood CD34(+) population was assessed by serial transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. When 400,000 cord blood CD34(+) cells were transplanted into primary recipients, all primary and secondary recipients contained and expressed the transgene. Over 50% of CFU-GM colonies derived from the bone marrow of these primary and secondary recipients contained the vector on average as determined by PCR. Transplantation of transduced cells in limiting dilution generated GFP(+) lymphoid and myeloid progeny cells that may have arisen from a single SRC. Inverse PCR analysis was used to amplify vector-chromosomal junctional fragments in colonies derived from SRC and confirmed that the vector was integrated. These results show that lentiviral vectors can efficiently transduce very primitive human hematopoietic progenitor and stem cells. (Blood. 2000;96:3725-3733)

Lentivirus vector gene expression during ES cell-derived hematopoietic development in vitro

The murine embryonal stem (ES) cell virus (MESV) can express transgenes from the long terminal repeat (LTR) promoter/enhancer in undifferentiated ES cells, but expression is turned off upon differentiation to embryoid bodies (EBs) and hematopoietic cells in vitro. We examined whether a human immunodeficiency virus type 1-based lentivirus vector pseudotyped with the vesicular stomatitis virus G protein (VSV-G) could transduce ES cells efficiently and express the green fluorescent protein (GFP) transgene from an internal phosphoglycerate kinase (PGK) promoter throughout development to hematopoietic cells in vitro. An oncoretrovirus vector containing the MESV LTR and the GFP gene was used for comparison. Fluorescence-activated cell sorting analysis of transduced CCE ES cells showed 99.8 and 86.7% GPF-expressing ES cells in the VSV-G-pseudotyped lentivirus (multiplicity of infection [MOI] = 59)- and oncoretrovirus (MOI = 590)-transduced cells, respectively. Therefore, VSV-G pseudotyping of lentiviral and oncoretrovirus vectors leads to efficient transduction of ES cells. Lentivirus vector integration was verified in the ES cell colonies by Southern blot analysis. When the transduced ES cells were differentiated in vitro, expression from the oncoretrovirus LTR was severely reduced or extinct in day 6 EBs and ES cell-derived hematopoietic colonies. In contrast, many lentivirus-transduced colonies, expressing the GFP gene in the undifferentiated state, continued to express the transgene throughout in vitro development to EBs at day 6, and many continued to express in cells derived from hematopoietic colonies. This experimental system can be used to analyze lentivirus vector design for optimal expression in hematopoietic cells and for gain-of-function experiments during ES cell development in vitro.

Val34Leu polymorphism of plasma factor XIII: biochemistry and epidemiology in familial thrombophilia

Val34Leu polymorphism of the A subunit of coagulation factor XIII (FXIII-A) is located in the activation peptide (AP) just 3 amino acids away from the thrombin cleavage site. This mutation has been associated with a protective effect against occlusive arterial diseases and venous thrombosis; however, its biochemical consequences have not been explored. In the current study it was demonstrated that the intracellular stability and the plasma concentration of FXIII of different Val34Leu genotypes are identical, which suggests that there is no difference in the rate of synthesis and externalization of wild-type and mutant FXIII-A. In contrast, the release of AP by thrombin from the Leu34 allele proceeded significantly faster than from its wild-type Val34 counterpart. By molecular modeling larger interaction energy was calculated between the Leu34 variant and the respective domains of thrombin than between the Val34 variant and thrombin. In agreement with these findings, the activation of mutant plasma FXIII by thrombin was faster and required less thrombin than that of the wild-type variant. Full thrombin activation of purified plasma FXIII of different genotypes, however, resulted in identical specific transglutaminase activities. Similarly, the mean specific FXIII activity in the plasma was the same in the groups with wild-type, heterozygous, and homozygous variants. Faster activation of the Leu34 allele hardly could be associated with its presumed protective effect against venous thrombosis. No such protective effect was observed in a large group of patients with familial thrombophilia.

Effect of Val34Leu polymorphism on the activation of the coagulation factor XIII-A

Coagulation factor XIII (FXIII) is a protransglutaminase involved in the last step of the coagulation cascade by stabilising the fibrin clot. Recently, a common variation (FXIII Val34Leu) has been associated with a decreased risk of myocardial infarction and deep venous thrombosis. Val34Leu is critically located near the thrombin activation site of FXIII-A. In this study we investigated its effects on the activation of FXIII. Both recombinant and platelet-derived FXIII Val34Leu variants were shown to be more susceptible to thrombin cleavage than the wild type FXIII. The rate of enzymatic activation of FXIII Val34Leu was found increased, however, the specific activity of fully activated wild type FXIII and the Val34Leu mutant did not differ. During the course of thrombin-induced activation of FXIII fibrin gamma-chain dimerisation and alpha-chain polymerisation developed more rapidly with the Val34Leu mutant. The increased rate of fibrin stabilisation brought about by the Val34Leu FXIII seems to be paradoxically associated with a protective effect against pathological thrombosis.

Arg506Gln factor V mutation and Val34Leu factor XIII polymorphism in Finnish patients with inflammatory bowel disease

BACKGROUND

Earlier studies have indicated that inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), is associated with thromboembolic complications, whereas inherited disorders of coagulation occur less often than expected in IBD patients. The point mutation Arg506Gln of factor V (Factor V Leiden), resulting in resistance to activated protein C, is the commonest inherited form of thrombophilia. Alterations in circulating levels of factor XIII (FXIII) have been reported among IBD patients. We investigated whether Factor V Leiden or inherited Val34Leu polymorphism of FXIII would associate with IBD or its clinical outcome.

METHODS

Factor V Leiden mutation and FXIII Val34Leu polymorphism were assayed in 328 unrelated Finnish patients with UC and 235 patients with CD by solid-phase minisequencing techniques. The control groups comprised 142 apparently healthy Finnish subjects and 120 Finnish blood donors.

RESULTS

The frequency of Factor V Leiden mutation among IBD patients (4.5%) was not significantly different from that in subjects living in the same area (2.1%). No significant differences could be observed in the FXIII Val34Leu polymorphism allele frequencies between patients and controls. Clinical features of IBD, including the disease extent, requirement for immunosuppressive drugs, and occurrence of complications, seemed to be independent of these two clotting factor variants analyzed.

CONCLUSIONS

Our data do not support an aetiologic or disease-modifying role for the factor V mutation or factor XIII Val34Leu polymorphism in IBD.

Association of FXIII Val34Leu with decreased risk of myocardial infarction in Finnish males

Factor XIII is a transglutaminase that crosslinks fibrin in the last steps of the coagulation process. A few polymorphic sites have been identified in this gene, one of them being a point mutation (FXIII Val34Leu), leading to an amino acid change of valine to leucine. Recently, in British patients, FXIII 34Leu allele was suggested to be associated with a decreased incidence of myocardial infarction (MI). PAI-1 4G/4G genotype seemed to lessen the beneficial effect of FXIII 34Leu allele. The aim of our study was to further investigate the possible protective role of the FXIII 34Leu allele against MI and its suggested interaction with the PAI-1 4G/5G polymorphism. We carried out genotype analyses for FXIII Val34Leu using solid-phase minisequencing in two independent Finnish study groups. In our study, the FXIII 34Leu allele was associated with a lower risk of MI (P = 0.009), however, the PAI-1 4G allele showed no interaction with this polymorphism. To establish the population frequency of the FXIII 34Leu allele and to study the possible variations in Finland four DNA pools from different geographical areas of Finland were genotyped. No significant differences in the allele frequencies were observed (21-28%) except in the Eastern Kainuu area (13%), an area with an increased risk of mortality from coronary artery disease (CAD), supporting the results presented above. The association of FXIII 34Leu variant with a lower incidence of myocardial infarction suggests a new role for FXIII in a polygenic thrombotic disease.

Peripheral blood platelets express VEGF-C and VEGF which are released during platelet activation

VEGF-C is a recently characterised endothelial growth factor structurally related to vascular endothelial growth factor (VEGF). We studied the expression of VEGF-C and VEGF in the cells of peripheral blood and in the umbilical cord blood CD 34+ cells, representing haematopoietic progenitor cells. Expression of VEGF-C was detected in the CD34+ cells. In peripheral blood VEGF-C mRNA was restricted to platelets and T-cells. In contrast to the expression pattern of VEGF-C, VEGF mRNA was detected in all peripheral blood cell fractions studied, and also in CD34+ cells. VEGF-C mRNA was also detected in fresh bone marrow samples of acute leukaemia patients, but the expression did not show lineage specificity. VEGF-C and VEGF polypeptides were present in platelets and they were released from activated platelets together with the release of beta-thromboglobulin, suggesting that VEGF-C and VEGF reside in the alpha-granules of platelets. VEGF-C and VEGF, released from activated platelets, may have a role in angiogenesis during wound healing, and possibly also in other pathological conditions, such as atherosclerosis, tumour growth, and metastasis formation.

Association of a common polymorphism in the factor XIII gene with myocardial infarction

Factor XIII when activated by thrombin, crosslinks fibrin, however its role in thrombotic disorders is unknown. A common point mutation (G-->T) in exon 2 of the A-subunit gene which codes for an amino acid change three amino acids from the thrombin activation site (Factor XIIIVal34Leu) is a candidate for a role in the pathogenesis of acute myocardial infarction. Factor XIII genotype frequencies were determined in a case-control study of 398 caucasian patients and 196 healthy controls. Patients had undergone angiography for investigation of coronary artery disease and were evaluated for a history of myocardial infarction. The prevalence of the mutation was lower in patients with myocardial infarction than without (32% vs. 50%), p = 0.0009 and than in controls (32% vs. 48%), p = 0.005. Patients possessing the mutation with a history of myocardial infarction had higher PAI-1 concentrations (mean, 27.9 vs. 16.7 ng/ml, p = 0.004) and the PAI-1 4G/4G genotype was commoner (43% vs. 26%, p = 0.03). There was no difference in PAI-1 4G/4G genotype (33% vs. 32%) and PAI-1 levels (mean, 21.0 vs. 20.9 ng/ml) in patients possessing wild type with MI compared to those without MI. These results indicate that the G-->T mutation coding for factor XIIIVal34Leu is protective against myocardial infarction and suggest a mechanism whereby elevated levels of PAI-1 may contribute to vascular risk.

Transglutaminase 1 mutations in autosomal recessive congenital ichthyosis: private and recurrent mutations in an isolated population

Autosomal recessive congenital ichthyosis (ARCI) is a rare, heterogenous keratinization disorder of the skin, classically divided into two clinical subtypes, lamellar ichthyosis (LI) and nonbullous congenital ichthyosiformis erythroderma (CIE). Recently, strong evidence for the involvement of the transglutaminase 1 gene (TGM1) in LI has evolved. We have studied ARCI in the isolated Finnish population, in which recessive disorders are often caused by single mutations enriched by a founder effect. Surprisingly, five different mutations of TGM1 (Arg141His, Arg142Cys, Gly217Ser, Val378Leu, and Arg395Leu) were found in Finnish ARCI patients. In addition to affected LI patients, we also identified TGM1 mutations in CIE patients. Moreover, haplotype analysis of the chromosomes carrying the most common mutation, a C-->T transition changing Arg142 to Cys, revealed that the same mutation has been introduced twice in the Finnish population. In addition to this Arg142Cys mutation, three other mutations, in Arg141 and Arg142, have been described elsewhere, in other populations. These findings suggest that this region of TGM1 is more susceptible to mutation. The corresponding amino acid sequence is conserved in other transglutaminases, but, for example, coagulation factor XIII (FXIII) mutations do not cluster in this region. Protein modeling of the Arg142Cys mutation suggested disruption or destabilization of the protein. In transfection studies, the closely related transglutaminase FXIII protein with the corresponding mutation was shown to be susceptible to degradation in COS cells, further supporting evidence of the destabilizing effect of the Arg142Cys mutation in TGM1.

Molecular mechanisms of mutations in factor XIII A-subunit deficiency: in vitro expression in COS-cells demonstrates intracellular degradation of the mutant proteins

Factor XIII deficiency is an autosomal recessive bleeding disorder that is largely caused by various mutations in FXIII A-subunit gene. Characteristically, the patients lack both A-subunit activity and antigen in the circulation. Here we have analysed the consequences of four missense mutations (Met242-->Thr, Arg252-->Ile, Arg326-->Gln, Leu498 to Pro) and one stop mutation (Arg661-->Stop) in the FXIII A-subunit gene by expression in COS-cells. After transient transfection each mutant cDNA expressed mRNA at an equal level to the wild type FXIII. However, the mutant polypeptides accumulated in the cells in significantly reduced quantities and demonstrated only very low enzymatic activity. Analysis of immunoprecipitated metabolically labelled polypeptides demonstrated remarkable instability and intracellular degradation of all mutant FXIII proteins. These results verify the deleterious nature of the individual amino acid changes and confirm that protein instability and susceptibility to proteolysis are consequences of the mutations, as predicted from the three-dimensional model of crystallised FXIII A-subunit.

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 gamma-gamma 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 gamma-gamma 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.

Gene defects in congenital factor XIII deficiency

The knowledge of the molecular basis of factor XIII deficiency has improved significantly in recent years. Almost 20 different mutations have been described in the gene coding for the factor A-subunit and 3 mutations in the gene coding for the B-subunit. Half of the mutations in the factor XIIIa A-subunit gene are nonsense mutations that result in premature termination of translation. Three of them are frameshift mutations that are caused by minor deletions. Two of them are splicing mutations and 3 are stop mutations that are caused by single nucleotide substitutions. Ten of the mutations are missense mutations caused by nucleotide transitions leading to amino acid substitutions. In the factor XIII B-subunit gene, the 3 mutations are an amino acid substitution, a splicing mutation, and a trinucleotide insertion. These mutations explain the disease in the two families reported to have XIII B-subunit deficiency. In factor XIII A-subunit deficiency, the genetic defects have been characterized so far only in a minority of cases. In most of the reports of factor XIII A-subunit mutations, each family carries its own mutation/mutations. However, in some populations such as Finns and Arabs some enrichment of specific mutations has occurred. Some international migration of a few mutations has also been noted. The structural and functional effects of the mutations have been analyzed by studying the expression of the factor XIII subunits on mRNA and protein levels in vivo or in vitro, and by utilizing the three-dimensional model of crystallized factor XIII A-subunit in modeling of the missense mutations.

Novel aspects of blood coagulation factor XIII. I. Structure, distribution, activation, and function

Blood coagulation factor XIII (FXIII) is a protransglutaminase that becomes activated by the concerted action of thrombin and Ca2+ in the final stage of the clotting cascade. In addition to plasma, FXIII also occurs in platelets, monocytes, and monocyte-derived macrophages. While the plasma factor is a heterotetramer consisting of paired A and B subunits (A2B2), its cellular counterpart lacks the B subunits and is a homodimer of potentially active A subunits (A2). The gene coding for the A and B subunits has been localized to chromosomes 6p24-25 and 1q31-32.1, respectively. The genomic as well as the primary protein structure of both subunits has been established, and most recently the three-dimensional structure of recombinant cellular FXIII has also been revealed. Monocytes/macrophages synthesize their own FXIII, and very likely FXIII in platelets is synthesized by the megakaryocytes. Cells of bone marrow origin seem to be the primary site for the synthesis of subunit A in plasma FXIII, but hepatocytes might also contribute. The B subunit of plasma FXIII is synthesized in the liver. Plasma FXIII circulates in association with its substrate precursor, fibrinogen. Fibrin(ogen) has an important regulatory role in the activation of plasma FXIII. The most important steps of the activation of plasma FXIII are the proteolytic removal of activation peptide by thrombin, the dissociation of subunits A and B, and the exposure of the originally buried active site on the free A subunits. The end result of this process is the formation of an active transglutaminase, which cross-links peptide chains through epsilon(gamma-glutamyl)lysyl isopeptide bonds. Cellular FXIII in platelets becomes activated through a nonproteolytic process. When intracytoplasmic Ca2+ is raised during platelet activation, the zymogen--in the absence of subunit B--assumes an active configuration. The protein substrates of activated FXIII include components of the clotting-fibrinolytic system, adhesive and contractile proteins. The main physiological function of plasma FXIII is to cross-link fibrin and protect it from the fibrinolytic plasmin. The latter effect is achieved mainly by covalently linking alpha 2 antiplasmin, the most potent physiological inhibitor of plasmin, to fibrin. Plasma FXIII seems to be involved in wound healing and tissue repair, and it is essential to maintaining pregnancy. Cellular FXIII, if exposed to the surface of the cells, might support or perhaps take over the hemostatic functions of plasma FXIII; however, its intracellular role has remained mostly unexplored.

Four novel mutations in deficiency of coagulation factor XIII: consequences to expression and structure of the A-subunit

The characterization of naturally occurring mutations is one way to approach functionally significant domains of polypeptides. About 10 mutations have been reported in factor XIII (FXIII) A-subunit deficiency, but very little is known about the effects of the mutations on the expression or the structure of this enzyme. In this study, the recent crystallization of FXIII A-subunit and determination of the three-dimensional model were used for the first time to pursue the structural consequences of mutations in the A-subunit. The molecular analysis of four families from Sweden, Germany, and Denmark revealed four previously unreported point mutations. Three of the mutations were missense mutations, Arg326-->Gln, Arg252-->Ile, and Leu498-->Pro, and one was a nonsense mutation, a deletion of thymidine in codon for Phe8 resulting in early frameshift and premature termination of the polypeptide chain. In the case of the nonsense mutation, delT Phe8, the steady-state mRNA level of FXIII A-subunit was reduced, as quantitated by reverse transcriptase-polymerase chain reaction and solid-phase minisequencing. In contrast, none of the missense mutations affected mRNA levels, indicating the possible translation of the mutant polypeptides. However, by enzyme-linked immunosorbent analysis and immunofluorescence, all the patients demonstrated a complete lack of detectable factor XIIIA antigen in their platelets. In the structural analysis, we included the mutations described in this work and the Met242-->Thr mutation reported earlier by us. Interestingly, in the three-dimensional model, all four missense mutations are localized in the evolutionarily conserved catalytic core domain. The substitutions are at least 15 A away from the catalytic cleft and do not affect any of the residues known to be directly involved in the enzymatic reaction. The structural analyses suggest that the mutations are most likely interfering with proper folding and stability of the protein, which is in agreement with the observed absence of detectable FXIIIA antigen. Arg326, Arg252, and Met242 are all buried within the molecule. The Arg326-->Gln and Arg252-->Ile mutations are substitutions of smaller, neutral amino acids for large, charged residues. They disrupt the electrostatic balance and hydrogen-bonding interactions in structurally significant areas. The Met242-->Thr mutation is located in the same region of the core domain as the Arg252-->Ile site and is expected to have a destabilizing effect due to an introduction of a smaller, polar residue in a tightly packed hydrophobic pocket. The substitution of proline for Leu498 is predicted to cause unfavorable interatomic contacts and a disruption of the alpha-helix mainchain hydrogen-bonding pattern; it is likely to form a kink in the helix next to the dimer interface and is expected to impair proper dimerization of the A-subunits. In the case of all four missense mutations studied, the knowledge achieved from the three-dimensional model of crystallized FXIII A-subunit provides essential information about the structural significance of the specific residues and aids in understanding the biologic consequences of the mutations observed at the cellular level.

Deficiency in the A-subunit of coagulation factor XIII: two novel point mutations demonstrate different effects on transcript levels

Congenital deficiency in coagulation factor XIII is a rare autosomal recessive bleeding disorder. Although the defect was characterized over 30 years ago, little is known about the molecular basis of the disorder. Here, we show two novel point mutations in the gene of the A-subunit of factor XIII in the genetically isolated population of Finland. All eight factor XIII-deficient families identified in Finland were studied. The exons of the gene of A-subunit were amplified individually by polymerase chain reaction and subsequently screened by single-strand conformation polymorphism. Sequence analysis of the abnormally migrating fragments showed two point mutations resulting in an amino acid alteration. A C-to-T transition at Arg-661 in exon XIV created a premature stop codon. This mutation was detected in six of the eight families, thus being the major alteration causing FXIII deficiency in Finland. In two of the six families, the patients were compound heterozygotes with the Arg-661-Stop mutation in one allele and either a T-to-C point mutation in exon VI or a thus far uncharacterized mutation in the other allele. The T-to-C transition in exon VI resulted in a substitution of threonine for methionine 242. The transition was found in one family only, where it was in the heterozygote form combined with the Arg-661-Stop mutation. To evaluate the consequences of these mutations, steady-state FXIII mRNA levels were quantitated by solid-phase minisequencing. In addition to the termination of translation 70 amino acids before the initial stop codon, the Arg-661-Stop mutation causes a 10- to 30-fold reduction in FXIII mRNA levels. This is also likely to result in a low translation level in the truncated polypeptide. In contrast, Met-242-Thr mutation does not seem to affect the level of mRNA. Here, the absence of a functional and immunodetectable protein is probably caused by an altered conformation of the mutant polypeptide, resulting in early degradation of the defective protein.