Polymorphisms of serum proteins in Japanese patients with vascular diseases. I. Factor XIIIB, plasminogen and complement types in primary varicose veins

A novel polymorphism in the factor XIII B-subunit (His95Arg): relationship to subunit dissociation and venous thrombosis

BACKGROUND

Factor (F)XIII B-subunit, which plays a carrier role for zymogen FXIIIA, is highly polymorphic, but the molecular basis for these polymorphisms and their relationship to disease remains unknown.

OBJECTIVES

To screen the FXIIIB gene coding region for common variation and analyze possible functional effects.

METHODS AND RESULTS

We examined the FXIIIB gene by PCR-SSCP and identified three common single nucleotide polymorphisms: A8259G, C29470T and A30899G. A8259G results in substitution of His95Arg in the second Sushi domain. An FXIII tetramer ELISA was developed to analyze B-subunit dissociation from A-subunit (leading to access to the catalytic site of FXIII). Increased subunit dissociation, 0.51 vs. 0.45 (fraction of total tetramer), was found in plasma from subjects possessing the Arg-allele. However, when the variants were purified to homogeneity and binding was analyzed by steady-state kinetics, no difference was observed. The relationship between His95Arg and venous thrombosis was investigated in 214 patients and 291 controls from Leeds. His/Arg + Arg/Arg genotypes were more frequent in patients than controls (22.4% vs. 15.1%). His95Arg was also investigated in the Leiden Thrombophilia Study, in which a similar difference was observed for 471 patients vs. 472 controls (18.5% vs. 14.0%), for a pooled odds ratio (OR) of 1.5 (CI95 1.1-2.0).

CONCLUSIONS

We have identified three FXIIIB polymorphisms, one of which codes for substitution of His95Arg. The Arg95 variant associates with a moderately increased risk for venous thrombosis, and with increased dissociation of the FXIII subunits in plasma, although in vitro steady-state binding between purified subunits was not affected.

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.

Identification of a human specific Alu insertion in the factor XIIIB gene

The factor XIIIB gene was examined to determine the nature of a previously described 300 bp restriction fragment length polymorphism (RFLP) seen in the human population. Polymerase chain reaction analysis of different regions within the factor XIIIB gene was carried out to define a high resolution map of the region encompassing the polymorphism, followed by DNA sequence analysis. An Alu insertion was found to be the source of this variation. This Alu repeat is a member of the human specific-1 (HS-1) subfamily, although one of the five diagnostic nucleotides is a cattarhine specific (CS) subfamily mutation, suggesting that it may represent an intermediate form in the evolution between these two subfamilies. Subsequently, we developed a PCR-based assay to detect the polymorphism, rendering it a more useful marker for genetic linkage studies and genome mapping. This insertion is also a valuable polymorphism for human population studies, as demonstrated by the large variations in allele frequencies seen in three population groups.