The U.S. Thrombosis and Hemostasis Centers pilot sites program

Venous thromboembolism (VTE) is a common disorder associated with significant morbidity and mortality. Despite important advances in understanding the etiology of VTE, delivery of care to patients with thrombosis and thrombophilia is frequently incomplete and highly variable. A comprehensive model of health care has been used successfully to treat and prevent complications for people with hemophilia and other chronic disorders. The effectiveness of an integrated healthcare model for patients with all coagulation disorders has yet to be evaluated. The Division of Hereditary Blood Disorders of the Centers for Disease Control and Prevention (CDC) is collaborating with eight Thrombosis and Hemostasis Centers (pilot sites) to provide health-related services and conduct research directed toward the reduction or prevention of complications of thrombosis and thrombophilia. The initial objectives of the collaboration are to (1) determine the efficacy of integrated multidisciplinary care and prevention services for people with hemostatic disorders, (2) assess unmet needs for service delivery and identify outreach strategies to improve access to care, (3) develop effective messages aimed at disease management and prevention, and (4) foster the development of training programs to enhance provider skills for the delivery of patient care. To address these objectives, the investigators and CDC have developed and implemented a web-based patient registry to follow prospectively service allocation and patient outcomes. Funding for the program began in October 2001. All eight funded centers are affiliated with U.S. medical schools. Principal investigators at the centers are hematologists (five adult, two pediatric) or cardiologists. Faculty in obstetrics-gynecology, surgery, and multiple other specialties are integral to the model of care at the centers. Other critical components at the centers are clinical laboratory services, training programs, research networks, and education and outreach programs. From August 2003 to March 2006, over 2,600 patients were enrolled in the registry, accounting for a total of more than 5,000 visits to the centers. Immediate goals of the data collection at the centers are to characterize patients receiving care at centers and document the state of health services provided. Long-term goals are to evaluate prospectively clinical outcomes for patients receiving multidisciplinary care and prevention services at centers. The network of data collection across centers will facilitate future collaborative clinical and epidemiologic investigations and enhance collective expertise in hemostasis and coagulation disorders.

Characterization of Leu777Pro and Ile865Thr type IIA von Willebrand disease mutations

Type IIA von Willebrand disease (vWD) is an autosomal dominant bleeding disorder characterized by a qualitative defect in von Willebrand factor (vWF). A number of missense mutations responsible for type IIA vWD have recently been identified. This report examines the type IIA vWD mutations Leu777-->Pro and Ile865-->Thr by expression of recombinant vWF containing mutant and wild-type (WT) sequences. Recombinant vWF containing the L777P mutation (vWFL777P) showed markedly impaired secretion compared with that for wild-type vWF (vWFWT) after DNA transfection into mammalian cells. Multimer analysis of secreted vWFL777P showed predominantly low molecular weight forms. In contrast, recombinant vWF containing the I865T mutation (vWFI865T) was processed in a pattern similar to vWFWT, with secretion of the full spectrum of vWF multimers. Thus, L777P and I865T are subclassified as type IIA group I and group II mutations, respectively. Analysis of platelet vWF from a patient heterozygous for the L777P mutation shows reduced large vWF multimers in a pattern similar to plasma, consistent with the intracellular transport defect predicted for a group I mutation. An increase in the proportion of high molecular weight multimers observed in type IIA vWD patient plasma, after renal transplantation from a normal donor, suggests that the kidney endothelium may be a major source of plasma vWF.

Coagulation status during aortic aneurysm surgery: comparison of thrombelastography with standard tests

A prospective comparison of thrombelastography to standard coagulation tests was undertaken in ten patients undergoing routine, uncomplicated abdominal aortic aneurysm surgery in order to explore potential clinical utility and establish normal patterns of change. Thrombelastograph k values increased (7.1 vs 5.4 min baseline, P < or = .01), and alpha angle (43 vs 52 degrees baseline, P < or = .001) and ma (39 vs 52 mm baseline, P < or = .01) values decreased following graft placement, while r values remained unaffected (6.4 vs 7.5 min baseline, P > .05). Weak correlations were observed between alpha angle and fibrinogen, prothrombin time, and partial thromboplastin time (aPTT), as well as between k and aPTT (0.70 < r < 0.79 for all). Systemic fibrinolysis was suggested by thrombelastography in 25% of samples, although euglobulin lysis times were abnormal in only 5% (chi 2 = 4.80, P < or = .05). Fibrin degradation product detection increased through the fifth postoperative day in all patients. Variations in thrombelastographic parameters and their correlation to standard coagulation tests in patients undergoing uncomplicated abdominal aortic aneurysm repair were documented. In such a setting, no clear advantages to thrombelastography were defined. Further observations will be necessary to establish the role for thrombelastography in the management of patients experiencing clinically significant perioperative coagulation disorders.

Binding and covalent cross-linking of purified von Willebrand factor to native monomeric collagen

We have analyzed the interaction of the adhesive glycoprotein, von Willebrand factor (vWF), with native monomeric collagen monolayers by adsorbing acid soluble Types I and III collagen derived from calf skin to polystyrene microtiter wells and incubating the wells with purified human 125I-vWF. The binding of 125I-vWF was saturable, reversible, specific, and was abolished by heat denaturation of the collagen monomers. Binding was half-maximal at 5 micrograms/ml, and, at saturation, 7.5 ng 125I-vWF were bound to each microgram of immobilized collagen. 125I-vWF did not bind to wells coated with other extracellular matrix or plasma proteins such as fibronectin, fibrinogen, gelatin, or the q subunit of the first component of complement (C1q). In addition, bound 125I-vWF could not be displaced from collagen by the addition of either fibronectin or fibrinogen. After incubation with Factor XIIIa, plasma transglutaminase, 125I-vWF bound to collagen could no longer be displaced by vWF, which suggests covalent cross-linking of vWF to collagen monomers. Factor XIIIa-dependent covalent cross-linking of vWF to collagen, but not to fibronectin or laminin, was also demonstrated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate.

Covalent crosslinking of von Willebrand factor to fibrin

Factor XIIIa crosslinks a limited number of substrates via epsilon(gamma-glutamyl)-lysyl bond formation. It crosslinks fibrin to itself, alpha 2-plasmin inhibitor and fibronectin to fibrin, and fibronectin to collagen. Results presented here show that plasma von Willebrand factor (vWF) is a substrate for factor XIIIa and can be crosslinked to fibrin during gel formation. vWF-fibrin crosslinking was studied in purified systems and in plasma with 125I-vWF and 131I-fibrinogen. vWF incorporation into fibrin increased with time or increasing factor XIIIa. After electrophoresis of dissolved clots, distribution of 125I and 131I was measured and showed that vWF was crosslinked to the alpha chain of fibrin and entered the high-mol-wt alpha polymer. vWF-fibrin crosslinking decreased the initial rate of alpha polymer formation. Crosslinking of vWF polymer to itself could not be demonstrated under physiologic conditions but occurred if vWF was reduced first. Factor XIIIa catalyzed incorporation of putrescine into both monomeric and polymeric vWF. Altogether, these studies indicate that factor XIIIa can readily form covalent bonds between glutamine in vWF and lysine in fibrin alpha chains. This reaction occurs readily in vitro when plasma clotting is slow and may occur in vivo under similar conditions.

Structural basis of von Willebrand factor binding to platelet glycoprotein Ib and collagen. Effects of disulfide reduction and limited proteolysis of polymeric von Willebrand factor

von Willebrand factor (vWF) is a large, multimeric glycoprotein that helps platelets adhere to vascular subendothelium. Although vWF binding to platelet receptors and connective tissue constituents is of fundamental importance in adhesion, there is little information regarding the nature of these vWF binding sites. In this paper, we have compared the structural requirements for vWF binding with platelet glycoprotein Ib (GpIb), heparin, and collagen and have shown that fragments derived from large vWF multimers retain biologic activity. We have shown that a 440,000-D subunit produced by disulfide reduction and alkylation of vWF polymer binds to platelet GpIb. When analyzed by polyacrylamide gel electrophoresis and Sepharose CL6B chromatography, the 440,000-D vWF oligomer is a dimer of the 220,000 subunit of fully reduced native vWF. This vWF dimer competes with 125I-vWF for binding to GpIb with an IC50 of 100 micrograms/ml (227 nM). The GpIb binding domain on vWF was further localized by digestion of native vWF polymers with Staphylococcal V8 protease. A 285,000-D fragment of vWF multimer was separated from heterogeneous 210,000-225,000-D fragments by its ability to bind to heparin. The 285,000-D fragment that bound to heparin Sepharose was composed of two disulfide-linked 175,000- and 115,000-D polypeptides. The heterogeneous fragments contained disulfide-linked 96,000, 66,000, and 53,000-D polypeptides when analyzed on polyacrylamide gel electrophoresis. The 285,000-D fragment competed with 125I-vWF for binding to GpIb with an IC50 of 22 micrograms/ml (77 nM), while the other fragments did not compete for binding. Neither the vWF dimer nor the proteolytic fragments competed with native 125I-vWF polymer for binding to collagen.

Native conformation of human von Willebrand protein. Analysis by electron microscopy and quasi-elastic light scattering

von Willebrand factor (vWF) was analyzed by electron microscopic and quasi-elastic light scattering techniques in order to evaluate the size and shape of this heterogeneous polymeric plasma glycoprotein. Electron micrographs demonstrated that native vWF molecules are flexible, linear polymers, ranging in contour length from 100 to 1300 nm. In their typical configuration, the polymers were coiled upon themselves with maximal diameters ranging from 60 to 200 nm. Individual repeating protomeric subunits were discernible in occasionally noted, uncoiled polymers and measured 100 nm X 1.5-2.0 nm. Quasi-elastic light scattering analysis confirmed that measurements of the size and shape of purified vWF molecules in solution were similar to those obtained with electron microscopic techniques. In addition, the mean Stokes radius and mean radius of gyration assessed by quasielastic light scattering were directly related over a wide range of values, as were the diameter and contour length measured from electron micrographs, suggesting that the overall shape of polymers does not change with increasing size. This study supports the concept that native vWF molecules are flexible, linear polymers. In addition, this study clearly shows that the polymer configuration assessed from electron micrographs is a valid representation of the configuration of the polymer in solution. The data presented also provide the first evidence for a well-defined, repeating protomeric subunit.