Molecular basis for fibrinogen Dusart (A alpha 554 Arg-->Cys) and its association with abnormal fibrin polymerization and thrombophilia

Fibrinogen Bonn (p. Arg510Cys) in the Aα-Chain Is Associated with High Risk of Venous Thrombosis

INTRODUCTION

 Inherited dysfibrinogenemia is a qualitative defect of fibrinogen caused by various mutations among three fibrinogen genes. Dysfibrinogenemia can be associated with an increased risk of thrombosis, bleeding, or both. Here, we report a 36-year-old female with dysfibrinogenemia who experienced two successful pregnancies under thromboprophylaxis after cerebral venous sinus thrombosis (CVST).

PATIENTS AND METHODS

 In addition to plasmatic coagulation tests, fibrinogen genes FGA, FGB, and FGG were screened using direct genomic DNA sequencing. The structural-functional implications of the detected mutation were analyzed in silico.

RESULTS

 Inherited dysfibrinogenemia was diagnosed in an index patient after CVST in a risk situation. Anticoagulation with warfarin was stopped after 12 months when the first pregnancy was planned. Pregnancy and spontaneous delivery (2020) was uncomplicated. A second pregnancy was interrupted because of acute cytomegalovirus infection and the third pregnancy was successful in 2022. Pregnancies were accompanied by thromboprophylaxis with enoxaparin 40 mg once daily until 6 weeks postpartum. Substitution of fibrinogen has not become necessary in the index patient so far. Genetic analysis revealed a novel missense mutation (p. Arg510Cys) in the FGA gene ("fibrinogen Bonn") in the index patient, as well as an asymptomatic sister, and their father who experienced recurrent pulmonary embolism. Surface exposure of wild-type Arg510 suggested the mutated Cys510 to form nonnative disulfide bonds with surface-exposed reactive cysteines from other plasma proteins like albumin leading to formation of aggregates and impaired fibrinolysis.

CONCLUSIONS

 Fibrinogen Bonn might be associated with an increased risk of thrombosis, possibly due to impaired polymerization.

Germline VWF/MPRIP and somatoplasm FGA variants synergically confer susceptibility to non-traumatic osteonecrosis of the femoral head

Non-traumatic osteonecrosis of the femoral head (ONFH) relies on multiple pathogenic factors, including intravascular coagulation, osteoporosis and lipid metabolism disorders. Despite extensively explored from various aspects, genetic mechanism underlying non-traumatic ONFH has not been fully elucidated. We randomly collected blood and necrotic tissue samples from 32 patients with non-traumatic ONFH as well as blood samples from 30 healthy individuals for whole exome sequencing (WES). Germline mutation and somatic mutation were analyzed to identify new potential pathogenic genes responsible for non-traumatic ONFH. Three genes might correlate with non-traumatic ONFH: VWF, MPRIP (germline mutations) and FGA (somatic mutations). Germline or somatic mutations in VWF, MPRIP and FGA correlate with intravascular coagulation, thrombosis, and consequently, ischemic necrosis of the femoral head.

Diagnosis and classification of hereditary fibrinogen disorders

Hereditary fibrinogen disorders (HFDs) are rare bleeding disorders with a wide spectrum of biological and clinical features. While most patients with HFDs are at risk to suffer from mild to severe, sometimes life-threatening bleeding, thrombotic events are also common. Therefore, an appropriate diagnosis is needed to offer the optimal treatment. Diagnosis of HFDs can be challenging and plenty of pitfalls. The sensitivity and specificity of hemostasis routine test are depending on the reagents, the methods, and the fibrinogen variants. To distinguish subtypes of HFDs additional tests are often required. Historically based on the assessment of fibrinogen levels, a recent classification also considers the clinical phenotype and the genotype. In this short review, diagnosis strategies and HFDs classification are reviewed.

One Hundred Years of Congenital Fibrinogen Disorders

Congenital fibrinogen disorders encompass a broad range of fibrinogen defects characterized by a wide molecular and clinical spectrum. From the first clinical description of afibrinogenemia in 1920, many major achievements have contributed to a better understanding of these complex disorders. The finding of causative mutations in all three fibrinogen genes has contributed to reveal the molecular mechanisms involved in biosynthesis of the fibrinogen molecule and to clarify the basic processes of fibrin polymerization and fibrinolysis. The compilation of abundant cases with detailed genetic, biological, and clinical features has enabled the classification of congenital fibrinogen disorders into several types and subtypes. Thus, the recent classification of congenital fibrinogen disorder is based not only on the clottable and antigenic fibrinogen levels but also on the patient's clinical phenotype and genotype. Fibrinogen supplementation is the cornerstone of bleeding management in fibrinogen disorders. Since the discovery of blood fractionation, the method of production of fibrinogen concentrate has been progressively modified to significantly improve purity and safety. Nevertheless, the availability of such products is still limited to a few countries and the optimal threshold of fibrinogen to target is still not established. In this review, we describe the major advances that have characterized 100 years of congenital fibrinogen disorders, focusing on afibrinogenemia and dysfibrinogenemia.

Congenital (hypo-)dysfibrinogenemia and bleeding: A systematic literature review

Ranging from bleeding to thrombosis, the clinical features of congenital fibrinogen qualitative disorders, including dysfibrinogenemia and hypodysfibrinogenemia, are highly heterogeneous. Although the associations between some specific fibrinogen mutations and the thrombotic phenotypes have been well elucidated, the underlying mechanism between fibrinogen variants and bleeding events remains underestimated. After systematically reviewing the literature of (hypo-)dysfibrinogenemia patients with bleeding phenotypes, we identified several well-characterized bleeding-related fibrinogen variants in those patients. Several possible pathomechanisms are proposed to explain the genotype-phenotype associations: 1, mutations in the NH₂-terminal portion of the Aα chain hamper fibrinogen fitting into the active site cleft of thrombin and drastically slow the conversion of fibrinogen into monomeric fibrin; 2, mutations adding new N-linked glycosylation sites introduce bulky and negatively charged carbohydrate side chains and undermine the alignment of fibrin monomers during polymerization; 3, mutations generating unpaired cysteine form extra disulfide bonds between the abnormal fibrinogen chains and produce highly branched and fragile fibrin networks; 4, truncation mutations in the fibrinogen αC regions impair the lateral fibril aggregation, as well as factor XIII crosslinking, endothelial cell and platelet binding. These established relationships between specific variants and the bleeding tendency will help manage (hypo-)dysfibrinogenemia patients to avoid adverse bleeding outcomes.

Thromboses and Hemostasis Disorders Associated with COVID-19: The Possible Causal Role of Cross-Reactivity and Immunological Imprinting

By examining the issue of the thromboses and hemostasis disorders associated with severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) through the lens of cross-reactivity, it was found that 60 pentapeptides are shared by SARS-CoV-2 spike glycoprotein (gp) and human proteins that— when altered, mutated, deficient or, however, improperly functioning— cause vascular diseases, thromboembolic complications, venous thrombosis, thrombocytopenia, coagulopathies, and bleeding, inter alia. The peptide commonality has a relevant immunological potential as almost all of the shared sequences are present in experimentally validated SARS-CoV-2 spike gp-derived epitopes, thus supporting the possibility of cross-reactions between the viral gp and the thromboses-related human proteins. Moreover, many of the shared peptide sequences are also present in pathogens to which individuals have previously been exposed following natural infection or vaccinal routes, and of which the immune system has stored imprint. Such an immunological memory might rapidly trigger anamnestic secondary cross-reactive responses of extreme affinity and avidity, in this way explaining the thromboembolic adverse events that can associate with SARS-CoV-2 infection or active immunization.

Dysfibrinogenemia-Potential Impact of Genotype on Thrombosis or Bleeding

The congenital dysfibrinogenemias, most often associated with bleeding disorders, encompass mutations in the amino-terminal end of fibrinogen α-chain consisting of Gly17-Pro18-Arg19-Val20, known as knob A, which is a critical site for fibrin polymerization. Here we review the studies reporting dysfibrinogenemia due to mutations affecting fibrinogen knob A and identified 29 papers. The number of reports on dysfibrinogenemias related to residues Gly17, Pro18, Arg19, and Val20 is 5, 4, 18, and 2, respectively. Dysfibrinogenemias related to residues Gly17, Pro18, and Val20 are exclusively associated with bleeding tendency. However, the clinical picture associated with dysfibrinogenemia related to residue Arg19 varies, with most patients suffering from bleeding tendencies, but also transitory ischemic attacks and retinal thrombosis may occur. The reason for this variation is unclear. To elaborate the genotype-phenotype associations further, we studied a Danish family with knob A-related dysfibrinogenemia caused by the Aα Arg19Gly (p.Arg19Gly) mutation using whole-exome sequencing and fibrin structure analysis. Our family is the first reported carrying the p.Arg19Gly mutation combined with one or more single nucleotide polymorphisms (SNP)s in FGA, FGB, and/or FGG and increased fibrin fiber thickness and fibrin mass-to-length ratio suffering from pulmonary emboli, suggesting that compound genotypes may contribute to the thrombogenic phenotype of these patients. Our review, accordingly, focuses on significance of SNPs, compound genotypes, and fibrin structure measures affecting the genotype-phenotype associations in fibrinogen knob A mutations.

Venous Thrombosis and Thrombocyte Activity in Zebrafish Models of Quantitative and Qualitative Fibrinogen Disorders

Venous thrombosis occurs in patients with quantitative and qualitative fibrinogen disorders. Injury-induced thrombosis in zebrafish larvae has been used to model human coagulopathies. We aimed to determine whether zebrafish models of afibrinogenemia and dysfibrinogenemia have different thrombotic phenotypes. Laser injuries were used to induce venous thrombosis and the time-to-occlusion (TTO) and the binding and aggregation of fluorescent Tg(itga2b:EGFP) thrombocytes measured. The fga-/- larvae failed to support occlusive venous thrombosis and showed reduced thrombocyte binding and aggregation at injury sites. The fga+/- larvae were largely unaffected. When genome editing zebrafish to produce fibrinogen Aα R28C, equivalent to the human Aα R35C dysfibrinogenemia mutation, we detected in-frame skipping of exon 2 in the fga mRNA, thereby encoding AαΔ19-56. This mutation is similar to Fibrinogen Montpellier II which causes hypodysfibrinogenemia. Aα+/Δ19-56 fish had prolonged TTO and reduced thrombocyte activity, a dominant effect of the mutation. Finally, we used transgenic expression of fga R28C cDNA in fga knock-down or fga-/- mutants to model thrombosis in dysfibrinogenemia. Aα R28C expression had similar effects on TTO and thrombocyte activity as Aα+/Δ19-56. We conclude that thrombosis assays in larval zebrafish can distinguish between quantitative and qualitative fibrinogen disorder models and may assist in anticipating a thrombotic phenotype of novel fibrinogen mutations.

The genetics of venous thromboembolism: a systematic review of thrombophilia families

Genetic risk factors are important for the occurrence and prognosis of venous thromboembolism (VTE). The studies of thrombophilia families are important for dissecting the genetic background of the thrombotic disease. We conducted the systematic review of all published family-based studies on VTE genetics across all racial groups through PubMed and Embase prior to 13th April 2020. This systematic review of 287 families (including 225 Caucasian families, 52 East Asian families, and families of other ethnicities) revealed a total of 21 different genes; the five most reported mutated genes were F5 (88/287, 30.7%), SERPINC1 (67/287, 23.3%), PROC (65/287, 22.6%), F2 (40/287, 13.9%) and PROS1 (48/287, 16.7%). For Caucasian families, F5 mutations were most frequently reported at 37.8% (85/225), while PROS1 mutations were most frequently reported, at 40.4% (21/52), for East Asian families (Chinese, Japanese and Korean). Factor V Leiden was reported more frequently in Caucasians than in East Asians. Missense mutations were reported frequently in the SERPINC1, PROC and PROS1 genes. In conclusion, our study found the most likely mutated genes associated with VTE among different ethnic groups and provided indications for VTE genetic testing and research in the future.

Missing regions within the molecular architecture of human fibrin clots structurally resolved by XL-MS and integrative structural modeling

Fibrinogen hexamers are major components of blood clots. After release of fibrinopeptides resulting in fibrin monomers, clot formation occurs through fibrin oligomerization followed by lateral aggregation, packing into fibrin fibers, and consequent branching. Shedding light on fibrin clots by in situ cross-linking mass spectrometry and structural modeling extends our current knowledge of the structure of fibrin with regard to receptor-binding hotspots. Further restraint-driven molecular docking reveals how fibrin oligomers laterally aggregate into clots and uncovers the molecular architecture of the clot to albumin interaction. We hypothesize this interaction is involved in the prevention of clot degradation. Mapping known mutations validates the generated structural model and, for a subset, brings their molecular mechanisms into view.

Upon activation, fibrinogen forms large fibrin biopolymers that coalesce into clots which assist in wound healing. Limited insights into their molecular architecture, due to the sheer size and the insoluble character of fibrin clots, have restricted our ability to develop novel treatments for clotting diseases. The, so far resolved, disparate structural details have provided insights into linear elongation; however, molecular details like the C-terminal domain of the α-chain, the heparin-binding domain on the β-chain, and other functional domains remain elusive. To illuminate these dark areas, we applied cross-linking mass spectrometry (XL-MS) to obtain biochemical evidence in the form of over 300 distance constraints and combined this with structural modeling. These restraints additionally define the interaction network of the clots and provide molecular details for the interaction with human serum albumin (HSA). We were able to construct the structural models of the fibrinogen α-chain (excluding two highly flexible regions) and the N termini of the β-chain, confirm these models with known structural arrangements, and map how the structure laterally aggregates to form intricate lattices together with the γ-chain. We validate the final model by mapping mutations leading to impaired clot formation. From a list of 22 mutations, we uncovered structural features for all, including a crucial role for βArg’169 (UniProt: 196) in lateral aggregation. The resulting model can potentially serve for research on dysfibrinogenemia and amyloidosis as it provides insights into the molecular mechanisms of thrombosis and bleeding disorders related to fibrinogen variants. The structure is provided in the PDB-DEV repository (PDBDEV_00000030).

Fibrinogen αC domain: Its importance in physiopathology

ABSTRACT

Fibrinogen, involved in coagulation, is a soluble protein composed of two sets of disulfide-bridged Aα, Bβ, and γ-chains. In this review, we present the clinical implications of the αC domain of the molecule in Alzheimer's disease, hereditary renal amyloidosis and a number of thrombotic and hemorrhagic disorders. In Alzheimer's disease, amyloid beta peptide (Aβ) is increased and binds to the αC domain of normal fibrinogen, triggering increased fibrin(ogen) deposition in patients' brain parenchyma. In hereditary renal amyloidosis, fibrinogen is abnormal, with mutations located in the fibrinogen αC domain. The mutant αC domain derived from fibrinogen degradation folds incorrectly so that, in time, aggregates form, leading to amyloid deposits in the kidneys. In these patients, no thrombotic tendency has been observed. Abnormal fibrinogens with either a point mutation in the αC domain or a frameshift mutation resulting in absence of a part of the αC domain are often associated with either thrombotic events or bleeding. Mutation of an amino acid into cysteine (as in fibrinogens Dusart and Caracas V) or a frameshift mutation yielding an unpaired cysteine in the αC domain is often responsible for thrombotic events. Covalent binding of albumin to the unpaired cysteine via a disulphide bridge leads to decreased accessibility to the fibrinolytic enzymes, hence formation of poorly degradable fibrin clots, which explains the high incidence of thrombosis. In contrast, anomalies due to a frameshift mutation in the αC connector of the molecule, provoking deletion of a great part of the αC domain, are associated with bleeding.

c.259A>C in the fibrinogen gene of alpha chain (FGA) is a fibrinogen with thrombotic phenotype

Introduction

Dysfibrinogenemia is a rare inherited disease that results from mutation in one of the three fibrinogen genes. Diagnosis can be misleading since it may present as a bleeding tendency or thrombosis and a specific coagulation test for diagnosis is not routinely available

Aim

To search for a new candidate gene of thrombophilia in a family with three generations of arterial and venous thrombosis.

Methods

Whole exome sequencing followed by Sanger validation and segregation analysis was carried out. In addition, structural modeling was performed. Screening for thrombophilia along with blood counts, prothrombin time, activated partial thromboplastin, thrombin, reptilase time, and fibrinogen was done in each patient.

Results and discussion

A missense c.259A>C, p.K87Q (g.chr4: 155510050A-C) (rs764281241) in FGA gene was found in all three siblings without any other known thrombophilia marker to explain thrombosis in all three siblings. It is expected to be damaging by six out of seven prediction programs and is very rare in the entire population with Exac=0.000008.

Conclusion

The occurrence of the c.259A>C mutation in FGA may well explain the thrombosis phenotype of the affected family and is suggested as a new marker for thrombophilia phenotype.

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Human Fibrinogen: Molecular and Genetic Aspects of Congenital Disorders

Congenital fibrinogen disorders can be quantitative (afibrinogenemia, hypofibrinogenemia) or functional (dysfibrinognemia). To date, several genetic variants have been identified in individuals with fibrinogen disorders. The complexity of the fibrinogen molecules, formed by three non-identical chains and with a trinodal organization, renders the identification of molecular causes and of clinical and biochemical phenotypes very challenging. However, the acknowledgement of the type of molecular defect is crucial for a safer therapy, which is going to improve the clinical management of these patients. In this review, some aspects concerning molecular and clinical findings available on congenital fibrinogen disorders will be discussed.

Hereditary Fibrinogen Aα-Chain Amyloidosis in Asia: Clinical and Molecular Characteristics

Hereditary fibrinogen Aα-chain amyloidosis (Aα-chain amyloidosis) is a type of autosomal dominant systemic amyloidosis caused by mutations in fibrinogen Aα-chain gene (FGA). Patients with Aα-chain amyloidosis have been mainly reported in Western countries but have been rarely reported in Asia, with only five patients with Aα-chain amyloidosis being reported in Korea, China, and Japan. Clinically, the most prominent manifestation in Asian patients with Aα-chain amyloidosis is progressive nephropathy caused by excessive amyloid deposition in the glomeruli, which is similar to that observed in patients with Aα-chain amyloidosis in Western countries. In molecular features in Asian Aα-chain amyloidosis, the most common variant, E526V, was found in only one Chinese kindred, and other four kindred each had a different variant, which have not been identified in other countries. These variants are located in the C-terminal region (amino acid residues 517–555) of mature Aα-chain, which was similar to that observed in patients with Aα-chain amyloidosis in other countries. The precise number of Asian patients with Aα-chain amyloidosis is unclear. However, patients with Aα-chain amyloidosis do exist in Asian countries, and the majority of these patients may be diagnosed with other types of systemic amyloidosis.

VLITL is a major cross-β-sheet signal for fibrinogen Aα-chain frameshift variants

The first case of hereditary fibrinogen Aα-chain amyloidosis was recognized >20 years ago, but disease mechanisms still remain unknown. Here we report detailed clinical and proteomics studies of a French kindred with a novel amyloidogenic fibrinogen Aα-chain frameshift variant, Phe521Leufs, causing a severe familial form of renal amyloidosis. Next, we focused our investigations to elucidate the molecular basis that render this Aα-chain variant amyloidogenic. We show that a 49-mer peptide derived from the C-terminal part of the Phe521Leufs chain is deposited as fibrils in the patient's kidneys, establishing that only a small portion of Phe521Leufs directly contributes to amyloid formation in vivo. In silico analysis indicated that this 49-mer Aα-chain peptide contained a motif (VLITL), with a high intrinsic propensity for β-aggregation at residues 44 to 48 of human renal fibrils. To experimentally verify the amyloid propensity of VLITL, we generated synthetic Phe521Leufs-derived peptides and compared their capacity for fibril formation in vitro with that of their VLITL-deleted counterparts. We show that VLITL forms typical amyloid fibrils in vitro and is a major signal for cross-β-sheet self-association of the 49-mer Phe521Leufs peptide identified in vivo, whereas its absence abrogates fibril formation. This study provides compelling evidence that VLITL confers amyloidogenic properties to Aα-chain frameshift variants, yielding a previously unknown molecular basis for the pathogenesis of Aα-chain amyloidosis.

Study on the influence of oxidative stress on the fibrillization of fibrinogen

Human fibrinogen is an important coagulation factor as well as an independent predictor of coronary heart disease and stroke. Analysis of dysfibrinogens may provide useful information and help us to understand the molecular defects in fibrin polymerization. In the present study, we investigated the influence of oxidative stress of fibrinogen induced by H2O2 on the polymerization state of fibrin. UV absorbance spectroscopy, circular dichroism, ζ-potential, dynamic light scattering and steady shear viscosity were all employed to study the influence of oxidative stress on the molecular structure, the surface charges, and the size and shape of fibrinogen molecules. The fibrin morphology obtained was imaged and investigated using atomic force microscopy. The results demonstrated that the cross-linking, branching and height distribution of formed fibrin will be influenced by the oxidative stress of fibrinogen. This study presents new insights into the aggregation behaviour of fibrinogen and will be helpful to understand the formation mechanism of thrombosis under oxidative stress.

Dysfibrinogenemia: from molecular anomalies to clinical manifestations and management

Congenital dysfibrinogenemia is a qualitative congenital fibrinogen disorder characterized by normal antigen levels of a dysfunctional fibrinogen. The diagnosis is usually based on discrepancies between fibrinogen activity and antigen levels, but could require more specialized techniques for the assessment of fibrinogen function, owing to some limitations in routine assays. Molecular abnormalities, which are frequently heterozygous missense mutations localized in exon 2 of FGA and exon 8 of FGG, lead to defects in one or more phases of fibrinogen to fibrin conversion, fibrin network formation, and other important functions of fibrinogen. The clinical phenotype is highly heterogeneous, from no manifestations to bleeding and/or thrombotic events. Asymptomatic propositi and relatives with the predisposing genotype are at risk of developing adverse outcomes during the natural course of the disease. Correlations between genotype and phenotype have not yet been clearly established, with the exception of some abnormal fibrinogens that severely increase the risk of thrombosis. Functional analysis of polymerization and fibrinolysis, structural studies of the fibrin network and the viscoelastic properties of fibrin clot could help to predict the phenotype of congenital dysfibrinogenemia, but have not yet been evaluated in detail. The management is essentially based on personal and family history; however, even individuals who are still asymptomatic and without a family history should be carefully assessed and monitored. Particular situations, such as pregnancy, delivery, and surgery, require a multidisciplinary approach.

Natural history of patients with congenital dysfibrinogenemia

We conducted a multicenter study of 101 patients with congenital dysfibrinogenemia (CD) to characterize the incidence of hemorrhagic and thrombotic events as well as complications of pregnancy and surgery. At the time of diagnosis, 10.9% and 13.9% had experienced major bleeding and thrombotic events, respectively. During a mean follow-up of 8.8 years after CD diagnosis, the incidence of major bleeding and thrombotic events was 2.5 and 18.7 per 1000 patient-years, respectively, with estimated cumulative incidences at age 50 years of 19.2% and 30.1%. We identified 111 pregnancies with an overall incidence of spontaneous abortions and postpartum hemorrhage of 19.8% and 21.4%, respectively. The risk of postpartum hemorrhage was associated with a previously identified bleeding phenotype (odds ratio, 5.8; 95% CI, 1.2 to 28.0). Among 137 surgical procedures analyzed, 9 (6.5%) were complicated by abnormal bleeding. Propositi vs relatives, sex, mutation hotspots, fibrinogen levels, and activity:antigen ratios were not associated with the risk of thrombotic or bleeding outcomes. In conclusion, the results of our study, the largest in genotyped CD and the first including long-term history, indicate that propositi with CD and their relatives carry not only a high risk of major bleeding, including postpartum hemorrhage, but also of thrombotic event.

Roles of serum fibrinogen α chain-derived peptides in Alzheimer's disease

OBJECTIVE

To find a blood biomarker and disease-related peptides in Alzheimer's disease (AD), we comprehensively detected serum peptides.

METHODS

Ion intensity of serum peptides from 62 AD patients and 82 control subjects was measured by mass spectrometry.

RESULTS

A total of 157 peptides were detected from 30 AD patients and 30 healthy control (HC) subjects. Sixty out of the 157 peptide profiles discriminated between the AD and HC groups. Sixteen out of the 60 peptides were identified, 10 out of which were fragments of a fibrinogen α chain (FIBA). Among the 10 peptides, four and six peptides were derived from fibrinopeptide A (FPA, Aα1-16) and the C-terminal region of the αC-domain (αCDC, Aα557-610), respectively. The profile of 10 FIBA-derived peptides combined with age discriminated between the two groups with an area under the receiver operating characteristic curve (AUROC) of 0.940. Validation of this model using a testing set of 32 AD patients and 19 HC subjects showed an AUROC of 0.717, sensitivity of 65.6%, and specificity of 73.7% by a cutoff value of 0.56420. Another value, 0.04029, showed sensitivity of 96.9%, suggesting that subjects with values less than 0.04029 rarely possess AD. FPA and αCDC showed increased ion intensity in the AD group compared with the HC group (p < 0.05).

CONCLUSIONS

The profile of 10 FIBA-derived peptides combined with age would be a candidate biomarker for AD, which facilitates screening of the disease. The significant release of FPA and αCDC may be involved in the aberrant coagulation that leads to vascular damage in AD.

Dysfibrinogen Kagoshima with the amino acid substitution γThr-314 to Ile: Analyses of molecular abnormalities and thrombophilic nature of this abnormal molecule

INTRODUCTION

Emerging lines of evidence have suggested that certain dysfibrinogens present a significant risk of thrombosis.

PATIENT/METHODS

The thrombophilic nature of a new-type of dysfibrinogen Kagoshima identified in a 36-year-old female with deep vein thrombosis during the postpartum period was studied.

RESULTS/DISCUSSION

Based on the analyses of the patient fibrinogen and the fibrinogen genes, fibrinogen Kagoshima was shown to have the amino acid substitution of gammaThr-314 to Ile that resulted in impaired function and hypofibrinogenemia. Polymerization of fibrin monomers derived from patient fibrinogen was severely impaired with a partial correction in the presence of calcium ions, causing very low clottability and delayed cross-linking of patient fibrin catalyzed by activated factor XIII. Because of the low clottability, a large amount of soluble fibrin was formed upon thrombin treatment, resulting in an increase of thrombin in the soluble fraction. Additionally, tPA-mediated plasmin generation on fibrin was impaired and calcium-ion-dependent integrity of the gamma-chain D domain of Kagoshima fibrinogen was perturbed. The presence of many tapered-fiber ends inside the tangled fibrin networks, observed by scanning electron microscopy, suggested early termination of fibrin polymerization and the structural alteration.

CONCLUSION

These data suggest that fibrinogen Kagoshima is dysfunctional, giving rise to formation of fibrinolysis-resistant soluble fibrin polymers and entrance of soluble fibrin associating with thrombin to the circulation, partly accounting for the thrombophilic nature of the affected fibrinogen and fibrin molecules.

Common variation in the C-terminal region of the fibrinogen beta-chain: effects on fibrin structure, fibrinolysis and clot rigidity

Fibrinogen BbetaArg448Lys is a common polymorphism, positioned within the carboxyl terminus of the Bbeta-chain of the molecule. Studies suggest that it is associated with severity of coronary artery disease and development of stroke. The effects of the amino acid substitution on clot structure remains controversial, and the aim of this study was to investigate the effect(s) of this polymorphism on fibrin clot structure using recombinant techniques. Permeation, turbidity, and scanning electron microscopy showed that recombinant Lys448 fibrin had a significantly more compact structure, with thin fibers and small pores, compared with Arg448. Clot stiffness, measured by means of a novel method using magnetic tweezers, was significantly higher for the Lys448 compared with the Arg448 variant. Clots made from recombinant protein variants had similar lysis rates outside the plasma environment, but when added to fibrinogen-depleted plasma, the fibrinolysis rates for Lys448 were significantly slower compared with Arg448. This study demonstrates for the first time that clots made from recombinant BbetaLys448 fibrinogen are characterized by thin fibers and small pores, show increased stiffness, and appear more resistant to fibrinolysis. Fibrinogen BbetaArg448Lys is a primary example of common genetic variation with a significant phenotypic effect at the molecular level.

Structural analyses of fibrinogen amyloid fibrils

Hereditary fibrinogen amyloidosis is characterized by deposition of amyloid fibrils in renal glomeruli. The subunit protein of the amyloid fibrils is a proteolytic fragment of the fibrinogen Aalpha-chain. To investigate the structure of fibrinogen amyloid, fibrils were extracted from the tissues of a patient and studied by X-ray fiber diffraction and electron microscopy. We have carried out a full structural characterization of amyloid fibrils taken from disease tissue. These studies revealed that ex vivo fibrinogen amyloid fibrils have a cross-beta structure similar to other chemical types of amyloid fibrils.

Direct evidence for specific interactions of the fibrinogen alphaC-domains with the central E region and with each other

The carboxyl-terminal regions of the fibrinogen Aalpha chains (alphaC regions) form compact alphaC-domains tethered to the bulk of the molecule with flexible alphaC-connectors. It was hypothesized that in fibrinogen two alphaC-domains interact intramolecularly with each other and with the central E region preferentially through its N-termini of Bbeta chains and that removal of fibrinopeptides A and B upon fibrin assembly results in dissociation of the alphaC regions and their switch to intermolecular interactions. To test this hypothesis, we studied the interactions of the recombinant alphaC region (Aalpha221-610 fragment) and its subfragments, alphaC-connector (Aalpha221-391) and alphaC-domain (Aalpha392-610), between each other and with the recombinant (Bbeta1-66)2 and (beta15-66)2 fragments and NDSK corresponding to the fibrin(ogen) central E region, using laser tweezers-based force spectroscopy. The alphaC-domain, but not the alphaC-connector, bound to NDSK, which contains fibrinopeptides A and B, and less frequently to desA-NDSK and (Bbeta1-66)2 containing only fibrinopeptides B; it was poorly reactive with desAB-NDSK and (beta15-66)2 both lacking fibrinopeptide B. The interactions of the alphaC-domains with each other and with the alphaC-connector were also observed, although they were weaker and heterogeneous in strength. These results provide the first direct evidence for the interaction between the alphaC-domains and the central E region through fibrinopeptide B, in agreement with the hypothesis given above, and indicate that fibrinopeptide A is also involved. They also confirm the hypothesized homomeric interactions between the alphaC-domains and display their interaction with the alphaC-connectors, which may contribute to covalent cross-linking of alpha polymers in fibrin.

NMR solution structure, stability, and interaction of the recombinant bovine fibrinogen alphaC-domain fragment

According to the existing hypothesis, in fibrinogen, the COOH-terminal portions of two Aalpha chains are folded into compact alphaC-domains that interact intramolecularly with each other and with the central region of the molecule; in fibrin, the alphaC-domains switch to an intermolecular interaction resulting in alphaC-polymers. In agreement, our recent NMR study identified within the bovine fibrinogen Aalpha374-538 alphaC-domain fragment an ordered compact structure including a beta-hairpin restricted at the base by a 423-453 disulfide linkage. To establish the complete structure of the alphaC-domain and to further test the hypothesis, we expressed a shorter alphaC-fragment, Aalpha406-483, and performed detailed analysis of its structure, stability, and interactions. NMR experiments on the Aalpha406-483 fragment identified a second loose beta-hairpin formed by residues 459-476, yielding a structure consisting of an intrinsically unstable mixed parallel/antiparallel beta-sheet. Size-exclusion chromatography and sedimentation velocity experiments revealed that the Aalpha406-483 fragment forms soluble oligomers whose fraction increases with an increase in concentration. This was confirmed by sedimentation equilibrium analysis, which also revealed that the addition of each monomer to an assembling alphaC-oligomer substantially increases its stabilizing free energy. In agreement, unfolding experiments monitored by CD established that oligomerization of Aalpha406-483 results in increased thermal stability. Altogether, these experiments establish the complete NMR solution structure of the Aalpha406-483 alphaC-domain fragment, provide direct evidence for the intra- and intermolecular interactions between the alphaC-domains, and confirm that these interactions are thermodynamically driven.

Pro-thrombotic and pro-oxidant effects of diet-induced hyperhomocysteinemia

Elevated plasma homocysteine levels are associated with the risk of atherosclerosis and arterial and venous thrombosis. We have previously demonstrated that rabbits rendered hyperhomocysteinemic by parenteral administration of homocysteine develop a dysfibrinogenemia that is associated with the formation of fibrin clots that are abnormally resistant to fibrinolysis. We suggested that this acquired dysfibrinogenemia contributes to the thrombotic tendency in hyperhomocysteinemia. However, it was possible that the homocysteine-associated dysfibrinogenemia was an artifact of the parenteral administration model. Therefore, the goals of the current study were to develop a diet-induced model of homocysteinemia in rabbits and determine whether a dysfibrinogenemia and evidence of oxidative stress develop in this model as they do when homocysteine is injected. We found that rabbits fed a diet severely deficient in folate and mildly deficient in choline develop mild hyperhomocysteinemia: 14.8+/-4.0 microM in deficient rabbits compared to 9.0+/-1.7 microM in controls. The deficient rabbits also develop evidence of oxidant stress: increased lipid peroxidation in liver, impaired mitochondrial enzyme activities in liver and elevated caspase-3 levels in plasma. Most importantly, the deficient rabbits also develop a dysfibrinogenemia characterized by increased resistance to fibrinolysis. We believe that this dietary model of homocysteinemia is clinically relevant and reproduces many features associated with hyperhomocysteinemia in previous work using in vitro and in vivo models. Our findings suggest that an acquired dysfibrinogenemia could play a role in the increased risk of atherothrombotic disease in mildly hyperhomocysteinemic human subjects.

Molecular weight fibrinogen variants determine angiogenesis rate in a fibrin matrix in vitro and in vivo

BACKGROUND

During wound repair, fibrin acts both as a barrier to prevent blood loss and as a temporary matrix for the invasion and ingrowth of endothelial and tissue cells. A well-controlled angiogenesis process in the fibrinous exudate matrix is crucial for optimal wound healing. The composition and structure of the fibrin matrix are important determinants of the invasion of endothelial cells and capillary-tube formation into the matrix.

OBJECTIVE

Fibrinogen circulates in a high and low molecular weight form (HMW and LMW, respectively) and the purpose of this study was to investigate how fibrin matrices from these naturally occurring fibrinogen variants influence angiogenesis. Angiogenesis was studied using an in vitro model in which human microvascular endothelial cells (hMVEC) were cultured on three-dimensional fibrin matrices from different fibrinogen forms, and using two in vivo mouse models.

RESULTS

The in vitro angiogenesis in an HMW-fibrin matrix shows increased cell and tubular structure ingrowth compared with unfractionated fibrin matrix (median increase 58%, range 46-234%). The ingrowth of tubular structures in an LMW-fibrin matrices is decreased when compared with unfractionated fibrin (median decrease 70%, range 67-100%). Similar results were observed for in vivo angiogenesis.

CONCLUSIONS

The naturally occurring fibrinogen variants HMW- and LMW-fibrin modulate the angiogenic capacity of endothelial cells in fibrin matrices. The different effects of the molecular weight fibrinogen variants provide further insight in the matrix characteristics in angiogenesis and could possibly be applied in the context of tissue engineering and wound healing.

Modification of fibrinogen by homocysteine thiolactone increases resistance to fibrinolysis: a potential mechanism of the thrombotic tendency in hyperhomocysteinemia

We have previously shown functional differences in fibrinogen from hyperhomocysteinemic rabbits compared to that in control rabbits. This acquired dysfibrinogenemia is characterized by fibrin clots that are composed of abnormally thin, tightly packed fibers with increased resistance to fibrinolysis. Homocysteine thiolactone is a metabolite of homocysteine (Hcys) that can react with primary amines. Recent evidence suggests that Hcys thiolactone-lysine adducts form in vivo. We now demonstrate that the reaction of Hcys thiolactone with purified fibrinogen in vitro produces fibrinogen (Hcys fibrinogen) with functional properties that are strikingly similar to those we have observed in homocysteinemic rabbits. Fibrinogen purified from homocysteinemic rabbits and Hcys fibrinogen are similar in that (1) they both form clots composed of thinner, more tightly packed fibers than their respective control rabbit and human fibrinogens; (2) the clot structure could be made to be more like the control fibrinogens by increased calcium; and (3) they both form clots that are more resistant to fibrinolysis than those formed by the control fibrinogens. Further characterization of human fibrinogens showed that Hcys fibrin had similar plasminogen binding to that of the control and an increased capacity for binding tPA. However, tPA activation of plasminogen on Hcys fibrin was slower than that of the control. Mass spectrometric analysis of Hcys fibrinogen revealed twelve lysines that were homocysteinylated. Several of these are close to tPA and plasminogen binding sites. Lysines are major binding sites for fibrinolytic enzymes and are also sites of plasmin cleavage. Thus, modification of lysines in fibrinogen could plausibly lead to impaired fibrinolysis. We hypothesize that the modification of lysine by Hcys thiolactone might occur in vivo, lead to abnormal resistance of clots to lysis, and thereby contribute to the prothrombotic state associated with homocysteinemia.

Fibrinogen Guarenas, an abnormal fibrinogen with an Aalpha-chain truncation due to a nonsense mutation at Aalpha 467 Glu (GAA)-->stop (TAA)

Fibrinogen Guarenas is a dysfibrinogenemia with a nonsense mutation at G4731T that causes an Aalpha-chain truncation at Ser 466. This abnormal fibrinogen is associated with a bleeding diathesis, severe in the proposita and mild in one brother, even though the fibrinogen levels in plasma are normal. All other family members are asymptomatic. Fibrinogens from the proposita and one family member, the mother of the proposita, both heterozygous for the mutation, were studied. Turbidity curves of fibrin polymerization showed that the lateral association of protofibrils was impaired and the maximum rate of polymerization was slightly diminished. The binding of albumin to fibrinogen was increased compared to control due to the presence of a free sulfhydryl group because of the missing disulphide bridge between Aalpha-Cys 442-472 in the mutated molecules. The abnormal fibrinogen formed much less alpha-polymer, and gamma-dimer formation was delayed compared to the control. Plasminogen activation by t-PA in the presence of fibrin was decreased. When Guarenas clots were perfused with fibrinolytic enzymes, clot degradation was retarded. Clot structure studied by confocal 3D microscopy showed that the fibrin network was dense, made up of thin and highly branched fibers, which accounted for the decreased flow rates by buffer permeation and increased rigidity of the fibrin clots, measured using a torsion pendulum. It seems that the increased clot rigidity, decreased porosity, hypofibrinolysis and t-PA induced fibrinolysis, by itself are not necessarily associated with thrombotic disorders in dysfibrinogenemia.

In vitro fibrin clot formation and fibrinolysis using heterozygous plasma fibrinogen from γAsn319, Asp320 deletion dysfibrinogen, Otsu I

INTRODUCTION

We have reported a heterozygous dysfibrinogenemia, fibrinogen Otsu I, caused by the deletion of gammaAsn319 and gammaAsp320, which was originally identified in the dysfibrinogen Vlissingen/Frankfurt IV (V/FIV) associated with thrombosis. Unlike the V/FIV family, the Otsu propositus showed no thrombotic tendencies. To analyze the relationship between thrombosis and the heterozygous plasma variant fibrinogen, we used purified plasma fibrinogen from the Otsu patient and compared it with a normal control.

MATERIALS AND METHODS

Thrombin-induced fibrin clot formation and clot structure were observed by fibrin polymerization and scanning electron microscopy, respectively. For in vitro observation of fibrinolysis, plasmin generation and clot lysis assays were performed by the addition of tissue type plasminogen activation (tPA) and plasminogen.

RESULTS AND CONCLUSIONS

Polymerization of Otsu was markedly impaired, while fibrin fibers were much thicker and the density of the bundles of fibrin fibers was less and porous compared with normal. Lysis of the Otsu clot was not significantly different from normal when a tPA and plasminogen mixture was overlaid onto the clots. For Otsu, the penetration of the tPA/plasminogen mixture into the clot was much faster than normal and the protection against plasmin cleavage was impaired; however, tPA-induced plasmin activation of the Otsu fibrin was slower than that of normal fibrin, resulting in a clot lysis of Otsu similar to normal.

Natively unfolded regions of the vertebrate fibrinogen molecule

Although it has long been realized that a large portion of the fibrinogen alpha chain has little if any defined structure, the physiological significance of this flexible appendage remains mysterious.

The alphaC domains of fibrinogen affect the structure of the fibrin clot, its physical properties, and its susceptibility to fibrinolysis

The functions of the alphaC domains of fibrinogen in clotting and fibrinolysis, which have long been enigmatic, were determined using recombinant fibrinogen truncated at Aalpha chain residue 251. Scanning electron microscopy and confocal microscopy revealed that the fibers of alpha251 clots were thinner and denser, with more branch points than fibers of control clots. Consistent with these results, the permeability of alpha251 clots was nearly half that of control clots. Together, these results suggest that in normal clot formation, the alphaC domains enhance lateral aggregation to produce thicker fibers. The viscoelastic properties of alpha251 fibrin clots differed markedly from control clots; alpha251 clots were much less stiff and showed more plastic deformation, indicating that interactions between the alphaC domains in normal clots play a major role in determining the clot's mechanical properties. Comparing factor XIIIa cross-linked alpha251 and control clots showed that gamma chain cross-linking had a significant effect on clot stiffness. Plasmin-catalyzed lysis of alpha251 clots, monitored with both macroscopic and microscopic methods, was faster than lysis of control clots. In conclusion, these studies provide the first definitive evidence that the alphaC domains play an important role in determining the structure and biophysical properties of clots and their susceptibility to fibrinolysis.

The mechanical properties of fibrin for basic scientists and clinicians

In this review, I set forth some basic information about the mechanical properties of fibrin clots and attempt to identify the big questions remaining. The intent is to make this topic understandable to both basic scientists who are interested in blood clotting and to hematologists or cardiologists, since I believe that this is something everyone working in these fields should know. The viscoelastic properties of fibrin are remarkable and unique among polymers. Moreover, these properties are essential to the physiology of blood clotting and are important for understanding and therefore preventing and treating thrombosis.

Two novel fibrinogen variants found in patients with pulmonary embolism and their families

BACKGROUND

The occurrence of dysfibrinogen is quite rare in comparison with other hemostatic defects, specially in cases of venous thrombosis.

OBJECTIVES

Fibrinogen is known to have multiple functions, which are not evaluated by simple coagulation testing. We have used gel electrophoresis to search for new mutations.

PATIENTS AND METHODS

Specimens of purified fibrinogen from 217 consecutive patients with familial or recurrent or early thrombosis and from 490 control subjects were evaluated by electrophoresis. Plasma fibrinogen levels and coagulation-dependent tests (electromechanical and optical coagulometric determinations, immunological measurement, thrombin and Reptilase(R) times) were normal.

RESULTS

Two novel familial variants were detected. For a 42-year-old patient, an in-frame 117 base pair insertion in the Aalpha-chain gene caused a 5-kDa mobility shift of the Aalpha chain. This corresponds to a 39 amino acid duplication in the connector domain (fibrinogen Champagne au Mont d'Or). This pattern was also found in the patient's mother and child. A second 31-year-old patient presented an extra band under non-reducing conditions, 30 kDa larger than HMW fibrinogen and reacting with antifibrinogen antibodies (fibrinogen Lozanne). A heterozygous 5909A-->G mutation was found on the Bbeta-chain gene leading to heterozygous Bbeta Tyr236--> stop codon. The predicted truncated Bbeta chain could participate in chain assembly. Two family members were also affected, one of whom had suffered early venous thrombosis.

CONCLUSIONS

Electrophoretic testing of apparently normal fibrinogens can reveal new variants which may be clinically relevant.

Elevated plasma homocysteine leads to alterations in fibrin clot structure and stability: implications for the mechanism of thrombosis in hyperhomocysteinemia

Elevated plasma homocysteine is associated with an increased risk of atherosclerosis and thrombosis. However, the mechanisms by which homocysteine might cause these events are not understood. We hypothesized that hyperhomocysteinemia might lead to modification of fibrinogen in vivo, thereby causing altered fibrin clot structure. New Zealand White rabbits were injected intraperitoneally (i.p.) every 12 h through an indwelling catheter with homocysteine or buffer for 8 weeks. This treatment raised the plasma homocysteine levels to about 30 micro mol L(-1) compared with 13.5 micro mol L(-1) in control rabbits by the end of the treatment period. The fibrinogen levels were 3.2 +/- 0.6 in homocysteine-treated and 2.5 +/- 1.1 mg mL(-1) in control rabbits. The reptilase time was prolonged to 363 +/- 88 for plasma from homocysteine-treated rabbits compared with 194 +/- 48 s for controls (P < 0.01). The thrombin clotting time (TCT) for the homocysteine-treated rabbits was significantly shorter, 7.5 +/- 1.7 compared with 28.6 +/- 18 s for the controls (P < 0.05). The calcium dependence of the thrombin clotting time was also different in homocysteinemic and control plasmas. Clots from plasma or fibrinogen of homocysteinemic rabbits were composed of thinner fibers than control clots. The clots formed from purified fibrinogen from homocysteine-treated rabbits were lyzed more slowly by plasmin than comparable clots from control fibrinogen. Congenital dysfibrinogenemias have been described that are associated with fibrin clots composed of thin, tightly packed fibers that are abnormally resistant to fibrinolysis, and recurrent thrombosis. Our results suggest that elevated plasma homocysteine leads to a similar acquired dysfibrinogenemia. The formation of clots that are abnormally resistant to fibrinolysis could directly contribute to the increased risk of thrombosis in hyperhomocysteinemia.

Structure and function of human fibrinogen inferred from dysfibrinogens

Fibrinogen is a 340-kDa plasma protein that is composed of two identical molecular halves, each consisting of three non-identical subunit polypeptides designated as A alpha, B beta- and gamma-chains held together by multiple disulfide bonds. Fibrinogen has a trinodular structure, i.e., one central E domain comprizing the amino-terminal regions of paired individual three polypeptides, and two identical outer D domains. These three nodules are linked by two coiled-coil regions [1,2]. After activation with thrombin, a tripeptide segment consisting of Gly-Pro-Arg is exposed at the amino-terminus of each alpha-chain residing at the center of the E domain and combines with its complementary binding site, called the 'a' site, residing in the carboxyl-terminal region of the gamma-chain in the outer D domain of another molecule. By crystallographic analysis [3], the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between the carboxyl group of gamma Asp-364 and the carboxyamide of Gln-329 in the 'a' site. Half molecule-staggered, double-stranded fibrin protofibrils are thus formed [4,5]. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280 and Ser-300 of the gamma-chain on the surface of the abutting two D domains [3]. Thereafter, carboxyl-terminal regions of the fibrin a-chains are thought to be untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and interwoven networks after appropriate branching [6-9]. Although many enigmas still remain regarding the mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In my talk, I would like to discuss these molecular interactions of fibrinogen and fibrin based on the up-date data provided by analyses of normal as well as hereditary dysfibrinogens, particularly in the latter by introducing representative molecules at each step of fibrin clot formation.

Fibrinogen alpha C domains contain cryptic plasminogen and tPA binding sites

Surface plasmon resonance and ELISA experiments revealed that recombinant fibrinogen alpha C fragment (residues A alpha 221-610) corresponding to the alpha C domain binds tPA and plasminogen with high affinity. This binding was found to be Lys-dependent and occurred via independent binding sites. Study with truncated variants of the alpha C fragment located these sites in its COOH-terminal half. Binding of tPA and plasminogen to these sites stimulated activation of the latter whereas proteolytic degradation of the alpha C fragment reduced this effect substantially, suggesting the importance of the alpha C domains in regulation of fibrinolysis.

The structure and function of the alpha C domains of fibrinogen

The alpha C domains have been localized on fibrinogen and fibrin. Several model systems have been developed to study their functions. Analysis of the amino acid sequence of the alpha C domains suggested that each is made up of a globular and an extended portion. Microcalorimetry confirmed this result and showed that the two alpha C domains interact intramolecularly. Electron microscopy of fibrinogen with a monoclonal antibody to the alpha C domains demonstrated that these regions normally interact with the central portion of the molecule. In the conversion from fibrinogen to fibrin there is a large scale conformational change, such that the alpha C domains dissociate from the central region and are available for intermolecular interaction. Experiments with highly purified and well characterized fragment X monomer, missing either one or both of the alpha C domains, indicate that intermolecular interactions between alpha C domains are important for the enhancement of lateral aggregation during fibrin polymerization. Isolated alpha C fragments polymerized at neutral pH and interacted with the alpha C domains of fibrin monomer to influence clot formation. Several dysfibrinogenemias in which there are amino acid substitutions in, or truncations of, the alpha C domains revealed that these changes can have dramatic effects on polymerization and clot structure. The polymerization of A alpha 251 recombinant fibrinogen, that contains A alpha chains truncated at residue 251, was altered, as were the mechanical properties and the rate of fibrinolysis of the clots. Altogether, these results help to define the role of the alpha C domains in determining the structure and properties of clots.