Anticoagulant activities of heparin oligosaccharides and their neutralization by platelet factor 4

Oligosaccharides of well-defined molecular size were prepared from heparin by nitrous acid depolymerization, affinity chromatography on immobilized antithrombin III (see footnote on Nomenclature) and gel chromatography on Sephadex G-50. High affinity (for antithrombin III) octa-, deca-, dodeca-, tetradeca-, hexadeca- and octadeca-saccharides were prepared, as well as oligosaccharides of larger size than octadecasaccharide. The inhibition of Factor Xa by antithrombin III was greatly accelerated by all of these oligosaccharides, the specific anti-Factor Xa activity being invariably greater than 1300 units/mumol. The anti-Factor Xa activity of the decasaccharide was not significantly decreased in the presence of platelet factor 4, even at high platelet factor 4/oligosaccharide ratios. Measurable but incomplete neutralization of the anti-Factor Xa activities of the tetradeca- and hexadeca-saccharides was observed, and complete neutralization of octadeca- and larger oligo-saccharides was achieved with excess platelet factor 4. The octa-, deca-, dodeca-, tetradeca- and hexadeca-saccharides had negligible effect on the inhibition of thrombin by antithrombin III, whereas specific anti-thrombin activity was expressed by the octadeca-saccharide and by the larger oligosaccharides. An octadecasaccharide is therefore the smallest heparin fragment (prepared by nitrous acid depolymerization) that can accelerate thrombin inhibition by antithrombin III. The anti-thrombin activities of the octadecasaccharide and larger oligosaccharides were more readily neutralized by platelet factor 4 than were their anti-Factor Xa activities. These findings are compatible with two alternative mechanisms for the action of platelet factor 4, both involving the binding of the protein molecule adjacent to the antithrombin III-binding site. Such binding results in either steric interference with the formation of antithrombin III-proteinase complexes or in displacement of the antithrombin III molecule from the heparin chain.

The central role of thrombin in hemostasis

Following vascular injury, blood loss is controlled by the mechanisms of hemostasis. During this process, the serine proteinase, thrombin, is generated both locally and rapidly at sites of vessel damage. It plays a pivotal role in clot promotion and inhibition, and cell signaling, as well as additional processes that influence fibrinolysis and inflammation. These functions involve numerous cleavage reactions, which must be tightly coordinated. Failure to do so can lead to either bleeding or thrombosis. The crystal structures of thrombin, in combination with biochemical analyses of thrombin mutants, have provided insight into the ways in which thrombin functions, and how its different activities are modulated. Many of the interactions of thrombin are facilitated by exosites on its surface that bind to its substrates and/or cofactors. The use of cofactors not only extends the range of thrombin specificity, but also enhances its catalytic efficiency for different substrates. This explains a paradox (i.e. thrombin is a specific proteinase, and yet one that has multiple, and sometimes opposing, substrate reactions). In this review, we describe the context in which thrombin acts during hemostasis and explain the roles that its exosites and cofactors play in directing thrombin function. Thereafter, we develop the concept of cofactor competition as a means by which the activities of thrombin are controlled.

A comparison of risk stratification schemes for stroke in 79,884 atrial fibrillation patients in general practice

BACKGROUND

Anticoagulation management of patients with atrial fibrillation (AF) should be tailored individually on the basis of ischemic stroke risk. The objective of this study was to compare the predictive ability of 15 published stratification schemes for stroke risk in actual clinical practice in the UK.

METHODS

AF patients aged ≥ 18 years in the General Practice Research Database, which contains computerized medical records, were included. The c-statistic was estimated to determine the predictive ability for stroke for each scheme. Outcomes included stroke, hospitalizations for stroke, and death resulting from stroke (as recorded on death certificates).

RESULTS

The study cohort included 79,844 AF patients followed for an average of 4 years (average of 2.4 years up to the start of warfarin therapy). All risk schemes had modest discriminatory ability in AF patients, with c-statistics for predicting events ranging from 0.55 to 0.69 for strokes recorded by the general practitioner or in hospital, from 0.56 to 0.69 for stroke hospitalizations, and from 0.56 to 0.78 for death resulting from stroke as reported on death certificates. The proportion of patients assigned to individual risk categories varied widely across the schemes, with the proportion categorized as moderate risk ranging from 12.7% (CHA(2) DS(2)-VASc) to 61.5% (modified CHADS(2)). Low-risk subjects were truly low risk (with annual stroke events < 0.5%) with the modified CHADS(2), National Institute for Health and Clinical Excellence and CHA(2) DS(2) -VASc schemes.

CONCLUSION

Current published risk schemes have modest predictive value for stroke. A new scheme (CHA(2) DS(2) -VASc) may discriminate those at truly low risk and minimize classification of subjects as intermediate/moderate risk. This approach would simplify our approach to stroke risk stratification and improve decision-making for thromboprophylaxis in patients with AF.

Epidemiology of coagulation factors, inhibitors and activation markers: the Third Glasgow MONICA Survey. I. Illustrative reference ranges by age, sex and hormone use

Coagulation factor activity (fibrinogen, VII, VIII and IX), coagulation inhibitor activity (antithrombin, protein C, protein S), and coagulation activation markers (prothrombin fragment F1, 2; thrombin-antithrombin complexes) were measured in 747 men and 817 women aged 25-74 years, randomly sampled from the north Glasgow population in the Third MONICA Survey. Significant effects of age, sex, menopause and hormone use were observed and specific reference ranges are presented to illustrate these effects. Significant correlations were observed between several coagulation factors and inhibitors. Increased levels of factors VII, VIII and IX and decreased levels of protein C were associated with increased coagulation activation. In general, increases in coagulation factors with age were greater than increases in coagulation inhibitors, especially in men; this imbalance may favour increased coagulation activation and hence increased thrombotic risk with age.

Adjusted prognostic association of depression following myocardial infarction with mortality and cardiovascular events: individual patient data meta-analysis

BACKGROUND

The association between depression after myocardial infarction and increased risk of mortality and cardiac morbidity may be due to cardiac disease severity.

AIMS

To combine original data from studies on the association between post-infarction depression and prognosis into one database, and to investigate to what extent such depression predicts prognosis independently of disease severity.

METHOD

An individual patient data meta-analysis of studies was conducted using multilevel, multivariable Cox regression analyses.

RESULTS

Sixteen studies participated, creating a database of 10 175 post-infarction cases. Hazard ratios for post-infarction depression were 1.32 (95% CI 1.26-1.38, P<0.001) for all-cause mortality and 1.19 (95% CI 1.14-1.24, P<0.001) for cardiovascular events. Hazard ratios adjusted for disease severity were attenuated by 28% and 25% respectively.

CONCLUSIONS

The association between depression following myocardial infarction and prognosis is attenuated after adjustment for cardiac disease severity. Still, depression remains independently associated with prognosis, with a 22% increased risk of all-cause mortality and a 13% increased risk of cardiovascular events per standard deviation in depression z-score.

Quality of life in adults with congenital heart disease

OBJECTIVE

To examine the quality of life of adults with congenital heart disease.

DESIGN AND SETTING

Observational, cross sectional study conducted at one general hospital in Birmingham, UK.

PATIENTS

All 471 patients registered at the adult congenital heart disease clinic were sent the 36 item short form health survey (SF-36) to assess their quality of life. Questionnaires were completed by 276 (58.6%) patients (41.7% men; median (interquartile range) age 31.0 (26.3-36.0) years, range 16-85 years).

RESULTS

Surprisingly, patients deemed surgically cured (for example, atrial septal defect repair) had significantly poorer quality of life in all domains (all p < 0.05), except for pain, than the general population, as determined from population normative data. Patients who had received palliative treatment reported quality of life scores similar to those who had never required cardiac surgery and to the general population, although both patient groups had significantly poorer physical functioning and overall general health perception than the general population (all p < 0.01). Patients with inoperable conditions had significantly poorer physical functioning (all p < 0.01) and overall general health perception (all p < 0.05) than all other patients, and significantly worse quality of life in all domains than the similarly aged general population. Patients with cyanotic conditions had significantly worse quality of life than age and sex matched acyanotic patients (all p < 0.01).

CONCLUSIONS

Patients with inoperable or cyanotic conditions and, paradoxically, those deemed surgically cured, had the poorest quality of life among adults with congenital heart disease. However, all adults with congenital heart disease had significantly poorer levels of physical functioning and overall general health perception then similarly aged people in the general population.

Factor V Leiden (FV R506Q) in Families with Inherited Antithrombin Deficiency

We investigated the presence of the gene mutation of factor V, FV R506Q or factor V Leiden, responsible for activated protein C resistance, in DNA samples of 127 probands and 188 relatives from 128 families with antithrombin deficiency. The factor V mutation was identified in 18 families. Nine families were available to assess the mode of inheritance and the clinical relevance of combined defects.

The factor V and antithrombin genes both map to chromosome 1. Segregation of the defects on opposite chromosomes was observed in three families. Co-segregation with both defects on the same chromosome was demonstrated in four families. In one family a de novo mutation of the antithrombin gene and in another a crossing-over event were the most likely explanations for the observed inheritance patterns.

In six families with type I or II antithrombin deficiency (reactive site or pleiotropic effect), 11 of the 12 individuals with both antithrombin deficiency and the factor V mutation developed thrombosis. The median age of their first thrombotic episode was 16 years (range 0-19); this is low compared with a median age of onset of 26 years (range 20-49) in 15 of 30 carriers with only a defect in the antithrombin gene. One of five subjects with only factor V mutation experienced thrombosis at 40 years of age. In three families with type II heparin binding site deficiencies, two of six subjects with combined defects experienced thrombosis; one was homozygous for the heparin binding defect.

Our results show that, when thrombosis occurs at a young age in antithrombin deficiency, the factor V mutation is a likely additional risk factor. Co-segregation of mutations in the antithrombin and factor V genes provides a molecular explanation for severe thrombosis in several generations. The findings support that combinations of genetic risk factors underly differences in thrombotic risk in families with thrombophilia.

Dabigatran adherence in atrial fibrillation patients during the first year after diagnosis: a nationwide cohort study

BACKGROUND

There is a perception among physicians that lack of routine monitoring with non-vitamin K antagonist oral anticoagulants (NOACs) may lead to poor adherence to medication. We studied adherence during the first year of usage in a cohort of patients with newly diagnosed non-valvular atrial fibrillation (AF) started on the NOAC, dabigatran etexilate.

METHODS AND RESULTS

Nationwide Danish patient and prescription purchase registries were used to identify newly diagnosed AF patients taking dabigatran, comorbidities, and refill patterns under a twice-daily, one pill regimen. Adherence was characterized among remaining users (N = 2960) after 1 year using the proportion of days covered (PDC), gap rates and restart rates. The overall 1-year PDC was 83.9%, with 76.8% of patients having a 1-year PDC in excess of 80%. Patients with a CHA2 DS2 -VASc score ≥ 2 were more adherent to medication regimes than patients with a CHA2 DS2 -VASc score of 1 (PDC ratio, 1.12; 95% confidence interval [CI], 1.08-1.17) and generally patients with higher morbidity showed more adherence. Patients with prior bleeding were not less adherent to medication regimes than patients with no prior bleeding (PDC ratio, 1.02; 95% CI, 0.98-1.06). The overall gap rate was 1.4 gaps per year. There were no clear tendencies in gap rates among subgroups, although patients with higher morbidity tended to have slightly more, but shorter, gap periods.

CONCLUSIONS

More than 75% of patients were showed > 80% adherence to medication regimes during the first year. Patients with higher morbidity, including patients with a higher risk of stroke or bleeding, exhibited better adherence. This improvement may be attributable to more regular contact with the healthcare system.

Epidemiology of coagulation factors, inhibitors and activation markers: The Third Glasgow MONICA Survey. II. Relationships to cardiovascular risk factors and prevalent cardiovascular disease

Coagulation factor activity (fibrinogen, VII, VIII and IX), coagulation inhibitor activity (antithrombin, protein C, protein S), and coagulation activation markers (prothrombin fragment F1, 2; thrombin-antithrombin complexes) were measured in 746 men and 816 women aged 25-74 years, randomly sampled from the north Glasgow population in the Third MONICA Survey. After age-adjustment, significant associations with cardiovascular risk factors were observed. Serum cholesterol and triglyceride were associated with increases in factors VII and IX, as well as antithrombin, protein C and protein S; and with increased fibrinogen and factor VIII in women. Apart from factor VIII (related to blood pressure in men, but not in women), similar associations were observed for blood pressure and body mass index. Smoking status and/or smoking markers were related to fibrinogen, factor IX, antithrombin and protein S. Alcohol intake was related to protein S, and inversely to fibrinogen and antithrombin in men. Low social class was associated with fibrinogen, factor VIII, factor IX, and with antithrombin, protein S, and low protein C in men. Serum vitamin C was associated inversely with coagulation factors and coagulation inhibitors. The only associations of activation markers were with low serum vitamin C, and with alcohol consumption and low social class in men. Prevalent cardiovascular disease was associated only with fibrinogen. These associations of coagulation factors and inhibitors with cardiovascular risk factors are plausibly relevant to thrombotic risk in cardiovascular disease. In general, 'worse' values of risk factors are associated with increased plasma levels of both coagulation factors and inhibitors, without significant increase in coagulation activation markers. However, the association of lower serum vitamin C with increased coagulation activation markers is of potential therapeutic interest.

Is female sex a risk factor for stroke and thromboembolism in patients with atrial fibrillation? A systematic review and meta-analysis

BACKGROUND

Atrial fibrillation (AF) increases the risk of stroke, but this risk is not homogenous. Many risk factors contribute to stroke risk however, the evidence for female sex as a risk factor is less well-established.

AIM

To perform a systematic review and meta-analysis of the available evidence to establish if female sex is a risk factor for stroke/thromboembolism among patients with AF.

METHODS

A systematic literature search was conducted using Medline. The search term 'atrial fibrillation' was used in combination with 'stroke risk', 'thromboembolism', 'female' and 'gender differences' and returned 735 articles, of which 17 were appraised and included. Females with AF were compared with males with AF for the outcome of stroke/thromboembolism.

RESULTS

Seventeen studies, 5 randomized-controlled trials and 12 prospective observational studies were included; 10 demonstrated an increased risk of stroke in women. Meta-analysis of the 17 studies revealed a 1.31-fold (95% confidence intervals (CIs) 1.18-1.46) elevated risk of stroke in women with AF; the risk appearing greatest for women aged ≥75 years. Only three studies compared entirely anticoagulated populations; stroke rates among these patients varied from 1.2-1.44% per-patient year for men and 2.08-2.43% per-patient year for women. Risk of stroke in women appeared similar regardless of oral anticoagulation therapy [risk ratio (95% CI 1.29 (1.09-1.52) and 1.49 (1.17-1.90) in non-anticogulated vs. anticoagulated/mixed cohorts, respectively).

CONCLUSIONS

Women with AF are at increased risk of stroke, particularly elderly women. Comprehensive stroke risk assessment, including sex as a risk factor, should be undertaken in all AF patients.

Assessment of hypercoagulable states by measurement of activation fragments and peptides

Broad spectrum assays which measure a range of fibrinogen/fibrin derivatives (FDPs) in serum have become an established means of identifying activation of blood coagulation and/or fibrinolysis, such as occurs in disseminated intravascular coagulation (DIC). There is considerable interest in the application of these assays to the diagnosis of other hypercoagulable states, such as recurrent deep venous thrombosis and myocardial infarction. In recent years, more sensitive and specific FDP assays (e.g. for fragment E, fragment E neoantigen, D-dimer, fragment D neoantigen, fibrinopeptide A and fibrin fragment beta 15-42) have been devised, some of which allow measurement in plasma of FDPs without interference from fibrinogen or certain of its derivatives. It was predicted that these assays would both avoid the possibility of artifacts introduced as a consequence of serum preparation and improve detection of hypercoagulable states. In the light of these expectations we have reviewed data published on the use of assays to detect clinical hypercoagulability, giving prominence to assays of crosslinked fibrin derivatives and nothing particularly certain studies that have compared the performance of different assays on the same samples. The accumulating evidence indicates that all of the assays are adequate for detection of DIC. The same cannot be said for other hypercoagulable states. Here much variation is evident between different studies of similar patients in the ability of a particular marker to discriminate between a normal control group and patients determined to be hypercoagulable by an independent method. This variability would seem to be a function of patient group heterogeneity and selection, as assays that detect different antigenic determinants produce results on the same plasma samples that are well correlated. It appears that the precise antigenic determinant does not critically affect detection of hypercoagulability. Additionally, some studies have indicated that use of serum need not introduce artifacts. Despite there being no other obvious advantage, the convenience of some of the plasma assays may well encourage their widespread use. Assays have also been developed for measuring activation fragments of coagulation proteins (e.g. prothrombin fragment F1 + 2 and protein C activation peptide) and for proteinase inhibitor complexes (e.g. thrombin-antithrombin complex) generated during activation of coagulation. The latter assays have been useful in providing a biochemical definition of a 'prethrombotic state'.

Academic calculations versus clinical judgments: practicing physicians' use of quantitative measures of test accuracy

PURPOSE

To determine how often practicing physicians use the customarily recommended quantitative methods that include sensitivity, specificity, and likelihood ratio indexes; receiver operator characteristic (ROC) curves; and Bayesian diagnostic calculations.

PARTICIPANTS AND METHODS

A random sample of 300 practicing physicians (stratified by specialty to include family physicians, general internists, general surgeons, pediatricians, obstetrician/gynecologists, and internal medicine subspecialists) were briefly interviewed in a telephone survey. They were asked about the frequency with which they used the formal methods, the reasons for non-use, and if they employed alternative strategies when appraising tests' diagnostic accuracy.

RESULTS

Of the 300 surveyed physicians, 8 (3%) used the recommended formal Bayesian calculations, 3 used ROC curves, and 2 used likelihood ratios. The main reasons cited for non-use included impracticality of the Bayesian method (74%), and nonfamiliarity with ROC curves and likelihood ratios (97%). Of the 174 physicians who said they used sensitivity and specificity indexes, 165 (95%) did not do so in the recommended formal manner. Instead, the physicians directly estimated tests' diagnostic accuracy by determining how often the test results were correct in groups of patients later found to have, or to be free of, the selected disease.

CONCLUSIONS

The results indicate that most practicing physicians do not use the formal recommended quantitative methods to appraise tests' diagnostic accuracy, and instead report using an alternative direct approach. Although additional training might make physicians use the formal methods more often, the physicians' direct method merits further evaluation as a potentially pragmatic tool for the determination of tests' diagnostic accuracy in clinical practice.

Complete nucleotide sequence of the antithrombin gene: evidence for homologous recombination causing thrombophilia

Antithrombin is the principle regulator of thrombin and other blood coagulation proteinases. It is a member of the serpin family of proteinase inhibitors. The genomic sequence of the antithrombin locus has been completed, revealing a gene spanning 13,477 base pairs from the transcription start site to the poly(A) addition signal. Nine complete and one partial Alu repeat elements were identified within the introns of the gene, with all but one orientated in the reverse direction. Inherited deficiency of antithrombin is associated with a venous thrombotic tendency. Restriction fragment mapping of the antithrombin genes in an individual with type I antithrombin deficiency identified an intragenic deletion in one allele. Localization of the deletion breakpoints involved restriction analysis and direct sequencing of amplified DNA spanning the deletion site. The deletion removed 2761 base pairs, affecting exon 5 and flanking introns, with the deletion ends contained within the left components of two Alu elements. It is likely, therefore, that the deletion arose by homologous recombination between the two Alu elements.

Thrombomodulin gene mutations associated with myocardial infarction

BACKGROUND

Thrombomodulin is an important receptor for thrombin on the endothelial cell surface of most blood vessels, including those of the heart. Thrombin-bound thrombomodulin activates protein C, which inhibits thrombin generation by degrading factors Va and VIIIa. The aim of this study was to analyze the 5' region of the thrombomodulin gene to determine whether mutations contribute a risk for myocardial infarction.

METHODS AND RESULTS

We screened the promoter region of the thrombomodulin gene by single-stranded conformation polymorphism analysis in 104 patients with diagnosed myocardial infarction. Five mutations (three distinct) were identified (GG-9/-10AT, G-33A, and C-133A). The dinucleotide mutation GG-9/-10AT was identified in 3 individuals (2 heterozygous, 1 homozygous). Only one of the three different mutations was identified in 104 patient control subjects matched for age, sex, and race (G-33A in a single individual). All mutations identified were in close proximity to consensus sequences for transcription control elements within the thrombomodulin gene. In contrast, no difference was observed between patients and control subjects for the allelic frequency of a previously identified neutral polymorphism GCC/GTC coding for Ala/Val455, with 3 individuals homozygous for GTC (Val) in both groups.

CONCLUSIONS

The findings suggest that mutations in the promoter region of the thrombomodulin gene may constitute a risk for arterial thrombosis.

Evidence that high von Willebrand factor and low ADAMTS-13 levels independently increase the risk of a non-fatal heart attack

BACKGROUND

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS-13) may influence von Willebrand factor (VWF) levels and consequently the risk of myocardial infarction (MI). Moreover, ADAMTS-13 influences hemostatic plug formation in mouse models. We therefore studied their associations in the Glasgow MI Study (GLAMIS).

METHODS AND RESULTS

We measured ADAMTS-13 and VWF antigen levels by ELISAs in stored plasma from a case-control study of 466 MI cases and 484 age- and sex-matched controls from the same north Glasgow population. There was no correlation between ADAMTS-13 and VWF levels in cases or controls. ADAMTS-13 levels correlated positively with serum cholesterol and triglycerides and body mass index, and negatively with high-density lipoprotein-cholesterol. VWF levels correlated with age, fibrinogen and C-reactive protein. In multivariable analyses including risk factors, VWF correlated positively with risk of MI, and ADAMTS-13 correlated negatively with risk of MI. These associations were independent of each other. The association of ADAMTS-13 with risk of MI was observed only in multivariable analysis.

CONCLUSIONS

VWF and ADAMTS-13 levels were not associated in this study, and showed associations with MI risk in opposite directions but of similar strength. The association of ADAMTS-13 with MI is influenced by lipid levels, and consequently requires further investigation.

Factor V Leiden gene mutation and thrombin generation in relation to the development of acute stroke

To determine the prevalence of the factor V Leiden gene mutation in relation to the phenotypes of cerebral infarction and cerebral hemorrhage, we studied 386 randomly selected cases of acute stroke and 247 control subjects. Factor V genotype was determined by amplification of a 267-bp sequence of exon/intron 10 of the factor V gene. Levels of prothrombin fragment F(1 + 2), a marker of thrombin generation, were determined in both acute and convalescent stroke and related to factor V genotype. Prothrombin fragment F(1 + 2) was assessed by using an enzyme-linked immunosorbent assay. Sixteen stroke cases (4.1%) were identified as having the mutation compared with 14 (5.6%) control subjects. Prothrombin fragment F(1 + 2) levels were estimated in 191 cases and found to be elevated both acutely and after 3 months, but they were not related to factor V genotype. Prothrombin fragment F(1 + 2) is elevated in acute stroke and requires further evaluation in relation to cerebrovascular disease. These results suggest that the factor V Leiden gene mutation is not a risk factor for arterial thrombosis causing stroke.

Clarification of the risk for venous thrombosis associated with hereditary protein S deficiency by investigation of a large kindred with a characterized gene defect

BACKGROUND

Protein S is an important regulatory protein of the coagulation cascade. The risk for venous thrombosis associated with protein S deficiency has been uncertain because all previous risk estimates used phenotypic evaluation alone, which can be ambiguous.

OBJECTIVE

To quantitate the risk for thrombosis associated with a characterized protein S gene mutation that causes a Gly295-->Val substitution and protein S deficiency.

DESIGN

Retrospective study of a single extended family.

SETTING

University hospital referral center.

PARTICIPANTS

A 122-member protein S-deficient family, in which 44 members had a recently characterized gene defect.

MEASUREMENTS

Comprehensive history of thrombosis, history of exposure to acquired risk factors for thrombosis, levels of total and free protein S antigen, and genotype for the mutation causing the Gly295-->Val substitution.

RESULTS

Kaplan-Meier analysis of thrombosis-free survival showed that the probability of remaining free of thrombosis at 30 years of age is 0.5 (95% CI, 0.33 to 0.66) for carriers of the Gly295-->Val mutation compared with 0.97 (CI, 0.93 to 1.0) for normal family members (P < 0.001). In a multivariate Cox regression model that included smoking and obesity, the mutation was a strong independent risk factor for thrombosis (hazard ratio, 11.5 [CI, 4.33 to 30.6]; P < 0.001). For free (but not total) protein S antigen levels, the distributions of persons with and persons without the mutation did not overlap.

CONCLUSIONS

Protein S deficiency, as defined by the presence of a causative gene mutation or a reduced level of free protein S antigen, is a strong independent risk factor for venous thrombosis in a clinical affected family.

Relationships between homocysteine, factor VIIa, and thrombin generation in acute coronary syndromes

BACKGROUND

It has been suggested by clinical, epidemiological, and experimental in vitro studies that homocysteine potentiates thrombin generation. This prothrombotic effect however has not previously been demonstrated in patients presenting with acute coronary syndromes (ACS).

METHODS AND RESULTS

Patients with ACS (n =117) presenting with confirmed acute myocardial infarction (MI) (n =57) or unstable angina pectoris (UAP) (n =60) were consecutively recruited together with patients (n =18) in whom the presenting chest pain was not of cardiac origin (NCP), included as controls. Plasma samples were collected on admission and before clinical intervention. Homocysteine was assayed by high performance liquid chromatography, and both Factor VIIa and prothrombin fragment F1+2 were analyzed by ELISA. There were significant elevations in F1+2 in MI (P<0.001) and UAP (P=0.003), and modest elevations in Factor VIIa in UAP (P<0.05) compared with NCP but no differences in homocysteine levels among those groups. On dividing patients with ACS into quartiles of homocysteine, there was a stepwise increase in F1+2 (P<0.0001) and of Factor VIIa (P<0.05). There were significant correlations in ACS between homocysteine and F1+2 (r=0.46, P<0.0001), homocysteine and Factor VIIa (r=0.24, P<0.01), and F1+2 and Factor VIIa (r=0.41, P<0.0001). There was no correlation between homocysteine and either F1+2 (r=-0.15, P=0.57) or Factor VIIa (r=0. 22, P=0.37) in the NCP patients.

CONCLUSIONS

Elevated plasma homocysteine is associated with and may cause elevated Factor VIIa and thrombin generation in patients presenting with ACS. These findings suggest an explanation for the prothrombotic effect of homocysteine in ACS.

Pleiotropic effects of antithrombin strand 1C substitution mutations

Six different substitution mutations were identified in four different amino acid residues of antithrombin strand 1C and the polypeptide leading into strand 4B (F402S, F402C, F402L, A404T, N405K, and P407T), and are responsible for functional antithrombin deficiency in seven independently ascertained kindreds (Rosny, Torino, Maisons-Laffitte, Paris 3, La Rochelle, Budapest 5, and Oslo) affected by venous thromboembolic disease. In all seven families, variant antithrombins with heparin-binding abnormalities were detected by crossed immunoelectrophoresis, and in six of the kindreds there was a reduced antigen concentration of plasma antithrombin. Two of the variant antithrombins, Rosny and Torino, were purified by heparin-Sepharose and immunoaffinity chromatography, and shown to have greatly reduced heparin cofactor and progressive inhibitor activities in vitro. The defective interactions of these mutants with thrombin may result from proximity of s1C to the reactive site, while reduced circulating levels may be related to s1C proximity to highly conserved internal beta strands, which contain elements proposed to influence serpin turnover and intracellular degradation. In contrast, s1C is spatially distant to the positively charged surface which forms the heparin binding site of antithrombin; altered heparin binding properties of s1C variants may therefore reflect conformational linkage between the reactive site and heparin binding regions of the molecule. This work demonstrates that point mutations in and immediately adjacent to strand 1C have multiple, or pleiotropic, effects on this serpin, leading ultimately to failure of its regulatory function.

Role of factor XII in thrombin generation and fibrinolysis during cardiopulmonary bypass

During cardiopulmonary bypass, thrombin is generated, which is thought to be initiated by activation of factor XII on the surface of the bypass equipment. We present a patient with severe factor XII deficiency who underwent cardiac surgery. As much thrombin was formed during cardiopulmonary bypass (measured by the prothrombin activation fragment F1 + 2 and thrombin-antithrombin complexes) as in normal patients, showing that factor XII was not necessary for thrombin generation. Factor X, but not factor IX, was activated (as measured by their activation peptides), and this activation correlated with F1 + 2 and thrombin-antithrombin complexes, suggesting that the tissue-factor/factor-VIIa pathway is the trigger for thrombin formation.