Does Elevated Plasma Fibrinogen Increase the Risk of Coronary Heart Disease?

Coronary heart disease aetiology: associations and causality

Coronary heart disease (CHD) prevention has largely benefited in the past from the development of epidemiological research. However, the opposition association-causation is currently raised from observational data. We successively review, from some important examples, the classical methodological approach for discussing causality in epidemiology. The easy identification of DNA polymorphisms has prompted new CHD aetiological research in the past 10 years. Causality of the associations presents some special characteristics when genes are involved: necessity of replication, Mendelian randomization..., which might prove to be important in future research.

Analysis of the effect of multiple genetic variants of cardiovascular disease risk on insulin concentration variability in healthy adults of the STANISLAS cohort

INTRODUCTION

Given the hypothesis of a common soil for atherosclerosis, type 2 diabetes and metabolic syndrome, we tested the contribution of gene polymorphisms involved in cardiovascular diseases on fasting insulin concentration (FIC).

METHODS

The polymorphisms were investigated by a multiplex assay in 308 apparently healthy French middle-aged men and women, taken from the STANISLAS cohort. FIC was measured by a microparticular enzymatic immunoassay.

RESULTS

After a series of regression analyses involving 34 polymorphisms, FGB -455G/A was the only polymorphism that remained significantly associated with FIC when adjusting the analyses for multiple testing. Stepwise models showed that FGB polymorphism accounted for 4.39% of FIC variability in men. Additionally, interactions between FGB and with environmental factors (alcohol and smoking in men, and BMI in women) were found.

DISCUSSION

To our knowledge, this is the first study reporting an influence of FGB polymorphism on FIC in a healthy population. Our results concord with the already shown link between fibrinogen concentration and FIC, and support the hypothesis of a relationship between fibrinogen and endothelium in FIC homeostasis whose alteration may induce several metabolic disorders. The contribution of this gene, although modest, is consistent with the polygenic nature of insulin levels.

Associations of plasma fibrinogen and factor VII clotting activity with coronary heart disease and stroke: prospective cohort study from the screening phase of the Thrombosis Prevention Trial

BACKGROUND

As with 'conventional' risk factors such as cholesterol and smoking, there is a need for large, long-term prospective studies on hemostatic factors.

OBJECTIVES

To investigate the prospective relationship of fibrinogen and factor VII clotting activity (FVIIc) with risk of coronary heart disease (CHD) and stroke in a study with a large number of outcomes over a period of 15 years.

PATIENTS/METHODS

A cohort of 22 715 men aged 45-69 years was screened for participation in the Thrombosis Prevention Trial. Men were followed up for fatal and non-fatal CHD and stroke events. There were 1515 CHD events (933 CHD deaths) and 391 strokes (180 stroke deaths). Hazard ratios (HRs) and 95% confidence intervals are expressed per standardized increase in log fibrinogen and log FVIIc, adjusting for age, trial treatment group, conventional CHD risk factors and regression dilution bias.

RESULTS

Hazard ratios for fibrinogen were 1.52 (1.37-1.70) for all CHD events, and 1.36 (1.09-1.69) for all strokes. Exclusion of events within the first 10 years showed a persistent association between CHD and fibrinogen, with an adjusted HR of 1.93 (1.42-2.64). The HRs for FVIIc, adjusting for age and trial treatment, were 1.07 (1.01-1.12) for all CHD events and 1.07 (0.97-1.20) for all strokes, and the fully adjusted HRs were, respectively, 0.97 (0.84-1.05) and 1.07 (0.85-1.33).

CONCLUSIONS

The persisting association between fibrinogen and CHD beyond 10 years may imply a causal effect. There is a small effect of FVIIc on CHD, after adjustment for age and trial treatment, but no association independent of other risk factors.

Fibrinogen and coronary heart disease: test of causality by 'Mendelian randomization'

BACKGROUND

Blood concentrations of fibrinogen have been associated with coronary heart disease risk in epidemiological studies, but it is uncertain whether this association is causal or reflects residual confounding by other risk factors. We investigated the relationship between the single nucleotide polymorphism at position -148 in the beta-fibrinogen gene promoter (beta - 148C/T), blood fibrinogen levels, and risk of myocardial infarction (MI) in sufficiently large numbers of coronary disease cases to reliably address this question.

METHODS

Genotyping and measurement of blood fibrinogen concentration were carried out in 4,685 cases of confirmed MI and 3,460 controls with no history of coronary disease. A meta-analysis of ISIS and 19 other studies of beta-fibrinogen genotypes involving a total of 12,220 coronary disease cases and 18,716 controls was conducted.

RESULTS

Among the ISIS controls, mean plasma fibrinogen concentrations with the C/C, C/T and T/T genotypes were 3.34 (SE 0.015), 3.48 (0.022), and 3.60 (0.064) g/l, respectively, corresponding to an increase of 0.14 (0.024) g/l per T allele (trend P < 0.0001). In the case-control comparison, 0.14 g/l higher usual plasma fibrinogen concentration was associated with an age-adjusted and sex-adjusted risk ratio for MI of 1.17 [95% confidence interval (95% CI) 1.14-1.19; P < 0.0001]. But, after further adjustment for smoking, body mass index, and plasma apolipoprotein B/A(1) ratio, this risk ratio fell to 1.03 (95% CI 1.00-1.05; P = 0.05). Moreover, fibrinogen genotype was not significantly associated with MI incidence: risk ratio of 1.06 (95% CI 0.96-1.16) per higher-fibrinogen allele in ISIS alone and of 1.00 (95% CI 0.95-1.04) per allele in the meta-analysis.

CONCLUSIONS

Genotypes that produce lifelong differences in fibrinogen concentrations do not materially influence coronary disease incidence. As these genotype-dependent differences in fibrinogen were allocated randomly at conception (Mendelian randomization), this association is not likely to be confounded by other factors. Consequently, these genetic results provide strong evidence that long-term differences in fibrinogen concentrations are not a major determinant of coronary disease risk.

Additive value of immunoassay-measured fibrinogen and high-sensitivity C-reactive protein levels for predicting incident cardiovascular events

BACKGROUND

Current guidelines suggest measuring high-sensitivity C-reactive protein (hs-CRP) as an aid to coronary risk assessment in adults without cardiovascular disease (CVD). Whether other inflammatory biomarkers, such as fibrinogen, add further prognostic information is uncertain.

METHODS AND RESULTS

In a prospective study of 27,742 initially healthy middle-aged women, the associations of baseline immunoassay fibrinogen and hs-CRP measurements with incident CVD were examined over a 10-year follow-up period. Compared with women in the bottom biomarker quintile, age-adjusted hazard ratios (95% confidence intervals [CIs]) for incident CVD for quintiles 2 to 5 of fibrinogen were 1.10 (0.86 to 1.41), 1.30 (1.03 to 1.65), 1.46 (1.16 to 1.85), and 2.43 (1.95 to 3.02); for hs-CRP they were 1.48 (1.06 to 2.05), 1.70 (1.24 to 2.33), 2.20 (1.63 to 2.96), and 3.24 (2.43 to 4.31). After further adjustment for established risk factors, both biomarkers remained associated (P for trend < or = 0.001) with incident CVD (hazard ratio, 1.35; 95% CI, 1.07 to 1.71 for top fibrinogen quintile; and hazard ratio, 1.68; 95% CI, 1.22 to 2.29 for top hs-CRP quintile compared with the bottom quintiles). Further adjustment for the other biomarker resulted in hazard ratios of 1.23 and 1.56 (P for trend = 0.02 and 0.002), respectively. Although fibrinogen correlated positively with hs-CRP (rs = 0.41, P < 0.001), the highest CVD risk was associated with elevated levels of both fibrinogen and hs-CRP: age-adjusted hazard ratio of 3.45 (95% CI, 2.60 to 4.57) for women with fibrinogen > 393 mg/dL and hs-CRP > 3 mg/L compared with < 329 mg/dL and < 1 mg/L, respectively.

CONCLUSIONS

In this cohort of initially healthy women, baseline levels of fibrinogen measured with a high-quality immunoassay provided additive value to hs-CRP and traditional risk factors in predicting incident CVD.

Can haematological tests predict cardiovascular risk? The 2005 Kettle Lecture*

The risk of venous or arterial thrombosis is routinely assessed by clinical variables (risk factors) supplemented by measurement of blood lipids and glucose for arterial thrombotic events. Haematological tests that might play a role in risk prediction include haemostatic variables, haematocrit and inflammatory markers (erythrocyte sedimentation rate, plasma viscosity, white cell count). Recent epidemiological studies of these phenotypes and related genotypes are reviewed. For the risk prediction of first venous thrombosis, screening for thrombophilias in 'high-risk' situations does not appear clinically effective or cost-effective; with the possible exception of women considering oral hormone replacement therapy. General screening after a first venous event to predict recurrence (or risk in asymptomatic relatives) does not appear effective; with the possible exception of d-dimer, which requires further study. For risk prediction of first arterial thrombosis, screening adds little to prediction by current clinical risk scores. Screening of persons after a first arterial event, or with atrial fibrillation (e.g. with D-dimer for stroke prediction), requires further study. In conclusion, haematological tests have very limited roles in the prediction of cardiovascular risk, and should only be used according to evidence-based guidelines. The need for management studies is highlighted.

Insight into the nature of the CRP-coronary event association using Mendelian randomization

BACKGROUND

It is unclear wheather the association between C-reactive protein (CRP) and incident coronary events is free from bias and confounding. Individuals homozygous for a +1444C>T polymorphism in the CRP gene have higher circulating concentrations of CRP. Since the distribution of this polymorphism occurs at random during gamete formation, its association with coronary events should not be biased or confounded.

METHODS

We calculated the weighted mean difference in CRP between individuals with variants of the +1444C>T polymorphism in the CRP gene among 4,659 European men from six studies (genotype-intermediate phenotype studies). We used this difference together with data from previous observational studies to compute an expected odds ratio (OR) for non-fatal myocardial infarction (MI) among individuals homozygous for the T allele. We then performed four new genetic association studies (6,201 European men) to obtain a summary OR for the association between the +1444C>T polymorphism and non-fatal MI (genotype-disease studies).

RESULTS

CRP was 0.68 mg/l [95% confidence interval (95% CI) 0.31-1.10; P = 0.0001] higher among subjects homozygous for the +1444-T allele, with no confounding by a range of covariates. The expected ORs among TT subjects for non-fatal MI corresponding to this difference in CRP was 1.20 (95% CI 1.07-1.38) using the Reykjavik Heart study data and 1.25 (1.09-1.43) for all observational studies to 2004. The estimate for the observed adjusted-OR for non-fatal MI among TT subjects was 1.01 (95% CI 0.74-1.38), lower than both expected ORs.

CONCLUSIONS

A common CRP gene polymorphism is associated with important differences in CRP concentrations, free from confounding. The null association of this variant with coronary events suggests possible residual confounding (or reverse causation) in the CRP-coronary event association in observational studies, though the confidence limits are still compatible with a modest causal effect. Additional studies of genotype (or haplotype) and coronary events would help clarify whether or not the link between CRP and coronary events in observational studies is causal.

Seven haemostatic gene polymorphisms in coronary disease: meta-analysis of 66,155 cases and 91,307 controls

BACKGROUND

Variants of certain haemostatic genes (such as that encoding factor V Leiden) are involved in the development of venous thrombosis, but studies of such variants in coronary disease have reported apparently conflicting results. We did meta-analyses on seven such haemostatic genetic variants for which the available evidence on each comprises at least 5000 coronary disease cases and at least 5000 controls.

METHODS

Meta-analyses were done of 191 studies in relation to factor V G1691A (ie, factor V Leiden), factor VII G10976A, prothrombin G20210A, plasminogen activator inhibitor-1 (PAI-1) [-675] 4G/5G, and three platelet glycoprotein (GP) receptor variants (GPIa C807T, GPIbalpha T[-5]C, GPIIIa C1565T), involving a total of 66 155 coronary disease cases and 91 307 controls. We explored potential sources of heterogeneity.

FINDINGS

In a combined analysis of all studies, the per-allele relative risks (RR) for coronary disease of factor V 1691A and of prothrombin 20210A were 1.17 (95% CI 1.08-1.28) and 1.31 (1.12-1.52), respectively. Combined analyses of studies of the PAI-1 [-675] 4G variant yielded a per-allele relative risk for coronary disease of 1.06 (1.02-1.10), but there was an indication of publication bias in these studies. Combined analyses of the factor VII 10976A, GPIa 807T, GPIbalpha [-5]C, and GPIIIa 1565T variants showed no significant overall associations with coronary disease, yielding per-allele RRs of 0.97 (0.91-1.04), 1.02 (0.97-1.08), 1.05 (0.96-1.13), and 1.03 (0.98-1.07), respectively.

INTERPRETATION

The 1691A variant of the factor V gene and the 20210A variant of the prothrombin gene, both of which increase circulating thrombin generation, might each be moderately associated with the risk of coronary disease. Further studies are merited to assess these associations in greater detail (including any gene-gene and gene-environment interactions) and to determine any implications with regard to potential therapies designed to reverse patients' prothrombotic phenotype, such as selective plasma factor V or factor Xa inhibition.