title: Fibrinogen and coronary heart disease: test of causality by 'Mendelian randomization'.
abstract: 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.
authors.affiliation: Institute of Human Genetics, University of Newcastle-upon-Tyne, UK. b.d.keavney@ncl.ac.uk
authors.forename: Bernard
authors.initials: B
authors.lastname: Keavney
authors.forename: John
authors.initials: J
authors.lastname: Danesh
authors.forename: Sarah
authors.initials: S
authors.lastname: Parish
authors.forename: Alison
authors.initials: A
authors.lastname: Palmer
authors.forename: Sarah
authors.initials: S
authors.lastname: Clark
authors.forename: Linda
authors.initials: L
authors.lastname: Youngman
authors.forename: Marc
authors.initials: M
authors.lastname: Delépine
authors.forename: Mark
authors.initials: M
authors.lastname: Lathrop
authors.forename: Richard
authors.initials: R
authors.lastname: Peto
authors.forename: Rory
authors.initials: R
authors.lastname: Collins
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D003327:Coronary Disease / Q000097:blood / Q000235:genetics*; D005340:Fibrinogen / Q000235:genetics*; D005838:Genotype; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D011110:Polymorphism, Genetic; D011995:Recombination, Genetic / Q000235:genetics*; D012189:Retrospective Studies; D018570:Risk Assessment / Q000379:methods
chemical_list: D005340:Fibrinogen