Fibrinogen Genotyping in Peripheral Vascular Disease

Elevated plasma levels of fibrinogen have been associated with an increased risk of peripheral vascular disease and with elevated levels of blood pressure. Furthermore there is some evidence that plasma levels of fibrinogen may be determined at least in part by variation at the fibrinogen gene loci. This study was undertaken to investigate the contribution of variation at the fibrinogen loci with plasma fibrinogen levels and to seek association between fibrinogen gene variation and the determination of levels of blood pressure and the occurance of peripheral vascular disease. Allele frequencies at the alpha, beta and gamma fibrinogen loci were determined in 235 cases and controls from the peripheral vascular disease study. The values were alpha/TaqI 0.75 (for A the 2.4Kb fragment), 0.25 (for a the 1.6Kb fragment); beta/BclI 0.84 (for B the 5.3Kb fragment), 0.16 (for b the 4.2Kb fragment) and gamma/KpnI/SacI 0.76 (for D the 14Kb fragment), 0.24 (for d the 11Kb fragment). Allele frequencies at the beta fibrinogen locus for the HaeIII polymorphism were determined using the polymerase chain reaction (PCR) in a further 170 of these individuals. The values were 0.81 (for H1 the 0.575Kb and 0.383Kb fragments) and 0.19 (for H2 the 0.958Kb fragment). The overall genotype frequencies were compared with those predicted from the Hardy-Weinberg equilibrium and no significant difference was noted. Allele frequencies were compared with previously published studies and a significant difference was noted for beta/BclI between this study on Scottish subjects and that of Humphries et al. (1987) on English subjects (p0.80). Strong linkage disequilibrium was found between the alpha/TaqI and gamma/KpnI/SacI markers and the beta/BclI and beta/HaeIII markers which was highly significant at any level (p=1.1x10-27 and 2.24x10-8respectively). A lesser association was found between the alpha/TaqI and beta/BclI loci (p=6.6x10-4); beta/BclI and gamma/KpnI/SacI markers (p=2.5x10-3); alpha/TaqI and beta/HaeIII (p=1.3xl0-4) markers and the gamma/KpnI/SacI and beta/HaeIII markers (p=1.4x10 ). This is consistent with the known physical order of the loci and suggests a relative excess of recombination in the alpha/gamma-beta interval. Fibrinogen levels were determined in the peripheral vascular disease case/control study by clotting or nephelometric assays and the relationship of fibrinogen level to genotype analysed. No statistically significant association was detected for any of the four markers TaqI, BclI, KpnI/SacI and HaeIII with respect to fibrinogen levels. The 235 individuals in this study had one of 13 different haplotypes with respect to the TaqI, BclI and KpnI/SacI markers. No Statistically significant association was detected for any of these haplotypes in respect of fibrinogen levels. The percentage of phenotypic variability was calculated and varies from 2.4-4.9% in the different analyses. 155 individuals were genotyped with respect to the HaeIII-beta polymorphism. There was no association between plasma fibrinogen level and any genotype (p=0.58). Differences in genotype distribution between cases and controls were analysed using X2 . No significant difference in distribution was observed (p=0.55). The contribution of genotype and smoking status to fibrinogen level was analysed. There was no relationship between the HaeIII RFLP, smoking and fibrinogen level (p=0.58). The contribution of fibrinogen haplotype to the occurrence of peripheral vascular disease was assessed by multiple logistic regression using variables which represent the difference in the log odds of disease for each haplotype relative to haplotype 1. Haplotype was found to be a significant predictor of disease after accounting for other risk factors and there is some consistency in the results for haplotypes 3 (aa Bb DD) and 5 (Aa Bb Dd) suggesting that they are significant predictors of disease (p<0.05), over and above the level of fibrinogen. (Abstract shortened by ProQuest.).