Capacidad Antioxidante

Genetic polymorphisms contribute to the understanding and m anagem ent of haemostatic problems in varying ways. Polymorphisms in or near a gene of interest, such as the factor VIII gene in haemophilia A, are used to track an abnormal gene through a family in order to predict with greater certainty whether family m embers will be affected by the disease or are carriers of the particular trait. T h e polymorphism itself is not the cause of the disease state but is a m arker to identify on which of the alleles the causal mutation lies. More recently, genetic polymorphisms have been used in population studies to see if genotype can be used to predict levels of a particular protein. The main difference in these circumstances is that the levels of the protein are within the normal range and the genetic variation detected by the polymorphism is, by definition, common (ie the frequency of the rare allele is >1% ). Instead of a few people having a major defect, a large number of people have a small change in protein levels. The overall impact of this change in a population can be significant, as has been shown by the increased incidence of IHD, in those with a plasma fibrinogen level raised one standard deviation above the mean (M eade et al, 1986). As has been described earlier in this chapter, such a rise in plasma fibrinogen levels resulted in an 84%

increased risk of an episode of IHD within the following five years. Genetic- environment interactions can be studied using this approach. The polymorphisms used in these associations may or may not be functional themselves, however, they help to pinpoint the functional changes. This is important for several reasons. Firstly in the understanding of normal control and function of a gene and secondly in the future because of the possibility of directed therapeutic strategies with genetic specificity. If gene-interaction is shown, advice on avoidance of certain lifestyles can be targetted more effectively.

The extent to which genetic factors are involved in determining plasma fibrinogen levels is of interest since individual and environmental factors account for only around 2 0 -2 8 % of the population variance in fibrinogen (M eade et al, 1976; Thompson et al,1987). The level of fibrinogen observed in the plasma of an individual at any particular time is determined by interaction between a number of specific environmental factors experienced by the individual and their genetic make-up. M any environmental factors affect plasma fibrinogen levels as described previously, but of these, smoking is the single, target environmental factor determining fibrinogen levels in individuals in the general population. It has been suggested that a large part of the relationship between smoking and IHD is mediated through the rise of fibrinogen levels (M eade et al,1987). In one small study of fourteen healthy individuals, the degree of within-individual variation for fibrinogen was estimated to be at least twice that for cholesterol (Thompson et al,1987), and this is confirmed by other larger studies (M eade and North, 1977);

thus a single measure of an individual's plasma fibrinogen level will result in a significant underestimate of the true relationship between fibrinogen and, for exam ple, subsequent risk of disease.

There is less information on the relative contribution of genetic variation to the determination of plasma fibrinogen levels in the general population. O ne study using path analysis in families of healthy individuals and smokers estimated a heritability of 0.5 (Hamsten et al,1987) and a recent study by Bara and coworkers (1994 ) investigating plasma fibrinogen levels in young adults with a paternal history of premature myocardial infarction support the hypothesis that fibrinogen is a transmissible risk factor of coronary artery disease in males. Livshits and coworkers (1995) using segregation analysis found that total genetic effects explain about 80% of phenotypic variance in plasma fibrinogen levels. However, two studies in twins reported a low heritability of 0.3 (Berg and Keirulf,1989; Reed et al,1994). It is likely that variation at the gene locus coding for the fibrinogen protein may contribute to the genetic component determining plasma fibrinogen levels. Since synthesis of the BR chain is the rate-limiting step in the production of m ature fibrinogen (Yu et al,1983; Yu et al,1986; Roy et al,1990), this has prompted investigation of genetic variation in this region, especially the

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fibrinogen promoter, by exploiting DMA polymorphisms. A report by Humpries and coworkers (1987) showed an association between such variation and differences in plasma levels of fibrinogen in healthy individuals but not all studies have shown this association (Berg and Keirulf,1989; Connor et al,1992). T h e sam e association

has been shown in some but not all studies of patients with peripheral arterial disease (Monsalve et al,1988; Fowkes et al,1992). In the study by Fowkes and coworkers (1992) R-fibrinogen genotype was an independent predictor of risk of peripheral arterial disease (Fowkes et al,1992), but this was not observed in the EC T IM study of myocardial infarct survivors and controls from Belfast and France (Scarabin et al,1993). Determination of the size of the effect of genetic variation at the fibrinogen gene locus on plasma fibrinogen levels and interaction with environmental factors are the subjects of this thesis.