The angiotensin converting enzyme gene in cardiovascular disease.

In every individual there are two copies of each gene (one of paternal and the other of maternal origin). The gene structure is of sequences of the nucleic acid bases adenine, guanine, cytosine, and thymine. The numbers of these base pairs in any single gene may run into thousands. Determination of the genetic code-that is, the sequence of these bases in a gene-is now commonplace. Substitutions of a base with another base either in one gene copy (heterozygous) or both (homozygous) are known as point mutations. If they lead to the production of an abnormal protein, clinical sequelae may occur-for example, sickle cell disease. Insertion of an extra length of bases or deletion of a segment of the gene can also lead to an altered product and clinical sequelae. It is, however, now clear that many changes in the base sequences have no apparent sequelae. Thus there are alternative forms of the gene with some changes in the base sequences but no apparent effect on the subject. The advent of gene sequencing on a large scale has shown the existence of numerous polymorphisms of many genes. Epidemiological studies have shown that different polymorphisms can confer different risks for developing diseases. The increase or decrease in risk may be minor and may be detected only by large population studies. For many polymorphisms how this effect is mediated is unknown and one of the greatest current challenges for molecular cardiology is to understand the mechanism. The concept of an association between a gene encoding a component of the renin angiotensin system and a cardiac disease phenotype is particularly attractive in view of the accumulating evidence that ACE inhibition reduces morbidity and mortality from left ventricular dysfunction. ' When we consider such studies it is important to remember that an association between different variants of a gene and a disorder does not necessarily indicate an aetiological role. In association studies where the genotypes of unrelated cases and controls are contrasted these markers may be acting as a beacon indicating that there is a causative mutation somewhere within a 10 000-100 000 base pair range. Recently an intriguing series of genetic associations with cardiovascular disease have been reported for an insertion/deletion polymorphism, located within a noncoding region (intron 16) of the angiotensin converting enzyme gene (ACE). The polymorphism relies upon the presence (insertion) or absence (deletion) of a 287 base pair fragment, which can be amplified from genomic DNA using the polymerase chain reaction and genotyped after gel electrophoresis. A further stimulus to study has been the observation of a major effect of the ACE gene upon plasma ACE activity with enzyme activity being highest in those white Europeans who are homozygous for the deletion allele (DD genotype) of this insertion/deletion polymorphism.2 Perhaps the greatest incentive to study this polymorphism has been the report by Cambien et al that the deletion variant (DD genotype) associates with myocardial infarction, particularly among those at apparently low risk of this complication.3 Though this is an interesting study we must be careful not to extrapolate to a whole population observations in a retrospective, cross sectional study on survivors of myocardial infarction. By definition because 40% of those who have a myocardial infarction die in the immediate post-infarct period they were not available for genotyping in this study. Therefore the claim that the ACE gene operates an independent risk factor for infarction is diluted. Furthermore, the same polymorphism has also been associated with longevity, which raises the possibility that the DD genotype might be a marker for survival rather than a deleterious allele.4 Cambien et al recognised this problem and have developed circumstantial evidence for their risk factor hypothesis by demonstrating association of the DD genotype with a family history of death from infarction and also in a postmortem study.56 Interestingly, a study on 1226 men from the Caerphilly Heart Study has reported association of the DD genotype with coronary artery disease (CAD) in those at apparently low risk as defined by classic risk factors. This potentially widens the phenotypic relation beyond infarction in an homogenous male population, although the study emphasises that any influence of this polymorphism is likely to be quantitatively small.7 The mechanisms by which ACE might influence coronary artery disease include hypertension, cardiovascular remodelling, and atherogenesis. Genetic influence upon ACE activity may alter not only the renin-angiotensin system but also the kalikrein-kinin system or the balance between them. Several studies have not shown a linkage or association of the ACE gene with hypertension.8 9 The problems with these negative studies is that with one exception they make no statement of the power they offer to rule out a remote contribution of this locus to hypertension.9 For a study to demonstrate a 12% difference in allelic frequency of the ACE I/D polymorphism (such as seen between myocardial infarction cases and controls in the study by Cambien et ao would require 250 infarcts v 250 controls to achieve a significant difference at the 5% level with 80% power. Despite this caveat, however, the 207