DNA Polymorphism Studies. Approaches to Elucidating Multifactorial Ischaemic Heart Disease: the Apo B Gene as an Example

It is well established that elevated plasma levels of low-density lipoprotein (LDL) particles are a risk factor for ischaemic heart disease with the distribution in LDL levels seen in the general population being the result of interaction between environmental factors, such as dietary fat intake, and genetic variation that is present in different individuals. One of the candidate genes where such variation is likely to occur, is the gene coding for apolipoprotein B (apo B). Many studies have reported an association between a common polymorphism of the apo B gene, detected using the restriction enzyme Xbal, and differences in plasma lipid levels, explaining 3-5% of the variance in LDL-cholesterol levels in samples representative of the healthy population. It has been proposed that the mechanism of this association is due to functional amino acid changes within the apo B protein, that affect LDL catabolism by altering binding affinity to the LDL-receptor. Several amino acid substitutions in the apo B gene have now been characterized, and these form the basis of the different epitopes that create the Ag marker system. Previous studies have reported that the Ag(x) epitope is associated with lower plasma lipid levels, and until recently the molecular basis for this association has been unclear. We have determined that the Ag(x) epitope is associated with both a Pro-Leu2712, and Asn-Ser4311 substitution, with the Leu-Ser allele being associated with significantly lower levels of plasma lipids in a sample of healthy individuals from Sweden. The association between these two amino acids is present in all ethnic groups so far studied, and appears to have been maintained throughout the recent evolutionary history of the human species. Since this amino acid polymorphism explains only part of the differences in plasma lipid levels detected using the Xbal polymorphism, this suggests that there are other functionally important sequence changes that remain to be detected, which will be associated with lipid-raising effects. The identification of these functional changes, and the mechanism by which they act will help in our understanding of the role of gene-environment interaction in determining plasma lipid levels in healthy individuals and in patients.