4.3 Genes and ageing
It is often assumed that ageing is genetically determined, because each species has a characteristic, well-defined maximum lifespan. Such differences between species could lead us to look for ‘ageing genes’, inherited in some human families (and certain other species) but not in others. Indeed, genes are activated throughout the lifetime of every individual, but the pattern of gene expression within a particular cell type may change during the lifespan. It is now widely accepted, however, that genes that solely affect the ageing process do not exist; rather the evidence suggests that any effects of particular genes on ageing and/or longevity exert these effects because they were selected for beneficial roles earlier, during reproductive life. Hence a gene that confers reproductive advantage early in life, but a negative effect later in life (a phenomenon known as antagonistic pleiotropy) could persist in the population. What has become apparent over the many years that ageing has been studied is the variation in the rate and extent of the process; even between genetically similar strains of laboratory animals. Gene-environment interactions can affect age associated changes in cells, tissue functions and overall lifespan. There is no doubt that certain styles of living may predispose one to an early death. One simple but striking example of this effect is the difference in the lifespan between honey-bee queens and workers in a beehive. Even though both have a similar genetic make-up, the lifespan of a worker is only few months, whereas a queen may live for a decade or more. Evidence suggests that this difference is indeed due to the difference in lifestyle. Another simple example of gene–environment interactions is reflected in the trends in mortality due to specific diseases among men of Japanese ancestry who live in the USA or Japan. Widespread screening for cardiovascular disease has shown that the prevalence of coronary heart disease (CHD) among men of Japanese ancestry living in the USA is about twice that among those living in Japan. This is unlikely to be due simply to the fact that the Japanese in Japan have some intrinsic biological advantage. These differences in incidence of CHD must be due to differences in environment or lifestyle. Such considerations are also very relevant to the addiction strand of the course, as an important issue for modern medicine is the influence of a drug user's lifestyle on his or her longevity.