WHAT ARE SOME EXAMPLES OF ENVIRONMENTAL EFFECTS ON PHYSIOLOGY THAT ARE MEDIATED THROUGH EPIGENETICS?
Both the environment and age can directly or indirectly affect DNA methylation and/or chromatin modifications in ways that have yet to be fully understood. Parental nutritional status has been associated with DNA methylation at specific loci in their children. Returning to the example of season-dependent birth weight of Gambian children, several gene promoters were found to be differentially methylated in children conceived during the rainy or dry seasons. It is hypothesized that these differences cause changes in gene expression, which eventuate in different birth weights. Differences that emerge between identical twins (who share the same genomic DNA) in physical features and health outcomes as they get older are believed to be due in part to epigenetic “drift,” the process by which different environmental exposures result in different epigenetic changes.
Exercise has long been known to have an effect on health, lowering the incidence of cardiovascular disease, obesity, and type 2 diabetes. Recently, it has been found that exercise can have a direct effect on DNA methylation and gene expression. A group of 23 subjects exercised one leg but not the other for 45 minutes four times a week for three months. At the end of this period, the methylation and gene expression patterns were different in the exercised leg relative to the one that did not exercise, indicating a direct effect of exercise on epigenetics.
Aging is also associated with specific changes in our DNA methylation patterns. Research suggests that it is possible to predict chronological age within a five-year window based on the methylation pattern in our cells. This has possible applications for forensic analysis—biological samples from a crime scene could be used to estimate the chronological age of the person from whom the tissue came.
It is important to note that many of these links between epigenetic modifications and conditions are associations and not necessarily causations, that is, it is not always clear what is cause and what is effect. Additional research, often in model organisms, will be important to fully understand the biological effects of epigenetic modifications.