Most complex diseases have genetic causes, but observations from families and especially from twins indicate that other factors are almost always necessary for the actual appearance of disease. One such factor appears to be the proportion of DNA units that have methyl groups attached to them, or more accurately the methylation status at important specific loci. The level of methylation is known to vary between individuals and has been linked to several chronic diseases. The level has been shown to change with increasing age, even within the members of identical twin pairs. Some of these changes are probably the result of endogenous forces and therefore are likely to have appeared at random. Others, however, are likely to be the result of the environmental exposures that cause disease or the risk factors that precede its appearance. This study seeks to evaluate the linkage between the DNA methylation status and environmental factors known to predict disease occurrence. Methylation will be assayed using the more accurate pyrosequencing method, and studied both in blood cells, upon which much of the current understanding is based, and squamous epithelial cells from the mouth, likely to be more heavily exposed to high levels of environmental agents. The level of methylation will be compared in twins who give a history of pertinent environmental exposure, to that in their relatively unexposed identical co-twins. The list of such exposures includes smoking, alcohol usage, obesity, consumption of foods known to contain folic acid and food known to contain carcinogens, and occupational and residential exposure to various environmental chemicals and toxins. The plan is to also select those sets of identical twins least likely to have been exposed to environmental toxins, based on residence, occupational exposure, and diet, in order to measure the age-specific levels of DNA methylation, and the degree to which those levels vary in relation to sex and twin pair. These levels and indices of variation will be compared to the same findings from pairs selected on the basis of sex, educational status, and maternal educational status (a measure of childhood social class). Finally, any evidence of a heritable propensity to change methylation status, based on identical and like sex fraternal twins having the least evidence of cumulative toxic exposure, will be investigated.
