WHAT IS EPIGENETICS?
Another way that environmental factors impact the genome is indirectly, through epigenetic (“beside the genome”) changes. Epigenetic changes refer to stable, heritable changes in the expression of genes that occur without alterations to the actual DNA sequence. Epigenetic changes are important in normal human development and health. For example, these changes in aggregate, that is, the epigenome, contribute to the process by which the tissues in one’s body develop different characteristics although all are composed of cells that share the same genome. Epigenetic changes also have an important role in a person’s physiologic response to the environment (discussed in detail later). It is likely that epigenetic changes have an equal or even greater effect on our health than genetic changes.
Two common types of epigenetic changes are DNA methylation and chromatin modification (Figure 1). DNA methylation is a process by which one of the nucleotide bases, cytosine, is directly modified by an enzyme that transfers a methyl group onto the base. Cytosine methylation often occurs in the upstream regulatory regions of genes (i.e., the promoters) and usually results in gene inactivation. DNA methylation also can occur at other locations outside of promoters. Chromatin modification is another common type of epigenetic modification. Chromatin is genomic DNA packaged with proteins. These proteins, called histones, can be modified in various ways, such as by methylation or acetylation. Simply put, these modifications affect whether the DNA regions are accessible (euchromatin) and the genes therein are expressed; alternatively, the DNA may be packaged in a closed conformation (heterochromatin) and the genes lie dormant. DNA accessibility changes within cells throughout organismal development to give rise to cells of different tissues.
Figure 1. Both DNA methylation and histone modification can affect gene regulation. DNA methylation in promoter regions generally turns genes off. Modifications on histone proteins can either activate or inactivate gene expression, depending upon the particular modification.