ABSTRACT
INTRODUCTION Epigenetics is the study of gene regulation that is independent of DNA sequence. The term, reintroduced from classical embryology by Conrad Waddington in the 1950s, is what we now refer to as “developmental biology,” in that an organism’s phenotype arises from its genotype through programmed “epigenetic” events. For example, cells from reproductive tract tissues and hepatocytes from the same individual have identical genomes at the level of the nucleotide sequence, yet these cells have large differences in gene expression as evidenced by their differentiated cell types and function; epigenetic mechanisms are responsible for providing stable (or semi-stable) regulation of gene expression that is separate from nucleotide sequence. In fact, differentiation processes described in developmental biology largely depend on epigenetic mechanisms to orchestrate the formation of different tissues and organs, which come from a single fertilized egg at conception and grow to a fully formed individual at the end of gestation who is ready to be born, because all cells, except just a few, have the same nucleotide sequence. However, the modern definition of epigenetics used today is more narrow and specific (1) and refers to stable and heritable changes in gene expression that do not involve a change in DNA sequence or create mutations. These changes encompass an array of molecular modifications to both DNA and chromatin; the most extensively investigated are DNA methylation and histone modifications.
