Epigenomics is the study of the sum of epigenetic modifications that can act on a cell's genome (1). Epigenetics is a relatively new field, and refers to the process that turns on/off sections of DNA or RNA. . These genetic modifications are thought to be a result of the organism's environment. Epigenetic modifications are also reversible; so a methyl group can be removed just as easily as it was added (2). Epigenomics is a field that aims to study these epigenetic modifications.
Two types of epigenetic modifications are histone modification and DNA methylation. The most common epigenetic modification is DNA methylation; a methyl group is added to DNA, which then "turns off" that section of DNA so it cannot be transcribed to RNA (3). Histone modification is a broader category of modifications than DNA methylation. Histones are modified by acetylation, which is similar to DNA methylation. An acetyl group is added to the histone protein, similar to how a methyl group is added to a DNA sequence. Adding/removing the acetyl group changes the intensity of how tight the DNA is bound to the histone. Since DNA is wound around histones to form nucleosomes, entire sequences of DNA can be turned on/off based on how tight the DNA is bound (4).
There are various methods when it comes to sequencing DNA, such as Sanger sequencing, pyrosequencing, shotgun sequencing,etc. While you can sequence an organism's DNA, epiegentic modifications, however, are harder to test for. Sequencing reactions cannot distinguish between sequences that are epigenetically modified and those that are not. For example, to assay for DNA methylation, DNA is digested by 2 endonucleases: one is methylation sensitive and the other is methylation insensitive. If both enzymes were able to digest a specific site, then that DNA was not methylated, but if only the insensitive enzyme could digest the DNA, then that sequence was methylated (1).
Epigenomics is very similar to proteomics and genomics. Proteomics is the study of all proteins, with respects to both their structures and functions. Genomics is the study of sequencing and assembling the complete genome of various organisms. Epigenomics is similar to these two fields because epigenomics aims to study each epigenetic modification and determine how they interact with DNA/RNA (1).
1. Epigenomics. Wikipedia
2. Epigenetic Modifications Regulate Gene Expression. SabioSciences
3. The Role of Methylation in Gene Expression. Phillips, Theresa. Nature
4. Histone Modifications and Variants. Life Technologies