Epigenetics is the study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence — a change in phenotype without a change in genotype. Epigenetic change is a regular and natural occurrence but is also influenced by age, the environment, and disease state. Epigenetic modifications include acetylation, methylation, phosphorylation and ubiquitination and alter the accessibility of DNA to transcription machinery and so affect gene expression.
Proteins that carry out these epigenetic modifications are considered to be either writers, readers or erasers. Epigenetic writers catalyze the addition of chemical groups onto either histone tails or the DNA itself, these modifications being known as epigenetic marks. Epigenetic readers are effector proteins that identify and interpret these modifications. Epigenetic erasers are enzymes that remove epigenetic tags. Consequently epigenetic marks are not necessarily permanent modifications but can be removed to reverse the influence of a mark on gene expression.
Diseases may be caused by direct changes in epigenetic marks, such as DNA methylation, commonly found to affect imprinted gene regulation. Cancer was the first human disease to be linked to epigenetics as DNA hypomethylation can activate oncogenes and initiate chromosome instability, whereas DNA hypermethylation can silence tumour suppressor genes. Epigenetic changes are also linked to metabolic and autoimmune diseases and adult psychiatric, autistic, and neurodegenerative disorders