What is Epigenetics?
Since its use in the 1940s by Conrad H. Waddington to refer to the study of epigenesis, the term "epigenetics" has undergone several transitions in its meaning. A broad definition understands epigenetics as the study of the changes that happen to the expression of genes without modifying the actual sequences within the gene.
Epigenetic changes interfere with the usual function of a DNA sequence, such as by preventing it from being transcribed or by exposing it when it would typically be hidden-essentially controlling if and how a gene works in the organism.
How Does Epigenetics Work?
l DNA Methylation
DNA methylation works by adding a chemical group to DNA. Typically, this group is added to specific places on the DNA, where it blocks the proteins that attach to DNA to "read" the gene. This chemical group can be removed through a process called demethylation. Typically, methylation turns genes "off" and demethylation turns genes "on".
l Histone Modification
DNA wraps around proteins called histones. DNA wrapped tightly around histones cannot be accessed by proteins that "read" the gene. Some genes are wrapped around histones and are turned "off" while some genes are not wrapped around histones and are turned "on". Chemical groups can be added or removed from histones and change whether a gene is unwrapped or wrapped ("on" or "off").
l Non-coding RNA
Your DNA is used as instructions for making coding and non-coding RNA. Coding RNA is used to make proteins. Non-coding RNA helps control gene expression by attaching to coding RNA, along with certain proteins, to break down the coding RNA so that it cannot be used to make proteins. Non-coding RNA may also recruit proteins to modify histones to turn genes "on" or "off".
Fig.2 Three levels at which epigenetic activity takes place[1].
Epigenetics and Human Health
Abnormal epigenetic states can predispose to or cause human diseases. Epimutations may derive from genetic mutations affecting components of the epigenetic machinery or cis-acting regulatory elements and from stochastic or environmentally induced epigenetic aberrations without an underlying genetic mutation.
Epimutations triggered by genetic mutations are found, for instance, in the Rett syndrome (caused by mutations in the X-chromosomal MECP2 gene) and the Fragile-X syndrome (caused by expansion and methylation of a CGG trinucleotide repeat). Epimutations of all types may contribute to imprinting disorders such as the Beckwith–Wiedemann, Angelman, and Prader–Willi syndromes and also play a prominent role in neoplastic transformation. In fact, essentially all cancers have been shown to be associated with major epigenetic alterations, such as general DNA hypomethylation and regional DNA hypermethylation, and aberrations in histone modifications. Such alterations can contribute to the genomic instability typical of cancer cells and to the inactivation of tumor suppressor genes or the activation of oncogenes, and may represent early and causal events in the development of some tumors[2].
References
[1] Matouk C C, Marsden P A . Epigenetic Regulation of Vascular Endothelial Gene Expression[J]. Circulation Research, 2008, 102(8): 873-887.
[2] J Casadesús, M Noyer-Weidner. Epigenetic. Brenner’s Encyclopedia of Genetics, 2013, (2): 628-637.
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