Epigenetics

Epigenetic (Greek: after, behind, around, in addition) refers to all processes in a cell that are considered "additional" to the processes of genetics. In distinction to the term "gene regulation", epigenetics defines all meiotically and mitotically heritable changes in gene expression that are not encoded in the DNA sequence itself, i.e. that are not based on mutation.

Epigenetic regulation includes the marking of DNA segments by means of methylation, histone modifications as well as non-coding microRNA (miRNA). Here, DNA methylation is the best studied epigenetic mechanism to date.

While genetic processes take many generations to become established, the epigenome can change rapidly in response to environmental stimuli. These changes can be passed on to the next generation.

As an example, the term "epigenesis" covers all gradual processes of embryonic morphogenesis of organs. These are based on epigenetic processes during cell division of the progenitor cells, cell differentiation (see below mosaic, cutaneous).

Epigenetic processes include:

• paramutation
• Bookmarking
• Imprinting
• Gene Silencing
• X inactivation
• Position effects
• maternal effects
• the process of carcinogenesis (see also oncogenesis)
• many effects of teratogenic substances.

Epigenetics and other drugs: It is sometimes believed that many drugs have epigenetic effects. This is known from best. Psychotropic drugs like fluoxetine, also from morphines. Some of these gene expression adaptations are the result of an altered DNA structure caused by chromatin transformations: epigenetic modification of histones and gene silencing by DNA methylation due to increased expression of various drugs. Methyl binding proteins (e.g. MeCP2 and MBD1; MeCP2 = methyl-CP binding protein 2) is important for the normal function of mature nerve cells and belongs to a group of nuclear proteins).

Epigenetic dysregulation is important for the development of immunological (e.g. systemic lupus erythematosus, atopic eczema) and various neurological diseases. For example, certain DNA methylation patterns are characteristic for genes encoding Il-4- imd IL-5 receptors, which play a role in allergic processes. Differences in methylation patterns can be detected in patients with atopic eczema and atopic asthma compared to the non-atopic population.

Basically important (also therapeutically interesting) is their reversibility and manipulability (e.g. in the immunotherapy of peanut allergy).

1. Guo Y et al(2014) Epigenetics in the treatment of systemic lupus erythematosus: potential clinical
   application. Clin Immunol 155:79-90.
2. Kabesch M (2014) Epigenetics in asthma and allergy. Curr Opin Allergy Clin Immunol 14:62-68.