Epigenetics - Etymology and Definitions

Etymology and Definitions

There exist several definitions of epigenetics, and as a result, there are disagreements as to what epigenetics should mean. Epigenetics (as in "epigenetic landscape") was coined by C. H. Waddington in 1942 as a portmanteau of the words genetics and epigenesis. Epigenesis is an old word that has more recently been used (see preformationism for historical background) to describe the differentiation of cells from their initial totipotent state in embryonic development. When Waddington coined the term the physical nature of genes and their role in heredity was not known; he used it as a conceptual model of how genes might interact with their surroundings to produce a phenotype.

Robin Holliday defined epigenetics as "the study of the mechanisms of temporal and spatial control of gene activity during the development of complex organisms." Thus epigenetic can be used to describe anything other than DNA sequence that influences the development of an organism.

The more recent usage of the word in science has a stricter definition. It is, as defined by Arthur Riggs and colleagues, "the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence." The Greek prefix epi- in epigenetics implies features that are "on top of" or "in addition to" genetics; thus epigenetic traits exist on top of or in addition to the traditional molecular basis for inheritance.

The term "epigenetics", however, has been used to describe processes which haven't been demonstrated to be heritable such as histone modification, there are therefore attempts to redefine it in broader terms that would avoid the constraints of requiring heritability. For example, Adrian Bird defined epigenetics as ""the structural adaptation of chromosomal regions so as to register, signal or perpetuate altered activity states." This definition would be inclusive of transient modifications associated with DNA repair or cell-cycle phases as well as stable changes maintained across multiple cell generations, but exclude others such as templating of membrane architecture and prions unless they impinge on chromosome function. Such redefinitions however are not universally accepted and are still subject to dispute.

The similarity of the word to "genetics" has generated many parallel usages. The "epigenome" is a parallel to the word "genome", and refers to the overall epigenetic state of a cell. The phrase "genetic code" has also been adapted—the "epigenetic code" has been used to describe the set of epigenetic features that create different phenotypes in different cells. Taken to its extreme, the "epigenetic code" could represent the total state of the cell, with the position of each molecule accounted for in an epigenomic map, a diagrammatic representation of the gene expression, DNA methylation and histone modification status of a particular genomic region. More typically, the term is used in reference to systematic efforts to measure specific, relevant forms of epigenetic information such as the histone code or DNA methylation patterns.

The psychologist Erik Erikson used the term epigenetic in his book Identity: Youth and Crisis (1968). Erikson writes that the epigenetic principle is where "anything that grows has a ground plan, and that out of this ground plan, the parts arise, each part having its time of special ascendancy, until all parts have arisen to form a functioning whole." That usage, however, is of primarily historical interest.