Summary: Epigenetics—from Phenomenon to Field
This extract was created in the absence of an abstract.
Excerpt
When Bruce Stillman made his opening remarks at the69th Cold Spring Harbor Symposium, one of the thingshe said he hoped to learn was a way to easily explain what"epigenetics" meant to his wife Grace. After a week ofdiscussions, it became clear that such a request was akinto asking someone to define "family values"—everyoneknew what it meant, but it had a different meaning foreach person. Part of the reason for the range of opinionsmay be understood from the etymology of "epigenetics"as explained by David Haig: The word had two distinctorigins in the biological literature in the past century andthe meaning has continued to evolve. Waddington firstcoined the term for the study of "causal mechanisms" bywhich "the genes of the genotype bring about phenotypiceffects" (see Haig). Later, Nanney used it to explain hisrealization that cells with the same genotype could havedifferent phenotypes that persisted for many generations.I define an epigenetic phenomenon as a change in phenotype that is heritable but does not involve DNA mutation.Furthermore, the change in phenotype must be switchlike, ON or OFF, rather than a graded response, and itmust be heritable even if the initial conditions that causedthe switch disappear. Thus, I consider epigenetic phenomena to include the lambda bacteriophage switch between lysis and lysogeny (Ptashne 2004), pili switchingin uropathogenic Escherichia coli (Hernday et al. 2003),position-effect variegation in Drosophila (Haynes et al.),heritable changes in cortical patterning of Tetrahymena(Frankel 1990), prion diseases (Wickner et al.), and Xchromosome inactivation (Huynh and Lee; Heard et al.)...
