Sunday, May 27, 2007

Everything and nothing   posted by amnestic @ 5/27/2007 10:47:00 AM

Redefinition / turnin' your play into a tragedy / exhibit level degree on the mic / passionately - Kweli

Nature has an Insight section up ostensibly about epigenetics starting with an article by Adrian Bird suggesting a re-definition. It's free. I think the term is useful to the extent that you can predict something about a phenomenon or mechanism by knowing that it is 'epigenetic'. Bird's suggestion is to make the term less useful and more inclusive. The last section is labeled 'Refining a definition' when in fact he is doing just the opposite:
...there might be a place for a view of epigenetics that keeps the sense of the prevailing usages but avoids the constraints imposed by stringently requiring heritability. The following could be a unifying definition of epigenetic events: the structural adaptation of chromosomal regions so as to register, signal or perpetuate altered activity states. This definition is inclusive of chromosomal marks, because transient modifications associated with both DNA repair or cell-cycle phases and stable changes maintained across multiple cell generations qualify.

He wants to allow transience and yet use epigenetics to explain stable phenomena:
A growing idea is that functional states of neurons, which can be stable for many years, involve epigenetic phenomena, but these states will not be transmitted to daughter cells because almost all neurons never divide.
Without such epigenetic mechanisms, hard-won changes in genetic programming could be dissipated and lost;
With this refinement, epigenetics is everything and nothing. The only thing you can infer about an epigenetic event is that it doesn't change the DNA sequence. Bird wants to claim that a unifying trait is that epigenetics is 'responsive' rather than 'proactive'. I don't understand. If you're going to introduce new terms, why not choose to bring into to broader use the distinction between meiotically and mitotically heritable or force people to be specific about which chromatin modification they are referring to instead of saying 'epigenetic modifications'? Responsive and proactive are more loosely defined concepts destined to muddy waters and lead away from insight.