Epigenetics is one of those words that means entirely different things to different people. P.Z. Myers has put up a nice description of the term on his blog [Epigenetics]. Here's how he defines epigenetics ...
Epigenetics is the study of heritable traits that are not dependent on the primary sequence of DNA.In fairness, he then goes on to explain that this is an unsatisfactory definition. That's an understatement.
Now, as it turns out, those scientists who work on animal development employ a definition of epigenetics that looks very much like what we used to call developmental regulation of gene expression. That's why PZ can say ...
... developmental biology basically takes epigenetics entirely for granted — development is epigenetics in action! Compare an epidermal keratinocyte and a pancreatic acinar cell, and you will discover that they have exactly the same genome, and that their profound morphological, physiological, and biochemical differences are entirely the product of epigenetic modification. Development is a hierarchical process, with progressive epigenetic restriction of the fates of cells in a lineage — a dividing population of cells proceeds from totipotency to pluripotency to multipotency to a commitment to a specific cell type by heritable changes in gene expression; those cases where there is modification of the DNA, as in the immune system, are the exception.Here's the problem. If this is epigenetics then what's the point? When I was growing up we had a perfectly good term for these phenomena—it was regulation of gene expression. Why is there a movement among animal developmental biologists to use "epigenetics" to refer to a well-understood phenomenon?
I've been bugging my colleagues today by asking them to tell me whether certain examples of gene regulation are epigenetic or not.1 The answers are mixed so I thought I'd submit the questions to Sandwalk readers. Which of the following are "epigenetic"?
- Consider an E. coli cell that grows and divides for hundreds of generations in the absence of any exogenous β-galactosides (e.g. lactose). Under those conditions the lac operon is repressed and this state is heritable from generation to generation due to the presence of lac repressor.
- Consider mating type in yeast. In an α cell the a gene is suppressed from generation to generation. This is heritable regulation of gene expression. All daughter cells inherit the ability to express the α gene and suppress the a gene.
- During a bacteriophage infection certain genes are turned on in a definite sequence. In the simplest cases there is a set of "early" genes that are expressed as soon as the 'phage DNA enters the cell. After a few minutes the expression of the "early" genes triggers the expression of the "late" genes. Note that the "late" genes are not transcribed initially even though they are present.
- Right now your major heat shock genes (e.g. Hsp70 genes) are transcriptionally silent. However, if you are stressed by heat those genes will become active and will be transcribed at a very high rate.
- During oogenesis in fruit flies the bicoid gene is expressed in nurse cells and bicoid mRNA is deposited in the egg. In males, the bicoid gene is never expressed.
- One of the nucleotides at an EcoR1 restriction endonuclease site in E. coli is methylated. This blocks cleavage at that site, thus protecting the bacteria from degrading its own genome. The methylation pattern is inherited from generation to generation by the action of a methylase enzyme.
- Globin genes are expressed in erythroblasts but not in brain cells. During development the globin genes are activated in erythroblast stem cells because certain activator proteins are synthesized. The globin genes are not activated in any other tissues.
- During development in mammalian females one of the X-chromosomes is randomly inactivated [Calico Cats]. Once this occurs the pattern is inherited in (almost) all cells that descend from the initial embryonic cell where the inactivation first occurred. The same X-chromosome is inactivated in all daughter cells.
1. And they are quite annoyed about it. Many of them are avoiding me because they don't know how to answer the questions.
[Image Credit: The cartoon is from Mark Hill's website at the University of New South Wales, Australia. It appeared originally in Nature. The figure represents a different definition of "epigenetics"—one that focuses on modifications to DNA and histones.]