Ok, so we all know what the genome is: the entire sequence of As, Ts, Gs, and Cs (all 6 billion or so of them) that make up the DNA sequence in each of our cells. As you all now know, we’ve had the complete human genome mapped out for ten years now (of course, we’re still trying to figure out what it all does). We also know that whenever there is a change in the DNA molecule (a mutation), that change can become permanent and, in some cases, will be passed down to offspring.
Well, there are ways that changes can be heritable WITHOUT actually changing the underlying DNA sequence…which is where the epigenome comes into play (“epi” comes from the Greek for “above” or “over”). The epigenome is the host of non-genetic factors that cause genes to change the way that they behave. The classic example is cell differentiation: how is it that a single fertilized egg can differentiate into heart cells, liver cells, skin cells, etc., even though the DNA molecule remains the exact same in all the cells? What happens is that epigenetic factors turn on only the genes that are needed for each cell type to carry out their specific functions.
Now, most of these epigenetic changes occur only within an individual’s lifetime and are thus only passed from one cell to the next as they divide. However, and this is where it gets really cool, if these changes occur in a sperm or egg cell, then some could be inherited from one generation to the next. This should sound familiar because, in effect, this would be the inheritance of acquired characteristics, which is Jean-Baptiste Larmarck’s oft-ridiculed mechanism for evolution!
Scientists are now on the hunt for a map of the epigenome. For starters, they have been mapping the relatively small epigenomes of the fruit fly and the round worm, and an ongoing study now has a basic epigenomic map for both species. What can this tell us? From an interview with one of the team leaders on the project, Dr. Sarah C.R. Elgin (Washington University, St. Louis), from ScienceDaily:
“We learned many things from the Human Genome Project,” Elgin says, “but of course it didn’t answer every question we had!
“Including one of the oldest: We all start life as a single cell. That cell divides into many cells, each of which carries the same DNA. So why are we poor, bare, forked creatures, as Shakespeare put it, instead of ever-expanding balls of identical cells?
“This work,” says Elgin, “will help us learn the answer to this question and to many others. It will help us to put meat on the bones of the DNA sequences.”
There is actually a conference on human epigenetics, Environmental Epigenetics and Disease Susceptibility, March 27-April 1 in Asheville, NC. Just a smattering of the papers that will be given:
“Epigenetics, Brain Evolution and Behavior”; “Transgenerational Epigenetic Inheritance”; “Epigenetics at the Interface of Genetic and Environmental Risk Factors for Autism Spectrum Disorders”; “The Imprinted Brain Theory: How Genes Set the Balance between Autism and Psychosis”