I attended the Society for Neuroscience 2008 annual conference for work, as a member of the media. Although I didn't formally write anything up, I did attend two press conferences. One of which Tiptoe wrote about here on addiction and impaired insight (via press release, which was accurate to what was said during the conference), and one that I'm going to summarize here.
In some sense, your genes are your genes. You're born with 'em, you die with 'em. Other than in cases like cancer, they don't change. Clearly, however, our environment effects us. When fair skinned people stand out in the sun, they get tan (or they burn). When we diet, our metabolisms slow. If we keep dieting, they stay slow. And, researchers are finding out, traumatic experiences early in life can permanently change the brain.
The body can turn genes on and off via different environmental cues by adding and subtracting methyl groups ( teensy little molecules) that they attach to the DNA. Because DNA is super-long, it has to coil around itself like a huge ball of yarn. But if a gene is going to make a protein, it can't be in the tightly wound part, or else all of the other proteins in the cell can't get to the gene. Adding a methyl group (among other things) alters the ability of the cell to read the gene, literally silencing it and preventing access to the protein-making machinery of the cell.
This whole process is known as epigenetics. Some genes that have been epigenetically changed during fetal development (and therefore before the formation of the sex cells) can be inherited. Most aren't.
And it turns out that trauma in early life can prevent a specific gene ( BDNF - brain derived neurotrophic factor ) from ever turning on. BDNF is one of the big master switches in the brain. Among other things, BDNF is active in areas of the brain responsible for learning, memory and higher thinking. Abused rats showed decreased levels of BDNF long after abuse and neglect had stopped, and they also showed many anxiety traits. These epigenetic changes were primarily found in the amydala, the center of the brain that controls anxiety and fear.
For a full summary click here and scroll down to page two.
Earlier, unrelated research on the brains of people who had died by suicide found numerous epigenetic changes in the GABA genes when compared to those who had died from other causes. GABA is a neurotransmitter linked to mood regulation.
"...in the face of adversity, certain genes in the brain will be methylated, effectively shutting them down. Once they occur, these changes are difficult to reverse, creating stable disabilities, perhaps for the remainder of a life. Bad experience gives mammals a different genetics and different brains."
Childhood abuse is generally thought of as a HUGE risk factor for pretty much any sort of mental illness. It has also been tentatively linked to bulimia and bulimic behaviors. These studies, among many others, only help to emphasize that a biologically-based illness can be influenced by the environment. And with this research, they're finding out how it might happen at the biochemical level, and how these changes might one day be reversed.