Most people know that DNA contains the data that programs all living things. What is less known is how the shape of the DNA affects the ultimate expression of proteins. The mechanisms that regulate if and when proteins are expressed by the DNA template is a very hot topic of research. Not only the packing but the chemical markers attached to the DNA appear to be important in gene expression or inhibition. Vast areas of DNA that had been labelled as “junk” are increasingly found to be important in the regulation of protein production. Only a few percent of a person’s DNA is actually a blueprint for protein production. The remainder is an intricate network of feedback and feed forward mechanisms that start and stop protein production. Tightly packed DNA tends to decrease protein transcription, and relaxed DNA strands increases the chance a gene will be expressed. Two groups of chemical markers have been discovered that regulate the DNA packaging, and hence the ultimate expression of any particular gene. So, one may ask, what has this to do with mental health?
The chemical groups that regulate gene expression are critically important to the understanding of addiction and depression. Within an hour of injecting mice with cocaine, over one hundred genes become activated. If cocaine is used everyday, particular genes are actually inhibited from expressing proteins. Prolonged use may render some genes over-activated for weeks and even months, whereas others become chronically inhibited. The ingestion of this one chemical causes profound genetic alterations in the brain’s reward centers that may persist long after the drug is discontinued. Many genes remain highly sensitized to the effects of cocaine for several weeks after the mouse was last injected. The brain is ready and waiting for the next dose of cocaine. The cocaine causes the epigentic chemicals to loosen the strands of DNA; priming them to be activated.
In depression, the the epigenetic influence on DNA is nearly opposite to that of cocaine abuse. Depression appears to be the consequence of repressed gene activation in the reward centers of the brain. Environments that are abusive will tend to make the DNA strands tightly bound; decreasing gene activation. For example, a mouse that is not able to escape the domination of a more powerful mouse will display decreased activation in twelve hundred genes! Depression appears to inhibit the activation of DNA in the reward centers that allow an animal to feel good. Just as many humans are resistant to depression, about one third of the mice in the bully experiment did not manifest symptoms of depression. The resilient group of mice did not develop the the inhibited gene expression that infected the larger depressed group. This sizable group of genes in the reward center of higher animals is implicated in the treatment effectiveness of tricyclic antidepressants. Some antidepressants may actually boost the brain’s natural mechanism to confer resilience.
Addiction and depression are not the only psychological manifestations of epigenetic modulation. As described by Eric Nestler in Scientific American (2011), epigenetic “modifications can promote behavioral changes that last a lifetime.” Maternal rat behavior is partially or completely modulated by epigenetics, and this has lasting effects on the offspring. The memory area of the mother’s brain is inhibited, and this epigenetic reduction increases the stress response of the mother over their lifetime. Anxious and fearful mothers produce a change in the epigenetic regulation of their pups, and this effect will reverberate down the generations. The behavior of the mother will alter gene expression in their children, and their children’s children.
As with so many discoveries in neuroscience, what works in a mouse may not generalize to a human. It is likely that humans have the same epigenetic marks that influence gene expression, but it may deviate from what is observed in mice and rats. Additionally, the complexity of the human brain often makes it difficult to reduce an observation to a few simple rules of organization. It is nearly impossible to tease out the influence of genetic inheritance from the effects of the environment. The relative influence of environment versus heredity has been hotly debated for decades, if not centuries. If this research proves valid in humans, it renders the nature versus nurture debate practically moot. Increasingly it appears that the environment has a profound and lasting effect on gene expression. The role of gene expression can no longer be considered in isolation, as if it is the last word in the life story of the organism. Perhaps the duality of genes and environment will have the same fate as that of the mind and body. One can only be understood in relation to the other.