Saturday, November 26, 2011

Nature and Nurture: The Role of Epigenetic “Marks” on Chromosomes in Mental Illnesses

Recent research has shown that a person’s experiences may contribute to mental illness by causing epigenetic changes (as opposed to simply genetic influences), which turn genes "on" or "off" without changing the genes themselves. The causes for neurological problems like addiction, depression, autism, schizophrenia, and the like seem to be mostly genetic, but the addition of environmental factors such as drug use or high amounts of stress in a person’s lifetime play an equally strong role. The way our experiences are processed in the brain has to do with nerve cells, which exchange information by releasing and identifying chemicals called neurotransmitters. It has been discovered that neurotransmitters can activate or inhibit different nerve cells and then switch a group of responsive genes on or off -- over time, the particular genes a neurotransmitter affects can help in predicting how a nerve cell will respond to an experience, which then determines how an individual behaves.

[caption id="attachment_2748" align="alignright" width="239" caption="Image source: Nestler, Eric J. "Hidden Switches in the Mind." Scientific American Dec. 2011: 79. Print."]Click on me! :)[/caption]

However, the aforementioned effects of neurotransmitters on behavior are not permanent, which is why the role of epigenetic marks are so interesting in that they do cause more lasting behavioral changes. Essentially, these marks are chemical tags that are located on either DNA or histone proteins around which DNA is normally wrapped on genes, and they influence how tightly packed chromatin should be. (The more tightly wrapped DNA strands are around histone proteins, the more inactive genes are and the less likely they are to turn on. When a protein needs to be made, however, the section of DNA the gene that is needed to create it is on unfurls slightly so that it can be transcibed into a strand of RNA and the encoded protein can be produced.) These epigenetic changes are made by different enzymes, some of which add chemical tags, and others which remove them; researcher C. David Allis calls them the "writers" and "erasers," respectively, of the epigenetic code.

Studies have been done showing the impact of epigenetic markers on various types of behavior, including maternal behavior. These studies were conducted with rats and showed that epigenetically-affected behavior can be passed down in generations by affecting only the pups', or baby rats', brains because they are born with methyl marks on particular genes that enhance sensitivity to stress and if they are raised by relaxed, nurturing mothers, these methyl groups disappear and they are calmer pups and later, parents; however, if they are raised by passive, fearful mothers, then their genes actually acquire more methyl groups, and they grow up to be similarly passive, fearful parents themselves. Using this information is and will continue to be important in the future to improve methods for treating various psychiatric problems by developing drugs and attempting to ascertain to what degree epigenetic changes that occur with these psychiatric conditions are heritable.

For more information about the article used as the primary source for this post, listen to the author's own thoughts about it.

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