Glued to our DNA are thousands of molecules that shut some genes off and allow other genes to be active. Our experiences can physically rearrange the pattern of those switches and, in the process, change the way our brain cells work. . . . Two families of molecules perform that kind of genetic regulation. One family consists of methyl groups, molecular caps made of carbon and hydrogen. A string of methyl groups attached to a gene can prevent a cell from reading its DNA sequence. As a result, the cell can’t produce proteins or other molecules from that particular gene. The other family is made up of coiling proteins, molecules that wrap DNA into spools. By tightening the spools, these proteins can hide certain genes; by relaxing the spools, they can allow genes to become active.
Together the methyl groups and coiling proteins—what scientists call the epigenome—are essential for the brain to become a brain in the first place. An embryo starts out as a tiny clump of identical stem cells. As the cells divide, they all inherit the same genes but their epigenetic marks change. As division continues, the cells pass down not only their genes but their epigenetic marks on those genes. Each cell’s particular combination of active and silent genes helps determine what kind of tissue it will give rise to—liver, heart, brain, and so on. Epigenetic marks are remarkably durable, which is why you don’t wake up to find that your brain has started to turn into a pancreas.
Our experiences can rewrite the epigenetic code, however, and these experiences can start even before we’re born. In order to lay down the proper pattern of epigenetic marks, for example, embryos need to get the raw ingredients from their mothers.
Spina bifida and fetal alcohol syndrome may be examples of disorders associated with a flawed epigenome. Child abuse appears to leave epigenetic marks, and there are also epigenetic marks found more commonly in people who have committed suicide and in rats suffering from a depression-like condition. Epigenetic marks in the brain may be one way the experience influences people's personalities.
Changing someone's epigenome in some kinds of cells, without changing the genetic material itself, could be one means of treating many disorders. For example, if cocaine addiction leaves an epigenetic mark, changing that part of a person's epigenome could end the addiction. Also, even if epigenetic therapy doesn't prove possible, tests to determine what epigenetic marks are present in a person could be used to diagnose conditions and for forensic purposes (like determining whether suicide or child abuse were likely causes of a person's death, or determining if someone was a drug addict).
Understanding the epigenome could also be critical in making stem cell therapies work, and for linking particular parts of the genome to the biochemical results that they code.
We might also learn to determine which parts of the genome code for epigenetic elements that are part of the developmental process. While most of a person's genome can be fairly summarized as a cook book of biochemical recipes that go into building a person, the part of the genome that codes for the epigenetic activity seen in the developmental process might be a good place to look for bigger picture issues in the blue print of life -- instructions on what tissues go into a heart muscle instead of instructions on the proteins that can be combined into a variety of tissues, for example.