The standard explanation of genetics explains that DNA provides the instructions to make RNA, and RNA creates proteins that control all cell activity. Recent research shows things aren't quite so simple.
How Epigenetics Works and DNA Methylation
Epigenetics includes anything affecting the genome not encoded in DNA itself. According to Dr. Jean-Pierre Issa at the M.D. Anderson Cancer Center in an interview with Sarah Holt for PBS' Nova series, only 10-20% of genes are active in any cell. This prevents genes of one cell type from being expressed in another. For example, the gene for eye color only expresses in the eyes, not the liver, skin or brain.
Control of gene expression can be handled in different ways. Sometimes, small molecules bind to DNA, changing its ability to give instructions. These molecules originate as proteins, protein complexes or small bits of RNA. For example, in times of drought, the body produces molecules to modify DNA and turn on or off genes that help it endure difficult circumstances.
In other cases, interfering molecules come from the environment. Molecules called methyl groups are present in foods, household chemicals and environmental pollutants. These can modify the structure of DNA in a process called DNA methylation, turning genes on and off and affecting what gets translated into RNA and proteins.
What Epigenetics Affects
Epigenetics affects many areas of biology. In animals, one of the most important changes happens during embryonic development. Genes use epigenetics to guide proper development of stem cells into different cells of the body.
In some cases, different DNA methylation effects from the mother and father compete to determine which parent contributes the trait. For example, when a donkey and horse mate, the resulting mule is different depending on which species was mother or father. This also explains why individuals with the same genome, such as identical twins, exhibit different characteristics, depending on whose epigenetic effects - mom's or dad's- won out in each baby.
Epigenetic Effects Pass From Mother to Baby
Many epigenetic effects stem from the mother's activities during pregnancy. If mom is overweight, for example, it can affect weight control mechanisms in her child, leading to obesity or diabetes years down the road.
These effects can even be passed down through multiple generations, so eating particular foods or being exposed to environmental factors could lead to effects in grandchildren and great-grandchildren.
Cancer Epigenetics and Other Disease
Growing evidence shows that environmental hits affect both genetics and epigenetics in diseases such as cancer. Smoking, for example, is known to damage DNA, but new research shows it also causes epigenetic changes. Even undamaged DNA can be affected by smoke molecules that bind to it via DNA methylation and prevent proper functioning. Future cures may include injections to alter the epigenetic profile of cancer cells and revert them to normal cells.
Behavioral Epigenetics
One of the most stunning discoveries surrounding epigenetics was a study showing that mice who spent more time grooming their young made those offspring braver and more resilient to stress. The infant mice actually changed their behavior due to epigenetic effects when mother's grooming caused a particular methylation pattern in the babies' brain DNA.
Researchers are investigating whether epigenetic influences might be a contributor to mental illnesses, which could lead to new potential treatments.
As scientists discover more about the role of epigenetics, this is sure to remain a hot topic in biology for years to come.
Sources:
Epigenetic mechanisms mediating the long-term effects of maternal care on development, Frances A Champagne and James P Curley, Neuroscience and Biobehavioral Reviews, 18 January 2008
PBS Nova website, interview with Dr. Jean-Pierre Issa at the M.D. Anderson Cancer Center "Ghost in Your Genes"
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