Epigenetics: An Overview

Epigenetics: An Overview

Genetics is a concept most of us are in some way familiar with. When we're young, we realize children look similar to their parents. As we get older, we start hearing about DNA. In high school, we are taught about chromosomes, Mendel's Laws, and other patterns of inheritance. In AP Biology at Notre Dame, our class has been taught to recognize that the characteristics of every living thing are determined by genetics, specifically by an organism's genome. But what is epigenetics?

Our genetics are determined by the chromosomes our parents pass onto us. (Credit: Yourgenome.org)

Epigenetics, to put it simply, is the study of how environmental factors affect can alter the gene expression of an organism. The genome consists of all DNA contained in the cells of a particular organism. The human genome consists of approximately 3 billion base pairs, their order contributing to what traits an organism expresses throughout its lifetime. Hence, the epigenome is the genome as it exists when acted upon by external factors. If it helps, you can think of the epigenome as the software of a computer and the genome as the hardware.

"Why Are We Genetically Modifying Humans?" is a 2022 documentary that discusses the current advancements in epigenetics, discussing the history of the field and the studies that have proven essential to our understanding of how the environment impacts our DNA. The genome is altered through DNA methylation, a process by which a methyl group is added to a strand of DNA to either invite proteins to help repress a gene or inhibit transcription factors from binding. By doing so, the expression of a gene is either activated or inactivated.

The image above showcases DNA methylation. A methyl group (M) is removed from S-Adenosyl methionine (SAM) by the enzyme DNA methyltransferase (DNMT) and attached to a DNA strand at a cytosine (highlighted in teal) to form 5-methylcytosine. A methyl binding domain (MBD protein) attached to the 5-methylcytosine to allow co-repressor proteins, in this case HDAC, to bind to the gene. (Credit: ScienceDirect)

One of the first studies to make a breakthrough in epigenetics was a study on agouti mice run by Dr. Randy Jirtle and Dr. Robert Waterland of Duke University Medical Center. In this study, the diet of two pregnant mice was controlled. Although the resulting offspring were genetically identical, one was larger and with a yellow coat and diabetes while the other was smaller and healthy with a brown coat. The success of this study sparked the established of many more in recent years, performed on mice, ants, and humans.

The mice from the agouti mice study presented a number of different phenotypes, each with a corresponding level of health. (Credit: Taylor & Francis Online)

The field of genetics is ever expanding, as researchers observe more and more external factors that impact how an individual may present in terms of gene expression. Scientists now know that extreme stress during pregnancy can lead to developmental delays in children and that nurturing upbringings lead to more nurturing parental strategies when those children grow up. Studies leave us with implications of epigenetic drugs being used as a treatment for mental disorders such as PTSD, depression, and substance use disorders. Each day, epigeneticists are working towards improving the