Introduction

Epigenomics, or epigenetics, literally means above, or on top of, genetics.  In fact, the epigenome is a multitude of chemical compounds that tells the genome what to do.  The human genome contains over three billion letters of DNA, called nucleotides.  These nucleotides make up a genetic code that make you, you.  For more information on what the genome is, look at my previous blog, https://eportfolios.isucomm.iastate.edu/bwhite/2016/09/14/blog/#more-8.  Chemical compounds and proteins that attach to DNA and direct actions like turning genes on or off and controlling the production of proteins in particular cells are what makes up the epigenome.  Epigenomic compounds “mark” the genome by attaching compounds to DNA and modifying its function.  This does not change the DNA sequence, but changes how the cells use the DNA’s instructions.  Epigenetics is a vital key in understanding the genome and how human health works, and should be researched in depth as a way to tailor treatments to specific patients.

                                                                   https://www.sciencenews.org/article/year-review-epigenome-makes-its-debut?mode=topic&context=87

Figure 1.  Some of the tissues and cells that can be epigenetically marked and mapped.

What does the epigenome do?

Every cell in the human body is specific for different functions in organs, bones, and muscles.  These cells carry the same genome in its nucleus.  The epigenome makes up the chemical compounds that have been added to all of one’s DNA, which comprises one’s genome.  These chemical compounds are a way to control the expression of every gene in the genome.  The chemical compounds involved with the epigenome are not part of the DNA sequence, but are attached to the DNA.  Many people believe that you are born with genes inherited from your parents, and that your genetics can not change.  While this may be true on many levels, it is not true about epigenetics.  Changes in the epigenome are considere a natural process of development and tissue differentiation.  A person’s epigenome can change based on environmental factors, like: smoking, diet, and exposure to pollutants.  It can also change due to disease.  The changes made to the epigenome will remain as the cells divide and can be inherited through generations.  Whether genes are turned on or off, the production of certain proteins in cells, and ensuring that cells only produce the proteins needed are all things epigenetic changes determine.  An example of this, proteins that promote muscle growth would not be made in bone cells.  For more on what epigenetics is and how it works, watch the following video:

Epigenomic modifications

Methylation is a typical epigenomic modification.  During methylation a methyl group is attached to a specific segment of DNA.  When the methyl groups added during methylation attach to a gene, they can turn off the gene.  This results in no protein being produced from that particular gene.  The methyl group can also turn on a gene, leading to protein being produced by that gene.  Methylation allows the cells in the body to remember which genes are on or off.

                                                                                                                                   https://www.quora.com/How-is-DNA-methylation-related-to-senescence

Figure 2.  Cytosine made into 5-Methylcytosine through methylation.  The newly added methyl group is in the red box.  

The second most common epigenomic modification is histone modification.  In histone modification, DNA in cells is wrapped around histone proteins, which then make spool-like structures.  This enables DNA’s molecules to be wound into chromosomes inside the cell nucleus.  From there, proteins can add many different chemical markers to histones.  Proteins from other cells can detect these markers and decide whether that region of DNA should be ignored or used in that cell.

                                                                                             genome.gov/images/content/epigenomics_factsheet.jpg

Figure 3.  Histone modification.

Errors in the epigenetic process

Many errors can occur in the epigenetic process.  These errors are what lead to some diseases.  The modification of the wrong gene or the failure to add a compound to the gene is an example of and epigenetic error.  These two errors can lead to unusual activity or inactivity of the gene, which causes genetic disorders.  Some of the genetic disorders caused by errors in the epigenetic process includes: cancers, degenerative disorders, and metabolic disorders.

Conclusion

While the genome is a very complex structure that is difficult to comprehend, scientists are using epigenetics to understand it a little better.  Today, researchers are using epigenetics to map the locations and understand the functions of all the chemical tags that mark the genome.  For more information on current research in epigenetics, look at:  http://www.roadmapepigenomics.org/.  An interactive tutorial that might help in your understanding of the epigenome can be found:  http://learn.genetics.utah.edu/content/epigenetics/.  Until recently it was thought that human diseases occurred largely due to changes in DNA sequence, infectious agents such as bacteria and viruses, or environmental agents.  Researchers have now found that changes in the epigenome can also lead to disease.  Many doctors hope to one day be able to use epigenomic maps to evaluate patients health status and determine a patient’s response to treatments.  It is now more important than ever to continue studying epigenetics in an effort to better understand the human body and, ultimately, improve medicine and human health.