Over the past two decades, our understanding of genetics and of the human genome has expanded exponentially. Today, epigenomics has emerged as one of the most important and fastest-moving fields of biomedical research. Read on for an overview of the epigenome, its role in human health, and the current focus of epigenomics research.
What is the epigenome?
If the genome is the term for a living organism’s complete set of DNA, the epigenome refers to the multitude of chemical compounds that are responsible for directing the functioning of the genome. In other words, the epigenome describes the compounds and proteins that tell the genome what to do.
What does the epigenome do?
Our DNA is like the instruction manual for our cells: it directs the building of particular proteins, which in turn influence how different cells develop and what functions they carry out. Epigenomic compounds fit into this equation by attaching to DNA and directing particular actions like turning different genes on or off, or controlling protein production. We can think of this as something like a “manual override” function, where the epigenome changes the way that different cells respond to the instructions they receive from our DNA. When epigenomic compounds modify DNA function like this, scientists refer to the genome as being “marked,” but it is important to understand that the DNA sequence itself has not been changed.
What is the epigenome composed of?
The chemical modifications to DNA and DNA-associated proteins that constitute the epigenome occur as part of a natural process of tissue differentiation and development, but they can also be altered as a result of disease or other environmental factors.
When we talk about the epigenome, we are primarily talking about two different types of “marks” or modifications. The first one, DNA methylation, has a direct impact on the DNA in a particular genome. Methyl groups, a kind of chemical tag, attach to particular parts of the DNA molecule and turn different genes on or off. The second mark is known as histone modification. This mark affects DNA indirectly by attaching chemical tags to histone proteins, around which DNA is wrapped, rather than to the DNA itself. When these tags are detected, other proteins determine whether that section of DNA should be used in the cell or ignored.
Is the epigenome hereditary?
The genome is passed on in two main ways: from parents to their offspring, and as a result of cells dividing to produce the next generation. In the parent-to-offspring genome transfer, most of the epigenome is reset and the offspring begin with a “blank slate” epigenome. There are some circumstances under which the “marks” or chemical tags described above are inherited along with the base genomic information. In cell division, however, most of the epigenome is passed on to the next cell generation, as this information is what helps the cells remain differentiated and fulfil specific functions.
Can the epigenome change?
The short answer is yes—the epigenome can change over the course of a person’s lifetime. However, we’re not yet completely sure what causes these changes. Sometimes they can result from lifestyle or environmental factors that put pressure on the body and prompt chemical responses. Smoking, diet, and exposure to infectious diseases are some of the most commonly-studied reasons behind epigenomic change. But what’s most interesting is that some of these changes to the epigenome seem to be a required and “normal” part of maintaining human health, while some changes to the epigenome can be damaging, causing protein malfunctions that lead to illness and disease.
What is the connection between the epigenome and diseases like cancer?
The link between the epigenome and cancer has come under great scrutiny in recent years. In general terms, cancers are caused by changes to either the genome, the epigenome, or both together. The reason cancer can result from epigenomic changes is that these changes (as described above) often influence the functioning of genes responsible for cell growth or the immune response. In other words, changes to the epigenome that switch “on” the cell growth gene can cause uncontrolled growth, one of the major characteristics of cancer, while changes that switch “off” the immune response gene can inhibit the immune system from destroying tumors.
What is epigenomics research currently focused on?
One of the principal aims of current epigenomics research is simply understanding more about what epigenomic changes can lead to cancer. At present, researchers from the Cancer Genome Atlas Network are working to compare the genomes and epigenomes of healthy cells with those of cancerous cells, in order to gain a better idea of the factors that turn a normal cell into a cancerous cell. Other researchers supported by the National Institutes of Health are working with human organs and tissues to create epigenomic maps of different cell types, which are intended to boost our understanding of the future role that epigenomics could play in disease prevention, diagnosis, and treatment.