Epigenetics is the study of chemical reactions that influence gene expression, through activation or deactivation of genes at specific times and locations. These changes in gene expression do not involve a change in the DNA sequence, meaning there is a change in phenotype, without any change in genotype.
Although epigenetic changes occur regularly and naturally, they can be influenced by various factors such as the environment, lifestyle, age, and disease state. Epigenetic modifications can sometimes lead to adverse effects that can cause diseases such as cancer. There are at least three processes that are known to begin and sustain epigenetic modification, namely histone modification, DNA methylation, and gene silencing.
Epigenomics is the study of the epigenome, the set of chemical compounds and proteins that can attach to the genome and control or modify the expression of genes by turning them on or off.
In this way, the epigenome can tell the genome what to do and control the production of proteins in cells. While epigenetics focuses on processes that regulate gene expression, epigenomics is focused on the analysis of epigenetic changes across the entire genome.
Marking the DNA
Epigenomic compounds “mark” the genome by attaching to the DNA in order to alter its function. Rather than changing the DNA sequence, the marks change how the DNA instructions are used by cells. These marks are sometimes passed on from one cell to another during cell division and can also be inherited by offspring.
DNA methylation marks the DNA directly, while histone modification marks it indirectly. In DNA methylation, methyl groups are attached by proteins to bases in DNA at specific places and these groups activate or deactivate genes by altering interactions between the DNA and other proteins.
In histone modification, proteins attach chemical tags to histones, the proteins DNA is tightly wrapped around in order to allow the very long DNA molecule to be neatly coiled into chromosomes inside the nucleus of a cell. Once the histones are marked with these tags, they are detected by other proteins, which can influence whether a DNA region should be used or ignored.
Epigenetics has been linked to many human diseases, including cancer. Studies on tumor tissues showed that genes coding for colorectal cancer cells were significantly hypomethylated compared with those coding for normal tissues. While DNA hypomethylation can cause chromosome instability by activating oncogenes, DNA hypermethylation silences tumor suppressor genes.
Researchers have performed many studies to examine the link between errors in DNA methylation and diseases such as cancer, muscular dystrophy, lupus, and various birth defects. This is hoped to provide valuable insights that will improve how these disorders are treated and prevented.
Epigenetic changes are also linked to changes in brain function and brain diseases. Brain function is influenced at the cellular level by gene expression changes and mechanisms such as DNA methylation and histone modification ensure that gene expression remains stable.
Abnormalities in these processes have therefore also been linked to brain diseases including mental illness and addiction problems. Understanding the role of epigenetics in brain disease could open the door for more effective treatments in brain disease.
Reviewed by Sally Robertson, BSc