Our ability to sequence the genes of organisms has undoubtedly paved the way for modern day genetics. Recent advances in this field have led to so-called ‘next-generation’ sequencing, where lengthy genomes such as our own can speedily be sequenced at a fraction of the previous cost. Yet despite this wealth of genetic information, many questions regarding the expression of these genes still remain unanswered, and now more than ever we must begin to appreciate the role epigenetics has in this complex life of ours.
A Recap on Genetics
As you may well know, genetics refers to the exact coding sequence of our DNA. The whole genome (that’s all our genetic information) is essentially just a long string of the letters A (for adenine), T (thymine), C (cytosine) and G (guanine). These letters make up our genes which encode the proteins we need to function.
So if genetics refers to the coding sequence of our DNA, what is epigenetics?
‘Epigenetics’ encompasses factors that affect how our genes are expressed, but which aren’t a change in the actual coding sequence. These are often modifications to the structure of the DNA – it’s like having the same string of letters but some are in different sizes, or are in bold font, and so are more pronounced (and therefore that modified gene is expressed more than the other genes).
Why is Epigenetics Important?
Epigenetics may go a long way to explaining why organisms with identical DNA coding sequences can still appear different – like identical twins. Epigenetics can be altered by environmental factors, like diet and lifestyle, which could explain why these factors can affect our risk of developing diseases like cancer.
A Common Example: DNA Methylation
DNA methylation is simply the addition of a methyl group (a carbon atom and three hydrogen atoms [-CH3]) to the letters in the DNA sequence. These can be added to the ‘C’ letters in the sequence, but not all of the Cs are methylated.
In the regions that control the amount that a gene is expressed, the amount of methylated Cs is particularly important. Generally, hypomethylation (lower levels of methylation) are associated with increasing the amount of gene expression. On the other hand hypermethylation (lots of methylation) is linked with decreased expression, or ‘silencing’ where the gene is basically not expressed at all.
So, we can therefore imagine that hypomethylation of pro-cancer genes might lead to an increase in the amount of that gene being expressed, and could promote cancer growth. Equally, hypermethylation of anti-cancer genes could lead to ‘silencing’ of those genes, and therefore also promote cancer progression.
Other Epigenetic Factors
Other epigenetic factors include modifications to the proteins (called histones) around which the DNA is wrapped and packaged. This influencing how easy it is to get to the genetic information. The production of short coding sequences which can work to block access to the full letter sequence (by essentially covering it up) can also influence levels of gene expression.
Epigenetics may well start to answer some of our most pressing questions about human genes and their expression. As our knowledge of epigenetic mechanisms increases, so will our understanding of their roles in the development of some of the most prevalent diseases in our society.
Hamilton, J.P. (2011) ‘Epigenetics: principles and practice’, Dig Dis, 29(2), pp. 130-5.