Epi means ‘on the top of’ in Greek. This is why the study of that which is ‘on the top of’ the genes is called epigenetics. In other words it is an additional layer of information that controls gene regulation but which is not codified in the DNA sequence, the part of inheritance that does not involve changes in the underlying DNA sequence.
One of the most well known epigenetic phenomena is DNA methylation. DNA is formed by combinations of four nucleotides:
- adenine (A)
- thiamine (T)
- guanine (G)
- cytosine (C)
This last one, cytosine, can be modified by the addition of a methyl group (CH3). This is what is called methylation, and its consequence is a change in the expression of the gene whose sequence has been modified. DNA methylation generally leads to transcriptional silencing of that gene.
The modification of histones– DNA-associated proteins that form the chromatin – is another type of epigenetic change. Histones, as DNA, can be methylated, but they can also be acetylated, by the addition of an acetyl group (CH3CO), or phosphorylated with a phosphate group (PO4). Any of these modifications changes the structure of histones and therefore of chromatin, which compacts DNA up to 10,000 times, leading to changes in the expression of genes.
Epigenetic modications such as DNA methylation or histone phosphorylation, acetylation and methylation, can inhibit or activate gene expression.
A combination of the above described epigenetic modifications plays an important role in “imprinting”, a type of ‘mark’ that implies that a gene will be expressed or not, depending on whether it comes from the female or male gamete.
And what is the importance of these phenomena? On the one hand epigenetics helps explaining the differences amongst identical twins, who share the exact same DNA sequence yet they can have different epigenetic variants (only one of them could have some genes inhibited by methylation, for example).
But the epigenetic ‘layer’ of information is crucial for the development, the growth and the aging of all people. Epigenetic changes are also involved with cancer. Indeed, in the past few years, increasing evidence has suggested that perturbations of chromatin structure and of other epigenetic mechanisms can cause inappropriate gene expression, which can result in malignant growth and cancer.