Identical twins, despite being biologically identical at birth, grow up to become unique individuals. Sure they may have a lot more things in common than two randomly picked individuals, yet there are many characteristics which belong only to one or the other. If the twins have the exact same DNA, then what is that makes them different?
The common answer to this question is it’s the environment that they live in which shapes them differently. Researchers have found that such environmental factors cause chemical modifications to the genome without affecting the nucleotide sequence, leading to the unique characteristics that we observe. This field of research is called epigenetics, and beyond the DNA, it’s what shapes our lives.
Rat mothers nurture their pups by licking and grooming. Researchers in Canada studying epigenetic changes found that rats whose mothers licked them more than normal expressed hundreds of genes differently from those who were licked less than normal. These differences were consistent and predictable, and led to a number of behavioural changes among the rats, including one where highly licked rats’ response to stress was a lot better than the less‐licked rats’.
Epigenetic changes don’t just occur through environmental factors but are also a different form of inheritance, one that doesn’t have to suffer from the randomness of natural selection. The licking of the rat encodes specific information onto her pup’s DNA without modifying to the sequence of base pairs. Mom’s behaviour programs the pup’s DNA in a way that will make it more likely to succeed. Such information is stored in the DNA in many ways, one of which is through DNA methylation. Through this process methyl groups are attached on to the DNA, and their attachment at specific positions leads to genes being turned on or off. This makes epigenetic changes reversible. For example, you can take a low‐nutured rat, inject its brain with a drug that removes methyl groups, and make it act like a high‐nurtured rat.
DNA methylation also plays a key role in cell division and cancer cells are known to divide faster than normal cells. Researchers in the US have developed drugs to interfere with DNA methylation as a treatment for cancer. They use molecules that mimic cytosine, one of the four bases of DNA. In cell replication, the fake cytosine swaps places with real cytosine in the growing stand of DNA, which then in turn traps DNA methyltransferase. When used in low enough doses, the drug allows the formation of the cell but with less methylated DNA. These drugs are currently being used to treat myelodysplastic syndrome, a prelukemia condition.
As Brona McVittie says, like the conductor of an orchestra controls the performance of musicians, epigenetic factors govern how the cell plays the notes in DNA. A better understanding of these factors has the potential of revolutionising evolutionary and developmental biology, thus affecting practices from medicine to agriculture.
- Learn Genetics, The University of Utah
- Introduction to epigenetics from Science magazine
- More ways to fight cancer through epigenetics, The Economist