Most people are aware that monozygotic (identical) twins share the exactly the same DNA, but it might be surprising to know that traits and diseases with genetic components can vary between these twins. In the case of some psychiatric disorders with strong genetic components, there are many pairs of identical twins in which only one twin actually develops the disease. In bipolar disorder for example a monozygotic twin has only a 40%-70% chance of also having bipolar disorder if their twin has been diagnosed. If bipolar disorder really has a strong genetic component, then why isn’t this number 100%?

Of course, we also know that environment can play a strong role in determining a disease outcome. For identical twins in their first environment,  the womb, each individual had slight environmental differences the moment the zygote split into two.  As identical twins develop, there will always be some environmental differences, many of which are still not fully understood. Before we explain this mystery of differences between identical (or more accurately “semi-identical”) twins as a matter of nature vs. nurture, there is another component that requires consideration.

There are other ways that DNA can be modified which don’t involve changes in DNA sequence (the order of the DNA’s A, C, T, G chemical “spelling”). These modifications often involve enzymes that can, for example, alter how, or if DNA is transcribed into mRNA. One type these DNA modifying enzymes is methyltransferase, which can add methyl modifications to cytosine (C), often resulting in the suppression of a gene. These types of non-sequence based changes are referred to as epigenetic modifications. Taking the sum of all of the epigenetic modifications gives us the term epigenome, the additional heritable information content of the DNA genome.

In a recent study published in the journal Human Molecular Genetics a paper by E.L. Dempster at the Institute of Psychiatry Kings College in London showed that epigenetic DNA modifications between sets of identical twins can vary by as much as 20%. This study is highly significant in its demonstrating why researches will have to go beyond sequencing the DNA genome of patients and start paying more attention to the epigenome. The Kings College study is the first wide-scale study that brings recent technological advances in epigenetic investigation to understanding psychiatric disorder. The study found epigenetic modifications not only in gene regions already known to be involved in disorders like schizophrenia and bipolar disorder, but has revealed new genes that could be potential targets for drugs. Because epigenetic modification involves chemical modification of DNA, aberrant epigenetic modifications can often be targeted by drugs.  Hopefully further exploration of the epigenome will  yield more clues about these often devastating conditions.