Twins on Trial

When someone hears of twins, their mind usually flies right to the thought of two individuals that look alike and 50% of the time you would be right. There are two different types of twins: fraternal and identical. Fraternal twins are what you would call dizygotic twins. This means during fertilization there were two eggs produced and fertilized. These twins develop within separate placentas and usually come out looking different. On the other hand identical twins, or monozygotic twins, happen when one egg is fertilized and splits into two embryos. These twins share the same placenta and are born exact replicas.

Now with this information in mind, let's bring it down to a more molecular level. Monozygotic twins share the exact set of DNA since they are in a sense replicas of one another while fraternal twins have their own specific set of DNA. So with all the fun things that happens to our DNA throughout our lifetime, DNA methylation is really neat. This is the process when DNA methyltransferase (pretty neat name because it literally gives you a hint for what I'm about to say it does) adds a methyl group on the 5' position of the pyrimidine ring of the cytosine nucleotide. This methylation usually only occurs when there is a cytosine followed by a guanine. You might be familiar with the term CpG sites. Methylation is a cool process that usually affects the nature of a gene; it acts like a switch to turn active genes off or, during stem cell differentiation, methylation helps guide the stem cell to become a certain type of cell based on the methylation pattern. In mammals, this methylation happens pretty uniformly but the patterns can be variable from person to person based on the different environments we have encountered throughout our lifetime.

I probably have you asking by now what does methylation have to do with twins, well my friend, CRIME. We have always been told that if a biological sample is found a scene that holds DNA that that DNA can be sequenced and a suspects DNA can be sequenced to form a match with a high statistical probability. While this is true, it isn't so much true in the instances where monozygotic twins are involved for obvious reasons. Researchers have thought of a sort of solution, while it isn't perfect and there is a lot of testing to be done, of taking a look at not only the DNA but of the DNA methylation patterns to differentiate between the two individuals. The study is based on the principle that the methylation patterns will be different because the two individuals, even though they are twins, could have experienced different environmental stimuli at one time or another in their lifetime.

Using melt curve analysis Stewart et al. tested this hypothesis at the Alu sites of 5 sets of monozygotic twins. Alu sites are highly conserved throughout the human species and they are what you would call a short interspersed nuclear element meaning that they are about 300 nucleotides long and the nucleotides are in a repeated sequence (these sites are commonly mutated because of the repetition). Even though this method seems pretty solid, it is very variable. One set of twins did show a large differentiation between their methylation patterns while the others showed subtle or no differences. Age and environmental stimuli play a huge role for the success of using this kind of study. While the test itself is relatively cheap, the results might not be substantial for the use in a court of law. If anything, it could be used as a preliminary test before moving on to the more expensive mutation tests in a forensic laboratory. I believe that this there are other sites that could be tested for the differences in methylation that might yield more substantial results. On the other hand, a larger pool of twins might need to be used in order to get a truer result.

For further reading: http://www.sciencedirect.com/science/article/pii/S0003269715000500