By Shelley Farrar, MSc, BSc
In the future, it may be possible to create a profile of a criminal suspect from a drop of blood that provides details of their age, diet, smoking status, the drugs they have consumed, the polluted environment they live in, and even if they have a traumatic history of abuse.
This is because of epigenetics, in which one studies the effect of environmental factors on the activity of genes through various molecular mechanisms. Epigenetic programming begins during fetal development, can be inherited from one generation to the next, and changes during the lifetime of an individual. Current research is being developed to use tests of epigenetic markers in forensic applications.
Age prediction in forensic cases
One epigenetic molecular mechanism is DNA methylation, the process by which a methyl (CH3) group is added to DNA at the cytosine nucleobase. As there is a decrease in DNA methylation levels with advancing age, new methods that detect DNA methylation levels can determine the age of an individual. Current methodologies require whole genome analysis but a recent study aimed to identify the best CpG sites for predicting age from DNA methylation in a blood sample.
A CpG site is an area in the DNA sequence where the nucleobases, cytosine and guanine, are found next to each other. An age prediction model was formed using 16 CpG sites that provided accurate age estimations with a mean absolute error of 4.6 years. The study found the method has the potential to be applied to a wide age range and the results were consistent when tested with non-blood tissues.
Crime scene body fluid identification
The ability to identify body fluids is important to forensic investigations as DNA from skin cells may indicate innocent transfers of DNA but body fluids can signify that a crime has occurred. It can be difficult to identify body fluids at crime scenes as the stain left may be similar to other fluids and trace levels may be impossible to determine through either visualisation or serological testing.
Epigenetic testing can provide a better methodology as it provides quantitative results and exhibits longer stability than methods requiring RNA and protein markers. Three CpG sites have been identified that produce specific patterns of DNA methylation for semen, saliva and blood. The markers were found to be versatile with human specificity and provided consistent results for low concentrations of 0.1-10 ng. The method was also effective when samples were degraded by heat and inhibitors were added.
Monozygotic twin differentiation
Criminal cases that require the differentiation of two suspects who are monozygotic twins can be aided through the use of epigenetic marker testing. As monozygotic twins develop from a single fertilized egg, they share the same gene sequence and therefore cannot be differentiated by DNA sequence analysis. Monozygotic twins can, however, be identified through epigenetic differences, with both locus-specific DNA methylation and whole genome DNA methylation varying between monozygotic twins.
The development of standardised detection methods is currently underway. A 2015 study evaluated the use of detecting methylation levels of LINE-1 elements in monozygotic twins. LINEs or Long Interspersed Nuclear Elements, are a group of highly repeated sequences which do not contain long terminal repeats (LTRs). As LINE-1 elements are present at greater than 50 million copies, or around 17% of the human genome, methylation levels of LINE-1 are representative of the methylation level of the whole genome.
The study found that LINE-1, when used as a methylation marker, was able to differentiate between 12.61% of monozygotic pairs in the study. This confirms that epigenetic markers can be used to differentiate between monozygotic twins, but other methylation markers will need to be used with higher discrimination percentages.
The future of epigenetics
Tests using epigenetic markers need to be standardised with high sensitivity before they are admissible in court. There is also the difficulty of many current methods requiring large samples of tissue which will rarely be available at crime scenes. Fortunately, this emerging field may soon be used to enhance current forensic technologies. As the best CpG sites for determining DNA methylation levels are discovered, the forensic methodologies will become easier to put into practice.
- Khader, F. and Ghai, M. 2015. DNA methylation and application in forensic sciences. Forensic Science International, 249, pp. 255-265. https://www.ncbi.nlm.nih.gov/pubmed/25732744
- Silva, D. et al. 2016. Developmental validation studies of epigenetic DNA methylation markers for the detection of blood, semen and saliva samples. Forensic Science International, 23, pp. 55-63. https://www.ncbi.nlm.nih.gov/pubmed/27010659
- Vidaki, A. et al. 2017. DNA Methylation-based forensic age prediction using artificial neural networks and next generation sequencing. Forensic Science International, 28, pp. 225-236. https://www.ncbi.nlm.nih.gov/pubmed/28254385
- Xu, J. et al. 2015. LINE-1 DNA methylation: A potential forensic marker for discriminating monozygotic twins. Forensic Science International, 19, pp. 136-145. https://www.ncbi.nlm.nih.gov/pubmed/26223032