Approximately 60 000 traces of DNA from crime scenes are analysed each year at the Swedish National Forensic Centre in Linköping. However, many are contaminated and not possible to use. A new thesis is paving the way for more robust methods of analysis, which in turn means that more crimes can be solved.
A crime has been committed. The perpetrator has left traces. It might be leftover food, clothes, blood or other bodily fluids. By taking a DNA swab or lifting cells with adhesive tape from surfaces, the traces can be secured to obtain the perpetrator’s DNA.
One problem is that proteins in blood often ruin the DNA analysis. When it comes to sperm or other bodily fluids in soil, the problem is that substances in soil spoil the possibilities for analysis.
Discovery of which molecules cause DNA analyses to fail
A few research groups around the world are working on improving the techniques so that these kinds of crime scene traces can be used in forensic investigations in the future. Their discoveries and improvements have today led to it being possible to obtain a DNA profile with just a small number of cells.
One of the research groups is based at the Faculty of Engineering at Lund University and at the Swedish National Forensic Centre in Linköping. Maja Sidstedt is one of the researchers in the group and recently completed her doctoral thesis. In her thesis, she demonstrates what happens when soil and blood substances impair, or completely ruin, DNA analyses.
“I have discovered that certain substances such as haemoglobin in blood and humus in soil knock out the enzyme that is used to copy the DNA. Some of these also knock out the fluorescent signals that are used to detect the replicated DNA”, she says.
Previously, the erroneous conclusion was drawn that the copying had failed, when in reality it was just the signal that had been quenched. However, it is possible to correct this by using another signal system. Enzyme identified that can handle nearly fifty times as much contamination
The enzyme is the motor in the DNA analysis. The discovery at the crime scene may contain just a few cells, which is not sufficient to be able to identify a person directly. The DNA must be copied first and it is the enzyme that is responsible for that, explains Maja Sidstedt. She has studied the most common technique used for forensic DNA analysis, PCR (polymerase chain reaction), which identifies and multiplies DNA fragments so that a person may be identified. She has also studied the newer techniques MPS (massive parallel sequencing) which produces a broader DNA analysis, for example determining eye colour, as well as digital PCR.
“My investigations show that the modern technologies are just as sensitive to contamination as PCR. However, there are ways to solve this, including by adding a specific protein that can bind to the interfering molecules so that the enzyme is not disturbed.”
During her investigations, Maja Sidstedt also identified an enzyme that can handle nearly fifty times more contamination than the enzyme usually used for this kind of analysis.
“Further development on this is needed, based on the inhibition mechanisms that I have mapped”, she says.
Could be used for food analysis and bioterrorism
Some of Maja Sidstedt’s results have already started to be applied by the Swedish National Forensic Centre, among others. Her research could also be used to improve DNA analyses in other areas, such as medicine, food and bioterrorism.
Text: Kristina Lindgärde – published on 6 September 2019 Photo: Johannes Hedman
