DNA samples often convict criminals. But many of today's forensic tests are so polluted by soil, tobacco and food remains, for example, that they can not be used. Now researchers at Lund University in Sweden, working together with the Swedish National Laboratory of Forensic Science, SKL, have improved a critical part of the analysis process.
The first findings, published in the latest issue of the journal Biotechniques, indicate that the new method strengthens the DNA analysis so that previously negative samples yield positive and usable DNA profiles.
"The results are overwhelming. In my study I selected 32 truly difficult samples from the SKL archive, that is, with few cells, little DNA, and many so-called inhibitors, meaning lots of junk. With current methods it was impossible to get acceptable DNA profiles from any of them. But with the new method, 28 of the samples yielded more usable DNA profiles," says Johannes Hedman, an industrial doctoral candidate from SKL doing research at the Faculty of Engineering, Lund University.
Genetic information has become more and more common in forensic analyses. The analysis flow usually starts with taking a sample with a swab from a drinking glass or a blood spot, for example. The cells from the swab are then dissolved in water, and the DNA is extracted.
In forensics all over the world, much work has been done to improve the taking and handling of samples.
"The DNA analysis, on the other hand, has been something of a black box, since it is purchased as a finished product. No one has tried to improve it to be able to deal with dirty samples. But this is absolutely crucial, since the samples often have extremely small amounts of DNA. In this phase you copy certain parts of the DNA strands and then obtain a DNA profile that is unique to each person.
In the copying step, I have optimized the chemical environment and replaced a key enzyme, a so-called DNA polymerase. This yields a clearer genetic footprint, a DNA profile, to work with," explains Johannes Hedman. He has also devised a new mathematical model that makes it easy to interpret the DNA analysis.
If the copying phase is improved, stronger DNA evidence can be shown from crime scenes that today provide only partial or entirely blank DNA profiles. In other words, the chances are greater that a person can be found and linked to a particular crime.
The reason Johannes Hedman wound up at Lund is the fact that Peter Rådström, a professor of microbiological engineering, has been working since the late 1980s to improve DNA-based infection diagnostics and microbiological analyses for food. SKL was eager to find out whether these research findings could be applied to improving forensic DNA analysis.
"This collaboration opens new vistas for both SKL and Lund University, and we hope to be able to continue to work together with Peter Rådström's team. We have truly seen cross-fertilization," says Birgitta Rasmusson, research director at SKL.
- Johannes Hedman, Anders Nordgaard, Birgitta Rasmusson, Ricky Ansell, and Peter Ra%u030Adstro%u0308m. Improved forensic DNA analysis through the use of alternative DNA polymerases and statistical modeling of DNA profiles. Biotechniques, November 2009 DOI: 10.2144/000113256
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