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Knox walks free due to unreliable DNA analysis

Amanda Knox and her ex-boyfriend Raffaele Sollecito were yesterday cleared of killing Knox’s housemate Meredith Kercher, a British exchange student at the University of Perugia in Italy. The judge delivering the appeal verdict said the DNA evidence linking the pair to Kercher’s murder was “not reliable”. (New scientist , BBC News, Guardian, New York Times )

Knox, from Seattle, Washington, and Sollecito, an Italian, walked free after forensic scientists said the DNA evidence on which the prosecution’s case depended could be contaminated. Very small amounts of Knox’s DNA were found on a knife located at the crime scene 46 days after Kercher’s murder. The police believed the knife was the murder weapon. Tiny amounts of Sollecito’s DNA were found on a bra clasp belonging to Kercher.

Speaking at the request of the defence, two forensic scientists, Stefano Conti and Carla Vecchiotti from Sapienza University in Rome, said police investigators failed to follow international protocols for collecting and handling evidence and conducting tests on small genetic samples known as low-copy-number (LCN) DNA analysis. For example, officers were not wearing protective masks or hair caps at the crime scene. (See Nature’s feature on the controversies around the LCN fingerprinting.) In addition, Conti said police often used plastic bags, rather than paper, to wrap evidence, heightening the risk of contamination.

Scientists have developed LCN technique to enable them to produce profiles from just a few cells, rather than the 33 cells’ worth required for standard DNA fingerprinting. This more sensitive analysis can be achieved by running more PCR (polymerase chain reaction) cycles to copy more DNA, or purifying the sample after PCR. But this technological advance has its down sides.

During analysis of any sample, large or small, random fluctuations occur that distort the results. DNA present in the original sample might not show up. In addition, profiles can show DNA that is not present in samples. This effect can be caused by contamination.

“There are various circumstances do not adhere to protocols and procedures,” Conti told the court.

Consequently, the independent experts concluded that they could not rule out the possibility that the knife and bra had been contaminated by other sources of Knox’s and Sollecito’s DNA, such as other evidence at the crime lab where forensic testing was taking place.

These random effects are easily spotted and ruled out from standard analysis, but not so for LCN. So what appears to be a strong indication that DNA is present could in fact be contamination. The sensitivity of the approach requires strict rules for handling DNA evidence to minimize the risk of contamination and the misreading of results. And this seems to be where the case against Knox and Sollecito fell short. This is not the first case to be questioned over the LCN technique, and it probably won’t be the last.


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