Isolating DNA from sexual assault cases: a comparison of standard methods with a nuclease-based approach
1 Confarma France SARL, Zone Industrielle Canal d’Alsace, Hombourg, 68490, France
2 Landeskriminalamt Baden-Württemberg, Crime Laboratory, Taubenheimstrasse 85, Stuttgart, 70372, Germany
3 Molecular Diagnostics Laboratory, Genetic Forensic Unit, Via Tesserete 48, Lugano, CH, 6900, Switzerland
4 Forensic Genetic Unit, University Center of Legal Medicine Lausanne and Geneva, Rue du Bugnon 21, Lausanne, CH, 1011, Switzerland
5 University of Bern, Institute of Legal Medicine, Sulgenauweg 40, Bern, CH, 3007, Switzerland
6 Los Angeles County Sheriff‘s Department, Forensic Biology, 1800 Paseo Rancho Castilla, Los Angeles, CA, 90032, USA
Investigative Genetics 2012, 3:25 doi:10.1186/2041-2223-3-25Published: 4 December 2012
Profiling sperm DNA present on vaginal swabs taken from rape victims often contributes to identifying and incarcerating rapists. Large amounts of the victim’s epithelial cells contaminate the sperm present on swabs, however, and complicate this process. The standard method for obtaining relatively pure sperm DNA from a vaginal swab is to digest the epithelial cells with Proteinase K in order to solubilize the victim’s DNA, and to then physically separate the soluble DNA from the intact sperm by pelleting the sperm, removing the victim’s fraction, and repeatedly washing the sperm pellet. An alternative approach that does not require washing steps is to digest with Proteinase K, pellet the sperm, remove the victim’s fraction, and then digest the residual victim’s DNA with a nuclease.
The nuclease approach has been commercialized in a product, the Erase Sperm Isolation Kit (PTC Labs, Columbia, MO, USA), and five crime laboratories have tested it on semen-spiked female buccal swabs in a direct comparison with their standard methods. Comparisons have also been performed on timed post-coital vaginal swabs and evidence collected from sexual assault cases.
For the semen-spiked buccal swabs, Erase outperformed the standard methods in all five laboratories and in most cases was able to provide a clean male profile from buccal swabs spiked with only 1,500 sperm. The vaginal swabs taken after consensual sex and the evidence collected from rape victims showed a similar pattern of Erase providing superior profiles.
In all samples tested, STR profiles of the male DNA fractions obtained with Erase were as good as or better than those obtained using the standard methods.