Abstract
BackgroundProfiling 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.MethodsThe 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.ResultsFor 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.ConclusionsIn 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.
Highlights
Profiling sperm DNA present on vaginal swabs taken from rape victims often contributes to identifying and incarcerating rapists
DNA purification, DNA quantitation, and Short tandem repeat (STR) profiling The methods used by each participating laboratory for DNA purification, DNA quantitation, and STR profiling are given in Table 1 and in each case were the methods used by these laboratories to process routine casework and followed the manufacturer’s instructions
All 40 swab cuttings, processed either with the standard method or Erase, provided abundant amounts of DNA in the female fractions, and profiling of these fractions gave accurate female profiles in every case. This is not surprising, as the swab cuttings each contained a large number of female epithelial cells and these cells provide high nanogram to low microgram amounts of female DNA, which is far more than needed for profiling
Summary
Profiling sperm DNA present on vaginal swabs taken from rape victims often contributes to identifying and incarcerating rapists. The standard method for obtaining male DNA from a vaginal swab for autosomal profiling was first described by Gill and colleagues in 1985 [7] and is still being used in modified forms [8,9,10] by crime laboratories today. This method is commonly called differential lysis because the non-sperm cells are selectively lysed with detergent and Proteinase K, while the sperm are not lysed due to the heavily disulfide cross-linked proteins in the sperm head that resist protease treatment. The washing steps are tedious, difficult to automate, and result in sperm loss, and many attempts have been made to circumvent the need to wash the sperm pellet such as collecting the sperm by filtration [11,12], flow cytometry [13], and laser dissection [14-16], none of these methods have been applied to routine casework
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