Multiple studies on DNA transfer have shown that when an object is touched, skin epithelial cells, as well as sweat, are left behind. While the average person sheds ≈400,000 skin cells/day, the cells of the outer dermis are highly keratinized and typically lack nuclei. Still, it is widely recognized that DNA is readily deposited onto the surface of a touched item. It has been subsequently shown that what is commonly referred to as “touch DNA” is primarily extracellular DNA termed Cell-free Nucleic Acids (CNAs) from the surface of the skin cells. A smaller number of nucleated cells carried in sweat are also thought to be a source of DNA. The physical contact required to handle a weapon or load a handgun magazine is sufficient to transfer CNAs and sweat associated epithelial cells from skin to a non-porous metal surface.
Efficient recovery of this DNA, however, has presented practitioners with a significant and ongoing challenge – one that must be overcome in order to generate interpretable genetic profiles. The most frequently offered explanation for the difficulty of obtaining STR profiles from handled items such as firearms, cartridge cases and explosives evidence is simply that the touch nature of DNA transfer leaves too little DNA for successful profiling by traditional technologies. Additionally, intense heat and reactive metal species may contribute to poor DNA recovery. Given the prevalence of these types of evidence, it is necessary to develop and rigorously optimize methodologies both for the initial extraction as well as the downstream profiling of DNA present on challenging sample types.
Much of the research published on improving the amplification of DNA from touch-type samples has focused on Low Copy Number (LCN) techniques which employ an increased cycle number during the polymerase chain reaction (PCR). LCN techniques are capable of generating profiles from as little as 30-60pg of DNA but data interpretation is often complicated by increased stochastic effects including excessive allele drop-in / drop-out and highly variable stutter peak height ratios. Instead of focusing on increased PCR cycle number approaches, the CFSRE has focused on three specific areas of research that have the potential to significantly improve the results obtained from challenging sample types. These are: methods for maximizing the amount of DNA in the final extract; characterizing and circumventing the impact on GSR and metallic species on STR profile quality; and evaluating the potential utility of Next Generation Sequencing (NGS) to increase the sensitivity of allele detection and the quantitative resolution of DNA mixtures. Advances in these areas has the potential both to improve DNA testing of challenging sample types currently being handled by forensic analysts as well as to lay the foundation for future improvements through the introduction of NGS approaches to DNA analysis of challenging samples.
Publications and Presentations
Evaluation and Optimization of DNA Recovery from Bullet Cartridge Cases
Meghan Troy, Christian G. Westring, Phillip Danielson, Heather E. Mazzanti. AAFS, Platform Presentation, February 2015.
Evaluation and Optimization of DNA Recovery and Amplification from Bullet Cartridge Cases
Heather E. Mazzanti, Meghan Troy, Christian G. Westring, Phillip Danielson. IAFS, Platform Presentation, October 2014.
Developing STR Profiles from Fired and Unfired Brass and Nickel-plated Cartridge Cases
Meghan Troy, Christian G. Westring, Al Stewart, Jill Yeakel, Heather E. Mazzanti. AAFS, Platform Presentation, February 2014.
Recovery of touch DNA from pipe bombs and their containers following various rendering-safe techniques
Megan Boll, Christian G. Westring, Al Stewart, Jill Yeakel, Heather E. Mazzanti. AAFS, Platform Presentation, February 2014.