This study reports the development of a two-step protocol using pressure cycling technology (PCT) and alkaline lysis for differential extraction of DNA from mixtures of sperm and vaginal epithelial cells recovered from cotton swabs.
In controlled experiments, in which equal quantities of sperm and female epithelial cells were added to cotton swabs, 5 min of pressure pulsing in the presence of 0.4 M NaOH resulted in 104 ± 6 percent recovery of female epithelial DNA present on the swab. Following the pressure treatment, exposing the swabs to a second 5-min alkaline treatment at 95 °C without pressure resulted in the selective recovery of 69 ± 6 percent of the sperm DNA. The recovery of the vaginal epithelia and sperm DNA was optimized by examining the effect of sodium hydroxide concentration, incubation temperature, and time. Following the alkaline lysis steps, the samples were neutralized with 2 M Tris (pH 7.5) and purified with phenol-chloroform-isoamyl alcohol to permit downstream analysis. The total processing time to remove both fractions from the swab was less than 20 min. Short tandem repeat (STR) analysis of these fractions obtained from PCT treatment and alkaline lysis generated clean profiles of female epithelial DNA and male sperm DNA for 1:1 mixtures of female and male cells and predominant male profiles for mixtures up to 5:1 female to male cells. By reducing the time and increasing the recovery of DNA from cotton swabs, this new method presents a novel and potentially useful procedure for forensic differential extractions. (publisher abstract modified)