"Getting blood from a stone": ultrasensitive forensic DNA profiling of microscopic bio-particles recovered from "touch DNA" evidence.

In forensic casework analysis it is sometimes necessary to obtain genetic profiles from increasingly smaller amounts of biological material left behind by persons involved in criminal offenses. The ability to obtain profiles from trace biological evidence is routinely demonstrated with the so-called touch DNA evidence (generally perceived to be the result of DNA obtained from shed skin cells transferred from donor to an object or a person during physical contact). The current method of recovery of trace DNA employs cotton swabs or adhesive tape to sample an area of interest. While of practical utility, such a "blind-swabbing" approach will necessarily co-sample cellular material from the different individuals whose cells are present on the item, even if the individuals' cells are located in geographically distinct locations on the item. Thus some of the DNA mixtures encountered in such touch DNA samples are artificially created by the swabbing itself. Therefore, a specialized approach for the isolation of single or few cells from "touch DNA evidence" is necessary in order to improve the analysis and interpretation of profiles recovered from these samples. Here, we describe an optimized and efficient removal strategy for the collection of cellular microparticles present in "touch DNA" samples, as well as enhanced amplification strategies to permit the recovery of short tandem repeat profiles of the donor(s) of the recovered microparticles.