Direct STR typing from bullet casings

Abstract

It is not uncommon for cartridges, bullets, and casings (CBCs) to be left at a crime scene involving shootings. During loading of guns, the handler touches the CBCs and hence they can be a valuable source of touch DNA. Previous studies on development of STR profiles from CBCs resulted in very low success rates. Direct PCR has shown increased success rates with low-level DNA but has not been applied to STR typing from casings. Here, we quantified the effect of firing on DNA quantity and also evaluated direct PCR as an alternative method to STR typing from bullet casings. Buffy coat was deposited on 9mm bullets and DNA was extracted from the casings of fired and unfired bullets (N=30 each). Firing the bullets decreased the amount of DNA recovered by around 30%. For comparison of STR typing protocols, ten volunteers touched three ammunition types (9mm, 5.56mm, and 7.62mm; N=60 bullets). Casings were swabbed with EO swab moistened with PBS. Total DNA obtained from fired casings were 207±523pg. In terms of the number of alleles typed, the direct PCR protocol did not differ statistically from the conventional extraction-STR typing protocol (95% Bayesian credible intervals of 3.0–7.8 and 3.9–10.3 alleles, respectively). This study showed that direct PCR can be used as an alternative method for STR typing from bullet casings. As the direct PCR protocol is quicker and cheaper than the conventional protocol, forensic DNA laboratories may benefit from using direct PCR for bullet casings.