Casework direct kit as an alternative extraction method to enhance touch DNA samples analysis

Abstract

Latent fingerprints are commonly found in crime scenes, and currently used in forensic analysis to obtain STR profiles from DNA recovered from finger contact. Analysis of STR profiles obtained from touch DNA has been very useful to elucidate crimes and the extraction method may be determinant for the recovery of genetic material collected from different surfaces. This study aimed to verify and compare the efficiency of two different extraction kits for processing touch DNA samples obtained from fingerprints deposited on computer keyboards, knife handles and exterior door handles and steering wheels of cars.One hundred and four experiments were conducted to simulate crime scenes and evaluate the efficiency of two extraction kits for touch DNA samples: the DNA IQ™ System and the Casework Direct Kit (both Promega Corporation). Each experiment was conducted with two individuals in order to obtain a mixture profile. The genetic material deposited was collected by double swab method (Sweet et al. 1997) and DNA quantification was conducted using Quantifiler Trio™ (ThermoFisher Scientific). Samples were amplified by PowerPlex® Fusion System kit (Promega).It was possible to obtain STR profiles for 32 (61.5%) out of the 52 extracted using DNA IQ and 51 (98.1%) out of the 52 extracted using the Casework Direct Kit. Samples extracted by DNA IQ had higher average of quantification values for long targets (>200bp) across all tested surfaces. That seems to be due to an incompatibility between the Quantifiler Trio and the Casework Direct Kit. Samples with positive quantification but without STR profile, as well as samples without quantification but with STR profiles were also observed.Statistical analysis showed that the Casework Direct Kit produced significantly more useful profiles than DNA IQ (p-value = 0.001), since these profiles had more STR markers with allelic correspondence to second donators present in the mixture.This study provides insights about the effect of different surfaces and extraction methods on recovery and generation of STR profiles. Limitations for the quantification step for these samples with a low quantity of DNA were highlighted as well. We concluded that the Casework Direct Kit was much more efficient for processing touch DNA samples than DNA IQ.