Evaluating the effect of multiple displacement amplification upon low-level multiple source DNA profiles

The interpretation of low-level DNA mixtures, where there is DNA present from more than one individual in a sample, is of great difficulty. This can be due to the low-level nature of the result and the complexity inherent in DNA results with multiple sources. These two elements in one sample only further compound the difficulty. Multiple displacement amplification (MDA) techniques, with or without a macromolecular crowding (MDA-MC) agent, have been suggested as potential tools for improving these results. The aim of this study was to investigate whether the proposed techniques could be of use to forensic genetics casework. Buccal swabs underwent extraction, and then mixtures of samples were prepared with a range of known mixing ratios, from 1:1 through to 50:1. These samples were split into three prior to undergoing standard DNA analysis, MDA or MDA-MC. DNA quantification was carried out using real-time PCR and the Human DNA Quantifiler kit, and STR analysis was carried out using the AMPfSTR NGMSElect kit. For each set of results, the mean mixing ratio was calculated and compared with the actual mixing ratio. The incorporation of MDA and MDA-MC did not sufficiently improve the mixed DNA results for all mixing ratios. Although there were significant improvements at mixing ratios of 10:1 and 50:1, this is insufficient, as knowledge of the actual mixing ratio prior to analysis is required. Consequently, as it stands, MDA and MDA-MC is not suitable for forensic casework. However, despite a range of actual mixing ratios from 1:1 through to 50:1, no mixing ratios above ~10:1 could be obtained, suggesting that this is the threshold at which the major contributor masks the minor contributor. This is thought to be due to drop-outs from the minor contributor, which increases as the mixing ratio increases.