The optimization of extracting foreign DNA from fingernails
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Citation
Abstract
Fingernail samples are submitted to a forensic laboratory for the analysis of endogenous deoxyribonucleic acid (DNA) for purposes of human identification or for the analysis of exogenous, or foreign, DNA on the nail surface. The presence of foreign DNA on nail samples may be a result of physical contact between two individuals, for example a victim and their attacker. The purpose of this study was to optimize the recovery of foreign DNA from fingernails. Phase 1 focused on length of incubation for fingernail samples to evaluate recovery of endogenous DNA; this informed Phase 2, which aimed at comparing the recovery of foreign DNA from fingernail samples when using a soaking or swabbing collection method.In Phase 1, fingernails of three donors were used to assess the average recovery of DNA in nanograms per milligram of nail (ng DNA/mg nail) after extraction incubation times of 15, 30, 45, and 60 minutes (min). The recovery of DNA in nanograms per microliter of whole blood (ng DNA/μl whole blood) after extraction was also evaluated using neat blood controls. The objective was to select an incubation time for nail samples that would effectively extract foreign DNA with minimal release of endogenous DNA from the sample. An incubation time of 15 minutes was chosen for Phase 2 to minimize the potential of endogenous DNA recovery from the nail samples.
In Phase 2, the comparison of DNA recovery was assessed between the soaking and swabbing collection methods using nails spiked with various volumes and dilutions of blood. Results indicate that the soaking method recovers more total DNA (both nail DNA and blood DNA) present in the sample compared to the swabbing method; however, the difference in DNA recovery between the collection methods was not statistically significant.
The DNA profiles of the blood-spiked nail samples were interpreted to compare each collection method’s efficiency at extracting foreign DNA while also minimizing the recovery of endogenous DNA. The difference in average peak height from unique nail alleles and unique foreign alleles, observed across loci where there were no shared alleles between nail and blood donors, were both not statistically significant between collection methods. However, the difference in the average percent recovery of unique nail alleles for nail samples spiked with 10, 5, and 2 microliters (μl) of whole blood was statistically significant between collection methods (p= 0.017), with 69.1% recovery using the soaking method and 32.7% recovery using the swabbing method. These results suggest that the potential for recovering endogenous DNA from samples is lower when using the swabbing method as compared to the soaking method.
However, there was no statistical significance in the difference of percent recovery of unique nail alleles for the samples spiked with the second set of blood volumes and concentrations (2 μl of whole blood, 2 of μl 1:2 blood dilution, and 2 μl of 1:5 blood dilution). Furthermore, similar to the first set of blood-spiked nails, both the soaking and swabbing collection methods recovered 100% of foreign DNA alleles from the fingernail samples. As a result, no conclusion was made on the efficiency of recovery foreign DNA between collection methods.
The percent recovery of unique foreign alleles was 100% across all samples of Phase 2, indicating that the amount of whole blood and blood dilution samples chosen to be aliquoted onto the nail samples may have been too large to effectively assess the recovery of foreign DNA from nail samples between collection methods. As a result, no conclusion was made on the effectiveness of the soaking and swabbing collection methods on the recovery of foreign DNA from fingernail samples.
Description
2025