Emily C. Lennert
DNA, touch DNA, fingerprint, PCR, STR, mixed DNA
- Templeton, J. E.L.; Taylor, D.; Handt, O.; Linacre, A. Typing DNA profiles from previously enhanced fingerprints using direct PCR. Forensic Science International: Genetics. 2017, 29, 276-282.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
Fingerprint and DNA analysis are both valuable investigative tools that offer significant advantages in human identification. Powdering is a common method used for developing a fingerprint at a crime scene for subsequent analysis. Powdering allows the print to be developed for visual examination and characterization of friction ridges and minutiae which may be used to identify individuals due to the unique nature of fingerprints. Several types of powders are used to develop prints. On dark surfaces, silver, aluminum, or white powder can be used. On lightly colored surfaces, black carbon powder or black magnetic powder may be used. When fingerprint analysis is not enough, such as in cases where a partial or smudged print is obtained, DNA analysis may be performed after powdering.
Touch DNA, i.e. where skin cells, containing DNA, are deposited when a surface is touched, may provide a partial or complete DNA profile to aid investigators. However, touch DNA is often present in small amounts and may be degraded, making touch DNA analysis challenging. Standard DNA extraction procedures may result in loss of DNA due to several washing steps and tube transfers that occur. This is problematic for touch DNA, where the amount of DNA present is already small. Therefore, a direct polymerase chain reaction (PCR) approach is suggested in which the extraction and quantification processes are skipped, and DNA is immediately amplified by PCR. (NOTE: PCR is a process through which amplify the number of DNA molecule, by copying the DNA strands over and over again.) A previous study conducted and cited by the author found that direct PCR yielded the highest amount of DNA that could be amplified from a single fingerprint when compared to other published works. This study by Templeton aimed to evaluate the effect of fingerprint powders on touch DNA, to determine whether the fingerprint powders produced any negative effect on the subsequent DNA profile recovery.
Several steps were taken to prevent cross contamination of samples in this study. These steps are outlined in detail in section 2.2 of the study. One key step was that all fingerprints were dusted with a separate disposable brush that was previously irradiated with UV light to prevent any cross contamination.
To examine the effect of the powder on direct PCR and DNA profiling, a simulated sample was first set up. Using a sterile, DNA free swab, the swab head was dipped into one of four powders: white, silver, black, and black magnetic. After application of the power to the swab, the swab was cut and placed into a PCR tube. The tube was spiked with a control DNA and STR typing by direct PCR amplification was performed. A positive control containing only control DNA, i.e. no powder, was also set up to allow for recovery comparison. After PCR, products were centrifuged to force the powder down to the bottom of the tube. The supernatant, i.e. top liquid layer, was carefully removed to prevent any powder from being transferred. PCR products were then analyzed using a 3130xl Genetic Analyzer to develop DNA profiles. Analysis of the results revealed that no powder inhibited the ability to generate full DNA profiles when a control DNA was used. No apparent negative effect of fingerprint powders on PCR amplification was observed.
Ten volunteers participated in this study. Reference buccal DNA swabs were collected from the volunteers. Volunteers washed their hands 15 minutes prior to depositing fingerprints on plastic slides. During the 15 minute period, volunteers resumed normal daily activities, including typing, touching mobile phones and personal items, reading, watching television, etc. Fingerprints were deposited by placing four fingers of the dominant hand onto a sterile, DNA free plastic slide using medium pressure for 15 seconds. This was repeated four times per volunteer to allow for each powder to be tested. After collection, samples were placed in individual sterile petri dishes for storage at 4 °C.
Fingerprints were dusted the following day, approximately 16 hours after fingerprint deposition. Slides were lightly dusted with one of each of the four powders, such that four fingerprints were developed per powder for each volunteer. Fingerprints were swabbed immediately after dusting. A small section of fibers was cut from the tip of a swab, then held in sterile forceps and premoistened with 2 µL of 0.1% Triton™ X-100. The fingerprint was then swabbed and the swab section was placed in a PCR tube. A second swabbing was then performed following the same procedure, and the second section of fibers was placed in the same PCR tube. STR typing was performed with the AmpF/STR® NGM SElect™ PCR amplification kit. PCR products were centrifuged to separate the PCR product from the powder, and the supernatant, i.e. PCR product, was removed and analyzed using a 3130xl.
DNA profiles were considered full when all alleles were detected above the threshold; a threshold of 50 relative fluorescence units was selected. Profiles were considered informative or “up-loadable” if 12 alleles or more were observed for an individual. The term “up-loadable” was used to refer to profiles that were sufficient for upload to the Australian National Criminal Investigation DNA Database.
One way ANOVA statistical analysis showed no significant difference between the four powders in the mean values of the DNA data. No powder appeared to have a negative effect on PCR amplification. Overall, 98 out of 160, 61%, of samples produced profiles that were considered up-loadable, i.e. informative, when analyzed. Full profiles were obtained from 23 samples out of 160, approximately 14%. The highest success rate was observed for silver powder samples, with 75% of samples producing up-loadable profiles. White powder was second best, with 65% of samples producing up-loadable profiles. Black magnetic powder followed at 57% of samples producing up-loadable profiles, and black powder was least successful with 47% of samples producing up-loadable profiles.
Some mixed profiles were observed, with 31% of samples producing mixed profiles. In one sample, the volunteer was not the major contributor to the DNA mixture observed. These mixtures were attributed to secondary or tertiary transfer, due to the strict anti-contamination procedures that were followed. Volunteers reported touching objects that were touched by other people, such as door knobs, before depositing fingerprints. This action may lead to transfer of DNA from the object to the individual, and thus the individual’s fingerprint contained mixed DNA.
- Direct PCR allowed for the recovery of informative profiles from touch DNA samples.
- Direct PCR can be applied to touch DNA samples, provided that strict procedures are followed to prevent loss of sample or cross contamination of samples.
- None of the fingerprint powders tested showed any significant negative effect on the recovery of DNA from a processed fingerprint.
Fingerprints are often processed with powders to allow for visualization and characterization of the fingerprint. If additional information is needed, DNA may be recovered post-processing.
The fingerprint powders tested in this study do not appear to inhibit DNA analysis of processed fingerprints.