Detection of latent DNA

Author: Piyamas Kanokwongnuwut

Kanokwongnuwut, Piyamas, 2021 Detection of latent DNA, Flinders University, College of Science and Engineering

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Latent DNA present on items or surfaces at crime scenes is invisible and therefore it is not possible to locate and collect this potentially crucial forensic evidence. Forensic practitioners, therefore, collect latent DNA based on their assumption where contact may have been made. The chosen media for collection of DNA, such as a swab or a tape, regularly contain no DNA or insufficient amounts of DNA for subsequent analysis. Consequently, low success rates of DNA profiling from latent DNA samples have been reported internationally.

Over the past decades, a number of forensic DNA technologies have been developed that increase the sensitivity of DNA profiling, such as improved enzymes and buffers, although the traditional blind collection of latent DNA on evidential material has not altered. Prior to this project, a range of potential fluorescent nucleic acid dyes for detection biological samples have been reported. Visualisation of latent DNA and dye complex required a standard fluorescence microscope; these are non-mobile, require the sample be on a microscope slide, and can only be performed in a darkroom (Chapter 1). Prior to this project, it was impossible to visualise latent DNA on forensic relevant items at a scene via a portable system that can be used in ambient light; as a result of research presented here, this is now possible.

This thesis outlines the first development of latent DNA detection using the fluorescent nucleic acid dye “Diamond™ Nucleic Acid Dye” (DD) for staining and visualised by a portable microscope “Dino-Lite” with fluorescence capability. This technique was tested on various biological samples (e.g. hair, saliva and fingermark) from which an optimised dye solution for use on different substrate types was developed. Touch DNA within fingermarks could be visualised for the first time using this technique. This project confirmed that these visualised residues were human cells, specifically corneocytes. The technique was applied to detect latent touch DNA on a range of items encountered typical in forensic investigations such as bullet cartridges, mobile phones and credit cards (Chapter 2, Publication I).

DD staining now allows cellular material visualisation allowing a targeted approach to latent DNA collection, however, the question remained “How many cells are required to generate an informative DNA profile?”. Chapter 3 (Publication II) reports on examination how many visualised cells are required to generate an STR DNA profile when recovered from either saliva or touch samples and processed through various processes for collection (swabbing/tapelifting) and analyses (DNA extraction/direct PCR). The recommended number of visualised buccal cells and corneocytes for each workflow are reported in this chapter. The correlation between the number of visualised cells and percent success of DNA profiling, and also correlations between the number of cells and total relative fluorescent unit (RFU) value, are reported.

There are several applications of forensic relevance incorporating the use of DD staining and visualised by the microscope (Chapter 4-6). First, in Chapter 4, this method can be used to assess key factors affecting touch DNA deposition: shedder status (Publication III); the speed of accumulation of cells on an individuals’ hand (Publication IV); and deposited by different body parts such as handprints (Publication V) and lip-prints (Publication VI). Second, in Chapter 5, this method has been tested to determine whether or not detection of latent DNA is possible on realistic items such as fingermarks after fingermark enhancement, and vice versa (Publication VII); and touched samples that have been placed outside exposed to the environment up to four weeks (Publication VIII). Furthermore, Chapter 6, describes whether the detection of latent DNA can be applied as a method for triaging to determine whether there are sufficient cells present on collecting media such as swabs (Publication IX) and tapes (Publication X and XI).

The visualisation of latent DNA, achieved through DD staining and visualised by a Dino-Lite digital microscope, is a novel technique for whether the targeting latent DNA on evidence types prior to sampling or even on collecting media after sampling. This technique is simple to perform, rapid, inexpensive, portable and can be performed in ambient light. It provides a real potential to be a routine step in a forensic workflow to improve greatly means of targeting and collecting the challenging invisible biological evidence at evidence recovery laboratories and crime scenes. The implications are immense for both evidence recovery and crime scene investigations.

Keywords: latent DNA, shedder status, Diamond dye, touch DNA

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Prof. Adrian Linacre