How many cells are required for successful DNA profiling?

Through advances in fluorescent nucleic acid dye staining and visualisation, targeted collection of cellular material deposited, for example by touch or within a saliva deposit, is possible. In regard to the potential evidentiary value of the deposit the questions remain: 'How many cells are required to generate an informative DNA profile?'; 'How many visualised corneocytes within a touch deposit compared to typical nucleated cells are required in order to achieve successful DNA profiling?'. Diamond TM Nucleic Acid Dye (DD) staining of cellular material, and subsequent visualisation utilising portable fluorescence microscopy, was performed for touch and saliva samples to target defined numbers of cells for collection, by swab and tapelift, and subsequent processing via direct PCR and PCR post-extraction. The resulting DNA quantification data and alleles generated within subsequent DNA profiles could be correlated to the number of cells initially collected to determine cellular threshold requirements for DNA profile generation for each workflow. Full profiles were consistently generated using direct PCR when the template was ≥40 buccal cells collected by either a swab or tapelift. By contrast ≥800 corneocytes collected by swabbing or ≥4,000 corneocytes collected by a tapelift were required to generate same number of STR alleles from touch samples. When samples were processed through a DNA extraction workflow, ≥80 buccal cells were required to generate full profiles from both swab and tapelift, while touch samples required ≥4,000 corneocytes collected by a swab and >8,000 corneocytes collected by a tapelift. The data presented within this study allow for informative sample triage and workflow decisions to be made to optimise STR amplification based on the presence and visual quantification of stained cellular material.
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