Nucleic Acids from Human Nails and the Post-mortem Interval

Abstract

The determination of the time since death (post-mortem interval, PMI) is often critically important to homicide investigations. Many techniques have been developed for determining the PMI however these depend upon taphonomic influences that can greatly alter the rate at which decay occurs. One possible solution has been proposed; the use of nucleic acid decay in the determination of the PMI. This project explored using DNA, ribosomal RNA (rRNA) and messenger RNA (mRNA) from human nail material to infer the PMI. Nail material is relatively resistant to decay compared to soft tissue and is easy to collect in a non-invasive manner, but has not been used extensively in forensic science. This work has shown it is possible to co-extract RNA and DNA from human nail material, with a quantitative PCR assay developed to examine the decay of the nucleic acids over time using the KRT31 and KRT86 DNA genes and the 18S rRNA, mRNA KRT85 and mRNA KRT86 transcripts. This project has shown, using nail clippings from live participants and cadavers, that nucleic acids in nails degrade over time in a detectable manner. Variation is seen between decay in fingernails exposed to air, soil and water, between fingernails and toenails and between the nail clippings from live participants and cadavers, although none of these differences are statistically significant. Additional testing showed that there are different amounts of nucleic acid within adult and child nails although the relative stabilities were similar. Nail polish and nail polish remover were shown to have a detrimental effect upon the nucleic acid quantity and quality extracted from human nail material. Statistical regression developed models for the prediction of the PMI, measured in days since death, for live participant clippings exposed to air and soil and cadaver nails, with the respective adjusted R2 values of 0.738, 0.528 and 0.499. These parsimonious models contained a range of factors and a substantial amount of statistical error reflective of the exploratory nature of the work. This indicates that, with further development and research, the rate of nucleic acid decay in human nail material has potential for use as a PMI determinant.