Novel cellular signatures for determining time since deposition for trace DNA evidence

Description:
With the increase in sensitivity of DNA profiling, questions about how and when the DNA was deposited have become a driving issue in forensic cases. To address this, we developed a novel method to determine time-since-deposition of trace DNA samples based on morphological and autofluorescence properties of epithelial cells which change as the sample ages. To develop this signature, a series of trace DNA samples were generated by contact/handling a substrate and then allowed to age anywhere between one day and more than one year prior to collection. Imaging flow cytometry (IFC) was then used to characterize the morphology and autofluorescence profiles of individual cells within each sample followed by multivariate modelling and predictive classification.
Results showed that epithelial cell populations could be classified with high accuracy (~90%) into one of three time-since-deposition groups: <1 week, between 1 week and 2 months, and >2months. To further test this approach for forensic casework, 47 individual donor cell populations spanning each time deposition group were classified blindly against the remaining data set. Samples containing at least 75 cells and a posterior probability greater than 0.90 showed classification accuracies ~95%. Classification accuracies for individual cells varied slightly across time groups: ~97% (<1 week), ~92% (1week-2months), and ~98% (>2 months). The average posterior probability for all time groups was ~0.96. This workflow was also tested in mock casework samples containing mixtures of epidermal cells representing multiple time-since depositions. Cell populations representing recent depositions (i.e., <4 days old) were successfully detected (>50 cells, posterior probability> 0.90) in each of the mixture samples when present. This indicates that autofluorescence and morphological analyses may provide probative information regarding time since deposition for many types of trace DNA samples in forensic casework.

Speaker: Christopher Ehrhardt - Virginia Commonwealth University
Christopher Ehrhardt is currently a Professor in the Forensic Science Department at Virginia Commonwealth University in Richmond VA. He received his PhD from UC-Santa Barbara in Earth and Environmental Sciences and completed postdoctoral appointments at the FBI Laboratory’s Research Unit (Quantico, VA) and the National Security Directorate at Pacific Northwest National Laboratory (Richland WA). At VCU, his research group studies the biochemistry, optical properties, and genetics of trace biological samples as well as front end methods for resolving cell mixtures for DNA casework.