Testing Hormone-specific Antibody Probes for Presumptive Detection and Separation of Contributor Cell Populations in Trace DNA Mixtures

Abstract

"Touch" or trace DNA evidence represent a significant proportion of samples analyzed by forensic science laboratories. Because these samples frequently contain multiple contributors and are often challenging to analyze due to low DNA concentrations and frequent degradation, front end techniques to simplify the mixture prior to DNA profiling could significantly impact case processing and enhance success rates. The goal of this study was to investigate whether targeting hormone molecules within the cell with antibody probes could be used to selectively label and then physically isolate contributor cell populations in trace biological samples. The separation of male and female cells into distinct fractions could reduce the complexity of the mixture prior to DNA profiling. To accomplish this, we first tested the specificity of fluorescently labelled anti-testosterone and anti-dihydrotestosterone antibody probes to epidermal cells from both male and female individuals. Results show that male and female cell populations can be effectively labeled using anti-testosterone and anti-dihydrotestosterone antibody probes and that distinct differences in binding efficiency and resulting median fluorescence of cell populations were observed between several individuals. These differences were then used to design sorting criteria for physically isolating each cell population in two-person epidermal cell mixtures using fluorescence activated cell sorting (FACS). DNA profiling of separated fractions in combination with probabilistic modeling demonstrated that some cell mixtures could be enriched for one contributor in separated cell fractions and yielded statistically more discriminating profiles compared to those generated from the original mixtures. Other mixtures tested showed less evidence of effective cell separation possibly due to a number of factors including imbalance of contributor DNA ratio, intra-sex variation of antibody binding efficiency, and contributions of extracellular or cell-free DNA in the mixture sample. Screening and separation of trace DNA samples with this approach may be presumptive and ultimately constrained by specific parameters of the original mixture, however, antibody binding optimization may mitigate some of these influences.