Development of a Simple Cell Harvesting Method to Maximise DNA Recovery from Historic Microscope Slides for Sexual Assault Investigations

Abstract

Microscopy slides are routinely created as part of sexual assault workflows for screening purposes and retained indefinitely with instances, such as cold cases, where they are the only remaining source of evidence. To date, no method has been developed to harvest the cells from these slides for differential extraction using the i-sep® DL column or Intimate extraction using the PrepFiler™ Express chemistry supplemented with 60 mM DTT. This study used mock sexual assault slides from 2010 to develop a potential cell harvesting method, then tested both the extraction methods on historic casework slides collected in the 1980s and 1990s. Key findings included the necessity to re-screen slides microscopically to assess current cellular loading and the utility of using phase contrast to enhance visualisation of spermatozoa on historic slides. Both extraction methods successfully recovered DNA and generated partial profiles from the 1990s slides, but 1980s slides were too degraded to provide informative profiles. Differential extraction provided additional contextual information by separating contributors into sperm and non-sperm fractions, resulting in cleaner sperm profiles for interpretation. The study successfully established methods for historic microscope slides to be used as viable sources of evidence, potentially aiding in the progression and resolution of sexual assault investigations.