Use of laser microdissection greatly improves the recovery of DNA from sperm on microscope slides

Traditionally, sperms are isolated from vaginal cell mixtures by preferential extraction methods. Although these methods work well when there is a reasonable amount of DNA present, they are problematic when there are limited amounts (ca. 250 pg). In particular, the analysis of sperm from microscope slides has proven difficult. Here, we describe the use of laser capture microdissection (LM) for the isolation of spermatozoa from microscope slides containing sperms and vaginal cells. Such slides are frequently an important source of evidential material during the forensic investigation of rape and other sexual assaults. Low copy number (LCN) PCR was used to compare profiles of sperm DNA prepared using LM and preferential lysis. LM was found to outperform preferential lysis in 15 out of 16 samples. The application of LM to the processing of actual casework slides, and in particular the potential use of LM for the analysis of old cases, is discussed. Finally, 77 post-coital slides were processed in order to accurately assess the robustness of the technique. There was a significant association between the quality of the male profile recovered and time since intercourse that was independent of the number of sperms analysed, suggesting that the DNA was degraded even though the spermhead was intact.