Abstract

Genetic profiling of sperm from complex biological mixtures such as sexual assault casework samples requires isolation of a pure sperm population and the ability to analyze low abundant samples. Current standard procedure for sperm isolation includes preferential lysis of epithelial contaminants followed by collection of intact sperm by centrifugation. While effective for samples where sperm are abundant, this method is less effective when samples contain few spermatozoa. Laser capture microdissection (LCM) is a proven method for the isolation of cells biological mixtures, even when found in low abundance. Here, we demonstrate the efficacy of LCM coupled with on-chip low volume PCR (LV-PCR) for the isolation and genotyping of low abundance sperm samples. Our results indicate that this method can obtain complete profiles (13–16 loci) from as few as 15 sperm cells with 80% reproducibility, whereas at least 40 sperm cells are required to profile 13–16 loci by standard ‘in-tube’ PCR. Further, LCM and LV-PCR of a sexual assault casework sample generated a DNA genotype that was consistent with that of the suspect. This method was unable, however, to analyze a casework sample from a gang rape case in which two or more sperm contributors were in a mixed population. The results indicate that LCM and LV-PCR is sensitive and effective for genotyping sperm from sperm/epithelial cell mixtures when epithelial lysis may be insufficient due to low abundance of sperm; LCM and LV-PCR, however, failed in a casework sample when spermatozoa from multiple donors was present, indicating that further study is necessitated.

Highlights

  • Genetic profiling in forensic science often requires the isolation of a population of cells from complex biological mixtures

  • We demonstrate the efficacy of Laser capture microdissection (LCM) coupled with on-chip low volume PCR (LV-PCR) for the isolation and genotyping of low abundance sperm samples

  • We have previously demonstrated the utility of on-chip low volume PCR (LV-PCR)

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Summary

Introduction

Genetic profiling in forensic science often requires the isolation of a population of cells from complex biological mixtures. The DNA profiling following FISH/LMD, requires a minimum of seventy-five diploid cells in the 50 mL of in-tube PCR reaction [1]. Laser capture microdissection (LCM) is an effective method for the isolation of low abundant cells from biological mixtures, such as postcoital vaginal swabs and chorionic villi of abortion material [2,3]. The difference in size and morphology of sperm compared to vaginal epithelium makes LCM an ideal method to isolate sperm from sexual assault cases. Retrospective analysis of mock casework samples validated genetic profiles obtained following LCM cell isolation in many cases [4,5,6,7]

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