DNA profiling of spermatozoa by laser capture microdissection and low volume-PCR

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.

Figure 1. Haploid-type electropherograms derived from a single sperm.

Panels A, B and C indicate electropherograms of replicate 1, 2 and 6 listed in Table 2. Red circles indicate allele dropout, arrows indicate allele drop-in. Allele call and peak height are shown under each peak.

Figure 2. Sperm cell separation and collection by LCM.

Samples were resuspended and placed on a PEN membrane slide. Sperm were identified by light microscopy (400× magnification), and removed by laser cutting. Finally, sperm cells were catapulted and collected onto a low-volume PCR slide.

Figure 3. Genetic profiling of sample No. 1.

(A) Preferential epithelial lysis followed by ‘in-tube’ PCR generated a mixed genetic profile from the casework sample. (B) Electropherogram profile of replicate 1 obtained by sperm isolation by LCM and profiling by LV-PCR. This method generated an individual profile with consensus to the known sample. Red circles indicate allele dropout.