The work of one of our students was featured in the University of Auckland's UniNews magazine. Read the story on Eletra Willaims which was reproduced with their permission below:
What Am I Discovering – Resolving Cell Mixtures
Crime is rarely clean. The very nature of violent and sexual crime means that biological samples recovered may be mixtures of multiple cell types from multiple donors. This causes two problems; where did the cells come from, and who?
These are two questions that forensic biologists strive to answer. Knowing the biological origin of samples, such as from semen, blood, vaginal fluid or saliva, can help rebuild the series of events which have occurred, and may confirm the occurrence of sexual assault. Knowing who the cells came from enables investigators to identify both the perpetrator and victim. Unfortunately, the biological samples recovered from crimes may be a mixture of different body fluids from different donors. Using genetic methods, it is possible to use DNA profiling and messenger RNA profiling to identify the individuals and body fluid origins of cells within the sample.
While there are statistical methods (such as ESR’s STRmix™) to analyse DNA mixtures, it is not possible to know which DNA profile is related to which body fluid, when multiple fluids are indicated by messenger RNA profiling. My research project has aimed to overcome this problem using a two-part approach; labelling forensically relevant cell types and then separating these labelled cells to enable DNA analysis of sorted single source isolates.
I have developed several methods for labelling forensic cell types using Fluorescent in situ Hybridisation (FISH). FISH is a technique where a fluorescent probe is used to label nucleic acids, such as DNA or messenger RNA, within the cell. This means that when the cells are viewed through a microscope using fluorescence excitation and specific filters, the cells fluoresce bright colours. If nucleic acids specific to one cell type can be targeted, we can cause the desired cell types to be easily distinguished from others within a mixture.
The main focus of my work has been considering epithelial cells from saliva and vaginal fluid, because these appear identical under the microscope, and their similar function in lining body cavities means that they have a lot of the same proteins and messenger RNAs. Although I have successfully developed protocols which label these cells, I am yet to identify targets which label either cell type specifically. Future research incorporating massive parallel sequencing may be able to achieve this by screening for markers on a huge scale.
The second aspect of my work is separating these labelled cells, which creates isolated single source samples and means that statistical analysis of mixtures is not required. I have compared two techniques, laser microdissection (LMD) and fluorescence activated cell sorting (FACS). Using FISH labelled forensic epithelial cells, I’ve successfully performed DNA profiling on 100 cells isolated using LMD and 150 cells isolated using FACS. This shows that the application of these techniques may be very useful to forensic casework, and the combination of FISH labelling with cell separation will greatly enhance the resolution of cell mixtures.