Estimating the age of biological evidence recovered from a crime scene by assessing RNA degradation in the sample

The forensic investigation of crime involves answering four basic questions: who, what, where, and when.  Tools exist to answer questions of who, what and where, but the question when is often important in criminal investigations and is the one question for which tools don’t exist to produce an answer. If a suspect or victim of a crime frequents the scene of a crime because they reside there, it might be expected that their DNA could be found.  If it were possible however to establish that the age of a stain found in the house coincided with the time a crime was committed, the value of the stain for the investigation is strengthened.  It is also often impossible to estimate the time of death for a victim of homicide whose skeletal remains are found in a clandestine grave. For the past several years we have been working on developing a method to estimate the age of body fluid stains through the use of molecular methods.  The approach we have taken is to use the extent of degradation of a collection of messenger RNA (mRNA) molecules present in blood or semen stains.  We have shown that RNA degradation is predictable and can be used to create standard degradation curves that enables the age of an unknown stain to be estimated with high accuracy.  In more recent studies, the effect of environmental conditions have been investigated on the rate of RNA degradation.  Both heat and relative humidity accelerate degradation, and current efforts are aimed at incorporating changes in the degradation rate constant due to environmental conditions into an algorithm for estimating sample age. Early studies are also underway to explore rhythmic changes in gene expression during the daily cycling of males and females of varying age.  The goal of these studies is to explore the possibility of determining the time of day biological evidence was created at a crime scene.

Learning Objectives:

1. Be able to describe the nature of RNA degradation in dried blood stains

2. Understand the environmental effects of heat, relative humidity, and substrate, on the rate of RNA degradation in a dried blood stain

3. Understand the relationship between gene expression during the circadian rhythm and estimating the time of day a blood stain was created