Determining postmortem interval (PMI), the time which has elapsed since a person has died, as exactly as possible is a vital component of many forensic cases. Currently, body temperature is the most accurate method of determining PMI, but depending on the environmental temperature, clothing, age, as well as a number of other factors, this method is only useful for up to 36 hours after death, leaving the door wide open to techniques which can accurately work after this time (1). So it’s no surprise that methods for determining PMI are constantly being re-evaluated and refined.
Now, a team headed by Peter Steinbacher at the University of Salzburg, Austria think they might have made a breakthrough. They have identified a series of changes in the presence and activity of a number of proteins present in skeletal muscle which could be used to accurately determine the PMI up to 10 days from the time of death (2). “The breakdown products are present for a specific time – so if you know which of these products are present... then you know when the individual died”, explains Steinbacher, whose work built upon pre-existing research into meat tenderness, which demonstrated that a number of muscle proteins reproducibly degenerated into the same products, even across different species (3,4). This degradation of muscle protein is down to a group of calcium-dependent proteolytic enzymes called calpains, which are activated once the integrity of the sarcolemma is compromised, increasing intracellular calcium. Using skeletal muscle has several benefits when it comes to forensics; it is the most abundant tissue in the body and comes with a greater delay in postmortem change when compared with other tissues in the body, vital for that post 36 hour PMI reading. Using Western blotting and SDS-PAGE gel electrophoresis, Steinbacher assessed the degradation and appearance of a number of muscle proteins over a period of 240 hours postmortem. The experiments revealed protein changes of varying significance; some proteins, such as titin dp2 appeared in a relatively large time range, whilst others, such as the degradation of titin 1, occurred in a much more limited timeframe. Steinbacher claims that when used in combination, these patterns could be used to effectively characterize certain time points, allowing a more precise determination of PMI. As a similar protein degradation pattern appears in several other vertebrate species, the research team analyzed over 60 human tissue samples from the university’s forensic department where they found related patterns. “Research with human samples is always very difficult, as there is no way to influence any variables or to standardize experimental conditions”, says coauthor of the associated study Pittner Stefan. Despite this, Steinbacher remains positive, claiming that once implemented, the accuracy will steadily improve as the database compiling the information expands. The team now intends to continue its work by examining the effect of other variables on pig muscle protein, including temperature, gender, body mass and humidity.
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References
- J Claridge, “Measuring body temperature”, available at: http://bit.ly/1JRCWaC. Accessed on July 29, 2015. P Steinbacher, et al., “Postmortem degradation of skeletal muscle proteins: a novel approach to determine the time since death”, Int J Legal Med, [Epub ahead of print], (2015). PMID: 26041514. M Szalata, et al., “Titin and troponin T changes in relation to tenderness of meat from pigs of various meatiness”, Pol J Food Nutr Sci, 14, 139–144, (2005). G Wu, et al., “LC MS/MS identification of large structural proteins from bull muscle and their degradation products during post mortem storage”, Food Chem, 150, 137–144, (2014). PMID: 24360430.
