Last Month's Most Accessed Feature: Postmortem interval: Forensic proteomic pointers

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  • Published: Jun 1, 2018
  • Author: Steve Down
  • Categories: Proteomics & Genomics / Proteomics
thumbnail image: Last Month's Most Accessed Feature: Postmortem interval: Forensic proteomic pointers

Estimating postmortem interval

A proteomics study of decaying bones has thrown up a number of proteins that might prove to be valuable markers of the skeletal postmortem interval as an aid to forensic scientists.

One of the most crucial aspects of forensic science when a human body is discovered is estimation of the postmortem interval (PMI): the time lapsed since the person died. For fresh deaths, factors like body temperature and lividity are a guide but different criteria come into play for older bodies that have been decaying for weeks, months or years. In these cases, the physical size of the person and their pathology and trauma affect the decomposition rate which is compounded by external factors such as temperature, humidity, burial conditions and scavengers.

For older specimens in which flesh is scarce, forensic scientists can resort to the bones because they too are subject to particular degradation processes. However, bones are also affected by environmental conditions that can skew the results. The same limitations apply to residual DNA.

Another option is to examine bone proteins to see if they could provide a better way to estimate PMI. Some proteins are tightly bound to the inorganic hydroxyapatite matrix that constitutes the majority of bone and protects them from degradation after death. The timescale can be very long: some proteoglycans have even been isolated from archaeological specimens. The bone proteome does become less complex over geological time but in the relative shorter term of a few years or decades, proteins might still be a solution to the PMI problem.

Bone proteomics

In 2017, a group of scientists in the UK published their findings on the use of bone proteins to estimate biological age at death following proteome analysis. They found one protein in particular, fetuin-A, that was negatively correlated with biological age, reducing in abundance with time. However, there are no published studies that correlate the proteome profiles of forensic bones with known PMIs.

So, Michael Buckley, Noemi Procopio and Andrew Chamberlain from The University of Manchester and Anna Williams from the University of Huddersfield undertook a search for protein biomarkers in bone from animal carcasses that had been buried for known amounts of time. Four juvenile pigs (Sus scrofa, the wild boar) of various ages, common surrogates for humans in decomposition studies, were buried together in a special facility and protected from scavengers and birds while allowing arthropods to carry out their normal activities.

After one, two, four or six months, one tibia from each pig was carefully removed with as little disturbance as possible. After one year, the carcasses were removed completely and a second tibia from each animal was taken for analysis. The proteins were extracted from portions of powdered, demineralised bones and processed by conventional methods for mass spectrometric analysis. The proteins were identified by database matching of their constituent tryptic peptides.

Protein candidate biomarkers

The first set of samples that were collected over six months generated almost 35,000 spectra which were refined to give a subset of 48 proteins which had statistically significant abundance variations between the four time periods. A principal components analysis clustered the four interval groups, indicating substantial changes in the bone proteome between months one and two, with minor changes thereafter up to month six.

From this set, 28 proteins fell in abundance between the first and fourth months but changed only marginally thereafter. This group was divided into four subclasses which behaved differently over time, decreasing at different rates over different periods.

One group consisted of plasma proteins: haemoglobin alpha and beta, transferrin, lactoferrin and triosephosphate isomerase. Haemoglobin has been the subject of the classic luminol test as well as the benzidine test, being sensitive over about 70 and 100-350 years postmortem, respectively. However, in the present study no haemoglobin peptides were detected after a PMI of one year, so its utility appears to be limited.

A second sub-group comprised mainly muscle proteins including two types of myosin, beta-enolase and creatinine kinase M-type, as well as the plasma protein haptoglobin. The third group included other plasma proteins as well as collagen type 5, matrix Gla protein and protein disulphide isomerase and the final group comprised annexin and fructose bisphosphate aldolase A.

A further time-related decomposition was observed with deamidations over six months of the asparagine residues in the protein biglycan which plays a role in bone growth and mineralisation. The protein was still observed after one year and the researchers believe that it could become a new biomarker for PMI over the short term.

The value of fetuin-A was also confirmed. It displayed no variation between individual subjects and was relatively stable after death, prompting the team to continue to support its use in estimating PMI.

Given the promising results in this proof-of-concept study, the potential of bone proteome analysis for estimating the time since death in forensic science has been emphasized. Future work will focus on expanding the PMI and investigating the effects of environmental conditions such as weather.

Related Links

Journal of Proteomics 2018, 177, 21-30: "Forensic proteomics for the evaluation of the post-mortem decay in bones"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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