On statistical analysis of forensic DNA: Theory, methods and computer programs

Identification of sample donor by 24-plex short tandem repeat in a post-race equine plasma containing dexamethasone

BACKGROUND

Animal sport such as horseracing is tainted with drug abuse as are human sports. Treatment of racehorses on race day with therapeutic medications in most cases is banned, and thus, it is essential to monitor the illicit use of drugs in the racing horse to maintain integrity of racing, ensure fair competition and protect the health, safety and welfare of the horse, jockeys and drivers. In the event of a dispute over the identity of the sample donor, if the regulator can provide evidence that the DNA genotype profile of the post-race sample matched that of the alleged donor, then the potential drug violation case might be easily resolved without legal challenges.

CASE DESCRIPTION

We present a case study of a racehorse sample that tested positive for dexamethasone in a post-race plasma sample in Pennsylvania (PA) but the result was challenged by the trainer of the horse. Dexamethasone is a synthetic glucocorticoid widely used in the management of musculoskeletal problems in horses but its presence in the horse during competition is banned by the PA Racing Commissions. The presence of dexamethasone in the post-competition plasma sample was confirmed using liquid chromatography-tandem mass spectrometry. However, this finding was challenged by the trainer of the horse alleging that the post-race sample was not collected from his/her horse and thus petitioned the Commission to be absolved of any wrong-doing. To resolve the dispute, a DNA test was ordered by the PA Racing Commission to identify the correct donor of the dexamethasone positive sample. For this purpose, a 24-plex short tandem repeat analysis to detect 21 equine markers and three human markers was employed. The results indicated that all the samples tested had identical DNA profiles and thus, it was concluded that the samples were collected from the same horse and that the probability of drawing a false conclusion was approximately zero (1.5 × 10(-15)).

CONCLUSIONS

The plasma sample confirmed for the presence of dexamethasone was collected from the alleged horse.

Current genetic methodologies in the identification of disaster victims and in forensic analysis

This review presents the basic problems and currently available molecular techniques used for genetic profiling in disaster victim identification (DVI). The environmental conditions of a mass disaster often result in severe fragmentation, decomposition and intermixing of the remains of victims. In such cases, traditional identification based on the anthropological and physical characteristics of the victims is frequently inconclusive. This is the reason why DNA profiling became the gold standard for victim identification in mass-casualty incidents (MCIs) or any forensic cases where human remains are highly fragmented and/or degraded beyond recognition. The review provides general information about the sources of genetic material for DNA profiling, the genetic markers routinely used during genetic profiling (STR markers, mtDNA and single-nucleotide polymorphisms [SNP]) and the basic statistical approaches used in DNA-based disaster victim identification. Automated technological platforms that allow the simultaneous analysis of a multitude of genetic markers used in genetic identification (oligonucleotide microarray techniques and next-generation sequencing) are also presented. Forensic and population databases containing information on human variability, routinely used for statistical analyses, are discussed. The final part of this review is focused on recent developments, which offer particularly promising tools for forensic applications (mRNA analysis, transcriptome variation in individuals/populations and genetic profiling of specific cells separated from mixtures).

Determining the source of equine bloodstains by dinucleotide repeats

A novel multiplex of independent dinucleotide tandem repeat (DTR) loci was previously described that is capable of not only discriminating human and equine DNA, but of identifying a single equine source. We report a case in which a bloodstained syringe and two needles were found during inspection of a barn by inspectors of the Pennsylvania Racing Commissions. Using the multiplex and single-locus detection, all 21 equine DTR markers were detected in a suspect horse and two evidence samples, indicating the evidence samples came from the suspect animal. Only six markers were detected in the third evidence sample because the volume of blood was limited. Following whole-genome amplification and single-locus PCR, the third evidence sample detected a total of 17 markers and the likelihood of identity (probability from suspect horse/probability from a random pacer) was 7.0 × 10⁶. The DTR multiplex has some technical limitations, but it is already practical for casework.