A comparative study of different laboratory storage conditions and DNA extraction methods for enhanced forensic analysis of soil-human blood mixed sample

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June, 2018
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Crime scene investigation is an important step in the entire criminal investigation process because this is where evidence is gathered. Blood from the perpetrator or victim of a crime can be left at crime scenes or transferred to other materials such as clothing, knives and guns. Most often, this body fluid is contaminated with soil at outdoor crime scenes but this might be the only or the most important evidence in solving a crime. This study aimed at identifying the most appropriate method of storing crime scene soil-human blood mixed sample prior to analysis. The best DNA extraction method for this soil-blood mixed sample was also studied. Three commercial DNA extraction kits (PrepFiler Forensic DNA Extraction kit, Promega DNA IQ Kit, Blood Miniprep kit) that have been claimed by the manufacturers to be effective in extracting DNA from soil contaminated samples were used for the DNA extractions. Hemastix and Hexagon OBTI kits were used for the serological analysis in this study. Human blood was mixed with soil and stored at three different storage conditions (i.e., Room temperature/25℃, 4℃ and - 20℃). Hemastix and Hexagon OBTI serological tests for blood and human blood, respectively were positive for soil-blood mixed samples at all storage conditions throughout the 12 week study period. Samples stored at room temperature saw significant reduction in DNA concentration as storage time increased (P=0.001 and 0.0055 for Prepfiler and DNA IQ extractions, respectively). Samples stored at 4℃ saw a drastic decrease in DNA concentration just after two weeks of storage. By the eighth week of storage at 4℃, there was no detectable DNA (P=0.000 for all extraction methods). Samples stored at -20℃ recorded no specific pattern in decrease or increase in DNA concentration for the entire 12 week storage (P=0.324 and 0.161 for PrepFiler and DNA IQ extractions respectively). The PrepFiler kit yielded more DNA than the DNA IQ and Blood Miniprep kits at all storage conditions with no significant difference between PrepFiler and DNA IQ (P=0.603). The PrepFiler kit and DNA IQ kit were successful at removing possible PCR inhibitors from the soil during DNA extraction with no significant difference (p=0.887). The Blood Miniprep kit performed poor in terms of removing possible PCR inhibitors. There were full STR Profiles generated for room temperature stored samples and -20℃ stored samples extracted with PrepFiler and DNA IQ kits throughout the study. There were no allele recorded for room temperature stored samples and -20℃ stored samples extracted with Blood Miniprep kit. There were full, partial and null Profiles generated for 4℃ stored samples extracted with PrepFiler and DNA IQ kits depending on the sample storage duration. There were no alleles recorded for 4℃ stored samples extracted with Blood Miniprep kit. In conclusion, the -20℃ and PrepFiler Forensic DNA extraction kit were identified as the best storage condition and extraction method, respectively.
This thesis is presented to the Department of Biochemistry and Biotechnology in partial fulfilment of the requirements for the award of an Msc Degree in Forensic Science.