Evaluation of storage conditions on DNA used for forensic STR analysis

Abstract

Short tandem repeat (STR) analysis is currently the most common method for processing biological forensic evidence. STRs are highly polymorphic and allow for a strong statistical power of discrimination when comparing deoxyribonucleic acid (DNA) samples. Since sample testing and court proceedings occur months, if not years apart, samples must be stored appropriately in the event additional testing is needed. There are generally accepted methods to store DNA extracts long-term; however, one universally recognized method does not exist. The goal of this project was to examine various methods of storage and make recommendations for a universal storage method that maintained DNA integrity over time. Four variables were evaluated: storage buffer, storage temperature, initial storage concentration and the effects of repeated freeze-thaw cycles. DNA quantity was assessed using real-time polymerase chain reaction and DNA quality was evaluated using STR genotyping. Overall, the Tris-EDTA (TE) buffer outperformed nuclease free water as a long-term storage buffer for DNA extracts. Stock tubes stabilized concentration better than single use aliquots when eluted with TE while tube type was not significant when water was the buffer. For samples stored in TE, temperature had no effect on DNA integrity over time, but samples stored in water were largely affected at room temperature. Additionally, the greater the initial DNA concentration, the less likely it was to degrade in water. As a result of this research, DNA extracts from forensic samples should be stored long-term in TE buffer with a minimum concentration of 0.1 ng/μL. When water is the buffer, frozen storage is recommended.