A few weeks ago, I posted an article about best practices for long term storage of unpurified Oragene•DNA samples on The Genetic Link. The response to the article was positive so I'm following up with today's story about long term storage of DNA purified from Oragene•DNA samples. The preservation and storage of DNA is an important consideration for molecular epidemiology and population studies. As the makers of the Oragene•DNA family of products, we have prepared these recommendations for the long-term storage of DNA purified from saliva samples collected with Oragene to help our customers achieve optimal results.
Preventing DNA Degradation
There are three major causes of DNA degradation in a purified sample (ref. 1). Samples may be accidentally contaminated by bacteria, but storage at 4°C or lower will minimize bacterial metabolism and the release of nucleases. DNases may be inadvertently introduced from the skin, but this can be minimized by wearing gloves when handling samples. Repeated cycles of freezing and thawing may also contribute to DNA degradation. This may be minimized by splitting the purified DNA into multiple aliquots and thawing one at a time.
There are two major causes of degradation of purified DNA:
- Acid hydrolysis - due to storage in a solution with no buffering capacity, explained in "Comparison of TE and water" section
- Nuclease activity - DNases may come from the skin of the person handling the sample, contaminated labware or bacterial contamination of the purified sample. Use of gloves can minimize contamination from the person and use of nuclease-free labware and working in a clean environment can prevent both nuclease and bacterial contamination. Additionally, inclusion of a metal ion-chelating agent, such as EDTA, in the storage buffer can effectively inhibit the activity of any DNases present in the sample.
Comparison of TE and water
Kasper and Lenz (ref. 2) performed an 8-year study of DNA stored in water or Buffer AE (10 mM TrisHCl; 0.5 mM EDTA, pH 9.0). DNA stored in Buffer AE at -20°C or 2-8°C showed no degradation by gel electrophoresis and amplified well in a PCR assay. DNA in water remained intact when stored at -20°C but samples were degraded when stored at 2-8°C and performed poorly in a PCR assay. Pure water lacks buffering capacity and an acidic pH may lead to DNA hydrolysis.
An EU workshop on Biobanks (ref. 3) recommends freezing DNA samples to prevent bacterial contamination and to minimize evaporation of the sample. Tris-EDTA (TE) buffer contains sufficient buffering capacity to prevent acid hydrolysis of DNA. Similarly, the UK Biobank (ref. 4) recommends the storage of DNA in a nuclease-inhibiting environment at a temperature of -20°C, since no significant increase in stability is observed at temperatures below -20°C.
We hope these best practices will help you optimize the long term storage of all your purified Oragene samples.
 O'Brien, D. (2002). High-throughput DNA purification. Modern Drug Discovery. 5(3), 25-26.
 Kasper, Y. et Lenz, C. (2004). Stable 8-year storage of DNA purified with the QIAamp DNA Blood Mini Kit. QIAGEN News. 2004 e10.
 Biobanks for health: Optimising the use of European biobanks and health registries for research relevant to public health and combating disease. Report and recommendations from an EU workshop held at Voksenåsen Hotel, Oslo. January 28-31, 2003.
 Sample handling and storage: Subgroup protocol and recommendations. Version 1.0. UK Biobank. July 7, 2004