Josh P. Roberts has an M.A. in the history and philosophy of science, and he also went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology.
It’s a maxim as old as molecular biology itself: You don’t store DNA on the bench. DNA typically is stored frozen, at -20°C, -80°C or even in liquid nitrogen, requiring significant capital and recurring costs as well as space—not to mention a frantic scramble when freezers go down. Similarly, shipping frozen DNA with cooling blocks or dry ice is expensive, may require special paperwork and handling and runs the risk of spoilage if there is a delay.
But there is a warmer, simpler alternative: systems designed to store and ship DNA at room temperature. Here we explore your options.
Membranes
One way to preserve DNA without freezing it is to bind it to some kind of fibrous material, notes Natalia Ivanova, lead DNA scientist at the Biodiversity Institute of Ontario at the University of Guelph.
Whatman FTA cards, for example, employ a coated cellulose membrane into which the DNA will entwine. Starting sample can be as varied as blood, bacterial cultures or purified DNA, and the chemical coating will lyse the cells and protect the sample DNA from nucleases and oxidative, ultraviolet and microbial degradation. According to the product literature (PDF) from GE Healthcare Life Sciences, which now owns Whatman, “data from genomic DNA stored on FTA cards at room temperature shows DNA stability of over 20 years for blood samples.”
But Ivanova cautions that “eluting the DNA is not that easy,” requiring a series of lab-based washes to get rid of the contaminants; instead, punches typically are taken from the FTA cards and used directly in downstream applications such as PCR.
IntegenX’s GenPlates are microtiter plates containing a 6-mm FTA disc in each well; they are available in various configurations, such as three 96-well regions, six 40-well regions and 12 16-well regions, for example. After samples are added to the wells and dried, the plate is sealed with an adhesive cover for storage or transport. GenPlates can be used with commercially available liquid handlers to aid with automation.
Other products, such as Macherey-Nagel’s NucleoCard®, can similarly be used to lyse cells and to bind DNA.
Whatman recently introduced a new line of FTA Elute cards. Here, in contrast to standard FTA cards in which the DNA remains bound to the matrix, “the chemistry of FTA Elute allows the proteins[, from lysed cells, for example,] to remain tightly bound to the card matrix,” says the product brochure (PDF), enabling “a quick and easy elution step involving water and heat to obtain a solution of purified, single-stranded DNA.”
Glass
Anhydrobiotic organisms such as brine shrimp and tardigrades can survive century-long droughts by preserving their DNA and other biomolecules in a glassine form—essentially shrink-wrapping them. In the presence of high concentrations of disaccharide sugars such as trehalose, the organisms are able to reversibly stabilize these polar molecules through hydrogen bonding.
Trehalose and the polymeric polyvinyl alcohol (PVA) have been used successfully for cryopreservation of nucleic acids in forensics and other laboratory sciences. Ivanova and Masha Kuzmina tested DIY versions of these, and the commercially available synthetic Biomatrica DNAstable® product, for their relative abilities to preserve DNA for four years [1]. They found that storage on DNAStable plates provided better protection at room temperature and 56°C (to emulate accelerated aging) than the DIY solutions, but that all were sufficiently stable for shipping purposes.
Biomatrica’s DNAStable is available is a variety of formats, including dried down in tubes and 96- and 384-well plates to which DNA is added, as well as a liquid (DNAStable LD) that can be added to samples in the customer’s labware of choice. The samples are dried under a laminar flow hood or using a vacuum concentrator. Special apparatus isn’t required to store the samples, notes Michael Vengrow, marketing specialist at Biomatrica. “However, for the sake of erring on the side of caution, we provide moisture bags and some desiccants with the products.”
IntegenX offers a similar product, GenTegra DNA, as does Qiagen with its QIAsafe (discontinued at the end of 2013). For both DNAStable and GenTegra DNA, recovery simply involves reconstituting the sample, typically in water. No cleanup or purification is required after rehydration, and the proprietary ingredients apparently do not affect downstream applications such as PCR, sequencing and microarrays, writes UCSF’s Eunice Wan and her colleagues in a 2009 publication [2]. However, they do alter the A230/280 ratio, presumably because the products themselves preferentially absorb at 230 nm.
Biomatrica recently introduced a new product, DNAStable Plus, which allows the sample to be stabilized and stored in liquid format for up to six months before being used or dried down, Vengrow notes. The company also offers products specifically formulated for RNA.
Sampling
Freeze/thaw cycles are known to have a deleterious effect on DNA integrity, and researchers generally are encouraged to minimize freeze/thaw by preparing samples in aliquots.
DNA stored dry at room temperature on Whatman cards can be “reused” by taking a fresh punch from the card each time. Similarly, individual wells from GenPlates can be sampled without disturbing other wells.
In the case of chemically preserved DNA, “you’re able to do multiple hydration/dehydrations. We give recommendations for how many in each of the protocols that we have,” Vengrow says. Storing dried DNA in aliquots can alleviate the issue, but, warns Ivanova, dilute samples don’t hold up to multiple hydration/dehydrations as well as more concentrated ones.
Bottom line
For labs with overcrowded freezers or archives of samples from post-docs long past, room-temperature storage systems may seem a welcome relief. But Ivanova doesn’t recommend moving all your existing samples to room-temperature storage just yet. The data just aren't all in. “For really long-term storage, I would still go with -80 [°C],” she says. But for labs that don’t have such a freezer, “storing DNA in a dry condition can add an additional layer of security at -20 [°C] storage, as well.”
References
[1] Ivanova, NV, Kuzmina, ML, “Protocols for dry DNA storage and shipment at room temperature,” Molecular Ecology Resources, 13:890-8, 2013. [PubMed ID: 23789643]
[2] Wan, E, et al., “Green technologies for room temperature nucleic acid storage,” Curr Issues Mol Biol, 12:135-42, 2009. [PubMed ID: 19801719]