High preservation of DNA standards diluted in 50% glycerol

Standard curves are important tools in real-time quantitative polymerase chain reaction (PCR) to precisely analyze gene expression patterns under physiologic and pathologic conditions. Handling of DNA standards often implies multiple cycles of freezing and thawing that might affect DNA stability and integrity. This in turn might influence the reliability and reproducibility of quantitative measurements in real-time PCR assays. In this study, 3 DNA standards such as murine tumor necrosis factor (TNF) alpha, interferon (IFN) gamma, and kainat-1 receptor were diluted in 50% glycerol or water after 1, 4, and 16 cycles of freezing and thawing and amplified copy numbers after real-time PCR were compared. The standards diluted in water showed a reduction to 83%, 55%, and 50% after 4 cycles, to 24%, 5%, and 4% after 16 cycles for kainat-1 receptor, TNF alpha, and IFN gamma standards, respectively, when compared with a single cycle of freezing and thawing. Interestingly, all cDNA samples diluted in 50% glycerol were amplified in comparable copy numbers even after 16 cycles of freezing and thawing. The effect of the standards undergoing different cycles of freezing and thawing on sample values was demonstrated by amplifying cDNA obtained from Borna disease virus infected and noninfected TNF-transgenic mice brain. This revealed significant differences of measured cDNA copy numbers using water-diluted DNA standards. In contrast, sample values did not vary using glycerol-diluted standards that were frozen and thawed for 16 times. In conclusion, glycerol storage of DNA standards represents a suitable tool for the accurate and reproducible quantification of cDNA samples in real-time PCR analysis.