Storage Stability of Solutions of DNA Standards

High accuracy, reliability, and reproducibility of genetic analyses in various applications require optimized and validated protocols and standards. Optimal procedures for storing the genetic material extracted from biological samples are equally important. In this study, we investigated the stability of dilute (4000 cp/μL, nominal concentration, equivalent to 0.02 ng/mL) DNA solutions stored at 4, -20, and -80 °C in the presence or absence of nucleic acid carriers. As representative examples, we used different formulations of a linearized plasmid DNA solution considered for characterization as reference materials (RMs) for specific applications. Employing droplet digital PCR, a highly accurate and precise method for quantification of nucleic acid not requiring a calibrant, we demonstrated that inclusion of a carrier nucleic acid in the formulation (at 50 ng/μL) improved the plasmid stability at -20 and -80 °C. For the case of a DNA standard used in real-time PCR assays for human erythropoietin gene, cDNA or transcript, we found that inclusion of yeast RNA in the formulation was preferred over salmon testes DNA as it had no effect on PCR amplification and provided the lowest relative expanded uncertainty for the characterized RM. RNA background may also be preferred as it is applicable to a broader range of DNA RMs. Our findings are important in production of reliable, stable DNA standards, including DNA RMs. These results can be used when selecting protocols for stable storage of DNA either extracted from biological samples or synthesized in a laboratory.