Stability of microRNAs in Canine Serum—A Prerequisite for Use as Biomarkers in Tumour Diagnostics

Since microRNAs are released into the bloodstream and miRNA profiles are supposed to differ between healthy and tumour patients, miRNAs seem to have potential as biomarkers. An essential prerequisite for biomarkers in a routine diagnostic setup is their stability in serum over time. In this study, serum samples from 10 healthy dogs were analysed at different time points and under various temperature conditions (after 24 and 48 h, at 4° or 20 °C) for the copy number of eight miRNAs (miR-20b, 21, 122, 126, 192, 214, 222, 494) using ddPCR. The miR-21 had the highest copy number, whereas miR-494 had the lowest copy number in canine blood samples. The values of each miRNA varied individually between the dogs, showing a 5 to 10-fold range. Stability differed between the miRNAs, with miR-192 having the best stability. The copy number of miR-20b, miR-126 and miR-214 decreased not significantly during 48 h storage time. In contrast, miR-21, miR-122 and miR-222 were stable for 24 h only but decreased significantly after 48 h. The (in)stability of individual canine miRNAs must be considered when transferring study results into veterinary routine diagnostics, as the transport and storage conditions are variable. As far as possible, standardisation of sampling, storage and quantification of miRNAs is needed.