Circulating
miRNAs are intensively evaluated as promising blood-based
biomarkers. This growing interest in developing assays for circulating
miRNAs necessitates careful consideration of the effects of preanalytical and analytical parameters on the isolation, stability, and quantification of circulating
miRNAs. By using quantitative stem-loop RT-PCR, we compared the relative efficiencies of four
miRNA isolation systems and different storage conditions. The effect of the data normalization procedure on the quantification of circulating
miRNA levels in plasma from 30 healthy individuals and 30 patients with
non-small cell lung carcinoma was estimated by measuring endogenous hsa-miR-21 and hsa-miR-16 and exogenous cel-miR-39 that was spiked in all samples at the same concentration.
Silica column-based
RNA extraction methods are more effective and reliable with respect to
TRIzol LS. Endogenous circulating
miRNA levels are unstable when plasma is stored at 4°C, and samples should be kept at -70°C, where the extracted
miRNAs remain stable for up to 1 year. When normalization is based on combined endogenous and exogenous control
miRNAs, differences in
miRNA recovery and differences in
cDNA synthesis between samples are compensated. Using this normalization procedure and hsa-miR-21 as a
biomarker, we could clearly discriminate healthy individuals from patients with
cancer. Experimental handling and the use of exogenous and endogenous controls for normalization are critical for the reliable quantification of circulating
miRNA levels in plasma.