Evaluation of miRNA expression in 14 different tissues by the RAKE technology

The short length of miRNAs results in a high dynamic range of melting temperatures and therefore impedes a proper selection of detection probes or optimized PCR primers. While miRNA microarrays allow for massive parallel and accurate relative measurement of all known miRNAs, they have so far been less useful as an assay for absolute quantification. Here we developed a new method based not only to the hybridization process that presents the limits before described, but integrating the hybridization to an enzymatic reaction. Moreover we introduced spike-in in the hybridization-enzymatic reaction allowing the quantification of miRNAs respect to them, canceling biases related to sequence, labeling, or hybridization. An alternative method for the absolute miRNA quantization was recently proposed by Bissels (Absolute quantification of microRNAs by using a universal reference. RNA). It was based on the Absolute quantification of microRNAs by using a universal reference consisting of 954 synthetic human, mouse, rat, and viral miRNAs, with each individual oligoribonucleotide present in equimolar concentrations with tested miRNAs. Thereby, any single miRNA detected on a microarray can be quantified by directly comparing its signal intensity with the one obtained by the same miRNA sequence present in the universal reference adjusting for biases related to sequence, labeling, hybridization, or signal detection. Our method allowed the detection of a comparable concentration of miRNA (10-18 moles to 10-14 moles in a linear range) (see Figure), but allows controlling the hybridization quality and reproducibility basing on the results of the interpolation of the spike-in dependent curve. Moreover, our method does not influenced by phenomena imputable to different labeling process due to different sequences because labeling was due only to the incorporation of biotin-d(A) if the hybridized miRNA acted as primer for the klenow enzyme. This method allowed the discussion of miRNA genes expression in 14 different tissues relating it with tissue anatomical proximity and functional similarity. 2K microarray was hybridized with small RNAs from 14 different tissues (Atrium Sinister; Skin; Liver; W_B_A: White Blood Cells from Arteriosum blood; W_B_V: White Blood Cells from Venosum blood; Lymph Node; Tongue; Spleen; Skeletal Muscle; Lung; Kidney; Stomach; Adipose Tissue; Ventricle Sinister). Each experiment was replicated to have 28 experiments and miRNA gene expression was correlated with mRNA gene expression in the same samples.