VALIDATION OF REFERENCE microRNAs FOR NORMALIZING EXPRESSION DATA GENERATED BY QUANTITATIVE PCR

Abstract Small noncoding RNAs such as microRNAs (miRNA) act as posttranscriptional regulators of numerous gene targets. The level of expression of these epigenetic factors can regulate the stability and translation of target mRNA molecules. Quantification of miRNA expression levels can therefore help us understand biological processes such as those regulating oocyte and pre-implantation embryo development. To date, most studies that have examined miRNA expression levels have used expression of either reference mRNAs or U6 spliceosomal RNA or miRNA Let-7a for normalization. We analysed the suitability of several miRNAs as potential expression normalizers in bovine oocytes and early embryos. Bovine oocytes at the germinal vesicle and metaphase II stages, and zygotes, 2-cell, 4-cell and 8-cell embryos, morulae, and blastocysts were collected during three in vitro fertilization replicates. qPCR was performed to quantify expression of miR-93, miR-103a, miR-26a, miR-191, miR-23b, Let-7a, and U6. The average starting material for each sample was determined with the help of specific standard curves made for each primer set. Subsequently, geNorm software was used to identify a set of stably transcribed miRNAs. Surprisingly, U6 spliceosomal RNA and Let-7a, which are widely used to normalise miRNA expression levels, were not stably expressed and were therefore considered unsuitable for normalization. Stepwise removal to determine the optimal number of reference miRNAs identified miR-93 and miR-103a as the most stably expressed. It is concluded that the combination of miR-93 and miR-103a can be used for normalizing miRNA expression for qPCR experiments on bovine oocytes and pre-implantation embryos.