MicroRNA Stability in Postmortem FFPE Tissues: Quantitative Analysis Using Autoptic Samples from Acute Myocardial Infarction Patients

MicroRNAs (miRNAs) are very short (18-24 nucleotides) nucleic acids that are expressed in a number of biological tissues and have been shown to be more resistant to extreme temperatures and pH compared to longer RNA molecules, like mRNAs. As miRNAs contribute to diverse biological process and respond to various kinds of cellular stress, their utility as diagnostic biomarkers and/or therapeutic targets has recently been explored. Here, we have evaluated the usefulness of miRNA quantification during postmortem examination of cardiac tissue from acute myocardial infarction (AMI) patients. Cardiac tissue was collected within one week of the patient's death and either frozen (19 samples) or fixed in formalin for up to three years (36 samples). RNA integrity was evaluated with an electropherogram, and it appears that longer RNAs are fragmented after death in the long-term fixed samples. Quantitative PCR was also performed for seven miRNAs and three other small RNAs in order to determine the appropriate controls for our postmortem analysis. Our data indicate that miR-191 and miR-26b are more suitable than the other types of small RNA molecules as they are stably detected after death and long-term fixation. Further, we also applied our quantitation method, using these endogenous controls, to evaluate the expression of three previously identified miRNA biomarkers, miR-1, miR-208b, and miR-499a, in formalin-fixed tissues from AMI patients. Although miR-1 and miR-208b decreased (1.4-fold) and increased (1.2-fold), respectively, in the AMI samples compared to the controls, the significance of these changes was limited by our sample size. In contrast, the relative level of miR-499a was significantly decreased in the AMI samples (2.1-fold). This study highlights the stability of miRNAs after death and long-term fixation, validating their use as reliable biomarkers for AMI during postmortem examination.