RNA-Integrity and 8-Isoprostane Levels Are Stable in Prostate Tissue Samples Upon Long-Term Storage at -80°C

Sampling of prostate tissue (n = 97) was performed in conjunction with planned radical prostatectomies, in collaboration with Biobank1^®. The tissue used in this study was collected during the period 2003-2016, quickly frozen, and kept at -80°C until assayed in 2018. RNA extraction was performed with two different protocols (miRNeasy and mirVana™), and RNA quality was determined by measuring the RNA Integrity Number (RIN). The level of isoprostanes is widely recognized as a specific indicator of lipid peroxidation both in vitro and in vivo. The level of 8-isoprostane was measured because it is the main oxidation product of arachidonic acid, the most abundant phospholipid fatty acid. The level of 8-isoprostane was measured using enzyme immunoassay. There was no statistically significant difference in yield between the samples isolated with the mirVana protocol compared to the miRNeasy protocol. Average RIN was 2.8 units higher with the mirVana extraction protocol compared to the miRNeasy protocol (p < 0.001). For miRNeasy extractions, RINs were 7.1 for prostatectomies in 2005-2007 and 6.2 for those in 2018 (p < 0.001). For mirVana extractions, the difference in RIN score between the two groups regarding years of collection was not statistically significant. There was no significant increase in the levels of 8-isoprostane between the 2005-2007 samples and the 2018. The conclusion is that there is no oxidation of phospholipids with increasing storage time up to 15 years.