Deleterious effects of formalin-fixation and delays to fixation on RNA and miRNA-Seq profiles

The National Cancer Institute conducted the Biospecimen Pre-analytical Variables (BPV) study to determine the effects of formalin fixation and delay to fixation (DTF) on the analysis of nucleic acids. By performing whole transcriptome sequencing and small RNA profiling on matched snap-frozen and FFPE specimens exposed to different delays to fixation, this study aimed to determine acceptable delays to fixation and proper workflow for accurate and reliable Next-Generation Sequencing (NGS) analysis of FFPE specimens. In comparison to snap-freezing, formalin fixation changed the relative proportions of intronic/exonic/untranslated RNA captured by RNA-seq for most genes. The effects of DTF on NGS analysis were negligible. In 80% of specimens, a subset of RNAs was found to differ between snap-frozen and FFPE specimens in a consistent manner across tissue groups; this subset was unaffected in the remaining 20% of specimens. In contrast, miRNA expression was generally stable across various formalin fixation protocols, but displayed increased variability following a 12 h delay to fixation.