Targeted RNA-sequencing of aged archival formalin-fixed paraffin-embedded tissue samples

Formalin-fixed paraffin-embedded (FFPE) samples are the only remaining biological archive for many toxicological and clinical studies, yet their use in genomics has been limited due to variability in RNA quality from formalin fixation. Older FFPE samples, which often have highly degraded RNA, pose a particularly difficult technical challenge. Recent developments in probe-based targeted sequencing technologies show promise in addressing this issue, while also being more cost effective than traditional RNA-sequencing (RNA-seq) methods. In the current study, we evaluated dose-dependent transcriptional changes from paired frozen and archival FFPE liver samples stored for over 20 years using targeted resequencing (TempO-Seq) and whole-genome RNA-seq methods. Samples were originally collected from male mice exposed to a reference chemical (dichloroacetic acid, DCA) at 0, 198, 313 and 427 mg/kg-day, (n=6/dose) by drinking water for 6 days. TempO-Seq showed high overlap in differentially expressed genes (DEGs) between matched FFPE and flash-frozen (FROZ) samples and concordance in fold-changes across the highest two dose levels of DCA vs. control (R2 = 0.94). Similarly, high concordance in fold-changes was observed between TempO-Seq FFPE and RNA-seq FROZ results (R2 = 0.92). Only 28 DEGs or fewer were identified in RNA-seq FFPE samples. Modeling of DCA-dependent changes in gene sets identified benchmark doses from TempO-Seq FROZ and FFPE samples that were within 1.4-fold of RNA-seq FROZ samples (93.9 mg/kg-d), whereas RNA-seq FFPE samples were 3.3-fold higher (310.3 mg/kg-d). This work demonstrates that targeted sequencing may provide a more robust method for quantifying gene expression profiles from aged archival FFPE samples. Overall design: Gene expression from paired frozen (FROZ) and formalin-fixed paraffin-embedded (FFPE) liver samples collected in 1994 from mice exposed to 0, 198, 313, and 427 mg/kg-d DCA via drinking water (n=6/dose/preservation) using the TempO-seq platform.