Gene expression analysis in sections and tissue microarrays of archival tissues by mRNA in situ hybridization

Scalable in situ hybridization on tissue arrays for validation of novel cancer and tissue-specific biomarkers

Tissue localization of gene expression is increasingly important for accurate interpretation of large scale datasets from expression and mutational analyses. To this end, we have (1) developed a robust and scalable procedure for generation of mRNA hybridization probes, providing >95% first-pass success rate in probe generation to any human target gene and (2) adopted an automated staining procedure for analyses of formalin-fixed paraffin-embedded tissues and tissue microarrays. The in situ mRNA and protein expression patterns for genes with known as well as unknown tissue expression patterns were analyzed in normal and malignant tissues to assess procedure specificity and whether in situ hybridization can be used for validating novel antibodies. We demonstrate concordance between in situ transcript and protein expression patterns of the well-known pathology biomarkers KRT17, CHGA, MKI67, PECAM1 and VIL1, and provide independent validation for novel antibodies to the biomarkers BRD1, EZH2, JUP and SATB2. The present study provides a foundation for comprehensive in situ gene set or transcriptome analyses of human normal and tumor tissues.

Evaluation and validation of total RNA extraction methods for microRNA expression analyses in formalin-fixed, paraffin-embedded tissues

Histopathology archives of well-annotated formalin-fixed, paraffin-embedded (FFPE) tissue specimens are valuable resources for retrospective studies of human diseases. Since recovery of quality intact mRNA compatible with molecular techniques is often difficult due to degradation, we evaluated microRNA (miRNA), a novel class of small RNA molecules with growing therapeutic and diagnostic potential, as an alternative analyte for gene expression studies of FFPE samples. Analyzing total RNA yield, miRNA recovery, and robustness of real-time polymerase chain reaction for miRNA, mRNA, and rRNA species, we compared the performance of commercially available RNA isolation kits and identified a preferred methodology. We further implemented modifications to increase tissue throughput and incorporate a quantitative Armored RNA process control to monitor RNA recovery efficiency. The optimized process was tested for reproducibility as well as interoperator and interday variability, and was validated with a set of 30 clinical samples. In addition, we demonstrated that, independent of FFPE block age and RNA quality, miRNAs generate quantitative reverse transcription-polymerase chain reaction signals that are more robust and better correlate with expression levels from frozen reference samples compared with longer mRNAs. Our broad study, including a total of 272 independent RNA isolations from 17 tissue types and 65 FFPE blocks up to 12 years old, indicates that miRNAs are not only suitable but are also likely superior analytes for the molecular characterization of compromised archived clinical specimens.