Abstract PD02-04: Automated Quantitative RNA In Situ Hybridization for Resolution of Equivocal and Heterogeneous ERBB2 (HER2) Status in Invasive Breast Carcinoma
Authors: Wang Z , Portier BP , Gruver AM , Bui S , Wang H , Su N , Vo H-T , Ma X-J , Luo Y , Budd GT , Tubbs RR 
Published: August 2014

Background: Breast carcinomas that demonstrate a heterogeneous ERBB2 (HER2) status or equivocal results by both immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) present diagnostic challenges for which there is neither a standard methodology to achieve resolution in the clinical laboratory nor a uniform approach to management. We assessed the feasibility of using a novel automated and quantitative HER2 mRNA bright field in situ hybridization (ISH) assay capable of single molecule detection to determine HER2 status in invasive breast carcinomas that demonstrated significant tumor heterogeneity or failed to be resolved by standard IHC and FISH algorithmic testing.

Design: Formalin-fixed, paraffin-embedded (FFPE) breast carcinomas from a non-consecutive series of 163 patients were analyzed for HER2 mRNA using a fully automated bright field RNA ISH assay (RNAscope, Advanced Cell Diagnostics, Hayward, CA). Cases were assigned into either a training set (n = 34) or a validation set (n = 129) and analyzed by both Q-RT-PCR and RNAscope. automated image analysis was used to numerate the punctate signal dots per cell in RNAscope-stained slides. A HER2 mRNA score based on single-cell quantification by RNAscope was developed and correlated to HER2 FISH and HER2 mRNA Q-RT-PCR results. A simple cutoff value was derived using the training set and applied to the validation set.

Results: Evaluable HER2 results were obtained for 154 cases (94.5%) by RNAscope and 163 cases (100%) by Q-RT-PCR. In the training set, both FISH/IHC positive and negative cases were definitively separated by both Q-RT-PCR and RNAscope. HER2 mRNA dots per cell correlated strongly to FISH (Spearman r=0.77) and Q-RT-PCR (r = 0.81). Application of both methods to the validation set resulted in correct identification of 31/31 positive cases and 41/43 negative (overall concordance=97.3%) for both RNAscope and Q-RT-PCR. RNAscope showed a significant advantage over Q-RT-PCR in correctly identifying cases equivocal by FISH that were resolved by reflex IHC testing. RNAscope classified 7 of 26 (26.9%) FISH/IHC double equivocal cases as positives. In cases with HER2 protein heterogeneity, RNAscope showed a 100% concordance with FISH results, whereas Q-RT-PCR showed a 42.9% concordance.

Conclusion: RNAscope analysis of HER2 mRNA is an effective means to resolve HER2 status in double equivocal cases and cases that demonstrate heterogeneity. Automation and image analysis-based quantification minimize analytical and post-analytical variability. Quantification of single RNA transcripts in situ at single-cell level demonstrates superiority over qRTPCR and great potential in predictive biomarker analysis. Further studies of larger cohorts correlating clinical response with HER2 mRNA expression in situ are warranted.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD02-04.