Breast cancers that overexpress human epidermal growth factor receptor 2 (HER2) tend to be aggressive and have a poor prognosis. Most diagnostic pathology laboratories utilise silver, chromogenic or fluorescent in situ hybridisation (SISH, CISH or FISH respectively) to assess HER2 gene amplification. However, gene amplification techniques lack consistency, have variable failure rates and can result in masked signals. The aim of this work was to determine whether in situ mRNA expression of HER2 could be used as an alternative approach to assess HER2 status.
We analysed 133 cases of breast cancer on a TMA (0.6mm cores in duplicate). Immunohistochemistry (IHC) and SISH were performed according to standard diagnostic protocols. ViewRNA assays (Affymetrix) were performed for HER2 mRNA transcript detection according to the manufacturer’s instructions. Briefly, FFPE sections were deparaffinised and dehydrated before heat and protease treatment. Target hybridization was performed using specific probe sets directed against HER2 followed by signal amplification and chromogenic visualization.
HER2 mRNA was detected in 107 (80%) cases. Of these 107 cases, 16 were dual positive for protein and gene amplification and 26 expressed either HER2 protein or had gene amplification. There were 21 (16%) cases that were negative by all methods.
Our results show that HER2 transcript is detectable by a sensitive in situ method (80% cases) even when there is no evidence of gene amplification and protein expression. A recent study has demonstrated that increased HER2 protein expression is mediated by NF-kB (RANK)-ligand in the bone microenvironment and is not a result of gene amplification1. This novel mechanism may explain the discordance between HER2 gene amplification, transcript and protein. Our data may shed light on why some “apparent” HER2 negative tumours respond well to adjuvant trastuzumab treatment. It also raises the question of what defines “HER2 positivity”.