[HER2 mRNA expression in breast cancers with equivocal immunohistochemical results using in situ mRNA hybridization]

To investigate in situ mRNA expression of HER2 oncogene in breast cancers with equivocal immunohistochemical results, and to explore the potential feasibility of RNAscope technique in evaluating HER2 status in breast cancers.

Sixty-nine FFPE samples of invasive ductal breast cancer with equivocal HER2 immunohistochemistry results (IHC 2+) were collected from surgical excisions from Peking Union Medical College Hospital between June 2010 and June 2013. HER2 status and in situ mRNA expression were tested by fluorescence in situ hybridization (FISH) and RNAscope respectively using tissue microarray constructed from tumor paraffin blocks. The results of HER2 mRNA expression were scored 0 to 4 (from low to high levels) according to mRNA expression in 100 cancer cells. HER2 mRNA expression was evaluated in two groups of patients, with positive and negative FISH results.

Twenty-three of the 69 samples were FISH positive, including 16 samples that were scored 4 by RNAscope (70%, 16/23), 6 samples were scored 3 (26%, 6/23) and one sample was scored 2 (4%, 1/23). High in situ mRNA expression (score 4 or 3) were observed in 96% of HER2 FISH positive samples. All of samples that were scored 4 by RNAscope were FISH positive. Forty-six samples were FISH negative, including 17 samples that were scored 3 by RNAscope (37%, 17/46), 25 samples were scored 2 (54%, 25/46), and 4 samples were scored 1 (9%, 4/46).

Breast cancer with HER2 IHC 2+ could be further classified according to in situ mRNA expression status. Among them, RNAscope score of 4 could be one of the interpretation criteria for re-testing IHC 2+ samples. In situ detection of HER2 mRNA may be an additional candidate method of confirmation for HER2 gene amplification or protein overexpression, and has potential clinical utility.