[Quantitative HER2 mRNA assay in breast cancer with HER2 immunohistochemistry 0]

Objective: To investigate HER2 mRNA expression in breast cancer with HER2 immunohistochemistry (IHC) 0 and to analyze the feasibility of distinguishing between the tumor with HER2 μltra-low expression and the one without expression of HER2 (no staining by IHC) by HER2 mRNA level preliminarily. Methods: HER2 mRNA was analyzed by reverse transcription digital PCR in 41 cases of formalin-fixed paraffin-embedded surgical tissue samples of invasive breast cancer obtained between January 2020 and March 2023 at Peking Union Medical College Hospital. The cohort included 21 HER2 IHC 1+ and 20 IHC 0 (12 ultra-low and 8 non-expression of HER2). HER2 mRNA expression level was quantitatively evaluated by the FAM (HER2)/VIC (reference gene) ratio. Results: The expression of HER2 mRNA for the cases with 1+, ultra-low, and non-expression of HER2 by IHC was 0.30 to 1.78 (average 0.90, median 0.82), 0.55 to 1.51 (average 0.93, median 0.90) and 0.22 to 0.78 (average 0.41, median 0.36), respectively. For the mean and median HER2 mRNA levels, there was no significant difference between HER2 IHC 1+ and HER2 ultra-low expression diseases (P=0.757). A remarkable difference in HER2 gene expression was found between the tumors with 1+ and non-expression of HER2 by IHC (P=0.002). And, HER2 ultra-low cases contained statistically higher levels of HER2 mRNA compared with non-expression of HER2 subgroup by IHC (P=0.001). Conclusions: Based on HER2 mRNA, HER2 non-expression and HER2 weak expression (including HER2 IHC 1+ and ultra-low) belong to two different types of the tumor and the disease with HER2 IHC 1+ and HER2 ultra-low expression may be the same. It is necessary to further test the performance of HER2 mRNA detection for stratifying the HER2 weak expression subgroup and to determine the threshold.