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.

Objective:To investigate the clinical and pathological characteristics of breast cancer with HER2 low expression.Methods:The data from 3 422 patients with invasive breast cancer which archived in Peking Union Medical College Hospital between April 2019 and July 2022 were retrospectively reviewed. Among them, 136 patients were treated with neoadjuvant chemotherapy. The tumor size, histological type, tumor differentiation, lymph node metastasis, Ki-67 index, the status of estrogen receptor, progesterone receptor and HER2 as well as pathological complete response (pCR) rate were collected.Results:The HER2 status of 3 286 patients without neoadjuvant therapy, 616 (616/3 286, 18.7%) score 0, 1 047 (1 047/3 286, 31.9%) score 1+, 1 099 (1 099/3 286,33.4%) score 2+ and 524 (524/3 286,15.9%) score 3+ by immunohistochemistry (IHC). Among the 1 070 IHC 2+ cases, 161 were classified as HER2 positive by reflex fluorescence in situ hybridization (FISH) assay. In our cohort, 1 956 cases of HER2-low (IHC 1+ and IHC 2+/FISH-) breast cancer were identified. Compared to the HER2 IHC 0 group, HER2-low tumors more frequently occurred in patients with hormone receptor (HR) positive ( P<0.001), Ki-67 index below 35% ( P<0.001), well or moderate differentiation ( P<0.001) and over the age of 50 ( P=0.008). However, there were no significant differences in histological type, tumor size, and lymph node metastasis between HER2-low and HER2 IHC 0 group. For patients who had neoadjuvant therapy, the pCR rate in the patients with HER2-low was lower than those with HER2 IHC 0 (13.3%, 23.9%), but there was no significant difference. Although HER2-low breast cancers showed a slightly lower pCR rate than HER2 IHC 0 tumors, no remarkable difference was observed between tumors with HER2-low and HER2 IHC 0 regardless of hormone receptor status. Conclusions:The clinicopathological features of HER2-low breast cancers are different from those with HER2 IHC 0. It is necessary to accurately distinguish HER2-low breast cancer from HER2 IHC 0 and to reveal whether HER2-low tumor is a distinct biological entity.