Expression of miR-21, miR-378a, miR-205, and Their Targets in ER-Positive Breast Tumors with Different HER2 Protein Levels

Tyrosine protein kinase HER2 plays an important role in carcinogenesis. In breast cancer (BC), the HER2 gene is amplified in approximately 20% of cases. Trastuzumab is used to treat BC with HER2 gene amplification. Trastuzumab is not effective in treating tumors with low HER2 expression, but trastuzumab deruxtecan was recently found to significantly improve prognosis in these patients. Nonetheless, there are still difficulties with accurate diagnosis of HER2-low BC, especially at the preoperative stage. In this study, when comparing the results between core needle biopsies and resection specimens using three immunohistochemistry scores (0, 1+, and 3+) of the HER2 protein level, we observed only moderate agreement (66.2%, κ = 0.486) in patients who did not undergo neoadjuvant therapy (n = 71). Other miRNA- or protein-coding genes regulated by HER2 signaling pathways may be additional markers for the scoring of the HER2 level. We measured levels of HER2-regulated miR-378a, -205, and -21 and mRNA levels of their target genes TRPS1, ITGA2, BCL6, and PTEN in BC surgical specimens. For estrogen receptor-positive BC (n = 64), we confirmed that the expression of miR-21, miR-378a, TRPS1, PTEN, and BCL6 is associated with the HER2 protein level. Moreover, the expression profile of miR-378a, BCL6, and PTEN differed between HER2 1+ and HER2 3+ tumors. TRPS1 expression was significantly higher in HER2 3+ tumors compared with HER2 0 tumors, but there was no difference in the amount of TRPS1 mRNA between HER2 1+ and HER2 3+ tumors. Thus, an analysis of the expression of miR-21, miR-378a, TRPS1, PTEN, and BCL6 may help to distinguish between HER2-negative, HER2-positive, and HER2-low BCs.