Clinical and biological heterogeneities in triple-negative breast cancer reveals a non-negligible role of HER2-low

BACKGROUND: HER2-low could be found in some patients with triple-negative breast cancer (TNBC). However, its potential impacts on clinical features and tumor biological characteristics in TNBC remain unclear. METHODS: We enrolled 251 consecutive TNBC patients retrospectively, including 157 HER2-low (HER2(low)) and 94 HER2-negtive (HER2(neg)) patients to investigate the clinical and prognostic features. Then, we performed single-cell RNA sequencing (scRNA-seq) with another seven TNBC samples (HER2(neg) vs. HER2(low), 4 vs. 3) prospectively to further explore the differences of tumor biological properties between the two TNBC phenotypes. The underlying molecular distinctions were also explored and then verified in the additional TNBC samples. RESULTS: Compared with HER2(neg) TNBC, HER2(low) TNBC patients exhibited malignant clinical features with larger tumor size (P = 0.04), more lymph nodes involvement (P = 0.02), higher histological grade of lesions (P < 0.001), higher Ki67 status (P < 0.01), and a worse prognosis (P < 0.001; HR [CI 95%] = 3.44 [2.10–5.62]). Cox proportional hazards analysis showed that neoadjuvant systemic therapy, lymph nodes involvement and Ki67 levels were prognostic factors in HER2(low) TNBC but not in HER2(neg) TNBC patients. ScRNA-seq revealed that HER2(low) TNBC which showed more metabolically active and aggressive hallmarks, while HER2(neg) TNBC exhibited signatures more involved in immune activities with higher expressions of immunoglobulin-related genes (IGHG1, IGHG4, IGKC, IGLC2); this was further confirmed by immunofluorescence in clinical TNBC samples. Furthermore, HER2(low) and HER2(neg) TNBC exhibited distinct tumor evolutionary characteristics. Moreover, HER2(neg) TNBC revealed a potentially more active immune microenvironment than HER2(low) TNBC, as evidenced by positively active regulation of macrophage polarization, abundant CD8(+) effector T cells, enriched diversity of T-cell receptors and higher levels of immunotherapy-targeted markers, which contributed to achieve immunotherapeutic response. CONCLUSIONS: This study suggests that HER2(low) TNBC patients harbor more malignant clinical behavior and aggressive tumor biological properties than the HER2(neg) phenotype. The heterogeneity of HER2 may be a non-negligible factor in the clinical management of TNBC patients. Our data provide new insights into the development of a more refined classification and tailored therapeutic strategies for TNBC patients. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13058-023-01639-y.