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.