Distinct clinical and somatic mutational features of breast tumors with high-, low-, or non-expressing human epidermal growth factor receptor 2 status

BACKGROUND: HER2-low breast cancers were reported to have distinct clinicopathological characteristics from HER2-zero; however, the difference in their genetic features remains unclear. This study investigated the clinical and molecular features of breast tumors according to HER2 status. METHODS: We analyzed the clinicopathological and genomic data of 523 Chinese women with breast cancer. Genomic data was generated by targeted next-generation sequencing (NGS) of breast tumor samples using a commercial 520 gene panel. The cohort was stratified according to HER2 status as HER2-zero (n = 90), HER2-low (n = 231), and HER2-positive (n = 202) according to their immunohistochemistry and fluorescence in situ hybridization results. RESULTS: HER2-low breast tumors were enriched with hormone receptor-positive tumors, and who had lower Ki67 expression levels. Genes were differentially mutated across HER2 subgroups. HER2-low tumors had significantly more mutations involved in PI3K-Akt signaling than HER2-positive (p < 0.001) and HER2-zero breast tumors (p < 0.01). HER2-zero tumors had more mutations in checkpoint factors (p < 0.01), Fanconi anemia (p < 0.05), and p53 signaling and cell cycle pathway (p < 0.05) compared to HER2-low breast tumors. Compared with HER2-zero tumors, HER2-low tumors had significantly lower pathological complete response rates after neoadjuvant therapy (15.9% vs. 37.5%, p = 0.042) and proportion of relapsed/progressed patients across follow-up time points (p = 0.031), but had comparable disease-free survival (p = 0.271). CONCLUSION: Our results demonstrate the distinct clinical and molecular features and clinical outcomes of HER2-low breast tumors. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12916-022-02346-9.