Classification of HER2-negative breast cancers by ERBB2 copy number alteration status reveals molecular differences associated with chromosome 17 gene aberrations

BACKGROUND: Recently, HER2-negative breast cancers have been reclassified by protein expression into ‘HER2-low’ and ‘HER2-zero’ subgroups, but the consideration of HER2-low breast cancer as a distinct biological subtype with differing prognoses remains controversial. By contrast, non-neutral ERBB2 copy number alteration (CNA) status is associated with inferior survival outcomes compared to ERBB2 CNA-neutral breast cancer, providing an alternative approach to classification. METHODS: Here, we investigated the molecular landscape of non-metastatic HER2-negative BCs in relation to ERBB2 CNA status to elucidate biological differences. Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) and The Cancer Genome Atlas (TCGA) TCGA-BRCA datasets (n = 1875) were analyzed. RESULTS: Nearly two-fifths of the cohort harbored ERBB2 CNAs (39.4%), which were significantly enriched within hormone receptor-negative (56.1%) than within hormone receptor-positive BCs (35.5%; p < 0.0001). Globally, CNAs across the genome were significantly higher in ERBB2 non-neutral compared to neutral cohorts (p < 0.0001). Notably, genetic aberrations on chromosome 17 – BRCA1, NF1, TP53, MAP2K4, and NCOR1 – were widespread in the ERBB2 non-neutral cases. While chromosome 17q arm-level alterations were largely in tandem with ERBB2 CNA status, arm-level loss in chromosome 17p was prevalent regardless of ERBB2 gain, amplification, or loss. Differential gene expression analysis demonstrated that pathways involved in the cell cycle, proteasome, and DNA replication were upregulated in ERBB2 non-neutral cases. CONCLUSION: Classification of HER2-negative BCs according to ERBB2 CNA status reveals differences in the genomic landscape. The implications of concurrent aberrations in other genes on chromosome 17 merit further research in ERBB2 non-neutral BCs.