Molecular and clinicopathological characteristics of invasive breast cancers with HER2-ultralow expression

The identification of HER2-low and HER2-ultralow breast cancer (BC) subgroups has garnered considerable attention following the demonstrated clinical efficacy of HER2-targeted antibody-drug conjugates (ADCs). This study investigated whether these subgroups represent distinct biological subtypes compared to HER2-undetected tumors. Analyzing 297 HER2-negative BCs stratified by immunohistochemistry (50.2 % HER2-low, 35.7 % HER2-ultralow, 14.1 % HER2-undetected), we systematically compared clinicopathological and molecular profiles (AmoyDx® HANDLE Classic Panel). HER2-detected tumors (combined low/ultralow cohort) showed significant differences in hormone receptor (HR) positivity versus HER2-undetected tumors (p = 0.001). The mutation frequencies of the three most frequently altered genes (TP53, PIK3CA, and PTEN) in HER2-ultralow tumors did not significantly differ from those in the two other subgroups. However, HER2-detected tumors exhibited different molecular alterations compared to HER2-undetected tumors, with increased PIK3CA mutations in the former (p = 0.024) and TP53 mutation enrichment in the latter (p = 0.007). These differences attenuated when tumors were stratified by HR status. In addition, HER2 expression positively correlated with ERBB2 copy number, showing higher mean values in HER2-detected subgroups versus undetected counterparts (2.01 ± 0.31 vs 1.88 ± 0.23; p = 0.011). While no significant survival differences emerged across subgroups, most mortality events occurred in HER2-low (6/12, 50.0 %) and HER2-ultralow (5/12, 41.7 %) cases. Our findings indicate that while HER2-low and HER2-ultralow breast cancers may not represent independent biological subtypes, their clinical distinction from HER2-undetected tumors remains crucial due to potential ADC therapeutic implications.