Correlation between HER2 gene copy number and immunohistochemistry categories in HER2-negative breast cancer: diagnostic utility for differentiating HER2-null, ultralow, and low tumors.

[BACKGROUND] The recent recognition of human epidermal growth factor receptor 2 (HER2)-low and HER2-ultralow breast cancers (BCs) has expanded the therapeutic relevance of HER2 testing in the antibody-drug conjugate era. However, the biological continuum of HER2 expression measured by immunohistochemistry (IHC) and its relationship with the HER2 gene copy number remain unclear.

[METHODS] We retrospectively analyzed 135 HER2-negative invasive BCs and reclassified them as HER2-null (IHC 0), HER2-ultralow (0+), or HER2-low (1+ or 2+ without amplification). HER2 gene copy number was determined using silver-enhanced in situ hybridization. Statistical analyses were performed to compare HER2 copy number among IHC categories and evaluate the discriminatory value of HER2 copy number for distinguishing IHC subgroups.

[RESULTS] The mean HER2 copy number increased stepwise across IHC categories: 1.95 ± 0.54 (null), 2.03 ± 0.43 (ultralow), 2.25 ± 0.65 (low, 1+), and 3.29 ± 1.05 (low, 2+). Significant differences were observed between the ultralow and low groups (p = .003) and between the null and low groups (p < .001), but not between the null and ultralow groups or between the ultralow and 1+ groups.

[CONCLUSIONS] HER2 gene copy number was positively correlated with protein expression as reflected by IHC categories. Although HER2 gene copy number was statistically higher in HER2-low than in HER2-null tumors, the substantial overlap in copy number ranges likely limits its utility in distinguishing HER2-low from HER2- null BCs.