Background. Primary breast cancers that overexpress human epidermal growth factor receptor 2 have variable biological features and clinical outcomes. A subgroup of HER2-overexpressing tumors that express basal-like immunohistochemical markers - the so-called basal-HER2+ subtype - is associated with poor prognosis.We investigated the clinical relevance of this basal-HER2+ subtype within HER2-overexpressing breast tumors. Methods. Database review identified consecutive patients with HER2-overexpressing breast cancer. Archival tumor specimens from these patients were immunostained for estrogen receptor (ER), HER2, and basal cytokeratin (CK) expression, then subtyped as luminal-HER2+ (ER positive and basal CK negative), HER2+ (ER negative and basal CK negative), and basal-HER2+ (ER negative and basal CK positive). Subtypes were correlated with clinicopathologic features and overall survival. Results. Immunohistochemical assessment of 131 HER2-overexpressing breast tumors identified 79 (60%) luminal-HER2+ tumors, 40 (31%) HER2+ tumors, and 12 (9%) basal-HER2+ tumors. There was no difference in the use of adjuvant trastuzumab and chemotherapy among patients with these subtypes. Five-year overall survival was 65% for patients with basal-HER2+ tumors versus 94% (P = 0.0035) and 96% (P = 0.0031) for patients with luminal-HER2+ and HER2+ tumors, respectively. The basal-HER2+ subtype was associated with the worst prognosis after adjusting for age, tumor size, lymph node status, and adjuvant treatment (hazard ratio 5.06, 95% confidence interval 1.1-23.2, P = 0.037). Conclusions. The basal-HER2+ subtype highlights the heterogeneous biology of HER2-overexpressing breast cancer. The basal-HER2+ subtype is independently associated with poor survival and may provide insight into breast cancer cell response to anti-HER2 therapy.
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