Relationship between HER2 gene status and selected potential biological features related to trastuzumab resistance and its influence on survival of breast cancer patients undergoing trastuzumab adjuvant treatment

Background: The aim of the study was to investigate if parameters associated with human epidermal growth factor receptor type 2 (HER2) status (HER2 gene copy number, HER2/CEP17 ratio or polysomy of chromosome 17) are related to various biological features potentially responsible for trastuzumab resistance (PTEN, IGF-1R, MUC4, EGFR, HER3, HER4, and mutation status of PIK3CA) as well as their influence on survival of HER2-positive breast cancer patients treated with adjuvant chemotherapy and trastuzumab. Patients and methods: The investigated group consisted of 117 patients with invasive ductal breast cancer (T >= 1, N >= 0, M0) with overexpression of HER2, who underwent radical surgery between 2007 and 2014. Status of ER, PR, and HER2 expression was retrieved from patients' files. HER2 gene copy number was investigated by fluorescence in situ hybridization using PathVysion HER-2 DNA Probe Kit II. Expression of PTEN, IGF-1R, MUC4, EGFR, HER3, and HER4 was assessed immunohistochemically on formalin-fixed paraffin-embedded tissue sections. PIK3CA mutation status was determined by qPCR analysis. Results: Overexpression of HER2 protein (IHC 3+) and ER negativity corresponded to higher HER2 copy number and HER2/CEP17 ratio (p < 0.001). Tumors with polysomy were characterized by higher HER2 gene copy number but lower HER2/CEP17 ratio (p < 0.026,p < 0.001). Patients with tumors featuring HER3 immunonegativity or low HER2/CEP17 ratio (<= 4) were characterized by 100% metastasis-free survival (p=0.018, p=0.062). Conclusion: Presence of both unfavorable factors, ie, HER3 expression and high HER2/CEP17 ratio, allowed to distinguish a group of patients with worse prognosis (p=0.001).