BACKGROUND: We examined tumor genotype characteristics of human epidermal growth factor receptor 2 (HER2)-positive relapsed (R-) and de novo (dn-) metastatic breast cancer (MBC) in trastuzumab-treated patients who were previously not exposed to this agent.
MATERIALS AND METHODS: We analyzed genotypes obtained upon deep sequencing from 113 HER2-positive primary tumors from 69 patients with R-MBC and 44 patients with dn-MBC.
RESULTS: Mutations were observed in 90 (79.6%) tumors, 56 R-MBC and 34 dn-MBC (median number per tumor: 2; mean: 11.2; range: 0-150). The top mutated gene was TP53 (63.7%) followed by PIK3CA (24.8%) and others that were mostly co-mutated with TP53 (eg, 22 of 28 PIK3CA mutated tumors were co-mutated in TP53, 17 of these were R-MBC [P = .041]). dn-MBC had higher CEN17 average copies (P = .048). Tumor mutational burden inversely correlated with average HER2 copies (rho -0.32; P < .001). In all patients, PIK3CA mutations and higher proliferation rate were independent unfavorable prognosticators. In R-MBC, longer disease-free interval between initial diagnosis and relapse conferred lower risk for time-to-progression (P < .001) and death (P = .009); PIK3CA mutations conferred higher risk for death (P = .035). In dn-MBC, surgical removal of the primary tumor before any other therapy was favorable for time-to-progression (P = .002); higher tumor mutational burden was unfavorable for survival (P = .026).
CONCLUSIONS: Except for the overall unfavorable prognostic effect of PIK3CA mutations in trastuzumab-treated MBC, our exploratory findings indicate that the outcome of patients with R-MBC is related to patient benefit from the preceding adjuvant chemotherapy and provide initial evidence that tumor mutational burden may be related to prognosis in dn-MBC, which is of potential clinical relevance and merits further investigation.