Abstract PD3-1: Serial next generation sequencing (NGS) of poor prognosis luminal tumors across treatment history reveals both de novo and acquired alterations potentially associated with endocrine resistance

Cancer therapies exert selective pressure on genomically heterogeneous breast cancers and may select for alterations in genes associated with drug resistance or metastasis. We evaluated this hypothesis by comparing sequential primary, post-neoadjuvant chemotherapy, and recurrent metastatic tumor samples from 5 patients with aggressive hormone receptor positive (HR+) breast cancer. Triplet samples (primary untreated, primary post-neoadjuvant chemotherapy, metastatic recurrence) were assessed in 5 patients with HR+ breast cancer. All patients were treated with cyclophosphamide and doxorubicin followed by paclitaxel or docetaxel, and adjuvant radiation and hormonal therapy. They all recurred during adjuvant antiestrogen treatment (12-43 months post-surgery). Metastatic sites included liver (2), ovary, brain, and pleural fluid. In these 5 tumor sets, we queried genomic alterations across 3,769 exons from 236 cancer genes using the FoundationOne™ NGS assay. Gene amplifications were frequently discordant among the triplet biopsies; all patients showed copy number alterations and 3 of 5 patients had two or more additional gene amplifications confirmed only in the biopsy of the metastasis. ESR1 (ERa) mutations were identified in the metastatic lesions of 2/5 patients but not in the diagnostic biopsy or the post-neoadjuvant therapy mastectomy specimen; both also harbored de novo and concurrent GATA3 truncations. One of the ESR1 mutations, Y537S, is known to result in an estrogen-independent, transcriptionally active receptor. The same patient showed amplification of 11q13 genes CCND1, EMSY, and FGF19, an amplicon known to be associated with endocrine resistance. Concordant FGFR1 and ZNF703 amplifications, both at 8p12, were observed de novo in 2 other patients; these alterations have beenassociated with the luminal B subtype and resistance to antiestrogens. FGFR1/ZNF703 alterations were mutually exclusive with ESR1 mutations. The 5th tumor contained a PIK3CA ‘hot spot’ mutation in all 3 sequential biopsies. This tumor also exhibited IGF1R, IRS2, MYC and MCL1 gene amplification only in the metastasis. Overall, IGF1R amplifications were observed in 3/5 patients. In addition, amplification of ZNF217 at 20q13.2, an amplicon with known association with poor prognosis, was observed de novo in 2 patients that exhibited rapid recurrence on adjuvant therapy (12 and 17 months). Acquired somatic alterations that deregulate ERα signaling may allow for escape from estrogen deprivation leading to treatment resistance. We speculate ESR1 mutations, rarely observed in primary breast tumors, are selected for during adjuvant hormonal treatment in aggressive HR+ breast cancer. Testing for de novo and acquired alterations throughout the clinical course of patients with ER+ cancers may identify targetable alterations, such as FGFR1 and IGF1R gene amplifications and others, against which targeted drugs are available. Studies are underway to validate these findings in additional patients with sequential samples and to investigate the function and therapeutic implications of ESR1 mutations. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr PD3-1.