Purpose High-grade serous ovarian cancer (HGSOC) is an aggressive disease with few available targeted therapies. Despite high expression of estrogen receptor-alpha in ~80% of HGSOC and some small but promising clinical trials of endocrine therapy, estrogen receptor-alpha has been understudied as a target in this disease. We sought to identify hormone-responsive, estrogen receptor-alpha-dependent HGSOC. Experimental Design We characterized endocrine response in HGSOC cells across culture conditions (2-D, 3-D, forced suspension) and in patient-derived xenograft (PDX) explants, assessing proliferation and gene expression. Estrogen-regulated transcriptome data were overlapped with public datasets to develop a comprehensive panel of estrogen receptor-alpha target genes. Expression of this panel and estrogen receptor-alpha H-score were assessed in HGSOC samples from patients who received endocrine therapy. Time on endocrine therapy was used as a surrogate for clinical response. Results Proliferation is estrogen receptor-alpha-regulated in HGSOC cells in vitro and in vivo, and is partly dependent on 3-D context. Transcriptomic studies identified genes shared by cell lines and PDX explants as estrogen receptor-alpha targets. The selective estrogen receptor-alpha down-regulator (SERD) fulvestrant is more effective than tamoxifen in blocking estrogen receptor-alpha action. Estrogen receptor-alpha H-score is predictive of efficacy of endocrine therapy, and this prediction is further improved by inclusion of target gene expression, particularly IGFBP3. Conclusions Laboratory models corroborate intertumor heterogeneity of endocrine response in HGSOC but identify features associated with functional ERα and endocrine responsiveness. Assessing ERα function (e.g. IGFBP3 expression) in conjunction with H-score may help select patients who would benefit from endocrine therapy. Preclinical data suggest that SERDs might be more effective than tamoxifen.

Invasive lobular carcinoma (ILC) is an understudied subtype of breast cancer that requires novel therapies in the advanced setting. To study acquired resistance to endocrine therapy in ILC, we have recently performed RNA-Sequencing on long-term estrogen deprived cell lines and identified FGFR4 overexpression as a top druggable target. Here, we show that FGFR4 expression also increases dramatically in endocrine-treated distant metastases, with an average fold change of 4.8 relative to the paired primary breast tumor for ILC, and 2.4-fold for invasive ductal carcinoma (IDC). In addition, we now report that FGFR4 hotspot mutations are enriched in metastatic breast cancer, with an additional enrichment for ILC, suggesting a multimodal selection of FGFR4 activation. These data collectively support the notion that FGFR4 is an important mediator of endocrine resistance in ILC, warranting future mechanistic studies on downstream signaling of overexpressed wild-type and mutant FGFR4.

DNA sequencing has identified a limited number of driver mutations in metastatic breast cancer beyond single basepair mutations in the estrogen receptor (ESR1). However, our previous studies and others have observed that structural variants, such as ESR1 fusions, may also play a role. Therefore, we expanded upon these observations by performing a comprehensive and highly sensitive characterization of copy-number (CN) alterations in a large clinical cohort of metastatic specimens. NanoString DNA hybridization was utilized to measure CN gains, amplifications, and deletions of 67 genes in 108 breast cancer metastases, and in 26 cases, the patient-matched primary tumor. For ESR1, a copyshift algorithm was applied to identify CN imbalances at exon-specific resolution and queried large data sets (>15,000 tumors) that had previously undergone next-generation sequencing (NGS). Interestingly, a subset of ER þ tumors showed increased ESR1 CN (11/82, 13%); three had CN amplifications (4%) and eight had gains (10%). Increased ESR1 CN was enriched in metastatic specimens versus primary tumors, and this was orthogonally confirmed in a large NGS data set. ESR1-amplified tumors showed a site-specific enrichment for bone metastases and worse outcomes than nonamplified tumors. No ESR1 CN amplifications and only one gain was identified in ER À tumors. ESR1 copyshift was present in 5 of the 11 ESR1-amplified tumors. Other frequent amplifications included ERBB2, GRB7, and cell-cycle pathway members CCND1 and CDK4/6, which showed mutually exclusivity with deletions of CDKN2A, CDKN2B, and CDKN1B. Implications: Copy-number alterations of ESR1 and key CDK pathway genes are frequent in metastatic breast cancers, and their clinical relevance should be tested further.