Association of FGFR1 with ERα Maintains Ligand-Independent ER Transcription and Mediates Resistance to Estrogen Deprivation in ER+ Breast Cancer

Abstract

<b>Purpose:</b><i>FGFR1</i> amplification occurs in approximately 15% of estrogen receptor-positive (ER⁺) human breast cancers. We investigated mechanisms by which <i>FGFR1</i> amplification confers antiestrogen resistance to ER⁺ breast cancer.<b>Experimental Design:</b> ER⁺ tumors from patients treated with letrozole before surgery were subjected to Ki67 IHC, FGFR1 FISH, and RNA sequencing (RNA-seq). ER⁺/<i>FGFR1</i>-amplified breast cancer cells, and patient-derived xenografts (PDX) were treated with FGFR1 siRNA or the FGFR tyrosine kinase inhibitor lucitanib. Endpoints were cell/xenograft growth, FGFR1/ERα association by coimmunoprecipitation and proximity ligation, ER genomic activity by ChIP sequencing, and gene expression by RT-PCR.<b>Results:</b> ER⁺/<i>FGFR1</i>-amplified tumors in patients treated with letrozole maintained cell proliferation (Ki67). Estrogen deprivation increased total and nuclear FGFR1 and FGF ligands expression in ER⁺/<i>FGFR1-</i>amplified primary tumors and breast cancer cells. In estrogen-free conditions, FGFR1 associated with ERα in tumor cell nuclei and regulated the transcription of ER-dependent genes. This association was inhibited by a kinase-dead FGFR1 mutant and by treatment with lucitanib. ChIP-seq analysis of estrogen-deprived ER⁺/<i>FGFR1</i>-amplified cells showed binding of FGFR1 and ERα to DNA. Treatment with fulvestrant and/or lucitanib reduced FGFR1 and ERα binding to DNA. RNA-seq data from <i>FGFR1</i>-amplified patients' tumors treated with letrozole showed enrichment of estrogen response and E2F target genes. Finally, growth of ER⁺/<i>FGFR1-</i>amplified cells and PDXs was more potently inhibited by fulvestrant and lucitanib combined than each drug alone.<b>Conclusion</b>s<b>:</b> These data suggest the ERα pathway remains active in estrogen-deprived ER⁺/<i>FGFR1</i>-amplified breast cancers. Therefore, these tumors are endocrine resistant and should be candidates for treatment with combinations of ER and FGFR antagonists. <i>Clin Cancer Res; 23(20); 6138-50. ©2017 AACR</i>.