Abstract P5-04-21: FGFR4 is a novel druggable target for recurrent ER-positive breast cancers
Authors: Levine KM , Ding K , Priedigkeit N , Sikora MJ , Tasdemir N , Zhu L , Tseng GC , Jankowitz RC , Dabbs DJ , McAuliffe PF , Lee AV , Oesterreich S 
Published: May 2019

Abstract
Background
Breast cancer recurrence is a major clinical problem for estrogen receptor positive (ER+) disease, even decades after initial surgery. These long-term recurrences are a challenge for invasive ductal carcinoma (IDC), and are particularly frequent for the histological subtype of invasive lobular carcinoma (ILC). To study the long-term endocrine resistance seen in ILC patients, our lab recently generated six long-term estrogen deprivation (LTED) models of ILC cells and performed RNA-Sequencing to identify differentially expressed genes that ostensibly allow these cells to grow in the absence of estrogen. We overlapped these results with a previously published microarray dataset of tamoxifen-resistant cells, and found that FGFR4 is the most consistently overexpressed gene in the setting of acquired resistance to endocrine therapy in ILC cells. From a recent publication of RNA-Seq from other LTED models, FGFR4 RNA overexpression is also seen in all five IDC cell lines.
Hypothesis
FGFR4 is an important mediator of acquired endocrine resistance in breast cancer.
Methods
To study the role of FGFR4 in vitro, we used multiple shRNAs and specific small molecule inhibition for growth assays. To study the role of FGFR4 in de novo resistance to endocrine therapy, we collected 129 well curated ER+ ILC tumor specimens and performed gene expression analysis on the pre-treatment samples using a custom NanoString panel. To study the role of FGFR4 in acquired resistance, we collected over 50 pairs of primary-metastatic ER+ tumors and performed exon capture based RNA-Sequencing.
Results
FGFR4 inhibition decreases parental and LTED cell growth in classic 2D conditions and in colony formation assays. The LTED cells, with higher FGFR4 expression, are more sensitive to its inhibition. For the parental cells, combination FGFR4 and ER-targeting drugs results in synergistic decreases in growth. In our database of primary ILC clinical samples, increased expression of FGFR4 is predictive of shorter time to distant recurrence. Among primary-recurrent tumor pairs, FGFR4 is an outlier expression gain in 20/50 (40%), spanning all recurrence sites studied (i.e. local recurrences, and metastases to the brain, bone, ovaries, and GI tract). Finally, in analyzing large cohorts of metastatic tumors, there is a significant enrichment of hotspot FGFR4 mutations in tumors originating in the breast, with &gt;2% of metastatic ILC tumors containing such a mutation.
Conclusion/Future studies
FGFR4 may play an important role in de novo resistance to endocrine therapy in ILC and acquired resistance in both ILC and IDC. Ongoing studies include overexpression of wild-type and FGFR4 hotspot mutations in ILC and IDC cell lines to determine growth and metastatic phenotypes.
Citation Format: Levine KM, Ding K, Priedigkeit N, Sikora MJ, Tasdemir N, Zhu L, Tseng GC, Jankowitz RC, Dabbs DJ, McAuliffe PF, Lee AV, Oesterreich S. FGFR4 is a novel druggable target for recurrent ER-positive breast cancers [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-04-21.