FGFR4 is a key regulator of tumor subtype differentiation in luminal breast cancer and metastatic disease [set 2]

Mechanisms driving tumor progression from less aggressive subtypes to more aggressive states represent key targets for breast cancer therapy. We observed that a subset of Luminal A primary breast tumors give rise to HER2-enriched (HER2E) subtype metastases, but remain clinically HER2 negative (cHER2-). By testing the unique genetic and transcriptomic features of these cases, we developed the hypothesis that FGFR4 drives this subtype switching. To evaluate this, we developed two FGFR4 signatures using a PDX model treated with a FGFR4 inhibitor (BLU9931), which inhibited PDX growth in vivo. Examining patient outcomes in the METABRIC breast cancer cohort showed that the FGFR4-induced and FGFR4-repressed signatures each predicted overall survival (OS) (HR=6.30, P<0.0001; HR=0.33; P<0.0001, respectively). Multivariate analysis showed that the FGFR4-induced signature was also an independent prognostic factor beyond subtype and stage for OS (HR=2.34, P=0.014). Supervised analysis of 77 primary tumors with paired metastasis revealed that the FGFR4-induced signature was significantly higher in luminal/ER+ tumor metastases compared with their primaries. Finally, multivariate analysis demonstrated that the FGFR4-induced signature also predicted site-specific metastasis for lung, liver and brain, but not for bone or lymph nodes. These data identify a link between FGFR4-regulated genes and metastasis, suggesting a treatment options for FGFR4-positive patients, whose high expression is non-genetically determined. There are 13 samples in this set. We analyzed 5 replicates of the untreated tumors (control) plus 4 replicates of the BLU9931 treated and Lapatinib treated PDX tumors. [set1]