ESR1 fusions invoke breast cancer subtype-dependent enrichment of ligand independent oncogenic signatures and phenotypes.

Breast cancer is a leading cause of female mortality and despite advancements in personalized therapeutics, metastatic disease largely remains incurable due to drug resistance. The estrogen receptor (ER, ESR1) is expressed in two-thirds of all breast cancer, and under endocrine stress, somatic ESR1 mutations arise in ∼30% of cases that result in endocrine resistance. We and others reported ESR1 fusions as a mechanism of ER mediated endocrine resistance. ER fusions, which retain the AF1 and DNA binding domains, harbor ESR1 exons 1-6 fused to an in-frame gene partner resulting in loss of the ER ligand binding domain (LBD). We demonstrate that in a no-special type (invasive ductal carcinoma (IDC)-NST) and an invasive lobular carcinoma (ILC) cell line, ER fusions exhibit robust hyperactivation of canonical ER signaling pathways independent of estradiol or anti-endocrine therapies. We employ cell line models stably overexpressing ER fusions with concurrent endogenous ER knockdown to minimize endogenous ER influence. Cell lines exhibited shared transcriptomic enrichment in pathways known to be drivers of metastatic disease, notably MYC signaling. Cells expressing the 3' fusion partners SOX9 and YAP1 consistently demonstrated enhanced growth and cell survival. ILC cells expressing the DAB2 fusion led to enhanced growth, survival, and migration; phenotypes not appreciated in the IDC-NST DAB2 model. Herein, we report that cell line activity is subtype-, fusion-, and assay-specific suggesting that LBD loss, the fusion partner, and the cellular landscape all influence fusion activities. Therefore, it will be critical to assess fusion frequency, in the context of the tumor clinicopathology.