Abstract 5677: Recurrent transcriptionally active ESR1 fusions render therapeutic vulnerabilities to kinase inhibition in advanced breast cancer
Authors: Gou Xuxu , Lei Jonathan , Kim Beom-Jun , Anurag Meenakshi , Seker Sinem , Rehman Saif , Lee Adrian V. , White Kevin , Caldwell Michael , Ball Jonathan , Robinson Dan R. , Ellis Matthew J. 
Published: September 2020

Abstract
Background: We recently reported two ESR1 fusions (ESR1-YAP1 and ESR1-PCDH11X) that drive endocrine therapy (ET) resistance and metastasis in estrogen receptor positive (ER+) metastatic breast cancer (MBC) (PMC6171747). Here we report additional ESR1 fusions with diverse C-terminal partner genes - ESR1-DAB2, ESR1-GYG1, ESR1-SOX9, ESR1-ARNT2, ESR1-PCMT1 and ESR1-ARID1B. Their functional characteristics and effects on kinase biology will be described.
Methods: ESR1 fusions were identified by RNA-seq in MBC using ChimeraScan to detect fusion junction reads. ESR1 fusion cDNA constructs were expressed in ER+ breast cancer cell lines. An alamarBlue assay assessed cell proliferation. RNA-seq followed by mRNA-qPCR assessed the transcriptional properties. A scratch wound assay assessed cell motility. A Kinase Inhibitor Pulldown (KIP) mass spectrometry-based assay was conducted to examine ESR1 fusion-driven druggable kinases.
Results: All fusions retained the first 6 exons (e6) of ESR1, fused in-frame to C-terminal sequences of diverse partner genes. In addition to ESR1-YAP1 and ESR1-PCDH11X fusions, ESR1-SOX9 and ESR1-ARNT2 drove fulvestrant-resistant growth. RNA-seq revealed fusion-specific transcriptional signatures indicating enrichment of estrogen responsive and epithelial-to-mesenchymal transition (EMT) pathways that were confirmed by mRNA-qPCR. Transcriptionally active ESR1 fusions also promoted hormone-independent cell motility. KIP profiling demonstrated an increase in protein abundance of multiple receptor tyrosine kinases including RET and insulin like growth factor 1 receptor (IGF1R) in T47D cells expressing active ESR1 fusion proteins, both of which have been previously implicated in driving ET and MBC. Both proteins were also elevated in a patient-derived xenograft naturally harboring the ESR1-YAP1 fusion. Combinatorial inhibition of RET and IGF1R significantly suppressed ESR1 fusion-driven cell growth in vitro. Neither transcriptional activation nor kinase upregulation was observed in other ESR1-e6 fusions. Lastly, we report recurrent examples of specific known active ESR1 fusions: ESR1-PCDH11X (3 examples) (TEMPUS unpublished data) and ESR1-ARNT2 (2 examples) (PMC6872491).
Conclusion: A subset of ESR1-e6 fusions are active, drive ET resistance and metastasis/EMT in experimental models. When ESR1 is fused in-frame with another transcriptional regulator, activity is predictable. However, recurrent partners that have non-transcriptional roles (PCDH11X) suggests cDNA expression-based functional screens should be continued. A common pattern of kinase activation indicates that ESR1 fusion specific therapeutic strategies could be devised.
Citation Format: Xuxu Gou, Jonathan Lei, Beom-Jun Kim, Meenakshi Anurag, Sinem Seker, Saif Rehman, Adrian V. Lee, Kevin White, Michael Caldwell, Jonathan Ball, Dan R. Robinson, Matthew J. Ellis. Recurrent transcriptionally active ESR1 fusions render therapeutic vulnerabilities to kinase inhibition in advanced breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5677.