Abstract 5741: Evaluating the role of recurrent ESR1-CCDC170 fusion in breast cancer endocrine resistance
Authors: Hu Yiheng , Veeraraghavan Jamunarani , Wang Xian , Tan Ying , Kim Jin-Ah , Schiff Rachel , Wang Xiao-Song 
Published: July 2017

Abstract
Background: Recurrent gene fusions resulting from chromosome rearrangements are central to the formation of cancer. Previously, our lab identified recurrent rearrangements between the estrogen receptor α (ERα) gene ESR1 and its neighbor gene CCDC170, in ~6-8% of luminal B tumors, a more aggressive form of ER+ breast cancer that has worse clinical outcome after endocrine therapy. ESR1-CCDC170 rearrangements enable the expression of different-sized N-terminally truncated CCDC170 (ΔCCDC170) under ESR1 promoter, such as the E2-E7 or E2-E10 variants in which exon 2 of ESR1 fused with exon 7 or exon 10 of CCDC170. Consistent with the behavior of luminal B tumors, ectopic ΔCCDC170 expression in ER+ breast cancer cells transformed cells to a more aggressive form and led to reduced endocrine sensitivity in vitro, as well as enhanced xenograft growth in vivo. In the present study, we examined the role of ESR1-CCDC170 in breast cancer endocrine resistance in vivo and further elucidated the potential engaged mechanisms of its actions.
Results: Our in vivo endocrine sensitivity study showed that, while tamoxifen (Tam) treatment rendered tumor regression in the vector-expressing T47D xenograft tumors, it made the E2-E7 overexpressing T47D xenografts static and less likely to regress, whereas the E2-E10 overexpressing xenografts, though grew initially upon Tam treatment, became cytostatic and showed significant reduction in tumor regression. Kaplan-Meier analysis revealed a significantly worse progression-free survival for E2-E7 (p&lt;0.01) and E2-E10 (p&lt;0.001) overexpressing tumors treated with Tam compared to the control tumors. These data suggest that ESR1-CCDC170 variants differ in level of reduced responsivity to Tam, and may render the T47D xenografts less sensitive or resistant to Tam in vivo. Further studies suggest that ΔCCDC170 protein preferentially localizes to cytoplasm (different from wild-type CCDC170 protein enriched in nucleus), physically interacts with ERα and HER2, and forms homodimers. RPPA analysis showed that silencing of ESR1-CCDC170 repressed ERα and BCL2 protein levels, as well as total/phospho-HER2 levels, consistent with the findings of ΔCCDC170 interactions with ERα and HER2. Importantly, analysis of ΔCCDC170 protein sequence revealed a potential high-affinity ATP-binding pocket at its C-terminus, suggesting that ΔCCDC170 may be directly druggable.
Conclusion: These data suggest a potential role of ESR1-CCDC170 in mediating breast cancer endocrine responsivity, which may possibly through cytoplasmic mislocalization of ΔCCDC170 and interactions with ERα and HER2. Further studies will affirm and elucidate the role of overexpressed or endogenous ESR1-CCDC170 in breast cancer endocrine resistance, and pinpoint the precise mechanisms of its oncogenic function, in order to develop a novel target therapy against it for the fusion-carrying patient community around the world.
Citation Format: Yiheng Hu, Jamunarani Veeraraghavan, Xian Wang, Ying Tan, Jin-Ah Kim, Rachel Schiff, Xiao-Song Wang. Evaluating the role of recurrent ESR1-CCDC170 fusion in breast cancer endocrine resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5741. doi:10.1158/1538-7445.AM2017-5741