Abstract 4474: Novel neoplastic RAD51AP1-DYRK4 fusion transcript in aggressive luminal breast cancers

Abstract Background: Estrogen receptor positive (ER+) breast cancer is known as luminal breast cancer, which can be classified into A and B intrinsic subtypes. While the luminal A tumors have a favorable outcome following endocrine therapy, the luminal B tumors are characterized by higher proliferation index, aggressive clinical behavior, early relapse following endocrine therapy, and high risk of metastatic dissemination. Clinically the treatment options are limited, and it is even difficult to clearly define these deadly tumors. The underlying pathological molecular events remain poorly understood, and recent genome sequencing studies have revealed a paucity of actionable oncogenic drivers, which hinders the development of new treatment strategies. Experimental design and methods: Large-scale analyses of breast cancer RNAseq data from The Cancer Genome Atlas (TCGA) were performed to identify the driver gene fusions. ER+ breast tumor tissues were screened by RT-PCR. To test the function of RAD51AP1-DYRK4 transcripts in breast cancer, we engineered the fusion cDNA containing the E9-E2 chimeric ORF together with endogenous5’ translation start sequences into a doxycycline-inducible lentiviral vector, which was then transduced into T47D luminal breast cancer cells. The overexpression (OE) and endogenous fusion knockdown (KD) models were then subjected to diverse functional assays including MTS, Clonogenic, soft agar colony formation, migration and invasion. Results: In this study, a large-scale analysis of breast cancer transcriptome revealed a tumor-specific RAD51AP1-DYRK4 fusion transcript preferentially overexpressed in luminal B tumors. Molecular analysis of 200 ER-positive breast cancer tissues detected strong RAD51AP1-DYRK4 expression in 19 tumors (9.5%), which is markedly enriched in the luminal B subtype (17.5%). The fusion encodes c-terminal truncated RAD51AP1 protein fused to an out frame peptide from DYRK4, which leads to the loss of the RAD51 interacting domain. Ectopic expression of RAD51AP1-DYRK4 but not wild-type RAD51AP1 significantly increased invasiveness of luminal breast cancer cells. Further, we have identified the endogenous RAD51AP1-DYRK4 protein in fusion-overexpressing cells, silencing of which leads to decreased cell viability. Conclusions: In summary, this study identifies the first tumor-specific transcription-induced chimera that is preferentially overexpressed in the luminal B breast cancer, and the underlying mechanism. The results suggest that RAD51AP1-DYRK4 transcript may drive the more aggressive form of luminal breast cancers. Citation Format: Chia Chia Liu, Jamunarani Veeraraghavan, Ying Tan, Jin-Ah Kim, Xian Wang, Rachel Schiff, Xiao-Song Wang. Novel neoplastic RAD51AP1-DYRK4 fusion transcript in aggressive luminal breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4474.