A Novel Neoplastic Fusion Transcript, , Confers Sensitivity to the MEK Inhibitor Trametinib in Aggressive Breast Cancers.

Purpose: Luminal B breast tumors are more aggressive estrogen receptor-positive (ER) breast cancers characterized by aggressive clinical behavior and a high risk of metastatic dissemination. The underlying pathologic molecular events remain poorly understood with a paucity of actionable genetic drivers, which hinders the development of new treatment strategies.

Experimental Design: We performed large-scale RNA sequencing analysis to identify chimerical transcripts preferentially expressed in luminal B breast cancer. The lead candidate was validated by reverse transcription PCR in breast cancer tissues. The effects of inducible ectopic expression or genetic silencing were assessed by phenotypic assays such as MTS, transwell, and transendothelial migration assays, and by clonogenic assays to assess MEK inhibitor sensitivity. Subcellular fractionation, Western blots, and immunoprecipitation were performed to characterize the protein products and elucidate the engaged mechanisms.

Results: Here we report a novel tumor-specific chimeric transcript preferentially expressed in luminal B tumors. Analysis of 200 ER breast tumors detected overexpression in 19 tumors (9.5%), which is markedly enriched in the luminal B tumors (17.5%). Ectopic expression of , but not wild-type , leads to marked activation of MEK/ERK signaling, and endows increased cell motility and transendothelial migration. More importantly, RAD51AP1-DYRK4 appears to endow increased sensitivity to the MEK inhibitor trametinib through attenuating compensatory activation of HER2/PI3K/AKT under MEK inhibition.

Conclusions: This discovery sheds light on a new area of molecular pathobiology of luminal B tumors and implies potential new therapeutic opportunities for more aggressive breast tumors overexpressing this fusion.