ADAR1 and RNA editing in triple-negative breast cancer

Abstract

The family of Adenosine Deaminases Acting on RNA (ADARs) regulate global gene expression output by catalyzing Adenosine-to-Inosine (A-to-I) editing of double-stranded RNA and through interacting with RNA and other proteins. ADARs play important roles in development and disease, and previous work has shown that ADAR1 is oncogenic in a growing list of cancer types. ADAR1 overexpression and over-editing are prevalent in triple-negative breast cancer and correlate with poor prognosis. Given the unmet medical need for targeted therapies in this breast cancer subtype, we performed an in-depth analysis of ADAR1 function and mechanism in triple-negative breast cancer in order to demonstrate its potential as a cancer target. By genetic perturbation of ADAR1, we show that it is critical for promoting growth (including viability and cell cycle progression), invasion, and mammosphere formation in triple-negative breast cancer cells. Whole transcriptome sequencing analyses reveal ADAR1 regulation of both coding and non-coding targets via gene expression level, A-to-I editing, and splicing. Furthermore, ADAR1-regulated targets are enriched for pathways involved in growth and invasion, as well as cytokine and immune signaling. We investigate an edit site in the coding sequence of filamin B (FLNB chr3:58156064) and determine that editing lowers the tumor suppressive function of the protein to promote growth and invasion. We also demonstrate that the tumor suppressor microRNAs miR-27a-5p and miR-4485-3p are upregulated upon ADAR1 loss and suppress cell cycle progression and invasion. This work describes a role for ADAR1 in promoting cancer hallmarks in triple-negative breast cancer, profiles broad transcriptomic alterations in this context upon ADAR1 loss, and identifies several novel mechanisms of ADAR1-mediated oncogenesis, contributing evidence to promote treatment strategies targeting ADAR1 in triple-negative breast cancer, an aggressive subtype with few treatment options.