Abstract P2-05-04: Deregulation of A-to-I RNA editing is associated with poor prognosis in HER2+ breast cancers in the neoALTTO trial

Background A-to-I RNA editing, a post-transcriptional modification of the RNA catalyzed by the ADAR family of enzymes, is emerging as a widespread phenomenon in breast cancer (BC). A-to-I RNA editing is more frequent in the highly repetitive Alu regions but can affect both coding and non-coding regions. It has been shown to greatly impact cell functionality. In a recent report, we have shown that A-to-I RNA editing is regulated both by ADAR copy number and type I interferon response (Fumagalli et al. Cell Rep 2015). The main aim of the current study was to investigate the extent and profile of A-to-I RNA editing in HER2+ BC patients (pts) treated in the NeoALTTO trial, and to explore its impact on pathologic complete response (pCR) and survival. Methods Aligned RNAseq reads of sufficient quality and quantity were obtained for 252 of the 455 pts enrolled in the study, as described previously (Fumagalli et al. JAMA Oncol 2016). Editing sites from the rediPortal database were assessed. The editing level at a given site was computed by counting the number of Gs and As. Sites with coverage more than 10 were considered for further analyses. Editing in normal tissues was obtained from the GTEx project of the rediPortal database. Tumor infiltrating lymphocytes (TILs) and copy number aberrations were previously reported. Correlations between different parameters were assessed using Spearman correlations (ρ). The Mann-Whitney test was used to relate binary and numerical features. Event-free survival (EFS) analysis was performed using the Cox proportional hazard model. Results There was a median of 71470 edited sites per sample. As expected, mean editing per sample correlated with ADAR expression (ρ=59%, p Conclusions Our study shows for the first time that deregulated RNA editing, as compared to editing in normal tissues, is a widespread phenomenon in HER2+ BC patients treated in the NeoALTTO trial and is associated with poor outcome. These results may provide new perspectives for the treatment of HER2+ disease by developing therapies targeting RNA editing. Citation Format: Venet D, Rothé F, Dupont F, Maetens M, Fumagalli D, Salgado R, Bradbury I, Pusztai L, Harbeck N, Izquierdo M, de la Pena L, Ignatiadis M, de Azambuja E, Huober J, Nuciforo P, Baselga J, Piccart M, Loi S, Sotiriou C. Deregulation of A-to-I RNA editing is associated with poor prognosis in HER2+ breast cancers in the neoALTTO trial [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-05-04.