Endogenous interaction profiling identifies DDX5 as an oncogenic coactivator of transcription factor Fra-1 [ChIP-seq]

Fra-1, a member of the activator protein 1 (AP-1) family, is overexpressed in triple-negative breast cancer (TNBC) and plays crucial roles in tumor progression. However, a systematic analysis of the composition of the Fra-1 protein network specifically on chromatin is still missing. Here we performed endogenous purification of Fra-1 transcriptional complex under ChIP conditions, followed by mass spectrometry, to identify chromatin-bound partners of Fra-1 in TNBC cells. This study allowed the identification of 118 interactors, highlighting DDX5 as the high ranking of Fra-1 interacting proteins. DDX5, a previously unrecognized protein in the network, is recruited globally to Fra-1 binding sites and shares a substantial set of Fra-1 target genes required for the TNBC cell growth. We provide evidence that DDX5 expression enhances Fra-1 transcriptional activity, thereby enhancing Fra-1-driven tumorigenesis. By integrating ChIP-seq and RNA-seq, we show that DDX5 target gene signature predicts poor clinical outcome in breast cancer patients. DDX5 protein level was higher in triple-negative basal like tumors than in non-basal like tumors, including luminal A, luminal B and HER2-enriched subtypes. Collectively, this comprehensive Fra-1 interaction profiling provides a broad and deep view of Fra-1 chromatin interaction landscape, which will help in deciphering mechanisms of AP-1 regulation of gene expression. Identifiation of Fra-1 and DDX5 specific and common binding events in BT549 cells.