Transposon Mutagenesis Reveals RBMS3 Silencing as a Promoter of Malignant Progression of BRAF(V600E)-Driven Lung Tumorigenesis

Mutationally activated BRAF is detected in approximately 7% of human lung adenocarcinomas, with BRAF(T1799A) serving as a predictive biomarker for treatment of patients with FDA-approved inhibitors of BRAF(V600E) oncoprotein signaling. In genetically engineered mouse (GEM) models, expression of BRAF(V600E) in the lung epithelium initiates growth of benign lung tumors that, without additional genetic alterations, rarely progress to malignant lung adenocarcinoma. To identify genes that cooperate with BRAF(V600E) for malignant progression, we used Sleeping Beauty–mediated transposon mutagenesis, which dramatically accelerated the emergence of lethal lung cancers. Among the genes identified was Rbms3, which encodes an RNA-binding protein previously implicated as a putative tumor suppressor. Silencing of RBMS3 via CRISPR/Cas9 gene editing promoted growth of BRAF(V600E) lung organoids and promoted development of malignant lung cancers with a distinct micropapillary architecture in BRAF(V600E) and EGFR(L858R) GEM models. BRAF(V600E)/RBMS3(Null) lung tumors displayed elevated expression of Ctnnb1, Ccnd1, Axin2, Lgr5, and c-Myc mRNAs, suggesting that RBMS3 silencing elevates signaling through the WNT/β-catenin signaling axis. Although RBMS3 silencing rendered BRAF(V600E)-driven lung tumors resistant to the effects of dabrafenib plus trametinib, the tumors were sensitive to inhibition of porcupine, an acyltransferase of WNT ligands necessary for their secretion. Analysis of The Cancer Genome Atlas patient samples revealed that chromosome 3p24, which encompasses RBMS3, is frequently lost in non–small cell lung cancer and correlates with poor prognosis. Collectively, these data reveal the role of RBMS3 as a lung cancer suppressor and suggest that RBMS3 silencing may contribute to malignant NSCLC progression. SIGNIFICANCE: Loss of RBMS3 cooperates with BRAF(V600E) to induce lung tumorigenesis, providing a deeper understanding of the molecular mechanisms underlying mutant BRAF-driven lung cancer and potential strategies to more effectively target this disease.