HGG-13. ONCOGENIC STAT3 AND SHP2 ACTIVATION IN ROS1-FUSION DRIVEN INFANT-TYPE HEMISPHERIC GLIOMAS

Genomic analyzes identified a subgroup of gliomas – infant-type hemispheric gliomas – that are driven by oncogenic fusions of, amongst others, ROS1 receptor tyrosine kinase with a variety of fusion partners. These fusions result in ligand independent constitutively active oncogenic kinases. In the present study we aim to identify the, yet poorly characterized, alterations in cellular signaling driven by ROS1 fusions leading to tumor formation to enhance our understanding of IHG formation and to identify complementary targets for therapy. To answer these questions, we deploy a combination of biochemical characterization of cellular models as well as in vivo experiments and transcriptomic and phosphoproteomic analyzes. Expression of different ROS1-fusions results in robust tumor formation in two different mouse models, proving their tumorigenic potential. The impact of the fusion partner, which dictates subcellular localization, as shown by immunofluorescence microscopy, on differences in tumor formation has not yet been established. Transcriptomic and phosphoproteomic analyses of ROS1-fusion cellular models revealed diverging effects of the fusions on transcriptomic and proteomic alterations. A common denominator, however, is a significant hyper-phosphorylation of the tyrosine phosphatase SHP2, which we established to be a direct substrate of ROS1. ROS1 fusions, irrespective of the fusion partner, drive non-canonical STAT3 signaling, indicated by direct phosphorylation of STAT3 by ROS1 and upregulation of STAT3 target genes. Characterizing the main signaling pathways using immunoblots did not detect alterations in the PI3K/AKT/mTOR or MAPK pathways. STAT3 signaling is a well characterized pathway for promoting pro-survival, proliferation and stemness cues in development and cancer, making it a paramount candidate for ROS1 driven tumorigenesis as well as an excellent target for targeted therapies.