CSIG-12. ANNEXIN A7 ISOFORMS DIFFERENTIALLY REGULATE TUMORIGENIC EGFR SIGNALING IN GLIOBLASTOMA MULTIFORME

Dysregulation of receptor tyrosine kinase genes is a frequent genetic abnormality in GBM. A common genetic alteration is amplification and/or overexpression of the epidermal growth factor receptor (EGFR) gene. Annexin A7 (ANXA7) is a hydrophilic protein that binds membranes in a calcium dependent manner; it is important for membrane scaffolding and vesicle trafficking. ANXA7 is alternatively spliced and expressed as two isoforms – unspliced ANXA7 Isoform 1 (I1), containing cassette exon 6 and spliced Isoform 2 (I2). Our lab has determined that I1 is tumor suppressive in GBM and that loss of I1 promotes tumorigenicity by perpetuating EGFR signaling. However, if restored, I1 but not I2, reduces EGFR levels, activation and downstream pathways and diminishes tumorigenicity. In patients, I1 levels directly correlate with survival and prognosis. We propose that I1 inhibits EGFR signalling by binding to and promoting degradation of EGFR in a calcium dependent manner. In GBM, I1 is absent, preventing endosomal degradation of EGFR and leading to perpetual signalling. Our approach is novel; rather than address how/whether EGFR is activated, we focus on why EGFR fails to be downregulated afterwards. Herein, we present a mechanism that explains, in part, how I1 mediates endosomal degradation of EGFR. Further studies are underway to identify exactly where and how I1 modifies endosomal trafficking and degradation of EGFR. We are also exploring the role of I1 in regulating the degradation of other receptor tyrosine kinases (RTKs), such as MET and PDGFRA. Collectively, understanding how I1 functions will provide the foundation for future, selective therapies that restore I1 expression and/or function to reduce GBM tumorigenicity.