CSIG-21. LINEAGE-SPECIFIC SPLICING OF ANNEXIN A7 MODULATES RECEPTOR TYROSINE KINASE TRAFFICKING THROUGH ISOFORM-SPECIFIC INTERACTIONS IN GLIOBLASTOMA: STRUCTURAL AND THERAPEUTIC INSIGHTS

Abstract Alternative splicing in the brain is lineage-specific, maintains tissue identity and creates distinct protein isoforms. Glioblastomas exploit exon skipping to generate cell-distinctive isoforms and alter isoform-specific protein interactions to sustain tumor phenotypes. Annexin A7 (ANXA7), a tumor suppressor involved in membrane scaffolding and vesicle trafficking, retains cassette exon 6 in post-mitotic neurons (ANXA7 isoform I1), which is skipped in precursor cells and their glioma-initiating descendants (ANXA7 isoform I2) by polypyrimidine tract-binding protein 1 (PTBP1). PTBP1 is upregulated in glioblastomas promoting splicing in favor of I2. In this study we used a combination of protein homology-modeling prediction and intracellular assays to elucidate the structural and functional differences between I1 and I2 and their impact on multiple oncogenic receptor tyrosine kinases (RTK) in glioblastoma. We show how I1 acts as a master regulator of RTK fate by interacting with various components of the endocytic machinery and sorting for degradation. By contrast, RTKs were diverted for recycling by I2 to permit unabated signaling. Knockdown of exogenous I1 reverted glioblastoma cells to an I2 phenotype. Protein structure prediction revealed that the cassette exon in I1 forms a secondary structure enclosing a domain that imparts the unique ability to interact with the endocytic machinery. To restore I1 in glioblastoma cells and revert to a tumor suppressive phenotype, we are currently testing a newly identified stapled peptide PTBP1 inhibitor in preclinical models. Our work provides critical insight into how glioblastoma cells exploit lineage-specific splicing in favor of isoforms that perpetuate tumorigenesis and how aberrant splicing of ANXA7 can be therapeutically targeted.