Abstract 3747: Dysregulated splicing of Annexin A7 impairs receptor tyrosine kinase trafficking in glioblastoma
Authors: Nair Sindhu , Rajbhandari Rajani , Dates-Hurt Centdrika , Bredel Markus 
Published: September 2020

Abstract
Alternative splicing (AS), a tightly regulated process, contributes to proteome diversification essential for tissue development and identity. During brain development, splicing targets include genes involved in several cellular processes like endocytosis, cytoskeleton dynamics and vesicle-mediated transport. Deregulation of AS in cancers enables tumor cells to alter the transcriptome in favor of isoforms that drive tumor progression. 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 by PTBP1 and expressed as two isoforms - unspliced ANXA7 Isoform 1 (I1), containing cassette exon 6 and spliced Isoform 2 (I2). We determined that this splicing of ANXA7 is lineage-specific in the brain - I1 is highly expressed in post-mitotic mature neurons while I2 is expressed in neural and glial precursors. In order to understand the functional impact of these isoforms, we generated GBM cell lines that express endogenous I2 (I2 cells), or endogenous I2 + exogenous I1 (I1 cells). We evaluated the influence of I1 and/or I2 expression on RTK levels, activation and downstream events using western blot and co-immunoprecipitation assays. Receptor trafficking was observed using flow cytometry and immunofluorescence assays tagging different components of the endocytic pathway. In GBM, we observed high levels of PTBP1 with a subsequent increase in I2 levels indicative of dysregulated AS. Consequently, we observed upregulated activation of several receptor tyrosine kinases (RTKs) such as EGFR, MET, PDGFRα and EGFR vIII in the I2 cells. In I1 cells, however, we observed reduced levels and activation of all aforementioned RTKs along with the diminished activation of downstream pathways. Using EGFR signaling as a model, we found that in I1 cells EGFR co-localized with markers of lysosomal degradation indicating efficient trafficking of activated EGFR for endosomal degradation; co-localization was absent in I2 cells indicative of impaired trafficking. We propose that I1, by virtue of the amino acids encoded by cassette exon 6, binds to conserved motifs within multiple RTKs and targets them for endosomal degradation thereby attenuating tumorigenic signaling in GBM. Our results show that ANXA7 I1 is tumor suppressive in GBM and that loss of I1 promotes tumorigenicity by perpetuating RTK signaling. We present a mechanism that explains, in part, how I1 mediates the degradation of multiple tumorigenic RTKs by spatio-temporally regulating their endosomal trafficking. A refined understanding of how I1 functions will provide the foundation for future, selective therapies that target dysregulated AS in GBM.
Citation Format: Sindhu Nair, Rajani Rajbhandari, Centdrika Dates-Hurt, Markus Bredel. Dysregulated splicing of Annexin A7 impairs receptor tyrosine kinase trafficking in glioblastoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3747.