All ETDs from UAB

Advisory Committee Chair

Markus Bredel

Advisory Committee Members

Susan Bellis

Chenbei Chang

James Collawn

Stuart Frank

Document Type

Dissertation

Date of Award

2021

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Alternative splicing (AS) is a tightly regulated process essential for lineage specification in complex tissues like the brain. Dysregulated splicing in glioblastoma (GBM) is a mechanism exploited by tumor cells to retain or splice out exons consequently rewiring isoform-specific protein interactions to sustain tumor phenotypes. Receptor tyrosine kinases (RTK) amplifications are frequent events in GBM driving tumor growth and progression and are key targets for chemotherapy. However, RTK targeting in GBM has achieved limited success predominantly due to adaptive mechanisms of resistance in a constantly evolving tumor microenvironment. Clonal populations and crosstalk between RTKs sustain heterogeneity within a tumor leading to the failure of targeted RTK therapies. We previously found that monosomy of chromosome 10 caused haploinsufficiency of tumor suppressor ANXA7 with a concurrent amplification of EGFR indicating an inhibitory effect of ANXA7 on EGFR signaling in GBM. ANXA7, a member of the annexin family, binds membranes in a calcium dependent manner and regulates endo- and exocytosis. ANXA7 is alternatively spliced by PTBP1 into either isoform 1 (I1), containing a cassette exon, or isoform 2 (I2) lacking the cassette exon. In GBM, high levels of PTBP1 ensure splicing of ANXA7 in favor of I2 with a subsequent elevation of EGFR signaling. Reintroducing I1 into GBM cells lead to a decrease in tumor growth and angiogenesis along with an inhibition of EGFR signaling. How I1 mediates EGFR downregulation is not clear. In this dissertation we dissect the mechanism by which ANXA7 isoforms have divergent impacts on RTK signaling in GBM. I1 mediates the sorting of multiple RTKs such as EGFR, MET, PDGFRα and EGFRvIII for lysosomal degradation thereby abrogating signaling while RTKs are recycled in I2 expressing cells. Using predictive structural modeling, we show that the cassette exon region in I1 encloses a domain that potentially interacts with RTKs as well as components of the endocytic machinery conferring it with the unique ability to target RTKs for lysosomal degradation. The overarching goal of this study is to better understand the functional impact of AS in GBM and how targeting AS to retain tumor suppressive isoforms could offer an alternative approach to target GBM.

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