Glioblastoma (GBM) is a devastating cancer, due to both our narrow understanding of its molecular drivers and limited therapeutic strategies. One potential mechanistic driver is alternative splicing. The brain contains the most alternatively spliced transcripts of any organ, and many splicing factors are upregulated between normal brain and GBM. While chemotherapeutic options are limited by the physical blood brain barrier (BBB), the DNA- damaging agent temozolomide (TMZ) is able to cross into the brain. However, most patients rapidly become resistant to TMZ and TMZ-resistant GBM is uniformly fatal. An initial goal of my PhD research was to establish novel TMZ-resistant cellular models in order to identify pathways that could be targeted for GBM treatment. My comprehensive characterization of the cell growth, motility, and metabolic phenotypes of my two new TMZ- resistant GBM models forms the basis for my initial first-author paper. During my dissertation research (Aim 1), I have conducted two complementary studies that identify novel approaches to targeting alternative splicing events in GBM.
The first (Aim 1. 1) is to target the alternatively spliced estrogen-related receptor beta (ERR?). I have started to define with in silico and in vitro methods how the pro-apoptotic isoform of this gene, ERR?2, is processed. I found that the serine/arginine (SR) rich splicing factor SRSF6 plays a role in ERR?2 production and that inhibition of Cdc-like kinases (CLKs, which phosphorylate SR proteins) with TG-003 in combination with the ERR? synthetic agonist DY-131 potently inhibits TMZ-resistant GBM cells in vitro and in intracranial xenografts.
The second (Aim 1. 2) is a broader study of splicing inhibition and regulation in TMZ-resistant GBM. I found that TMZ decreases the phosphorylation (p) of SR proteins in TMZ-sensitive, but not TMZ-resistant models. This is accompanied by mis-localization of pSR proteins, and increased baseline levels of DNA damage. In TMZ- resistant GBM cells, the RNA binding protein EWS also mis-localizes and forms aggregates that are stabilized by tubulin. My working hypothesis is that because of the increased DNA damage in TMZ-resistant GBM, the DNA damage response becomes reprogrammed which causes splicing factors (like EWS and pSR proteins) to be displaced from their normal cellular compartments and poised for aberrant aggregation. Also, that this new splicing factor/DNA damage repair axis can be therapeutically targeted with novel splicing inhibitors. During the postdoctoral training period (Aim 2), I will address a key gap in our understanding of the GBM transcriptome: the role of non-coding RNAs, specifically the noncanonical back-spliced or circular RNAs (circRNAs). I propose to define the circRNA landscape of GBM, to determine the regulatory functions and to propose potential therapeutic applications of these abundant and dynamic regulators of splicing and transcription. Together, my pre- and postdoctoral research experiences will have prepared me to balance both big picture ideas and focused studies of mechanism when I establish my own research program as an independent cancer researcher.
In the transition from normal brain to glioblastoma (GBM), tumor cells modify many cellular processes in order to survive, including those that repair DNA and regulate the alternative splicing of gene products. This proposal seeks to determine how the DNA repair and splicing machinery are reorganized in GBM that no longer responds to chemotherapy. The ultimate goal of this research is to translate this knowledge into new treatment strategies for drug-resistant GBM.
