Therapeutic targeting of the SWI/SNF chromatin remodeler to regulate GBM chemosensitivity

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Glioblastoma (GBM) is the most common primary malignancy of the adult brain and is among the most devastating cancers as it is invariably lethal. The perivascular niche and necrotic regions of GBM tumors is enriched for stem-like tumor cells called GBM stem cells (GSCs) that are highly resistant to therapy. Within GBM tumors there are also differentiated cells that are intrinsically resistant, or acquire resistance to, therapy. Thus, there is a critical unmet need to identify and target the molecular pathways that promote GBM cancer stemness, which will enhance GBM sensitivity to currently approved therapies. This proposal is premised on several of our key findings: 1) the BRG1 catalytic subunit of the SWI/SNF chromatin remodeling complex promotes the GBM GSC phenotype and resistance to DNA alkylating agents such as temozolomide (TMZ) used to treat GBM patients; 2) PFI-3, a small molecule BRG1 bromodomain (BRD) inhibitor (BRI), improves GSC sensitivity to DNA alkylating agents by reducing GSC stemness; 3) next generation BRIs were developed that overcome the resistance of TMZ-resistant GBM cell lines, including a potent BRG1-specific BRI (IV-255); and 4) BRIs increase expression of a subclass of interferon (IFN) response genes that are predicted to enhance the anti-tumoral host immune response. Our central hypothesis is that targeting the BRD of BRG1 selectively disrupts GBM GSCs maintenance, which enhances GBM sensitivity to approved therapies and increases GBM immunoreactivity. In Aim 1 we will restore BRG1 expression in BRG1-KD (knockdown) GSCs and in TMZ resistant GBM cells with either wild-type BRG or mutant BRG1 BRD that prevents chromatin binding. We will examine the role of the BRG1 BRD in maintaining the GBM malignant phenotype and therapeutic sensitivity in vitro and in vivo. In Aim 2 we will refine the BRI structure and develop additional small molecule BRG1 BRIs. We will define BRI activity on cancer stemness and therapeutic sensitivity on GBM in vitro and in vivo, and assess selectivity for BRD binding, drug-like properties and optimize BBB penetrance. The anticancer efficacy of BRIs on intracranial GBM tumors will be tested in both immunocompromised and immunocompetent GBM mouse models to define their impact on tumor-associated immune cells. In Aim 3 we will define how BRD targeting of BRG1 reprograms transcription to promote GBM immunoreactivity and inhibit tumorigenesis by performing global transcriptome analysis (RNA-seq), which will be coupled with BRG1 and RNA polymerase II specific ChIP. These BRD-regulated genes will define the BRG1- dependent transcriptome in GSCs, and it also will identify candidate molecular pathways that could be pharmacologically targeted with BRG1 inhibition to get cancer cell synthetic lethality. Our overarching goal is to target BRG1 therapeutically with BRIs combined with presently employed therapies as a novel and urgently required therapeutic approach for this devastating form of brain cancer.