Interrogating the role of arginine methyltransferases in radiation response in glioma

Radiotherapy (RT) is an important survival-prolonging treatment for glioblastoma (GBM), a largely incurable primary malignant brain tumor. The treatment failure relies on the adaptation of tumor cells to treatment and resistance, which is a major reason for inevitable recurrence. Understanding the molecular mechanisms behind this adaptive resistance and designing effective therapeutic strategies are of utmost priority. Therefore, finding epigenetic modulators of RT response, in other words, "radiosensitizers", is crucial for GBM patients. This thesis investigates the effects of Arginine methyltransferase family (PRMT) inhibition on the RT response of glioblastoma cells. PRMTs are among histone-modifying enzymes, which provides methylation of arginine residues on histones, thereby regulating transcriptional activity. While PRMTs have been functionally implicated in cancer and there is a considerable effort for various PRMT inhibitors as anti-cancer drugs, combinatorial effects of PRMT inhibitors and RT have not been well-characterized in GBM. Our interest in PRMT inhibitors stems from our lab's previously conducted unbiased chemical screen with epigenetic drugs, which identified several PRMT inhibitors as radiosensitizers in GBM cells. In this thesis, this finding was further explored in detail. First, effects of PRMT inhibitors as single agents were tested on GBM cell lines and optimal doses of drugs were selected to be further explored. To assess the combined efficacy of PRMT inhibitors and RT, cell viability analyses were conducted on U373 GBM cells treated with various radiation doses and various PRMT inhibitors (MS023, MS049, SGC707, and LLY283). Particularly, the drug MS023, targeting the Type-I PRMT family, exhibited a significant impact on survival and reduced proliferation capacity of GBM cells when combined with RT. Additionally, as a complement to the chemical approach, CRISPR/Cas9-based genetic ablation was employed to reduce the expression of genes encoding relevant PRMT enzyme family members. The combined effect of PRMT inhibition and RT was comprehensively examined through these diverse experimental approaches. In the subsequent stage of the thesis, the molecular mechanisms underlying the relationship between PRMT inhibition and RT response were explored by using RNA sequencing and the effects of MS023 and ionizing radiation on the transcriptome of GBM cells were defined. The results indicate that PRMT inhibition modulates the expression of genes related to the functioning of cell cycle and mitosis, which may partly account for the radiosensitization potential of these drugs in GBM. These findings establish a foundational framework for identifying RT resistance mechanisms and devising innovative combination therapy strategies tailored for GBM. The observed effects of PRMT inhibition on RT response offer promising avenues for future research and potential clinical applications.