ABSTRACT While radiation treatment is the major adjuvant therapy for patients with advanced stages of oral squamous cell carcinoma (OSCC), the acquired resistance of OSCC cells against ionizing radiation (IR) reduces its efficacy. It was reported that cancer stem-like cells (CSLCs) were more resistant to IR than other cells, but the mechanism by which CSLCs acquire IR resistance remains elusive. A crucial CSLC factor that drives IR resistance needs to be characterized, as studies have failed to observe IR resistance of cells with known CSLC markers such as CD133 and CD44. Recent evidence indicates that PRMT5 (protein arginine methyl transferase 5) is crucial not only for maintaining CSLC status, but also for enhancing DNA double-strand break repair (DSBR), and thus may be an important target to increase efficacy of IR-based therapy. However, the molecular alterations in OSCC that increase PRMT5 expression are not known. In addition, DNA single-strand breaks (SSB) and oxidative DNA damage are more abundantly produced by IR than DSB, but whether high PRMT5 expression is also associated with increased repair of these lesions has not been elucidated. The PRMT5 gene is in chromosome 14 where APE1, a crucial repair factor for oxidative DNA damage, is closely localized. By scrutinizing genomics data, we found that expression of PRMT5 and APE1 are highly coordinated, which is likely caused by the local copy number variation. Furthermore, we identify high levels of both PRMT5 and APE1 in OSCC as an important indication of poor radiotherapy outcomes. These observations led us to the central hypothesis of this project, that the expressional co-regulation of PRMT5 and APE1 is caused by the local copy number variation in the OSCC tissues prior to radiotherapy, and levels of these genes synergistically influence cellular sensitivity against IR. In Aim 1 of this project, we will analyze the copy number variation (CNV) of the genomic region in OSCC tissues and the levels of PRMT5 and APE1 proteins by immunohistochemistry, to investigate whether (a) CNV at the gene level in this region occurs in early OSCC development before radiotherapy and (b) the CNV is associated with the PRMT5 and APE1 protein levels. In Aim 2, we will elucidate whether simultaneous knockdown or inhibition of PRMT5 and APE1 effectively sensitizes OSCC cells to IR. The long-term goal of this project is to delineate the novel mechanism of the PRMT5-driven IR resistance of cells by the gene level CNV, which should help us develop not only a reliable biomarker, but also a novel strategy of synthetic lethality that effectively targets OSCC with high levels of PRMT5.