A Genomic Locus Associated with Radiation Resistance of Oral Cancer Cells

While radiation treatment is the major adjuvant therapy for patients with advanced stages of oral squamous cell carcinoma (OSCC), acquired resistance of OSCC cells against ionizing radiation (IR) foreshadows its outcome. It was reported that cancer stem-like cells (CSLCs) were more resistant to IR than the rest of the cells, but its mechanism remains elusive. It is likely that a crucial CSLC factor that drives IR resistance is still missing, as some studies 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 the CSLC status, but also enhances DNA double-strand break repair (DSBR), and thus may be an important target to increase efficacy of IR-based therapy. However, what molecular alterations in OSCC increases PRMT5 expression is not known. In addition, DNA single-strand breaks (SSB) and oxidative DNA damage are more abundantly produced by IR than DSB, but whether PRMT5 also enhances repair of these lesions has not been elucidated at all. The PRMT5 gene is in chromosome 14 where APE1, a crucial repair factor for oxidative DNA damage, is closely localized. Through genomics data analysis, we found that expression of PRMT5 and APE1 are highly coregulated, which is caused by the local copy number variation. Furthermore, high levels of both PRMT5 and APE1 in OSCC are indication of poor outcome of radiotherapy. These observations led us to the central hypothesis of this project, that the expressional coregulation 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 the cellular sensitivity against IR. In Aim 1 of this project, we will analyze CNV of the genomic region and the levels of PRMT5 and APE1 proteins in OSCC tissues, to investigate whether (a) the gene level CNV in this region occurs in OSCC tissues 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 establish the mechanism of the PRMT5-driven IR resistance of cells by the gene level CNV, which may help develop not only a reliable biomarker but also a novel strategy of synthetic lethality that effectively targets OSCC that show high expression of both PRMT5 and APE1