Manipulating translesion synthesis proteins MAGE-A4 and RAD18 in a murine model of oral cancer

Head and neck squamous cell carcinomas (HNSCCs) account for 6% of cancers worldwide. Because of their link to carcinogens such as alcohol and tobacco, HNSCCs are characterized by a high mutational burden and genome instability. Cancer recurrence due to chemoresistance, or a tolerance of chemotherapy-induced mutations, is the major cause of mortality in HNSCCs. DNA damage response pathways, such as translesion synthesis (TLS), are activated in response to genotoxic stress, including from conventional chemotherapy. The TLS pathway facilitates repair of certain types of mutations and thus is important for DNA damage tolerance. However, TLS polymerases are error-prone and therefore, paradoxically, may promote genome instability. In this way, elevated or “pathologic” TLS in the context of a cancer cell may be a “double-edged sword,” conferring damage tolerance but at the expense of introducing additional mutations that could fuel tumor growth. We have previously shown that melanoma antigen A4 (MAGE-A4) recruits the E3 ubiquitin ligase Rad18 to stalled replication forks where it monoubiquitinates PCNA to recruit TLS polymerases to bypass the lesion. In HNSCC, MAGE-A4 overexpression correlates with both increased mutational burden and worse prognosis. We hypothesize that aberrant high MAGE-A4 expression tolerizes cancer cells to excessive mutations, making them invulnerable to the DNA-damaging effects of traditional chemotherapies. In order to gain a better understanding of the consequences of MAGE-A4 overexpression and TLS activation in oral tumors in vivo, we created a genetically engineered mouse model that expresses MAGE-A4 under the control of an epithelial-specific Cre driver. When treated with the potent mutagen 4NQO in their drinking water, both transgenic and control mice developed oral tumors. The collected squamous cell carcinomas were analyzed using whole-exome sequencing and immunofluorescence to examine whether MAGE-A4+ tumors gained more mutations and were more invasive than MAGE-A4- or RAD18 KO tumors. Together, these data provide a greater understanding of MAGE-A4 and RAD18 biology and TLS in oral cancers. Citation Format: Bethany L. Wagner, Jason Guo, Yanzhe Gao, Cyrus Vaziri, Scott E. Williams. Manipulating translesion synthesis proteins MAGE-A4 and RAD18 in a murine model of oral cancer [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; 2019 Apr 29-30; Austin, TX. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_2):Abstract nr B10.