Abstract 1652: P16INK4a over-expression sensitizes HPV(-) HNSCC to radiation through down-regulation of USP7

Background: Head and neck squamous cell carcinoma (HNSCC) tumorigenesis induced by human papillomavirus (HPV) shows a greater clinical response to radiation therapy when compared to HPV negative patients. The underlying mechanism for this more favorable outcome is unknown, but the presumption is that DNA repair response must play a large role. P16INK4a is a known surrogate marker for HPV positivity; however the relationship between radiation sensitivity and P16INK4a has not been completely elucidated in this context. Ubiquitin specific protease 7 (USP7), also known as HAUSP, is a deubiquitylating enzyme that cleaves ubiquitin from substrates to stabilize target proteins. USP7 overexpression is a predictor of poor prognosis in lung carcinomas and correlates with disease progression and lower survival in gliomas. More recent studies have shown USP7 has a role in the stability of DNA damage response proteins such as RNF168, BRCA1 and Chk1; and USP7 knockdown has been shown to radio-sensitize breast cancer cells to radiation. We have investigated the possible role of USP7 in the radio-response of HNSCC cells in the context of HPV status. Methods: Four HPV(+) and four HPV(-) HNSCC were chosen for this study. The radio-response was determined by clonogenic survival assay. P16INK4a overexpression was generated using lentivirus. Western blot analysis was used for visualizing protein expression. The Cancer Genome Atlas was interrogated to examine the relationship between USP7 and clinical outcome. Results: Our data suggest P16INK4a negatively regulates USP7 and leads to the more radiosensitive phenotype associated with HPV(+)HNSCC. Radio-resistant HPV(-) HNSCC cell lines show higher levels of USP7 than HPV(+) lines and over-expression of P16INK4a in HPV(-) HN5 cells leads to down-regulation of USP7 and radio-sensitization. Conversely, P16INK4a siRNA knockdown in HPV(+) UMSCC-47 shows an increase of USP7 protein expression and radio-resistance. P22077, an inhibitor of USP7 activity, was also shown to radiosensitize a HPV(-) cell line. This shows proof of principle that inhibiting USP7 can be a viable approach to sensitizing HPV(-) HNSCC to radiation. USP7 overexpression was also associated with poorer overall survival in HNSCC. Conclusion: These results suggest that USP7 may be a marker of clinical radioresistance in HNSCC and that its inhibition has the potential to improve the treatment outcome of patients with HPV(-) HNSCC when combined with radiotherapy. Citation Format: David Molkentine, Li Wang, Kathleen Bridges, Kathy Mason, Raymond Meyn, Heath D. Skinner. P16INK4a over-expression sensitizes HPV(-) HNSCC to radiation through down-regulation of USP7. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1652.