p16 Expression Represses DNA Damage Repair via a Novel Ubiquitin-Dependent Signaling Cascade.

Human papillomavirus (HPV) drives the development of squamous cell carcinoma at several sites, including the oropharynx. Generally, the presence of HPV renders a tumor more sensitive to DNA-damaging therapies such as radiation; however, the mechanism behind this phenomenon is elusive. Previous studies have shown that p16, the clinically utilized surrogate for HPV tumor positivity, can render cells more sensitive to radiation. In the current manuscript, using a combination of immunoprecipitation mass spectrometry (IP/MS), in vivo and in vitro modulation and clinical tumor profiling, we identify a novel ubiquitin-dependent signaling pathway linking p16 to increased activity of the transcription factor SP1 leading to increased HUWE1 transcription and degradation of ubiquitin-specific protease 7 (USP7). This pathway is activated in HPV-positive tumor cells, leading to an absence of TRIP12, decreased DNA damage repair and increased mitotic death following radiation. As USP7 inhibitors are currently in clinical trials, this pathway provides a novel means by which radioresistant tumors may be targeted to increase response and improve outcome.