Abstract 3792: Restoring DREAM assembly in HPV-positive cancer cell lines promotes growth suppression
Authors: Sesay Fatmata , Saini Siddharth , James Claire , Felthousen Jessica , Morgan Iain , Litovchick Larisa 
Published: August 2019

Abstract
Human Papillomaviruses (HPV) 16 and 18 are recognized as causative factors in 4-5% of all human cancers, including the majority of cervical and oropharyngeal carcinomas. The HPV vaccine is expected to reduce the incidence of HPV-driven cancers in the future. However, millions of people in the US already infected with high-risk HPV are still at risk. Therefore, it remains a priority to improve our understanding of the mechanism of progression of the initial HPV infections to cancer. Furthermore, there are currently no therapies that specifically target HPV-positive cancers, despite evidence that viral genes continue to contribute to growth and survival of established cancer cell lines. In high risk HPV (those that cause cancer), viral protein E7 binds to pRb via a LxCxE motif to relieve repression of E2F1-mediated transcription of S-phase genes, whereas the E6 protein interacts with the tumor suppressor p53 protein and targets it for degradation. Therefore, the consequence of E6 and E7 expression in the host cell is an aberrant induction of cellular proliferation and suppression of apoptosis, which contribute to oncogenic transformation. Recently, it has also been demonstrated that E7 interaction with the pRb-like protein p130 disrupts the transcriptional repressor DREAM (dimerization partner, RB-like, E2F and MuvB) complex. In G0/G1, p130 serves as a scaffold to recruit E2F4/5-DP1/2 transcription factors, and the MuvB core complex of five proteins, to the promoters of approximately 800 cell-cycle regulated genes. In S-phase, DREAM dissociates and the MuvB core binds to B-Myb to promote expression of mitotic genes. The LIN52 subunit of the MuvB core serves as an adaptor that mediates the binding between p130 and MuvB core proteins. Our data show that LIN52 binds to the LxCxE-binding cleft in p130, and that the E7 protein can compete for p130 binding to LIN52 therefore disrupting the DREAM complex. Indeed, we found that the DREAM assembly was impaired in human cancer cell lines HeLa and SiHa containing integrated viral genomes of HPV18 and HPV16, respectively. Using our recently solved structure of the p130-LIN52 complex, we identified a LIN52 mutant (LIN52 S20C) that can compete for HPV E7 binding to p130. Stable expression of S20C-LIN52 in HeLa and SiHa cells resulted in significantly increased DREAM complex assembly as well as suppression of proliferation and colony formation in 2D and 3D cultures. Our findings identify the DREAM complex as an important tumor suppressor disrupted by high risk HPV, and support the rationale for targeting the interaction between the oncogenic HPV proteins and host cell factors as a therapeutic approach for treatment of HPV-positive cancers and pre-cancerous lesions.
Citation Format: Fatmata Sesay, Siddharth Saini, Claire James, Jessica Felthousen, Iain Morgan, Larisa Litovchick. Restoring DREAM assembly in HPV-positive cancer cell lines promotes growth suppression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3792.