High-risk human papillomaviruses (HPVs) are associated with over 99% of cervical carcinomas. During carcinogenic progression, the HPV genome frequently integrates into the host chromosome, and as a result, E6 and E7 are the only viral proteins that are consistently expressed in HPV-positive cervical cancers. During the viral life cycle, E6 and E7 function to establish and/or maintain a cellular milieu that supports viral genome replication. Because HPV genome replication and virion production occur in the most differentiated layers of the epithelium, E6 and E7 subvert pathways that signal growth arrest during differentiation. One of the best-characterized functions of HPV E7 is its ability to associate with and destabilize the retinoblastoma protein (pRB) tumor suppressor, and consequently inactivate the pRB-E2F repressor complex allowing for uncontrolled cell cycle progression. HPV16 E7 has also been reported to directly interact with E2F1 to activate E2F1-dependent transcription, thus allowing for cell cycle progression. E2F6, a recently identified member of the E2F family, lacks domains involved in transactivation and binding to the pRB family members. E2F6 is able to repress the transcription of known E2F-responsive genes, suggesting a role in controlling cell cycle progression by balancing the function of activating E2Fs. Utilizing the tandem affinity purification (TAP) method, our laboratory identified E2F6 and members of the polycomb group (PcG) as potential E7 interacting proteins. Intriguingly, E2F6 has been shown to be a component of the Bmi1-containing polycomb transcriptional repressor complex. We are currently analyzing these interactions to determine what role they play in HPV-associated carcinogenesis. The interaction between HPV16 E7 and E2F6 has been confirmed by co-immunoprecipitations, and the interactions between E7 and the other members of the PcG group are currently being investigated. We have found that HPV16 E7 interacts with the transcriptional repressor E2F6 and relieves transcriptional repression on E2F response elements. We hypothesize that E7 relieves E2F6-mediated transcriptional repression, thus providing yet another layer of control by which HPV ensures that the infected host cell remains S-phase competent, which is vital for viral genome replication and likely contributes to cellular transformation. We are currently investigating what domains and functions of E7 and E2F6 are essential for E7 release of E2F6-mediated transcriptional repression and are determining the effect of E7 on the ability of E2F6 to repress E2F-dependent transcription during G0 and G1/S. Results from these studies will shed further light on the mechanisms by which HPV E7 abrogates the cell cycle, allowing for cervical carcinogenesis.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]