Human papillomavirus-mediated carcinogenesis and tumor progression

High-risk human papillomaviruses (HPV) cause 5% of all human cancers and are primary etiologic agents of cervical, anal, and oropharyngeal cancer. HPV infection is necessary, but not sufficient per se to produce cancer: additional changes must occur that transform HPV-infected cells to malignancy. The HPV oncoproteins E6 and E7 immortalize human keratinocytes, cervical cells, and fibroblasts in culture. Each oncoprotein interacts with a variety of cellular binding partners; most important for transformation are E6 and E7’s interactions with p53 and RB (respectively) which lead to degradation of p53 and RB through the ubiquitin pathway. Inactivation of p53 and RB leads to inactivation of pivotal cell cycle checkpoints, thereby stimulating cell proliferation and allowing cell division to occur independently of the presence of DNA damage, replicative stress, and other such insults, leading to genome instability. Continuous expression of E6/E7 drives the proliferation and progression of most HPV-mediated cancers of the cervix and a substantial fraction of those of the oropharynx. However, at both cancer sites, “HPV-inactive” tumors that contain HPV DNA, but do not express E6/E7 arise. We propose that these HPV-inactive cancers begin as HPV-driven lesions, but lose E6/E7 expression at some point during progression. We have recently shown that p53 deletion in HPV-immortalized, premalignant cells allows for the emergence of cell populations that no longer express E6/E7. These findings corroborate the notion of a pivotal role of p53 in the context of HPV-mediated transformation, both at the initiation and progression stages of cancer development.