The human papillomaviruses (HPVs) are associated with specific human cancers. Over 70 different HPVs have now been identified, of which, over 25 have been associated with genital tract lesions. HPVs can be further classified as either """"""""high risk"""""""" (HPV 16,18) or """"""""low risk"""""""" (HPV 6,11) based on the clinical lesions with which they are associated and the relative propensity for these lesions to progress to cancer. Approximately 85-90 percent of human cervical cancers harbor the DNA of a """"""""high risk"""""""" HPV type and express the two well characterized viral oncoproteins, E6 and E7. One characteristic of HPV related carcinogenic progression is the frequent integration of the viral genome into the human chromosome in the cancer cells. This integration occurs in a manner that results in the loss of expression of the viral E2 gene leading to high levels of E6/E7 expression. Several observations suggest that the loss of E2 expression may be an important step in HPV associated carcinogenic progression. First, the E2 regulatory protein encoded by the genital HPVs can repress the E6/E7 promoter. Second, HPV16 genomes with mutations in the E2 open reading frame have an increased immortalization capacity in primary keratinocytes. Finally, the reintroduction of the E2 gene into HPV positive cervical carcinoma cell lines leads to suppression of growth and cell cycle arrest. The current model for the role of E2 in this process involves the down regulation of the E6/E7 promoter, resulting in the loss of expression of the viral oncoproteins, the induction of p53 and p21, and the consequent inhibition of the cyclin dependent kinases, leading ultimately to a G1 cell cycle arrest. However, recent data suggest that E2 transcriptional repression of E6/E7 is necessary but not sufficient to cause cellular growth suppression. Comparison of the activities of various BPV1 E2 proteins to those of the oncogenic and """"""""low risk"""""""" HPV E2 proteins indicates that E2 mediated growth arrest involves an additional activity that is independent of E2s ability to repress transcription of the viral E6/E7 promoter. In this grant application, we propose to examine the mechanisms by which E2 causes cellular growth suppression.
