Long-term genome stability and minimal genotypic and phenotypic alterations in HPV16 E7-, but not E6-, immortalized...

Parameters of genome instability and morphological alterations associated with cell transformation were studied in an isogeneic set of clonal human uroepithelial cell (HUC) lines immortalized by the human papilloma virus 16 (HPV16) E6 and/or E7 gene(s). HPV16 E6 binds p53, leading to rapid degradation of p53, whereas E7 binds and alters pRb and other proteins. We report that two independent E7-immortalized HUC lines showed minimal phenotypic or genotypic alterations, except that both lines contained amplification of 20q DNA sequences and a greater polyploidization at an early passage. The E7-immortalized HUC line resembled normal HUC lines, except that they failed to senesce. In contrast, the E6-immortalized HUC lines were morphologically altered, contained numerous random chromosome aberrations, and showed unstable evolving karyotypes with passage in culture. No amplified DNA sequences were detected in E6-immortalized HUC lines. Instead, clonal losses of chromosome regions (i.e., -3p, -6q, -9p), putatively containing tumor suppressor or senescence genes, accompanied the E6-HUC immortalization event. E6-immortalized HUC lines showed transformed phenotypes similar to E6/E7-HUC lines. The difference in genome stability between E6- and E7-immortalized HUC was highly significant statistically (p-value < 10(-6). Thus, the HPV16 E7 gene led to HUC immortalization by a pathway that blocked cellular senescence, but did not disrupt genome stability. These results implicate p53 loss, but not pRb alteration, in genome destabilization.