High-risk human papillomavirus (HPV) is a known risk factor in the etiology of cervical intraepithelial neoplasia (CIN) I-III and invasive cervical carcinoma (ICC). The most severe preinvasive lesion is CIN III, and it is still not entirely understood why some cases progress to invasion, whereas others do not. Our hypothesis that this could be predicted by intratype variation of the immortalizing and transforming early proteins E6 and E7 was tested. Because HPV16 is frequently detected in cervical neoplastic lesions, 25 CIN III and 17 ICC cases from Swedish women, all positive for this genotype, were selected to investigate the E6 and E7 genes for mutations. PCR-amplified products were sequenced by the fluorescent dideoxy termination method. ICC harbored almost exclusively HPV16 E6 variants (94%) and rarely harbored the prototype (6%), whereas CIN III demonstrated a more uniform distribution of variants (56%) and prototype (44%; P = 0.013). All variants contained variations that were identified in areas likely to be important for protein-protein interaction with p53 or in areas of immunological significance. The most frequent E6 variation was seen at residue 83. This polymorphism was detected alone or in combination with others in 88% of ICC and 44% of CIN III cases. E7 variations were extremely rare and were only detected together with E6 variations in 4% of CIN III and in 6% of ICC cases, suggesting that the HPV16 E7 but not the HPV16 E6 oncoprotein is highly conserved in vivo. This indicates that HPV16 E6 variants, specifically those containing the substitution at residue 83, may be more oncogenic than the prototype and thus carry a higher risk for the development of invasive cervical disease. This may be due to subtle differences in the type of transformation produced or to evasion of host immune defenses. These results might have implications for future in vitro studies, diagnostics, treatment, and vaccine design.
The work reported in this paper was undertaken during the tenure of a Research Training Fellowship awarded by the IARC (to I. Z.).