Disruption of either the E1 or the E2 regulatory gene of human papillomavirus type 16 increases viral immortalization capacity

Abstract

The "high-risk" human papillomavirus types 16 (HPV-16) and 18 (HPV-18) have been etiologically implicated in the majority of human cervical carcinomas. In these cancers, the viral DNAs are often integrated into the host genome so that expression of the E1 and the E2 genes is lost, suggesting that disruption of these regulatory genes plays an important role in carcinogenic progression. Previous studies defining the viral genes affecting HPV-16 transformation functions have used the "prototype" viral genome, which was cloned from a human cervical carcinoma and later discovered to harbor a mutation in the E1 gene. In this study, we have corrected this mutation and have evaluated the effect of mutations of either the E1 or the E2 gene on the efficiency of HPV-16 immortalization of human keratinocytes. Mutation of either the E1 gene or the E2 gene in the background of a "wild-type" HPV-16 genome markedly increased immortalization capacity. Mutations were also generated in the E2-binding sites located upstream of the P97 promoter, which directs synthesis of the viral E6 and E7 transforming genes. E2 negatively regulates the P97 promoter through binding at adjacent sites. Surprisingly, the mutation of these sites only partially relieved the negative effect of E2 on viral immortalization, implicating additional mechanisms in the E2 repression of viral immortalization functions. Our results provide genetic evidence that the E1 and E2 gene products each can repress HPV-16 immortalization and support the hypothesis that a selective growth advantage is provided by integration of the viral genome in a manner that causes the loss of expression of either E1 or E2.