Role of the Human Papillomaviruses in Human Cancer1

The papillomaviruses associated with human anogenital carcinomas encode two transforming genes, E6 and E7. The oncoprotein products of these two genes complex with the tumor suppressor gene products p53 and pRB, respectively. The loss of the normal function of these tumor suppressor gene products, either as a consequence of their association with l-'.fiand E7 or by mutation, appears to be a common event in human cervical carcinogenesis. Compelling evidence now associates specific human papillo maviruses with certain human anogenital cancers, most notably cervical cancer. Numerous epidemiological studies have impli cated a venereally transmitted agent in human cervical carci noma (1, 2). Sexual transmission of an agent with prolonged latency of perhaps 20 to 25 years is suggested by these epide miological studies. The suggestion of an infectious agent in the etiology of cervical cancer has led investigators to evaluate genital pathogenic organisms as potential candidate factors. Approximately 20 years ago, HSV2 type 2 was considered a potential etiological factor (3, 4). Molecular biological studies, however, failed to provide any convincing evidence for the presence of HSV nucleic acids in cervical cancers (5). A pro spective epidemiological study carried out by Vonka et al (6, 7) failed to support an involvement of HSV-2 infection as a principal etiological factor in cervical cancer or in preneoplastic lesions. At this point, however, it is not possible to totally exclude HSV as a potential cofactor perhaps working in concert with the HPVs. The first concrete evidence suggesting a role of HPV infection with cervical cancer was the recognition that morphological abnormalities that constitute cervical dysplasia (also referred to as CIN) were the cytopathic effects of a papillomavirus infection (8-10). Further support for a papillomavirus etiology in cervical dysplasia came from the demonstration of papillomavirus capsid proteins in the nuclei of some cells in approximately 50% of cases of cervical dysplasia examined (11, 12). Initial experi ments examining for the presence of HPV DNA sequences in cervical cancers were generally negative. In retrospect, however, these negative data can be explained by the marked plurality of HPV types which are now recognized (13) and the fact that the papillomavirus DNA probes used at that time were not appro priate. The recognition, however, that cervical dysplasia (CIN) was a papillomavirus infection of the cervix, which is accepted as a preneoplastic lesion which may in some cases proceed to cervical cancer, inspired a search of cervical cancers for the evidence of HPV DNA. There are approximately 65 different types of HPVs which have now been described (13). Approximately 20 of these HPVs are associated with anogenital lesions, and these HPVs can be further classified as either "high risk" or "low risk" based on whether or not the genital tract lesions with which these HPVs ' Presented at the Symposium, "Discoveries and Opportunities in Cancer Research: A Celebration of the 50th Anniversary of the Journal Cancer Research" May 15, 1991, during the 82nd Annual Meeting of the American Association for Cancer Research, Houston, TX. 2The abbreviations used are: HSV, herpes simplex virus; HPV, human papil lomavirus; CIN, cervical intraepithelial neoplasia. are associated have a risk for malignant progression. The "low risk" viruses such as HPV-6 and HPV-11 are associated with venereal warts or condyloma acuminata and these rarely prog ress to malignancy. In contrast, the "high risk" viruses such as HPV-16 and 18 are associated with CIN, which are at risk for progression to malignancy. It was the laboratory of Harald zur Hausen that initially cloned HPV-16 and HPV-18 DNAs from cervical carcinoma tissues (14, 15) and, using these two DNAs as probes, were able to demonstrate that approximately 70% of the human cervical carcinomas contain DNA of these HPV types (16). HPV-33 was subsequently cloned and appears to be present in a subset of invasive cervical carcinomas (17), and other specific HPVs now are recognized as part of this "high risk" HPV group. In general, HPV types 16, 18, 31, 33, 35, 39, 45, 51, 52, and 56 are found in cases of moderate and severe dysplasia and invasive cervical carcinomas and therefore con stitute the "high risk" types (18). A recent study has demonstrated that approximately 84% of cervical carcinomas contained a "high risk" HPV DNA (19). Studies of cervical cancers and of cervical cancer cell lines that are HPV positive have shown that the DNA is usually found to be integrated (16) although there are some cases where the DNA is apparently extrachromosomal. In those cases where the DNA is integrated, the pattern of integration is clonal indicating that the association of the HPV precedes the clonal outgrowth of the tumor. In contrast to the integrated state of the HPV DNA that is generally found in cervical cancers, the viral DNA is usually extrachromosomal in the premalignant CIN lesions (20). Inte gration appears to be a random event as far as the site of integration in the host chromosome is concerned since the viral genome can be found in different locations in different cancers and cell lines. In some cell lines, however, the integration has occurred in the vicinity of known oncogenes. For instance in the HeLa cell line (which is a HPV-18 positive cervical carci noma cell line) the integration of the viral genome has occurred within approximately 50 kilobases of the c-myc locus on human chromosome 8 (21). Whether such an integration event pro vides a selective advantage to the progression of a preneoplastic lesion to a cancer is currently unknown. In those cancers or derived cell lines in which the number of integrated viral ge nomes is low enough to permit a detailed analysis, the integra tion pattern reveals a remarkable specificity with respect to where the double-stranded circular viral genome is opened and disrupted as a consequence of the integration event. Integration generally occurs in the E1/E2 region of the viral genome (22, 23) disrupting the E2 viral transcriptional regulatory circuit. The E2 open reading frame encodes a transcriptional regulatory protein, which is a DNA-binding protein. Products of the E2 open reading frame have been most extensively studied in the bovine papillomavirus and its products have been shown to have both positive and negative acting transcriptional regula tory functions (24). For HPV-16 and HPV-18, E2 appears to act principally as a repressor of the promoter from which the E6 and E7 genes are transcribed (25-27). The HPV genomes in cervical cancers and in derived cell lines are transcriptionally active, and the patterns of viral RNA expression are specific