Changes in RNA expression pattern during the malignant progression of cottontail rabbit papillomavirus-induced tumors in rabbits.

Cottontail rabbit papillomavirus induces strictly epithelial tumors in both cottontail and domestic rabbits. A high proportion of the initial benign papillomas progress within 8 to 14 months to invasive carcinomas. With the help of mRNA-specific riboprobes for E6, E7, E1, E2, L1 and L2, we investigated by in situ hybridization the RNA expression pattern of cottontail rabbit papillomavirus in tissue sections of biopsies from different stages of tumor development. Common features of all lesions were high levels of E6 and E7 mRNAs and low levels of E1 and E2 mRNAs. In agreement with earlier reports, there was no evidence for a major mRNA class equivalent to the prominent E1-E4 RNA of human papillomavirus types 6/11 and 16. In cottontail rabbit papillomas, high levels of E6 and E7 mRNAs were present in the upper differentiated epithelial layers. These layers also contained most of the E1 and E2 mRNAs and the viral DNA. In contrast, papillomas of domestic rabbits revealed the opposite differentiation-dependent expression pattern for the E6 and E7 mRNAs; there were strong signals in the basal layers, and these declined with increased differentiation. Transcripts encoding the L1 mRNA were detected only in a few isolated cells of the granular layer. There was no difference between the amounts of E6, E7, E1, and E2 mRNAs present in highly dysplastic tissue and those present in adjacent normal papillomatous epithelium within a progressing papilloma. However, late transcripts and viral DNA detectable only in the upper layers of the papilloma were present throughout the thickness of the dysplastic tissue, indicating a newly acquired permissiveness of the dysplastic cells for viral DNA replication and late transcription. Carcinomas in general had the same expression patterns for E6, E7, and E1 but were dissimilar in the levels of expression of E2 and late transcripts.