Animal papillomaviruses

Abstract

Significant progress in papillomavirus research has been made in recent years. However, numerous aspects of the biology of this unique group of tumor viruses remain to be elucidated. Although the genomes of many of the papillomaviruses have been molecularly cloned and detailed physical maps of the viral DNA have been constructed, a major impediment for analysis of this virus genus is the lack of a culture system permissive for their replication. Transcription of portions of the viral genome in tumor and transformed cells indicates the potential for the synthesis of virus-specific proteins. However, reasons for the inability to detect unique proteins in these cells remain obscure. The rejection of papillomas clearly is associated with a cell-mediated phenomenon directed against the tumor cell. The cellular determinant that is recognized by the immune system is unknown. The finding of multiple, minimally related types within a single species would suggest that the papillomaviruses have evolved in response to pressure of the host immune system since regions of the capsid which would not normally be exposed to the immune system have remained conserved throughout their evolution. The papillomaviruses may offer an interesting system for the analysis of evolution at the polynucleotide level, since a large number of viruses have been identified from diverse species and all show conserved regions of DNA sequence homology. Information as to why some animals can reject papillomas while lesions persist (and progress to carcinoma in some instances) in other animals could provide insights into host immune defense against neoplasia. The transformation of papillomas to carcinoma appears to involve not only virus but other extrinsic factors such as genetic background or environmental factors. Multifactorial carcinogenesis is thought to be involved in the generation of many forms of cancer. Lack of integration of papillomavirus genomes in transformed cells would suggest that these viruses may have evolved an alternative mechanism for maintenance of the transformed state.