7 Papilloma Viruses

Abstract

The first viruses to be isolated and characterized were those that occur naturally in large amounts and are not easily inactivated. Rabbit papilloma virus, discovered by Shope (1933), exhibits both these characteristics and was one of the very first viruses to be studied by physicochemical techniques. Shope found that warts or papillomas of cottontail rabbits, in contrast to those borne by domestic rabbits, contain large amounts of papilloma virus; simple extraction procedures yield 0.01 mg to 1 mg of virus per gram of wart tissue. Not surprisingly, rabbit trappers, notably those in Kansas, have found a brisk market for any cottontail rabbits with warts.

Since Shope’s pioneering work, papilloma viruses have been isolated from human warts and warts of many other animals (see below). By virtue of their structure and biochemical composition, the many papilloma viruses are now classified as one of the two genera of the papovavirus family, the second genus comprising polyoma virus, SV40, and their close relatives. However, although rabbit papilloma virus was discovered some 20 years before polyoma virus, we know enormously more about the latter (and SV40) than about any papilloma virus. The reason for this is simply that, although warts yield large amounts of papilloma virus particles, no one has yet discovered a permissive host cell for these viruses that can be grown in culture.

VIRION COMPOSITION AND STRUCTURE
Papilloma virus particles contain only DNA and protein. The single DNA molecule in each virion is double-stranded, covalently closed, circular, and supercoiled and has a...