4 Transformation by Simian Virus 40 and Polyoma Virus

Abstract

Cell division is controlled in multicellular organisms. Tumors grow because cancer cells continue to multiply in conditions that regulate the multiplication of normal cells. Polyoma virus and SV40 can effect a change in the growth control of a normal cell; as a result, these viruses can cause primary tumors when injected into susceptible animals. They can also cause changes in the growth control of cells growing in vitro; they transform the cells. Such transformation is believed to be analogous to the induction of primary tumors in animals, because cells transformed in vitro by polyoma virus or SV40 acquire a set of properties (see Table 4.1), some of which (including increased malignancy) are general characteristics of tumor cells.

Polyoma virus and SV40 are not oncogenic in their natural host species. They must be injected in large amounts into susceptible rodents to induce tumors, and even then, the tumors do not usually metastasize. All this is also true of the tumors that develop when cells transformed in vitro by these viruses are injected into susceptible host animals. These reservations not-withstanding, transformation of cultivated cells by tumor viruses is the best model system we have for studying, in a quantitative way, at least some of the several cellular events that lead to the development of primary tumors in natural populations of animals and man. In addition, of the transforming viruses available, polyoma viruses and SV40 have the smallest genomes, and the complete nucleotide sequences of their genomes have been determined (see Appendixes A...