Evolution and modes of transmission of RNA tumor viruses. Parke-Davis Award lecture.

Most vertebrates contain sets of gene sequences (virogenes) which are an integral part of the chromosomal DNA and which can code, in some instances, for the production of Type C RNA tumor viruses. These genes are transmitted from parent to progeny along with other cellular genes, and their activation from a normally reressed state may be part of the mechanism by which RNA tumor viruses produce cancer. Isolates of endogenous genetically transmitted baboon Type C viruses are morphologically and biochemically related to other mammalian Type C viruses but can clearly be distinguished from the other groups (mouse, rat, cat, etc.) by immunologic and nucleic acid hybridization criteria. Within the primates, Type C viral gene sequences have evolved as the species have evolved, with virogenes from the most closely related genera and families showing the most sequence homology; all higher primate, including man, however, do have detectable virogene sequences in their normal tissues. Type C viruses have also been transferred under natural conditions between species only remotely related phylogenetically. The results show three clear examples where viral genes from one group of animals have become incorporated into the germ line of genetically distant groups of animals (inheritance of acquired genes). Infectious Type C viruses of primates, distinct from the endogenous primate virus group, have also been isolated (woolly monkey and gibbon isolates) and can be shown to produce tumors in other primates. Related viral information (nucleic acid sequences, enzymes, and antigens) have been reported in human tumors. The significance of infectious and/or genetically transmitted viruses in naturally occurring cancer is a major focus of current research. The presence of genetically transmitted viral genes in so many vertebrate species and the evidence that they have been conserved in several distinct vertebrate lineages suggests that they may provide some normal function(s) advantageous to the species carrying them and that their potential to cause cancers is a pathologic manifestation of normal, as yet undefined, physiologic processes.