10 Endogenous Viruses

Abstract

I. INTRODUCTION
One of the most striking features that distinguishes retroviruses from all other animal viruses is the presence, in the chromosomes of normal uninfected cells, of genomes closely related to, or identical with, those of infectious viruses. Endogenous viruses are extremely widespread in animal populations and have been described in species as diverse as reptiles, birds, and many mammals, quite possibly including man. Their study has occupied a major portion of RNA tumor virus research, with the result that it has recently been possible to reconcile very complex biological and genetic observations into a quite clear virological picture of the phenomenon. We begin with a brief historical background of the subject. More detail can be found in numerous excellent reviews (Gross 1970; Aaronson and Stephenson 1976; Levy 1978; Robinson 1978; Pincus 1980; Todaro 1980; Weinberg 1980).

A. Historical Survey
Endogenous viruses were postulated to be elements of importance in cancer long before their existence was conclusively demonstrated. Although this rationale stimulated research in the area, it should be emphasized that evidence for such a connection presently exists only in some laboratory mouse systems, which may represent special cases. Andrewes (1939) speculated on the possible activation of latent viral infections in cancerous tissues, and it was postulated by Darlington (1948) that such viruses could arise from cellular genetic elements, which he named proviruses. Gross (1958) and Lieberman and Kaplan (1959) observed that lymphoid tumors induced in mice by X rays contained murine leukemia virus (called radiation leukemia virus [RadLV]), which...