Summary and Conclusion
There are two groups of avian ribodeoxyviruses, the avian leukosis-sarcoma viruses and the reticuloendotheliosis viruses. Virions of these viruses contain RNA, and the viruses replicate through a DNA intermediate. There is wide variation from none to great among these viruses in their ability to cause cancer. The viruses that cause cancer seem to have genes for cancer. Carcinogenesis by these strongly transforming viruses involves the formation in infected cells of genes for cancer copied from the viral genome. Normal chicken cells do not contain infectious DNA for transformation or for virus production. Therefore, there is not an inactive provirus of a strongly transforming virus in normal chicken cells.
The protovirus hypothesis for cancer states that the genes for neoplastic transformation arise in an organism as a result of misevolution of a normal system of DNA to RNA to DNA information transfer. The protovirus hypothesis for the origin of ribodeoxyviruses states that ribodeoxyviruses arise as a result of misevolution of normal cellular genes. There is evidence from experiments involving nucleic acid hybridization and study of DNA polymerase relationships for the protovirus hypothesis for the origin of ribodeoxyviruses.
Presented at the 1974 meeting of the American Association for Cancer Research in Houston, Texas. The work in my laboratory was supported by USPHS Research Grant CA-07175 from the National Cancer Institute and Research Grant VC-7 from the American Cancer Society. I formerly held Research Career Development Award CA-8182 from the National Cancer Institute and now am an American Cancer Society Research Professor.