Human studies following animal models of tumorigenesis by oncornaviruses

Similarities have been observed for some time between oncornavirus-induced malignancies in laboratory animals and leukemias and solid tumors in man. Particles similar to type C oncornaviruses have been detected by electron microscopy both in cells or plasma from leukemia patients and in solid-tumor human malignancies such as Hodgkin's lymphoma, lymphosarcomas, and sarcomas. Likewise, particles resembling type B oncornaviruses in shape and appearance have been found in human breast cancer. In neither case has the infectious nature of the particles been confirmed. However, DNA synthesized in vitro by the enzyme of murine mammary tumor virus was found to hybridize with polysomal RNA obtained from human mammary adenocarcinomas. The presence of RNA complementary to RNA from the Rauscher strain of murine leukemia virus has been observed in other human malignancies unrelated to breast cancer. It has also been found that cells of patients with myelogenous leukemia possess an oncornaviral-type reverse transcriptase that is distinguishable from other cell DNA polymerases and serologically related to the reverse transcriptase of primate oncornaviruses.