Interactions Between Viral and Genetic Factors in the Origin of Mammary Tumors in Mice

Resistance to the oncogenic effect of a murine mammary tumor is controlled by a few genes. It is associated with poor replication of the virus. A battery of tests, including electron microscopy, immunodiffusion, immunofluorescence, and bioassay, demonstrated the presence of a mammary tumor virus in 7 genetically diverse mouse strains. The virus was often expressed in an incomplete form and usually at a late age. Switching on of virus release correlated well with spontaneous tumor incidences. Carcinogens, such as X rays and urethan, induced the early appearance of viral antigens. These findings strongly suggest an all-viral etiology of mammary cancer in mice. It is theorized that every mouse contains genetic information for a mammary tumor virus, which is in the form of a DNA copy of the viral RNA integrated into one of its chromosomes. The transcription of this germinal provirus is regulated by a repressor, but other controlling mechanisms are also involved, leading sometimes to partial expression.