Strain-dependent expression of endogenous mouse-tropic leukemia viruses in chemically induced murine leukemias

The effects of two chemical carcinogens, nitrosobutylurea and 7,12-dimethylbenz-(a) anthracene, on the expression of endogenous N- and B-tropic viruses were studied. The mice used were from two inbred strains, C57BL/6 and DDD-Fvr, and from 13 partially inbred strains derived from the cross of C57BL/6 with DDD-Ffr. These mouse strains are classified into three groups by the pattern of expression of endogenous viruses in normal, aged mice: (1) negative for both viruses; (2) positive for N-tropic virus only; and (3) positive for both viruses. Mice were given the carcinogens, and were tested at various intervals by the UV-XC procedure for the viruses in the spleens and other enlarged organs. Irrespective of the presence or absence of leukemias, the carcinogen-treated mice showed the same pattern of expression of endogenous viruses as that of non-treated normal mice. It can be concluded that the activation of the endogenous viruses is dependent on the mouse strains and that the growth of the activated viruses is not a necessary step for the chemical induction of leukemias.