Radiation leukemia in C57BL/6 mice. II. Lack of ecotropic virus expression in the majority of lymphomas.

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Abstract
The expression of endogenous ecotropic viruses in radiation-induced thymomas of C57BL/6 mice was examined. Competition radioimmunoassays for AKR MuLV [murine leukemia virus] gp71, p30 and p12 were used for viral antigen expression. Of 40 lymphomas, 3 had readily detectable ecotropic gp71 at levels of 95-689 ng/mg protein; the remainder of the tumors had no detectable gp71 (< 1.0 ng/mg protein). In 30 thymomas MuLV p30 was present at levels of 1-10 ng/mg protein, levels that were comparable to those found in thymus extracts from age-matched, nonirradiated control. In 10 tumors p30 levels were 10-30 ng/mg protein. In 1 tumor significant levels of AKR MuLV p12 were detectable. Since B-tropic and N-tropic viruses from C57BL/6 mice have glycoproteins (gp71) indistinguishable from AKR MuLV gp71 and the N-tropic virus had a p12 serologically identical to AKR MuLV p12, these results demonstrate that overt endogenous B-tropic virus was detectable in 2 of 40 thymomas and endogenous N-tropic virus was detectable in 1 of 40 thymomas. The lack of overt expression of gp71 or p12 was also confirmed by cytotoxicity assays using monospecific antisera to these viral proteins. Radiation-induced lymphomas were also examined for the presence of reverse transcriptase after chromatography of tissue extracts on poly G-Sepharose. One tumor, which was characterized by the lack of gp71, also had no detectable reverse transcriptase; 1 tumor with gp71 was characterized by readily detectable levels of reverse transcriptase in cellular extracts. The presence of viral RNA was examined using AKR c[complementary]DNA. Low levels of RNA capable of hybridizing with AKR cDNA were found in age-matched, nonirradiated mice; these hybrids had Tm [melting points] of 72.degree. C, while hybrids with AKR MuLV 70S RNA had Tm of 80.degree. C. In 1 of 12 thymomas the concentration of hybridizable RNA and the Tm of the hybrids were identical to control values. In 9 of 12 thymomas the concentration of hybridizable sequences increased approximately 3 to 5-fold and the Tm of these hybrids varied from 73-75.degree. C. In 1 of 12 thymomas the concentration of hybridizable sequences increased over 100-fold, hybridized completely with AKR MuLV cDNA, and the hybrids had Tm of 79.degree. C. This thymoma was also characterized by the presence of the AKR MuLV type of gp71 and p12. One tumor was characterized by a 10 to 100-fold increase in hybridizable sequences, which only partially hybridized with AKR MuLV cDNA, and hybrids had a Tm of 73.degree. C. This tumor was characterized by the presence of AKR MuLV gp71 but not AKR MuLV p12. Apparently overt endogenous ecotropic virus expression is only rarely detectable in radiation-induced thymomas of C57BL/6 mice.