Activation of Protooncogenes in Spontaneously Occurring Non-Liver Tumors from C57BL/6 x C3H F, Mice

The C57BL/6 x C3H F, (hereafter called B6C3F,) mouse is an important animal model for long-term carcinogenesis studies. Maintained under normal laboratory conditions, these mice develop various types of spontaneous tumors during their lifetime. Activated I la-rav genes have been detected in 66% of spontaneous hepatocellular tumors in the B6C3F] mouse (Reynolds et a/., Science (Washington DC), 237:1309, 1988]. In this study 49 spontaneous non-liver tumors were investigated for oncogene activation by DNA transfection techniques. Of the 49 tumor DNAs analyzed, only 5 yielded multiple foci in the NIH 3T3 focus assay: 2 of 10 pulmonary adenocarcinomas; 0 of 25 lymphomas; 2 of 2 Harderian gland adenomas; 0 of 1 adenocarcinoma of the small intestine; 1 of 6 malignant skin tumors; 0 of 4 hemangiosarcomas; and 0 of 1 lung metastasis of a hepatocellular carcinoma. DNA from six lymphomas which were negative in the NIH 3T3 focus assay were further analyzed for transforming genes by the nude mouse tumorigenicity assay. One of the five lymphomas tested positive with this assay. Southern blot analysis identified five activated ra.vgenes: I I-ruv in two Harderian gland adeno mas; K-ra.v in one pulmonary adenocarcinoma and in one s.c. adenocar cinoma; and \-ruv in one lymphoma. The mutations involved were CG to AT and AT to TA in codon 61 of the 11-ruv genes, GC to AT or TA in codon 12 of the Yi-ras genes, and a GC to AT mutation in codon 12 of the N-ras gene. Transformant DNA from a pulmonary adenocarcinoma which yielded multiple foci in the transfection assay did not hybridize to DNA probes specific for the K-, H-, and N-ras, raf, neu, and met genes. Thirteen additional tumor DNAs yielded a single focus in the NIH 3T3 transfection assay. The transformant DNAs retransmitted in a second cycle transfection assay. Rearranged and/or amplified ruf genes were detected in six of the transformant DNAs. At present we do not know whether these activated ruf genes were present in the original tumor DNA. The other seven transformant DNAs did not hybridize with any of the above mentioned specific DNA probes utilized in Southern blot analysis. Unlike liver tumors, the activation of ras protooncogenes is not a frequent event in the development of spontaneous non-liver tumors of the B6C3F| mouse. The results from this study should aid in understand ing the neoplastic development associated with exposure to chemical carcinogens in the B6C3F| mouse.