Description:
Amplification of the c-myc oncogene has been detected by Southern blotting in the DNA of radiation-induced skin cancers in the rat. In the current work the localization of oncogene amplification within specific cells in the different cancers and in multiple biopsies of the same cancer was studied by in situ hybridization. The amount of amplification was measured by counting grains on tissue sections hybridized in situ to biotin-labeled human c-myc third exon, rat v-H-ras, and rat v-Ki-ras probes. The in situ estimates of c-myc amplification were generally correlated with previous findings using the Southern blot method, but within each cancer only a fraction of cells exhibited amplification. Multiple biopsies of a squamous carcinoma showed amplification of v-H-ras and c-myc but not v-Ki-ras during tumor growth, but none of these oncogenes were amplified during tumor regression. The c-myc-positive cells were distributed uniformly within the cancers and exhibited a more uniform nuclear structure in comparison to the more vacuolated c-myc-negative cells. A high [<sup>3</sup> H]thymidine labeling index was found in irradiated epidermal cells on Day 7 after exposure, and yet no evidence of c-myc oncogene amplification was found in situ. No c-myc amplification was found in unirradiated normal epidermis or in irradiated epidermal cells in the vicinity of radiation-induced cancers. The data indicate that c-myc amplification is cell-specific within radiation-induced carcinomas and does not occur in epidermal cells proliferating in response to radiation exposure.