Molecular analysis of DNA isolated from the different stages of X‐ray‐lnduced transformation in vitro

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Abstract
A major challenge in radiation carcinogenesis is to identify the cellular gene or genes involved in initiating the process. We examined the transforming activities of DNAs obtained from C3H10T1/2 cells during x‐ray‐induced morphological transformation. DNAs extracted from mass cultures of 10T1/2 cells at different times after irradiation with 600 rad and from type III‐transformed foci were transfected into NIH 3T3 cells. The results indicate that certain oncogenes are activated beginning 3 wk after irradiation, well before the appearance of macroscopically visible transformed foci. For DNA isolated from x‐ray‐transformed 10T1/2 cells (type III foci), the frequencies of transfection were 0.003–0.11 foci/μg of genomic DNA with NIH 3T3 cells and 0.004–0.04 foci/μg genomic DNA using 10T1/2 cells as recipients. Southern blot analysis of DNAs obtained from 23 primary transfectants and from 23 x‐ray‐transformed cell lines indicated no gross rearrangements or amplification of any of the 14 oncogenes screened (v‐Ha‐ras, v‐Ki‐ras, N‐ras, v‐myc, v‐raf, v‐src, v‐fes, v‐abl, v‐mos, v‐erbA, v‐erbB, M‐myb, v‐fos, v‐sis). This suggests that x‐irradiation may activate as yet unidentified oncogenes. The occurrence of positive transfection 3 wk after irradiation is discussed in terms of the hypothesis that transformation may not occur as a direct consequence of the exposure to x‐rays but develops as a rare event in the progeny of the irradiated cells at some later time, as a consequence of the delayed activation of certain genes.