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Induction and Repair of DNA Double Strand Breaks in Radiation-resistant Cells Obtained by Transformation of Primary Rat Embryo Cells with the Oncogenes H-ras and v-myc¹

Department of Radiation Oncology and Nuclear Medicine, Thomas Jefferson University, Philadelphia, Pennsylvania 19107 [G. I., L. M.], and Departments of Pathology and Laboratory Medicine [R. J. M.] and of Radiation Oncology [W. G. M.], University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104

Rat embryo cells (REC) transformed by the H-ras oncogene plus the cooperating oncogene v-myc are highly resistant to ionizing radiation as compared with the nontransformed parent cells, REC, or immortalized REC. In an attempt to understand the potential mechanism of resistance in these cells, the induction and repair of double strand breaks (dsb) in DNA were measured in a H-ras plus v-myc transformed (3.7) and an immortalized REC (mycREC) line using pulsed field gel electrophoresis. Cells were irradiated in the exponential phase of growth, and the amount of DNA dsb present was quantified by measuring the fraction of DNA activity released from the agarose plugs in which cells were embedded. Similar values of the fraction of DNA activity released were measured for both cell lines at equal X-ray doses, after correction for differences in cell cycle distribution, suggesting a similar induction of DNA dsb per Gy. Repair of DNA dsb measured after exposure to 40 Gy of X-rays was similar in both cell lines and displayed a fast and a slow component. The fast component had a 50% repair time of approximately 12 min, and the slow component, 50% repair time of about 3 h. These results suggest that the relative radioresistance of 3.7 cells is not conferred by a decrease in the amount of DNA dsb induced per Gy per dalton or by alterations in the capacity of the cells to repair DNA dsb. It is hypothesized that alterations in the expression of potentially lethal damage underlie this phenomenon.

¹ This work was supported by National Cancer Institute Grants CA 45557 and CA 42026 awarded from NIH, Department of Health and Human Services. Presented in part at the 38th Annual Meeting of the Radiation Research Society, held in New Orleans, LA, on April 7–12, 1990.

² To whom requests for reprints should be addressed.

Received 5/ 4/90. Accepted 7/13/90.

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