G2 chromatid breaks in murine SCID cells

The G2 chromosomal radiosensitivity of murine SCID (severe combined immunodeficient) and normal fibroblasts has been investigated. We have also investigated the G2 response of these cell lines to the restriction endonuclease PvuII. We show that chromatid breaks are induced linearly with radiation dose in both cell lines and SCID cells are approximately 1.6 times as radiosensitive as normal murine fibroblasts when tested using a G2 assay with a 2 h sampling time. The disappearance of chromatid breaks with time after irradiation was first order with a half-time of approximately 1.5 h in both cell lines. Thus, although SCID cells are deficient in the rejoining of double-strand breaks (dsb), they show similar kinetics of disappearance of chromatid breaks with time as normal CB17 cells, indicating that the 'rejoining' of chromatid breaks does not reflect dsb repair. When CB17 and SCID cells were treated with PvuII, which generates dsb in cellular DNA in the presence of streptolysin O (as a porating agent), approximately 3 times more chromatid breaks were observed in SCID than CB17 cells. We conclude that SCID cells convert a higher number of dsb into chromatid breaks than do CB17 cells. The conversion process is interpreted in terms of the recently proposed 'signal' model, whereby a signal, resulting from a single dsb, triggers the cell to make a recombinational exchange which, if incomplete, gives rise to a visible chromatid break. In terms of the signal model, elevated conversion of dsb into chromatid breaks results from altered signalling and the disappearance of chromatid breaks with time following irradiation represents the completion of recombinational exchanges rather than repair of dsb.