G2 CHROMATID BREAKS IN MURINE SCID CELLS

The G2 chromosomal radiosensitivity of murine SCID (severe combined im munodeficient) and normal fibroblasts has been investigated. We have a lso investigated the G2 response of these cell lines to the restrictio n endonuclease PvuII, We show that chromatid breaks are induced linear ly with radiation dose in both cell lines and SCID cells are similar t o 1.6 times as radiosensitive as normal murine fibroblasts when tested using a G2 assay with a 2 h sampling time. The disappearance of chrom atid breaks with time after irradiation was first order with a half-ti me of similar to 1.5 h in both cell lines. Thus, although SCID cells a re deficient in the rejoining of double-strand breaks (dsb), they show similar kinetics of disappearance of chromatid breaks with time as no rmal CB17 cells, indicating that the 'rejoining' of chromatid breaks d oes not reflect dsb repair. When CB17 and SCID cells were treated with PvuII, which generates dsb in cellular DNA in the presence of strepto lysin O (as a porating agent), similar to 3 times more chromatid break s were observed in SCID than CB17 cells. We conclude that SCID cells c onvert a higher number of dsb into chromatid breaks than do CB17 cells . The conversion process is interpreted in terms of the recently propo sed 'signal' model, whereby a signal, resulting from a single dsb, tri ggers the cell to make a recombinational exchange which, if incomplete , gives rise to a visible chromatid break. In terms of the signal mode l, elevated conversion of dsb into chromatid breaks results from alter ed signalling and the disappearance of chromatid breaks with time foll owing irradiation represents the completion of recombinational exchang es rather than repair of dsb.