Specific-locus mutations response to unequal, 1 + 9 Gy X-ray fractionations at 24-h and 4-day fraction intervals

Abstract

The specific-locus mutation frequency obtained from mouse spermatogonial stem cells following unequal, 1 + 9 Gy X-ray fractionation with a 24-h fractionation interval is slow, and consistent with the two fractions acting additively. The response is therefore markedly different from the augmented mutation frequencies obtained with 500 + 500 R and 100 + 500 R, 24-h fractionations. The lower yield compared with the 100 + 500 R response also indicates a clear difference from the translocation data which demonstrate increase in yield with increasing second dose over the same dose range. The decline in specific locus mutation yield with the increase in the second dose from 500 R to 9 Gy suggests that the stem cells surviving the first fraction are heterogeneous in their sensitivities to this class of genetic damage. A similar, additive specific locus mutation frequency is obtained with unequal, 1 + 9 Gy X-irradiation when the interval between fractions is 4 days. Thisiis consistent with 500 + 500 R, 4-day and 7-day interval responses obtained previously but again differs from the sub-additive translocation responses obtained with such X-ray fractionation. Taken together with the data from previous studies the present results suggest that (1) 24 h after the first fraction, (a) the surviving stem cell have two components; survivors of the formerly radiosensitive, cycling component of the normal stem cell population and the formerly radioresistant, G 0 or arrested G 1 cells, which are being ‘triggered’ into a rapid cell cycle to achieve repopulation of the testis; (b) these two components are near-equal sensitivity to translocation induction and cell killing, hence the additive translocation yields with equal X-ray fractionations and yields consistent with those extrapolated from lower doses with higher, unequal fractionations, e.g. 1 + 7 Gy, 1 + 9 Gy; but (c) the formerly radioresistant, triggered component is much more sensitive than surviving cycling component to specific locus mutations and cell killing, hence the augmented mutation response with 500 + 500 R fractionation and the drop in yiled with 1 + 9 Gy compared with 100 + 500 R X-irradiation. (2) 4 days after the first fraction, (a) a sten cell population has changed but still comprises two polulations; rapidly-dividing cells which are highly sensitive to killing and newly-established radioresistant cells equivalent to those of a normal stem cell population; (b) translocations recovered from the second fraction derive primarily from the G 1 phase of the cell of the rapidly-dividing cells; hence, since G 1 is shorter in these cells than in the cycling cells of a normal cell population, the translocation yield is lower; (c) specific-locus mutations recovered from the second fraction derive primarily from the more constant S, G 2, M phases of the cell cycle of the rapidly-dividing cells that are common to the cycling component of a normal stem cell population; hence, the additive response for this clas of genetic damage.