STRUCTURAL DIFFERENCES BETWEEN SPECIFIC-LOCUS MUTATIONS INDUCED BY DIFFERENT EXPOSURE REGIMES IN MOUSE SPERMATOGONIAL STEM-CELLS

It was first shown by W.L. Russell (1962), and confirmed by him and ot hers, that a 24-h interval between dose fractions (but not shorter or longer ones) elevates the rate of radiation-induced spermatogonial spe cific-locus mutations to levels considerably above the linear extrapol ation made from lower-dose results. We have now analyzed the nature of mutations induced either in previously undisturbed or in ''sensitized '' spermatogonial stem cells, i.e., those that received a challenging dose of X-rays 24 h following a priming dose. Results are based on mol ecular studies of a large set of viable albino mutations using probes derived from the tyrosinase (c) gene and from the regions surrounding c!, and on retrospective classifications of mutations at c and two ad ditional loci into LL (large lesions), IG (intragenic mutations), and OL (other lesions), utilizing criteria developed earlier. A significan t difference (P = 0.016) was found between previously undisturbed and sensitized stem-cell spermatogonia; the latter have a higher LL/IG rat io, similar to the ratio observed for mutations induced in poststem-ce ll stages. This finding of a qualitative difference indicates that the additional mutations produced by a 24-h fractionated treatment are th e result of the second (challenging) dose. The qualitative difference, further, indicates that the mutation-rate-augmenting effects of 24-h fractionation are not due, merely, to an increase (caused by the primi ng dose) of a normally responsive component of the spermatogonial popu lation. The finding that the additional mutations that are produced by the challenging dose are primarily large DNA lesions suggests that th e nuclear state of sensitized stem-cell spermatogonia may be different from the state of previously undisturbed spermatogonia. This state, w hich appears to be similar to that of postspermatonial stages, may be conducive to the formation of LLs, even by agents that are not LL indu cers in other systems. The results further indicate that the relative paucity of LLs characteristic of treated (previously undisturbed) sper matogonial stem cells is probably not the result of selection against such mutations during subsequent germ-cell development.