Ionizing radiation and genetic risks III. Nature of spontaneous and radiation-induced mutations in mammalian in vitro systems and mechanisms of induction of mutations by radiation

Abstract

This paper (1) presents an analysis of published data on the molecular nature of spontaneously arising and radiation-induced mutations in mammalian somatic cell systems and (2) examines whether the molecular nature and mechanisms of origin of radiation-induced mutations, in mammalian in vivo and in vitro systems, as currently understood, are consistent with expectations based on the biophysical and microdosimetric properties of ionizing radiation. Depending on the test system (CHO cells, human T lymphocytes and human lymphoid cell line TK6), 80–97% of spontaneous HPRT mutations show normal Southern patterns; the remainder is due to gross changes, predominantly partial (intragenic) deletions. Total gene deletions at the HPRT locus are rare except in the TK6 cell line. At the APRT locus in CHO cells, 80–97% of spontaneous mutations are due to base-pair changes, the remainder being, mostly, partial delections. The latter can extend upstream in the 5′ direction but not beyond the APRT gene in the 3′ direction. At the human HLA-A locus (T lymphocytes), the percentage of mutations with normal Southern patterns is lower than that for HPRT, and in the range of 50–60%. At the HLA-A locus, mitotic recombination contributes substantially to the mutation spectrum (∼ 30% of mutations recovered) and this is likely to be true of the TK locus in the TK6 cell line as well. With a few exceptions, most of the radiation-induced mutations show altered Southern patterns and are consistent with their being deletions and/or other gross changes (HPRT, 70–90% (CHO); 50–85% (TK6); 50–75% (T lymphocytes); TK, 60–80% (TK6); HLA-A, 80% (T lymphocytes); DHFR, 100% (CHO)). The exceptions are APRT mutations in CHO cells (16–20% of mutants with deletions or other changes) and HPRT mutations in T lymphocytes from A-bomb survivors (15–25%); the latter finding is consistent with the occurrence of in vivo selection against HPRT mutant cells. In cases of HPRT intragenic deletions analyzed (CHO cells and V79 Chinese hamster cells), there is evidence for a non-random distribution of breakpoints. The spontaneous mutation frequencies vary widely, from about 0.04/10 6 cells (sickle cell mutations at the human HBB locus) to 30.8/10 6 cells HLA-A mutations in T lymphocytes) and are dependent on the locus, the system employed and a number of other factors. Those for the other loci fall between these limits. The rates of induction by X-rays again are quite low for the HBB locus (0.07–1.5/10 6 cells/Gy; only two types of specific point mutations were scored), low for DHFR (1.6/10 6 cells/Gy) and APRT (4.3/10 6 cells/Gy), in the range of about 10–30/10 6 cells/Gy for most others, and are highest for the HLA-A, and glycophorin A locus (the NØ variant) (about 60/10 6/cells/Gy). Gene size alone cannot explain the differences between either spontaneous frequencies or induction rates. On the basis of chromatin and DNA organization in cells and the biophysical and microdosimetric properties of ionizing radiation, one can qualitatively explain the predominance of deletions among radiation-induced mutations. For spontaneously arising and radiation-induced point mutations, there may be common elements in mechanisms, but for spontaneously arising and induced deletions, the extent of overlap in mechanisms is difficult to discern at present.