A role for long-lived radicals (LLR) in radiation-induced mutation and persistent chromosomal instability: counteraction by ascorbate and RibCys but not DMSO.

Miazaki, Watanabe, Kumagai and their colleagues reported that induction of HPRT(-) mutants by X-rays in cultured human cells was prevented by ascorbate added 30min after irradiation. They attributed extinction of induced mutation to neutralization by ascorbate of radiation-induced long-lived mutagenic radicals (LLR), found using spectroscopy to have half-lives of minutes or hours. We find that post-irradiation treatment with ascorbate reduces, but does not eliminate, induction of CD59(-) mutants in human-hamster hybrid A(L) cells exposed to high-LET carbon-ions (LET of 100KeV/microm). A(L) cells contain a standard set of Chinese hamster ovary (CHO) chromosomes and a single copy of human chromosome 11 containing the CD59 gene which encodes the CD59 cell surface antigen, a convenient marker for mutation. RibCys [2(R, S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)thiazolidine-4(R)-carboxylic acid] a 'prodrug' of l-cysteine which also scavenges LLR, had a similar but lesser effect on induced mutation. DMSO, which scavenges classical radicals like H* and OH* but not LLR, also reduced mutation, but only when it was present during irradiation. The lethality of carbon-ions was not altered by ascorbate, RibCys no matter when added. Post-radiation addition of ascorbate and RibCys also affected the quality of CD59(-) mutations induced by carbon-ions. The major change in mutant spectra was a reduction in the prevalence of small, intragenic mutations (mutations not detected by PCR) and in the prevalence of unstable, complicated mutants, which display high levels of persistent chromosomal instability. Thus, ascorbate and RibCys may suppress some kinds of mutations induced by ionizing radiation including those displaying aspects of radiation-induced genomic instability. Countering the effects of both classical radicals and LLR may be important in preventing genetic diseases.