Caffeine enhanced measurement of mutagenesis by low levels of gamma-irradiation in human lymphocytes

Health Risks of Low Dose Ionizing Radiation in Humans: A Review

Radiobiologists have been struggling to estimate the health risks from low doses of radiation in humans for decades. Health risks involve not only neoplastic diseases but also somatic mutations that may contribute to other illnesses (including birth defects and ocular maladies) and heritable mutations that may increase the risk of diseases in future generations. Low dose radiation-induced cancer in humans depends on several variables, and most of these variables are not possible to correct for in any epidemiologic study. Some of the confounding factors include (i) interaction of radiation with other physical (UV light), chemical, and biological mutagens and carcinogens in a synergistic manner; (ii) variation in repair mechanisms that depend on dose; (iii) variation in sensitivity of bystander cells to subsequent radiation exposure that depends on whether they have been pre- or postirradiated; and (iv) variation in adaptive response that depends on radiation doses and protective substances (antioxidants). In our opinion, both the linear no-threshold-response and the threshold-response models might not be suitable in predicting cancer risk at low radiation doses in a quantitative sense. Low doses of ionizing radiation should not be considered insignificant for risks of somatic and heritable, mutations and neoplastic and nonneoplastic diseases in humans.

Rationale for using multiple antioxidants in protecting humans against low doses of ionizing radiation

Health risks of low doses of ionizing radiation (10 cGy or less) may not be accurately estimated in humans by epidemiological study or mathematical modelling because of several inherent confounding factors including environmental, dietary and biological variables that cannot be accounted for in any radio-epidemiological study. In addition, the expression of radiation-induced damage in humans not only depends upon total dose, dose rate, linear energy transfer (LET), and fractionation and protraction of total doses, but also on repair mechanisms, bystander effects, and exposure to chemical carcinogens, tumour promoters and other toxins. It also depends upon the levels of anti-carcinogenic and anti-tumour promoting agents. Low doses of ionizing radiation should not be considered insignificant with regard to increasing the incidence of somatic mutations (neoplastic and non-neoplastic diseases) and heritable mutations in humans owing to its interaction with other toxins that can enhance damage produced by irradiation. It is very prudent to continue to support the well-established radiobiological concept that no radiation dose can be considered completely safe, and that all efforts must be made to reduce both the radiation dose and biological damage, no matter how small that damage might be, without sacrificing the benefits of radiation. Based on the results of many scientific experiments, formulations containing multiple antioxidants for biological protection against radiation damage in humans can be developed, and this strategy together with the existing physical concept of radiation protection, should further reduce potential risks of low doses of ionizing radiation in humans.

Mutagenesis and repair by low doses of alpha radiation in mammalian cells

Low doses of alpha radiation in basements have been causally implicated in lung cancer. Previous studies have concentrated on high dose effects, for which no significant repair was found. In the present study, the methodology for measuring mutation by quantitating mitotic breaks and gaps was found to be applicable to G2-phase Chinese hamster ovary cells irradiated with 10-50 cGy of alpha radiation. The mutation yield in such cells closely resembles that of gamma irradiation. Caffeine, which inhibits repair, produces the same straight line increase of alpha and gamma mutation yields plotted against the dose. In the absence of caffeine, the repair of alpha radiation lesions is almost twice as great as for gamma radiation. Mitotic index changes substantiate these interpretations. It is proposed that the higher ion density associated with alpha radiation may result in fewer lesions being missed by the repair processes. The quantitation of chromosomal lesions for G2 cells exposed to low doses of alpha radiation, gamma radiation, or chemical mutagens in the presence and absence of caffeine is a rapid and reproducible methodology. Protection from mutational disease in a fashion similar to the use of sanitation for infectious disease appears practical.

Caffeine does not potentiate gamma-radiation induced DNA damage in ataxia telangiectasia lymphoblastoid cells

Ataxia telangiectasia (AT) cells display a profound sensitivity to ionizing radiation, exhibiting more frequent chromosomal breaks, increased micronuclei formation and abnormal DNA repair kinetics following exposure. Despite the recent cloning of the ATM gene there remains a need for a simple and rapid means of discriminating AT heterozygotes from normal individuals. Caffeine (1,3,7-trimethyl xanthine), known to inhibit the repair of double-strand DNA breaks following ionizing radiation, increases the frequency of radiation induced chromosomal breaks in normal cells. Here we report that caffeine potentiates the induction of chromosomal breaks in G2 arrested AT heterozygote and normal lymphoblastoid cells, but not in homozygous AT lymphoblastoid cells. This observation parallels the findings reported by others that caffeine fails to potentiate the effect of ionizing radiation in radiation-sensitive yeast strains and radiation sensitive CHO cells. It also suggests that caffeine may somehow mimic the effect of the ATM gene product in normal cells. We also report that caffeine is unlikely to be useful in helping to discriminate AT heterozygotes from normal individuals.

Mutation measurement in mammalian cells. IV: Comparison of gamma-ray and chemical mutagenesis

The interaction of chemical mutagens with mammalian cells is much more complex than that of gamma-irradiation because of the different ways in which chemical agents react with cell and medium components. Nevertheless, the system previously described for analysis of mutagenesis by gamma-radiation appears applicable to chemical mutagenesis. The approach involves measurement of cell survival, use of caffeine to inhibit repair, analysis of mitotic index changes, and quantitation of microscopically visible structural changes in mitotic chromosomes. The behavior of a variety of chemical mutagens and nonmutagens in this system is described and compared with that of gamma-irradiation. The procedure is simple and the results reasonably quantitative though less so than those of gamma-irradiation. The procedure can be used for environmental monitoring, analysis of mutational events, and individual and epidemiological testing. Mutational events should be classified as primary or secondary depending on whether they represent initial genomic insult, or genomic changes resulting from primary mutation followed by structural changes due to metabolic actions. While caffeine has multiple effects on the mammalian genome, when used under the conditions specified here it appears to act principally as an inhibitor of mutation repair, and so affords a measure of the role of repair in the action of different mutagens on cells in the G2 phase of the life cycle.

A system for mutation measurement in mammalian cells: application to gamma-irradiation

Monitoring of mutagenesis by environmental agents for the purpose of preventing genetic disease including cancer must include quantitation of cell killing, sensitive measurement of mutation production by appropriate doses of each agent, and assessment of mutation repair effects in mammalian cells. A four-step procedure, in the presence and absence of a repair suppressor, is proposed: (i) determination of the survival curve; (ii) measurement of the mitotic index in cells collected after treatment with colcemid; (iii) construction of a mutagenesis yield curve in the presence and absence of a repair suppressor, like caffeine; and (iv) assessment of the effect of test agents on the repair of mutations produced by other mutagens. The procedure is quantitative, reproducible, and reasonably rapid. It involves measurement of mutations causing visible chromosomal aberrations. Numerical parameters are proposed defining quantitatively mutation, cell killing, and mutation repair capacity. The procedure is applied to gamma-irradiation and can detect the effects of doses as low as 2-5 cGy. Theoretical analysis of the underlying processes is presented, using the concept of D(0)E, the effective dose of mutagen after repair mechanisms and neutralizing agents have acted. Microscopically visible chromosome aberrations are due to mutations that distort the process of mitotic chromosome condensation, with or without actual chromosome breakage.

Gamma-ray mutagenesis measurement in mammalian cells

Application of conventional in vitro mutagenesis testing has so far failed to result in marked reduction of the total incidence of cancer. At least part of the reason may lie in the frequent use of a cell target too small to yield adequate sensitivity, and in failure to take into account the effects of cell killing in the assessment of mutagenic action. A single theoretical analysis fits the results of experimental data on gamma-irradiation applied to single marker gene testing in bacteria and to cytogenetic analysis of irradiated mammalian cells, and permits determination of the mean mutagenic dose, DoM, without complication due to cell killing. Cytogenetic monitoring of human lymphocytes which can detect mutagenic effects of gamma-radiation down to doses of < 0.1 Gy (10 rad) will also furnish an estimate of repair effectiveness at these low levels and may well be a useful tool in a program for prevention of cancer and other genetic disease.