Ionizing radiation and genetic risks. I. Epidemiological, population genetic, biochemical and molecular aspects of Mendelian diseases

A comparative study of genotoxicity endpoints for women exposed to different levels of indoor radon concentrations

BACKGROUND AND PURPOSE

Radon and its radioactive progenies are the most important source of ionizing radiation of natural origin, being classified as a Group 1 carcinogen. The aim of this study is to investigate the genotoxic effects of a wide range of indoor radon concentrations, as well as the kinetics of the process of repairing DNA-induced lesions by a challenging dose of gamma irradiation.

MATERIAL AND METHODS

Female subjects residing in the Băiţa-Ştei radon priority area were selected as the exposed group. The reference group was comprised of women from the same county (Bihor), but located in an area with an average indoor radon concentration typical of the county from which they were taken. Radon concentration values of 300 Bq/m³ and 148 Bq/m³, respectively, were chosen as a threshold in order to capture the impact of radon exposure between the groups. The alkaline comet assay was used in order to measure the DNA damage, as well as the repair kinetics at 2 and 24 h after 2 Gy challenging doses of gamma irradiation using peripheral blood lymphocytes. From the serum of the subjects, the oxidative damage by 8-hydroxydeoxyguanosine as well as the PARP induction was evaluated. The chromosomal aberrations were evaluated using the Cytokinesis Block MicroNucleus Assay.

RESULTS

A statistically significant increase was observed in terms of DNA-induced lesions assessed by comet assay for the exposed group compared to the reference group. A positive correlation was obtained between DNA damage and the annual effective dose, respectively with the radon progenies concentrations. A statistically significant difference was also observed for the frequency of the micronuclei between the exposed - reference groups. Significantly faster repair kinetics of DNA-induced lesions was recorded for the first 2 h after gamma irradiation in the reference group compared to the exposed group. Using the threshold of 300 Bq/m³ for radon concentration, faster kinetics of DNA damage repair for people exposed to low radon concentrations, compared to those exposed to higher concentrations for the second phase of DNA repair kinetics was observed.

CONCLUSION

An increased radiosensitivity of lymphocytes, as well as slower repair kinetics, may be associated with exposure to higher indoor radon concentrations.

The dose-, LET-, and gene-dependent patterns of DNA changes underlying the point mutations in spermatozoa of Drosophila melanogaster. I. Autosomal gene black

Sequence analysis of 7 spontaneous, 27 γ-ray- and 20 neutron/neutron+γ-ray-induced black (b) point mutants was carried out. All these mutants were isolated as non-mosaic transmissible recessive visibles in the progeny of irradiated males from the wild-type high-inbred laboratory D32 strain of Drosophila melanogaster. Among spontaneous mutants, there were two (28.5 %) mutants with copia insertion in intron 1 and exon 2, three (42.8 %) with replacement of b^+D32 paternal sequence with maternal b¹ sequence (gene conversion), one (14.3 %) with 142-bp-long insertion in exon 2, and one (14.3 %) with a short deletion and two single-base substitutions in exon 3. Among γ-ray-induced mutants, there were 1 (3.7 %) with copia insertion in intron 2, 6 (22.2 %) with gene conversion, and the remaining 20 (74.1 %) mutants had 37 different small-scale DNA changes. There were 20 (54.1 %) single- or double-base substitutions, 7 (18.9 %) frameshifts (indels), 9 (24.3 %) extended deletions or insertions, and 1(2.7 %) mutant with a short insertion instead of a short deletion. Remarkably, clusters of independent small-scale changes inside the gene or within one DNA helical turn were recovered. The spectrum of DNA changes in 20 neutron/ neutron+γ-ray-induced mutants was drastically different from that induced by γ-rays in that 18 (90.0 %) mutants had the b¹sequence. In addition, 2 (10.0 %) with gene conversion had 600- or 19-bp-long deletion in exon 3 and 1 (5.0 %) mutant with a short insertion instead of a short deletion. Analysis of all 27 mutants with gene conversion events shows that 20 (74.1 %) had full b¹ sequence whereas 7 others (25.9 %) contained a partial b¹ sequence. These data are the first experimental evidence for gene conversion in the early stages of animal embryogenesis in the first diploid cleavage nucleus after male and female pronuclei have united. The gene conversion, frameshifts (indels), and deletions between short repeats were considered as products of a relevant DNA repair pathways described in the literature. As the first step, the gametic doubling doses for phenotypic black point mutations and for intragenic base substitution mutations in mature sperm cells irradiated by 40 Gy of γ-rays were estimated as 5.8 and 1.2 Gy, respectively, showing that doubling dose for mutations at the molecular level is about 5 times lower than that at the phenotypic level.

Heritable Anomalies among the Inhabitants of Regions of Normal and High Background Radiation in Kerala: Results of a Cohort Study, 1988–1994

In a genetic epidemiological and fertility survey among 70,000 inhabitants in a high-background radiation region (HBRR) and normal radiation region (NRR) in Kerala, India, 985 persons were found to have heritable anomalies. Suggested etiologies for the anomalies were chromosomal and Mendelian, 15 percent; multifactorial, 60 percent; and congenital, 25 percent. There was a statistically significant increase of Down syndrome, autosomal dominant anomalies, and multifactorial diseases and an insignificant increase of autosomal recessive and X-linked recessive anomalies in the HBRR. The total fertility rate was 3.85 per couple; 9 percent of live-born children were reported dead. The rate of untoward pregnancy outcome—death of the offspring or presence of an anomaly in a living child—was 6.4 percent among the unrelated couples in the NRR, with one spouse born outside the area of current residence (“migrant”). Considering this as the base, the excess relative risks in the other groups are: “NRR-nonmigrant,” 35 percent; “HBRR-nonmigrant,” 69 percent; “NRR-consanguineous,” 76 percent; and “NBRR-consanguineous,” 157 percent. Ionizing radiation, consanguinity, and nearness of birthplace of the spouse are risk factors for the death of offspring and for anomalies. The higher risk among the “nonmigrant” couples may be due to geographic inbreeding. The findings are suggestive of an autosomal recessive etiology for the majority of the multifactorial anomalies.

Mouse models for induced genetic instability at endogenous loci

Exposure to environmental factors and genetic predisposition of an individual may lead individually or in combination to various genetic diseases including cancer. These diseases may be a consequence of genetic instability resulting in large-scale genomic rearrangements, such as DNA deletions, duplications, and translocations. This review focuses on mouse assays detecting genetic instability at endogenous loci. The frequency of DNA deletions by homologous recombination at the pink-eyed unstable (p(un)) locus is elevated in mice with mutations in ATM, Trp53, Gadd45, and WRN genes and after exposure to carcinogens. Other quantitative in vivo assays detecting loss of heterozygosity events, such as the mammalian spot assay, Dlb-1 mouse and Aprt mouse assays, are also reviewed. These in vivo test systems may predict hazardous effects of an environmental agent and/or genetic predisposition to cancer.

Germ cell mutagenesis in lacZ transgenic mice treated with methyl methanesulfonate

Mutagenesis induced by methyl methanesulfonate (MMS), a germ cell mutagen, in the testis and the sperm isolated from epididymis and vas deferens have been investigated using lacZ transgenic mice (Muta Mouse). Male Muta Mice were injected intraperitoneally with MMS at a dose of 80 mg/kg, a potent dominant lethal dose. Animals were killed on days 3 and 7 (Experiment 1) or days 10 and 14 (Experiment 2) after the treatment. Mutant frequencies (MFs) in the testis, sperm and spleen (Experiment 2 only) were analyzed by the positive selection system using E. coli C (GalE-) strain and phenyl beta-D-galactoside. The spontaneous MFs in the testis and sperm were 2.0-3.1 x 10(-5). No induction of mutation in the testis or sperm of the MMS-treated groups was observed at any sampling point. In the spleen, the spontaneous MF was approximately twice as high as that in the germ cells although the MF at each sampling point was almost the same as the spontaneous MF. MMS is known as a potent clastogen from the results of the dominant lethal assay and the micronucleus assay. The reason for the discrepancy between the results of these assays and the present results may have been insensitivity of the in vitro packaging to large deletion due to the failure to rescue the large deleted gene. It is suggested that the transgenic mouse assay using the in vitro packaging can not replace the dominant lethal assay in the case of MMS.

The molecular nature of spontaneous mutations at the hprt locus in the radiosensitive CHO mutant xrs-5

The radiosensitive mutant xrs-5, a derivative of the Chinese hamster ovary (CHO) K1 cell, is defective in DNA double-strand break rejoining ability and in V(D)J recombination. The radiosensitivity and defective repair phenotype are complemented by the 80-kDa subunit of the Ku protein. We determined the nature of the mutations that develop spontaneously at the hprt locus in this cell line using both multiplex PCR deletion screening and DNA sequencing. Ninety-two independent spontaneous mutants were analyzed and the results were compared to the mutation spectrum of 64 previously analyzed hprt spontaneous mutants isolated from the parental CHO-K1 cell line. More than 50% of the spontaneous xrs-5 mutants had lost one or more exons while less than 25% of spontaneous CHO-K1 mutants had lost one or more exons. Most of the deletions in xrs-5 cells involved the loss of multiple exons while single exon deletions predominated in CHO-K1. There was also a nonrandom distribution of breakpoints in both CHO-K1 and xrs-5. Most of the deletion breakpoints were 3' to exon 9, around exons 4-6, or near exon 1. Although the frequency of base substitutions was lower in xrs-5, the spectrum of base substitutions was qualitatively similar to that of CHO-K1. There was no significant difference in the spontaneous mutant frequency in xrs-5 and CHO-K1. The results suggest that in certain regions of the hprt gene, base alterations can be converted to large deletions, and that alterations in the Ku protein complex can influence this process.

Spontaneous and X-ray-induced deletion mutations in a LacZ plasmid-based transgenic mouse model

Transgenic mouse mutation models carrying bacterial marker genes in bacteriophage lambda shuttle vectors have been applied to study spontaneous or induced mutations in vivo. However, due to the nature of the shuttle vector these models are insensitive to large deletions. Clastogenic agents, which predominantly induce large deletions, were therefore found to yield very low responses in these assays. Here we report the use of LacZ plasmid-based transgenic mice, allowing the detection of a broad spectrum of mutations. Treatment of mice with X-rays (5 x 50 rads) resulted in induction of up to about 5-fold higher mutation frequencies in lung, spleen and liver. Analysis of spontaneous and induced mutant LacZ genes indicated that at least 40-50% of all mutations were caused by deletions. The possibility of detecting a broad spectrum of mutations with this system suggests that the LacZ plasmid-based transgenic mouse may be the mammalian model of choice for studying spontaneous and induced mutations in vivo.

Biological basis of germline mutation: comparisons of spontaneous germline mutation rates among drosophila, mouse, and human

Spontaneous mutation rates per generation are similar among the three species considered here--Drosophila, mouse, and human--and are not related to time, as is often assumed. Spontaneous germline mutation rates per generation averaged among loci are less variable among species than they are among loci and tests and between gender. Mutation rates are highly variable over time in diverse lineages. Recent estimates of the number of germ cell divisions per generation are: for humans, 401 (30-year generation) in males and 31 in females; for mice, 62 (9-month generation) in males and 25 in females; and for Drosophila melanogaster, 35.5 (18-day generation) in males and 36.5 (25-day generation) in females. The relationships between germ cell division estimates of the two sexes in the three species closely reflect those between mutation rates in the sexes, although mutation rates per cell division vary among species. Whereas the overall rate per generation is constant among species, this consistency must be achieved by diverse mechanisms. Modifiers of mutation rates, on which selection might act, include germline characteristics that contribute disproportionately to the total mutation rates. The germline mutation rates between the sexes within a species are largely influenced by germ cell divisions per generation. Also, a large portion of the total mutations occur during the interval between the beginning of meiosis and differentiation of the soma from the germline. Significant genetic events contributing to mutations during this time may include meiosis, lack of DNA repair in sperm cells, methylation of CpG dinucleotides in mammalian sperm and early embryo, gonomeric fertilization, and rapid cleavage divisions.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 5. Impact of the molecular spectrum of mutational lesions on estimates of germinal gene-mutation rates

Review of the molecular characteristics of the variants identified at a series of disease loci suggests significant differences among loci in the relative frequency of nucleotide substitutions versus more complex events such as deletions. Some common features are repeatedly observed in each class of variant. For example, a high proportion of the nucleotide substitutions involve transitions of deoxycytidine and are suggested to result from deamination of cytosine at 5-methyl-CpG sites. Similarly, deletions of three or fewer nucleotides are relatively common in the non-nucleotide substitution class and these deletions are often associated with a seven-nucleotide core sequence. A significant fraction of the larger deletions and rearrangements may be associated with repetitive elements. Many of the deletion events do not appear to involve a chromosomal recombination mechanism. Mechanisms involving transcription slippage and chromatid exchange have been suggested as possible alternative mechanisms for generating deletion events. The spectrum of mutational events identified, e.g. nucleotide substitutions versus deletions, differs between loci and is probably a reflection of both the gene structure and the selective pressure to generate a disease phenotype. This locus specificity (at both the biological and molecular level) would appear to have significant potential to compromise estimates of increases in the gene germinal mutation rate following exposure to mutagenic agents.

Radiation risks from imaging of intestinal and abdominal inflammation

Most abdominal imaging procedures involve some radiation dose to the patient. The types of radiation-induced harm from these techniques are described. The dose received from each of these techniques in clinical practice is thought to be below the threshold required to cause developmental abnormalities in pregnancy. According to the present state of knowledge, there is no significant risk of genetic damage to the patient from any of these procedures. Organs vary considerably in their sensitivity to radiation-induced cancer. The effect of radiation on any organ depends on the radiation dose to that particular organ only. The most radiosensitive organs are bone marrow, lung, stomach and large bowel. The macroscopic and microscopic distribution of radiation dose is discussed for all of the various imaging techniques. There is a large dose variation between individual patients and the mean dose received from each procedure is similar for each of the imaging techniques described, the mean radiation dose is low and the decision as to which test to choose depends more on the diagnostic value of each particular procedure.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 6. Estimation of genetic risks of exposure to chemical mutagens: relevance of data on spontaneous mutations and of experience with ionizing radiation

This paper examines the impact of advances in knowledge on the molecular biology of human Mendelian diseases on the estimation of genetic risks of exposure to ionizing radiation and to chemical mutagens. More specifically, it addresses the question of whether and to what extent naturally occurring Mendelian diseases can be used as a baseline for efforts in this area. Data on the molecular nature and mechanisms of origin of spontaneous mutations underlying naturally occurring Mendelian diseases and on radiation-induced mutations in experimental systems suggest that for ionizing radiation, naturally occurring Mendelian diseases may not constitute an entirely adequate frame of reference and that current risk estimates for this class of diseases are conservative; these estimates however provide a margin of safety in formulating radiation protection guidelines. Currently available data on mechanisms and specificities of action of chemical mutagens, molecular dosimetry, repair of chemically induced adducts in the DNA, adduct-mutation relationships etc., permit the tentative conclusion that naturally occurring Mendelian diseases may provide a better baseline for genetic risk estimation for chemical mutagens than for ionizing radiation. With both ionizing radiation and chemical mutagens, the question of which Mendelian diseases are potentially inducible will become answerable in the near future when more molecular data on human genetic diseases become available. It is therefore essential that risk estimators keep abreast of advances in human genetics and integrate these into their conceptual framework. However, induced Mendelian diseases (especially the dominant ones which are of more immediate concern) are likely to represent a very small fraction of the adverse genetic effects of induced mutations. More attention therefore needs to be devoted to studies on the heterozygous effects of induced mutations.

Ionizing radiation, genetic risk estimation and molecular biology: impact and inferences

In recent years, a substantial amount of information has been obtained on the molecular nature of spontaneous mutations underlying human mendelian diseases, and on the mechanisms that give rise to these mutations. These data, when considered together with data on mutations induced by ionizing radiation in mammalian experimental systems, support the view that current radiation risk estimates for mendelian diseases (which are based on mouse data) are conservative.

Ionizing radiation and genetic risks. II. Nature of radiation-induced mutations in experimental mammalian in vivo systems

This paper reviews data on the nature of spontaneous and radiation-induced mutations in the mouse. The data are from studies using a variety of endpoints scorable at the morphological or the biochemical level and include pre-selected as well as unselected loci at which mutations can lead to recessive or dominant phenotypes. The loci used in the morphological recessive specific-locus tests permit the recovery of a wide spectrum of induced changes. Important variables that affect the nature of radiation-induced mutations (assessed primarily using tests for viability of homozygotes) include: germ cell stage, type of irradiation and the locus. Most of the results pertain to irradiated stem cell spermatogonia. The data on morphological specific-locus mutations show that overall, more than two-thirds of the X- or gamma-ray-induced mutations are lethal when homozygous. This proportion may be lower for those that occur spontaneously, but the numbers of tested mutants are small. For spontaneous mutations, there is evidence for the occurrence of mosaics and for proviral insertions. Most or all tested induced enzyme activity variants, dominant visibles (recovered in specific-locus experiments) and dominant skeletal mutations are lethal when homozygous and this is true of 50% of dominant cataract mutations, but again, the numbers of tested mutants are small. Electrophoretic mobility variants, which are known to be due to base-pair changes, are seldom induced by irradiation. At the histocompatibility loci, no radiation-induced mutations have been recovered, presumably because deletions are incompatible with survival even in heterozygotes. All these findings are consistent with the view that in mouse germ cells, most radiation-induced mutations are DNA deletions. Some mutations (in the morphological specific-locus tests) which had previously been inferred to be deletions on the basis of genetic analyses have now been shown to be DNA deletions by molecular methods. However, the possibility cannot be excluded that at least a small proportion of induced mutations may be intragenic changes. The data on the rates of induction of recessive lethals and of dominant skeletal and dominant cataract mutations (and proportions of the latter two which are homozygous lethal) can be used to estimate the proportions of recessive lethals which are expressed as skeletal abnormalities or cataracts. These calculations show that about 10% of recessive lethals manifest themselves as skeletal and less than 0.2% as cataract mutations.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionizing radiation and genetic risks. IV. Current methods, estimates of risk of Mendelian disease, human data and lessons from biochemical and molecular studies of mutations

This paper is aimed at a synthesis of conclusions and concepts from the first three papers of this series and an inquiry of their relevance to the estimation of the risk of autosomal dominant and X-linked diseases in man, due to exposure to ionizing radiation. For a population under conditions of continuous irradiation, the doubling-dose method (DD method) enables the prediction of the excess risk of dominant and X-linked diseases at equilibrium. Per unit dose, this quantity is the product of the natural prevalence of these diseases (assumed to be 10,000/10(6) livebirths) and the reciprocal of the DD. The DD currently used is 1 Gy and is based primarily on data on the induction of recessive specific-locus mutations in male mice. The estimate of risk to the first generation is derived from that at equilibrium; the figure is about 15% of the equilibrium value (i.e., 15 cases/10(6) livebirths/cGy). With the direct method, the first-generation risk of dominant disease is estimated using data on the induction of dominant skeletal and cataract mutations in male mice and a number of correction factors. The estimates are about 10-20 cases and 0-9 cases, respectively, for irradiation of males and females, per 10(6) livebirths/cGy. In the Japanese studies, no significant adverse genetic effects, attributable to exposure of the parents to the atomic bombs, could be demonstrated with respect to any of the endpoints used. Most of the latter are clinically and socially relevant but mutationally insensitive. On the basis of these data, Neel and colleagues have estimated that the gametic DD for genetic effects of radiation in man is at least about 4-5 times the 1 Gy value thus far used. The concepts, assumptions, and the data-base used with the DD method have been re-examined. Arguments are advanced to support the thesis that ionizing radiation is probably not very efficient in inducing the very specific molecular changes that are known to underlie spontaneous mutations which cause naturally occurring dominant genetic diseases. It is suggested that (i) the DD estimate of 1 Gy that is used to estimate risk for autosomal dominant and X-linked diseases is conservative and (ii) the 1% prevalence figure for these diseases that is used for this purpose may be too high. If these suggestions are correct, then the estimate of risk for the dominant and X-linked diseases may need to be revised downwards.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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 deletions. 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 (approximately 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.(ABSTRACT TRUNCATED AT 400 WORDS)