A Genome Scanning Approach to Assess the Genetic Effects of Radiation in Mice and Humans

A systematic review of human evidence for the intergenerational effects of exposure to ionizing radiation

PURPOSE

To provide a synthesis of the published evidence pertaining to the intergenerational health effects of parental preconceptional exposure to ionizing radiation in humans.

METHODS

The study populations are the descendants of those who were exposed to ionizing radiation prior to conception. A Boolean search identified publications for review in accordance with Office of Health Assessment and Translation guidelines. Initially, a risk of bias assessment was conducted for each published study and relevant data extracted. Information was organized into adverse health outcome groups and exposure situations. To make an assessment from the body of evidence within each group, an initial confidence rating was assigned, before factors including inconsistencies between studies, magnitude of effect, dose response and confounders were considered. From this, 'an effect', 'no effect' or whether the evidence remained 'inadequate' to determine either effect or no effect, was ascertained. This assessment was based primarily upon the author's conclusions within that evidence-base and, by binomial probability testing of the direction of effect reported.

RESULTS

2441 publications were identified for review which after screening was reduced to 127. For the majority of the adverse health groups, we find there to be inadequate evidence from which to determine whether the health effect was, or was not, associated with parental preconceptional radiation exposure. This was largely due to heterogeneity between individual study's findings and conclusions within each group and, the limited number of studies within each group. We did observe one health grouping (congenital abnormalities) in occupationally exposed populations, where an increase in effect relative to their controls or large magnitude of effects, were reported, although it is noted that the authors of these studies interpreted their findings as most likely not to be associated with parental radiation exposure.

CONCLUSIONS

We find there to be a lack of evidence to enable the formal assessment of radiation-related adverse effects in offspring of exposed humans. This is not the same as there being no clear evidence that effects may occur but does infer that if adverse health effects do arise in children of exposed parents, then these effects are small and difficult to reproducibly measure. Inconsistencies in designing studies are unavoidable, however we highlight the need for an element of standardization and, more sharing of primary datasets as part of open access initiatives, in order for future reviews to make reasonable conclusions. Overall, there is a need for future work to ensure comparable measures between studies where possible.

Three major reasons why transgenerational effects of radiation are difficult to detect in humans

PURPOSE

Ionizing radiation can induce mutations in germ cells in various organisms, including fruit flies and mice. However, currently, there is no clear evidence for the transgenerational effects of radiation in humans. This review is an effort to identify possible reasons for the lack of such observations.

METHODS

Literature search and narrative review.

RESULTS

1) In both mice and humans, resting oocytes locate primarily in the cortical region of the ovary where the number of blood vessels is very low especially when young and extra-cellular material is rich, and this region is consequently hypoxic, which probably leads to immature oocytes being resistant to the cell killing and mutagenic effects of radiation. 2) In studies of spermatogonia, the mouse genes used for specific locus test (SLT) studies, which include coat color genes, were hypermutable when compared to many other genes. Recent studies which examined over 1000 segments of genomic DNA indicate that the induction rate of deletion mutation per segment was on the order of 10^-6 per Gy, which is one order of magnitude lower than that obtained from the SLT data. Therefore, it appears possible that detecting any transgenerational effects of radiation following human male exposures will be difficult due to a lack of mutable marker genes. 3) Fetal malformations were examined in studies in humans, but the genetic component in such malformations is low, and abnormal fetuses are prone to undergo miscarriage which does not occur in mice, and which leads to difficulties in detecting transgenerational effects.

CONCLUSION

The lack of clear evidence for radiation effects in humans probably does not result from any problem in the methodologies used but may be due largely to biological properties. Currently, whole genome sequencing studies of exposed parents and offspring are planned, but ethical guidelines need to be followed to avoid discrimination, which had once happened to the atomic bomb survivors.

Cytoplasmic DNA: sources, sensing, and role in aging and disease

Endogenous cytoplasmic DNA (cytoDNA) species are emerging as key mediators of inflammation in diverse physiological and pathological contexts. Although the role of endogenous cytoDNA in innate immune activation is well established, the cytoDNA species themselves are often poorly characterized and difficult to distinguish, and their mechanisms of formation, scope of function and contribution to disease are incompletely understood. Here, we summarize current knowledge in this rapidly progressing field with emphases on similarities and differences between distinct cytoDNAs, their underlying molecular mechanisms of formation and function, interactions between cytoDNA pathways, and therapeutic opportunities in the treatment of age-associated diseases.

Why Genetic Effects of Radiation are Observed in Mice but not in Humans

Genetic effects from radiation have been observed in a number of species to date. However, observations in humans are nearly nonexistent. In this review, possible reasons for the paucity of positive observations in humans are discussed. Briefly, it appears likely that radiation sensitivity for the induction of mutations varies among different genes, and that the specific genes that were used in the past with the specific locus test utilizing millions of mice may have simply been very responsive to radiation. In support of this notion, recent studies targeting the whole genome to detect copy number variations (deletions and duplications) in offspring derived from irradiated spermatogonia indicated that the mutation induction rate per genome is surprisingly lower than what would have been expected from previous results with specific locus tests, even in the mouse. This finding leads us to speculate that the lack of evidence for the induction of germline mutations in humans is not due to any kind of species differences between humans and mice, but rather to the lack of highly responsive genes in humans, which could be used for effective mutation screening purposes. Examples of such responsive genes are the mouse coat color genes, but in human studies many more genes with higher response rates are required because the number of offspring examined and the radiation doses received are smaller than in mouse studies. Unfortunately, such genes have not yet been found in humans. These results suggest that radiation probably induces germline mutations in humans but that the mutation induction rate is likely to be much lower than has been estimated from past specific locus studies in mice. Whole genome sequencing studies will likely shed light on this point in the near future.

Genome-Wide Deletion Screening with the Array CGH Method in Mouse Offspring Derived from Irradiated Spermatogonia Indicates that Mutagenic Responses are Highly Variable among Genes

Until the end of the 20th century, mouse germ cell data on induced mutation rates, which were collected using classical genetic methods at preselected specific loci, provided the principal basis for estimates of genetic risks from radiation in humans. The work reported on here is an extension of earlier efforts in this area using molecular methods. It focuses on validating the use of array comparative genomic hybridization (array CGH) methods for identifying radiation-induced copy number variants (CNVs) and specifically for DNA deletions. The emphasis on deletions stems from the view that it constitutes the predominant type of radiation-induced genetic damage, which is relevant for estimating genetic risks in humans. In the current study, deletion mutations were screened in the genomes of F1 mice born to unirradiated or 4 Gy irradiated sires at the spermatogonia stage (100 offspring each). The array CGH analysis was performed using a "2M array" with over 2 million probes with a mean interprobe distance of approximately 1 kb. The results provide evidence of five molecularly-confirmed paternally-derived deletions in the irradiated group (5/100) and one in the controls (1/100). These data support a calculation, which estimates that the mutation rate is 1 × 10^-2/Gy per genome for induced deletions; this is much lower than would be expected if one assumes that the specific locus rate of 1 × 10^-5/locus per Gy (at 34 loci) is applicable to other genes in the genome. The low observed rate of induced deletions suggests that the effective number of genes/genomic regions at which recoverable deletions could be induced would be only approximately 1,000. This estimate is far lower than expected from the size of the mouse genome (>20,000 genes). Such a discrepancy between observation and expectation can occur if the genome contains numerous genes that are far less sensitive to radiation-induced deletions, if many deletion-bearing offspring are not viable or if the current method is substandard for detecting small deletions.

Evidence relevant to untargeted and transgenerational effects in the offspring of irradiated parents

In this article we review health effects in offspring of human populations exposed as a result of radiotherapy and some groups exposed to chemotherapy. We also assess risks in offspring of other radiation-exposed groups, in particular those of the Japanese atomic bomb survivors and occupationally and environmentally exposed groups. Experimental findings are also briefly surveyed. Animal and cellular studies tend to suggest that the irradiation of males, at least at high doses (mostly 1Gy and above), can lead to observable effects (including both genetic and epigenetic) in the somatic cells of their offspring over several generations that are not attributable to the inheritance of a simple mutation through the parental germline. However, studies of disease in the offspring of irradiated humans have not identified any effects on health. The available evidence therefore suggests that human health has not been significantly affected by transgenerational effects of radiation. It is possible that transgenerational effects are restricted to relatively short times post-exposure and in humans conception at short times after exposure is likely to be rare. Further research that may help resolve the apparent discrepancies between cellular/animal studies and studies of human health are outlined.

The genetic risk in mice from radiation: an estimate of the mutation induction rate per genome

Restriction Landmark Genome Scanning (RLGS) is a method that uses end-labeled (32)P NotI sites that are mostly associated with coding genes to visualizes thousands of DNA fragments as spots in two-dimensional autoradiograms. This approach allows direct detection of autosomal deletions as spots with half normal intensity. The method was applied to mouse offspring derived from spermatogonia exposed to 4 Gy of X rays. A genome-wide assessment of the mutation induction rate was estimated from the detected deletions. Examinations were made of 1,007 progeny (502 derived from control males and 505 from irradiated males) and 1,190 paternal and 1,240 maternal spots for each mouse. The results showed one deletion mutation in the unirradiated paternal genomes of 502 offspring (0.2%) and 5 deletions in the irradiated paternal genomes of 505 offspring (1%). The difference was marginally significant, with the deletion sizes ranged from 2-13 Mb. If the frequencies are taken at face value, the net increase was 0.8% after an exposure of 4 Gy, or 0.2% per Gy per individual if a linear dose response is assumed. Since the present RLGS analysis examined 1,190 NotI sites, while the mouse genome contains ∼25,000 genes, the genomic probability of any gene undergoing a deletion mutation would be 25× 0.2%, or 5% per Gy. Furthermore, since the present RLGS screened about 0.2% of the total genome, the probability of detecting a deletion anywhere in the total genome would be estimated to be 500 times 0.2% or 100% (i.e., 1 deletion per Gy). These results are discussed with reference to copy number variation in the human genome.

LINE-1 retrotransposition events regulate gene expression after X-ray irradiation

Long interspersed nuclear element-1 (LINE-1) retrotransposons are mobile elements that insert into new genomic locations via reverse transcription of an RNA intermediate. The mechanism of retrotransposition is not entirely understood. The integration of these elements occurs by target-primed reverse transcription (TPRT), which initiates double-strand breaks (DSBs) during the LINE-1 integration. Also, X-ray is known to induce DNA damage. The aim of this study was to evaluate the potential effects of LINE-1 de novo retrotransposition on the expression of different genes after X-ray irradiation in human endothelial cells. After stable transfection of the human hybrid endothelial cell line EA.hy926 with the human LINE-1 element, we analyzed the expression of different genes after irradiation with 5 Gy X-rays by reverse transcription-polymerase chain reaction (RT-PCR). We determine the expression level of phosphorylated p53 and γ-histone H2AX protein levels upon X-ray irradiation with 5 Gy for 24 h. Our results showed that EA.hy926 LINE-1 cell clones react with a strong upregulation of phosphorylated p53 protein, already 15 min after irradiation compared to the wild type (WT) cells. Also, the expression of γ-histone H2AX protein was elevated in the cell clones with retrotransposition events 15 min after irradiation, whereas the WT cells have a delayed expression of phosphorylated histone H2AX protein. Taken together, our findings provide that LINE-1 retrotransposition events regulate different gene expression after irradiation in the EA.hy926 cell line.

Molecular logic underlying chromosomal translocations, random or non-random?

Chromosomal translocations serve as essential diagnostic markers and therapeutic targets for leukemia, lymphoma, and many types of solid tumors. Understanding the mechanisms of chromosomal translocation generation has remained a central biological question for decades. Rather than representing a random event, recent studies indicate that chromosomal translocation is a non-random event in a spatially regulated, site-specific, and signal-driven manner, reflecting actions involved in transcriptional activation, epigenetic regulation, three-dimensional nuclear architecture, and DNA damage-repair. In this review, we will focus on the progression toward understanding the molecular logic underlying chromosomal translocation events and implications of new strategies for preventing chromosomal translocations.

DNA damage and L1 retrotransposition

Barbara McClintock was the first to suggest that transposons are a source of genome instability and that genotoxic stress assisted in their mobilization. The generation of double-stranded DNA breaks (DSBs) is a severe form of genotoxic stress that threatens the integrity of the genome, activates cell cycle checkpoints, and, in some cases, causes cell death. Applying McClintock's stress hypothesis to humans, are L1 retrotransposons, the most active autonomous mobile elements in the modern day human genome, mobilized by DSBs? Here, evidence that transposable elements, particularly retrotransposons, are mobilized by genotoxic stress is reviewed. In the setting of DSB formation, L1 mobility may be affected by changes in the substrate for L1 integration, the DNA repair machinery, or the L1 element itself. The review concludes with a discussion of the potential consequences of L1 mobilization in the setting of genotoxic stress.

Restriction landmark genome scanning for the detection of mutations

There is a continuing need for more efficient methods to examine mutations in humans and other species resulting from exposures to environmental toxins and radiation. Environmental genomic studies, which often utilize nonmodel system species and as a result, there is a particular need for a method that does not rely on the availability of genome sequence information. Restriction landmark genome scanning (RLGS) is a two-dimensional electrophoresis (2-DE) of end-labeled DNA fragments. A vertical giant gel 2-DE system has been developed and applied to the RLGS. On a single RLGS pattern of mouse or human DNA, approx 2000 DNA fragments (spots) varying in size from 1.0 to 5.0 kb in the first dimension and 0.2 to 3.0 kb in the second dimension are visualized. In principle, this system will detect genomic alterations of two types: (1) that due to gain or loss of a cut site for the three restriction fragment enzymes employed in the study and (2) that due to insertion/deletion/rearrangement (I/D/R) events. After optimization of the sample preparation and electrophoresis conditions, the gel quality reached a level such that the electrophoresis patterns derived from a single DNA sample gave distribution patterns of spots able to be superimposed. This technology can visualize up to 3000 DNA fragments per gel without using any probes, and thus should be highly efficient in monitoring for mutations resulting in I/D/R events in DNA fragments distributed throughout the genome. This method relies on direct labeling of DNA fragments rather than hybridization and therefore precise information on genome sequences is not required. As a result this method is applicable to any species.

Estimation of mutation induction rates in AT-rich sequences using a genome scanning approach after X irradiation of mouse spermatogonia

We have previously used NotI as the marker enzyme (recognizing GCGGCCGC) in a genome scanning approach for detection of mutations induced in mouse spermatogonia and estimated the mutation induction rate as about 0.7 x 10(-5) per locus per Gy. To see whether different parts of the genome have different sensitivities for mutation induction, we used AflII (recognizing CTTAAG) as the marker enzyme in the present study. After the screening of 1,120 spots in each mouse offspring, we found five mutations among 92,655 spots from the unirradiated paternal genome, five mutations among 218,411 spots from the unirradiated maternal genome, and 13 mutations among 92,789 spots from 5 Gy-exposed paternal genome. Among the 23 mutations, 11 involved mouse satellite DNA sequences (AT-rich), and the remaining 12 mutations also involved AT-rich but non-satellite sequences. Both types of sequences were found as multiple, similar-sequence blocks in the genome. Counting each member of cluster mutations separately and excluding results on one hypermutable spot, the spontaneous mutation rates were estimated as 3.2 (+/- 1.9) x 10(-5) and 2.3 (+/- 1.0) x 10(-5) per locus per generation in the male and female genomes, respectively, and the mutation induction rate as 1.1 (+/- 1.2) x 10(-5) per locus per Gy. The induction rate would be reduced to 0.9 x 10(-5) per locus per Gy if satellite sequence mutations were excluded from this analysis. The results indicate that mutation induction rates do not largely differ between GC-rich and AT-rich regions: 1 x 10(-5) per locus per Gy or less, which is close to 1.08 x 10(-5) per locus per Gy, the current estimate for the mean mutation induction rate in mice.

Gender differences in germ-cell mutagenesis and genetic risk

Current international classification systems for chemical mutagens are hazard-based rather than aimed at assessing risks quantitatively. In the past, germ-cell tests have been mainly performed with a limited number of somatic cell mutagens, and rarely under conditions aimed at comparing gender-specific differences in susceptibility to mutagen exposures. There are profound differences in the genetic constitution, and in hormonal, structural, and functional aspects of differentiation and control of gametogenesis between the sexes. A critical review of the literature suggests that these differences may have a profound impact on the relative susceptibility, stage of highest sensitivity and the relative risk for the genesis of gene mutation, as well as structural and numerical chromosomal aberrations in male and female germ cells. Transmission of germ-cell mutations to the offspring may also encounter gender-specific influences. Gender differences in susceptibility to chemically derived alterations in imprinting patterns may pose a threat for the health of the offspring and may also be transmitted to future generations. Recent reports on different genetic effects from high acute and from chronic low-dose exposures challenge the validity of conclusions drawn from standard methods of mutagenicity testing. In conclusion, research is urgently needed to identify genetic hazards for a larger range of chemical compounds, including those suspected to disturb proper chromosome segregation. Alterations in epigenetic programming and their health consequences will have to be investigated. More attention should be paid to gender-specific genetic effects. Finally, the database for germ-cell mutagens should be enlarged using molecular methodologies, and genetic epidemiology studies should be performed with these techniques to verify human genetic risk.

Genetic effects of radiation in atomic-bomb survivors and their children: past, present and future

Genetic studies in the offspring of atomic bomb survivors have been conducted since 1948 at the Atomic Bomb Casualty Commission and its successor, the Radiation Effects Research Foundation, in Hiroshima and Nagasaki. Past studies include analysis of birth defects (untoward pregnancy outcome; namely, malformation, stillbirth, and perinatal death), chromosome aberrations, alterations of plasma and erythrocyte proteins as well as epidemiologic study on mortality (any cause) and cancer incidence (the latter study is still ongoing). There is, thus far, no indication of genetic effects in the offspring of survivors. Recently, the development of molecular biological techniques and human genome sequence databases made it possible to analyze DNA from parents and their offspring (trio-analysis). In addition, a clinical program is underway to establish the frequency of adult-onset multi-factorial diseases (diabetes mellitus, high blood pressure, and cardiovascular disease etc) in the offspring. The complementary kinds of data that will emerge from this three-pronged approach (clinical, epidemiologic, and molecular aspects) promise to shed light on health effects in the offspring of radiation-exposed people.