History of radiation genetics: light and darkness

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.

Technical Report: A Simple and Robust Real-Time Quantitative PCR Method for the Detection of Radiation-Induced Multiple Exon Deletions of the Human HPRT Gene

The hypoxanthine-phosphoribosyltransferase (HPRT) mutation assay has been widely used to investigate gene mutations induced by radiation. Here, we developed a novel method detecting deletions of multiple exons of the HPRT gene based on real-time quantitative PCR (qPCR). Immortalized normal human fibroblasts (BJ1-hTERT) were irradiated at various doses with γ rays, subjected to the 6-thioguanine (6-TG) selection, and more than one hundred 6-TG-resistant (6-TGR) clones were isolated. High-molecular-weight genomic DNA was extracted, and real-time qPCR was performed with the nine exon-specific primers. Optimization of the primer concentration, appropriate selection of PCR enzyme and refinement of the reaction profiles enabled simultaneous quantitative amplification of each exon. We were able to identify 6-TGR clones with total deletions, which did not show any amplification of the nine exons, and partial deletion mutants, in which one or some of the nine exons were missing, within a few days. This novel technique allows systematic determination of multiple deletions of the HPRT exons induced by ionizing radiation, enabling high-throughput and robust analysis of multiple HPRT mutants.

Increased Frequency of Copy Number Variations Revealed by Array Comparative Genomic Hybridization in the Offspring of Male Mice Exposed to Low Dose-Rate Ionizing Radiation

There is very little information on the transgenerational or genetic effects of low dose-rate ionizing radiation. We report the detection of the transgenerational effects of chronic low dose-rate irradiation in mice, at the molecular level in the whole genome, using array comparative genomic hybridization technology. We observed that the number of the mice with de novo copy number variations (specifically, deletions) was significantly increased in the offspring of C57BL/6J male mice exposed to 20 mGy/day gamma-rays for 400 days (total dose: 8000 mGy), as compared to non-irradiated controls. We did not detect any difference in the size of the de novo deletions between the irradiated and the non-irradiated groups. An analysis of the life span of the offspring suggested a possibility that de novo copy-number variations may be associated with shorter life spans.

Let Chromosomes Speak: The Cytogenetics Project at the Atomic Bomb Casualty Commission (ABCC)

Hibakusha (atomic bomb survivors) are "witnesses" of the atomic bombings, not just in a standard sense but also in the instrumental sense. For medical and scientific experts, hibakusha are biological resources of unparalleled scientific value. Over the past seventy years, the hibakusha bodies have narrated what it means to be exposed to radiation. In this paper, I explore studies at the Atomic Bomb Casualty Commission (ABCC) that examined hibakusha bodies as sites where risk could be read. I focus on a period from the mid-1950s to 1975, during which new methods, practices, and technologies allowed ABCC scientists to investigate chromosomes as a way to study radiation exposure and human risk. By focusing on chromosomal aberrations, ABCC scientists connected their work directly to the emerging infrastructure for radiobiology at the time. ABCC administrators actively sought out such prestige, especially given their relationship with the Atomic Energy Commission (AEC). The shift in approach would also alleviate some public relations problems with which the institution was struggling. Launching a cytogenetics program required some older practices that had assumed American privilege and dominance to be abandoned. Eventually, the decision to let chromosomes speak of radiation exposure brought about fundamental changes in ABCC, which came to symbolize the model for future studies at the organization, especially as ABCC was transitioning to a US-Japan binational organization. More broadly, this case highlights the intricate scientific negotiation of radiation risk where uncertainties necessarily prevail.

Epidemiological studies of atomic bomb radiation at the Radiation Effects Research Foundation

Epidemiological studies of people who were exposed to atomic bomb radiation and their children who were conceived after parental exposure to radiation (F₁) have investigated late health effects of atomic bomb radiation and its transgenerational effects. Those studies were initiated by the Atomic Bomb Casualty Commission (ABCC) in the 1950s. ABCC was reorganized to the Radiation Effects Research Foundation (RERF) in 1975, which continued the work of the ABCC. Follow-up of vital status and cause of death is performed for all RERF cohorts, including the atomic bomb survivors (the Life Span Study: LSS), in utero survivors, and the children of the survivors (F₁). Cancer incidence is investigated for accessible subpopulations of the cohorts. Health examinations for subcohorts of the LSS and in utero survivors are conducted as the Adult Health Study (AHS); a program of health examinations for a subcohort of the F₁ study is called the F₁ Offspring Clinical Study (FOCS). Participants of all clinical programs are asked to donate their blood and urine for storage and future biomedical investigations. Epidemiological studies have observed increased radiation risks for malignant diseases among survivors including those exposed in utero, and possible risks for some noncancer diseases. No increased risks due to parental exposure to radiation have been observed for malignancies or other diseases in F₁, but continuing investigations are required.