Induction of lympho-haemopoietic malignancy: impact of preconception paternal irradiation

Male-mediated developmental toxicity

Male-mediated developmental toxicity has been of concern for many years. The public became aware of male-mediated developmental toxicity in the early 1990s when it was reported that men working at Sellafield might be causing leukemia in their children. Human and animal studies have contributed to our current understanding of male-mediated effects. Animal studies in the 1980s and 1990s suggested that genetic damage after radiation and chemical exposure might be transmitted to offspring. With the increasing understanding that there is histone retention and modification, protamine incorporation into the chromatin and DNA methylation in mature sperm and that spermatozoal RNA transcripts can play important roles in the epigenetic state of sperm, heritable studies began to be viewed differently. Recent reports using molecular approaches have demonstrated that DNA damage can be transmitted to babies from smoking fathers, and expanded simple tandem repeats minisatellite mutations were found in the germline of fathers who were exposed to radiation from the Chernobyl nuclear power plant disaster. In epidemiological studies, it is possible to clarify whether damage is transmitted to the sons after exposure of the fathers. Paternally transmitted damage to the offspring is now recognized as a complex issue with genetic as well as epigenetic components.

Epigenetics in radiation biology: a new research frontier

The number of people that receive exposure to ionizing radiation (IR) via occupational, diagnostic, or treatment-related modalities is progressively rising. It is now accepted that the negative consequences of radiation exposure are not isolated to exposed cells or individuals. Exposure to IR can induce genome instability in the germline, and is further associated with transgenerational genomic instability in the offspring of exposed males. The exact molecular mechanisms of transgenerational genome instability have yet to be elucidated, although there is support for it being an epigenetically induced phenomenon. This review is centered on the long-term biological effects associated with IR exposure, mainly focusing on the epigenetic mechanisms (DNA methylation and small RNAs) involved in the molecular etiology of IR-induced genome instability, bystander and transgenerational effects. Here, we present evidence that IR-mediated effects are maintained by epigenetic mechanisms, and demonstrate how a novel, male germline-specific, small RNA pathway is posited to play a major role in the epigenetic inheritance of genome instability.

Some important questions connected with non-targeted effects

This paper briefly reviews the highlights of experimental evidence that led to the adoption of the term "non-targeted" to describe new effects induced by ionising radiation that did not fit the classical radiobiological paradigm, principally genomic instability and bystander effect, identifying the reports that were most influential on the subsequent course of radiobiological research. The issue of appropriate terminology for the new effects is discussed. Particular emphasis is placed on the inheritance of genomic instability, where there are issues concerning which effects should be considered as transgenerational. Finally, in respect of the question as to whether these new effects are likely to have an impact on human health is addressed. It is concluded that there is a need for a clearer terminology to facilitate research progress, that real health effects cannot be ruled out and that therefore there is a need for new paradigms not only for radiobiology but also for risk assessment and radiological protection.

The offspring of irradiated parents, are they stable?

Mutation induction in directly exposed cells is currently regarded as the main component of the genetic risk of ionising radiation for humans. However, recent studies showing that exposure to ionising radiation results in elevated mutation rates detectable in the non-irradiated progeny of exposed cells challenge the existing paradigm in radiation biology. This review describes some recent data on radiation-induced genomic instability in vitro and mainly focuses on the in vivo phenomenon of transgenerational instability, where elevated mutation rates are detected in the non-exposed offspring of irradiated parents. The possible mechanisms and implications of transgenerational instability are also discussed.

Radiation-induced genomic instability is associated with DNA methylation changes in cultured human keratinocytes

The mechanism by which radiation-induced genomic instability is initiated, propagated and effected is currently under intense scrutiny. We have investigated the potential role of altered genomic methylation patterns in the cellular response to irradiation and have found evidence for widespread dysregulation of CpG methylation persisting up to 20 population doublings post-irradiation. Similar effects are seen with cells treated with medium from irradiated cells (the 'bystander effect') rather than subjected to direct irradiation. Using an arbitrarily primed methylation sensitive PCR screening method we have demonstrated that irradiation causes reproducible alterations in the methylation profile of a human keratinocyte cell line, HPV-G, and have further characterised one of these sequences as being a member of a retrotransposon element derived sequence family on chromosome 7; MLT1A. Multiple changes were also detected in the screen, which indicate that although the response of cells is predominantly hypermethylation, specific hypomethylation occurs as well. Sequence specific changes are also reported in the methylation of the pericentromeric SAT2 satellite sequence. This is the first demonstration that irradiation results in the induction of heritable methylation changes in mammalian cells, and provides a link between the various non-radiological instigators of genomic instability, the perpetuation of the unstable state and several of its manifestations.

Genetic counselling following paternal therapeutic irradiation

We discuss a patient who received adjuvant radiotherapy for stage I seminoma. He was advised to avoid conception for 6 months following treatment. However, his partner became pregnant only shortly after he completed his radiotherapy (i.e. with sperm that had been irradiated). We estimated the dose received by the remaining testis as 30 cGy. Here, we review the information available to advise patients on the risks to the fetus from paternal preconception irradiation. For the population, a doubling dose for hereditary effects of 1 Gy has recently been reaffirmed (United Nations Scientific Committee on the Effects of Atomic Radiation 2001). However, a range of animal studies suggest conception with postmeiotic sperm carries a greater risk of genetic damage than conception with sperm derived from irradiated stem cells. We have attempted to quantify the risks in this particular case. Lead shielding of the testes may reduce radiation received from the primary beam, but internal scatter still produces a risk. In male patients who are potentially fertile, the best advice remains to delay conception after radiotherapy for as long as 6 months. Our case illustrates the need to reinforce such advice.

Is embryo-cryopreservation really neutral? A new long-term effect of embryo freezing in mice: protection of adults from induced cancer according to strain and sex

BACKGROUND

Beneficial or harmful effects of embryo freezing have been described in man and animals, raising the question of the neutrality of this technique.

OBJECTIVE

We examined, in mice, the possibility that embryo freezing influences the probability of the emergence, in adults, of an induced urinary bladder cancer.

METHODS

The experiment was conducted in mice derived from embryos of two different genotypes. Females receiving embryos were parsed into two groups according to whether these embryos were cryopreserved or not. The derived adults received the chemical carcinogen N-butyl-N-4hydroxybutylnitrosamine (BBN), in the drinking water. Time to death since the onset of treatment was measured for each animal until the 300th day.

RESULTS

In females from one of the two strains tested, embryo freezing led to a favorable long-term effect on the probability of resistance to induced cancer.

CONCLUSION

This beneficial effect, taken together with other effects reported in the literature be they beneficial or harmful, suggests that embryo freezing in mice may not be neutral.

Establishing the possible radiogenicity of morbidity and mortality from participation in UK nuclear weapons development

To succeed in compensation claims for radiogenic injuries induced by their participation in the United Kingdom's atomic and nuclear weapons tests of the 1950s and 1960s, the 22,000 nuclear test veterans will have to satisfy Hill's Principles of Causation. Although they are blocked by legal restraints from suing the Crown, the evidential case for the veterans has been strengthening in recent years to a point where it would probably succeed in a UK court of law. The delay in mounting such a case has worked in favour of the veterans; if the legal constraints are removed, their chances of success are greater than if they had gone into court a decade ago. The article examines the evidence in support of their claims in relation to each of Hill's Principles.

Genetic effects of radiotherapy for childhood cancer

Radiation-induced heritable diseases have not been demonstrated in humans and estimates of genetic risks for protection purposes are based on mouse experiments. The most comprehensive epidemiologic study is of the Japanese atomic bomb survivors and their children, which found little evidence for inherited defects attributable to parental radiation. Studies of workers exposed to occupational radiation or of populations exposed to environmental radiation appear too small and exposures too low to convincingly detect inherited genetic damage. In contrast, survivors of childhood cancer form the largest group of people exposed to high doses of ionizing radiation before reproduction and offer unique advantages for studying trans-generation effects. A wide range of gonadal doses are possible, several comparison groups are readily available (including siblings), and there is a strong willingness among cancer survivors to participate in health studies. Cancer patients also have detailed medical records that facilitate both the accurate estimation of gonadal doses and the assessment of potentially confounding factors, such as intercurrent illness, personal and family medical histories, lifestyle characteristics such as tobacco use, and circumstances at delivery. An international study is nearing completion of over 25,000 survivors of childhood cancer in the United States and Denmark who gave birth to or fathered over 6,000 children. Doses to gonads are being reconstructed from radiotherapy records with 46% over 100 mSv and 16% over 1,000 mSv. Adverse pregnancy outcomes being evaluated include major congenital malformations, cytogenetic abnormalities, stillbirths, miscarriages, neonatal deaths, total deaths, leukemia and childhood cancers, altered sex ratio, and birth weight. The main analyses are based on dose-response evaluations. Blood studies of trios (cancer survivor, spouse or partner and offspring) have been initiated to evaluate mechanistic evidence for the transmission of any radiation-induced genetic damage such as minisatellite mutations. Markers of cancer susceptibility such as chromosomal radiosensitivity and genotype profile will also be examined. In the United States series to date, 4,214 children were born to cancer survivors among whom 157 (3.7%) genetic diseases were reported in contrast to 95 (4.1%) reported conditions among 2,339 children born to sibling controls. In the Denmark series the comparable figures were 82 (6.1%) birth defects among 1,345 children of cancer survivors and 211 (5.0%) among 4,225 children of sibling controls. Coupled with prior studies, these preliminary findings, if sustained by ongoing dose-response analyses, provide reassurance that cancer treatments including radiotherapy do not carry much if any risk for inherited genetic disease in offspring conceived after exposure.

Non-targeted and delayed effects of exposure to ionizing radiation: II. Radiation-induced genomic instability and bystander effects in vivo, clastogenic factors and transgenerational effects

The goal of this review is to summarize the evidence for non-targeted and delayed effects of exposure to ionizing radiation in vivo. Currently, human health risks associated with radiation exposures are based primarily on the assumption that the detrimental effects of radiation occur in irradiated cells. Over the years a number of non-targeted effects of radiation exposure in vivo have been described that challenge this concept. These include radiation-induced genomic instability, bystander effects, clastogenic factors produced in plasma from irradiated individuals that can cause chromosomal damage when cultured with nonirradiated cells, and transgenerational effects of parental irradiation that can manifest in the progeny. These effects pose new challenges to evaluating the risk(s) associated with radiation exposure and understanding radiation-induced carcinogenesis.

Induction of stem cell cycling in mice increases their sensitivity to a chemical leukaemogen: implications for inherited genomic instability and the bystander effect

Preconception paternal irradiation (PPI) modifies haemopoietic and stromal tissues of offspring and increases risk of generating lympho-haemopopietic malignancy if those offspring are then exposed to a leukaemogen. We hypothesised that this increased risk was related to inherited damage which had caused increased stem cell proliferation rates. To test for this link, in vivo, rapid stem cell proliferation was established by giving sub-lethal irradiation (3Gy gamma-rays) and allowing 3 days recovery. At this stage, 60% of haemopoietic spleen colony-forming units (CFU-S) were in DNA-synthesis, compared to <10% in unirradiated controls. Two groups of mice, unirradiated controls and irradiated animals, were then injected with 50mg/kg methyl nitrosourea (MNU) and observed daily for onset of lympho-haemopoietic malignancy. In a further control group of 60 mice, irradiated but not injected with MNU, only one leukaemia developed. In unirradiated controls, 20% of the mice developed malignancies between 3 and 8 months later: in the irradiated, MNU-treated groups, 95% developed malignancies between 2 and 7 months later. Thus, at least one powerful potentiating mechanism for induction of lympho-haemopoietc malignancy following inherited damage can be related to haemopoietic stem cell proliferation. Genomic instability is exposed by cell proliferation and has been implicated in this type of damage. However, a regulatory stromal microenvironment plays a part in inducing that proliferation. Thus, the microenvironment is the effective "bystander" which is thought to promote and amplify genomic instability, and thereby influence the induction of malignancy both in PPI offspring and in mice with induced stem cell proliferation.

Transgenerational effects of preconception paternal contamination with (55)Fe

The conjecture that germline mutations induced by radiation exposure before conception may predispose subsequent offspring to cancer remains contentious. Previous experimental studies have shown that preconception paternal irradiation with (239)Pu induces perturbations in the hemopoietic systems of offspring and influences sensitivity to a secondary carcinogen. In the present study, male DBA2 mice were injected intravenously with the Auger electron emitter (55)Fe (4 kBq g(-1)) 18 or 84 days before mating with normal females. Comet analysis showed an increased incidence of DNA strand breaks in sperm from contaminated animals after 84 days, but not after 18 days, indicating spermatogonial rather than spermatid damage. Offspring were either assayed for changes in bone marrow stem cells and committed progenitors or challenged with the chemical carcinogen methyl nitrosourea (MNU, 50 mg/kg) at 10 weeks of age and monitored for the onset of malignancy. Offspring from irradiated fathers had normal peripheral blood profiles, although the stem cell population was amplified in offspring arising from those exposed to (55)Fe at 84 days before conception. Exposure to MNU significantly increased the incidence of lympho-hemopoietic malignancies in offspring from the 84-day group, but not in those from the 18-day group. These findings support the hypothesis that aberrations that are potentially leukemogenic may be transmitted to offspring after radiation damage to the paternal germline.

Paternal transmission of genetic damage: findings in animals and humans

The concept that mutations can be induced in the male germ-line and result in adverse effects in the offspring has achieved only limited acceptance despite considerable theoretical appeal. This is partly because fetal malformations are generally perceived to be induced solely as a result of maternally mediated events during gestation and partly because the low incidence of the end-points concerned make experimental approaches costly and time-consuming. Nonetheless, a substantial body of work relating to the hypothesis has accumulated in the last 20 years, which has never been reviewed in its entirety. A consideration of the available evidence indicates that preconceptional paternal exposure to mutagens (particularly radiation, cyclophosphamide and ethylnitrosourea) can indeed, under certain conditions, have adverse effects on offspring. The results suggest two principal mechanisms by which such effects may be induced: the induction of germ-line genomic instability or the suppression of germ cell apoptosis.

Identification and characterization of three subtypes of radiation response in normal human urothelial cultures exposed to ionizing radiation

In an attempt to assess genetic variation underlying the variation in human responses to radiation exposure, measurements of apoptosis, necrosis and induction of key proteins were made in primary explant cultures of human normal urothelium and correlated with growth post- exposure to a range of doses of (60)Co. These data were validated by similar experiments using CBA/H and C57/BL6 mouse strains, known to exhibit genetically determined differences in response to radiation. The data for human tissues show a wide variation in response with three broad categories being identifiable. The commonest had a hypersensitive response involving considerable apoptosis in the low dose region, followed by 'induction' of a survival response at higher doses involving the persistence of abnormal cells. The pattern of gene expression was consistent with suppression of apoptosis. The second category showed no induction of survival and considerable necrosis was seen in the progeny. The rarest category showed an extremely hypersensitive low dose response and despite induction of a survival response, the sensitivity to higher doses was very severe. Considerable apoptosis and necrosis were seen in these cultures. In the mouse experiments, strain CBA/H (mice known to exhibit genetic instability post-irradiation) had lower levels of delayed cell death and apoptosis than C57/BL6 mice (which exhibit significantly less instability). It is concluded that there is a variation in response to radiation between human patient cultures which is detectable in this system and which is consistent with a pattern of radiation- induced delayed death/apoptosis correlating with long-term genomic stability. The mouse experiments demonstrate the importance of genetic factors in determining these responses.

Preconception urethane or chromium(III) treatment of male mice: multiple neoplastic and non-neoplastic changes in offspring

Increase in neoplasia in offspring after preconception exposure of parents presents puzzling features such as high frequency of effects and lack of Mendelian inheritance. The present study examined the hypothesis that preconception carcinogenesis involves an increase in the rate of occurrence of neoplasms with a spontaneous incidence. Male NIH Swiss mice (12 per group) were exposed 2 weeks before mating (once, ip) to urethane (1.5 g/kg) or chromium(III) chloride (1 mmol/kg). Offspring (48-78/sex/group) were examined for all grossly apparent changes when moribund or at natural death, followed by histopathological diagnosis and statistical analysis. Significant exposure-related changes occurred in multiple organs. Ten to 20 percent of offspring showed changes related to paternal exposure, including at least one sired by most treated males. Pheochromocytomas occurred in both male and female offspring after both treatments, with none in controls. These neoplasms are rare in mice and suggest endocrine dysfunction as a component of preconception carcinogenesis. This was supported by increases in thyroid follicular cell and Harderian gland tumors, ovarian cysts, and uterine abnormalities. Lung tumors were increased in female offspring only. Effects seen in offspring only after paternal urethane exposure were an increase in preneoplasia/neoplasia in the glandular stomach (males) and in females, increased lymphoma but decreased incidence of histiocytic sarcoma. Increases in incidence of male reproductive gland tumors and of renal non-neoplastic lesions occurred only after chromium exposure. Thus, preconception exposure of fathers to toxicants had a significant impact on both neoplastic and non-neoplastic changes in almost all tissues in which these lesions often occur naturally during the aging process.