A Hypothesis: Radiation-Related Leukemia is Mainly Attributable to the Small Number of People who Carry Pre-existing Clonally Expanded Preleukemic Cells

Reasons why the idea that radiation exposures induce cancer needs to be revisited

PURPOSE

It has long been thought that the carcinogenic effect of radiation resulted from the induction of oncogenic mutations which then led to an increase in the proportion of cancer-bearing individuals. However, even as early as the 1960s, there were indications that the carcinogenic effect of radiation might result from the induction of an earlier onset of cancer. Recently, the former notion was challenged by its inability to explain time-dependent decline of the relative risk following an exposure to radiation, and a parallel shift of mouse survival curves toward younger ages following an exposure to radiation. The two observations are clearly understood if it is assumed only that a radiation exposure causes an earlier onset of spontaneously occurring cancers.

METHOD

In the present study, a critical review was conducted which examined papers that showed dose responses which apparently supported the mutation induction theory of radiation carcinogenesis.

RESULTS

It was found that there were two types of misleading experimental designs: one consisted of studies in which observations were prematurely terminated, and which consequently hid a complete story of radiation carcinogenesis. The other set of papers used age adjustments which were derived from the idea that the life shortening effect of radiation needs to be compensated for since tumor mortality becomes higher among older subjects. This type of adjustment appeared reasonable but was found actually to be a different form of description on an earlier onset of cancer following radiation exposures.

CONCLUSION

In mouse experiments, radiation exposures did not lead to the induction of a large increase in the proportion of tumor deaths when life-long observations were made. Human epidemiologic data are also in line with the earlier onset hypothesis of radiation action. It should be cautioned, however, that the earlier onset model applies only to malignancies whose mortality increases rapidly with the increase of age and does not apply to diseases of short latency such as childhood leukemia and thyroid cancers.

Radiation-Related Thyroid Cancer

Radiation is an environmental factor that elevates the risk of developing thyroid cancer. Actual and possible scenarios of exposures to external and internal radiation are multiple and diverse. This article reviews radiation doses to the thyroid and corresponding cancer risks due to planned, existing, and emergency exposure situations, and medical, public, and occupational categories of exposures. Any exposure scenario may deliver a range of doses to the thyroid, and the risk for cancer is addressed along with modifying factors. The consequences of the Chornobyl and Fukushima nuclear power plant accidents are described, summarizing the information on thyroid cancer epidemiology, treatment, and prognosis, clinicopathological characteristics, and genetic alterations. The Chornobyl thyroid cancers have evolved in time: becoming less aggressive and driver shifting from fusions to point mutations. A comparison of thyroid cancers from the 2 areas reveals numerous differences that cumulatively suggest the low probability of the radiogenic nature of thyroid cancers in Fukushima. In view of continuing usage of different sources of radiation in various settings, the possible ways of reducing thyroid cancer risk from exposures are considered. For external exposures, reasonable measures are generally in line with the As Low As Reasonably Achievable principle, while for internal irradiation from radioactive iodine, thyroid blocking with stable iodine may be recommended in addition to other measures in case of anticipated exposures from a nuclear reactor accident. Finally, the perspectives of studies of radiation effects on the thyroid are discussed from the epidemiological, basic science, and clinical points of view.

Preleukemic Fusion Genes Induced via Ionizing Radiation

Although the prevalence of leukemia is increasing, the agents responsible for this increase are not definitely known. While ionizing radiation (IR) was classified as a group one carcinogen by the IARC, the IR-induced cancers, including leukemia, are indistinguishable from those that are caused by other factors, so the risk estimation relies on epidemiological data. Several epidemiological studies on atomic bomb survivors and persons undergoing IR exposure during medical investigations or radiotherapy showed an association between radiation and leukemia. IR is also known to induce chromosomal translocations. Specific chromosomal translocations resulting in preleukemic fusion genes (PFGs) are generally accepted to be the first hit in the onset of many leukemias. Several studies indicated that incidence of PFGs in healthy newborns is up to 100-times higher than childhood leukemia with the same chromosomal aberrations. Because of this fact, it has been suggested that PFGs are not able to induce leukemia alone, but secondary mutations are necessary. PFGs also have to occur in specific cell populations of hematopoetic stem cells with higher leukemogenic potential. In this review, we describe the connection between IR, PFGs, and cancer, focusing on recurrent PFGs where an association with IR has been established.

Molecular pathogenesis of pediatric thyroid carcinoma

There has been little understanding of the molecular pathogenesis of pediatric thyroid cancers. Most of them are histologically classified as papillary thyroid carcinoma (PTC). Ionizing radiation is the most important environmental factor to induce PTC, especially in children. Particularly, radiation-related pediatric PTCs after the Chernobyl accident provided invaluable information. In addition, the recent accumulation of sporadic pediatric PTC cases, partly due to advances in diagnostic imaging, has also provided insight into their general pathogenesis. In PTC development, basically two types of genetic alterations, fusion oncogenes, mainly RET/PTC, and a point mutation, mainly BRAFV600E, are thought to play a key role as driver oncogenes. Their frequencies vary depending on patient age. The younger the age, the more prevalent the fusion oncogenes are. Higher incidence of fusion oncogenes was also observed in cases exposed to radiation. In short, fusion oncogenes are associated with both age and radiation and are not evidence of radiation exposure. The type of driver oncogene is shifted toward BRAFV600E during adolescence in sporadic PTCs. However, until about this age, fusion oncogenes seem to still confer dominant growth advantages, which may lead to the higher discovery rate of the fusion oncogenes. It has been postulated that RET/PTC in radiation-induced PTC is generated by ionizing radiation; however, there is an interesting hypothesis that thyroid follicular cell clones with pre-existing RET/PTC were already present, and radiation may play a role as a promoter/progressor but not initiator. Telomerase reverse transcriptase gene (TERT) promoter mutations, which are the strongest marker of tumor aggressiveness in adult PTC cases, have not been detected in pediatric cases; however, TERT expression without the mutations may play a role in tumor aggressiveness. In this paper, the recent information regarding molecular findings in sporadic and radiation-associated pediatric PTCs is summarized.

Early induction and increased risk of precursor B-cell neoplasms after exposure of infant or young-adult mice to ionizing radiation

Epidemiological studies of atomic-bomb survivors have revealed an increased risk of lymphoid neoplasm (i.e. acute lymphoblastic leukemia) associated with radiation exposure. In particular, children are more susceptible to radiation-induced precursor lymphoid neoplasm than adults. Although ~75% of human lymphoid tumors are B-cell neoplasms, the carcinogenic risk associated with each stage of differentiation of B-cells after radiation exposure is poorly understood. Therefore, we irradiated mice at infancy or in young adulthood to investigate the effect of age at exposure on the risk of developing B-cell neoplasms. Histopathology was used to confirm the presence of lymphoid neoplasms, and the population of B-cell neoplasms was classified into the precursor B-cell (pro-B and pre-B cell) type and mature B-cell type, according to immunophenotype. The data revealed that precursor B-cell neoplasms were induced soon after radiation exposure in infancy or young adulthood, resulting in a greater risk of developing the neoplasms. This was particularly the case for the pro-B cell type after young adult exposure. Our findings suggest that exposure to radiation at young age increases the risk of developing precursor B-cell neoplasms in humans.

Age Dependence of Radiation-Induced Genomic Instability in Mouse Hematopoietic Stem Cells

Age at exposure is a critical factor that influences the risk of radiation-induced leukemia. Accumulating evidence suggests that ionizing radiation can induce genomic instability and promote leukemogenesis in hematopoietic stem cells (HSCs); however, the influence of age on this phenomenon has not been elucidated. In this study, infant (1-week-old) or adult (14-week-old) C3H/He mice received sham or 4 Gy whole-body irradiation, and bone marrow cells were transplanted to recipients at day 1 or 60 postirradiation. Twelve days after bone marrow transplant, we analyzed the radiation-induced genomic instability by scoring the frequency of DNA damage and micronucleus formation in colony-forming units-spleen (CFU-Ss). We observed significant increases in DNA damage and micronucleus formation in CFU-Ss of the 4 Gy irradiated adult cells transplanted at day 1 or 60 postirradiation. However, the frequency of DNA damage focus and micronucleus formation in CFU-Ss of 4 Gy irradiated infant cells transplanted at day 1 or 60 postirradiation was relatively decreased. Quantitative differences in the reactive oxygen species and cells expressing inducible nitric oxide synthase in CFU-Ss suggested that age-dependent radiation-induced genomic instability may result from chronic oxidative stress by pro-inflammatory states in HSC descendants after radiation exposure.

Influence of diet and metabolism on hematopoietic stem cells and leukemia development following ionizing radiation exposure

PURPOSE

The review aims to discuss the prominence of dietary and metabolic regulators in maintaining hematopoietic stem cell (HSC) function, long-term self-renewal, and differentiation.

RESULTS

Most adult stem cells are preserved in a quiescent, nonmotile state in vivo which acts as a "protective state" for stem cells to reduce endogenous stress provoked by DNA replication and cellular respiration as well as exogenous environmental stress. The dynamic balance between quiescence, self-renewal and differentiation is critical for supporting a functional blood system throughout life of an organism. Stress-conditions, for example ionizing radiation exposure can trigger the blood forming HSCs to proliferate and migrate through extramedullary tissues to expand the number of HSCs and increase hematopoiesis. In addition, a wealth of investigation validated that deregulation of this balance plays a critical pathogenic role in various different hematopoietic diseases including the leukemia development.

CONCLUSION

The review summarizes the current knowledge on how alterations in dietary and metabolic factors could alter the risk of leukemia development following ionizing radiation exposure by inhibiting or even reversing the leukemic progression. Understanding the influence of diet, metabolism, and epigenetics on radiation-induced leukemogenesis may lead to the development of practical interventions to reduce the risk in exposed populations.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Analysis of chromosome translocation frequency after a single CT scan in adults

We recently reported an increase in dicentric chromosome (DIC) formation after a single computed tomography (CT) scan (5.78-60.27 mSv: mean 24.24 mSv) and we recommended analysis of 2000 metaphase cells stained with Giemsa and centromere-FISH for dicentric chromosome assay (DCA) in cases of low-dose radiation exposure. In the present study, we analyzed the frequency of chromosome translocations using stored Carnoy's-fixed lymphocyte specimens from the previous study; these specimens were from 12 patients who were subject to chromosome painting of Chromosomes 1, 2 and 4. Chromosomes 1, 2 and 4 were analyzed in ∼5000 cells, which is equivalent to the whole-genome analysis of almost 2000 cells. The frequency of chromosome translocation was higher than the number of DICs formed, both before and after CT scanning. The frequency of chromosome translocations tended to be higher, but not significantly higher, in patients with a treatment history compared with patients without such a history. However, in contrast to the results for DIC formation, the frequency of translocations detected before and after the CT scan did not differ significantly. Therefore, analysis of chromosome translocation may not be a suitable assay for detecting chromosome aberrations in cases of low-dose radiation exposure from a CT scan. A significant increase in the frequency of chromosome translocations was not likely to be detected due to the high baseline before the CT scan; the high and variable frequency of translocations was probably due to multiple confounding factors in adults.

ICRP Publication 131: Stem cell biology with respect to carcinogenesis aspects of radiological protection

Current knowledge of stem cell characteristics, maintenance and renewal, evolution with age, location in 'niches', and radiosensitivity to acute and protracted exposures is reviewed regarding haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. The identity of the target cells for carcinogenesis continues to point to the more primitive and mostly quiescent stem cell population (able to accumulate the protracted sequence of mutations necessary to result in malignancy), and, in a few tissues, to daughter progenitor cells. Several biological processes could contribute to the protection of stem cells from mutation accumulation: (1) accurate DNA repair; (2) rapid induced death of injured stem cells; (3) retention of the intact parental strand during divisions in some tissues so that mutations are passed to the daughter differentiating cells; and (4) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the vital niche. DNA repair mainly operates within a few days of irradiation, while stem cell replications and competition require weeks or many months depending on the tissue type. This foundation is used to provide a biological insight to protection issues including the linear-non-threshold and relative risk models, differences in cancer risk between tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age.

Personalized Cancer Risk Assessments for Space Radiation Exposures

Individuals differ in their susceptibility to radiogenic cancers, and there is evidence that this inter-individual susceptibility extends to HZE ion-induced carcinogenesis. Three components of individual risk: sex, age at exposure, and prior tobacco use, are already incorporated into the NASA cancer risk model used to determine safe days in space for US astronauts. Here, we examine other risk factors that could potentially be included in risk calculations. These include personal and family medical history, the presence of pre-malignant cells that could undergo malignant transformation as a consequence of radiation exposure, the results from phenotypic assays of radiosensitivity, heritable genetic polymorphisms associated with radiosensitivity, and postflight monitoring. Inclusion of these additional risk or risk reduction factors has the potential to personalize risk estimates for individual astronauts and could influence the determination of safe days in space. We consider how this type of assessment could be used and explore how the provisions of the federal Genetic Information Non-discrimination Act could impact the collection, dissemination and use of this information by NASA.

Genetically engineered mouse models of human B-cell precursor leukemias

B-cell precursor acute lymphoblastic leukemias (pB-ALLs) are the most frequent type of malignancies of the childhood, and also affect an important proportion of adult patients. In spite of their apparent homogeneity, pB-ALL comprises a group of diseases very different both clinically and pathologically, and with very diverse outcomes as a consequence of their biology, and underlying molecular alterations. Their understanding (as a prerequisite for their cure) will require a sustained multidisciplinary effort from professionals coming from many different fields. Among all the available tools for pB-ALL research, the use of animal models stands, as of today, as the most powerful approach, not only for the understanding of the origin and evolution of the disease, but also for the development of new therapies. In this review we go over the most relevant (historically, technically or biologically) genetically engineered mouse models (GEMMs) of human pB-ALLs that have been generated over the last 20 years. Our final aim is to outline the most relevant guidelines that should be followed to generate an “ideal” animal model that could become a standard for the study of human pB-ALL leukemia, and which could be shared among research groups and drug development companies in order to unify criteria for studies like drug testing, analysis of the influence of environmental risk factors, or studying the role of both low-penetrance mutations and cancer susceptibility alterations.

Exposure-response relationships for select cancer and non-cancer health outcomes in a cohort of U.S. firefighters from San Francisco, Chicago and Philadelphia (1950-2009)

OBJECTIVES

To examine exposure-response relationships between surrogates of firefighting exposure and select outcomes among previously studied US career firefighters.

METHODS

Eight cancer and four non-cancer outcomes were examined using conditional logistic regression. Incidence density sampling was used to match each case to 200 controls on attained age. Days accrued in firefighting assignments (exposed-days), run totals (fire-runs) and run times (fire-hours) were used as exposure surrogates. HRs comparing 75th and 25th centiles of lagged cumulative exposures were calculated using loglinear, linear, log-quadratic, power and restricted cubic spline general relative risk models. Piecewise constant models were used to examine risk differences by time since exposure, age at exposure and calendar period.

RESULTS

Among 19,309 male firefighters eligible for the study, there were 1333 cancer deaths and 2609 cancer incidence cases. Significant positive associations between fire-hours and lung cancer mortality and incidence were evident. A similar relation between leukaemia mortality and fire-runs was also found. The lung cancer associations were nearly linear in cumulative exposure, while the association with leukaemia mortality was attenuated at higher exposure levels and greater for recent exposures. Significant negative associations were evident for the exposure surrogates and colorectal and prostate cancers, suggesting a healthy worker survivor effect possibly enhanced by medical screening.

CONCLUSIONS

Lung cancer and leukaemia mortality risks were modestly increasing with firefighter exposures. These findings add to evidence of a causal association between firefighting and cancer. Nevertheless, small effects merit cautious interpretation. We plan to continue to follow the occurrence of disease and injury in this cohort.

Mortality and ionising radiation exposures among workers employed at the Fernald Feed Materials Production Center (1951-1985)

OBJECTIVES

To examine mortality patterns and dose-response relations between ionising radiation and mortality outcomes of a priori interest in 6409 uranium workers employed for at least 30 days (1951-1985), and followed through 2004.

METHODS

Cohort mortality was evaluated through standardised mortality ratios (SMR). Linear excess relative risk (ERR) regression models examined associations between cause-specific mortality and exposures to internal ionising radiation from uranium deposition, external gamma and x-ray radiation, and radon decay products, while adjusting for non-radiologic covariates.

RESULTS

Person-years at risk totalled 236 568 (mean follow-up 37 years), and 43% of the cohort had died. All-cause mortality was below expectation only in salaried workers. Cancer mortality was significantly elevated in hourly males, primarily from excess lung cancer (SMR=1.25, 95% CI 1.09 to 1.42). Cancer mortality in salaried males was near expectation, but lymphohaematopoietic malignancies were significantly elevated (SMR=1.52, 95% CI 1.06 to 2.12). A positive dose-response relation was observed for intestinal cancer, with a significant elevation in the highest internal organ dose category and a significant dose-response with organ dose from internal uranium deposition (ERR=1.5 per 100 μGy, 95% CI 0.12 to 4.1).

CONCLUSIONS

A healthy worker effect was observed only in salaried workers. Hourly workers had excess cancer mortality compared with the US population, although there was little evidence of a dose-response trend for any cancer evaluated except intestinal cancer. The association between non-malignant respiratory disease and radiation dose observed in previous studies was not apparent, possibly due to improved exposure assessment, different outcome groupings, and extended follow-up.

The incidence of leukemia, lymphoma and multiple myeloma among atomic bomb survivors: 1950-2001

A marked increase in leukemia risks was the first and most striking late effect of radiation exposure seen among the Hiroshima and Nagasaki atomic bomb survivors. This article presents analyses of radiation effects on leukemia, lymphoma and multiple myeloma incidence in the Life Span Study cohort of atomic bomb survivors updated 14 years since the last comprehensive report on these malignancies. These analyses make use of tumor- and leukemia-registry based incidence data on 113,011 cohort members with 3.6 million person-years of follow-up from late 1950 through the end of 2001. In addition to a detailed analysis of the excess risk for all leukemias other than chronic lymphocytic leukemia or adult T-cell leukemia (neither of which appear to be radiation-related), we present results for the major hematopoietic malignancy types: acute lymphoblastic leukemia, chronic lymphocytic leukemia, acute myeloid leukemia, chronic myeloid leukemia, adult T-cell leukemia, Hodgkin and non-Hodgkin lymphoma and multiple myeloma. Poisson regression methods were used to characterize the shape of the radiation dose-response relationship and, to the extent the data allowed, to investigate variation in the excess risks with gender, attained age, exposure age and time since exposure. In contrast to the previous report that focused on describing excess absolute rates, we considered both excess absolute rate (EAR) and excess relative risk (ERR) models and found that ERR models can often provide equivalent and sometimes more parsimonious descriptions of the excess risk than EAR models. The leukemia results indicated that there was a nonlinear dose response for leukemias other than chronic lymphocytic leukemia or adult T-cell leukemia, which varied markedly with time and age at exposure, with much of the evidence for this nonlinearity arising from the acute myeloid leukemia risks. Although the leukemia excess risks generally declined with attained age or time since exposure, there was evidence that the radiation-associated excess leukemia risks, especially for acute myeloid leukemia, had persisted throughout the follow-up period out to 55 years after the bombings. As in earlier analyses, there was a weak suggestion of a radiation dose response for non-Hodgkin lymphoma among men, with no indication of such an effect among women. There was no evidence of radiation-associated excess risks for either Hodgkin lymphoma or multiple myeloma.

Risk of leukaemia mortality from exposure to ionising radiation in US nuclear workers: a pooled case-control study

OBJECTIVE

To follow-up on earlier studies of the leukaemogenicity of occupational ionising radiation exposure.

METHODS

We conducted a nested case-control analysis of leukaemia mortality in a pooled cohort of US nuclear workers followed through 2005. Each case was matched to four controls on attained age. Exposures were estimated from available records. General relative risk models were used to estimate the excess relative risk (ERR) of leukaemia, excluding chronic lymphocytic (CLL), acute myeloid leukaemia, chronic myeloid leukaemia and CLL while controlling for potential confounders. Preferred exposure lags and time-windows of risks were calculated using joint maximum likelihood. Dose-response was also examined using linear, linear-quadratic, categorical and restricted cubic spline models.

RESULTS

There were 369 leukaemia deaths in 105 245 US nuclear workers. The adjusted ERR for non-CLL leukaemia was 0.09 (95% CI -0.17 to 0.65) per 100 mGy. Elevated non-CLL risks were observed from exposures occurring 6-14 years prior to attained age of cases (ERR per 100 mGy=1.9; 95% CI <0 to 8.0). Lagged models indicated non-linearity of risk at very low (<10 mGy) and high (>100 mGy) doses, which contributed to the imprecision of results in linear models. Similar risk attenuation was not evident in time-windows-based models.

CONCLUSIONS

Risk estimates were in reasonable agreement with previous estimates, with the temporality of non-CLL leukaemia risk as a dominant factor in dose-response analyses. Future research should focus on methods that improve evaluations of the dose-response, particularly in the low-dose range.

Quantitative modeling of chronic myeloid leukemia: insights from radiobiology

Mathematical models of chronic myeloid leukemia (CML) cell population dynamics are being developed to improve CML understanding and treatment. We review such models in light of relevant findings from radiobiology, emphasizing 3 points. First, the CML models almost all assert that the latency time, from CML initiation to diagnosis, is at most ∼10 years. Meanwhile, current radiobiologic estimates, based on Japanese atomic bomb survivor data, indicate a substantially higher maximum, suggesting longer-term relapses and extra resistance mutations. Second, different CML models assume different numbers, between 400 and 10(6), of normal HSCs. Radiobiologic estimates favor values>10(6) for the number of normal cells (often assumed to be the HSCs) that are at risk for a CML-initiating BCR-ABL translocation. Moreover, there is some evidence for an HSC dead-band hypothesis, consistent with HSC numbers being very different across different healthy adults. Third, radiobiologists have found that sporadic (background, age-driven) chromosome translocation incidence increases with age during adulthood. BCR-ABL translocation incidence increasing with age would provide a hitherto underanalyzed contribution to observed background adult-onset CML incidence acceleration with age, and would cast some doubt on stage-number inferences from multistage carcinogenesis models in general.

Roles of stem cells in tissue turnover and radiation carcinogenesis

Radiation research has its foundation on the target and hit theories, which assume that the initial stochastic deposition of energy on a sensitive target in a cell determines the final biological outcome. This assumption is rather static in nature but forms the foundation of the linear no-threshold (LNT) model of radiation carcinogenesis. The stochastic treatment of radiation carcinogenesis by the LNT model enables easy calculation of radiation risk, and this has made the LNT model an indispensable tool for radiation protection. However, the LNT model sometimes fails to explain some of the biological and epidemiological data, and this suggests the need for insight into the mechanisms of radiation carcinogenesis. Recent studies have identified unique characteristics of the tissue stem cells and their roles in tissue turnover. In the present report, some important issues of radiation protection such as the risk of low-dose-rate exposures and in utero exposures are discussed in light of the recent advances of stem cell biology.

Radiation-induced leukemia among children aged 0-5 years at the time of the Chernobyl accident

This case-control study was conducted to estimate the radiation-induced risk of acute leukemia during the period from 1987 to 1997 among residents 0-5 years of age at the time of the Chernobyl accident in the most radioactively contaminated territories of the Ukraine (Rivno, Zhytomyr, Chernihiv and Cherkasy regions). Data were collected from 246 leukemia cases diagnosed between 1 January, 1987, and 31 December, 1997. Each case was verified and interviewed. Verified cases were compared to 492 randomly selected controls matched by age, sex, type of settlement (rural, semirural and urban) and administrative region of residency. The cumulative level of radiation exposure from the time of the Chernobyl accident to the date of diagnosis was assessed for each case and corresponding controls. Four dose-range groups were selected for statistical analysis (0-2.9, 3-9.9, 10-99.9 and 100-313.3 mGy). The risk of leukemia was significantly increased (-2.4 [95%CI: 1.4-4.0]) among those with radiation exposure doses higher than 10 mGy (p = 0.01). The association between radiation exposure and risk was stronger among males (-2.8 [95%CI: 1.4-5.5, p = 0.01]), and for cases of acute leukemia that were diagnosed during the period from 1987 to 1992 (-2.5 [95%CI: 1.2-5.1, p = 0.05]), particularly acute myeloid leukemia (-5.8 [95%CI: 1.4-24.6, p = 0.05]). The influence of possible confounders and methods of selecting controls on the leukemia risk assessment was analyzed. The evaluated risk per unit dose is discussed.

LTA 252GG and GA genotypes are associated with diffuse-type noncardia gastric cancer risk in the Japanese population

BACKGROUND

There are limited numbers of reports on the association of lymphotoxin-alpha (LTA) genotypes with gastric cancer.

METHODS

A nested case-control study was carried out in the longitudinal cohort of atomic bomb survivors using stored sera before diagnosis (mean, 2.3 years) and blood cells. Enrolled were 287 cases with noncardia gastric cancer of diffuse and intestinal types and three controls per case selected from cohort members matched on age, gender, city, and time and type of serum storage and counter-matched on radiation dose.

RESULTS

LTA 252GG and GA genotypes were associated with the prevalence of Helicobacter pylori IgG seropositivity and higher antibody titer against H. pylori cytotoxin-associated gene A (CagA) protein in controls and they were an independent risk factor for noncardia gastric cancer of diffuse type (RR = 2.8 (95% CI: 1.3-6.3), p = .01, and RR = 2.7 (95% CI: 1.5-4.8), p < .001), but not for intestinal type, after adjusting for H. pylori IgG seropositivity, CagA antibody titers, chronic atrophic gastritis, smoking, and radiation dose. Cessation of smoking (RR = 0.4 (95% CI: 0.2-0.7), p < .001) and never smoking (RR = 0.4 (95% CI: 0.3-0.6), p < .001) were both protective for future noncardia gastric cancer. Radiation dose was associated with noncardia gastric cancer in subjects with both the LTA 252G-allele and never smoking/quit smoking histories (RR = 3.8 (95% CI: 1.7-5.9), p = .009).

CONCLUSION

The LTA 252 genotype is associated with noncardia gastric cancer of diffuse type in Japan and interacted with radiation dose.

Ionizing radiation and leukemia mortality among Japanese Atomic Bomb Survivors, 1950-2000

This paper provides the first comprehensive report on mortality by type of leukemia among the Japanese atomic bomb survivors in the Life Span Study (LSS). Analyses include 310 deaths due to leukemia during the period 1950-2000 among 86,611 people in the LSS. Poisson regression methods were used to evaluate associations between estimated bone marrow dose and leukemia mortality. Attention was given to variation in the radiation dose-leukemia mortality association by time since exposure, age at exposure, city and sex. The excess relative rate per gray of acute myeloid leukemia was best described by a quadratic dose-response function that peaked approximately 10 years after exposure. Acute lymphatic leukemia and chronic myeloid leukemia mortality were best described by a linear dose-response function that did not vary with time since exposure. Adult T-cell leukemia was not associated with estimated bone marrow dose. Overall, 103 of the 310 observed leukemia deaths were estimated to be excess deaths due to radiation exposure. In the most recent decade of observation (1991-2000), the estimated attributable fraction of leukemia deaths among those survivors exposed to >0.005 Gy was 0.34, suggesting that the effect of the atomic bombings on leukemia mortality has persisted in this cohort for more than five decades.

Implication of replicative stress-related stem cell ageing in radiation-induced murine leukaemia

BACKGROUND

The essential aetiology of radiation-induced acute myeloid leukaemia (AML) in mice is the downregulation of the transcription factor PU.1. The causative mutation of the PU.1-endocing Sfpi1 gene consists mostly of C:G to T:A transitions at a CpG site and is likely to be of spontaneous origin. To work out a mechanism underlying the association between radiation exposure and the AML induction, we have hypothesised that replicative stress after irradiation accelerates the ageing of haematopoietic stem cells (HSCs), and the ageing-related decline in DNA repair could affect the spontaneous mutation rates.

METHODS

Mathematical model analysis was conducted to examine whether and to what extent the cell kinetics of HSCs can be modified after irradiation. The haematopoietic differentiation process is expressed as a mathematical model and the cell-kinetics parameters were estimated by fitting the simulation result to the assay data.

RESULTS

The analysis revealed that HSCs cycle vigourously for more than a few months after irradiation. The estimated number of cell divisions per surviving HSC in 3 Gy-exposed mice reached as high as ten times that of the unexposed.

INTERPRETATION

The mitotic load after 3 Gy irradiation seems to be heavy enough to accelerate the ageing of HSCs and the hypothesis reasonably explains the leukaemogenic process.

Factors that elevate the internal radionuclide and chemical retention, dose and health risks to infants and children in a radiological-nuclear emergency

The factors that influence the dose and risk to vulnerable population groups from exposure and internal uptake of chemicals are examined and, in particular, the radionuclides released in chemical, biological, radiological, nuclear and explosive events. The paper seeks to identify the areas that would benefit from further research. The intake and body burdens of carbon and calcium were assessed as surrogates for contaminants that either act like or bind to hydrocarbons (e.g. tritium and (14)C) or bone-seeking radionuclides (e.g. (90)Sr and (239)Pu). The shortest turnover times for such materials in the whole body were evaluated for the newborn: 11 d and 0.5 y for carbon and calcium, respectively. However, their biokinetic behaviour is complicated by a particularly high percentage of the gut-absorbed dietary intake of carbon (approximately 16%) and calcium (approximately 100%) that is incorporated into the soft tissue and skeleton of the growing neonate. The International Commission on Radiological Protection dose coefficients (Sv Bq(-1)) were examined for 14 radionuclides, including 9 of concern because of their potential use in radiological dispersal devices. The dose coefficients for a 3-month-old are greater than those for adults (2-56 times more for ingestion and 2-12 times for inhalation). The age-dependent dose and exposure assessment of contaminant intakes would improve by accounting for gender and growth where it is currently neglected. Health risk is evaluated as the product of the exposure and hazard factors, the latter being about 10-fold greater in infants than in adults. The exposure factor is also approximately 10-fold higher for ingestion by infants than by adults, and unity for inhalation varying with the contaminant. Qualitative and quantitative physiological and epidemiological evidence supports infants being more vulnerable to cancer and neurological deficit than older children.

Cancer and non-cancer effects in Japanese atomic bomb survivors

The survivors of the atomic bombings in Hiroshima and Nagasaki are a general population of all ages and sexes and, because of the wide and well characterised range of doses received, have been used by many scientific committees (International Commission on Radiological Protection (ICRP), United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Biological Effects of Ionizing Radiations (BEIR)) as the basis of population cancer risk estimates following radiation exposure. Leukaemia was the first cancer to be associated with atomic bomb radiation exposure, with preliminary indications of an excess among the survivors within the first five years after the bombings. An excess of solid cancers became apparent approximately ten years after radiation exposure. With increasing follow-up, excess risks of most cancer types have been observed, the major exceptions being chronic lymphocytic leukaemia, and pancreatic, prostate and uterine cancer. For most solid cancer sites a linear dose response is observed, although in the latest follow-up of the mortality data there is evidence (p = 0.10) for an upward curvature in the dose response for all solid cancers. The only cancer sites which exhibit (upward) curvature in the dose response are leukaemia, and non-melanoma skin and bone cancer. For leukaemia the dose response is very markedly upward curving, indeed largely describable as a pure quadratic dose response, particularly in the low dose (0-2 Sv) range. Even 55 years after the bombings over 40% of the Life Span Study cohort remain alive, so continued follow-up of this group is vital for completing our understanding of long-term radiation effects in people. In general, the relative risks per unit dose among the Japanese atomic bomb survivors are greater than those among comparable subsets in studies of medically exposed individuals. Cell sterilisation largely accounts for the discrepancy in relative risks between these two populations, although other factors may contribute, such as the generally higher underlying cancer risks in the medical series than in the Japanese atomic bomb survivors. Risks among occupationally exposed groups such as nuclear workforces and underground miners are generally consistent with those observed in the Japanese atomic bomb survivors. In general, consistent patterns of variation of risk with age at exposure are also seen in all studies-risks for all cancer types diminish with increasing age at exposure. There are also excess risks of various types of non-malignant disease in the Japanese atomic bomb survivors, in particular cardiovascular, respiratory and digestive diseases. Indeed, risks are elevated to much the same degree for a number of non-malignant disease endpoints, suggestive of bias. However, in contrast with the cancer data, there is much less consistency in the pattern of risk between the atomic bomb survivors and other exposed groups; for example, radiation-associated respiratory and digestive diseases have not been seen in these other groups. Although cardiovascular risks have been seen elsewhere, particularly in medically exposed groups, in contrast with the cancer data there is much less consistency in risk between studies: risks per unit dose in epidemiological studies vary over at least two orders of magnitude, possibly as a result of confounding factors. In the absence of a convincing mechanistic explanation of epidemiological evidence, at present a cause-and-effect interpretation of the reported statistical associations for cardiovascular disease is unreliable but cannot be excluded. Further epidemiological and biological evidence will allow a firmer conclusion to be drawn.

Gene expression profiling distinguishes between spontaneous and radiation-induced rat mammary carcinomas

The ability to distinguish between spontaneous and radiation-induced cancers in humans is expected to improve the resolution of estimated risk from low dose radiation. Mammary carcinomas were obtained from Sprague-Dawley rats that were either untreated (n = 45) or acutely gamma-irradiated (1 Gy; n = 20) at seven weeks of age. Gene expression profiles of three spontaneous and four radiation-induced carcinomas, as well as those of normal mammary glands, were analyzed by microarrays. Differential expression of identified genes of interest was then verified by quantitative polymerase chain reaction (qPCR). Cluster analysis of global gene expression suggested that spontaneous carcinomas were distinguished from a heterogeneous population of radiation-induced carcinomas, though most gene expressions were common. We identified 50 genes that had different expression levels between spontaneous and radiogenic carcinomas. We then selected 18 genes for confirmation of the microarray data by qPCR analysis and obtained the following results: high expression of Plg, Pgr and Wnt4 was characteristic to all spontaneous carcinomas; Tnfsf11, Fgf10, Agtr1a, S100A9 and Pou3f3 showed high expression in a subset of radiation-induced carcinomas; and increased Gp2, Areg and Igf2 expression, as well as decreased expression of Ca3 and non-coding RNA Mg1, were common to all carcinomas. Thus, gene expression analysis distinguished between spontaneous and radiogenic carcinomas, suggesting possible differences in their carcinogenic mechanism.

A multi-compartment cell repopulation model allowing for inter-compartmental migration following radiation exposure, applied to leukaemia

There is much uncertainty about cancer risks at the high radiation doses used in radiotherapy (RT). It has generally been assumed that cancer induction decreases rapidly at high doses due to cell killing. However, this is not seen in all RT groups, and a model recently developed by Sachs and Brenner [2005. Solid tumor risks after high doses of ionizing radiation. Proc. Natl Acad. Sci. USA 102, 13040-13045] proposed a mechanism for repopulation of cells after radiation exposure that explained why this might happen, at least for solid tumours. In this paper, this model is generalized to allow for heterogeneity in the dose received, and various alternate patterns of repopulation are also considered. The model is fitted to the Japanese atomic bomb survivor leukaemia incidence data, and data for various therapeutically irradiated groups. Two sets of parameters from these model fits are used to assess the sensitivity of model predictions. It is shown that in general allowing for heterogeneity in dose distribution and haematopoietic stem cell migration results in lower risks than the same average dose administered uniformly and without such migration, although this does not hold in the limiting case of complete stem cell repopulation between radiation dose fractions. We also investigate the difference made by assuming a compartmental repopulation signal, and a global repopulation signal. In general we show that in the absence of stochastic extinction, compartmental repopulation always predicts a larger number of mutated cells than global repopulation. However, in certain dose regimes stochastic extinction cannot be ignored, and in these cases the numbers of mutated cells predicted with global repopulation can exceed that for compartmental repopulation. In general, mutant cell numbers are highly overdispersed, with variance much greater than the mean.

Clonally Expanded T-Cell Populations in Atomic Bomb Survivors Do Not Show Excess Levels of Chromosome Instability

Radiation-induced genomic instability has been studied primarily in cultured cells, while in vivo studies have been limited. One major obstacle for in vivo studies is the lack of reliable biomarkers that are capable of distinguishing genetic alterations induced by delayed radiation effects from those that are induced immediately after a radiation exposure. Here we describe a method to estimate cytogenetic instability in vivo using chromosomally marked clonal T-cell populations in atomic bomb survivors. The basic idea is that clonal translocations are derived from single progenitor cells that acquired an aberration, most likely after a radiation exposure, and then multiplied extensively in vivo, resulting in a large number of progeny cells that eventually comprise several percent of the total lymphocyte population. Therefore, if chromosome instability began to operate soon after a radiation exposure, an elevated frequency of additional but solitary chromosome aberrations in clonal cell populations would be expected. In the present study, six additional translocations were found among 936 clonal cells examined with the G-band method (0.6%); the corresponding value with multicolor FISH analysis was 1.2% (4/333). Since these frequencies were no higher than 1.2% (219/17,878 cells), the mean translocation frequency observed in control subjects using the G-band method, it is concluded that chromosome instabilities that could give rise to an increased frequency of persisting, exchange-type aberrations were not commonly generated by radiation exposure.

Ionizing radiation and leukaemia: more questions than answers

The mechanisms underlying the unequivocal association between ionizing radiation and the development of leukaemia remain unknown. Recent progress in defining sub-cellular events has contributed to our understanding of the production of genetic lesions in irradiated cells but the importance of tissue effects in response to radiation damage has attracted much less attention. Thus, genetic lesions induced by radiation are considered to result from the deposition of energy in the cell nucleus and the initiating lesion for radiation-induced transformation has been similarly attributed to direct DNA damage. Recently, however, there have been many reports of radiation effects, characteristically associated with the consequences of energy deposition in the cell nucleus, arising in non-irradiated cells as a consequence of communication with irradiated cells. These, so-called, non-targeted radiation effects pose major challenges to current views of the mechanisms of radiation-induced DNA damage and the mechanisms underlying radiogenic malignancies. Considered together with data obtained from laboratory model systems, a rather complex picture of radiation leukaemogenesis is emerging in which, additional to any damage induced directly in target stem cells, the haemopoietic microenvironment can be a source of damaging signals and cellular interactions make important genotype-dependent contributions to determining overall outcome after radiation exposures.