R Julian Preston's research while affiliated with The Ohio Environmental Protection Agency and other places

Purpose The overall aim of this contribution to the “Second Bill Morgan Memorial Special Issue” is to provide a high-level review of a recent report developed by a Committee for the National Council on Radiation Protection and Measurements (NCRP) titled “Approaches for Integrating Information from Radiation Biology and Epidemiology to Enhance Low-Dose Health Risk Assessment”. It derives from previous NCRP Reports and Commentaries that provide the case for integrating data from radiation biology studies (available and proposed) with epidemiological studies (also available and proposed) to develop Biologically-Based Dose- Response (BBDR) Models. In the present review it is proposed for such models to leverage the Adverse Outcome Pathways (AOP) and Key Events (KE) approach for better characterizing radiation-induced cancers and circulatory disease (as the example for a noncancer outcome). The review discusses the current state of knowledge of mechanisms of carcinogenesis, with an emphasis on radiation-induced cancers, and a similar discussion for circulatory disease. The types of the various informative BBDR models are presented along with a proposed generalized BBDR model for cancer and a more speculative one for circulatory disease. The way forward is presented in a comprehensive discussion of the research needs to address the goal of enhancing health risk assessment of exposures to low doses of radiation. Conclusion The use of an AOP/KE approach for developing a mechanistic framework for BBDR models of radiation-induced cancer and circulatory disease is considered to be a viable one based upon current knowledge of the mechanisms of formation of these adverse health outcomes and the available technical capabilities and computational advances. The way forward for enhancing low-dose radiation risk estimates will require there to be a tight integration of epidemiology data and radiation biology information to meet the goals of relevance and sensitivity of the adverse health outcomes required for overall health risk assessment at low doses and dose rates.

The overall aim of this Report is to provide input for the development of biologically based dose-response (BBDR) models for radiation- induced cancers and circulatory disease that use an adverse outcome pathways and key-events approach for providing parameters for these models. These mechanistic data can be integrated with the most recent epidemiologic data to develop overall doseresponse curves for radiation-induced adverse health outcomes. This integration of the findings from radiation biology and epidemiology will enhance the risk assessment process by reducing uncertainties in estimated risk following exposure to low doses and low dose rates of ionizing radiation. For many decades the basis for setting radiation protection guidance for exposure to low absorbed doses and low absorbed-dose rates of ionizing radiation has been the estimation of the risk of radiation-induced cancer. In addition, there is ongoing discussion concerning risks of radiation-induced noncancer effects1 (particularly circulatory disease). The estimates for radiation-induced cancer have been derived primarily from exposure to higher doses and higher dose rates of ionizing radiation and assumptions on how to extrapolate to low doses and low dose rates. For the purpose of this Report, for low linear-energy transfer (LET) radiation, a low absorbed dose is <100 mGy delivered acutely, and a low absorbeddose rate is <5 mGy h–1 for any accumulated absorbed dose (NCRP 2015). This Report addresses the conclusions and recommendations from three previous National Council on Radiation Protection and Measurements (NCRP) reports and commentaries on the topic of the risks of adverse health outcomes at low doses and low dose rates of ionizing radiation (NCRP 2012, 2015, 2018a). In this context, the present Report proposes a path forward to enhance the estimation of risk at low doses and low dose rates. Such a modified approach is needed to supplement the information that can be obtained from the conduct of even large epidemiologic studies such as the One Million U.S. Workers and Veterans Study of Low-Dose Radiation Health Effects (million U.S. workers and veterans study) 1For this Report the term noncancer is restricted to somatic noncancer outcomes and does not include heritable effects. (Bouville et al. 2015; Boice et al. 2019), the International Nuclear Workers Study (Leuraud et al. 2015; Richardson et al. 2015), the European pooled study of radiation-induced cancer from pediatric computed tomography (Bernier et al. 2019), or other low-dose pooling studies (Lubin et al. 2017; Little, Kitahara et al. 2018). This Report presents such an approach based upon the integration of data from epidemiology and radiation biology. An essential component of the integration process is the use of BBDR models with parameters being developed from analyzing adverse outcome pathways and their associated key events. In principle, an adverse outcome pathway is the series of necessary steps that result in an initial molecular event leading to an adverse health outcome (for this Report, either cancer or circulatory disease). Definitions of adverse outcome pathways and key events are given in Section 2 and can be found also in recent reviews (Edwards et al. 2016; Preston 2017). Also, when considering mechanistic data underlying the induction of adverse health outcomes, it is important to distinguish between potential bioindicators and biomarkers of these outcomes. A bioindicator is a cellular alteration that is on a critical pathway to the disease endpoint itself (i.e., necessary, but not by itself sufficient for the endpoint), such as a specific mutation in a target cell that is associated with tumor formation. Thus, a bioindicator can be perceived as informing on the shape of the dose-response curve for the disease outcome or on cancer frequency itself, and therefore, is equivalent to a key event. A biomarker is a biological phenotype [e.g., chromosome alteration, deoxyribonucleic acid (DNA) adduct, gene expression change, specific metabolite] that can be used to indicate a response to an exposure at the cell or tissue level. In this regard, a biomarker is generally a measure of the potential for development of an adverse health outcome such as cancer (e.g., a predictor of exposure level). This Report expands upon this general approach of adverse outcome pathways, key events, and BBDR models to enhance the process of low-dose, low dose-rate risk estimation. The arrangement of this Report for the application of this general approach is: here is what we know, here is what we need to know, and this is how we can obtain the necessary knowledge. A synopsis of Sections 2 through 7 is given below. Section 2 (Introduction) provides an overview of current approaches to radiation risk assessment, the associated uncertainties and possible ways forward for enhancing the estimation of risks of cancer and circulatory disease at low doses and low dose rates. Section 3 (Epidemiology, Biosamples and Biomarkers: Cancer and Circulatory Disease) presents a review of the radiation epidemiologic studies for which biomarker data or biological samples were used. For noncancer effects it was clear that the only adverse health outcome for which significant data from radiation biology are available for use in BBDR models is circulatory disease and so this forms the basis for the discussion on noncancer effects. There are a large number of radiation epidemiologic studies available that are very informative for estimating risks at higher doses but that can only be used with a fairly high degree of uncertainty for predicting low-dose risks. A review of the main radiation epidemiologic studies has been provided in NCRP Commentary No. 27 (NCRP 2018a). Section 3.1 briefly describes the major epidemiologic radiation studies with associated biosamples that potentially can be employed to conduct investigations of bioindicators of the pathogenesis of radiation-induced cancer and other health endpoints. While none of the current investigations has yet been able to identify definitive bioindicators, there are several suggestions of biomarkers that merit confirmation through further investigations and might be informative in the absence of more definitive bioindicator studies. The details and references for these studies are provided in Section 3.1.3. Section 3.2 indicates that it is likely that bioindicators of radiation- induced noncancer effects at low doses will be restricted to circulatory disease and so this is the sole topic reviewed for biomarkers associated with noncancer responses. With current knowledge, substantive biomarker information is only available in two major radiation studies: the Japanese atomic-bomb survivors, and the Mayak Production Association workers (Mayak workers), although little use has been made of this latter population in analyses to date. In summary, there is a paucity of radiation-specific bioindicators of cancer and circulatory disease and a relative lack of radiation- specific biomarkers predictive of adverse health outcomes. Thus, it is necessary to consider the mechanisms of formation of cancers and circulatory disease, especially for radiation-induced responses, to aid with the identification of bioindicators of adverse health outcomes and to a lesser extent, biomarkers of association with an adverse health outcome. Section 4 (Radiation-Induced Biological Effects Related to Cancer and Circulatory Disease) reviews the underlying mechanisms of carcinogenesis and circulatory disease with the aim of identifying potential bioindicators of the adverse health outcome, and if possible radiation-associated bioindicators of such responses. There has been an increased understanding of the underlying mechanisms of human diseases as a result of new molecular, cellular and computational approaches, further enhanced by informative experimental animal systems that model human disease. To a lesser extent such approaches have been used to better understand the etiology of radiation-induced diseases. There is a description of the types of studies that have identified pathways and potential key events in the carcinogenesis process (Section 4.1). While currently there are no fully validated bioindicators or biomarkers of radiation-induced cancer, there is a substantial and increasing body of knowledge on radiation-induced cancer mechanisms, particularly in experimental animal systems. Quantification of inflammation and generation of persistently elevated reactive oxygen species (ROS) holds promise as a further bioindicator that is also recognized as an enabling hallmark of cancer in the context of Hanahan and Weinberg (2011). In addition, cell-survival parameters can be of importance in mechanistic models of carcinogenesis. The use of data from experimental animal systems provides opportunities to demonstrate the added value of building and applying mechanistic models of radiation-induced cancer. There are additional opportunities to apply similar models in some human radiation-induced cancers, most notably thyroid, where some work utilizing knowledge of the CLIP2 marker is already available. The incorporation of quantitative mechanistic data into appropriate cancer models (discussed in Sections 5 and 6) is likely to increase the precision of estimated risks, particularly at low-dose levels and so continued efforts to identify and validate bioindicators of radiation- induced cancers will assist in refining risk estimation. Section 4.2 outlines the biology of circulatory disease, a significant radiation-induced noncancer disease2 and the one which currently offers the best opportunity for bioindicator identification given the mechanistic data already available. The complex inflammatory processes underlying most major types of circulatory disease are reviewed, specifically those associated with atherosclerosis. The possible ways that low-dose radiation exposure and other 2NCRP (2018a) stated that radiation-induced cardiovascular disease (a circulatory disease) remains an area where further investigation is necessary. Although there is evidence that cardiovascular disease may be a factor at exposures lower than previously estimated, that evidence was not yet sufficient to allow for development of an approach to including cardiovascular disease in NCRP’s overall system of radiation protection published in NCRP (2018b). biological stressors might affect the circulatory system are also reviewed. While it is not possible yet to identify bioindicators of radiation-induced circulatory disease, it appears feasible to build upon the rapidly increasing knowledge of the mechanisms of formation of circulatory disease to develop adverse outcome pathways and at least some of the associated key events. Section 5 (Biologically Based Dose-Response Models) assesses biomathematical models of chronic disease, especially those for cancer and circulatory disease (with particular emphasis in circulatory disease on models of atherosclerosis). First, general material outlining the overall goals of biomathematical models is presented, followed by discussion of modeling considerations, particularly application of specific models using human, animal or cell data to cancer and circulatory disease. Biologically based modeling of radiation- induced cancers of the breast, colon, lung, and thyroid gland have been conducted. After considerations of some general features of BBDR models of cancer development in Section 5.1, a number of BBDR models and their application to various human and animal datasets are presented in Section 5.2. Despite some shortcomings (e.g., the fact that different models might explain the available data using different mechanistic assumptions), multiple pathway models are considered a promising conceptual approach to developing a general model framework for the complex process of carcinogenesis in various tissues. In certain cases, multiple pathway models may allow predictions that can be validated against experimental data. Circulatory disease models are considered in Section 5.3. These are less well developed than those that have been constructed to model cancer. A number of candidate models of atherosclerosis are considered. Atherosclerosis is the disease process underlying the main types of circulatory disease, specifically ischemic heart disease (IHD) and stroke, which is thought to have a largely inflammatory etiology. A number of atherosclerosis models, which share certain features, have been proposed for these inflammatory processes, specifically the adhesion and transport of monocytes through the epithelial cell layer, and diffusion through the intima. However, it is not yet clear what the radiation-associated mechanisms may be for most types of circulatory disease. Having identified the types of BBDR models that could possibly be used to enhance the estimation of low-dose, low dose-rate radiation adverse health outcomes, it is necessary to determine whether there is a generalized model that can be used for: all radiationinduced cancer types, or circulatory disease as a class. It was concluded that it would be unlikely that a single model structure could be used for describing cancer and circulatory disease. Also, it appears likely that there may be different responses even for different types of circulatory disease. The concept of a generalized model is discussed in Section 6 (Proposed Generalized Model Framework of Cancer and Circulatory Disease). It is proposed that a form of multistage clonal expansion model would be appropriate for integrating data from epidemiology and radiation biology for estimating low-dose, low dose-rate cancer risk. The parameters for such a model structure are proposed to be developed from an adverse outcome pathways and key-events approach. In such an approach the key events are considered to be bioindicators of the adverse health outcome itself. In support of this proposal to utilize generalized multistage clonal expansion models, there has been considerable recent discussion on the use of such parameterized models for environmental chemicals (OECD 2020). The Organization for Economic Co-operation and Development (OECD 2020) website provides a considerable amount of information on developing adverse outcome pathways and their use in risk assessment and ultimately in risk management practice. This general approach is also described and applied in the research program of the U.S. Environmental Protection Agency (EPA 2018). A description of biologically detailed models of specific cancers that have been applied with some levels of success is provided to indicate the viability of the use of BBDR models for estimating adverse health outcomes at low doses and low dose rates. While not definitive at this time, the approach certainly has a real likelihood of being successful. Section 7 (Research Needs) provides specific examples of research activities, both large and small that are designed for developing adverse outcome pathways and their associated key events. These include epidemiologic, human sample, laboratory animal, cellular, and molecular studies. Such research activities include investigating some general but critical responses, in order to derive greater insight into the parameters of most importance for further model development. Currently, one can envisage the following to be of high relevance: • target cell population numbers and characteristics; • survival parameters for these populations after radiation exposure; • target gene(s) critical for pathogenesis and their mutation or epimutation frequency as a function of radiation dose; • proliferation characteristics in normal and mutation-carrying cell populations; and • timing and frequency of acquisition of further mutational events in key genes and the impact of these on survival and proliferation characteristics. For enhanced model development, it is necessary to more fully identify the mechanisms of cancer development in response to radiation. The following are of importance in this regard: • Mechanisms in the development of a radiation-induced disease may differ from those in sporadic disease. • Does radiation initiate or accelerate the same processes that lead to sporadic disease, or are distinct molecular pathways involved? • BBDR models have the potential to address such questions if appropriate bioindicators become available for specific types of cancer or other diseases. • For transcriptomics, proteomics, metabolomics and epigenomics, adequate BBDR models ideally might require measurements at several time points because the profiles of phenotypic alterations may differ by stage in the pathogenesis of a disease. Clearly, the overarching need is the furthering of research targeted at the underlying mechanisms of radiation-induced adverse health outcomes (cancer and noncancer disease) leading to the identification of truly informative bioindicators of the apical endpoint (i.e., the adverse health outcome). The framework for such an approach can be the characterization of adverse outcome pathways for specific outcomes and the identification of key events from the initial event to adverse health outcome. In this context, a key-event or informative bioindicator is a true surrogate for the adverse health outcome. This approach will require the integration of data from epidemiology and radiation biology to maximize the information for estimating low-dose responses for adverse health outcomes. A particularly important result will be the ability to better describe the form of the dose-response curve for different types of radiation- induced cancer, for example, and thereby avoid the need to rely on application of the linear-nonthreshold (LNT) model without sufficient biological substantiation. A concerted effort will be needed; this is going to require a well-defined and quite extensive research effort. The need for this effort is recognized by many in the risk assessment and risk management arena.

National Council on Radiation Protection and Measurements Commentary 27 examines recent epidemiologic data primarily from low-dose or low dose-rate studies of low linear-energy-transfer radiation and cancer to assess whether they support the linear no-threshold model as used in radiation protection. The commentary provides a critical review of low-dose or low dose-rate studies, most published within the last 10 y, that are applicable to current occupational, environmental, and medical radiation exposures. The strengths and weaknesses of the epidemiologic methods, dosimetry assessments, and statistical modeling of 29 epidemiologic studies of total solid cancer, leukemia, breast cancer, and thyroid cancer, as well as heritable effects and a few nonmalignant conditions, were evaluated. An appraisal of the degree to which the low-dose or low dose-rate studies supported a linear no-threshold model for radiation protection or on the contrary, demonstrated sufficient evidence that the linear no-threshold model is inappropriate for the purposes of radiation protection was also included. The review found that many, though not all, studies of solid cancer supported the continued use of the linear no-threshold model in radiation protection. Evaluations of the principal studies of leukemia and low-dose or low dose-rate radiation exposure also lent support for the linear no-threshold model as used in protection. Ischemic heart disease, a major type of cardiovascular disease, was examined briefly, but the results of recent studies were considered too weak or inconsistent to allow firm conclusions regarding support of the linear no-threshold model. It is acknowledged that the possible risks from very low doses of low linear-energy-transfer radiation are small and uncertain and that it may never be possible to prove or disprove the validity of the linear no-threshold assumption by epidemiologic means. Nonetheless, the preponderance of recent epidemiologic data on solid cancer is supportive of the continued use of the linear no-threshold model for the purposes of radiation protection. This conclusion is in accord with judgments by other national and international scientific committees, based on somewhat older data. Currently, no alternative dose-response relationship appears more pragmatic or prudent for radiation protection purposes than the linear no-threshold model.

The recently published NCRP Commentary No. 27 evaluated the new information from epidemiologic studies as to their degree of support for applying the linear nonthreshold (LNT) model of carcinogenic effects for radiation protection purposes [1]. The aim was to determine whether recent epidemiologic studies of low-LET radiation, particularly those at low doses and/or low dose rates (LD/LDR), broadly support the LNT model of carcinogenic risk or, on the contrary, demonstrate sufficient evidence that the LNT model is inappropriate for the purposes of radiation protection. An updated review was needed because a considerable number of reports of radiation epidemiologic studies based on new or updated data have been published since other major reviews were conducted by national and international scientific committees. The Commentary provides a critical review of the LD/LDR studies that are most directly applicable to current occupational, environmental and medical radiation exposure circumstances. This Memorandum summarizes several of the more important LD/LDR studies that incorporate radiation dose responses for solid cancer and leukaemia that were reviewed in Commentary No. 27. In addition, an overview is provided of radiation studies of breast and thyroid cancers, and cancer after childhood exposures. Non-cancers are briefly touched upon such as ischemic heart disease, cataracts, and heritable genetic effects. To assess the applicability and utility of the LNT model for radiation protection, the Commentary evaluated 29 epidemiologic studies or groups of studies, primarily of total solid cancer, in terms of strengths and weaknesses in their epidemiologic methods, dosimetry approaches, and statistical modeling, and the degree to which they supported a LNT model for continued use in radiation protection. Recommendations for how to make epidemiologic radiation studies more informative are outlined. The NCRP Committee recognizes that the risks from LD/LDR are small and uncertain. The Committee judged that the available epidemiologic data were broadly supportive of the LNT model and that at this time no alternative dose-response relationship appears more pragmatic or prudent for radiation protection purposes.

This is an update of R.J. Preston and J.A. Ross, 1.16 - DNA-Reactive Agents, In Comprehensive Toxicology (Second Edition), edited by Charlene A. McQueen, Elsevier, Oxford, 2010, Pages 349–360.

The process of setting radiation protection standards requires the interaction of a number of international and national organizations that in turn require the input of scientific and regulatory experts. Bill Morgan served in an expert capacity for several of these organizations particularly for the application of radiation biology data to risk assessment. He brought great enthusiasm and dedication to these committee efforts. In fact, he really enjoyed this type of service. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), for example, provides comprehensive reviews of the input data for radiation risk assessments. In this context, they do not conduct risk assessments. In Europe, a research component of the risk assessment process is provided by the Multidisciplinary European Low Dose Initiative (MELODI). Specific issue areas are identified for which additional research can aid in reducing uncertainty in risk assessments. The International Commission for Radiological Protection (ICRP) uses these types of input data to develop nominal cancer risk estimates as input data for establishing dose limits for the public and workers. A similar regulatory role is provided in the US by the National Council on Radiation Protection and Measurements (NCRP). The NCRP Reports address the underlying principles for setting regulatory dose limits for the US public and workers; these differ to a limited extent from those of ICRP. The implementation of dose limits is conducted by individual countries but with significant guidance by the International Atomic Energy Agency (IAEA) through its Basic Safety Standards. The role of other national and international organizations are discussed in this same framework.

Purpose: This review is a contribution to the memory of Dr William (Bill) Morgan and highlights an area of research and deliberation that he considered extremely important in support of the setting of protective radiation dose limits. Biological research has generally played a minor role in the estimation of adverse health outcomes following exposure to low doses and low dose rates of radiation. The reliance has been on the available, quite extensive data base of epidemiology studies. The major concern is that such studies are for moderate to high doses requiring risk extrapolation methodologies for estimating low dose effects. There are significant uncertainties associated with this approach. This review will discuss how radiation biology studies can potentially reduce this uncertainty through the use of a key events/adverse outcome pathways approach to identify bioindicators of cancer and noncancer effects for use as parameters in biologically-based dose-response (BBDR) models. Such models would allow for an improved extrapolation approach for estimating health effects at low doses and low dose rates of radiation. Conclusion: Based on reported and ongoing studies for environmental chemicals, the adverse outcome/key events approach is a viable one for enhanced risk assessment (and risk management practice). The identification of informative bioindicators of adverse health effects will be a challenge but with modern molecular and advanced computational techniques, it is certainly feasible. This approach provides a framework for defining a low dose radiation research program; something that was of great importance to Bill Morgan.