J. Carl Barrett,1 Harri Vainio,2 David Peakall,3 and Bernard D. Goldstein4
Recent advances in the understanding of molecular biology, the human genome, toxicology, and disease mechanisms and ecosystem functions have led to significant advances in the study of susceptibility factors for environmental hazards. These methods can be used to identify environmental causes of human diseases and harm to ecosystems; to identify susceptible subpopulations; and to understand interindividual and interethnic differences in response to environmental hazards, which, we hope, will lead to disease prevention, an important translation of molecular medicine. The complex interplay between genes and environment represents a tremendous challenge to scientists but also an important opportunity to reduce the burden of disease and dysfunctions to humans and the ecosystem.
The number of biomarkers available to study responses of biological systems to environmental factors is growing rapidly. The term "biomarker" is a general term for specific measurements of an interaction of a biological system and an environmental agent (1,2). Biomarkers of exposure measure an exogenous substance or its metabolite and its interaction with a biological molecule. Because a number of factors determine whether a chemical exposure reaches its biological target for a toxic response, the most accurate measurement of "dose" is the biologically effective dose at the target tissue, which can be more reliably measured by biomarkers of exposure than estimated by measurements of administered or ambient chemical exposure.
Biomarkers of effect are measurable biochemical, physiological, behavior, or other alterations within an organism (1). Biomarkers of effect are primarily concerned with adverse effects, although the level of evidence varies for the relationship between a given measured effect and specific, pathological responses, which occur often after a long period and after chronic exposures. There is an overlap between biomarkers of exposure and biomarkers of effect, because the same biomarker can be used for both measurements. Some of the same biomarkers are also used to measure interindividual differences in response and thus further serve as biomarkers of susceptibility.
Figure 1. The role of susceptibility in the continuum between biological markers of exposure and of effect.
Figure 1 was reproduced from Figure 1 in the Journal of Occupational and Evironmental Medicine [37:9199 (1995)]. No attribution was provided. The authors apologize to K. Van Damme, L. Casteleyn, E. Heseltine, A. Huici, M. Sorsa, N. van Larebeke, and P. Vineis for this oversight.
There is a continuum between biological markers of exposure and those of effect (Figure 1). Exploring the relationship between biological markers on this continuum can be a useful means for detecting susceptibility. Biological markers of susceptibility can be defined as indicators of the mechanistic processes that cause variability among the compartments in the continuum between exposure and effect.
As a simplification, there are five different mechanistic avenues through which factors affecting susceptibility can influence the interaction between biological systems and environmental exposures. These avenues are body uptake, metabolism, target cell uptake, subcellular or molecular interaction, and the baseline status of the individual or ecosystem. The mechanistic avenues are bounded by compartments in which exposure or effect can be quantitated. These compartments are external exposure level, internal exposure level, extracellular level of toxic agent or metabolite, cellular level of toxic agent or metabolite, target cell toxicity, and adverse effect to the individual or ecosystem. In each case one or more of the mechanisms responsible for susceptibility produce a variation in the relation between compartments that is potentially detectable through use of biomarkers.
In the simplest formulation, susceptibility can be defined as a variation in the quantity of exposure/effect in a compartment among individuals or populations who have similar levels of exposure/effect in the preceding compartment. Of necessity, this can only be accounted for by mechanisms operative in the avenue between these two compartments. Examples of compartments and mechanistic avenues are the following:
Each mechanistic avenue can have multiple inputs in different directions, which influence the extent to which a quantity in one compartment affects the quantity in the next compartment. For example, even when the relation between the absorbed dose of a xenobiotic and the level of a toxic metabolite is the result of a single enzyme, the activity of this enzyme in any individual may be a function of a genetic polymorphism as well as the presence of other pollutants or of dietary factors that affect enzyme activity. The potential order of the compartments may differ depending upon the mechanistic pathways; e.g., metabolism may occur in a nontarget cell and precede uptake of the toxic metabolite into the target cell (reversal of the second and third sections in the above example). The relationship between compartments can be modeled mathematically so as to explore the extent of variability in the relationship between two compartments among a specified population group, i.e., the distribution of susceptibility due to this particular mechanistic avenue.
In essence, the true distribution of susceptibility in a population is the relation between the first compartment and the last one--between external exposure and disease outcome, with the intervening mechanistic avenues contributing to this distribution. For a given individual it is possible that a mechanism that increases susceptibility to a disease outcome in one avenue may be counterbalanced by a mechanism that decreases susceptibility in another avenue. It is thus important to recognize that analysis of only one mechanistic avenue in the pathway between exposure and disease does not necessarily provide definitive information on the susceptibility of any single individual in the population.
Biomarkers can be divided into those that measure compartments, e.g., blood levels, DNA adducts, or tissue damage; and those that measure mechanisms, e.g., enzyme activity levels or gene polymorphisms. Each has its advantages and its limitations. Measurement of the compartment tells little about the reasons for the level; measurement of the mechanism is not always a definitive indicator of what will be happening in the next compartment. For individuals or populations at risk, it is preferable to have measurements both from mechanistic avenues and from related compartments to more effectively target prevention strategies.
Genetic Susceptibility
Individuals may vary in terms of their responses to environmental hazards due to differences in their genetic constitution. The human genome encodes for 50,000 to 100,000 genes, some of which are key regulators of biological processes. Mutations (heritable alterations in the primary coding sequences of genes or their controlling elements) in specific genes ("disease genes") greatly predispose an individual to a disease. For example, mutations that inactivate the retinoblastoma gene, a central regulator of cell growth and division, can result in a 105-fold increased probability of a rare eye cancer in children (retinoblastoma). Due to recent advances in gene mapping and isolation, a number of major disease genes have been discovered, including genes involved in cystic fibrosis; Huntington's disease; Alzheimer's disease; and breast, colon, lung, and many other cancers. Mutations in disease genes lead to a very high probability of disease development, often approaching a 100% incidence of disease in carriers of the mutations.
Mutations in disease genes are usually rare; depending on the gene, 1 in 200 to 1 in a million people are affected. However, multiple genes may regulate a specific disease, therefore, the percentage of disease cases for which there is a genetic component may be high. For example, in certain cancers (e.g., colon, breast, and prostate) it is estimated that up to 10% of the cancers are due to a genetic predisposition. These very strong susceptibility genes may not be influenced by environmental factors, but there is evidence for environmental influences in some diseases even with a strong genetic predisposition.
Many of the genes in the genome of humans and other species influence the impact of environmental agents on the organism. Genetic controls on the uptake, activation, detoxification, or repair of environmental insults are known. The exact number of genes involved in the organism's response to environmental hazards is unknown but could be very large. For example, the estimates of the number of the individual P450 genes in any mammalian species range from 60 to 200 (3).
All genes commonly have variations in their sequences that may or may not have functional consequences. Changes in DNA sequence that occur frequently (in >1% of the population) are called genetic polymorphisms. Polymorphisms often affect the function of a gene but some may change the level of expression of a gene or change the activity of the gene product, for example, an enzyme. Genetic polymorphisms that are functionally significant are quite important when the gene controls the response of an organism to environmental hazards. For example, polymorphisms in the genes that metabolize carcinogens can affect the response of individuals to that carcinogen. Given that a large number of genes are involved in responses to environmental hazards and that a large number of polymorphisms exist in these genes, genetic differences are important susceptibility factors in environmental responses. Many of these commonly occur in human populations.
Recent advances in the identification and cloning of specific genes and methods for the detection of mutations and polymorphisms in these genes have led to significant advances in our understanding of genetic susceptibility to environmental hazards. There is an important difference between individuals with genetic alterations that lead to disease susceptibility and individuals with genetic susceptibility to environmental factors. Individuals who inherit a mutation in a disease susceptibility gene have a high risk of developing that disease regardless of environmental exposures, although environmental factors may increase the incidence or rate of disease development. Individuals who have a mutation or polymorphism in genes involved in response to environmental hazards will only have an increased risk of disease development when they are exposed to specific environmental hazards. Therefore, risk to these individuals is influenced strongly by gene-environment interaction. Also, because multiple genes are involved in response to the same environmental hazards, two individuals with the same genetic susceptibility and environmental exposure may have different risks because of the interplay between genes involved in response to xenobiotics. For example, two individuals may both have a polymorphism in a gene that increases the rate of carcinogen activation but different polymorphisms in a gene that inactivates the same carcinogen. Because many environmental response genes remain to be identified and characterized and because many mixtures of environmental exposures are present in the environment, it is difficult to determine an individual's risk. Great advances have been made in the identification of susceptibility factors for subpopulations exposed to environmental hazards.
Susceptible Populations versus Susceptible Individuals
Environmental and occupational medicine, like medicine in general, is traditionally concerned with individuals and their health. It may also seek to identify individuals who are liable to develop a specific disease (predictive medicine). This approach implies the possibility of dividing the people into two groups: those who are disease prone, and the healthy remainder. This is, of course, a considerable oversimplification, because susceptibility is rarely confined to a distinct high-risk minority and because our ability to predict the disease outcome for these individuals is therefore weak.
Epidemiology, as a discipline of public health, includes the study of the distribution of diseases and risk factors in populations. Epidemiologic research focuses on populations in order to elucidate a broad range of risk factors for disease. A distinction exists between the populations that epidemiologists study and the individuals comprising those populations. Within a low risk population, there are individuals who develop disease, and within high-risk populations there are disease-free individuals. An individual's risk will be determined by a complex interplay of multiple genes, multiple exposures, diet, age, and chance. The application of biomarkers of exposure, effect, and susceptibility to the assessment of an individual's risk from environmental hazards is not conceptually different from the application of common clinical measures in medical practice. For example, blood pressure, blood lipids, relative weight, and smoking habits are applied to the assessment of an individual's risk for coronary heart disease in clinical medicine. Combining biomarkers with epidemiological study design has led to the development of molecular epidemiology, an approach that generates a great deal of interest and expectation. The use of biomarkers in human studies raises challenging ethical questions that must be addressed in advance of applying new technologies in this area.
Susceptible Populations and Ecosystems
Ecologists are concerned with understanding differences in susceptibility among individuals within communities of thousands of species. One important methodological approach is the development of biomarkers of exposure and effect that are applicable across a broad range of species that range from invertebrates to birds and mammals. However, ecologists also are concerned with understanding and predicting--through the use of biological indicators--differences in susceptibility of communities and ecosystems. Here, vulnerability refers to changes in the structure and function of ecosystems. The challenge for ecologists is to develop a broad range of methodologies from molecular through ecosystem measures that can be used to assess susceptibility.
In many cases, cellular and molecular biomarkers that can be used to assess the susceptibility of a range of indicator species will be most useful for communities and ecosystems as well. Such biomarkers could be used to assess both the health of the ecosystem and the potential of the ecosystem to support human populations and provide ecological services.
Ethics
The study of susceptibility in human populations and in ecosystems poses a number of ethical challenges. Regarding humans, both the manner in which information is obtained and the uses to which it could be put raise urgent concerns. Regarding ecosystems, the ethical duty to work towards protecting our shared environment and its life support systems requires ongoing attention and the development of clear guidelines.
Specifically relevant to susceptibility studies in humans is the sensitivity of the information and the likelihood of labeling certain individuals or subgroups in ways that could cause unanticipated harm. Scientists and public health professionals investigating susceptibility or implementing related preventive screening programs must be aware of the potential for harmful social and psychological side effects arising from susceptibility studies. For example, denial of employment, denial of insurance, fear for the premature onset of illness or death, social marginalization, reconsideration of family planning and of both medium and longer term life goals, and the need for psychological counseling are possible untoward effects that may arise from the identification of susceptibility in individuals.
Scientists also must remain sensitive to cross-cultural differences in perception of personal risks associated with susceptibility studies and must strive for equity in their research and in its potential consequences. For example, there may be greater need to focus resources on populations subjected to more severe pollution than on those populations experiencing lower levels of exposure.
Scientists must understand ethical issues to be able to address the challenges presented by susceptibility studies and to learn how to resolve them. Preparation would include defining the key ethical issues, identifying the principles behind them, and then attempting to resolve the ethical dilemmas that emerge. This objective requires a certain amount of formal training in ethics as well as ongoing dialogue involving a broad range of stakeholders who try to anticipate ethical dilemmas. Stakeholders include, among others, other professionals as well as patient advocates.
Markers of susceptibility are of a particularly sensitive nature. Therefore, serious attention must be paid to quality assurance so that data integrity can be even more highly assured than in most other types of studies. In addition, existing guidelines that protect confidentiality and ensure the participants' "right to know" must be revisited and made more specific in light of the potential consequences of susceptibility studies.
Where legal protections are not clearly in place to ensure confidentiality of personal information gathered in the conduct of research into susceptibility, and to protect the various stakeholders against retroactive actions, the risks associated with such studies must be made clear to any research participants in advance of their commitment to participate. Such risks include birth with inherited negative traits, and illness induced or associated with environmental/occupational exposures only subsequently recognized as having been associated with susceptibility traits.
Finally, susceptible ecosystems exist, the destruction of which can have both direct and indirect negative consequences for human health and well-being. For example, the susceptibility of the earth's atmosphere has consequences for global warming associated with, among other things, increasing skin cancer rates, coastal flooding disasters, and crop failures. Scientists engaged in such work must bring to the attention of human populations their stewardship duty and responsibility to protect the ecosphere.
Conclusions and Recommendations
The following conclusions and recommendations have been formulated in recognition of human and ecosystem vulnerability to environmental hazards:
a) Determination of susceptibility to chemicals in the workplace and general environment is becoming increasingly feasible through rapid advances in biological sciences, particularly molecular biology. Parallel advances have occurred in epidemiology, ecology, toxicology, and related sciences which have greatly facilitated understanding and measurement of susceptibility.
b) Increased understanding of the pathways leading to susceptibility of individuals, of populations, and of ecosystems to chemical and physical agents is of value in protecting human health and the environment.
c) Identification of biological markers of exposure and of effect is a useful avenue for determining susceptibility. Markers of susceptibility in essence operate in the pathway between the various compartments of exposure and effect, reflecting mechanisms responsible for variations in response to the levels in the previous compartment.
d) There is an important distinction between studies of the susceptibility of populations at risk and studies of the susceptibility of individuals. It is important not to misinterpret from population studies the risk to an individual.
e) The ethical, legal, and social dimensions should be recognized in studies of individuals, populations, communities, and ecosystems at risk. Ethical dilemmas must be considered in advance of research activities or of any use of susceptibility markers in evaluating populations at risk. In some cases public debate and legislation will be required to clarify social norms and to provide legal protection for the participant. Ethical issues should be incorporated in the development of methodologies to study and evaluate communities and ecosystems and should be reflected in research proposals.
f) Studies of biomarkers of susceptibility should not be undertaken in circumstances in which confidentiality and privacy safeguards cannot be assured.
g) Training programs for professionals engaging in susceptibility studies of biomarkers need to integrate formal ethics education. Continuing education should include an ongoing discourse through ethics workshops, symposia, and discussion in the journals of the respective professions.
h) Numerous other methodological issues must be taken into account before accepting a marker of susceptibility as being potentially useful in protecting public health and the environment. These issues include technical feasibility, cost, quality assurance and control, and cultural and logistic issues related to sample availability, collection, and storage.
i) The determination and use of susceptibility markers pose both technical and ethical issues in developing countries. These need to be taken into account when designing studies or applying the use of susceptibility markers validated in industrialized countries.
j) Additional research is needed to focus biological advances on increasing understanding of factors affecting the susceptibility of human and nonhuman components of ecosystems, and to determine how best to apply this understanding to protect public health and the environment. Good science should be tied to good ethics and vice versa.
Last Update: June10, 1997