The word biomarker is short for biological marker, a term that refers to a measurable event occurring in a biological system, such as the human body. This event is then interpreted as a reflection, or marker, of a more general state of the organism or of life expectancy. In occupational health, a biomarker is generally used as an indicator of health status or disease risk.
Biomarkers are used for in vitro as well as in vivo studies that may include humans. Usually, three specific types of biological markers are identified. Although a few biomarkers may be difficult to classify, usually they are separated into biomarkers of exposure, biomarkers of effect or biomarkers of susceptibility (see table 1).
Table 1. Examples of biomarkers of exposure or biomarkers of effect that are used in toxicological studies in occupational health
|Adipose tissue||Dioxin||Dioxin exposure|
|Exhaled breath||Toluene||Toluene exposure|
|Blood||Carboxyhaemoglobin||Carbon monoxide exposure|
|Red blood cells||Zinc-protoporphyrin||Lead exposure|
|White blood cells||DNA adducts||Mutagen exposure|
Given an acceptable degree of validity, biomarkers may be employed for several purposes. On an individual basis, a biomarker may be used to support or refute a diagnosis of a particular type of poisoning or other chemically-induced adverse effect. In a healthy subject, a biomarker may also reflect individual hypersusceptibility to specific chemical exposures and may therefore serve as a basis for risk prediction and counselling. In groups of exposed workers, some exposure biomarkers can be applied to assess the extent of compliance with pollution abatement regulations or the effectiveness of preventive efforts in general.
Biomarkers of Exposure
An exposure biomarker may be an exogenous compound (or a metabolite) within the body, an interactive product between the compound (or metabolite) and an endogenous component, or another event related to the exposure. Most commonly, biomarkers of exposures to stable compounds, such as metals, comprise measurements of the metal concentrations in appropriate samples, such as blood, serum or urine. With volatile chemicals, their concentration in exhaled breath (after inhalation of contamination-free air) may be assessed. If the compound is metabolized in the body, one or more metabolites may be chosen as a biomarker of the exposure; metabolites are often determined in urine samples.
Modern methods of analysis may allow separation of isomers or congeners of organic compounds, and determination of the speciation of metal compounds or isotopic ratios of certain elements. Sophisticated analyses allow determination of changes in the structure of DNA or other macromolecules caused by binding with reactive chemicals. Such advanced techniques will no doubt gain considerably in importance for applications in biomarker studies, and lower detection limits and better analytical validity are likely to make these biomarkers even more useful.
Particularly promising developments have occurred with biomarkers of exposure to mutagenic chemicals. These compounds are reactive and may form adducts with macromolecules, such as proteins or DNA. DNA adducts may be detected in white blood cells or tissue biopsies, and specific DNA fragments may be excreted in the urine. For example, exposure to ethylene oxide results in reactions with DNA bases, and, after excision of the damaged base, N-7-(2-hydroxyethyl)guanine will be eliminated in the urine. Some adducts may not refer directly to a particular exposure. For example, 8-hydroxy-2´-deoxyguanosine reflects oxidative damage to DNA, and this reaction may be triggered by several chemical compounds, most of which also induce lipid peroxidation.
Other macromolecules may also be changed by adduct formation or oxidation. Of special interest, such reactive compounds may generate haemoglobin adducts that can be determined as biomarkers of exposure to the compounds. The advantage is that ample amounts of haemoglobin can be obtained from a blood sample, and, given the four-month lifetime of red blood cells, the adducts formed with the amino acids of the protein will indicate the total exposure during this period.
Adducts may be determined by sensitive techniques such as high-performance lipid chromatography, and some immunological methods are also available. In general, the analytical methods are new, expensive and need further development and validation. Better sensitivity can be obtained by using the 32P post labelling assay, which is a nonspecific indication that DNA damage has taken place. All of these techniques are potentially useful for biological monitoring and have been applied in a growing number of studies. However, simpler and more sensitive analytical methods are needed. Given the limited specificity of some methods at low-level exposures, tobacco smoking or other factors may impact significantly on the measurement results, thus causing difficulties in interpretation.
Exposure to mutagenic compounds, or to compounds which are metabolized into mutagens, may also be determined by assessing the mutagenicity of the urine from an exposed individual. The urine sample is incubated with a strain of bacteria in which a specific point mutation is expressed in a way that can be easily measured. If mutagenic chemicals are present in the urine sample, then an increased rate of mutations will occur in the bacteria.
Exposure biomarkers must be evaluated with regard to temporal variation in exposure and the relation to different compartments. Thus, the time frame(s) represented by the biomarker, that is, the extent to which the biomarker measurement reflects past exposure(s) and/or accumulated body burden, must be determined from toxicokinetic data in order to interpret the result. In particular, the degree to which the biomarker indicates retention in specific target organs should be considered. Although blood samples are often used for biomarker studies, peripheral blood is generally not regarded as a compartment as such, although it acts as a transport medium between compartments. The degree to which the concentration in the blood reflects levels in different organs varies widely between different chemicals, and usually also depends upon the length of the exposure as well as time since exposure.
Sometimes this type of evidence is used to classify a biomarker as an indicator of (total) absorbed dose or an indicator of effective dose (i.e., the amount that has reached the target tissue). For example, exposure to a particular solvent may be evaluated from data on the actual concentration of the solvent in the blood at a particular time following the exposure. This measurement will reflect the amount of the solvent that has been absorbed into the body. Some of the absorbed amount will be exhaled due to the vapour pressure of the solvent. While circulating in the blood, the solvent will interact with various components of the body, and it will eventually become subject to breakdown by enzymes. The outcome of the metabolic processes can be assessed by determining specific mercapturic acids produced by conjugation with glutathione. The cumulative excretion of mercapturic acids may better reflect the effective dose than will the blood concentration.
Life events, such as reproduction and senescence, may affect the distribution of a chemical. The distribution of chemicals within the body is significantly affected by pregnancy, and many chemicals may pass the placental barrier, thus causing exposure of the foetus. Lactation may result in excretion of lipid-soluble chemicals, thus leading to a decreased retention in the mother along with an increased uptake by the infant. During weight loss or development of osteoporosis, stored chemicals may be released, which can then result in a renewed and protracted “endogenous” exposure of target organs. Other factors may affect individual absorption, metabolism, retention and distribution of chemical compounds, and some biomarkers of susceptibility are available (see below).
Biomarkers of Effect
A marker of effect may be an endogenous component, or a measure of the functional capacity, or some other indicator of the state or balance of the body or organ system, as affected by the exposure. Such effect markers are generally preclinical indicators of abnormalities.
These biomarkers may be specific or non-specific. The specific biomarkers are useful because they indicate a biological effect of a particular exposure, thus providing evidence that can potentially be used for preventive purposes. The non-specific biomarkers do not point to an individual cause of the effect, but they may reflect the total, integrated effect due to a mixed exposure. Both types of biomarkers may therefore be of considerable use in occupational health.
There is not a clear distinction between exposure biomarkers and effect biomarkers. For example, adduct formation could be said to reflect an effect rather than the exposure. However, effect biomarkers usually indicate changes in the functions of cells, tissues or the total body. Some researchers include gross changes, such as an increase in liver weight of exposed laboratory animals or decreased growth in children, as biomarkers of effect. For the purpose of occupational health, effect biomarkers should be restricted to those that indicate subclinical or reversible biochemical changes, such as inhibition of enzymes. The most frequently used effect biomarker is probably inhibition of cholinesterase caused by certain insecticides, that is, organophosphates and carbamates. In most cases, this effect is entirely reversible, and the enzyme inhibition reflects the total exposure to this particular group of insecticides.
Some exposures do not result in enzyme inhibition but rather in increased activity of an enzyme. This is the case with several enzymes that belong to the P450 family (see “Genetic determinants of toxic response”). They may be induced by exposures to certain solvents and polyaromatic hydrocarbons (PAHs). Since these enzymes are mainly expressed in tissues from which a biopsy may be difficult to obtain, the enzyme activity is determined indirectly in vivo by administering a compound that is metabolized by that particular enzyme, and then the breakdown product is measured in urine or plasma.
Other exposures may induce the synthesis of a protective protein in the body. The best example is probably metallothionein, which binds cadmium and promotes the excretion of this metal; cadmium exposure is one of the factors that result in increased expression of the metallothionein gene. Similar protective proteins may exist but have not yet been explored sufficiently to become accepted as biomarkers. Among the candidates for possible use as biomarkers are the so-called stress proteins, originally referred to as heat shock proteins. These proteins are generated by a range of different organisms in response to a variety of adverse exposures.
Oxidative damage may be assessed by determining the concentration of malondialdehyde in serum or the exhalation of ethane. Similarly, the urinary excretion of proteins with a small molecular weight, such as albumin, may be used as a biomarker of early kidney damage. Several parameters routinely used in clinical practice (for example, serum hormone or enzyme levels) may also be useful as biomarkers. However, many of these parameters may not be sufficiently sensitive to detect early impairment.
Another group of effect parameters relate to genotoxic effects (changes in the structure of chromosomes). Such effects may be detected by microscopy of white blood cells that undergo cell division. Serious damage to the chromosomes—chromosomal aberrations or formation of micronuclei—can be seen in a microscope. Damage may also be revealed by adding a dye to the cells during cell division. Exposure to a genotoxic agent can then be visualized as an increased exchange of the dye between the two chromatids of each chromosome (sister chromatid exchange). Chromosomal aberrations are related to an increased risk of developing cancer, but the significance of an increased rate of sister chromatid exchange is less clear.
More sophisticated assessment of genotoxicity is based on particular point mutations in somatic cells, that is, white blood cells or epithelial cells obtained from the oral mucosa. A mutation at a specific locus may make the cells capable of growing in a culture that contains a chemical that is otherwise toxic (such as 6-thioguanine). Alternatively, a specific gene product can be assessed (e.g., serum or tissue concentrations of oncoproteins encoded by particular oncogenes). Obviously, these mutations reflect the total genotoxic damage incurred and do not necessarily indicate anything about the causative exposure. These methods are not yet ready for practical use in occupational health, but rapid progress in this line of research would suggest that such methods will become available within a few years.
Biomarkers of Susceptibility
A marker of susceptibility, whether inherited or induced, is an indicator that the individual is particularly sensitive to the effect of a xenobiotic or to the effects of a group of such compounds. Most attention has been focused on genetic susceptibility, although other factors may be at least as important. Hypersusceptibility may be due to an inherited trait, the constitution of the individual, or environmental factors.
The ability to metabolize certain chemicals is variable and is genetically determined (see “Genetic determinants of toxic response”). Several relevant enzymes appear to be controlled by a single gene. For example, oxidation of foreign chemicals is mainly carried out be a family of enzymes belonging to the P450 family. Other enzymes make the metabolites more water soluble by conjugation (e.g., N-acetyltransferase and μ-glutathion-S-transferase). The activity of these enzymes is genetically controlled and varies considerably. As mentioned above, the activity can be determined by administering a small dose of a drug and then determining the amount of the metabolite in the urine. Some of the genes have now been characterized, and techniques are available to determine the genotype. Important studies suggest that a risk of developing certain cancer forms is related to the capability of metabolizing foreign compounds. Many questions still remain unanswered, thus at this time limiting the use of these potential susceptibility biomarkers in occupational health.
Other inherited traits, such as alpha1-antitrypsin deficiency or glucose-6-phosphate dehydrogenase deficiency, also result in deficient defence mechanisms in the body, thereby causing hypersusceptibility to certain exposures.
Most research related to susceptibility has dealt with genetic predisposition. Other factors play a role as well and have been partly neglected. For example, individuals with a chronic disease may be more sensitive to an occupational exposure. Also, if a disease process or previous exposure to toxic chemicals has caused some subclinical organ damage, then the capacity to withstand a new toxic exposure is likely to be less. Biochemical indicators of organ function may in this case be used as susceptibility biomarkers. Perhaps the best example regarding hypersusceptibility relates to allergic responses. If an individual has become sensitized to a particular exposure, then specific antibodies can be detected in serum. Even if the individual has not become sensitized, other current or past exposures may add to the risk of developing an adverse effect related to an occupational exposure.
A major problem is to determine the joint effect of mixed exposures at work. In addition, personal habits and drug use may result in an increased susceptibility. For example, tobacco smoke usually contains a considerable amount of cadmium. Thus, with occupational exposure to cadmium, a heavy smoker who has accumulated substantial amounts of this metal in the body will be at increased risk of developing cadmium-related kidney disease.
Application in Occupational Health
Biomarkers are extremely useful in toxicological research, and many may be applicable in biological monitoring. Nonetheless, the limitations must also be recognized. Many biomarkers have so far been studied only in laboratory animals. Toxicokinetic patterns in other species may not necessarily reflect the situation in human beings, and extrapolation may require confirmatory studies in human volunteers. Also, account must be taken of individual variations due to genetic or constitutional factors.
In some cases, exposure biomarkers may not at all be feasible (e.g., for chemicals which are short-lived in vivo). Other chemicals may be stored in, or may affect, organs which cannot be accessed by routine procedures, such as the nervous system. The route of exposure may also affect the distribution pattern and therefore also the biomarker measurement and its interpretation. For example, direct exposure of the brain via the olfactory nerve is likely to escape detection by measurement of exposure biomarkers. As to effect biomarkers, many of them are not at all specific, and the change can be due to a variety of causes, including lifestyle factors. Perhaps in particular with the susceptibility biomarkers, interpretation must be very cautious at the moment, as many uncertainties remain about the overall health significance of individual genotypes.
In occupational health, the ideal biomarker should satisfy several requirements. First of all, sample collection and analysis must be simple and reliable. For optimal analytical quality, standardization is needed, but the specific requirements vary considerably. Major areas of concern include: preparation of the in- dividual, sampling procedure and sample handling, and measurement procedure; the latter encompasses technical factors, such as calibration and quality assurance procedures, and individual- related factors, such as education and training of operators.
For documentation of analytical validity and traceability, reference materials should be based on relevant matrices and with appropriate concentrations of toxic substances or relevant metabolites at appropriate levels. For biomarkers to be used for biological monitoring or for diagnostic purposes, the responsible laboratories must have well-documented analytical procedures with defined performance characteristics, and accessible records to allow verification of the results. At the same time, nonetheless, the economics of characterizing and using reference materials to supplement quality assurance procedures in general must be considered. Thus, the achievable quality of results, and the uses to which they are put, have to be balanced against the added costs of quality assurance, including reference materials, manpower and instrumentation.
Another requirement is that the biomarker should be specific, at least under the circumstances of the study, for a particular type of exposure, with a clear-cut relationship to the degree of exposure. Otherwise, the result of the biomarker measurement may be too difficult to interpret. For proper interpretation of the measurement result of an exposure biomarker, the diagnostic validity must be known (i.e., the translation of the biomarker value into the magnitude of possible health risks). In this area, metals serve as a paradigm for biomarker research. Recent research has demonstrated the complexity and subtlety of dose-response relationships, with considerable difficulty in identifying no-effect levels and therefore also in defining tolerable exposures. However, this kind of research has also illustrated the types of investigation and the refinement that are necessary to uncover the relevant information. For most organic compounds, quantitative associations between exposures and the corresponding adverse health effects are not yet available; in many cases, even the primary target organs are not known for sure. In addition, evaluation of toxicity data and biomarker concentrations is often complicated by exposure to mixtures of substances, rather than exposure to a single compound at the time.
Before the biomarker is applied for occupational health purposes, some additional considerations are necessary. First, the biomarker must reflect a subclinical and reversible change only. Second, given that the biomarker results can be interpreted with regard to health risks, then preventive efforts should be available and should be considered realistic in case the biomarker data suggests a need to reduce the exposure. Third, the practical use of the biomarker must be generally regarded as ethically acceptable.
Industrial hygiene measurements may be compared with applicable exposure limits. Likewise, results on exposure biomarkers or effect biomarkers may be compared to biological action limits, sometimes referred to as biological exposure indices. Such limits should be based on the best advice of clinicians and scientists from appropriate disciplines, and responsible administrators as “risk managers” should then take into account relevant ethical, social, cultural and economic factors. The scientific basis should, if possible, include dose-response relationships supplemented by information on variations in susceptibility within the population at risk. In some countries, workers and members of the general public are involved in the standard-setting process and provide important input, particularly when scientific uncertainty is considerable. One of the major uncertainties is how to define an adverse health effect that should be prevented—for example, whether adduct formation as an exposure biomarker by itself represents an adverse effect (i.e., effect biomarker) that should be prevented. Difficult questions are likely to arise when deciding whether it is ethically defensible, for the same compound, to have different limits for adventitious exposure, on the one hand, and occupational exposure, on the other.
The information generated by the use of biomarkers should generally be conveyed to the individuals examined within the physician-patient relationship. Ethical concerns must in particular be considered in connection with highly experimental biomarker analyses that cannot currently be interpreted in detail in terms of actual health risks. For the general population, for example, limited guidance exists at present with regard to interpretation of exposure biomarkers other than the blood-lead concentration. Also of importance is the confidence in the data generated (i.e., whether appropriate sampling has been done, and whether sound quality assurance procedures have been utilized in the laboratory involved). An additional area of special worry relates to individual hypersusceptibility. These issues must be taken into account when providing the feedback from the study.
All sectors of society affected by, or concerned with carrying out, a biomarker study need to be involved in the decision-making process on how to handle the information generated by the study. Specific procedures to prevent or overcome inevitable ethical conflicts should be developed within the legal and social frameworks of the region or country. However, each situation represents a different set of questions and pitfalls, and no single procedure for public involvement can be developed to cover all applications of exposure biomarkers.