Genetic toxicity assessment is the evaluation of agents for their ability to induce any of three general types of changes (mutations) in the genetic material (DNA): gene, chromosomal and genomic. In organisms such as humans, the genes are composed of DNA, which consists of individual units called nucleotide bases. The genes are arranged in discrete physical structures called chromosomes. Genotoxicity can result in significant and irreversible effects upon human health. Genotoxic damage is a critical step in the induction of cancer and it can also be involved in the induction of birth defects and foetal death. The three classes of mutations mentioned above can occur within either of the two types of tissues possessed by organisms such as humans: sperm or eggs (germ cells) and the remaining tissue (somatic cells).
Assays that measure gene mutation are those that detect the substitution, addition or deletion of nucleotides within a gene. Assays that measure chromosomal mutation are those that detect breaks or chromosomal rearrangements involving one or more chromosomes. Assays that measure genomic mutation are those that detect changes in the number of chromosomes, a condition called aneuploidy. Genetic toxicity assessment has changed considerably since the development by Herman Muller in 1927 of the first assay to detect genotoxic (mutagenic) agents. Since then, more than 200 assays have been developed that measure mutations in DNA; however, fewer than ten assays are used commonly today for genetic toxicity assessment. This article reviews these assays, describes what they measure, and explores the role of these assays in toxicity assessment.
Identification of Cancer HazardsPrior to the Development of the Fieldof Genetic Toxicology
Genetic toxicology has become an integral part of the overall risk assessment process and has gained in stature in recent times as a reliable predictor for carcinogenic activity. However, prior to the development of genetic toxicology (before 1970), other methods were and are still being used to identify potential cancer hazards to humans. There are six major categories of methods currently used for identifying human cancer risks: epidemiological studies, long-term in vivo bioassays, mid-term in vivo bioassays, short-term in vivo and in vitro bioassays, artificial intelligence (structure-activity), and mechanism-based inference.
Table 1. Advantages and disadvantages of current methods for identifying human cancer risks
|Epidemiological studies||(1) humans are ultimate indicators of disease;
(2) evaluate sensitive or susceptible populations;
(3) occupational exposure cohorts; (4) environmental sentinel alerts
|(1) generally retrospective (death certificates, recall biases, etc.); (2) insensitive, costly, lengthy; (3) reliable exposure data sometimes unavailable or difficult to obtain; (4) combined, multiple and complex exposures; lack of appropriate control cohorts; (5) experiments on humans not done; (6) cancer detection, not prevention|
|Long-term in vivo bioassays||(1) prospective and retrospective (validation) evaluations; (2) excellent correlation with identified human carcinogens; (3) exposure levels and conditions known; (4) identifies chemical toxicity and carcinogenicity effects; (5) results obtained relatively quickly; (6) qualitative comparisons among chemical classes; (7) integrative and interactive biologic systems related closely to humans||(1) rarely replicated, resource intensive; (3) limited facilities suitable for such experiments; (4) species extrapolation debate; (5) exposures used are often at levels far in excess of those experienced by humans; (6) single-chemical exposure does not mimic human exposures, which are generally to multiple chemicals simultaneously|
|Mid- and short-term in vivo and in vitro bioassays||(1) more rapid and less expensive than other assays; (2) large samples that are easily replicated;
(3) biologically meaningful end points are measured (mutation, etc.); (4) can be used as screening assays to select chemicals for long-term bioassays
|(1) in vitro not fully predictive of in vivo; (2) usually organism or organ specific; (3) potencies not comparable to whole animals or humans|
|Chemical structure–biological activity associations||(1) relatively easy, rapid, and inexpensive; (2) reliable for certain chemical classes (e.g., nitrosamines and benzidine dyes); (3) developed from biological data but not dependent on additional biological experimentation||(1) not “biological”; (2) many exceptions to formulated rules; (3) retrospective and rarely (but becoming) prospective|
|Mechanism-based inferences||(1) reasonably accurate for certain classes of chemicals; (2) permits refinements of hypotheses; (3) can orient risk assessments to sensitive populations||(1) mechanisms of chemical carcinogenesis undefined, multiple, and likely chemical or class specific; (2) may fail to highlight exceptions to general mechanisms|
Rationale and Conceptual Basisfor Genetic Toxicology Assays
Although the exact types and numbers of assays used for genetic toxicity assessment are constantly evolving and vary from country to country, the most common ones include assays for (1) gene mutation in bacteria and/or cultured mammalian cells and (2) chromosomal mutation in cultured mammalian cells and/or bone marrow within living mice. Some of the assays within this second category can also detect aneuploidy. Although these assays do not detect mutations in germ cells, they are used primarily because of the extra cost and complexity of performing germ-cell assays. Nonetheless, germ-cell assays in mice are used when information about germ-cell effects is desired.
Systematic studies over a 25-year period (1970-1995), especially at the US National Toxicology Program in North Carolina, have resulted in the use of a discrete number of assays for detecting the mutagenic activity of agents. The rationale for evaluating the usefulness of the assays was based on their ability to detect agents that cause cancer in rodents and that are suspected of causing cancer in humans (i.e., carcinogens). This is because studies during the past several decades have indicated that cancer cells contain mutations in certain genes and that many carcinogens are also mutagens. Thus, cancer cells are viewed as containing somatic-cell mutations, and carcinogenesis is viewed as a type of somatic-cell mutagenesis.
The genetic toxicity assays used most commonly today have been selected not only because of their large database, relatively low cost, and ease of performance, but because they have been shown to detect many rodent and, presumptively, human carcinogens. Consequently, genetic toxicity assays are used to predict the potential carcinogenicity of agents.
An important conceptual and practical development in the field of genetic toxicology was the recognition that many carcinogens were modified by enzymes within the body, creating altered forms (metabolites) that were frequently the ultimate carcinogenic and mutagenic form of the parent chemical. To duplicate this metabolism in a petri dish, Heinrich Malling showed that the inclusion of a preparation from rodent liver contained many of the enzymes necessary to perform this metabolic conversion or activation. Thus, many genetic toxicity assays performed in dishes or tubes (in vitro) employ the addition of similar enzyme preparations. Simple preparations are called S9 mix, and purified preparations are called microsomes. Some bacterial and mammalian cells have now been genetically engineered to contain some of the genes from rodents or humans that produce these enzymes, reducing the need to add S9 mix or microsomes.
Genetic Toxicology Assays and Techniques
The primary bacterial systems used for genetic toxicity screening are the Salmonella (Ames) mutagenicity assay and, to a much lesser extent, strain WP2 of Escherichia coli. Studies in the mid-1980s indicated that the use of only two strains of the Salmonella system (TA98 and TA100) were sufficient to detect approximately 90% of the known Salmonella mutagens. Thus, these two strains are used for most screening purposes; however, various other strains are available for more extensive testing.
These assays are performed in a variety of ways, but two general procedures are the plate-incorporation and liquid-suspension assays. In the plate-incorporation assay, the cells, the test chemical and (when desired) the S9 are added together into a liquefied agar and poured onto the surface of an agar petri plate. The top agar hardens within a few minutes, and the plates are incubated for two to three days, after which time mutant cells have grown to form visually detectable clusters of cells called colonies, which are then counted. The agar medium contains selective agents or is composed of ingredients such that only the newly mutated cells will grow. The liquid-incubation assay is similar, except the cells, test agent, and S9 are incubated together in liquid that does not contain liquefied agar, and then the cells are washed free of the test agent and S9 and seeded onto the agar.
Mutations in cultured mammalian cells are detected primarily in one of two genes: hprt and tk. Similar to the bacterial assays, mammalian cell lines (developed from rodent or human cells) are exposed to the test agent in plastic culture dishes or tubes and then are seeded into culture dishes that contain medium with a selective agent that permits only mutant cells to grow. The assays used for this purpose include the CHO/HPRT, the TK6, and the mouse lymphoma L5178Y/TK+/- assays. Other cell lines containing various DNA repair mutations as well as containing some human genes involved in metabolism are also used. These systems permit the recovery of mutations within the gene (gene mutation) as well as mutations involving regions of the chromosome flanking the gene (chromosomal mutation). However, this latter type of mutation is recovered to a much greater extent by the tk gene systems than by the hprt gene systems due to the location of the tk gene.
Similar to the liquid-incubation assay for bacterial mutagenicity, mammalian cell mutagenicity assays generally involve the exposure of the cells in culture dishes or tubes in the presence of the test agent and S9 for several hours. The cells are then washed, cultured for several more days to allow the normal (wild-type) gene products to be degraded and the newly mutant gene products to be expressed and accumulate, and then they are seeded into medium containing a selective agent that permits only the mutant cells to grow. Like the bacterial assays, the mutant cells grow into visually detectable colonies that are then counted.
Chromosomal mutation is identified primarily by cytogenetic assays, which involve exposing rodents and/or rodent or human cells in culture dishes to a test chemical, allowing one or more cell divisions to occur, staining the chromosomes, and then visually examining the chromosomes through a microscope to detect alterations in the structure or number of chromosomes. Although a variety of endpoints can be examined, the two that are currently accepted by regulatory agencies as being the most meaningful are chromosomal aberrations and a subcategory called micronuclei.
Considerable training and expertise are required to score cells for the presence of chromosomal aberrations, making this a costly procedure in terms of time and money. In contrast, micronuclei require little training, and their detection can be automated. Micronuclei appear as small dots within the cell that are distinct from the nucleus, which contains the chromosomes. Micronuclei result from either chromosome breakage or from aneuploidy. Because of the ease of scoring micronuclei compared to chromosomal aberrations, and because recent studies indicate that agents that induce chromosomal aberrations in the bone marrow of living mice generally induce micronuclei in this tissue, micronuclei are now commonly measured as an indication of the ability of an agent to induce chromosomal mutation.
Although germ-cell assays are used far less frequently than the other assays described above, they are indispensable in determining whether an agent poses a risk to the germ cells, mutations in which can lead to health effects in succeeding generations. The most commonly used germ-cell assays are in mice, and involve systems that detect (1) heritable translocations (exchanges) among chromosomes (heritable translocation assay), (2) gene or chromosomal mutations involving specific genes (visible or biochemical specific-locus assays), and (3) mutations that affect viability (dominant lethal assay). As with the somatic-cell assays, the working assumption with the germ-cell assays is that agents positive in these assays are presumed to be potential human germ-cell mutagens.
Current Status and Future Prospects
Recent studies have indicated that only three pieces of information were necessary to detect approximately 90% of a set of 41 rodent carcinogens (i.e., presumptive human carcinogens and somatic-cell mutagens). These included (1) knowledge of the chemical structure of agent, especially if it contains electrophilic moieties (see section on structure-activity relationships); (2) Salmonella mutagenicity data; and (3) data from a 90-day chronic toxicity assay in rodents (mice and rats). Indeed, essentially all of the IARC-declared human carcinogens are detectable as mutagens using just the Salmonella assay and the mouse-bone marrow micronucleus assay. The use of these mutagenicity assays for detecting potential human carcinogens is supported further by the finding that most human carcinogens are carcinogenic in both rats and mice (trans-species carcinogens) and that most trans- species carcinogens are mutagenic in Salmonella and/or induce micronuclei in mouse bone marrow.
With advances in DNA technology, the human genome project, and an improved understanding of the role of mutation in cancer, new genotoxicity assays are being developed that will likely be incorporated into standard screening procedures. Among these are the use of transgenic cells and rodents. Transgenic systems are those in which a gene from another species has been introduced into a cell or organism. For example, transgenic mice are now in experimental use that permit the detection of mutation in any organ or tissue of the animal, based on the introduction of a bacterial gene into the mouse. Bacterial cells, such as Salmonella, and mammalian cells (including human cell lines) are now available that contain genes involved in the metabolism of carcinogenic/mutagenic agents, such as the P450 genes. Molecular analysis of the actual mutations induced in the trans-gene within transgenic rodents, or within native genes such as hprt, or the target genes within Salmonella can now be performed, so that the exact nature of the mutations induced by the chemicals can be determined, providing insights into the mechanism of action of the chemical and allowing comparisons to mutations in humans presumptively exposed to the agent.
Molecular advances in cytogenetics now permit more detailed evaluation of chromosomal mutations. These include the use of probes (small pieces of DNA) that attach (hybridize) to specific genes. Rearrangements of genes on the chromosome can then be revealed by the altered location of the probes, which are fluorescent and easily visualized as colored sectors on the chromosomes. The single-cell gel electrophoresis assay for DNA breakage (commonly called the “comet” assay) permits the detection of DNA breaks within single cells and may become an extremely useful tool in combination with cytogenetic techniques for detecting chromosomal damage.
After many years of use and the generation of a large and systematically developed database, genetic toxicity assessment can now be done with just a few assays for relatively small cost in a short period of time (a few weeks). The data produced can be used to predict the ability of an agent to be a rodent and, presumptively, human carcinogen/somatic-cell mutagen. Such an ability makes it possible to limit the introduction into the environment of mutagenic and carcinogenic agents and to develop alternative, nonmutagenic agents. Future studies should lead to even better methods with greater predictivity than the current assays.