Toward a molecular equivalent dose: use of the medaka model in comparative risk assessment

Recent changes in the risk assessment landscape underscore the need to be able to compare the results of toxicity and dose-response testing between a growing list of animal models and, quite possibly, an array of in vitro screening assays. How do we compare test results for a given compound between vastly different species? For example, what dose level in the ambient water of a small fish model would be equivalent to 10 ppm of a given compound in the rat's drinking water? Where do we begin? To initially address these questions, and in order to compare dose-response tests in a standard rodent model with a fish model, we used the concept of molecular dose. Assays that quantify types of DNA damage that are directly relevant to carcinogenesis integrate the factors such as chemical exposure, uptake, distribution, metabolism, etc. that tend to vary so widely between different phyletic levels. We performed parallel exposures in F344 rats and Japanese medaka (Oryzias latipes) to the alkylating hepatocarcinogen, dimethylnitrosamine (DMN). In both models, we measured the DNA adducts 8-hydroxyguanine, N(7)-methylguanine and O(6)-methylguanine in the liver; mutation frequency using lambda cII transgenic medaka and lambda cII transgenic (Big Blue(R)) rats; and early morphological changes in the livers of both models using histopathology and immunohistochemistry. Pulse dose levels in fish were 0, 10, 25, 50, or 100 ppm DMN in the ambient water for 14 days. Since rats are reported to be especially sensitive to DMN, they received 0, 0.1, 1, 5, 10, or 25 ppm DMN in the drinking water for the same time period. While liver DNA adduct concentrations were similar in magnitude, mutant frequencies in the DMN-exposed medaka were up to 20 times higher than in the Big Blue rats. Future work with other compounds will generate a more complete picture of comparative dose response between different phyletic levels and will help guide risk assessors using "alternative" models.

Chronic Mycobacterium marinum infection acts as a tumor promoter in Japanese Medaka (Oryzias latipes)

An accumulating body of research indicates there is an increased cancer risk associated with chronic infections. The genus Mycobacterium contains a number of species, including M. tuberculosis, which mount chronic infections and have been implicated in higher cancer risk. Several non-tuberculosis mycobacterial species, including M. marinum, are known to cause chronic infections in fish and like human tuberculosis, often go undetected. The elevated carcinogenic potential for fish colonies infected with Mycobacterium spp. could have far reaching implications because fish models are widely used to study human diseases. Japanese medaka (Oryzias latipes) is an established laboratory fish model for toxicology, mutagenesis, and carcinogenesis; and produces a chronic tuberculosis-like disease when infected by M. marinum. We examined the role that chronic mycobacterial infections play in cancer risk for medaka. Experimental M. marinum infections of medaka alone did not increase the mutational loads or proliferative lesion incidence in all tissues examined. However, we showed that chronic M. marinum infections increased hepatocellular proliferative lesions in fish also exposed to low doses of the mutagen benzo[a]pyrene. These results indicate that chronic mycobacterial infections of medaka are acting as tumor promoters and thereby suggest increased human risks for cancer promotion in human populations burdened with chronic tuberculosis infections.

The cytotoxicity and genotoxicity of hexavalent chromium in medaka (Oryzias latipes) cells

Chromium is an increasing health concern for aquatic environments, however, the mechanism of chromium toxicity in aquatic species is yet unknown. We used a medaka (Oryzias latipes) fin cell line to investigate the cytotoxicity and genotoxicity of sodium chromate, a soluble form of hexavalent chromium. We used a clonogenic cytotoxicity assay to measure sodium chromate cytotoxicity, gamma-H2A.X immunofluoresence to measure DNA double-strand breaks, and chromosome damage to measure clastogenicity. We found that sodium chromate is cytotoxic to medaka fin cells, with toxicity increasing in a concentration-dependent manner. Treatments of 0.5, 1, 5, 10, 25, 50 and 100 microM sodium chromate caused 100, 103.5, 87.8, 77.5, 40.9, 15 and 2.7% survival, respectively, relative to the control. We visualized DNA double-strand breaks in medaka cells through the formation of gamma-H2A.X foci. Breaks could be detected at concentrations as low as 1 microM. We also found that sodium chromate induces chromosomal aberrations, causing chromatid lesions and exchanges that increase with concentration. Treatments of 0, 1, 5, 10 and 25 microM sodium chromate damaged 10.3, 17, 32.3, 43 and 51.6% of metaphases and induced 13, 23, 44, 69 and 118 total aberrations in 100 metaphases, respectively. These data show that hexavalent chromium is both cytotoxic and genotoxic to fish cells. Our results set the context for future work in the medaka cell culture model and provide important tools for investigating mechanisms of toxicity in aquatic organisms.

Transgenic lambda medaka as a new model for germ cell mutagenesis

To address the need for improved approaches to study mutations transmitted to progeny from mutagen-exposed parents, we evaluated lambda transgenic medaka, a small fish that carries the cII mutation target gene, as a new model for germ cell mutagenesis. Mutations in the cII gene in progeny derived from ethyl-nitrosourea (ENU)-exposed males were readily detected. Frequencies of mutant offspring, proportions of mosaic or whole body mutant offspring, and mutational spectra differed according to germ cell stage exposed to ENU. Postmeiotic germ cells (spermatozoa/late spermatids) generated a higher frequency of mutant offspring (11%) compared to premeiotic germ cells (3.5%). Individuals with cII mutant frequencies (MF) elevated more than threefold above the spontaneous MF (3 x 10(-5)) in the range of 10(-4) to 10(-3) were mosaic mutant offspring, whereas those with MFs approaching 1 x 10(-2) were whole body mutant offspring. Mosaic mutant offspring comprised the majority of mutant offspring derived from postmeiotic germ cells, and unexpectedly, from spermatogonial stem cells. Mutational spectra comprised of two different mutations, but at identical sites were unusual and characteristic of delayed mutations, in which fixation of a second mutation was delayed following fertilization. Delayed mutations and prevalence of mosaic mutant offspring add to growing evidence that implicates germ cells in mediating processes postfertilization that contribute to genomic instability in progeny. This model provides an efficient and sensitive approach to assess germ cell mutations, expands opportunities to increase understanding of fundamental mechanisms of mutagenesis, and provides a means for improved assessment of potential genetic health risks.

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

Accumulation and metabolism of the anticancer flavonoid 5,7-dimethoxyflavone compared to its unmethylated analog chrysin in the Atlantic killifish

The use of dietary flavonoids as potential chemopreventive agents is a concept of increasing interest. Recent findings indicate that methylated flavones have the advantage of increased metabolic stability. One such compound, the naturally-occurring 5,7-dimethoxyflavone (5,7-DMF), has been shown to be a potential chemopreventive agent in human cancer originating from the liver, mouth, esophagus and lung. As bioavailability is a key issue for potential in vivo effects, the tissue accumulation and biliary elimination of 5,7-DMF and its non-methylated analog chrysin were examined in a small fish model (Fundulus heteroclitus). The fish were exposed to 5,7-DMF, chrysin or vehicle control (DMSO<0.01%) in seawater for 8h. Toxicity was not observed at the 5microM exposure level. Tissues and bile were harvested and analyzed by HPLC and LC/MS for quantitation and identification of parent compound and metabolites. 5,7-DMF accumulated 20-fold to 100-fold in all tissues examined, with the highest accumulation in liver and brain, whereas chrysin was barely detectable in any tissues except the liver. The bile of chrysin-exposed fish contained very low concentrations of unchanged chrysin but high concentrations of two glucuronic acid conjugates. In the bile of 5,7-DMF-exposed fish, the parent compound was detectable in significant amounts along with glucuronic acid conjugates of O-demethylated 5,7-DMF. In conclusion, our study demonstrated high tissue accumulation and limited metabolism of 5,7-DMF compared to chrysin in vivo, making this flavone a promising chemopreventive molecule.

Sub-chronic exposure to 1,1-dichloropropene induces frameshift mutations in lambda transgenic medaka

1,1-Dichloropropene (1,1-DCP) is a contaminant present in both ground and surface waters used as sources for drinking water. Structural similarity to several compounds with known mutagenicity and carcinogenicity, and recent demonstration of mutagenicity in vitro, suggest this compound may be similarly mutagenic in vivo. A transgenic fish model, the lamda transgenic medaka, was used to evaluate the potential mutagenicity of this contaminant in vivo following sub-chronic exposure for 6 weeks. Mutant frequencies of the cII target gene (MF) increased six-fold in the livers of fish exposed to the lowest 1,1-DCP exposure concentration (0.44 mg/L, MF = 18.4 x 10(-5), and increased with each treatment, culminating in a 32-fold induction in fish from the highest 1,1-DCP treatment (16.60 mg/L, MF = 96.3 x 10(-5). Mutations recovered from treated fish showed a distinctive mutational spectrum comprised predominantly of +1 frameshift mutations, induced 166-fold above that of untreated animals. The majority of frameshifts were +1 insertions at thiamine and adenine. These results represent the first evidence of mutagenicity of 1,1-DCP in vivo, and of the highly characteristic spectrum of induced mutations dominated by +1 frameshift mutations. Based upon results from previous in vitro studies, the similar role of glutathione S-transferase (GSTT1-1) in the activation of 1,1-DCP to a mutagen in vivo is also suggested. This study further illustrates the utility of the lamda transgenic medaka as a model for identifying and characterizing potential genetic health risks associated with chemical exposures in the environment.

Antimutagenicity of green tea polyphenols in the liver of transgenic medaka

We examined the ability of a mixture of the predominant green tea polyphenolic compounds (GTP) to reduce benzo[a]pyrene (B[a]P)-induced mutations in the cII gene of the lambda transgenic medaka. Fish were treated with 50 ppb B[a]P for 24 hr, followed by exposure to 2 ppm or 10 ppm GTP for 28 days. cII mutations in livers of fish exposed to B[a]P were increased significantly, 2.6-fold above controls. In contrast, the addition of GTP significantly reduced the frequency of cII mutants by 84%, comparable to that of controls. The frequencies of mutations at G:C basepairs, mutations that are highly characteristic of B[a]P exposure, were elevated significantly in treated fish. By comparison, B[a]P-exposed fish also treated with GTP showed reductions in these mutations, demonstrating a protective effect of GTP against B[a]P-induced mutagenesis. The antioxidant mechanism of GTP possibly played an important role in the reduction of B[a]P mutagenicity. These results corroborate findings from rodent models, showing that the protective effects of green tea extend to different species, and suggesting that similar mechanisms of B[a]P mutagenesis and GTP antimutagenesis are shared among the models. These studies illustrate the utility of lambda transgenic medaka for in vivo mutation analyses and suggest that this fish may be a valuable model in chemoprevention studies.

MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone], a drinking-water carcinogen, does not induce mutations in the liver of cII transgenic medaka (Oryzias latipes)

Mutagenicity assays with Salmonella have shown that 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), a drinking-water disinfection by-product, is a potent mutagen, accounting for about one-third of the mutagenic potency/potential of chlorinated drinking water. The ability of MX to induce mutations was investigated in the liver of medaka (Oryzias latipes), a small fish model, utilizing the cII transgenic medaka strain that allows detection of in vivo mutations. Methylazoxymethanol acetate (MAMAc), a carcinogen in medaka, served as a positive control. Fish were exposed to MX at 0, 1, 10, or 30 mg/L for 96 h, whereas the MAMAc exposures were for 2 h at 0, 0.1, 1, or 10 mg/L. Both exposures were conducted under static water conditions and with fasted medaka. Following exposure, fish were returned to regular culture conditions to allow mutation expression for 15 or 40 d for MX or for 15 or 32 d for MAMAc. Mutations were not induced in medaka exposed to MX for 96 h. However, a concentration- and time-dependent increase in mutations was observed from the livers of fish exposed to 1 and 10 mg/L MAMAc. In conclusion, mutation induction was not observed in the livers of cII medaka exposed to MX for 96 h; however, studies are planned to examine mutation induction in the gills and skin to explore the possibility that MX-induced DNA damage occurs primarily in the tissues of initial contact.

Generating new marine cell lines and transgenic species--conference summary

Marine species offer a tremendous diversity of life histories, physiologies, genetics, behaviors, and biologies, reflecting myriad adaptations to the water environment. Historically, marine vertebrates, particularly fish, have played significant roles in a wide range of disciplines, including environmental toxicology, genetics, developmental biology, and physiology, among others. Much still remains to be learned from these animals, and there is a growing need for new marine models. Models for expression of marine animal genes have been limited to heterologous expression systems. While there is still a great deal to gain from heterologous expression systems, the interactions of genes with one another can best be determined in homologous expression systems where appropriate interactions are possible. This has become particularly important with the development of functional genomics in marine models. These homologous gene expression systems will be key to the use of functional genomics for marine animal molecular physiology and toxicology.

Trangenic fish as models in environmental toxicology

Historically, fish have played significant roles in assessing potential risks associated with exposure to chemical contamination in aquatic environments. Considering the contributions of transgenic rodent models to biomedicine, it is reasoned that the development of transgenic fish could enhance the role of fish in environmental toxicology. Application of transgenic fish in environmental studies remains at an early stage, but recent introduction of new models and methods demonstrates progress. Rapid advances are most evident in the area of in vivo mutagenesis using fish carrying transgenes that serve as recoverable mutational targets. These models highlight many advantages afforded by fish as models and illustrate important issues that apply broadly to transgenic fish in environmental toxicology. Development of fish models carrying identical transgenes to those found in rodents is beneficial and has revealed that numerous aspects of in vivo mutagenesis are similar between the two classes of vertebrates. Researchers have revealed that fish exhibit frequencies of spontaneous mutations similar to rodents and respond to mutagen exposure consistent with known mutagenic mechanisms. Results have demonstrated the feasibility of in vivo mutation analyses using transgenic fish and have illustrated their potential value as a comparative animal model. Challenges to development and application of transgenic fish relate to the needs for improved efficiencies in transgenic technology and in aspects of fish husbandry and use. By taking advantage of the valuable and unique attributes of fish as test organisms, it is anticipated that transgenic fish will make significant contributions to studies of environmentally induced diseases.

Investigation of the radioadaptive response in brain and liver of pUR288 lacZ transgenic mice

The radioadaptive response, where a small priming dose of ionizing radiation can lessen the effects of subsequent exposure to a higher radiation challenge dose, was investigated in brain and liver within transgenic mice. Although it is well characterized in models in vitro, current radioadaptive response research has focused on particular cell types (i.e., lymphocytes) and does not provide comparative data for responses of multiple tissues within an organism. Transgenic animals are useful for such comparisons, because the transgene is integrated into all cells in the body. The pUR288 lacZ plasmid-based transgenic mouse model utilizes a plasmid vector allowing highly efficient recovery of mutational targets, including large size-change mutations that result from radiation exposure. Female C57BI/6 pUR288 lacZ mice were exposed to priming doses of 0.075- to 0.375-Gy x-rays over a 3-d period. After 3 wk, they received an acute challenge dose of 2.5-Gy x-rays. Spontaneous mutant frequencies in lacZ were significantly higher in liver than in brain (6.62 x 10(-5) vs. 3.51 x 10(-5)). In the absence of a priming dose, the 2.5-Gy challenge doubled the mutant frequency of both liver and brain (13.38 x 10(-5), and 7.63 x 10(-5) respectively). Priming doses of 0.15, 0.225, and 0.375 Gy significantly reduced (by 40%) the mutagenic effects of the 2.5-Gy challenge in brain. Restriction enzyme analysis of rescued mutant plasmids revealed a decrease in large size-change mutations at the three priming doses in brain. This study demonstrates the utility of this model for the investigation of radiological processes of large size-change mutations, as well as showing a radioadaptive response in brain, but not liver, of mice in vivo.

Bacteriophage lambda and plasmid pUR288 transgenic fish models for detecting in vivo mutations

We adapted transgenic rodent mutation assays based on fish carrying bacteriophage lambda and plasmid pUR288 vectors to address the needs for improved methods to assess health risks from exposure to environmental mutagens and also to establish new animal models to study in vivo mutagenesis. The approach entails separating the vectors from fish genomic DNA and then shuttling them into specialized strains of E. coli bacteria to analyze spontaneous and induced mutations in either lacI and cII or lacZ mutational targets. Fish exhibited low frequencies of spontaneous mutants comparable to the sensitivity of transgenic rodent models. Mutations detected after treating fish with chemical mutagens showed concentration-dependent, tissue-specific, and time-dependent relationships. Spontaneous and induced mutational spectra also were consistent with the specificity of known mutagens, further supporting the utility of transgenic fish for studies of in vivo mutagenesis.

Aquaria fish models of human disease: reports and recommendations from the working groups

On September 21-24, 2000, the National Cancer Institute, Southwest Texas State University, and the Roy and Joan Mitte Foundation sponsored an international conference entitled "Aquaria Fish Models of Human Disease" at Southwest Texas State University (SWT), San Marcos, Texas, USA. Over 100 scientists, representing various fish model systems, participated in four roundtable working groups. We considered the first step in promoting the exciting research with fish models was to unify the efforts within this scientific community towards accomplishing specific goals. With this objective in mind, the following four working groups were convened: (1) fish cancer models: sustenance and enhancement; (2) fish genomics and transgenics: resources and technology; (3) fish pathology: standards for tumor pathology classification; and, (4) resources underpinning aquaria fish research. Each working group was charged with preparing a report of their discussions with recommendations on how researchers and funding agencies might best direct and strengthen research support to ensure a healthy future for such work. Included are the final reports from these working groups, together with a brief summary of the discussions held during the sessions and the consensus recommendations from each group.

Four resource centers for fishes: specifies, stocks, and services

A conference on "Aquaria Fish Models of Human Disease" was held September 20-23, 2000, at Southwest Texas State University, San Marcos, Texas, USA. The meeting was sponsored by the National Cancer Institute (National Institutes of Health), the Roy and Joan Mitte Foundation, and Southwest Texas State University, home of the Xiphophorus Genetic Stock Center. In conjunction with the meeting, the conference organizers asked several participants to describe those components of their research programs that provide services and information to other researchers. This article summarizes their responses.

Detection of mutations in transgenic fish carrying a bacteriophage lambda cII transgene target

To address the dual needs for improved methods to assess potential health risks associated with chemical exposure in aquatic environments and for new models for in vivo mutagenesis studies, we developed transgenic fish that carry multiple copies of a bacteriophage lambda vector that harbors the cII gene as a mutational target. We adapted a forward mutation assay, originally developed for lambda transgenic rodents, to recover cII mutants efficiently from fish genomic DNA by lambda in vitro packaging. After infecting and plating phage on a hfl- bacterial host, cII mutants were detected under selective conditions. We demonstrated that many fundamental features of mutation analyses based on lambda transgenic rodents are shared by transgenic fish. Spontaneous mutant frequencies, ranging from 4.3 x 10(-5) in liver, 2.9 x 10(-5) in whole fish, to 1.8 x 10(-5) in testes, were comparable to ranges in lambda transgenic rodents. Treatment with ethylnitrosourea resulted in concentration-dependent, tissue-specific, and time-dependent mutation inductions consistent with known mechanisms of action. Frequencies of mutants in liver increased insignificantly 5 days after ethylnitrosourea exposure, but increased 3.5-, 5.7- and 6. 7-fold above background at 15, 20, and 30 days, respectively. Mutants were induced 5-fold in testes at 5 days, attaining a peak 10-fold induction 15 days after treatment. Spontaneous and induced mutational spectra in the fish were also consistent with those of lambda transgenic rodent models. Our results demonstrate the feasibility of in vivo mutation analyses using transgenic fish and illustrate the potential value of fish as important comparative animal models.