Cloning of the Retinoblastoma cDNA from the Japanese medaka (Oryzias latipes) and preliminary evidence of mutational alterations in chemically-induced retinoblastomas

Susceptibility of polar cod (Boreogadus saida) to a model carcinogen

Studies that aim to characterise the susceptibility of the ecologically relevant and non-model fish polar cod (Boreogadus saida) to model carcinogens are required. Polar cod were exposed under laboratory conditions for six months to control, 0.03 μg BaP/g fish/week and 0.3 μg BaP/g fish/week dietary benzo(a)pyrene (BaP), a reference carcinogen. The concentrations of the 3-OH-BaP bile metabolite and transcriptional responses of genes involved in DNA adduct recognition (xpc), helicase activity (xpd), DNA repair (xpf, rad51) and tumour suppression (tp53) were assessed after 0, 1, 3 and 6 months of exposure, alongside body condition indexes (gonadosomatic index, hepatosomatic index and condition factor). Micronuclei and nuclear abnormalities in blood and spleen, and liver histopathological endpoints were assessed at the end of the experiment. Fish grew steadily over the whole experiment and no mortality was recorded. The concentrations of 3-OH-BaP increased significantly after 1 month of exposure to the highest BaP concentration and after 6 months of exposure to all BaP concentrations showing the biotransformation of the mother compound. Nevertheless, no significant induction of gene transcripts involved in DNA damage repair or tumour suppression were observed at the selected sampling times. These results together with the absence of chromosomal damage in blood and spleen cells, the subtle increase in nuclear abnormalities observed in spleen cells and the low occurrence of foci of cellular alteration suggested that the exposure was below the threshold of observable effects. Taken together, the results showed that polar cod was not susceptible to carcinogenesis using the BaP exposure regime employed herein.

Linking pollution and cancer in aquatic environments: A review

Due to the interconnectedness of aquatic ecosystems through the highly effective marine and atmospheric transport routes, all aquatic ecosystems are potentially vulnerable to pollution. Whilst links between pollution and increased mortality of wild animals have now been firmly established, the next steps should be to focus on specific physiological pathways and pathologies that link pollution to wildlife health deterioration. One of the pollution-induced pathologies that should be at the centre of attention in ecological and evolutionary research is cancer, as anthropogenic contamination has resulted in a rapid increase of oncogenic substances in natural habitats. Whilst wildlife cancer research is an emerging research topic, systematic reviews of the many case studies published over the recent decades are scarce. This research direction would (1) provide a better understanding of the physiological mechanisms connecting anthropogenic pollution to oncogenic processes in non-model organisms (reducing the current bias towards human and lab-animal studies in cancer research), and (2) allow us to better predict the vulnerability of different wild populations to oncogenic contamination. This article combines the information available within the scientific literature about cancer occurrences in aquatic and semi-aquatic species. For the first aim, we use available knowledge from aquatic species to suggest physiological mechanisms that link pollution and cancer, including main metabolic detoxification pathways, oxidative damage effects, infections, and changes to the microbiome. For the second aim, we determine which types of aquatic animals are more vulnerable to pollution-induced cancer, which types of pollution are mainly associated with cancer in aquatic ecosystems, and which types of cancer pollution causes. We also discuss the role of migration in exposing aquatic and semi-aquatic animals to different oncogenic pollutants. Finally, we suggest novel research avenues, including experimental approaches, analysis of the effects of pollutant cocktails and long-term chronic exposure to lower levels of pollutants, and the use of already published databases of gene expression levels in animals from differently polluted habitats.

Western Pacific ALS-PDC: Evidence implicating cycad genotoxins

Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS-PDC) is a disappearing neurodegenerative disorder of apparent environmental origin formerly hyperendemic among Chamorros of Guam-USA, Japanese residents of the Kii Peninsula, Honshu Island, Japan and Auyu-Jakai linguistic groups of Papua-Indonesia on the island of New Guinea. The most plausible etiology is exposure to genotoxins in seed of neurotoxic cycad plants formerly used for food and/or medicine. Primary suspicion falls on methylazoxymethanol (MAM), the aglycone of cycasin and on the non-protein amino acid β-N-methylamino-L-alanine, both of which are metabolized to formaldehyde. Human and animal studies suggest: (a) exposures occurred early in life and sometimes during late fetal brain development, (b) clinical expression of neurodegenerative disease appeared years or decades later, and (c) pathological changes in various tissues indicate the disease was not confined to the CNS. Experimental evidence points to toxic molecular mechanisms involving DNA damage, epigenetic changes, transcriptional mutagenesis, neuronal cell-cycle reactivation and perturbation of the ubiquitin-proteasome system that led to polyproteinopathy and culminated in neuronal degeneration. Lessons learned from research on ALS-PDC include: (a) familial disease may reflect common toxic exposures across generations, (b) primary disease prevention follows cessation of exposure to culpable environmental triggers; and (c) disease latency provides a prolonged period during which to intervene therapeutically. Exposure to genotoxic chemicals ("slow toxins") in the early stages of life should be considered in the search for the etiology of ALS-PDC-related neurodegenerative disorders, including sporadic forms of ALS, progressive supranuclear palsy and Alzheimer's disease.

Genetic alterations and cancer formation in a European flatfish at sites of different contaminant burdens

Fish diseases are an indicator for marine ecosystem health since they provide a biological end-point of historical exposure to stressors. Liver cancer has been used to monitor the effects of exposure to anthropogenic pollution in flatfish for many years. The prevalence of liver cancer can exceed 20%. Despite the high prevalence and the opportunity of using flatfish to study environmentally induced cancer, the genetic and environmental factors driving tumor prevalence across sites are poorly understood. This study aims to define the link between genetic deterioration, liver disease progression, and anthropogenic contaminant exposures in the flatfish dab (Limanda limanda). We assessed genetic changes in a conserved cancer gene, Retinoblastoma (Rb), in association with histological diagnosis of normal, pretumor, and tumor pathologies in the livers of 165 fish from six sites in the North Sea and English Channel. The highest concentrations of metals (especially cadmium) and organic chemicals correlated with the presence of tumor pathology and with defined genetic profiles of the Rb gene, from these sites. Different Rb genetic profiles were found in liver tissue near each tumor phenotype, giving insight into the mechanistic molecular-level cause of the liver pathologies. Different Rb profiles were also found at sampling sites of differing contaminant burdens. Additionally, profiles indicated that histological "normal" fish from Dogger sampling locations possessed Rb profiles associated with pretumor disease. This study highlights an association between Rb and specific contaminants (especially cadmium) in the molecular etiology of dab liver tumorigenesis.

A novel population health approach: Using fish retinoblastoma gene profiles as a surrogate for humans

Retinoblastoma, a tumor suppressor gene, is frequently mutated in diverse types of human tumors. We have previously shown that two types of fish tumor, eye and liver, also possess mutant Rb genes. Our aim is to determine if the Rb allele status is linked to environmentally-induced cancer and whether this information in fish can be used to predict future phenotype. This is a proof-of-concept investigation to elucidate if fish may act as surrogates in assessing pollution-induced tumor incidence and inform regulatory authorities of potential long-term population health consequences. Marine flatfish, Limanda limanda, that display either normal liver histopathology, liver adenoma or liver hepatocellular carcinoma were analysed for the presence of Rb gene alterations. Several Rb alterations were detected in the fish displaying adenoma and carcinoma, and not in the surrounding normal tissue from the same individuals. The profile is similar to that reported in humans in that they spread across the gene, particularly exons 8-23, and a functionally important region of the protein. This Rb allele data was then used to build statistical classifier sets, linking Rb status with tumor pathology. Further flatfish caught from coastal-water areas of differing contaminant burden around the UK were subsequently analysed for the presence of Rb alterations. Using novel pattern matching statistics of the classifier sets compared with the coastal samples, the coastal fish were considered more similar to the characterised disease phenotype than the normal phenotype. Preliminary data suggests that using a statistical approach, based on classifying sets of histopathologically-defined tumor states, makes it possible to predict the phenotype of wild fish based on the status of the Rb allele. Since the Rb gene is orthologous, fish populations could act as surrogates for human populations in an eco-epidemiological investigation of the combined roles of genetics and environmental exposures in the tumorigenesis process.

Regulation of CDKN2A/B and Retinoblastoma genes in Xiphophorus melanoma

Xiphophorus interspecies hybrids provide several well-characterized genetic models of melanoma susceptibility. The Xiphophorus CDKN2A/B gene, homologous to mammalian CDKN2A/B cyclin-dependent kinase inhibitors (p16 and p15), is a candidate tumor susceptibility gene in these models. Using real-time PCR and Western blot analysis, we analyzed expression of CDKN2A/B in spontaneous and UV-induced primary melanomas from individual backcross hybrid fish. We found that CDKN2A/B mRNA is highly expressed in melanomas (18-fold), relative to other fish tissues. Expression is also elevated, to a lesser extent (9.5-fold), in melanized skin from tumor-bearing fish. However, quantitative levels of CDKN2A/B mRNA in tumors varied considerably and positively correlated with expression of the Xmrk oncogene, suggesting possible functional interaction between Xmrk and CDKN2A/B expression. As a homolog corresponding to members of the mammalian CDKN2 family which regulate cell cycle progression at the G1 checkpoint, the CDKN2A/B p13 protein is a putative regulator of the G1 checkpoint apparatus in Xiphophorus. Since CDKN2A is often observed to be inversely regulated compared to RB in some human tumors, and is capable of transcriptionally regulating RB in human ovarian tumors, we cloned the Xiphophorus maculatus RB cDNA and analyzed RB expression by real-time PCR and Western blot analysis in the fish melanomas. These experiments were designed to ascertain whether CDKN2A/B and RB expression were inversely correlated. Our results indicate that RB mRNA was consistently expressed at only a 2-fold higher level in both tumors and melanized skin than in muscle. Qualitatively similar results were obtained for protein expression. These results collectively suggest that (i) Xmrk and CDKN2A/B may be co-regulated at the transcriptional level, and (ii) there is little, if any, alteration of RB expression in Xiphophorus melanomas.

Isolation of the retinoblastoma cDNA from the marine flatfish dab (Limanda limanda) and evidence of mutational alterations in liver tumors

We have isolated a dab (Limanda limanda) homologue of the human retinoblastoma (Rb) tumor suppressor gene. The L. limanda partial Rb cDNA encodes a partial predicted protein of 753 amino acids. DNA sequence analysis with other vertebrate Rb sequences demonstrates that the L. limanda Rb cDNA is highly conserved in regions of functional importance. The sequence reported herein, combined with the high degree of conservation observed in critical domains, has also facilitated an investigation of the molecular etiology of environmentally induced liver tumor samples in a feral fish species. Mutational alterations were detected in liver adenoma samples, also in apparently "normal" regions of liver samples dissected from fish displaying adenoma, but not in normal liver samples from otherwise healthy feral fish. These results are the first reporting the appearance of Rb mutations in wild-caught fish and suggest that the molecular etiology of fish cancer appears to involve Rb-implicated tumorigenesis. The ecotoxicological relevance of the Rb mutations in feral fish liver tumors, in terms of future genome instability and possible development of a genotoxicity biomarker, is discussed.

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.

Structure, expression and activation of fish ras genes

Ras genes encode proteins that play a central role in cell growth signaling cascades. The fish ras genes characterized to date, have a high degree of nucleotide sequence and deduced amino acid similarity with the mammalian ras gene counterparts. A large proportion and wide variety of mammalian tumors possess mutant forms of ras. In such cases, the localization of ras mutations has been restricted to exons I and II, and to codons 12, 13 and 61. Experimental exposure of fish to a range of genotoxic compounds has similarly led to the production of a ras mutational profile for selected species. The inducing compound, tissue investigated and the fish species studied affect the ras mutational spectrum and incidence observed, despite the apparent conserved sequence homology. Furthermore, the fish ras mutational profile differs from that observed in rodent models, including a novel codon (16) mutation. The role of ras genes in tumor formation in feral fish has been investigated using several species collected from areas of high hydrocarbon contamination. Tomcod (Microgadus tomcod), winter flounder (Pseudopleuronectes americanus) and dragonet (Callionymus lyra) liver samples display evidence of ras gene mutations, though for the latter species the codon affected is not characteristic of ras gene mutational profiles. English sole (Pleuronectes vetulus) and European flounder (Platichthys flesus) liver tumor samples so far examined, on the other hand, do not display ras gene mutations. Thus, the pattern and incidence of ras gene mutations in environmentally-induced tumors also appear to be species specific. In determining the basis of both the species susceptibility observed in the field and species differences in effects of laboratory controlled exposures, the interaction of fish ras genes with other components of the cell growth signaling cascade (such as protein kinase C, additional oncogenes and tumor suppressor genes) are discussed. The effect of promoting agents following contaminant-induced initiation could similarly provide answers in unraveling the question of species susceptibility.

Isolation and characterization of the retinoblastoma protein from fish

The retinoblastoma (Rb) gene represents the first tumor suppressor gene characterized. The encoded protein, pRb, plays a crucial role in cell cycle control, preventing malignant cell proliferation. Recently, homologues of the Rb gene have been isolated in fish and the pocket domain, which is central to Rb function, was conserved. In our studies, using coelocanth (Latimeria chalumnae), rainbow trout (Oncorhynchus mykiss), medaka (Oryzias latipes) and English sole (Parophrys vetulus), we have developed a simple protocol for the isolation of the Rb tumor suppressor protein and determined its' tissue and cellular localization. Fish Rb proteins display apparent molecular weights in the range of 100-110 kDa, similar to the human pRb. The protein was detected in all tissues examined, consistent with the proteins' universal role in cellular signalling. An interesting pattern of immunoreactive bands was detected in each of the cells' two main compartments, suggesting differential proteolysis. Immuno-analysis of the pRb in trout liver tumor material revealed an additional Rb reactive product that was absent in normal liver cell extracts.