A mutated EGFR is sufficient to induce malignant melanoma with genetic background-dependent histopathologies

Evidence of epidermal growth factor receptor expression in uveal melanoma: inhibition of epidermal growth factor-mediated signalling by Gefitinib and Cetuximab triggered antibody-dependent cellular cytotoxicity

Despite advances in surgery and radiotherapy of uveal melanoma (UM), many patients develop distant metastases that poorly respond to therapy. Improved therapies for the metastatic disease are therefore urgently needed. Expression of the epidermal growth factor receptor (EGFR), a target of kinase inhibitors and humanised antibodies in use for several cancers, had been reported. Forty-eight human UMs were analysed by expression profiling. Signalling was tested in three EGFR expressing UM cell lines by Western blotting using phosphorylation specific antibodies for EGFR and the downstream mediators AKT (v-akt murine thymoma viral oncogene homolog) and extracellular signal-regulated kinase (ERK). Evidence for signalling in tumours was obtained through the application of a UM-specific EGF-signature. The EGFR specific kinase inhibitor, Gefitinib and the humanised monoclonal antibody, Cetuximab, were tested for their effect on EGFR signalling. Natural killer cell mediated antibody-dependent cellular cytotoxicity (ADCC) and tumour necrosis factor α (TNF-α) release was analysed for Cetuximab. Fourteen of 48 UMs and three of 14 cell lines (over-)express EGFR, at least in part due to trisomy of the EGFR locus on chromosome 7p12. EGFR and the downstream mediator, AKT, are phosphorylated upon stimulation with EGF in EGFR expressing cell lines. EGFR over-expressing tumours but not EGFR negative tumours show an activated EGF-signature. Gefitinib inhibits EGFR and AKT phosphorylation and Cetuximab induces EGFR phosphorylation but inhibits signalling to AKT induced with EGF. Cetuximab triggers natural killer (NK) cells to lyse EGFR+ cell lines and to release TNF-α. EGFR appears suited as a novel molecular drug target for therapy of uveal melanoma.

Differential evolution of duplicated medakafish mitf genes

Gene duplication is a major force of evolution. One whole genome duplication (WGD) event in the fish ancestor generated genome-wide duplicates in all modern species. Coloration and patterning on the animal body surface exhibit enormous diversity, representing a mysterious and ideal system for understanding gene evolution. Surface colors and patterns are determined primarily by pigment cells in the skin and eye. Thus, microphthalmia-associated transcription factor (Mitf) as a master regulator of melanocyte development is excellent for studying the evolution of WGD-derived gene duplicates. Here we report the evolution of mitf duplicate, mitf1 and mitf2, in the fish medaka (Oryzias latipes), which encode medaka co-homologs Mitf1 and Mitf2 of the mouse Mitf. Compared to mitf1, mitf2 exhibits an accelerated sequence divergence and loses melanocytic expression in embryos at critical developmental stages. Compared to a Xiphophorus counterpart, the medaka Mitf2 displayed a reduced activity in activating melanogenic gene expression by reporter assays and RT-PCR analyses. We show that the medaka Mitf2 has the ability to induce melanocyte differentiation in medaka embryonic stem cells but at a remarkably reduced efficiency compared to the Xiphophorus counterpart. Our data suggest differential evolution of the medaka mitf duplicate, with mitf1 adopting conservation and mitf2 employing degeneration, which is different from the duplication-degeneration-complementation proposed as the mechanism to preserve many gene duplicates in zebrafish. Our finding reveals species-specific variations for mitf duplicate evolution, in agreement with enormous diversity of body coloration and patterning.

p53 gene targeting by homologous recombination in fish ES cells

Background

Gene targeting (GT) provides a powerful tool for the generation of precise genetic alterations in embryonic stem (ES) cells to elucidate gene function and create animal models for human diseases. This technology has, however, been limited to mouse and rat. We have previously established ES cell lines and procedures for gene transfer and selection for homologous recombination (HR) events in the fish medaka (Oryzias latipes).

Methodology and Principal Findings

Here we report HR-mediated GT in this organism. We designed a GT vector to disrupt the tumor suppressor gene p53 (also known as tp53). We show that all the three medaka ES cell lines, MES1∼MES3, are highly proficient for HR, as they produced detectable HR without drug selection. Furthermore, the positive-negative selection (PNS) procedure enhanced HR by ∼12 folds. Out of 39 PNS-resistant colonies analyzed, 19 (48.7%) were positive for GT by PCR genotyping. When 11 of the PCR-positive colonies were further analyzed, 6 (54.5%) were found to be bona fide homologous recombinants by Southern blot analysis, sequencing and fluorescent in situ hybridization. This produces a high efficiency of up to 26.6% for p53 GT under PNS conditions. We show that p53 disruption and long-term propagation under drug selection conditions do not compromise the pluripotency, as p53-targeted ES cells retained stable growth, undifferentiated phenotype, pluripotency gene expression profile and differentiation potential in vitro and in vivo.

Conclusions

Our results demonstrate that medaka ES cells are proficient for HR-mediated GT, offering a first model organism of lower vertebrates towards the development of full ES cell-based GT technology.

Establishment of HRAS(G12V) transgenic medaka as a stable tumor model for in vivo screening of anticancer drugs

Most targeted anticancer drugs have been identified by screening at the molecular or cellular level in vitro. However, many compounds selected by such costly and time-consuming screening do not prove effective against tumors in vivo. The development of anticancer drugs would thus be facilitated by the availability of an in vivo screening system based on a multicellular organism. We have now established a transgenic line of the freshwater fish medaka in which melanophores (melanocytes) proliferate in a manner dependent on heat shock–induced signaling by a human RAS oncoprotein. The human HRAS^G12V oncogene was expressed under the control of a melanophore-specific gene promoter in order to allow visualization of tumor growth in live fish maintained in a water tank. The expression of HRAS^G12V was induced as a result of Cre-mediated recombination by exposure of the fish to a temperature of 37°C for 30 min, given that the Cre gene was placed under the control of a medaka heat shock promoter. One of the stable transgenic lines developed abnormal pigment cell proliferation in the eyes and epidermis with 100% penetrance by 6 months postfertilization. Sorafenib, an inhibitor of RAS signaling, was administered to the transgenic fish and was found both to reduce the extent of melanophore proliferation and to improve survival. The transgenic medaka established here thus represents a promising in vivo system with which to screen potential anticancer drugs that target RAS signaling, and this system can readily be adapted for the screening of agents that target other oncogenes.

Hyperactivation of constitutively dimerized oncogenic EGF receptors by autocrine loops

The epidermal growth factor (EGF) receptor (EGFR) has a key role in normal embryonic development, adult tissue homeostasis and many pathological processes, in particular tumour formation. Aberrant EGFR activation occurs in many cancer types, and inhibition of this receptor is a promising anti-tumour strategy. Besides overexpression of the wild-type receptor, mutated oncogenic EGFR variants are often associated with malignant transformation. In human non-small-cell lung cancers, kinase mutants of the EGFR are rather common. Human glioblastoma often express the truncated EGFRvIII version as well as other dimerized and permanently activated mutants of the receptor, which are considered as tumour drivers. Similarly, the mutated and dimerized EGFR variant Xiphophorus melanoma receptor kinase (Xmrk) is causative for the development of malignant pigment cell tumours in medaka and Xiphophorus melanoma models. It is generally believed that oncogenic receptors that are active due to dimerizing mutations are ligand independent. Here, we show that different EGFR variants from fish and human efficiently induce autocrine loops by inducing EGFR ligands such as amphiregulin and HB-EGF. Importantly, the pre-dimerized oncogenic EGFR versions Xmrk from Xiphophorus and human EGFR(C600F), though already active in absence of ligands, respond to ligand stimulation with enhanced oncogenic signalling. In summary, our data show that autocrine or paracrine loops are still acting on pre-dimerized oncogenic EGFRs and contribute to their pro-tumorigenic signalling.

Conserved expression signatures between medaka and human pigment cell tumors

Aberrations in gene expression are a hallmark of cancer cells. Differential tumor-specific transcript levels of single genes or whole sets of genes may be critical for the neoplastic phenotype and important for therapeutic considerations or useful as biomarkers. As an approach to filter out such relevant expression differences from the plethora of changes noted in global expression profiling studies, we searched for changes of gene expression levels that are conserved. Transcriptomes from massive parallel sequencing of different types of melanoma from medaka were generated and compared to microarray datasets from zebrafish and human melanoma. This revealed molecular conservation at various levels between fish models and human tumors providing a useful strategy for identifying expression signatures strongly associated with disease phenotypes and uncovering new melanoma molecules.

Inducible and repressable oncogene-addicted hepatocellular carcinoma in Tet-on xmrk transgenic zebrafish

BACKGROUND & AIMS

Liver cancer, mainly hepatocellular carcinoma, is a major malignancy and currently there are no effective treatment protocols due to insufficient understanding of hepatocarcinogenesis. As a potentially high-throughput and cost-effective experimental model, the zebrafish is increasingly recognized for disease studies. Here, we aim at using the zebrafish to generate a convenient hepatocellular carcinoma model.

METHODS

Using the Tet-on system for liver-specific expression of fish oncogene xmrk, a hyperactive version of epidermal growth factor receptor homolog, we have generated transgenic zebrafish with inducible development of liver cancer.

RESULTS

Liver tumors were rapidly induced with 100% penetrance in both juvenile and adult xmrk transgenic fish. Histological examination indicated that they all showed features of hepatocellular carcinoma. The induced liver tumors regressed rapidly upon inducer withdrawal. During the tumor induction stage, we detected increased cell proliferation and activation of Xmrk downstream targets Erk and Stat5, which were important for liver tumorigenesis as proved by inhibition experiments. When tumors regressed, there were decreased phosphorylated Erk and Stat5 accompanied with an increase in apoptosis.

CONCLUSIONS

Our zebrafish model demonstrates the potential of a hyperactivated epidermal growth factor receptor pathway in initiating heptocarcinogenesis. It provides clear evidence for the requirement of only a single oncogene for HCC initiation and maintenance and is thus a convenient model for further investigation of oncogene addiction and future anti-cancer drug screening.

Expression regulation triggers oncogenicity of xmrk alleles in the Xiphophorus melanoma system

The Xiphophorus melanoma model has gained attention in biomedical research as a genetic model for tumor formation. Melanoma development in interspecific hybrids of Xiphophorus is connected to pigment cell specific overexpression of the mutationally activated receptor tyrosine kinase Xmrk. In purebred fish the oncogenic function of xmrk is suppressed by a so far unknown regulator locus R. To test the hypothesis that R is involved in transcriptional regulation of xmrk and consequently acts upstream of the xmrk signal, we performed a quantitative analysis of xmrk transcript levels in normal and melanoma tissues of different Xiphophorus genotypes carrying either a highly tumorigenic or a non-tumorigenic xmrk allele. Our results demonstrate that expression of the tumorigenic xmrk allele is highly increased in malignant melanomas compared to benign lesions, macromelanophore spots, and healthy skin. Transcription of the non-tumorigenic xmrk allele in pigment cells, in contrast, is not influenced by the presence or absence of R. These findings strongly indicate that differential transcriptional regulation of the xmrk promoter determines the tumorigenic potential of xmrk alleles in the Xiphophorus melanoma system, thereby supporting the hypothesis that R suppresses the oncogenic function of xmrk on the level of transcriptional control.

Genome evolution and meiotic maps by massively parallel DNA sequencing: spotted gar, an outgroup for the teleost genome duplication

Genomic resources for hundreds of species of evolutionary, agricultural, economic, and medical importance are unavailable due to the expense of well-assembled genome sequences and difficulties with multigenerational studies. Teleost fish provide many models for human disease but possess anciently duplicated genomes that sometimes obfuscate connectivity. Genomic information representing a fish lineage that diverged before the teleost genome duplication (TGD) would provide an outgroup for exploring the mechanisms of evolution after whole-genome duplication. We exploited massively parallel DNA sequencing to develop meiotic maps with thrift and speed by genotyping F(1) offspring of a single female and a single male spotted gar (Lepisosteus oculatus) collected directly from nature utilizing only polymorphisms existing in these two wild individuals. Using Stacks, software that automates the calling of genotypes from polymorphisms assayed by Illumina sequencing, we constructed a map containing 8406 markers. RNA-seq on two map-cross larvae provided a reference transcriptome that identified nearly 1000 mapped protein-coding markers and allowed genome-wide analysis of conserved synteny. Results showed that the gar lineage diverged from teleosts before the TGD and its genome is organized more similarly to that of humans than teleosts. Thus, spotted gar provides a critical link between medical models in teleost fish, to which gar is biologically similar, and humans, to which gar is genomically similar. Application of our F(1) dense mapping strategy to species with no prior genome information promises to facilitate comparative genomics and provide a scaffold for ordering the numerous contigs arising from next generation genome sequencing.

CrossQuery: a web tool for easy associative querying of transcriptome data

Enormous amounts of data are being generated by modern methods such as transcriptome or exome sequencing and microarray profiling. Primary analyses such as quality control, normalization, statistics and mapping are highly complex and need to be performed by specialists. Thereafter, results are handed back to biomedical researchers, who are then confronted with complicated data lists. For rather simple tasks like data filtering, sorting and cross-association there is a need for new tools which can be used by non-specialists. Here, we describe CrossQuery, a web tool that enables straight forward, simple syntax queries to be executed on transcriptome sequencing and microarray datasets. We provide deep-sequencing data sets of stem cell lines derived from the model fish Medaka and microarray data of human endothelial cells. In the example datasets provided, mRNA expression levels, gene, transcript and sample identification numbers, GO-terms and gene descriptions can be freely correlated, filtered and sorted. Queries can be saved for later reuse and results can be exported to standard formats that allow copy-and-paste to all widespread data visualization tools such as Microsoft Excel. CrossQuery enables researchers to quickly and freely work with transcriptome and microarray data sets requiring only minimal computer skills. Furthermore, CrossQuery allows growing association of multiple datasets as long as at least one common point of correlated information, such as transcript identification numbers or GO-terms, is shared between samples. For advanced users, the object-oriented plug-in and event-driven code design of both server-side and client-side scripts allow easy addition of new features, data sources and data types.

Generation and characterization of neurogenin1-GFP transgenic medaka with potential for rapid developmental neurotoxicity screening

Fish models such as zebrafish and medaka are increasingly used as alternatives to rodents in developmental and toxicological studies. These developmental and toxicological studies can be facilitated by the use of transgenic reporters that permit the real-time, noninvasive observation of the fish. Here we report the construction and characterization of transgenic medaka lines expressing green fluorescent protein (GFP) under the control of the zebrafish neurogenin 1 (ngn1) gene promoter. Neurogenin (ngn1) is a helix-loop-helix transcription factor expressed in proliferating neuronal progenitor cells early in neuronal differentiation and plays a crucial role in directing neurogenesis. GFP expression was detected from 24 h post-fertilization until hatching, in a spatial pattern consistent with the previously reported zebrafish ngn1 expression. Temporal expression of the transgene parallels the expression profile of the endogenous medaka ngn1 transcript. Further, we demonstrate that embryos from the transgenic line permit the non-destructive, real-time screening of ngn1 promoter-directed GFP expression in a 96-well format, enabling higher throughput studies of developmental neurotoxicants. This strain has been deposited with and maintained by the National BioResource Project and is available on request (http://www.shigen.nig.ac.jp/medaka/strainDetailAction.do?quickSearch=true&strainId=5660).

Role of the growth hormone-IGF-1 axis in cancer

A substantial body of evidence supports a role for the growth hormone (GH)-IGF-1 axis in cancer incidence and progression. This includes epidemiological evidence relating elevated plasma IGF-1 to cancer incidence as well as a lack of cancers in GH/IGF-1 deficiency. Rodent models lacking GH or its receptor are strikingly resistant to the induction of a wide range of cancers, and treatment with the GH antagonist pegvisomant slows tumor progression. While GH receptor expression is elevated in many cancers, autocrine GH is present in several types, and overexpression of autocrine GH can induce cell transformation. While the mechanism of autocrine action is not clear, it does involve both STAT5 and STAT3 activation, and probably nuclear translocation of the GH receptor. Development of a more potent GH receptor antagonist or secretion inhibitor is warranted for cancer therapy.

Generating and analyzing fish models of melanoma

Melanoma is the most deadly form of skin cancer and incidence continues to rise rapidly (Gray-Schopfer et al., 2007). Melanoma develops from melanocytes, the pigmented cells that color our skin, hair, and eyes. Fish also have melanocytes, among other pigment cell types, and the fish and human developmental programme are highly conserved (Kelsh, 2004). The first fish models of melanoma were established in Xiphophorus, and more recently, transgenic melanoma models in zebrafish and medaka have been developed (Meierjohann and Schartl, 2006; Patton et al., 2010; Schartl et al., 2010). In this Chapter, we describe the basic techniques to generate genetic, environmental, and transgenic models of melanoma, discuss diagnoses, and describe standard molecular analysis techniques.

Kita driven expression of oncogenic HRAS leads to early onset and highly penetrant melanoma in zebrafish

BACKGROUND

Melanoma is the most aggressive and lethal form of skin cancer. Because of the increasing incidence and high lethality of melanoma, animal models for continuously observing melanoma formation and progression as well as for testing pharmacological agents are needed.

METHODOLOGY AND PRINCIPAL FINDINGS

Using the combinatorial Gal4-UAS system, we have developed a zebrafish transgenic line that expresses oncogenic HRAS under the kita promoter. Already at 3 days transgenic kita-GFP-RAS larvae show a hyper-pigmentation phenotype as earliest evidence of abnormal melanocyte growth. By 2-4 weeks, masses of transformed melanocytes form in the tail stalk of the majority of kita-GFP-RAS transgenic fish. The adult tumors evident between 1-3 months of age faithfully reproduce the immunological, histological and molecular phenotypes of human melanoma, but on a condensed time-line. Furthermore, they show transplantability, dependence on mitfa expression and do not require additional mutations in tumor suppressors. In contrast to kita expressing melanocyte progenitors that efficiently develop melanoma, mitfa expressing progenitors in a second Gal4-driver line were 4 times less efficient in developing melanoma during the three months observation period.

CONCLUSIONS AND SIGNIFICANCE

This indicates that zebrafish kita promoter is a powerful tool for driving oncogene expression in the right cells and at the right level to induce early onset melanoma in the presence of tumor suppressors. Thus our zebrafish model provides a link between kita expressing melanocyte progenitors and melanoma and offers the advantage of a larval phenotype suitable for large scale drug and genetic modifier screens.

MMP13 mediates cell cycle progression in melanocytes and melanoma cells: in vitro studies of migration and proliferation

Background

Melanoma cells are usually characterized by a strong proliferative potential and efficient invasive migration. Among the multiple molecular changes that are recorded during progression of this disease, aberrant activation of receptor tyrosine kinases (RTK) is often observed. Activation of matrix metalloproteases goes along with RTK activation and usually enhances RTK-driven migration. The purpose of this study was to examine RTK-driven three-dimensional migration of melanocytes and the pro-tumorigenic role of matrix metalloproteases for melanocytes and melanoma cells.

Results

Using experimental melanocyte dedifferentiation as a model for early melanomagenesis we show that an activated EGF receptor variant potentiates migration through three-dimensional fibrillar collagen. EGFR stimulation also resulted in a strong induction of matrix metalloproteases in a MAPK-dependent manner. However, neither MAPK nor MMP activity were required for migration, as the cells migrated in an entirely amoeboid mode. Instead, MMPs fulfilled a function in cell cycle regulation, as their inhibition resulted in strong growth inhibition of melanocytes. The same effect was observed in the human melanoma cell line A375 after stimulation with FCS. Using sh- and siRNA techniques, we could show that MMP13 is the protease responsible for this effect. Along with decreased proliferation, knockdown of MMP13 strongly enhanced pigmentation of melanocytes.

Conclusions

Our data show for the first time that growth stimuli are mediated via MMP13 in melanocytes and melanoma, suggesting an autocrine MMP13-driven loop. Given that MMP13-specific inhibitors are already developed, these results support the evaluation of these inhibitors in the treatment of melanoma.

The zebrafish as a model for cancer

For the last three decades significant parts of national science budgets, and international and private funding worldwide, have been dedicated to cancer research. This has resulted in a number of important scientific findings. Studies in tissue culture have multiplied our knowledge of cancer cell pathophysiology, mechanisms of transformation and strategies of survival of cancer cells, revealing therapeutically exploitable differences to normal cells. Rodent animal models have provided important insights on the developmental biology of cancer cells and on host responses to the transformed cells. However, the rate of death from some malignancies is still high, and the incidence of cancer is increasing in the western hemisphere. Alternative animal models are needed, where cancer cell biology, developmental biology and treatment can be studied in an integrated way. The zebrafish offers a number of features, such as its rapid development, tractable genetics, suitability for in vivo imaging and chemical screening, that make it an attractive model to cancer researchers. This Primer will provide a synopsis of the different cancer models generated by the zebrafish community to date. It will discuss the use of these models to further our understanding of the mechanisms of cancer development, and to promote drug discovery. The article was inspired by a workshop on the topic held in July 2009 in Spoleto, Italy, where a number of new zebrafish cancer models were presented. The overarching goal of the article is aimed at raising the awareness of basic researchers, as well as clinicians, to the versatility of this emerging alternative animal model of cancer.

Genetic and environmental melanoma models in fish

Experimental animal models are extremely valuable for the study of human diseases, especially those with underlying genetic components. The exploitation of various animal models, from fruitflies to mice, has led to major advances in our understanding of the etiologies of many diseases, including cancer. Cutaneous malignant melanoma is a form of cancer for which both environmental insult (i.e., UV) and hereditary predisposition are major causative factors. Fish melanoma models have been used in studies of both spontaneous and induced melanoma formation. Genetic hybrids between platyfish and swordtails, different species of the genus Xiphophorus, have been studied since the 1920s to identify genetic determinants of pigmentation and melanoma formation. Recently, transgenesis has been used to develop zebrafish and medaka models for melanoma research. This review will provide a historical perspective on the use of fish models in melanoma research, and an updated summary of current and prospective studies using these unique experimental systems.

Xmrk in medaka: a new genetic melanoma model

Melanoma is the most lethal form of skin cancer, and its incidence is rising rapidly (Chin et al., 2006; Linos et al., 2009). Highly aggressive, metastatic melanoma is notoriously resistant to chemotherapy, and early detection is critical for surgical excision (Gray-Schopfer et al., 2007). A detailed knowledge of the cellular, molecular, and genetic events during melanoma progression is highly relevant to both diagnosis and the development of new therapies. Animal models, such as the one described in this issue by Schartl and colleagues, are important tools for identifying the genetic and environmental factors that contribute to melanoma development, as well as identifying and testing new therapeutic strategies.

Zebrafish have a competent p53-dependent nucleotide excision repair pathway to resolve ultraviolet B-induced DNA damage in the skin

Ultraviolet (UV) light is a primary environmental risk factor for melanoma, a deadly form of skin cancer derived from the pigmented cells called melanocytes. UVB irradiation causes DNA damage, mainly in the form of pyrimidine dimers (cis-syn cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts), and organisms have developed complex multiprotein repair processes to cope with the DNA damage. Zebrafish is becoming an important model system to study the effects of UV light in animals, in part because the embryos are easily treated with UV irradiation, and the DNA damage repair pathways appear to be conserved in zebrafish and mammals. We are interested in exploring the effects of UV irradiation in young adult zebrafish, so that we can apply them to the study of gene-environment interactions in models of skin cancer. Using the Xiphophorus UV melanoma model as a starting point, we have developed a UV irradiation treatment chamber, and established UV treatment conditions at different ages of development. By translating the Xiphophorus UV treatment methodology to the zebrafish system, we show that the adult zebrafish skin is competent for nucleotide excision DNA damage repair, and that like in mammalian cells, UV treatment promotes phosphorylation of H2AX and a p53-dependent response. These studies provide the groundwork for exploring the role of UV light in melanoma development in zebrafish.