BRAF Mutations Are Sufficient to Promote Nevi Formation and Cooperate with p53 in the Genesis of Melanoma

Defining cancer stem cells by xenotransplantation in zebrafish

The zebrafish (Danio rerio) has become an increasingly utilized and relevant model organism in the study of cancer. The use of transgenic and reverse genetic approaches has yielded several strains that model a variety of human neoplasms. In addition to modeling human disease, these strains provide a platform for the analysis of tumor stem cells. Here we describe the basic technique for the isolation and transplantation of tumor tissue in the zebrafish. This technique was designed to study metastasis and invasive potential of zebrafish tumor cells. Additionally, the basic protocol can be modified in order to describe cancer stem cell characteristics, including proliferative capacity, self-renewal, and the minimum number of tumor cells required for engraftment.

[Inactivation of failsafe programs by Twist oncoproteins and tumor progression]

Multicellular organisms have developed innate defense mechanisms to prevent the expansion of abnormal cells with significant proliferative potential. The two major safeguard mechanisms are premature senescence, which is characterized by definitive cell cycle arrest, and apoptosis, the most common form of programmed cell death. In normal and premalignant cells, the control of these processes is coupled to the regulation of cell proliferation, mainly through the p16 (Ink4A) -Rb and ARF-p53 intracellular signaling pathways. Hence, in benign tumors, aberrant mitogenic activity is counterbalanced by the induction of these oncosuppressive pathways, leading to either apoptosis or senescence which both limit tumor outgrowth. Progression towards malignant and potentially metastatic tumors requires the inhibition of these failsafe programs. Based on our work on Twist oncoproteins, we propose a presentation of recent data on cellular mechanisms by which cancer cells override the surveillance machinery and escape senescence and apoptosis, and we will describe the biological impact of this process on tumor metastasis.

Melanoma genetics and therapeutic approaches in the 21st century: moving from the benchside to the bedside

Metastatic melanoma is notoriously one of the most difficult cancers to treat. Although many therapeutic regimens have been tested, very few achieve response rates greater than 25%. Given the rising incidence of melanoma and the paucity of effective treatments, there is much hope and excitement in leveraging recent genetic and molecular insights for therapeutic advantage. Over the past 30 years, elegant studies by many groups have helped decipher the complex genetic networks involved in melanoma proliferation, progression and survival, as well as several genes involved in melanocyte development and survival. Many of these oncogenic loci and pathways have become crucial targets for pharmacological development. In this article we review: (1) our current understanding of melanoma genetics within the context of signaling networks; (2) targeted therapies, including an extensive discussion of promising agents that act in the Bcl-2 signaling network; (3) future areas of research.

Oxidative stress plays a critical role in inactivating mutant BRAF by geldanamycin derivatives

The geldanamycin derivatives 17-allylamino-17-demethoxygeldanamycin (17-AAG) and 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) are promising chemotherapeutic drugs that inhibit heat shock protein 90 (HSP90) function. Previous studies have shown that 17-AAG/DMAG treatment induces the degradation of mutant BRAF (V600E) and inhibits the activation of mitogen-activated protein/extracellular signal-regulated kinase 1/2 (MEK1/2). We have found, however, that HSP90 inhibition alone is not sufficient for efficient BRAF(V600E) degradation in some cells. HSP90 inhibitors structurally unrelated to geldanamycin, radicicol and novobiocin, while inducing the degradation of the HSP90 client protein RAF-1 fail to induce BRAF(V600E) degradation or inhibit MEK1/2 activation in HT29 human colon cancer cells. Moreover, after treatment with 17-DMAG, the kinase activity of residual, undegraded BRAF(V600E) was also lost. Incubation of cells with a reactive oxygen species (ROS) scavenger, N-acetyl cysteine, partially restored kinase activity and also partially prevented BRAF(V600E) degradation due to 17-DMAG treatment. Conversely, treatment with the ROS producing drug menadione clearly inhibited MEK1/2 and reduced BRAF(V600E). These results suggest that in addition to direct inhibition of HSP90, the antitumor effect of geldanamycin and its derivatives is also mediated though the production of ROS, which may directly inactivate tumorigenic mutant BRAF(V600E).

Co-injection strategies to modify radiation sensitivity and tumor initiation in transgenic Zebrafish

The zebrafish has emerged as a powerful genetic model of cancer, but has been limited by the use of stable transgenic approaches to induce disease. Here, a co-injection strategy is described that capitalizes on both the numbers of embryos that can be microinjected and the ability of transgenes to segregate together and exert synergistic effects in forming tumors. Using this mosaic transgenic approach, gene pathways involved in tumor initiation and radiation sensitivity have been identified.

Akt3 and mutant V600E B-Raf cooperate to promote early melanoma development

B-Raf is the most mutated gene in melanoma; however, the mechanism through which it promotes early melanomas remains uncertain. Most nevi contain activated (V600E)B-Raf but few develop into melanoma, and expression in melanocytes is inhibitory with low protein levels present in surviving cells, suggesting unknown cooperative oncogenic events are necessary for melanoma development. Because many melanomas have (V600E)B-Raf and active Akt3, it is possible that these proteins cooperatively facilitate melanocyte transformation. In this study, Akt3 is shown to phosphorylate (V600E)B-Raf to lower its activity as well as that of the downstream mitogen-activated protein kinase (MAPK) pathway to levels promoting early melanoma development. Expression of active Akt3 in early melanoma cells containing (V600E)B-Raf reduced MAPK signaling and promoted anchorage-independent growth. Furthermore, expression of both (V600E)B-Raf and active Akt3 in melanocytes promoted a transformed phenotype. Mechanistically, aberrant Akt3 activity in early melanomas serves to phosphorylate Ser(364) and Ser(428) on (V600E)B-Raf to reduce activity of (V600E)B-Raf to levels that promote rather than inhibit proliferation, which aids melanocytic transformation. Inhibition of (V600E)B-Raf or Akt3 in advanced melanoma cells in which both pathways were active reduced anchorage-independent growth and tumor development in a cooperatively acting manner. Inhibition of Akt3 alone in these cells led to increased MAPK signaling. In summary, these results suggest that activating B-Raf mutations initially promote nevi development, but the resulting high, intense activation of the MAPK pathway inhibits further tumor progression requiring Akt3 activation to bypass this barrier and aid melanoma development.

Oncogenic KRAS induces progenitor cell expansion and malignant transformation in zebrafish exocrine pancreas

BACKGROUND & AIMS

Although the cell of origin for pancreatic cancer remains unknown, prior studies have suggested that pancreatic neoplasia may be initiated in progenitor-like cells. To examine the effects of oncogene activation within the pancreatic progenitor pool, we devised a system for real-time visualization of both normal and oncogenic KRAS-expressing pancreatic progenitor cells in living zebrafish embryos.

METHODS

By using BAC transgenes under the regulation of ptf1a regulatory elements, we expressed either extended green fluorescent protein (eGFP) alone or eGFP fused to oncogenic KRAS in developing zebrafish pancreas.

RESULTS

After their initial specification, normal eGFP-labeled pancreatic progenitor cells were observed to actively migrate away from the forming endodermal gut tube, and subsequently underwent characteristic exocrine differentiation. In contrast, pancreatic progenitor cells expressing oncogenic KRAS underwent normal specification and migration, but failed to differentiate. This block in differentiation resulted in the abnormal persistence of an undifferentiated progenitor pool, and was associated with the subsequent formation of invasive pancreatic cancer. These tumors showed several features in common with the human disease, including evidence of abnormal Hedgehog pathway activation.

CONCLUSIONS

These results provide a unique view of the tumor-initiating effects of oncogenic KRAS in a living vertebrate organism, and suggest that zebrafish models of pancreatic cancer may prove useful in advancing our understanding of the human disease.

Zebrafish as a cancer model

The zebrafish has developed into an important model organism for biomedical research over the last decades. Although the main focus of zebrafish research has traditionally been on developmental biology, keeping and observing zebrafish in the lab led to the identification of diseases similar to humans, such as cancer, which subsequently became a subject for study. As a result, about 50 articles have been published since 2000 in which zebrafish were used as a cancer model. Strategies used include carcinogenic treatments, transplantation of mammalian cancer cells, forward genetic screens for proliferation or genomic instability, reverse genetic target-selected mutagenesis to inactivate known tumor suppressor genes, and the generation of transgenics to express human oncogenes. Zebrafish have been found to develop almost any tumor type known from human, with similar morphology and, according to gene expression array studies, comparable signaling pathways. However, tumor incidences are relatively low, albeit highly comparable between different mutants, and tumors develop late in life. In addition, tumor spectra are sometimes different when compared with mice and humans. Nevertheless, the zebrafish model has created its own niche in cancer research, complementing existing models with its specific experimental advantages and characteristics. Examples of these are imaging of tumor progression in living fish by fluorescence, treatment with chemical compounds, and screening possibilities not only for chemical modifiers but also for genetic enhancers and suppressors. This review aims to provide a comprehensive overview of the state of the art of zebrafish as a model in cancer research. (Mol Cancer Res 2008;6(5):685-94).

BRAF mutation associated with other genetic events identifies a subset of aggressive papillary thyroid carcinoma

PURPOSE

BRAF(V600E) mutation represents the most common oncogenic event in sporadic papillary thyroid cancer (PTC). There are, however, significant discrepancies regarding the overall frequency, its prevalence in PTC-variants, and its relationship with clinico-pathological parameters of poor outcome. Moreover, the impact of BRAF(V600E) mutants on tumour-related patient's death has not been evaluated.

DESIGN

We analysed, by PCR-SSCP and/or PCR-direct sequencing, exons 8, 10, 11 and 15 of BRAF in 113 tumour samples from 49 PTC-patients. Matched lymph node metastases and/or distant metastases (DMs) were screened in 35 patients. Focal changes in the growth pattern or microscopic grade within the primary tumour (Pt) or the metastases were separately genotyped. Mutations at H-, K-, N-ras and PIK3CA exons 9 and 20 were also investigated. For comparison with PTC cases, the BRAF and Ras mutational status was evaluated in 89 specimens obtained from 24 poorly differentiated thyroid carcinomas (PDCs) and 36 anaplastic thyroid carcinomas (ATCs).

RESULTS

BRAF(V600E) was found in 13/16 classical PTCs (CL-PTCs), 6/17 follicular variant PTCs (FV-PTCs) and 8/16 mixed (papillary/follicular) PTCs (Mx-PTCs), being significantly associated with CL-PTCs (P = 0.015). BRAF(V600E) segregated with metastatic PTC-cells in 43% of the patients, but only one DM disclosed the mutation. PTC-tumours featuring concurrent less-differentiated foci were BRAF wild-type in both components. Noteworthy, the frequency of BRAF mutations among PDCs and ATCs resulted considerably lower (16.6% and 25%, respectively) than in PTCs (55%). The prevalence of Ras mutations among PDCs and ATCs (46% and 36%, respectively) was, however, much higher than in PTCs (14%). Five (71%) of the patients who died of PTC displayed somatic mutations. Four of them had other gene alteration associated with BRAF(V600E) and the only one that did not, BRAF(V600E) was restricted to the Pt. The occurrence of BRAF(V600E) associated with other genetic events was an independent predictor of DMs during follow-up, recurrence and tumour-related death. Remarkably, two PDCs (8.3%) and five ATCs (14%) revealed concurrent BRAF and Ras mutations.

CONCLUSION

BRAF(V600E)'alone' does not represent a marker for poor outcome, however, when associated with alterations in other genes identifies a subset of PTCs with increased risk of recurrence and decreased survival.

How to create the vascular tree? (Latest) help from the zebrafish

The cardiovascular system provides oxygen, nutrients and hormones to organs, it directs traffic of metabolites and it maintains tissue homeostasis. It is one of the first organs assembled during vertebrate development and it is essential to life from early stages to adult. For these reasons, the process of vessel formation has being studied for more than a century, but it is only in the late eighties that there has been an explosion of research in the field with the employment of various in vitro and in vivo model systems. The zebrafish (Danio rerio) offers several advantages for in vivo studies; it played a fundamental role in new discoveries and helped to refine our knowledge of the vascular system. This review recapitulates the zebrafish data on vasculogenesis and angiogenesis, including the specification of the haemangioblasts from the mesoderm, their migration to form the vascular cord followed by axial vessels specification, the primary and secondary sprouting of intersomitic vessels, the formation of the lumen, the arterial versus venous specification and patterning. To emphasize the strengths of the zebrafish system in the vascular field, we summarize main tools, such as gene expression and mutagenesis screens, knock down technologies, transgenic lines and imaging, which played a major role in the development of the field and allowed significant discoveries, for instance the recent visualization of the lymphatic system in zebrafish. This information contributes to the prospective of drug discovery to cure human diseases linked to angiogenesis, not last tumours.

Investigator profile. An interview with A. Thomas Look, M.D. Interview by Vicki Glaser

Dr. Look received his M.D. degree and postgraduate training in Pediatrics from the University of Michigan, Ann Arbor, and completed his fellowship training in Pediatric Oncology at St. Jude Children's Research Hospital in Memphis, Tennessee. He then accepted a faculty position at St. Jude and remained there for 20 years, ultimately becoming the Chair of the Experimental Oncology Department. In June 1999, he joined the Dana-Farber Cancer Institute in Boston, Massachusetts, as Vice-Chair for Research in Pediatric Oncology and Professor of Pediatrics at Harvard Medical School. Work in Dr. Look's laboratory focuses on the molecular pathogenesis of leukemia. His group has been credited with the identification and functional analysis of several chimeric oncogenes activated by chromosomal translocations, including the E2A-HLF transcription factor, which was shown to act through an evolutionarily conserved genetic pathway to promote leukemia cell survival. Their efforts in human T-cell acute lymphoblastic leukemia have revealed key multistep mutational pathways that drive the pathogenesis of this disease and demonstrated that NOTCH1 receptors are mutationally activated in a majority of these cases. More recently, Dr. Look's laboratory developed the first transgenic model of leukemia in the zebrafish, opening the way for chemical and genome-wide genetic modifier screens in a vertebrate disease model.

Gene expression signature associated with BRAF mutations in human primary cutaneous melanomas

With the aim to correlate BRAF mutation status with gene expression in human primary cutaneous melanomas, and thus to get more insight on the consequences of BRAF mutation on cell biology, we analyzed all expression data obtained in melanomas from which DNA was extracted from the same tissue slides that were used for the expression study. A cohort of 69 frozen primary melanoma whose oligonucleotide micro-array expression data were available, were genotyped for BRAF and NRAS genes. The expression data from these melanomas were re-analyzed according to BRAF mutational status. A set of 250 probes representing 209 genes that were significantly (raw P< or =0.001) associated with BRAF mutation status was identified and 17 of these were previously shown to be implicated in cutaneous melanoma progression or pigmentation pathway-associated genes driven by the microphthalmia transcription factor (MITF). The list of 34 top probes contained no more than 1% of false discoveries with a probability of 0.95. Among the genes that differentiated most strongly between BRAF mutated and non-mutated melanomas, there were those involved in melanoma immune response such as MAGE-D2, CD63, and HSP70. These findings support the immunogenicity of BRAF(V600E), eliciting patients T-cell responses in various in vitro assays. The genes whose expression is associated with BRAF mutations are not simply restricted to the MAPK/ERK signaling but also converge to enhanced immune responsiveness, cell motility and melanosomes processing involved in the adaptative UV response.

Distribution of BRAF T1799A(V600E) mutations across various types of benign nevi: implications for melanocytic tumorigenesis

PURPOSE

The BRAF mutation is common in melanomas, but variation in rates across melanoma subtypes points to a complex interplay between BRAF activation and other factors (eg, sun exposure). Nevi also harbor the BRAF mutation. A description of mutation distribution in nevi could provide insight into the significance of this event in melanocytic tumorigenesis.

EXPERIMENTAL DESIGN

One hundred thirty-five nevi from 116 patients were evaluated for the T-->A mutation at nucleotide 1799. The nevi were inclusive of congenital (n = 34) and acquired (n = 101) nevi, dysplastic (n = 11) and nondysplastic (n = 124) nevi, and anogenital (n = 24) and common cutaneous (n = 111) nevi.

RESULTS

The overall mutation rate was 81%. The rate varied only slightly by anatomic site: BRAF mutations were detected in 21 of 21 (100%) nevi of the head and neck, 62 of 76 (82%) nevi of the trunk, 8 of 14 (62%) nevi of the extremities, and 18 of 24 (75%) anogenital nevi. For acquired nevi, there was no association between BRAF mutations and sun exposure as inferred from anatomic site. There were no significant differences in the mutation rates between congenital and acquired nevi (76% versus 81%; P = 0.5).

CONCLUSIONS

The BRAF mutation is uniformly distributed in various types of nevi. Its presence in congenital and anogenital nevi points to mechanisms of induction other than sun exposure. Its ubiquitous presence suggests that it poses no significant threat of malignant transformation, raising doubts about its relevance in melanoma development and its suitability as a target of directed therapy in patients with melanoma.

Issues affecting molecular staging in the management of patients with melanoma

Prediction of metastatic potential remains one of the main goals to be pursued in order to better assess the risk subgroups of patients with melanoma. Detection of occult melanoma cells in peripheral blood (circulating metastatic cells [CMC]) or in sentinel lymph nodes (sentinel node metastatic cells [SNMC]), could significantly contribute to better predict survival in melanoma patients. An overview of the numerous published studies indicate the existence of several drawbacks about either the reliability of the approaches for identification of occult melanoma cells or the clinical value of CMC and SNMC as prognostic factors among melanoma patients. In this sense, characterization of the molecular mechanisms involved in development and progression of melanoma (referred to as melanomagenesis) could contribute to better classify the different subsets of melanoma patients. Increasing evidence suggest that melanoma develops as a result of accumulated abnormalities in genetic pathways within the melanocytic lineage. The different molecular mechanisms may have separate roles or cooperate during all evolutionary phases of melanocytic tumourigenesis, generating different subsets of melanoma patients with distinct aggressiveness, clinical behaviour, and response to therapy. All these features associated with either the dissemination of occult metastatic cells or the melanomagenesis might be useful to adequately manage the melanoma patients with different prognosis as well as to better address the different melanoma subsets toward more appropriate therapeutic approaches.

Characterization of Kras-mediated pancreatic tumorigenesis in zebrafish

Activating Kras mutations are a pervasive and characteristic feature of human pancreatic cancer. In order to examine the earliest in vivo effects of oncogenic Kras expression in the exocrine pancreas, we generated two lines of zebrafish expressing eGFP alone or eGFP fused to human Kras with an activating mutation in codon 12 (Kras G12V) driven by ptf1a regulatory elements using a BAC recombineering strategy (Park et al., 2008). In this review, we describe the techniques that we used to observe the effects of eGFP-Kras G12V expression in pancreatic progenitor cells of the zebrafish embryo, as well as techniques used to characterize malignant pancreatic tumors in the adult zebrafish. This zebrafish model of pancreatic neoplasia provides a unique view of the effects of oncogenic Kras in the embryonic pancreas and suggests that the zebrafish will be a useful model organism in which to study the biology of Kras-initiated pancreatic neoplasia.

Genetics of melanoma tumorigenesis

Melanoma therapy is moving away from combinatorial approaches and towards newer targeted strategies. With the identification of mutations in various RAS pathway genes, there are now tremendous opportunities to bring inhibitors of RAS signalling to the clinical arena.

Differential gene expression in melanocytic nevi with the V600E BRAF mutation

We studied gene expression in 18 melanocytic nevi with and four nevi without the V600E mutation in the BRAF gene using HG-U133A 2.0 microarray with 22,277 transcripts. Data analysis revealed 92 genes up-regulated and 105 genes down-regulated in nevi with the mutation compared to nevi without mutation. Pathway analysis showed that differentially regulated genes mapped to 10 genetic networks. The major network included genes involved in cell death, cell cycle, and cellular growth and proliferation. Up-regulated genes in nevi with the mutation included CDKN2A, CDKN1C, and MITF; whereas down-regulated genes included those involved in apoptotic and other pathways. Principal component analysis identified 22 probe sets (20 genes) that caused separate segregation of nevi with and without mutations. In conclusion, our data showed differences in gene expression between nevi with and without the V600E BRAF mutation. Moreover, nevi with mutations showed over-expression of genes involved in melanocytic senescence and cell cycle inhibition.

The INT6 cancer gene and MEK signaling pathways converge during zebrafish development

BACKGROUND

Int-6 (integration site 6) was identified as an oncogene in a screen of tumorigenic mouse mammary tumor virus (MMTV) insertions. INT6 expression is altered in human cancers, but the precise role of disrupted INT6 in tumorigenesis remains unclear, and an animal model to study Int-6 physiological function has been lacking.

PRINCIPAL FINDINGS

Here, we create an in vivo model of Int6 function in zebrafish, and through genetic and chemical-genetic approaches implicate Int6 as a tissue-specific modulator of MEK-ERK signaling. We find that Int6 is required for normal expression of MEK1 protein in human cells, and for Erk signaling in zebrafish embryos. Loss of either Int6 or Mek signaling causes defects in craniofacial development, and Int6 and Erk-signaling have overlapping domains of tissue expression.

SIGNIFICANCE

Our results provide new insight into the physiological role of vertebrate Int6, and have implications for the treatment of human tumors displaying altered INT6 expression.

Models and mechanisms in malignant melanoma

Human melanoma represents the fastest growing malignancy in the US. The etiology of melanoma is highly debated as is the role of ultraviolet (UV) radiation in the initiation and progression of melanoma. This article discusses data from UV exposure and its relationship to the development of melanoma from various models of melanoma as well as various genetic alterations seen in oncogenic transformation of melanocytes. Genetic alterations such as the p16(INK4a) deletion, melanocortin 1 receptor (MC1R), RAS, and v-raf murine sarcoma viral oncogene homolog B1 (BRAF) may be indicative of a predisposition to melanoma development. Historical research as well as current data on the significance of the hot spot mutation in BRAF is discussed in its relative potential to the activating mutation in RAS.

BRAF(E600) in benign and malignant human tumours

Of the RAF family of protein kinases, BRAF is the only member to be frequently activated by mutation in cancer. A single amino acid substitution (V600E) accounts for the vast majority and results in constitutive activation of BRAF kinase function. Its expression is required to maintain the proliferative and oncogenic characteristics of BRAF(E600)-expressing human tumour cells. Although BRAF(E600) acts as an oncogene in the context of additional genetic lesions, in primary cells it appears to be associated rather with transient stimulation of proliferation. Eventually, BRAF(E600) signalling triggers cell cycle arrest with the hallmarks of cellular senescence, as is illustrated by several recent studies in cultured cells, animal models and benign human lesions. In this review, we will discuss recent advances in our understanding of the role of BRAF(E600) in benign and malignant human tumours and the implications for therapeutic intervention.

FishNet: an online database of zebrafish anatomy

BACKGROUND

Over the last two decades, zebrafish have been established as a genetically versatile model system for investigating many different aspects of vertebrate developmental biology. With the credentials of zebrafish as a developmental model now well recognized, the emerging new opportunity is the wider application of zebrafish biology to aspects of human disease modelling. This rapidly increasing use of zebrafish as a model for human disease has necessarily generated interest in the anatomy of later developmental phases such as the larval, juvenile, and adult stages, during which many of the key aspects of organ morphogenesis and maturation take place. Anatomical resources and references that encompass these stages are non-existent in zebrafish and there is therefore an urgent need to understand how different organ systems and anatomical structures develop throughout the life of the fish.

RESULTS

To overcome this deficit we have utilized the technique of optical projection tomography to produce three-dimensional (3D) models of larval fish. In order to view and display these models we have created FishNet http://www.fishnet.org.au, an interactive reference of zebrafish anatomy spanning the range of zebrafish development from 24 h until adulthood.

CONCLUSION

FishNet contains more than 36,000 images of larval zebrafish, with more than 1,500 of these being annotated. The 3D models can be manipulated on screen or virtually sectioned. This resource represents the first complete embryo to adult atlas for any species in 3D.

Detection of the p53 response in zebrafish embryos using new monoclonal antibodies

The zebrafish has many advantages as a vertebrate model organism and has been extensively used in the studies of development. Its potential as a model in which to study tumour suppressor and oncogene function is now being realized. Whilst in situ hybridization of mRNA has been well developed in this species to study gene expression, antibody probes are in short supply. We have, therefore, generated a panel of anti-zebrafish p53 monoclonal antibodies and used these to study the p53 response in zebrafish embryos. By immunohistochemistry, we show that the exposure of zebrafish embryos to p53-activating agents such as R-roscovitine and gamma-irradiation results in the accumulation of p53 protein in the gut epithelium, liver and pancreas. A combination of R-roscovitine and gamma-irradiation results in massive p53 induction, not only in the pharyngeal arches, gut region and liver but also in brain tissues. Induction of apoptosis and expression of p53 response genes are seen in regions that correspond to sites of p53 protein accumulation. In contrast, although zebrafish tp53(M214K) mutant embryos showed a similar accumulation of p53 protein, a complete lack of a downstream p53-dependent response was observed. In this system the p53 gene is identified as a p53-responsive gene itself. Our results demonstrate that zebrafish p53 protein can readily be induced in embryos and detected using these new antibody tools, which will increase the usefulness of zebrafish as a model in compound-based screening for novel drugs in cancer research.

Oncogenic BRAF(V600E) inhibits BIM expression to promote melanoma cell survival

Somatic activating mutations of BRAF are the earliest and most common genetic abnormality detected in the genesis of human melanoma. However, the mechanism(s) by which activated BRAF promotes melanoma cell cycle progression and/or survival remain unclear. Here we demonstrate that expression of BIM, a pro-apoptotic member of the BCL-2 family, is inhibited by BRAF-->MEK-->ERK signaling in mouse and human melanocytes and in human melanoma cells. Trophic factor deprivation of melanocytes leads to elevated BIM expression. However, re-addition of trophic factors or activation of a conditional form of BRAF(V600E) leads to rapid inhibition of BIM expression. In both cases, inhibition of BIM expression was dependent on the activity of MEK1/2 and the proteasome. Consistent with these observations, pharmacological inhibition of BRAF(V600E) or MEK1/2 in human melanoma cells (using PLX4720 and CI-1040 respectively) led to a striking elevation of BIM expression. Re-activation of BRAF-->MEK-->ERK signaling led to phosphorylation of BIM-EL on serine 69 and its subsequent degradation. Interestingly, endogenous expression of BIM in melanoma cells was insufficient to induce apoptosis unless combined with serum deprivation. Under these circumstances, inhibition of BIM expression by RNA interference provided partial protection from apoptosis. These data suggest that regulation of BIM expression by BRAF-->MEK-->ERK signaling is one mechanism by which oncogenic BRAF(V600E) can influence the aberrant physiology of melanoma cells.

Melanoma invasion - current knowledge and future directions

The acquisition of invasive behaviour is the key transition in the progression of benign melanocyte hyperplasia to life threatening melanoma. Understanding this transition and the mechanisms of invasion are the key to understanding why malignant melanoma is such a devastating disease and will aid treatment strategies. Underlying the invasive behaviour is increased cell motility caused by changes in cytoskeletal organization and altered contacts with the extra-cellular matrix (ECM). In addition, changes in the interactions of melanoma cells with keratinocytes and fibroblasts enable them to survive and proliferate outside their normal epidermal location. Proteomic and genomic initiatives are greatly increasing our knowledge of which gene products are deregulated in invasive and metastatic melanoma; however, the next challenge is to understand how these genes promote the invasion of melanoma cells. In recent years new models have been developed that more closely recapitulate the conditions of melanoma invasion in vivo. It is hoped that these models will give us a better understanding of how the genes implicated in melanoma progression affect the motility of melanoma cells and their interactions with the ECM, stromal cells and blood vessels. This review will summarise our current understanding of melanoma invasion and focus on the new model systems that can be used to study melanoma.

Ultrasound biomicroscopy permits in vivo characterization of zebrafish liver tumors

Zebrafish are a valuable vertebrate model to study carcinogenesis, but noninvasive imaging is challenging because adult fish are not transparent. Here we show that tumors could be readily detected in vivo using high-resolution microscopic ultrasound in zebrafish. We successfully obtained tissue perfusion calculations and cellular aspirates, and analyzed tumor progression and response to treatment. Ultrasound biomicroscopy allows longitudinal studies of tumor development and real-time assessment of therapeutic effects in zebrafish.

Molecular markers of circulating melanoma cells

Of all skin cancers, cutaneous malignant melanoma (CMM) is the most aggressive and the life expectancy of patients with lymphatic or systemic metastases is dramatically reduced. Understandably therefore, scientists and clinicians have focused on improving diagnostic and prognostic techniques. Of these, perhaps the most promising are multimarker real-time RT-PCR and microarray for detection of circulating CMM cells in peripheral blood. While the optimal set of markers is still to be identified that can accurately assess disease severity and progression at all clinical stages of the disease, recent progress has been dramatic. Here we provide an exhaustive review of recent studies in which a variety of markers are assessed. Moreover, the efficacy of the markers relative to clinical stage is discussed in light of experimental findings. From these studies, it is apparent that researchers are now much closer to defining a set of markers of circulating cells that can be utilized in routine diagnostic tests.

Generation of medaka gene knockout models by target-selected mutagenesis

A reverse genetics approach for the routine generation of medaka (Oryzias latipes) gene knockouts is described and applied to create a cryopreserved resource containing knockouts for most medaka genes.

We have established a reverse genetics approach for the routine generation of medaka (Oryzias latipes) gene knockouts. A cryopreserved library of N-ethyl-N-nitrosourea (ENU) mutagenized fish was screened by high-throughput resequencing for induced point mutations. Nonsense and splice site mutations were retrieved for the Blm, Sirt1, Parkin and p53 genes and functional characterization of p53 mutants indicated a complete knockout of p53 function. The current cryopreserved resource is expected to contain knockouts for most medaka genes.

Raf kinases: function, regulation and role in human cancer

The Ras-Raf-MAPK pathway regulates diverse physiological processes by transmitting signals from membrane based receptors to various nuclear, cytoplasmic and membrane-bound targets, coordinating a large variety of cellular responses. Function of Raf family kinases has been shown to play a role during organism development, cell cycle regulation, cell proliferation and differentiation, cell survival and apoptosis and many other cellular and physiological processes. Aberrations along the Ras-Raf-MAPK pathway play an integral role in various biological processes concerning human health and disease. Overexpression or activation of the pathway components is a common indicator in proliferative diseases such as cancer and contributes to tumor initiation, progression and metastasis. In this review, we focus on the physiological roles of Raf kinases in normal and disease conditions, specifically cancer, and the current thoughts on Raf regulation.

Heat shock-inducible Cre/Lox approaches to induce diverse types of tumors and hyperplasia in transgenic zebrafish

RAS family members are among the most frequently mutated oncogenes in human cancers. Given the utility of zebrafish in both chemical and genetic screens, developing RAS-induced cancer models will make large-scale screens possible to understand further the molecular mechanisms underlying malignancy. We developed a heat shock-inducible Cre/Lox-mediated transgenic approach in which activated human kRASG12D can be conditionally induced within transgenic animals by heat shock treatment. Specifically, double transgenic fish Tg(B-actin-LoxP-EGFP-LoxP-kRASG12D; hsp70-Cre) developed four types of tumors and hyperplasia after heat shock of whole zebrafish embryos, including rhabdomyosarcoma, myeloproliferative disorder, intestinal hyperplasia, and malignant peripheral nerve sheath tumor. Using ex vivo heat shock and transplantation of whole kidney marrow cells from double transgenic animals, we were able to generate specifically kRASG12D-induced myeloproliferative disorder in recipient fish. This heat shock-inducible recombination approach allowed for the generation of multiple types of RAS-induced tumors and hyperplasia without characterizing tissue-specific promoters. Moreover, these tumors and hyperplasia closely resemble human diseases at both the morphologic and molecular levels.

Zebrafish: an emerging model system for human disease and drug discovery

In vivo studies represent an essential step in drug development and currently rely largely on mice, yet limitations of mammalian models motivated the search for complementary vertebrate model systems. This review focuses on zebrafish, Danio rerio, as a facile model system to study human disease and drug responses. Zebrafish are particularly suited for this purpose because they represent a vertebrate species, their genome is sequenced, and a large number of synchronously developing, transparent embryos can be produced. Zebrafish embryos are permeable to drugs and can easily be manipulated using well-established genetic and molecular approaches. Here, we summarize recent work on drug discovery and toxicity in zebrafish embryos. In addition, we provide a synopsis of current efforts to establish disease models in zebrafish focusing on neoplasia. The results of these studies highlight the potential of zebrafish as a viable addition to established animal models by offering medium and, potentially, high throughput capabilities.

Melanoma, long wavelength ultraviolet and sunscreens: Controversies and potential resolutions

Although sunlight is known to cause melanoma, there has been considerable controversy as to the importance of short (UVB) and long (UVA) ultraviolet (UV) wavelengths in causing melanoma, leading to uncertainty in how best to prevent this cancer. This uncertainty has been compounded by the difficulties in assaying the UVA protection abilities of sunscreens, as compared to widely accepted measures of UVB screening by the sun protection factor (SPF). This review discusses the controversies surrounding UVA causation of melanoma in both human and animal models and the use of sunscreens to prevent melanoma. In addition, it details the development of an electron paramagnetic resonance (EPR) technique, initially used to determine the wavelength dependence (or action spectrum) of intramelanocyte radical generation to resolve these controversies in the Xiphophorus model. It is shown how this EPR technique allows a sunscreen protection factor to be determined, that is weighted to the melanocyte, and how this also allows study of the wavelength-dependent screening ability of sunscreens.

Animal models of human disease: zebrafish swim into view

Despite the pre-eminence of the mouse in modelling human disease, several aspects of murine biology limit its routine use in large-scale genetic and therapeutic screening. Many researchers who are interested in an embryologically and genetically tractable disease model have now turned to zebrafish. Zebrafish biology allows ready access to all developmental stages, and the optical clarity of embryos and larvae allow real-time imaging of developing pathologies. Sophisticated mutagenesis and screening strategies on a large scale, and with an economy that is not possible in other vertebrate systems, have generated zebrafish models of a wide variety of human diseases. This Review surveys the achievements and potential of zebrafish for modelling human diseases and for drug discovery and development.

Conditional N-rasG12V expression promotes manifestations of neurofibromatosis in a mouse model

Human clinical neurofibromatosis type 1 (NF1) and type 2 (NF2) result from mutations and inactivation of neurofibromin and merlin genes, respectively, which negatively regulate Ras pathways. To evaluate the contribution of N-Ras activity to the development of NF, we generated a novel transgenic mouse expressing oncogenic N-ras specifically in central nerve cells, neural crest-derived cells and lens epithelial cells. Soon after birth, the mouse skin showed hyperpigmentation of the epidermis and melanin-laden macrophages in the dermis, as observed in the café-au-lait spots of human cases. At 3 months of age, all the mice had neurofibromas in the skin and neurofibroma-like tumors with structure similar to Wagner-Meissner bodies in the adrenal medulla. At 4 months of age, all the mice developed subcapsular cataract. In the 5th month, some developed protruding dermal neurofibromas involving subcutaneous fat. However, plexiform neurofibroma, schwannoma, astrocytoma and pheochromocytoma were not observed in the mice, suggesting a requirement for signal(s) other than the activated N-Ras pathway to induce these tumors. Thus, the activated N-Ras signal may be a main pathway for the development of the disease phenotypes characteristic of NF.

Point mutations of protein kinases and individualised cancer therapy

The treatment of cancer is rapidly changing, with an increasing focus on converting our improved understanding of the molecular basis of disease into clinical benefit for patients. Protein kinases that are mutated in cancer represent attractive targets, as they may result in cellular dependency on the mutant kinase or its associated pathway for survival, a condition known as 'oncogene addiction'. Early clinical experiences have demonstrated dramatic clinical benefit of targeting oncogenic mutations in diseases that have been largely resistant to traditional cytotoxic chemotherapy. Further, mutational activation of kinases can indicate which patients are likely to respond to targeted therapeutics. However, these experiences have also illuminated a number of critical challenges that will have to be addressed in the development of effective drugs across different cancers, to fully realise the potential of individualised molecular therapy. This review utilises examples of genetic activation of kinases to illustrate many of the lessons learned, as well as those yet to be implemented.

NRAS and BRAF mutations in melanoma tumours in relation to clinical characteristics: a study based on mutation screening by pyrosequencing

We have previously demonstrated the use of pyrosequencing to investigate NRAS [neuroblastoma RAS viral (v-ras) oncogene homolog] mutations in melanoma biopsies. Here, we expanded the analysis to include BRAF (V-raf murine sarcoma viral oncogene homolog B1), another member of the Ras-Raf-mitogen-activated protein kinase (MAPK) signalling pathway, and analysed a total of 294 melanoma tumours from 219 patients. Mutations in BRAF exons 11 and 15 were identified in 156 (53%) tumours and NRAS exon 2 mutations in 86 (29%) tumours. Overall, mutations in NRAS or BRAF were found in 242 of 294 tumours (82%) and were found to be mutually exclusive in all but two cases (0.7%). Multiple metastases were analysed in 57 of the cases and mutations were identical in all except three, indicating that BRAF and NRAS mutations occur before metastasis. Association with preexisting nevi was significantly higher in BRAF mutated tumours (P=0.014). In addition, tumours with BRAF mutations showed a significantly more frequent moderate to pronounced infiltration of lymphocytes (P=0.013). NRAS mutations were associated with a significantly higher Clark level of invasion (P=0.022) than BRAF mutations. Age at diagnosis was significantly higher in tumours with NRAS mutations than in those with BRAF mutations (P=0.019). NRAS and BRAF mutations, however, did not influence the overall survival from time of diagnosis (P=0.7). In conclusion, the separate genotypes were associated with differences in several key clinical and pathological parameters, indicating differences in the biology of melanoma tumours with different proto-oncogene mutations.

Differential regulation of B-raf isoforms by phosphorylation and autoinhibitory mechanisms

The B-Raf proto-oncogene encodes several isoforms resulting from alternative splicing in the hinge region upstream of the kinase domain. The presence of exon 8b in the B2-Raf(8b) isoform and exon 9b in the B3-Raf(9b) isoform differentially regulates B-Raf by decreasing and increasing MEK activating and oncogenic activities, respectively. Using different cell systems, we investigated here the molecular basis of this regulation. We show that exons 8b and 9b interfere with the ability of the B-Raf N-terminal region to interact with and inhibit the C-terminal kinase domain, thus modulating the autoinhibition mechanism in an opposite manner. Exons 8b and 9b are flanked by two residues reported to down-regulate B-Raf activity upon phosphorylation. The S365A mutation increased the activity of all B-Raf isoforms, but the effect on B2-Raf(8b) was more pronounced. This was correlated to the high level of S365 phosphorylation in this isoform, whereas the B3-Raf(9b) isoform was poorly phosphorylated on this residue. In contrast, S429 was equally phosphorylated in all B-Raf isoforms, but the S429A mutation activated B2-Raf(8b), whereas it inhibited B3-Raf(9b). These results indicate that phosphorylation on both S365 and S429 participate in the differential regulation of B-Raf isoforms through distinct mechanisms. Finally, we show that autoinhibition and phosphorylation represent independent but convergent mechanisms accounting for B-Raf regulation by alternative splicing.

A genome-wide scan for naevus count: linkage to CDKN2A and to other chromosome regions

High numbers of melanocytic naevi (moles), and mutations in the p16 gene (CDKN2A), are two strong risk factors for cutaneous malignant melanoma. We have previously reported linkage of mole count to the CDKN2A locus. Here, we report genome-wide scans for mole counts (differentiated into flat, raised and atypical subtypes) with a total of 796 microsatellite markers for 424 families with 1024 twins and siblings, plus genotypes for 690 parents. Inclusion of 221 pairs of MZ twins enabled separation of shared environmental and polygenic influences, so placing an upper limit to estimates of QTL variance. Maximum likelihood multipoint variance component methods were used to assess linkage of naevus count. Sex, age, body surface area, skin colour, hair colour, sunburn and facial freckles were included as covariates. Peak linkage of flat mole count was to regions on chromosomes 2, 9, 8 and 17 with lod scores 2.95, 2.95, 2.50 and 2.15, respectively. The support for linkage to the CDKN2A gene region (9p21) increased to 3.42 when additional fine mapping markers were added. For raised mole count, there was suggestive evidence of linkage in our sample to chromosome 16 (lod=1.87), and for atypical mole count on chromosomes 1, 6 and X with lod scores of 2.20, 2.00 and 2.00, respectively. The multivariate linkage peaks generally match those from individual trait analyses, with the exception of a new peak on chromosome 4 (point-wise empirical P-value=0.001). We replicate our earlier finding of linkage to CDKN2A and discovering linkage to several novel regions that may also influence risk of the development of malignant melanoma.