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Keywords:

  • melanoma;
  • mouse models;
  • mutation;
  • histopathology

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Phenotypic and molecular heterogeneity in human melanoma has impaired efforts to explain many of the clinically important features of melanoma. For example, many of the underlying mechanisms that might predict age-of-onset, time to metastasis and other key elements in melanoma progression remain unknown. Furthermore, melanoma staging used to predict outcome and treatment has not yet moved beyond a basic phenotypic classification. While molecularly targeted therapies show great promise for melanoma patients, establishing accurate animal models that recapitulate human cutaneous melanoma progression remains a priority. We examine the relevance of mice as models for human melanoma progression and for key molecular and histopathologic variants of melanoma. These mice may be used as preclinical models to probe the relationships between causative mutations, disease progression and outcome for molecularly targeted therapeutics. We ask how new mouse models, or more detailed histopathologic and molecular analyses of existing mouse models, may be used to advance our understanding of genotype–phenotype correlations in this tumour type. This necessarily involves a consideration of the utility of mice as models for ultraviolet radiation-induced melanoma, and how this might be improved.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

For many cancer types, mice have been used effectively to demonstrate mechanistic proof-of-principle concepts that are invaluable in informing carefully selected, and often more difficult and expensive experiments using human subjects. Nevertheless, in preclinical melanoma research, the power of the experimental mouse has not been optimally utilized because of the key differences in phenotypic presentation of cutaneous melanomas. One reason for this is that murine lesions do not generally exhibit the epidermal changes typical of most human melanomas. In addition to reduced epidermal thickness, nocturnal and heavily hair-covered species like mice have no need for skin pigmentation, and most often have no melanocytes (MCs) in the epidermis, except in tail, and to a lesser extent ear skin. In adult mice, the great majority of MCs are in the hair bulb. Why epidermal MCs are localized in the human but not in the murine epidermis is poorly understood. However, over-expression of a single-cytokine, stem cell factor, also termed Kit ligand (Kitlg), in keratinocytes is sufficient to support epidermal localization of MCs, suggesting that ‘humanization’ of MC localization within mouse skin is relatively easy to achieve (Kunisada et al., 1998). As will be discussed, significant progress has been made towards generating epidermal melanomas in various mouse models. However, not all melanomas are epidermal. Notably, some mouse models (e.g. those carrying hypermorphic Gnaq and Gna11 mutations) develop dermal lesions that are very similar to those observed in humans. In these mice, the location of neoplastic MCs within the skin is comparable to human malignant blue naevi (that carry constitutively activating GNAQ and GNA11 mutations) (Van Raamsdonk et al., 2009, 2010).

Despite these differences in mouse and human MC biology, mice have been widely used to examine the key principles of MC development and to study molecular pathways that lead to melanoma in vivo. Our aim is not to exhaustively review murine models of melanoma [this has been done recently by Larue and Beermann (2007) and Damsky and Bosenberg (2010)], but to assess the utility of mice, particularly genetically engineered mouse models (GEMMs), as a blueprint for the mechanism of action of alterations that drive progression of melanoma. In this review, we examine how closely mouse models parallel key aspects of human melanomagenesis, including the potential of different mouse models to produce premalignant melanocytic naevi in addition to melanomas. We discuss how mouse models may be used to study the molecular mechanisms that determine whether a melanoma may arise de novo or via a naevus-dependent pathway. Human primary melanomas have a diverse histological presentation encapsulated in the American Joint Committee on Cancer (AJCC) subtypes that record body site of the tumour and the histological location and features within the skin. We consider how comparable features of human melanoma subtypes appear in various mouse melanoma models and how the emerging molecular classification of human melanomas, including somatic mutational signatures that appear in each histologic subtype, may be used to inform the development of new mouse models that better parallel the key histologic features of human melanomagenesis and progression. Finally, ultraviolet radiation (UVR) plays a key role in the initiation and progression of human melanoma, with childhood and adolescent UV exposures and intermittent rather than chronic UV exposures carrying the greatest risk of later melanoma development. Here, we consider whether these UV exposure patterns may play a similar role in extant mouse melanoma models. In sum, this work provides a comprehensive summary of the strengths and weaknesses of mice as models for human melanoma, and areas where advances are needed to improve mouse models are highlighted. With this in mind, it is likely that increased attention to the phenotypic characterization of mouse melanoma models, and/or the engineering of these genotype–phenotype correlations, will reveal a much greater relevance to human melanomas than previously suspected.

Molecular pathways defective in melanoma and their modelling in mice

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

First we consider the major signalling networks deregulated in melanoma by mutation or other genomic alterations. These pathways can be summarized in four major groups: the RAS-RAF-MAPK, PI3K-AKT (incorporating PTEN), CDK4-INK4A-pRB and ARF-p53 pathways (reviewed in Hocker et al., 2008). Mutations in the oncogenic RAS-RAF-MAPK pathway (mainly BRAF and NRAS) are present in most melanomas. In superficial spreading and nodular melanoma (NM), 66 and 68% of lesions, respectively, have either BRAF or NRAS mutations (Lee et al., 2011). The fact that most naevi also carry such mutations suggests shared mechanisms of progression even if melanomas do not always emanate from naevi. The common BRAFV600E mutation acts principally through MAPK pathway activation. Mice with a MC-specific BrafV600E mutation develop melanocytic lesions reminiscent of dermal naevi or malignant blue naevi, and in one study, malignant tumours are observed (Dankort et al., 2009; Dhomen et al., 2009; Goel et al., 2009). Animals with MC-specific activating mutations in either Hras (Broome Powell et al., 1999), Nras (Ackermann et al., 2005) or Kras (Monahan et al., 2010) appear to develop dermal MC proliferations, with only rare conversion to malignancy. Although there has not been a detailed comparison, there appears to be no major morphological differences in naevus-like lesions developing in these models. Alternative ways of activating signalling cascades, such as the MAPK pathway, have been investigated by over-expression of oncogenes in mice such as hepatocyte growth factor (Hgf), metabotropic glutamate receptor 1 (Grm1) and Ret proto-oncogene (Ret), in each case resulting in cellular transformation and invasive growth in different cell types, including MCs (Chin et al., 1998; Kato et al., 1998; Noonan et al., 2001; Pollock et al., 2003; Santoro et al., 2005; Shin et al., 2008). Although mutations in RET, HGF and GRM1 are very rare in human melanoma, mice carrying corresponding mutations have been important in investigating oncogenic pathways important in melanoma development (e.g. Ha et al., 2007; Namkoong et al., 2007).

It should be noted that activating mutations in NRAS, KRAS and HRAS result in the activation of both the MAPK and PI3K/AKT pathways. The latter is the second important oncogenic pathway that is frequently perturbed in various other ways in melanoma (Hocker et al., 2008). In particular, inactivation of the upstream inhibitor, PTEN, by deletion, mutation or methylation occurs in 15–50% of melanomas. Phospho-AKT is expressed in the majority of melanocytic tumours (Fecher et al., 2007). The importance of the PI3K-AKT pathway is confirmed in mouse models carrying constitutional or MC-specific deletion of Pten, where susceptibility to the development of melanoma is greatly exacerbated (reviewed in Damsky and Bosenberg, 2010). Mice carrying activated Akt have not been reported.

The third critical pathway mutated in melanoma is the CDK4-CYCLIN D-p16INK4A-pRB cascade that has an important role in tumour suppression. It is well known that melanoma-prone families carry germ-line mutations affecting either, or both of the two transcripts of the CDKN2A locus (INK4A and ARF), or in the INK4A binding partner CDK4. This pathway is also frequently deregulated somatically in human melanoma (reviewed in Hocker et al., 2008). Remarkably, in mice with deletions of Ink4a or Arf, melanoma is extremely rare (e.g. Sharpless et al., 2001), although the development of these tumours is enhanced after neonatal UVR of Ink4a/Arf-null mice (Yang et al., 2007).

The ARF-p53 signalling axis constitutes a fourth major melanoma pathway. The tumour suppressor p53 is critical in transducing signals to activate DNA damage repair and other responses after various kinds of cellular stress (Box and Terzian, 2008). p53 mutations are observed in about 10% of all melanomas (most commonly in nodular or lentigo melanoma) (Hocker and Tsao, 2007). ARF-specific mutations have been identified in several melanoma-prone families (Meyle and Guldberg, 2009). ARF is somatically inactivated both genetically and epigenetically in about 43% of metastases (Freedberg et al., 2008). Studies with p53 or Arf knockout mice have shown that both genes are strong tumour suppressors in melanoma. In the context of UVR-induced melanoma in mice, the role of Arf as a tumour suppressor is particularly evident (Ferguson et al., 2010; Ha et al., 2007; Kannan et al., 2003).

Most cancers appear to be driven by oncogene activation in combination with loss of a major tumour suppressor. This is also true for melanoma where loss of p53, Arf or Rb pathway members in conjunction with activating Ras or Raf mutations results in the development of aggressive tumours in murine models (reviewed in Damsky and Bosenberg, 2010). We can conclude that the molecular pathways altered in human melanoma are also involved in the induction of melanoma in mice.

Naevus formation in humans and in mice

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Naevogenesis is an area of intense investigation in humans because of the realization that the presence of many melanocytic naevi is a strong phenotypic risk indicator for melanoma. There are multiple subtypes of naevi. Broadly, these include dermal (blue naevi), compound (common acquired, spitz and congenital naevi) and epidermal (e.g. reed naevi) lesions (Argenziano et al., 2007). The great majority of naevi carry BRAF mutations, chiefly BRAFV600E (Pollock et al., 2003). Mice carrying this mutation also develop naevus-like lesions (Dankort et al., 2009; Dhomen et al., 2009). Of interest, BRAF mutations may be more important in some naevus subtypes as they occur with varying frequencies within different dermoscopic and histopathological subtypes (Zalaudek et al., 2011). GNAQ may be another gene that is important in stratifying naevi by subtype. This gene was discovered from studies on hyperpigmented mice with dermal melanocytosis caused by hypermorphic mutations in the G-protein-coupled receptors Gna11 and Gnaq (Van Raamsdonk et al., 2004). Subsequent investigation of human lesions shows that GNAQ is somatically mutated in up to 80% of blue naevi and that GNA11 is also mutated, albeit less frequently (Van Raamsdonk et al., 2009). While these mutations would have been eventually identified by whole genome sequencing of human lesions, the discovery of this genotype–phenotype correlation demonstrates the value of mutant mice in revealing specific molecular signatures present in human naevus and melanoma subtypes. Mice carrying a MC-specific activating Gnaq (or Gna11) mutation would appear to be a good model for blue naevus and related malignant forms (called malignant blue naevi or animal-type melanoma).

However, the finding of somatic BRAF and GNAQ mutations in naevi does not inform about the inherent susceptibility to develop naevi. The strongest signal from genome wide association studies on naevus count is single-nucleotide polymorphism on chromosome 9p21 located in the 5′UTR of MTAP (a gene involved in nucleoside metabolism), located next to CDKN2A on chromosome 9p21 (Falchi et al., 2009). Detailed mapping and sequence analysis may confirm the relative importance of CDKN2A and MTAP in naevogenesis. If no causative variants are found, functional studies will be necessary and GEMMs carrying specific mutations of these genes may be an avenue to this end. A murine model for MTAP (Mtap+/−) is prone to lymphoma (Kadariya et al., 2009), suggesting that MTAP is a tumour suppressor in its own right. What of GEMMs carrying CDKN2A and CDK4 mutations? Naevi can result after carcinogen and/or UVR treatment of Cdk4R24C or Ink4a/Arf-null mice (van Schanke et al., 2006; Sotillo et al., 2001). Our recent study provides more functional evidence for a role for INK4A-CDK4 pathway. Tyr-Nras::Cdk4R24C/R24C, but not Tyr-Nras::p53−/− or Tyr-Nras::Arf−/− mice develop dermal naevi (Ferguson et al., 2010). Activated Cdk4 appears to support the formation of multiple spontaneous naevi in the Mt-Hgf::Cdk4R24C/R24C model also (Landsberg et al., 2010). In some ways, these models may mimic the familial atypical multiple mole melanoma syndrome in humans (Lynch et al., 1980); where many types of naevi, including common and atypical junctional and dermal melanocytic naevi, are observed (Tucker et al., 2002). We are not aware of any murine models for epidermal naevi, although it will be intriguing to study ‘early’ lesions appearing on Tyr-NrasQ61K::Cdk4R24C/R24C::K14-Kitlg mice. In the Mt-Hgf model, the slow-growing early lesions (naevi) may be similar to some human junctional or compound naevi.

Melanoma development de novo or via naevus intermediates

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

It estimated that about 25–50% of melanomas arise from pre-existing naevi (Bevona et al., 2003; Garcia-Cruz et al., 2009), depending upon whether this is determined histopathologically (naeval remnants associated with lesions), by following the lesion in vivo over time dermoscopically or through the patient’s own recall of a pre-existing lesion (not to be heavily relied on as the pre-existing lesion may already been a melanoma). Most melanoma subtypes can arise from naevi, although rarely in the case of lentigo and acral melanoma (Garcia-Cruz et al., 2009). However, the majority of melanomas probably arise de novo from normal skin. It would be an important development for dermatologists to be able to predict which naevus is likely to progress to malignancy, as well as to be able to identify very small de novo lesions. Whether melanomas form via the naevus or de novo pathway may depend stochastically upon whether the critical mutation(s) occurs in a naevus cell or normal MC. Ways of modelling such disparate modes of progression in mice are becoming apparent. New data have come from the Mt-Ret mice, which develop melanoma progressively via benign-premalignant-malignant stages based on histopathology and Ki-67 index (Kumasaka et al., 2010). As the malignant lesions express lower levels of the endothelin receptor B (Ednrb), Mt-Ret::Ednrb+/− mice were generated to assess the role of decreased Ednrb expression on melanomagenesis. These mice developed tumours significantly earlier than the Mt-Ret alone, and the lesions were all either premalignant or malignant, with no benign tumours present. Thus, haploinsufficiency of Ednrb resulted in the skipping of the benign stage of progression. It is too early to speculate on the mechanism of this effect, but these animals clearly provide an experimental system that warrants further investigation.

We also shed some light on the mechanisms by which melanomas may develop from benign precursors (Terzian et al., 2010). We used a model carrying MC-specific activated HrasG12V (Tpras) with melanomagenesis driven by the carcinogen dimethylbenzanthracene (DMBA). When crossed a heterozygous Mdm4 background (which increases p53 expression), the Tpras::Mdm4+/− mice developed fewer melanomas with a delay in age-of-onset. Furthermore, there was a dramatic decrease in tumour growth, and a lack of metastasis in the compound mutants. Thus, elevated p53 levels effectively prevented the conversion of small benign lesions to melanomas. Assessment of the mechanisms of growth suppression by high p53 expression using in vitro studies indicated that cell cycle regulation through the p21WAF1/CIP1 signalling network may be key to its anti-melanomagenic activity. Hence, increased endogenous p53 may well prevent the progression of naevi to melanoma. In support of this notion, mice carrying p53 deletion, along with NrasQ61K, exhibit aggressive histological features even in the earliest detectable lesions, perhaps indicating that the transition to melanoma occurs before a visible lesion is present (Ferguson et al., 2010). Again, careful assessment of the histopathology and growth characteristics of melanomas in GEMMs can yield surprising information as to the molecular mechanisms that determine whether melanoma arises via a naevus intermediate, or de novo.

Metastatic melanoma

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Modelling of the melanoma metastatic process in mice has been reviewed by Zaidi et al. (2008). Genetically engineered mouse models that develop metastases predictably indicate that mutation burden is important in invoking the metastatic phenotype. For instance, most of the models tested form metastases after exposure to DMBA, which is much more mutagenic than UVR (Damsky and Bosenberg, 2010). In general, while many murine models have been reported to have atypical MCs in lymph nodes, relatively few develop visceral metastases. It is clear that Braf or Ras mutations are not sufficient to cause spread throughout the body and invasion of other organs (Goel et al., 2009; Dhomen et al., 2009; Dankort et al., 2009), consistent with the fact that very few human naevi progress to a malignant phenotype (Tsao et al., 2003). Most GEMMs of melanoma do not present with visceral metastases even when the mice also carry very destabilizing cell cycle mutations (such as in Cdkn2a, Tp53). Although MC-specific Nras activation in conjunction with Ink4a loss is somewhat effective at inducing visceral metastases (Ackermann et al., 2005), the loss of Pten and BrafV600E seems very effective (Dankort et al., 2009). Thus, Pten loss appears to be very destabilizing to MCs and effectively induces metastatic spread. This has been further demonstrated recently in another genetically modified murine model with Pten deletion (Nogueira et al., 2010). On the other hand, transgenics overexpressing Grm1, Ret and Hgf in the skin form visceral metastases spontaneously (Kato et al., 1998; Otsuka et al., 1998; Zhu et al., 1998). Hence, these mutations appear to be more destabilizing to the MC than MAPK pathway activation alone. Another possible reason for the increased malignant conversion in these models is that unlike in the Ras/Raf models, the mutations are expressed in both the MC and its microenvironment. For instance, MC-specific activation of Grm1 (Pollock et al., 2003) does not seem to be as effective at inducing metastases as when the mutation is also expressed constitutionally (Zhu et al., 1998).

Another example of ‘cross-species’ gene discovery using GEMMs is the identification of NEDD9 as a regulator of melanoma metastasis (Kim et al., 2006). Using mice engineered to switch Ras on or off by administering doxycycline to the animals, Chin et al. (1999) showed that oncogenic Hras expression was needed for both the initiation of melanomas and their maintenance, setting the groundwork for the idea of targeting BRAFV600E in melanoma. As is likely to occur in humans, some ‘escaper’ tumours lost their dependence on Ras and presumably developed a complement of mutations necessary for metastatic spread. Array comparative genomic hybridization (aCGH) studies on ‘escaper’ lesions led to the discovery of NEDD9, a gene amplified in murine and human metastases. NEDD9 has been shown to be involved in regulating cell shape and plasticity (Croft and Olson, 2008; Sanz-Moreno et al., 2008), but there are as yet no reports on its targeted suppression of metastases. This study reinforces the idea that at least some of the fundamental genetic changes in murine melanoma progression are conserved between mouse and man.

In general, linking particular mutations carried by GEMMs of melanoma to the propensity for metastatic spread is in its infancy, and measures of metastatic propensity such as lesion size at spread, mitotic rate, ulceration, time to spread, site of metastasis etc. are rarely reported. When they have been (e.g. Landsberg et al., 2010; Monahan et al., 2010; Terzian et al., 2010), important findings have been forthcoming. Genetically engineered mouse models with inducible mutations provide excellent systems to assess the effects of mutational events during different stages of tumorigenesis. Many mice carrying inducible MC-specific mutations are available for detailed studies examining the relationship between the timing of mutational events and tumour progression (e.g. Dankort et al., 2009; Dhomen et al., 2009; Huijbers et al., 2006; Jeong et al., 2008; Monahan et al., 2010; VanBrocklin et al., 2010).

Relevance of mouse melanoma to human clinical and phenotypic classification

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Arguably the most important aspect of melanoma classification is the appraisal of the stage of progression. This would include assessment of clinical (e.g. change in shape, size, pigmentation etc.) and histological signs of malignant progression, such as variable cell size, abnormal nuclear size and shape, condensed chromatin, enlarged nucleoli, mitotic figures, in concert with overall architectural features such as asymmetry, epidermal contour, type of pagetoid spread, circumscription and dermal invasion (Breslow thickness) (Balch et al., 2009, 2010). Current clinical staging, including the Tumour, Node, Metastasis system and the AJCC staging system, integrates information on thickness, ulceration, mitotic rate, presence of regional nodal metastasis and presence and site of distant metastasis to determine how far a lesion has progressed. Given the prognostic significance of these phenotypic staging systems, and the fact that many of the same features appear in melanomas arising on GEMMs, it is possible that the mouse may be used to fully explore the molecular mechanisms and pathways underlying key phenotypic transitions during human melanoma progression.

Over the last 10 yr, ways of looking at melanoma classification have evolved to include linking histopathological and phenotypic features of tumour progression to specific genetic mutations (Miller and Mihm, 2006). Viros et al. (2008) have shown that BRAF-mutant lesions exhibit increased scatter and nesting within the epidermis and that cells are larger and more rounded compared to those without BRAF mutations. These histological parameters are better predictors of BRAF status than subtype. Yet other features, including anatomical site, age-of-onset and relationship with sun exposure, provide additional ways of stratifying melanomas apart from the classical subtypes (reviewed in Whiteman et al., 2011). The melanoma classification system began with the initial recognition of different subtypes of melanoma by matching clinical presentation with histopathological features, and the subsequent integration of molecular features has validated and refined the initially proposed taxonomy (e.g. Viros et al., 2008).

Unlike most human melanomas, lesions from GEMMs, or carcinogen-treated wild-type mice, at least on the commonly used C57BL6 strain background, are dermal, generally with very high levels of pigment (e.g. Florell et al., 2007; Shapiro et al., 1996). In addition, mouse models carrying multiple mutations including deletion of cell cycle regulators can sometimes develop lesions with extreme cellular atypia not frequently seen in human melanomas. Nonetheless, murine melanomas often share histopathologic and phenotypic features with their human counterparts (summarized in Table 1). Many of these features vary in different mouse melanoma models and therefore they may represent an informative means for meaningful comparison with human tumours. To improve the relevance of murine melanoma models, we propose that clinical and morphological features of human melanomas may also be used as a guide to enhance the characterization of murine lesions. We approach this by assessing the utility of GEMMs as models for human melanoma progression, in the context of different subtypes of primary lesion.

Table 1.   Mouse melanoma models
 Histopathology/behaviorPotential mouse modelReferences
  1. Hgf, hepatocyte growth factor; LMM, lentigo maligna melanoma; LSL, Lox-Stop-Lox; NM, nodular melanoma; SSM, superficial spreading melanoma; UVR, ultraviolet radiation.

  2. aAmerican Joint Committee on Cancer % positive sentinal lymph node shown for SSM; data is similar for NM and LM (Balch et al., 2009).

  3. bSpontaneous or UVR-induced melanomas only. Arf, Ink4a and Pten refers to the null genotype for these genes.

Premalignant lesionsJunctional naevusMt-Hgf, Tyr-Nras::Arf::Kitlg?Noonan et al., 2001; Walker et al., 2011
Dermal naevusLSL-Gnaq; most onco-transgenics 
Compound naevusMt-Hgf, Tyr-Nras::Arf::Kitlg?Noonan et al., 2001; Walker et al., 2011
Dysplastic/atypical naevusMt-Hgf, Tyr-Nras::Arf::Kitlg?Noonan et al., 2001; Walker et al., 2011
Primary MelanomaHair covered skinMost mouse models 
Soles, palms, mucosal membranesLSL-KitL576P, LSL-BrafV600EDhomen et al., 2009; Dankort et al., 2009
Metastatic melanoma
 SizeBreslow thicknessa (<1 mm; >1 mm)Tpras, Mt-Ret (5–10 mm2)Terzian et al., 2010; Kumasaka et al., 2010
 SiteLymph nodesbTyr-Nras::Ink4a, Tyr-BrafV600E, LSL-BrafV600E, Tyr-Hras::Ink4a/Arf, Dct-Grm1, Mt-Hgf::Ink4a/Arf, Mt-Ret, Tyr-SV40E, TprasDamsky and Bosenberg, 2010
 BrainMt-Retvon Felbert et al., 2005
LiverTyr-Nras::Ink4a, Mt-Hgf, Mt-Hgf::Ink4a/ArfDamsky and Bosenberg, 2010
LungTyr-Nras::Ink4a, Dct-Grm1, LSL-BrafV600E::Pten, Tyr-SV40EDamsky and Bosenberg, 2010;
Histological subtypes of melanomaSSM-likeMt-Hgf, Tyr-Nras::Arf::KitlgNoonan et al., 2001; Walker et al., 2011
NM-likeMost mouse models 
LMM-likeKitlg::Xpa (+chronic UVR)Yamazaki et al., 2005
UVR exposureChronic exposureK14-Kitlg, K14-Kitlg::XpaYamazaki et al., 2005
Intermittent exposureMost models (neonatal UVR) 

Human melanoma histologic subtypes and their recapitulation in mouse models

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Little attention has been given to modelling the phenotypic and histopathologic features of each major subtype in mice. Given that specific mutation spectra are being elucidated for human melanoma subtypes (e.g. Romano et al., 2011), it is important to consider how well the reverse could be true and that the induction of specific mutations in mice will lead to specific murine melanoma subtypes morphologically resembling their human counterparts. Here, we describe the major histological subtypes of cutaneous melanoma, their best available ‘molecular signatures’, and discuss how these subtypes may be modelled in mice.

Superficial spreading melanoma (SSM)

Superficial spreading melanoma is the most common histologic subtype in Caucasians, accounting for around 70% of newly diagnosed disease. It generally follows a radial growth phase, beginning in the epidermis (melanoma in situ), and spreading laterally with minimal extension into the dermis. The presence of atypical MCs, either as single cells or nests at all levels of a usually slightly thickened epidermis, essentially defines SSM (Smoller, 2006). Over time, vertical growth can be initiated, with melanoma cells extending, usually in dense sheets, throughout the dermis. Clinically, the depth of extension into the dermis, and rarely into the subcutis, is highly predictive of outcome (Balch et al., 2001). Hence, it is critical to understand how melanoma cells behave in the dermis, where they are totally separated from any possible restraining influence of keratinocytes. Superficial spreading melanoma can arise from a pre-existing common or atypical naevus (in about 35% of cases), or de novo, skipping a premalignant naevus phase altogether (Bevona et al., 2003).

As BRAFV600E is the predominant oncogenic event in SSM (Table 2, Lee et al., 2011), to recapitulate SSM, we might begin by expressing this mutation in mice. Several such models have been generated. Two transgenic mouse lines expressing BrafV600E constitutively in MCs have been created by Goel et al. (2009). The higher expressing line developed senescent naevi, and melanoma, whereas the low expressing line had naevi and only few melanomas. In addition, ‘knock-in’ models that enable conditional expression of BrafV600E under control of its endogenous promoter have been reported. One model, from Martin McMahon’s laboratory, had hyperpigmentation and naevi with heterozygous and homozygous expression of BrafV600E and no melanomas. Deletion of Pten was required for overt neoplasia (Dankort et al., 2009). The second model, utilized by Richard Marais’ group, also developed naevi, with melanomas appearing in approximately 50% of heterozygous BrafV600E mice by 1 yr of age (Dhomen et al., 2009). Disparities between these two studies could pertain to BrafV600E expression level or genetic background differences. Interestingly, increased copy numbers of mutant BRAFV600E may be found in a proportion of human lesions showing that variation in mutant BRAF expression level is important in human melanomagenesis (e.g. Willmore-Payne et al., 2006). Despite some differences between these models, they both confirm the role of BrafV600E as a melanoma initiator.

Table 2.   Summary of key oncogenic and tumor suppressor mutations in human melanoma subtypes
Gene mutatedSSM (%)NM (%)LMM (%)ALM (%)Mucosal (%)Uveal (%)
  1. ALM, acral lentiginous melanoma; LMM, lentigo maligna melanoma; NM, nodular melanoma; SSM, superficial spreading melanoma.

BRAF49412225100
NRAS1727141750
KITNil∼1∼217a17a0–33a
PTEN621 0  
P5361850 300
GNAQ<1<1<10046
GNA110000032

Nonetheless, mouse BrafV600E lesions are located almost totally within the dermis, and therefore like most other models, they do not recapitulate some of the key features of human SSM. For example, a radial growth phase is absent prior to tumour extension into the dermis. This is likely related to the underlying absence of epidermal MCs, at least in hair-covered mouse skin, and the fact that in virtually all mouse models, the introduction of mutations into MCs, by genetic engineering (Damsky and Bosenberg, 2010), or via carcinogen treatment (Shapiro et al., 1996), results in the appearance of melanocytic proliferations in the interfollicular dermis, apparently the preferred location of ‘initiated’ MCs in mouse skin. However, it should be noted that some epidermal involvement does occur, particularly in the ear, tail and footpads, when mutant Braf MCs are further destabilized by concomitant deletion of Pten (Dankort et al., 2009). One potentially very good murine model for SSM is the Mt-Hgf transgenic, in which melanomas often exhibit pagetoid spread (Noonan et al., 2001). After neonatal UVR exposure, these mice tend to develop epidermal lesions (Noonan et al., 2001), suggesting that the Mt-Hgf mice may model SSM arising after intermittent sun exposure. K14-Kitlg mice, which have epidermal MCs throughout life (Kunisada et al., 1998), may represent another. When combined with Tyr-NrasQ61K::Arf−/− mice that had only dermal lesions (Figure 1A), Tyr-NrasQ61K::Arf−/−::K14-Kitlg mice developed epidermal lesions reminiscent of SSM (Figure 1B), characterized by atypical MCs within all levels of the epidermis. Atypical MCs were to some extent also present beneath the epidermis in the papillary dermis. In addition, we have generated Tyr-NrasQ61K::Cdk4R24C/R24C::K14-Kitlg mice (G. Walker, unpublished) and found that lesions once more exhibit significant epidermal involvement (Figure 1C) and are evocative of human SSM (Figure 1D). In these models, the localization of the murine lesions is totally controlled by only one keratinocyte cytokine, Kitlg. The presentation of melanomas in the epidermis of K14-Kitlg and Mt-Hgf mice suggests that dermal location of melanoma no longer presents an insurmountable barrier for mouse models to recapitulate key features of human melanoma. As reproducing SSM in mice is in its infancy, modelling of the vertical growth phase has not yet been performed. Interestingly, mice carrying K14-Kitlg along with MC-specific Grm1 activation (Abdel-Daim et al., 2010) developed melanomas earlier than MC-specific Grm1 alone, but all lesions were dermal. Why this is different from the above models is unknown, but genetic modification design differences and strain background differences must be examined. One important complication is that in the Tyr-NrasQ61K model, all MCs express the mutation developmentally, whereas in the Grm1 model, the oncogene is only activated in adult mice. However, it is tempting to consider that Grm1 activation may override the effects of K14-Kitlg, again underlining the idea that particular genes specify either dermal or epidermal location of MCs and lesions. Notably, Gnaq and Gna11, which act in the same signalling pathway as Grm1, seem to confer dermal location of both murine and human melanocytic lesions (Van Raamsdonk et al., 2010).

image

Figure 1.  Histopathology of dermal and epidermal melanomas in genetically modified mice. Dotted yellow arrows denote atypical melanocytes (MCs) in the epidermis. Dotted white arrows indicate atypical MCs in the dermis. (A) Three magnifications of an H&E section of a deep dermal melanoma (MM) from an Arf−/−::Tyr-NrasQ61K mouse. The NM is clearly separated from the epidermis by layers of collagen. The solid yellow arrow denotes a NM composed of sheets of pleomorphic MCs. (B) Bleached H&E section of a melanoma from an Arf−/−::Tyr-NrasQ61K::K14-Kitlg mouse. The dermis is highly pigmented without bleaching and contains many melanophages (Walker et al., 2011). There is an increased number of MCs, varying in size and shape, not only at the dermo-epidermal junction but also at all levels of the epidermis. (C) Bleached H&E section of melanoma from Cdk4R24C/R24C::Tyr-Nras::K14-Kitlg mouse. Again there are atypical MCs at all levels of the epidermis and also in the hair follicle outer root sheath. (D) Human superficial spreading melanoma. Yellow scale bars – 500 μM. White scale bars – 100 μM.

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Nodular melanoma

Nodular melanoma is primarily situated in the dermis and frequently has some epidermal involvement. Both epidermal and dermal components are sharply circumscribed, with a clear lack of epidermal radial extension (Smoller, 2004). Nodular melanoma was initially defined as originating from the epidermis. In fact it was recently suggested that NM is simply an accelerated form (one in which the transition from radial growth phase to vertical growth phase happens quicker) of other melanoma types [i.e. SSM, lentigo maligna melanoma (LMM), etc.], thus placing it on a lower taxonomic level than these types (Broekaert et al., 2010). However, the initial view that all NM is derived from epidermal MCs has been challenged with the proposal that some cases of NM may in fact arise from putative dermal MCs (Zalaudek et al., 2008), a notion supported by mouse models where melanoma origin from a rare dermal MC population is not disputed. The extensive dermal component of NM is characterized by marked cellular atypia, composed of either epitheloid cells, spindle cells or a mixture of both. Approximately 11% of NMs have associated naeval remnants (Bevona et al., 2003) that appear to be parts of dermal naevi, not compound dysplastic naevi. Notwithstanding any debate about epidermal versus dermal origin of NM, it is typically well circumscribed with an overwhelmingly dermal location. Thus, it is possible that the great majority of mouse melanomas mimic NM reasonably well, although better cross-species morphological comparison of melanomas is needed to substantiate this. We do know that as with human NM, murine dermal melanomas often exhibit mixed epitheloid (e.g. Dhomen et al., 2009) or spindle-shaped cell morphology (e.g. Huijbers et al., 2006; Landsberg et al., 2010; Monahan et al., 2010; Yang et al., 2007) in the same mouse, or even in the same tumour (e.g. Landsberg et al., 2010; Yang et al., 2007). Some of the murine lesions are reminiscent of blue naevi, malignant blue naevi or naevoid melanoma (e.g. Dhomen et al., 2009; Ferguson et al., 2010; Landsberg et al., 2010).

It is difficult to define a clear mutational signature that differentiates SSM from NM. Nodular melanoma tends to carry more p53 mutations, although the difference is only marginally significant (Hocker and Tsao, 2007). Forty-one per cent of all NMs carry BRAFV600E versus 49% in SSM (Lee et al., 2011), suggesting that oncogenic BRAF is important in both subtypes and that the Lox-Stop-Lox (LSL)-BrafV600E mice may also be useful as a model for NM. Moreover, 27% of NMs carry NRAS mutations, implying that mice with these mutations may model NM. Given that similar molecular pathways appear in SSM and NM, it is interesting to speculate that epidermal or dermal site of origin may not be such a critical factor at least in terms of later stages in melanoma progression and that the key lessons to be learned from largely dermal mouse melanomas may be more relevant to SSM progression than is currently appreciated. Notwithstanding the similarities, Rose et al. (2011) have detected genomic deletions of various genes that occur in SSM, but not NM, suggesting that there are some independent pathways in the respective subtypes.

Clearly the most important question is whether we can learn anything about genotype–phenotype correlations from murine dermal NMs. An exciting example has arisen in comparing mouse melanomas with NrasQ61K and Cdk4, Arf or p53 mutations on the same strain background (Ferguson et al., 2010). The resulting dermal lesions showed enormous genotype-specific differences in cellular and nuclear size, shape, proliferative index and the expression of pigmentation markers. Here, melanomas with alterations in different molecular pathways exhibit diverse phenotypic and behavioural characteristics. It is possible that such murine studies may help to further classify NM based on histopathology and mutational status. This may improve prognostic evaluation of this subtype and may also be important for melanoma therapy considerations.

Lentigo maligna melanoma

Lentigo maligna melanoma is the only subtype unequivocally associated with chronic sun exposure. Lesions display confluent spread of MCs along the epidermal basal layer and in the upper portion of the hair follicle and are invariably associated with moderate to severe solar elastosis in the underlying dermis. Lentigo maligna melanoma is thought to arise from epidermal MCs, although follicular derivation is also proposed (Zalaudek et al., 2008; Box et al., 2010). Progression in LMM also entails radial spread, which can proceed for many years. As with SSM, it is only after vertical spread into the dermis that the lesions present a serious threat (Smoller, 2006). Nevertheless, LMM is very different from SSM. Lentigo maligna melanoma is less frequently derived from naevi, has a much later age-of-onset, exhibits characteristic changes in the epidermis (atrophy) and dermis (solar elastosis), and shows a distinct pattern of MC distribution (lentiginous versus pagetoid spread) (reviewed in McKenna et al., 2006). Hence, LMM appears to develop via a unique pathogenic mechanism dominated by chronic sun damage (CSD) over many decades. Molecularly, LMMs are also very different from SSM, with only 10–22% harbouring BRAFV600E mutation (Maldonado et al., 2003; Curtin et al., 2005; Hocker et al., 2007; Lee et al., 2011), and they appear to carry p53 mutations much more often (Hocker et al., 2007). In addition, some LMM, but not SSM, carry KIT mutations (Curtin et al., 2006). It is likely that the single most important risk factor driving the development of LMM is the cumulative sun damage rather than a gene signature or genetic susceptibility per se, although mutational targets of UVR such as p53 may be important.

In mice, critical features of LMM such as solar elastosis and follicular involvement are rarely seen. The results of chronic UVR damage on interfollicular epidermal MCs may be largely redundant in mouse models, as they generally do not have many epidermal MCs that will acquire UVR-induced DNA adducts. Therefore, at this stage, no good murine models for LMM have been developed. Nevertheless, the key feature of LMM, solar elastosis, is recapitulated in mice after chronic UVR exposure (Kligman and Sayre, 1991). One model, carrying stabilized β-Catenin along with NrasQ61K (Tyr-NrasQ61K::Bcatsta), develops melanomas that frequently exhibit involvement of the upper portion of the hair follicle (Delmas et al., 2007). Thus, this particular feature of LMM is in fact seen in mice, and thorough histopathologic assessment of early lesions in other models might also reveal signs of follicular origin of melanomas.

Acral lentiginous melanoma (ALM)

Acral lentiginous melanoma may occur on palmoplantar skin and in the nail apparatus unit. Histopathologically, they exhibit a confluent array of atypical MCs along the epidermal basal layer before extending into the dermis. They can also show pagetoid spread, although less pronounced than in SSM. Acral lentiginous melanoma accounts for approximately 10% of melanomas (Piliang, 2011) and it occurs at the same rate in all ethnic backgrounds, independently of pigmentation. Acral lentiginous melanoma is thought to occur independently of UVR-induced damage, and the predominant feature differentiating it from other lesions is the anatomic location. A major difference between palmoplantar skin and skin from most other sites is the very low numbers of MCs. This may be due to mesenchymal fibroblast expression of the Dickopf gene (DKK1), a Wnt/β-Catenin pathway antagonist critical in suppressing MC number (Yamaguchi et al., 2004) and/or the very low levels of KITLG and EDN1, in acral epidermal keratinocytes (Hasegawa et al., 2008).

Molecularly, ALM is less likely to harbour BRAF mutations than SSM (25 versus 49%); rather they frequently have activating KIT mutations (Beadling et al., 2008; Curtin et al., 2006; Lee et al., 2011). Somatic KIT mutations (mainly D816V) are frequently found in mast cell proliferations (mastocytosis) (Metcalfe, 2008). KIT is also overexpressed in many non-mutated lesions (Wang et al., 2009). In a mastocytosis transgenic mouse model, expression of the KitD816V variant mimicked the human disease (Zappulla et al., 2005), indicating that the biological consequences of KIT activation are similar in mouse and human. To accurately model ALM in mice, the knock-in LSL system that was used in the inducible MC-specific Braf models needs to be developed. Given that the most common KIT mutation in melanoma is the L576P variant (Garrido and Bastian, 2010), we would predict that an LSL-KitL576P mouse would be desirable. Normally, mice have few MCs in the footpads, although K14-Kitlg and ‘high p53’ mice have hyperpigmented footpads (Kunisada et al., 1998; McGowan et al., 2008; Terzian et al., 2011), with MCs in the acral epidermis. It is possible with the right study design that these models may be useful in for generating ALM-like melanomas in mice. In the Tyr-NrasQ61K::Arf−/−::K14-Kitlg mice atypical MCs can be seen throughout the acral epidermis (Walker et al., 2011), although it is as yet unclear whether or not such lesions mimic ALM in situ.

Mucosal melanoma

Mucosal melanomas share many of the histopathological characteristics of ALM, but differ mainly in the anatomic sites involved. Like SSM, they are characterized by the presence of MCs singly or in nests, presumably beginning in the basal layer and then extending into the dermis (Smoller, 2006). Mucosal melanoma stands out from the other four subtypes (SSM, NM, LMM and ALM), in that these melanomas have a very low frequency of BRAF and NRAS mutations (Table 2). Around 16.5% on average carry activating changes in the KIT gene (reviewed in Garrido and Bastian, 2010). Several murine models develop melanomas on mucosal surfaces, although the lesions are invariably dermal, reminiscent of congenital naevi (Dhomen et al., 2009; Pollock et al., 2003; Umansky et al., 2008). However, as the best available mutation signature for mucosal melanoma is KIT mutation, a murine model for mucosal melanoma also awaits the generation of LSL-KitL576P mice.

Uveal melanoma (UM)

Uveal melanoma is the predominant form of ocular melanoma. Uveal melanoma occurs in the uveal tract of the eye, including the choroids, ciliary body and the iris. Conjunctival melanoma, appearing in the thin lining, or conjunctiva, of the eye, or in the eyelid, occurs much less frequently. Uveal melanoma is the most common cancer of the eye, and its incidence increases with age with the most cases diagnosed in people in their 50s and older. There is evidence for pigmentary and UVR risk factors for UM (Meyle and Guldberg, 2009; Schmidt-Pokrzywniak et al., 2009; Shields and Shields, 2007). Subjects with Xeroderma Pigmentosum or with developmental oculodermal melanocytosis (naevus of Ota) are also at greatly elevated risk for UM (Johnson et al., 1989; Singh et al., 1998). Up to 50% of patients with UM will have metastatic disease, the majority of which will spread initially to the liver (Shields and Shields, 2006). Uveal melanoma appears to have a distinct molecular signature (Table 2), with frequent activating mutations in the GNAQ and GNA11 oncogenes that effectively induce RAS/RAF/MAPK signalling in uveal MCs (Van Raamsdonk et al., 2009, 2010). This mechanism appears to substitute for BRAF and NRAS mutations, which are virtually absent during uveal melanomagenesis (Cruz et al., 2003; Rimoldi et al., 2003; Saldanha et al., 2004; Zuidervaart et al., 2005). Similarly MAPK pathway activation via mutations in KIT was initially thought to be rare in UM (Beadling et al., 2008; Hofmann et al., 2009; Pache et al., 2003), although a recent study has detected them in around 33% of primary UM (Wallander et al., 2011), and overexpression of the KIT protein is frequently observed (Hofmann et al., 2009). p53 mutation and overexpression is also rarely observed in UM (Chana et al., 1999; Kishore et al., 1996), while studies of PTEN mutations have been restricted to UM cell lines (Abdel-Rahman et al., 2006; Naus et al., 2000).

Morphologically, mouse MCs appear in similar locations in the eye to their corresponding locations in human eyes, including in the uveal tract and in the retinal pigmented epithelium (RPE). In the original UM transgenic mouse model, expressing the oncogenic simian virus 40 early region (SV40E) under the control of the tyrosinase promoter (Tyr-SV40E), UMs arise in similar locations to human UM, with melanomas appearing in the choroid, ciliary body and iris (Bradl et al., 1991). Nevertheless, a preponderance of melanomas had a tendency to arise in the RPE in these mice, a location rarely involved in human ocular melanoma (Bradl et al., 1991; Mintz and Klein-Szanto, 1992). Retinal pigmented epithelium-derived lesions were also predominant in the independently generated Tyr driven SV40 large T antigen Tyr-Tag A and Tyr-Tag B mice (Syed et al., 1998), as was the case with the TPras melanoma model that expresses mutant HRAS under the Tyrosinase promoter (Kramer et al., 1998), although UMs were also reported in this line. In contrast, around 15% of Tyr-HrasG12VInk4a/Arf+/− mice had spontaneous UM by 6 months of age (Tolleson et al., 2005; Latendresse et al., 2007) highlighting the importance of RAS pathway activation in UM, and indicating that mice can faithfully replicate the histological origin of human melanoma. While the Tyr-SV40E RPE tumours may metastasize to the liver, none of the reported UM cases in these mouse models have demonstrated metastasis to the liver, as happens in humans. It is clear that further work is needed to advance mouse models that genotypically and morphologically recapitulate human UM genesis and progression. One could imagine that loss of Ink4a/Arf in Gnaq- or Gna11-mutant mice with activating mutations corresponding to the frequent mutational hotspots in human UM would be a good model for this subtype.

Mutation-defined melanoma subtypes

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Earlier we have focused on major cutaneous histological subtypes, as these distinctions are critical in assessing prognosis and treatment options. The mutations signatures in each subtype are rarely seen in more than half of the tumours in each subtype. It is therefore likely that each subtype develops as a consequence of environmental factors and/or unknown innate factors together with additional destabilizing mutations not seemingly common to all lesions in each subtype. Thus, it may be necessary to classify melanomas on the basis of the mutations they carry (Vidwans et al., 2011; Viros et al., 2008). For example, most melanoma subtypes carry BRAF mutations (Table 2), although they are absent in UM and blue naevus associated melanoma and rare in mucosal melanoma. Therefore, most subtypes are potentially treatable with BRAF-inhibiting drugs, and thus, it may now be necessary to use a new treatment-dependent classification of melanomas in addition to or in place of conventional staging. Mouse models may prove to be key in preclinical assessment of molecularly targeted therapies in different genotype sub-groups to ensure that drugs are not mis-applied. For example, studies in mice have identified a role for specific p53 activating drugs such as Nutlin-3 in the therapy of some cancer types (Vassilev et al., 2004); nevertheless, in the presence of p53 mutations, use of such drugs may actually promote tumour progression and worsen the outcome for the patient (Terzian et al., 2008). On another note, it is possible that a new disease model linking tumour genotype with various parameters measured clinically and by non-invasive surveillance techniques may assist clinicians in predicting mutation status and concomitant treatment and outcome, thus improving standard of care. Vidwans et al. (2011) have elegantly outlined the concept of specific therapies for each melanoma genotype, and given the ability of GEMMs to recapitulate these mutations, we need to partner our mouse models with these concepts. This may include better ‘clinical’ observation of murine melanoma growth and/or the use of non-invasive techniques such as in vivo microscopy to properly assess melanomas in mice carrying a specific mutation. In this way, mutation status and clinical parameters could be better matched with lesion appearance.

Use of GEMMs to discover novel melanoma pathways

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Genetically engineered mouse models are readily used as a discovery tool to investigate the in vivo consequences of alterations to specific genetic pathways. Often in such studies, unanticipated findings arise. With this in mind, one would expect that mice carrying an activated Ras mutation with Cdkn2a deletion would under all circumstances develop melanoma. However, when an additional mutation is created in their MCs, such as deletion of the Atf2 transcription factor (Shah et al., 2010) or deletion of Ikkb, a signalling kinase that causes constitutive activation of the NF-κB pathway (Yang et al., 2010), melanoma development is greatly diminished. These results highlight the power of the mouse for mechanistic studies of melanomagenesis and offer insights into the MC intracellular pathways that could provide potential targets for chemoprevention or therapy. Other GEMMs have shown that keratinocyte-specific mutations can dramatically influence melanoma progression. For example, keratinocyte-specific deletion of the nuclear receptor RXRα, or the TFIID subunit TAF4, (Fadloun et al., 2007; Hyter et al., 2010), or overexpression of Kitlg (Abdel-Daim et al., 2010), all enhance melanomagenesis. Evidently there is tremendous scope to use mouse models to target mutations to specific cell types within the skin to help us understand how changes in the interplay between MCs and their microenvironment can influence melanoma behaviour.

Sunlight and melanoma, a complex relationship

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Although epidemiological evidence is overwhelming that sun exposure increases melanoma risk, it is only true for intermittent rather than chronic exposures (Gandini et al., 2005). Examples of intermittent sun exposures include number of waterside vacations and number of severe sunburns, whereas information on chronic exposures can be captured by recording the number of hours outdoors during a summer day, or outdoors occupations. Unlike melanoma, keratinocyte-based skin cancers like squamous cell carcinoma (SCC) are clearly linked to chronic sun exposures. One melanoma subtype, LMM, is invariably linked to chronic sun exposure. Both melanoma and non-melanoma skin cancer risk is intricately dependent on pigmentation characteristics. The fact that there are different UVR exposure profiles linked to melanoma and non-melanoma skin cancers is not surprising. MCs are long-lived cells, resistant to apoptosis, whose principal function is to produce melanin. In contrast, the primary function of keratinocytes is to provide a protective barrier, the epidermis, which is in a continual state of regeneration, supplied by proliferation of epidermal basal layer keratinocytes in a programmed process of differentiation and apoptosis as needed. Of note, melanoma cannot be induced by chronic UVR treatment of wild-type mice, whereas SCC can. All mice where UVR induces melanoma are genetically modified to carry mutations that affect oncogenic, cell cycle or DNA damage repair pathways.

UVR and divergent pathways to melanoma

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

The divergent pathway hypothesis (Whiteman et al., 2003) is one plausible explanation for the unusual relationship between UVR exposure and melanoma. Two fundamental pathways are postulated. The first is characterized by an older age-of-onset, and lesions predominantly emerge from chronic sun-exposed sites (e.g. head and neck). Histologically, the surrounding non-lesional skin has solar elastosis and other signs of CSD. In the second pathway, melanomas emerge with an earlier age-of-onset, presumably after low or intermittent sun exposure and frequently on less sun-exposed sites (e.g. the trunk). Melanoma arising via this pathway is thought to result from a strong genetic risk component, as evidenced by accompanying high naevus counts in many of these subjects (Whiteman et al., 2003). This idea is gleaned from epidemiological studies, although support may be found in a number of molecular studies. Array CGH experiments on a cohort of melanomas have found significant differences in genomic instability at specific chromosomal locations including amplification of the CCND1 gene (encoding cyclin D1) in CSD but not non-CSD tumours (Curtin et al., 2005). Nearly all CSD lesions were LMM and nearly all non-CSD lesions were SSM. It is clear that BRAFV600E is a signature for the non-CSD pathway (Lee et al., 2011). Further studies discriminate CSD and non-CSD melanomas using many parameters such as BRAF status, age-of-onset, body site and association with naevi (e.g. Bauer et al., 2011; Hacker et al., 2010; Maldonado et al., 2003; Viros et al., 2008). All of these studies include LMM in their analyses, and to some extent, the distinction of CSD versus non-CSD melanoma relates to LMM versus SSM. However, there is a group of SSMs that are associated with CSD (Viros et al., 2008), and the CSD group may contain lesions that are difficult to assign histologically to either SSM or LMM. At this point, it may be that we are best served by modelling LMM for CSD melanoma in mice. For LMM, decades of UVR are presumably essential to produce chronic changes in the skin, and the mutagenic/carcinogenic process could be very different than that for CSD-associated SSM, where UVR exposure is not necessarily needed but probably exacerbates the process.

Modelling non-CSD melanoma in mice

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

As non-CSD melanomas have a relatively early age-of-onset, intermittent UVR exposure in childhood and adolescence, known to increase melanoma risk, appears to be an important factor. By far the most effective way of using UVR exposure to induce melanoma in mice is via a single neonatal treatment, and one can argue that this may mimic childhood sun exposure quite well (Noonan et al., 2001). Whether neonatal UVR exacerbates murine melanomagenesis in mice carrying the non-CSD BrafV600E signature has not yet been reported. But, as mentioned earlier, mice carrying K14-Kitlg and Mt-Hgf transgenes may be potential non-CSD melanoma models as they can develop SSM-like tumours, and it has been shown at least in the latter, that a single neonatal sunburn is sufficient to induce melanoma (Noonan et al., 2001).

It is not yet known why neonatal UVR so efficiently induces melanoma in GEMMs, however, a number of factors may play a role: (i) there is a muted inflammatory response to UVR in neonates (McGee et al., 2011; Wolnicka-Glubisz et al., 2007), (ii) there is a heightened sensitivity of neonatal MCs to proliferation following UVR (Walker et al., 2009) or (iii) neonatal mice have epidermal MCs while adult mice do not. In terms of MC behaviour, neonatal mouse and human skin differ somewhat (Gleason et al., 2008), but in both cases MCs are naturally proliferating to accommodate increasing skin area. Although thought to be immature, murine neonatal epidermal MCs express MC differentiation markers like Tyrp1 (Walker et al., 2009) and produce pigment (Hirobe, 1984). Hence, they appear to be a reasonable model for a differentiated MC, albeit within a dynamic keratinocyte microenvironment dictated by the developing hair follicle. Supporting this view, experiments using inducible p53 and Ink4a knockouts in cooperation with MC-specific Kras found that both neonatal and adult MCs are ‘transformable’ with similar efficiencies (Monahan et al., 2010). We therefore conclude that neonatal MCs are not entirely different from MCs observed in human skin.

Neonatal UVR has been used with murine models such as Mt-Hgf to provide important information about UVR carcinogenesis in melanoma. With lesions induced by a single sunburn, such models provide tractable experimental systems to determine a melanoma action spectrum, to assess which type of UVR-induced DNA adducts are required, and to study the role of UVR-induced oxidative stress, inflammation and immunosuppression. For instance, the wavelength dependence of melanoma induction in albino Mt-Hgf mice is overwhelmingly within the ultraviolet-B (UVB) range (De Fabo et al., 2004), similar to the action spectrum for SCC in mice and the inferred action spectrum for human SCC (de Gruijl et al., 1993). These mice have been used to show that sunscreens protect against neonatal UVR-induced melanoma, in support of human data on this subject (Green et al., 2011; Klug et al., 2010). Goodson et al. (2009) showed that decreasing oxidative stress using an oral antioxidant can reduce the age-of-onset of UVR-induced melanoma in this model. Moreover, the proliferative burst of murine MCs following neonatal UVR has revealed a novel interaction between the macrophages and MCs in melanomagenesis. Zaidi et al. (2011) utilized the power of the GEMM by inducibly expressing green fluorescent protein in neonatal MCs. Immediately after UVB exposure, MCs were isolated by flow cytometry at various time points. Gene expression array analysis detected a strong signature of interferon-gamma (IFNγ)-induced genes that coincided with the appearance of MCs in the epidermis. It was subsequently shown that the MC response is largely driven by IFNγ released from infiltrating macrophages. These macrophages also contribute to the pro-tumourigenic inflammatory microenvironment of melanomas and enhance tumour growth (Zaidi et al., 2011).

Mice carrying mutations in genes involved in human melanoma susceptibility and progression have yielded valuable insights into the complex roles of UVR damage in melanomagenesis. Tyr-Hras::Arf−/− and Tyr-Hras::Ink4a−/− mice both form melanoma spontaneously, but neonatal UVR treatment increases penetrance only in Tyr-Hras::Arf−/− mice (Kannan et al., 2003). Spontaneous lesions developing on both backgrounds show a high degree of chromosomal instability, while the UVR-induced lesions were cytogenetically relatively intact. The strong effect of Arf compared to Ink4a loss in exacerbating UVR-induced melanoma has been confirmed in other studies (Ha et al., 2007; Ferguson et al., 2010). Interestingly, the effect of Arf loss may not be in decreased DNA damage repair capacity (G. Walker, unpublished; Ferguson et al., 2010), but because Arf is a very powerful tumour suppressor in mouse MCs. Work using inducible p53 mouse models of radiation-induced lymphoma (Christophorou et al., 2006) supports the notion that loss of p53 during the immediate damage response is inconsequential to tumorigenesis and that the effect of p53 in protecting against tumour development is only during the latency period. It is possible that Arf exerts its effects in UVR-induced melanoma development mainly on tumour latency. These studies again highlight the ability of GEMMs to provide insights regarding the timing of important mutations during tumorigenesis.

Genetically engineered mouse models may be used to assess whether UVR may somehow induce the oncogenic BRAF or NRAS mutations that are prevalent in non-CSD melanoma. Arguments for and against this are discussed elsewhere (Walker, 2008). Current models (e.g. Mt-Hgf) already carry oncogene activation at the time of UVR exposure. However, GEMMs without pre-existing oncogenic mutations have provided some evidence in this debate. Yang et al. (2007) UV irradiated neonates to generate melanomas in Ink4a/Arf−/−::Xpc−/− mice. Ink4a/Arf abrogation was indispensible for tumour initiation, whereas the DNA repair defect merely exacerbated it. Importantly, the resulting lesions frequently carried Kras mutations. Although KRAS mutations are rare in human melanoma, mouse models show that oncogenic Ras mutations can occur as a result of UVB-induced damage, whether it be via a mechanism related to incorrect repair of DNA adducts (the common BRAF and NRAS nucleotide changes in melanoma are not classical pyrimidine dimer UV signature mutations) or the creation of an environment permissive of such mutations forming, such as increased levels of oxidative stress (Saxowskya et al., 2008). Possibly the oxidative mechanism is more likely given that BRAF and RAS mutations are also observed in internal cancers.

Modelling CSD melanoma in mice

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Hairless mice

The pigmented hairless mouse is a potential model for chronic UVR-induced naevi and possibly melanoma. These animals carry a mutation in the hairless (Hr) gene and have immature hair follicles and some epidermal MCs – although most appear to be located in the dermis (van Schanke et al., 2006). In response to repeated UVB exposures, they become hyperpigmented. When irradiation is ceased, the skin colour reverts to that of non-irradiated skin. Intriguingly, at about 25 weeks after UVR cessation, pigmented spots, reminiscent of solar lentigines, develop on the dorsal skin of mice (Aoki and Moro, 2005). The spots contain large numbers of proliferating MCs in the epidermis. This suggests that there is a ‘molecular memory’ in the skin many months after the cessation of UVR treatment that drives MC proliferation. Investigation of the mechanisms of the delayed MC response suggest that IFNγ and other chemokines produced in the epidermis-induce migration of inflammatory cells, including macrophages, are part of a network that can influence MC proliferation (Aoki and Moro, 2005). This is somewhat reminiscent of the study of Zaidi et al. (2011) mentioned earlier. It would be interesting to determine whether these lesions are clonal, which would indicate that the molecular memory is carried within the MC(s), or polyclonal, more suggestive of keratinocytes driving the delayed MC proliferation. There may be ways to answer these questions by cell fate mapping using GEMMs carrying single (e.g. Clayton et al., 2007) or multiple (e.g. Livet et al., 2007) inducible fluorescent MC lineage markers that allow one to label single cells and subsequently trace their clonal cell lineages. Such studies may also be transferable to studies on the clonality of other melanocytic lesions developing in GEMMs after UVR exposure, including melanomas.

Hairless mice have not been reported to develop melanoma after UVR, and further mutations must be necessary for this to happen. van Schanke et al. (2006) have performed extensive UVR carcinogenesis studies on pigmented hairless mice carrying Ink4a/Arf deletion (sometimes with co-deletion of the nucleotide excision repair gene Xpa). Animals developed naevi at a low rate spontaneously which was greatly increased by UVB treatment (how much depended upon the protocol and genotype). However, the naevi rarely progressed. Naevus formation was dramatically increased by Xpa deletion, implicating pyrimidine dimer-type mutations in the pathogenesis of the lesions. Notably, high-dose intermittent erythemal exposures were much more effective at driving naevi than the same dose delivered daily as suberythemal exposures. The naevi (and presumably melanomas) were located in the dermis (van Schanke et al., 2006). Although these mice seemingly fail to model CSD melanoma, it is unclear how to model decades of sun exposure in mice, although possibly this could be performed with much longer periods of low-level UVR exposure than have so far been attempted. Perhaps a model is needed that better supports long-term MC localization in the epidermis.

K14-Kitlg mice

Although they have epidermal MCs, K14-Kitlg mice do not develop melanoma after chronic UVR exposures (Yamazaki et al., 2004). Even when K14-Kitlg mice were crossed with DNA repair defective Xpa-null mice, no lesions were detected in K14-Kitlg::Xpa−/− progeny when a chronic UVB exposure protocol consisting of a total dose of 72 kJ/m2 (over 30 weeks) was used. But when the radiation was increased to 150 kJ/m2 (over 10 weeks), just over one half of the K14-Kitlg::Xpa−/− mice developed melanomas. Two subtypes are observed. The first were epidermal lesions reminiscent of LMM, and the second nodular vertical growth phase melanomas (Yamazaki et al., 2005). Unfortunately, very high, almost physiologically irrelevant doses of UVB were required, in addition to the DNA repair defect, to induce transformation of the epidermal MCs in these mice. Interestingly, these mice developed very few SCCs, in contrast to wild-type mice submitted to the same UVR protocol (Yamazaki et al., 2004). These data suggest that the presence of the K14-Kitlg transgene is protective against SCC development. This is probably due to the protective effects of hyperpigmentation and may explain why such high UVB doses are required to induce melanoma. Hence, the murine epidermal MCs may not be resistant to the transformation by UVR per se, and LMM can develop. We believe that the K14-Kitlg model has not yet been exploited to its full potential and that albino K14-Kitlg mice exposed to long-term low-level UVR may be a potential model for CSD melanoma (i.e. LMM).

For UVR carcinogenesis studies, mice can provide information where epidemiological studies suffer from biases and confounding factors. There are many ways UVR damage can potentially initiate or promote melanoma, including through oxidative stress, inflammation and immunosuppression as well as through the formation of DNA adducts. It is clearly necessary to define the precise wavelengths (or type of DNA adducts) that induce melanoma to enable the optimum design of sunscreens and the implementation of improved regulations for solaria use. For the foreseeable future mouse models will be used for mechanistic studies of UVR-induced melanoma to inform such efforts. Notably, variation in many pigmentation genes (e.g. SLC45A2, MC1R, TYRP1, TYR and ASIP) confers increased risk for the melanoma (Gerstenblith et al., 2010). There are numerous murine coat colour mutants carrying known variants in these genes (http://www.espcr.org/micemut/), and such strains may prove useful, in conjunction with GEMMs, to assess how the pigmentation genes influence melanoma risk.

Are there better animal models than mice?

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

Grey horses and certain strains of pigs are models for genetic susceptibility to melanoma. The former is an interesting example of melanoma linked to a vitiligo phenotype. The causative mutation is a duplication effecting the expression of two genes, NR4A3 and STX17 (Rosengren Pielberg et al., 2008), which do not have a known role in pigmentation or melanoma. Nevertheless, NR4A3 is a downstream target of MC1R in MCs (Smith et al., 2008). Various strains of pigs, for instance Sinclair swine, are also susceptible to melanoma (Misfeldt and Grimm, 1994). Some other mammals also develop sporadic melanoma. In dogs, mucosal melanoma is predominant, although incidence is the strongest in heavily pigmented breeds (Smith et al., 2002). Opossom, guinea pigs and Angora goats can develop melanocytic lesions (Chan et al., 2001; Green et al., 1996; Menzies et al., 2004). Guinea pigs could be excellent models for SSM given that they form junctional naevi after UVB exposure (Menzies et al., 2004) and melanomas with nesting in the epidermis after carcinogen treatment (Pawlowski and Lea, 1983). The major problem with all of these species is that animal husbandry is extremely expensive, and they are not easily genetically modifiable to test the role of various mutations, and there is a general limitation in resources for genetic and biochemical analyses. Thus, while it would be desirable to develop these models, it is unclear how significant a role they will play as experimental animal models for human melanoma. Various strains of fish are also excellent and emerging models for melanoma, providing many if not all features of experimental manipulation provided by mice (reviewed in Patton et al., 2010), although they are somewhat further removed evolutionarily from humans. Finally, although not discussed here, the use of cultured human MCs and melanoma cell lines xenotransplanted into immunodeficient mice can also provide insights into the mechanisms of MC transformation (reviewed in Becker et al., 2010; Herlyn and Fukunaga-Kalabis, 2010).

Future directions

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References

There are undoubtedly differences between the behaviour of murine and human MCs. Moreover, MCs and melanoma in human skin exhibit significant variation between different anatomical location, in histopathology and in the levels of sun exposure that may play a role in the development of different types of melanoma. To make the best use of mouse models such differences should be taken into consideration. Expanded characterization of individual mouse tumours may enable greater comparison with corresponding genotype-defined human tumours. The emerging success of molecularly targeted chemotherapies presages the need to move beyond the present phenotypic staging methods for melanoma, and to ultimately develop a molecularly defined staging system where each subtype is identified on the basis of combined phenotypic and molecular properties. In pursuit of this, genotype–phenotype correlations for melanoma subtypes are presently being sought (e.g. Viros et al., 2008; Romano et al., 2011; Vidwans et al., 2011). More than ever, mouse models are needed for true in vivo preclinical evaluation of therapeutic strategies, their strengths and weaknesses and consequences when misapplied, and to identify the mechanisms of action of genes that are potential drug targets in melanoma.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Molecular pathways defective in melanoma and their modelling in mice
  5. Naevus formation in humans and in mice
  6. Melanoma development de novo or via naevus intermediates
  7. Metastatic melanoma
  8. Relevance of mouse melanoma to human clinical and phenotypic classification
  9. Human melanoma histologic subtypes and their recapitulation in mouse models
  10. Mutation-defined melanoma subtypes
  11. Use of GEMMs to discover novel melanoma pathways
  12. Sunlight and melanoma, a complex relationship
  13. UVR and divergent pathways to melanoma
  14. Modelling non-CSD melanoma in mice
  15. Modelling CSD melanoma in mice
  16. Are there better animal models than mice?
  17. Future directions
  18. Acknowledgements
  19. References
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