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Animal Models of Melanoma

  • Linan Ha
    Affiliations
    Laboratory of Cell Regulation and Carcinogenesis, National Cancer Institute, Bethesda, Maryland, USA
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  • Frances P. Noonan
    Affiliations
    Laboratory of Photobiology and Photoimmunology, Department of Environmental and Occupational Health, School of Public Health and Health Services, The George Washington University Medical Center, Washington, District of Columbia, USA
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  • Edward C. De Fabo
    Affiliations
    Laboratory of Photobiology and Photoimmunology, Department of Environmental and Occupational Health, School of Public Health and Health Services, The George Washington University Medical Center, Washington, District of Columbia, USA
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  • Glenn Merlino
    Correspondence
    Laboratory of Cell Regulation and Carcinogenesis, National Cancer Institute, Bethesda, Maryland 20892, USA.
    Affiliations
    Laboratory of Cell Regulation and Carcinogenesis, National Cancer Institute, Bethesda, Maryland, USA
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      HGF/SF
      hepatocyte growth factor/scatter factor
      UV
      ultraviolet
      Cutaneous melanoma is noteworthy for its therapeutic resistance, aggressive clinical behavior, and predisposition for late metastasis. In contrast to the declining incidence for many types of cancer, recent studies indicate that the incidence of melanoma has increased steadily in the last three decades (
      • Howe H.L.
      • Wingo P.A.
      • Thun M.J.
      • Ries L.A.
      • Rosenberg H.M.
      • Feigal E.G.
      • Edwards B.K.
      Annual report to the nation on the status of cancer (1973 through 1998), featuring cancers with recent increasing trends.
      ), which demands an improved understanding of the molecular mechanisms involved in the genesis and progression of melanoma. Although a causal role for the ultraviolet (UV) portion of solar radiation in melanoma etiology is well accepted (
      • Marks R.
      Epidemiology of melanoma.
      ), the functional relationship between genes and environmental sunlight in the pathogenesis of melanoma remains unclear. Basic melanoma research would benefit greatly from the availability of relevant animal models of melanoma, in which outstanding questions could be experimentally addressed. Unfortunately, investigations into the initiation and progression of melanoma have been hampered by the paucity of such animal models.
      An “ideal” animal model would accurately recapitulate human disease, particularly the UV-based etiology, the molecular genetics, and the histopathological architecture of cutaneous melanoma. Furthermore, it should be amenable to genetic and immunologic manipulation. Although a number of animal melanoma models have been described (reviewed in
      • Jhappan C.
      • Noonan F.P.
      • Merlino G.
      Ultraviolet radiation, and cutaneous malignant melanoma.
      ), which will briefly be recapitulated here, the histopathological appearance and graded progression of the arising melanocytic malignancies have been, for the most part, distinct from human cutaneous melanoma. Large animal models of melanoma have been reported, which include Sinclair swine and Camargue horse (
      • Fleury C.
      • Berard F.
      • Balme B.
      • Thomas L.
      The study of cutaneous melanomas in Camargue-type gray-skinned horses (1): Clinical–pathological characterization.
      ); however, neither develops melanomas with a sunlight-based etiology. The non-mammalian Xiphophorus fish model has been used to study the photobiology and genetics of melanoma. Although the Xiphophorus fish develops melanomas spontaneously and is also responsive to UV radiation (
      • Walter R.B.
      • Kazianis S.
      Xiphophorus interspecies hybrids as genetic models of induced neoplasia.
      ), the tumor histology differs substantially from human melanomas. The opossum Monodelphis domestica is an interesting model that also responds to UV to initiate melanoma (
      • Kusewitt D.F.
      • Applegate L.A.
      • Ley R.D.
      Ultraviolet radiation-induced skin tumors in a South American opossum (Monodelphis domestica).
      ). But the inability to derive inbred opossums ultimately limits the utility of this model for genetic and immunologic investigations. Melanomas have been reported in several guinea-pig models and in the Syrian hamster; however, the tumors were initiated by exposure to the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA) (reviewed in
      • Bardeesy N.
      • Wong K.K.
      • DePinho R.A.
      • Chin L.
      Animal models of melanoma: Recent advances and future prospects.
      ) an agent of unknown etiological relevance, and not by UV alone.
      Mice in general represent an outstanding animal model, due in large part to the extensive understanding of mouse genetics; unfortunately, melanomas are extremely difficult to initiate in mice. Moreover, like other animal models, melanomas that do arise are typically dermal in origin, and thus do not show histopathologic similarities to human disease. The dermal origin of mammalian melanomas is likely a consequence of the normal location of melanocytes within the skin. In most mammals, including mice, melanocytes are confined to the hair follicles within the dermis. In contrast, melanocytes in human skin reside in the basal layer of the epidermis. A number of mouse models of cutaneous melanoma have now been generated (reviewed in
      • Bardeesy N.
      • Wong K.K.
      • DePinho R.A.
      • Chin L.
      Animal models of melanoma: Recent advances and future prospects.
      ;
      • Merlino G.
      • Noonan F.P.
      Modeling gene–environment interactions in malignant melanoma.
      ). The model that has been most thoroughly analyzed at the molecular level arises because of expression of activated Ras and inactivation of the ink4a/arf locus (
      • Chin L.
      • Tam A.
      • Pomerantz J.
      Essential role for oncogenic Ras in tumour maintenance.
      ), already implicated as encoding a melanoma tumor suppressor.
      Recently, we have derived a melanoma model in neonatally UV-irradiated mice transgenic for hepatocyte growth factor/scatter factor (HGF/SF) (
      • Takayama H.
      • La Rochelle W.J.
      • Anver M.
      • Bockman D.E.
      • Merlino G.
      Scatter factor/hepatocyte growth factor as a regulator of skeletal muscle and neural crest development.
      ;
      • Noonan F.P.
      • Recio J.A.
      • Takayama H.
      Neonatal sunburn and melanoma in mice.
      ). HGF/SF is a multifunctional cytokine that can elicit mitogenic, motogenic, and/or morphogenic responses in a variety of cells, including melanocytes, that express its receptor tyrosine kinase c-Met. The c-Met receptor is highly expressed in many mouse and human tumors and has been implicated in oncogenesis (
      • Jeffers M.
      • Rong S.
      • Vande Woude G.F.
      Hepatocyte growth factor/scatter factor-Met signaling in tumorigenicity and invasion/metastasis.
      ;
      • Danilkovitch-Miagkova A.
      • Zbar B.
      Dysregulation of Met receptor tyrosine kinase activity in invasive tumors.
      ). A key feature of the HGF/SF-transgenic mouse is the ectopic localization of HGF/SF-expressing melanocytes to the epidermis, upper regions of the dermis, and at the epidermal–dermal junction, phenotypically more akin to human skin (
      • Noonan F.P.
      • Recio J.A.
      • Takayama H.
      Neonatal sunburn and melanoma in mice.
      ;
      • Recio J.A.
      • Noonan F.P.
      • Takayama H.
      Ink4a/arf deficiency promotes ultraviolet radiation-induced melanoma genesis.
      ). Cutaneous melanomas then arise in the UV-irradiated HGF/SF-transgenic mouse in distinct stages that resemble human disease, including grossly identifiable premalignant lesions, intermediate radial and vertical growth stages of heterogeneous histopathologies, and late metastatic spread to a variety of distant organs Figure 1 (
      • Noonan F.P.
      • Recio J.A.
      • Takayama H.
      Neonatal sunburn and melanoma in mice.
      ;
      • Recio J.A.
      • Noonan F.P.
      • Takayama H.
      Ink4a/arf deficiency promotes ultraviolet radiation-induced melanoma genesis.
      ). A single neonatal dose of mild erythemal (skin reddening) UV radiation was necessary and sufficient to induce melanoma in the HGF/SF-transgenic mouse with relatively high penetrance, arising with histopathologic and molecular pathogenetic profiles reminiscent of human melanoma (
      • Noonan F.P.
      • Recio J.A.
      • Takayama H.
      Neonatal sunburn and melanoma in mice.
      ). These data experimentally support the hypothesis based on long-debated epidemiological evidence that childhood sunburn is in fact a high melanoma risk factor (
      • Whiteman D.C.
      • Whiteman C.A.
      • Green A.C.
      Childhood sun exposure as a risk factor for melanoma: A systematic review of epidemiologic studies.
      ). When placed on a background devoid of Ink4a/Arf, the median time to melanoma development induced by UV irradiation of HGF/SF-transgenic mice was significantly reduced (
      • Recio J.A.
      • Noonan F.P.
      • Takayama H.
      Ink4a/arf deficiency promotes ultraviolet radiation-induced melanoma genesis.
      ), demonstrating that ink4a/arf plays a critical role in UV-induced melanoma development and strongly suggesting that sunburn is a significant risk factor in human kindreds harboring germ-line mutations in INK4a/ARF (
      • Bishop D.T.
      • Demenais F.
      • Goldstein A.M.
      Melanoma Genetics Consortium. Geographical variation in the penetrance of CDKN2A mutations for melanoma.
      ). Taken together, our data show that the HGF/SF-transgenic mouse represents an authentic animal model for human cutaneous melanoma.
      Figure thumbnail gr1
      Figure 1Histopathological comparison between a panel of human melanomas (left), and melanocytic lesions arising in ultraviolet (UV)-irradiated hepatocyte growth factor/scatter factor (HGF/SF)-transgenic mice (right). (a) Early human melanoma in situ in which solitary atypical melanocytes (arrow) are scattered into the upper epidermis. (b) Lesion arising in HGF/SF-transgenic mouse with same features shown in (a) (arrow). Inset: Identification of scattered epidermal cells as melanocytes (TRP1 positive). (c) Two panels showing more advanced human radial growth phase melanoma exhibiting proliferation of atypical melanocytes as solitary units (arrowhead) or nests (arrow), distributed along and above the dermal–epithelial junction. (d) Two lesions arising in HGF/SF-transgenic mice highly similar to (c) showing large nests of atypical melanocytes (arrow), and “pagetoid spread” melanoma cells into the keratinized layer (arrowhead). Inset: Identification of nested cells as melanocytes (TRP1 positive). (e) Representative human vertical growth phase melanoma exhibiting broad growth, asymmetry, and irregular distribution of melanocytes along the junction; lower magnification. (f) Invasive lesion from transgenic mice comparable with (e) exhibiting broad growth, asymmetry, and chaotic melanocyte distribution in the epidermis; lower magnification. Inset displays S100 stain of same tumor. (g) Lymph node metastasis from melanoma patient. Inset: Lower magnification of S100-positive metastatic cells. (h) Lymph node metastasis from UV-irradiated transgenic mouse. Inset: lower magnification of TRP1-positive metastatic cells. All tissue sections shown were stained with H&E unless otherwise indicated. This figure is reproduced from
      • Recio J.A.
      • Noonan F.P.
      • Takayama H.
      Ink4a/arf deficiency promotes ultraviolet radiation-induced melanoma genesis.
      , with permission from the American Association of Cancer Research.
      Although UV exposure is strongly implicated in the etiology of cutaneous melanoma, there are conflicting opinions on the roles of UVB and UVA, which differ in their ability to initiate DNA damage, stimulate cell-signaling pathways, and induce immune alterations. To address this issue, HGF/SF-transgenic mice were exposed as neonates to light from specialized optical sources, emitting isolated or combined UVB or UVA wavebands or solar-simulating radiation (
      • De Fabo E.C.
      • Noonan F.P.
      • Fears T.
      • Merlino G.
      Ultraviolet B, but not ultraviolet A radiation initiates melanoma.
      ) Figure 2. All UVB-containing light sources were found to effectively initiate melanoma. In sharp contrast, the response of transgenic mice irradiated with either isolated UVA, or a sunlamp filtered to remove the UVB, was the same as unirradiated animals. These data show that in this albino mouse model UVB is responsible for the induction of melanoma, whereas UVA is ineffective even at doses considered physiologically relevant. Data from this animal model may be relevant with respect to risk assessment from exposure to both solar and artificial UVB, and may facilitate the development of more efficacious sun protection strategies.
      Figure thumbnail gr2
      Figure 2Melanoma induction in neonatally ultraviolet (UV)-irradiated albino hepatocyte growth factor/scatter factor (HGF/SF)-transgenic mice. Shown are the Kaplan–Meier melanoma-free survival curves for HGF/SF-transgenic mice treated with pure UVA (black) or pure UVB (red). Note that UVA exposure in these mice was alone, incapable of inducing melanoma genesis; see
      • De Fabo E.C.
      • Noonan F.P.
      • Fears T.
      • Merlino G.
      Ultraviolet B, but not ultraviolet A radiation initiates melanoma.
      for details.
      In summary, the HGF/SF-transgenic mouse appears to represent a useful surrogate for human melanoma. This animal model should permit a relevant assessment of both the genetic basis for the pathogenesis and progression of melanoma in different in vivo stages and the risk associated with exposure to various candidate etiological agents and conditions. It is anticipated that identification of novel factors and/or pathways in melanoma genesis through use of this animal model will provide insight into mechanisms underlying cutaneous melanoma, as well as new avenues for more effective treatment.

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