Table 2.

Animal models in melanoma research.

Models			Advantages/Drawbacks	References
Xenografts	Human cell lines	Nude mice	Easily available and propagated after subcutaneous transplantation. Melanoma cell lines established under non-physiological conditions for several years may result in selection of clones that differ significantly from the originating cells and are no longer representative of the original tumor. Poorly predictive of clinical outcome: drugs showing efficacy in this model often fail in clinical trials. Growth in inadequate tumor microenvironment, including the lack of an immune system.	[151,152,153,154,155,156,157,158,159]
	PDTXs	Immunosuppressed mice Nude athymic (nu/nu) mice SCID mice NSG mice	Good availability and affordability Good representation of a comprehensive patient population with different mutation burden. Helpful in guiding clinical management of the patient’s tumor: the process from target identification to validation and then to efficacy screening can be rationalized around the same model, from the patient to the mouse and then back to the patient. Useful to determine mutations required for melanocyte transformation and melanoma cell invasion. Technically challenging and time-consuming process: time for palpable tumor to develop typically ranges from three to nine months, and in many cases, tumors fail to develop. Failure in modelling immune responses: tumors do not grow in the context of an intact immune system.	[151,160,161,162,163,164]
UVR induced		HGF⁄SF transgenic mice	Useful for simulating the natural progression of melanoma development as it occurs in human beings. Strong epidermal component, or junctional activity, with a variety of histopathologies .Neonatal UVR irradiation sufficient for induction of junctional melanoma. Exposure of neonatal animals to UVR resulting in the development of lesions resembling RGP/ VGP melanoma and invasive melanoma with junctional and dermal components. Adult UVR irradiation unable to initiate melanoma but able to increase the multiplicity of melanocytic lesions in neonatally irradiated animals. Progression from early proliferative lesion to metastasis, such as that observed for k4a deletions in humans. Useful to determine the most implicated UVR band in melanomagenesis. Difference between localization of melanocytes within the mice and human skin. Spontaneous melanoma in nearly 22% of HGF/SF transgenic mice with a mean onset of 15,6 months	[165,166,167,168,169,170,171]
Chemically induced		Mice DMBA induced TPA induced	Fully functional immune system (useful for immunotherapeutic strategies). Used in combination with other models to decrease the latency of developing melanoma. DMBA alone can induce nevi in pigmented mice, useful to study mechanism(s) of malignant transformation Lack of clinical relevance to the human disease	[172,173,174,175,176,177,178]
Syngeneic		Harding-Passey cells in BALB/c × DBA/2F1 mice S91 cells in DBA/2 mice B16 cells in C57BL/6 mice	Due to melanin production, useful to study the effects of melanin content on the metabolic function of melanoma. Intact immune system. No spontaneous metastases.	[171,172,179,180]
GEM	CDKN2A loss	Mice	Loss of CDKN2A locus located at 9p21 encoding two well-identified tumor suppressor proteins, p16INK4A and p14ARF (p19ARF in mouse) Melanomas predominantly originating in the eyes, skin melanomas infrequent and mostly benign. Ink4a or ARF loss not enough to trigger melanoma development but makes animals susceptible to UVR or carcinogen-induced melanomagenesis.	[181,182,183,184,185,186,187,188,189,190,191]
	RAS mutated	HRASV12G mice UVR/DBA induced HRASV12G mice/p16INK4a/p19ARF knockout mice (Cross breeding)	Tyrosinase-driven expression of activated HRASV12G not able to trigger spontaneous melanoma development. Induction of melanoma in a relatively short latency with concurrent UVR or DMBA treatment. A new model with the capability of developing a large number of spontaneous cutaneous melanomas with a shorter latency obtained by crossbreeding between HRASV12G and p16INK4a/p19ARF knockout mice	[192,193,194,195,196]
	PTEN loss/BRAF mutated	Mice	Tyr:Cre-ERT2 transgenic mice useful to investigate BRAF V600E mutated and PTEN deleted melanomas.	[197,198]
	RCAS/TVA system	Mice	Multiple genetic alterations introduced, through retroviral-vector delivery systems, rapidly and in a sequential manner, without the requirement of crossing multiple mice strains Rapid assessment of newly identified genes on disease progression and maintenance. Well mimicked tumor microenvironment, as cancer develops from few modified surrounded by normal cells.	[151,172,199,200]
	RET controlled	Mice	Stepwise melanoma development, RET expression driven, under the control of metallothionein-1 promoter. No tumors for several months after birth, followed by growth of multiple benign melanocytic tumors that eventually become malignant and metastasize to distant organs.	[151,199,200,201]
	GRM1	Mice	Melanocyte-specific expression of GRM1 via Dct promoter able to trigger development of spontaneous, highly pigmented melanomas in skin, eyes and ear of the animals with 100% penetrance. Useful as a spontaneous uveal melanoma model.	[202,203,204]
	GNAQ mutated	Mice	When crossed with mice defective in p16INK4A and p19ARF genes, 50% of the mice developed cutaneous melanoma with a latency of ~35 weeks, following doxycycline treatment. Melanoma not developed up to 40 weeks.	[205,206]
Fish		Platyfish and swordtails Transgenic Zebrafish	Short generation time, large number of progenies, low cost, small housing Simple genetic manipulation, with transgenes or morpholinos injected into the embryo In Zebrafish, due to the transparency of their embryos, which develop externally, high-resolution visualization of transplanted fluorescent melanoma cells in vivo is possible with relative ease	[207,208,209,210,211,212,213,214,215,216]
Avian and other mammalian		Hamster Swine Horse Gray short-tailed opossum Chick embryo	In the chick embryo model: -the neural tube transplant used as a model for spontaneous neural crest migration: while there is spontaneous neural crest migration of melanoma cells, this is not the case for primary human melanocytes -the optic cup transplant used as a model for melanoma invasion -the rhombencephalon transplant used as a model for brain metastasis	[217,218,219,220,221,222,223,224,225,226,227,228,229]

Abbreviations: ARF: alternate reading frame; BRAF: v-raf murine sarcoma viral oncogene homolog B1; CDKN2A: cyclin-dependent kinase inhibitor 2A; Dct: dopachrome tautomerase; DMBA: 7,12-dimethylbenz(a)anthracene; GEM: genetically engineered models; GNAQ: guanine nucleotide-binding protein G(q) subunit alpha; GRM1: metabotropic glutamate receptor 1; HGF ⁄ SF: hepatocyte growth factor / scatter factor; NSG: non obese diabetic (NOD) SCID gamma; PDTXs: patient-derived tumor xenografts; PTEN: phosphatase and tensin homolog; RCAS/TVA: replication-competent avian sarcoma-leukosis virus long terminal repeat with splice acceptor/tumor virus A; RET: rearranged during transfection; RGP: radial growth phase; SCID: severe immuno-deficient; TPA: 12-o-tetradecanoylphobol-13-acetate; Tyr:Cre-ERT2: melanocyte-specific inducible Cre recombinase; UVR: ultraviolet rays; VGP: vertical growth phase.