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In human cutaneous malignant melanoma, a predominance of activated mutations in the N-ras gene has been documented. To obtain a mouse model most closely mimicking the human disease, a transgenic mouse line was generated by targeting expression of dominant-active human N-ras (N-RasQ61K) to the melanocyte lineage by tyrosinase regulatory sequences (Tyr::N-RasQ61K). Transgenic mice show hyperpigmented skin and develop cutaneous metastasizing melanoma. Consistent with the tumor suppressor function of the INK4a locus that encodes p16INK4A and p19ARF, >90% of Tyr::N-RasQ61K INK4a−/− transgenic mice develop melanoma at 6 months. Primary melanoma tumors are melanotic, multifocal, microinvade the epidermis or epithelium of hair follicles, and disseminate as metastases to lymph nodes, lung, and liver. Primary melanoma can be transplanted s.c. in nude mice, and if injected i.v. into NOD/SCID mice colonize the lung. In addition, primary melanomas and metastases contain cells expressing the stem cell marker nestin suggesting a hierarchical structure of the tumors comprised of primitive nestin-expressing precursors and differentiated cells. In conclusion, a novel mouse model with melanotic and metastasizing melanoma was obtained by recapitulating genetic lesions frequently found in human melanoma.

We describe herein a new transgenic mouse tumor model in which fibroblast growth factor (FGF) receptor activity is selectively inhibited. Tyrp1-Tag mice that develop early vascularized tumors of the retinal pigment epithelium were crossed with tyrp1-FGFR1-DN mice that express dominant-negative FGF receptors in the retinal pigment epithelium to generate bigenic mice. Initial angiogenesis-independent tumor growth progressed equally in tyrp1-Tag and bigenic mice with no significant differences in the number of dividing and apoptotic cells within the tumor. By contrast, at a later stage when tyrp1-Tag tumors rapidly expanded to fill the entire eye posterior chamber and migrate along the optic nerve toward the chiasma, bigenic tumors remained small and were poorly vascularized. Secondary tumors of small size developed in only 20% of bigenic mice by 1 month. Immunohistochemical analysis of secondary tumors from bigenic mice showed a reduction of angiogenesis and an increase in apoptosis in tumor cells. Tumor cells from bigenic mice expressed high levels of truncated FGF receptors and did not induce endothelial tube formation in vitro. All in all, this indicates that the tyrp1-Tag mouse may be a useful model to study selective tumor inhibition and the effect of antitumor therapy that targets a specific growth factor pathway. FGF receptors are required at the onset of tumor invasion and angiogenesis in ocular tumors and are good therapeutic targets in this model. The bigenic mouse may also constitute a useful model to answer more fundamental questions of cancer biology such as the mechanism of tumor escape.

Phosphate and tensin homologue deleted on chromosome 10 (PTEN) is a tumor suppressor gene inactivated in numerous sporadic cancers, including melanomas. To analyze Pten functions in melanocytes, we used the Cre-loxP system to delete Pten specifically in murine pigment-producing cells and generated DctCrePtenflox/flox mice. Half of DctCrePtenflox/flox mice died shortly after birth with enlargements of the cerebral cortex and hippocampus. Melanocytes were increased in the dermis of perinatal DctCrePtenflox/flox mice. When the mutants were subjected to repeated depilations, melanocyte stem cells in the bulge of the hair follicle resisted exhaustion and the mice were protected against hair graying. Although spontaneous melanomas did not form in DctCrePtenflox/flox mice, large nevi and melanomas developed after carcinogen exposure. DctCrePtenflox/flox melanocytes were increased in size and exhibited heightened activation of Akt and extracellular signal–regulated kinases, increased expression of Bcl-2, and decreased expression of p27Kip1. Our results show that Pten is important for the maintenance of melanocyte stem cells and the suppression of melanomagenesis. [Cancer Res 2008;68(14):5760–8]

Immunization of mice with human dopachrome tautomerase (hDCT) provides greater protection against melanoma than immunization with the murine homologue (mDCT). We mapped the CD8+ and CD4+ T-cell epitopes in both proteins to better understand the mechanisms of the enhanced protection. The dominant CD8+ T-cell epitopes were fully conserved between both proteins, yet immunization with hDCT produced frequencies of CD8+ T cells that were 5- to 10-fold higher than immunization with mDCT. This difference was not intrinsic to the two proteins because comparable frequencies of CD8+ T cells were elicited by both antigens in DCT-deficient mice. Strikingly, only hDCT elicited a significant level of specific CD4+ T cells in wild-type (WT) mice. The murine protein was not devoid of CD4+ T-cell epitopes because immunization of DCT-deficient mice with mDCT resulted in robust CD4+ T-cell immunity directed against two epitopes that were not identified in WT mice. These results suggested that the reduced immunogenicity of mDCT in WT mice may be a function of insufficient CD4+ T-cell help. To address this possibility, the dominant CD4+ T-cell epitope from hDCT was introduced into mDCT. Immunization with the mutated mDCT evoked CD8+ T-cell frequencies and protective immunity comparable with hDCT. These results reveal a novel mechanism by which xenoantigens overcome tolerance. Our data also suggest that immunologic tolerance is more stringent for CD4+ T cells than CD8+ T cells, providing a mechanism of peripheral tolerance where autoreactive CD8+ T cells fail to be activated due to a lack of autoreactive CD4+ T cells specific for the same antigen. [Cancer Res 2007;67(13):6459–67]

We describe herein a new transgenic mouse tumor model in which fibroblast growth factor (FGF) receptor activity is selectively inhibited. Tyrp1-Tag mice that develop early vascularized tumors of the retinal pigment epithelium were crossed with tyrp1-FGFR1-DN mice that express dominant-negative FGF receptors in the retinal pigment epithelium to generate bigenic mice. Initial angiogenesis-independent tumor growth progressed equally in tyrp1-Tag and bigenic mice with no significant differences in the number of dividing and apoptotic cells within the tumor. By contrast, at a later stage when tyrp1-Tag tumors rapidly expanded to fill the entire eye posterior chamber and migrate along the optic nerve toward the chiasma, bigenic tumors remained small and were poorly vascularized. Secondary tumors of small size developed in only 20% of bigenic mice by 1 month. Immunohistochemical analysis of secondary tumors from bigenic mice showed a reduction of angiogenesis and an increase in apoptosis in tumor cells. Tumor cells from bigenic mice expressed high levels of truncated FGF receptors and did not induce endothelial tube formation in vitro. All in all, this indicates that the tyrp1-Tag mouse may be a useful model to study selective tumor inhibition and the effect of antitumor therapy that targets a specific growth factor pathway. FGF receptors are required at the onset of tumor invasion and angiogenesis in ocular tumors and are good therapeutic targets in this model. The bigenic mouse may also constitute a useful model to answer more fundamental questions of cancer biology such as the mechanism of tumor escape.