Animal model for ultraviolet radiation-induced melanoma: platyfish-swordtail hybrid

Sunlight exposure is strongly indicated as one of the important etiologic agents in human cutaneous malignant melanoma. However, because of the absence of good animal models, it has not been possible to estimate the wavelengths or wavelength regions involved. We have developed a useful animal model from crosses and backcrosses of platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri). Two strains of these fish are susceptible to invasive melanoma induction by exposure to filtered radiation from sunlamps in the wavelength ranges lambda greater than 290 nm and lambda greater than 304 nm. Multiple exposures on 5-20 consecutive days beginning on day 5 after birth or a single exposure of approximately 200 J/(m2.day) of lambda greater than 304 nm result in a tumor prevalence of 20% to 40% at 4 months of age compared with a background rate of 12% in one strain and 2% in another. Exposure of the fish to visible light after UV exposure reduces the prevalence to background. The melanomas are similar in many respects to mammalian melanomas, as judged by light and electron microscopy. The genetics of the crosses determined by others and the high sensitivity of the hybrids to melanoma induction indicate that the UV radiation probably inactivates the one tumor repressor gene (or a small number of tumor repressor genes) in the hybrid fish. The small size of the animals and their high susceptibility to melanoma induction make them ideal for action spectroscopy.