Four months after exposure to ultraviolet (UV) light, Xiphophorus hybrids had a melanoma prevalence of 20-40% compared with 2-12% in similar hybrids not exposed to UV light (Setlow et al., 1989). Wavelengths from 302 to at least 405 nm induced melanomas in Xiphophorus hybrids, even though the longer wavelengths are not absorbed directly by DNA (Setlow et al., 1993). Setlow et al. (1989) concluded that UV radiation probably inactivated tumour repressor gene(s) and thus allowed induction of melanomas. A CDKN2-like sequence is a candidate for this repressor gene (Nairn et al., 1996).
Thyroid tumours developed after thyroid cells were irradiated in vitro with UV radiation (254 nm) and then injected into Amazon mollies (Poecilia formosa) that were isogenic (Hart et al., 1977). Thyroid growths were found in most recipient fish injected with cells exposed to an average incident dose of 10-20 J m-2. Lower incidence of thyroid growths occurred in fish injected with cells having smaller or larger exposures to UV radiation. In vitro exposure of thyroid cells to photoreactivation light (360 nm) after UV irradiation prevented formation of tumours in recipient fish. Hart et al. (1977) presented several types of evidence, including transplantation of thyroid growths to Amazon mollies that were not isogenic, that these thyroid masses were neoplasms rather than goitres. However, the cells and follicles in the affected fish were well differentiated, with no indication of cellular atypia.
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