WAVELENGTHS EFFECTIVE IN INDUCTION OF MALIGNANT-MELANOMA

It is generally agreed that sunlight exposure is one of the etiologic agents in malignant melanoma of fair-skinned individuals. However, the wavelengths responsible for tumorigenesis are not known, although DNA is assumed to be the target because individuals defective in the repa ir of UV damage to DNA are several thousandfold more prone to the dise ase than the average population. Heavily pigmented backcross hybrids o f the genus Xiphophorus (platyfish and swordtails) are very sensitive to melanoma induction by single exposures to UV. We irradiated groups of five 6-day-old fish with narrow wavelength bands at 302, 313, 365, 405, and 436 nm and scored the irradiated animals for melanomas 4 mont hs later. We used several exposures at each wavelength to obtain estim ates of the sensitivity for melanoma induction as a function of exposu re and wavelength. The action spectrum (sensitivity per incident photo n as a function of wavelength) for melanoma induction shows appreciabl e sensitivity at 365, 405, and probably 436 nm, suggesting that wavele ngths not absorbed directly in DNA are effective in induction. We inte rpret the results as indicating that light energy absorbed in melanin is effective in inducing melanomas in this animal model and that, in n atural sunlight, 90-95% of melanoma induction may be attributed to wav elengths > 320 nm-the UV-A and visible spectral regions.