ELECTRON-MICROSCOPIC STUDY OF THE PHOTO-CHEMICAL CROSS-LINKING OF DNA IN GUINEA-PIG EPIDERMIS BY PSORALEN DERIVATIVES

Albino guinea pigs were treated with psoralen derivatives plus 320-400 nm ultraviolet radiation, and DNA was extracted from their epidermis. The DNA was assayed for the presence of interstrand cross-links by standard denaturation-renaturation assays and by a new technique, electron microscopy of the DNA under totally denaturing conditions. The latter method allows individual cross-links to be directly observed and counted. When either 4,5′,8-trimethylpsoralen or 8-methoxypsoralen was applied topically to the skin (8-20 μg/cm2) or administered orally (10-12 mg/kg body weight), followed by exposure to 320-400 nm ultraviolet radiation, most of the epidermal DNA was found to contain a high frequency of cross-links. For example, oral or topical trimethylpsoralen treatment gave an average of one cross-link per 250 nucleotide pairs or about 3 · 105 cross-links per guinea pig chromosome. When the dose of either drug was decreased 20-fold to the level used in the clinical treatment of psoriasis, however, no cross-links could be detected in the epidermal DNA. The electron microscopic assay is sensitive enough that we can put an upper limit of 1 cross-link per 106 nucleotide pairs (80 cross-links per chromosome) for the low dose studies. The significance of these findings to the understanding of the effectiveness of psoralens in psoriasis therapy is discussed. © 1979.