Pyrimidine dimer formation by UVA radiation: Implications for photoreactivation

The duality of biological and biochemical effects mediated by UVA radiation complicates evaluation of its biological role. On the one hand, UVA can drive photoreactivation and prevent inactivation of a UV-irradiated organism; on the other hand, UVA clearly kills cells. We have investigated the ability of UVA to induce pyrimidine dimers in human skin in situ. Results of these studies indicate that UVA induces easily quantifiable levels of pyrimidine dimers in the DNA of human skin exposed in situ; and significant levels of dimers are induced in skin exposed to biologically relevant UVA doses (0--1 minimal erythemal dose (MED)). Also, UVA doses appropriate for photorepair may induce sufficient dimer frequencies to mask photoreactivation in biological systems, including human skin. Therefore, careful design of photoreactivation experiments is essential. The UV lamp used must not reverse or convert photodamage, nor induce additional lesions in the DNA. 29 refs., 4 figs., 2 tabs.