UV-induced DNA strand breaks detected by in situ nick translation in human epidermis

A mineral sunscreen affords genomic protection against ultraviolet (UV) B and UVA radiation: in vitro and in situ assays

Ultraviolet (UV) radiation has been shown to be responsible for different biological effects on human skin, including the initiation of photocarcinogenesis. Both UVB and UVA have been described as mutagenic, but the processes by which they alter the DNA are different. Although cells can repair DNA damage, some deleterious mutations nevertheless appear and can promote cancer. The risk of photocarcinogenesis is acknowledged and the frequency of photogenodermatosis is increasing. In order to evaluate the protection efficacy of a high sun protection factor (SPF) mineral sunscreen against UVB- and UVA-induced genomic alterations, we have followed two approaches. First, we have tested the sunscreen for its ability to decrease the unscheduled DNA synthesis response in vitro in human fibroblasts, as an indirect measure of UVB-induced lesions (0.005 and 0.01 J/cm2), and second, we have verified its ability to reduce the in situ end-labelling intensity in human skin as a direct measure of UVA-induced single-strand breaks (10 J/cm2). Microscopic analysis clearly demonstrated the protective effect of the sunscreen against UVB and UVA. A dose-dependent effect of mineral sunscreens was observed. There was also a relationship between the SPF and genomic protection. By limiting the accumulation of UV-induced lesions on DNA, this mineral sunscreen could limit the mutation frequency.

Demonstration of DNA damage/repair in individual cells using in situ end labelling: association of p53 with sites of DNA damage

We describe the development and application of in situ end labelling (ISEL) to identity sites of damaged DNA in the nuclei of individual cells. In cell culture, exposure to a variety of genotoxic agents induced a dose and time-dependent increase in nuclear labelling. In addition, examination of histological sections of human skin exposed to solar-stimulated UV light showed ISEL in both keratinocytes and superficial dermal cells, with the same spatial and temporal distribution as that of a marker of DNA repair, PCNA (proliferating cell nuclear antigen). Using co-localization techniques and confocal microscopy, we found increased levels of p53 in many ISEL-positive cells in vitro, with a similar distribution of labelling in the nucleus. This observation provides further evidence for a direct role of p53 in the recognition of damaged DNA. Thus, ISEL should prove a convenient method for demonstrating genotoxic insult in individual cells and in histological material, and may have value in toxicological screening. This high-resolution microscopy technique can also be used to compare the spatial distribution of various proteins implicated in the response to DNA damage with the sites of the lesion.