An electron microscopic technique has been used to visualize crosslinks after total denaturation on DNA isolated from epidermis and dermis in patients with psoriasis treated with 8-methoxypsoralen (8-MOP) and irradiation with ultraviolet light at 360 nm (PUVA treatment). This technique facilitated accurate measurements of the number and density of DNA interstrand crosslinks. 30 biopsies from 14 patients were studied and a total of 9503 DNA molecules were scored in the electron-microscope, 6 patients were treated topically with 8-MOP and 10 were on systemic treatment. Two of the patients on topical treatment had previously been on systemic treatment. 1% of all DNA molecules contained 3 or more cross-links. The overall frequency of cross-links was almost identical in the epidermis (1.1%) and in the dermis (0.9%) and, furthermore, virtually the same in patients on topical and systemic PUVA treatment. The total number of crosslinks was of the same magnitude as that previously found in normal human skin. (V. Bohr et al., Acta Dermatovener, in press.) No significantly increased damage of the genetic material (DNA) was demonstrated in our study as a consequence of the PUVA treatment using 8-MOP. We have previously shown (V. Bohr & A. Lerche, Biochim Biophys Acta, in press) that 8-MOP induced crosslinks after irradiation at 360 nm in an in vitro system of pure DNA. In this system a correlation was established between the density of crosslinks formed and irradiation time, concentration of 8-MOP, and irradiation intensity.
DNA Epidermis