DNA interstrand crosslinks visualized by electron microscopy in PUVA-treated psoriasis.

An electron microscopic method has been used to visualize crosslinks after total denaturation of 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 enabled accurate measurement of the number and density of DNA interstrand crosslinks. 30 biopsies 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 and biopsies were obtained immediately after irradiation. 1% of the total DNA molecules examined contained 3 or more crosslinks. The overall frequency of crosslinks 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 those previously found by us in normal human skin. Thus, though the formation of interstrand DNA crosslinks can be considered as an indicator of damage of the genetic material, we were unable to demonstrate this as being a consequence of PUVA treatment using 8-MOP.